Carbon dioxide (CO2) is a nonflammable, harmless, and odorless refrigerant. It has an ozone depletion potential (ODP) of 0 and a global warming potential (GWP) of 1, and is environmentally friendly like ammonia.
In recent years, carbon dioxide is being studied in and outside Japan as a refrigerant for car coolers and hot water heat pumps. We developed and commercialized, a supercritical CO2 ice heat storage system (photo) using an oil injection type screw compressor.
Because a CO2 refrigerating cycle contains a supercritical area, engineering problems different from conventional refrigerating systems occur in compressors and heat exchangers. In order to achieve an efficient CO2 refrigerating cycle, a compressor capable of compressing at pressures 7.5 MPa or higher is required. This system adopts an oil injection type screw compressor which has a good track record in various types of gas compressors uses newly developed screw rotors to compress CO2 from 2.3 to 9.0 MPa. To assure pressure resistance, a shell and tube type heat exchanger was adopted for the condenser and evaporator. Brine cooled in the evaporator is used to generate ice in the storage tank. The refrigerating capacity is 141 kW, and the heat storage capacity is 2430 MJ.
3. Scope of Application
The applicable area of a CO2 refrigerator is from -56.6ºC, the triple point of CO2, to approximately 80ºC for hot water application.
The CO2 refrigerator, when used for cooling, has a COP value lower than that of an ammonia. However, at temperatures lower than the boiling point of ammonia, -33.3ºC, the difference becomes small and, at temperatures of -50ºC or under, the CO2 cascade system predominates the ammonia 2-stage refrigerating cycle.
With hot water supply conditions of approximately 80ºC, a higher COP can be obtained compared with other refrigerants and a hot water supply heat pump is being developed for commercialization.
CO2 and ammonia, which are typical natural refrigerants, are expected to be applied in a wide range of applications in the future.