CO2 Heat Pump System for Space Heating and Hot Water Heating in Low-Energy Houses and Passive Houses

Low-energy and passive houses are superinsulated and air-tight buildings where the space heating demand is considerably lower than that of buildings constructed in accordance with prevailing buildings codes. Due to the low space heating demand, the annual heating demand for domestic hot water (DHW) typically constitutes 50 to 85 % of the total annual heating demand for the residence. A heat pump system can be used to cover the heating demands in low-energy houses and passive houses. The heat pump can be designed as a stand-alone system, i.e. a heat pump water heater in combination with a separate unit for space heating, or be an integrated unit for combined space heating and hot water heating. Due to compact design, the latter system is most likely to achieve the lowest investment and installation costs and with that the best profitability. Integrated residential heat pump systems using carbon dioxide (CO2, R744) as a working fluid can achieve a high Coefficient of Performance (COP) due to the unique characteristics of the CO2 heat pump cycle. Integrated CO2 heat pumps for combined space heating and hot water heating can be designed to utilize different heat sources such as ground, exhaust ventilation air, ambient air or a combination of exhaust ventilation air and ambient air. Different integrated CO2 heat pump systems have been investigated, focusing on the design of the heat