Concentrator photovoltaic systems are known to convert light energy to electric energy at a high efficiency while Fresnel lenses are used for concentration. In the prior art, Fresnel lenses are manufactured by a templating method of heating and compressing thermoplastic resin in a template mold. This method has the drawback of time-consuming molding because the thermoplastic resin is heated and die-pressed, and then cooled below the heat deflection temperature. Since the mold is exposed to high temperature, the mold surface is oxidized. This causes changes to the prism topography of Fresnel lenses, adversely affecting the reproducibility of Fresnel lenses. It is thus recently proposed to fill the lens-forming mold with a UV- or EB-curable resin which can be processed under relatively mild conditions, and to cure the resin for shaping. This method had the benefit of high production efficiency because temperature changes are not utilised.
The method using curable resin, however, requires to laminate a transparent plastic film as a support substrate to one surface of the curable resin in order to increase the strength of Fresnel lenses. The production method is by filling a mold with a curable resin, placing a transparent plastic film so as to cover the surface of the curable resin, and then curing the resin. The Fresnel lens thus produced is likely to curl due to polymerization shrinkage of the curable resin, which raises problems including lenses of unacceptable quality and low yields of production. Additionally, a post-working step for curl correction is necessary. When the Fresnel lens is parted from the mold, smooth parting is obstructed because the stiff substrate is laminated thereto. This often leads to the drawback that opaque spots form on the Fresnel lens surface. Further, since the support substrate is in direct contact with the curable resin during production, a certain type of resin of which the support substrate is composed can undergo swelling or creasing under the influence of the UV- or EB-curable resin. Thus a choice of support substrate resin is extremely limited,
There are proposed various Fresnel lenses which address the above problems or eliminate the drawbacks. For example, one known silicone composition utilizes an acryloyl group as photo-curable functionality so that the composition may be cured by UV exposure. However, solar cells are mainly installed outdoor, resins of certain type encounter diverse troubles. For example, an acrylic resin is used in JP-A 2007-079082. Because of moisture permeability, the acrylic resin allows for penetration of water from air or rain. If water is frozen with a lowering of ambient temperature, the resin is cracked or otherwise damaged.