Geothermal Energy
Photos of US Geological Survey
Global heat flow map prepared from the database compiled by the
International Heat Flow Commission (H. N. Pollack, S. J. Hurter,
and J. R. Johnson, Reviews of Geophysics, Vol. 31, 1993.)
Global Geothermal Sites
http://www.deutsches-museum.de/ausstell/dauer/umwelt/img/geothe.jpg
25
50
75
100/150+
Ground Structures
Boyle, Renewable Energy, 2nd edition, 2004
Different Geothermal Energy
Sources
Normal Geothermal Gradient: At any place on the planet, there is a
normal temperature gradient of 25-300C per km dug into the earth (at the
surface!). Therefore, if one digs down 7 km the temperature will be about
1900C above the surface temperature. This difference will be enough to
produce electricity. However, no useful and economical technology has
been developed to extracted this large source of energy.
Hot Dry Rock: This type of condition exists in 5% of the US. It is similar
to Normal Geothermal Gradient, but the gradient is >400C/km underground.
Hot Water Reservoirs: As the name implies these are reservoirs of hot
underground water. There is a large amount of them in the US, but they
are more suited for space heating than for electricity production.
Natural Steam Reservoirs: In this case a hole dug into the ground can
cause steam to come to the surface. This type of resource is rare in the US.
Geopressured Reservoirs: In this type of reserve, brine completely
saturated with natural gas is stored under pressure from the weight of
overlying rock. This type of resource can be used for both heat and for
natural gas.
Molten Magma: No technology exists to tap into the heat reserves stored
in magma. The best sources for this in the US are in Alaska and Hawaii.
Geysers
Clepsydra Geyser in Yellowstone
http://en.wikipedia.org/wiki/Geyser
Hot Springs
Hot springs in Steamboat Springs area.
http://www.eia.doe.gov/cneaf/solar.renewables/page/geothermal/geothermal.html
Fumaroles
Clay Diablo Fumarole (CA)
http://lvo.wr.usgs.gov/cdf_main.htm
White Island Fumarole
New Zealand
http://volcano.und.edu/vwdocs/volc_images/img_white_island_fumerole.html
But how much geothermal power is available?
Geothermal power varies between:
a) an ordinary location on the earth’s crust
b) special hot spots like Iceland (figure 16.3).
Hot spots are obvious places, but what about ordinary locations?
But how much geothermal power is available?
Geothermal power varies between:
a) an ordinary location on the earth’s crust
b) special hot spots like Iceland (figure 16.3).
Hot spots are obvious places, but what about ordinary locations?
What happens to hot rock if you pump water into it??
But how much geothermal power is available?
Geothermal power varies between:
a) an ordinary location on the earth’s crust
b) special hot spots like Iceland (figure 16.3).
Hot spots are obvious places, but what about ordinary locations?
What happens to hot rock if you pump water into it??
It cools off!
But how much geothermal power is available?
Geothermal power varies between:
a) an ordinary location on the earth’s crust
b) special hot spots like Iceland (figure 16.3).
Hot spots are obvious places, but what about ordinary locations?
The difficulty with making sustainable geothermal power is that the conduction of heat
is very slow. If you try and extract the heat too quickly it will cool down the rock
before the heat can be replenished from below.
If you stick a pipe down a 15-km hole in the earth, it is easily hot enough to boil water
If you could stick two pipes down, pump cold water down one pipe and withdraw hot
water (or steam) from the other
But after a while, you will reduce the temperature of the rock, and the heating effect will
diminish.
You now have a long wait before the rock at the bottom of your pipe warms up again.
Geothermal power that would be sustainable forever
In a typical continent, the heat flow coming from the deep mantle is about 10 mW/m2.
The heat flow at the surface is typically ~50 mW/m2 = 0.05 W/m2 (below the global
average of 87 mW/m2 because a lot of heat comes out at mid-ocean ridges) (this is
compared to solar radiation of ~235 W / m2)
so the radioactive decay of elements in the continental crust has added an extra 40
mW/m2 to the heat flow from the deep mantle.
At a typical location, the maximum power we can get per unit area is ~50 mW/m2.
But that power is not high-grade power, it’s low-grade heat that’s trickling through at
the ambient temperature up here.
To make electricity, we must drill down and use a source that is at a higher
temperature than the ambient temperature.
Geothermal power that would be sustainable forever
The temperature increases with depth as shown, reaching
a temperature of about 500 ◦C at a depth of 40 km.
Between depths of 0 km where the heat flow is biggest but
the rock temperature is too low, and 40 km, where the
rocks are hottest but the heat flow is 5 times smaller
(because we are missing out on all the heat generated from
radioactive decay) there is an optimal depth at which we
should put a pipe.
The optimal depth depends on what sort of power station
machinery we use.
The maximum sustainable power is fixed by finding the
optimal depth assuming that we have an ideal engine for
turning heat into electricity, and that drilling to any depth is
free.
radioactivity
Maximum Geothermal Power
For the temperature profile shown, this gives the optimal depth at about 15 km.
