PERFORMANCE OF SOLAR POWERED COOLING SYSTEM USING PARABOLIC TROUGH

COLLECTOR IN UAE

Fadi A. Ghaith*, Haseeb-ul-Hassan Razzaq

School of Engineering and Physical Sciences,

Heriot-Watt University,

Dubai 294345, United Arab Emirates.

ABSTRACT

This paper addresses the potential of integrating the Parabolic Trough Collectors (PTC) with a

double-effect absorption chiller for the purpose of space cooling in residential buildings. The

proposed model was designed such to provide a continuous cooling. During the absence of sun,

the bio-mass heater was used as an auxiliary heating source. In this study, the thermal performance

of the proposed integrated system was investigated and a feasibility study was conducted in order

to assess the system's economic and environmental impacts. The obtained model was implemented

on a case study represented by a four-floored residential building based in Dubai with a net

cooling load requirement of 366 kW. The obtained results from the numerical simulation were

analyzed to identify the optimum configuration in terms of feasibility and potential savings. It was

found that the hybrid system with 30% solar contribution is the most viable compared to other

alternatives in terms of performance and cost. The proposed system achieved Annual Energy

Consumption (AEC) savings of about 519322 kWh and a reduction by 65% in the annual

operating costs. The payback period of the proposed system was found to be 2.5 years only.

Moreover; the system reduced the Carbon-dioxide emissions by 304 tons/year.

Keywords: Parabolic Trough Collector (PTC), Double-Effect Absorption Chiller, Feasibility.

* Corresponding author. Tel.:+971 4 4358713, Fax: +971 4 4477344.

Email Addresses: f.ghaith@hw.ac.uk (F. A. Ghaith), h.razzaq94@gmail.com (Haseeb-ul-Hassan Razzaq)

1. INTRODUCTION

As indicated by US energy information administrative (IE02013 report), the world energy demand

is anticipated to increase by 56% between 2010 and 2040 [1]. This is due to a continuous

development in the technological and economic sectors to provide the best quality of life for a

steeply increasing population. There is an astonishing energy demand which needs to be satisfied.

For the past few decades, fossil fuels have been used as the primary source of fulfilling this energy

demand [2]. Unfortunately, fossil fuels, while aptly fulfilling this energy demand, have

considerably adverse effects on the environment. The combustion products of these fossil fuels

continue to have an ever-growing impact on the climate and atmospheric conditions, yet they are

constantly being depleted. United Arab Emirates (UAE) happens to have the world's 10th

largest

electricity consumption per capita. The cooling load alone accounts for 40% of the total electricity

consumption [3]. Moreover, UAE is heavily reliant on the oil and gas resources for fulfilling this

energy demand; as a result, in 2010, UAE was ranked as having the largest ecological footprint

across the globe. Also the global warming effects are leading to a constantly increased demand for

cooling. Hence any saving in the cooling load is viewed as a critical measure in decreasing energy

utilization. Also there is a significant increase in the global awareness about the climate change

accompanied with serious collaborative efforts on the global level to accommodate it. One

example is represented by the surfacing of Paris Climate Accord, which has really sparked an

interest for utilizing the abundant and clean solar energy resource.

UAE has good solar exposure due to its geographical location. It is exposed to 10-12 hours of

sunshine/day throughout the year. The fact that most of the cooling is required when the solar

radiation availability is at the peak, along with the complementary advantages it offers such as

abundant energy, zero fuel cost and no greenhouse emissions, makes the solar integrated cooling

exceptionally appealing for implementation. Acting as substitute to oil and gas energy resources, it

will lead to major reduction in the power bill for consumers.

Solar powered cooling can be produced using both solar thermal and solar photovoltaic (PV)

ways. Lazarrin[4] conducted a thermodynamic analysis and economical comparison between

solar thermal and PV technology which indicated that the specific cost (€/m2) of the PV would be

higher in comparison to the alternative thermal collectors. Bijarniya et al. [5] reviewed the

Concentrated Solar Power (CSP) technology and its components for utilization in the power

generation sector in India. Authors reported that despite the reduction in the cost of PV panels

since 2014, CSP is still considered as a reliable candidate for solar energy utilization than the solar

electrical systems due to its energy storage capabilities. Bishoyi and Sudhakar [6] simulated the

performance of 100 MW Linear Fresnel Collector (LFR) based solar thermal power plant. They

identified that the CSP based system can generate more units of electricity per year compared to

solar electrical technology. Moreover they also reported the CSP technology to have higher

lifetime and efficiency compared to the PV based technology. A study conducted by Otanicar et al.

[7] showed that the thermal systems, unlike the electrical systems, are capable of utilizing more

than 65% of the incoming solar radiation and are far more convenient. Also it was reported by

Allouhi et al. [8] that thermal collectors with a maximum efficiency of 90% were presented by

Institut für Solartechnik (SPF). Meanwhile the solar-electrical (PV) systems can utilize just 35%

of the solar radiation for electricity generation[9]. Zhang et al. [10] reviewed different CSP

technologies and compared them on the basis of performance and cost. They identified the PTC to

be the most mature CSP technology available in the market having relatively less cost compared to

other CSP technologies such as Solar Tower collectors and Parabolic Dish Collectors. Lazzarin [4]

investigated the overall system efficiency and the investment costs using different solar collectors.

