A borehole heat exchanger with a "closed loop" collectorA borehole heat exchanger consists of a vertical borehole with a “closed loop” collector system. In Norway, the system is often referred to as an “energy well”. The majority of the borehole is usually drilled in solid rock at a typical diameter of 140 mm; the depth can range from 80 to 200 metres. A plastic (high density polythene) collector hose, filled with a “carrier fluid”, is installed in the borehole. This circulating carrier fluid (a solution of anti-freeze) absorbs heat from the surrounding bedrock, which is then extracted by the heat pump for use in space-heating.
If the water table is shallow, the natural groundwater filling the borehole acts as a thermal contact between the pipe and the borehole wall (see diagram above). If the water table is deep, however, the borehole may need to be backfilled with a “thermally enhanced grout” to provide a thermal contact.
There are three geological factors that affect the investment cost of this type of installation. These are:
- The thickness of the unconsolidated sediment cover over the bedrock surface.
- The temperature in the ground.
- The thermal conductivity of the bedrock.
(1) Depth to bedrock surface
“What is the depth to rock-head?” is by the far most common question that NGU’s thermogeologists are asked in relation to ground-source energy. For both large- and smaller-scale borehole heat exchanger systems, the thickness of the unconsolidated sediment cover over the bedrock is often decisive for the cost-effectiveness of ground-source energy. When drilling in unconsolidated sediments, a steel lining must be inserted into the hole in order to stabilise the sediments. This is around four times more expensive than drilling in solid rock. In Norway, the cover of unconsolidated sediments is generally thin, but in valleys and in parts of eastern Norway, Trøndelag, Jæren and Finnmark, the thickness of unconsolidated sediments can be considerable (several tens of metres). Many urban centres have developed on areas underlain by unconsolidated deposits.
Except in areas where the bedrock is very close to the surface, it can often be difficult to know the thickness of unconsolidated deposits. In this case, the best advice would be to check NGU’s maps and databases:
- The surface geology database gives access to maps on which the unconsolidated sediments are shown. They are categorised as different sediment types and are also subdivided into “thin” and “thick” cover.
- The national groundwater database, GRANADA, provides detailed information on boreholes drilled for energy and water supply purposes. The borehole log will often record the depth to bedrock.
If you require further information, you could also inquire about depth to bedrock via local authorities, road authorities, geotechnical drilling companies, local well drillers etc. Some rental companies offer equipment (for instance, the Pionjär hand drilling machine) which allow you to probe the subsoil to several metres’ depth yourself.
NGU is continually working to collect relevant data and provide web solutions for better information on ground conditions. If you are installing a large ground source heating / cooling system where the financial stakes are high, you can reduce uncertainty by using geophysical methods to map the subsurface or, ideally, by drilling a test borehole. Such a test borehole would not only confirm the geological conditions, but you can also carry out simple tests to measure the temperature and thermal conductivity of the ground (see below). Otherwise, it is simply a question of gritting your teeth, setting the drill-rig to “go” and hoping that it is not too far down to the bedrock surface!
(2 and 3) Temperature and the thermal conductivity of the bedrock
The temperature and the thermal conductivity of the bedrock are two important natural factors in the design of borehole heat exchanger systems. As the size of the ground source heating / cooling scheme increases, the more important it becomes to know these with some accuracy. As a general rule, the temperature in the ground is 1-2°C higher than the average annual air temperature at the location in question. This is partly due to the natural geothermal gradient, but also because snow insulates the ground from the worst extremes of winter temperature. The difference thus partly depends on the number of days of snow cover. In Norway, the ground temperature typically ranges from around 2°C (in mountainous or inland northern regions) to 8°C (near the coast and in the south).
The thermal conductivity of the bedrock typically varies between 2 and 4.5 W/m×K, and is a measure of how well the rock conducts heat (heat transport) towards the borehole. The thermal conductivity of the bedrock is largely dependent on the rock’s quartz content. Pure quartz can have a thermal conductivity in excess of 6 W/m×K. Layered rocks tend to conduct heat better parallel to the layering than perpendicular to it. In bedrock with a high thermal conductivity, the heat can be “sucked in” from further afield than if the rock has a low thermal conductivity, and greater quantities of heat can be obtained per drilled borehole metre. NGU has equipment for measuring the thermal conductivity of rocks, and has systematically determined the thermal conductivity of various rock types, primarily in the Oslo region.
In heating mode, the amount of heat that can be transferred from the ground to the borehole is typically 30-40 Watts per metre of thermally “active” borehole, although this figure can vary from 20 to 80 W/m. In cooling mode, the heat transfer from borehole to rock is higher, typically 90 W/m active borehole, because of the higher temperature gradients that are applied. The term “active” borehole refers to the water-filled part (or grouted part) of the borehole.
In general the groundwater level in bedrock approximately follows the terrain, and is usually around 1-10 metres below ground level. On hilltops or plateaux, the depth to the groundwater can be even deeper. In addition to the temperature and thermal conductivity of the bedrock, heat transport with flowing groundwater may be important for the performance of borehole heat exchanger systems. This effect generally enhances system performance and can be important especially for wells in sloping terrain where the bedrock is fractured and permeable.
Thermal response test
During the design of larger borehole heat exchanger systems: for example, systems combining both heating and cooling (underground thermal energy storage (UTES))), a thermal response test will be of great value. The test should be performed during an early phase of project planning and should provide three pieces of information:
- the thermal conductivity of the ground (average over the active borehole length)
- the thermal resistance of the borehole (a measure of how efficiently the borehole transfers heat)
- the natural average temperature of the ground over the active borehole length.
Knowledge of all three factors can assist in achieving an optimal system design. NGU’s equipment for the thermal response test was developed in Luleå in Sweden and can be hired out for research and training purposes.
