Extending the use of water-as-refrigerant based air-to-water heat pumps : solutions with a water-carbon dioxide cascade and water-ethanol mixtures
Recent regulatory changes concerning refrigerants are driving significant changes in the field of heat pumps (HP). The Fgas regulation requires a reduction in the use of hydrofluorocarbons (HFC). There is also a strong desire among certain European countries to tighten regulations on per- and polyfluoroalkyl substances (PFAS), also known as forever chemicals. Hydrofluoroolefin (HFO) refrigerants replace partially HFCs. However, since HFO decomposition products are part of PFAS, the risk of restrictions on their use in the future should not be overlooked. There are few alternatives to HFO. On the one hand, there are hydrocarbons, whose use is limited by their flammability. On the other, there are natural fluids. Water is one of them, and recent developments in high-speed electric motors have led to the emergence of compact high speed compressors with high volume flow rates. However, a HP using water alone is limited to temperate climates with mild winters due to the relatively high triple point temperature of water. In order to expand the range of uses for water as a refrigerant, the authors are working on two innovative solutions: a watercarbon dioxide cascade and water-ethanol mixtures. This article presents the modeling work on these two solutions. Regarding the cascade, the work focused on multi-variable optimization of the cycle. The results show very good HP performance for simultaneous heating and domestic hot water production (DHW). At 0°C ambient temperature the combined COP is 2.9°C for 47°C to 55°C heating regime and 55°C DHW production. In the case of water-ethanol mixtures, the work focused in particular on modeling fractionation in direct contact exchangers of the HP. The results