A Design Tool for Solar Thermal Remediation Using Borehole Heat Exchangers

Low temperature heating of the subsurface (increases of about 5-20 °C) can substantially increase rates of biotic and abiotic destruction of dissolved contaminants such as chlorinated solvents. Low-temperature heating can be sustainably and cost-effectively achieved using solar thermal collectors coupled with closed-loop borehole heat exchangers. This technology has been implemented at several sites in the United States and abroad with favorable results. The design of a solar thermal remediation system requires a quantitative understanding of heat transfer from the array of borehole heat exchangers. We present an easy-to-use design tool that is based on a transient three-dimensional analytical heat transfer solution and is programmed in Visual Basic in Excel. This tool can be used to quickly explore the effect of design variables on the forecast temperature field. Typical design variables would include the site-specific average and monthly solar insolation data, solar collector configuration, borehole heat exchanger geometry and spacing, and the effects of environmental variables such as groundwater velocity, background subsurface temperature, and thermal conductivity. The design tool has been verified by comparisons with the TOUGH2 multiphase heat transfer code for a three-dimensional multi-heater system with seasonally variable thermal power rates. The code has been validated by comparisons to observed temperatures measured at a solar thermal field remediation application at a site in Colorado.