Development of CO2 Heat Pump Water Heater for Residential Use

Michiyuki Saikawa, Central Research Institute of Electric Power Industry, Katsumi Hashimoto, Central Research Institute of Electric@Power Industry, Masahiko Itoh, Denso Corporation, Hisayoshi Sakakibara, Denso Corporation and@Tomoaki Kobayakawa, Tokyo Electric Power@Company

1. Outline
In Japan, the demand for hot tap water accounts for about 30% of residential final energy consumption, but most of this demand is met by direct combustion of fossil fuel. So it is expected to develop high performance heat pump water heater for energy saving. On the other hand, Developments of heat pumps with natural working fluids are expected for environmental protection. Under such a circumstance, the CO2 heat pump water heater for residential use was developed for the first time in the world.

2. Contents of techniques
Using CO2 as a working fluid for heat pump water heater, its cycle becomes trans-critical with super critical condition at high side pressure. So, high COP is expected because heat exchange with low temperature difference is possible in CO2-water heat exchanger. On the other hand, operating pressure and pressure difference between high side and low side of CO2 heat pump cycle are very high as compared with those of fluorocarbons. Therefore, many breakthroughs are needed to develop CO2 heat pump.

The emphasized important points of the development are as follows.
1. Development of highly efficient compressor operating under high pressure and high difference of pressure between low side and high side.
2. Development of compact and efficient heat exchanger for water heating
3. Development of control method of the heat pump with high COP
For these points, trial manufacturing and evaluation were made repeatedly and finally the heat pump system was developed. The developed system can achieve high yearly average COP (over 3 under Tokyo climate condition) and produce high 90 oC hot water.

Fig.1 shows the developed system. The heat pump unit has a simple cycle with a vapor-liquid separator and a rated heat capacity of 4.5kW. The tank unit has two 150 litters tanks for a total of 300 litters hot water storage. Because the heat pump is operated during night and the produced hot water is stored in the tank,  the system can use the cheap night rate electricity.

Fig.2 shows the developed compressor. It's an inverter-motor-driven, full-hermetic scroll type. Using rolling thrust bearings could decrease its mechanical loss. Also precision working and assembling could decrease its leakage loss. The highly efficient compressor was developed using these technologies.

Fig.3 shows the developed CO2-water heat exchanger. It's a countercurrent type. Using capillary tubes for CO2 flow path could achieve high withstand pressure of the heat exchanger and high heat transfer coefficient of CO2. Also using plate with off set inner fin for water flow path could achieve high heat transfer coefficient of water. With both technologies, the small and highly efficient heat exchanger with small temperature difference was developed.

For developing control method of the developed heat pump, effect of opening of the expansion valve on its performance was investigated. As a result, it was clarified that controlling the temperature difference between CO2 at the outlet and water at the inlet of the CO2 -water heat exchanger was important for operating the system with high COP.

3. Conclusion
Through the development mentioned above, the CO2 heat pump water heater for residential use was developed. It plays the leading role for energy saving in the 21st century.



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