How does geothermal heat work? Heat energy from groundwater

how does geothermal heat work

Rock heat is the form of heating energy that is produced from the bedrock's groundwater.

A rock heating system intended for a single-family house requires that one or more holes with a depth of 90 - 200 meters be drilled in the rock. The energy is extracted by sending cooled liquid through the rock in a pipe, where the liquid is heated by the groundwater in the rock, which at the same time cools the groundwater slightly. This liquid is called coolant or collector liquid. After the refrigerant has circulated through the rock, the liquid is led to an evaporator where it heats the refrigerant but the heat from the rock. When the brine has given off its heat, it is brought back down into the rock to be heated again. The refrigerant has a very low boiling point so that it evaporates easily. The gas is then fed into a compressor where it is compressed. This process makes the gas even hotter and when it has reached the right temperature, it is sent to the heat pump's condenser to be used to heat the water that the house's heating system uses to heat the house. After giving off its heat, the refrigerant cools and becomes liquid again by condensation. The refrigerant is returned to the evaporator to be reheated by the refrigerant.


  • 1 Background
  • 2 Execution
  • 3 Guide value for possible extracted energy for a rock heating hole
  • 4 Technical design
  • 5 See also
  • 6 References and footnotes
  • 7 External links


Rock heat uses geoenergy, which is energy created by solar energy. Rock heat is extracted from the depth where the bedrock and groundwater have the same temperature.

The groundwater is heated by the sun when the water is close to the ground and by water in nearby larger lakes and seas which are also heated by the sun. In Sweden, the groundwater maintains a relatively even temperature of 6 - 8 degrees throughout the year. The large supply of groundwater in the fracture zones of the bedrock, in combination with the relatively steady temperature of the groundwater, means that geothermal heat can be used all year round.

A misconception that exists regarding traditional rock heat is that the heat comes from the earth's interior, so-called geothermal energy. It would be very costly as in Sweden boreholes with a depth of at least 1000 - 2000 meters would be required to take advantage of the heat from the earth's interior.

There are some countries, such as El Salvador, Iceland and Kenya, where the earth's crust is thin enough for geothermal energy to be used for electricity production.



Rock heat means that you take heat energy from the groundwater by drilling holes in the bedrock. The borehole is around 90 - 200 meters deep depending on how the groundwater is distributed in the ground as you want the water to enclose as long a section of the borehole as possible.

When the geothermal heat pump absorbs the heat energy, it leads to the groundwater temperature being lowered by an average of 3 - 4 degrees. If the water turnover in the borehole is good, relatively large amounts of energy can be extracted from a single borehole.

In order for the rock heat to be as efficient as possible, it is important that as long a part of the borehole as possible is enclosed and that the water turnover of the borehole is very good. If these conditions do not exist, there is a risk that there will be rapid ice formation and so-called permafrost in the borehole. Permafrost means that the ice does not thaw during the hot part of the year.

This can lead to the lump of ice that normally forms around the borehole in winter, increasing in volume with each passing year. Over time, this leads to a drastic or complete cessation of the water flow around the borehole and the extracted heat energy.

It is normal for there to be some ice formation in the lower part of the borehole during the winter. However, this ice thaws during the hot part of the year and has no significance for the extraction of energy as this is controlled by the temperature difference. The temperature drop between -1 to - 4 degrees gives the same energy as the temperature drop between +8 to +5 degrees, both cases have a temperature difference of 3 degrees.

There is also a certain positive effect with ice formation, if it is not too extensive, as the energy-absorbing surface towards the groundwater increases.

In some areas, the groundwater is distributed in such a way that it is more efficient to drill the hole obliquely downwards, a so-called degree hole, instead of straight down. This is done so that the borehole has contact with the groundwater for a longer distance.

If you take into account the electricity that is used to power the heat pump and its various parts, you can count on reducing the energy requirement for other energy for heating and hot water by about 50%.

Guide value of possible energy extracted for a rock heat hole

For a rock heating hole with 140 mm in diameter (5.5 inches), the guideline value is that it is possible to take a maximum of about 140 kWh per meter of water-bearing boreholes and per year. If you calculate it in power (W), it means that the maximum power output per meter of water-bearing borehole is 50 W, in other words no more than a light bulb of 50 W.

