Low-Temperature Soil Heating Using Renewable Energy

Data from a pilot study, in which renewable energy was used for low-temperature subsurface heating in a northern climate, suggests that such an approach may be useful for remediating low permeable soils. Low-temperature soil heating is expected to enhance remediation effectiveness by increasing contaminant volatility, diffusion, desorption, and microbiological activity. Direct and indirect solar energy was harvested with a hybrid photovoltaic/wind electric system. The electrical energy generated by the hybrid renewable energy system was distributed to the subsurface using a control system and wire, then converted to heat energy using a resistive element emplaced in an unsaturated silty layer 2.3 m below grade. Renewable energy system performance, soil temperature, and environmental data were collected. Ambient soil temperatures fluctuated seasonally within the silt layer from 4 to 15°C. The small renewable energy system performed as predicted and injected 441 kWh of energy into the soil over the eight-month study. This energy input translated to increased soil temperatures ranging from 7.7 to 19.4°C and from 3.3 to 4.3°C above ambient at distances 0.3 and 0.9 m from the heating well, respectively. The system supplied sufficient heat to maintain soil temperatures above ambient even in winter in Vermont, where low direct solar energy was available and sustained low ambient temperatures prevail.