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Model Based Design and Performance Analysis of

Solar Absorption Cooling and Heating System

Ming Qu

Carnegie Mellon University

School of Architecture

Ph.D. Committee

Prof. Volker Hartkopf, Ph.D. (Chair)

Prof. David Archer, Ph.D.

Prof. Khee Poh Lam, Ph.D.


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Copyright Declaration

I hereby declare that I am the sole author of this thesis.

I authorize Carnegie Mellon University, Pittsburgh, Pennsylvania to lend this thesis to other

institutions or individuals for the purpose of scholarly research.

I authorize Carnegie Mellon University, Pittsburgh, Pennsylvania to reproduce this thesis by

photo copying or by other means, in total or in part, at the request of other institutions or

individuals for the purpose of scholarly research.

Copyright ©2008 by Ming Qu


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Acknowledgement

I wish to express my gratitude to my advisor, Dr. Volker Hartkopf, for his invaluable vision,

support, and encouragement. His enthusiasm and inspiration were essential to the success of this

research. Surely, I would like to extend my sincere appreciation and profound gratitude to Dr.

David Archer who has played a pivotal role in this thesis. He has far exceeded his duty as an

advisor. He gave me a deep understanding of mechanical engineering; he taught me how to

develop critical thinking and work effectively. He has been an ever-present source of guidance

and encouragement throughout my doctoral program. It gives me great pleasure to thank Dr. Khee

Poh Lam for providing valuable suggestions and carefully reviewing and constructively critiquing

of my work.

I owe many thanks to my dear colleague and husband, Hongxi Yin, who gave me continuous

support and took care of our babies, Ryan and David, who fill us with joy every day. This thesis is

dedicated to my parents in their confidence, their high expectations, and their hearty blessing.


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Model Based Design and Performance Analysis of

Solar Absorption Cooling and Heating System


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Table of Contents

1 Introduction........................................................................................................................... 1

1.1 Background and motivation........................................................................................... 2

1.1.1 Solar receivers ........................................................................................................... 3

1.1.2 Absorption Cycle....................................................................................................... 3

1.1.3 Solar collector coupled with absorption chillers........................................................ 5

1.2 Current studies on solar absorption cooling and heating systems.................................. 5

1.3 Research objective ......................................................................................................... 7

1.4 Research approach ......................................................................................................... 8

1.4.1 The planning of the test system ................................................................................. 9

1.4.2 The development of solar collector model ................................................................ 9

1.4.3 The development of annual system performance simulation .................................. 10

1.4.4 The installation of the test system ........................................................................... 11

1.4.5 The test program and experimental data gathering.................................................. 12

1.4.6 Data analyses, model validation and simulation evaluation .................................... 13

1.5 Chapter overview......................................................................................................... 13

2 Solar absorption cooling and heating test system and program..................................... 15

2.1 Parabolic trough solar collector ................................................................................... 15

2.1.1 Device description ................................................................................................... 15

2.1.2 Major components and characteristic ...................................................................... 17

2.2 Absorption chiller ........................................................................................................ 17

2.2.1 Device description ................................................................................................... 17

2.2.2 Major components and characteristics..................................................................... 18

2.3 Solar absorption cooling and heating test system ........................................................ 20

2.3.1 System description................................................................................................... 20

2.3.2 The solar collection loop ......................................................................................... 20

2.3.3 The load loop........................................................................................................... 22

2.3.4 Instrumentation, control and data acquisition system.............................................. 23

2.4 The test program .......................................................................................................... 27

2.4.1 The results of PTSC test at the transient states........................................................ 31

2.4.2 The results of PTSC test at a steady state ................................................................ 33


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2.4.3 The results of solar absorption cooling / heating daily test ..................................... 34

2.4.4 The results of solar heating daily test by using heat exchanger............................... 40

2.5 The interpretation of PTSC performance data ............................................................. 42

2.5.1 The energy balance of the PTSC ............................................................................. 42

2.5.2 The selection of experimental data .......................................................................... 44

2.5.3 PTSC performance predicted by statistic tool ......................................................... 44

2.6 Discussion of test program........................................................................................... 46

3 Solar collector performance model ................................................................................... 47

3.1 PTSC model assumption.............................................................................................. 47

3.2 Energy balance analysis ............................................................................................... 48

3.3 Heat transfer analysis................................................................................................... 51

3.4

Calculation procedure .................................................................................................. 53

3.5 Solar irradiation absorption.......................................................................................... 55

3.5.1 Direct normal solar radiation................................................................................... 56

3.5.2 Incident angle and incident modifier ....................................................................... 56

3.5.3 End-loss ................................................................................................................... 57

3.5.4 Shadow-loss............................................................................................................. 59

3.6 Conclusion ................................................................................................................... 60

4 Model-based experimental data analysis of PTSC........................................................... 61

4.1 Analytical method........................................................................................................ 61

4.1.1 Model validation...................................................................................................... 61

4.2 Model-based PTSC performance analysis ................................................................... 65

4.2.1 Temperature distribution in the receiver pipe.......................................................... 65

4.2.2 Thermal losses ......................................................................................................... 66

4.2.3 PTSC efficiency and solar radiation........................................................................ 67

4.2.4 PTSC efficiency and incident angle of solar beam.................................................. 68

4.2.5 PTSC efficiency and wind speed............................................................................. 68

4.2.6 PTSC efficiency and fluid type................................................................................ 69

4.2.7 PTSC efficiency and flow rate................................................................................. 70

4.2.8 PTSC efficiency and air in the annular space .......................................................... 70

4.2.9 PTSC efficiency and glass envelope........................................................................ 72

4.3 Recommendations on the PTSC’s design .................................................................... 73

4.3.1 Bellow design .......................................................................................................... 73


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4.3.2 Glass cover .............................................................................................................. 73

4.3.3 Diameter of the glass envelope................................................................................ 74

4.3.4 Diameter of the absorber pipe.................................................................................. 74

5 Solar absorption cooling and heating system simulation ................................................ 75

5.1 Model approach............................................................................................................ 76

5.2 Model assumptions ...................................................................................................... 77

5.2.1 Weather.................................................................................................................... 77

5.2.2 Assumptions in the model of solar energy supply system....................................... 79

5.3 System components and operation controls................................................................. 82

5.3.1 Components in the solar heating base-case ............................................................. 82

5.3.2 Operational controls in the solar heating base-case................................................. 83

5.3.3

Components and operation controls in the solar cooling base-case ........................ 84

5.4 Simulation evaluation .................................................................................................. 85

5.5 Base-case result of solar cooling and heating simulation ............................................ 90

5.5.1 Building simulation results ...................................................................................... 90

5.5.2 Solar energy system simulation results.................................................................... 91

6 Simulation-based design and performance analysis on solar cooling and heating ....... 96

6.1 Orientation of PTSC..................................................................................................... 96

6.1.1 Orientation of the PTSC for increased, effective solar energy recovery ................. 97

6.1.2 Orientation, tracking limitation, and solar beam irradiation on the PTSC ............ 101

6.1.3 Orientation and overall system performance ......................................................... 102

6.2 System operation and control..................................................................................... 102

6.2.1 Constant-flow or constant-outlet temperature control of the PTSC ...................... 102

6.3 Storage tank requirements.......................................................................................... 105

6.3.1 The volume of the storage tank ............................................................................. 106

6.3.2 Storage used for shifting energy for later use in solar heating .............................. 107

6.3.3 Storage used for shifting energy for later use in solar cooling .............................. 108

6.3.4 Storage used for preheating ................................................................................... 110

