The modelling of the long-wave irradiance dependence of the collector, as (c 4 ðE L rT 4aÞ) is made in a similar way as described in the basic ISO 9806-3, for testing of unglazed collectors, but here it is treated as a heat loss term. All additions to Eq. (2) inserted in Eq. (3)
are based on long agreed collector theory. The coefficients in Eq. (3) are explained below: