Seasonal Underground Thermal Energy Storage Using Smart Thermosiphon Technology

Abstract

In climates with hot summers and cold winters, it is thermodynamically possible to provide all heating and air-conditioning needs without significant fuel or electrical energy input if adequate thermal energy storage capacity exists. Because all buildings sit on massive volumes of soil, water and rock, capacity is not the problem. The obstacle in meeting that goal is the lack of technology to readily transfer heat to and from soil in a cost effective manner. Enhancement of heat transfer rates coupled with an order of magnitude drop in installation cost over current practices would likely lead to the widespread use of seasonal underground thermal energy storage (UTES). Smart thermosiphon technology may be a path to those technical and economic goals. This technology uses conventional passive thermosiphon technology to transfer energy out of soil, and controlled rate transfer of energy into the soil. In this paper, we describe how smart thermosiphon technology can facilitate ample seasonal energy storage to meet air conditioning and winter heating needs. Simulations of soil freezing within an array of thermosiphons and the use of those frozen soils to provide air conditioning demands will be presented. A comparison of heat transfer from looped tubes inserted in vertical wellbores with the calculated heat transfer from thermosiphons shows that the same total heat transfer rate can be realized using 40% of the borehole length if smart thermosiphon technology is used. Results of the testing of a lab-scale experimental smart thermosiphon demonstrate that uniform temperatures and heat fluxes can be maintained on the inside wall of the thermosiphon pipe, thus proving the potential for dramatic enhancements of heat transfer rates. Winter heating and summer air conditioning modes have been demonstrated. The first pilot-scale installation of smart thermosiphons for seasonal UTES has demonstrated the ability to install the devices using inexpensive direct push techniques.