Green Power Generation

Lecture 6

Geothermal

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• Geothermal energy is defined as heat from the Earth• Although areas with telltale signs like hot springs are

more obvious and are often the first places geothermal resources are used, the heat of the earth is available everywhere, and we are learning to use it in a broader diversity of circumstances

• It is considered a renewable resource because the heat emanating from the interior of the Earth is essentially limitless

• The heat continuously flowing from the Earth’s interior, which travels primarily by conduction, is estimated to be equivalent to 42 million megawatts (MW) of power, and is expected to remain so for billions of years to come, ensuring an inexhaustible supply of energy

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• A geothermal system requires heat, permeability, and water

• The heat from the Earth's core continuously flows outward

• Sometimes the heat, as magma, reaches the surface as lava, but it usually remains below the Earth's crust, heating nearby rock and water — sometimes to levels as hot as 700°F

• When water is heated by the earth’s heat, hot water or steam can be trapped in permeable and porous rocks under a layer of impermeable rock and a geothermal reservoir can form

• This hot geothermal water can manifest itself on the surface as hot springs or geysers, but most of it stays deep underground, trapped in cracks and porous rock

• This natural collection of hot water is called a geothermal reservoir.

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• Geothermal energy can be used for electricity production, for commercial, industrial, and residential direct heating purposes, and for efficient home heating and cooling through geothermal heat pumps.

Geothermal Electricity:

• To develop electricity from geothermal resources, wells are drilled into a geothermal reservoir

• The wells bring the geothermal water to the surface, where its heat energy is converted into electricity at a geothermal power plant

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Heating Uses:

•Geothermal heat is used directly, without involving a power plant or a heat pump, for a variety of applications such as space heating and cooling, food preparation, hot spring bathing and spas (balneology), agriculture, aquaculture, greenhouses, and industrial processes•Uses for heating and bathing are traced back to ancient Roman times•Currently, geothermal is used for direct heating purposes at sites across the United States•U.S. installed capacity of direct use systems totals 470 MW or enough to heat 40,000 average-sized houses

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• The Romans used geothermal water to treat eye and skin disease and, at Pompeii, to heat buildings

• Medieval wars were even fought over lands with hot springs

• The first known "health spa" was established in 1326 in Belgium at natural hot springs

• And for hundreds of years, Tuscany in Central Italy has produced vegetables in the winter from fields heated by natural steam

• A few examples of geothermal direct use applications today are at the Idaho Capitol Building in Boise Burgett Geothermal Greenhouses in Cotton City, New Mexico and Roosevelt Warm Springs Institute for Rehab in Warm Springs, Georgia

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Typical Direct Use Geothermal Heating System Configuration

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Geothermal Heat Pumps (GHPs):

• Geothermal heat pumps take advantage of the Earth’s relatively constant temperature at depths of about 10 ft to 300 ft

• GHPs can be used almost everywhere in the world, as they do not share the requirements of fractured rock and water as are needed for a conventional geothermal reservoir

• GHPs circulate water or other liquids through pipes buried in a continuous loop, either horizontally or vertically, under a landscaped area, parking lot, or any number of areas around the building

• The Environmental Protection Agency considers them to be one of the most efficient heating and cooling systems available.

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• Animals burrow underground for warmth in the winter and to escape the heat of the summer

• The same idea is applied to GHPs, which provide both heating and cooling solutions

• To supply heat, the system pulls heat from the Earth through the loop and distributes it through a conventional duct system

• For cooling, the process is reversed; the system extracts heat from the building and moves it back into the earth loop

• It can also direct the heat to a hot water tank, providing another advantage — free hot water. GHPs reduce electricity use 30–60% compared with traditional heating and cooling systems, because the electricity which powers them is used only to collect, concentrate, and deliver heat, not to produce it.

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How does a geothermal power plant work?

