Abstract: In a solar-to-thermal energy converter comprising an insulated frame, a solar energy absorber mounted in the frame, fluid flow heat exchanger means thermally coupled to the absorber to carry away as thermal energy the absorbed solar energy, and a window transmissive to solar energy mounted in the frame and spaced from the absorber, that improvement consisting of a heat loss suppressor comprising a multiplicity of walls extending between the window and the absorber, adjacent walls being at least in part non-parallel to each other to define a first set of elongated channels generally diverging and opening toward the window interleaved with a second set of elongated channels generally diverging and opening toward the absorber, the walls being of materal transmissive to solar energy and absorptive of thermal energy, the channels being narrow enough to reduce conversion heat loss and deep enough to reduce radiation heat loss.