The optimal charge of carbon dioxide in water–water heat pump systems with and without an internal heat exchanger

ABSTRACTAn experimental study was conducted on the optimal charge of carbon dioxide in water–water heat pump systems (WWHPSs) with and without an internal heat exchanger (IHX). An appropriate carbon dioxide charge standard was obtained from the experimental results. All tests were conducted with the inlet and outlet water temperatures set to 12°C and 7°C in the evaporator, respectively and 20°C and 55°C in the gas cooler, respectively. During the tests, the evaporation temperature was 0°C. The coefficient of performance of cooling (COPc) and coefficient of performance of heating (COPh) increased by 8.3% and 0.7% in the WWHPS with an IHX compared with the WWHPS without an IHX. Under the test conditions, the optimal carbon dioxide charges with and without an IHX were found to be 2.0 kg and 2.25 kg, respectively. The test results show that the optimal carbon dioxide charge decreases by 11.1% when an IHX is included in the WWHPS.