In most areas, including Denmark, geothermal energy can only be extracted at temperatures below 100 °C and can therefore not be used for electricity production. On the other hand, the heat from the underground can be used for heating purposes through district heating, and there are many places, where this heat can be produced. Normally, production includes pumping up the geothermal brine through one well (the production well), cooling the geothermal brine in the surface installations and pumping it back into the underground through a second well (the injection well).
In the surface installations the heat is extracted from the geothermal brine through heat exchanges and/or heat pumps. The geothermal brine, which is often highly saline, is not circulated as such in the district heating network – instead the energy is transferred to the district heating water.
The geothermal plant is often placed close to another district heating productions units such as a waste-to-energy plant, a combined heat and power (CHP) plant or a biomass boiler unit, that can the absorption heat pumps of the geothermal plant with process heat. Absorption heat pumps are run on heat or steam instead of electricity, which gives can give a lower heat production price as well as environmental benefits. There would though be great advantages – also for economics of society – by changing the taxation scheme, so that the use of large electrically driven heat pumps would become economically attractive for the district heating companies.
|The geothermal concept.|
Other than production of electricity and district heating, geothermal energy can also be used for a number og other purposes:
Such plants can be found abroad, but not yet in Denmark.
Before a geothermal plan can be found, sandstone layers with right combination of temperature and trasmissivity (the product of permeability and layer thickness) need to be located. This is done through existing geological and geophysical data from nearby wells and seismic surveys. Often it is necessary to improve or supplement the existing data with new a seismic survey in order to set up a geological model, which can be used for planning the wells.
Seismic data are collected by sending pressure waves into the underground. When these pressure waves hit a boundary between two layers, e.g. a claystone and a sandstone layer, a fraction of the pressure wave will be reflected. By measuring the time it takes for the reflected pressure wave to get back to the surface, the depth to the different layers can be estimated.
Onshore, the pressure waves are recorded by small, simple microphones called geophones. These are laid out in long chains of lengths up to several kilometers. The many geophones – up to several thousand – are connected to a recording unit, typically a truck loaded with advanced electronics and powerful computers.
The pressure waves can be created in several ways. In earlier times, dynamite was often used, but this technique is nowadays only used under special circumstances. Today, large vibrator trucks equipped with heavy pistons that, when they vibrate against the surface, can create pressure waves, are used. Often more vibrator trucks are used simultaneously in order to give the pressure waves enough energy, so that the geophones can record the reflections for deeper layers.
The vibrator trucks send most of the energy into the ground. You can therefore vibrate quite close to buildings without causing structural damages. As a precaution, the vibrations are constantly measured at nearby buildings and other vulnerable structures in order to make sure, that the vibrations are always kept below a safe and low level.
|The principle of seismic surveying.|
When the recording of the seismic data is done, these have to be processed, which again requires the use of powerful computers and advanced programs. The cross sections of the underground, that are the result of the processing, are called seismic profiles and can in the end be incorporated into the geologist’s interpretation of the underground.
The results of the seismic surveys provide a basis for choosing the optimal location of the wells and work out a well profile with the expected depths to and thicknesses of the underground layers.
|An example of a seismic profile and geological interpretation (courtesy of Hovedstadsområdets Geotermiske Samarbejde).|
Geothermal wells are quite similar to oil and gas wells. The same technology and equipment is used, but typically geothermal wells have a larger diameter, as the volumes that have to be pumped up and reinjected are relatively large.
As with oil and gas wells, geothermal wells can also be deviated. This means that the spacing between the wells can be approximately 10 meters on surface but up to 700-1,200 meters in reservoir depth. The spacing in reservoir depths is important to insure that the influence of the cooled geothermal brine being injected back into the reservoir is not seen in the production well until after 25 – 30 years of operation.
|Drilling rig in Sønderborg, Denmark – summer 2010.|
Immediately after the drilling of the first well the transmissivity of the reservoir is tested. If the reservoir properties meet expectations, the second well is drilled. When both wells are complete, they are once again tested to make sure, that they are hydraulically connected through the reservoir. The reservoir properties are used for designing the surface installations.
The extraction of geothermal energy from the underground is done by pumping warm water from 1,000-2,500 meters depth to the surface. This is done with a very powerful electrical submersible pump (ESP), which hangs in typically 500-700 meters depth in the so called production well.
On surface the energy is extracted from the warm water through heat exchangers and/or heat pumps. After filtering the water is pumped back into the underground in order to keep up the pressure in the reservoir. The highly saline water from the underground is there not mixed with the district heating water.
|An absorption heat pump arrives at the geothermal plant in Copenhagen.|
The use of heat pumps in the geothermal plant serves two purposes:
Increasing the temperature from e.g. 35-75 °C in the reservoir to the flow temperature of e.g. 80 °C in the district heating network.
Cooling the geothermal brine as much as possible, e.g. to 10 - 20 °C, before the brine is reinjected back into the reservoir; in this way as much energy as possible is extracted.
Instead of electrically driven heat pumps, which are typically used in shallow geothermal systems, we use so called absorption heat pumps.
In the evaporator of the heat pump, water is bailed by use of the hot geothermal brine from the underground. As the temperature of the geothermal brine is 35 - 75 °C and is cooled to 10 - 20 °C in the evaporator, this process takes place at a very low pressure.
From the evaporator the steam generated is sucked up into the absorber by the use of a mixture of lithium bromide (LiBr) and water (H2O). Lithium bromide is a salt, which is highly attractive to water. When the LiBr/H2O mixture absorbs the steam, which condenses into water, heat is given off and transferred to the district heating water. At the same time, the LiBr/H2O mixture is thinned.
To keep the process going it is necessary to continuously regenerate the concentration of the LiBr/H2O mixture. This takes place in the so called generator, where water is boiled off by the use of an external source of heat or steam. As LiBr holds on very tightly to the water, the boiling process takes place at a rater high temperature, typically 140-180 °C. The higher concentration LiBr/H2O mixture then flows back into the absorber, while the water is condensed by the use of the district heating water before flowing back into the evaporator.
The energy that is used in the generator to boil off the water from the LiBr/H2O mixture is also transferred to the district heating water and thereby comes to use. In case steam from a CHP plant with a low pressure steam turbine is used, the steam could otherwise have been used to produce electricity. If the steam or heat comes from a plant without a low pressure steam turbine, it would have ended up as district heating anyway and is therefore “for free”. The geothermal plant so to say “borrows” the energy, before it is returned to the district heating network together with the heat from the underground.