Geothermal Electricity Production

Most power plants need steam to generate electricity. The steam rotates a turbine that activates a generator, which produces electricity. Many power plants still use fossil fuels to boil water for steam. Geothermal power plants, however, use steam produced from reservoirs of hot water found a couple of miles or more below the Earth's surface. There are three types of geothermal power plants:dry steam, flash steam, and binary cycle.

Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming, where there's a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers.

Flash steam power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.

Binary cycle power plants operate on water at lower temperatures of about 225°-360°F (107°-182°C). These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

Small-scale geothermal power plants (under 5 megawatts) have the potential for widespread application in rural areas, possibly even as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.

Low Temerature and Direct Geothermal Energy Use

According to the National Renewable Energy Laboratory, "The Earth houses a vast energy supply in the form of geothermal resources. Domestic resources are equivalent to a 30,000-year energy supply at our current rate for the United States! In fact, geothermal energy is used in all 50 U.S. states today. But geothermal energy has not reached its full potential as a clean, secure energy alternative because of issues with resources, technology, historically low natural gas prices, and public policies. These issues affect the economic competitiveness of geothermal energy."

Besides use of geothermal energy in power plants, hot water from geothermal resources can be used for a number of purposes (also see Figure 1) including:

• Heating buildings or districts (either individually or whole towns)
• Raising plants in greenhouses
• Drying crops
• Heating water at fish farms
• Industrial processes, such as pasteurizing milk

Geothermal direct use dates back thousands of years, when people began using hot springs for bathing, cooking food, and loosening feathers and skin from game. Today, hot springs are still used as spas, but there are now more sophisticated ways of using this geothermal resource.

In modern direct-use systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical system -- piping, a heat exchanger, and controls -- delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it on the surface. In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii.

The direct-use geothermal reservoirs have relatively low to moderate temperatures - 68° to 302°F (20° to 150°C)

Direct use of geothermal energy in homes and commercial operations is much less expensive than using traditional fuels. Savings can be as much as 80% over fossil fuels. Direct use is also very clean, producing only a small percentage (and in many cases none) of the air pollutants emitted by burning fossil fuels.

District and Space Heating

The primary uses of low-temperature geothermal resources are in district and space heating, greenhouses, and aquaculture facilities. A 1996 survey found that these applications were using nearly 5.8 billion megajoules of geothermal energy each year - the energy equivalent of nearly 1.6 million barrels of oil!

In the U.S., more than 120 operations, with hundreds of individual systems at some sites, are using geothermal energy for district and space heating. District systems distribute hydrothermal water from one or more geothermal wells through a series of pipes to several individual houses and buildings, or blocks of buildings. Space heating uses one well per structure. In both types, the geothermal production well and distribution piping replace the fossil-fuel-burning heat source of the traditional heating system.

Geothermal district heating systems can save consumers 30% to 50% of the cost of natural gas heating. The tremendous potential for district heating in the western U.S. was illustrated in a 1980s inventory which identified 1,277 geothermal sites within 5 miles of 373 cities in 8 states.

Greenhouse and Aquaculture Facilities

Greenhouses and aquaculture (fish farming) are the two primary uses of geothermal energy in the agribusiness industry. Thirty-eight greenhouses, many covering several acres, are raising vegetables, flowers, houseplants, and tree seedlings in 8 western states. Twenty-eight aquaculture operations are active in 10 states.

Most greenhouse operators estimate that using geothermal resources instead of traditional energy sources saves about 80% of fuel costs - about 5% to 8% of total operating costs. The relatively rural location of most geothermal resources also offers advantages, including clean air, few disease problems, clean water, a stable workforce, and, often, low taxes.

Industrial and Commercial Uses

Industrial applications include food dehydration, laundries, gold mining, milk pasteurizing, spas, and others. Dehydration, or the drying of vegetable and fruit products, is the most common industrial use of geothermal energy. The earliest commercial use of geothermal energy was for swimming pools and spas. In 1990, 218 resorts were using geothermal hot water.

Permits for Direct Use

Direct use projects are not regulated by the California Energy Commission, and usually fall under the jurisdiction of local government unless on federal lands or tribal lands. Depending on the type of project and the specifics of the resource such as temperature, flow and chemistry there may be variations in the permitting requirements. The local government most likely would require a Conditional Use Permit. Additional permits would depend on the type of use. If there are structures, local building permits may be needed. Balneology may require a public pool permit from a local health department, district heating systems may need permits for disposal (National Pollution Discharge Elimination Permit - NPDES) of the resource from the Regional Water Quality Control Board if there is not an injection well, etc. These permits can take just as long as two years in the case of the NPDES permit and are in addition to the CEQA or National Environmental Policy Act environmental review process needed.

