of small-diameter boreholes equipped with heat exchangers made of plastic PEX pipe can be used to store heat or cold between opposing seasons in a mass of native rock. The technique is called seasonal thermal energy storage. Media that can be used for this technique ranges from gravel to bedrock. There can be a few to several hundred boreholes, and in practice, depths have ranged from 150 to 1000 ft.

History

Borehole drilling has a long history. By at least the Han dynasty

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