GEO THERMAL ENERGY.What is geothermal energy and how does it work?
GEO
THERMAL ENERGY
What is geothermal energy and how does it work?
ans:- Geothermal power plants, which use heat from deep inside the Earth to generate steam to make electricity. Geothermal heat pumps, which tap into heat close to the Earth's surface to heat water or provide heat for buildings.
We live
between two great sources of energy, the hot rocks beneath the surface of the
earth and the sun in the sky. Our ancestors knew the value of geothermal
energy; they bathed and cooked in hot springs. Today we have recognized that this
resource has potential for much broader application.
The term
geothermal comes Geo meaning earth and thermal meaning heat. Heat from the
Earth, or geothermal from the Greek-Geo (Earth) + thermal (heat) -energy can be
and already is accessed by drilling water or steam wells in a process similar
to drilling for oil.
The core
of the earth is very hot and it is possible to make use of this geothermal
energy in Greek it means heat from the earth). These are areas where there are
volcanoes, hot springs, and geysers, and methane under the water in the oceans
and seas. In some countries, such as in the USA water is pumped from
underground hot water deposits and used to heat people's houses.
Geothermal
energy is an enormous, underused heat and power resource that is clean (emits
little or no greenhouse gases), reliable (average system availability of 95%),
and homegrown (making us less dependent on foreign oil). The geothermal fields
were first discovered in 1847 by William Bell Elliot, an explorer surveyor who
was hiking in the mountains between Clover dale and Callisto, California, in
search of grizzly bears. He discovered steam seeping out of the ground along a
quarter of a mile on the steep slope of a canyon near colb Mountain, an extinct
volcano, now known as the Geysers. The first application of geothermal energy
was for space heating, cooking, and medicinal purposes.
The center
of the earth is estimated at temperature up to 10,000 K due to decay process of
radioactive isotopes. The total steady geothermal energy flow towards earth's
surface is 4.2 x 10" kW. But the average flow energy is only 0.063 W/m2.
The
utilization of geothermal energy for the production of electricity dates back
to the early part of the twentieth century. For 50 years the generation of electricity
from geothermal energy was confined to Italy and interest in this technology
was slow to spread elsewhere. In 1943 the use of geothermal hot water was
pioneered in Iceland.
The
following general objectives of geothermal energy:
(1)
Reduction
of dependence on nonrenewable energy and stimulation of the state's economy
through development of geothermal energy.
(2)
Mitigation
of the social, economic, and environmental impacts of geothermal development.
(3)
Financial
assistance to counties to offset the costs of providing public services and
facilities necessitated by the development of geothermal resources within their
jurisdictions.
(4)
Maintenance
of the productivity of renewable resources through the investment of proceeds
from these resources.
HOT SPRINGS
Earth tremors in the early Cenozoic
period caused the magma to come close to the earth's surfece in certain places
and crust fissures to open up. The hot magma near the surface thus causes
active volcanoes and hot springs and geysers where water exists. It also causes
steam to vent through the fissures
The hot magma near the
surface solidifies into igneous rock. The heat of the magma is conducted upward
to this igneous rock. Ground water that finds its way down to this rock through
fissures in it will be heated by the heat of the rock or by mixing with hot
gases and steam emanating from the magma. The heated water will then rise
convectively upward and into a porous and permeable reservoir above the igneous
rock. A layer of impermeable solid rock that traps the hot water in the
reservoir caps this reservoir. The solid rock, however, has fissures that act
as vents of the giant underground boiler. The vents show up at the surface as
geysers, fumaroles, or hot springs. The natural heat in the earth has
manifested itself for thousands of years in the form of hot springs. A well
taps steam from the fissure for use in a geothermal power plant.
Geothermal
power stations have been installed at a number of places around the world,
where geothermal steam is available.
The share
of geothermal produced electricity in the year 2000 is 0.3% of the total
electricity produced in the world. In India, there are more than 300 hot water
springs.
STEAM
EJECTION
Hot water
geothermal energy deposits are present in several locations around the earth.
