Solar Engineering a Condensed Course

345.3 Collector Overall Heat Loss CoefficientDB 6.4The heat loss coefficient UL[W/m 2 K] is the sum of the top, bottom, and edge losscoefficients:U = U + U + U(5.3.1)LtbeThe top loss coefficient is due to convection and radiation, and the two others to heatconduction. For an efficient collector, all three are kept low (as low as it is economical).Bottom and edge losses are minimized by means of adequate insulation.The top loss coefficient is more difficult to make low without decreasing S. An extra glass orplastic film between the glass and the absorber decreases convection losses, but also S getslower. A selective absorber surface gives much lower radiation losses than an absorbercovered with black paint.(One more way to minimize losses is to keep the average temperature of the absorber as lowas possible, since the heat losses from an absorber are proportional to the difference betweenthis temperature and the ambient temperature.)ULcan be calculated from the optical, geometrical and thermal properties of the collector,and is typically between 2 and 8 W/m 2 K. UL(or, rather, F RUL) can also be measured, andthis is always done by collector manufacturers. In the present treatment it will be assumed thatmeasured heat loss factors are available. The interested reader, who wants to learn theintricacies of calculating a collector's U , is referred to Duffie-Beckman.L