Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
Copyright © 2015 by Pearson Education, Inc.All Rights Reserved
Buchla, Kissell, Floyd
Renewable Energy
Systems10
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
Copyright © 2015 by Pearson Education, Inc.All Rights Reserved
10Geothermal Power
Generation
10-1 TYPES OF GEOTHERMAL RESOURCES
10-2 GEOTHERMAL ELECTRICAL POWER
10-3 LOW-TEMPERATURE GEOTHERMAL HEAT
10-4 GROUND-SOURCE HEAT PUMPS
10-5 ENVIRONMENTAL IMPACT
Renewable Energy SystemsBuchla, Kissell, Floyd
Chapter Outline
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
Copyright © 2015 by Pearson Education, Inc.All Rights Reserved
Three attributes of a high quality geothermal source are heat, water, and permeability of the ground. Locations with these attributes are called high-enthalpy resources.
Enthalpy is defined as the amount of energy in a system capable of doing mechanical work; it is a function of temperature, pressure, and volume.
10-1 Types of Geothermal Sources
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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The geothermal gradient refers to the increasing temperature at increasing depths within the earth. The thermal gradient varies widely according to location; it is about 25 oC/km in most locations but can be as much as 200 oC/km.
10-1 Types of Geothermal Sources
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Three temperature classifications of geothermal sources are:
10-1 Types of Geothermal Sources
1. High temperature sources are >150 oC. These
sources are useful for generating electricity.
2. Moderate temperature sources are between
90oC and 150 oC. These sources are useful for
space heating, drying, and industrial heat and in
some cases are used for electricity production.
3. Low temperature sources are <90 oC. Low
temperature sources are useful for heating
buildings, fish farms, and bathing.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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To exploit high temperature hot dry rock, water is generally injected from nearby wells. The water boils and the steam is used to drive a steam turbine.
10-1 Types of Geothermal Sources
One plant at the 750 MW Geysers in Northern CA, is shown here. Recycled water from nearby Santa Rosa is piped in to be injected to create steam for the turbine.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Geopressurized reservoirs consist of high-pressure, hot brine in deep underground reservoirs. They are characterized by dissolved methane. The best known areas for geopressurized reservoirs are along the Texas and Louisiana Gulf Coast.
10-1 Types of Geothermal Sources
Energy can be obtained from the thermal energy, from the hydraulic high pressure, or from burning the dissolved methane.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Hot water reservoirs are natural steam reservoirs that are typically found along fault lines in the earth. The temperatures are high enough to create steam, which can be used to drive turbines.
10-1 Types of Geothermal Sources
For moderate temperature reservoirs, Organic Rankine Cycle (ORC) generators are used. ORC generators are shown at the Chena Hot Springs Resort in AK.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Normal ground temperature sources are useful for underground geothermal heat pumps. In most locations in the world, the ground is a constant 10 oC at relatively shallow depths. This is sufficient to keep heat pumps working efficiently even when the outdoor temperature is freezing.
10-1 Types of Geothermal Sources
The photo shows drilling of a geothermal well for a heat
pump in Klamath Falls, OR.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-2 Geothermal Electrical Power
Three methods for generating geothermal electricity:
1. Dry steam
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-2 Geothermal Electrical Power
Three methods for generating geothermal electricity:
2a. Flash steam (single flash)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-2 Geothermal Electrical Power
Three methods for generating geothermal electricity:
2b. Flash steam (double flash)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-2 Geothermal Electrical Power
Three methods for generating geothermal electricity:
3. Binary-cycle
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-2 Geothermal Electrical Power
An Enhanced Geothermal System (EGS) is one in which a geothermal site is made productive by artificial means including fracturing networks to increase permeability and improved drilling methods including horizontal drilling and laser drilling.
EGS has successfully
produced an additional
1.7 megawatts from an
existing well field using
EGS fracturing methods at Brady Hot Springs, NV
(shown here). Other
successes are at
Newberry, OR and Desert
Rock, NV.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-2 Geothermal Electrical Power
Another innovative proposal is to harness the heat in underground coal seam fires. These fires represent an ecological disaster as they can burn for tens of decades in a very wide area and emit tons of pollutants as well as consume a valuable resource. Most are caused by man and they have been nearly impossible to extinguish.
One study has shown that there
is significant energy that can be
recovered by using standard
methods developed in
geothermal plants. The heat
would be used to drive ORC turbines.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-3 Low-Temperature Applications for Geothermal Heat
Low-temperature sources can be used for space heating and cooling. Cooling is accomplished with an absorption type refrigeration system in four steps:
1
•Generator: The generator heats the refrigerant (lithium bromide solution); this causes the water in solution to boil, creating water vapor at high pressure.
2
•Condenser: The high pressure vapor is condensed to a liquid at higher pressure. Heat from compressing is removed.
