The Effect of Using Secondary Reflectors on the Thermal Performance of Solar Collectors with Evacuated Tubes

The aim of this paper is to improve the thermal performance of the evacuated tube solar collectors by using secondary reflectors and covering the collector surface area with an aluminum foil. Adding a secondary reflecting surface in the form of a parabola fixed on the end of the parabolic trough, acts as an additional reflective surface, which increases the input energy to the solar collector and consequently improves the thermal output of the collector. Two solar collectors were manufactured, one with modifications and the other without modifications, which is taken as a reference for the sake of comparison. Four experiments were performed; the first and second experiments reported the influence of adding a horizontal and a vertical secondary reflector on the solar collector efficiency. In the third and fourth experiment the effect of covering the collector surface area with aluminum foil together with a secondary vertical reflector on the heat gained by the bulb is examined. It is found that the average temperature of the heat pipe bulb in case of a secondary reflector installed horizontally is about 2.5% greater than the average temperature of the reference collector, while in case of the secondary reflector installed vertically is 7% greater than the reference collector. Adding the secondary reflecting surface in the vertical position, and covering the surface area of main parabola and the secondary reflector with aluminum foil, has increased the temperature of the heat pipe bulb by 11%. Finally, adding a secondary vertical reflector to the parabolic trough collector and covering both reflectors, i.e. the primary and the secondary reflectors, with aluminum foil improve the heating capability of the solar collector.