Improving environmental conditions in a greenhouse by cooling the air and the root area using latent heat
2010
Yasuba, K., National Inst. of Vegetable and Tea Science, Taketoyo, Aichi (Japan)
In Japan, air temperatures in greenhouses may become too high during summer for cultivating some plants, so the greenhouses are often cooled by taking advantage of latent heat. The latent heat approach relies on the principle that when water is vaporized it draws heat out of the surrounding environment, producing a cooling effect. This approach to cooling is also useful for humidity control in greenhouses. However, an easy method of cooling the root area by means of latent heat has not yet been developed, and the threshold level of cooling that is required to promote crop growth is not clear. Misting is a useful method for cooling the air by taking advantage of latent heat, but it is difficult to control the cooling effect because the ventilation rate of the greenhouse affects the resulting cooling efficiency. To solve these problems, a method for controlling the degree of cooling of the root area and air in greenhouses was developed. Microporous film was used to cool the root area. Gas and water vapor can penetrate the microporous film, but liquid water cannot. When the hydroponic bed was created from microporous film, water in this bed was able to evaporate through the film, and oxygen from the ambient air passed through the film in the opposite direction and was dissolved in the root water. The relationship between water temperature and airflow velocity at the surface of the microporous film was investigated by connecting a duct to the bottom of the bed made of microporous film. A low airflow velocity (1.2 to 1.5 m/s) was more effective than other rates (0, 10, and 20 m/s). The cooling efficiency with microporous film was about 60% of the theoretical cooling effect predicted by latent heat calculations. The rate of oxygen supply through the microporous film was about 0.22 micromol/square m/sec.
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