Application of a Heat Transfer Model for Heated Substrates in Greenhouses to the Dimensioning of Spacing between Electric Heating Cables

Substrate heating in greenhouses shows important benefits for crop development. Yet, the energy costs of the system are high and demand the application of energy saving strategies. This study analyses the effect of heating cable spacing on the power of the cable and on temperature variability in the root zone. A general-purpose finite-element code was used to simulate the transient thermal behaviour of six configurations of substrate heating in greenhouses. The heating cable was placed at a depth of 225 mm, and at spacings of 100, 150, 200, 250, 300 and 350 mm. The analysis conducted reproduced the behaviour of the different configurations of heating during 24 h under the same air temperatures and operational conditions of the heating cable. The results obtained by the model were combined with dimensional analysis in order to obtain simple expressions that were useful in the dimensioning process. Based on the expressions obtained, the power per unit length can be dimensioned as a function of heating cable spacing in order to obtain 20 °C in the root zone at 75 mm depth, taking into consideration the depth of installation, the thermal properties of the substrate, and air temperature. In addition, this study quantifies the variations that occur in temperature at a depth of 75 mm. Temperature spatial variability is dependent on the spacing of the electric cable and on air temperature.