Energy and Exergy Analysis of a Compression and Absorption Chiller Coupled with Solar Energy

The performance of a water-ammonia air-cooled absorption chiller cycle is evaluated using a low-temperature source of solar thermal energy in accordance with Tehran's climatic conditions in different working conditions. Energy and exergy analysis of an absorption chiller with computer code written in EES software is performed. Exergy analysis showed that 71% of the exergy loss in the system is related to the generator and 24% is related to the absorber. The results showed that with increasing the generator temperature to a certain temperature, the coefficient of performance increased. The generator temperature of 70 degrees at low absorber temperatures performs better than other generator temperatures. As the generator temperature rises to about 70 ℃ , the exergy efficiency increases, and then the exergy efficiency decreases with increasing temperature. As the temperature of the generator decreases, the circulation ratio increases, and at temperatures below 70 degrees, this increase is seen as exponential and so undesirable that it makes it practically impossible to use the cycle at temperatures below 70 degrees. With the other negative effects seen at temperatures above 80 ℃ to reduce the exergy efficiency, the generator temperature between 70 and 80 ℃ seems appropriate for the proposed absorption cooling system.


Introduction
In order to provide the required energy for cooling devices in summer, a combination of absorption chillers with solar heating systems is used, which is called solar cooling. Since the input of absorption chillers is heat energy, so they can be combined with solar heating systems. Due to this advantage, the chiller heat production system (boiler or burner, etc.) can be used as a backup source and is only operated during the day and night when the solar system's energy is too low. In addition to replacing a renewable source with a fossil source, this method has also made efficient use of solar energy.
There has been a study carried out by Jafarkazemi and Ahmadifard [1] as well as another by Ge et al. [2] that studied the energy and exergy of flat plate collectors. Exergy efficiency of finned double-pass solar collectors has been performed by Fudholi et al. [3]. An analysis of the energy and exergy of a thermoelectric solar air collector with two passes was conducted by Khasee et al. [4]. In their study of porous baffles embedded in solar air heaters, Bayrak et al. [5] used energy and exergy analysis methods.
In this study, energy and exergy analysis and performance of coupled solar energy absorption and compression chiller system using EES software for a residential-commercial building in Tehran climate have been studied. This system consists of solar collectors, absorption or compression chillers, heat exchangers, evaporator components, and condensers that are considered in the energy-exergy analysis. The purpose of this paper is thermodynamic modeling and exergy analysis of compression chiller and absorption chiller cooling systems coupled with solar energy. In this paper, after designing the system and adjusting the arrangement and relationship of system components with each other, thermodynamic equations for each component for thermodynamic modeling and exergy analysis are written and solved. A solar cooling system is analyzed for energy and exergy. Using the energyexergy analysis method, the performance of this system and its sensitivity to performance parameters are determined. Also, the potentials for improving the performance of this cycle to reduce the rate of exergy degradation and increase the efficiency of the second law of thermodynamics are determined. Figure 1 presents states 1 to 4 as ammonia-based thermodynamic properties, whereas states 5 to 10 reflect the composition of water and ammonia.

Governing Equations
Temperature and equilibrium pressure of two ammonia phases, specific enthalpies of saturated ammonia liquid and saturated vapor in temperature terms, the relationship between saturation equilibrium pressure, concentration and temperature of a water-ammonia compound and the specific volume of the compound were calculated using EES software. All equations are solved with EES software and the entropy of waterammonia composition in the saturated liquid phase in terms of temperature and concentration is calculated using the same software. Every component of this system has been analyzed by applying mass and energy conversion laws, as well as the second law of thermodynamics. A steady state condition is used in this study.
The following equations are needed to determine the mass and energy conservation of each component in absorption systems in accordance with the first law of thermodynamics.

Σ̇− Σ̇= 0
(1) Σ̇= Σ̇ℎ − Σ̇ℎ +̇ (2) The coefficient of performance is the ratio of the useful energy utilized by the evaporator to the initial energy given to the generator plus the mechanical work performed by the system pump.

Results and Discussion
The percentage of dimensionless exergy loss of the 4 major components of the cycle is shown in Figure 2 under operating conditions, 10 kW cooling load and heat exchanger efficiency of 80%, = 30℃ , T gen = 80℃ , = 30℃ , and = 2℃. More than 71% of exergy wastes occur in the cycle generator section. The second and third exergy losses occur in the absorber and condenser. Irreversibility is mainly due to mixing losses in the generator and adsorbent and mass transfer with a high concentration gradient and hightemperature difference. In addition, when the ammonia output from the generator is superheated, a higher temperature under the same pressure is required, which results in more thermodynamic losses in the generator as well as the nature of the adsorbent. The superheated temperature also brings more cooling requirements to the condenser, which leads to a loss of exergy in the condenser.

Conclusions
The first and second laws of thermodynamics have been evaluated and calculated at different temperature conditions of different components. The results show that the coefficient of performance of the system increases significantly with increasing the temperature of the heat source and the temperature of the generator up to about 70 degrees, and after that, the changes in the coefficient of performance are not significant. This temperature value of 70 to 90 degrees, which is the coefficient of performance at the highest values, is quite suitable for a flat plate solar collector heating system that can provide this temperature. As the evaporator temperature increases, the system performance increases with an almost linear slope. But in the condenser and absorber, the opposite behavior is seen and with increasing temperature, a significant decrease in the performance coefficient of the system is created. Reducing the temperature to ambient temperature will not have much effect and changes in

Non-dimensional Exergy loss for each component
Generator Absorber

Condenser Evaprator
system performance coefficient in the ambient temperature range has a lower slope. Exergy efficiency decreases with increasing temperatures of the generator, condenser, evaporator, and absorber. Exergy analysis showed that the largest exergy loss in the system, which is about 71%, is related to the generator. The absorber also has the second largest portion of exergy loss in the cycle after the generator. For this reason, the greatest effort to improve the cycle efficiency should be to improve the generator and then the absorber. Energy and exergy analysis in written code shows where system losses have occurred and how system performance can be improved. It also provides insight into which components need to be modified in the design in order to achieve higher performance. These results can be used to optimize a solar absorption chiller cycle. An economic analysis will be possible using these results in future research. [1]  increases, and at temperatures below 70 degrees, this increase is seen as exponential and so undesirable that it makes it practically impossible to use the cycle at temperatures below 70 degrees. With the other negative effects seen at temperatures above 80℃ to reduce the exergy efficiency, the generator temperature between 70 and 80 ℃ seems appropriate for the proposed absorption cooling system.