Efficiency and annual gains of non-tracking solar thermal collectors can be increased by applying reflectors. This work investigates to which extent diffuse irradiance contributes to the output of such low-concentrating collectors. As an example, the RefleC collector is used. This double-covered process heat flat-plate collector has a one-sided booster reflector. RefleC is analysed theoretically by raytracing simulations based on measured collector material parameters. Furthermore, a collector gain simulation model for TRNSYS is described, which uses the raytracing results. Contrary to the state of the art, this model recalculates the incidence angle modifier (IAM) for diffuse irradiance in every time step, accounting for the varying anisotropy of diffuse irradiance. It is shown that the sky-diffuse IAM of RefleC can vary up to 25 % during one day. Different test samples of the collector are characterized by high-temperature outdoor efficiency tests and IAM measurements. Adding to that, the monitoring data of the RefleC pilot plant at a laundry in Germany with collector temperatures up to 130 °C are analyzed. The work concludes that instantaneous and annual gains of all non-focusing solar thermal collectors are undervalued when constant, isotropic diffuse-IAMs calculated from raytracing or steady-state collector tests are used. This is especially relevant for very incidence-angle-selective collectors operating at higher working temperatures.
