Geothermal Energy and its Uses

By Savio Vales(M.Sc Previous)

Content

• What is Geothermal Energy?• The Geothermal Gradient• Where is Geothermal Energy derived from?• Applications of Geothermal Energy• Geothermal Energy in India• Harmful Effects Of Geothermal Energy• Positive Attributes

What is Geothermal Energy? Geothermal energy (from the Greek roots geo, meaning

earth, and thermos, meaning heat) is energy from heat stored in the earth.

This geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface.

It has been used for bathing since hundred’s of years ago and for space heating since ancient Roman times, but is now better known for generating electricity.

The Geothermal Gradient

• The geothermal gradient is the rate at which the Earth's temperature increases with depth, indicating outward heat flows from a hot interior.

• Away from tectonic plate boundaries, it is 25-30°C per km of depth in most of the world.

• Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets.

• The Earth's internal heat comes from a combination of residual heat from planetary accretion and heat produced through radioactive decay.

Where is Geothermal Energy derived from?• Much of the heat is believed to be created by decay of

naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements within the mantle.

• Heat of impact and compression released during the original formation of the Earth by accretion of in-falling meteorites.

• Heat released as abundant heavy metals (iron, nickel, copper) descended to the Earth's core.

• 10 to 25% of the heat flowing to the surface may be produced by a sustained nuclear fission reaction in Earth's inner core.

• Heat may be generated by tidal force on the Earth as it rotates, since land cannot flow like water it compresses and distorts, generating heat.

• Some heat may be created by electromagnetic effects of the magnetic fields involved in Earth's magnetic field.

Heat generated due to radioactive disintegration:

• The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.

• At the centre of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 Gpa

• Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher.

• This extra heat production, which was twice that of present-day at approximately 3 billion years ago,would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatites that are not formed today.

• Heat flows constantly from its sources within the Earth to the surface ,so the earth loses heat on a daily basis.

• Geothermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock.

• The heat of the earth is replenished by radioactive decay at a rate of 30 TW. The global geothermal flow rates are more than twice the rate of human energy consumption from all primary sources.

Applications of Geothermal Energy:Direct Applications :Heating

• Direct heating in all its forms is far more efficient than electricity generation and places less demanding temperature requirements on the heat resource.

• Heat may come from co-generation with a geothermal electrical plant or from smaller wells or heat exchangers buried in shallow ground.

• 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 it can be produced by conventional furnaces . Geothermal heat pumps can be used for space heating essentially anywhere in the world.

Indirect Applications: Generation of Electricity

• Flash Steam Plants• Binary Cycle Power Plants• Hot Dry Rock Plants

Flash Steam Plants • These are the most common plants.

• These systems pull deep, high pressured hot water that reaches temperatures of 3600F or more to the surface.

• This water is transported to low pressure chambers, and the resulting steam drives the turbines.

• The remaining water and steam are then injected back into the source from which they were taken.

Source: http://www.greenearthenergy.com.au

Binary cycle power plant• A binary cycle power plant allows cooler geothermal reservoirs to be used

than with dry steam and flash steam plants.

• They are used when the temperature of the water is less than 175 °C.

• Pumps are used to pump hot water from a geothermal well, through a heat exchanger, and the cooled water is returned to the underground reservoir.

• A second "working" or "binary" fluid with a low boiling point, typically a butane or pentane hydrocarbon, is pumped at fairly high pressure (500 PSI) through the heat exchanger, where it is vaporized and then directed through a turbine.

• The vapor exiting the turbine is then condensed by cold air radiators or cold water and cycled back through the heat exchanger.

Source: http://www.greenearthenergy.com.au

Hot Dry Rock Plants

• The simplest models have one injection well and two production wells.

• Pressurized cold water is sent down the injection well where the hot rocks heat the water up. Then pressurized water of temperatures greater than 2000F is brought to the surface and passed near a liquid with a lower boiling temperature, such as an organic liquid like butane.

• The ensuing steam turns the turbines. • Then, the cool water is again injected to be heated. This system does

not produce any emissions.

http://www.quantecgeoscience.com/

Geothermal Energy in India

Province Surface Tempo C

Reservoir Tempo C

Heat FlowmW/m2

Thermal gradiento C/km

Godavari 50-60 175-215 93-104 60

Himalaya 90 260 468 243

Cambay 40-90 150-175 80-93 70

West coast 46-72 102-137 75-129 47-59

SONATA 60 - 95 105-217 120-290 60-90

Potential Geothermal provinces of India

Source: Values from Geological Survey Of India on the basis of their research

http://www.geos.iitb.ac.in/geothermalindia/pubs/IBC/IBCTALKweb.html

Harmful Effects Of Geothermal Energy Brine can salinate soil if the water is not injected back into the reserve

after the heat is extracted.

• Extracting large amounts of water can cause land subsidence, and this can lead to an increase in seismic activity. To prevented this the cooled water must be injected back into the reserve in order to keep the water pressure constant underground.

• Power plants that do not inject the cooled water back into the ground can release H2S, the “rotten eggs” gas. This gas can cause problems if large quantities escape because inhaling too much is fatal.

• One well “blew its top” 10 years after it was built, and this threw hundreds of tons of rock, mud and steam into the atmosphere.

Geothermal Energy: Positive Attributes• Useful minerals, such as zinc and silica, can be extracted from

underground water.

• Geothermal energy is “homegrown.” This will create jobs, a better global trading position and less reliance on oil producing countries.

• In large plants the cost is almost competitive with conventional energy sources.

• Geothermal plants can be online 100%-90% of the time. Coal plants can only be online 75% of the time and nuclear plants can only be online 65% of the time.

• Flash and Dry Steam Power Plants emit 1000x to 2000x less carbon dioxide than fossil fuel plants, no nitrogen oxides and little SO2.

• Geothermal Heat Pumps:

- produces 4 times the energy that they consume.

-initially costs more to install, but its maintenance cost is 1/3 of the cost for a typical conventional heating system and it decreases electric bill. This means that geothermal space heating will save the consumer’s money.

• Electricity generated by geothermal plants saves 83.3 million barrels of fuel each year from being burned world wide. This prevents 40.2 million tons of CO2 from being emitted into the atmosphere.

• Direct use of geothermal energy prevents 103.6 million barrels of fuel each year from being burned world wide. This stops 49.6 tons of CO2 from being emitted into the atmosphere.

• Geothermal electric plants produce 13.380 g of Carbon dioxide per kWh, whereas the CO2 emissions are 453 g/kWh for natural gas, 906g g/kWh for oil and 1042 g/kWh for coal.

• Binary and Hot Dry Rock plants have no gaseous emission at all.

• Geothermal plants do not require a lot of land, 400m2 can produce a gigawatt of energy.

References http:// en.wikipedia.org/wiki/Geothermal http:// www.usgs.gov/ http://www.geos.iitb.ac.in/geothermalindia/pubs/IBC/IBCTALKweb.html http://www.greenearthenergy.com.au http://www.quantecgeoscience.com/ Turcotte, D. L.; Schubert, G. (2002). "4". Geodynamics (2nd ed.).

Cambridge, England, Cambridge University Press. pp. 136–137 Stacey, Frank D. (1977). Physics of the Earth (2nd ed.). John Wiley & Sons.

pp. 183-4 Sleep, Norman H.; Kazuya Fujita (1997). Principles of Geophysics.Blackwell

Science. pp. 187-9

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