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At a low DHW heating demand ratio, the HFC heat pump system was more efficient than the CO2 systems due to their poor COP during operation in the SH mode. At an increasing DHW heating demand ratio, the SPF of the CO2 systems were gradually improved, since an increasing part of the heating demand was covered by operation in the Combined mode and the DHW mode. On the other hand, the SPF for the HFC heat pump system dropped quite rapidly with increasing DHW heating demand, since the COP during operation in the DHW mode was about 38% lower than that of the SH mode. At the actual operating conditions, the break-even for the prototype CO2 system occurred at a DHW heating demand ratio around 60%, whereas the break-even for the improved CO2 system was about 10 percentage points lower. In existing houses where the DHW heating demand ratio typically ranges from 10 to 30%, a high-efficiency HFC heat pump system will be more energy efficient than an integrated singe-stage CO2 heat pump system. However, in low-energy houses and passive houses, where the DHW heating demand ratio typically ranges from 50 to 80%, an integrated CO2 heat pump system will outperform the most energy efficient HFC heat pump system on the market. 4. CONCLUSION Integrated residential heat pump systems using carbon dioxide (CO2, R744) as a working fluid can achieve high Coefficient of Performance (COP) due to the unique characteristics of the transcritical cycle with heat rejection in a gas cooler at a gliding CO2 temperature. Integrated CO2 heat pumps for space heating and hot water heating can be designed to utilize different heat sources such as bedrock, 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 gas cooler and the DHW system. It was found that a counter-flow tripartite CO2 gas cooler in combination with an external single-shell hot water tank and a low-temperature heat distribution system, would enable production of DHW in the required temperature range from 60 to 85oC 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 laboratory measurements and compared with the performance of a high- efficiency HFC brine-to-water heat pump unit. At DHW heating demand ratios above approximately 50%, the CO2 heat pump system outperformed the HFC heat pump system. Consequently, integrated CO2 heat pump systems equipped with a tripartite gas cooler represent a very promising, high-efficiency system for combined space heating and DHW heating in low-energy and passive houses. The results presupposes the use of a space heating system with a low return temperature as well as optimized design of the DHW tank in order to minimize thermodynamic losses caused by mixing of hot and cold water and conductive heat transfer inside the tank. The tripartite gas cooler design is more complex than traditional gas cooler concepts, and a compact, low-cost tripartite water/CO2 heat exchanger with high performance need to be developed. 5. REFERENCES Breembroek, G., Dieleman, M., 2001: Domestic Heating and Cooling Distribution and Ventilation Systems and their Use with Residential Heat Pumps. IEA Heat Pump Centre Analysis Report HPC-AR8. ISBN 90-72741-40-8. Dokka, T.H., Hermstad, K., 2006: Energieffektive boliger for framtiden – en håndbok for planlegging av passivhus og lavenergihus (Energy Efficient Houses for the Future – A Handbook for Planning of Passive Houses and Low-Energy Houses). Norwegian final report from ECBCS Annex 38, Sustainable Solar Housing. SINTEF Building and Infrastructure. Stene, J., 2004: Residential CO2 Heat Pump System for Combined Space Heating and Hot Water Heating. Doctoral Thesis at the Norwegian University of Technology and Science, Dept. of Energy and Process Engineering, 2004:53. ISBN 82-471-6316-0. Stene, J., 2006: Residential CO2 Heat Pumps for Combined Space Heating and Hot Water Heating – System Design, Test Procedures and Calculation of SPF. Report no. TR A6102. SINTEF Energy Research, Norway. International Energy Agency (IEA) Heat Pump Programme – Annex 32 – Workshop in Kyoto, Japan – December 6th, 2007 14PDF Image | INTEGRATED CO2 HEAT PUMP SYSTEMS
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