6.4 Auxiliary heater for preheating in the solar collection loop....................................... 112

6.5 The length and diameter of collection loop pipe and solar system performance ....... 113

6.6 The area of solar collector and storage tank............................................................... 116

6.7 Guidelines for design and operation of solar cooling and heating system................. 117


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7 Contributions and areas of future research ................................................................... 120

7.1 Contributions.............................................................................................................. 120

7.2 Areas of future research ............................................................................................. 122

7.2.1 Improving the tracking system of the PTSC.......................................................... 122

7.2.2 Extending the operational controls of the PTSC, the absorption chiller, and the heat

recovery exchanger............................................................................................................. 123

7.2.3 Integrate thermal storage in the cooling/heating system........................................ 124

7.2.4 Cost model ............................................................................................................. 125

8 References.......................................................................................................................... 126


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List of Figures

Figure 1-1 Simplified system arrangement of solar absorption cooling and heating system.......... 2

Figure 1-2 Two types of solar collector .......................................................................................... 4

Figure 1-3 Electric chiller and absorption chiller............................................................................ 4

Figure 1-4 Solar collector efficiency and operating temperature required by absorption chiller.... 6

Figure 1-5 Research approach schematic chart ............................................................................. 10

Figure 1-6 Process and instrumentation diagram of the test solar absorption cooling and heating

system............................................................................................................................................ 11

Figure 2-1 The PTSC’s installed on the IW .................................................................................. 16

Figure 2-2 Broad BJ16A parabolic trough solar collectors and the receiver tube......................... 16

Figure 2-3 Absorption chiller ........................................................................................................ 19

Figure 2-4 Absorption chiller in cooling cycle.............................................................................. 19 Figure 2-5 Absorption chiller in heating cycle.............................................................................. 19

Figure 2-6 Broad pump and control package ................................................................................ 22

Figure 2-7 Overall solar absorption cooling and heating test system............................................ 23

Figure 2-8 Structure of control system.......................................................................................... 24

Figure 2-9 Interface of the WebCTRL .......................................................................................... 25

Figure 2-10 PTSC test diagram at transient state .......................................................................... 28

Figure 2-11 Solar absorption cooling /heating system daily test................................................... 29

Figure 2-12 Solar heating daily test by using heat exchanger....................................................... 29 Figure 2-13 Operating temperatures of the PTSC test at transient state on 29 March 07 ............. 31

Figure 2-14 Energy flows of the PTSC test at the transient state on 29 March 07........................ 32

Figure 2-15 Operating temperatures of the PTSC test at steady state on 22 April 07................... 34

Figure 2-16 Energy flows of the PTSC test at steady state on 22 April 07................................... 34

Figure 2-17 Operating temperatures of solar cooling test on 31 July 07....................................... 37

Figure 2-18 Operating temperatures of solar cooling test on 16 July 07....................................... 37

Figure 2-19 Cooling capacity of solar cooling system on 31 July 07 ........................................... 38

Figure 2-20 Cooling capacity of solar cooling system on 16 July 07 ........................................... 38 Figure 2-21 Operating temperatures of solar absorption heating test on 9 March 07 ................... 39

Figure 2-22 Heating capacity of solar absorption heating system on 9 March 07 ........................ 39

Figure 2-23 Operating temperatures of HX based solar heating system on 2 March 07............... 41

Figure 2-24 Heating Capacity of HX based solar heating system on 2 March 07 ........................ 41

Figure 2-25 Scatter plot of I*Aa*cos(theta) and m*(Cpo*To-Cpi*Ti)......................................... 45


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Figure 2-26 Scatter plot of average operation temperature and m*(Cpo*To-Cpi*Ti) .................. 45

Figure 3-1 Energy flow in the PTSC............................................................................................. 48

Figure 3-2 The thermal network.................................................................................................... 52

Figure 3-3 The connection between the PTSCs ............................................................................ 53

Figure 3-4 Incident angle of the PTSC.......................................................................................... 56

Figure 3-5 Incident angle modifier and incident angle.................................................................. 57

Figure 3-6 End-loss of the PTSC................................................................................................... 58

Figure 3-7 The length of the end-loss in the solar field................................................................. 58

Figure 3-8 Shadow loss from the adjacent solar collector array ................................................... 59

Figure 4-1 Measured temperature distribution of the glass envelope............................................ 62

Figure 4-2 Comparison between the measured data and calculation solutions ............................. 65

Figure 4-3 Temperature distribution in the receiver pipe.............................................................. 66

Figure 4-4 Thermal losses through the receiver pipe .................................................................... 67

Figure 4-5 PTSC’s efficiency and direct normal solar radiation at 0 incident angle..................... 67

Figure 4-6 PTSC’s efficiency and incident angle.......................................................................... 68

Figure 4-7 PTSC’s efficiency and direct normal solar radiation at 15 incident angle................... 69

Figure 4-8 PTSC’s efficiency and wind speed .............................................................................. 69

Figure 4-9 PTSC’s efficiency and fluid type................................................................................. 70

Figure 4-10 PTSC’s efficiency & flow rate................................................................................... 71

Figure 4-11 PTSC’s efficiency and air in the annual space .......................................................... 71

Figure 4-12 Thermal losses with Sun or No-sun........................................................................... 72 Figure 4-13 PTSC’s efficiency and glass cover ............................................................................ 72

Figure 4-14 New bellow design from SOLEL .............................................................................. 73

Figure 5-1 Information flow of TRNSYS simulation ................................................................... 77

Figure 5-2 Monthly average dry bulb temperature of Pittsburgh.................................................. 78

Figure 5-3 Direct normal solar radiation in Pittsburgh.................................................................. 78

Figure 5-4 Daily average solar radiation throughout a year in Pittsburgh .................................... 79

Figure 5-5 TRNSYS information flow diagram of solar cooling base case.................................. 80

Figure 5-6 TRNSYS information flow diagram of solar heating base case .................................. 80 Figure 5-7 Boiling temperature and pressure of aqueous propylene glycol solutions................... 81

Figure 5-8 BROAD PTSC tracking range..................................................................................... 82

Figure 5-9 Operation temperature comparison between solar heating evaluation simulation and

experiment..................................................................................................................................... 86


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Figure 5-10 Energy flow comparison between solar heating evaluation simulation and experiment

....................................................................................................................................................... 87

Figure 5-11 Operating temperature comparison between cooling evaluation simulation and

experiement ................................................................................................................................... 88

Figure 5-12 Energy flow comparison between solar cooling evaluation simulation and experiment

....................................................................................................................................................... 89

Figure 5-13 IW building heating and cooling load estimated by building simulation .................. 90

Figure 5-14 Useful solar energy and IW sensible heating load in January ................................... 92

Figure 5-15 Useful solar energy and IW sensible cooling on 30 December................................. 92

Figure 5-16 Useful solar energy, cooling load and energy provided by chiller in August............ 94

Figure 5-17 Useful solar energy, cooling load and energy provided by chiller on 09 August...... 94

Figure 6-1 Twelve orientations in the simulation.......................................................................... 97

Figure 6-2 Orientation and solar beam irradiation on a PTSC on 10 June in Pittsburgh .............. 98

Figure 6-3 Orientation and solar beam irradiation on a PTSC on 2 December in Pittsburgh ....... 98

Figure 6-4 Orientation and solar beam irradiation on a PTSC in summer of Pittsburgh .............. 99

