Abstract: A solar heat-collecting structure comprises a series of interconnected transparent lenses forming a dome-shaped roof for receiving direct solar radiation and for transmitting such radiation through the lenses into the enclosure formed thereby and focusing it on a shallow tray of water forming a floor of the enclosure. The inner surfaces of the lenses are coated with a solar radiation-reflective film so that radiation reflected from within the enclosure toward the roof is re-reflected toward the heat-collecting water surface. Means are provided for removing heated water from the tray and replacing it with water to be heated. Each lens in the roof lens system has a focal length dependent on its position on the roof with respect to the water surface so as to concentrate solar radiation at the water surface and thereby maximize the heating effect of such solar radiation in various sun positions.