General Overview
- The adjective geothermal originates from the Greek roots geo, meaning earth, and thermos, meaning heat. Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is energy that determines the temperature of matter. Earth’s geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, from volcanic activity, and from solar energy absorbed at the surface. 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.
- 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.
- 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.
- Generally speaking, the further down one drills, the hotter the temperatures. Most of the commercial-grade production geothermal energy is harvested along localized “geothermal systems”, where the heat flow is near enough to the surface that hot water or steam is able to rise either to the surface, or to depths that we can reach by drilling. Many of these regions occur within the “ring of fire“, a ring of geothermal sites.
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ORIGIN OF GEOTHERMAL ENERGY
Geothermal energy is generated in the earth‘s core, almost 4,000 miles beneath the earth‘s surface. The double-layered core is made up of very hot magma (melted rock) surrounding a solid iron center. Very high temperatures are continuously produced inside the earth by the slow decay of radioactive particles. This process is natural in all rocks. Surrounding the outer core is the mantle, which is about 1,800 miles thick and made of magma and rock. The outermost layer of the earth, the land that forms the continents and ocean floors, is called the crust. The crust is 3–5 miles thick under the oceans and 15–35 miles thick on the continents.
The crust of the earth is made up of several broken pieces, which are known as plates. The hot magma from deep down below rises up close to the surface of the earth at the junctures of these plates. These are the places where volcanoes are formed. The lava that spews from volcanoes is made up partly of magma. The heat from this magma is absorbed by the water and rocks that occur deep beneath the earth‘s surface. The temperature of the water and the rocks get increasingly hotter the deeper down you go below the earth‘s surface. Superheated substances in the form of magma, that contains enormous energy and power, which is quite evident every time a volcano erupts, can be tapped for creating geothermal power. Some of these substances also rise to the surface in the form of hot water and steam, which spew out from natural vents. When the rising hot water and steam is trapped in permeable and porous rocks under a layer of impermeable rock, it can form a geothermal reservoir. Therefore, we can make artificial vents as well as create containment chambers where the magma can be kept, and turn all this geothermal energy into electricity, which can be used to heat and light our homes. In order to set up a geothermal power plant, a well will have to be dug where there is a good source of superheated fluid or magma. Pipes would then be fitted, which would go down into the source, and then the fluids would be forced up to the surface in order to produce the required steam. This steam would then be used to rotate a turbine engine, thus generating electricity, or geothermal power.
There is more than one type of geothermal energy, but only one kind is widely used to make electricity. It is called hydrothermal energy. Currently, hydrothermal energy is being commercially used for electricity generation and for meeting thermal energy requirements. Hydrothermal resources have two common ingredients: water (hydro) and heat (thermal). Depending on the temperature of the hydrothermal resource, the heat energy can either be used for making electricity or for heating.
Geothermal energy resources
There are four major types of Geothermal energy resources.
1. Hydrothermal
2.Geopressurised brines
3.Hot dry rocks
4. Magma
Applications of Geothermal Energy
- Electricity Generation: 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.
- Direct Applications: In the geothermal industry, low temperature means temperatures of 300 °F (149 °C) or less. Low-temperature geothermal resources are typically used in direct-use applications, such as district heating, greenhouses, fisheries, mineral recovery, and industrial process heating. However, some low-temperature resources can generate electricity using binary cycle electricity generating technology. Direct heating is far more efficient than electricity generation and places less demanding temperature requirements on the heat resource. Heat may come from co-generation via a geothermal electrical plant or from smaller wells or heat exchangers buried in shallow ground. As a result, geothermal heating is economic at many more sites than geothermal electricity generation. Where natural hot springs are available, the heated water can be piped directly into radiators. If the ground is hot but dry, earth tubes or downhole heat exchangers can collect the heat. But even in areas where the ground is colder than room temperature, heat can still 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, and they frequently combine a variety of functions, including air conditioning, seasonal energy storage, solar energy collection, and electric heating. Geothermal heat pumps can be used for space heating essentially anywhere. Geothermal heat supports many applications. District heating applications use networks of piped hot water to heat many buildings across entire communities. In ReykjavÃk, Iceland, spent water from the district heating system is piped below pavement and sidewalks to melt snow. Geothermal desalination has been demonstrated.
