Borehole Thermal Energy Storage (BTES)
A borehole thermal energy storage (BTES) system is an underground structure for storing large quantities of solar heat collected in summer for use later in winter. It is basically a large, underground heat exchanger.
A BTES consists of an array of boreholes resembling standard drilled wells. After drilling, a plastic pipe with a “U” bend at the bottom is inserted down the borehole. To provide good thermal contact with the surrounding soil, the borehole is then filled with a high thermal conductivity grouting material.
Aerial view of Borehole Thermal Energy Storage (BTES)
The BTES in the Drake Landing Solar Community (DLSC) consists of 144 boreholes, each stretching to a depth of 37 meters and planned in a grid with 2.25 meters between them. The BTES field covers 35 metres in diameter. At the surface, the U-pipes are joined together in groups of six that radiate from the center to the outer edge, and then connect back to the Energy Centre building. The entire BTES field is then covered in a layer of insulation and then soil – with a landscaped park built on top.
When solar heated water is available to be stored, it is pumped into the centre of the BTES field and through the U-pipe series. Heat is transferred to the surrounding soil and rock, and the water gradually cools as it reaches the outer edge and returns to the Energy Centre.
Conversely, when the homes require heat, cooler water is pumped into the edges of the BTES field and as the water flows to the centre it picks up heat. The heated water passes to the short-term storage tank in the Energy Centre and is then circulated to the homes through the district heating loop. All pumps and control valves are housed in the neighbouring Energy Centre building.
Even with sunny Alberta weather, it took approximately three years to fully charge the BTES field. In the first years of operation, the field operated at relatively low temperatures, and the recoverable energy was largely depleted before the end of the heating season. However, after a few years of operation, the core temperature of the BTES field approached 80°C by the end of summer, with sufficient heat for almost an entire heating season.