31 December 2008

Heat Transfer Characteristics of CO2 as Refrigerant for Heat Pump Water Heaters

In the air-conditioning and refrigeration fields, systems based on CFC and HCFC refrigerants have been used for the last several decades for heat pumping and refrigeration applications. However, since Molina and Rowland (1) reported the destruction of the stratospheric ozone layers due to CFC refrigerants in 1974, the international movement to prevent ozone destruction has been enhanced. As a result, CFC refrigerants have been totally eliminated, and HCFC refrigerants have been restricted in the Montreal Protocol in 1987. This has been followed by promotion of HFC refrigerants, that have no effect on ozone layers, as replacements for CFCs and HCFCs. Moreover, at the Kyoto Conference on Prevention on Global Warming (COP3) held in 1997, HFC refrigerants were also subject to regulations from the viewpoint of prevention of global warming. In response to such an effort to reinforce international regulations, the development of high-performance heat pump and refrigeration systems using naturally occurring substances as working fluids such as carbon dioxide, ammonia, water, isobutane and so on, which have no or only negligible effect on global warming, has become the most important challenge recently. Heat pump water heaters that use carbon dioxide as the refrigerant, residential-use refrigerators that use isobutane as the refrigerant and vending machines that use carbon dioxide or isobutane as the refrigerant, are already available on the market, and the possibility of using carbon dioxide (CO2) refrigerant for air conditioners such as automotive air conditioners, packaged air conditioners, room air conditioners and others is now being considered.
In such a stream of technology development, our research group is now engaged in experimental research concerning heat transfer and flow characteristics in various heat transfer processes (forced convection cooling at supercritical pressure, boiling and evaporating heat transfer, and condensation) of CO2 in spiral-grooved tubes with the aim of obtaining basic data to design heat exchangers for heat pump and refrigeration systems. This report introduces and analyses the results obtained by our research group. Before describing the heat transfer characteristics of CO2, the report starts with an overview of a trans-critical heat pump cycle that uses CO2 as a refrigerant, complemented by a presentation of the basic thermophysical properties of CO2.