Lecture 6 2015-2016

Renewable Energy Systems David Buchla | Thomas Kissell | Thomas Floyd

Copyright © 2015 by Pearson Education, Inc. All Rights Reserved

Renewable Energy Systems 10



10 Geothermal 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

Chapter Outline



Three attributes of a high quality geothermal source are

•  heat

•  water

•  permeability of the ground

Permeability: a measure of the ability of a material to pass a fluid.

When ground is permeable, water can move through it easily.

10-1 Types of Geothermal Sources



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.

H = U + pV

U: internal energy of the system

p: pressure

V: volume

The total enthalpy, H, of a system cannot be measured directly. To measure the enthalpy of a system, we must refer to a defined reference point; therefore what we measure is the change in enthalpy, ΔH.

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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 near a tectonic plate. But it can be much higher when near hot igneous bodies.

In some cases, the geothermal gradient is much less than average, particularly in subduction zones where cold, water-filled sediments are driven underground.

In Iceland, it can be as much as 200 oC/km.




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A high geothermal gradient is one indictor of useful geothermal energy.

The best sources also have available fluids in the underground formation and porous rocks with high permeability.



The geothermal gradient is of value for geothermal heat pumps (GHPs), because they can use the source at shallow depths.

This is relatively low-tech application for geothermal energy that takes advantage of the relatively constant temperature of the ground at a depth of a few meters (up to 100 meters).

GHPs can move heat in cold weather from the ground and put it back during hot weather, so they are useful in both heating and cooling applications.

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Three temperature classifications of geothermal sources are:


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.



High-temperature resources

•  Efficient for producing electricity, which is their predominant application.

•  High-temperature resources include: –  Molten Magma –  Hot dry rock –  Geopressurized resources –  Hot water reservoirs

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Molten Magma

The hottest geothermal resource is approximately 650 oC to 1300 oC.

Found in the molten magma pools in and under volcanoes.

The lava pools contain significant energy but are difficult to tap.

Experimental holes are drilled in magma chambers in Hawaii and Iceland.

There is no commercial use of this resource.

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http://science.jrank.org/kids/pages/176/Three-Types-Rock.html



Hot dry rock

The rock above magma chamber is very hot, but it often lacks water content and frequently lacks permeable rock formations.

Underground hot dry rock formations are one of the most common forms of geothermal resource.

While they can serve as a consistent source of heat, the lack of water and permeable rock are significant drawbacks.

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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.


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http://www.hotrockenergy.com/images/geothermal-egs.jpg

Some projects enhance permeability artificially and water is piped to the region of hot dry rocks.

When water comes in contact with the hot rocks, it boils immediately, creating superheated steam at a temperature above 93oC.

The steam can be used to generate electricity.




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One plant at the 750 MW Geysers in Northern California, is shown here.

Recycled water from nearby Santa Rosa is piped in to be injected to create steam for the turbine.



Geopressurized reservoirs consist of high-pressure, hot brine (salt water) in deep underground reservoirs made of permeable rock that traps the water under an impermeable layer of cap rock.

Usable geopressurized reservoirs are typically 3 km to 4 km deep. A layer of impermeable cap rock covers the resource.


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It is possible to capture the natural gas from this resource and use it for co-generation. But the water itself is unusable for heating or bathing, because of the dissolved materials.

Currently geothermal energy and natural gas are being co-produced from deep sedimentary rocks in the Gulf Coast of the United States.

The best known areas for geopressurized reservoirs are along the Texas and Louisiana Gulf Coast.

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Energy can be obtained from the thermal energy, from the hydraulic high pressure, or from burning the dissolved methane.

The large reservoirs of natural gas have spurred the development of related work in conversion of natural gas into diesel, jet fuel, and other liquid hydrocarbons.

The process is called gas-to liquid (GTL).




Hot water reservoirs are natural steam reservoirs. They have geothermal heated underground water and steam.

They are typically found along fault lines in the earth. The temperatures are high enough to create steam, which can be used to drive turbines.

These reservoirs are the most common usable form of geothermal energy for electricity production.

The water typically reaches a temperature that is hot enough at pressure that is low enough to create steam.

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Intermedia- and low- temperature resources

•  Intermedia-temperature sources are too cool to power a steam generator directly, but they can still be useful for electricity production using an organic Rankine cycle (ORC) plant to take advantage of the lower boiling point for the working fluid.

•  Intermedia-temperature sources have provided hot water for heating offices and homes.

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•  Geysers and Hot Springs

•  Hot springs are naturally occurring water resources in which geothermally heated water rises to the surface.

•  The water is heated underground by hot rocks or magma.

•  Relatively few have been developed for geothermal power.

•  In general geysers and hot springs represent a very small fraction of the geothermal resource.

