Soil Temperature Increases Induced by Subsurface Line Heat Sources

Generating electricity using the steam cycle produces large quantities of waste heat. Conversion efficiencies range from 32 to 38% which means that for three units of energy input about one unit of electrical energy is produced and two units of waste heat must be disposed of. Circulating the condenser cooling water through a network of underground pipes would result in warming the soil. The present study was initiated to evaluate the effect of increased soil temperatures on crop growth, the energy balance of the proposed system, and the economic feasibility of the system. This report presents results of the energy balance studies. The proposed system was simulated by electrical heating cables, 92 cm deep with 183 cm spacing, maintained at temperatures ranging from 35 to 37 C. Soil temperatures were measured at hourly intervals over a 2-year period at points forming arrays in planes perpendicular to the heat sources. The temperature measurements were recorded on magnetic tape and later processed to obtain desired information, such as daily averages at various depths. Energy input was measured with watt-hour meters. With a bare soil surface the average daily soil Temperature at the 5 cm depth increased about 1.0 C during the summer and 1.5 C during the winter. At the 25 cm depth temperature increases were about 4.0 C and 5.5 C, respectively. The average daily temperature of the profile to a depth of 220 cm was increased about 10 C in the summer and 13 C in the winter. The rate of energy dissipation varied from about 0.012 cal/cm²/min in the summer to about 0.030 cal/cm²/min in the winter. During summer months heat flow from the soil to the atmosphere accounted for less than half of the total flux. During the winter the heat flux to the atmosphere accounted for most of the energy dissipation. Very large surface areas would be required to dissipate the waste eat generated by a power plant with an electrical output of 1,000 MW. The proposed system of soil warming is not an efficient primary heat sink. The rate of heat dissipation could be increased by placing the pipes closer to the soil surface and closer together.