BTES

Borehole Thermal Energy Storage (BTES) is a method of storing thermal energy in the subsurface using a field of vertical boreholes containing heat transfer loops. BTES systems store heat or cold for later use, typically in combination with heat pumps, solar thermal collectors, or other heat sources and sinks. The stored energy is retrieved by circulating a heat transfer fluid through the boreholes to a surface plant where a heat exchanger transfers energy to or from the building heating or cooling system.

A BTES field consists of numerous boreholes arranged in a grid or other pattern. Each borehole contains a loop or U-tube through which a circulating fluid absorbs or releases heat. The boreholes are filled with grout to maximize thermal contact with surrounding rock or soil, and the system is controlled by pumps and a thermal management system. The surrounding geology provides the ground medium for heat storage, with performance influenced by thermal conductivity, moisture content, and temperature gradients.

Typical applications include seasonal storage of heat for district heating or campus-scale buildings, as well as seasonal cooling storage in some climates. Temperatures stored for heating applications are commonly in the 20–60°C range, depending on design, while cooling storage may target near 5–15°C. System size varies from modest installations to large arrays with many boreholes spanning tens to hundreds of meters in depth.

Advantages of BTES include high energy density in the underground, long-term storage capability, and compatibility with renewable heat sources. Challenges involve site suitability, drilling costs, potential thermal losses, and careful management of subsurface heat transfer to avoid long-term environmental impacts.