Experimental Analysis of an Indirect Solar Assisted Heat Pump for Domestic Water Heating
Bridgeman, Andrew George
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Due to rising energy costs and increasing environmental awareness, various methods of energy conservation are being investigated. In the residential sector, both solar and heat pump systems have been extensively tested. Recently, combinations of solar and heat pump systems have been developed for both space heating and water heating. These systems have the capability to provide better performances than either of the systems operating on their own. The solar collector benefits from a lower fluid (glycol) inlet temperature which increases efficiencies, and the heat pump benefits from higher evaporator temperatures due to the additional solar energy collected. For this study, a prototype of an Indirect-Solar Assisted Heat Pump for use in the Canadian environment was constructed, instrumented and tested. Controlled, constant temperature tests, as well as solar profile tests were conducted and the heat transfer rates, natural convection flow rate, and heat pump coefficient of performance (COP) were calculated. These values were then compared to simulation results based on a model developed in a previous feasibility study. The experimental COPs calculated ranged from 2.3 to 3.3 throughout the full range of tests which were conducted with supply temperatures ranging from 0°C – 40°C. The simulated results consistently overestimated the experimental results by between 12% - 15%. New empirical heat exchanger relationships were developed for the model bringing the simulated results within 5% of the experimental results for tests run at glycol flow rates of 77 and 154 kg/hr. These relationships were used to refine the model, and produce updated annual performance values in the TRNSYS simulation software. Updated results showed a 5% drop in the annual free energy ratio in Toronto from 57% to 52%, but further optimization of the system using the TRNSYS software package improved annual results up to 56%. For the purposes of the study, a solar assisted heat pump prototype was constructed and tested successfully within the laboratory environment. Results showed that the system has a strong market potential, especially in regions without a strong solar resource. Based on these results further research is recommended, continuing with a full year outdoor test using unglazed solar collectors.