Measurement of the soil-air convective heat transfer coefficient from a greenhouse mulch soil.

摘要

The greenhouse industry has extended in coastal areas with mild-winter climates, such as the Almeria coast, in southeast Spain. In this area, most greenhouses consist of low-cost structures covered with plastic films and without climate control systems. Soils with superficial mulch layers of sand and fine gravel particles are widely used to improve thermal conditions, soil salinity and water use efficiency. This work analyzes the soil-air convective heat transfer coefficient of this soil in order to improve its function as energy exchanger during the daytime. Measurements were carried out in a three-span multi-tunnel greenhouse with an artificial layered soil, typical in this region, which was without crop. An isothermal hot box, insulated by extruded polystyrene and heated with an electric heating mat laid on the bottom, was integrated in the upper part of the mulch. A 1-cm layer of fine gravel, above all, and very coarse sand particles was placed over the heating mat. Nighttime measurements were carried out from 2 to 10 June 2008. Greenhouse air temperature, mulch surface temperature, and inside and outside surface box temperatures were measured. The energy dissipated by the heating mat to the mulch layer and the net radiation were also measured. The soil-air convective heat flux (H_s), determined by an energy balance, was used to calculate the soil-air convective heat transfer coefficient (h_s). A potential relationship between h_s and soil-air temperature gradient values was found (h_s=1.72 (T_s-T_a)0.33; R2=0.94; N=6). Convective heat transfer coefficient values remained stable during the measured night-time periods and were within the lower range of values reported in the literature.