An experimental investigation on a small-sized parabolic trough solar collector for water heating in cold areas

When used in cold environment, the conventional flat plate solar collectors and all-glass evacuated tube solar collectors will cause various problems, such as large heat loss, low efficiency, freeze and tube-burst, which severely limit their applications. In this paper, a special small-sized parabolic trough solar collector (PTC), which could overcome the shortcomings of conventional solar collectors, was proposed for water heating in cold areas. An experimental platform was developed, and extensive tests were conducted to evaluate the characteristics of the proposed PTC. It was found that the thermal efficiency of the proposed PTC reached about 67% even under the condition of solar radiation of less than 310W/m2, indicating that the PTC could collect solar radiation efficiently. The results also showed that when the fluid temperature was under 100°C, the thermal efficiency was improved with increased fluid temperature, due to the increase of Reynolds number caused by the great variation of thermophysical parameters of the heat transfer fluid (HTF), which resulted from the increasing fluid temperature. Both the frosting on the reflector and the ambient wind velocity weakened the thermal efficiency greatly. The highest thermal efficiencies were respectively about 26% and 56% under condition of frosting and wind velocity of 5.0m/s. The comparison test showed the coated double glazing receiver had a longer temperature response delay to the deflected sunlight than that of the uncoated one. The tested PTC system operated normally in low fluid temperature condition, showing great anti-freezing property in cold environment.