Flat Plate Collector for Process Heat with Full Surface Aluminium Absorber, Vacuum Super Insulation and Front Foil

A new flat plate collector is under development for economic process heat of temperatures between 80° and 150 °C. A vacuum super insulation (VSI) with micro-porous powders (perlite or fumed silica) in an evacuated (0.1 mbar) stainless steel envelope has been developed for the rear side of the collector which replaces the mineral wool insulation of standard collectors. CFD simulations of the equivalent thermal conductivity of the VSI including losses via the powder and edge losses have been performed depending on geometry. Applying a 100 μm thin VA foil as VSI cover, the collector losses can be reduced by about 0.6 W/m2K. Between glass cover and absorber the collector will be equipped with a transparent ETFE foil reducing the loss coefficient further by about 1.2 W/m2K. Additionally, a full surface aluminium absorber with a very high collector efficiency factor F’ of 0.97 is used, reaching maximum collector efficiencies of about 0.9. Up to a temperature difference of 110 K, the new collector is estimated to have higher efficiencies than standard flat plate and vacuum tube collectors. In the temperature range of 80° and 150 °C the efficiency is expected to lie between 65% and 35%. Four different and successively improved prototypes are under test and construction, respectively. The first prototype showed the principal feasibility of the concept and was the first VSI collector operated under outdoor conditions at temperatures up to 120 K over ambient.