3
•Expansion valve: High pressure liquid water passes through an expansion valve and becomes a cold liquid vapor mix. The pressure (and the boiling point) are reduced.
4
•Evaporator and Absorber: The cold water absorbs heat from the space to be cooled and is absorbed by the LiBr to form a weaker solution. The solution is returned to the generator.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-3 Low-Temperature Applications for Geothermal Heat
Snow melting is an application for low-temperature heat in areas with ample geothermal sources like Iceland. The idea is being tested also in other locations on bridges and overpasses.
Tubing for snow melting is installed before concrete
is poured. This installation
is in Klamath Falls, OR. The
city has more than 50,000
sidewalks and crosswalks
with district heating.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-3 Low-Temperature Applications for Geothermal Heat
Greenhouses and fish farms are another application for low-temperature geothermal heat. Farming operations include aquatic plants and animals as well as certain food drying applications (including onion, garlic, carrots, and celery dehydrators).
Alligators are raised in
Colorado at an elevation of
over 7500 ft (2300 m). These
are part of the Colorado
Gators Reptile Park, which
also has other reptiles and birds on display. A geothermal
well keeps the water warm
year round.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-4 Geothermal Heat Pumps
A conventional heat pump operates in a standard refrigeration cycle. Here it is operating in the heat cycle:
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-4 Geothermal Heat Pumps
A geothermal heat pump uses the same principle of operation but absorbs heat from the ground into a glycol solution.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-4 Geothermal Heat Pumps
The coefficient of performance (COP) is a measure of efficiency for heat pumps and is the ratio of heat energy delivered to energy supplied to the heat pump. Geothermal heat pumps are very efficient.
The energy supplied in 30 min is (5 kW)(0.5 h) = 2.5 kWh
A heat pump rated for 5 kW is on for 30 min and delivers
40,000 BTU of heat in that time. What is the COP?
The energy delivered = 40,000 BTU/(3413 BTU/kWh) = 11.7 kWh
The COP = 11.7 kWh/2.5 kWh = 4.69
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-5 Environmental Impact
Geothermal plants use steam and need to recycle water to maintain the source, so monitoring of the quality of injected water and flow is important. Plants like the Geysers use treated wastewater for injection.
Water
Monitoring of ground water can
be done by injecting chemical
tracers. Tracer tests are used to
evaluate the injected water flow
through the reservoir and the amount of injection-derived steam
that is produced.
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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10-5 Environmental Impact
Air pollution from geothermal plants is minor and is less that 1% of an equivalent fossil-fueled plant. Small amounts of hydrogen sulfide (H2S) may be in the steam; most can be removed at the plant. Other gases include small amounts of CO2 and NOx.
Air
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Selected Key Terms
Binary-cycle plant
Coefficient of
performance (COP)
Double-flash
steam plant
Dry-steam plant
A geothermal power plant that uses the brine water or steam from the geothermal reservoir to
heat and vaporize a secondary fluid with a lower
boiling point to drive the turbine and generator.
A measure of efficiency for a heat pump; it is the
ratio of the heat produced to the energy
consumed and it varies with the outside
temperature.
A geothermal electrical plant that uses superheated dry steam from a geothermal
reservoir and routes it directly to a steam turbine
and generator to produce electricity.
A geothermal plant with two pressure reducing
stages to create high pressure and low pressure
steam. The high and low pressure steam is routed
to two different turbines, which turn a generator.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Selected Key Terms
Enhanced
Geothermal
System
EnthalpySolar module
Flash steam
plant
A system in which a geothermal site that is
deficient in water or permeability is made to be
productive by artificial means.
The amount of energy in a system capable of
doing mechanical work; it is a function of
temperature, pressure, and volume.
A geothermal plant that creates steam from high
pressure hot water (brine) using a special control
valve or orifice plate to reduce the pressure and
cause some of the liquid to boil (or flash) into
steam, which is used to drive a steam turbine and
generator to produce electricity.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
1. A high enthalpy site for geothermal
power is one that has high heat,
water, and permeability.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
2. Low-temperature geothermal
sources are useful for generating
electricity.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
3. Hot water reservoirs are not found in
cold climates.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
4. Most locations in the world can use
geothermal energy for heat pumps.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
5. An advantage of a flash power
plant is that it isolates underground
water from the turbines.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
6. The Organic Rankine cycle is used
with double-flash power plants.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
7. Enhanced Geothermal Systems do
not use artificial means to improve
output.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
8. Low-temperature geothermal
sources can be used to cool
buildings.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
9. Geothermal heat pumps tend to be
more efficient than standard heat
pumps.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
10. Tracers can track the flow of water
through a reservoir and the amount
of steam that is produced.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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true/false quiz
Answers:
1.T
2.F
3.F
4.T
5.F
6.F
7.F
8.T
9.T
10. T