•There are four commercial types of geothermal power plants: •a. flash power plants•b. dry steam power plants•c. binary power plants•d. flash/binary combined power plants

•Flash Power Plant•Geothermally heated water under pressure is separated in a surface vessel (called a steam separator) into steam and hot water (called “brine” in the picture)•The steam is delivered to the turbine, and the turbine powers a generator•The liquid is injected back into the reservoir.

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Flash Power Plant Diagram

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b. Dry Steam Power Plant:

•Steam is produced directly from the geothermal reservoir to run the turbines that power the generator, and no separation is necessary because wells only produce steam

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The Geysers, CA, Dry Steam Plant

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c. Binary Power Plant: •Recent advances in geothermal technology have made possible the economic production of electricity from geothermal resources lower than 150°C (302°F)•Known as binary geothermal plants, the facilities that make this possible reduce geothermal energy’s already low emission rate to zero•Binary plants typically use an Organic Rankine Cycle system•The geothermal water (called “geothermal fluid” in the accompanying image) heats another liquid, such as isobutane or other organic fluids such as pentafluoropropane, which boils at a lower temperature than wate•The two liquids are kept completely separate through the use of a heat exchanger, which transfers the heat energy from the geothermal water to the working fluid•The secondary fluid expands into gaseous vapor•The force of the expanding vapor, like steam, turns the turbines that power the generators•All of the produced geothermal water is injected back into the reservoir.

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Binary Power Plant

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d. Flash/Binary Combined Cycle:

•This type of plant, which uses a combination of flash and binary technology, has been used effectively to take advantage of the benefits of both technologies•In this type of plant, the portion of the geothermal water which “flashes” to steam under reduced pressure is first converted to electricity with a backpressure steam turbine and the low-pressure steam exiting the backpressure turbine is condensed in a binary system•In addition to different power plant technologies in use today, additional applications and technologies continue to emerge:

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Flash/Binary Power Plant Diagram

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• Enhanced Geothermal Systems (EGS): • Although the deeper crust and interior of the Earth is

universally hot, it lacks two of the three ingredients required for a naturally occurring geothermal reservoir: water and interconnected open volume for water movement

• Producing electricity from this naturally occurring hot, but relatively dry rock requires enhancing the potential reservoir by fracturing, pumping water into and out of the hot rock, and directing the hot water to a geothermal power plant

• Research applications of this technology are being pursued in the U.S.,Australia, and Europe

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Mixed Working Fluid/ Kalina System:

•As of January 2009 the Kalina System was being used at two power plants•The first is a small demonstration power plant operated as part of Iceland's Husavik GeoHeat Project•The second plant to use the Kalina System is in Germany at the Unterhaching Power Station•The Kalina cycle uses an ammonia-water mixed working fluid for high efficiency•The Kalina cycle is only one of the possible mixed working fluid approaches to possibly achieving greater heat transfer efficiency and/or lower temperature production of power

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Distributed Generation:

•Geothermal applications can be sized and constructed at geographically remote sites in order to meet on-site electricity demands•Examples of remote geothermal power systems are at Chena Hot Springs in Alaska and at the Rocky Mountain Oil and Gas Testing Center (RMOTC) in Wyoming•In the first, the unit powers a remote resort, in the second the power supplies electricity to operate an oil field

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Supercritical Cycles :

•Supercritical fluids are at a temperature and pressure that can diffuse through solids•A supercritical fluid such as carbon dioxide can be pumped into an underground formation to fracture the rock, thus creating a reservoir for geothermal energy production and heat transport•The supercritical fluid used to form the reservoir can heat up and expand, and is then pumped out of the reservoir to transfer the heat to a surface power plant or other application

