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Geothermal energy is produced by converting Earth's inner heat into electricity or for heating. The generation of geothermal energy does not require fossil fuels and is a clean source of energy. Wells are drilled to access hot water at depth within Earth's crust. Heating occurs naturally because Earth's temperature rises with depth below the surface. The sources of this heat are energy left over from the formation of the planet and the decay of radioactive elements in Earth's deep interior. For continental crust, the average geothermal gradient is approximately 22.1° C per kilometer. This value is higher near to volcanoes, mid-ocean ridges, and hotspots. Near to magmatic intrusions, temperatures can exceed 600° C at relatively shallow depths (5-10 km). Oceanic crust is thinner than continental crust and has higher heat flow values. Accessible locations where the Earth is much hotter closer to the surface are ideal for the generation of geothermal energy.
Electricity produced from geothermal energy uses hot water or steam from Earth's interior to drive turbines and generate electricity. In geologically suitable regions, wells are drilled to depths of several kilometers into hot crustal rocks. Tectonic activity often breaks up the rock covering bodies of magma which allows water to pass through them and enhances their permeability. For this reason, hot springs and geysers occur, e.g. Old Faithful in Yellowstone National Park. Here, the waters can exceed 200° C (430° F). One way of tapping into this system is by collecting hot water and stream that rise naturally through the crust by "hydrothermal convection." This means that hot water rises because of its relatively lower density than the fluids that surround it. As it rises, it is replaced by cooler denser fluids. This is the fundamental mechanism in liquid hydrothermal systems.
Another way is to drill into "dry hot rocks" and then artificially create a "hot spring". This method involves drilling a bore hole to penetrate the hot body of rock. Following this, the rocks are hydraulically fractured by injecting water under great pressure to increase the permeability of the reservoir rocks. After this, a second well is drilled to allow the hot fluids to rise to the surface. Using an input well and an output well, a continuous supply of hot water passes through the system.
The sources of hydrothermal waters are often meteoric, i.e. they originate from the atmosphere. They feed into the ground water system and slowly percolate deep into the crust. Such waters are not always clean, and contain dissolved impurities and small particles. As a result, the steam is then cleaned to prevent damage and corrosion to the blades of turbines. The spent stream is then condensed back into water at the plant. The water is stored in a tank until it is redirected through drill holes back into the ground to recharge the hot rocks of the reservoir deep below the crust. Geothermal plants require a continuous supply of high volumes of hot water to produce electricity. Without water, plants "dry up". To alleviate such problems in Santa Rosa, California, 41.5 million liters of treated waste water flows through a 66 km pipeline to recharge the reservoir rocks.
Dry steam and wet steam are two common terms in hydrothermal systems. Wet steam occurs when boiling water contains both liquid and vapor. If all of the water is boiled away and only vapor exists, this is called dry steam. Because of the pressures involved, the temperature of underground steam can exceed the vaporization temperature of water, which is 100° C, and form superheated steam above 130° C.
There are three main types of geothermal plants. Dry steam plants use steam from the ground directly to turn the turbine. They do not require additional machinery to heat the water or other fluids. Because little natural steam is dry, this type of plant is rare. Flash steam power plants use water hotter than 150° C. Boiling geothermal water is collected in a reservoir where it flashes to steam under reduced pressures. The steam powers the turbines and is then condensed and returned to the reservoir. Binary steam power plants use hot geothermal water to heat another fluid which has a lower boiling point than water. This fluid vaporizes to steam and drives the turbines. The water from the ground never comes into contact with the turbine. Binary plants are the most common because they can utilize lower temperature geothermal fluids.
Direct heat is one of the oldest uses of geothermal energy. Such uses include bathing, space and district heating, agricultural applications, aquaculture and some industrial uses. In agriculture, geothermal fluids are used to heat greenhouses. This enables the cultivation of vegetables and flowers out-of-season, saving up to 35% of production costs. On farms, breeding animals in a temperature-controlled environment is beneficial to animal health. Sanitation and sterilization can make good use of hot fluids. In aquaculture, controlled breeding temperatures are an essential factor for maintaining yields. Other potential industrial uses include process heating, evaporation, drying, distillation, sterilization, washing, de-icing, and salt extraction.
