Design of a Solar Absorption Cooling System: Case Study

In this paper, the principles of the operation of an adsorption cooling circuit and its operating points are analyzed through both a thermodynamic analysis and with mathematical calculations of the whole circuit and using EES to design the solar system. The results of the program are discussed and highlighted by comparing them with the results of the theoretical and experimental studies in the references.


Introduction
Research on renewable energies is one of the most important issues of our present time, especially solar energy, as a clean energy and as an alternative solution to the polluting and depleting fossil energy. In addition, solar is considered available in the countries that have abundant solar radiation [1]. Since the beginning of his time, man has sought to seek comfort and to provide the best conditions for ideal living. Cooling and air-conditioning have become basic requirements of his daily life.
It was noted from the global statistics that researchers estimated in 2005 that there was a clear rise in the demand for the use of refrigeration and air conditioning. 25% of the energy used in the world was used for air conditioning and cooling [2]. Libya lies in the center of North Africa between latitudes 20˚N -33˚N and longitude 10˚E -25˚E. The country is located in the Sun Earth belt. How to cite this paper: Jenkins, P., Elmnifi, M., Younis, A., Emhamed, A. and Alshilmany, M. (2020) Design of a Solar Absorption Cooling System: Case Study. Journal of Power and Energy Engineering, 8, According to the report of the Institute of Thermodynamics Engineering at the German Space Center in Stuttgart [1], the direct natural solar radiation varies from 1900 kWh/m 2 a year in the far north of the country to more than 2800 kWh/m 2 a year in parts of the south-east.
Concentrated solar power plants can be considered economically valuable only for sites with direct solar radiation above 1800 kWh/m 2 a year [3]. All Libyan lands can meet this condition with higher potential than the southern parts of the country. The sector of buildings is, on a global scale, one of the largest energy consumers (together with transport and industry sectors). Over time solutions were developed to meet the needs of users. One solution was the widespread use of air conditioning systems based on electric driven compression technology, which have greatly improved the quality of indoor environments in buildings. However, these systems have high-energy consumption in heating and cooling. The costs associated with air conditioning have been increasing and it's expected that this growth will be even more pronounced in coming years, either due to the rising standards in comfort required by the occupants, or due to climate changes [3] [4].
Today in Libya, buildings account for about 60% of the electric energy consumption and about 30% of primary energy consumption [5], this makes this sector a target for the improvement of energy efficiency ratings. Thus, any measure to keep or improve standards in indoor comfort, and at the same time allowing for the reduction in the energy bill, should be an aim of research. This paper proposes to analyze the use of a solar-based system to obtain the required thermal energy for heating and cooling, as well as the production of domestic hot water (DHW).
The idea for this research was developed after reviewing what was available in this field of scientific experiments and research. Many references were studied of solar-powered cooling applications; among these experimental studies was the design of a solar powered refrigerator adsorption system in China that could absorb about 18 MJ/m 2 of solar radiation [6]. Using this research, we were able to design a cooling system based on the principles of adsorption and work on modeling the system variables and their impact on performance of the cooling system.

System and Process Description
Intermittent adsorption systems usually have a single bed adsorption cycle that has been improved for some applications, such as preservation of food and vaccine storage. The adsorption system consists of three main parts: a solar collector with adsorbent bed, where a porous solid material is placed, a condenser, and an evaporator, as shown in Figure 1. The operating cycle of the system has four processes. In Figure 1, line (1-2) represents the heating process and line (2)(3) represents the desorbing process. The cooling process is represented by line  period, the adsorbent bed receives heat from the solar energy that raises the temperature of the pair of adsorbent and adsorbate, as shown in Figure 1, line 1-2, which is an isosteric heating process at constant concentration of the adsorbate, x max . When the adsorbent bed pressure reaches the pressure of the condenser, the adsorbate vapor diffuses from the collector and is collected and condensed in the condenser (line 2-3, desorption process at condenser pressure) so that the concentration of the adsorbate in the reactor reaches the minimum value (x min ) at the end of this desorption process. This process is followed by cooling the generator (line 3-4, isosteric cooling process). Then, the liquid adsorbate flows from the condenser to the evaporator. After that, the adsorbent adsorbs the refrigerant that comes from the evaporator (line 4-1, adsorption process at evaporator pressure). As a result, the liquid water in evaporator is converted into ice [7].