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Geothermal electricity

•Electricity generated from geothermal energy•Technologies in use include dry steam power plants, flash steam power plants and binary cycle power plants•Geothermal electricity generation is currently used in 24 countries[1] while geothermal heating is in use in 70 countries•Estimates of the electricity generating potential of geothermal energy vary from 35 to 2000 GW•Current worldwide installed capacity is 10,715 megawatts (MW), with the largest capacity in the United States (3,086 MW),[3] Philippines, and Indonesia•Geothermal power is considered to be sustainable because the heat extraction is small compared with the Earth's heat content•The emission intensity of existing geothermal electric plants is on average 122 kg of CO2 per megawatt-hour (MW·h) of electricity, about one-eighth of a conventional coal-fired plant

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• Global geothermal electric capacity. Upper red line is installed capacity; lower green line is realized production

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• In the 20th century, demand for electricity led to the consideration of geothermal power as a generating source

• Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904 in Lardrello, Italy

• It successfully lit four light bulbs• Later, in 1911, the world's first commercial

geothermal power plant was built there• Experimental generators were built in Beppu, Japan

and the Geysers, California, in the 1920s, but Italy was the world's only industrial producer of geothermal electricity until 1958

• In 1958, New Zealand became the second major industrial producer of geothermal electricity when its Wairakei station was commissioned

• Wairakei was the first plant to use flash steam technology

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• In 1960, Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States at The Geysers in California

• The original turbine lasted for more than 30 years and produced 11 MW net power

• The binary cycle power plant was first demonstrated in 1967 in Russia and later introduced to the USA in 1981,following the 1970s energy crisis and significant changes in regulatory policie

• This technology allows the use of much lower temperature resources than were previously recoverable

• In 2006, a binary cycle plant in Chena Hot Springs, Alaska, came on-line, producing electricity from a record low fluid temperature of 57°C (135°F)

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• Geothermal electric plants have until recently been built exclusively where high temperature geothermal resources are available near the surface

• The development of binary cycle power plants and improvements in drilling and extraction technology may enable enhanced geothermal systems over a much greater geographical range

• Demonstration projects are operational in Landau-Pfalz, Germany, and Soultz-sous-Forets France, while an earlier effort in Basel, Switzerland was shut down after it triggered earthquakes

• Other demonstration projects are under construction in Australia, the United Kingdom, and the United States

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• The thermal efficiency of geothermal electric plants is low, around 10-23%,because geothermal fluids are at a low temperature compared with steam from boilers

• By the laws of thermodynamics this low temperature limits the efficiency of heat engines in extracting useful energy during the generation of electricity

• Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses, timber mills, and district heating

• The efficiency of the system does not affect operational costs as it would for a coal or other fossil fuel plant, but it does factor into the viability of the plant

• In order to produce more energy than the pumps consume, electricity generation requires high temperature geothermal fields and specialized heat cycles

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• Because geothermal power does not rely on variable sources of energy, unlike, for example, wind or solar, its capacity factor can be quite large – up to 96% has been demonstrated

• The global average was 73% in 2005.

• Because geothermal power does not rely on variable sources of energy, unlike, for example, wind or solar, its capacity factor can be quite large – up to 96% has been demonstrated

• The global average was 73% in 2005.

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Capacity factor •The net capacity factor of a power plant is the ratio of the actual output of a power plant over a period of time and its potential output if it had operated at full nameplate capacity the entire time•To calculate the capacity factor, take the total amount of energy the plant produced during a period of time and divide by the amount of energy the plant would have produced at full capacity•Capacity factors vary greatly depending on the type of fuel that is used and the design of the plant•The capacity factor should not be confused with the availability factor, capacity credit (firm capacity) or with efficiency.

