Radiation heat losses in solar collectors with plastic covers

The use of plastic covers instead of glass covers in solar collectors requires a modification of the calculation of radiation heat loss, because plastics are, in contrast to glass, partly transparent for thermal radiation. This article describes a simple method which enables calculation of radiation heat loss in collectors equipped with plastic covers. Comparison of this method with an incorrect method used by some authors shows that the latter leads to large errors in some cases. The results of the correct method indicate that in some situations the glass covers can be replaced by much cheaper plastic covers with a negligible increase in collector heat loss.