Experimental and numerical study of a directly PV-assisted domestic hot water system

The solar domestic hot water (SDHW) system is the most highly developed system for use of solar energy. The developments for the thermal regulation of buildings should reinforce this trend given the significant reduction of heating needs. Currently, the design of these SDHW installations is well controlled and the system performance is reasonably good. The annual average solar fraction is consistent with expected level (between 60% and 70%) according to a report of CSTB by evaluating 120 SDHW installations (Buscarlet and Caccavelli, 2006). However, the control mode of conventional SDHWs induces additional costs related to the consumption of auxiliaries and other risks of dysfunction of the circulation pump due to the temperature probes and controller setup which induces low annual productivity of solar collector (200 instead of 400 kW h/m² expected). From this point of view, the photovoltaic pumped system seems suitable since it eliminates the controller and temperature sensors. This paper focuses on an experimental and numerical study of the behavior of a PV-SDHW system, focusing on the start-up phase optimized through various electronic devices. A detailed model of a circulation pump was developed by considering a direct current (DC) circulation pump coupled with various electronic devices (linear current booster and maximum power point tracker). The developed models were then validated experimentally, to reveal the influence of the threshold solar radiation on the circulation pump start-up and the pump flow rate as a function of the solar radiation, and its effects on the annual energy performance of PV-SDHW systems.