A comparison of different solar powered thermal cooling techniques with respect to FPC, ETC and

PTC was conducted. Results revealed a better compatibility of the double effect absorption

cooling systems when integrated with the PTC rather than FPC or ETC. Cabrera et al.[11] studied

the potential of using PTC for solar cooling technique. PTC's indicated to be the most effective

collector type when coupled with the double effect absorption chiller. PTC's also appeared to have

the highest mean annual solar fraction in comparison to a Compound Parabolic Concentrator

(CPC) and static collectors (i.e. ETC and FPC). The aforementioned study conducted by Cabrera

et al.[11] was aimed towards identifying the true potential of the PTC rather than studying the

system performance of the PTC integrated with double-effect absorption cooling system.

Absorption cooling systems utilize the thermal energy to split a refrigerant from a

refrigerant/absorbent mixture. There are several refrigerant/absorbent pairs available for the

absorption cooling cycle, however, LiBr/H2O is the most appropriate working solution for the

solar absorption technique as specified by Mittal et al. [12] The absorption systems can further be

classified on the basis of their thermodynamic cycle of operation[13]. Kaushik and Arora [14]

conducted comparative study between single and double-effect absorption systems in which they

found that the Coefficient of Performance (COP) of a double-effect system was 60 -70% higher

than a single effect system. Their study also indicated a higher temperature requirement up to

150°C for the double effect absorption cooling system relative to the single effect system which

requires around 91 °C to achieve the optimum COP. This is due to the fact that the double-effect

chiller splits refrigerant/absorbent mixture in two stages. Aliane et al. [15] showed that the

separate operation of each component within the absorption cooling system under defined

environmental conditions differs from its operation as a part of the overall system because of the

varying temperature levels and heat transfer rates.

Many studies and research papers reported in the literature investigated the characteristics of solar

powered absorption cooling systems. Balgouthi et al. [16] simulated the performance of single

effect absorption cooling system integrated with 30 m2 Flat Plate Collectors (FPC) at 35° tilt

angle. Their results indicated a COP of 0.74. Darkwa et al. [17] studied analytically the

performance of a similar system employing Evacuated Tube Collectors (ETC). A COP of 0.69 was

calculated in comparison to the manufacturer's rating of 0.7. Their results concluded the system to

be a viable cooling technique for application in buildings. Auxiliary heating sources were

suggested for maintaining adequate supply of hot water during low solar radiation. Bellos et al.

[18] conducted analytical study using the Engineer Energy Solver (EES) software to study the

overall system performance when different collectors such as PTC, ETC, CPC and FPC were

integrated with a single stage absorption chiller. System evaluation showed the PTC to have the

best performance exergetically and energetically. However integration of ETC with a single-effect

absorption chiller was observed to be the most economical combination. Al-Alili et al.[19]

investigated the performance of ETC and single-effect absorption chiller integrated solar cooling

system at UAE weather conditions. The devised system showed the capability of reducing 60% of

the electricity consumption for producing the desired cooling load in comparison to a vapor

compression cycle. The same authors simulated the performance of a hybrid solar air conditioner

operated using both PV and thermal collectors and later they investigated it experimentally.

Promising results were obtained with achievable COP >1[20][21][22]. Ghaith and Abusitta [23]

developed an integrated solar heating and cooling system for two building configurations based in

the UAE. Investigation was carried out using the single-effect chiller incorporated with static

collectors (FPC and ETC). A hybrid system resulted in energy savings of up to 175648 kWh and

reductions in CO2 emissions equivalent to eliminating 30.7 cars from road. Tzivanidis and Bellos

[24] conducted an analytical study to investigate the application of PTC integrated single effect

absorption cooling system. The system showed satisfactory performance in the presence of

adequate radiation; producing 150 kWh of cooling load with a total operation period of 12.5 hours.

Powel et al. [25] discussed the potential of concentrating solar thermal technology when

hybridized with conventional systems for power generation. They concluded that sharing CSP

technology with conventional systems can bring huge benefits to the table providing significant

advantages such as increased efficiency, reduction in the capital costs as a result of equipment

sharing and flexibility in the system operation by rotating energy sources etc.

Up to this point and among investigating the existing literature, it was found that most of the

research on the subject is yet based on the combination of the single-stage absorption chillers with

the static collectors. Moreover, the majority of the research papers were limited to standalone solar

powered cooling systems with a limited operation period. To utilize the available solar energy in

the most efficient way, it is necessary to investigate alternative system combinations to identify the

most appropriate combination in terms of system performance along with economical and

environmental feasibility. Since the research in this area is still at a relatively early stage and the

prototyping costs are considerably high, this work is aimed to help implement the most

appropriate absorption cooling system in UAE and other regions with similar environmental

conditions. Hence, this paper endeavors to develop on the existing literature from the following

points of view:

Investigate the technical performance of the PTC integrated system with a double-effect

absorption chiller.

Design a cooling system that can provide a continuous cooling which covers 24 operating

hours and valid for both summer and winter seasons. This was achieved by incorporating the

bio-mass auxiliary heater.

Define and select the hybrid system parameters in order to achieve the most feasible system

configurations. The obtained model was implemented for a realistic case study represented by

a residential building located in UAE.

Conduct the feasibility study of the system in terms of economical and environmental

evaluation.

Conduct system simulation and validation using TRNSYS software.