Figure 6-5 Orientation and solar beam irradiation on a PTSC in winter of Pittsburgh ................. 99

Figure 6-6 Annual solar beam irradiation on PTSC with different orientations in Pittsburgh.... 100

Figure 6-7 Tracking angle and orientation of the PTSC on 21Jun.............................................. 100

Figure 6-8 Tracking angle and orientation of the PTSC on 21 Dec............................................ 100

Figure 6-9 Solar beam irradiation and orientation of the PTSC in Pittsburgh ............................ 101

Figure 6-10 System performance comparison of alternate controls on 9 August 2007 .............. 104 Figure 6-11 System operating temperature comparison of alternate controls on 9 August 2007 104

Figure 6-12 Trnsys information flow diagram of solar heating system with storage.................. 107

Figure 6-13 Solar energy collected, heating load, and energy provided on 14, 15 November 2007

..................................................................................................................................................... 108

Figure 6-14 Trnsys information flow diagram of solar cooling system with storage for shifting

energy .......................................................................................................................................... 109

Figure 6-15 Operating temperature of solar cooling system with and without storage on 9 August

..................................................................................................................................................... 109 Figure 6-16 Trnsys information flow diagram for solar cooling with storage for preheating..... 111

Figure 6-17 Effect of a heater on energy flow for solar cooling on 09 August........................... 113

Figure 6-18 Effect of a heater on operating temperature for solar cooling on 9 August............. 113

Figure 6-19 Solar fraction and pipe size under two control strategies ........................................ 115

Figure 6-20 System energy performance and pipe size on 9 August .......................................... 115


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Figure 6-21 Operating temperature and pipe size on 9 August................................................... 116

Figure 6-22 Effect of PTSC area and storage volume on the solar fraction in IW cooling and

heating ......................................................................................................................................... 117

Figure 6-23 Idealized IW solar cooling/heating system performance and system sensitivity

analysis ........................................................................................................................................ 119


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List of Tables

Table 2-1 Specifications of the parabolic trough solar collector installed .................................... 18

Table 2-2 Specifications of the absorption chiller installed .......................................................... 21

Table 2-3 Instrumentation of IW solar cooling and heating system.............................................. 26 Table 2-4 Four types of tests conducted........................................................................................ 28

Table 2-5 Heat capacity of the solar collection loop..................................................................... 33

Table 2-6 Operating condition in the PTSC performance tests..................................................... 33

Table 2-7 Heating system performance comparison between HX based and absorption chiller

based.............................................................................................................................................. 41

Table 3-1 Heat transfer correlations used in the PTSC model ...................................................... 54

Table 3-2 Parameters and values used in the PTSC model ........................................................... 55

Table 4-1 Glass temperature measurements in the test ................................................................. 62

Table 4-2 Comparison between measured values and model calculations.................................... 64

Table 5-1 Control mode in the base-case simulation of solar heating system............................... 83

Table 5-2 Control mode in the base-case simulation of solar cooling system .............................. 85

Table 5-3 System performance estimated by IW solar heating system base-case simulation....... 91

Table 5-4 System performance estimated by IW solar cooling system base-case simulation ...... 93

Table 6-1 Effect of PTSC’s orientation on overall system performance..................................... 102

Table 6-2 Effect of flow controls on overall system performance.............................................. 103

Table 6-3 Effect of storage volume on solar heating system performance ................................. 108

Table 6-4 Effect of storage volume on solar cooling system performance ................................. 110

Table 6-5 Effect of “Preheat” storage tank volume on solar cooling performance..................... 111

Table 6-6 Effect of collection loop volume on solar heating system performance ..................... 114

Table 6-7 Effect of collection loop volume on solar cooling system performance..................... 114


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Abstract

The work presented in this thesis deals with the question of how solar energy might most

effectively and efficiently be used in supplying energy for the operation of a building. The

approach to dealing with this question has involved a specific building space, Carnegie Mellon’s

Intelligent Workplace; a specific solar system, parabolic trough solar thermal receivers, Parabolic

Trough Solar Collector’s; and a specific building energy use, space cooling and heating. The

work has involved the design, installation, and test of a system incorporating PTSC’s, an

absorption chiller, a heat recovery exchanger, auxiliary equipment, instrumentation and controls.

Mathematical models based on fundamental scientific and engineering principles have been

developed and programmed for both the PTSC’s and the overall IW cooling and heating system,

These models have been improved and validated through comparisons of predicted and measured

PTSC and IW cooling and heating system performance. The work reported in this thesis has

developed suggestions and methods for the effective design and evaluation of PTSC’s and also

for the optimized design and operation of solar absorption cooling and heating systems, so that

the system is able to reduce building energy consumption, and achieve environmental benefits in

the operation of buildings by the use of renewable, solar energy.


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1 Introduction

HVAC systems are the major users of electricity in commercial buildings. In the United States,

commercial air conditioning makes up 40% of the summer time peak electrical demand (Kulkarni

1994). In recent years, the increasing power demand of building HVAC system, and the

increasing costs of energy have caused people to seek alternative cheaper, renewable energy

sources for building cooling and heating. In addition, the environmental issues such as global

warming, ozone depletion, and energy conservation, are other important factors, impelling people

to look for space cooling and heating without involving gas or electricity. The use of solar energy

for building cooling and heating can potentially provide the solution to these economic and

environmental problems.

An important motivation for research and development in solar cooling is the coincidence of

comfort cooling demand and the availability of solar radiation. Building cooling systems which

use solar thermal energy can make use of absorption cycles, desiccant cycles, on solar-mechanical

processes. Compared to solar desiccant cycles and solar-mechanical processes, solar absorption

cycle technology is more developed; solar thermally driven systems can provide reliable and

quiet cooling. In addition, combining solar heating and domestic hot water production with

cooling can improve economic performance of the system, compared to solar heating or solar

cooling alone.

Solar absorption cooling was a subject of significant research interest from 1970 to 1980, when anumber of demonstration projects were conducted in the United States. However, these systems

failed to establish a significant global market for cooling systems due to their high initial cost,

lack of commercial hot water driven absorption chillers, and scarcity of demonstrations and

impartial assessments by reputable institutions (Kulkarni 1994).

This thesis investigates the technical and energy efficient aspects of using high temperature solar

thermal receivers with a two stage absorption chiller to cool and heat a building space; it

reassesses the feasibility of commercializing solar absorption cooling technology by considering

the recent improvements in solar collection technologies, severe electric shortages, and the

environmental problems. This research contributes in depth knowledge and methods for the

design and operation of solar absorption cooling systems that reduce energy consumption,

decrease operational costs, and minimize air quality problems.


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The work reported in this thesis has developed, validated and applied a numerical performance

model, a parabolic trough solar collector (PTSC) model; this model provides a tool for the

analysis of performance data and for the design and operation of a solar absorption cooling and

heating system for a building. Additionally, the work developed a comprehensive simulation and

applied it to system optimization and sensitivity analyses. This thesis provides generic guidelines

on the design and operation of a solar absorption cooling and heating system in order to reduce

energy consumption and operation costs, as well as benefit the environment.

1.1 Background and motivation

Solar heating systems have been studied for almost 70 years since the Massachusetts Institute of

Technology began their studies in 1938 (Beckman 1980). Currently solar heating is relatively

mature. However, despite substantial research and development efforts on solar cooling, most

have focused on simulation and isolated experiments. Few involved the current state of hot water

driven, two stage absorption chillers and well designed solar chillers with effective control

systems. The requirements for effective operation and maintenance practices have not been

explored.