Sources (all accessed 9/22/08):

• NREL: Learning - Geothermal Direct Use, http://www.nrel.gov/learning/re_geo_direct_use.html
• Geothermal Direct-Use Case Studies, Geothermal Direct-Use Case Studies, http://geoheat.oit.edu/casestudies.htm
• Direct Use of Geothermal Energy, http://www1.eere.energy.gov/geothermal/pdfs/directuse.pdf
• Geothermal Technologies Program: Direct Use of Geothermal Energy, http://www1.eere.energy.gov/geothermal/directuse.html
• Geothermal Direct Use - Geothermal Energy, http://www.renewableenergyworld.com/rea/tech/geodirectuse

Types of Geothermal Power Plants

All geothermal power plants use steam to turn large turbines, which run electrical generators. In the Geysers Geothermal area, dry steam from below ground is used directly in the steam turbines. In other areas of the state, super-hot water is "flashed" into steam within the power plant, and that steam turns the turbine.

 

Direct Dry Steam

Steam plants use hydrothermal fluids that are primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine. (Also eliminating the need to transport and store fuels!)

This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904. Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal electricity. These plants emit only excess steam and very minor amounts of gases.

Flash and Double Flash Cycle

Hydrothermal fluids above 360°F (182°C) can be used in flash plants to make electricity.

Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator.

If any liquid remains in the tank, it can be flashed again in a second tank (double flash) to extract even more energy.

Binary Cycle

Most geothermal areas contain moderate-temperature water (below 400°F). Energy is extracted from these fluids in binary-cycle power plants.

Hot geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines.

Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. Moderate-temperature water is by far the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.

Text and graphics from U.S. Department of Energy's Energy Efficiency and Renewable Energy Program.

Geothermal Heat Pump Basics

Geothermal heat pumps take advantage of the nearly constant temperature of the Earth to heat and cool buildings. The shallow ground, or the upper 10 feet of the Earth, maintains a temperature between 50° and 60°F (10°–16°C). This temperature is warmer than the air above it in the winter and cooler in the summer.

Geothermal heat pump systems consist of three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground.

Heat pumps work much like refrigerators, which make a cool place (the inside of the refrigerator) cooler by transferring heat to a relatively warm place (the surrounding room), making it warmer. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system, moving heat from the ground to the building's interior. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger, effectively moving the heat from indoors to the ground. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water.

Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution.

Geothermal Heat Pumps

The shallow ground, the upper 10 feet of the Earth, maintains a nearly constant temperature between 50° and 60°F (10°-16°C). Like a cave, this ground temperature is warmer than the air above it in the winter and cooler than the air in the summer. Geothermal heat pumps take advantage of this resource to heat and cool buildings.

Geothermal heat pump systems consist of basically three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is basically a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground.

In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water.

Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution.

All areas of the United States have nearly constant shallow-ground temperatures, which are suitable for geothermal heat pumps.

Extreme Spring-Time Energy Savings with Geothermal Pool Heating

Spring is here, and the cooling season is quickly approaching. Pools that have been out of commission after our very cold winter are likely to stay that way unless we turn the heat on.  That can get expensive.

Stop for a moment and think of how many blow dryers, computers, cooking appliances, lights and people there are in your home that add to the cooling load.  These are all a potential source of energy that can provide domestic hot water and be used to heat spas and pools. With standard air source heat pumps or air conditioners, the heat generated inside your home typically goes out the return air ductwork and ultimately is exhausted through outside air exchange.

Like geothermal sourced cooling and heating systems, geothermal pool heat pumps bring your swimming pool and potable water heating in harmony with nature, while providing high energy efficiency. They do that by working together with the stable earth temperatures to provide heating for your pool throughout your desired swimming season.

With just a little twist to a home’s geothermal heating and cooling system, the waste heat from appliances and devices can be channeled into usable heat for domestic hot water needs, swimming pools, and spas.

There are a few different ways that a pool is normally heated; Fossil fuel, electric resistance, solar, or heat pump (either “air sourced” or “geothermal sourced”).

Solar-thermal is the most energy efficient and renewable source for potable water and pool heating, but solar is dependent upon the cooperation of the weather. Cloudy and cool days can mean a cold pool necessitating the need for backup heating sources during those times.

Fossil fuel heating of potable water, pools and spas is an old favorite. First cost is relatively low, but that comes at a higher price environmentally and monetarily as you move forward.  In addition to high costs for propane and other fuels, there are safety issues when fossil fuels are used as in this unfortunate story of carbon monoxide poisoning to guests at a hotel from a pool heater, March 21, in the LA Times.

Electric resistance heating uses raw electricity to warm heating elements over which the water passes, providing a clean and safe water heating alternative, but it can be extremely expensive. Using the Coefficient of Performance (COP) rating system (used internationally) for heating equipment, electric heating has a COP of 1.0, meaning that 1 unit of heat is provided for each unit of electricity, a one-to-one ratio, or 100% efficient in the COP rating system.

