This invention relates to solar energy reflectors and to processes for their manufacture.
The reflectors of this invention may be used in solar energy or heating installations, for example concentrating solar power plants. Such installations use the solar energy to first generate heat, which later can be converted into electricity or used for steam production. Concentrating solar power plants wherein reflectors according to the present invention may be used comprise, for example, parabolic trough power plants, central tower power plants (also called heliostat power plants), dish collectors and Fresnel reflector power plants. Solar energy reflectors according to the present invention may be used in such installations as flat or curved mirrors.
Conventionally, solar energy reflectors have been produced by forming a laminate comprising a thin mirror bonded to a supporting sheet. Best reflectivity for the mirror may be obtained if it is thin, so that less solar energy is absorbed when passing through the glass substrate of the mirror. However thin mirrors may be poor in terms of mechanical resistance, therefore it is necessary to laminate them on a supporting substrate, for example a metallic sheet. Mirrors used for this type of application have generally been produced as conventional domestic mirrors used for interior applications, i.e. as follows: a sheet of flat glass (float, soda-lime glass) was first of all polished and then sensitised, typically using an aqueous solution of SnCl2; after rinsing, the surface of the glass was usually activated by means of an ammoniacal silver nitrate treatment, and a silvering solution was then applied in order to form an opaque coating of silver; this silver coating was then covered with a protective layer of copper and then with one or more coats of leaded paint in order to produce the finished mirror. The combination of the protective copper layer and the leaded paint was deemed necessary to provide acceptable ageing characteristics and sufficient corrosion resistance. Solar energy reflectors manufactured with conventional mirrors comprising a layer of copper are for example described in GB 2 042 761 A.
More recently, mirrors were developed which dispensed with the need for the conventional copper layer, which could use substantially lead-free paints and yet which still had acceptable or even improved ageing characteristics and corrosion resistance. For example, U.S. Pat. No. 6,565,217 describes embodiments of a mirror with no copper layer which comprises in the order recited: a vitreous substrate; both tin and palladium provided at a surface of the vitreous substrate; a silver coating layer on said surface of the substrate; tin present at the surface of the silver coating layer which is adjacent to an at least one paint layer; and at least one paint layer covering the silver coating layer. Such mirrors provided a significant advance with respect to conventional coppered mirrors.
Solar energy reflectors comprising conventional coppered mirrors, as described in GB 2 042 761 A for example, may show good ageing properties and good corrosion resistance. However corrosion, in particular edge corrosion, of the mirrors may be responsible for a loss in the total reflective surface of a concentrating solar power plant with time, giving rise to a loss in the yield of the plant. Therefore, it may be necessary to replace from time to time the solar energy reflectors of the plant with new ones, which is time-consuming and expensive. There is thus a need for solar energy reflectors with increased lifetime, in particular with better corrosion resistance, especially edge corrosion resistance.
Solar energy reflectors incorporating new generation mirrors with no copper layer may provide good corrosion resistance, often better than when incorporating conventional coppered mirrors. However we have found that it may still be possible to improve their corrosion resistance, in particular their edge corrosion resistance.
Whilst it is quite clear that providing the edges of the mirror with an edge protection might increase the corrosion resistance of a solar energy reflector, we surprisingly found that using materials with very low permeability to water vapour for the edge protection, for example butyl-based or hot-melt adhesives, did not necessarily give the best results in terms of corrosion resistance for the finished solar energy reflector.