Application of the constant soil temperature layer for energy-saving control in the local environment of greenhouse crops II. Application to strawberry cultivation during the winter season

Greenhouse strawberry production is often limited by temperature and low solar radiation during the cold season, which depress photosynthesis in strawberry crops and by excessively high temperature during the warm season, which causes a delay in flower-bud differentiation (Sone et al., 2005; Dan et al., 2007; Hidaka et al., 2013). Furthermore, the steep increase in the price of oil has threatened the income during winter season, because this season requires a greater magnitude of greenhouse heating (Okimura, 2009). For stable year-round production of strawberry crops to maintain high profitability and sustainability, it is essential to establish a system for energy-saving and year-round environmental control by applying renewable energy resources. The constant soil temperature layer is an underground, widely occurring, and easily accessible renewable energy resource (Yamamoto, 1966; 1973; 1985; Takaura and Yamanaka, 1981). However, because of the low capacity of soil for heat storage and conduction, heat exchange with the constant soil temperature layer has been considered insufficient for controlling the temperature of the entire volume of air inside a greenhouse (Takami and Uchijima, 1977). To solve these problems in the greenhouse system, in our previous study (Miyoshi et al., 2013), we proposed a novel local environmental control system that related the constant soil temperature layer to the circulation of air and heat exchange between the soil and ambient air of strawberry crops. We examined the short-term performance of this system from the viewpoint of energy savings via the control of air conditions. The system enabled energysaving local control of the ambient air temperature and relative humidity of strawberry crops via heat exchange with the constant soil temperature layer, demonstrating a 50% reduction in the heating load for the ambient air of crops. The aim of the present study was to apply the system developed in our previous study to the elevated bed system of strawberry crops during the cold, winter season, and to examine the long-term effect of the system on energy-saving control of air condition and crop yield.