Abstract
A finite element method-based model of a substrate heated by an electric heating cable buried in a thermal isolated container was experimentally validated with root mean square error values of root zone temperature ranging 0.25 to 0.62 oC. The two-dimensional transient model allowed variations in the physical properties of the substrate with temperature, water content and depth. The operation of nine configurations of a heating cable buried in sand at different depths (50 to 450 mm, at 50 mm intervals) at 200 mm spacing was simulated and assessed. The validated model was used to perform 24-h simulations applying boundary conditions, and substrate moisture content was experimentally obtained at a mean substrate surface temperature of 13.98 oC. Such simulations reproduced the operation of the heating system by setting a reference temperature of 20 oC at the control point in the root zone. Burying the heating cable in the surface layers of the substrate caused large temperature gradients and high heat losses through the substrate surface. Accordingly, average temperature in the root zone increased with heating cable depth, up to the 200 mm depth. For greater depths, temperature in the root zone was constant. The ON/OFF control was most effective with the heating cable buried in the root zone and at control point temperatures of 20 ± 1 oC. Burying the heating cable in the surface layers required higher energy consumption, up to 28 % at 50 mm. The most efficient heating cable depth was 350 mm, with a daily energy consumption of 6750 kJ m-2.
Highlights
Root zone temperatures within a range of 18 oC to 24 oC are suggested for the development of plants (Roper et al, 2004; Gent and Ma, 2000; Dodd et al, 2000; Mortesen and Gislerod, 1996)
Our objective is to analyze the effects on root zone temperature (0-150 mm depth) and energy consumption of the depth of installation of an electric Heating cable (HC) buried at different depths in a sandy substrate without compromising production
The analysis presented in this paper is based on the model suggested by Fernandez and Rodriguez (2006), which was developed with the general-purpose code ANSYS v 6.0
Summary
Root zone temperatures within a range of 18 oC to 24 oC are suggested for the development of plants (Roper et al, 2004; Gent and Ma, 2000; Dodd et al, 2000; Mortesen and Gislerod, 1996). Models of localized heating systems for growing substrates have been developed and assessed. Some of these models use warm air (Boulard et al, 1989a,b; Kurpaska and Slipek, 1996), warm water (Ahmed et al, 1983; Kurpaska and Slipek, 2000), or electric cables (Fernandez et al, 2005 a,b; De La Plaza et al, 1999; Rikbost et al, 1975; Rodriguez et al, 2004). Heating cable (HC) spacing is one of the technical parameters that most affect the energy consumption of the heating system (Fernandez and Rodriguez, 2006; Fernandez et al, 2007). Likewise, choosing the appropriate HC depth may entail significant energy savings and changes in the distribution of temperatures affecting the plant
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