High-concentration solar energy systems for the built environment

Abstract

Transparent high-concentration solar energy systems may be applied in the built environment, for instance in roof lights and glass walls, and can generate electricity as well as heat, while allowing light to pass through into the building. These systems contain concentrators, which focus direct sunlight onto a photovoltaic cell, while heat is collected in a heat sink and diffuse light remains available for illumination. A solar-tracking system allows the concentrator to follow the position of the sun during the day and multiple concentrators can be combined in modules with the advantage of a centralized control system. In this project, a transparent pyramid-shaped concentrator was designed and constructed based on a flat spherical Fresnel lens of 30 × 30 cm2 and a 30 cm focal length. The photovoltaic cell was a 1 cm2 triple-junction cell with 40% efficiency. A secondary optical system consisting of aluminium mirrors was located close to the solar cell to increase efficiency as well as to protect the system from localized overheating. An aluminium heat sink with water as a coolant was placed directly behind the solar cell to collect thermal energy. A solar-tracking routine was developed, which allows for accurate focusing, and a measurement unit was designed and constructed to determine performance and efficiency. Results indicate that a gross electrical efficiency of 21.2 ± 2.0% and a gross heat efficiency of 29.3 ± 4.2% were obtained, with respect to the beam-normal irradiation. For application in the region of Arnhem, the concentrator would allow for a yearly electrical yield of 199 kWh per m2 of lens surface. Based on these results, two different prototypes for a modulair system for urban applications, such as roof lights, atria and greenhouses, were designed.