Experimental and computational study of indirect expansion solar assisted heat pump system with latent heat storage for domestic hot water production

Abstract

Solar assisted heat pump (SAHP) systems have been widely applied in domestic hot water (DHW) production due to their sustainability and stability in operations. However, their performance efficiency requires further improvement using advanced technologies such as energy storage with phase change materials (PCM) and optimal system controls. Undoubtedly, employing PCMs for latent heat storage (LHS) application has a great potential to improve a solar thermal application performance. Despite this fact, the use of PCM in this area is quite limited due to the poor thermal conductivity of available PCMs. Therefore, heat transfer enhancement is one of the essential strategies that can overcome this obstacle. Accordingly, a test rig of a new indirect expansion solar assisted heat pump (IDX-SAHP) system has been designed, built and instrumented. The system can handle heating capacity up to 9 kW. The IDX-SAHP system consists of three operational loops: solar thermal, solar-air assisted heat pump and load profile. A 2 kW PCM heat exchanger (HX) was purposely designed and installed in the system solar thermal loop to store solar energy, when applicable, and release heat when required by the heat pump. The PCM HX is employed with a novel heat transfer enhancement method. The maximum coefficient of performance (COP) of the IDX-SHAP system reached 4.99 during the sunny day with the PCM (HX) integration. However, the maximum energy saving was achieved during the cloudy day with the PCM HX integration. Moreover, the proposed heat transfer enhancement method has been modelled through CFD package and validated with the experimental results. This allows a clear understanding of the reasons for the longer discharging process compared with the charging process. Furthermore, the inlet flow rate and temperature variation of the PCM HX was simulated during charging and discharging processes. The optimum inlet flow rate for this application was found at 0.1 kg/s while the optimum inlet temperature was found at 40°C. Meanwhile, the whole system was modelled by the coupling of TRNSYS, EES and CFD to investigate the potential and advantages of using the system in locations with rich solar intensity such as Cairo and Madrid. The simulation shows that the solar thermal operation loop was called more frequently in these locations. This had a significant impact on the system energy consumption, especially during winter. The maximum COP and solar performance factor (SPF) of the modelled system were 5.3 and 0.83 respectively.