Experimental Investigation of the Thermal Performance of a Prototype Direct-Expansion Solar-Assisted Heat Pump System in a Cold Climate

Abstract Direct-expansion solar-assisted heat pumps (DX-SAHP) take advantage of solar energy to enhance the efficiency of conventional air-source heat pumps. However, their performance in very cold climates is not well understood. In this study, a prototype DX-SAHP is developed to experimentally characterize performance under realistic operating conditions. The system consists of a 2.3 m2 solar collector with a serpentine heat exchanger as an evaporator, a scroll compressor with a rated power input of 0.6 kW, an expansion valve, a coaxial coil heat exchanger as the condenser, and a 178 L water storage tank. A set of experiments was conducted under realistic outdoor air conditions to assess the system’s thermal performance. The system achieved an average coefficient of performance (COP) of 3.1 on a sunny day in winter to raise the water tank temperature from 16.1°C to 50.3°C in 4.5 hours. Conversely, the minimum average COP of 3.0 was recorded on an overcast summer day, raising the water temperature from 27.3°C to 49.5°C in 3.5 hours, while the system achieved its highest average COP of 3.44 on a sunny summer day. The highest instantaneous COP during the operation was 4.33, recorded in summer. However, the system was unable to operate continuously during overcast winter days, due to inadequate available thermal energy.