A typical solar absorption cooling and heating system is comprised primarily of the solar

receivers and the absorption chiller. The solar receivers convert solar radiation to thermal energy

in a heated fluid; the absorption chiller then uses this energy in summer to generate chilled water.

In the cooling cycle, the system acts as a heat pump; it gets the heat from chilled water and from

the sun and rejects heat by cooling water. In winter, instead of using cooling water, the system

directly transfer heat to the loop providing space heating. A simplified schematic of a solar

absorption cooling and heating system is illustrated in Figure 1-1.

Figure 1-1 Simplified system arrangement of solar absorption cooling and heating system

Chilled water

Hot water

s o l a

r c o l l e c

t o r s

absorption

chiller

building

cooling water

cooling cycle

heating cycle


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1.1.1 Solar receivers

Solar receivers are normally are classified in two groups: flat plate collectors and concentrating

collectors as shown in Figure 1-2. A flat plate collector can use both direct and diffuse solar

irradiation on a fixed receiving plate, while a concentrating device can only use the direct solar

irradiation since the diffuse solar irradiation from various directions focused by the reflector away

its focal line where the receiver pipe located. Concentrating collectors, with a relatively small

absorption area, can heat the heat transfer fluid to temperatures, far above those attainable by flat

plate collectors. Flat plate collectors are normally used in applications that require only low

temperatures, less than 100 ˚C; and concentrating collectors are utilized in medium or high

temperature applications, up to 600˚C.

1.1.2 Absorption Cycle

Absorption chillers can be thermally driven by using heat from the sun, from engine exhaust

gases, or from other variety of sources, to provide reliable and quiet cooling. They do not use

atmosphere harming halogenated refrigerants, and they can be used to reduce summer electric

peak demand. To illustrate how an absorption chiller works, a comparison between an absorption

chiller and an electrically driven vapor compression chiller is shown in Figure 1-3.

In the electrically driven vapor compression chiller, on the left of Figure 1- 3, a refrigerant vapor

is compressed to a higher pressure by a compressor and condensed by rejecting heat to the

ambient in the condenser. The refrigerant liquid then flows through the expansion valve to an

evaporator maintained at a low pressure mixture of liquid and vapor. This liquid refrigerant initial

flushes into a mixture of vapor and liquid. The liquid then vaporizes in the evaporator as it

absorbs heat from the water to be cooled thermally producing the cooling effect.

Instead of using a compressor, an absorption chiller, on the right of Figure 1- 3, produces the

same compression effect, raising the pressure of water vapor by absorbing vapor at low pressure

in the absorber and consequently desorbing this vapor at a high pressure in the regenerator. The

water vapor is absorbed at a low pressure by the concentrated sorbent solution in the absorber. A

solution pump then pumps up the diluted solution to a higher pressure and temperature in the


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Figure 1-2 Two types of solar collector

Electric chiller Absorption chiller

Figure 1-3 Electric chiller and absorption chiller

Condenser

Evaporator

from chilled water

expansion

Refrigerant

valve

Rejected heat

Work

Compressor

Heat absorbed

T

P

Condenser Regenerator

Absorber Evaporator

expansion

Refrigerant

valve

expansionSolution

valve

from chilled water

Heat absorbed

pumpSolution

T

P

Rejected heat

Rejected heat

Heat input

Flat-plate collector

Evacuated-tube collector

Parabolic trough solar collector (PTSC)(one axis tracking)

Integrated compound parabolic collector(fixed in a flat assembly)

Flat-plate collector Concentrating collector


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regenerator. The water vapor is boiled off from the diluted sorbent solution in the regenerator by

adding thermal energy, and the water vapor is condensed by rejecting heat to cooling water in the

condenser. The other processes in the absorption chiller, the vaporization of the water refrigerant

and the removal of heat from the chilled water in the evaporator, operate similarly to the ones in

the electric chiller.

If the temperature of the refrigerant water vapor (steam) produced in the regenerator, is high

enough, then it can be used to produce more refrigerant vapor from the weak solution. In return,

this vapor is then used in a chiller, which then serves as a double effect absorption chiller.

From the thermodynamic point of view, the absorption chiller is a combination of a heat engine

and heat pump. The heat engine absorbs heat at a high temperature, rejects heat at a lower

temperature, and produces work. The work drives a “heat pump” that absorbs heat at a low

temperature and rejects it to the ambient at a higher temperature.

1.1.3 Solar collector coupled with absorption chillers

All the varieties of solar collectors can be used for cooling, given the broad operating temperature

ranges of absorption chillers. Figure 1- 4 plots the efficiency of the four types of solar collectors -

- flat plate, evacuated tube without (ET) and with concentration (CPC), and linear parabolic

trough (PTSC) depending on their operating temperature. Typically, a single effect LiBr/H2O

absorption chiller requires a 75-100 °C heat source; and a double effect one requires a 150-180 °C

heat source to generate chilled water. Normally flat plat solar collectors are coupled with single

effect chillers due to their relatively high efficiency at low operating temperature; and evacuated

tube and parabolic trough collectors are used to provide a high temperature heat source for double

effect absorption chillers.

This thesis is focused on the solar absorption cooling system, comprised of the PTSC’s and the

double effect absorption chiller as indicated by the rectangular outline in Figure 1-4.

1.2 Current studies on solar absorption cooling and heating systems

Substantial research and development efforts on solar absorption cooling are in progress; and the

technology is evolving. Most of the research focuses on simulation analyses of solar driven single


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effect absorption cooling systems. In general, it has been concluded that this approach is the most

economic configuration with the highest system performance.

Although theoretically ET or CPC collectors can heat fluid to 150~160 °C, the required

temperature by the double effect absorption chiller, there are few successful studies showing thatthese two types of solar receivers can be successfully incorporated with double effect absorption

chillers.

Duff (Duff 2004) has reported that 106.5 m2 of integrated compound parabolic concentrating

collectors (ICPC) have been operated at 90-130 °C to serve a 70kW (20 ton) hot water driven

double effect McQuay/Sanyo chiller to serve a commercial building in Sacramento, California. At

the beginning of the operation in 1998, daily collection efficiencies were nearly 50% in the

operating range of 120-160 °C. While later, the highest operating temperature was at 110 °C with

55% daily collection efficiencies, due to the lower operating temperature with less heat loss.

Figure 1-4 Solar collector efficiency and operating temperature required by absorption chiller


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At U.S. Army’s Yuma Proving Ground in Arizona a 1245 m2 of Hexel PTSC’s solar system

supplying heated water to a 160 ton LiBr/H2O double effect absorption cooling system has been

successfully operated for nearly 14 years since its installation in 1979 (Hewett 1995). Hewett

discussed the technical and economic performances of this project and drew the conclusion that

the economics of solar absorption cooling systems are unattractive compared to conventional

alternatives (in 1995). The primary rationale included the high capital and operating costs of solar

collectors with absorption chillers, compared to those of electrically driven vapor compression

chillers.

Supplementing experimental research, many simulation based studies have been performed on

solar driven double effect absorption cooling systems. Lokurlu carried out system simulation

analyses in TRNSYS (Lokurlu 2002). He concluded that the combination of PTSC and a double

effect absorption chiller is promising for cooling systems with a load of at least 100 kW; Since

the PTSCs were not available for the study, this work remained in the simulation and preliminary

design stage.

