What is a ground source heat pump borehole?

We dig into the detail and explain how borehole arrays work, are installed and how much the system might set you back

- ROBIN WHITLOCK

There are two types of ground source heat pump (GSHP) arrays: a horizontal array that’s installed as straight or coiled pipes in trenches or a GSHP borehole, which is a vertical ground array sunk into the ground by a professional geothermal drilling specialist.

The role of the borehole is to collect heat from rock and deliver it to a ground source heat pump. Rock is the heat source because heat from the sun is stored in surface soil. From there it dissipates downwards through the rock strata.

THERE ARE TWO TYPES

Borehole arrays come as either closed loop or open loop. A closed loop borehole, the most common type, is a closed circuit which contains a fluid that transfers heat. An open loop borehole fulfils the same function but uses a natural water source instead of a special fluid. These boreholes are mostly used for district heating schemes, large commercial renewable energy projects with a high heat demand or smaller projects that need to save space, particularly where there isn’t enough space to install a horizontal array in a trench in a garden, for example.

HOW ARE THEY INSTALLED?

A drilling contractor is responsible for drilling the borehole. It is a specialist discipline, carried out by a geothermal drilling company.

The first task is to complete a geological survey, which will indicate the type of rock being drilled. This is important because the geology will affect the heat transfer characteristics, such that a borehole in loose stone will have an energy extraction rate of approximately 20 watts per metre (W/m) while a borehole in granite will have an extraction rate of 55 to 70W/m.