Development of modified bentonite slurry for use in subsurface thermal storage and insulation

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The use of ground source heat pumps (GSHPs) as an energy sink/source to save energy required for building heating and cooling has been increasing in recent years. According to Canadian Geothermal Coalition, the market in Canada for these heat pumps has been expanding approximately 44% per year from 2005 to 2009. As a result, innovative approaches/technologies are in high-demand to make the GSHPs more efficient and economical in industrial scale. In addition, because of environmental concerns about the potential for contamination of underlying ground water, jurisdictions commonly require boreholes in a GSHPs system be filled with nearly impervious bentonite grout. The thermal and hydraulic properties of such bentonite grout has significant impact on the economic and safe operation of GSHPs. Enhanced bentonite grout with high thermal conductivity allows more thermal energy to be transferred to or from the surrounding soil and hence increases the efficiency of the system. At the same time, materials with low thermal conductivity and low permeability are also required as insulation for subsurface storage of thermal energy. This research will develop new composite bentonite grouts that meet different requirements on thermal and hydraulic properties for ground source heat pumps. A theoretical study will be performed to help find the optimal combination of different types of additives with bentonite, while a series of laboratory tests will be carried out to verify the optimal combination of different types of additives. The findings will then be examined and verified in a large testing tank prior to full-scale field implementation. The expected outcomes of the project will accommodate the industry needs, and provide valuable tools to appropriately deal with special issues related to the design and safe operation of ground source heat pumps, which are considered to be more economical and able to reduce potential negative environmental impact.