A downhole heat exchanger, (DHE) also called a borehole heat exchanger, (BHE) is a heat exchanger

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 installed inside a borehole

Borehole

A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...

. It is used to capture or dissipate heat to or from the ground. DHT's are used for geothermal heating

Geothermal heating

Geothermal heating is the direct use of geothermal energy for heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ of geothermal heating in 2004...

, sometimes with the help of a geothermal heat pump.
The heat exchanger usually consists of one or two u-tubes through which the carrier fluid, usually water, circulates. The space around the u-tubes is filled with groundwater or backfilled with thermally conductive grout.
Another design use a single open pipe to flow water downward.