Air source heat pumps designed for pool and potable water heating are environmentally friendly and use outside air, pumping heat out of the ambient air into your pool or hot water tank. However, they too rely somewhat on cooperative weather conditions, that is; air temperatures being warm enough to facilitate extraction of heat to transfer to potable water and/or pool heating needs.  Air source heat pump efficiencies are in the 3.0 COP ranges (300% efficient).

For swimming pool and spa heating, the best scenario is attained with a geothermal sourced water to water heat pumps, pulling heat from a dependable, steady and renewable energy source; the earth.  Geothermalheat pumps can be about 5.0 COP (500% efficient).

Outside, temperatures fluctuate with the changing seasons, but underground temperatures don't change nearly as dramatically, thanks to the mass of the earth. Just 4 to 6 feet below the ground, the temperature remains relatively constant year round (about 45F to 75F in the US). A geothermal sourced water to water heat pump, which  can work in tandem with your geothermal HVAC system, typically consists of water sourced heat pump and a buried system of pipes called an earth loop, and/or a pump to reinjection (Class V thermal exchange process illustrated here) well. This geothermal source can be shared between the building’s HVAC and water heating systems.

Think of it like this: While providing power to run your home’s cooling system, you are also paying for the energy to run computers, lights, toasters, and home entertainment items like your big flat-screen TV. Your home’s cooling system must use power to remove the heat created by all of these internal gains on top of the occupant loads (one occupant presents a load of 1200 BTU’s each hour, or 10% of 1 ton of air). You pay for energy twice to remove this waste heat through the process of cooling your building. Why not channel that heat somewhere else where it’s needed? There is a way, and it’s easy with a geothermal sourced building:

Among the benefits that you can get from a geothermal HVAC system is the ability to channel and use this waste heat energy.  That’s because unlike the widely used air sourced cooling equipment (those that have an outside condenser that discharge waste heat), geothermal systems discharge the heat through a discharge water line.  Most manufacturers of geothermal heat pumps even have a factory installed hot water generator available.  This option gives you two extra connections labeled DHW (Domestic Hot Water) “In” and “Out” that may be connected to most any hot water tank, and the geothermal heat pump can turn waste heat into usable hot water.

There are thousands of geothermal heated pools around in the US.  There is a good chance that the local YMCA, hotel, health club or community pool near you already has geothermal sourced pool heating, because they maintain heated pools year round.  They know geothermal is dependable and ultra-efficient.   Surprisingly, many of these still have air sourced cooling systems that could be converted to geothermal (and likely will be) during the normal course of HVAC equipment attrition and upgrade.

Stop paying two and three times to move energy, and share the loads in your home; enjoy true thermal load sharing with a geothermal HVAC system.

Geothermal energy is classed as renewable energy. Renewable energy is generally described as energy obtained from sources that are essentially inexhaustible, in contrast to fossil fuels, of which there is a finite supply.

The term geothermal energy is often used to indicate that part of the earth's heat that may be recovered and utilised. Heat transferred from the earth's molten core to underground deposits of dry steam (steam with no water droplets), wet steam (a mixture of steam and water droplets), hot water, or rocks lying fairly close to the earth's surface. It is also generated locally within the earth's crust from the natural decay of the radiogenic elements that occur in rocks, and in certain granites they can be concentrated such that there is a marked elevation in the local surface heat flow.

GEOTHERMAL SYSTEMS

The characteristics of geothermal systems vary widely, but three components are essential:

• a subsurface heat source
• fluid to transport the heat
• faults, fractures or permeability within sub-surface rocks that allow the heated fluid to flow from the heat source to the surface

The amount of heat being generated by the earth (heat flow) is one of the key factors that determine the temperature gradient at any location. The other major factor is the thermal conductivity of the crustal rocks, which controls how well they trap the generated heat. High heat flow will result in a higher temperature gradient, while an insulating blanket of sedimentary rocks over the heat source will trap that heat. Some rocks make better insulators than others, but in general, fine grained sedimentary rocks such as shale are better insulators than sandstone. The highest thermal gradients are therefore found in regions with both high heat flow and low thermal conductivity.

THE BENEFITS OF GEOTHERMAL ENERGY

When properly developed and managed, geothermal systems are a clean, abundant, and reliable source of renewable energy. Use of geothermal energy for electricity generation or for direct use conserves non-renewable and more polluting resources. It is uniquely reliable, with conventional geothermal energy plants typically achieving much higher load factors compared to typical load factors for hydro and wind power plants. Geothermal energy is effectively a renewable resource that does not consume any fuel or produce significant carbon dioxide emissions.

Geothermal power sheds its hot-spring roots

GEOTHERMAL energy, long a poor relation among the more glamorous renewable technologies of wind and solar power, is poised to smarten its dowdy image. Piping-hot underground water and steam, percolating up through fissures in rocks fractured by seismic activity, have been a welcome feature of the European landscape since the Romans popularised bathing. On the other side of the globe, the Japanese have luxuriated since the Heian era in hot-spring onsen that dot their volcanic and quake-strewn archipelago. Even so, as a source of renewable energy, geothermal electricity has gone largely ignored as fortunes have been heaped on its rivals.

