Design, Construction, and Thermodynamic Analysis of a Direct-Expansion Solar Assisted Heat Pump for Cold Climates

Direct expansion solar assisted heat pump (DX-SAHP) systems have the potential to provide the heat load required for domestic hot water (DHW) sustainably and with minimum emissions. DX-SAHPs utilize a solar thermal collector to evaporate a working fluid. By using less energy in the process, these systems can achieve higher coefficients of performance (COP) than those afforded by conventional air source heat pumps. With Calgary possessing the highest solar potential in Canada of about 2396 hours of sunlight available 333 days a year [1], the implementation of such systems would make technical and economic sense. In this paper, the design, fabrication, and testing of a DX-SAHP system for cold climates is presented. A mathematical model representing the system was developed by combining the Hottel-Whillier-Bliss equation for the solar collector and a control volume analysis using the first law of thermodynamics for the heat pump cycle. Theoretical results demonstrate that a COP in the range of 3.4–4.5 is achievable. With the promising theoretical results, an experimental test setup was designed, constructed, and instrumented to determine the long-term performance of a DX-SAHP under local climatic conditions.