2. SYSTEM DESCRIPTION

The proposed solar integrated cooling system comprises of parabolic trough collectors, an

auxiliary biomass heater, a hot water storage tank and a Li-Br/H2O double effect absorption

chiller. The general configuration of the system is shown schematically in Fig.1. During the day

time, the energy is absorbed by the PTCs to generate hot water which is then fed into the generator

in absorption cooling machine for providing the space cooling. Whereas, a bio-mass auxiliary

heater is switched on during the night time to meet the cooling load requirements. Incorporating a

biomass heater enables cooling load production beyond the sunlight hours and allowing the system

to operate continuously for 24 hours valid for both summer and winter seasons. The function of

the storage tank is to store the thermal energy when the cooling load requirements are low. This

energy can be provided for domestic hot water applications or for an unexpected increment in the

cooling requirements. The proposed system was implemented for a case study represented by

residential building based in Dubai that comprising a roof of total area of 400 m2 and has a net

cooling load requirement of 366 kW. The building material properties and associated heat gains

are shown in Table 1. A hybrid system was proposed in order to meet the cooling requirement

which is then evaluated against both the conventional system and static collector integrated with

single-effect absorption cooling system. The primary objective behind this study was to minimize

the utilization of conventional chiller by implementing the most effective solar cooling system in

terms of energy savings and cost reduction.

3. METHODOLOGY

The numerical algorithm was developed using the MATLAB® software. It was based on the set of

equations resulted from the thermal model described in Section 4. The methodology of this

research work execution is depicted by Fig.2. Monthly meteorological data was obtained from

National Center of Meteorology and Seismology (NCMS) [26] and Helioclim-3 [27] on the basis

of which the average solar radiation is presumed to be 600 W/m2. The cooling load for the case

study under investigation was estimated to be 366 kW using the Hourly Analysis Program (HAP)

software. After identifying the building requirements, the components were selected from different

manufacturer catalogues with the aim of modeling a realistic system and obtaining dependable

results. Energy analysis was followed by a systematic feasibility study in order to identify the

energy and cost savings of the proposed system. Initial expenses for the system constituted the

setup costs of the system components such as the PTC's, storage tanks, auxiliary heater, absorption

chiller, cooling tower, circulatory pumps and other subsystems such as the piping, wiring,… etc.

Moreover, the operating costs required for running the system such as electricity costs, upkeep

costs and the other supplies (i.e. refrigerant, water and necessary materials) required for system

operation were considered. Environmental impact was likewise contemplated by finding the CO2

emissions after a year of the system operation. Also the hybrid solar powered cooling system was

simulated for the selected case study using Transient System Simulation System (TRNSYS)

software for the purpose of validation.

4. MATHEMATICAL MODELING

This section describes the set of equations that provide the structure of the proposed integrated

solar cooling system. Fig.3 shows the general arrangement of the system which consists basically

of the PTC, storage tank, biomass heater and double-effect absorption chiller. The main

assumption involved within the developed mathematical model that the system is working under

steady state conditions. Also the temperature of the water in the tank was assumed to be uniform.

4.1 Parabolic Trough Collectors

The overall heat loss coefficient UL in the PTC consists mainly of the convection and radiation

components[28]. Losses by conduction (hc) through supports are considered to be negligible.

Accordingly UL can be written as:

𝑈𝐿 = ℎ𝑤 + ℎ𝑟 + ℎ𝑐 (1)

Linearized radiation coefficient, hr can be expressed as [28]:

ℎ𝑟 = 4𝜎𝜀𝑇𝑟𝑒𝑐3 (2)

Also the losses by convection can be given by [28]

ℎ𝑤 = 4𝑑−0.42𝑣0.5 (3)

By considering the thermal and optical losses that occur in the PTC, the useful energy from the

collector is related by the succeeding equation [29].

𝑄𝑢 = 𝐹𝑅[𝐺𝐵ηoAa − Ar𝑈𝐿(𝑇𝑐,𝑖 − 𝑇𝑎)] (4)

Also the useful energy produced by the collector can be evaluated by

𝑄𝑢 = �̇�𝑐𝑝(𝑇𝑐,𝑜 − 𝑇𝑐,𝑖) (5)

The thermal efficiency of the collector (i.e. the ratio between the useful energy and the incident

radiation) can be expressed as

η =Qu

GB × Aa= �̇�𝑐𝑝(𝑇𝑐,𝑜 − 𝑇𝑐,𝑖)

GB × Aa (6)

4.2 Storage Tank

Energy balance in the tank can be used to determine the storage tank temperature [29].

(𝑀𝑠𝑐𝑝 , +𝑀𝑡𝑐𝑝, )𝑑𝑇𝑠𝑑𝑡= 𝑄 − 𝑈𝑠𝐴𝑠(𝑇𝑠 − 𝑇𝑎) − 𝑄𝑔 (7)

By referring to Fig.3, and assuming negligible heat losses between the collector and the storage

tank by ensuring good thermal insulation, the expression “𝑈𝑠𝐴𝑠(𝑇𝑠 − 𝑇𝑎)” can be equated to 0 and

in this case Q = Qu since 𝑇𝑠,𝑖 = 𝑇𝑐,𝑜 and 𝑇𝑠,𝑜 = 𝑇𝑐,𝑖.

Assuming negligible losses between the storage tank and generator gives

𝑄𝑔 = �̇�𝑔𝑐𝑝(𝑇𝑔,𝑖 − 𝑇𝑔,𝑜) (8)

4.3 Auxiliary Biomass Heater

Biomass heaters are implemented in the system as an auxiliary heat source for low level radiation

or at night.

