BRUNEL UNIVERSITY LONDON

PV/Thermal – Solar Assisted Heat Pump Systems in Southern Europe: Performance

Comparison of Different System Configurations Nikolaos – Alexandros Stogiannos

1037560

MSc Thesis - Building Services Engineering with Sustainable Energy

2011-2012

Abstract

Photovoltaic panels suffer performance degradation when cell temperature increases. PV/Thermal panels address this problem by cooling the cells while also preheating the water in the DHW storage tank. However, at low DHW load and/or high irradiance conditions, the temperature of the water in the tank increases excessively and the cooling advantage diminishes. PV-SAHP systems offer a solution that can deal with all issues, offering high-temperature water output and PV cell cooling simultaneously. In this project various system configurations are tested using TRNSYS simulations under the extreme Greek climate. An optimal design is established that is applicable to domestic buildings and is found to perform remarkably. An average annual COP of 4.4 and an average annual PVT efficiency of 57.3% were estimated by the simulations. The proposed system offers a 23% reduction in electrical energy, CO2 emissions and costs for a typical Greek domestic residence, compared with a conventional DHW electric system.

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Table of Contents Notation ............................................................................................................................................ 3

Glossary and Abbreviations .............................................................................................................. 4

1. Introduction .............................................................................................................................. 5

1.1. Context of the project ............................................................................................................. 5

1.2. Project Aim and Objectives .................................................................................................... 6

1.3. Brief description of contents ................................................................................................... 6

2. Technological and Theoretical Background .......................................................................... 7

2.1. The PVT panel, Past and Present .......................................................................................... 7

2.2. PVT – Basic Theoretical Background ..................................................................................... 8

2.2.1. PVT Design Aspects ....................................................................................................... 8

2.2.2. PVT Electrical Performance ............................................................................................ 9

2.2.3. Flat Plate Solar Collector Thermal Performance............................................................ 11

2.2.4. The Florschuetz PVT model .......................................................................................... 13

2.3. The Photovoltaic solar assisted Heat pump system (PV-SAHP) ........................................... 14

3. Literature Review ................................................................................................................... 16

3.1. PVT, Research and Development (R&D) ............................................................................. 16

3.2. PV-SAHP R&D ..................................................................................................................... 18

3.3. The Spiral sheet-and-tube heat exchanger........................................................................... 21

4. Data Acquisition Methodology and Result Discussion: Experimental Measurements ..... 25

4.1. Electrical Part 1: Performance characteristics ..................................................................... 25

4.1.1. Available data from the group report (Couch, et al., 2012) ............................................ 25

4.1.2. The new set of measurements ...................................................................................... 26

4.2. Thermal Part 1: Performance characteristics ....................................................................... 32

4.3. The correct procedure for proper operation of the System Rig ............................................. 37

4.4. Electrical Part 2: Pmax panel-temperature coefficient ......................................................... 37

4.5. Thermal Part 2: Improved ASHRAE curve............................................................................ 39

5. Theoretical Model Validation ................................................................................................ 45

6. Data Acquisition Methodology and Result Discussion: TRNSYS Simulations ................. 48

6.1. Introduction .......................................................................................................................... 48

6.2. Available PVT and Heat Pump TRNSYS ‘types’ ................................................................... 48

6.2.1. PVT types ..................................................................................................................... 48

6.2.2. Water-to-water Heat Pump types .................................................................................. 49

6.3. The ‘PVT-Storage Tank’ configuration .................................................................................. 49

6.3.1. Weather Data Component ............................................................................................. 49

6.3.2. Task 35 PVT component, type250 ................................................................................ 51

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6.3.3. Pump, Storage Tank and DHW profile components ...................................................... 52

6.4. The ‘directly connected PV-SAHP’ configuration .................................................................. 53

6.4.1. Initial design intention .................................................................................................... 53

6.4.2. Heat Pump implementation in TRNSYS ........................................................................ 53

6.4.3. A problematic configuration ........................................................................................... 55

6.5. The ‘Dual Tank PV-SAHP’ configuration .............................................................................. 56

6.6. Simulation Results-Discussion ............................................................................................. 57

6.6.1. Control Strategy Evaluation ........................................................................................... 57

6.6.2. System Performance Evaluation ................................................................................... 60

7. Economic Evaluation............................................................................................................. 68

8. Concluding Remarks ............................................................................................................. 69

8.1. Recapitulation ...................................................................................................................... 69

8.2. Conclusions ......................................................................................................................... 69

8.3. Recommendations for Future Work ...................................................................................... 70

9. References ............................................................................................................................. 71

Appendix A – Project Management ................................................................................................. 76

Original Aim and Broad Objectives ................................................................................................. 76

Review of Project Management ...................................................................................................... 77

Appendix B – Project Proposal ....................................................................................................... 81

1. Introduction .............................................................................................................................. 81

2. Background to the Project ........................................................................................................ 82

2.1. The PVT panel .................................................................................................................. 82

2.2. The Photovoltaic solar assisted Heat pump system (PV-SAHP) ....................................... 82

3. Aims and Broad Objectives ...................................................................................................... 83

4. Methods to be Adopted ............................................................................................................ 84

5. Specific Outcomes ................................................................................................................... 84

6. Time-plan ................................................................................................................................ 85

7. References .............................................................................................................................. 85

Appendix C – Raw Experimental Data and Graphs ......................................................................... 87

Acknowledgements

I would like to thank my supervisor Dr Dehouche for trusting me with this interesting,

practical and demanding project and I hope that I have stood up to his expectations. Moreover, I

would like to thank James Allan for his support whenever difficulties occurred throughout the course

of this project. Most importantly, I would like to thank my parents for their on-going support and the

unimaginable sacrifices that they had to make so that I could finish my MSc studies.

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Notation

Symbol Meaning Units (SI)

Pmax/QE Maximum power output W

Vmp Voltage at the maximum power point V

Imp Current at the maximum power point A

Isc Short-circuit current A

Voc Open-circuit voltage V

FF Fill Factor -

Ac Collector absorber area m2

G Total irradiance incident on a plane W/m2

ηPV Panel photovoltaic efficiency -

Glass transmittance -

Cell efficiency -

Cell efficiency at reference temperature -

Cell efficiency temperature coefficient -

Cell temperature degC

Reference cell temperature degC

Thermal efficiency of PVT -

PVT useful thermal energy J

Mass flow rate of water Kg/sec

Temperature at PVT outlet degC

Temperature at PVT inlet degC

Specific Heat J/kg K

Heat removal factor -

( ) Transmittance-absorptance product -

Overall loss coefficient W/m2 K

Ambient temperature degC

Collector efficiency factor -

Tube spacing m

Tube diameter m

Fin efficiency -

Bond conductance

Internal tube diameter m

Fluid heat transfer coefficient W/m2 K

Bond thermal conductivity W/m K

b Bond width m

γ Bond average thickness m

k Absorber plate thermal conductivity W/m K

δ Absorber plate thickness m

Cell efficiency at ambient temperature -

Tal Aluminium absorber plate temperature degC

DT Outlet-inlet temperature difference degC

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Glossary and Abbreviations

UN = United Nations

IEA = International Energy Agency

EU = European Union

EC= European Commission

FPC = Flat Plate solar Collector

PVT = Photovoltaic-Thermal panel

PV-SAHP = PVT solar assisted heat pump

MPP = Maximum Power Point

Group report = The Meng group report about the design and manufacture of a PVT panel in Brunel

University (Couch, et al., 2012)

Spiral Panel = the aforementioned PVT panel which employs a spiral sheet-and-tube heat

exchanger

System Rig = the portable rig containing all the necessary equipment to conduct experiments on

the Spiral Panel

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1. Introduction

1.1. Context of the project In today’s world there are two noticeable trends related to the energy market; the rapid

increase in global demand for energy and the continuous fluctuations of energy prices (electricity,

gas and oil primarily). The first can be witnessed in our everyday lives; world population, average

GDP and thus, energy demand, have been steadily increasing in the past and the situation isn’t

likely to change in the foreseeable future (UN, 2004; IEA, 2010). It is projected that, by 2030, global

energy demand will be approximately 40% higher compared to 2005 levels (ExxonMobil, 2010).

Fuel price fluctuations and their effects in electricity, gas and oil utility bills cannot go by

unnoticed. Such fluctuations are inevitable in a globalized market and depend on a vast number of

factors, ranging from political stability in the energy producing countries, to extreme weather

adversely affecting trade routes and wind power.

Energy importers like the EU, faced with diminishing local reserves, resort to increased

imports of traditional fossil fuels, in an attempt to boost their own power production. Import

dependency, however, has a critical consequence on the importing countries’ economies. An

extremely volatile energy market (i.e. unpredicted fluctuations in fossil fuel prices) is intensifying

these consequences and economic uncertainty among the population is the overall result.

The EU citizen’s summary of the energy 2020 targets (EU, 2010) is indicative of the global

response to the pressing issues of rising fuel prices and import dependency:

Green House Gas emissions reduction to combat climate change

Reduction in energy demands (i.e. efficient energy use)

Increase in production from alternative and sustainable energy sources

The construction sector is a renewable-application area of particular interest, as one third of

the primary global energy demand comes from commercial, institutional and residential buildings

(ASME CCTF, 2009). According to the EU’s Energy Performance of Buildings Directive, buildings

are responsible for more than 40% of the total energy consumption and related carbon dioxide

emissions (EU-2, 2010).

Among the various renewable sources, solar energy is characterized as the most abundant,

inexhaustible and clean. Policy-makers’ decisions such as the EU 2020 directive, have contributed

in driving the capital costs of solar systems down, as technologies are getting more mature and

scale production kicks in to cover the increasing demand (EC, 2010; ofgem, June 2011).

In line with the aforementioned points, this project investigates the performance of one of the

most modern building services systems. In particular, the project at hand focuses on a Combined

Heat and Power (CHP) system that can assist in covering the electricity and heat demands of a

building, while offering annual cost savings, energy and CO2 reductions to the building occupants.

The system combines two technologies considered renewable, a hybrid photovoltaic-thermal panel

(PVT) and a vapour-compression heat pump. It utilises the available solar energy to produce

electricity with increased efficiency through a cooled PVT array, as well as useful heat, by

employing the heat pump to “upscale” the thermal output of the PVT to the required temperature

levels.

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1.2. Project Aim and Objectives The primary aim of the project is to evaluate the performance of the aforementioned CHP

system, termed Photovoltaic-Solar Assisted Heat Pump (PV-SAHP), in respect with the

system’s ability to efficiently cover part or all of the electrical and domestic hot water (DHW)

needs of a typical residence in Greece.

The specific project objectives are the following:

The establishment of the performance characteristics of a novel PVT panel (referred

to onwards as Spiral Panel), that was designed and manufactured in Brunel

University (Couch, et al., 2012)

The investigation, via software simulation, of the performance of a PV-SAHP system

that is based on the Spiral Panel, while operating in Corfu Island, Greece and in

respect to a typical domestic building’s loads

The comparison of the above system with the conventional set up, in which a PVT

array is cooled by the DHW storage tank

The estimation of the annual energy, cost and CO2 savings that can be achieved by

the PVT-SAHP system in comparison to a system that is conventionally used in the

island

The conduct of an economic study to evaluate the expected capital cost and payback

period of the system

1.3. Brief description of contents Chapter 1 provides an introduction to the general context of the project and the broad

problem it addresses. Chapter 2 offers a comprehensive insight to the theoretical background

knowledge that relates with the project and serves as a preparation to the reader, in respect to what

will follow. Chapter 3 next, demonstrates the latest and most relevant scientific developments in the

field of PVT panels and PV-SAHP systems, while trying to focus on the operation and performance

problems that have been identified in the literature. Chapters 4,5 and 6 form the core of this project,

describing in detail the methodologies that were followed and the results that were retrieved,

discussing any problems that occurred and of course the meaning of the obtained results. Chapter 7

finally, presents an economic evaluation of the suggested system, in respect to the advantages it

can offer compared to a typical system. The project’s results and conclusions are presented in

chapter 8, along with recommendations for future work.

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2. Technological and Theoretical Background

2.1. The PVT panel, Past and Present The hybrid photovoltaic-thermal panel is not a new concept. The idea was first presented in

the 1970s by various researchers (Wolf, 1976; Evans & Florschuetz, 1975; Florschuetz, 1979;

Hendrie, 1979; Kern & Russel, 1978). The main idea was the creation of a hybrid panel that

combines the technologies of PV cells and flat plate solar collectors (see Figure 1).

Figure 1 – Network of different solar conversion technologies, adopted from (Zhang, et al., 2012)

The concept however, has only drawn the attention of the scientific community and decision

makers around the world in the last decade. Some significant catalysers that contributed to the

renewed interest and attention were the following:

The initiation of the PVT research program of the Energy research Centre of the

Netherlands (ECN, 2012) by the PhD Thesis of Douwe de Vries (de Vries, 1998)

The PVT Forum project, part of the EC’s PV Catapult Coordination Action (EC, 2006)

The Task 35 project, part of the IEA’s Solar Heating and Cooling Programme (IEA, 2005)

A few commercial products have appeared as a result of the above actions, but technological

advancement and market growth have been limited, mainly due to the general lack of research on

the subject and of relevant financial incentives. Nonetheless, the renewed interest of the scientific

community and the recent financial incentives provided by many countries worldwide, are very likely

to drive market and technological growth forward.

Indeed, perhaps the most interesting aspect of the PVT panel and the one most likely to drive

market growth, is the unique financial opportunity that it provides to exploit the latest incentives that

apply both to renewable electricity and heat production (e.g. Feed in Tariffs and Renewable Heat

Incentive in the UK (DECC, 2012)).

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2.2. PVT – Basic Theoretical Background

2.2.1. PVT Design Aspects The design of a PVT panel is rather simple. As defined in Task 35, a PVT device employs PV

cells or a PV panel to absorb solar radiation. A small part of this radiation, typically between 5-15%,

is converted to electrical power by the PV cells; while the remaining absorbed solar radiation, which

would otherwise be rejected as waste heat, is extracted from the PVT panel by a heat exchanger

and transferred to a circulating fluid to be utilised productively. The simplest, yet not as much

effective, design consists of a PV panel directly attached on the absorber of a conventional solar

collector.

Figure 2 - Section of a PVT panel showing the glass cover, metal frame, PV cells and liquid carrying tubes (EC, 2006)

More advanced designs involve PV cells encapsulated within a polymer resin, usually ethylene-

vinyl-acetate (EVA) and sandwiched between a low iron, high transmission, toughened glass cover

and a polyvinyl fluoride (PVF) film, usually Tedlar by DuPont (Du Pont, 2012). Behind the PVF film,

a metal plate is laminated, usually copper or aluminium, that acts as a heat sink for the PV cells.

Fluid carrying tubes, usually circular copper tubes carrying water, are attached on the back of the

metal plate and transfer the excess heat to the circulating fluid to be used elsewhere productively.

Figure 3 - The layers of a typical PVT module, adopted from (Zhang, et al., 2012)

Heat extraction allows the PV cells to operate at lower temperatures and thus higher efficiencies

(Kalogirou & Tripanagnostopoulos, 2006). In addition, the advantages in building-related

applications compared to an installation of both PV panels and solar thermal collectors are

significant:

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More effective use of valuable roof space

Modern aesthetic appearance and roof uniformity due to a single PV top surface

Lower installation costs, as less panels have to be installed

Figure 4 - Illustration of PVT advantages, adopted from (Van Helden, 2010)

2.2.2. PVT Electrical Performance The electrical performance of a PVT panel is described using the exact same equations that

are used for a typical PV panel. Accordingly, the maximum power output of a PVT panel is defined

as:

Where Vmp [Volts] and Imp [A] are the output values of voltage and current respectively when the

PVT panel is operating at its maximum power point (MPP) (see Figure 5).

Figure 5 – The I-V and P-V curves and associated parameters of a PV cell (Dehouche, 2012)

The Isc and Voc values indicated in Figure 5 correspond to the short-circuit current and the open

circuit voltage of the panel. The ratio between Pmax and the product Isc*Voc, which is the theoretical

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maximum power output, is defined as the fill factor of the panel. Higher PV cell quality yields a

higher fill factor and consequently better electrical efficiency for the PVT panel:

The electrical efficiency ( ) of a PVT panel is the ratio of the maximum power output of the

panel (Pmax) to the total incident radiation on the panel (Ac*G), where Ac [m2] is the PVT collector

absorber area and G [W/m2] is the total irradiance incident on the plane of the PVT panel:

Sometimes the detrimental effect of the glass cover of the PVT panel is taken into account in order

to assess the actual electrical efficiency of the cells ( ):

Kalogirou & Tripanagnostopoulos showed that the efficiency of monocrystalline and

polycrystalline silicon solar cells decreases by about 0.45% for every degree rise in cell temperature

(Kalogirou & Tripanagnostopoulos, 2006). This behaviour is typically modelled by the traditional

linear equation for the PV cell electrical efficiency:

( )

Where is the cell electrical efficiency, is the cell efficiency at the reference temperature

(usually 25 degC), is the efficiency temperature coefficient (usually around 0.45% as mentioned

above) and Tcell and Tref are the cell operating temperature and reference temperature

respectively. Each PV cell manufacturer provides values for and in their product

datasheets. Figure 6 below, shows the linear decrease in cell efficiency described above, for two

different PV cell types.

Figure 6 – The linear dependence of cell efficiency on cell temperature, adopted from (Zhang, et al., 2012)

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2.2.3. Flat Plate Solar Collector Thermal Performance Similarly with the equations describing the electrical performance of a PVT panel, the steady

state thermal performance is also described by the same equations that are used in traditional flat

plate solar collector analysis (FPC). In this case however, some corrections are introduced to

account for the difference in the absorber surface (i.e. the presence of the PV cells). These

corrections are the result of the work of (Florschuetz, 1979) and the latest recommended standard

on PVT panels, (IEA SHC - Task 35, 2012), suggests that they are used. The equations used in

FPC analysis are first presented below for easy reference and comparison.

The steady state thermal efficiency of a FPC is given by:

Where Qu is the thermal energy that is collected and transferred to the circulating fluid (useful

thermal energy) and is given by:

( )

Where is the mass flow rate of the circulating fluid, Cp [J/kgK] is the specific heat of the

fluid and Tfo-Tfi is the difference between the fluid temperatures at the inlet and outlet points.

The useful thermal energy is also given by the difference between the absorbed radiation

and the energy lost to the surroundings, by the mechanisms of conduction, convection and infrared

radiation:

( ) ( )

Where is the overall transmittance of the cover system (e.g. two glass covers), is the

absorptance of the FPC absorber plate, [W/m2K] is the collector loss transfer coefficient and Ta

is the temperature of the ambient. The last equation is known as the Hottel-Whillier equation and is

based on the analysis of a FPC with parallel tubes attached on the back of the absorbing surface,

commonly referred to as sheet and tube configuration, see Figure 7.

Figure 7 – A sheet-and-tube FPC with a section to show all components (Dehouche, 2012)

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The Heat Removal Factor is a measure of the FPC’s performance as a heat exchanger and is

affected only by the FPC’s design, the fluid type and the fluid flow rate. is given by:

F’ is the collector efficiency factor and which depends only on the geometric characteristics of the

FPC:

( )

Where W is the distance between the fluid carrying pipes’ centres, and are the external and

internal pipe diameters respectively, Cb is the conductance of the bond between the absorber plate

and the pipes, is the heat transfer coefficient of the circulating fluid and F is the standard fin

efficiency for straight fins with rectangular profile. Figure 8 below shows some of these parameters.

Figure 8 – Geometric parameters of a FPC (Dehouche, 2012)

The bond conductance Cb is calculated by:

Where , b and γ are the thermal conductivity, width and average thickness of the bond

respectively (see Figure 9).

Figure 9 – Close-up on the bond area and the associated parameters (Dehouche, 2012)

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The standard fin efficiency F is given by:

( )

( )

Where:

√

Here k is the thermal conductivity of the absorber plate and δ is the absorber plate thickness.

2.2.4. The Florschuetz PVT model A PVT panel’s thermal performance is worse than that of a conventional FPC, mainly due to

the following reasons:

The radiation converted to electrical power by the PV cells is ‘lost’ and cannot be converted

to thermal output

The absorptance of the PVT panel’s surface is lower than that of a typical FPC mainly due to

the presence of the PV cells instead of the selective coatings in a FPC

The heat resistance between the top most surface layer and the circulating fluid is greater

due to the additional layers of materials present in a PVT panel

As mentioned above, the equations used to describe the thermal performance of a PVT

panel are based on the classical Hottel-Whillier analysis and are the result of the work of

Florschuetz (Florschuetz, 1979). In general, all equations retain the same form with the only

difference being a modification to the values of and S ( )*G. The modifications are given by

the following equations:

(

)

( )

( )

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Where: is the cell efficiency at the ambient temperature and is the absorptance of the PVT

absorber plate. Florschuetz concluded in his work that the modified values , can be considered

identical to the unmodified values ’, for practical purposes, as their differences are negligible.

Moreover, Florschuetz provides an equation to directly calculate the electrical output of a

PVT panel based on parameters that can either be determined experimentally ( ) or

retrieved from the PV cell manufacturer ( ).

[ ( )

( )]

2.3. The Photovoltaic solar assisted Heat pump system (PV-SAHP) Heat pump technology has been available for many years and installations of both ground-

source and air-source systems can be found all over the world. However, while these devices are

potentially greener than burning fossil fuels, they do still use large amounts of electricity, especially

the air-source type in the northern and colder regions.

