The heat is on – everywhere…
Geysers are the most visible and best known naturally occurring form of geothermal energy. These are holes in the ground from which columns of water heated underground to above boiling point by the earth’s heat are ejected violently out of the earth’s surface, together with steam. Much of the hot water is trapped in permeable and porous rocks under a layer of impermeable rock, so forming geothermal reservoirs.
Although these well-known phenomena can be observed in a few places of volcanic activity, such as Iceland (from where the name geyser originates) or in the Yellowstone National Park in the US, geothermal energy is present everywhere. Its potential is being harnessed increasingly in a growing number of countries for a wide range of applications, from heating buildings to producing electricity in power plants, and in CHP (combined heat and power) cogeneration.
…from the surface…
Geothermal energy, in the form of heat, has been used in different civilizations and regions since ancient times. As one example, the Romans tapped geothermal energy found near the earth’s surface to heat buildings and baths. This direct use of low and moderate temperature heat (in the range of below 90°C to 150°C) is found in applications such as space and district heating, agriculture, aquaculture, hot spring bathing and spas or greenhouses.
Indirect use of geothermal energy for heating and cooling of buildings is widespread. It does not necessarily require hot sources but often relies on the constant temperatures found close to the surface., During the cold season, heat from the ground is absorbed by fluids circulating in underground pipes and is extracted using heat pumps. The process can be reversed in the summer to help with cooling: heat is transferred back into the ground, using it as a heat sink.
Geothermal heat pump systems are highly efficient and outperform other forms of heating and cooling that rely on fossil fuels or electricity. Although using geothermal energy for heating and cooling is not connected to energy generation, it relates directly to the overall energy picture as it cuts the consumption of fossil fuels and of electricity, owing to its wide availability.
…to greater depth
Although geothermal energy has been used over centuries to heat buildings or water, its application in power generation is relatively recent. Its use is now expanding rapidly throughout the world although its thermal efficiency is relatively low.
To produce electricity from geothermal resources, wells are drilled into geothermal reservoirs to bring steam or hot water to the earth’s surface. Here the heat is converted into electricity at a geothermal power plant. There are three main types of such plants and another one that combines two technologies.
- Dry steam plants feed dry steam (i.e. steam without water droplets) directly into steam turbines which drive generators to produce electricity. Dry steam sources are relatively rare. The first geothermal power generator was tested at the Larderello dry steam field in Italy in 1904 and the world's first geothermal power plant was built there in 1911.
- Flash steam plants channel the hot water flow into a steam separator (or flash tank) that removes excess (waste) water. The drier steam that results is used to power a turbine. Low steam pressure and efficiency loss of some 50% caused by the dissipation of thermal energy in the flashing process mean these plants are less efficient than dry steam systems. Waste water (also called brine) is pumped back into the reservoir. Flash steam plants are the most numerous since the majority of reservoirs are hot water based, not steam based.
- Binary plants enable electricity to be produced cost-effectively from geothermal resources with a temperature lower than 150°C. In these plants, a heat exchange is coupled with geothermal water to heat another fluid with a lower boiling point temperature, such as ammonia or butane. The process produces vapour that is used to power a turbine. Although relatively inefficient (with energy losses of around 50%) it allows electricity generation from temperatures too low to power steam-driven and flash steam plants. Flash/binary combined cycle plants take advantage of both these types of technology.
One well advanced and highly promising technology for tapping geothermal energy is EGS (Enhanced Geothermal Systems). It consists of injecting water into "intensely hot rocks, buried thousands of feet below the surface, that lack the permeability or fluid saturation found in naturally occurring geothermal systems" to capture their thermal energy, according to the US Office of Energy Efficiency & Renewable Energy.
EGS employs techniques developed for enhanced oil and gas recovery (also known as fracking). However, its huge potential is currently constrained by economic and technical factors, including uncertainties about cost and the technologies employed, coupled with potential negative environmental impact – for instance, concerns about induced seismicity and landslides.
Global interest heating up
Geothermal energy production is expanding throughout the world, growing at a sustained annual rate of 4% to 5%, with 700 projects under development in 76 countries, including in developing countries, according to a GEA (Geothermal Energy Association) April 2014 report.
The GEA forecast that geothermal installed capacity would have reached 12 000 MW by the end of 2013, with an additional 11 766 MW of planned capacity either in the early stages of development or under construction.
Over the years, an important development has been the addition of CHP generators to geothermal networks in a number of countries.
Geothermal resources have been exploited in a CHP installation at Paratunka, Kamchatka, in Russia since the late 1960s. It has 680 kW of installed power generation capacity and uses waste heat for soil and greenhouse heating. In Iceland a cogeneration CHP scheme provides hot water to 9 cities and to Kevlaﬁk International Airport, whilst also generating electricity, providing 45 MWe and 150 MWth power and heat capacities respectively.
In countries such as Iceland, Germany, Italy and the US, geothermal energy production is well established, but projects are also under way in parts of East Africa and the Middle East, which have promising geothermal resources.
For instance, the GEA reports that Djibouti has made progress on its first geothermal power plant. It has plans to meet the country's energy needs (estimated at 50 MW) and intends to explore the possibility of exporting electricity to other countries in the longer term. Ethiopia, Kenya, Rwanda and Tanzania have similar plans.
The South East Pacific region also has a substantial volume of geothermal resource both exploited and under development, according to the GEA, "with 5 209 MW in the pipeline and 7 206 MW of resources identified for development". Indonesia and the Philippines are ranked second and third in the world, behind the US, for installed geothermal capacity. According to IGA, (International Geothermal Association, geothermal) resources provided around 17% of the Philippines’ electricity production in 2009, a share that keeps growing.
Power-hungry industries warming up to renewable energy
Many countries ramp up their electricity production from renewable sources, including from geothermal energy, to cut consumption of fossil fuels and emissions of greenhouse gases.
One country which produces 100% of its electricity from renewable sources, Iceland, sees this resource, provided mainly by hydropower and geothermal energy, as a major asset for enticing energy-intensive industries to relocate plants to the country.
Iceland produces five times more energy than it needs for domestic consumption, according to Landsvirkjun, the country's national power company. This spare capacity, coupled with low and stable energy tariffs, has attracted industries such as aluminium smelting (where energy, i.e. the cost of electricity, represents between 30% and 40% of production expenses) and metallurgical grade silicon metal production.
Data centres represent another rapidly developing energy-intensive sector and are moving into areas where cheap renewable energy and favourable climatic conditions can be found. Data hosting company Verne Global has set up a data centre in Iceland that uses 100% renewable energy. It claims that environmental cooling and intelligent design result in a reduction in cooling costs of at least 80%.
IEC standardization work is essential to the development and correct operation of geothermal energy systems, even if the technologies may not be as well developed as with other renewable energies.
For geothermal heating used in buildings and in other applications, heat pumps play a central role in transferring heat from the soil and pumping it to another area inside the building where it is heated or cooled over a circulating coil system and is then transferred on to provide hot water, heating or cooling (using a heat exchanger). International Standards for heat pumps are prepared by IEC SC (Subcommittee) 61D: Appliances for air-conditioning for household and similar purposes.
Steam turbines are central to electricity generation from geothermal sources. IEC TC (Technical Committee) 5: Steam turbines, created in 1927, prepares International Standards for these (see article on steam turbines in this e-tech).
IEC TC 2: Rotating machinery, prepares International Standards with regard to specifications for rotating electrical machines, a category that includes motors and generators. Work from many other IEC TCs and SCs involved in the preparation of International Standards for energy generation, transmission and distribution is also central to the development and proper operation of the geothermal energy power chain, just as it is for other energy sources.