Optimum design for solar absorption refrigeration systems and comparison of the performances using ammonia-water, ammonia-lithium nitrate and ammonia-sodium thiocyanate solutions

Solar absorption refrigeration systems increasingly attract research interests. The most common cycles are H₂O-LiBr and NH₃-H₂O, absorption machines that have served as standards for comparison in studying and developing new cycles and new refrigerant-absorbent pairs. In recent years, research has been devoted to improvement of the performance of ammonia-water absorption refrigeration systems. In the present study, thermodynamic properties for ammonia-water, ammonia-lithium nitrate and ammonia-sodium thiocyanate solutions are compiled and are used in cycle simulation. Detailed thermodynamic design data and optimum design results are presented. Thermodynamic properties of these binary fluids are expressed in polynomial equations. The performances of these three cycles against various generator, evaporator, absorber and condenser temperatures are compared. The results show that the ammonia-lithium nitrate and ammonia-sodium thiocyanate cycles give better performance than the ammonia-water cycle, not only because of higher COP values, but also because of no requirement for analyzers and rectifiers. The ammonia-sodium thiocyanate cycle cannot operate at evaporator temperatures below -10°C for the possibility of crystallization. These results form a source of reference for developing new cycles and searching for new refrigerant-absorbent pairs. They can also be used in selecting operating conditions for existing systems and achieving automatic control for maintaining optimum operation of the systems.