𝑄𝐻 = �̇�𝐻𝑐𝑝,𝑤(𝑇𝐻,𝑜 − 𝑇𝐻,𝑖) (9)

Energy generated by burning the pellets is figured out by the following equation:

𝑄𝐻 = �̇�𝑝𝑐𝑝,𝑝(𝑇𝑝,𝑜 − 𝑇𝑝,𝑖) × ηH (10)

Where ηH is the efficiency of the heater.

During the summer, at night times, the heat balance equation in storage tank turn out to be

(𝑀𝑠𝑐𝑝 , +𝑀𝑡𝑐𝑝, )𝑑𝑇𝑠

𝑑𝑡= 𝑄𝐻 − 𝑈𝑠𝐴𝑠(𝑇𝑠 − 𝑇𝑎) − 𝑄𝑔 (11)

4.4 Absorption Chiller

The coefficient of performance (COP) of the absorption chiller is given by:

𝐶𝑂𝑃 = 𝑄𝑒𝑄𝑔 (12)

Where Qe is the energy produced by chiller's compressor.

4.5 Overall Model

By substituting equations (7) and (4) into equation (12), the overall thermal model can be

expressed by the following set of equations.

Cooling load with respect to the area of collectors during day time can be expressed as

𝑄𝑒 = 𝐶𝑂𝑃 × {[𝐹𝑅[𝐺𝐵ηoAa − Ar𝑈𝐿(𝑇𝑐,𝑖 − 𝑇𝑎)]] − (𝑀𝑠𝑐𝑝 , +𝑀𝑡𝑐𝑝, )𝑑𝑇𝑠

𝑑𝑡− 𝑈𝑠𝐴𝑠(𝑇𝑠 − 𝑇𝑎)} (13)

By incorporating the heat generated by the biomass heater represented by equation (10), the

cooling load can be evaluated by the following equation during the night time.

𝑄𝑒 = 𝐶𝑂𝑃 × {[�̇�𝑝𝑐𝑝,𝑝(𝑇𝑝,𝑜 − 𝑇𝑝,𝑖) × ηH] − (𝑀𝑠𝑐𝑝 , +𝑀𝑡𝑐𝑝, )𝑑𝑇𝑠

𝑑𝑡− 𝑈𝑠𝐴𝑠(𝑇𝑠 − 𝑇𝑎) } (14)

The uniqueness of the mathematical model can be viewed in light of the following perspectives:

The derived mathematical model can be used for sizing and selection of the system

components (i.e. PTC, absorption chillers, storage tanks, etc.) that satisfy both the cooling

load and available setup area requirements. The flexibility of this model allows kind of

freedom in selecting the system components according to market availability (i.e. it is not

restricted to the contents within the equipment lists of commercial software databases).

The mathematical model accounts for the optical losses which are commonly associated with

PTC and thermal losses in the storage tank. The review of existing literature reveals that the

available mathematical models were limited to the thermal analysis of each component

separately. Furthermore, most of the existing models did not consider the cooling load during

night hours.

The obtained model is considered fertile, allowing wide range of parametric studies to be

performed, either by changing the component specifications and/or altering the solar

penetration percentage in the hybrid setup. A parametric analysis of this nature is necessary to

procure an optimum thermal design that can satisfy the cooling load and also leads to short

payback period.

5. RESULTS AND DISCUSSION

The described equations presented in section 4 were solved numerically using the MATLAB®[30]

software and the EXCEL[31] worksheet. Table 2 illustrates the system parameters used for the

numerical solution. Due to the space limitations, the total collector area required for satisfying the

cooling load requirement of 366 kW should fit the total roof area of 400 m2. Hence, the solar

penetration must be carefully selected to ensure the maximum utilization of solar energy without

exceeding the roof area constraint.

5.1 Hourly Analysis of the Collector Area

In order to capture the effect of the solar radiation on the collector area requirement, hourly

analysis was performed to predict the collector area required to produce a total cooling load of 366

kW at different levels of radiation. This analysis was carried out for typical summer and winter

months (i.e. July and January, respectively). The obtained results were presented in Fig.4 and

Fig.5. The maximum average solar radiation of 936 W/m2 was observed at 2:00 pm in the month

of July, which resulted in the minimum collector area requirement of 496 m2 as illustrated by

Fig.4. Since the required collector area may exceed the total roof area at certain levels of radiation,

it is not feasible to setup the system on the basis of maximum solar radiation neither the minimum

one since the radiation varies throughout the year. Accordingly, the system was studied at an

average solar radiation of 600 W/m2. Also it was found that implementing a fully renewable

system is not practical for this case study since the collectors area exceeded the overall building

roof area even at the maximum solar radiation. Therefore, it was necessary to identify the

optimum hybrid solution which is capable of providing full cooling load of 366 kW throughout the

year despite the time dependent fluctuations in the solar radiation.

5.2 Evaluating the optimum solar configuration

Based on the hourly analysis of the collector area conducted in section 5.1, a fully renewable

system appeared to be not feasible in light of the estimated collector area. Hence, investigating the

hybrid configuration is required to come up with a viable solution. Different simulation runs were

carried out to estimate the required collector area at different solar penetrations as shown in Table

3. The obtained results showed that solar penetration of 30% is the most feasible in terms of the

generated useful energy and the available roof area. On the other hand, it was found that

implementing a fully renewable system is not applicable for this case study as it required very

high collector area (i.e. 969.5 m2) which is more than the available roof area of the building and

also it may lead to very high initial cost.

