Renewable Energy Systems CH-02 - University of Colorado ...€¦ · absorption type refrigeration system in four steps: 1 •Generator: The generator heats the refrigerant (lithium

Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd

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

Buchla, Kissell, Floyd

Renewable Energy

Systems10



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



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



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.




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.



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.


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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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.


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

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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.


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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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.


The photo shows drilling of a geothermal well for a heat

pump in Klamath Falls, OR.

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10-2 Geothermal Electrical Power

Three methods for generating geothermal electricity:

1. Dry steam





2a. Flash steam (single flash)





2b. Flash steam (double flash)





3. Binary-cycle




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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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 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.




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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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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10-4 Geothermal Heat Pumps

A conventional heat pump operates in a standard refrigeration cycle. Here it is operating in the heat cycle:




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




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



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

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.



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

Documents

Renewable Energy Systems CH-02 - University of Colorado ...€¦ · absorption type refrigeration system in four steps: 1 •Generator: The generator heats the refrigerant (lithium