Experimental study of the heat pump with variable speed compressor for domestic heat load applications

Capacity control for heat pump (HP) systems is essential to efficiently meet variable heating loads in domestic buildings. A low-to-medium temperature hot water supply within a 30 oC-55 oC range is commonly required for domestic heating load applications. In this study, the performance of a variable speed air source heat pump (ASHP) system was studied in the laboratory under a series of ambient conditions at -2 oC, 2 oC, 7 oC and 15oC, examining performance at different water supplying temperatures (WST) of 30 oC, 35 oC, 40 oC, 45 oC, 50 oC, and 55 oC. The performance of the variable speed HP system was evaluated in a conditioning chamber at the above-mentioned ambient temperatures and corresponding humidity levels as per BS EN 14511 standards. The HP performance at six different heating capacities of 18kW, 15kW,12kW, 9kW, 6kW, and 3kW was measured and evaluated via coefficient of performance (CoP) values. The CoP values changes according to the load, water supply temperature (WST) and ambient temperature conditions. The CoP values for fixed ambient temperature conditions and WST i.e., 7 oC and WST 30 oC (7A30W) at 15kW, 12kW, 9kW, 6kW, and 3kW were found to be 3.56, 3.83,4.35, 4.19, and 3.39, respectively. The corresponding frequency of operation was 112.25Hz, 105.87Hz, 81.57Hz, 61.05Hz, 37.64Hz, 18.75 Hz with an absolute percentage difference of 87.01%, 76.45%, 35.95%, 1.75%, 37.26%, and 68.75% to that of the nominal frequency of 60Hz. The trend was similar by changing WST to 35 oC, 40 oC, 45 oC, 50 oC and 55 oC, but with lower CoP values and a smaller variation range. The difference between water inlet and supply temperature (delta T) is another important parameter in domestic heat load applications, and this study also presents results of comparative testing of delta T of 5, 10 and 20 at ambient conditions of 15 oC for WST of 40 oC, 45 oC, 50 oC, and 55 oC. The experimental results indicate that this variable speed-based HP system has the potential to meet different domestic heating demands within a range of WST and ambient conditions.