NOVELTY - The method comprises contacting a surface of an optical element with an aqueous coating composition comprising water and spherical and elongated silica nanoparticles dispersed in the water, drying the coating composition in an ambient air to form a nanoparticle coating, and heating coated substrate at 300 degrees C. The coating composition: has a pH of greater than or equal to 5; and comprises an aqueous continuous liquid phase composition, an organic polymer binder and silica nanoparticles having a volume average particle diameter of less than or equal to 150 nm dispersed in the aqueous continuous liquid phase.    USE - The method is useful for providing a coating to a surface of an optical element of a solar energy conversion system.    ADVANTAGE - The method is capable of uniformly providing the coating to the surface of the optical element of the solar energy conversion system with desired cleaning and stain prevention properties thus enhancing the performance and efficiency of the solar energy conversion system and reducing the cost for electricity production.    DETAILED DESCRIPTION - The method comprises contacting a surface of an optical element with an aqueous coating composition comprising water and spherical and elongated silica nanoparticles dispersed in the water, drying the coating composition in an ambient air to form a nanoparticle coating, and heating coated substrate at 300 degrees C. The coating composition: has a pH of greater than or equal to 5; comprises an aqueous continuous liquid phase, an organic polymer binder and silica nanoparticles having a volume average particle diameter of less than or equal to 150 nm dispersed in the aqueous continuous liquid phase; and is heated during drying. The nanoparticles are free from a polymer core. The coating is rinsed prior to drying. The optical element is placed into the solar energy conversion system prior to its coating with the coating composition or the optical element is placed into the solar energy conversion system after it s coated with the coating composition. The organic polymer binder is an organic polymer latex such as an aliphatic polyurethane particle, and a water soluble polymer. An INDEPENDENT CLAIM is included for a solar energy conversion system.