5.3 Effect of different solar penetrations on the economic and environmental aspects

Beside the solar configuration selected for the case study (i.e. solar penetration of 30%), other

hybrid configurations were investigated in order to evaluate the influence of solar penetration on

potential energy savings as well as reduction in carbon-dioxide emissions. Fig.6 showed a

proportional relationship between the potential energy savings and the solar contribution of the

hybrid system. It was found that a fully powered solar system provided up to 776.5 MWh/yr

savings in energy compared to the conventional system. Fig.7 revealed that the fully renewable

system can reduce 586.57 tons/year of CO2 from being released into the atmosphere. Results

presented in Figures 6 and 7 can be useful where the similar cooling requirements are applicable

with the availability of a larger setup area such as hotels and residential villas.

5.4 Feasibility and comparison with other systems reported in the literature

In order to further investigate the potential and economical viability of the proposed system, a

feasibility study was conducted. The payback period of the system was calculated based on the

costs of the system components as presented in Table 4. The costs were estimated according to the

most recent data extracted from verified online resources [32-37]. The estimated costs took into

account the expected maintenance and installation costs. The proposed system was then compared

with two alternatives; the conventional fully electrical powered system and the static collector

integrated single-effect absorption cooling system (i.e. similar to the investigation performed in

[23]). Table 5 presented the obtained findings resulted from the comparative study between all

alternatives such as capital and operating costs, payback period, Annual Energy Consumption

(AEC), CO2 emissions…, etc. The calculations of CO2 emissions were based on the assumption

that each kWh electricity consumption is equivalent to 1.1 kg of CO2 emissions [38] and each

vehicle (on average) releases 4.7 metric tons of CO2 [39]. It was observed that the proposed PTC

integrated double effect absorption cooling system developed in this study was capable of

covering 30% of the maximum cooling load requirement while the alternative renewable system

[23] showed to fulfill only 20% of the cooling demand within the same roof area constraint. Fig.8

showed that the capital cost of the proposed system was found to be two times greater than

conventional system and 1.35 times more in contrast to the alternative solar cooling system.

However; the estimated savings in AEC was found to be 519322 kWh, which is 3 times more than

the alternative renewable system can provide. The large savings in energy is accounted by the fact

that the generator load requirement of the double-effect absorption chiller is significantly less than

the single-effect absorption chiller due to a higher coefficient of performance. Fig.9 showed that

the payback period associated with the proposed system is only 2.49 years while the static

collectors’ integrated system was found to have a payback period of 4.75 years. Fig.10 evaluated

the environmental impact of implementing each system by taking into consideration the amount of

carbon-dioxide emanated into the atmosphere. It was shown that the proposed system was more

eco-friendly while fulfilling the cooling load demand. The obtained findings represented by Fig.10

portrayed that the proposed system reduced carbon emissions by a significant 303 tons/ year

(equivalent to removing 65 cars from the road). This is almost 2.1 times more than the carbon

emission savings associated with the static collector integrated system.

Up to this point, the aforementioned analyses highlighted several factors which may assist in

stating the feasibility involved in the mass usage of the proposed system, especially for the

operation where similar cooling load requirements are applicable such as hotels, residential villas,

restaurants and other food processing establishments. The merits of the developed system in this

study can be viewed from the following perspectives:

Most areas of UAE receive abundant solar radiation making the proposed system an attractive

and viable solution for implementation.

The annual maintenance cost of the system is very low compared to the initial installation

costs.

The ceaseless rise in the electricity tariffs along with public environmental awareness has

aided the market for solar cooling systems.

Role of the government, such as providing incentives for the development of these solar based

systems is crucial in further stimulating interest in the development of these technologies.

5.5 System performance using the biomass heater

In order to illustrate the advantage of using the biomass heater for the proposed system over a

conventional heating source such as the electrical heater [40], the payback period was calculated

for each scenario as shown by Fig.11. It was found that the payback period for the system

incorporated the usage of biomass heater is almost half the payback period of that system powered

by electrical heaters at night time. This is due to large electrical energy required as input which

tremendously increases the operating cost of the system and increasing payback period while the

biomass heater utilizes a lower cost energy produced as a result of burning the pellets.

5.6 Effect of solar radiation on collector thermal efficiency

In order to study the influence of the solar radiation on the thermal efficiency of the PTC, the solar

radiation was varied over the range of 300-1100 W/m2. The ambient temperature was taken as

35°C and the inlet and outlet collector temperatures were kept constant at 160°C and 180°C,

respectively. Fig.12 showed that the collector thermal efficiency tends to increase at high levels of

solar radiation. Also it was observed that the change in thermal efficiency is relatively high within

the range of low levels of radiations (e.g. 300-600 W/m2) while it increased slightly at high levels

of radiation (800-1100 W/ m2).

5.7 Numerical Simulation using TRNSYS Software

This section represents the simulation of the hybrid solar powered cooling system (i.e. 4 floors residential

building) using Transient System Simulation System software (TRNSYS) [41] . The purpose of this study is

to analyze the temperature profiles across the system components in order to verify its capability to meet the

cooling load requirements. Also it aims to validate the accuracy of the results obtained from solving the

proposed mathematical model described in section 4. Fig.13 shows the schematic of the hybrid cooling

system developed by TRNSYS. The input system parameters for PTC, double effect absorption chiller and

auxiliary heater were selected carefully to be identical to the corresponding ones used in solving the

mathematical model and listed by Table 2. Fig.14 shows the collector outlet temperatures versus time for the

month of July. It was found that the maximum PTC outlet temperature may reach 200 °C at peak levels of

radiation. Fig. 15 shows the temperature profile of the water in the storage tank. The temperature profiles of

the chiller inlet temperatures versus time are shown in Fig. 16 for both systems with/without the auxiliary

heater. When the solar energy is not sufficient to run the absorption chiller, the auxiliary heater is switched

on which supplies the minimum inlet generator temperature to continue operation of the absorption chiller. It

was observed that the proposed system is capable of meeting the required generator temperature for almost 9

hours daily in July without the need of auxiliary heater. In order to predict the required power of the

auxiliary heater, the power versus the time across both day and night times were plotted as shown by Fig. 17.

