ESTIMATION OF THE ENERGY CONSUMPTION OF ELECTRICAL WIRE HEATING IN SAND SUBSTRATES ACCORDING TO THE TARGET ROOT ZONE TEMPERATURE

Abstract

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 energy consumption 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 target root zone temperature suitable for plant development. The objctive of this work is to analyze the influence of the target root temperature on the energy consumption of six configurations of soil heating system: 150, 225 and 300 mm depth with 100 and 150 mm spacings. An experimental test was arranged to measure ambient and root temperatures and operation times of the electrical heating cables. Dimensional analysis was applied to data obtained experimentally to relate all the involved variables: thermal properties of the substrates, depth, spacing, power and operation time of the heating cable, daily mean values of the measured temperatures. The obtained expressions permitted to quantify the energy consumption. An increase in target temperature in the root zone brings about increases in daily energy demand. An increase in air temperature brings about a decrease in energy requirements, but the slope of the curves increases as a function of target temperature. At a fixed heating cable depth, energy consumption is always higher for 100 mm cable spacing than for 150 mm spacing, and the differences become larger for shallower heating cable installation depths. The heating system with the cable installed at 150 mm spacing and 150 mm depth obtains the lowest energy consumption value, while the configuration with the cable installed at 100 mm spacing and 150 mm depth, and the configurations with the cable buried at 225 mm depth require the highest consumption.