Energy efficient and environmentally friendly heat pumping system using CO2 as working fluid; COHEPS
The "natural" refrigerant CO2 is an old refrigerant (Linde 1881) and has been used in the past for maritime cooling and for air conditioning systems in buildings, until it was replaced by the (H)CFCs. Taking the old "natural" fluids, only water (R-717) and CO2 (R-744) meet the requirements according to the EU regulation of December 1994. The limitations of water are the freezing temperature of 0°C and the low volumetric capacity at temperatures below 100°C; the limitations of CO2 have so far been the critical data, i.e. 31°C at 70 bar. For heat pumps with condensing temperatures exceeding 30°C, the trans-critical cycle with pressures up to 150 bar as proposed by G. Lorentzen has to be used. This trans-critical CO2 cycle offers a completely new characteristic: the "Lorentzen-cycle" has its favourable application in the case of an unlimited heat source and a limited heat sink with high temperature glides. Prototypes were developed, constructed and analysed as test rigs from several partners for the following applications: - Hot water heat pumps for residential applications (TUGRAZ.IFW). - Commercial heat pumps for water heating and heat recovery (SINTEF.RAC). - Heat pumps for space heating for retrofitting existing high-temperature systems (FKW). - Heat pumps for dehumidification and drying processes in residential and commercial applications (UESSEN.ATK). All results fulfilled the goals delineated in the proposal to this project and the correlation between modelling and test analyses was extremely high. This is very promising in order to continue in CO2 technologies and to make further efforts for the demonstration and dissemination of the results of this research project. Only one not so successful event occurred. Compressors could be used for test rig installations from two manufacturers (in Germany and Denmark). These compressors were prototypes and not produced within mass manufacturing. An Italian manufacturer presented a new design at the CO2 workshop in Mainz, March 1999, which will offer new possibilities for the next steps in development and dissemination. The responsible partners developed design guidelines for different applications. These give a condensed overview about the essential topics, which have to be pursued with the further development and construction of CO2 heat pumping systems. Conclusions resulting from extended analysis are that the risks related to the use of CO2 in heat pumps are low and acceptable if the charge is limited such that concentrations in rooms after release of the charge are less than 5%.