NOVELTY - Coating composition comprises: (a1) colloidal silica of 1-400 nm of particle diameters; (a2) photocatalyst of 1-200 nm of particle diameters; and (b) polymer emulsion particles of 10-800 nm of particle diameters, where component (b) comprises: (b1) hydrolysable silicon compound; (b2) vinyl monomer which has an at least one functional group selected from a hydroxyl group, a carboxyl group, an amide group, an amino group, and an ether group; (b3) emulsifier; and (b4) water.    USE - The coating composition is useful for manufacturing member for energy conversion apparatuses, reflector apparatus, and solar thermal power generation system (all claimed).    ADVANTAGE - The invention provides water-based coating composition, member for energy conversion apparatuses, reflector apparatus, and solar thermal power generation system which can be formed without baking on the coating film which can maintain high hydrophilicity for a long period of time at high temperature. The coating film using the coating composition has favorable transparency, and high hydrophilicity.    DETAILED DESCRIPTION - Coating composition comprises: (a1) colloidal silica of 1-400 nm of particle diameters; (a2) photocatalyst of 1-200 nm of particle diameters; and (b) polymer emulsion particles of 10-800 nm of particle diameters, where component (b) comprises: (b1) hydrolysable silicon compound; (b2) vinyl monomer which has an at least one functional group selected from a hydroxyl group, a carboxyl group, an amide group, an amino group, and an ether group; (b3) emulsifier; and (b4) water, where polymer emulsion particles are obtained by superposing components (b1) and (b2); and where mass ratio of component (a1) with respect to the total content is 85/100-99/100 and mass ration or components (a1) and (a2) with respect to content of component (b) is 60/100-480/100. INDEPENDENT CLAIMS are:    (1) a member for energy conversion apparatuses whose refractive indexes of the coating film are 1.3-1.5, where it is a member for energy conversion apparatuses provided with a base material and the coating film obtained by apply the coating composition on the base material, and making it dry at 100 degrees C or less;    (2) a reflector apparatus provided with the member for energy conversion apparatuses, where support body supports the member for energy conversion apparatuses from the opposite side to the coating film; and    (3) a solar thermal power generation system provided with the reflector apparatus which condenses sunlight, and the apparatus which converts the heat of the sunlight which condensed into an electrical energy.