Example of Direct Applications are:
i) Geothermal heating
ii) geothermal heat pump
Global Scenario
- Geothermal energy supplies more than 10,715 MW to 24 countries worldwide which is expected to generate 67,246 GWh of electricity in 2010 and produces enough electricity to meet the needs of 60 million people and another 22 countries will add to the list in 2010 (source: International Geothermal Association).
- An additional 28 gigawatts of direct geothermal heating capacity is installed for district heating, space heating, industrial processes, desalination and agricultural applications. This renewable energy source also has the potential to provide significant opportunities to do businesses both small and large.
- It is considered possible to produce up to 8.3% of the total world electricity with geothermal resources, serving 17% of the world population.
- According to market studies, investment in geothermal energy is growing globally at 24% a year. This exceptional investing growth rate is expected to continue – and increase even faster – for the foreseeable future.
- 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, with1,904 MW of capacity online. Geothermal power makes up approximately 18% of the country’s electricity generation. Also in Indonesia 5% of overall electricity generation is from geothermal energy.
- 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 inLandau-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.
Indian Scenario
- India has reasonably good potential for geothermal; the potential geothermal provinces can produce 10,600 MW of power.
- Though India has been one of the earliest countries to begin geothermal projects way back in the 1970s, but at present there are no operational geothermal plants in India. There is also no installed geothermal electricity generating capacity as of now and only direct uses (eg.Drying) have been detailed.
- Thermax, a capital goods manufacturer based in Pune, has entered an agreement with Icelandic firm ReykjavÃk Geothermal. Thermax is planning to set up a 3 MW pilot project in Puga Valley, Ladakh (Jammu & Kashmir). ReykjavÃk Geothermal will assist Thermax in exploration and drilling of the site.
- India’s Gujarat state is drafting a policy to promote geothermal energy
Direct Uses
i) Total thermal installed capacity in MW: 203.0
ii) Direct use in TJ/year: 1,606.3
iii) Direct use in GWh/year: 446.2
iv) Capacity factor: 0.25
Potential Sites
i) Puga Valley (J&K)
ii) Tatapani (Chhattisgarh)
iii) Godavari Basin Manikaran (Himachal Pradesh)
iv) Bakreshwar (West Bengal)
v) Tuwa (Gujarat)
vi) Unai (Maharashtra)
vii) Jalgaon (Maharashtra)
The various assessment studies and surveys undertaken so far have resulted in the identification of 340 hot springs across India. The discovery of vast geothermal reservoirs at Puga in the north-west of the Himalayas and Tatapani fields on the Narmada in central India also augurs well for the country.
| Potential Geothermal regions/sources in India
With India’s geothermal power potential of 10,600 MW, the following are the potential sources/ regions where geothermal energy can be harnessed in India.
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Power Generation Technology
Method of Heat Extraction
1. Borehole heat exchangers
2. Hydrothermal systems
3. Hot dry rock
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Process of Power Generation
- High Temperature Resources
High temperature geothermal reservoirs containing water and/or steam can provide steam to directly drive steam turbines and electrical generation plant. More recently developed binary power plant technologies enables more of the heat from the resource to be utilised for power generation. A combination of conventional flash and binary cycle technology is becoming increasingly popular.
High temperature resources commonly produce either steam, or a mixture of steam and water from the production wells. The steam and water is separated in a pressure vessel (Separator), with the steam piped to the power station where it drives one or more steam turbines to produce electric power. The separated geothermal water (brine) is either utilised in a binary cycle type plant to produce more power, or is disposed of back into the reservoir down deep (injection) wells. The following is a brief description of each of the technologies most commonly used to utilise high temperature resources for power generation.
- Dry steam Power Plant
Dry steam power plants use very hot (>455 °F, or >235 °C) steam and little water from the geothermal reservoir. The steam goes directly through a pipe to a turbine to spin a generator that produces electricity. This type of geothermal power plant is the oldest, first being used at Lardarello, Italy, in 1904.
- Flash Steam Power Plant
This is the most common type of geothermal power plant. The steam, once it has been separated from the water, is piped to the powerhouse where it is used to drive the steam turbine. The steam is condensed after leaving the turbine, creating a partial vacuum and thereby maximizing the power generated by the turbine-generator. The steam is usually condensed either in a direct contact condenser, or a heat exchanger type condenser. In a direct contact condenser the cooling water from the cooling tower is sprayed onto and mixes with the steam. The condensed steam then forms part of the cooling water circuit, and a substantial portion is subsequently evaporated and is dispersed into the atmosphere through the cooling tower. Excess cooling water called blow down is often disposed of in shallow injection wells. As an alternative to direct contact condensers shell and tube type condensers are sometimes used, as is shown in the schematic below. In this type of plant, the condensed steam does not come into contact with the cooling water, and is disposed of in injection wells.
