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

Abstract

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, in first and second experiments the influence of adding a horizontal and a vertical secondary reflector on the output of the solar collector are examined respectively, while in the third and fourth experiment the influence of covering the collector surface area with an aluminum foil together witha secondary vertical reflector 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 is installed vertically is 7% greater than the reference collector. It can be concluded that adding a secondary reflector atthe bottom of the PTSC should be vertically mounted and not horizontally, in order to maximize the energy reflected to the primary collector.Finally, 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%.