1920s – Collectors become flat14. December 2018First flat-plate collectors for solar water heating are used. https://www.sunpad.solar/wp-content/uploads/2019/08/sunpad-solar-water-heater-logo-2019.png 0 0 Webpunks Support https://www.sunpad.solar/wp-content/uploads/2019/08/sunpad-solar-water-heater-logo-2019.png Webpunks Support2018-12-14 15:29:472019-06-19 15:21:261920s – Collectors become flat