: Dengler, J.; Rommel, M.; Göttsche, J.; Wittwer, V.; Goetzberger, A.

Abstract
A new type of transparently insulated flat-plate collector was developed. It reaches higher efficiencies at low irradiation values or high operating temperatures than any other collector type known. Both sides of its absorber are covered with transparent insulation material and both sides are irradiated. Thus, the heat losses of the collector related to the total absorber area are distinctly reduced. An optical efficiency of Etasub0 is equal to 0.72 and a temperature dependent U-value of U(DeltaT) is equal to (0.95 plus 0.0076 DeltaTKhighminus1) W mhighminus2 Khighminus1 were measured with an outdoor test facility. The bifacial-absorber collector is considered to be the best option for the DHW system of the energetically self-sufficient solar house in Freiburg because of its outstanding winter performance.