Every night, nearly 100 water-free, energy-independent robots from Israel’s Ecoppia clean the panels at the Ketura Sun solar energy farm in Israel’s Negev desert. They prevent soiling of the panels, which previously reduced energy production by up to 35%.
http://israelnewtech.com/2014/03/ecoppia-announces-worlds-first-completely-autonomously-cleaned-solar-energy-park/