Electrical Wire Heating in Sand Substrates: Influence of Depth and Spacing on Power per Unit Length

Heat transfer processes occurring in greenhouse cultivation substrates are very complex due to the high number of environmental and soil factors involved. Few estimation methods allow establishing the capacity of the heating elements as a function of the design parameters (depth and spacing) and of environmental and cultivation criteria, such as the ambient temperature and the desired temperature for root development. The purpose of this study is to propose simple estimation formulae to determine the relationship between capacity and design parameters. It also aims to ensure that the target temperatures are achieved in the substrate. An experimental design of electrical wire heating in a sand substrate was constructed. New design materials and new programming and control technologies were used. Nine different heating designs were arranged as a result of combining three depths and three spacings of the heating wire. Dimensional analysis was applied to data obtained experimentally—substrate temperature at different depths—combined with substrate properties. Expressions for the estimation of the power per unit length of wire to be installed were obtained, allowing the design of a heating system capable of supplying the substrate with the appropriate temperatures for the development of the root system in an optimal thermal environment. In addition, an analysis was performed of the variations in the power per unit length required depending on different depths and spacings of installation of the heating wire.