Toward a standard testing methodology for solar thermal collectors with variable-geometry: The direct radiation incidence angle modifier issue

Solar thermal energy is able to provide a considerable fraction of the medium-temperature energy demand and should be more widely used in specific industrial sectors in 100-250 °C range. For energy collection within this temperature range, solar collector designs usually have optical concentration systems, and therefore, the optical behavior of these devices, especially Incidence Angle Modifier (IAM) term in the standard equations, is of paramount importance to the energy equation balance. In the last few decades various new designs have been developed specifically for industrial applications. At the end of 2013, the new revised version of International Standard ISO 9806 for solar thermal collector test methods was published for worldwide certification. This new Standard unifies all the former American, European and International Standards. However, some solar collector types still do not fit to the Standard testing methodology, in particular solar collectors with variable-geometry. This paper describes the points in International Standard ISO 9806 defined specifically for testing solar collectors with solar tracking and concentration, and present a theoretical model to specify the range of the incidence angles which should be tested on variable-geometry collectors to validate the IAM terms in the energy equation balance. Finally some issues are proposed to extend the scope of the current standards for testing collector designs with variable-geometry, in particular for medium-temperature applications.