Don't know about tarnishing, but for cleaning they most likely use water. From an article on solar thermal plants in the Middle East [1]: But harsh desert con

Don't know about tarnishing, but for cleaning they most likely use water. From an article on solar thermal plants in the Middle East [1]: But harsh desert conditions in parts of the MENA region generate large amounts of airborne dust which collects on the solar panels used in CSP systems, reducing their efficiency. They need regular cleaning, which consumes large amounts of water. 1: http://m.scidev.net/en/climate-change-and-energy/renewable-energy/news/abu-dhabi-bets-on-anti-dust-solar-panels.html http://m.scidev.net/en/climate-change-and-energy/renewable-e...