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In 2001, Denso Corporation Ltd. in Japan was the first company to start commercial production of a residential air-source CO2 heat pump water heater. The 4.5 kW unit delivers 85oC DHW, and the measured SPF in Tokyo climate is reported to be above 3 (Saikawa and Hashimoto, 2000). Due to the considerable market for heat pump water heaters in Japan, several other Japanese companies have also started production of CO2 heat pumps based on the same type of technology (www.shecco.com). 2.4.3.2 CO2 Heat Pumps for Retrofitting in High-Tempe- rature Heat Distribution Systems Brandes and Kruse (2000) investigated the possibility of using CO2 heat pumps for retrofitting of residential oil-fired boilers installed in high- temperature radiator systems. In the theoretical study, both single-stage and two-stage CO2 heat pumps were considered, but the 25% increase in COP could not be justified by the higher first costs of the two-stage system. A suction gas heat exchanger was not recommended owing to the marginal impact on the COP. The performance of the CO2 heat pump was calculated at different tempe- rature regimes in the radiator system, including 90/70oC, 70/50oC, 50/40oC and 35/30oC. In Berlin climate, the estimated SPF for an air-to- water CO2 heat pump installed in a 70/50°C system was about 2.8 when running the system at optimised high-side pressure, and about 2.6 when the high-side pressure was kept constant at 10.5 MPa. This was typically 10 to 25% lower than that of the state-of-the-art air-to-water heat pumps. However, by reducing the water flow rate in the heat distribution system by approximately 65%, the supply and return temperatures at design conditions were altered from 70/50oC to 93/40oC. Due to the reduced CO2 outlet temperature from the gas cooler, the SPF increased by about 15%. For the 93/40oC system, the optimum high-side pressure ranged from about 8.5 to 11.5 MPa at varying heat source and heat sink temperatures. A prototype CO2 air-to-water heat pump was designed and tested. The measured COP for the 93/40oC system was about 10 to 18% higher that that of the theoretical system. Hence, the CO2 unit was more energy efficient than the best residential air-to-water heat pump on the market. The main reason for the improved COP was that the prototype achieved a lower temperature approach (∆TA)2 than presupposed in the calculations. 2 The temperature approach is the difference between the CO2 outlet temperature and the inlet temperature of the heat sink for a counter-flow gas cooler. 2 – Technological Status 21PDF Image | Residential CO2 Heat Pump System for Combined
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