Influence of the source and sink temperatures on the optimal refrigerant charge of a water-to-water heat pump

This paper presents the results of a study carried out to elucidate the influence of the source and sink temperatures on the optimal charge of a propane water-to-water 16 kW heat pump which does not incorporate any liquid receiver. The unit had been fully tested along a previous experimental study, at various refrigerant charges and different condensing temperatures. A detailed mathematical model was then employed to simulate the unit performance. The predicted results were in very good agreement with the experiments, and furthermore, showed the same trends found in a similar unit tested at the KTH (Sweden) when the evaporation temperature was progressively decreased. Then, the model was employed to study the influence of the source and sink temperatures on the optimal charge of the unit. The simulation showed that the great variation of the optimal charge with the variation of the evaporation temperature is mainly due to the variation of the amount of refrigerant in the compressor oil.