A Spectral Selective Lamellae Concentrator System as BI-CPV/T System

Abstract

In warm periods the excess of incoming solar energy into a greenhouse is more than required for the growth of the crop. In 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. This earlier system was based on a circular covering geometry and an integrated filter for reflecting the NIR of the greenhouse. The reflected radiation was used in a solar energy system. In this feasibility study the new CPVT-system is simplified so more economically by avoiding the asymmetric greenhouse construction with bended glass and the large construction for solar tracking. All parts of the solar concentrating system will be mounted inside a standard Venlo type greenhouse. The concentrator consists of lamellae which only focus the NIR-part of the spectrum onto the CPVT–module. This module is mounted to or integrated into the ridge or gutter of the greenhouse. With this spectral selection the heat load inside the greenhouse will be reduced. The target is a maximal total annual electricity production. The optimization is done with a ray tracing model fed with actual radiation data. Two types of lamellae are compared: flat lamellae and trough shaped lamellae which focus the radiation individually. 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 servo 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 a part of the whole solar spectrum. The total effective concentration ration factor will be 23x, including the transmission losses of the greenhouse and the efficiency of the concentrator. The high geometric concentration factor will limit the shadowing effect of the cultivation area by the PV-cells with only 1%. Further optimalisation in the energy yield were performed on determination of the optimal focal length of the trough shaped lamellae. The highest 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