As was mentioned in the introduction, one of the project’s objectives is to investigate the

integration of a PVT panel with a vapour compression heat pump. This concept is fairly new and

research on the subject has only recently begun to develop. The general idea is to use a heat

pump to ‘upscale’ the thermal output of a PVT panel to useful temperatures in order to cover a

building’s hot water and space heating demands.

Although lately a great deal of research has been carried on PVT panels, attention is hardly

ever given to the fact that the thermal output of the device is low grade and water temperatures are

not likely to exceed 30 oC (Ibrahim, 2009). Temperatures like these are useful in a limited number of

applications, most notably perhaps in swimming pools. Thus, it can be understood that if PVT is to

be massively introduced to the global market, it must be coupled with another device that can

upgrade its low thermal output, so that the requirements of the more common applications can be

met. Some possible coupling configurations are illustrated in the following figures, along with the

term that the scientific community has assigned to them.

Figure 10 - PV-SAHP: the PVT panel acts as an evaporator for the Heat Pump. Two-phase refrigerant flow occurs inside the PVT

Figure 11 – ISAHP, Integrated Solar Assisted Heat Pump: the water output of the PVT is used as an input to the Heat Pump’s evaporator, directly or using a storage tank as a ‘buffer’

PVT-evaporator HeatPump StorageTank

PVT HeatPump StorageTank

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Figure 12 – SAHP, Solar Assisted Heat Pump: the PVT preheats the storage tank and the heat pump is used as an auxiliary heater

In addition, a heat pump is much more efficient in cooling the PVT panels compared to water

from a storage tank which is the usual way for PVT cooling. Unlike the alternatives, a heat pump

with a variable frequency compressor is able to maintain a set cell temperature throughout the year,

therefore ensuring high annual electrical efficiencies for the PV cells. This is only possible when the

water output of the PVT panel is used directly as an input to the evaporator of the heat pump or

when the PVT panel acts directly as the refrigerant evaporator for the Heat Pump.

The potential operation modes in such a configuration can be numerous, depending on

appropriate control systems. Constant evaporating temperature or condensing temperature can be

selected depending on the application requirements. For example, in a domestic residence where a

set hot water temperature is required, the condensing temperature would be set at around 50-60 oC.

This highly promising system has already been introduced to the UK market by Newform

Energy that claims to be the first company worldwide to offer a commercially available solution that

includes PVT panels integrated with a specifically designed water-to-water heat pump (Newform

Energy, 2012). The system has already secured eligibility for Feed-in-Tariffs (FiTs) and the

Renewable Heat Incentive (RHI), becoming thus, a very financially attractive option. Although not

mentioned by the company, it is very likely that such systems can also be eligible for financial

incentives such as the Renewable Heat Premium Payment scheme (DECC, 2012).

PVT StorageTank HeatPump

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3. Literature Review

3.1. PVT, Research and Development (R&D) As mentioned earlier, the existing literature on PV-SAHP systems is very limited.

Nonetheless, the direction and focus of the scientific community is by now apparent and some

important findings that form the basis for future work are already available. Current research in PV-

SAHP systems is heavily based on and reinforced by the existing literature on PVTs, Heat Pumps

and Solar Assisted Heat Pumps.

Although an extensive literature review on PVT technology is out of the scope of this project,

some important findings are highlighted below that are of particular interest. For a thorough

presentation of the works that led to the final design of the Spiral Panel that was created in Brunel

University, the reader is directed to the relevant Meng group report (Couch, et al., 2012).

For an up-to-date and comprehensive review of the R&D on PVT technology, the work of

Zhang provides an excellent approach (Zhang, et al., 2012). By evaluating the work that has already

been done, the writers identified the following fields as interesting future directions in R&D:

Studies on the dynamic performance of PVT systems

Feasibility studies on real buildings employing PVTs

They also noted the importance of the combination of PVTs with heat pumps and highlighted

the ability of these systems to achieve very high solar conversion efficiencies by offering a low and

steady working temperature for the evaporator-PVT. The writers also concluded that the available

theoretical PVT models are well established and can be used for research purposes.

Daghigh et al offer a more specialised review, concentrating on the advances in liquid-

cooled PVT technology (Daghigh, et al., 2011). The writers conclude that a system combining a

direct expansion heat pump, assisted by a PVT, is able to achieve a better cooling effect for the

PVT panel. They also highlighted the major problem of the conventional way of cooling PVTs via a

water storage tank (that usually also serves the hot water needs of the building); after a point in the

day, which depends on the solar irradiance at the particular area, the size of the storage tank and

the load profile, the whole volume of stored water will reach its desirable temperature and this will

then be the temperature of the cooling fluid for the PVT panel (typical water temperatures in storage

tanks vary between 40-80 degrees depending on use). The writers go on to note that when coupled

with a heat pump a steady set cooling temperature can be maintained.

Agarwal and Garg were the first to investigate the effect of the amount of water in the

storage tank that is conventionally used to draw the cooling water for PVTs (Agarwal & Garg, 1994).

The writers concluded that the mass of water affects the system performance significantly and that

cell efficiency is slightly increased with the increase in water mass. Santbergen et al also studied

the performance of a solar system based on a PVT panel for domestic hot water and power

(Santbergen et al, 2010). The writers concluded that there is a substantial loss of electrical

efficiency, compared to a pure PV installation, due to the high inlet fluid temperatures experienced

in the storage tank of the hot water system.

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Figure 13 - A classification of PVT liquid based panels by (Daghigh, et al., 2011)

With the intention of determining the absorption coefficient and the emissivity of a typical

crystalline-Silicon PV module (c-Si), Dupeyrat et al carried out reflection measurements (Dupeyrat,

et al., 2011). The global emissivity of a typical module was found to be around 0.9, whereas it is

only 0.05 for a selective-coated absorber of a solar FPC. The absorption coefficient of the PV

module was found to be around 0.85, whereas it is around 0.95 for a selective-coated absorber of a

thermal collector.

Figure 14 - Thermal efficiency curves of a standard FPC with selective coated absorber and of a typical PVT, operating at MPP and at open circuit conditions. The effect of the lower absorption coefficient for the PVT is

evident as well as the slight decrease due to the thermal power ‘lost’ to electrical conversion (Dupeyrat, et al., 2011)

Finally, a study by Christandonis, showed a remarkable potential for PVT technology in the

Greek Islands (Christandonis, et al., 2004). The writers highlighted the major problem of increasing

Air-Condition use in the EU and in particular in the Greek Islands, that experience very humid and

hot summers. This puts a lot of stress on the local autonomous diesel power stations, which are

often unable to respond to the increased load. A theoretical study using Rhodes Island climatic

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conditions showed that a PVT based system can cover a remarkable percentage of the domestic

heating and cooling demands.

3.2. PV-SAHP R&D In recent years PV-SAHP systems have become the focus of many research teams around

the world. In 2002, Badescu was one of the first researchers to use a vapour compression heat

pump in combination with PV panels and solar air heaters (Badescu, 2002). The writer noted that

during winter, the low temperature air from the solar air heaters can be used productively as a heat

source for the heat pump’s evaporator. Moreover, research results indicated that the heat pump

COP is increased and that the power consumption of the compressor is reduced up to 8%

compared to a standalone heat pump operation.

A few years later, Bakker et al simulated in TRNSYS (Klein, S.A et al, 2010) a 25 m2 PVT

system, combined with a ground coupled heat pump (Bakker et al, 2005). The system was found

able to cover 100% of the total heating load (space heating and hot water), as well as nearly all of

its own power consumption. Moreover, a capital cost evaluation showed that the system would cost

the same as a system consisting of 26 m2 of PV panels and 7 m2 of solar collectors, which

otherwise produces the same amount of electrical and thermal energy. Finally, the writers note that

the most promising market for PVTs is the residential. Domestic buildings have limited roof space

and require both heating and electricity, in ratios that correspond closely to the characteristics of

PVT panels.

After 2007, PV-SAHP systems started to attract more attention and many more studies were

conducted to investigate the performance of these systems. Ji et al experimentally analysed the

dynamic performance of a PV-SAHP system employing R22 refrigerant and a variable frequency

compressor (Ji, et al., 2008). The writers highlighted a major disadvantage of the conventional PVT

setup, using tank water for cooling, which is the reduction in performance as the temperature of the

stored water increases. They suggested that a PV-SAHP overcomes these problems. Additionally,

they highlighted one important advantage of the PV-SAHP system: the protection of the evaporator-

PVT from frosting during the winter, since it also acts as a solar absorber and thus operating

temperatures remain high. The experimental results indicated that the solar irradiance is the most

important factor for the thermal performance of the system and that higher ambient temperatures

are advantageous to the heating capacity as well. The writers concluded that the COP of the system

was found better compared to conventional heat pump systems and that PV efficiency was also

improved compared to standalone modules.

Continuing their research, Ji et al developed a mathematical model to simulate and further

study the system they build previously (Ji et al, 2007). Using their model, they conducted a

numerical analysis based on the distributed parameters approach, to better evaluate the

complicated thermal processes during the two-phase flow of the refrigerant in the copper tubes. The

simulation results were found in good agreement with the experimental measurements.

Furthermore, the model was found to provide higher precision than the lumped parameters

approach. The studies showed that the heat gain in the evaporator increases greatly with increasing

solar irradiance whereas the thermal efficiency decreases greatly. Additionally, the writers confirmed

that the advantage of PVT cooling diminishes at high irradiance levels, when conventional storage

tank water is employed for cooling. Moreover, by employing a refrigerant the operating temperature

is kept low and thus the losses to the ambient are reduced.

N.A. - Stogiannos

19

In their following work (Ji, et al., 2009), the writers present a very comprehensive literature

review on the relevant publications on two-phase refrigerant flow in evaporators, based on which

they further refined their model into greater detail and precision and compiled a new program using

the C++ programming language to perform the numerical simulations. By inputting the

instantaneous solar irradiance and ambient temperature, the numerical model is able to output the

spatial distributions of refrigerant conditions, including pressure, temperature, vapour quality and

enthalpy. A two-dimensional temperature distribution of the evaporator body is also computed. The

model was successfully validated with experimental results.

Figure 15 – Schematic diagram of the PV-SAHP set-up used by (Ji, et al., 2008)

Liu, who has also been working with Ji, extended the previous work of the group of

researchers and used the numerical model to test the performance of a PV-SAHP employing a

variable frequency compressor and an electricity-operated expansion valve (Liu, et al., 2009). The

frequency and mass flow rate of the system were set to vary accordingly, so as to follow the heat

load on the evaporator and maintain the refrigerant at a set superheated state. The writers note that

in low ambient temperatures and low solar irradiance conditions the evaporating temperature can

drop considerably and dew and frost problems can occur. This reduces the electrical and thermal

efficiency and can also damage the compressor. Moreover, at times with low solar irradiance the

refrigerant might remain in the two-phase state after exiting the evaporator due to insufficient

absorption of heat. Liquid refrigerant entering the compressor can prove very harmful to it.

Additionally, at high solar irradiance the refrigerant can get excessively super-heated with increased

losses to the ambient as a consequence. The study showed that significant improvements are

possible when using a variable-frequency compressor, with better life expectancy and energy

performance for the system.

Meanwhile, Pei, also a member of the same team, conducted experiments to investigate the

effect of an additional glass cover to the performance characteristics of a PV-SAHP system during

winter operation (Pei, et al., 2008). The study showed that although the PV exergy efficiency is

N.A. - Stogiannos

20

slightly reduced by the extra glass, the overall PVT Exergy efficiency, system COP and

photothermic exergy efficiency are improved considerably. Moreover, Chow used the numerical

model created by Ji et al to perform a dynamic simulation of the performance of a PV-SAHP system

operating in Hong Kong (Chow, et al., 2010). Weather data from a Typical Meteorological Year for

Hong Kong were used. The results were very encouraging and the system was found to have a

good potential for use in that area.

Almost at the same time, another group of researchers has been working intensely on PV-

SAHP systems. Xu et al in 2009 noted the difficulty of achieving a firm adhesion between the

absorber plate and the copper tubes in a conventional sheet-and-tube PVT collector (Xu, et al.,

2009). As an alternative, the writers developed a steady state model to study a PV-SAHP system

with an evaporator using multiport extruded aluminium tubes instead of the conventional round

copper tubes. As reported by the writers this kind of tube is already being used in automobile A/C

condensers to achieve better performance. Results indicated increased performance levels

compared to the conventional set up. Moreover, the system was shown to perform well under

annual weather data for two locations in China. Finally, variable compressor speed was found to

significantly improve system efficiency and heat output.

Fang et al designed a complex system integrating PVT, Heat Pump and Air Conditioning

technology (Fang, et al., 2010). The extensive experimental study established the stable operation

of the complex system and showed cell efficiency to improve by almost 24% compared to a

conventional PV module.

Following a more financial approach, Mastrullo & Renno also developed a numerical model

and a relevant MATLAB program to investigate the performance of a PV-SAHP and also to evaluate

the economic viability of such a system in the climatic conditions of Southern Italy (Mastrullo &

Renno, 2010).

Another major group of researchers has also focused on PVT systems, starting a bit later in

2010. Chen et al conducted an experimental study on the effect of various parameters on the

performance of a PV-SAHP system employing R134a to cool the PVTs (Chen, et al., 2011). The

writers noted the weakness of the uncovered PVT panels in general, being the increased thermal

losses during winter, when the ambient temperature can be very low. This is even more important in

the case of PVTs cooled by storage tank water, as the operating temperature in that scenario is

always high and thus, the losses always increased. A small PVT panel was therefore, constructed

consisting of six rows of PV cells, each laminated on a separate aluminium sheet-and tube

absorber. Each row was encased in a glass vacuum tube in order to increase the thermal resistance

to the ambient and thus, decrease the losses and improve the thermal efficiency. The PV rows were

connected in series to form the complete panel. The study showed that the COP increased with

increasing solar radiation but decreased with increasing condenser temperature and water flow rate.

The condenser temperature and water flow rate had little effect on the performance of the PV cells.

There is no mention however of the effect of the additional glass vacuum tube on the PV cell

performance regarding the reflection losses.

Chen & Riffat further performed a theoretical study on a PV-SAHP (Chen & Riffat, 2011).

The writers developed a numerical model to simulate the system’s performance and simulation

results indicated that both thermal efficiency and condenser heat capacity increased linearly with

increasing radiation. Electrical efficiency dropped linearly with increasing radiation and PV power

N.A. - Stogiannos

21

output increased linearly with increasing radiation. The COP of the hybrid system also increased

with increasing radiation.

The same model was used (Chen & Wei, 2011) for the theoretical study of a system which is

based on the glass vacuum tube design of Chen et al. The model was used to evaluate the

performance of the system under the climatic conditions of Nottingham, UK. The thermal and

electrical efficiencies were found to be higher compared to conventional PVT collectors. The

temperature of the PV module in particular varied between 7.7 and 27.6 oC, much lower than the

typical values reported in the literature for conventional PVT panels of about 30-50 oC. In general,

the energy performance of the whole system was not as good, mainly due to the low solar radiation

available in the particular area of Nottingham, UK. The writers note that the performance is

expected to be much higher in low latitude locations with better solar irradiation.

A different approach was presented by Zhao et al who designed a novel PV/e module that is

coupled with a heat pump and can be integrated in a building’s roof (Zhao, et al., 2011). A

theoretical model of the system was developed to examine the performance of the system. The

writers tried to investigate the effect of several parameters in order to optimise the configuration of

the system and its characteristics, such as the coil geometry, preferred evaporating and condensing

temperatures and mass flow rate of the refrigerant. The system was found to be very efficient. For

the UK climate, results indicated that lower evaporation temperature is beneficial to the overall

system efficiency, whereas the condensing temperature proved to have no effect. Additionally, the

writers suggest the use of Borosilicate as a material for the cover of the panel or alternatively

polycarbonate, with glass being the least favourable option.

Finally, Zhang et al designed a novel system combining PVTs and an air source heat pump

that uses two tanks for water storage and optimal control of the system (Zhang, et al., 2012). The

system’s high performance was established and its economic operation in comparison to

conventional systems demonstrated.

3.3. The Spiral sheet-and-tube heat exchanger The aforementioned PVT panel that was developed in Brunel employs a novel heat

exchanger that research indicated as the most efficient sheet-and-tube design (Ibrahim et al, 2009).

This presents a significant problem for the analytical modelling of the panel and the investigation of

its performance parameters. The reason is that the established Hottel-Whillier/Florschuetz model is

based on the header-riser sheet-and-tube design (see Figure 7) and the model’s assumptions are

based on the symmetry of this geometry. The most notable assumptions that do not hold true for the

Spiral Panel are the following:

The temperature gradient between the tubes is identical for each pair of tubes, as the fluid in

every tube is assumed to be at the same temperature for same height positions

The temperature gradient between the tubes is symmetrical about the centreline, effectively

meaning that the centreline is adiabatic (i.e. no heat transfer occurs on the x-axis direction)

The convection coefficient is calculated for the entry region as the length of the tubes is

generally small

These differences mean that the existing model needs to be validated against experimental data, to

establish whether it can be used for the new heat exchanger design. Furthermore, a literature

N.A. - Stogiannos

22

review is required to establish if there are other available models that are more appropriate for the

spiral design. The results of this review are presented below.

The available literature for the Spiral absorber was found to be extremely limited as far as

the author is aware. The first mention of the concept occurs in the MSc thesis project by M.A. Akgun

in 1980 titled “A Flat Plate Solar Collector with Spiral Tubing” (Akgun, 1980). The document is

referenced in (Akgun, 1988) and seems to have been published in the 3rd Miami International

Conference on Alternative Energy Sources in 1980, but the actual content could not be located by

the author.

Almost at the same time, Pillai and Agarwal investigated experimentally the thermal

performance of five different spiral designs (Pillai & Agarwal, 1980). The writers developed empirical

equations that predict the outlet temperature of the spiral collector for specif ic water inlet

temperatures. Although of limited practical use, these equations show a strong dependence of the

possible temperature rise on the spiral tube length.

Figure 16 – The Spiral heat exchanger, adopted from (Ibrahim, 2009)

The next source that could be located is much more recent and is the work of Ibrahim that

indicated the spiral design as the most effective (Ibrahim et al, 2009). Following these results, the

writers investigated in their next work (Ibrahim et al - 2, 2009) the effect of cooling fluid mass flow

rate on the performance of a PVT panel employing a spiral heat exchanger with rectangular tubes

and 90 degree bends. The writers concluded that a mass flow rate of 0.011 kg/sec at 55 oC surface

temperature provides optimal performance. In order to evaluate the thermal efficiency of the panel,

the writers used the modified Hottel-Whillier equations as suggested by Florschuetz (Florschuetz,

1979). Finally, in their latest work, the writers further investigated the performance of an array of

spiral tube panels. Results indicated a total PVT efficiency of 77% (Ibrahim et al, 2010).

Since no available theoretical models were found for a spiral absorber, the search was

extended to include models of another design, more similar to the spiral than the header-riser,

which can potentially be used either directly or after some modifications for the spiral absorber. The

closest design that is also supported by extensive literature is the serpentine design. The earliest

work to investigate this design is the work of Abdel-Khalik in 1976 (Abdel-Khalik, 1976). The author

considered the performance of the serpentine sheet-and-tube absorber and derived equations that

describe the variation of the fluid temperature in the different segments of the serpentine (See

Figure 17). Using these equations, the heat removal Factor (FR) was also determined.

N.A. - Stogiannos

23

Figure 17 – A serpentine heat exchanger (Abdel-Khalik, 1976)

The author presented the solution to the highly complex 2-dimensional problem for the case of N=2

(two pipe segments). To establish whether the solution form can be also used for higher values of

N, numerical comparisons were made. It was suggested that the maximum possible error is in the

order of 5%. In the case that the value of

is greater than one1, the error vanishes

completely. The author also concluded that for practical operating conditions this value is indeed

greater than one and thus the model can be used with increased accuracy.

Following these results, Zhang and Lavan further investigated the issue (Zhang & Lavan,

1985). The authors presented analytical solutions for N=3 and N=4, as well as an easy-to-use form

for the solution for N=2. Moreover, they showed that for N=1 the model reduces to the classic

Hottel-Whillier approach. The authors however disagree with the generalization for higher values of

N, suggested by (Abdel-Khalik, 1976) and that the error margin is only 5%. Instead, they show that

the value of FR is maximum for N=1 and minimum for N=2 and that the value increases at a

decreasing rate for higher values of N. Thus, the authors suggest that it is more appropriate to use

the FR Values for N=1 (i.e. Hottel-Whillier model) for N greater than 2, than it is to use N=2 values,

as suggested by (Abdel-Khalik, 1976).

The next researcher to deal with the matter was M.A. Akgun in 1988 (Akgun, 1988). In his

work, he presented a simple scalar solution for all values of N, produced by a simplifying

assumption. The assumption is similar to the HW assumption of no heat transfer across the

centreline between two tubes (adiabatic fins at the centreline). For the HW model this implies a

symmetric temperature distribution about the centreline. For the case of a serpentine tube absorber

this implies a steeper temperature gradient in the half-side of the colder pipe-segment (the one

closer to the inlet, see Figure 18). This assumption can be basically reproduced in reality by

thermally isolating the absorber between two pipe-segments (for example by cutting into the

absorber between the tubes and filling the cut with a thermally insulating material). The produced

1 F1 is a parameter that is a function of plate geometry and physical design

N.A. - Stogiannos

24

equation provides accurate results, in agreement with the works of the two previous researchers,

especially for N>4.