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Baseload power plant

•A base load power plant with a capacity of 1,000 megawatts (MW) might produce 648,000 megawatt-hours (MW·h) in a 30-day month•The number of megawatt-hours that would have been produced had the plant been operating at full capacity can be determined by multiplying the plant's maximum capacity by the number of hours in the time period•1,000 MW × 30 days × 24 hours/day is 720,000 MW·h•The capacity factor is determined by dividing the actual output with the maximum possible output. In this case, the capacity factor is 0.9 (90%)

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Base load power plant

•Baseload (also base load, or baseload demand) is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements•Baseload values typically vary from hour to hour in most commercial and industrial area

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Wind farm

•The Burton Would Wind Farmconsists of ten Enercon E70-E4 wind turbines @ 2 MW nameplate capacity for a total installed capacity of 20 MW• In 2008 the wind farm generated 43,416 MW·h of electricity. (Note 2008 was a leap year) •The capacity factor for this wind farm in 2008 was just under 25%:

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Baseload plant, (also baseload power plant or base load power station)

•Is an energy plant devoted to the production of baseload supply•Baseload plants are the production facilities used to meet some or all of a given region's continuous energy demand, and produce energy at a constant rate, usually at a low cost relative to other production facilities available to the system•Examples of baseload plants using nonrenewable fuels include nuclear and coal-fired plants•Among the renewable energy sources, hydroelectric, geothermal,

biogas, biomass, solar thermal with storage and ocean thermal energy conversion can provide baseload power•Baseload plants typically run at all times through the year except in the case of repairs or scheduled maintenance•Hydroelectric power also has the desirable attribute of dispatchability, but a hydroelectric plant may run low on its fuel (water at the reservoir elevation) if a long drought occurs over its drainage basin

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• Each baseload power plant on a grid is allotted a specific amount of the baseload power demand to handle

• The base load power is determined by the load duration curve of the system

• For a typical power system, the rule of thumb is that the base load power is usually 35-40% of the maximum load during the year

• Peaks or spikes in customer power demand are handled by smaller and more responsive types of power plants called peaking power plants, typically powered with gas turbines

• While historically large power grids have had base load power plant to exclusively meet the base load, there is no specific technical requirement for this to be so

• The baseload can equally well be met by the appropriate quantity of intermittent power sources and peaking power plant.

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Economics

•Power plants are designated baseload based on their low cost generation, efficiency and safety at rated output power levels.•Baseload power plants do not change production to match power consumption demands since it is more economical to operate them at constant production levels•Use of higher cost combined-cycle plants or combustion turbines is thus minimized, and these plants can be cycled up and down to match more rapid fluctuations in consumption.•Baseload generators, such as nuclear and coal, often have very high fixed costs, high plant load factor and very low marginal costs•On the other hand, peak load generators, such as natural gas, have low fixed costs, low plant load factor and high marginal costs•Typically baseload plants are large and provide a majority of the power used by a grid•Thus, they are more effective when used continuously to cover the power baseload required by the grid.

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Base load power plant usage

•Nuclear and coal power plants may take many hours, if not days, to achieve a steady state power output.•On the other hand, they have low fuel costs•Because they require a long period of time to heat up to operating temperature, these plants typically handle large amounts of baseload demand•Different plants and technologies may have differing capacities to increase or decrease output on demand: nuclear plants are generally run at close to peak output continuously (apart from maintenance, refueling and periodic refurbishment), while coal-fired plants may be cycled over the course of a day to meet demand•Plants with multiple generating units may be used as a group to improve the "fit" with demand, by operating each unit as close to peak efficiency as possible.

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• As of April 2011, the Danish wind farm Horns Rev 2

(the world's largest when it was inaugurated in September 2009 comprising 91 Siemens SWT-2.3-93 wind turbines each of 2.3 MW) with a nominal total capacity of 209 MW, has the best capacity factor at 46.7% having produced over 1.5 years 1,278 GW·h

• This corresponds to an average annual production of 0.85 TW·h (some two orders of magnitude less than the Three Gorges Dam example below of 79.47 TW·h for 2009).

