EFFECT OF SELECTIVE SURFACE DEGRADATION ON THE PERFORMANCE OF SOLAR WATER HEATING-SYSTEMS

Selective surfaces often degrade in the filed. Their solar absorptivity-alpha(s) and thermal emittance-epsilon change with time in service by some amount, say DELTA-alpha(s) and DELTA-epsilon, from their starting values. It is important to quantify the effect this degradation has on the annual fraction solar F(s). A given relative change in F(s) can be caused by different combinations of DELTA-alpha(s), and DELTA-epsilon. In this paper we use computer simulation of solar domestic hot water systems to graph these combinations, in a plot of DELTA-alpha(s) versus DELTA-epsilon, for relative changes in F(s), of 10% and 5%. The slope and intercepts of this plot, which is found to be linear, are studied for their dependence on a wide range of solar system parameters, such as geographical location, collector area, and set point temperature. We find that the slope, and - for starting values of F(s) less than about 0.5 - the intercepts, are relatively insensitive to the system parameters. We show that this result is consistent with a simple model. For F(s) greater-than-or-equal-to 0.5, the intercepts rise sharply with F(s), in a way that is strongly (and to some extent, only) dependent on the geographical latitude of location. These results have direct application to projecting the useful service life of a selective surface.