Figure 18 – Temperature gradients between the tubes for a serpentine and a header-riser absorber. Images from (Akgun, 1988)

Almost at the same time, Lund focused in his work on the comparison between header-riser

and serpentine designs (Lund, 1989). Again, among other findings, the writer concludes that the

number of segments in the serpentine has a very small effect to the value of FR.

Much more recently, in 1997, the issue was studied again extensively in the MSc Thesis of

M. Dayan (Dayan, 1997). The author presents an algorithm that employs a finite difference

technique to determine the value of FR for a serpentine absorber. The resulting values compare

favourably with the analytical solutions provided by the previous researchers (Abdel-Khalik, 1976;

Zhang & Lavan, 1985). Once more, the author confirms the tendency of FR to approach values for

N=1 as N increases. It was showed that the difference in the values of FR between a 15-turn

serpentine and a header-riser collector with the same tube spacing, is less than 3% for a flow rate of

0.002 [kg/s.m2] with the error decreasing for higher flow rates2. Based on these findings the author

suggests that for N>15 the analysis for a long straight (“Stretched”) collector (N=1) holds well, the

only exception being the difference in the value of the internal heat transfer coefficient, mainly due

to the greater length of the tube.

Finally, the established reference book on solar thermal processes ( (Duffie & Beckman,

1980)) adopts the solution offered by (Zhang & Lavan, 1985) for N=2 and points out that it can also

be used for N>2, provided

is greater than one. Moreover, the writers note that if a thermal

break is provided between the serpentine tubes, then the analysis for a normal collector can be

followed. If not, reduced performance is expected and the aforementioned solution should be used.

Following the results of the above literature review, it was decided to proceed with the

following:

Validity check of the classic HW-Florschuetz model against the experimental data for the

Spiral Panel

Same as above but with a modified fluid convection coefficient to account for the long

tubing

2 For the PVT panel developed in Brunel, the flow rate is 0.02 kg/s/m

2

N.A. - Stogiannos

25

Validation of Akgun’s (Akgun, 1988) solution (assuming a thermal break between the

tubes) against the experimental data

Validation of Zhang and Lavan’s solution in the case that

is greater than one

For all the above cases, the value of N must be decided (i.e. the number of serpentine

segments). Since the Spiral design is quite different, it was decided to use 3 different values for N

and check which one provides the best agreement with the experimental data. The first value is the

total number of straight tube segments in the spiral (e.g. for 2 bends, 3 straight segments exist, See

figure 16). The second value is the value of pipe segments in the long side of the panel and the 3 rd

is the number of pipe segments in the short side of the panel. For the Spiral Panel these values are:

Nt=21, N_long=11, N_short=11. Since the last two are the same only two different values need to

be examined. It is noted that in counting the number of tube segments, the central “S” tube is

counted as one tube and is included in both the long and short counts.

4. Data Acquisition Methodology and Result Discussion: Experimental Measurements

In order to evaluate the performance characteristics of the Spiral Panel and obtain values for

the parameters UL and FR it was necessary to perform a series of measurements. Apart from the

panel’s performance characterisation, these parameters are also required to establish the suitability

of the currently available models and to use the specific panel in any simulation studies (e.g.

TRNSYS). Since there are two different aspects in the PVT design, the electrical and thermal, both

were examined.

4.1. Electrical Part 1: Performance characteristics Regarding the electrical aspect, the goal was to establish the panel-temperature coefficients

for Pmax, Imp and Vmp and to assess the efficiency of the panel under the specific laboratory

conditions. A main interest is to show the decline in Pmax with increasing temperature so as to

demonstrate the gains that are achieved by cooling the PV cells. In addition, the cell-temperature

coefficient for Pmax ( ) is required for the complete performance characterisation of the PVT

panel (IEA SHC - Task 35, 2012).

4.1.1. Available data from the group report (Couch, et al., 2012) Some measurements were made in the context of that report and a set of results was

presented. Although many measurements were performed in cell level, no data is available for the

cell-temperature coefficients of Vmp, Imp and Pmax. There is mention however of values for Pmax,

Voc, and Isc (-0.44%, -0.329% and +0.038% respectively) of a commercial panel using the same

cells. Nonetheless, experimental measurements led to a value for the Voc cell-temperature

coefficient (-0.35%) which compares well with the reported value for the commercial panel.

A value for the Voc panel-temperature coefficient is also provided (-0.34%) which is very

close to both the cell value and the reported commercial panel’s value. It is noted that this value is

based on the aluminium absorber temperature (Tal) and not the actual cell temperature since the

N.A. - Stogiannos

26

latter is impossible to measure accurately owing to the specific design of the Spiral Panel. This is an

acceptable compromise nevertheless, as stated in (IEA SHC - Task 35, 2012).

Furthermore, although the necessary measurements were performed in the form of three

different I-V curves for 3 three different temperatures, no data is made available for the panel Pmax

other than what can be seen visually; that is that Pmax is decreasing for increasing temperature

(See Figure 19).

Figure 19 – Spiral Panel P-V curve as produced by the experiments in (Couch, et al., 2012)

4.1.2. The new set of measurements As the necessary data to continue with the objectives of this project were not available, it

was essential that new measurements were taken. Apart from the establishment of the panel

temperature coefficient for Pmax, the new measurements would also serve as an extra data set to

compare to the previous results.

The required temperature readings for the aluminium plate absorber were taken using seven

K-type thermocouples and a data logger (pico Technology, 2012). The thermocouples are spread to

the absorber’s back surface and were set to take readings at 5 second intervals, which were

summed and averaged to provide a value for the average aluminium plate temperature (see Figure

20). Special software provided by the data logger manufacturer was used for the monitoring and

recording of all measurements (pico Technology, 2012). Apart from temperature readings, an

external attachment (Terminal Board, see Figure 21) enables the data logger to measure voltage as

well. Thus, Voc readings were continuously recorded too.

Current and Voltage readings were recorded using a DC Electronic Load (see Figure 22),

referred to for convenience as ‘decade box’ (ARRAY, 2012). This instrument also enables the user

to set sequences of automatically alternating measuring points for faster I-V curve formation.

N.A. - Stogiannos

27

Figure 20 – The thermocouples that are attached on the back surface of the aluminium absorber (Couch, et al., 2012)

Figure 21 – The data logger (left) and the Terminal Board (right) that was used (pico Technology, 2012)

Figure 22 – The decade box that was used to monitor the current and voltage of the panel (ARRAY, 2012)

N.A. - Stogiannos

28

It is important to clarify here that the Spiral Panel is based on a portable rig that contains all

the necessary equipment to conduct experiments and retrieve measurements (i.e. circulating pump,

flow meter, refrigerating bath/chiller, storage tank, irradiance simulator and data logger, see Figure

23).

Figure 23 – The System Rig, image from (Couch, et al., 2012)

This rig, referred to onwards as System Rig, is quite complex and a significant amount of

time was spent to gain familiarity with its operation and to solve various problems that occurred

along the course of this project. The first one was the absence of any information on the

identification of every thermocouple reading that appears on the monitoring and logging software.

This means that although the readings were recorded normally and displayed by the logging

software on the computer screen, it was unknown to which thermocouple every reading

corresponded. Since no relevant information was available in the group report, the solution came

with the valuable help of Mr James Allan (EngD Student) who had set aside a relevant record. Even

with this record however, the rig was still explored cable-to-cable to make sure no changes have

been made since the record was made. Following the identification of all cables, a new record was

made, printed in clear and placed on the System Rig for easy reference to anyone interested.

The first measurement to be made was an I-V curve of the panel. Fifty resistance points

were chosen and sequenced on the decade box, as fifty is the maximum allowed by the instrument

for a single sequence. The points were spread on the three operating ranges of the instrument (i.e.

0.02 to 2 Ohms, 2 to 200 Ohms and 200 to 2000 Ohms). The produced I-V curve is shown on

N.A. - Stogiannos

29

Figure 24; for the complete data set please refer to Appendix C. As can be seen, almost half of the

chosen resistance points corresponded to a near-Voc state for the array of PV cells. Although the

maximum power point is sufficiently covered, there is a big gap between the MPP area and the Voc

area.

Figure 24 – The first I-V curve that was formed. The insufficient sequence point distribution is clearly shown

Consequently, a second I-V measurement was made to retrieve better results and produce a

proper curve (see Figure 25); 100 points were used this time (two sequences), taking into account

the experience from the previous experiment. It is noted here that for both experiments the absorber

temperature (Tal) was kept constant by circulating the cooling fluid at a set inlet temperature. This

way proved to be very effective in keeping a constant panel temperature, as reported in (Couch, et

al., 2012). The Power-Voltage Curve is also shown in Figure 26; the maximum power output

recorded was 14 Watts at 1.82 Ohms. For the complete data set, the reader is directed to appendix

C.

Figure 25 – The second I-V that was formed using an appropriate sequence of resistance points

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6 7

Cu

rre

nt

[A]

Voltage [V]

1st I-V Curve

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6 7

Cu

rren

t [A

]

Voltage [V]

2nd I-V curve, Isc=2.91 A, Voc=6.53 V, Pmax=14 Watt, FF=0.746,

(Tal=40 degC)

N.A. - Stogiannos

30

Figure 26 – The P-V Curve from the second I-V measurement

The normalised Voc vs. Tal curve is shown on Figure 27, along with the fitted linear equation

showing the calculated value for the Voc panel-temperature coefficient (-0.34%). This value is an

exact match to the reported Voc panel-temperature coefficient in (Couch, et al., 2012). This data set

was retrieved by turning the irradiance simulator (i.e. lamp) on, not cooling the panel and recording

temperature and Voc readings at 5 sec intervals using the data logger. It is noted here that data

sets retrieved through the data logger are not appended as they contain thousands of data points.

The reader is directed to the accompanying disc, that contains all retrieved raw data and excel

sheets that were used through the course of this project.

Figure 27 – Voc vs. Tal curve and corresponding linear-fitted curve

Figure 28 next, shows the power output of the panel vs. time for the cases ‘with and without

cooling’. The data can be found in the accompanying disc and was produced by maintaining a set

resistance value that corresponds to the MPP of the 2nd I-V curve (R=1.82 Ohm, Tal=40 degC). It

can be seen that power output decreases dramatically without any cooling, dropping a little more

than 4 watts after 2 hours (4/14 = 29% reduction in power output). It is noted that without cooling the

absorber reached almost 90 degC, while with cooling the average temperature was limited below 35

degC. For the ‘with cooling’ case, power output remains relatively the same, while a slight increase

is also evident.

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5 6 7

Po

we

r [W

atts

]

Voltage [Volts]

Power vs. Voltage

y = -0.0034x + 1.0622 R² = 0.9945

0.79

0.84

0.89

0.94

0.99

1.04

0.00 20.00 40.00 60.00 80.00 100.00

Per

cen

tage

Vo

c [d

im]

Tal [degC]

Percentage Voc vs. Tal

Linear(Percentage Voc -Tal )

N.A. - Stogiannos

31

This increase cannot be explained adequately. It is the author’s opinion that this effect is

most likely due to errors in the measurement procedure. It is also possible however that this is due

to the non-uniform cooling that creates temperature gradients throughout the panel. An image

provided by Mr Allan (See Figure 29) shows the temperature gradient as predicted by CFD

simulations and reveals that some cells are much hotter than others. This can potentially explain the

weird effect observed in the ‘with cooling’ case and it is the author’s opinion that it is worth

investigating further.

Figure 28 – Power output vs. time for the ‘with’ and ‘without cooling’ cases

Figure 29 – Temperature gradient as predicted with CFD simulations for the cooling scenario. Image kindly provided by Mr Allan.

The following figure (30), shows more clearly the reason behind the beneficial effect of

cooling in power output that is observed in Figure 28. The average temperature of the aluminium

absorber (i.e. very close to the actual cell temperature) is plotted against time for the two cases. A

huge difference of about 40 degC is evident. It is noted that this is a different data set than that in

Figure 28 and can also be found in the accompanying disc.

8

9

10

11

12

13

14

15

0 25 50 75 100

Po

wer

[W

atts

]

Time [mins]

Power@1.82 Ohm vs.Time

Pmax without cooling

Pmax with cooling

N.A. - Stogiannos

32

Figure 30 – Absorber temperature vs. time for the two cases

4.2. Thermal Part 1: Performance characteristics The established ASHRAE method was employed to evaluate the values of FR and UL for the

Spiral Panel. This simple method involves the plotting of the thermal efficiency of the panel (y-axis),

against the value of

, the so called reduced temperature difference (x-axis). The thermal

efficiency of a general flat-plate collector is given by (Duffie & Beckman, 1980):

( )

( )

Experimentally, the thermal efficiency can be determined by:

( )

Thus, by plotting the first equation using values from the 2nd, the parameters and can be

determined by fitting a linear equation to the plotted data. The y-intercept gives then the value for

( ) and the slope of the line gives the value for . Since ( ) is known, the two parameters

can be determined.

The above procedure was followed and the following graph was produced (Figure 31) using

a value of 0.0075 kg/sec for the mass flow rate of water ( ) as measured in (Couch, et al., 2012).

This value was adopted since no changes were made to the system rig that could alter the

measured flow rate. A value of 0.74 was used for ( ), adopted from (Zondag et al, 2003) as

suggested for single-glazed PVT panels. As can be seen in the graph, five thermal points,

corresponding to five different inlet temperatures, were used. These points were selected from five

corresponding 2-hour measurements with set inlet temperatures and when the system reached a

quasi-steady state. The relevant data sets can be found in the accompanying disc.

20

30

40

50

60

70

80

0 750 1500 2250 3000

Tem

per

atu

re [

deg

C]

Time [sec]

Tal 'With' and 'Without Cooling' over Time

Tin = 25

No Cooling

N.A. - Stogiannos

33

It is very important to note here that when Excel is used to fit the linear equation, the chart

type must be ‘scatter x-y’; otherwise the fitted equation is calculated incorrectly. A significant amount

of time was spent trying to figure this out, as it is not mentioned in Excel and initially, ‘line’ chart

types were used by the author.

Figure 31 – The first ASHRAE curve as calculated using 302 W/m2 as an input

Table 1 - The thermal points that were used for the first ASHRAE curve (Tamb=22.5 degC)

Tfi Tfo DT (Tfo-Tfi) Eff_th FR UL

22,09 29,58 7,49 2,12

2,8919 3,2313

27,13 34,52 7,39 2,09

30,87 37,23 6,36 1,80

35,13 41,36 6,23 1,76

38,61 44,67 6,06 1,72

Looking at the graph, the first observation is that the data do not fit very well with a linear

equation. This indicates improper measurement procedures and thus, inaccurate data readings.

Focusing next on the values of thermal efficiency, it is immediately clear that something is very

wrong, as the calculated value range is between 2.12 and 1.72, something impossible in reality.

Using the fitted linear equation shown in the graph, the values of FR and UL were found to be 2.89

and 3.23 respectively. Again, a value greater than 1 for FR is clearly wrong. This was a major

problem encountered during the course of this project, as no reason could be thought of for these

highly inaccurate results. The solution came once more from Mr Allan who informed the author that

the value for the available irradiance on the panel (302 W/m2), reported in the group project (and

adopted by the author), was wrong.

The initial, erroneous, belief of the author was that since nothing had changed in the System

Rig configuration (i.e. same lamp and at the same height and angle), the reported value for the

measured average irradiance would hold true. However, a thorough studying of the relevant group

report revealed that due to restrictions imposed by the lack of proper measuring equipment, there

was an error margin of 120% in the reported irradiance values (Couch, et al., 2012). In more detail,

y = -9.3445x + 2.14 R² = 0.8852

1.20

1.40

1.60

1.80

2.00

2.20

2.40

-0.0100 0.0100 0.0300 0.0500

Ther

mal

eff

icie

ncy

Reduced Temperature

ASHRAE - GT = 302 W/m2

eff th

Linear (eff th)

N.A. - Stogiannos

34

the irradiance measuring instrument that was used could not detect the substantial portion of

infrared radiation emitted by the tungsten-halogen lamp that is used in the System Rig. What’s

more, the spectrum of the lamp could not be accurately determined either, since the available

spectral radiometer could not detect the infrared radiation emitted by the lamp as well.

The value of 120% in the error margin was produced by using the available data from the

radiometer and a graph that provides the spectral output of another tungsten-halogen lamp, from a

different manufacturer, to fill the missing information. This graph was compared to the known

absorbance range of the measuring instrument so as to produce the error margin value (Couch, et

al., 2012). Using this correction factor (1+1.2=2.2) a new value of 664.4 W/m2 was assumed and the

ASHRAE graph remade (See Figure 32).

Figure 32 – The second ASHRAE curve, using the irradiance correction factor

Table 2 - The thermal points and extracted parameters from the second ASHRAE curve

Tfi Tfo DT (Tfo-Tfi) Eff_th Fr Ul

1,315 7,109

22,09 29,58 7,49 0,96

27,13 34,52 7,39 0,95

30,87 37,23 6,36 0,82

35,13 41,36 6,23 0,80

38,61 44,67 6,06 0,78

Again however, the values for the thermal efficiency were excessively high, although below 1

this time. The calculated values for FR and UL are 1.32 and 7.11 respectively. These are also more

realistic values than those first found, but still wrong. FR is still greater than one, something that

does not hold in reality.

Another consultation with Mr Allan revealed that the System Rig had been changed after all,

contrary to the initial belief. A metal protective grille from the lamp luminaire had been removed

since the group report measurements were made, something that obviously resulted in increased

y = -9.3445x + 0.9727 R² = 0.8852

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

-0.0050 0.0000 0.0050 0.0100 0.0150 0.0200 0.0250

Ther

mal

Eff

icie

ncy

Reduced Temperature

ASHRAE - GT = 664.4 W/m2

eff th

Linear (eff th)

N.A. - Stogiannos

35

irradiance on the PVT panel. Consequently, no irradiance value from the group report could be used

for this project’s new set of measurements.

A new irradiance measurement using the available instrument was not possible. The

instrument must be hold vertically to take readings and since its height is significant (about 15cm)

and comparable to the height of the lamp (70cm), it could not be used directly on top of the panel,

as this would result in greater (i.e. inaccurate) irradiance values than the actual ones on the PVT

surface. Combined with the unavoidable error mentioned above due to the spectral mismatch, this

approach was deemed unacceptable3.

Thus, it was decided at the time to wait for a new instrument that has been ordered, a PV

reference cell. This instrument has a very low height and can be used to take more accurate

measurements. Furthermore, since the data in any case did not fit well with a straight line, it was

decided to take completely new measurements, using the acquired experience to improve the data

gathering procedure and retrieve better results.

4.3. Thermal Part 1: Further results Apart from the above results, the first set of experiments produced a series of graphs that

helped in understanding the way the System Rig operates and also highlighted some issues that

require attention. Figure 33 shows the temperature difference (DT) of outlet and inlet temperatures

vs. time. It can be seen that the system reaches its maximum potential after some time and then

remains at an almost steady state. Seeing that the rise in DT is less than 0.5 degC after 25 minutes

(2000 to 3500 seconds), a steady state condition is a reasonable assumption. Although the

procedure was not designed for time constant measurements, a first rough indication is presented

too. The relevant data sets can be found in the accompanying disc.

Figure 33 – Graph demonstrating that quasi-steady state is eventually reached

3 It is noted here that the initial measurement of 302 W/m

2 was possible as the rig was not fully constructed

yet and the instrument could be positioned at the proper height level

-1

0

1

2

3

4

5

6

7

8

0 500 1000 1500 2000 2500 3000 3500

Tem

per

atu

re [

de

gC]

Time [sec]

DT vs. Time - Tfi avg = 25 - tc=765 sec

Tout-Tin °C

N.A. - Stogiannos

36

Figure 34 next, shows the expected decline in performance for increasing inlet temperatures.

Although the temperature range was small in this first set of measurements (see Table 1), the

decline is still evident.

The subsequent graph (Figure 35) reveals a problematic feature of the System Rig; that is

that the chiller is not powerful enough to maintain the temperature of the water in the tank to the

desired levels. In other words, the chiller capacity is smaller than the thermal output of the PVT

panel. Consequently, after some time has passed, the available volume of inlet water at the bottom

of the tank starts heating up. This affects the performance of the panel and prevents it from

reaching a steady state as the inlet temperature keeps rising. Unfortunately the thermal output is

dependent on the inlet temperature and keeps decreasing slowly as the inlet temperature is rising.

However, as can be seen in the linear fitted equations the rise is very small, in the order of 2 degC

per 4000 seconds. The exactly equal slope values, also show that this effect is purely due to the

increase in the tank, caused by the ineffective chiller.

Figure 34 – The decline in thermal performance as the inlet temperature increases

Figure 35 – Graph showing the dependence of inlet and outlet temperature rise on the rise in the storage tank

0

1

2

3

4

5

6

7

8

35.1330.8726.4327.1324.1622.09

DT

[deg

C]

AVG Inlet Temperature

DT at quasi-Steady State

DT

y = 0.0005x + 29.151 R² = 0.9914

y = 0.0005x + 21.656 R² = 0.9945

y = 0.0005x + 22.786 R² = 0.9735

15

17

19

21

23

25

27

29

31

33

0 1000 2000 3000 4000

(Tfi, Tfo, Ttank) vs. Time

392 Ch1 - Toutpanel °C392 Ch2 - TinPanel °C392 Ch4 - Ttankinside

N.A. - Stogiannos

37

4.4. The correct procedure for proper operation of the System Rig Although a lot of useful information has been acquired by the first set of measurements, the

required values for the performance characterisation of the PVT panel were still unknown. A main

issue was the lack of experience in operating the complex System Rig. Due to the intricate

interrelations between its components (i.e. the panel, pump, chiller, lamp and storage tank), the rig

operation requires some attention so as to ensure quasi-steady state conditions are reached. After

many measurements and tests however, the correct operation procedure was established and is

presented here for ease of reference4.

