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Refrigerants

In the past, ozone layer-depleting refrigerants have been mostly replaced with hydrofluorocarbons (HFCs), which, however, still harm the climate. For this reason, natural refrigerants with low global warming potential are becoming increasingly significant. For example, propane (R290) and propylene (R1270) could replace HCFCs and HFCs in heat pumps since they have similar thermodynamic properties. Their flammability, however, means that they are subject to several safety-related requirements, such as regarding the positioning and ventilation of the systems, the components used and possible safety devices.

In Germany there are only a few heat pumps with these refrigerants on the market. This is partly due to the product liability risk borne by the component manufacturers as a result of partly unclear legal provisions combined with the low sales potential. Their area of application is mostly limited to heat pumps used for heating domestic water with a refrigerant charge of less than 150 g and externally placed air-water heat pumps.

Significant research activities are carried out with the aim ofminimising the inner volume of components and to introduce new safety components. The BMU research project “Ersatz fluorierter Treibhausgase durch natürliche Kältemittel” (Replacement of fluorinated greenhouse gases with natural refrigerants) supports the market introduction of natural refrigerants.

From an ecological and safety point of view, carbon dioxide (CO2) is an almost ideal refrigerant. It is cheap, chemically inert, neither toxic nor flammable and has compared to almost all hydrofluorocarbons a negligible global warming potential. However, it requires considerable operating pressures that place particular requirements on technical components such as compressors and heat exchangers. A unique thermodynamic feature is its low critical temperature of 31 °C. With high sink temperatures the refrigerant no longer condenses but cools with a “strongly sliding” temperature (isobaric). CO2 is therefore particularly suitable when there is a large temperature spread between the supply and return line. Examples of this include domestic water heating as well as simultaneous hot water heating and low-temperature heating via two heat exchangers in series and air heating systems. One manufacturer currently offers CO2 heat pumps with a small heating capacity (5 and 9 kW) on the German market. Small heat pumps that would be suitable for the passive house sectors are not yet available.

Researchers at TU Braunschweig have investigated the heat supply for zero-energy and passive houses. CO2 heat pumps with a small heating capacity and a connected stratified storage tank demonstrated potential energy savings relative to conventional heat pumps.

Refrigerant research is also investigating new materials and material mixtures for cooling-based applications. The “Low-GWP-Kältemittel“ (Low-GHP refrigerant) project, which is being conducted by the Institute of Air Handling and Refrigeration (ILK) in Dresden with funding from the German Federal Ministry of Economics and Technology, shows promising results for a propane-CO2 mixture.

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