Conference Year



CO2 transcritical cycle, Optimal gas cooler pressure, Greywater, Subcooler


The use of CO2 heat pumps for water heating combined with energy recovery from greywater is a promising technology that can help to improve the efficiency of both domestic hot water (DHW) generation and space heating. A key aspect to keep in mind during the design of any CO2 refrigeration or heat pump system is the fact that the optimal gas cooler pressure in a transcritical CO2 cycle is mainly dependent on the refrigerant temperature at the gas cooler outlet. In space heating applications as well as DHW generation, operating conditions require high temperatures of refrigerant at the gas cooler outlet. That can lead to very high optimal pressures, in some cases, even higher than the maximum pressure of the system. The use of a subcooler fed by the same greywater used in the evaporator can help to reduce the optimal pressure and improve the efficiency of the system. When the greywater passes first through the subcooler, the evaporation temperature can be increased while the optimal pressure is reduced. When the greywater passes first through the evaporator, the evaporation temperature remains constant, but the refrigerant temperature at the gas cooler outlet can be reduced to a lower value. So, the order in which the water flows through subcooler and evaporator can affect the system’s efficiency and the best control strategy will depend on the operating conditions. First, a numerical model is used to model an experimental facility and model results are compared to some preliminary experimental results. Finally, this contribution analyses the influence that the greywater conditions (temperature, mass flow rate and flow order), as well as the subcooler efficiency have in the system’s efficiency depending on the operating conditions (DHW generation or space heating) in order to stablish the control strategy that optimize the system’s performance.