I. Initially the chiller must operate without the pump or lamp being on. This ensures a fast

cooling/heating of the water mass in the storage tank to the desired point

II. When the temperature in the storage tank reaches the desired set point (or is almost there),

the pump needs to be turned on to circulate the conditioned water around the rig. After a

short time the system will reach a steady state, with all points around the rig having almost

the same temperature. This also ensures a proper stratification in the storage tank and the

formation of an amount of water at its bottom, at a steady temperature, that can be used as

a constant-temperature inlet to the panel

III. Data recording begins and shortly after the lamp is turned on

IV. After around 1 hour the system reaches its maximum potential for thermal output (max

temperature difference in inlet-outlet). 90% capacity is reached much more quickly

V. At this point, enough data has been recorded to extract a thermal point (Tfi, Tfo) for use in

the ASHRAE chart. Moreover, at this point an I-V curve can be taken as the plate

temperature (i.e. cell temperature) remains almost constant

Using the acquired experience with the System Rig operation behaviour, and following the

above procedure, a new set of experiments was performed.

4.5. Electrical Part 2: Pmax panel-temperature coefficient In order to retrieve the necessary data to determine the maximum power temperature

coefficient, a series of measurements was performed. Six I-V curves were retrieved, at different

average aluminium plate temperatures, ranging from 38 to 70 degC. Since the plate is constantly

getting warmer as explained above, it was imperative that a quick procedure was followed to ensure

that the increase during the measurements was negligible. Consequently, the sequence function of

the decade box was employed. Using the experience acquired while taking the first set of I-V

curves, a sequence of 50 resistance points was created. These were selected to optimally cover the

operating range of the cells. The resulting I-V and P-V curves are shown below (Figures 36 and 37).

The relevant data sets can be found in Appendix C.

Looking at the I-V curve, the expected shifts in Isc and Voc are clearly shown. Turning the

attention to the P-V curve, the significant power loss occurring due to the increase in operating

temperature is evident. A 10.6% decrease in power output was calculated for a temperature rise of

31.83 degC.

4 It is suggested that this procedure is printed and placed at the rig in assistance to its future users

N.A. - Stogiannos

38

Figure 36 – Panel I-V Curves at different plate temperatures

Figure 37 – The loss in power output is clearly shown as the plate temperature increases

The next graph (Figure 38) shows the normalised Pmax vs. Tal curve. It is easily noticed that

there is some error in the data for the MPP. This was anticipated due to the sequence method that

was followed for reasons explained above. As the resistance points were fixed, the small shift of the

actual MPP could not be accurately detected. Thus, the selected maximum power points do not

correspond to the actual MPP of the curve but rather to the resistance point in the sequence that is

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3 4 5 6 7

Cu

rre

nt

[A]

Voltage [V]

I-V Curves at different plate temperatures

38,173

48,13

57,26

62,43

70,000

0

2

4

6

8

10

12

14

16

18

0 1 2 3 4 5 6 7

Po

wer

[W

]

Voltage [V]

P-V Curves at different plate temperatures

38,173

48,13

57,26

62,43

70,000

N.A. - Stogiannos

39

closest to the actual MPP. The direction of the MPP shift was noticed during the measurements and

the sequence was changed at the last 3 curves to better cover the expected range of values. Even

with the adjusted sequences however, errors are inevitable. There are many ways to retrieve a

more accurate I-V with the available equipment in the lab, however all of them are very time

consuming and cannot be followed for the Spiral Panel, as the rise in plate temperature would be

substantial and would distort the data. It is the author’s opinion that a MPP tracker would be the

ideal addition to the lab, as it is not overly expensive and it can provide the much needed accurate

values for Pmax vs. cell temperature.

In any case, the followed procedure produced a curve that fits well (R2=0.92) with a line and

the calculated MPP temperature coefficient is -0.36% as shown in the graph. The produced curves

for Imp and Vmp are also shown but these were more affected by the mismatch between the actual

MPP and the available point in the sequence and as such, the resulting coefficients are deemed

inaccurate.

Figure 38 – The produced curves and the corresponding temperature coefficients. Only the Power fitted line is shown to keep the graph clear

4.6. Thermal Part 2: Improved ASHRAE curve In order to retrieve sufficiently accurate data that cover the whole range of expected

operating conditions of the Spiral Panel, seven experiments were conducted, corresponding to

seven inlet water temperatures, spaced such as to cover a range of 17-to-65 degC. 17 degC is

more or less the lower limit with the available chiller. At lower temperatures the chiller cannot

maintain a quasi-steady state condition of the panel, as its capacity is reduced. The upper limit of 65

degC was chosen mainly for safety reasons, so as to limit the outlet temperature to under 75 degC.

It is important to note here, for future reference, that when operating the chiller at temperatures

above 35 degC, a switch needs to be turned, to shut down the refrigeration mode as instructed in

the device’s manual.

y = -0.0036x + 1.1354 R² = 0.9207

y = 0.0017x + 0.868 R² = 0.3012

y = -0.0051x + 1.2133 R² = 0.9119

0.8

0.85

0.9

0.95

1

1.05

30 40 50 60 70 80

No

rmal

ised

Val

ue

Plate Temperature [degC]

Normalised Pmax, Vmp and Imp vs. Tal

MPP

Imp

Vmp

N.A. - Stogiannos

40

Since the plate temperature is constantly rising, even at a low pace, a decision had to be

made concerning the selection of the data that would provide each of the seven thermal points to

plot the ASHRAE curve. Using the experience from the first set of measurements, it was deemed

appropriate to consider a range of data 10 minutes before and after the maximum DT value is

reached for each experiment. For all cases, DT reached a maximum after some time (usually 1hour)

and started decreasing afterwards due to the inlet water temperature rise. The selected timeframe

provides a good indication of the actual value for the maximum DT and was considered to be the

most accurate method for thermal point selection. Figure 39 shows the trend of the temperature

difference DT, when the chiller was set at 34 degC. As can be seen, after around one hour the

value remains fairly constant and close to its maximum value. All data sets can be found in the

accompanying disc.

Figure 39 – Graph showing the quasi-steady state that is reached after one hour

By the time the new set of experiments was performed, the new irradiance measuring

instrument had arrived (Figure 40). The reference PV cell from EETS (EETS, 2012) was used to get

a new, more accurate, irradiance value. 12 measurements were made, one for each cell position on

the PVT panel, with the instrument positioned on top of each cell. Isopropanol cleaning fluid was

used on the instrument to ensure accurate readings. A soft thin sheet was also placed underneath

the instrument to prevent any scratches on the glass cover of the panel (Figure 41). Two voltage

readings were taken each time, from the two pairs of cables of the instrument. One reading provides

the value of Isc for the reference cell and the other, the value of Voc. Using the equations provided

by the manufacturer, these readings were converted to an irradiance value. It is noted here that the

Voc reading also leads to the temperature of the cell which is used to provide a more accurate final

irradiance value.

-0.200

0.000

0.200

0.400

0.600

0.800

1.000

0 1000 2000 3000 4000 5000 6000 7000

Pe

rcen

tage

va

lue

Time [seconds]

DT Normalised, Tchiller=34 degC

N.A. - Stogiannos

41

Figure 40 – The pre-calibrated reference cell that measures the available solar irradiance (EETS, 2012)

Figure 41 – The PV reference cell, shown on top of the protective sheet and under the irradiance simulator on position No1

Figure 42 shows the identification convention for the 12 cells on the PVT panel. Figure 43

next, shows the corresponding irradiance values as measured and calculated in W/m2.

outlet

1 4 7 10

2 5 8 11

3 6 9 12

inlet Figure 42 – Number identification for every cell position

N.A. - Stogiannos

42

603,18 689,56 664,35 640,16

1005,29 977,61 930,49 840,65

555,38 575,18 551,46 485,99 Figure 43 – The corresponding irradiance values at every cell position in W/m

2

Figure 44 finally, shows the temperature recorded for each position. It is a reasonable

assumption that the actual uncooled PVT cell temperature is very close to this value, as the height

of the reference cell is very small and both devices have a single glass cover. Figure 45 also shows

the temperature distribution as predicted by a CFD simulation (image provided by Mr Allan). The

image has been cropped and rotated for easy comparison with Figure 44. The agreement between

the images, in respect to the hotter areas in the panel, is clearly visible.

63,2 77,3 92,0 102,3

63,2 81,1 90,6 95,3

81,6 95,3 104,7 100,0 Figure 44 – The recorded reference cell temperatures in degC

Figure 45 – Temperature distribution as predicted by CFD simulations (image provided by Mr Allan)

The average irradiance on the panel was measured to be 723 W/m2 and this value was used

for the ASHRAE method. The seven thermal points that were selected are shown in Table 3, along

with the corresponding set temperature of the chiller. The average ambient temperature during the

experiments was recorded to be 22.5 degC.

Table 3 - The thermal data pairs that were used to form the ASHRAE curve

Tfi Tfo Tchiller

1st 17,23 27,37 11,48

2nd 26,29 35,75 20,94

3rd 30,61 39,73 25,32

4th 38,69 47,09 33,8

5th 45,89 52,70 45,24

6th 55,71 61,78 54,55

7th 65,26 70,35 64,1

N.A. - Stogiannos

43

Figure 46 shows the produced ASHRAE curve; the accuracy of the retrieved data and

selected thermal points is evident by the excellent fit with a straight line (R2=0.98). The calculated

values for FR and UL were 1.564 and 6.186 respectively. Once more, the values calculated for the

thermal efficiencies and the parameter FR are clearly wrong. The reasons for this were explained in

the previous sections; although the PV reference cell is more accurate for solar irradiance

measurements than the instrument used in the group report, it is still ineffective for measuring the

output of a tungsten-halogen lamp which produces an excessive amount of deep infrared radiation,

well beyond the sensitivity limit of the PV cell (1100 nm).

Figure 46 – The ASHRAE curved produced by the seven experiments

To account for the shortcomings of the equipment, the correction factor (2.2) was used, as is

suggested in the group report (Couch, et al., 2012). The decision was based on the fact that this

factor was calculated taking into account the absorption range of the measuring instrument that was

used at the time, which has a silicon-based sensor (i.e. same absorbing surface as the reference

cell).

Thus, a new irradiance value was calculated (1591 W/m2) and used to produce another

ASHRAE curve (Figure 47).The re-calculated values for FR and UL were 0.711 and 13.614

respectively. The thermal efficiency at ambient temperature was calculated to be 53% which is

considerably low. All three parameters indicate a very inefficient panel, an outcome that was not

expected. Comparing with literature, very simple and inefficient PVT designs have better

performance parameters than the above. Since there is no apparent reason for such bad

performance (especially with zero wind conditions at the lab the value of UL should be much

smaller), it is reasoned that the irradiance value is wrong again, this time too excessive.

y = -9.6733x + 1.1571 R² = 0.9821

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

-0.0200 0.0000 0.0200 0.0400 0.0600 0.0800

Thermal efficiency vs. reduced temperature (Gt=723 W/m2)

eff th

Linear (eff th)

N.A. - Stogiannos

44

Figure 47 – The new ASHRAE curve produced with the corrected irradiance value

After all the above, it was realised that even if a more accurate correction factor is calculated

(more appropriate to the new instrument), the error margin would still be very high. The reason for

this is that the correction factor calculation is unavoidably based on diagrams of the spectral output

of the lamp and of the absorption range of the PV cell. There are two main reasons that this

approach is inaccurate; first, no diagram is available for the specific lamp that is used in the System

Rig (a 1000 kW tungsten-halogen lamp that was ordered from eBay). The only alternative is to use

a diagram of a similar lamp which obviously incurs errors. Moreover, no information is available for

the absorption range of the new instrument. However, since the absorber is a normal PV cell,

values from literature can be used, but again at a reduced accuracy. The second and much more

important reason is that information in the aforementioned diagrams is limited and specific values

must be chosen arbitrary, based on the user perception (See Figure 48 for example, for the

suggested lamp diagram). All the above will inevitably lead to an inaccurate value for the available

irradiance, even if more realistic (or desirable) values are achieved for the Spiral Panel. Thus, it was

decided to end this part of the project, concluding that with the available equipment and timeframe,

the FR and UL parameters cannot be accurately evaluated.

It is the author’s opinion that an accurate determination of the PVT panel’s performance

characteristics can only be achieved by conducting measurements under direct sunlight using the

PV reference cell. In this way, the accuracy of the instrument is guaranteed and realistic values can

be retrieved. The alternative is to acquire new equipment, a much more costly solution.

y = -9.6733x + 0.5258 R² = 0.9821

0.000

0.100

0.200

0.300

0.400

0.500

0.600

-0.0060 -0.0010 0.0040 0.0090 0.0140 0.0190 0.0240 0.0290

Thermal efficiency vs. reduced temperature (Gt=1591 W/m2)

eff th

Linear (eff th)

N.A. - Stogiannos

45

Figure 48 – The spectral output of a 1000 kW tungsten-halogen lamp. It is obvious that individual values for each wavelength band must be selected completely arbitrary (Newport.com, 2012)

5. Theoretical Model Validation

As described in section 3.3, the literature review on the available theoretical models for spiral

and serpentine absorber designs, produced a set of four action points, in order to compare the

available models against the experimental data that was produced.

Although the exact course of actions was already planned and all necessary equations

available at the relevant papers (Florschuetz, 1979; Dayan, 1997; Akgun, 1988; Zhang & Lavan,

1985), the validation procedure could not proceed. Even though the experiments produced data for

the inlet and outlet temperatures of the Spiral Panel, the required values for FR and UL could not be

accurately evaluated. Without these, model validation was not possible.

This unfortunate development does not mean of course that the work done is in vain. The

literature review and course of actions that it produced can be readily used, once the FR and UL

parameters are established by future projects. Moreover, if the system rig is not altered in any way,

the produced experimental data of this project can also be readily used, provided that an accurate

value for the lamp’s irradiance is obtained by appropriate instruments.

The list of parameters presented below (Table 4) is another outcome of the preliminary work

done on model validation. These parameters can be either found scattered in the group report

(Couch, et al., 2012), calculated or taken from literature and are presented here for the convenience

of future System Rig users. The cells in the table are colour-coded: Orange means that the value is

based on the arbitrary selected water temperature, brown that the value is a constant design

parameter and blue that the value depends on the mass flow rate of water in the System Rig. The

source of the values is also given: ‘Lit’ corresponds to values commonly found in any heat transfer

handbook (fluid properties etc.); ‘G.R’ refers to the group report (Couch, et al., 2012) and ‘C’ means

N.A. - Stogiannos

46

that the value is calculated from other parameters in the table. Calculated values that depend both

on fluid temperature and mass flow rate have both colour-codes.

As a final note, the equations used for the calculated values are not repeated here as can be

found in any heat transfer or solar engineering handbook. For easy reference, the reader is directed

to the group report that conveniently presents all equations (Couch, et al., 2012).

Table 4 – Spiral Panel and System Rig Parameters

Parameter Symbol Value Units Source

Inlet temperature Tfi 15 degC Arbitrary

Outlet temperature Tfout 25 degC Arbitrary

Mean fluid temp Tfm 20.00 degC Arbitrary

Water conductivity kw 0.603 W/mK Lit

Water sp heat cap Cp 4183 J/kgK Lit

water density ρ 998.2032 kg/m3 Lit

water viscosity μ 0.001002 kg/m/sec Lit

Glass Thickness 0.004 m G.R

EVA Layer Thickness 0.0004 m G.R

Solar Cell Layer Thickness 0.00018 m G.R

EVA Layer Thickness 0.0004 m G.R

Tedlar Layer Thickness 0.0005 m G.R

Aluminium Plate Thickness δ_abs 0.002 m G.R

Total Thickness of panel 0.00748 m C

Spacing between pipes W 0.0377 m G.R

Aluminium Conductivity k_abs 237 W/mK Lit

Collector Area A_col 0.366432 m2 G.R

Glass transmittance τ_glass 0.91 G.R

Total Tube Length L 8.5812 m G.R

External tube diameter D 0.0127 m G.R

Tube wall thickness δ_tube 0.000914 m G.R

Internal tube diameter Di 0.010872 m C

Tube cross sectional area A_tube 0.0000928 m G.R

mass flow rate water mdw 0.0075 kg/sec G.R

mass flow rate water mdw 27 kg/hr C

mass flow rate water mdw 73.6835 kg/hr/m2 C

water velocity V 0.081 m/sec C

water volume flow rate Vdw 0.000007514 m3/sec C

water volume flow rate Vdw 0.45081 l/m C

bond conductivity k_b 2 W/mK G.R

bond average thickness γ 0.001 m G.R

bond width b 0.0127 m G.R

Bond Conductance Cb 25.4 C

Insulation Resistance Rins 4.2 m2K/W G.R

Copper conductivity k_cop 401 W/mK Lit

Absorber plate Absorptance a_p 0.9

(IEA SHC

N.A. - Stogiannos

47

Absorber plate Emittance e_p 0.8 - Task 35,

2012)

Extinction Coefficient-thickness product KL 0.0648 dim C

K for Glass K 16.2 1/m (IEA SHC - Task 35,

2012)

Cell area Acell 0.292032 m2 G.R

Gintech PV Cell Nominal efficiency 0.164 G.R

Packing factor 0.797 dim C

Transmittance-absorptance product (τα) 0.74 dim (Zondag

et al, 2003)

Reynolds Number Re 876.91 C

Prandtl Number Pr 6.95 Lit

(Di/L)*Re*Pr 7.72 C

Nusselt Number Nu 4.81 C

Water heat transfer coefficient hfi 266.64 W/m2K C

Overal Loss Coefficient UL ? W/m2K

Heat Removal Factor FR ?

N.A. - Stogiannos

48

6. Data Acquisition Methodology and Result Discussion: TRNSYS Simulations

6.1. Introduction The second and core part of this project is the investigation of a PV-SAHP system’s

performance, in respect to covering all of the DHW needs and part of the electricity needs of a

typical domestic residence in Corfu Island, Greece. Initially, it was also desired to include space

heating requirements in the simulations. Yet, no space heating profiles, typical of a residence in

Greece or in any location with similar climate, could be found by the author. Unfortunately, the

source for the DHW and electrical profiles, IEA’s Annex 42 (IEA, 2012), does not offer space

heating profiles too. Moreover, a published paper refers to space heating profiles (Hawkes, et al.,

2009), based on the information found in Annex 42 but no information whatsoever is provided as to

how the profile was created and of course neither is the profile itself (such profiles are offered in

excel or text file format and extend to hundreds of pages).

In any case, various system configurations were tested to identify any real-world-operation

related problems and to conclude on a configuration that is both efficient and applicable to the

Greek domestic sector. After establishing an efficient configuration, the effective annual efficiencies

of the two components (i.e. PVT array and heat pump) were evaluated while operating in realistic,

non-laboratory, conditions.

As revealed by the literature review, a main problem related to non-laboratory applications of

PVT panels, is the availability of a low temperature cooling fluid. While the advantageous effects of

PV-cooling are well established, few studies deal with this matter as mentioned in section X. It is

easily understood that if the cooling supply is not sufficient, the temperature of the inlet water will

rise excessively and PVT efficiency, both thermal and electrical, will drop dramatically.

A typical system configuration in a domestic residence would include the PVT panels and a

hot water storage tank. While in winter the system might be sufficiently efficient, in summer and

especially at low DHW load times the water temperature in the tank (i.e. cooling fluid for PVT)

increases excessively. Thus, either a bigger tank needs to be employed, or another cooling source

be found. The problem remains even if space heating is added to the load, as this is non-existent

during summer. Hotels and other applications that have a significant hot water load throughout the

daytime might not face the same issues. Domestic residences, however, require a different

approach. A mixing valve can always be used to ensure a low inlet temperature but using potable

water to cool the PVT panels cannot be considered a sustainable and efficient solution.

6.2. Available PVT and Heat Pump TRNSYS ‘types’ An important first step before starting a TRNSYS project is to make sure that the software

can model sufficiently all system components.

6.2.1. PVT types Regarding the PVT panel, the basic TRNSYS suite offers a component, namely ‘type50’, as

coded and referred to by TRNSYS users. Additionally, ‘type560’ is also offered as part of the

optional ‘Electrical Component’ TESS Library (trnsys.com, 2012). The component that was decided

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to be used, ‘type250’, is the suggested type by Task 35 and the relevant PVT standard (IEA SHC -

Task 35, 2012). This component is basically an improved version of type50 and employs the Hottel-

Whillier analysis to model a PVT collector. To use this component, an external file was added to the

TRNSYS installation which is provided in the Task 35 website, accompanied by the relevant

installation instructions. Additionally, a complete TRNSYS project for the ‘PVT-Storage Tank’

configuration was created in (Couch, et al., 2012); nevertheless this was based in type50 and it was

decided to recreate the project from the beginning; this would also serve as a practice with TRNSYS

use.