•  However, an exception in the United States is at Chena Hot Springs resort in Alaska.

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For moderate temperature reservoirs, Organic Rankine Cycle (ORC) generators are used.

ORC generators are shown at the Chena Hot Springs Resort in Alaska.

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Geysers are natural hot springs that reach temperatures high enough to boil and intermittently eject a column of water and steam into the air.

They are found in regions where magma is deep underground, heating layers of rock.

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Surface water entering the location of the hot rocks boils and builds pressure.

When pressure builds sufficiently, the water is ejected violently from a surface vent.




One of the most active geyser regions in the world is in Yellowstone National Park.

Yellowstone is located over a large, hot magma region, the iconic geyser in Yellowstone is Old Faithful, a major tourist attraction.

This geyser can erupt to a height of 56 meters.

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http://www.mountainscenes.com/images/yellowstone/Castle_Geyser_Yellowstone_rainbow_YEL.122.jpg



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.

To take advantage of this heat, underground piping is installed for recirculating fluid, which adds to the initial cost; however, the long-term benefit of reduced electric usage pays for the initial higher cost.


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The photo shows drilling of a geothermal well for a heat pump in Klamath Falls, Oregon.



10-2 Geothermal Electrical Power

Capturing geothermal energy to produce electrical power is accomplished through dry steam flash and through binary-cycle steam power plants.

The most common way: tap into naturally occurring hydrothermal convection systems.

The geothermal systems produce hot water or steam and use the steam to drive turbines that turn electric generators.

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A general description on the process:

•  Very hot water and steam naturally rise to the surface, where they are captured.

•  The steam is directed across the blades of a turbine to drive it and spin the electric generator.

Other applications:

•  Holes are drilled into the rock to depths that allow water to be injected into them, and the steam that is produced is captured.

•  When the source is moderately hot but not hot enough for creating steam, the hot water can be sent through a heat exchanger to head a secondary fluid.

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Geothermal power plants can be based on three systems:

•  Dry steam plants

•  Flash steam plants

•  Binary cycle plants

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Dry steam (also called superheated steam) is steam so hot that it contains no liquid water in suspension.

When a geothermal reservoir is located in a place that has steam at the surface, the steam can be piped directly to a steam turbine that drives an electric generator – Dry-Steam Plant

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The steam at the Geysers in Northern California has a temperature greater than 235oC, so it has sufficient heat energy to turn the turbine and generator.

Basic processes in a dry-steam plant: 1.  Dry steam is tapped by the production well.

2.  A rock catcher is inserted between the steam pipe and the turbine to avoid damaging the turbine.

3.  The steam enters the turbine while it is very hot, and the energy in the steam is released.

4.  As the steam passes over the turbine blades, it loses energy.

5.  The steam becomes less pressure and cooler when leaving turbine.

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6. Some of steam may start to condense in the turbine to create water droplets.

7. The water vapor completes the condensation process in a unit called the condenser, which extracts most of the remaining water from the air.

8. The water is reinjected in the second well (injection well) to replenish the geothermal reservoir.

9. Adequate amounts of fresh water are introduced to restore the reservoir

10. The water is pumped out of the bottom of the condenser, causing the steam to condense into water.

11. Water vapor is vented out the top to the atmosphere.

12. Additional water is added to the injection well to keep producing sufficient steam.

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Dry steam




Flash steam is a mixture of pressurized hot water and steam that converts to steam when pressure is released.

This combination of liquid and vapor in an underground reservoir is called a liquid-dominated reservoir.

This occurs when large volumes of water from artesian wells come into contact with extremely hot rocks deep in the earth.

The temperature of this resource for the flash-steam plant is greater than 182 oC.

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Processes in the flash-steam plant: 1.  The mixture of vapor and high pressure hot water is pumped

through a special control valve, reducing the pressure and causing some of the liquid to ‘flash’ instantly into steam.

2.  Once the steam is separated from the remaining hot water and brine, the steam then goes to the turbine.

3.  The turbine is rotated to generate electricity.

4.  The steam then goes to condenser and injected to supply the water in reservoir.

5.  The water vapor is emitted and additional water is input.

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2a. Flash steam (single flash)




Double-flash steam plants

When the source is very hot, the water that remains from the separator may be much hotter than the normal boiling temperature of 100 oC because it is still under pressure that is higher that atmospheric pressure.

Additional steam can be created from the hot water by expanding it through a second separation system.

In a double-flash system, the second expansion creates lower pressure steam, which is routed to a separate low-pressure turbine.

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•  About 20% more electrical power can be produced in a double-flash system but at greater cost due to the second turbine, which is a special low-pressure turbine.

•  In addition, a double-flash system has extra piping and valves.

•  The double flash steam plant is used in several locations in California in spite of the additional cost.

