Root temperature and energy consumption at different cable depths in electrically heated substrates

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

Solute-travel time estimates for tile-drained fields. III. Removal of a geothermal brine spill from soil by leaching

The time required to leach a slug of saline, sodic geothermal brine from the point of injection to the tile outlet of an artificially drained field is calculated. Sprinkler, complete, and partial ponding leaching methods are compared as a function of drain spacing and initial location of the spill, with ponding requiring more water but less time to leach brine out of the system for all situations except where the brine spill occurs near the midpoint between tile lines. Calculation results are presented in dimensionless parameters which scale the drainage system dimensions and the soil water transport properties. A simple calculation is proposed to estimate the volume of leaching fluid required to remove excess Na/sup +/ from the exchange complex, and was found to be in good agreement with the results of laboratory soil column experiments. For fine-textured soils in the Imperial Valley of California it may require up to 30 pore volumes of leaching fluid to lower Na/sup +/ concentrations if saturated gypsum solution is used in reclamation. The results of these calculations suggest that reclamation of fine textured soils could require a prohibitive amount of time unless the brine spill is localized around a drain