6.2.2. Water-to-water Heat Pump types As a heat pump is a much more complex device, with intricate interrelations between its sub-

components, TRNSYS uses a different approach to model it’s a behaviour. Since manufacturers

readily provide diagrams or tables with performance data, the software makes use of these data to

simulate a specific commercial heat pump. Thus, the device’s behaviour is not predicted by a

theoretical model, as is in the PVT case, but by using actual performance measurements. The most

usual form of these data gives the heating capacity (condenser), cooling capacity (evaporator) and

power consumption of the device, as a function of inlet water temperatures in the evaporator and

condenser. The relevant basic TRNSYS component is type42. Another version also exists, type927,

part of the TESS Libraries (trnsys.com, 2012). As far as the author is aware, the two components

operate on the same principle with the only difference being that the second offers a greater degree

of usability and convenient features. For the purposes of this Project, type42 was decided to be

used.

6.3. The ‘PVT-Storage Tank’ configuration The first step of the simulation studies was the correct setup of this simple configuration. The

configuration includes a PVT panel array that is cooled by a DHW storage tank, a pump to circulate

the water in the circuit and a temperature-differential controller to decide when the pump operates

(see Figure 49). The figure below shows only the basic components of the system; other auxiliary

components such as equations, graph plotters and data printers are not shown to keep the image

clear. It was decided that the pump would operate whenever there is a minimum of 2 degC

temperature difference between the PVT cell temperature and the temperature at the bottom of the

storage tank (i.e. the PVT inlet temperature). This ensures that the pump only operates when there

is useful thermal energy to be collected from the PVT array.

6.3.1. Weather Data Component In order to simulate the specific weather conditions of Corfu Island, component type54 was

employed. This component acts as a weather generator, in that it takes weather data based on

monthly averages and creates hourly data using special algorithms. The use of this component was

necessary as there is no weather file available in the literature for the island of Corfu, as far as the

author is aware. As mentioned in the component’s description in TRNSYS, ‘The data are generated

in a manner such that their associated statistics are approximately equal to the long-term statistics

at the specified location’. The final outcome of this component is a Typical Meteorological Year

(TMY) of data for the specified location. It is important to note here that the algorithms of this

component are based on a temperate climate, as is the climate in Greece.

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Figure 49 – The basic components of the ‘PVT – Storage Tank’ TRNSYS project

The weather data that was inputted to this component were retrieved from a technical

guideline that is published by the Technical Chamber of Greece (referred to as ‘TEE’ from its Greek

initials (TEE, 2012)). This institution is the equivalent of the Engineering Council in the UK, in that it

encompasses all engineering disciplines and acts as the state’s advisor on all things technical. The

technical guidelines it publishes provide the ‘good practice’ standard in Greece and are widely used

by the country’s engineers. The particular document is ‘TOTEE 20701-3/2010, Climatic Data of

Greece’. It is noted here that this document is not made available freely but provided to the users of

TEE’s own software for building energy performance studies. As the author owns this software, a

copy of the document was available. Table 5 below shows the data for Corfu Island, as provided in

the document; these were used as input parameters to the TRNSYS weather generator.

Table 5 – Weather Data for Corfu Island, as provided in the technical guideline TOTEE 20701-3/2010

Month Mean Daily

Temperature [degC]

Mean Daily Wind Speed

[m/s]

Mean Global Horizontal Radiation

[kwh/m2/month]

Mean Daily Global

Horizontal Radiation

[kJ/m2]

1 9.7 2.5 57.7 6701

2 10.3 2.8 73.5 9450

3 12 2.6 116.7 13552

4 15 2.2 149.9 17988

5 19.8 1.8 195.4 22692

6 24 1.9 213.6 25632

7 26.5 1.8 221 25665

8 26.5 1.8 197.8 23736

9 22.7 1.7 148.2 17210

10 18.5 2.1 103.1 12372

11 14.3 2.6 64.4 7479

12 11.1 2.7 50.7 6084

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A radiation processor, type16, is also used in the simulation. This component receives as an

input the Global Daily Horizontal Radiation from the weather generator and outputs the total

radiation on a selected tilted surface; this total includes the diffuse, direct and ground reflected

radiation on the plane as calculated by the component’s algorithms. The values of 30 degrees for

the slope of the PVT collectors and 0.15 for ground reflectance were adopted from the

aforementioned technical guideline, as suggested for the island of Corfu. The location’s latitude was

also set to 39 degrees corresponding to Corfu’s latitude.

6.3.2. Task 35 PVT component, type250 This component simulates an array of PVT panels. The initial intent was to use an array of

Spiral Panel’s for all simulations. However, as the performance characteristics could not be

accurately evaluated, as already mentioned, a mix of parameters were used for the PVT array.

Essentially, the intent was to use all the available parameters of the Spiral Panel and complete the

rest from literature.

For the Collector Efficiency Factor (F’) the value of 0.96, reported in (Couch, et al., 2012),

was used. For the PV cell efficiency, the value of 16.4% was used, adopted from the same source.

For the maximum power temperature coefficient, the default TRNSYS value (-0.004) was employed,

as this is a reasonable compromise between the value indicated by the experimental studies of this

project (-0.36%, see section 4.4) and the reported value (-0.44%) for a commercial panel in (Couch,

et al., 2012). The value of the extinction coefficient-thickness product (KL) was set at 0.0648

corresponding to a glass thickness of 4 [mm] (Couch, et al., 2012) and a K-value for glass of 16.2

[1/m], adopted from (IEA SHC - Task 35, 2012). A value of 0.797 was used for the PV packing

factor (Couch, et al., 2012) and finally, the total area of the array was selected to be 4.76 m2,

corresponding to 13 Spiral Panels. All other values for which no information was available were left

at the component’s default values, that are considered typical by (IEA SHC - Task 35, 2012). Table

6 below shows all the parameters that were used in the model.

Table 6 – Input parameters to the PVT component

Parameter Value

Collector Area [m2] 4.763616

Collector Efficiency Factor (FP) 0.96165216

Fluid Heat Capacity [kJ/kg/K] 4.190

Number of Glass Covers 1

KL Product 0.0648

Back and Edge Loss Coefficient [W/m2/K] 10

PV Absorptivity 0.9

PV Emissivity 0.8

PV Efficiency 0.164

PV Temperature Coeff -0.004

PV Ref Temperature [degC] 25

PV Packing Factor 0.797

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52

6.3.3. Pump, Storage Tank and DHW profile components A 50W capacity pump was selected, similar to the one used at the System Rig (Couch, et

al., 2012). The flow rate was set at 0.011 kg/sec, a value adopted from (Ibrahim et al - 2, 2009) who

tested a spiral collector and concluded that at this flow rate the capacity is maximised.

It is very important to note here that the components used in TRNSYS to simulate PVT

panels, do not model the fact that the thermal output reaches a maximum at a certain mass flow

rate value. Thus, the user must carefully select the mass flow rate value as a high value will result in

an excessively high and unrealistic performance. In other words, as long as the user increases the

flow rate value, TRNSYS will also increase the capacity of the panel, something that does not hold

true in reality.

The hot water load profile was acquired from (IEA, 2012) . In the context of that project,

namely Annex 42, hot water and electrical load profiles were formed, that correspond to a typical

European domestic residence (see Figure 50). The DHW load selected for the Project’s simulations

is based on a 200L consumption of a single family house at 45 degC and is provided at one minute

intervals. TRNSYS component type9 was used to read an external data file that contains the DHW

profile information and feed it in the tank storage as a load.

Figure 50 – Annual average DHW load for a weekday. The morning and evening peaks related with the domestic occupancy pattern can be clearly seen (IEA, 2012)

Finally, the tank in the configuration was selected to be of 300L capacity and 2m high. The

volume selection is based on the daily DHW load of 200L. Ten different nodes model the

temperature stratification in the tank and an auxiliary heater (i.e. electric resistance) was used to

ensure that the temperature of the top five nodes (i.e. the water to the user) is maintained at 45

degC. The replacement fluid coming from the ‘tap’ is set at 18.1 degC as is suggested for Corfu

Island in the technical guide mentioned above.

The ‘PVT-Storage Tank’ configuration was simulated to gain familiarity with TRNSYS before

proceeding with the more complex configurations and primarily for comparison reasons with the

configurations presented below.

IEA Annex 26 - Weekday (annual average)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00

DH

W C

on

su

mp

tio

n [

l]

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6.4. The ‘directly connected PV-SAHP’ configuration

6.4.1. Initial design intention As mentioned in section 3.2, the latest interest of the scientific community is to use heat

pump refrigerant to directly cool the PVT panels. Initially this was the intended configuration to be

simulated and evaluated. However, it was soon clear that TRNSYS does not offer the necessary

components to model each part of a heat pump separately, especially the ‘unusual’ evaporator/PVT

panel. As such, it was impossible to assess this configuration with the selected software and other,

simpler set ups were preferred. It is important to note here that even if this configuration could be

modelled, it is far from being commercially available and as such, of limited interest as a solution for

the residential market.

Consequently, it was decided to model a system employing a water-to-water heat pump that

uses a PVT array as a heat source (or alternatively an array that is cooled by a heat pump’s

evaporator). The difference with the initially intended approach is that the evaporator is now part of

the heat pump (i.e. any commercially available heat pump can be used) and water, instead of

refrigerant, is used as a heat exchanging fluid between the PVT array and heat pump evaporator

(see Figure 51). As can be seen, all components remain the same as with the ‘PVT-Storage Tank’

configuration, with the exception that the heat pump has been added between the PVT array and

the storage tank.

Figure 51 – The directly connected PVT array and heat pump

6.4.2. Heat Pump implementation in TRNSYS The heat pump was particularly problematic to implement as it cannot be modelled by a

single TRNSYS component. The main component is type42, as mentioned above, that reads an

external file containing the performance data of the heat pump and outputs the heat pump cooling

and heating capacities depending on the condenser and evaporator water inlet temperatures, which

it receives as an input.

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54

When type42’s operation was understood, it became apparent that performance data for an

actual, commercially available heat pump should be acquired. Thus, a specific heat pump was

selected, keeping in mind that the application is domestic and that the desired water temperature is

45 degC. The market research revealed that water-to-water heat pumps are not very common, with

systems designed for residential applications even more rare. A particular issue was with the

available capacities, the smallest being 10 kW. The reason for this is that domestic heat pumps are

intended to cover both DHW and space heating loads. With the only load being the DHW in this

project’s simulations, 10 kW is considered excessive.

Figure 52 – The ClimateMaster Tranquillity High Water heat pump

Nonetheless, the model ‘Tranquillity High Temperature (THW)’ from ClimateMaster

(ClimateMaster, 2012) was selected, as the available data for this model were very detailed and the

operating range of temperatures one of the most broad in the market. The minimum evaporator inlet

temperature is -6.6 degC and the maximum 43.33 degC. For the condenser, the values are 10 and

48 degC respectively. These limits are provided by the manufacturer for safety reasons but they

also mark the range of the published performance data.

This means that the device can only be modelled correctly by TRNSYS in the

aforementioned temperature range. It is noted here that values are interpolated by type42 for the

temperature range between the limits, but cannot be extrapolated. The manufacturer also

specifically states that data extrapolation is not allowed. This means that if a situation occurs during

the simulation in which a water stream of 50 degC (i.e. higher than the 48 degC limit) enters the

condenser, TRNSYS will provide an output that is wrongly calculated. In fact, TRNSYS will treat the

water stream as if it was 48 degC, since it only has data up to this maximum temperature. If this

occurs often during a simulation, it will lead to an overestimation of the heat pump’s performance,

as it would be modelled like operating at more favourable conditions.

As such, it is required to control the condenser and evaporator inlet conditions during the

simulation to avoid miscalculations. This control of course, also models the integrated safety feature

of the heat pump that shuts down the device if these limits are violated.

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6.4.3. A problematic configuration At this point, it was realised that this configuration presents issues, as it is impossible to

control the inlet temperature at the evaporator. The condenser conditions are controlled by design,

since it is required to maintain the water in the top of the storage tank at 45 degC. Thus the water at

the bottom of the storage tank, which serves as the inlet to the condenser, will always be at lower

temperatures, well below the 48 degC upper limit for the condenser.

The lower limit of the evaporator however, cannot be maintained. Whenever the heat pump

operates (i.e. at times of DHW use by the occupants), the evaporator needs a heat source to absorb

heat from. During sunny conditions this heat can be provided by the PVT array. During low

irradiance conditions, though, and especially during night-time, there is no heat source available to

the evaporator. What happens at these times, is that the evaporator keeps cooling the volume of

water in the evaporator-PVT circuit, well below its operating limits and well beyond freezing

conditions (see Figure 53). The small volume of water (compared to the storage tank for example)

that circulates in the circuit gets cooled extremely fast by the evaporator of the 10-kW heat pump.

The problem remains of course even if a heat pump with smaller capacity is used, as at low

irradiance and low ambient temperature conditions the PVT array cannot serve as a heat source for

any reasonable sized of evaporator.

Figure 53 – Evaporator inlet temperature for the first week of January. Y-axis range is from -130 degC to 80 degC

This is a very serious problem with this configuration that severely limits its application range

to only sunny periods in a day. It is also easy to understand that this problem also holds true (even

more so) when refrigerant flows directly through the PVT panel. For the above reasons, this

configuration is deemed ineffective (if not impossible) for domestic applications. A dual source

evaporator (Ji, et al., 2008) can be used to avoid this problem, using air as a heat source whenever

there is not enough solar irradiance. A combination of a PVT and a ground loop (Bakker et al, 2005)

can also provide a solution; nonetheless these options are well beyond the scope of this project and

most importantly, are considered too expensive for the first case and too demanding in land area for

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56

the second, as the majority of residencies in Greece do not have the required land for a ground loop

(excluding new constructions that can use the space underneath the building).

6.5. The ‘Dual Tank PV-SAHP’ configuration As the ‘directly connected’ configuration proved inappropriate, a new configuration was

thought of that can deal with the encountered problems. With the previous configuration, when the

heat pump is operating, the strong cooling capacity of the evaporator freezes the small water

volume in the circuit to an excessive degree. This cooling capacity vanishes when the heat pump is

inactive, which creates an extra problem: the lack of available cooling for the PVT array when the

heat pump is not operating (i.e. at low or no load periods). Thus, a solution needed to be found that

reduces the excessive cooling provided by the evaporator, while at the same time offers a steady

supply of cool water to the PVT array. Moreover this had to be accomplished using equipment

appropriate for a domestic application. As mentioned before, ground loops, open water loops and

cooling towers are not an option for domestic applications in Greece.

A simple idea that seemed to provide a solution to all problems was the use of a secondary

water storage tank that would exploit the high heat capacity of the stored water, to both absorb the

excessive cooling of the evaporator and provide a steady supply of cool water to the PVT array. The

simple mechanism of stratification in the storage tank can create the much needed water sources; a

high temperature heat source for the evaporator at the top of the tank, supplied by the PVT array

during daylight and/or high ambient temperature conditions and a low temperature heat sink

(cooling supply) for the PVT array at the bottom of the storage tank, cooled by the evaporator

whenever the heat pump is operating.

It is highlighted here that these temperatures cannot be controlled. As the thermal output of

the PVT array depends on the solar radiation and the operation of the heat pump on the DHW

profile, it is understood that the temperatures in the storage tank are random during the day.

Nonetheless, the volume of water in the storage tank helps in maintaining an average temperature,

not too low for the heat pump and not too high for the PVT array.

A clarification perhaps is needed here about the high ambient temperature conditions

mentioned above. As shown by the simulation of the ‘directly connected’ configuration (Figure 53),

the evaporator’s capacity over-cools the available water. Even when using a storage tank, the

temperature can drop below the safe limit of the evaporator at low irradiance/high DHW load

conditions and even if the safe limit is not reached, lower evaporating temperatures result in lower

heat pump efficiency. Thus, when the water in the bottom of the storage tank reaches a temperature

below the ambient by some degrees, the PVT arrays can act as a heat source even during night

time. It is reminded here that ambient temperatures in Greece can be very high especially during

spring/summer seasons and even at night time.

The following image (Figure 54) shows the complete TRNSYS project for this configuration.

All components, including the auxiliary ones, are shown here to demonstrate the complexity of the

project.

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Figure 54 – The complete TRNSYS project for the final configuration

In Figure 54 a flow diverter can be seen above the secondary storage tank. This was added

to control the temperature of the top of the tank (i.e. the inlet to the evaporator), so as not to exceed

the operating limit of the heat pump (43.33 degC). Moreover, this helps in fully exploiting the

potential of the PVT array even during the sunniest summer days: whenever the temperature of the

top storage tank part reaches 43 degC, the flow is diverted to the primary storage tank that contains

the water to cover the DHW load. This means that heat pump use is significantly reduced during the

summer months and when it is used (to cover perhaps sudden high load demands); it operates at

an increased efficiency, since the evaporating temperature is very high.

Although TRNSYS can monitor and output a multitude of variables and parameters, the

performance indicators that best describe the system’s overall performance are the effective annual

efficiencies of the PVT array and heat pump. In order to evaluate these values, a TRNSYS

component was employed that integrates the selected power outputs during a desired time-step.

Thus, the thermal and electrical outputs of the PVT array were monitored, in addition to the power

consumption of the circulating pump and the available solar irradiance. Moreover, the heating

output and power consumption of the heat pump were also monitored. These variables were

integrated on a monthly and annual basis to provide both seasonal and annual performance

information. To ensure increased accuracy, the simulations were run on 1-minute intervals.

6.6. Simulation Results-Discussion

6.6.1. Control Strategy Evaluation A series of graphs were produced and are presented below to demonstrate the system’s

performance. The first concern was to establish that the simulation is working as intended and that

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58

no errors are encountered during the simulation. Regarding the operation of the heat pump, it was

necessary to ensure that its operating limits are not exceeded.

The following graph (Figure 55) shows the annual variation of the evaporator inlet

temperature. The Y-axis values (temperatures) are between -10 and 45 degC. As can be seen, the

flow diversion strategy described above successfully maintains the inlet temperature between the

limit ranges of -6 to 43.33 degC, with the exception of only three spikes that reach almost -10 degC.

This rare occasion however is considered to have a negligible effect on the simulation.

Figure 55 – The evaporator inlet temperature during the year

It is important to note here that TRNSYS cannot plot, or output in general, the values of

variables that are used as an input to a component. Consequently, the above shown temperature is

actually the temperature of the water at the top of the secondary storage tank, which is fed into the

evaporator whenever the heat pump works. Since the heat pump is not operating 24/7 throughout

the year, the actual evaporator inlet temperature corresponds to whatever temperature the water in

the storage tank is at the time the heat pump operates. As the actual inlet temperature cannot be

examined, it is sufficient to examine the temperature of the heat source (i.e. top of secondary

storage tank). Since this remains between the limits throughout the year, so does the actual inlet

temperature.

The next graph (Figure 56) shows the condenser inlet temperature throughout the year with

the Y-axis range between 15 and 60 degC. The upper limit of the condenser inlet temperature is

satisfied by design. The reason is that the heat pump only operates whenever the temperature to

the load drops below 45 degC, as this is the assumed load temperature in the Annex 42 load profile

(IEA, 2012). Consequently, the inlet temperature to the condenser (i.e. the temperature of the

bottom part of the primary tank) will always be below the 48 degC upper limit. Indeed it will be much

lower due to stratification, considering that the tank is 2m high and holds 300L of water. The lower

limit of 10 degC is also satisfied as can be seen in the graph.

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Figure 56 – The annual variation of condenser inlet temperature

The satisfaction of the upper limit is better shown in the following graph (Figure 57). The

green spikes correspond to the heating power of the heat pump and basically mark the specific time

that the device operates. The size of the image is not helping, but if carefully examined, it can be

seen that whenever the heat pump operates, the inlet temperature is well below the limit of 48

degC.

Figure 57 – Graph showing the actual condenser inlet temperature when the heat pump operates

Lastly, the following graph (Figure 58) shows the temperature of the water available to the

users. It can be seen that the temperature generally remains between 45 and 48 degC. The

substantial increase during summer, when the PVT array has increased thermal output, is also

evident. This however hints to a weakness of the created TRNSYS model. It can be easily

understood that if the temperature of the available water is increased, the users will inevitably use

less hot water and more cold (from the ‘tap’), as they are only interested in achieving the same

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comfortable temperature, regardless of the temperature of the hot water. This reduction in demand

was not modelled in the simulation and it basically means that the system’s overall efficiency will be

underestimated. A special algorithm could be created perhaps, that adds this behaviour in the

simulation; however this was not possible in the timeframe of this project.

Figure 58 – Annual DHW temperature variation

6.6.2. System Performance Evaluation Continuing with the actual performance indicators, Figure 59 below shows the annual

variation of the inlet temperature to the PVT array (i.e. the temperature at the bottom of the

secondary storage tank). The Y-axis range is -10 to 40 degC to clearly demonstrate that the

configuration succeeds in providing a steady supply of cool water to the PVT array. It can be seen

that the temperature mostly remains between 0 and 30 degC, with the range increasing to 10 to 35

during the summer season.

The consequences of the above are presented in the next graph (Figure 60) that shows the

cell temperature annual variation, as calculated by TRNSYS type250 component. The Y-axis range

is between -10 to 65 degC. It can be seen that except summer, the cell temperature remains below

50 degC, while during summer, it is limited below 65 degC. For comparison reasons, in (Couch, et

al., 2012) it is reported that cell temperature reaches 90 degC during noon in January and 120 degC

in July, for uncooled PV panels under the much less sunny UK climate.

Moreover, Figure 61 shows the annual cell temperature variation for the simple ‘PVT-

Storage Tank’ configuration. The upwards shift is clearly visible, as temperature peaks are more

often close to 50 degC and exceed 65 degC during summer. The lower temperature range remains

the same for both cases as this corresponds to the night-time cell temperatures which are equal to

the ambient for both cases, as this is the way that type250 treats a zero-irradiance situation.

