Checking the performance of absorption refrigeration systems with 3 levels and 4 different temperature levels

Dissertation for M.Sc.

Treatment: Energy Conversion

Abstract

In this research, a study on irreversible absorption refrigeration systems based on internal and external irreversibility is presented with regard to the limited thermal capacities of external tanks. To optimize the system, three scenarios were defined in which objective functions such as the coefficient of performance (COP), environmental function (E) and thermoeconomic criterion ( ) and the specific entropy production rate are simultaneously involved in the optimization process. The first scenario, which includes two objective functions, is to maximize the environmental performance coefficient ECOP and minimize the specific entropy production rate at the same time. Multi-objective evolutionary algorithms (MOEAs) based on the NSGA-II algorithm have been used, while the temperature of the working fluid in the generator ( ), the temperature of the working fluid in the evaporator ( ), the temperature of the working fluid in the condenser and the temperature of the working fluid in the absorber ( ) have been considered as decision variables. ([1]COP), environmental function (E) and thermoeconomic criterion (F) in which these functions were optimized simultaneously and the results obtained were compared with previous researches. Multi-objective evolutionary algorithms ([2]MOEAs) based on the NSGA[3]-II algorithm have been used, while the temperature rate of the working fluid ( , ) and the rate of the heat transfer surface ( , ) have been considered as decision variables. The optimal Pareto frontier has been carried out and a final optimal solution has been selected using various decision-making methods such as LINMAP method, TOPSIS method and Fuzzy method. style="direction: rtl;">According to the fact that the facilities of a building are considered as the heart of that building and the design and selection of its equipment is of great importance, and also considering the importance of reducing energy consumption, in this article, an attempt has been made to discuss the advantages of absorption chillers compared to compression and how they work to provide cold water for cooling, hot water for heating, and hot water for consumption in order to save energy consumption.

Chillers are among the most important equipment in cooling, which can generally be divided into two categories: compression chillers and absorption chillers. In general, compression chillers use electrical energy and absorption chillers use thermal energy as the main source of cooling.

Edwin Andersen[5] in the book "Refrigeration: Domestic and Commercial" about the refrigerant and absorbent pair of absorption chillers that have liquid absorbent material, he enumerates 9 important characteristics of refrigerant and absorbent that can play a decisive role in the selection for use in such systems as follows:

First: lack of solid state - refrigerant pair And the adsorbent should not become solid during the interactions and the normal temperature range of the absorption cooling operation. Because the occurrence of solid phase leads to the slow movement of the solution or even blockage of the fluid paths. Second: high volatility ratio - the volatility of the refrigerant must be much more than the volatility of the absorbent material so that it is possible to separate them easily during the condensation operation that takes place in the generator. The possibility of easy separation of the refrigerant from the adsorbent, which enters the generator as a solution, has a direct effect on reducing the amount of heat energy and reduces the costs related to the condensation operation. Such a desire leads to a kind of dependence and attachment when living together with another. Therefore, the speed of mixing and merging increases and the activity coefficient of the refrigerant becomes less than unity, and on the other hand, the amount of adsorbent material to absorb the refrigerant is reduced, and as a result, the amount of heat energy required is reduced. Also, the size of the heat exchanger, which enables the exchange of heat between the concentrated solution of the adsorbent coming out of the generator and the dilute solution of the adsorbent and refrigerant under the pressure of the pump, becomes smaller. At the same time, the research of Jacob [6], Albright and Tucker [7] , [1-4] shows that the strong desire of the absorbent material for the refrigerant leads to the problem of condensation in the generator, because in the generator, more heat energy must be spent to separate these two materials, which, of course, with that strong desire to connect, such a consequence can be predicted.

Fourth: medium pressure - the operating pressure of the refrigerant and The absorber must be medium in order to carry out the process of absorption and then separation, which leads to cooling. Because the need for high pressure increases the thickness of the device's walls and the use of high-pressure equipment and fittings, which add to its weight and costs. On the other hand, the need for very low pressures and vacuum also leads to an increase in the volume of the device for absorption operations and requires special equipment to maintain the vacuum inside the device. Fifth: Stability - Absorbent and refrigerant materials must have good stability and chemical stability and maintain their original properties over many years. Chemical stability reduces the possibility of the formation of gases and solid materials and minimizes corrosion. Sixth: low corrosion and erosion - absorbents and coolants cause more or less corrosion and erosion of the metal surfaces of the device, and naturally, among these, materials that are more stable and have less erosive effects are more suitable. Sometimes, in order to prevent the erosive effects of materials, it is necessary to add other chemical compounds as inhibitors to the absorbent and refrigerant pair.

Seventh: Safety - The absorbent and refrigerant pair should not be toxic or combustible, and their use should not have harmful environmental effects. The fewer side effects and the higher the safety of their use, the higher the user score.

Eighth: Low viscosity - the smoother and less viscous the absorbent and refrigerant materials are, the faster and better they move, and as a result heat and mass are transferred more easily, and the pumps use less energy to move them.

Ninth: high latent heat of the refrigerant - the higher the latent heat of the refrigerant, the lower the circulation rate of the absorbent material. The high latent heat of the refrigerant leads to an increase in efficiency. Known adsorbent and refrigerant couples do not have all the above properties completely, but among them, the adsorbent couple of lithium bromide / water and also water / ammonia have better conditions and according to the above, they meet more expectations.