Heat is always a problem in greenhouses during summer months and even during winter in some areas. Extreme high temperatures cause stress in plants and reduce yields, quality and increases pest and disease pressure thereby increasing costs. Overall, the economic viability of a greenhouse decreases if temperature is not controlled sufficiently. One of the latest advances in cooling greenhouses is the use of thermal screens. The aim of thermal screens is to reduce radiation (sun light) heating the interior of the greenhouse.
If there is less radiation reaching objects, they are cooler, and so are the plants. Thermal screens reduce the effort a plant must exert to cool itself down. 95% of all radiation from the sun heats the plant up. The other 5% is called photosynthetic active radiation or PAR. A thermal screen breaks up a large part of the heat radiation and converts it to PAR light making the plant more efficient converting light into yield. You cannot remove all the radiation and have only PAR light since the plant requires a specific temperature range to grown optimally.
Thermal screens are not only used to reduce temperatures during the day, but they also increase temperatures during the night in preventing the heat radiating out of the tunnel. The thermal screen bounces the radiation coming from inside the tunnel back to the plants, thus re-using energy on a passive way. Not any material can be used, the ideal material should have a high thermal resistance in order to provide a barrier which is large enough to make a difference in day and night temperatures inside the greenhouse (Öztürk & Basçetinçelik, 1997)1)Öztürk, H. H. and Basçetinçelik, A. 1997. The nocturnal heat loss and internal temperatures in plastic tunnel greenhouses with thermal screens. Acta Horticulturae. 443: 79 – 84.. Near-ambient temperatures inside the greenhouse can be achieved through the use of thermal screens, however the biggest influence in achieving optimal results is the designing of the greenhouse and thereafter the type of screen installed (Desmarais & Raghavan, 1997)2)Desmarais, G. and Raghavan, G. S. V. 1997. Thermal characteristics of screenhouse configurations in a West-African tropical climate. Acta Horticulturae. 443: 39 – 46..
So a badly designed greenhouse will not benefit from the installation of thermal screens. It is needless to say that greenhouses using thermal screens should be well ventilated, either through forced methods or natural methods.
|↑1||Öztürk, H. H. and Basçetinçelik, A. 1997. The nocturnal heat loss and internal temperatures in plastic tunnel greenhouses with thermal screens. Acta Horticulturae. 443: 79 – 84.|
|↑2||Desmarais, G. and Raghavan, G. S. V. 1997. Thermal characteristics of screenhouse configurations in a West-African tropical climate. Acta Horticulturae. 443: 39 – 46.|