A variety of techniques are used during the exploration phase for geothermal resources. For any potential site, an accurate geothermal model is needed to evaluate its scale, heat production, and expected lifetime. Many of the tools used are similar to those employed in the exploration for oil and gas.
In early stages, exploration involves field studies and the mapping of bedrocks and hydrology. Geologists read the structures in exposed rocks to understand the architecture and shape of the reservoir below. Rocks are studied under powerful microscopes to determine their porosity, which affects how water passes through them. Careful field studies, prior to the drilling of boreholes, saves money and time later in the project.
Geochemical studies are used to determine whether a geothermal system is water- or vapor-dominated. Samples of geothermal waters are taken from hot springs, geysers, and fumaroles. They provide information on the expected temperature at depth, the uniformity of the water supply, and the source of the water that naturally recharges the system. Studying isotopes helps scientist to find out about different behaviours and characteristics of hydrothermal waters at depth. Again, geochemical studies combined with geological and hydrological studies provide valuable insights before the use of more expensive geophysical methods.
Geophysical methods are employed either at the surface or down boreholes. Measurements include: temperature, electrical conductivity, magnetism, density, and response to seismic waves. Sensitive electrical measurements are very important for the detection of hydrothermal fluids. The application of these can often provide a good approximation of the temperature at the top of the reservoir. To keep costs down, geophysical methods must be selected very carefully. In the final phase of exploration, exploratory wells are selectively drilled in key places. The data from wells is used to test the models and hypothesis made earlier during the investigation.
More than 70 countries now have an interest in geothermal energy. In 2007, approximately 40 countries had projects either under development or were actively planning. According to the International Geothermal Energy Association, both the capacity and production of geothermal energy had increased by 20 percent between 2005 and 2010. As of early 2012, the online capacity in the 24 major producing countries was 11,224 MW and is expected to rise to 18,500 MW by 2015. For context, one market analyst has reported that global electricity consumption is likely to increase from 10,500-billion kilowatt hours in 1990 to over 30,100-billion kilowatt hours in 2030.
Some countries have a greater dependency on geothermal energy than others. Geologic factors play a large role. For example, Iceland has a unique geologic position and sits on top of the mid-Atlantic ridge. It is rich in geysers and hot springs. Geothermal surveys began 250 years ago and its oldest geothermal plant is more than a century old. Geothermal energy provides 66% of Iceland's energy supply. Hot geothermal waters are used to heat waters directly for central heating to over 35,000 homes in Reykjavik.
The United States is ranked number one in both the capacity and production of geothermal energy. In 2013, the United States had a capacity of 3,386 MW. This nearly equals the combined capacity of second-ranked Philippines (1,904 MW) and third-ranked Indonesia (1,222 MW). The development of geothermal resources in the United States has been stimulated by an increased focus on energy independence and clean energy production. Nine U.S. states produce geothermal energy: Alaska, California, Hawaii, Idaho, Nevada, New Mexico, Oregon, Utah, and Wyoming. New states that are developing installations include Colorado, Louisiana, Mississippi, and Texas. Geothermal energy contributes only 4% of the total energy requirement of the United States. This is partly due to the fact that geothermal resources are not evenly distributed.
California is located in the volcanically active region of the Pacific Rim. Here, magma is found at just 6 km below the surface in continental rocks. This geologic setting helps California to be the largest producer of geothermal energy in the United States. In 2013, California had 2,634 MW of installed capacity and more is currently under development. This is greater than any other country in the world. In a 110 km² region known as "The Geysers" lies the largest complex of geothermal power plants (about 40) in the world. In 2013, geothermal energy supplied 6.28% of California's energy needs. This is enough energy for about 725,000 homes, or a city about the size of San Francisco.
Ground source heat pumps are a smaller scale method for using geothermal energy to supply hot water. They access shallow depth geothermal heat in the first few meters beneath the surface. To do this, hundreds of meters of pipes are laid at shallow depths beneath the ground. Aquifers and soils vary in temperature from 5 to 30 ° C. Water is fed into the pipes where it continues to flow before arriving back at a storage tank a few degrees warmer than before it entered the series of pipes. These small-scale devises offset the amount of gas or electricity used to supply households with hot water. In summer, heat pumps can be used for space cooling by moving warm air from buildings into the relatively cooler ground. Ground source heat pumps are presently installed in more than 30 countries; mostly in the United States, but they are also popular in Canada and Western Europe.
