Design procedures for subsurface soil-warming pipe systems

Soil warming has the potential to be an alternative to conventional heating methods in agriculture. It has been proven that crop yield is enhanced and growing periods are shortened with relatively little energy input. A subsurface soil-warming system consists of a network of parallel horizontal hot-water pipes buried at an appropriate depth in the soil to maintain a favourable temperature field throughout the root zone. Theoretical steady-state, constant property soil-warming models for four pipe configurations are presented in dimensionless form. Experimental verification of one of the configurations is presented. A procedure for establishing optimal pipe depth and spacing according to root-zone heating requirements is described and an example is illustrated. Effects of diurnal temperature fluctuations are also considered. Applications for these models include greenhouse crops and small unprotected crops of several hectares.