How to use geothermal energy?
Geothermal energy is heat that comes from underground. The heat is transferred from the centre of the Earth through its layers to the surface. Geothermal heat might be used for generating electricity or heating buildings and tap water. The scale of use varies from large power stations, district heating plants to single residential systems.
Electricity can be generated if hot water or steam are brought from underground reservoirs to the power plant. The steam is used directly in a turbine generator which makes electricity. If hot water is brought to the surface it is first turned into steam which then drives a turbine to generate electricity. At the end of the cycle the steam condenses to water and is injected back into the ground.
For heating purposes geothermal heat can be used directly when hot underground water is pumped to the surface or flows out naturally as hot springs. The water is pumped through a heat exchanger, which transfers the heat into a building's heating system or district heating plant.
Figure 1. Geothermal district heating
Another way to capture ground’s energy is by using ground-source heat pumps. They are installed below the ground’s surface at depths where the temperature is nearly constant all year long – higher than the air temperature in winter and lower in summer. The heat exchanger is a system of pipes called a loop, which is buried in the ground. In the pipes circulates a fluid (water or antifreeze liquid) which absorbs heat from the ground and transfers it to the heat pump unit. This heat is then distributed within a building. In summer, the system does the opposite and can be used to cool building’s interiors. The loops can be installed vertically or horizontally in the ground. Horizontal loops are buried in the shallow ground and are not considered to be true geothermal energy since they rather use solar energy adsorbed by the ground instead of energy generated by the Earth itself.
Figure 2. Horizontal ground-source heat pump system
Why use geothermal energy?
Geothermal energy does not produce pollutants contributing to the greenhouse effect.
It is not influenced by weather condition in contrast to solar or wind energy (except for shallow ground-source heat pumps).
Geothermal power plants or other geothermal installations do not affect the landscape with highly visible equipment such as solar panels or turbines.
Once a geothermal power station is built, the energy is almost free. The geothermal installations for heating purposes significantly reduce cost of heating.
Geothermal energy is a source that will not run out.
Where to use geothermal energy?
In general, the use of geothermal energy is not restricted geographically since it is present everywhere. However, easy to reach geothermal water (or steam) is not found everywhere. Regions in the world, where geothermal reservoirs are conveniently located in scope of economical conditions and currently available technology, are limited. For instance high temperature resources (above 150°C) are usually required to generate electricity. As a consequence, the majority of the geothermal power plants in Europe are located in Iceland and Italy, where geothermal resources of such temperatures are available. On the other hand, new geothermal power plants producing electricity from low- and medium-temperature resources (between 90°C and 150°C), have been developed in Germany and Austria.
Low temperature resources (below 90°C) can be found everywhere. Ground-source heat pumps can operate anywhere that the loops can be buried in the ground.
What are potential obstacles?
Main disadvantage of geothermal energy is a high installation cost in case of both industrial and residential use. Particularly large costs are generated by drillings. Moreover, present drilling technology is able to reach these geothermal resources, which are located relatively close to the surface.
In case of industrial use there exists a possibility of running out of steam or water. This may happen due to different reasons, for example injecting too much cold water back to the reservoir and cooling the rocks down. That is why geothermal resources must be carefully managed in order to avoid local depletion. There is also the risk that some hazardous gases and minerals may be released from the geothermal reservoirs.
In case of ground-source heat pumps or geothermal water pumps electricity is required to operate a pump so the extracted heat is not entirely “for free”. Geothermal water wells or vertical loops of heat pump systems are generally more expensive to install because of the drilling costs but need a limited space. Cheaper horizontal loops, however, require relatively large area which is not always available in the vicinity of buildings. There is also the risk of mistakes in designing, installing and/or operating a heat pump system which can lead to damaging the unit.