Under these conditions, an ideal heat engine would deliver 17mW/m2.
At the world population density of 47 people per square km (7,000,000,000 people,
and 149,000,000 sq km of continent), that’s 9 kWh per person per day, if all land area
is used.
This is the sustainable-forever figure, ignoring hot spots, assuming perfect power
stations, assuming every square meter of continent is exploited, and assuming that
drilling is free.
….and that it is possible to drill 15-km-deep holes!
For reference:
World: 60 kWh per day per person
United States: 240 kWh per day per person
Geothermal power as mining
In “enhanced geothermal extraction” from hot dry rocks
(figure 16.5), the steps are:
1) drill down to a depth of 5 or 10 km, and fracture the
rocks by pumping in water.
1) drill a second well into the fracture zone.
2) pump water down one well and extract superheated
water or steam from the other.
This steam can be used to make electricity or to deliver
heat.
For a country like England, the biggest estimate of the
hot dry rock resource is that it could conceivably
contribute 1.1 kWh per day per person of electricity
for about 800 years.
Methods of Heat Extraction
http://www.geothermal.ch/eng/vision.html
Geothermal Electrical Power Generation
• World production of ~10 GW
– ~3 GW in US
• The Geyers (US) is world’s largest site
– Produces ~2 GW
• Other attractive sites
– Rift region of Kenya, Iceland, Italy, France,
New Zealand, Mexico, Nicaragua, Russia,
Phillippines, Indonesia, Japan
http://en.wikipedia.org/wiki/Geothermal
A short glimpse at
geothermal power
First experiment to produce geothermal power, done in Italy in 1904
by prince Ginori Conti
Photo courtesy of ENEL/ERGA, Italy
Photos: Lund
Modern geothermal power
plants in Larderello, Italy
Hydrothermal
Power Systems
There are three geothermal
power plant technologies being
used to convert hydrothermal
fluids to electricity.
The major conversion
technologies are
1) DRY STEAM,
2) FLASH
3) BINARY CYCLE.
The type of conversion used
depends on the state of the
fluid (whether steam or water)
and its temperature.
Dry Steam Power Plants
• “Dry” steam extracted from natural reservoir
– 180-225 ºC ( 356-437 ºF)
– 4-8 MPa (580-1160 psi)
– 200+ km/hr (100+ mph)
• Steam is used to drive a turbo-generator
• Steam is condensed and pumped back into the
ground
• Can achieve 1 kWh per 6.5 kg of steam
– A 55 MW plant requires 100 kg/s of steam
Boyle, Renewable Energy, 2nd edition, 2004
Dry Steam Schematic
Boyle, Renewable Energy, 2nd edition, 2004
Geysers dry steam field in
northern California
© 2000 Geothermal Education Office
Geysers Geothermal Plant
The Geysers is the largest producer of geothermal
power in the world.
http://www.ece.umr.edu/links/power/geotherm1.htm
What happens if you remove the pressure from
hot, pressurized water?
What happens if you remove the pressure from
hot, pressurized water?
It flash-expands and turns to steam!
Single Flash Steam Power Plants
• Steam with water extracted from ground
• Pressure of mixture drops at surface and more
water “flashes” to steam
• Steam separated from water
• Steam drives a turbine
• Turbine drives an electric generator
• Generate between 5 and 100 MW
• Use 6 to 9 tonnes of steam per hour
Single Flash Steam Schematic
Boyle, Renewable Energy, 2nd edition, 2004
Flash steam plant in Japan
© 2000 Geothermal Education Office
Binary Cycle Power Plants
• Low temps – 100o and 150oC
• Use heat to vaporize organic liquid
– E.g., iso-butane, iso-pentane
• Use vapor to drive turbine
– Causes vapor to condense
– Recycle continuously
• Typically 7 to 12 % efficient
• 0.1 – 40 MW units common
http://www.worldenergy.org/wec-geis/publications/reports/ser/geo/geo.asp
Binary Cycle Schematic
Boyle, Renewable Energy, 2nd edition, 2004
Binary plant in Nevada
© 2000 Geothermal Education Office
Combined Cycle Plants
• Combination of conventional steam turbine
technology and binary cycle technology
– Steam drives primary turbine
– Remaining heat used to create organic vapor
– Organic vapor drives a second turbine
• Plant sizes ranging between 10 to 100+ MW
• Significantly greater efficiencies
– Higher overall utilization
– Extract more power (heat) from geothermal resource
http://www.worldbank.org/html/fpd/energy/geothermal/technology.htm
Technology vs. Temperature
Reservoir
Temperature
Reservoir
Fluid
Common
Use
High Temperature
>220oC
(>430oF).
Water or
Steam
Power Generation
Water
Low Temperature
50-150oC
(120-300oF).
