22 May 2002
Amidst concerns over global environmental issues, social controls and restrictions on the use of gases that could not only deplete the ozone layer but also possess a greenhouse effect are accelerating. As a result, attention is focusing on natural refrigerants. Among them, CO2 is a non-inflammable, non-toxic, safe refrigerant. Applications that could make the best use of its properties in favor of heat pump heating are under intensive development.
CO2-based refrigeration cycles would involve three to five times higher pressures than do any existing HFC (hydrofluorocarbon)-based cycles, urging the development of a compressor customized for CO2. The CO2 compressor developed by the authors uses two-rolling piston two-stage compression to provide a high discharge pressure while suppressing both vibrations and noises. Moreover, an internal intermediate pressure scheme was employed under which the gas discharged from the first-stage compression unit is introduced into the compressor chamber in order to help reduce the design pressure and reduce compressor dimensions.
As an application system incorporating the CO2 compressor system, a heat pump water heater has been developed. Other developments are drink vending machines and a secondary refrigerant heat transfer systems designed for refrigerated supermarket showcases.
The heat pump water heater can be used in a broad range of geographic regions, including cold districts, because it not only efficiently produces hot water at 90? or above, but involves little capacity loss in a low-temperature environment.
Regarding the drink vending machines, as they use hot water, chilled water, and ice, the simultaneous processes of hot water supply and ice making could make significant savings in energy requirements.
The heat transfer systems that use liquefied CO2 as a secondary refrigerant could not only cut their HFC-based primary refrigerant requirements drastically but would allow for the use of copper piping with a smaller outside diameter secondary piping, contributing to a cost reduction on the whole.
This paper describes a CO2 compressor and its system applications with respect to their characteristics and status of development.