Underground water collects heat from surrounding hot rocks. Such hot water
reserves are with small content of steam. Rain water collected over the land
areas of several hundreds of square kilometers percolates through the ground to
the depths of 1 to 6 km where it is heated by thermal conduction from the
surrounding hot rocks. The hot water moves upwards through the defects of
restricted areas in the rocks. The defects' are of fractures and highly
permeable portions in the rock. The hot water moves upwards to the surface with
relatively little or no storage in between. If however a zone of geothermal
energy deposits is covered by a impermeable rock with a few fractures or
defects, the energy deposits will be stored under ground readily available for
extraction.
The energy available in such
deposits can be extracted by means of production wells drilled through the
impermeable rocks. In hydro-geothermal energy deposits, the geothermal fluid
are in form of geothermal brine, hot mineral water and steam. Steam deposits
are very few in number.
However,
largest geothermal energy reserve of petro-geothermal type and are called Hot
Rock type that is without underground water. HDR deposits have largest
geothermal energy poten in the world.
Extraction of geothermal
energy through these hot dry rocks requires injection of water artificially
created fractured rock cavities in hot dry rock and extraction of hot water and
stea means of production wells. Cold water is injected into the well by means
of injection wells an water and steam is extracted by the production wells.
Water injected into the well acts as a heat co ing and heat transporting
medium. Cavity in hot dry rock acts like a boiler steam generator. Cold we is
injected and hot water/steam is obtained.
SITE
SELECTION
India has
about 150 known geothermal sites having geothermal fluid of moderate and low
tem perature (< 160°C). The geothermal fields in India are in the form of
hot water springs (40 to 98°C) and shallow water reservoir temperatures are
less than 160°C.
The
important hydro-geothermal resource locations are Puga Hydro-Geothermal Field,
Jammu and Kashmir West-Coast Hydro-Geothermal Field, Maharashtra, Gujarat.
Tattapani-Hydro-Geothermal Field, Madhya Pradesh.
Due to
moderate and low temperatures of geothermal fluids, the prospects of
geothermally elec trical power plants in India are very low. However,
Geothermal Hydrothermal Energy is likely to have several applications in the
temperature range of 30°C to 190°C.
For site
selection of geothermal energy, the following factor may be considered. (1)
Borax deposits present (2) Some locations with water at 120°C at 200 to 500 m
depths (3) Na-Ca-Cl-SO4 contents (4) Some shallow depth reservoir with water at
80 to 110°C.
GEOTHERMAL
POWER PLANTS
The first
mechanical conversion was in 1897 when the steam of the field at Larderello,
Italy, was used to heat a boiler producing steam which drove a small steam
engine. The first attempt to produce electricity also took place at Larderello
in 1904 with an electric generator that powered four light bulbs
This was
followed in 1912 by a condensing turbine; and by 1914, 8.5 mW of ity was being
produced. By 1944 Larderello was producing 127 mW. The plant was destroyed end
of World War II, but was fortunately rebuilt and expanded and eventually
reached 360 mW 1981.
In the
United States, the first attempt at developing the geysers field was made in
1922. Steam was successfully tapped, but the pipes and turbines of the time
were unable to cope with the corrosive and abrasive steam. The effort was not
revived until 1956 when two companies, Magma Power and Thermal Power, tapped
the area for steam and sold it to Pacific Gas and Electric Company. By that
time stainless steel alloys were developed that could withstand the corrosive
steam, and the first electricgenerating unit of 11 mW capacity began operation
in 1960. Since then 13 generally progressively larger units have been added to
the system. The latest is a 109 mW unit that began operation in September 1982
and which brought the Geysers total capacity to 909 mW. Two more units are
under construction and four more are planned, which will bring the total
capacity to 1514 mW by the late 1980s.
Other electric-generating
fields of note are in New Zealand (where the main activity at Wairakei dates
back to 1958), Japan, Mexico (at Cerro Prieto), the Phillipines, the Soviet
Union, and Iceland (a large space-heating program).
Future world
projections for geothermal electric production, based on the decade of the
1970s, are 7 percent per year. In the last four years of that decade, however,
the growth rate was 19 percent per year. In the United States, the projections
are for growth between 13.5 and 22 percent per year through the 1980s, which is
2.5 to 4 times the 5.3 percent per year growth rate of the total
electric-generating capacity. This includes the steam field at the Geysers and
other fields of different types.
