Heat loss optimisation of a concentric cylindrical solar collector employing a cobalt oxide selective absorber

Experimental and theoretical investigations to optimise the geometrical design parameters of a tubular collector under atmospheric and evacuated (～ 10^?3 torr) conditions are reported. The results indicate that a maximum stagnation temperature, which corresponds to the minimum heat loss, is obtained for a gap width of ～ 12 mm. An absorber tube of 35 mm diameter concentric with a cover glass tube of 59 mm diameter was observed to be the best combination. An improvement in the collector performance was observed by increasing the illuminated area of the absorber tube and by evacuating the annular gap in the collector.