Study of a photovoltaic-thermal system—Thermosyphonic solar water heater combined with solar cells

Abstract

The theoretical study of a solar hybrid system is reported in this work. A photovoltaic thermal (PV/T) hybrid system combines two different solar systems (thermal and photovoltaic) in one unit. The end products of such a system are both thermal and electrical energy. For the study, a thermosyphon water heater combined with solar cells is chosen. Experiments were performed for several days on the thermosyphon water heater alone. A finite difference method is used to simulate the performance of the solar water heater. The one day experimental data are used to determine the heat loss coefficients of the collector unit and water tank. The predictions for other days are in excellent agreement with experimental observations. The experimentally validated model is used to simulate the performance of the system when it is converted into a hybrid system, i.e. solar cells are pasted directly over the absorber plate. The solar cells are assumed to be in perfect thermal contact with the absorber and, at the same time, electrically insulated from the absorber plate. Since the solar cell efficiency depends on temperature, a linear relation has been used to calculate its variation with mean absorber plate temperature. The cell efficiency and the absorber plate temperature are determined iteratively. The effect of some design parameters on the daily cell efficiency is studied. The average cell efficiency is more or less independent of the cell area. However, the thermal efficiency does depend on the cell area. The amount of water in the tank affects the system performance significantly. Simulations are done for different water masses in the tank and also for different amounts of water withdrawn at different times. The cell efficiency increases slightly with the increase in water mass. It is shown that domestic hybrid solar water heaters (area > 2 m 2) can generate sufficient electrical energy round the year to run tube lights, television etc. for 5–6 h during the night.