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TECHNICAL REPORTS saves energy by making use of CO2 characteristics. The development target is to achieve an annual oper- ating efficiency equal to that of the current models, or higher. Figure 1 shows the major component devices and their characteristics. 4.2 Technological challenges of developing CFC-free refrigerant heat pump component de- vices (1) Expansionpowerrecoverycircuit One of the means of recovering expansion power is with an expander. An expander is driven by the ex- pansion of high-pressure refrigerant. The resultant driving force is used for compression or other types of power. The most difficult challenge with this mechanism is to control the operating conditions, such as the rotat- ing speed of the expander and the compressor, for maximum power recovery. (2) Heat exchangers 1) A gas cooler for water heaters During heat exchange within the supercritical range, the temperature of CO2, which is incondensable, gradually lowers while passing heat to water, thus there is no range where the temperature remains constant. For this particular characteristic, a gas cooler for water heaters can affect ideal counter- current heat exchange with little temperature dif- ference between the water and the refrigerant. Mitsubishi employed a twisted-tube gas cooler (2) in which a CO2-flow pipe is wrapped around the slot in the twisted water piping. Mitsubishi will improve the pipe diameter, joining method, and insulation method for improved performance of the gas cooler. 2) A gas cooler for air-conditioners Most gas coolers for air-conditioners employ a single-direction heat exchange method for the air. It is difficult to use a complete counter-flow type method like that employed in water heaters. How- ever, with a structure containing a number of mi- crochannels laid out in parallel to provide counter- flow channels for heat exchange with the air, the ef- ficiency of the gas cooler can be improved. (3) Compressor When using CO2 as natural refrigerant, compared with the current HFC refrigerant, the discharge pressure of the compressor nearly triples to 10 MPa and the rate of discharge pressure to suction pressure almost triples. As a result, internal losses in the compressor such as leakage loss and mechanical loss increase. Naturally, it becomes necessary to improve the performance against loss and also reliability, by increasing the structural strength as well as wear/friction durability. Mitsubishi has developed small-capacity rotary type compressors (3),(4),(5) for household heat pump water heaters. In addition, for large-capacity applica- tions, the development of a CO2 scroll compressor to be used in business-use heat pump water heaters or building-type multi-air conditioners is underway. Refrigerant circuit for multi-air conditioner Remarkable simplification of piping arrangement Heat exchanger High efficiency is achieved by increased radiation Outdoor unit Multiple indoor units Power recovery expander High efficiency is achieved by recovering expansion power Scroll compressor Large capacity and high reliability. Fig. 1 Development points of a multi-air conditioner for buildings with CO2 refrigerant 4PDF Image | Heat Pump with Natural Refrigerants 3041
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