DESIGN OF A NIR-CONCENTRATOR SYSTEM INTEGRATED IN A GREENHOUSE

In warm periods the excess of incoming solar energy into a greenhouse is more than required for the growth of the crop. Particular the near infrared radiation (NIR) part of the incoming radiation is not necessarily. In a previous research project a new type of greenhouse with an integrated concentrated photovoltaic system with thermal energy output (CPVT-system) was developed. The system is based on a circular covering geometry and an integrated filter for reflecting the NIR of the greenhouse and exploiting this radiation in a solar energy system. In this feasibility study the CPVT-system is further optimized in order to avoid the asymmetric greenhouse construction with bended glass, the large construction for solar tracking and the high investments. Hereto all parts of the solar concentrating system will be inside a standard Venlo type greenhouse. NIR-reflective lamellae serve as NIR-reflector and the CPVT-module is mounted to or integrated into the ridge or gutter. With the spectral selective lamellae, which reflect a part of the NIR radiation, the heat load inside the greenhouse can be reduced and the reflected NIR radiation is focused onto the CPVT-module. The optimization process is based on a maximal total annual electricity production and is done with a ray tracing model and actual radiation data. Two types of lamellae are compared: flat lamellae and trough shaped lamellae which focus the radiation individually. Compared to flat lamellae, trough shaped lamellae have the advantage of reducing the number of lamellae in combination with a high concentration ratio. This will lower the costs for the drive of the lamellae. The reflected NIR radiation can be focused with a geometric concentration factor of 100x. The lamellae will not only reflect 49% of the NIR radiation but also reflect a part of the whole solar spectrum. This contribution will result in an effective concentration factor of 46x. The high geometric concentration factor will limit the shadowing effect by the PV-cells with only 1%. Further optimalisations in the energy yield were performed on determination of the optimal focal length of the trough shaped lamellae. The maximal annual electrical output was found for lamellae with individual optimized focal lengths. In that case the annual output for Dutch climate conditions can be over 29 kWh/m².