The Development of Heat Pump Water Heaters Using CO2 Refrigerant

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discharged from the discharge side of the expander. In the expander system, the refrigerant’s
expansion energy generated when it expands is recovered and converted into rotation torque to use it
as part of the energy for the compressor.
Table 3 shows specifications of a CO 2 water heater system using an expander-compressor. From Table
3, the heating capacity of the heat pump is set at 10 kW. The system’s performance was evaluated by
relative COP under conditions shown in Figure 8 with equal capacity with a conventional compressorbased
system. Figure 8 shows the results of a performance test of a CO 2 water heater system using an
expander-compressor. Figure 8 also shows relative evaluation of performance of an expander and a
compressor, tested separately with and without an expander. Figure 8 shows that the performance
(COP) of a system with an expander-compressor was 6.4% higher than that of a conventional
compressor. The result is confirmed in number between square brackets, in which the efficiency
calculated from the separate test results of an expander and a compressor is 108% of the efficiency of
the compressor, which agrees with the test result of the expander. This is probably caused by a heatloss
caused by systematization of the heat pump equipment and a difference of optimization between
ideal control and actual control. The results showed that energy requirements for the compressor were
reduced. The engineering development work for this CO 2 water heater, expected to be complete in
March 2008, has been carried out by Daikin together with NEDO since June 2005. The work has
focused on downsizing the water heater for the purpose of expanding its use in high density urban
areas, and on improving system efficiency. However, as there are still cost barriers, more advanced
development of this system is needed for market-oriented commercialization.
4. CONCLUSION
The development of heat pump water heaters using CO 2 natural refrigerant can be summarized as
follows:
- The heat transfer performance of the dimpled tube used in this water heat exchanger is twice
that of a smooth tube.
- The mechanism of the improved performance was confirmed in a visualizing test.
- This water heat exchanger with improved performance is 10-30% lighter and smaller in
volume than the conventional double-tube heat exchanger, allowing a compact, lightweight
design.
- The CO 2 expander for residential Eco Cute products recovers the energy loss of the
compressor. Single-unit and system performance tests have demonstrated about 6-8%
improvement in efficiency.
- Thanks to development of water heat exchangers, expander-compressors, and other key
components and improvement of system technologies, heat pumps may contribute
significantly in future to addressing the challenge of global warming.
2010 International Symposium on Next-generation Air Conditioning and Refrigeration Technology,
17 – 19 February 2010, Tokyo, Japan