The U.S. Geological Survey predicts a
U.S. potential from currently iden-tified sources to be around 23,000 mW of
electric power and around 42 x 10 Sk of space and process heat for 30 years
with existing technology, and 72,000 to 127,000 mW of electricity and 144 to
294 x 10') Btu of heat trom unidentified sources. Areas of geothermal potential
in the North American continent, Geysers Region in Northern California, the
Imperial Valley in Southern California, and the Yellowstone Region in Idaho, Montana,
and Wyoming.
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. seothermal 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.
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.
Where is geothermal energy found
Ans :- the Geo thermal power generation has found as far down to the earth's hot molten rock, magma.
Dry Steam
Power Plant (Vapour Dominated System).
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.
Power
plants using dry steam systems were the first type of geothermal power
generation plants built. They use the steam from the geothermal reservoir as it
comes from wells, and route it directly through turbine/generator units to
produce electricity. It is the rarest form of geothermal energy but the most
suitable generation and the most developed of all geothermal resources or
system.
schematic
diagram of a dry steam power system also called vapour dominated system. Dry
steam from the turbine at perhaps 200°C is used. It is near saturated at the
bottom of the wall and may have a shut-off pressure up to 35 bar. Pressure
drops through the well cause it to slightly superheated at the well head. An
example of a dry steam generation operation is at the Geysers in northern
California.
Flash
Steam Power Plant (Liquid Domain System). Flash steam power plants are the
most common. They use geothermal reservoirs of water with temperatures greater
than 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.
Flash-steam
power plants built in the 1980s tapped into reservoirs of water with temperatur
oreater than 182°C The hot water flows un through wells in the ground under its
own pressure As it flows upward, the pressure decreases and some of the hot
water “flashes" into steam. The Geysers in thern California, which uses
steam piped directly from wells, produces the world's largest single source of
geothermal power.
Flash steam plants are the
most common type of geothermal power generation plants in operaon today. They
use water at temperatures greater than 182°C that is pumped under high pressure
to the
neration
equipment at the surface. Upon reaching the generation equipment the pressure
is suddenly aduced, allowing some of the hot water to convert or "flash"
into steam. This steam is then used to wer the turbine/generator units to
produce electricity. The remaining hot water not flashed into steam, and the
water condensed from the steam is generally pumped back into the reservoir. An
example of an ore using the flash steam operation is the Cal Energy Navy I
flash geothermal power plant at the Coso geothermal field.
Binary
Cycle Power Plant (Liquid Dominatd Systems). Binary cycle power plants operate
on water at lower temperatures of about 107°-182°C. These plants use the heat
from the hot water to boll 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.
binary
cycle power plant operates on water at lower temperatures of about 107 degrees
Celsius 102 degrees Celsius. These plants use the heat from the hot water to
boil a fluid, usually an organic compound with a low boiling point.
Binary
cycle geothermal power generation plants differ from Dry Steam and Flash Steam
sysons in that the water or steam from the geothermal reservoir never comes in
contact with the turbine/ generator units. In the Binary system, the water from
the geothermal reservoir is used to heat another Working fluid” which is
vaporized and used to turn the turbine/generator units. The geothermal water,
and the working and the "working fluid" are each confined in separate
circulating systems or "closed loops" and never come in contact with
each other. The advantage of the binary cycle plant is that they can operate
with lower temperature waters (225°F-360°F), by using working fluids that have
an even lower boiling noint than water They also nroduce no air
Hybrid
Geothermal Power Plant-Fossil System. The concept of hybrid geotherm
fuel systems utilizes the relatively low-tem-perature heat of geothermal
sources in the low-ten end of a conventional cycle and the high-temperature
heat from fossil-fuel combustion in temperature end of that cycle. The concept
thus combines the high-efficiency of a high-ter cycle with a natural source of
heat for part of the heat addition, thus reducing the consumptio expensive and
nonrenewable fossil fuel.
There are
two possible arrangements for hybrid plants. These are
(1)
Geothermal
preheat, suitable for low-temperature liquid-dominateted systems, and
(2) Fossil superheat, suitable for
vapor-dominated and high-temperature liquid-domin
systems.
