11 May 2004

Selected Issues on CO2 in Compression Systems

Carbon Dioxide (CO2) has shown promising results as an alternative working fluid compared to the CFCs, HFCs and HCFCs. CO2 provides an environmental friendly alternative in a number of heat pump applications, in automobile air conditioning, and as a secondary fluid in refrigeration systems. The physical and the thermodynamic properties of CO2 differ considerably from the more conventional working fluids and offer new possibilities as well as design challenges for systems and components.

On this background IEA Heat Pump Programme’s Annex 27 was established. The main objective of the Annex has been to bring the CO2 heat pump technology closer to commercialization, by addressing critical issues of both basic and applied character.

The scope of the work under this Annex includes compression heat pump, refrigeration and air-conditioning systems and components, with the main emphasis on heat pumps, using CO2 as working fluid. The term “compression heat pump” covers vapor compression circuits with phase change. The term ”system” includes all the components used in a heating/cooling system from the heat pump to the inside unit, controls included.

Results from 12 different research projects together with an extensive literature survey are presented. The projects are carried out as independent research projects, and the findings and the results are the sole responsibility of the authors. The following projects are presented:

· Feasibility of transcritical CO2 systems for mobile space conditioning applications
· Use of CO2- and propane thermosyphons in combination with compact cooler in domestic freezer
· Heat transfer of carbon dioxide in an evaporator
· Correlating the heat transfer coefficient during in-tube cooling of turbulent supercritical CO2
· Heat transfer and pressure drop characteristics of super-critical CO2 in microchannel tubes under cooling
· Flow vaporization of CO2 in microchannel tubes
· Two-phase flow patterns during microchannel vaporization of CO2 at near-critical pressures
· Small oil-free piston type compressor
· Some safety aspects of CO2 vapor compression systems
· Boiling liquid expanding vapor explosions (BLEVE) in CO2 vessels: initial experiments
· Experimental study on boiling liquid expansion in a CO2 vessel
· CO2 as secondary refrigerant (Sweden)

Participating countries were: Japan, Norway, Sweden, Switzerland, and USA. The report, which during the first two years is available for Annex 27 member countries only, can be obtained from the Heat Pump Center (http://www.heatpumpcentre.org).

Keywords: Natural working fluids, Carbon dioxide, heat pumping equipment, IEA