Wardono developed a computer model of the double effect LiBr/H2O absorption cooling system

coupled with tilted flat plate solar collectors for an application in Albuquerque, NM (Wardono

1996). He calculated that the total solar energy input depends upon the ambient temperature, sky

clearness index, and system design; and he indicated that the solar contribution for supplying

energy to the double effect LiBr/H2O cooling system did not significantly vary for various

latitudes.

1.3 Research objective

The primary purpose of this research program is the development of systems which reduce the

energy requirements for the operation of buildings by a factor of two or greater, and the provision

of techniques and tools for the design and evaluation of such systems. This thesis re-assesses the

technical aspect of a solar driven double effect absorption cooling system by means of

experimental equipment, modeling and system simulation.

The aim of this research is to develop methods and tools for the effective design and optimization

of a solar absorption cooling and heating system that reduces energy consumption, decreases


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capital, operation, and maintenance costs, and also benefits the environment. The special

objectives of this thesis are:

• the establishment of a unique experimental set up and procedures for investing a solar

absorption cooling and heating system

• the carrying out a test program on both a parabolic trough solar collector and an

absorption cooling and heating system

• the construction of a computerized performance model of a parabolic trough solar

collector based on energy balances and heat transfer correlations and validation of the

model with data obtained from the test program

• the development of annual overall solar cooling and heating system simulation, the use of

this simulation to design the system and to optimize its performance; and validation of

the simulation by data obtained from the test program

• the analysis of the experimental data, refinement of the model, and improvement of the

design and operation on the basis of the simulations

The solar collector model and overall solar system simulations developed are now being used as

tools to adapt parabolic trough solar collectors and double effect absorption chillers to various

climate zones, building applications, and system configurations in order to provide optimized

design and operation guidelines. In both its practical and theoretical aspects, this study

contributes important knowledge for the application of the solar absorption cooling and heating

systems. The practical observation and operation of the particular solar cooling and heating

system has laid the groundwork for improvement of the control and integration of solar collectors

and double effect absorption chillers; thus the analytical methods provide a platform to analyze

and improve systems.

1.4 Research approach

To achieve the thesis objectives, this research is focused on solar collector model development,

system performance simulation, system installation and test of a high temperature based solar

absorption cooling and heating system as illustrated in Figure 1-5.


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In this research, mathematical modeling, system simulation, equipment testing, system testing,

and data analyses were combined to provide a deeper understanding of the system, to discover the

possible improvements in the solar collector design, to optimize the system design and operation,

and to provide a framework to design and evaluate solar absorption cooling and heating systems.

The research has been carried out by the following steps.

1.4.1 The planning of the test system

Prior to the design and installation of the solar cooling and heating system, studies were

performed regarding the solar field location, the structure for supporting the solar collectors,

system energy balances, the piping, and other engineering issues. In the mean time, a

comprehensive parabolic trough solar collector model and annual system performance

simulations were developed to assist the system design and operation.

1.4.2 The development of solar collector model

The performance model of the solar collector is focused on a coated absorber pipe enclosed in an

evacuated glass envelope: the receiver of the parabolic trough solar collector (PTSC). This steady

state, single dimensional model is based on fundamental material and energy balances together

with heat transfer correlations programmed in the Engineering Equation Solver (EES).

Incident solar energy on the solar collector is distributed among useful energy gain, optical losses,

and thermal losses. To represent different optical losses, coefficients have been introduced in the

model. This model deals with the thermal losses resulting from conduction, convection and

radiation heat transfer to the surroundings, from the receiver. This PTSC model is based on

energy balance relations for the absorber pipe and the glass envelope together with heat transfer

correlations for the various the energy streams among them and the surroundings. It has 192

variables and 160 equations. This model predicts how the efficiency of the PTSC is influenced by

direct normal solar radiation, the incidence angle, collector dimensions, material properties, the

operating temperature, the presence of air in the annular space, the wind speed, the type of the

fluid, and the operating flow rate.

This model was used to select the proper operating conditions, to detect the possible problems in

the operation of the collectors such as incipient boiling at the tube surface, and to select a

circulation pump. Additionally the estimated performance of PTSC has been used to optimize

design parameters and operating conditions of the solar absorption cooling and heating system.


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1.4.3 The development of annual system performance simulation

The system performance simulations were modeled in the TRNSYS transient simulation program

by two sections; the building simulation and the solar energy system simulation. The

sophisticated building simulation calculates the building heating and cooling loads based on the

inputs of the configuration of the building; weather conditions; the schedules for occupancy,

lighting, equipment; and set points for temperature and humidity in the test building space. The

thermal system simulations estimated the required energy input to meet the calculated building

loads, either as available solar radiation or as natural gas auxiliary fuel. The thermal system

simulation included all of major system components and operation strategies. Most of the major

components in the test solar energy supply system are available in the simulation library. Only

two new components, the solar receiver and the system controls, were written to integrate

controls for PTSC, chiller, pumps and fan coils.

Figure 1-5 Research approach schematic chart


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The system simulation has been used as a generic system model to optimize system design and

operation, and to assess the impact of these parameters on the system performance: the

orientation of the PTSC; the location and volume of the storage tank; the piping diameter, length,

thickness, and the insulation; as well as the operating strategy.

1.4.4 The installation of the test system

The solar thermal system was designed to cool or heat the south zone of the Robert L. Preger

Intelligent Workplace (IW), an office space for multiple uses – class rooms, laboratories, meeting

spaces, offices for faculty and students, at Carnegie Mellon University. To meet the cooling and

heating loads of this space, a 16 kW double effect absorption chiller was selected and installed.

Figure 1-6 Process and instrumentation diagram of the test solar absorption cooling and heating

system

This chiller is driven either by hot water or by natural gas to provide cooling in the summer and

heating in the winter. The chiller switches between the cooling mode and heating mode by

adjusting a two-position valve. This chiller incorporates a cooling tower to reject heat from its

operation as required in the cooling cycle. To satisfy the requirement of the double effect

absorption chiller, 52 m2 of linear parabolic trough solar thermal receivers, (PTSC) were

installed, including a circulating propylene glycol water mixture, instrumentation for flow,

temperature, pressure and direct normal solar radiation; circulation pumps, an expansion/pressure

tank; and a drain/ filling apparatus. A web-based automation system was also installed to operate


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the solar collector, heat exchanger and the absorption chiller with their auxiliary system, monitor

the overall system status, and collect experimental data. In addition, there is a heat exchanger

installed parallel to the absorption chiller to compare the system performance between the solar

heating systems using an absorption chiller and the heat exchanger. The process instrumentation

diagram of the test solar absorption cooling and heating system is shown in Figure 1-6.

1.4.5 The test program and experimental data gathering

The experiments provided significant knowledge and understanding of the PTSC’s and the solar

absorption cooling and heating system. The test program characterized the equipment and the

systems, validated the mathematic model of the solar collector, and the system simulation for

evaluating the annual system performance. The experiments in the test program are classified into

three groups:

• Solar collector performance – testing the solar collectors operated at a steady state, the

inlet temperature of heat transfer fluid (HTF) entering the solar collector receiver arrays

and the direct normal solar radiation are constant; testing the solar collection at a transient

state, when the operation temperature of the solar collectors increases with time until an

elevated temperature is researched.

