CALCULATION OF NIR EFFECT ON GREENHOUSE CLIMATE IN VARIOUS CONDITIONS

In Northern regions of Europe glass is mainly used as greenhouse covering material whereas in Southern regions plastic films are commonly used. The develop-ment of covering material optical properties focuses on high light transmission, reduction of heating energy losses (higher latitudes) and reduction of cooling energy load by radiation (lower latitudes). Solar radiation can be divided into photosynthetic active radiation PAR and near infrared radiation NIR. Whereas the PAR is needed for crop growth and development, the energy fraction of the NIR heats the greenhouse and crop and contributes to transpiration, which is not necessarily always desirable during periods with high radiation. Materials or additives for greenhouse covers that reflect or absorb a fraction of the NIR radiation have become available on the market. Excluding NIR from the greenhouse will reduce the greenhouse air temperature. However, there are several side-effects of the reduction of NIR, which are important to consider, such as a higher energy use in winter and a smaller impact on reduction of heat load as expected because of the high NIR reflection of the crop itself. In this study model calculations of the effect on greenhouse climate of different NIR-absorbing prototype plastic films for the climatic conditions of Southern Spain are presented. The effect on greenhouse air temperature and humidity, and on crop transpiration are analyzed. The most appropriate application of a NIR-selective greenhouse cover for that climate is presented.