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Hydroelectric dam

•As of 2010, Three Gorges Dam is the largest power generating station in the world by nameplate capacity•In 2009, not yet fully complete, it had 26 main generator units @ 700 MW and two auxiliary generator units @ 50 MW for a total installed capacity of 18,300 MW•Total generation in 2009 was 79.47 TW·h, for a capacity factor of just under 50%:

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• Hoover Dam has a nameplate capacity of 2080 MW and an annual generation averaging 4.2 TW·h• (The annual generation has varied between a high

of 10.348 TW·h in 1984, and a low of 2.648 TW·h in 1956)

• Taking the average figure for annual generation gives a capacity factor of:

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Reasons for reduced capacity factor

•There are several reasons why a plant would have a capacity factor lower than 100%•The first reason is that it was out of service or operating at reduced output for part of the time due to equipment failures or routine maintenance•This accounts for most of the unused capacity of base load power plants•Base load plants have the lowest costs per unit of electricity because they are designed for maximum efficiency and are operated continuously at high output•Geothermal plants, nuclear plants, coal plants and biorenergy plants that burn solid material are almost always operated as base load plants

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• The second reason that a plant would have a capacity factor lower than 100% is that output is curtailed because the electricity is not needed or because the price of electricity is too low to make production economical

• This accounts for most of the unused capacity of peaking power plants

• Peaking plants may operate for only a few hours per year or up to several hours per day

• Their electricity is relatively expensive• It is uneconomical, even wasteful, to make a peaking

power plant as efficient as a base load plant because they do not operate enough to pay for the extra equipment cost, and perhaps not enough to offset the embodied energy of the additional components

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• A third reason is a variation on the second: the operators of a hydroelectric dam may uprate its nameplate capacity by adding more generator units. Since the supply of fuel (i.e. water) remains unchanged, the uprated dam obtains a higher peak output in exchange for a lower capacity factor

• Because hydro plants are highly dispatchable, they are able to act as load following power plants

• Having a higher peak capacity allows a dam's operators to sell more of the annual output of electricity during the hours of highest electricity demand (and thus the highest spot price)

• In practical terms, uprating a dam allows it to balance a larger amount of intermittent energy sources on the grid such as wind farms and solar power plants, and to compensate for unscheduled shutdowns of baseload power plants, or brief surges in demand for electricity.

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Load following power plants

•Load following power plants, also called intermediate power plants, are in between these extremes in terms of capacity factor, efficiency and cost per unit of electricity•They produce most of their electricity during the day, when prices and demand are highest•However, the demand and price of electricity is far lower during the night and intermediate plants shutdown or reduce their output to low levels overnight

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Capacity factor and renewable energy

•When it comes to several renewable energy sources such as solar power, wind power and hydroelectricity, there is a third reason for unused capacity•The plant may be capable of producing electricity, but its fuel (wind, sunlight or water) may not be available•A hydroelectric plant's production may also be affected by requirements to keep the water level from getting too high or low and to provide water for fish downstream•However, solar, wind and hydroelectric plants do have high availability factors, so when they have fuel available, they are almost always able to produce electricity•When hydroelectric plants have water available, they are also useful for load following, because of their high dispatchability. A typical hydroelectric plant's operators can bring it from a stopped condition to full power in just a few minutes

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• Wind farms are variable, due to the natural variability of the wind

• For a wind farm, the capacity factor is mostly determined by the availability of wind

• Transmission line capacity and electricity demand also affect the capacity factor

• Solar energy is variable because of the daily rotation of the earth and because of cloud cover

• However, according to the SolarPACES program of the International Energy Agency (IEA), solar power plants designed for solar-only generation are well matched to summer noon peak loads in areas with significant cooling demands, such as Spain or the south-western United States., although solar PV does not reduce the need for generation of network upgrades given that air conditioner peak demand often occurs in the late afternoon or early evening when solar output is zero

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• SolarPACES states that by using thermal energy storage systems the operating periods of solar thermal power (CSP) stations can be extended to meet baseload needs

• The IEA CSP Technology Roadmap (2010) suggests that "in the sunniest countries, CSP can be expected to become a competitive source of bulk power in peak and intermediate loads by 2020, and of base-load power by 2025 to 2030“

• Geothermal has a higher capacity factor than many other power sources, and geothermal resources are available 24 hours a day, 7 days a week. While the carrier medium for geothermal electricity (water) must be properly managed, the source of geothermal energy, the Earth's heat, will be available for the foreseeable future

• Geothermal power can be looked at as a nuclear battery where the heat is produced via the decay of radioactive elements in the core and mantle of the earth.