Today, a handful of countries that sit astride seismic belts or have active volcanoes in their midst, such as Iceland, the Philippines, Costa Rica and New Zealand, get a significant proportion of their heat and electricity from geothermal sources. America actually has more geothermal generating capacity (3.4 gigawatts) than any other country. But because of its huge resources of coal and natural gas, along with heavy investments in hydroelectricity and nuclear power, geothermal juice contributes a mere 0.4% of its electrical output.

There is, however, much to like about geothermal energy. It is reasonably clean; leaves behind little in the way of waste; does not suffer the vagaries of the weather or the inevitability of sunset; makes the tiniest of footprints on the land; and is pretty well inexhaustible. Above all, it is more or less free for the taking. Yet, lacking the political clout of wind and solar power, geothermal electricity has never received the attention it deserves.

Over the past five years, for example, the American government has spent close on $150 billion on clean energy, through a combination of grants and tax credits. Half went on handouts for electrical vehicles, advanced batteries, high-speed rail, electricity distribution, nuclear power and new fossil-fuel technologies. A further sixth was spent on subsidising biofuels. Of the remaining third, devoted to various forms of renewable electricity, wind and solar took the lion’s share. The Department of Energy’s 2014 budget for solar research, for instance, amounts to $257m, while geothermal’s is a modest $45m. Overall, geothermal has received around a thirtieth of the federal support—in terms of research grants, matching funds and tax credits—that has been handed out to wind and solar.

Whether such parsimony has truly hindered innovation in geothermal engineering is hard to say, for in a sense it is two different industries. In volcanic areas the heat comes to you. It is just a question of corralling it and using it—not a matter that needs state subsidy. Elsewhere, though, you have to dig deep to get at useful amounts of heat, and it is certainly true that exploration and drilling costs have remained stubbornly high for the deeper wells needed outside hot-spring regions, and that developers have been slow to devise better ways of extracting heat from such rocks, even if wells are sunk. Here, a little financial lubrication for research might pay dividends.

One important advance has been made—or, rather, borrowed from the oil and gas industry. This is the use of hydraulic fracturing ("fracking"), in which, in the case of oil or gas, water is injected into rocks whose hydrocarbons are too tightly bound to the rocky matrix to gush to the surface of their own accord. The high-pressure water shatters the matrix, releasing the bound payload.

Fracking is the technique behind the new “enhanced geothermal systems” that extract energy from rocks which are hot enough, but too dry, to produce steam. In such cases, developers bore two wells several kilometres down to the basement rocks and fracture the matrix between them with either high-pressure water or explosives. Water is then pumped down one of the boreholes and rises, heated, up the other. The pressure drops as the boiling water approaches the surface, causing it to flash into steam. This steam is used to drive turbines for generating electricity.

A report prepared several years ago by scientists at Massachusetts Institute of Technology (MIT), which examined the potential for enhanced geothermal systems, reckoned $1 billion spent over 15 years on research and development could lead to 100 gigawatts of geothermal generating capacity being established by 2050 in the United States alone. Worldwide, the amount of geothermal energy that might be extracted this way could exceed 200 zettajoules (ie, over 50 million-billion kilowatt-hours). With further refinement, the MIT researchers estimated that ten times more geothermal energy could be made available—enough to meet the world’s current needs for several thousand years.

The reason enhanced geothermal wells have to be deep is that the thermal efficiency (and thus the profitability) of geothermal generation is particularly sensitive to the temperature of the water brought to the surface. That temperature needs to be 150ºC or more to produce steam powerful enough to drive electrical turbines. Away from places where tectonic plates abut, the temperature of the underlying rocks increases by roughly 25º-30ºC per kilometre (23º-26ºF per 1,000 feet) of depth. This means that to get water hot enough to raise steam, you have to drill down several kilometres.

Short of sinking wells to unprecedented depths (the deepest so far is 12.3km), the water temperature is unlikely to be high enough to produce the quality of steam found in, say, a boiler heated by fossil fuel. At best, the thermal efficiency of geothermal power generation is around 23%—about half that of a coal-fired power station.

This does not mean geothermal electricity is uncompetitive. The capital costs of geothermal plants are high—as much as $2m-7m per megawatt of capacity. But with the “fuel” being essentially free and maintenance and environmental problems minimal, operating costs are particularly low. Typically, geothermal generating plants produce a kilowatt-hour of electricity for around five cents (the same as coal), compared with eight cents for wind and 13 cents for solar.

And unlike wind or solar, geothermal generating stations can run day and night, year in and year out. Their average capacity factor (a measure of the amount of electricity produced compared with the capacity installed) is 73%, though some operate as high as 96%. The average capacity factor for solar-generating arrays is no more than 12%, while wind farms manage around 23%. In many ways, geothermal plants are similar to nuclear-power stations (with a capacity factor of 90%), albeit on a smaller scale and without the radiation or waste-disposal problems.

