Cold storage to agricultural products preservation is maintained with low temperature and high humidity which is formed by evaporator in refrigerator. However, frost deposition is unavoidable on the surface of evaporator when the surface temperature of the evaporator is below 0°C during air cooling. Especially, in fin-tube heat exchanger, frost brings about air-side pressure drop that leads to decrease the amount of air which passes through the evaporator and decreases the heat transfer between air and refrigerant. And the decrease of the heat transfer cuts COP of the refrigerator by 30%. Generally, EHD(Electric Heater Defrost) method is used to remove the frost in cold storage system in the merit of stable heat source and installation convenience. However, EHD method requires higher power consumption than other methods. Furthermore, the supply of excessive heat source increases the temperature in cold storage and reduces the evaporator life.
In this study, Thermal Storage Defrost(TSD) method is suggested, which uses condensing exhaust heat(alternative heat source) instead of electric heater as defrost heat to remove the frost on the surface of the evaporator. The defrost heat is stored to a phase change material(PCM, NMP: 30, 42 and 52°C) in thermal storage tank, and supplied to evaporator by circulation loop of brine periodically.
In order to find the characteristic of frost and defrost cycle, three types of refrigeration system are equipped with EHD and TSD method, respectively.
First, frost and defrost characteristics of EHD and TSD method are analyzed using refrigeration system with 1/2HP compressor. In the case of applying EHD method, array of heater causes a deviation of defrost heat source supply and excessive heat source for defrost reduces defrost efficiency. When refrigeration system is operated with TSD method under high temperature of brine, the defrost efficiency increases to pump power consumption. However the defrost efficiency decreases to the amount of supplied heat. Moreover, the end of defrost cycle is predicted through the analysis of surface temperature.
Second, refrigeration system(2HP) is applied to thermal storage tank with NMP 30, 42, 52°C PCM, and observed to frost and defrost cycle, respectively. As a result, the supplied heat during defrost cycle increases with increasing the NMP of PCM and Coefficient of Defrost Performance(CDP) decreases, which is same trend as the refrigeration system with 1/2HP compressor.
Third, in order to evaluate the performance, EHD and TSD method is applied to proto type cold storage. As a result, the time for first defrost cycle by TSD method decreases 45% and the total power consumption decreases to 21% in comparison of that by the EHD one.