Evaluation of an Indirect Solar Assisted Heat Pump Water Heater in the Canadian Environment
Elliott, Bryn Davis
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Solar Domestic Hot Water systems and air-source heat pumps offer the potential for energy savings in residential hot water production, however their performance is limited in cold climates, where the low ambient temperature reduces the collector efficiency or the heat pump coefficient of performance. Combining these systems into a Solar-Assisted Heat Pump can alleviate these limitations by reducing the required collector temperature and by providing an increased heat pump evaporator temperature. This study is a continuation of the development of an Indirect Solar-Assisted Heat Pump undertaken at the Queen’s University Solar Calorimetry Laboratory. Previously, a numerical study compared its performance to existing technology, and based upon this feasibility analysis, a prototype was constructed for controlled laboratory tests using simulated solar input. In the current study, the prototype was modified to include a novel hybrid collector such that its performance under actual weather conditions throughout the year could be assessed. On sunny days, the system experienced daily averaged collector efficiencies between 0.47 and 0.88, depending on the flow rate and season. Averaged heat pump coefficients of performance of 2.54 to 3.13 were observed. Overcast days experienced reduced coefficients of performance, between 2.24 and 2.44. However, on overcast days, upwards of 76% of the collected energy gain was from convection with the surroundings. Based upon these experimental results, a model for the hybrid collector was developed. Annual simulations of the system were conducted to compare the performance of the solar heat pump system when fitted with the hybrid collector relative to cases with more conventional glazed and unglazed collectors commonly used in solar thermal systems. Results were produced for three Canadian cities: Toronto, Vancouver and Winnipeg. The heat pump with the hybrid collector outperformed the other collectors in the Toronto climate, with a free energy ratio of 0.548. Adding a thermally controlled valve to the hybrid collector was proposed to further increase the annual free energy ratio, and was shown to perform best in all three cities, with free energy ratios of 0.558, 0.576 and 0.559 for Toronto, Vancouver and Winnipeg, respectively. It is proposed that additional improvements could be achieved by allowing the collectors to deliver heat directly to the storage tank, by circumventing the heat pump if the conditions were favorable.