Experimental study and simulation model of a direct expansion solar assisted heat pump to CO2 for water heating: Inventory, coefficient of performance and total equivalent warming impact

This work is a theoretical and experimental study about the refrigerant charge inventory in a CO2 Direct Expansion Solar Assisted Heat Pump to heating residential water. The effects of variation in the irradiation levels were analyzed for 72 experimental points, and a mathematical model was developed to predict the amount of refrigerant in several parts of the system. The conditions of the supercritical fluid and two-phase flow were considered, and eight void fraction correlations were tested. The model revealed that changes in the solar irradiation levels had a significant effect on the mass amount, and the accumulator was responsible for compensating these variations. Nevertheless, in percentage terms, the mass distribution remained practically constant in the heat pump components. The best results for mass prediction were obtained by the correlations of Hughmark and Filimonov, with an average error of 6.38% and 6.88% respectively. Considering a tolerance margin of +/- 2%, Hughmark reached total precision in 19.7% of the predictions against 9.2% achieved by Filimonov, and all the tested correlations had 100% accuracy for a tolerance of +/- 18%. A growth of around 0.3 kW/ m2 in the solar irradiation produced an increase of 30% in Coefficient of Performance (COP) and a reduction of 20% in Total Equivalent Warming Impact (TEWI). Among the conditions analyzed, the best operating point occurred with a COP of 3.67, TEWI of 733 kg CO2-eq and a mass of 589.9 g, with an error of 8.54%.