It was observed that the auxiliary heater is required from 7:00 PM till 10:00 AM.

5.8 Model Validation

In order to verify the accuracy of the obtained model, a comparative study was established

between the manufacturer parameters for a standard PTC and the calculated corresponding

parameters resulted from solving the mathematical model obtained in this study. The

manufacturer model used in this comparison was NEP PolyTrough 1800[42]. The manufacturer

parameters for this PTC type are given by Table 6. These parameters were entered as input to the

proposed mathematical model. Moreover, in order to match the same simulation conditions used

by the manufacturer, the input parameters of the current study were simulated at the same ambient

temperature of 25°C and solar radiation of 1000 W/m2. The efficiency curves resulted from the

manufacturer data and calculated ones are shown in Fig. 18. It was found that there is very close

proximity between the manufacturer data curve and the corresponding one (resulted from solving

the mathematical model of the proposed system) with a percentage of difference not exceeding

1.3%. This finding proved that the proposed model developed in this paper was capable to predict

the actual performance of the commercial PTC system accurately.

6 CONCLUSION

In this paper, the energy and environmental performance of integrated PTC with a double

absorption chiller were investigated. The proposed system was numerically simulated and

comparative studies were established against the conventional electrical system and alternative

renewable system which is based on the integration of flat plate collectors with a single effect

absorption chiller. The obtained results showed that the proposed system reduced significantly

both AEC and relevant operational costs. In summary, the outcomes obtained from this paper are

summarized below:

The payback period was found to be 2.49 years which is almost half the payback period of the

alternative renewable system reported recently in the literature [23].

The proposed system provided 519322 kWh savings of annual energy consumption which is

almost three times greater in comparison to the FPC integrated single effect absorption

cooling system.

Integrating an electrical heater instead of a biomass heater showed a significant increase in the

payback period due to large electrical energy requirements.

The developed system was found financially attractive in region with grid connected areas

where the supply of conventional energy is reliable. Hot water is available during the winter

season and can be provided for domestic hot water applications (i.e. directly from the storage

tank), while it can be used to fulfill the cooling requirements throughout the summer season

or as per the consumer requirements.

The obtained findings proved that the proposed system is effective and more viable technique with

better energy and environmental performance. In light of the obtained results, it can be stated that

the system has ample potential of upgrading the current UAE cooling systems and other regions

with similar environmental conditions to the hybrid solar systems. This will not only help to

achieve the UAE dream of evolution to green energy but will also help to reduce the electricity

cost for consumers.

NOMENCLATURE

hw Losses coefficient by convection (W/m2K)

hr Losses coefficient by radiation

UL Overall heat transfer coefficient

𝜎 Boltzmann's constant (5.67 × 10− 𝑚−2 −4) 𝜀 Emissivity (of the PTC absorber tube)

Q Energy (Watt)

d PTC absorber diameter (m)

v Wind speed

GB Solar beam radiation (W/m2)

FR Heat removal factor

𝑜 Optical efficiency

Aa Aperture area (m2)

Ar Absorber area (m2)

cp Specific Heat Capacity of water (J/kg °C)

Ms Mass of water in storage tank (kg)

Ts Temperature of storage tank (°C)

cp,t Specific Heat Capacity of tank (J/kg °C)

Mt Mass of empty tank (kg)

Us Loss coefficient of tank (W/m2K)

�̇� Mass flow rate (kg/s)

T Temperature

𝑡 Time (seconds)

Subscript

Ambient

Inlet

Outlet

𝑐 Collector

Fluid

Generator

ℎ Heater

Pellet

𝑢 Useful

rec Receiver

Abbreviations

AEC Annual Energy Consumption (kWh)

AED Dirham (UAE Currency)

Li-Br Lithium Bromide

H2O retaW

CO2 Carbon Dioxide

PTC Parabolic Trough Collector

FPC Flat Plate Collector

ETC Evacuated Tube Collector

HAP Hourly Analysis Program

NCMS National Center of Meteorology & Seismology

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List of Tables

Table 1 - Residential building heat gains and material properties [23]

Table 2 - System Parameters

Table 3 - Optimum solar system configuration for the selected case study

Table 4 - Component costs used in the feasibility Study

Table 5 - Comparative study between the proposed system against the conventional and single

effect absorber chiller hybrid system

Table 6 - PolyTrough-1800 Parameters

Table 1 - Residential building heat gains and material properties [23]

Component Material Thickness

(mm)

Density

(kg/m3)

Thermal

Conductivity

(W/m K)

Specific

Heat

Capacity

(kJ/kg K)

