Choice Of Working Pairs And Experimental Research On A Two Stage Chemical Adsorption Freezing Cycle

Generally, the heat source temperature of the thermochemicaladsorption refrigeration system is above50oC and the heat is usually fromindustrial waste heat or solar energy, etc. In order to fully use the heat, theproposed two stage adsorption freezing system can produced coolingpower under the condition of less than-15oC when the driven temperatureis lower than100oC.Under the same working pressure, the equilibrium temperature of tworeactive salts that were reacting with the same refrigerant （such asammonia） is different. By the temperature range of the heat source the twosalts are called as middle temperature salt （MTS） and low temperature salt（LTS）. The selection of the working pairs is the key factor for improvingthe utilization rate of the low grade thermal energy and the coolingperformace of the two stage adsorption freezing system. Based on thecharacteristics, kinetic properties and thermodynamic equilibrium of thesalts, the proper reactive salts were searched on the basis of the constrictpressure of condensation pressure which can satisfy practical requirementsof two stage adsorption freezing system were proposed. Combined withthe utilization requirments of the working pairs and the purpose for coolingat-15oC or lower than-15oC when the heat source temperature is lowerthan100oC, two reasonable salts i.e. CaCl₂and SrCl₂were picked up asMTS, and three suitable salts i.e. NaBr, and NH4Cl, BaCl₂were chosen asthe available salts as LTS. The natural working medium ammonia wasthe refrigerant used in the system. In order to understand the performanceof such a cycle in detail, different working pairs i.e. CaCl₂/NaBr/NH₃, SrCl₂/NH4Cl/NH₃and CaCl₂/BaCl₂/NH₃, were matched. The adsorptionand desorption performances of CaCl₂/NaBr/NH₃and SrCl₂/NH4Cl/NH₃aswell as the equilibrium adsorption quantity were tested at differentcondensation temperature and evaporation temperature. The refrigerationperformance of the working pair of CaCl₂/BaCl₂/NH₃is also researched bya refrigeration test unit. The adsorption quantities were more than87%oftheoretical values for the working pairs of CaCl₂/NaBr/NH₃andSrCl₂/NH4Cl/NH₃under the conditions of the heat source temperature of90oC, the cooling temperature of25oC and the evaporation temperature of5oC. The theoretical COP of the cycle was from0.237to0.277, and COPof SrCl₂/NH4Cl/NH₃is8%less than that of CaCl₂/NaBr/NH₃. Acomprehensive comparison of the performance between the working pairsindicates that the CaCl₂/NaBr/NH₃is better at the COP and SCP. So it waschosen for simulation. The simulation results showed that the maximumSCP is for the working pair of CaCl₂/NaBr/NH₃, and the values are520,352,272W·kg-1when the resorption time is300s,600s,900s, respectively.In the practical application, the COP is about0.07⁰.16and theoutput cooling power is about0.85².1kW with the heat source range of70⁹0℃and the evaporater outlet temperature range of-16.77⁸.38℃.The SCP is about58.4¹61.2W·kg-1. So the system’s performance isstable. Especially, when the hot water outlet temperature is66.4oC, theoutlet temperature of the cooling water is30.2oC and the chilled wateroutlet temperature is-16.15oC, the cooling power can reach to1.02kWwith the COP of0.14and SCP of78.8W·kg^-1.