Performance evaluation of water as a heat pump fluid using calculations and its fundamental properties
Industrial processes require a significant amount of heat, which can be supplied by high-temperature heat pumps (HTHP) enabling a carbon-neutral operation. The best working fluid (i.e. refrigerant) for these heat pumps has been addressed earlier by many authors, both experimentally and through simulation. The results do not always coincide and show anomalous behaviour for water. This work assesses the performance of various natural refrigerants (i.e., water, methanol, acetone, heptane, hexane, pentane, butane, and ammonia) based on model calculations and explains the results from fundamental theoretical considerations. A standard heat pump vapor compression cycle was modelled, and the coefficient of performance (COP) was used as the key performance indicator. These calculations have been performed for superheating taken at the exit of the evaporator, as usual, and at the exit of the compressor. Next, building on a theoretical expression for COP, the characteristics of the refrigerants were analysed. The sources of irreversibility of the cycle were analysed for its dependence on the fundamental characteristics of a medium. The results reproduce literature results that water is the best performing refrigerant at heat delivery temperatures above 160 °C. A new finding is that water is also the best performing refrigerant at lower temperatures, provided that wet compression is used, which is standard industrial practice. Further, the theoretical analysis shows that the performance of a refrigerant is connected to the ratio of the specific heat capacity and the heat of evaporation. Among all known natural refrigerants, this ratio is lowest for water. Therefore, this conclusion is fundamental.