Borehole Thermal Energy Storage (or BTES) is basically an underground structure, usually quite large, which is used to store large amounts of solar heat that was collected in the summer, for use in the winter time. You could think of it as being a massive underground heat exchanger.
Wherever boreholes can be drilled, these systems can be constructed. The system is usually composed of anywhere from one to a few hundred vertical boreholes. Warm temperature seasonal heat stores can be created using borehole fields to store surplus heat which is captured in the summer to actively raise the temperature of large thermal banks of soil, so that heat can be extracted more easily (and more cheaply) during the winter.
A ground source heat pump will be used in the winter, extracting the warmth from the thermal bank (the soil which absorbed the summer heat) to provide heating to a space via underfloor heating. Boreholes are filled with grout or water, dependent on the geological conditions, and they usually have a life expectancy in excess of 100 years. They don’t impair the use of the land in any way, as they can exist underneath buildings, agricultural fields and car parks.
In terms of heat storage, the flow of heat transport liquid in the summer comes from the center of the structure, and travels to the outer edges, by doing it this way the heat is transferred through conduction to the surrounding soil. When winter comes along, the flow direction is reversed, so the liquid flows from the outer edge of the system to the center, drawing in heat. By traveling in the reverse direction, the heat which was stored in the soil is then transferred back to the liquid running through the system.
The ground heat exchanger for BTES systems is designed and operated in such a way, so that the heat is stored or abstracted, seasonally whereas conventional GSHP systems are designed to simply dissipate heat or cold into the subsurface. BTES essentially uses the Earth as a giant thermal battery as opposed to a radiator.
This system typically involves design and operation of the Ground Heat Exchanger (GHX) in a manner so that the heat is sequentially built up in, or abstracted from a cylindrical column of rock or soil. This system usually consists of a number of boreholes resembling dry wells, and after drilling this hole, a plastic “U” pipe is inserted down the borehole. To provide a good thermal contact with the surrounding soil, the borehole is filled with a high thermal conductivity grouting material.
When solar heated water is available to be stored, it is then pumped into the center of the BTES field and through the U pipes. Heat is transferred to surrounding soil and rock, and the water gradually cools as it reaches the edge of the structure and returns to the energy center.
Written by Jade Turney – Building Tectonics Ltd.