Since the cell temperature graphs are not clear enough for our purposes, as they also

contain the night-time temperatures, Figure 62 also shows the annual variation of the PVT array

inlet temperature for the ‘PVT – Storage Tank’ case. The Y-axis range is the same as in the ‘Dual

Tank PV-SAHP’ case for easy comparison (at -10 to 40 degC). The difference between the

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effectiveness of the two configurations is now clearly visible (see Figures 59 and 62), as for the

second case the temperature constantly remains between the 20-40 degC temperature range.

Figure 59 – Annual variation of the PVT array inlet temperature

Figure 60 – PVT cell temperature variation throughout the year

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Figure 61 – Cell temperature for the ‘PVT-Storage Tank’ case

Figure 62 – PVT Array inlet temperature for the ‘PVT – Storage Tank’ case

Moving on to the energy outputs and effective efficiencies of the two configurations, Figure

63 shows the total thermal energy added to the primary storage tank, by the PVT array and the heat

pump, versus the thermal output of the heat pump, in a monthly basis and in kWh units (left axis).

Thus, the area difference between the two lines shows the share of the PVT array’s thermal output

in heating the primary storage tank. The blue line corresponds to the heat pump’s share and it is

apparent that the heat pump alone covers the load during winter and autumn. At these seasons, the

thermal output of the PVT array is used exclusively as a heat source for the heat pump’s

evaporator. During summer however, heat pump use is limited and the biggest part of the load is

covered by the PVT array.

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Figure 63 – Heat pump thermal output vs. the total energy provided to the primary storage tank

The next graph (Figure 64) reiterates this, showing the thermal output of the heat pump

(blue) vs. the thermal output of the PVT array (green). Again, the dominance of the PVT array

during the sunnier seasons is clearly visible. It can also be seen that during winter, the output of the

PVT array is almost equal to the heat pump one. This might lead someone to think that the PVT

array could cover the load during the winter on its own. Moreover, comparing Figure 63 and 64, it

can be seen that for most months, it looks as if the PVT array could cover the load without any help

from the heat pump. This however, is a misconception as the operation of the heat pump and the

cooling it provides to the secondary storage tank is exactly the reason why the thermal output of the

PVT array is so high. One need only look at the huge difference between the PVT inlet

temperatures for the two configurations mentioned above, to realise how much worse the PVT

thermal output is without the heat pump.

Figure 64 –Heat pump vs. PVT thermal output in a monthly basis

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The last graph on a monthly basis shows a very important and interesting result (Figure 65).

The graph shows the electrical energy output of the PVT array during each month (light blue) vs. the

consumption of the heat pump (orange) and the consumption of the pump (pink). Excluding the

winter months, it is clear that the PVT array can fully cover at least the power consumption of the

heat pump. During the winter months, the electrical output is very close to the heat pump

consumption. This indicates that whether the PVT electrical output is stored in batteries or sold to

the grid, it can cover the expenses of the whole system in an annual basis.

Figure 65 – PVT electrical output vs. Heat Pump consumption and Pump consumption

This is much more clearly shown in the total annual graph in Figure 66, showing the same

variables. The PVT electrical output clearly surpasses the consumption of the system. To be exact,

the simulation indicated that the total electrical energy produced by the PVT array throughout the

year is 876 kWh, whereas HP consumption is at 446 kWh and pump consumption is at 242 kWh.

Thus, a total of 688 kWh of electrical energy are consumed, leaving 188 kWh extra to be utilised

elsewhere. This result is of major importance, as it effectively means that the system can fully cover

its own consumption and consequently, freely cover the DHW load, while also offering 188 kWh of

free electrical energy. Of course, this does not mean that the system will not draw electrical power

from the grid, but that at the end of the year the building occupants will not only pay for the system’s

operation but indeed gain a bit of extra income.

Another conclusion that is derived from these results is that the consumption of the 50 Watt

pump is comparable to that of the 10 kW HP. The reason for this is that the pump operates for many

hours throughout a day, while the HP only when there is a significant DHW load.

Finally, Figure 67 shows the annual DHW energy consumption (red) vs. the output of the

heat pump (blue) and the total energy delivered to the tank (pink). This graph effectively confirms

that the DHW load is fully covered by the system.

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Figure 66 – Total annual electrical output vs. HP and pump consumption

Figure 67 – Annual total energy required for the DHW load vs. the HP and total energy inputs to the primary tank

To proceed to a more quantitative representation of the system’s performance, all relevant

values regarding the annual performance of the system are presented in Table 7 below. For

comparison, the relevant parameters are shown in Table 8 for the ‘PVT – Storage Tank’ case. The

ineffectiveness of the second configuration is evident.

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Table 7 – Annual performance indicators for the ‘Dual Tank PV-SAHP’ configuration

Parameter Energy [kWh]

Total Energy delivered to tank 2535

Energy delivered to tank from the PVT array 1007

Heat Pump Thermal Output 1968

Heat Pump Consumption 446

Heat Pump COP 4.41

PVT thermal output 3847

PVT electrical output 876

Solar energy delivered to PVT array 8239

PVT thermal efficiency 46.70%

PVT electrical efficiency 10.60%

PVT total efficiency 57.30%

Pump consumption 242

Electrical output in excess to HP and pump consumption 188

Table 8 - Annual energy data for the ‘PVT-Storage Tank’ configuration

Parameter Energy [kWh]

Solar Energy delivered to the PVT array 8239

PVT Electrical Output 838

PVT Thermal Output 2378

Pump Consumption 74

Thermal efficiency 28.9 %

Electrical Efficiency 10.2 %

Total PVT Array Efficiency 39.1 %

The monthly PVT array efficiencies are shown in Figure 68 and the monthly PVT array

outputs in Figure 69. Finally, the monthly variation of the HP’s COP is shown in Figure 70. As can

be seen, the system achieves excellent performance, with total PVT efficiency remaining above

50% throughout the year and heat pump COP remaining above 4. Considering the difficult climate

of Greece with extreme solar irradiances and ambient temperatures, the achieved performance is

remarkable. Comparing the consumed electrical energy by the system (688 kWh) and its total useful

output (Total energy delivered to primary tank and electrical output=2535+876=3411 kWh), the

system’s efficiency is 496% or 4.96, in an energy paid vs. energy delivered basis.

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Figure 68 – Monthly variation of PVT array efficiencies

Figure 69 – Monthly variation of PVT array output and Pump consumption

Figure 70 – Heat Pump COP monthly variation

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

1 2 3 4 5 6 7 8 9 10 11 12

Month in Year

PVT array monthly efficiencies

PVT eff_th

PVT eff_tot

PVT eff_el

0.00

100.00

200.00

300.00

400.00

500.00

1 2 3 4 5 6 7 8 9 10 11 12

Ener

gy [

kWh

]

Month in Year

PVT array monthly outputs and Pump consumption

Eth

Eel

Epump

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1 2 3 4 5 6 7 8 9 10 11 12

Month in Year

Heat Pump COP monthly variation

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7. Economic Evaluation

The final part of this project is a simple economic viability assessment of an investment on a

‘Dual Tank PV-SAHP’ system. Since the simulations have accurately produced the annual energy

outputs and consumptions of the system; and the DHW and electrical loads are known from (IEA,

2012), the only parameters required for the economic evaluation, are the capital costs of the system

components and the cost of the electrical kWh in Greece.

The manufacturer of PVT panels ‘Solimpeks’ was contacted to enquire about typical panel

prices (Solimepks, 2012) and they were kind enough to respond. A price of 350 euros per panel

was quoted as typical. This price concerns a 170-Watt panel and thus, the price per watt is 2.06

euros/watt. For the simulated system of 13 Spiral Panels, the total power output is: 13 panels * 17

Watts/panel = 221 Watt. The value of 17 as maximum power output is based on the experimental

studies (see Section 4.4).Thus, a capital cost of 455 Euros would be expected for the array of PVT

panels. Adding to that 150 euros for a 300 watt, 12V inverter (UnlimitedSolar, 2012), the total comes

to 755 euros.

To acquire a typical value for a 10 kW water-to-water heat pump, one of the largest HVAC

dealers in Corfu was contacted. They responded that prices are anywhere between 3000 and

10,000 euros, depending on manufacturer brand and available features (e.g. integrated controls that

improve efficiency). For a typical domestic application however, they suggested the price of 6000

euros. Thus, a total of 6755 euros can be considered a typical capital cost for the simulated system.

The suggested typical electrical load for a 5-member family by Annex 42 is 8387 kWh (IEA,

2012). The annual DHW load of 200 kg/day*365 days=73000kg , at 45 degC is calculated to 2294

kWh, using a temperature of 18.1 degC as ‘tap’ temperature, as mentioned in section 6.3.3. The

following equation is used:

( )

The total electrical consumption of a typical 5-member family is thus 10,681 kWh, assuming

here an electric water heater which is the typical system in Greece. The simulations indicated that

the system not only fully covers its consumption, but also provides 188 kWh of extra electrical

energy. Thus, the 2294 kWh DHW load is freely covered, assuming that the price for exported

electricity is the same as the cost of electricity, to take into account the difference in time between

the output of the PVT array and the actual system consumption.

As such, the new annual load with a PV-SAHP system would be: 10681-2294-188=8199

kWh. This means that a total of 2482 kWh are saved, equal to a 23.2% reduction in electrical

energy use. The price of the electrical kWh in Greece is 0.165 euros/kwh (DEI, 2012) and the CO2

emission factor for electricity is 0.981 kgCO2/kWh (ypeka.gr, 2012).

Consequently, it is calculated that 409 euros and 2455 kg of CO2 are saved annually. Since

all variables are in electrical kWh, the same reduction holds true for the cost and CO2 savings

(23.2%). With a capital cost of 6755 euros, the simple payback period for the investment is 16.5

years. No grants have been taken into account in this economic study, as with the current

unfortunate economic situation in Greece, all such grants have been revoked.

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The major reason nonetheless for this unfavourable outcome, is that the heat pump is

designed for greater loads (i.e. to cover both space and DHW loads). Consequently, obtaining such

a system to only cover the DHW load would be controversial. As mentioned before, commercial

heat pumps rarely come at smaller capacities, as they are primarily intended to cover all heating

loads of a residence. Unfortunately, as there is no typical profile available for the space heating

demands, the true economic potential of the ‘Dual Tank PV-SAHP’ system cannot be assessed.

It is the author’s opinion that a case study with actual space heating loads, either domestic

or commercial, would be very interesting to be conducted in the future.

8. Concluding Remarks

8.1. Recapitulation The literature review that was conducted revealed some problems with the real-world

application of PVT panels, the two most important being the detrimental effect of the temperature

rise of the water in the storage tank that is used to cool the panels and the inability of the panels to

produce high-temperature water. Moreover, the review resulted in a set course of actions to validate

the available theoretical models of serpentine absorbers, against the experimental data for the

Spiral Panel. This course of actions however was impossible to follow, as the required performance

parameters FR and UL of the Spiral Panel, could not be accurately determined.

The experimental studies led to the identification of the System Rig shortcomings and their

cause. The determination of the correct operation procedure which results in accurate data retrieval

was also achieved. Although a set of accurate thermal data was retrieved, these did not lead to the

determination of the thermal performance parameters, due to the inability to accurately measure the

available irradiance. The experiments also established that the System Rig reaches a quasi-steady

state condition, which is sufficient for the retrieval of accurate data. Finally, it was assessed that the

available irradiance in the System Rig cannot be accurately evaluated with the available equipment

in the laboratory.

The simulation studies revealed that a typical commercially available heat pump cannot be

directly connected with a PVT array, as this inevitably leads to freezing conditions in the evaporator.

Two other configurations were simulated and their performance compared

8.2. Conclusions The electrical performance parameters of the Spiral Panel were determined: a value

of -0.34% was found for the Voc panel-temperature coefficient and a value of -0.36%

for the maximum power panel-temperature coefficient. The Spiral Panel was also

found to produce 17 Watts at a plate temperature of 38 degC. The thermal

parameters could not be accurately determined

The proposed ‘Dual Tank PV-SAHP’ system overcomes all obstacles and was found

to be very efficient. The system can more than cover its own annual power

consumption while fully covering the DHW load. Annual PVT-array efficiency was

estimated at 57.3% (46.7% thermal and 10.6% electrical) and annual heat pump

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COP at 4.41. PVT efficiency never drops below 50% and heat pump COP never

drops below 4. Considering the harsh Greek climate of extremely high solar

irradiances and ambient temperatures, the annual performance of the system is

remarkable

The typical ‘PVT-Storage Tank’ configuration achieved an annual PVT efficiency of

only 39.1% (28.9% thermal and 10.2% electrical). The proposed system’s superiority

over the typical ‘PVT-Storage Tank’ configuration is thus established

The proposed system was found to offer annual cost savings of 409 euros, annual

energy savings of 2482 kWh and annual CO2 emission savings of 2455 kgCO2. The

corresponding reduction compared to a typical system is 23.2% for all three

parametersh

The capital cost of the system was estimated at 6755 euros, with a simple payback

period of 16.5 years. The long payback period found is attributed to the oversized

heat pump, which nonetheless, is one of the smallest available in the market

8.3. Recommendations for Future Work The thermal performance characteristics of the Spiral Panel can be established by

accurately measuring the available irradiance in the System Rig, using appropriate equipment. The

readily available data retrieved during this project, can then be used to determine the two

parameters FR and UL. To avoid expenses, experimental measurements can be conducted under

direct sunlight, employing the new PV reference cell to accurately determine the available solar

irradiance.

After determining the two parameters, validation of the serpentine models can be completed

using the initial plan that was formed for this project and the parameter table presented in section 5.

Additionally, further investigation of the increase in power output while plate temperature

increases for the ‘with cooling’ case is suggested.

Finally, further refinement of the TRNSYS model for the ‘Dual Tank PV-SAHP’ system is of course

possible and desirable. It would also be of particular interest to conduct simulations including an

actual space heating load, if such data can be found.

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Appendix A – Project Management

Original Aim and Broad Objectives The primary aim of the project was to perform a feasibility study for a PV-SAHP system, in

order to establish the system’s ability to efficiently cover part or all of the electrical and heating

loads of a typical domestic residence in Greece.

The initial broad objectives were the following:

The design, modelling and analytical as well as numerical testing (using TRNSYS) of

a PV-SAHP system based on a novel PVT module that was manufactured in Brunel

(Couch, et al., 2012)

the experimental investigation of the PVT module’s performance under the actual

(reproduced in the lab) weather conditions of the particular building site

the analytical and numerical investigation of the PV-SAHP system’s performance

under the same weather conditions and in respect to the actual building’s loads

the establishment of the annual energy, cost and CO2 savings that can be achieved

by the system, in comparison to the present system that is used in the building

an economic study to evaluate the expected capital cost and payback period for the

PV-SAHP system, taking into account any financial incentives provided at present

time from the local government

the provision of information on novel sustainable systems to the local authorities,

stake holders and community

The specific expected outcomes of the project were the following:

Gain an understanding of the current research on PV-SAHP systems and on the

available analytical and numerical models in the literature.

Gain an insight in the methods used in the literature in using real world load and

weather data in evaluating the performance of PV-SAHP systems.

Perform a full experimental study on the PVT’s electrical and thermal performance

under the particular weather conditions. Acquire I-V curves, temperature coefficients,

loss coefficients and efficiencies of the PV and thermal components and in general

explore the behaviour of all relevant parameters.

Perform analytical and numerical simulations to establish the energy performance

and saving potential of the proposed PV-SAHP system.

Assess the financial viability of the system, including expected capital cost, payback

period and annual cost savings.

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Review of Project Management Many unexpected setbacks and problems were encountered through the course of this

project that nonetheless did not lead to a noteworthy departure from the initial project aim and

objectives. The most important issue is found in the inability to accurately determine the thermal

performance parameters of the Spiral Panel and thus, sufficiently characterise its thermal

performance.

Considering things from the start of the project, the initial interest was to investigate the

performance of a system combining a PV/Thermal panel and a vapour compression heat pump. A

second important aspect was added to this that concerned the establishment of the performance

parameters of the Spiral Panel, both for validation of the initial set of results from (Couch, et al.,

2012) and also to be able to use the specific PVT panel in the investigation of the PV-SAHP

system. As a case study, a typical domestic residence in Greece was selected; this satisfied the

interests of the author while also being an ideal case to evaluate and demonstrate the advantages

of the system, at a climate where normal PV cells suffer a severe degradation in their performance.

The first setback occurred at the very beginning of the experimental measurements. The 14

thermocouples at the System Rig provided readings to the data-logging and monitoring software,

but it was unknown to what parameter they corresponded (e.g. inlet temperature, tank temperature

etc.). The readings come with an identification number that corresponds to one of the eight input

ports at the data logger. However, it was not known to what parameter the thermocouple at every

port corresponded to, as the cables were mixed and twisted together and various covers and

protective casings on the rig made it impossible to see where each thermocouple led to (i.e. what it

measured). No relevant information could be found anywhere on the System Rig or the relevant

report (Couch, et al., 2012). Some attempts were made to identify the readings by varying the

experiment conditions and noticing what readings changed, but only a few could be identified in this

way. The solution came with the help of Mr James Allan, EngD student, who was also working with

the System Rig, both at the time of the group report and simultaneously with the author: a hand-

written record was kept with all necessary information. Even though the record seemed to

correspond well with the shown readings on the logging software, it was decided to explore and

check every cable one by one to remove any doubt. Thus, the cables were separated and any

obstacles removed from the System Rig so that a proper identification could be done.

The next setback occurred due to the lamp in the System Rig solar simulator being blown. A

replacement lamp was readily available on the rig, so the problem was dealt with immediately. The

lamp blew again however, about a week after and no more lamps were available this time. A

decision was taken at that time to directly order two more lamps from eBay and override the

relevant university procedures to save time. This was vital both to this project but also to Mr Allan as

he was also working on the rig. Consequently, the author ordered 2 lamps and these were soon

received and used.

A major problem that had an effect on the direction of this project was the inability to

accurately measure the solar simulator’s output, or in other words, the available incident irradiance

on the Spiral Panel. Initially the reported value in (Couch, et al., 2012) of 302 W/m2 was adopted as

it was thought that no change had occurred in the rig since this value was measured. This value

however, led to inexplicable results for the thermal performance characteristics of the panel (e.g.

efficiency greater than one). A more thorough reading of (Couch, et al., 2012) revealed that the

reported value contained an error margin of 120% due to the mismatch between the measuring

instrument’s absorption (i.e. sensitivity) spectrum and the output spectrum of the lamp. This meant

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that the actual value could be more than two times higher (2.2 to be exact) than the reported. This

factor was taken into account but the results were wrong again. As no other explanation could be

thought of, it was apparent that something had changed in the rig setup. Indeed, Mr Allan confirmed

that he had removed a metal protective grille that was in front of the lamp and thus the irradiance

would be much higher than when the first experiments were made in (Couch, et al., 2012).

This of course, meant that either the grille should be replaced and all experiments remade,

or that the irradiance should be measured again. The first option was considered too time

consuming and inaccurate, as the grille could be replaced in a slightly different position, which

would affect the resulting incident irradiance on the panel. The second option was impossible with

the available instrument; the 15cm high instrument had to be placed vertically to take readings,

meaning that it could only provide values for the plane 15cm above the actual panel plane. If the

panel were to be removed so that the instrument could be placed at the proper plane, it would mean

that the whole System Rig would be drained and refilled with water, a very time-consuming

procedure. Moreover, the 120% error margin would still hold true, so in any case using the available

instrument was rejected as an option.

To deal with this issue a new irradiance measuring instrument was ordered by 4 MSc

students, using the funds available to MSc projects; a PV reference cell. At this time, the

experiments were paused to wait for the arrival of the new instrument. Unfortunately, although the

measurements were later repeated using the new instrument and employing a much more improved

and accurate procedure, the resulting values for the thermal parameters were still wrong. It was

then understood that a similar mismatch existed between the spectra of the new instrument and the

solar simulator. Most importantly, it was realised that an accurate correction factor could not be

produced for the new instrument, as the required data is not available. In particular, the diagrams

that contain the spectra of both the lamp and the silicon cell (i.e. the absorbing surface of the new

instrument) contain very limited information that is not sufficient to provide an accurate correction

factor. Thus, it was accepted that the irradiance value could not be determined with the available

means and at the available timeframe and it was decided to proceed with the other aspects of the

project.

This of course resulted in a direction change for the project. As the thermal parameters could

not be determined, the Spiral Panel could not be used in the simulations for the PV-SAHP system.

A compromising approach was decided, using the available parameters and completing the rest

from typical values suggested by the literature. Moreover, no analytical investigation could be

performed on the Spiral Panel and no theoretical model validation either, as both require accurate

values of the thermal performance parameters. Although the last two aspects were not included in

the original project objectives, they were suggested by the project supervisor as they are of

particular interest and could lead to important research outcomes. As such, preliminary work had

started while waiting for the new measuring instrument, the results of which are presented in the

relevant chapter of this report. Unfortunately, the work could not be completed but it is hoped that

what has already been done will be of help to future projects.