Unfortunately, these apparent advantages have turned into an incubus. What the geothermal industry needs, more than any subsidy, is to change the message it gives out. Until recently, it has boasted that, unlike other renewables, such as wind or solar, it is a base-load resource similar to coal, gas or even nuclear, but without greenhouse gases or radiation fears. Such a claim, far from being a virtue, has become something of a curse. The problem, as Dave Olsen of the California Independent System Operator Corporation sees it, is that utilities are hobbled by the inflexibility of their base-load generating stations.

Ironically, when Southern California Edison was forced last year to retire its San Onofre nuclear power station that served the greater San Diego area, the local grid became more stable rather than less so, despite losing its biggest chunk of steady, base-load capacity (see “Too hot to handle” June 17th 2013). Meanwhile, the amount of renewable power delivered to households increased.

What this incident revealed was that, as more and more wind and solar power are added to the grid, many utilities are facing a severe over-supply of electricity during the middle of the day. In 2013, California had to dump over 19 gigawatt-hours of pre-purchased renewable energy, because it could not throttle its inflexible base-load supplies sufficiently. As more renewables are mandated into existence (California plans to get a third of its electricity from renewable sources by 2020), the base-load problem can only get worse.

Recently, Mr Olsen told a meeting of the Geothermal Resources Council that the last thing they should be promoting their product as is a carbon-free alternative for base-load power. What the utilities are crying out for is more flexible power that can be ramped up  quickly. Rather than be seen as part of the problem, geothermal needs to present itself as a cheaper, cleaner, more reliable and efficient form of auxiliary power that can provide utilities with the flexibility they urgently seek.

It is premature to declare, as some have, that base-load power is dead. It still provides the cheapest electricity—and will continue to do so for decades to come. But the virtue of geothermal electricity is that it can provide base-load power, flexible power or anything in between. In other words, it can be a base-load producer that runs all the time, a "load-follower" that operates during the day and into the early evening, or even a "peaking power" plant that ramps up quickly to meet sudden spikes in demand. If the geothermal industry manages to get that message across, the days of wind and solar could well be numbered.

Disadvantages Of Geothermal Energy

Energy created from geothermal power is safe, clean, simple, reliable and environment friendly as it is extracted from deep within the earths surface. But despite these advantages, geothermal energy is not being used widely. Geothermal energy suffers from its disadvantages as described below.

Geothermal Energy Disadvantage

1. Not Widespread Source of Energy : Since this type of energy is not widely used therefore the unavailability of equipment, staff, infrastructure, training pose hindrance to the installation of geothermal plants across the globe. Not enough skilled manpower and availability of suitable build location pose serious problem in adopting geothermal energy globally.

2. High Installation Costs : To get geothermal energy, requires installation of power plants, to get steam from deep within the earth and this r

equire huge one time investment and require to hire a certified installer and skilled staff needs to be recruited and relocated to plant location. Moreover, electricity towers, stations need to set up to move the power from geothermal plant to consumer.

3. Can Run Out Of Steam : Geothermal sites can run out of steam over a period of time due to drop in temperature or if too much water is injected to cool the rocks and this may result huge loss for the companies which have invested heavily in these plants. Due to this factor, companies have to do extensive initial research before setting up the plant.

4. Suited To Particular Region : It is only suitable for regions which have hot rocks below the earth and can produce steam over a long period of time. For this great research is required which is done by the companies before setting up the plant and this initial cost runs up the bill in setting up the geothermal power plant. Some of these regions are near hilly areas or high up in mountains.

5. May Release Harmful Gases : Geothermal sites may contain some poisonous gases and they can escape deep within the earth, through the holes drilled by the constructors. The geothermal plant must therefore be capable enough to contain these harmful and toxic gases.

6. Transportation : Geothermal Energy can not be easily transported. Once the tapped energy is extracted, it can be only used in the surrounding areas. Other sources of energy like wood, coal or oil can be transported to residential areas but this is not a case with geothermal energy. Also, there is a fear of toxic substances getting released into the atmosphere.

Geothermal energy

Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. The geothermal energy of the Earth's crust originates from the original formation of the planet (20%) and from radioactive decay of minerals (80%). The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface. 

              Steam rising from the Nesjavellir Geothermal Power Station in Iceland.

Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core–mantle boundary may reach over 4000 °C (7,200 °F). The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of mantle convecting upward since it is lighter than the surrounding rock. Rock and water is heated in the crust, sometimes up to 370 °C (700 °F).

From hot springs, geothermal energy has been used for bathing since Paleolithic times and for space heating since ancient Roman times, but it is now better known for electricity generation. Worldwide, 11,700 megawatts (MW) of geothermal power is online in 2013. An additional 28 gigawatts of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications in 2010.

Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications such as home heating, opening a potential for widespread exploitation. Geothermal wells release greenhouse gases trapped deep within the earth, but these emissions are much lower per energy unit than those of fossil fuels. As a result, geothermal power has the potential to help mitigate global warming if widely deployed in place of fossil fuels.

The Earth's geothermal resources are theoretically more than adequate to supply humanity's energy needs, but only a very small fraction may be profitably exploited. Drilling and exploration for deep resources is very expensive. Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, subsidies, and interest rates. Pilot programs like EWEB's customer opt in Green Power Program  show that customers would be willing to pay a little more for a renewable energy source like geothermal. But as a result of government assisted research and industry experience, the cost of generating geothermal power has decreased by 25% over the past two decades. In 2001, geothermal energy cost between two and ten US cents per kWh.

Geothermal Electricity:

The International Geothermal Association (IGA) has reported that 10,715 megawatts (MW) of geothermal power in 24 countries is online, which was expected to generate 67,246 GWh of electricity in 2010. This represents a 20% increase in online capacity since 2005. IGA projects growth to 18,500 MW by 2015, due to the projects presently under consideration, often in areas previously assumed to have little exploitable resource.

In 2010, the United States led the world in geothermal electricity production with 3,086 MW of installed capacity from 77 power plants. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California. The Philippines is the second highest producer, with 1,904 MW of capacity online. Geothermal power makes up approximately 27% of Philippine electricity generation.

Installed geothermal electric capacity

Country	Capacity (MW) 2007	Capacity (MW) 2010	Percentage of national electricity production	Percentage of global geothermal production
United States	2687	3086	0.3	29
Philippines	1969.7	1904	27	18
Indonesia	992	1197	3.7	11
Mexico	953	958	3	9
Italy	810.5	843	1.5	8
New Zealand	471.6	628	10	6
Iceland	421.2	575	30	5
Japan	535.2	536	0.1	5
Iran	250	250
El Salvador	204.2	204	25
Kenya	128.8	167	11.2
Costa Rica	162.5	166	14
Nicaragua	87.4	88	10
Russia	79	82
Turkey	38	82
Papua-New Guinea	56	56
Guatemala	53	52
Portugal	23	29
China	27.8	24
France	14.7	16
Ethiopia	7.3	7.3
Germany	8.4	6.6
Austria	1.1	1.4
Australia	0.2	1.1
Thailand	0.3	0.3
TOTAL	9,981.9	10,959.7

Geothermal electric plants were traditionally built exclusively on the edges of tectonic plates where high temperature geothermal resources are available near the surface. The development of binary cycle power plants and improvements in drilling and extraction technology enable enhanced geothermal systems over a much greater geographical range. Demonstration projects are operational in Landau-Pfalz, Germany, and Soultz-sous-Forêts, 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 of America.

The thermal efficiency of geothermal electric plants is low, around 10–23%, because geothermal fluids do not reach the high temperatures of steam from boilers. The laws of thermodynamics limits the efficiency of heat engines in extracting useful energy. Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses, timber mills, and district heating. System efficiency does not materially affect operational costs as it would for plants that use fuel, but it does affect return on the capital used to build the plant. In order to produce more energy than the pumps consume, electricity generation requires relatively hot fields and specialized heat cycles. 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.

Types

Geothermal energy comes in either vapor-dominated or liquid-dominated forms. Larderello and The Geysers are vapor-dominated. Vapor-dominated sites offer temperatures from 240-300 C that produce superheated steam.

Liquid-dominated plants

Liquid-dominated reservoirs (LDRs) were more common with temperatures greater than 200 °C (392 °F) and are found near young volcanoes surrounding the Pacific Ocean and in rift zones and hot spots. Flash plants are the common way to generate electricity from these sources. Pumps are generally not required, powered instead when the water turns to steam. Most wells generate 2-10MWe. Steam is separated from liquid via cyclone separators, while the liquid is returned to the reservoir for reheating/reuse. As of 2013, the largest liquid system is Cerro Prieto in Mexico, which generates 750 MWe from temperatures reaching 350 °C (662 °F). The Salton Sea field in Southern California offers the potential of generating 2000 MWe.

Lower temperature LDRs (120-200 C) require pumping. They are common in extensional terrains, where heating takes place via deep circulation along faults, such as in the Western US and Turkey. Water passes through a heat exchanger in a Rankine cycle binary plant. The water vaporizes an organic working fluid that drives a turbine. These binary plants originated in the Soviet Union in the late 1960s and predominate in new US plants. Binary plants have no emissions.

Thermal energy

Lower temperature sources produce the energy equivalent of 100M BBL per year. Sources with temperatures from 30-150 C are used without conversion to electricity for as district heating, greenhouses, fisheries, mineral recovery, industrial process heating and bathing in 75 countries. Heat pumps extract energy from shallow sources at 10-20 C in 43 countries for use in space heating and cooling. Home heating is the fastest-growing means of exploiting geothermal energy, with global annual growth rate of 30% in 2005 and 20% in 2012.