Typically, flash condensing geothermal power plants vary in size from 5 MW to over 100 MW. Depending on the steam characteristics, gas content, pressures, and power plant design, between 6000 kg and 9000 kg of steam each hour is required to produce each MW of electrical power. Small power plants (less than 10 MW) are often called well head units as they only require the steam of one well and are located adjacent to the well on the drilling pad in order to reduce pipeline costs. Often such well head units do not have a condenser, and are called backpressure units. They are very cheap and simple to install, but are inefficient (typically 10-20 tonne per hour of steam for every MW of electricity) and can have higher environmental impacts.
- Binary Cycle Power Plants
In reservoirs where temperatures are typically less than 220o C. but greater than 100o C binary cycle plants are often utilised. The reservoir fluid (either steam or water or both) is passed through a heat exchanger which heats a secondary working fluid (organic) which has a boiling point lower than 100o C. This is typically an organic fluid such as Isopentane, which is vaporised and is used to drive the turbine. The organic fluid is then condensed in a similar manner to the steam in the flash power plant described above, except that a shell and tube type condenser rather than direct contact is used. The fluid in a binary plant is recycled back to the heat exchanger and forms a closed loop. The cooled reservoir fluid is again re-injected back into the reservoir.
Binary cycle type plants are usually between 7 and 12 % efficient, depending on the temperature of the primary (geothermal) fluid. Binary Cycle plant typically vary in size from 500 kW to 10 MW.
- Combined Cycle (Flash and Binary)
Combined Cycle power plants are a combination of conventional steam turbine technology and binary cycle technology. By combining both technologies, higher overall utilization efficiencies can be gained, as the conventional steam turbine is more efficient at generation of power from high temperature steam, and the binary cycle from the lower temperature separated water. In addition, by replacing the condenser-cooling tower cooling system in a conventional plant by a binary plant, the heat available from condensing the spent steam after it has left the steam turbine can be utilized to produce more power.
ADVANTAGES OF GEOTHERMAL ENERGY
- The first advantage of using geothermal heat to power a power station is that, unlike most power stations, a geothermal system does not create any pollution. It may once in a while release some gases from deep down inside the earth, that may be slightly harmful, but these can be contained quite easily. Geothermal power plants have sulphur-emissions rates that average only a few percent of those from fossil -fuel alternatives. The newest generation of geothermal power plants emits only ~135 gm of carbon (as carbon dioxide) per megawatt-hour (MW-hr) of electricity generated. This figure compares with 128 kg /MW-hr of carbon for a plant operating on natural gas (methane) and 225 kg/MW-hr of carbon for a plant using bituminous coal. Nitrogen oxide emissions are much lower in geothermal power plants than in fossil power plants. Nitrogen-oxides combine with hydrocarbon vapours in the atmosphere to produce ground-level ozone, a gas that causes adverse health effects and crop losses as well as smog.
- The cost of the land to build a geothermal power plant on, is usually less expensive than if you were planning to construct an; oil, gas, coal, or nuclear power plant. The main reason for this is land space, as geothermal plants take up very little room, so you don’ t need to purchase a larger area of land.
- Another factor that comes into this is that because geothermal energy is very clean, you may receive tax cuts, and/or no environmental bills or quotas to comply with the countries carbon emission scheme (if they have one).
- No fuel is used to generate the power, which in return, means the running costs for the plants are very low as there are no costs for purchasing, transporting, or cleaning up of fuels you may consider purchasing to generate the power.
- The overall financial aspect of these plants is outstanding, you only need to provide power to the water pumps, which can be generated by the power plant itself anyway. Because they are modular, then can be transported conveniently to any site. Both baseline and peaking power can be generated.
- Construction time can be as little as 6 months for plants in the range 0.5 to 10 MW and as little as 2 years for clusters of plants totalling 250 MW or more.
Disadvantages of Geothermal Energy
- 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 kilograms (269 lb) 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.
- In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic chemicals 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.
- Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei field in New Zealand and in Staufen im Breisgau, Germany. 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.
Sustainability
Geothermal power is considered to be sustainable because any projected heat extraction is small compared to the Earth’s heat content. The Earth has an internal heat content of 10 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.
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 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.