Exterior

walls

Plaster (light weight) 0.02 2699 0.16 0.9

Light weight, dry, 750 kg/m3

0.07 30 0.83 1.12

Extruded polystyrene 0.06 25 0.03 1.2

Light weight dry, 750 kg/m3

0.07 30 0.83 1.12

Plaster (light weight) 0.02 2699 0.16 0.9

Floor

Concrete Slab 150 2500 1.95 0.9

Sand cement screed 50 2080 1 0.84

Tiles 20 2284 1.104 0.8

Roof

Tiles 0.028 1.1 0.8

Cement mortat 0.01 0.72 0.4

Alluvial Clay, 40% sands 0.058 1.21 6

Polyisocyanate 0.05 0.021 0.8

Fiber board, wet feltred 0.004 0.051 1.12

Foamed, 700 kg/m3 0.05 0.15 1.507

Dense, reinforced 0.27 1.9 1.1

Plaster (light weight) 0.02 0.16 0.9

Internal Heat Gains and Zone Infiltration Flat Type Office type

Number of people 8 6

Lighting (W/m2) 26 26

Electrical equipment (W/m2) 100 150

Zone infiltration (ACH) 1 0.5

Table 2 - System Parameters

Parabolic Trough Collector (PTC) [32]

Optical Efficiency 82.7 %

Aperture Area 78.09 m2

Absorber Diameter 0.0318 m

Collector Length 36.66 m

Rim Angle 90° Type of tracking Single - Axis

Orientation 25° tilt towards south

Double-Effect Absorption Chiller [33]

Hot Water Inlet Temperature 180 °C

Hot Water Outlet Temperature 160 °C

Hot Water Flow Rate 12.9 m3/h

Cooling Water Inlet Temperature 35°C

Cooling Water Outlet Temperature 40.5 °C

Cooling Water Flow Rate 102 m3/h

Chilled Water Inlet Temperature 12.2 °C

Chilled Water Outlet Temperature 6.7 °C

Chilled Water Flow Rate 57.2 m3/h

Coefficient of Performance (COP) 1.3

Biomass Heater [34]

Efficiency 95.4 %

Input Power 4.7625 kW

Table 3 - Optimum solar system configuration for the selected case study

Absorption

Chiller

Percentage

Solar : Conventional

Chiller Qgen Q load PTC

Solar Load

(kW)

Conventional Load

(kW)

Generator Load

(kW) Solar Collectors load (kW)

Area

(m2)

366 100%, 0% 0 281.5 581.6 969.5

329.4 90%, 10% 36.6 253.4 523.6 872.6

292.8 80%, 20% 73.2 225.2 465.3 775.6

256.2 70%, 30% 109.8 197.1 407.2 678.7

219.6 60%, 40% 146.4 168.9 349.0 581.7

183 50%, 50% 183 140.8 290.9 484.8

146.4 40%, 60% 219.6 112.6 232.6 387.8

109.8 30%, 70% 256.2 84.5 174.6 290.9

73.2 20%, 80% 292.8 56.3 116.3 193.9

36.6 10%, 90% 329.4 28.2 58.3 97.0

Table 4 - Component costs used in the feasibility Study

Component Cost (AED)

Double-Effect Absorption Chiller [33] 528884

Biomass Heater [34] 251282

PTC [35] (40$ per m2) 56929

Storage Tank [35] 18350

Cooling Tower [35] 73400

All Circulatory Pumps [35] 26384

Conventional Chiller [36] 260000

Operational Cost (0.38 fils / kWh) [37] 127684

Table 5 – Comparative study between the proposed system against the conventional and single effect absorber chiller hybrid system

Parameters PROPOSED SYSTEM VALUES TAKEN FROM LITERATURE [23]

Double-Effect Hybrid System Single-Effect Hybrid System Fully conventional

Chiller Brand and Model No: YHAUCHW180

EXSB Johnson [33] RTAF 125N

Trane [36] EACM-C20

Ecotherm

YCALEE50

Ecotherm

YLAHEE50

York

Solar : Conventional Energy Share

Percentage

Renewable(%) Conventional(%) Renewable Conventional Conventional

30% 70% 20% 80% 100%

Type of Chiller Absorption Air cooled Absorption Air cooled Air cooled

Load Consumption (kW) 109.80 256.20 76.00 290.00 366 kW

Initial Costs (AED) 940995.63 260000.00 471201.50 420388.60 620274.40

Installation Costs (15%) 141149.34 39000.00 70680.20 63058.30 93041.20

Total (initial + installation) 1082144.97 299000.00 541881.70 483446.80 713315.60

Total renewable vs. conventional 1381144.97 1025328.60 713315.60

Operating Cost 141644.8780 344600.40 410284.60

Operating Cost savings 268639.72 65684.10

Payback Period 2.49 4.75

Annual Energy Consumption (kWh) 34672.08 338077.60 7446.00 708977.20 892071.50

AEC savings (kWh) 519321.82 175648.30

Annual CO2 emissions (kg) 38139.29 371885.36 6745.20 567181.70 713657.20

CO2 footprints (ton/year) 38.14 371.89 6.70 567.18 713.70

CO2 savings (ton/year) 303.68 139.70

Equivalent to X cars not used 64.61 30.70

N.B. Savings in Carbon-dioxide emission is based on the calculation that each kWh consumption generates 1.1 kg of Carbon-dioxide emissions [38]

and each vehicle (on average) releases 4.7 metric tons of Carbon-dioxide [39].

Table 6 - PolyTrough-1800 Parameters

PolyTrough 1800 [42]

Aperture Area 36.99 m2

Absorber Diameter 0.0104 m

Collector Length 20.9 m

Height 1.75 m

Rim Angle 71° Optical Efficiency 77%

List of Figures

Fig. 1 - Schematic of PTC integrated double-effect absorption cooling system

Fig. 2 - System analysis methodology

Fig. 3 - General schematic of the proposed system

Fig. 4 - Solar radiation and corresponding collector area for the month of July.