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2b. Flash steam (double flash)




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An important field is Yangbajain Geothermal Field,羊八井 with eight double flash units for a total capacity of 24 MW, fueled from a water dominated shallow reservoir at 140 °C – 160 °C with 18 wells of average depth 200 m. (Wiki)




Binary-cycle plant

•  The hot brine water and steam from the geothermal reservoir goes directly to a heat exchanger and never comes in contact with the turbine that drives the generator.

•  A secondary fluid is heated and vaporizes.

•  Secondary fluid: organic fluid (contains carbon) that has a lower boiling point than water.

•  Binary-cycle plants are also known as organic Rankine cycle (ORC) plants.

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3. Binary-cycle




Advantages of Binary-cycle plant:

1.  Produce power with lower quality heat sources. Organic fluid vaporizes at lower temperature operation. It allows ORC systems to operate in cases where a standard steam plant cannot.

2.  Closed secondary loop. Geothermal sources frequently contain dissolved materials that may be corrosive and also may contain small debris. The binary-cycle plant avoid the hazardous to turbines.

3.  Less waste and lower water usage as compared with other geothermal systems.

4.  No pollutant and no emission except for a small amount of water vapor.

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Applications of binary-cycle electrical plant:

•  First introduced in Russia in late 1960s.

•  First used in United States in early 1980s.

•  Today more systems are being brought online in locations where intermediate temperature geothermal resources are available.

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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.

An engineered underground reservoir is created where there is a heat source such as hot rock that lacks either the permeability or water to use for geothermal electrical production.

Water is injected under controlled conditions to increase the permeability by fracturing rock and creating water storage.

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EGS has successfully produced an additional 1.7 megawatts from an existing well field using EGS fracturing methods at Brady Hot Springs, Nevada (shown here). Other successes are at Newberry, Oregon and Desert Rock, Nevada.

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Drilling techniques for Enhanced geothermal Systems:

•  Drilling is to make unproductive areas economically viable.

•  Modern drilling technology allows wells to be drilled over 4 km deep.

•  Hydrospallation drilling: a steam of superheated high-pressure water is directed at the rock surface.

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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.



10-3 Low-Temperature Applications for Geothermal Heat

Low-temperature sources are used around the world for various heating applications.

Hot water from geothermal springs can be used to heat buildings, heat greenhouses and other agricultural applications, an provide process heat for industry.

Lower temperature sources are also used for resorts and spas.




Space heating

•  Geothermal steam from a geothermal reservoir or hot spring can be used for space heating.

•  Hot water or steam is routed through piping and heat exchangers and returned to the reservoir.

•  The secondary loop in the heat exchanger has clean water running through it, the clean water is pumped through radiators or fan coil systems mounted in ductwork throughout the building or homes.




Applications of geothermal steam in space heating:

•  Widely used in Iceland,.

•  Szentlörinc Heat Plant, Hungary (Mannvit)

•  heating and hot water demand for most of the homes in Szentlorinc, and has excess capacity for further expansion.

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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.




Snow melting is an application for low-temperature heat in areas with ample geothermal sources like Iceland.

When geothermal energy is used for melting snow, large pumps move hot water from the source through piping that is installed beneath the surface to be cleared.

The idea is being tested also in other locations on bridges and overpasses.

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Tubing for snow melting is installed before concrete is poured. This installation is in Klamath Falls, Oregon. The city has more than 50,000 sidewalks and crosswalks with district heating.

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The problems of snow melting systems:

•  Freezing the water in piping

•  Requiring large reservoir of hot water

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Greenhouses

Using geothermal hot water or steam technology to heat greenhouses.

The main expense in the geothermal greenhouse is for the pumps, heat exchangers, and storage tanks.

In some locations, warm water is pumped through tubing belowground and used to warm ground where vegetables are planted.

The warm ground allows seeds to be planted earlier and helps plants grow faster.

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A geothermal greenhouse in Netherlands.

http://www.greenhousecanada.com/energy-edge/renewables/adegeest-greenhouse-completes-geothermal-installations




Fish farms

Geothermal hot-water and steam systems are used in fish farms.

The warm waters help the fish to grow more quickly.

•  Aquaculture: –  Farming of aquatic plants and animals, including fish, shrimp,

oysters and seaweed.

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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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Food drying

Moderate-temperature geothermal heat can be used to dry the air, which is then used for drying the food.

Resorts

•  Hot springs for spa, bath –  Chena Hot Springs Resort (Alaska)

(Wiki)

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10-4 Geothermal Heat Pumps

Heat pump:

A mechanical system that uses a compressor to change the state of refrigerant so that heat can be removed or added to a conditioned space.

The geothermal heat pump is also called the ground source heat pump, it has liquid glycol running through tubing that is beneath the earth’s surface.

For most locations, the temperature of the earth stays constant at approximately 10 oC.