• Solar heating – in the morning, circulating the heat transfer fluid through the bypass until

the desired temperature is reached, and then diverting the HTF to the absorption chiller/

heat exchanger to produce hot water for space heating. The building load, heat exchanger

HX-1 maintains the hot water from the chiller / HX-2 in the reasonable range by the

available flow of chilled water. Ultimately, when the solar energy is no longer adequate to

operate chiller / HX-2 due to the heat loss from the system and the reduction of the direct

normal solar radiation, the HTF is then diverted through the bypass.

• Solar cooling – in the morning, circulating the HTF through bypass until the temperature

required by the absorption chiller is reached, then diverting the HTF flow through the

absorption chiller to produce chilled water for space cooling. Ultimately, when the

amount of solar energy supply is no longer adequate to operate the absorption chiller, the

HTF is then diverted through the bypass.

During these tests, as indicated in Figure 1.6, data on temperature, pressure, flow, and direct

normal solar radiation were gathered by the data acquisition system for further data analyses.


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1.4.6 Data analyses, model validation and simulation evaluation

After the data were collected, they were analyzed using the basic steady state heat balance of the

receiver and the balance of the system to determine fundamental characteristics of the system

such as the solar collector efficiency, the collector and system heat capacities, and the heat losses

from the system making use of statistical and other mathematical tools. The data then were used

to validate the PTSC model and to evaluate system simulation by comparisons with the solutions

of the solar collector model or system simulations.

There were two observations from the comparison. First, the initial collector model predicted

higher collector efficiency than the measurements. This discrepancy proved to be due to the

absorptivity of the glass envelope, a significant parameter impacting on the collector efficiency.

This absorptivity was apparently much higher than the value given by Broad, the equipment

manufacturer. After the properties of the glass envelope in the initial collector model were

adjusted, the deviation between the experimental data and the model solution was minimized.

Second, initial system performance simulation estimated much shorter preheat time than those

observed at the beginning of each day. This discrepancy proved to be due to the pipe component

in the TRNSYS modeled as an empty pipe without the heat capacity. A small storage tank,

inserted in the system simulation, added appropriate heat capacity to the piping. As a

consequence, the experimental data and the system simulation solution came into agreement.

1.5 Chapter overview

This thesis contains seven chapters followed by references, appendixes, and nomenclature.

Chapter 1, Introduction introduces the background and motivation of this dissertation and

summarizes the research objectives and approach.

Chapter 2, Solar absorption cooling and heating test system and program introduces the

overall experimental system setups of solar absorption cooling and heating test system. It presents

detailed information on the system devices, instrumentation and control. The test program and

data acquisition are introduced. The experimental data are analyzed.

Chapter 3, Solar collector performance model addresses the model assumptions, energy balance

and heat transfer and calculation procedure. It also introduces the facts of the absorption of solar

irradiation impact on the solar collector efficiency.


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Chapter 4, Experimental data analysis on PTSC model assesses and validates the solar model

by using experimental data. The model is used to analyze the PTSC performance under various

weather and operational condition.

Chapter 5, Solar absorption cooling and heating system simulation introduces the simulation

assumptions and in depth description of operation and control in the base case of the solar coolingand heating simulation. This chapter also includes the assessment and evaluation of the system

performance model by using the experimental daily data.

Chapter 6, Simulation-based design and performance analysis on absorption cooling and

heating system presents the system optimization and system sensitivity analysis by serial system

comparison simulations. The guidelines of design and operation for solar cooling and heating

system are provided.

Chapter 7, Conclusions and recommendations summarizes the finding and contributions of this

thesis and suggests future areas for research and the issues involved, including: the thermal

storage equipment, an advanced control system, and the integration of cooling and heating

devices.


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2 Solar absorption cooling and heating test system and program

A solar absorption cooling and heating test system has been designed, installed, and tested in the

IW at Carnegie Mellon University of Pittsburgh. As shown in Figure 1-6, the system consists of

52 m2 of parabolic trough solar collectors, PTSC’s; a 16 kW double effect absorption chiller; a

heat recovery exchanger; and a variable, simulated building load exchanger to measure the

performance of the solar collector and the overall solar cooling and heating system. A web-based

data acquisition and control system was developed and installed to operate the solar system while

storing and displaying the test measurement data. The PTSC was tested at various operating

conditions: direct solar irradiation, wind load, heat transfer fluid, flow rate, and temperature.

Daily tests on the solar cooling and heating were conducted at various weather conditions: clear

day, mostly sunny day, mostly cloudy and overcast in winter and summer, respectively. In the

future, this solar absorption cooling and heating test system will be integrated with the cooling

and heating units of the IW and incorporated in the campus cooling and heating grids.

2.1 Parabolic trough solar collector

2.1.1 Device description

The solar collectors installed in the IW are single axis tracking solar concentrators: parabolic

trough solar collectors (PTSCs). They track the altitude of the sun as it travels from east to west

during the day to ensure that the radiation from the sun is continuously focused on the linear

receiver. These PTSC’s, provided by Broad Air Conditioning Co., have a 52m2 aperture surface,

which is the total open cross sectional area of four modules of parabolic reflectors.

Figure 2-1 shows two arrays of PTSC’s installed in series in the two valleys of the IW saw-tooth

roof. They are connected by the supply and return lines of the heat transfer fluid (HTF), an

aqueous solution containing 50% propylene glycol, with the absorption chiller installed on the

southeast platform adjacent to the IW. The tracking axes of the PTSC’s are oriented 15° east of

true north because this orientation minimizes the collector height, wind loading, and structural

requirements of the installation.


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Figure 2-1 The PTSC’s installed on the IW

Figure 2-2 Broad BJ16A parabolic trough solar collectors and the receiver tube

The PTSCs

The IW

Pipelines

Receiver tube

Parabolic trough

reflector

Supportstructure

Tracking

mechanism

(Stine 1987)


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2.1.2 Major components and characteristic

The installed PTSC, shown in Figure 2-2, comprises a parabolic trough reflector mirror; a

receiver tube, a surface treated absorber pipe at its focal line surrounded by an evacuated

transparent tube; supporting structure; and a tracking mechanism.

A module of the Broad PTSC receiver weighs 200 kg. It is designed to withstand a 31 m/sec

wind load. The 13.34 m2 aperture area and 0.68 m2 receiver area corresponds to a 19.6

concentration ratio. The parabolic reflector trough, according to Broad, has a reflectance of 0.8.

Most of solar radiation on the reflector is focused on the receiver tube, after it impinges on this

mirror. The receiver tube consists of an absorber pipe at its center surrounded by a glass envelope

as shown in the right bottom of Figure 2-2. The absorber pipe is coated with selective blackened

nickel, which has a high absorptivity of 0.96 for short wave length solar radiation and a low

emissivity of 0.14 at 100°C for long wave length heat radiation. The glass envelope plays an

important role in reducing convective and the radiative losses from the receiver tube to the

atmosphere. Its surface temperature is much lower than that of the absorber pipe, and it is opaque

to thermal radiation from that tube. In addition, the annular space between the absorber pipe and

the glass envelope is evacuated in order to decrease the conduction and convection between them.

The Broad PTSC tracking drive is a large semi circular gear engaged with a small gear powered

by a 24V servo motor. PTSC’s typically have a higher efficiency than plate solar collectors such

as a flat solar collector or an evacuated-tube collector when the operation temperature is high.

The characteristics of the PTSC installed in the IW are addressed in Table 2-1.