Approximately 270 petajoules (PJ) of geothermal heating was used in 2004. More than half went for space heating, and another third for heated pools. The remainder supported industrial and agricultural applications. Global installed capacity was 28 GW, but capacity factors tend to be low (30% on average) since heat is mostly needed in winter. Some 88 PJ for space heating was extracted by an estimated 1.3 million geothermal heat pumps with a total capacity of 15 GW.

Heat for these purposes may also be extracted from co-generation at a geothermal electrical plant.

Heating is cost-effective at many more sites than electricity generation. At natural hot springs or geysers, water can be piped directly into radiators. In hot, dry ground, earth tubes or downhole heat exchangers can collect the heat. However, even in areas where the ground is colder than room temperature, heat can often be extracted with a geothermal heat pump more cost-effectively and cleanly than by conventional furnaces. These devices draw on much shallower and colder resources than traditional geothermal techniques. They frequently combine functions, including air conditioning, seasonal thermal energy storage, solar energy collection, and electric heating. Heat pumps can be used for space heating essentially anywhere.

Iceland is the world leader in direct applications. Some 92.5% of its homes are heated with geothermal energy, saving Iceland over $100 million annually in avoided oil imports. Reykjavík, Iceland has the world's biggest district heating system. Once known as the most polluted city in the world, it is now one of the cleanest.

Enhanced geothermal

Enhanced geothermal systems (EGS) actively inject water into wells to be heated and pumped back out. The water is injected under high pressure to expand existing rock fissures to enable the water to freely flow in and out. The technique was adapted from oil and gas extraction techniques. However, the geologic formations are deeper and no toxic chemicals are used, reducing the possibility of environmental damage. Drillers can employ directional drilling to expand the size of the reservoir.

Small-scale EGS have been installed in the Rhine Graben at Soultz-sou-Forects in France and at Landau and Insheim in Germany.

Economics

Geothermal power requires no fuel (except for pumps), and is therefore immune to fuel cost fluctuations. However, capital costs are significant. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet (extraction and injection wells) in Nevada can support 4.5megawatts (MW) and costs about $10 million to drill, with a 20% failure rate.

Geothermal power is highly scalable: from a rural village to an entire city.In total, electrical plant construction and well drilling cost about €2–5 million  per MW of electrical capacity, while the break–even price 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 break–even above $0.054 per kW·h in 2007. Direct heating applications can use much shallower wells with lower temperatures, so smaller systems with lower costs and risks are feasible. Residential geothermal heat pumps with a capacity of 10 kilowatt (kW) are routinely installed for around $1–3,000 per kilowatt. District heating systems may benefit from economies of scale if demand is geographically dense, as in cities and greenhouses, but otherwise piping installation dominates capital costs. The capital cost of one such district heating system in Bavaria was estimated at somewhat over 1 million € per MW. Direct systems of any size are much simpler than electric generators and have lower maintenance costs per kW·h, but they must consume electricity to run pumps and compressors. Some governments subsidize geothermal projects.

The most developed geothermal field in the United States is The Geysers in Northern California.

Geothermal projects have several stages of development. Each phase has associated risks. At the early stages of reconnaissance and geophysical surveys, many projects are cancelled, making that phase unsuitable for traditional lending. Projects moving forward from the identification, exploration and exploratory drilling often trade equity for financing.

Resources

The Earth's internal thermal energy flows to the surface by conduction at a rate of 44.2 terawatts (TW), and is replenished by radioactive decay of minerals at a rate of 30 TW. These power rates are more than double humanity’s current energy consumption from all primary sources, but most of this energy flow is not recoverable. In addition to the internal heat flows, the top layer of the surface to a depth of 10 meters (33 ft) is heated by solar energy during the summer, and releases that energy and cools during the winter.

Outside of the seasonal variations, the geothermal gradient of temperatures through the crust is 25–30 °C (77–86 °F) per kilometer of depth in most of the world. The conductive heat flux averages 0.1 MW/km². These values are much higher near tectonic plate boundaries where the crust is thinner. They may be further augmented by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation or a combination of these.

A geothermal heat pump can extract enough heat from shallow ground anywhere in the world to provide home heating, but industrial applications need the higher temperatures of deep resources. The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. The more demanding applications receive the greatest benefit from a high natural heat flux, ideally from using a hot spring. The next best option is to drill a well into a hot aquifer. If no adequate aquifer is available, an artificial one may be built by injecting water to hydraulically fracture the bedrock. This last 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.

Estimates of the potential for electricity generation from geothermal energy vary sixfold, from .035to2TW depending on the scale of investments. Upper estimates of geothermal resources assume enhanced geothermal wells as deep as 10 kilometres (6 mi), whereas existing geothermal wells are rarely more than 3 kilometres (2 mi) deep. Wells of this depth are now common in the petroleum industry. 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.