However, the time and the water-carrying length of the borehole mean that considerable amounts of heat energy can be extracted during a year. In Sweden, groundwater is on average 10 meters or more below ground level. If you have a depth of 10 meters until you reach the groundwater, you can by drilling a 200 meter deep borehole get an active borehole depth of 190 meters where energy can be absorbed.


  • Borehole diameter = 140 mm (5.5 ″)
  • Maximum energy extracted per meter hole and year = 140 kWh
  • Borehole depth 200 m. Effective water-bearing hole depth = 190 m
  • Possible extracted heat energy = 190 mx 140 kWh = 26 600 kWh.

This means that with two boreholes of 200 m, a potential 53,200 kWh of heat energy can be extracted. If you intend to drill more than one hole, leave a distance of 20 m between the boreholes so as not to cause too great an average lowering of the groundwater temperature. In addition, by law it must be at least 10 m to the next property plot. Due to this, relatively large ground areas are required if many boreholes are required for the facility.

A geothermal heat pump provides about 4 times as much energy as it consumes. In other words, the heat pump draws 1 part energy to extract 4 parts energy from the groundwater.

A smaller geothermal heating plant has some advantages compared to other energy sources. Advantages such as high operational reliability, not too large investment costs and that it provides a stable supply of energy regardless of weather and season.

In comparison with the cost of heating with oil or electricity, the investment cost for a smaller geothermal heating system can start to pay off after already 4 - 5 years. If you compare with the cost of heating with district heating, it can start to pay off after 8 - 10 years, this calculated with current energy costs.

Rock heat also has the advantage that so far there are no charges for energy extraction. In the future, however, it may be relevant to charge for large energy outlets in densely populated areas. This is because there can be negative environmental effects as the average groundwater temperature drops in larger areas.

Extracting heat from seawater is an alternative to rock heat as the seawater has practically infinite amounts of energy and the temperature tends to be higher than in the groundwater. There are currently a number of larger plants that use this in combination with other extraction methods. However, as this requires large investment costs and can have negative environmental effects in the local water area in the event of large energy outlets, the use of seawater heating has not increased significantly.


Technical design

The borehole has a closed system surrounded by groundwater consisting of flexible hoses with good thermal conductivity. In these hoses, a special liquid (brine liquid) circulates which is liquid even at minus degrees.

The depth required on the borehole varies depending on where the groundwater is located, but the borehole is on average 90 - 100 meters. Drilling is rarely deeper than 200 meters due to drilling technical reasons. However, it may be more economical to drill a 200 meter borehole instead of drilling two 100 meter boreholes.

There are also heat pumps with an open system. In one, the water in the borehole is pumped straight into the heat pump's heat exchanger or into a connected external heat exchanger. This type of system has the advantage that, because the water has a higher temperature, you can get a better efficiency. If you also have a good water supply, you can have a higher energy consumption. The disadvantage, however, is that you have to dispose of the pumped water in a suitable place and that it must be a good water quality to avoid the heat exchanger becoming clogged. This type of system is also called groundwater heating.

In a closed system, the length of the water-carrying part of the borehole is directly proportional to how much energy it is possible to extract from the borehole, while in an open system it is controlled how much energy can be extracted from the water supply.

In the closed system, the brine liquid is pumped via the hose into the borehole enclosed by groundwater. When the liquid passes through the borehole, it heats up by 3-4 degrees. Then the heated liquid is pushed up to the heat pump's heat exchanger where its heat energy, via another closed system, is transferred to the hot water that will later be used to, for example, heat the home. In order for the water to reach the desired temperature of about +50 degrees, it must be built up step by step. The water is stored in accumulator tanks and recirculated to the heat exchanger to be supplied with more energy until it reaches the desired temperature.

Source: wikipedia

Registration & application

Registration - outside water protection area

All heat pumps (except air source heat pumps) must be reported to the Operating Environment by you who are the property owner before installation. This is because the heat pumps that use heat from soil, rock, surface or groundwater can possibly affect the groundwater and thus have a negative effect on the environment.