2.2 Absorption chiller

2.2.1 Device description

The absorption chiller installed in IW is a dual fired two-stage, water-LiBr chiller with a cooling

tower. This chiller, also fabricated by Broad Air Conditioning Co., has a 16 kW rated cooling /

heating capacity driven by either hot water or natural gas. Figure 2-3 is the absorption chiller

installed on the southeast platform of the IW. It is connected with solar collection loop, and its

chilled / hot water supply and return lines. Its working schematic flow diagrams1 are illustrated in

Figure 2-4 and Figure 2-5.

1 From the absorption chiller brochure of Broad Air Conditioning Co.


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Table 2-1 Specifications of the parabolic trough solar collector installed

Parabolic trough solar collector features (BJ16A)

Manufacturer:

Operating temperature:

Module size:

Module operating weight:

Drive group size:

Delta-T loop size:

Rim angle, 2:

Reflectors:

Focal length:

Receiver:

Sun tracking

Tracking drive System

Wind loads

Broad Air Conditioning

Broad Town, Chang Sha, Hu Nan, CHINA

60-180C

2.3 * 5.75 m; 13.225 m2

200 kg

2 modules; 26.45 m2

2 drive groups; 52.9 m2

73°

Typical reflectance 0.8

81.8 cm

Absorber OD: 3.8 cm

Base material: Stainless steel 304L

Coating: Black nickel

Typical absorptivity: 0.96

Typical emittance: 0.14@100C

Pyrex glass cover OD: 10.2 cm

Transmissivity: 0.91

Vacuum in the annular space

Single-axis elevation tracking based on the calculated sun position

24 V powered

Servo motor

Small gear, big gear.

16 m/sec (tracking)

31 m/sec (stowed)

2.2.2 Major components and characteristics

The absorption chiller consists of five major and minor heat transfer components, three pumps, a

cooling tower, and other associated valves and pipe fittings in Figure 2-4.

The five major components include: an evaporator (marked 4), an absorber (marked 5), a high-

temperature regenerator (marked 2), a low-temperature regenerator (marked 1), and a condenser

(marked 3). The three minor heat transfer components are: a high temperature heat interchanger

2 Rim angle is the angle between the line from vertex to focus and the line from focus to the parabola ridge

point.


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(marked 6), a low-temperature heat interchanger (marked 7), and a refrigerant by-pass heat

exchanger (marked 7a).

Figure 2-3 Absorption chiller

The three pumps are: a solution pump (marked 8), a

chilled / hot water pump (marked 12), and a

refrigerant pump (marked 9).Table 2-2 lists the chiller

specifications from the manufacturer; these were the

commissioning data before the chiller sent out. The

chiller specification data are useful in evaluating the

experiment data of the chiller.

Both of these two flow diagrams intend to address the

general working principle; they do not include the

heating coils inside the regenerator. In the cooling

cycle, the water vapor is absorbed into concentrated

LiBr solution in the Absorber shown in Figure 2-4. A

solution pump then pumps up the dilute LiBr solution

Figure 2-4 Absorption chiller in cooling cycle Figure 2-5 Absorption chiller in heating cycle


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to the Regenerator that operates at a higher pressure and a higher temperature to vaporize water

from the solution making use of thermal energy from the solar collectors or from the natural gas

burner. The water vapor is condensed by rejecting heat to cooling water in the Condenser. Next,

condensate water is passed through an expansion nozzle into the Evaporator. The water is

vaporized there at a low pressure, absorbing heat transferred from chilled water flow.

The installed double effect absorption chiller has a coefficient of performance of 1.2 when

operated in cooling mode, driven by either fluid heated in solar receivers or by natural gas. A

single valve, marked 24 on both Figures 2-4 and Figure 2-5, can be opened to switch the chiller

from the cooling to heating mode. In the heating mode, the water vaporized from the LiBr

solution in the Regenerator, directly flows into the Evaporator, as shown in Figure 2-5. The

Evaporator now acts as a Condenser and heats a second water stream that is used for heating the

IW. At the design operating conditions in the cooling mode, the chiller cools 9 gpm of 14°C

return chilled water to 7°C and rejects the heat to cooling water at 30°C from its integral cooling

tower. In the heating mode, the chiller heats 9 gpm of 50°C return hot water to 57°C by

condensing vapor from the Regenerator in its Evaporator.

2.3 Solar absorption cooling and heating test system

2.3.1 System description

A solar absorption cooling and heating test system in the IW was set up to test the parabolic

trough solar collector and system performance. This test system, shown in Figure 1-6, consists of

two loops: the solar collection loop and the variable load loop. The absorption chiller / the heat

exchanger are in the middle. It is connected with the HTF supply system on the left and the

cooling / heating load system on the right. The necessary instrumentation and control for the

system have been installed to operate the system and to process the experimental data.

2.3.2 The solar collection loop

The solar collection loop comprises two main circulation pumps, an expansion tank, and three-

way valve in addition to the PTSC’s, the absorption chiller, and heat exchanger HX-2. One of two

pumps, marked S1 in Figure 1-6, was provided by Broad to circulate the HTF in the solar loop

including the absorption chiller. It is packed with three-way valve, Broad instrumentation, supply

and return lines, and the PTSC control panel together in a metal cabinet, shown in Figure 2-6. To


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reduce the difficulties of the system integration and control, another pump, marked as S4, and a

heat exchanger, HX-2, were selected and installed in the solar loop. The three-way valve was

installed in Broad pump & control package for adjusting the amount of the HTF flowing though

by-pass and the absorption chiller. Its control will be discussed in the control section of the test

system.

The volume of the HTF varies with the temperature changes in a closed system due to thermal

expansion and contraction. An expansion / compression tank is required to accommodate the

varying quantity of liquid and operate the HTF within a selected pressure range. Since the

operation temperature of the absorption solar cooling and heating test system is between -20°C to

180°C, an expansion tank, provided by Zilmet in Italy, was installed in the mechanical room to

maintain a controlled pressure and to prevent vaporization in the closed solar collection loop. It is

located at near the point of lowest pressure in the system and shared by the two solar collection

loops (solar absorption chiller loop and solar heat exchanger loop). Its maximum working

pressure is 10 bars and its pre-charge is 36 to 44 psi (2.5 to 3 bar).

Table 2-2 Specifications of the absorption chiller installed

Name Quantity Unit

Cooling capacity 15.212 kW

Chilled water return temperature 14.17 °C

Chilled water supply temperature 7.63 °C

C o o

l i n g w a t e r

Chilled water flow rate 2 m3/h

Heating capacity 4.5 kW

Heating water return temperature 50 °C

Heating water supply temperature 52 °C

H e a t i n g w a t e r

Heating water flow rate 2 m3/h

12.629 kWHot source consumptions

2.366 m3/h

Power voltage 220 V

Power frequency 60 HZ

P o w e

r

Maximum power consumption 1.749 kW

Water-LiBr sorbent solution mass 70 kg

S o l u t i o n

Water-LiBr sorbent concentration 56 %


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The heat exchanger, HX-2, was installed in the solar collection loop parallel to the absorption

chiller to compare the solar heating system performance of two configures by the absorption

chiller and by a heat exchanger. This heat exchanger from Bell and Gossett is a brazed plate heat

exchanger, which offers the highest level of thermal efficiency and durability in a compact, low

cost unit.

Figure 2-6 Broad pump and control package

According to the specification sheet of the provider, the overall heat transfer coefficient is 359

Btu/hr-ft2-F (2.05 kW/m2-C) and the total heat transfer surface area is 11.31 ft2 (1.05 m2). The

Maximum temperature is around 232°C, and the maximum pressure is 435 psi (30 bar).