Water
http://www.worldbank.org/html/fpd/energy/geothermal/technology.htm
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•
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Flash Steam
Combined (Flash
and Binary) Cycle
Direct Fluid Use
Heat Exchangers
Heat Pumps
Power Generation
Direct Use
•
•
•
•
Binary Cycle
Direct Fluid Use
Heat Exchangers
Heat Pumps
Direct Use
Intermediate
Temperature
100-220oC
(212 - 390oF).
Technology
commonly chosen
Direct Use
•
•
Direct Fluid Use
Heat Exchangers
Hot Dry Rock (HDR) Technology
• Wells drilled 3-6 km into crust
– Hot crystalline rock formations
• Water pumped into formations
• Water flows through natural fissures
picking up heat
• Hot water/steam returns to surface
• Steam used to generate power
http://www.ees4.lanl.gov/hdr/
Hot Dry Rock Technology: Enhanced Geothermal
System (EGS)
Fenton Hill plant, NM – First EGS test facility, abandoned in 1992.
http://www.ees4.lanl.gov/hdr/
Enhanced Geothermal System
25 MW Demonstration Plant at Cooper Basin, Australia
 Estimated to have 5-10 GW potential
 Useful life might be 20-30 years, before rock cools off too much
Enhanced Geothermal System: Projects Underway
Enhanced Geothermal System
Could potentially use carbon dioxide instead of water:
 Could be a way to sequester CO2
Technological Issues of Geothermal Energy
• Geothermal fluids can be corrosive
– Contain gases such as hydrogen sulphide
– Corrosion, scaling
• Requires careful selection of materials and
diligent operating procedures
• Typical capacity factors of 85-95%
http://www.worldbank.org/html/fpd/energy/geothermal/technology.htm
Costs of Geothermal Energy
• Costs highly variable by site
– Dependent on many cost factors, but mostly
well depth
• High exploration costs
• High initial capital, low operating costs
– Fuel is “free”
• Significant exploration & operating risk
– Adds to overall capital costs
– “Risk premium”
http://www.worldbank.org/html/fpd/energy/geothermal/
Geothermal’s Harmful Effects
 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. [Like the Arkansas
Earthquakes!!]
• 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’s Positive Attributes
• Useful minerals, such as zinc and silica, can be extracted from
underground water.
• Geothermal energy is “homegrown.” This creates local jobs, a
better global trading position and less reliance on oil producing
countries.
• US geothermal companies have signed $6 billion worth of contracts
to build plants in foreign countries in the past couple of years.
• In large plants the cost is 4-8 cents per kilowatt hour. This cost is
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 electric plants release ~13 g of Carbon dioxide per kWh,
whereas the CO2 emissions are 450 g/kWh for natural gas, 900 g/kWh for
oil and >1000 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 GW
of energy over 30 years.
1932
2000
© 2000 Geothermal Education Office
“Blue Lagoon,” Iceland (Svartsengi Geothermal Power Plant)
“Blue Lagoon,” Iceland (Svartsengi Geothermal Power Plant)
Direct uses of geothermal energy are
appropriate for sources below 1500C
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space heating
air conditioning
industrial processes
drying
Greenhouses
Aguaculture
hot water
resorts and pools
melting snow
Geothermal Greenhouses
Geothermal greenhouse
in Nigrita, Greece
Cultivation of spirulina
(red) algae using
geothermal heat
Geothermal application in
the food industry
Geothermal “sun-dried”
tomatos drying in
Northern Greece
The finished
product
Geothermal application in
the food industry
Fish factory in Laugar, Iceland
Geothermal fish drying in
Northern Iceland
The finished product
Daily St. Louis High and Low Temperatures
How can we save the heat from the summer and use it in the winter?
Residential Geothermal Heat Pump
Heat vs. Depth Profile
Boyle, Renewable Energy, 2nd edition, 2004
A geothermal heat pump or ground source heat pump (GSHP)
•
This is not the same as geothermal power itself (which is generally for deeper
systems).
•
Refers to a central heating and/or cooling system that pumps heat to or from the
ground.
•
the Earth is a heat source (in the winter) or a heat sink (in the summer).
•
moderate temperatures in the ground boost efficiency and reduce the operational
costs of heating and cooling systems, and may be combined with solar heating to form
a geosolar system with even greater efficiency.
•
the core of the heat pump is a loop of refrigerant pumped through a vaporcompression refrigeration cycle that moves heat. Heat pump can work both directions.
•
seasonal variations drop off with depth and disappear below seven meters due to
thermal inertia (in Missouri, the frost depth is about 2 meters).
•
even shallow ground temperature is warmer than the air above during the winter and
cooler than the air in the summer.
•
Deployment: 400,000 units in U.S. Cost of heat pump is ~$3000-5000.
•
Yield: 15 thermal GW of heating and cooling
A typical household can save $1500 a year
or more.
This can give most systems a payback
period of three to five years.
GSHP's are more than three times as
efficient as the most efficient fossil fuel
furnace.
They deliver three units of energy for every
one unit used to power the heat-pump
system.