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Typical capacity factors

•Wind farms 20-40%•Photovoltaic solar in Massachusetts 12-15%•Photovoltaic solar in Arizona 19range of 20% - 75% depending on water availability. •Nuclear energy 70% (1971-2009 average of USA's plants).•Nuclear energy 91.2% (2010 average of USA's plants)

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Resources

•Enhanced geothermal system•1:Reservoir•2:Pump house•3:Heat exchanger•4:Turbine hall•5:Production well•6:Injection well•7:Hot water to district heating•8:Porous sediments•9:Observation well•10:Crystalline bedrock

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• The earth’s heat content is 1031 joules• This heat naturally flows to the surface by

conduction at a rate of 44.2 terawatts, (TW,) and is replenished by radioactive decay at a rate of 30 TW

• These power rates are more than double humanity’s current energy consumption from primary sources, but most of this power is too diffuse (approximately 0.1 W/m2 on average) to be recoverable

• The Earth's crust effectively acts as a thick insulating blanket which must be pierced by fluid conduits (of magma, water or other) to release the heat underneath

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• Electricity generation requires high temperature resources that can only come from deep underground

• The heat must be carried to the surface by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation, oil wells, drilled water wells, or a combination of these

• This circulation sometimes exists naturally where the crust is thin: magma conduits bring heat close to the surface, and hot springs bring the heat to the surface

• If no hot spring is available, a well must be drilled into a hot aquifer

• Away from tectonic plate boundaries the geothermal gradient is 25-30°C per kilometer (km) of depth in most of the world, and wells would have to be several kilometers deep to permit electricity generation

• The quantity and quality of recoverable resources improves with drilling depth and proximity to tectonic plate boundaries

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• In ground that is hot but dry, or where water pressure is inadequate, injected fluid can stimulate production

• Developers bore two holes into a candidate site, and fracture the rock between them with explosives or high pressure water

• Then they pump water or liquefied carbon dioxide down one borehole, and it comes up the other borehole as a gas

• This approach is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America

• Much greater potential may be available from this approach than from conventional tapping of natural aquifers

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• Estimates of the electricity generating potential of geothermal energy vary from 35 to 2000 GW depending on the scale of investments

• This does not include non-electric heat recovered by co-generation, geothermal heat pumps and other direct use

• A 2006 report by MIT, that included the potential of enhanced geothermal systems, estimated that investing 1 billion US dollars in research and development over 15 years would allow the creation of 100 GW of electrical generating capacity by 2050 in the United States alone

• The MIT report estimated that over 200 zettajoules (1021 joules) (ZJ) would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements - sufficient to provide all the world's present energy needs for several millennia

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• At present, geothermal wells are rarely more than 3 kilometers (2 mi) deep

• Upper estimates of geothermal resources assume wells as deep as 10 kilometers (6 mi)

• Drilling at this depth is now possible in the petroleum industry, although it is an expensive process

• The deepest research well in the world, the Kola superdeep borehole, is 12 kilometres (7 mi) deep

• This record has recently been imitated by commercial oil wells, such as Exxon's Z-12 well in the Chayvo field, Sakhalin

• Wells drilled to depths greater than 4 kilometres (2 mi) generally incur drilling costs in the tens of millions of dollars

• The technological challenges are to drill wide bores at low cost and to break larger volumes of rock.