The other issue related with the lack of proper equipment in the lab is the inability to take fast I-V

curves. A particular problem with the System Rig, in addition to the lack of equipment, is that the

chiller used in the rig is of insufficient capacity, which results in a slow temperature increase in the

water of the storage tank. This increase is transferred to the inlet water temperature and ultimately

in an increase to the plate temperature and to the outlet temperature. In other words, although the

chiller is set to keep a set temperature, this cannot be achieved and after some time the

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temperature starts increasing. The inability to keep a constant set plate temperature (i.e. cell

temperature) means that a proper I-V curve cannot be formed, as half readings would be taken at

25 degC for example and the other half at 27 degC. This is due to the long time that it takes to

manually take all required readings, while manually changing the resistance at every step. To

overcome this, a special feature of the DC electronic load (decade box) was employed; the

particular instrument offers the ability to set sequences of resistance values that are automatically

changed, so as to form a proper I-V curve. The speed of the switch can be as low as a few

milliseconds. Although this seemed as an absolutely acceptable and accurate method, it was soon

discovered that the instrument does not offer automatic data logging, neither to its own memory nor

to an external computer terminal. Thus, although the sequence points switch automatically, the user

needs to manually observe the reading value and record it somewhere. Unfortunately this meant

that the minimum time between point-switch could not be less than 20 seconds, to ensure an

accurate data retrieval and recording process. Consequently, a substantial time was still needed to

take a full I-V curve, which meant that a temperature rise in the cells was inevitable. To keep the

rise negligible, only 50 resistance points were set to the sequence.

The implication of this is that the maximum power points of every curve cannot be accurately

determined. Instead, the maximum power corresponding to one of the set resistance points in the

sequence is assumed to be the MPP (i.e. if the true MPP was at 2 ohms and the sequence points

were at 1.9 and 2.1 ohms, the higher value between the two would be recorded as the maximum

power). Nonetheless, the followed procedure produced results with satisfying accuracy.

During the simulation phase, finally, it was realised that TRNSYS cannot model the specific

PV-SAHP system configuration that the literature review revealed to be the focus of the scientific

community (with refrigerant flowing through the PVT panel array). At the same time, it was realised

that this configuration is of no immediate interest as an application, since it is still in the very early

development stages. Consequently, other designs were thought of that combine applicability in the

domestic sector and scientific interest. This process led to the ‘dual tank PV-SAHP’ configuration

that is suggested as an efficient system, as an alternative to the typical ones used in Greece.

All dates of the initial time plan were shifted about a week to 10 days forward, mainly due to

the problems occurred in the experiment phase. This did not affect the project, besides that less

time was left between the draft thesis version and the final version for corrections and polishing.

Reflecting upon the initial objectives compared to the final outcomes, it can be said that, for

the best part, the outcomes cover sufficiently the intended aims and goals of this project. The

analytical aspect was combined with the numerical, as TRNSYS accomplishes both with a far

greater efficiency and accuracy than the author could achieve using excel sheets. The experimental

testing of the Spiral Panel under the specific weather conditions of Greece was impossible due to

the lack of proper equipment: the irradiance cannot be changed and neither can the ambient

temperature in the lab. Most importantly however, it was realised that there is no need anyway to

test the Spiral Panel under specific weather conditions but only to determine its performance

parameters. Other than the above changes, the major part of the project that concerns the

investigation of the PV-SAHP system’s performance was fully completed.

An important lesson learned, is that better planning is required when problems arise and

before solutions are adopted. The ordering of a new instrument that did not, in the end, prove

suitable was something that should have been avoided. That being said, the instrument was

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ordered as it was considered useful for many projects, current and future and not only for the needs

of this project.

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Figure 71 – The original project time-plan

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Figure 72 – The final project outline

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Appendix B – Project Proposal

1. Introduction In today’s world the most noticeable trends related to the energy market are the rapid

increase in global demand for energy and the continuous fluctuations of energy prices (electricity,

gas and oil primarily).

The first can be witnessed in our everyday lives; world population, average GDP and thus

energy demand have been steadily increasing in the past and the situation isn’t likely to change in

the foreseeable future (United Nations, 2004) (International Energy Agency, 2010). It is projected

that by 2030, global energy demand will be approximately 40% higher compared to 2005 levels

(ExxonMobil, 2010).

Fuel price fluctuations are more directly ‘felt’ by the effects in electricity and gas/oil bills.

These fluctuations are inevitable in a globalized market and depend on a vast number of factors

ranging from political stability in the energy producing countries, to extreme weather adversely

affecting trade routes and wind power.

Energy importers, such as the EU, faced with diminishing local reserves, resort to increased

imports of traditional fossil fuels in an attempt to boost their own power production, or to electricity

imports through grid connections to neighbouring countries. Import dependency has a critical

consequence on the importing countries’ economies. An extremely volatile energy market (i.e.

unpredicted fluctuations in fossil fuel prices) is intensifying this and economic uncertainty among the

population is the overall result.

The EU citizen’s summary on the energy 2020 targets (EU, 2010) is indicative of the global

trends in dealing with the pressing issues of rising fuel prices and import dependency:

Reduce GHG emissions to combat climate change

Reduce energy demands (efficient energy use)

Increase production from alternative and sustainable energy sources

Among the various renewable sources, solar energy is characterized as the most abundant,

inexhaustible and clean. Policy-makers’ decisions such as the EU 2020 directive, have contributed

in driving the capital costs of solar systems down, as technologies are getting more mature and

scale production kicks in to cover the demand (European Commission, 2010) (ofgem, June 2011).

The building sector is a renewable-application area of particular interest as one third of the

primary global energy demand comes from commercial, institutional and residential buildings

(ASME Climate Change Task Force, 2009). According to the EU’s Energy Performance in Buildings

Directive (EPBD), buildings are responsible for more than 40% of the total energy consumption and

related carbon dioxide emissions (European Union, May 2010).

This project intends on designing, analyzing, evaluating and optimizing a CHP system that will

assist in covering the electricity and heat demands of a typical residential building in Greece while

offering annual cost savings as well as energy and CO2 reductions to the building owners. The

system will combine two technologies, a photovoltaic-thermal panel (PVT) and a vapour

compression heat pump. It will utilize the available solar energy to produce electricity with increased

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efficiency through the PVT panel, as well as useful heat, by employing the heat pump to “upscale”

the thermal output of the PVT to the required levels.

2. Background to the Project

2.1. The PVT panel The hybrid photovoltaic-thermal panel is not a new concept. The idea was first presented in the

1970s by various researchers ( (Wolf, 1976; Evans & Florschuetz, 1975; Florschuetz, 1979;

Hendrie, 1979; Kern & Russel, 1978). The concept however, has only recently drawn the attention

of the scientific world and decision makers around the world. A few commercial products have

appeared in the last years but technological breakthroughs and market growth have been limited,

mainly due to the lack of research on the subject and of relevant financial incentives. Significant

catalysers to the technological development and market introduction of the PVT have been the

International Energy Agency’s Task 35 (Internationl Energy Agency, 2005) and the PVT Forum

(European Commission, 2006), part of the PV Catapult project of the E.C.

The design of the PVT panel is rather simple. As defined in Task 35, a PVT device employs PV

cells or a PV panel to absorb solar radiation. A small part of this radiation is converted to electricity

by the PV cells. The remaining absorbed solar radiation, which would otherwise be rejected as

waste heat, is then extracted from the PVT panel by a heat exchanger and transferred to a

circulating fluid to be utilised productively.

Heat extraction allows the PV cells to operate at lower temperatures and thus higher efficiencies

(Kalogirou & Tripanagnostopoulos, 2006). Moreover, the total efficiency of the device is increased

as it utilises a greater part of the absorbed solar radiation. The advantages to building related

applications compared to an installation of both PV and thermal collectors are significant:

More effective use of valuable roof space

Better aesthetic appearance and roof uniformity

Lower installation costs as less panels have to be installed

Perhaps the most interesting aspect of the PVT panel however, and the one most likely to drive

market growth, is the unique financial opportunity that it provides to exploit the latest incentives that

apply both to renewable electricity and heat production (e.g. Feed in Tariffs and Renewable Heat

Incentive in the UK (DECC, 2012)).

2.2. The Photovoltaic solar assisted Heat pump system (PV-SAHP) Heat pump technology has been available for many years and installations of both ground-

source and air-source systems can be found all over the world. However, while these devices are

potentially greener than burning fossil fuels, they do still use large amounts of electricity, especially

the air-source type in the northern and colder regions.

As was mentioned in the introduction, the focus of the project will be the integration of a PVT

panel with a vapour compression heat pump. This concept is fairly new and research on the subject

has only recently begun to develop. The general idea is to use a heat pump to ‘upscale’ the thermal

output of a PVT panel to useful temperatures in order to cover a building’s hot water and space

heating demands.

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Although lately a great deal of research has been carried on PVT panels, attention is hardly

ever given to the fact that the thermal output of the device is low grade and water temperatures are

not likely to exceed 30 oC (Ibrahim, 2009). Temperatures like these are useful in a limited number of

applications, most notably perhaps in swimming pools. Thus, it can be understood that if PVT is to

be massively introduced to the global market, it must be coupled with another device that can

upgrade its low thermal output, so that the requirements of the more widespread applications can

be met.

In addition, a heat pump is much more efficient in cooling the PVT panels compared to tap

water or water from storage tanks which are the usual ways for PVT cooling. Unlike the alternatives,

a heat pump is able to maintain a set cell temperature throughout the year, therefore ensuring high

annual electrical efficiencies for the PV cells.

A number of researchers have recently started to investigate the performance of the basic

system which has been given the name ‘photovoltaic solar assisted heat pump or PV-SAHP’ by the

scientific community. Attention has been given in the investigation of the potential advantages of

using refrigerants instead of water as the cooling medium (Chen, et al., 2011; Ji, et al., 2008),

variable frequency compressors instead of conventional ones (Liu, et al., 2009; Ji, et al., 2008; Xu,

et al., 2009) and in the effect of alternative designs for the cover of the panel (Chen, et al., 2011)

and the heat exchanger on the back of it (Xu, et al., 2009).

Furthermore, the important parameters that affect the performance of the system have been

identified and numerical models have been developed and validated that predict the dynamic

behaviour of the system (Chen & Wei, 2011; Ji, et al., 2009; Ji, et al., 2008). Lastly, weather data

have been used to test the performance of the system in the real world and under realistic loads

(i.e. building heating and electrical loads) (Xu, et al., 2009; Chow, et al., 2010; Chen & Wei, 2011).

Many more studies are available on the more general system of SAHP that uses a normal

solar collector (not PVT) as an evaporator. These can also be the source of useful information since

the thermal behaviour of PVT and a solar collector is very similar.

As last note, UK based Newform Energy claims to be the first company worldwide to offer a

commercially available solution that includes PVT panels integrated with a specifically designed

water/water heat pump (Newform Energy, 2012). The system has already secured eligibility for FiTs

and the RHI, making it therefore a very financially attractive option. Although not mentioned, it is

very likely that such systems can also be eligible for financial incentives such as the Renewable

Heat Premium Payment scheme (DECC, 2012).

3. Aims and Broad Objectives The primary aim of the project is to perform a feasibility study for a PV-SAHP system in

order to establish the system’s ability to efficiently cover part or all of the electrical and heating

loads of a typical domestic residence in Greece.

The broad objectives include:

The design, modelling and analytical as well as numerical testing (using TRNSYS) of

a PV-SAHP system based on a novel PVT module that was manufactured in Brunel

(Couch, et al., 2012)

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the experimental investigation of the PVT module’s performance under the actual

(reproduced in the lab) weather conditions of the particular building site

the analytical and numerical investigation of the PV-SAHP system’s performance

under the same weather conditions and in respect to the actual building’s loads

the establishment of the annual energy, cost and CO2 savings that can be achieved

by the system in comparison to the present system that is used in the building

an economic study to evaluate the expected capital cost and payback period for the

PV-SAHP system, taking into account any financial incentives provided at present

time from the local government

the provision of information on novel sustainable systems to the local authorities,

stake holders and community

4. Methods to be Adopted The following methods will be adopted to accomplish the desirable objectives:

1. A comprehensive literature review will be carried out, that will focus on studies that

investigate the performance of PV-SAHP systems and the parameters that affect it.

Particular attention will be given to studies that utilise real weather and building load data.

The review will also include, to a certain extent, simple SAHP systems.

2. Daily or monthly dynamic data will be acquired for the electrical and heating needs of a

typical domestic residence located in Greece. The extreme solar irradiance and temperature

conditions of Greece make it a very good location to investigate the improvement potential in

the electrical efficiency of the PV cells due to the reduction in operating temperature

provided by the heat pump.

3. Based on the literature review and on the climatic data, experiments will be carried out in the

lab to evaluate the PVT performance under these conditions and by employing the

recommended methods in the literature.

4. Using the most appropriate analytical and numerical models according to the literature

review, relevant analysis will take place to establish the annual performance and potential

energy and CO2 savings. TRNSYS will be used for the numerical simulation and RET

screen for the CO2 and financial evaluation of the proposed project.

5. Specific Outcomes Gain an understanding of the current research on PV-SAHP systems and on the

available analytical and numerical models in the literature.

Gain an insight in the methods used in the literature in using real world load and

weather data in evaluating the performance of PV-SAHP systems.

Perform a full experimental study on the PVT’s electrical and thermal performance

under the particular weather conditions. Acquire I-V curves, temperature coefficients,

loss coefficients and efficiencies of the PV and thermal components and in general

explore the behaviour of all relevant parameters.

Perform analytical and numerical simulations to establish the energy performance

and saving potential of the proposed PV-SAHP system.

Assess the financial viability of the system, including expected capital cost, payback

period and annual cost savings.

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7. References ASME Climate Change Task Force, 2009. files.asme.org. [Online]

Available at:

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CFQQFjAB&url=http%3A%2F

%2Ffiles.asme.org%2Fasmeorg%2FNewsPublicPolicy%2FGovRelations%2FPositionStatements%2F17971.p

df&ei=xzrXT_q9MYbj8QOIydi7Aw&usg=AFQjCNH4jauRydvSL334QHY4wZSZPxuwRg&si

[Accessed 06 2012].

Chen, H., Riffat, S. B. & Fu, Y., 2011. Experimental study on a hybrid photovoltaic/heat pump system. Applied

Thermal Engineering, Vol 31, 08, pp. 4132-4138.

Chen, H. & Wei, P., 2011. Numerical Study on a Novel Photovoltaic/Thermal Heat Pump System. Chengdu,

China, Elsevier Ltd.- Energy Procedia, Vol 12, pp 547-553.

Chow, T. et al., 2010. Potential use of photovoltaic-integrated solar heat pump system in Hong Kong. Applied

Thermal Engineering, Vol 30, pp. 1066-1072.

Couch, W., Cook, N., Borton, J. & Mustapha, R., 2012. Development of an optimised solar hybrid PV-T

technology for the simultaneous production of electricity and thermal energy, Uxbridge: Brunel University,

School of Engineering and Design.

DECC, 2012. Renewable Energy Policy. [Online]

Available at: http://www.decc.gov.uk/en/content/cms/meeting_energy/renewable_ener/renewable_ener.aspx

[Accessed 2012].

EU, 2010. Citizens' summary - ENERGY 2020: A strategy for competitive, sustainable and secure energy.

s.l.:European Union.

European Commission, 2006. PVT Forum. [Online]

Available at: http://www.pvtforum.org/index.html

[Accessed 10 06 2012].

European Commission, 2010. EU energy and transport in figures, s.l.: Publications office of the EE.

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European Union, May 2010. DIRECTIVE 2010/31/EU OF THE EUROPEAN PARLIAMENT AND OF THE

COUNCIL. s.l.:Official Journal of the EU.

Evans, D. & Florschuetz, L. W., 1975. Cost studies on terrestrial photovoltaic power systems with sunlight

concentration. Solar Energy Vol. 19, 15 September, pp. 255-262.

ExxonMobil, 2010. The Out look for Energy: A View to 2030, s.l.: ExxonMobil.

Florschuetz, L. W., 1979. Extension of the Hottel-Whillier model to the analysis of combined

photovoltaic/thermal flat palte collectors. Solar Energy vol. 22, 1 November, pp. 361-366.

Hendrie, S., 1979. Evaluation of combined photovoltaic/thermal collectors. Atlanta, International Solar Energy

Society.

Ibrahim, 2009. Performance of Photovoltaic Thermal Collector (PVT) with Different Absorber Designs.

WSEAS Transactions on Environment and Development, Vol 5, pp. 321-330.

International Energy Agency, 2010. World Energy Outlook , s.l.: IEA.

Internationl Energy Agency, 2005. PV / Thermal Solar Systems. [Online]

Available at: http://www.iea-shc.org/task35/index.html

[Accessed 06 2012].

Ji, j. et al., 2009. Distributed dynamic modeling and experimental study of PV evaporator in a PV/T solar-

assisted heat pump. International Journal of Heat and Mass Transfer, Vol 52, pp. 1365-1373.

Ji, J. et al., 2008. Performance analysis of a photovoltaic heat pump. Applied Energy, Vol 85, pp. 680-693.

Kalogirou, S. & Tripanagnostopoulos, Y., 2006. Hybrid PV/T solar systems for domestic hot water and

electricity production. Energy Conversion and Management vol 47, 30 01, pp. 3368-3382.

Kern, E. C. & Russel, M., 1978. Combined photovoltaic and thermal hybrid collector systems. Washington DC,

USA, IEEE photovoltaic specialists conference, pp. 1153-1157.

Liu, K. et al., 2009. Performance study of a photovoltaic solar assisted heat pump with variable-frequency

compressor – A case study in Tibet. Renewable Energy, Vol 34, pp. 2680-2687.

Newform Energy, 2012. The Hybrid Solar Solution. [Online]

Available at: http://www.newformenergy.com/hybrid-solar-solution

[Accessed 2012].

ofgem, June 2011. Electricity and Gas Supply Market Report, s.l.: ofgem.

United Nations, 2004. World Population to 2300, s.l.: UN.

Wolf, M., 1976. Performance Analysis of Combined Heating and Photovoltaic Power Systems for Residences.

Energy Conversion, Vol 16, 10 June, pp. 79-90.

Xu, G. et al., 2009. Simulation of a photovoltaic/thermal heat pump system having a modified

collector/evaporator. Solar Energy, Vol 83, pp. 1967-1976.

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Appendix C – Raw Experimental Data and Graphs

Table 9 – Resistance sequence points and corresponding I, V, P values for the first I-V curve measurement

Resistance - R Current - I Voltage - V Power - P Ohm Amp Volt Watt 0.02 2.9 0.6 1.74 0.14 2.899 0.407 1.179893 0.26 2.894 0.753 2.179182 0.38 2.893 1.098 3.176514 0.5 2.898 1.446 4.190508

0.62 2.889 1.79 5.17131 0.74 2.891 2.145 6.201195 0.86 2.892 2.484 7.183728 0.98 2.892 2.83 8.18436 1.1 2.88 3.172 9.13536

1.22 2.88 3.51 10.1088 1.34 2.88 3.86 11.1168 1.46 2.88 4.21 12.1248 1.58 2.87 4.53 13.0011 1.7 2.87 4.867 13.96829

1.82 2.82 5.12 14.4384 1.94 2.717 5.265 14.30501

2 2.662 5.317 14.15385 14 0.461 6.42 2.95962 26 0.252 6.5 1.638 38 0.174 6.49 1.12926 50 0.133 6.545 0.870485 62 0.108 6.553 0.707724 74 0.091 6.558 0.596778 86 0.078 6.563 0.511914 98 0.069 6.566 0.453054

110 0.062 6.568 0.407216 122 0.056 6.57 0.36792 134 0.051 6.57 0.33507 146 0.047 6.572 0.308884 158 0.044 6.574 0.289256 170 0.041 6.574 0.269534 182 0.038 6.575 0.24985 194 0.035 6.575 0.230125 200 0.0235 6.579 0.154607 306 0.0181 6.581 0.119116 412 0.0147 6.582 0.096755 518 0.0126 6.582 0.082933 624 0.0111 6.583 0.073071 730 0.0099 6.583 0.065172 836 0.009 6.584 0.059256 942 0.0083 6.583 0.054639

1048 0.0077 6.584 0.050697 1154 0.0072 6.584 0.047405 1260 0.0068 6.584 0.044771 1366 0.0065 6.584 0.042796 1472 0.0061 6.584 0.040162 1578 0.0057 6.584 0.037529 1790 0.0057 6.584 0.037529

N.A. - Stogiannos

88

Table 10 - Resistance sequence points and corresponding I, V, P values for the second I-V curve measurement

Resistance - R

Current - I Voltage - V

Power - P

Resistance - R

Current - I

Voltage - V

Power - P Ohm Amp Volt Watt Ohm Amp Volt Watt

0 2.91 0 0 1 0.020 2.910 0.060 0.175 51 9.920 0.636 6.281 3.995 2 0.140 2.910 0.408 1.187 52 10.160 0.621 6.287 3.904 3 0.260 2.910 0.757 2.203 53 10.400 0.608 6.292 3.826 4 0.380 2.910 1.108 3.224 54 10.640 0.594 6.297 3.740 5 0.500 2.910 1.459 4.246 55 10.880 0.582 6.302 3.668 6 0.620 2.910 1.805 5.253 56 11.120 0.570 6.306 3.594 7 0.740 2.910 2.150 6.257 57 11.360 0.558 6.311 3.522 8 0.860 2.910 2.500 7.275 58 11.600 0.547 6.315 3.454 9 0.980 2.900 2.850 8.265 59 11.840 0.536 6.320 3.388