•  The natural flow of heat is from a warmer region to a cooler one. A heat pump is a device that can reverse this natural direction using a basic refrigeration cycle.

•  The heat pump can operate as a cooling system in the summer or as a heating system in the winter.

•  A conventional heat pump operates in a standard refrigeration cycle.

•  The refrigeration cycle works by expanding a refrigerant material, which absorbs the heat of vaporization, or compressing vapor into liquid and releasing the heat of vaporization.

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Four main stages of conventional heat pumps: 1.  The refrigerant vapor is compressed and flows into a condenser

coil. When air passes over the condenser, the vapor cools.

2.  The vapor changes to a liquid as it moves through the condenser and gives up heat of vaporization. This causes the condenser coil to become hot; the heat is then released to the surroundings.

3.  The liquid passes through an expansion valve and flows through an evaporator coil, where it suddenly expands from a liquid to a vapor.

4.  When the refrigerant flows through the evaporator and changes from a liquid to a vapor, it absorbs heat from the air that moves over the evaporator.

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Here it is operating in the heat cycle:




Geothermal heat pumps:

•  It uses a standard refrigeration cycle but receives heat from glycol that is circulated through a large amount of piping buried in the ground.

•  Glycol picks up heat from the earth as it circulates through the piping. The piping remains at a fairly constant temperature that is equal to the ground temperature.

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A geothermal heat pump uses the same principle of operation but absorbs heat from the ground into a glycol solution.




Advantages of geothermal heat pumps:

•  The conventional heat pump cannot work efficiently in freezing temperatures because it used the outside air as a heat source.

•  Because of the constant ground temperature, the efficiency of the geothermal heat pump remains the same even with large variations in air temperature including freezing temperatures.

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Four basic arrangements of piping:

•  Closed-loop horizontal system

•  Closed-loop vertical system

•  Closed-loop system that uses a pond or lake

•  Open-loop system that uses two wells (One supplies water, the other returns it to a location below the earth’s surface)

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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.

COP = hh/hw

COP: coefficient of performance (efficiency)

Hh: heat delivered by the heat pump

Hw: equivalent electric energy supplied to heat pump




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

1 kWh = 3,413 BTU




•  COP for a heat pump is not a fixed number.

•  It varies according to outside temperature.

•  COP becomes smaller as the outdoor temperature drops.

•  When COP <1, it is better to turn the compressor off.

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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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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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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.



Selected Key Terms

Enhanced Geothermal

System

Enthalpy Solar 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.



true/false quiz

1.  A high enthalpy site for geothermal power is one that has high heat, water, and permeability.



true/false quiz

2.  Low-temperature geothermal sources are useful for generating electricity.



true/false quiz

3.  Hot water reservoirs are not found in cold climates.



true/false quiz

4.  Most locations in the world can use geothermal energy for heat pumps.



true/false quiz

5.  An advantage of a flash power plant is that it isolates underground water from the turbines.



true/false quiz

6.  The Organic Rankine cycle is used with double-flash power plants.



true/false quiz

7.  Enhanced Geothermal Systems do not use artificial means to improve output.



true/false quiz

8.  Low-temperature geothermal sources can be used to cool buildings.



true/false quiz

9.  Geothermal heat pumps tend to be more efficient than standard heat pumps.



true/false quiz

10.  Tracers can track the flow of water through a reservoir and the amount of steam that is produced.



true/false quiz

Answers: 1. T 2. F 3. F 4. T 5. F

6. F 7. F 8. T 9. T 10.  T



Multiple-choice quiz

1.  The geothermal gradient refers to the

A.  Increasing density of the earth as a function of depth

B.  Temperature increases in the earth as a function of depth

C.  Variations in temperatures of hot-water pools

D.  Differences between summer and winter temperatures

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2. Geothermal resources are commonly classified by the

A.  Depth and volume of magma present

B.  Amount of hydrocarbons present

C.  Porosity of rock at the source

D.  Temperature and amount of fluid present

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3. The most useful type of generating electricity has

A.  Geysers

B.  Molten magma

C.  Low permeability

D.  Dry steam

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4. The type of geothermal plant that does not have steam piped directly to the turbine is the

A.  Binary-cycle plant

B.  Dry-steam plant

C.  Flash steam plant

D.  Double-flash steam plant

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5. A geothermal heat pump

A.  Operates only as a heater

B.  Operates only as an air conditioner

C.  Can act as a heater or an air conditioner

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6. A disadvantage to geothermal heat pumps is that they

A.  Cannot be used to cool

B.  Require a very hot geothermal source

C.  Require underground plumbing and glycol

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Answers:

1.  B

2.  D

3.  D

4.  A

5.  C

6.  C

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Lecture 6 2015-2016

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Transcript of Lecture 6 2015-2016