Capacity modulation of a cascade heat pump with the variation of compressor speed and electronic expansion valve opening

Abstract

With the increased awareness on excessive energy consumption, increasing fuel cost and negative environmental impact (ozone depletion and global warming) of the major heat generation systems and processes, the ability of heat pumps to generate high temperature with very efficient use of energy and minimal environmental impact has gained lots of awareness over the years. Some domestic and most industrial heating and refrigeration applications demand very high temperature lifts. Single-stage cycles do not practically support these applications since their performance and reliability decrease greatly. High temperature generation can be successfully executed through the use of cascade heat pump. This study analyzes the effects of compressor speed and electronic expansion valve (EEV) opening on the performance of the cascade heat pump in heating mode. The rate of increase of capacity with high-stage (HS) EEV opening at higher HS compressor speed is larger than it at high low-stage (LS) compressor speed. The slope of the capacity with the HS EEV opening was much steeper at small opening range than that at large opening range. When the LS compressor speed increased, the maximum Coefficient of Performance (COP) occurred at larger HS EEV opening than the standard opening. The COP variation at the range of low HS compressor speed had smaller dependency on the LS EEV opening than that at the range of high HS compressor speed.