PERFORMANCE OF SOLAR POWERED COOLING SYSTEM USING ... · PERFORMANCE OF SOLAR POWERED COOLING...
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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: [email protected] (F. A. Ghaith), [email protected] (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