At the design conditions, the expansion tank sets the initial pressure in the system. The HTF

circulates through the three way valve into the by-pass line back to the solar collection loop.

When the temperature of the HTF meets the requirements of the absorption chiller, the HTF flows

through the absorption chiller and solar energy is then used to cool / heat the space.

2.3.3 The load loop

Systematic testing, of the PTSC and solar cooling and heating system, requires a load that can

readily be adjusted. This load is provided by a shell-and-tube heat exchanger HX-1 fed with

chilled water or hot water from the campus grids for heating or cooling tests, respectively. The

Supply and

return lines

Broad pump S-1

Control panel

ack view

Three-

a valve

Temperature and

pressure sensors


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flow of the chilled / heated water is controlled by a valve to achieve a desired set point

temperature and cooling / heating load. Figure 2-7 shows the pictures of the overall system.

The solar collection loop

The load loop

Figure 2-7 Overall solar absorption cooling and heating test system

2.3.4 Instrumentation, control and data acquisition system

For operation of IW solar cooling and heating system, an instrumentation, control and data

acquisition systems has been provided by the Automated Logical Co. (ALC). The ALC control

system collects the measurement data from the operation of the solar collector, the absorption

chiller, heat exchangers and variable load heat exchanger to evaluate the device and the system

performance. The ALC control system is a web-based, BACnet as protocol, control and data

display system. The system can be operated and observed in anywhere of the world through astandard web browser, without the need for special software on the workstation. This system

works together with two individual control systems from Broad, which are used to operate the

PTSC’s and the absorption chiller, respectively as indicated in Figure 2-8. The PTSC control

system and the chiller control system, provided by Broad, are used to operate the devices

separately by different portable control panels and also to communicate and corporate with each


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other in the solar driven cooling or heating operation. The data collected in both of the Broad

control systems are in a local domain; it is necessary to copy them to a working computer in order

to analyze the data. In addition to operating devices other than the PTSC and the absorption

chiller in the system, the ALC control system also serves as the major data acquisition and data

analysis system.

2.3.4.1 Data acquisition and display

The ALC control software, called the Web Control Server (WebCTRL), is used to program the

system operation control logic, set or change control parameters, and display the system

operation. Figure 2-9 is the graphic interface of WebCTRL designed for the solar thermal system.

The WebCTRL plots and presents historical data in various forms, such as graphics, trends, and

reports. The measurement data can be sampled in any time step from a second to a year.

2.3.4.2 Instrumentation in ALC system

Figure 1-6 shows the process and instrumentation diagram for the solar absorption cooling and

heating system. This figure includes all of components and instrumentation installed in the

system. There were a total of 21 sensors installed in ALC control system including flow rate

sensors, RTD temperature sensors, pressure sensors, and pyrheliometer for direct normal solar

radiation, as listed in Table 2-3.

Figure 2-8 Structure of control system


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2.3.4.3 Controls for the solar cooling and heating test system

There are three control systems in the test system as addressed before. To integrate two Broad

control systems into the ALC system, third-party integration has to be used so that the PTSC

control and the chiller control can directly communicate with ALC control system through a

standard communication part. However, since this integration requires that Broad open their

private communication protocol to ALC, the control system integration was not implemented.

Therefore an educated operator is required to integrate the operation of the overall system. None

the less, in all of these individual control systems, the control is electronic based, not like

traditional pneumatic or electric controls, with advanced control algorithms making the individual

system operation efficient and reliable.

The control elements for the PTSC system include

• Tracking the sun

• Startup and shutdown the circulation pump

• Startup and shutdown automatically tracking the sun

• Defocusing to prevent any hazards

The details of the PTSC control principles are discussed in Appendix 1.

Figure 2-9 Interface of the WebCTRL


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Table 2-3 Instrumentation of IW solar cooling and heating system

Label Sensor location Mediu

m

Range Manufacturer accuracy

T1 PTSC’s inlet HTF (-45.5 °C) to 260 °C

T1b PTSC’s inlet HTF (-45.5 °C) to 260 °C

T2 PTSCs outlet HTF (-45.5 °C) to 260 °C

T2b PTSCs outlet HTF (-45.5 °C) to 260 °C

T17 middle PTSCs HTF (-45.5 °C) to 260 °C

T3 Chiller inlet HTF (-45.5 °C) to 260 °C

T4 Chiller outlet HTF (-45.5 °C) to 260 °C

T15 HX2 inlet HTF (-45.5 °C) to 260 °C

T e m p e r a t u r e

T16 HX2 outlet HTF (-45.5 °C) to 260 °C

Sensor: ±0.3°C at 0 °C;

±0.8°C at 100 °C;±1.3 °C at 200 °C;Transmitter: ±0.1% of span

F1 PTSCs inlet HTF 0 to 10 gpm

f l o w

F2 Chiller inlet HTF 0 to 10 gpm

±0.2% at 100% flow, ±0.1%at 40% flow

P1 middle PTSCs HTF 0 to 250 psi

T h e s o l a r c o l l e c t i o n l o o p

P r e s s u r e

P2 PTSCs outlet HTF 0 to 250 psi

±0.25% of full span

T5 Chiller CHW/HW inlet water (-10 °C) to 110 °C

T6 Chiller CHW/HW outlet water (-10 °C) to 110 °C

±0.10% at 25% of span±0.40% others

T7 HX1 outlet to grids water (-45.5 °C) to 260 °C

T e m p e r a t u r e

T8 HX1 inlet from grids water (-45.5 °C) to 260 °C

Sensor: ±0.3°C at 0 °C;±0.8°C at 100 °C;±1.3 °C at 200 °C;Transmitter: ±0.1% of span

F3 Chiller CHW/HW inlet water 0 to 10 gpm T h e l o a d l o o

p

f l o w F4 HX1 outlet to grids water 0 to 10 gpm

±0.2% at 100% flow, ±0.1%

at 40% flow

pyrheliometer IW roof Irrad. 0 to 1400 W/m2 ±0.5% of full span

O t h e r s

gas meter Chiller gas inlet gas 1.4 cfh to 200 cfh ±0.8% of full span


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The control elements for the chiller system are

• Startup and shutdown

• Chilled water / Hot water temperature control

• Cooling water control

• Natural gas volume control

• Hot source flow control

• Three-way valve control

• Vacuum maintenance

• Crystallization judgment and de-crystallization

• Safety and alarm

The details of the chiller control principles can be found in Appendix 2.A. of Hongxi Yin’s thesis

“An Absorption Chiller in a Micro BCHP Application: Model based Design and Performance

Analysis” (Yin 2006).

In addition to the controls by above control system, controls in ALC are follows:

• Startup and shutdown the circulation pump S-4, S-5

• Switch the operation operations: ALC solar collection loop only, Broad solar collection

loop only, Broad solar cooling and heating system, ALC solar heating system

• Adjust variable load

• Safety and alarm

With the control, instrumentation, and data acquisition systems, the solar absorption cooling and

heating system can be operated to complete various test programs. The performance of the PTSC

and solar thermal system has been investigated, and the results and approaches are addressed in

the following sections.

2.4 The test program

The test program was planned, and then conducted in 37 days from February to September of

2007. The weather conditions for the tests included clear days, mostly sunny days, mostly cloudy

and overcast days in both winter and summer. There were four varieties of tests used to evaluate

the PTSC and the system performance: PTSC tests at transient states, PTSC tests at a steady state,


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