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• Geothermal power is considered to be sustainable because the heat extraction is small compared to the Earth's heat content, but extraction must still be monitored to avoid local depletion

• Although geothermal sites are capable of providing heat for many decades, individual wells may cool down or run out of water

• The three oldest sites, at Larderello, Wairakei, and the Geysers have all reduced production from their peaks

• It is not clear whether these plants extracted energy faster than it was replenished from greater depths, or whether the aquifers supplying them are being depleted

• If production is reduced, and water is reinjected, these wells could theoretically recover their full potential

• Such mitigation strategies have already been implemented at some sites

• The long-term sustainability of geothermal energy has been demonstrated at the Lardarello field in Italy since 1913, at the Wairakei field in New Zealand since 1958, and at The Geysers field in California since 1960

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Dry steam plant

• Power station types

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Flash steam plant

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• Geothermal power stations are not dissimilar to other steam turbine thermal power stations - heat from a fuel source (in geothermal's case, the earth's core) is used to heat water or another working fluid

• The working fluid is then used to turn a turbine, which in turn a generator to produce electricity. The fluid is then cooled and returned to the heat source

Dry steam power plants• Dry steam plants are the simplest and oldest design• They directly use geothermal steam of 150°C or more to turn

turbines.[2]

Flash steam power plants• Flash steam plants pull deep, high-pressure hot water into

lower-pressure tanks and use the resulting flashed steam to drive turbines

• They require fluid temperatures of at least 180°C, usually more. This is the most common type of plant in operation today

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Binary cycle power plants •Binary cycle power plants are the most recent development, and can accept fluid temperatures as low as 57°C•The moderately hot geothermal water is passed by a secondary fluid with a much lower boiling point than water•This causes the secondary fluid to flash to vapor, which then drives the turbines•This is the most common type of geothermal electricity plant being built today•Both Organic Rankine and Kalona cycles are used•The thermal efficiency of this type plant is typically about 10-13%.

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Environmental impact

•Fluids drawn from the deep earth carry a mixture of gases, notably carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4), and ammonia (NH3)•These pollutants contribute to global warming, acid rain, and noxious smells if released•Existing geothermal electric plants emit an average of 122 kg of CO2 per megawatt-hour (MW·h) of electricity, a small fraction of the emission intensity of conventional fossil fuel plants•Plants that experience high levels of acids and volatile chemicals are usually equipped with emission-control systems to reduce the exhaust•Geothermal plants could theoretically inject these gases back into the earth, as a form of carbon capture and storage.

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• In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic chemicals, such as mercury, arsenic, boron, antimony, and salt

• These chemicals come out of solution as the water cools, and can cause environmental damage if released

• The modern practice of injecting geothermal fluids back into the Earth to stimulate production has the side benefit of reducing this environmental risk

• Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei fieldin New Zealand

• Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing

• The project in Basel, Switzerland was suspended because more than 10,000 seismic events measuring up to 3.4 on the Richter Scale occurred over the first 6 days of water injection

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• Geothermal has minimal land and freshwater requirements

• Geothermal plants use 3.5 square kilometers per gigawatt of electrical production (not capacity) versus 32 and 12 square kilometres for coal facilities and wind farms respectively

• They use 20 liters of freshwater per MW·h versus over 1000 litres per MW·h for nuclear, coal, or oil

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Economics

•Geothermal power requires no fuel, and is therefore immune to fuel cost fluctuations, but capital costs tend to be high•Drilling accounts for over half the costs, and exploration of deep resources entails significant risks•A typical well doublet in Nevada can support 4.5 megawatt (MW) of electricity generation and costs about $10 million to drill, with a 20% failure rate•In total, electrical plant construction and well drilling cost about 2-5 million € per MW of electrical capacity, while the levelised energy cost is 0.04-0.10 € per kW·h•Enhanced geothermal systems tend to be on the high side of these ranges, with capital costs above $4 million per MW and levelized costs above $0.054 per kW·h in 2007•Geothermal power is highly scalable: a large geothermal plant can power entire cities while a smaller power plant can supply a rural village•Chevron Corporation is the world's largest private producer of geothermal electricity•The most developed geothermal field is the Geysers in California. In 2008, this field supported 15 plants, all owned by Calpine, with a total generating capacity of 725 MW