Borehole thermal energy storage (BTES) is an underground “structure” for storing large quantities of solar heat collected in summer, and utilizing it in winter, that is, seasonal storage. The concept of solar seasonal storage has been around since the mid-1970’s; with the earliest demonstration plant completed that same decade, in Sweden . The International Energy Agency (IEA) formed a task group to investigate solar seasonal storage in 1979. Even so, few solar seasonal storage systems were built until recent years. Building from the European successes the technology has been brought to Okotoks , Canada for the first time in North America .
A borehole storage field is an array of boreholes, initially identical to standard drilled wells. After drilling, a plastic pipe with a “U” bend at the bottom is inserted down the borehole (think of pushing the middle of a garden hose down the hole, leaving both ends at the surface). To keep everything stable, the borehole is then filled with a high thermal conductivity grouting material. In Okotoks, 144 of these holes, each to a depth of 35 meters, are planned in a grid, with 2.25 meters between them. At the surface, the U-pipes are joined together in groups of 6 (called a series) that radiate from the center to the outer edge, and then connected back to the Energy Centre building.
Once the drilling and plumbing is complete, the entire borehole field is covered in a layer of insulation and then soil – with a landscaped park built on top.
The operation of the BTES is very simple: when solar heated water is available, it is pumped into the centre of the BTES field, through the U-pipe series, transferring the heat to the soil and rock, and gradually cooling as it reaches the outer edge. 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 is then circulated to the homes through the district distribution loop. The BTES field is ensured a long life through the selection of durable materials, and because it contains no moving parts (other than flowing water). All pumps and control valves are housed in the neighboring Energy Centre building.