Microclimate in naturally ventilated tunnel greenhouses: effects of passive heating and greenhouse cover

Passive solar heating using heat storage is a well-known method for nighttime heating of greenhouses. However, the associated physical processes are less explored. The purpose of this study was to explore several approaches for heat storage in tunnel greenhouses. The study was conducted in unheated tunnel greenhouses growing a basil crop, located in the Beit Shean valley of Northern Israel. Some of the tunnels were equipped with a row of black plastic water sleeves (WS) positioned vertically inside the tunnel adjacent to the northern sidewall to store solar energy. Four treatments were investigated: (i) control ‒ without WS, single plastic cover; (ii) WS, double plastic cover; (iii) WS, double cover: plastic & “Agril” (non-woven plastic cloth or net, internal cover); (iv) without WS, double plastic cover. Microclimatic measurements included air temperature, air humidity, solar and net radiation, water temperature within the sleeves, and soil temperature. The results indicated that the nighttime air temperature in tunnels with water sleeves and double plastic cover was highest of all treatments, and 4.8°C (average data from 5~11 Jan, 2014) higher than the control tunnel. Hence, water sleeves stored a significant amount of heat which could balance the uneven distribution of solar energy between day and night. Nighttime temperatures in treatments (iii) and (iv) were roughly the same, and lower than treatment (ii), while the control tunnel temperature was lowest during the night. Water sleeves accumulated energy during the daytime (7:00~15:00); during the rest hours of the day the sleeves dissipated heat into the greenhouse air. Vertical profiles of water temperature inside the sleeves indicated a generally stable thermal stratification, with warmer water at upper levels within the sleeve. We conclude that water sleeves with double plastic cover provided best nighttime conditions for basil production in this region of the country.