Adsorption Chiller
The adsorption system in Figure 2 can be compared to a conventional air conditioner or refrigerator with a thermally driven adsorption compressor. The ability to be driven by heat used for desorption, instead of an electric powered compressor, makes adsorption cycles attractive for electrical energy savers. Also, since fixed adsorbent beds are usually employed, these cycles can be operational without moving parts other than magnetic valves; this results in low vibration system with high reliability, and very long life. The uses of fixed beds also result in intermittent cycle operation, with adsorbent beds changing between adsorption and desorption stages [8] [9].

Case Study
For this study, one of Al-Marj University's buildings was selected. The building was composed by two floors with a total surface area of 1450 m 2 . The building did not have any conditioning systems. The building was conditioned using electrical power. Due to the non-existence of a conditioning system, it was Journal of Power and Energy Engineering considered that the cooling of the building was achieved by using an electrical compression chiller. Table 1 lists the heating and cooling periods considered for this study. The image of the building with solar thermal collectors is presented in Figure 3.

Methodology
To supply the energy for air conditioning system thermal energy was supplied by a parabolic trough solar collectors (PTC). The system was combined with an adsorption system (for cold air production). Thus, several approaches were evaluated for sizing of the system. These approaches took into account the energy required to meet the energy needs of the building, considering monthly average area of collectors, average area of collectors in the period of heating, average area of collectors in the period of cooling, and the month in which greater area of collectors was needed. Another aspect to consider was that the installed collector power was equal to the power needed to satisfy the energy demand of the building. It was expected that total energy needs are not always satisfied due to the fluctuation of the availability of solar energy during the day.

Solar Radiation
The solar radiation parameters for Al-Marj city that were used for this study are presented in Table 2

Energy Needs of the Building
The heating and cooling needs, presented in Table 3, were determined by using equations for calculating both cooling and heating loads. Table 4 shows the design of the solar system that was needed to meet the energy demands of the building.

Design the Model
This design required 1200 NepSolar Poly Trough solar modules with a surface area of 140 m 2 that delivered 77 kW of installed power. An SorTec adsorption chiller was considered for providing the cold production of 48 kW.

Mathematical Model
In this section, a theoretical study evaluated the principles of the cooling circuit with absorption and its importance, compared to other cooling circuits using solar energy, in addition to the stages of its work with the working pairs. The basic adsorption refrigeration cycle can be presented by two methods: ideal cycle

Thermodynamic Analysis of Solar Adsorption Cooling System
This study was the basis that was used in the computer program for the determination of the equations needed to design the model system according to the fallowing order:

Thermal Analysis of the Adsorption System
The amount of heat added and was determined within the stages of the adsorp- Quantity of added heat during the process of generation (kJ/h) AD Q .
The heat of adsorption is a latent heat for adsorption and evaporation (kJ/kg) Amount of heat during cooling process (kJ/h) DF Q .

Amount of Heat in the Condenser
The amount of heat that the cooling medium put into the condenser was cooled and condensed into liquid in one of two ratios: Journal of Power and Energy Engineering fg h -Enthalpy evaporation is also given by the type of medium employed in the cycle.

Amount of Heat Absorbed in the Evaporator
The amount of heat required to evaporate the cooling medium within the evaporator, where Q eva was the convection of the place to be cooled, was known in advance according to the relationship:

Mass Transmission Equations
The system used three blocks for mass transmission: the mass of the adsorption receptor (which is often neglected when the vessel was well designed), the mass of adsorption (m nt ) and the mass of the absorbent (m at ) substance are formed by the working pair.
h h − -Enthalpy values on evaporator input and output.

Performance Analysis of the Cooling System by Absorption
The COP represents the ratio of the amount of heat needed to evaporate the cooling medium in the evaporator to the amount of heat provided to heat and generate the medium in the generator or the adsorption vessel according to the relationship in Equation (7).
g Q -The quantity of heat provided to the adsorption receptor was given by:

Amount of Heat Required from the Solar System
The thermal capacity required to cover the load of the adsorption vessel from the solar system was g S con coll Q Q η η ⋅ = (9) coll η -Solar Collector.
. con η -Connecting pipes. Journal of Power and Energy Engineering

Required Area of Solar Collectors
The area of the collector was determined from Equation (10) by the amount of heat required from the solar system and the value of the intensity of the solar radiation (G).
From Equation (11), the number of solar collectors (n) was determined as, LxW-Length and width of solar collector.