Fig. 5 - Solar radiation and corresponding collector area for the month of January.

Fig. 6 - Energy savings at different solar contributions

Fig. 7 - Reduction in CO2 emissions at different solar contributions

Fig. 8 – Comparative energy and cost analyses

Fig. 9 - Payback period of two different renewable systems

Fig. 10 - Carbon-dioxide emanation

Fig. 11 - Comparison between the payback periods for biomass and electrical heater.

Fig. 12- Thermal efficiency & solar radiation relationship

Fig. 13 - TRANSYS schematic of the proposed system

Fig. 14 – PTC outlet temperature versus time for the month of July

Fig. 15 – Storage tank outlet temperature versus time for the month of July

Fig. 16 – The inlet chiller temperatures versus time for the systems with/without auxiliary

heater

Fig. 17 – The power of the auxiliary heater versus time for the hybrid cooling system

Fig. 18 - Thermal efficiency versus temperatures

1

2

3

4 Fig. 1 - Schematic of PTC integrated double-effect absorption cooling system 5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

Fig. 2 - System analysis methodology 32

33 34

35

Devise a mathematical model for

the PTC integrated cooling

system

Extract solar radiation data for

UAE

Select a realistic case study for

investigating the proposed

system

Select the system components

from the manufacturer

catalogues

Carry out the energy analysis

Identify the optimum hybrid

system configuration matching

the case study constraints

Conduct the feasibility study and

calculating the payback period

Compare the proposed system

with conventional & static

collector integrated absorption

cooling system

Assess the environmental impact

(CO2 emissions) of the system

System simulation and validation

using TRANSYS

36

37

38

39 40

Fig. 3 - General schematic of the proposed system 41

42

43

44

45

46

47

48

49

50

51

Fig. 4 - Solar radiation and corresponding collector area for the month of July. 52

53

54

55

56

Fig. 5 - Solar radiation and corresponding collector area for the month of January. 57

58

59

60

61

62

63

64

65

66

67

68 Fig. 6 - Energy savings at different solar contributions 69

70

71

72

73

74

0

100

200

300

400

500

600

700

800

900

En

erg

y S

ain

gs

(MW

h/y

r)

Renewable : Conventional Ratio (%)

75 Fig. 7 – Reduction in CO2 emissions at different solar contributions 76

77

78

79

80

81

82

83

84

85 86

87

88

0

100

200

300

400

500

600

700

CO

2 s

av

ing

s (t

on

/yea

r)

Renewable : Conventional Ratio (%)

89 Fig. 8 - Comparative energy and cost analyses 90

91

92

93

94

95

96

97

98

0

200000

400000

600000

800000

1000000

1200000

1400000

1600000

Conventional FPC & ETC integrated20% Renewable

PTC Integrated 30%Renewable

Co

sts

(AE

D)

Initial Costs (AED) AEC (kWh) Operating Costs (AED)

99 Fig. 9 - Payback period of two different renewable systems 100

101

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

FPC & ETC integrated 20%Renewable

PTC Integrated 30%Renewable

4.75

2.49

Pa

yb

ack

Per

iod

(y

ears

)

102

103 Fig. 10 - Carbon-dioxide Emissions 104

105

0

200

400

600

800

Conventional FPC & ETC

integrated 20%

Renewable

PTC Integrated

30% Renewable

Car

bo

n D

ioxid

e E

mis

sio

ns

(to

ns/

yea

r)

106

Fig. 11 – Comparison between the payback periods for biomass and electrical heaters. 107

108

109

110

111

112

113

114

115

116

117

0

1

2

3

4

5

Solar system includingBiomass Heater

Solar system includingelectrical heater

2.49

4.8

Pa

yb

ack

Per

iod

(y

ears

)

118 Fig. 12 - Thermal efficiency versus solar radiation 119

120

0

10

20

30

40

50

60

70

80

90

100

200 400 600 800 1000 1200

Co

llec

tor

Ther

mal

Eff

icie

ncy

(%

)

Solar Radiation (W/m2)

121

122

Fig. 13 - TRANSYS schematic of the proposed system 123

124

125

126

127

128

129

130

131

132

Fig. 14 – PTC outlet temperature versus time for the month of July 133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

0102030405060708090

100110120130140150160170180190200210220

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Tem

per

atu

re (

C)

Time (h)

148

149

Fig. 15 – Storage tank outlet temperature versus time for the month of July 150

151

152

153

154

155

156

157

158

159

160

161

162

163

0

20

40

60

80

100

120

140

160

180

200

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Tem

per

atu

re (

C)

Time (h)

164

165

166

167

Fig. 16 – The inlet chiller temperatures versus time for the systems with/without auxiliary heater 168

169

170

171

172

173

174

175

176

177

178

179

0

20

40

60

80

100

120

140

160

180

200

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Tem

per

atu

re (

C)

Time (h)

T inlet chiller-without aux. heater T inlet chiller-with aux. heater

180

181

182

183

Fig. 17 – The power of the auxiliary heater versus time for the hybrid cooling system 184

185

186

187

188

189

190

191

192

193

194

195

-1

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Au

xilli

ary

Po

wer

(kW

)

Time (h)

196

197

198

199 Fig. 18 – Thermal efficiency versus temperatures 200

201

202

203

204

205

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300

Ther

mal

Eff

icie

ncy

Tavg-Tamb(°C)

Manufacturer Curve Mathematical model using Polytrough Parameters