At its birth about 4.5 billion years ago, the Earth was an immense reservoir of heat. That heat was gradually lost to space and the surface cooled to form an insulating crust that reduced the rate of heat loss, but did not stop it. Heat is still flowing out at an estimated average rate of 56-67 mW/m2, partly on account of the initial storage, but also to a significant extent because of replenishment through the decay of radioactive isotopes.
In certain regions, heat loss is significantly greater than the average flux owing to the dynamic forces that are at work within the Earth. This is particularly evident at tectonic plate boundaries and at hot spots where rising mantle plumes from the Earth‘s interior transport heat to the surface. Areas like these are linked to active volcanism, where magma chambers lurk at relatively shallow depths below the surface as particularly powerful sources of heat. However, in order to harness the thermal energy, a medium is needed to bring the heat to the surface where it can be utilized for the generation of electricity or for direct use.
Hydrothermal systems develop where water of meteoric origin, that seeps into the ground through tectonic fractures, comes into contact with a heat source and in turn is convected back to the surface. Strong circulation systems may be formed in volcanically active areas, where the heat source is extremely powerful. In other areas, the circulation may be weaker due to less dominant heat sources or limited water supply. In yet other areas, water is present without much circulation at depth due to the lack of a heat source and restrictions to flow. In these cases, large quantities of stored water may be heated up over long periods of time by Earth’s average heat flux.
Geothermal systems are commonly divided into high-temperature and low-temperature systems. High temperature systems are defined as systems where the temperature at 1 km depth exceeds 200°C and low-temperature systems as those where the temperature is lower than 150°C in the uppermost kilometre.
The most common way of harnessing geothermal energy is to tap hydrothermal reservoirs by drilling wells to bring the fluid to the surface. With current drilling technology water with themperatures of up to 360°C can be retrieved from the upper 2-3 km of the earth´s crust.
High temperature geothermal fluid may be brought to the surface as steam, or more commonly as a two phase mixture of water and steam. A common way of utilization is to separate the two phases and use the steam to drive turbines for electrical generation. In combined heat and power plants, the waste heat from this process is used for direct heating applications, thus allowing for more efficient use of the energy. Electricity can also be generated from geothermal fluids of medium temperatures by using the heat to evaporate a working fluid, such as isopentane or an ammonia-water mixture that has different thermal properties than water. This fluid runs in a closed loop within the power plant and is used to drive a turbine. Low temperature fluids are commonly used for direct heat applications such as space heating, aquaculture, horticulture, industrial balneology and swimming pools, industry and snow melting.
In many cases, the geothermal effluent is injected back into the reservoir from which it was extracted to increase the rate of recharge and to avoid possible negative impacts of discharge on the surface.
Another large potential in geothermal is Hot Dry Rock or Enhanced Geothermal Systems, where energy is stored in dry non-porous rock. Exploitation of this energy is more challenging than the more conventional hydrothermal exploitation, but research into such systems is well under way. If these resources can be harnessed economically, they have the potential to vastly increase the use of geothermal energy globally and to make it accessible to people that at present have to make do with other energy sources that are significantly less friendly to the environment.
In summary: Geothermal energy originates within the Earth and is brought to the surface by the planet‘s internal dynamic forces at varying rates depending on location. In specific regions, where the heat flux is anomalously high, the energy can be harnessed and used for electrical production as well as direct use. In other areas, geothermal energy is used exclusively for direct applications, such as space heating and recreational purposes.