Borehole fields: vertical ground heat exchangers
A borehole field is a set of vertical holes drilled into the ground, each holding a sealed loop of pipe, joined together so that fluid can carry heat into or out of the earth. Fields supply ground-source heat pumps and, in some projects, store summer heat for winter.
What is inside a borehole?
Each borehole is a narrow vertical hole, drilled to a depth that depends on the site and the design. A U-shaped loop of plastic pipe goes to the bottom, and the hole is then filled with grout so heat can pass between the pipe and the surrounding rock or soil and so groundwater cannot travel along the hole. Many boreholes connect to a shared header, where the fluid streams join and travel to the building. The fluid, usually water with antifreeze, is circulated by pumps and never enters the ground itself. The loop options are compared in ground loop types.
How are boreholes arranged in a field?
Boreholes are spaced so they do not draw heat from each other too strongly. For a heat pump field serving a house or a building, that usually means spacing of several metres, and designers choose the number, depth and layout from the heating and cooling load and the ground properties. Geothermal drilling depth and spacing explains the usual trade-offs. A thermal response test on a trial hole, in which heat is injected and the temperature recorded, tells the designer how well the ground conducts heat.
Fields can be set in a grid, a line or a ring. Shape matters less than total drilled length and spacing, and your installer's design drawings show the layout. Surface space needed depends on the layout. See ground temperature by depth in Canada for how the earth behaves below the frost line.
How does a borehole field differ for heat storage?
A heat pump field mainly extracts heat from a large mass of ground over the winter. A storage field is operated more like a battery: heat is injected in summer and taken back in winter, and the boreholes are packed closer together so the heat stays within a compact volume. A cap over the field and insulation at the surface can help limit losses. The idea is described in borehole thermal energy storage.
Drake Landing Solar Community used a field of 144 boreholes, each roughly 35 metres deep, spaced 2.25 metres apart in a circle about 35 metres across. Summer solar heat was injected, and the core approached 80 degrees Celsius in late summer. The borehole storage page shows the field in detail, and storage in general is covered in what is thermal energy storage.
What should you know before drilling one?
Drilling needs a licensed contractor, and many regions require permits or notification, covered in geothermal drilling permits in Canada. Geology matters: rock, clay, sand and groundwater change the drilling method and the cost. Ask for a design based on a load calculation, a plan for grouting, pressure testing of loops before burial and a record of where each borehole sits. How to pick a driller is in how to choose a geothermal drilling contractor.
What does a field cost and how long does it last?
Drilling usually makes up a large share of a ground-source project, and the total depends on geology, depth, access and region. Prices vary from year to year, so quotes decide. See geothermal heat pump cost in Canada for the cost drivers. The pipe in the ground is designed to last for decades, while the heat pump itself has a shorter life, which is the subject of ground-source heat pump lifespan. Fields that are heavily imbalanced, with much more heat taken than returned, can slowly cool the ground, so designers check the long-term balance.
Quick answers
What is a borehole field?
It is a group of vertical boreholes, each with a sealed pipe loop, connected so that fluid can exchange heat with the ground for a heat pump or for thermal storage.
How deep is a borehole?
It depends on the site and the design load. The Drake Landing storage boreholes were roughly 35 metres deep, while other fields differ.
Is a borehole the same as a water well?
No. A borehole for heat exchange is a closed loop that does not draw water from the ground.
How many boreholes does a house need?
The number depends on the heat load, the ground properties and the depth of each hole. A designer calculates it, and a thermal response test can refine the design.