The use of geothermal energy extends from the paleolithic era to the modern day. The earliest uses of hydrothermal waters were for warmth, healing, and cooking. Archeological evidence shows that hot springs have been used as the sources of settlements in North America for more than 10,000 years.
The industrial use of geothermal energy extends back nearly 190 years. Italy has had a long relationship with geothermal energy. In 1827, in Larderello, geothermal waters were brought to the surface through boreholes to help produce boric acid by evaporation. In 1904, Italian engineers used steam from hot springs to generate electricity from a turbine generator. In 1919, the first geothermal wells in Japan were drilled at Beppu. In 1928, Iceland began exploiting its geothermal fluids (mostly hot waters) for domestic heating purposes. By 1942, the capacity of its geothermal field reached 127,650 kW. New Zealand began its program in 1958 as did Mexico in 1960.
In the United States, in 1847, a surveyor came upon the geothermal springs in California which he named the Geysers (though there are no geysers there). In 1862, the region that what would later become the largest geothermal energy field in the world was used as a luxury spa. In 1892, in Boise, Idaho, 200 homes and 40 businesses were heated by water piped from hot springs. Today, this system warms about half-a-million square meters of property in the area. In 1921, a first attempt to generate electricity at the Geysers failed. Soon after, the first geothermal power station was successfully built in a neighboring valley. However, the plant could not compete with other sources of energy and shut down. In Boise, Idaho in 1930, a commercial greenhouse was heated by a 300 m well. Technological developments in groundwater heating pumps allowed heating of larger commercial buildings in Oregon. In 1960, a major breakthrough occurred with the first commercial geothermal electricity plant being developed at the Geysers. The first turbine lasted for more than 30 years and today the plant produces 11 MW of electricity.
By the mid 1970's, the Geothermal Energy Association had been formed and the National Science Foundation began federal geothermal programs. The Department of Energy began in 1977. In 1978, the first geothermal food processing plant for drying crops opened in Brady Hot Springs, Nevada. Over the following 20 years, significant developments in geothermal energy occurred in California, Nevada, Utah, and Hawaii. In 2000, the Department of Energy initiated the "Geopowering the West" project, where partnerships with industry were funded to develop new geothermal technologies with working groups in Nevada, Idaho, New Mexico, Washington and Oregon.
Geothermal power can be used directly as a source of heating, for cooking, and for health. Electricity can be produced cleanly without burning fossil fuels that release pollutants into the atmosphere. Geothermal plants are relatively small compared to power plants and the space needed for refineries. They are also relatively cheaper to build and operate. Geothermal energy is relatively inexpensive and approximately 80% cheaper than energy from fossil fuels. Geothermal plants are also self-sustaining and the power required to pump water from and into the reservoir rocks is generated geothermally. Suppliers do not need to pay for the transport of fuel from suppliers. Geothermal energy is not prone to price swings because it is not dependent on the global price of oil. Suppliers of geothermal power are often assisted by tax cuts by governments. Increased use of geothermal power reduces the reliance on fossil fuels. Thus, geothermal energy promotes energy independence from imports. Development of geothermal resources creates jobs both directly and through the provision of infrastructure.
Geothermal energy requires access to deep crustal rocks. The geologic conditions required for producing geothermal energy do not occur everywhere. Rocks at depth must be suitably fractured to enable water to pass through them. Also, the rocks must be hot enough and shallow enough to be accessed by drilling down from the surface. The source of heat has to be sustainable for the intended duration of the power plant. Although local volcanic activity is useful for producing hot crustal rocks, it is also a potential hazard to investment and development.
Geothermal plants do pose a threat of releasing gases from hydrothermal waters that rise from deep in the crust. Hydrogen sulfide is a toxic gas and detectable by an odor, similar to rotten eggs. Some geothermal fluids contain high concentrations of chemicals such as boron, fluoride or arsenic. Care must be taken to ensure they are treated and re-injected into the reservoir.
Small scale ground source heat pumps are initially expensive, which is prohibitive to many households. However, over many years the investment would be recovered. The area required for pipe systems may not work well for smaller developments.