Heating and cooling methods for the subirrigation of strawberry plants using air and geothermal heat pumps

Temperature control with conserving nutrients and irrigation water reduces energy loss, leading to economic and sustainable greenhouse farming. A subirrigation system was applied to strawberry cultivation by placing tubes containing temperature-controlled water (obtained using heat pumps with air or geothermal heat sources) under the soilless substrate of a cultivation container. A ball tap supplied water gravitationally, avoiding the need for a water reservoir, thereby minimizing the volume of water required for heating and cooling. The substrate temperature in the heat pump treatments retained an optimal temperature for growth (averaging 18.4 ℃ in all seasons). The coefficient of performance of the heat pump with the geothermal heat source during heating operations was approximately 1.8 times higher than that with the air heat source. However, the geothermal heat source reduced overall electricity consumption by 14.9% for five months of the experimental period. Production was increased by 21% and 36% in the geothermal and air-source heat pump treatments, respectively, compared to that of the control, owing to the preferable condition of the substrate for strawberry growth. Although salinization of the substrate due to the closed irrigation system may be of concern and damage plant growth, the subirrigation system minimized the loss of water, nutrients, and heat owing to the omission of a reservoir. Furthermore, with minor modifications, the proposed system could be converted to a dual heat pump system to enhance energy use efficiency.