10 1.100 2.900 3.200 9.280 60 12.080 0.526 6.324 3.326 11 1.220 2.910 3.540 10.301 61 12.320 0.516 6.326 3.264 12 1.340 2.900 3.890 11.281 62 12.560 0.507 6.329 3.209 13 1.460 2.890 4.230 12.225 63 12.800 0.497 6.332 3.147 14 1.580 2.890 4.570 13.207 64 13.040 0.488 6.337 3.092 15 1.700 2.870 4.880 14.006 65 13.280 0.480 6.340 3.043 16 1.820 2.790 5.080 14.173 66 13.520 0.472 6.343 2.994 17 1.940 2.680 5.210 13.963 67 13.760 0.464 6.346 2.945 18 2.000 2.630 5.260 13.834 68 14.000 0.457 6.360 2.907 19 2.240 2.423 5.417 13.125 69 26.000 0.250 6.442 1.611 20 2.480 2.235 5.533 12.366 70 38.000 0.173 6.470 1.119 21 2.720 2.072 5.625 11.655 71 50.000 0.132 6.485 0.856 22 2.960 1.930 5.702 11.005 72 62.000 0.107 6.493 0.695 23 3.200 1.805 5.765 10.406 73 74.000 0.090 6.499 0.585 24 3.440 1.695 5.819 9.863 74 86.000 0.078 6.503 0.507 25 3.680 1.597 5.866 9.368 75 98.000 0.069 6.507 0.449 26 3.920 1.510 5.907 8.920 76 110.000 0.062 6.510 0.404 27 4.160 1.432 5.944 8.512 77 122.000 0.056 6.512 0.365 28 4.400 1.361 5.976 8.133 78 134.000 0.051 6.514 0.332 29 4.640 1.297 6.005 7.788 79 146.000 0.047 6.516 0.306 30 4.880 1.239 6.031 7.472 80 158.000 0.044 6.517 0.287 31 5.120 1.186 6.055 7.181 81 170.000 0.041 6.517 0.267 32 5.360 1.136 6.075 6.901 82 182.000 0.038 6.518 0.248 33 5.600 1.091 6.095 6.650 83 194.000 0.036 6.519 0.235 34 5.840 1.050 6.114 6.420 84 200.000 0.035 6.519 0.228 35 6.080 1.011 6.130 6.197 85 306.000 0.024 6.523 0.157 36 6.320 0.975 6.146 5.992 86 412.000 0.018 6.524 0.117 37 6.560 0.942 6.159 5.802 87 518.000 0.015 6.525 0.098 38 6.800 0.910 6.173 5.617 88 624.000 0.013 6.525 0.085 39 7.040 0.881 6.185 5.449 89 730.000 0.011 6.526 0.072 40 7.280 0.854 6.196 5.291 90 836.000 0.010 6.526 0.065 41 7.520 0.828 6.206 5.139 91 942.000 0.009 6.527 0.059 42 7.760 0.804 6.215 4.997 92 1048.000 0.008 6.527 0.052 43 8.000 0.781 6.226 4.863 93 1154.000 0.008 6.527 0.052 44 8.240 0.759 6.234 4.732 94 1260.000 0.007 6.527 0.046 45 8.480 0.739 6.243 4.614 95 1366.000 0.007 6.527 0.046 46 8.720 0.720 6.251 4.501 96 1472.000 0.007 6.527 0.042 47 8.960 0.701 6.258 4.387 97 1578.000 0.006 6.527 0.039 48 9.200 0.684 6.265 4.285 98 1790.000 0.006 6.527 0.039 49 9.440 0.667 6.271 4.183 99 1896.000 0.006 6.527 0.037 50 9.680 0.651 6.275 4.085 100 0 6.531 0

N.A. - Stogiannos

89

Table 11 - Resistance sequence points and corresponding I, V, P, Tal values for Tal=38.17 and Tal=41.43 degC

Resistance - R

Current - I

Voltage - V

Tal Power - P

Resistance - R

Current - I

Voltage - V

Tal Power - P Ohm Amp Volt deg C Watt Ohm Amp Volt deg C Watt

0

3.56 0

0.000 0

3.46 0

0.000 1 0.020 3.560 0.073 41.350 0.260 1 0.020 3.460 0.071 37.900 0.246 2 0.260 3.560 0.930 41.340 3.311 2 0.260 3.460 0.900 37.900 3.114 3 0.500 3.550 1.780 41.330 6.319 3 0.500 3.460 1.730 37.890 5.986 4 0.740 3.550 2.620 41.330 9.301 4 0.740 3.470 2.560 37.890 8.883 5 0.980 3.540 3.470 41.300 12.284 5 0.980 3.470 3.390 37.890 11.763 6 1.220 3.530 4.300 41.000 15.179 6 1.220 3.450 4.200 37.890 14.490 7 1.460 3.430 5.000 41.310 17.150 7 1.460 3.410 4.970 37.890 16.948 8 1.580 3.260 5.140 41.310 16.756 8 1.580 3.270 5.160 37.900 16.873 9 1.700 3.090 5.240 41.300 16.192 9 1.700 3.110 5.280 37.910 16.421

10 1.754 3.020 5.290 41.290 15.976 10 1.754 3.040 5.320 37.920 16.173 11 1.808 2.950 5.330 41.260 15.724 11 1.808 2.970 5.360 37.930 15.919 12 1.862 2.880 5.360 41.270 15.437 12 1.862 2.900 5.400 37.940 15.660 13 1.916 2.820 5.400 41.270 15.228 13 1.916 2.840 5.430 37.940 15.421 14 1.970 2.760 5.430 41.270 14.987 14 1.970 2.780 5.470 37.930 15.207 15 2.024 2.700 5.450 41.270 14.715 15 2.024 2.720 5.500 37.940 14.960 16 2.078 2.640 5.480 41.260 14.467 16 2.078 2.660 5.530 37.950 14.710 17 2.132 2.590 5.510 41.280 14.271 17 2.132 2.610 5.550 37.960 14.486 18 2.186 2.540 5.530 41.300 14.046 18 2.186 2.560 5.580 37.970 14.285 19 2.240 2.490 5.560 41.300 13.844 19 2.240 2.510 5.600 37.980 14.056 20 2.480 2.280 5.650 41.310 12.882 20 2.480 2.300 5.700 37.990 13.110 21 3.440 1.720 5.890 41.320 10.131 21 3.440 1.730 5.950 38.020 10.294 22 4.400 1.370 6.030 41.330 8.261 22 4.400 1.390 6.090 38.040 8.465 23 5.360 1.140 6.120 41.360 6.977 23 5.360 1.160 6.180 38.040 7.169 24 6.320 0.980 6.180 41.370 6.056 24 6.320 0.990 6.240 38.060 6.178 25 7.280 0.890 6.230 41.380 5.545 25 7.280 0.870 6.290 38.090 5.472 26 8.240 0.760 6.260 41.380 4.758 26 8.240 0.770 6.320 38.110 4.866 27 9.200 0.686 6.290 41.380 4.315 27 9.200 0.690 6.350 38.130 4.382 28 10.160 0.624 6.312 41.410 3.939 28 10.160 0.630 6.370 38.150 4.013 29 11.120 0.572 6.331 41.450 3.621 29 11.120 0.580 6.390 38.170 3.706 30 12.080 0.528 6.347 41.480 3.351 30 12.080 0.530 6.410 38.180 3.397 31 13.040 0.490 6.360 41.490 3.116 31 13.040 0.500 6.420 38.240 3.210 32 14.000 0.457 6.365 41.520 2.909 32 14.000 0.460 6.430 38.270 2.958 33 20.000 0.323 6.414 41.520 2.072 33 20.000 0.330 6.480 38.290 2.138 34 26.000 0.250 6.440 41.530 1.610 34 26.000 0.250 6.510 38.320 1.628 35 50.000 0.132 6.483 41.550 0.856 35 50.000 0.130 6.550 38.330 0.852 36 74.000 0.090 6.498 41.540 0.585 36 74.000 0.091 6.563 38.370 0.597 37 98.000 0.068 6.505 41.560 0.442 37 98.000 0.069 6.571 38.370 0.453 38 122.000 0.055 6.510 41.570 0.358 38 122.000 0.056 6.574 38.380 0.368 39 146.000 0.047 6.513 41.580 0.306 39 146.000 0.047 6.577 38.420 0.309 40 170.000 0.040 6.514 41.590 0.261 40 170.000 0.041 6.579 38.440 0.270 41 194.000 0.036 6.515 41.620 0.235 41 194.000 0.036 6.580 38.470 0.237 42 306.000 0.023 6.519 41.620 0.150 42 306.000 0.024 6.583 38.490 0.158 43 518.000 0.015 6.522 41.620 0.095 43 518.000 0.015 6.586 38.500 0.099 44 730.000 0.011 6.523 41.630 0.071 44 730.000 0.011 6.586 38.530 0.072 45 942.000 0.009 6.524 41.660 0.058 45 942.000 0.009 6.586 38.560 0.059 46 1154.000 0.008 6.524 41.660 0.050 46 1154.000 0.008 6.586 38.590 0.053 47 1472.000 0.006 6.524 41.690 0.042 47 1472.000 0.007 6.586 38.620 0.046 48 1578.000 0.006 6.523 41.690 0.040 48 1578.000 0.006 6.586 38.630 0.040 49 1790.000 0.006 6.523 41.670 0.037 49 1790.000 0.006 6.586 38.650 0.040 50 1896.000 0.005 6.523 41.680 0.035 50 1896.000 0.006 6.584 38.660 0.040 51 0.000 6.525 0.000 51 0.000 6.586 0.000

N.A. - Stogiannos

90

Table 12 - Resistance sequence points and corresponding I, V, P, Tal values for Tal=48.13 and Tal=57.26 degC

Resistance - R

Current - I

Voltage - V

Tal Power - P

Resistance - R

Current - I

Voltage - V

Tal Power - P Ohm Amp Volt deg C Watt Ohm Amp Volt deg C Watt

0 3.56 0 0.000 0 3.6 0 0.000 1 0.020 3.560 0.073 48.19

0 0.260 1 0.020 3.600 0.073 56.39

0 0.263

2 0.260 3.560 0.920 48.170

3.275 2 0.260 3.600 0.930 56.410

3.348 3 0.500 3.560 1.770 48.15

0 6.301 3 0.500 3.590 1.800 56.42

0 6.462

4 0.740 3.550 2.610 48.150

9.266 4 0.740 3.590 2.660 56.500

9.549 5 0.980 3.550 3.460 48.13

0 12.28

3 5 0.980 3.590 3.510 56.55

0 12.60

1 6 1.220 3.530 4.320 48.110

15.250

6 1.220 3.570 4.350 56.560

15.530 7 1.460 3.350 4.890 48.08

0 16.38

2 7 1.460 3.270 4.760 56.62

0 15.56

5 8 1.520 3.260 4.950 48.060

16.137

8 1.520 3.170 4.820 65.650

15.279 9 1.580 3.170 5.010 48.01

0 15.88

2 9 1.580 3.087 4.873 56.68

0 15.04

3 10 1.700 3.010 5.110 48.000

15.381

10 1.700 2.924 4.964 56.700

14.515 11 1.754 2.940 5.150 47.97

0 15.14

1 11 1.754 2.855 5.001 56.72

0 14.27

8 12 1.808 2.870 5.190 47.960

14.895

12 1.808 2.789 5.036 56.800

14.045 13 1.862 2.810 5.220 47.94

0 14.66

8 13 1.862 2.726 5.069 56.84

0 13.81

8 14 1.916 2.750 5.250 47.960

14.438

14 1.916 2.665 5.100 56.890

13.592 15 2.024 2.630 5.310 47.96

0 13.96

5 15 2.024 2.551 5.155 56.88

0 13.15

0 16 2.078 2.570 5.340 47.970

13.724

16 2.078 2.496 5.179 56.870

12.927 17 2.132 2.520 5.360 47.96

0 13.50

7 17 2.132 2.443 5.201 56.87

0 12.70

6 18 2.186 2.470 5.390 47.970

13.313

18 2.186 2.395 5.225 56.900

12.514 19 2.240 2.420 5.410 47.97

0 13.09

2 19 2.240 2.347 5.248 56.89

0 12.31

7 20 2.480 2.220 5.500 48.000

12.210

20 2.480 2.155 5.334 56.890

11.495 21 3.440 1.670 5.740 48.07

0 9.586 21 3.440 1.620 5.562 56.91

0 9.010

22 4.400 1.340 5.870 48.120

7.866 22 4.400 1.297 5.695 56.930

7.386 23 5.360 1.110 5.960 48.15

0 6.616 23 5.360 1.081 5.780 56.96

0 6.248

24 6.320 0.955 6.020 48.180

5.749 24 6.320 0.927 5.839 56.960

5.413 25 7.280 0.836 6.070 48.18

0 5.075 25 7.280 0.811 5.883 56.96

0 4.771

26 8.240 0.743 6.110 48.190

4.540 26 8.240 0.720 5.917 56.990

4.260 27 9.200 0.669 6.132 48.20

0 4.102 27 9.200 0.648 5.944 57.01

0 3.852

28 10.160 0.608 6.155 48.200

3.742 28 10.160 0.589 5.965 57.040

3.513 29 11.120 0.557 6.175 48.21

0 3.439 29 11.120 0.540 5.983 57.09

0 3.231

30 12.080 0.514 6.190 48.210

3.182 30 12.080 0.499 5.997 57.100

2.993 31 13.040 0.478 6.203 48.20

0 2.965 31 13.040 0.463 6.010 57.13

0 2.783

32 14.000 0.446 6.213 48.190

2.771 32 14.000 0.432 6.020 57.190

2.601 33 20.000 0.315 6.263 48.19

0 1.973 33 20.000 0.305 6.066 57.21

0 1.850

34 26.000 0.244 6.288 48.180

1.534 34 26.000 0.236 6.091 57.250

1.437 35 50.000 0.129 6.332 48.16

0 0.817 35 50.000 0.125 6.132 57.28

0 0.767

36 74.000 0.088 6.341 48.170

0.558 36 74.000 0.085 6.146 57.310

0.522 37 98.000 0.067 6.350 48.19

0 0.425 37 98.000 0.065 6.152 57.34

0 0.400

38 122.000 0.054 6.357 48.190

0.343 38 122.000 0.052 6.156 57.390

0.320 39 146.000 0.046 6.360 48.19

0 0.293 39 146.000 0.044 6.157 57.46

0 0.271

40 170.000 0.040 6.361 48.200

0.254 40 170.000 0.038 6.159 57.480

0.234 41 194.000 0.035 6.362 48.20

0 0.223 41 194.000 0.034 6.160 57.51

0 0.209

42 306.000 0.023 6.368 48.200

0.146 42 306.000 0.022 6.162 57.580

0.136 43 518.000 0.014 6.370 48.22

0 0.089 43 518.000 0.014 6.165 57.63

0 0.086

44 730.000 0.011 6.370 48.230

0.069 44 730.000 0.010 6.165 57.660

0.062 45 942.000 0.009 6.370 48.24

0 0.056 45 942.000 0.009 6.166 57.67

0 0.052

46 1154.000 0.008 6.371 48.220

0.048 46 1154.000 0.007 6.166 57.710

0.044 47 1472.000 0.006 6.371 48.24

0 0.038 47 1472.000 0.006 6.166 57.74

0 0.037

48 1578.000 0.006 6.371 48.230

0.038 48 1578.000 0.006 6.164 57.800

0.036 49 1790.000 0.006 6.371 48.24

0 0.036 49 1790.000 0.005 6.163 57.80

0 0.033

50 1896.000 0.005 6.371 48.230

0.034 50 1896.000 0.005 6.163 57.830

0.031 51 0.000 6.370 0.000 51 0.000 6.163 0.000

N.A. - Stogiannos

91

Table 13 - Resistance sequence points and corresponding I, V, P, Tal values for Tal=62.43 and Tal=70.0 degC

Resistance - R

Current - I

Voltage - V

Tal Power - P

Resistance - R

Current - I

Voltage - V

Tal Power - P Ohm Amp Volt deg C Watt Ohm Amp Volt deg C Watt

0 3.72 0 0.000 0 3.72 0 0.000 1 0.020 3.720 0.076 62.07

0 0.283 1 0.020 3.720 0.076 69.79

0 0.283

2 0.260 3.720 0.967 62.170

3.597 2 0.260 3.730 0.970 69.790

3.618 3 0.500 3.720 1.865 62.20

0 6.938 3 0.500 3.720 1.870 69.82

0 6.956

4 0.740 3.730 2.760 62.180

10.295

4 0.740 3.720 2.760 69.820

10.267 5 0.980 3.720 3.640 62.18

0 13.54

1 5 0.980 3.720 3.650 69.80

0 13.57

8 6 1.220 3.600 4.390 62.200

15.804

6 1.076 3.710 3.980 69.780

14.766 7 1.460 3.210 4.680 62.18

0 15.02

3 7 1.172 3.600 4.210 69.78

0 15.15

6 8 1.520 3.116 4.730 62.160

14.739

8 1.268 3.430 4.350 69.790

14.921 9 1.580 3.028 4.778 62.14

0 14.46

8 9 1.364 3.270 4.450 69.80

0 14.55

2 10 1.700 2.866 4.865 62.160

13.943

10 1.460 3.110 4.540 69.800

14.119 11 1.754 2.798 4.901 62.22

0 13.71

3 11 1.490 3.070 4.570 69.80

0 14.03

0 12 1.808 2.732 4.933 62.230

13.477

12 1.520 3.020 4.590 69.800

13.862 13 1.862 2.670 4.963 62.23

0 13.25

1 13 1.580 2.940 4.640 69.82

0 13.64

2 14 1.916 2.610 4.994 62.210

13.034

14 1.700 2.780 4.720 69.820

13.122 15 2.024 2.498 5.047 62.21

0 12.60

7 15 1.754 2.710 4.750 69.87

0 12.87

3 16 2.078 2.445 5.071 62.280

12.399

16 1.808 2.650 4.780 69.880

12.667 17 2.132 2.393 5.094 62.39

0 12.19

0 17 1.862 2.590 4.810 69.89

0 12.45

8 18 2.186 2.345 5.116 62.410

11.997

18 1.916 2.530 4.840 69.890

12.245 19 2.240 2.298 5.138 62.40

0 11.80

7 19 2.024 2.420 4.890 69.90

0 11.83

4 20 2.480 2.109 5.222 62.420

11.013

20 2.078 2.370 4.920 69.930

11.660 21 3.440 1.587 5.447 62.47

0 8.644 21 2.132 2.320 4.940 69.93

0 11.46

1 22 4.400 1.270 5.576 62.470

7.082 22 2.186 2.270 4.960 69.940

11.259 23 5.360 1.059 5.661 62.45

0 5.995 23 2.240 2.230 4.980 69.94

0 11.10

5 24 6.320 0.908 5.720 62.410

5.194 24 2.480 2.050 5.060 69.950

10.373 25 7.280 0.794 5.764 62.39

0 4.577 25 3.440 1.539 5.280 69.98

0 8.126

26 8.240 0.706 5.798 62.390

4.093 26 4.400 1.232 5.411 70.000

6.666 27 9.200 0.635 5.824 62.39

0 3.698 27 5.360 1.027 5.490 70.01

0 5.638

28 10.160 0.578 5.846 62.400

3.379 28 6.320 0.881 5.550 70.000

4.890 29 11.120 0.529 5.864 62.41

0 3.102 29 7.280 0.771 5.600 70.03

0 4.318

30 12.080 0.489 5.880 62.410

2.875 30 8.240 0.685 5.630 70.050

3.857 31 13.040 0.454 5.892 62.42

0 2.675 31 9.200 0.617 5.656 70.05

0 3.490

32 14.000 0.424 5.903 62.440

2.503 32 10.160 0.561 5.678 70.060

3.185 33 20.000 0.299 5.950 62.46

0 1.779 33 11.120 0.514 5.690 70.09

0 2.925

34 26.000 0.232 5.976 62.470

1.386 34 12.080 0.475 5.710 70.090

2.712 35 50.000 0.122 6.016 62.48

0 0.734 35 13.040 0.441 5.722 70.11

0 2.523

36 74.000 0.083 6.030 62.500

0.500 36 14.000 0.412 5.730 70.110

2.361 37 98.000 0.063 6.036 62.52

0 0.380 37 20.000 0.291 5.778 70.10

0 1.681

38 122.000 0.051 6.040 62.530

0.308 38 26.000 0.225 5.800 70.100

1.305 39 146.000 0.043 6.043 62.56

0 0.260 39 50.000 0.119 5.843 70.13

0 0.695

40 170.000 0.037 6.044 62.580

0.224 40 74.000 0.081 5.857 70.160

0.474 41 194.000 0.033 6.044 62.59

0 0.199 41 98.000 0.062 5.860 70.15

0 0.363

42 306.000 0.022 6.048 62.610

0.133 42 122.000 0.050 5.868 70.180

0.293 43 518.000 0.013 6.049 62.63

0 0.079 43 146.000 0.042 5.871 70.18

0 0.247

44 730.000 0.010 6.050 62.660

0.061 44 170.000 0.036 5.873 70.160

0.211 45 942.000 0.008 6.050 62.72

0 0.048 45 194.000 0.032 5.874 70.18

0 0.188

46 1154.000 0.007 6.050 62.750

0.042 46 306.000 0.021 5.880 70.220

0.123 47 1472.000 0.006 6.050 62.81

0 0.036 47 518.000 0.013 5.880 70.23

0 0.076

48 1578.000 0.006 6.049 62.910

0.036 48 942.000 0.008 5.881 70.240

0.047 49 1790.000 0.005 6.048 62.99

0 0.030 49 1472.000 0.006 5.882 70.28

0 0.035

50 1896.000 0.005 6.048 62.980

0.030 50 1790.000 0.005 5.882 70.310

0.029 51 0.000 6.049 0.000 51 0.000 5.883 0.000