GROUND SOURCED ENERGY
Ground-source energy is about harnessing the heat stored in the basement rock, soil or ground water - an energy source that is a local, environmentally-friendly. With the help of a heat pump, energy from the ground can be used to heat homes and tap water. The stable temperature of the ground and groundwater over the course of one year provides good operating conditions for heat pumps. Approximately 70% of the heat that is distributed in a building may come from the ground, while the remaining 30% is electricity needed to power the heat pump. The heat stored in the rocks, is mostly from the sun, along with a small contribution from the decay of radioactive elements in the bedrock.
Many buildings may also need cooling. By retrieving the heat from the ground in the winter and cold from the ground during the summer, you get good value for your system and will have a fast return on your investment. This applies to the water-to-water heat pumps; too, assuming the building is equipped with under-floor heating pipes or radiators.
Geothermal heating systems extract heat stored in the rock via boreholes. Boreholes are usually drilled 14 centimeters in diameter and at an 80 to 200 m depth. An anti-freeze liquid circulates in plastic tubing in the borehole, and is removed by heat pump. There four geological factors that affect investment cost for this type of system:
- the thickness of the sediment lying over the solid rock
- the temperature of the ground
- the heat-bearing properties of the rock
- the groundwater level
“At what depth is the surface of the bedrock?” is by the far most common question received at NGU related to ground-source energy. For both larger and smaller borehole heat exchange systems, the thickness of the unconsolidated sediment over bedrock is often decisive factor in deciding whether or not ground-source energy is chosen. 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 as expensive as drilling in solid rock. In Norway, there may be only thin layers of unconsolidated sediment over the bedrock to be reached, but in valleys and in parts of eastern Norway, Trøndelag, Jæren and Finnmark, the thicknesses of unconsolidated sediments over bedrock is much greater. Many villages and urban areas are rich in such sediments.
It may often be difficult to determine the depths of these sediments, except in areas with exposed bedrock. To help the public, NGU provides online maps and databases determine the depth of the sediment layer:
Except in areas where the bedrock is at the surface, it can often be difficult to determine the thickness of sediment layer. In this case, the best advice would be to check NGU’s maps and databases:
- The quaternary geology database (www.ngu.no/kart/losmasse) gives users access to maps that show various types unconsolidated sediments, some of which are subdivided into "thick" and "thin".
- The national groundwater database, GRANADA (http://geo.ngu.no/kart/granada) provides detailed information of boreholes drilled for energy and water supply purposes, including distance downwards to the bedrock surface.
You may also seek advice and inforation from your municipality, the Norwegian Public Roads Administration (Statens vegvesen), geotechnical drilling companies and local drillers. Some equipment rental companies may have handheld drilling instruments that allow you to conduct simple subsurface probing yourself.
The temperature of the ground is 1-2 Celsius higher than the average annual air-temperature at the site; this difference partly depends on the number of days with snow cover.
The thermal conductivity of the bedrock often varies between 2 og 4,5 Watt per meter Kelvin (W/m K), and is a measure of how well the rock conducts heat (heat transport) towards the borehole. Where there is high thermal conductivity, the heat is extracted from greater distances and greater heat extraction is obtained per borehole metre. The thermal conductivity of the bedrock is largely linked to the content of quartz. Pure quartz can have a thermal conductivity in excess of 6 Watt per meter Kelvin (W/m K). In addition, when the bedrock is layered, heat is conducted better parallel to the layering than perpendicular to the layering. NGU has measured the thermal conductivity of many different rock types throughout Norway.
In heating mode, typical values for the effect transfer, depending on temperature and thermal conductivity of the bedrock, are 30-40 Watt per meter of active borehole varying from 20 to 80 Watt per meter. In cooling mode, the effect transfer is higher, typically 90 Watt per meter of active borehole. The term "active borehole" refers part of the borehole below the water table. In general the groundwater level follows the terrain, and is usually around 1-10 metres below the terrain surface. The distance down to groundwater level may be farther. The groundwater flow route can under certain conditions play a role for the effectiveness of a heat pump.
NGU is continually working to collect relevant data and provide web solutions for better information about the geology of the rocks and soil throughout Norway. When installing a large ground source energy system where more detailed mapping may be necessary, the use of geophysical methods and, ideally, a test borehole is recommended.
In the case where bedrock is far below the surface, soil sourced heat pumps can be effective if there enough area is accessible. Ordinary systems for collecting heat from the soil involve horizontally laid collector hoses (40 mm diameter) that are buried in 0.8-1.5 meters deep trenches in the soil. The heat extraction depends of the soil type and varies from 15 to 30 Watt per meter of collector hose. The heat extraction depends of the soil type and varies from 15 to 30 W/m collector hose. The major extraction is related to the freezing of water (phase transition from liquid to ice). High moisture content in the soil is favourable and favours soil types as bog, top-soil, clay etc. Dry sandy soil is less suited. Be aware of ground settlement. The collector tubes must be placed in such a way to avoid excessive settlements which can result in structural damage to a homes, roads and foundations.