Experimental Analysis of Various Refrigerant Circuit Component Combinations for Low Charge Propane Heat Pumps

In the last years the choice of the refrigerant had become more important to fit the F-gas regulations. Natural refrigerants, such as propane, are one option, but e.g. flammability and safety requirements for installation must be complied. 150g refrigerant charge of propane keeps being focused as the critical amount of charge regarding indoor applications. Taking these 150g of propane as target value for the operating charge, the project LC150 has been started together with a consortium of nine heat pump manufacturers. The goals are the identification of charge and energy efficient components as well as their combination in refrigerant cycles. This has been achieved by concentration on charge efficiency of heat exchangers, compressors, oil and pipes. A broad experimental cross evaluation of these components was set up, where they are combined into various refrigerant circuits. The results were used to identify charge efficient and thermodynamically efficient components. Additionally critical cycle characteristics were identified. The precisely determined overall refrigerant charge of each circuit has been varied to find an optimal charge of each circuit component combination. Presented circuits are in the capacity range of 8kW focusing on brine-to-water heat pump systems and using 100g to 260g of propane. The optimal charge for the tested circuits lies between 160g and 200g with a SCOP of up to 5. More charge would not increase the performance. A SCOP of 4.7 could be reached for 150g refrigerant charge. The methods of charge reduction and the technics of determining the charge are presented as well as some of the available results of the project in terms of heating capacity, COPs, SCOPs, actual and potential refrigerant charge.