Performance Coefficient of the Solar System
The coefficient of solar performance was the ratio between the amount of heat required to meet the required cooling load and the amount of heat produced from the solar system.

Results and Discussion
Basic data used: Working fluid used: lithium bromide/water (standard air conditioning).
Type of collector: Parabolic trough with a highest temperature of 150˚C.
Enthalpy steam and absorption were calculated using the steam tables. After determining the basic data, the following results were reached.

Absorption Cooling Cycle
The cooling circuit was represented by adsorption after knowing the four points of pressure and temperature, and was obtained on more than one equivalent curve for these evaluations by changing their assigned values calculated within the program, as shown in Figure 4, in terms of pressure and temperature. Three cases were taken to change the data in each case; the degree of condensation and cooling with the degree of generation of generation and absorption, as shown in the Table 5.

Effect of Condensation Temperature
The cooling temperature of the cooling medium used to condense the vapor of the substance played a major role in determining the conditions of the system.  The effect of the condensation temperature on both the thermal performance coefficient and the specific cooling capacity was studied using four variable values of the evaporating temperature of the generation and absorption temperatures, as shown in Figure 5. It was determined that with the increase of the temperature of the condensation, both the performance coefficient and the qualitative capacity decrease. The objective of decreasing the performance factor was to reach the required generating temperature (T g ), performance coefficient COP = 0.5 at a temperature of condensation T con = 33˚C.

Effect of Evaporation Temperature
This study showed that as the evaporation temperature decreased, the performance factor of the cycle decreases, as shown in Figure 6. This means that the required amount of heat will increase in order to reach a constant generating temperature and, consequently, required a larger system size.

Effect of Concentration Ratios (x)
The decrease in the concentration ratio resulted in a shift of the curve to the right. Therefore, it was noted that the coefficient of performance decreased when  this concentration ratio increased as shown in Figure 7. This means that there was a need for an increased amount of heat for the system to perform the generation process.

Effect of Both Condensation Pressure and Evaporation on System Performance
The values of vaporization and condensation pressure play an important role in determining the effectiveness of the adsorbed circuits. It was found that an improvement occurred with a decreasing pressure of the condenser, which resulted in an increase in the productivity, as shown in Figure 8. Condensation Figure 7. Effects of concentration ratios on the thermal performance coefficient at the two generation levels.

Solar Performance Factor of the System
The effect of the generation temperature on the solar coefficient associated with the radiation intensity and the area of the polarization and expressed by the solar thermal energy are shown in Figure 9.

Effect of Solar Radiation Intensity
This factor has a significant impact on the design of the system because its value Figure 9. Solar performance coefficient schemes with generation and evaporation temperature. Figure 10. Changes in the solar performance coefficient with the intensity of solar radiation. will determine the amount of heat required, providing the required load for cooling. Figure 10 shows that changes in the monthly solar radiation values with the solar performance coefficient. With the increase in solar radiation an increase in the coefficient of solar performance was noticed to settle at a steady state value.

Economic Analysis
For the economic analysis, it was considered that the system lifetime was 25 years. The analysis was carried out at constant pricing (without considering the rate of inflation) with a nominal discount rate of 3%. The costs associated with the maintenance of the system were not considered, and it was considered that an annual cost of € 2.692 was needed for the backup energy fossil fuels. The prices mentioned in Table 6 refer to PTCs and to the adsorption system that were obtained directly from the manufacturer [15].

Conclusions
Through the analytical and thermal analysis and the design of the system, the results of this research can be summarized as follows: 1) A new cooling system was designed for the adsorption system between two materials that were used to generate a medium and solar thermal energy was used to operate the system.
2) The possibility of working at a low generation temperature and a pressure of 70˚C -80˚C, 4 -8 kpa, respectively, with a coefficient of performance (COP = 0.4 -0.8) during the summer months in the coastal zone, were reasonable.
3) The possibility of applying a system of air conditioners that operate with a small capacity and space with fixed and continuous loads, such as in steering or control rooms, and with engines used with heavy electric winches.
4) It was determined from the analysis that the system can function effectually and continuously without interruption during daytime and night.