A study of Transient Performance of A Cascade Heat Pump System

Abstract

Transient performance of a cascade heat pump system is experimentally investigated. The cascade heat pump consists of 4 systems: the chilled water system, the low-temperature refrigeration cycle using R22, the high-temperature heat pump cycle using R134a and the hot water system. The cascade heat pump produces both chilled water for cooling application and hot water for heating application. A transient performance is observed when the hot water system is in a closed-loop operation. As the hot water temperature increases, the condensing pressure of the heat pump cycle also increases but the other refrigerant pressures have marginal changes. The operation of the test rig is terminated when the condensing pressure reaches the limit of the system. The condensing pressure at high hot water flow rate increases at a lower rate than that in case of the low hot water flow rate, so the working period is higher. The compressor power of the heat pump cycle increases continuously with time whereas the compressor power of the refrigeration cycle is almost constant. The condensation heat and cooling load rapidly increase at the beginning, and the condensation heat gradually increases whereas the cooling load is almost constant. The COP of the heat pump cycle gradually decreases with time after the rapid increase at the beginning, but the COP of the refrigeration cycle is almost constant. It can be concluded that the variation of the condensing pressure of the heat pump cycle with time has a little influences to the refrigeration cycle. By comparing the high and low hot water flow rate, the COP of the heat pump cycle and the final hot water temperature in the storage tank are higher whereas the others are the same.