If you do not have the room to install a ground loop, then a Borehole (Vertical Loop) system can be used. Boreholes are drilled to a pre-determined depth (Geothermal boreholes are typically 50 to 100 metres deep and approximately 150mm in diameter).
Contrary to popular belief, the boreholes are not accessing "hot" rocks at this depth. and a pipe is installed into the borehole, normally being either 32mm or 40mm pipe. The pipe is a loop that is connected at both ends to a manifold that in turn connects to the heat pump.
The heat pump process is identical to that of Air Source Heat Pumps and Ground Source Heat Pumps where water and antifreeze are circulated through the system and the difference in temperature between leaving and re-entering the pump is sufficient to pass through a compressor and be converted into useful heat.
It is important that the borehole is drilled and constructed correctly, paying particular attention to the grouting of the hole. A badly grouted Ground Source Heat Pump Borehole can mean that voids are left around the pipe and the system will not work as effectively as it should.
In a 'closed loop' system a single or double 'probe' is inserted into the full depth of the borehole and held in place with a thermal grout to ensure good heat conductivity between the probe and the surrounding area. The boreholes are joined to a manifold, and the manifold connected to the heat pump.
Boreholes can also be used in 'open loop' systems to extract water from underground aquifers, pass the water through a heat exchanger and return the water to the aquifer via a second borehole some distance away - essentially just borrowing the water to extract low grade heat.
The drilling rigs used for geothermal drilling are now very compact and boreholes are normally spaced at least 6m apart. The number of boreholes required for a given sized heat pump depends on the underlying geology.
The amount of heat that can be extracted per meter of borehole varies from 20W/m to 70W/m. As with all installations of ground source heat pumps, design of the ground collector (borehole & probes in this case) is critical to the long-term efficiency of the final installation.