30 May 2005


Recently, CO2 as a refrigerant has been attracting attention because of its little negative
environmental impact, non-flammable and non-toxic. The properties of this refrigerant are very favorable
for heat pump applications such as residential heat pump water heaters. Due to the low critical point of
CO2, the heat pump cycle becomes trans-critical. Hence, the high pressure doesn’t depend on condensing
temperature. The high pressure has large effects on the COP. By the way, CO2 systems are the higher
costs of production than HCFCs or HFCs systems, because CO2 is required high-pressure resistance. So,
it is also important to reduce manufacturing cost. In this paper, we investigated the possibility of three
simple CO2 heat pump cycles, paying attention to the relation between the initial refrigerant charge
amount and the COP when the ambient conditions such as water inlet temperature and outdoor
temperature change. As a result, we proposed the CO2 cycles without an accumulator, or with an internal
heat exchanger, which were achieved not only cost saving but also high COP.