Production

According to the Geothermal Energy Association (GEA) installed geothermal capacity in the United States grew by 5%, or 147.05 MW, since the last annual survey in March 2012. This increase came from seven geothermal projects that began production in 2012. GEA also revised its 2011 estimate of installed capacity upward by 128 MW, bringing current installed U.S. geothermal capacity to 3,386 MW.

Renewability and sustainability

Geothermal power is considered to be renewable because any projected heat extraction is small compared to the Earth's heat content. The Earth has an internal heat content of 10³¹ joules (3·10¹⁵ TW·hr). About 20% of this is residual heat from planetary accretion, and the remainder is attributed to higher radioactive decay rates that existed in the past. Natural heat flows are not in equilibrium, and the planet is slowly cooling down on geologic timescales. Human extraction taps a minute fraction of the natural outflow, often without accelerating it.

Geothermal power is also considered to be sustainable thanks to its power to sustain the Earth’s intricate ecosystems. By using geothermal sources of energy present generations of humans will not endanger the capability of future generations to use their own resources to the same amount that those energy sources are presently used. Further, due to its low emissions geothermal energy is considered to have excellent potential for mitigation of global warming.

Even though geothermal power is globally sustainable, extraction must still be monitored to avoid local depletion. Over the course of decades, individual wells draw down local temperatures and water levels until a new equilibrium is reached with natural flows. The three oldest sites, at Larderello, Wairakei, and the Geysers have experienced reduced output because of local depletion. Heat and water, in uncertain proportions, were extracted faster than they were replenished. If production is reduced and water is re injected, 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.

Falling electricity production may be boosted through drilling additional supply boreholes, as at Poihipi and Ohaaki. The Wairakei power station has been running much longer, with its first unit commissioned in November 1958, and it attained its peak generation of 173MW in 1965, but already the supply of high-pressure steam was faltering, in 1982 being derated to intermediate pressure and the station managing 157MW. Around the start of the 21st century it was managing about 150MW, then in 2005 two 8MW isopentane systems were added, boosting the station's output by about 14MW. Detailed data are unavailable, being lost due to re-organisations. One such re-organisation in 1996 causes the absence of early data for Poihipi (started 1996), and the gap in 1996/7 for Wairakei and Ohaaki; half-hourly data for Ohaaki's first few months of operation are also missing, as well as for most of Wairakei's history.

Environmental effects

Fluids drawn from the deep earth carry a mixture of gases, notably carbon dioxide (CO
2), hydrogen sulfide (H
2S),methane (CH
4) and ammonia (NH
3). These pollutants contribute to global warming, acid rain, and noxious smells if released. Existing geothermal electric plants emit an average of 122 kilograms (269 lb) of CO
2 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.

In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic elements such as mercury, arsenic, boron, and antimony. These chemicals precipitate as the water cools, and can cause environmental damage if released. The modern practice of injecting cooled geothermal fluids back into the Earth to stimulate production has the side benefit of reducing this environmental risk.

Direct geothermal heating systems contain pumps and compressors, which may consume energy from a polluting source. This parasitic load is normally a fraction of the heat output, so it is always less polluting than electric heating. However, if the electricity is produced by burning fossil fuels, then the net emissions of geothermal heating may be comparable to directly burning the fuel for heat. For example, a geothermal heat pump powered by electricity from a combined cycle natural gas plant would produce about as much pollution as a natural gas condensing furnace of the same size. Therefore the environmental value of direct geothermal heating applications is highly dependent on the emissions intensity of the neighboring electric grid.

Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei field in New Zealand. In Staufen im Breisgau, Germany, tectonic uplift occurred instead, due to a previously isolated anhydrite layer coming in contact with water and turning into gypsum, doubling its volume.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.

Geothermal has minimal land and freshwater requirements. Geothermal plants use 3.5 square kilometres (1.4 sq mi) per gigawatt of electrical production (not capacity) versus 32 square kilometres (12 sq mi) and 12 square kilometres (4.6 sq mi) for coal facilities and wind farms respectively. They use 20 litres (5.3 US gal) of freshwater per MW·h versus over 1,000 litres (260 US gal) per MW·h for nuclear, coal, or oil.

ADVANTAGES OF GEOTHERMAL POWER
1. Geothermal energy is relatively environmentally friendly. Pollution in the form of fumes are not produced although usually drilling of the earths surface takes place. The surrounding environment is not harmed with the exception of the land required for the power plant and transport links. 2. Unlike wind power, geothermal power can be relied on as it provides constant power. 3. The use of conventional polluting fuels such as oil and coal can be reduced if geothermal and other alternative energy forms are used (reducing pollution). 4. Geothermal power can take different forms. For instance, it can be used to produce electricity or the hot water can be used directly to heat homes and businesses.