Ray Tracing for Optimization of a Double Reflector System for Direct Illumination of a Heat Storage

• Published in: Energy procedia 2014, Vol.57, p.2211-2220
• Main Author: Nydal, Ole Jørgen
• Format: Article
• Language: English
• Subjects: concentrating solar thermal; double reflector; parabolic dish; Ray tracing; solar tracking
• ISSN: 1876-6102; 1876-6102
• DOI: 10.1016/j.egypro.2014.10.188

“Solar salt” (potassium and sodium nitrate mixture) is an option for a latent heat storage to be used with solar heat collectors. As the salt melts at about 210 degrees C it can be suitable as a heat storage for cooking applications. A solar salt container to be illuminated directly by the sun rays in should be heated from the top. One way of achieving this is to position the storage below a primary parabolic dish, this having a center hole to allow the rays from a secondary reflector to hit the top plate of the storage. A double reflector system becomes more sensitive to surface imperfections and to the solar tracking accuracy. A 2m diameter parabolic dish with a secondary 0.2 m diameter reflector has been considered as a test system for direct illumination of a heat storage. The optical performance of the system is investigated by means of a ray tracer. The ray tracer is described and a sensitivity study is made with respect to solar tracking accuracy, the effect of mirror tiles instead of smooth surfaces, the positioning and size of the secondary reflector and the effect of illumination of a the top plate from the side (the top plate remaining horizontal during the daily sun movements). A focusing effect is obtained by slightly lifting the secondary absorber above the point of the common focus of the primary and the secondary. The primary reflector should be as smooth as possible, the secondary reflector is less sensitive and could be surfaced with 1-2cm mirror tiles with acceptable performance. A 10cm diameter absorber dish is the minimum size, a 20cm dish is more tolerant for tracking accuracy and for low solar angles. A large absorber dish also reduces the risk of superheating the top plate which has conduction based heat transfer to a salt storage.