Application - within water protection area

It is extra sensitive to install geothermal heating if you live in a water protection area. Therefore, you must apply for a permit before installing a heat pump in such an area.

How to register / apply

Registration / application must be made in good time on a special form and you must not start the installation until you have received a response to the registration / application. The installer of the heat pump can usually be helpful with the information needed.

Map with marked borehole

When you make an application, you must also send in a scaled and clear map where buildings, boreholes and plot boundaries are marked. The application must also state whether there are individual water sources and sewage systems that are located within 100 meters of the drilling site. These should be marked on the attached map. Remember that even in areas with municipal water and sewage there may be individual water sources.


Operations environment charges a fee of SEK 1,700 for its processing of the notification and the processing of an application within the water protection area has a fee of SEK 2,550.


It is important that the installers and drillers you hire have liability insurance and sufficient competence as it is you as the property owner who is responsible for ensuring that no negative environmental effects occur in connection with the installation. Information on certified well drillers is available on SP Certification's website.

Electricity, telecommunications and water supply and sewerage lines

You are also responsible for checking whether there are electrical, telecommunications or water supply and sewerage lines or easements and tableware in the ground that the installation may affect. At there is information and measurement of electricity and telephone lines.

Neighbors' water sources

When installing geothermal heating, you must protect the neighbors' water sources and compensate for any damage to them. Be sure to inform the neighbors about the planned drilling. There should be at least 20 meters between two energy wells so as not to risk the boreholes "stealing" heat from each other. Therefore, the distance between the property boundary and the borehole must be at least 10 meters. You must send a certificate from the neighbors when registering / applying if it is not possible to achieve that distance.

Noise from heat pumps

It is important to think about how much noise the heat pump causes. This is especially important when installing an air source heat pump. The technical term for how much noise is emitted is source noise and you should choose a heat pump with a low one. Unfortunately, the manufacturers do not state noise in a uniform way, so the easiest way to find a pump with low source noise is to look at the Swedish Energy Agency's website. There are the tests they did of the pumps' efficiency and noise.

The noise of the air heat pump is mainly caused by the noise from the circulation pump, but there can also be noise if the air heat pump is poorly set up.

As the owner, it is your responsibility to ensure that the heat pump does not cause disturbances. Before installing the pump, it is therefore important that you discuss it with the neighbors so that the pump is placed in the best possible place. You can also hire a professional to investigate the most suitable location if there is a risk of disturbance, for example if the plot is small and the neighbors are close.

If the noise from the heat pump disturbs a neighbor, he can complain to the Business environment. As an owner, you must then prove that the noise level does not exceed the applicable guideline values. This may in some cases require a qualified sound level measurement.

    Wikipedia information about rock heat

    Bergvärme Bergvärme är ett tekniskt system för uppvärmning av byggnader. Lågvärdig energi, i form av kall vätska, hämtas från grundvattnet i berggrunden. En bergvärmepump höjer temperaturen till en nivå lämplig för byggnadens vattenburna uppvärmningsystem. För detta utför bergvärmepumpen ett mekaniskt arbete vars energi även tillförs byggnaden. En vanlig anläggning för uppvärmning av enfamiljshus består av ett, eller flera, 90–200 meter djupa hål som borras ner i berget. Ur berggrunden utvinner man sedan energi genom att en nedkyld vätska som passerar berget genom en ledning värms av grundvattnet i berget. Detta medför också då en viss nedkylning av detta grundvatten. Kollektorvätskan, som kallas köldbärare, cirkulerar genom berget och leds sedan till en förångare där köldbärarens temperatur kan vara väldigt låg, till och med under nollgradigt. I förångaren värmer vätskan upp köldmediet som finns i det slutna systemet i värmepumpen, man använder ett medium som har väldigt låg kokpunkt. Köldmediet förångas och förs vidare till en kompressor där gasen komprimeras, vilket gör den ännu varmare. Den heta gasen går vidare till värmepumpens kondensor där den avger sin värme till vattnet som cirkulerar i husets värmesystem. När köldmediet har avgett sin värme svalnar den och blir genom kondensation flytande igen. Köldbäraren kyls ned av det avsvalnade köldmediet och förs därefter ner i berget för att återigen bli uppvärmt.

    How does geothermal heat work?