Low energy greenhouse - A system approach

The increase of energy costs and the task of reducing CO ₂-emission of fossil fuels require solutions for low energy greenhouses. Until now greenhouses are mainly covered with single glazing providing high light transmittance for a good crop performance. In the meantime, several solutions have been developed to reduce the energy consumption of greenhouses like double-glazing, thermal screens and specific control algorithms. Such measures influence the light transmittance of the covering material and the greenhouse microclimate - especially air humidity. This may lead to a reduction of yield and an increase of fungi infections. The control of air humidity can increase the energy consumption, thus reducing the energy saving potential. Hence it is necessary to develop a low energy greenhouse concept as a system approach combining energy saving methods with an improved control of greenhouse microclimate. In the past, several low greenhouse concepts had been developed with a focus on covering materials and thermal screens. Recently, the closed greenhouse concept has been developed and propagated as an energyproducing greenhouse. The closed greenhouse is an example for a concept to reduce energy consumption and at the same time to increase yield and crop quality. The energy saving potential of such a greenhouse has been reported to be in the range of 20 to 33% and the increase of yield up to 20% (Opdam et al., 2005). A low energy greenhouse can have a higher energy saving potential of up to 90%. This is rendered possible by combining different methods, which have been developed in the past, with new approaches. The greenhouse should be operated semi-closed, allowing a better application of CO₂ in order to improve crop performance, reduce the application of pesticides and promote the use of solar energy for heating. New cladding materials with anti-reflective coating are available with high light transmittance allowing double-glazing with the same light transmittance as conventional single glazing. In combination with triple thermal screens a maximum insulation is possible. Using new concepts of climate control strategies such as temperature integration, a further reduction of energy consumption is possible. Since all these methods influence the greenhouse climate, the crop response must be investigated by phytomonitoring in order to determine limitations and to avoid negative effects. Furthermore, improving the cropping system and using new cultivars can contribute to the low energy greenhouse concept.