CO₂ heat pump system for space heating and hot water heating in low-energy houses and passive houses

Different integrated CO₂ heat pump systems for space heating and heating of domestic hot water (DHW) have been investigated, focusing on the design of the heat rejection heat exchanger (gas cooler) and the DHW system. It was found that a counter-flow tripartite CO₂ gas cooler in combination with an external single-shell DHW tank and a low-temperature heat distribution system would enable production of DHW from 60 to 85ºC without electric reheating, and contribute to the highest possible COP for the CO₂ heat pump system. The Seasonal Performance Factor (SPF) for a prototype brine-to-water CO₂ heat pump was calculated on the basis of extensive laboratory measurements and compared with the performance of a stateof-the-art high-efficiency brine-to-water heat pump. At DHW heating demand ratios above approx. 50%, the CO₂ heat pump outperformed the state-of-the-art heat pump. Consequently, an integrated CO₂ heat pumpsystem equipped with a tripartite gas cooler represents a promising, high-efficiency system for combined space heating and DHW heating in low-energy and passive houses. The results presupposes the use of a lowtemperature space heating system and optimized design of the DHW tank in order to minimize thermodynamic losses caused by mixing of hot/cold water and conductive heat transfer inside the tank.