US Patent 4,186,726
United States Patent and Trademark Office
University of Iowa Research Foundation (Iowa City, IA, US)
An absorber panel for solar energy is formed of two sheets with spacer means extending therebetween to maintain a spaced relationship between the sheets and provide a flow passage for a heat exchange liquid such as water. A pressure differential biases the sheets toward each other so that their spacing is maintained by the spacer means. The upper sheet which is exposed to solar energy may be thin, and the sheets are free to move away from each other at the location of the spacer means so that permanent damage will not result when the heat exchange liquid expands by freezing or when there is excessive flow of liquid through the flow passage in the absorber panel.
Spencer, Donald L. Apparatus and method for collecting solar energy. US Patent 4,186,726 filed December 30, 1977 and issued February 05, 1980. https://ir.uiowa.edu/patents/19