Geothermal-Preheat
Hydrid Systems. In these
systems the low-temperature geothermal ergy is used for feed water heating of
an otherwise conventional fossil-fueled steam plant. Geothe heat replaces some,
or all, of the feed water heaters, depending upon its temperature. A cycle
operatia on this principle is illustrated in Fig. 2.37. As shown, geothermal
heat heats the feed water through the low-temperature end prior to an open-type
deaerating heater. The DA is followed by a boiler for pump and three
closed-type feed water heaters with drains cascaded backward. These receive her
from steam bled from higher-pressure stages of the turbine. No steam is bled
from the lower-pressum stages because geothermal brine fulfills this function.
Fossil-Superheat
Hybrid Systems. In
these systems, the vapor-dominated steam, or the vape obtained from a flash
separator in a high-temperature liquid-dominated system, is superheated fossil
fired super heater.
It comprises a double-flash
geothermal steam system. Steve pro-duced at 4 in the first-stage flash
separator is preheated from 4 to 5 in a regenerator by exhau steam from the
high-pressure turbine at 7. It is then superheated by a fossil fuel fired super
heater to and expands in the high pressure turbine to 7 at a pressure near that
of the second stage steam separav It than enters the regenerator, leaves it at
8, where it mixes with the lower pressure steam produces the second stage flash
separator at 15, and produces steam at 9, which expands in the lower pressure turbine
to 10. The condensate at 11 is pumped and reinjected into ground at 12. The
spent brine from tage evaporator is also reinjected in to ground at 16.
ADVANCED
CONCEPTS
Geothermal
energy technologies use the heat of the earth for direct-use applications,
geothermal heat pumps, and electrical power production. Research in all areas
of geothermal development is helping to lower costs and expand its use.
Advanced
technologies will help manage geothermal resources for maximum power produccon,
improve plant-operating efficiencies, and develop new resources such as hot dry
rock, geopressured brines, and magma.
Encouraged
by the findings of nearly 340 hot springs in the country, a systematic
collaborative, research, development and demonstration programme was undertaken
with different organizations viz.
IIT,
Delhi, National Aeronautic Limited, Bangalore, Geological Survey of India,
National Geoph Research Institute (NGRI), Hyderabad, Oil & Natural Gas
Corporation etc. and the use of geoth energy was demonstrated in the country
for small scale power generation and thermal application
After
ascertaining the existence of potential reservoirs in Tattpani and Puga
geothermal fie Chhattisgarh and Jammu & Kashmir respectively through
Magneto-telluric investigations by Hydrabad, the Ministry is planning to
develop these fields for power generation. Sutluj-Spiti, Bra Parbati valley in
Himachal Pradesh, Badrinath-Tapovan in Uttranchal and Surajkund in Jharkhan
have some potenital sites for power generation. A programme for ascertaining
the existence and tial of the reservoir at these sites is being planned by the
Ministry through NGRI, Hyderabad NHPC. As most of the geothermal sites in the
country are in low and moderate temperature ran some demonstration projects
with direct heat utilization are being planned to be taken up at differ places
in the country. About 1400-3600 Mwe plants are under operation/under
construction. Wor wide non-electric applications amounts to 6000 MWt. boils and
turns into steam. Since this steam trapped between rocks, its pressure
increases. Identifying such places and inserting pipes there to brine out the
steam to drive a turbine, electricity can be produced.
Geothermal Heat Pumps use Shallow Ground Energy to Heat and Cool Buildings. Almost everywhere, the upper 10 feet of Earth's surface maintains a nearly constant temperature between 50 and 60 degrees F (10 and 16 degrees C). A geothermal heat pump system consists of pipes buried in the shallow ground near the building, a heat exchanger, and ductwork into the building. In winter, heat from the relatively warmer ground goes through the heat exchanger into the house. In summer, hot air from the house is pulled through the heat exchanger into the relatively cooler ground. Heat removed during the summer can be used as no-cost energy to heat water.
The Future
of Geothermal Energy. The three technologies
discussed above use only a tiny fraction of the total geothermal resource.
Several miles everywhere beneath Earth's surface is hot, dry rock being heated
by the molten magma directly below it. Technology is being developed to drill
into this rock, inject cold water down one well, circulate it through the hot, fractured
rock, and draw off the heated water from another well. One day, we might also
be able to recover heat directly from the magma.
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