Technologies and Applications
The potential of geothermal energy can be used practically everywhere. In countries such as Germany, Italy, Indonesia, the Philippines, Mexico, the USA and Iceland, the use of geothermal energy for heating and electricity generation has been part of the energy concept for many years.
At the end of 2015, global installed geothermal power capacity reached 13.2 GW. The countries with the largest installed capacity were the US, Philippines, Indo-nesia, Mexico and New Zealand. Especially in regions which fulfill the geological requirements (for example regions of volcanic activity, temperature > 200 °C), geothermal energy forms a sound basis for environmentally friendly and cost-effective energy production.
The geothermal energy available in the earth’s crust originates mainly from the residual heat from the time of the earth’s formation and processes of radioactive decay. In addition, the topmost layers (up to 2 m deep) partly store energy from the sun’s radiation.
Deep geothermal energy
Deep geothermal energy is used both to generate electricity in power plants and for heat in large heat grids for industrial production or for heating buildings. Deep geothermal energy assumes the exploitability of heat reservoirs at high temperatures. Depending on the availability of deep water, the water permeability and the system concept, a distinction is made between hydrothermal and petrothermal geothermal energy, as well as deep borehole heat exchangers.
Hydrothermal geothermal energy
• Hot water bearing strata at great depths are used directly (400 m).
• The water-bearing bed needs to have the widest possible vertical and lateral spread to ensure long-term use.
• Heat and energy generation depends on the flow rate and temperature of the thermal water.
• The water temperature must be above 100 °C.
• The steam drives a steam turbine and can also be used for other heat consumers such as households or industry.
• The cooled thermal water is then returned underground through a second borehole, known as the reinjection borehole.
Petrothermal geothermal energy
• Use of deep-lying heat reservoirs, which have no water flow or only a negligible water flow. • Reservoirs can be hot, dry layers of rock at a depth of three to six km with correspondingly high temperatures of more than 150 °C.
• They are developed by drilling two or more holes. Hydraulic and chemical stimulation processes (enhanced geothermal systems, EGS) create cracks and fissures in the rock. Using an injection borehole, water is pressed into the rock at high pressure where it is heated before rising through the production well.
• The hot water is used to heat agents with a low boiling point to generate steam for a turbine.
• Heat can also be fed into district heating grids via a heat exchanger.
Deep borehole heat exchangers
• Deep borehole heat exchangers refer to a closed system of energy production.
• Comprising a single borehole at depths of 400 m to several thousand metres.
• Double pipe exchangers are inserted into the borehole up to a depth of 4,000 m.
• Water circulates through the exchangers in a closed circuit.
• The heat from the water heated at a depth is then extracted at the surface and delivered to a heat pump circuit.
• In the case of high temperatures, the recovered energy can be used, for example, as process heat for in-dustrial applications or for agricultural applications in the case of low temperatures. It does not typically make economic sense to generate electricity with this process.
Near-surface geothermal energy for heating and cooling buildings
Near-surface geothermal energy makes use of boreholes going down to about 400 m and temperatures of up to 25 °C for heating and cooling buildings, technical plants or infrastructural facilities, as well as for water heating. It can be used in many regions of the world and is particularly suitable for single- or multi-family houses, blocks of flats, public buildings, central administration offices, hospitals, schools and business enterprises, as well as for heating road surfaces for ice prevention and improving road safety.
Thermal development of the soil
Borehole heat exchangers, geothermal heat collectors or energy piles are commonly used to thermally develop the soil:
Geothermal heat collectors are laid horizontally at a depth of 80 to 160 cm and are affected by the weather conditions prevailing on the surface. An area of about 200 – 250 m² is required for a single-family house. The heat transfer medium circulating through the piping loop transports the energy extracted from the ground to the heat pump. Investing in geothermal heat collectors is less expensive than investing in heat exchangers, but less efficient because they operate at a lower depth.
Borehole heat exchangers are used at depths of 50 – 250 m. Their use is widespread in Central and Northern Europe. They have a low space requirement and make use of a constant temperature level. The exchangers are typically implemented as vertical boreholes in which plastic (HDPE) tubing is installed. Within the tubes, a heat transfer fluid circulates that absorbs heat from the surrounding ground and feeds it to the heat pump. Using this technology, plants of different sizes – from small residential units to complete residential and office complexes – can be supplied with heating or cooling.
Energy piles are deep-reaching concrete piles, slurry walls or other structurally necessary concrete components built underground, which are fitted out with plastic pipes. Water is used as the main medium for exploiting the geothermal heat. The cold water is heated by geothermal energy in the concrete piles. An intermediate heat pump uses the warm water to heat the building. In summer, borehole heat exchangers, geothermal heat collectors and energy piles can be used for moderate cooling.
Groundwater can also be used under certain circumstances as a supply of heat. In Germany, the groundwater temperature is 7 to 14 °C, depending on region, depth and season. At a depth of 20 to 30 m, it is constant at roughly 10 °C. Thus groundwater still supplies enough energy to heat a house, even in extreme winter weather.
Using near-surface geothermal energy and the ambient air with heat pumps to generate heat or to cool buildings. Heat pumps in combination with energy piles, geothermal heat collectors, borehole heat exchangers or other ground contact concrete structures enable the use of near-surface geothermal energy. A heat pump relies on electricity – rarely gas – as its driving energy.
Depending on the type of driving energy used, a distinction is made between compression and sorption heat pumps. Compression heat pumps are the most common type of heat pump. They use the heat generated by the evaporation of a liquid. A refrigerant circulates in a compression heat pump; driven by a compressor, the refrigerant alternately adopts liquid and gaseous states. The basic principle is similar to a refrigerator: heat is extracted from the ground in order to evaporate a refrigerant. This vapour is heated strongly in a compressor and transfers its heat energy to the heating system, cooling and liquefying again in the process. The sorption heat pump uses thermal drive energy. It can be powered by gas, oil, waste heat and solar heat and is characterised by a high level of efficiency in terms of primary energy use. A distinction is made between two physical/chemical processes, those of absorption and adsorption. Absorption involves a liquid or gas being taken up by another liquid, whereas in adsorption the liquid is retained on the surface of a solid, as a function of pressure and temperature.
Cooling and heating with heat pumps
To enable active cooling, the functional principle of the heat pump is simply reversed. Cooling can be achieved through reversible operation. Heat pumps can use various sources of energy to provide heating or cooling:
• Geothermal heat: brine-to-water heat pumps,
• Heat stored in the groundwater: water-to-water heat pumps,
• Heat stored in the ambient air: air-to-water heat pumps.