Geothermal Energy Lecture Notes.pdf

7/25/2019 Geothermal Energy Lecture Notes.pdf

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Buchla, Kissell, Floyd

Renewable EnergySystems 10


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10Geothermal PowerGeneration

10-1 TYPES OF GEOTHERMAL RESOURCES10-2 GEOTHERMAL ELECTRICAL POWER10-3 LOW-TEMPERATURE GEOTHERMAL HEAT

10-4 GROUND-SOURCE HEAT PUMPS10-5 ENVIRONMENTAL IMPACT

Renewable Energy SystemsBuchla, Kissell, Floyd

Chapter Outline


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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-enthalpyresources.

Enthalpy is defined as the amount of energy in a

system capable of doing mechanical work; it is afunction of temperature, pressure, and volume.

10-1 Types of Geothermal Sources


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The geothermal gradient refers to the increasingtemperature at increasing depths within the earth. Thethermal gradient varies widely according to location; itis about 25 oC/km in most locations but can be asmuch as 200 oC/km.



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Three temperature classifications of geothermalsources are:


1. High temperature sources are >150 oC. Thesesources are useful for generating electricity.

2. Moderate temperature sources are between90oC and 150 oC. These sources are useful forspace heating, drying, and industrial heat and insome cases are used for electricity production.

3. Low temperature sources are


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To exploit high temperature hot dry rock, water isgenerally injected from nearby wells. The water boilsand the steam is used to drive a steam turbine.


One plant at the 750 MWGeysers in Northern CA, isshown here. Recycledwater from nearby SantaRosa is piped in to be

injected to create steamfor the turbine.

Source:NREL


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Geopressurized reservoirsconsist of high-pressure, hotbrine in deep underground reservoirs. They arecharacterized by dissolved methane. The best knownareas for geopressurized reservoirs are along the Texas

and Louisiana Gulf Coast.


Energy can be obtainedfrom the thermal energy,from the hydraulic high

pressure, or from burningthe dissolved methane.

Coprid/Fotolia


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Hot water reservoirs are natural steam reservoirs thatare typically found along fault lines in the earth. Thetemperatures are high enough to create steam, whichcan be used to drive turbines.


For moderate temperaturereservoirs, Organic RankineCycle (ORC) generatorsare used. ORC generators

are shown at the ChenaHot Springs Resort in AK.

Source:CourtesyofC

henaHotSprings


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Normal ground temperature sourcesare useful forunderground geothermal heat pumps. In mostlocations in the world, the ground is a constant 10 oC atrelatively shallow depths. This is sufficient to keep heat

pumps working efficiently even when the outdoortemperature is freezing.


The photo shows drilling of ageothermal well for a heat

pump in Klamath Falls, OR.

Source:NREL


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10-2 Geothermal Electrical Power

Three methods for generating geothermal electricity:

1. Dry steam


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2a. Flash steam (single flash)


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2b. Flash steam (double flash)


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3. Binary-cycle


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An Enhanced Geothermal System (EGS) is one in which ageothermal site is made productive by artificial meansincluding fracturing networks to increase permeabilityand improved drilling methods including horizontaldrilling and laser drilling.

EGS has successfullyproduced an additional1.7 megawatts from anexisting well field usingEGS fracturing methodsat Brady Hot Springs, NV(shown here). Othersuccesses are atNewberry, OR and DesertRock, NV.

Source:DavidBuchla


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Another innovative proposal is to harness the heat in

underground coal seam fires. These fires represent anecological disaster as they can burn for tens of decadesin a very wide area and emit tons of pollutants as well asconsume a valuable resource. Most are caused by man

and they have been nearly impossible to extinguish.One study has shown that thereis significant energy that can berecovered by using standardmethods developed in

geothermal plants. The heatwould be used to drive ORCturbines.


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10-3 Low-Temperature Applications forGeothermal Heat

Low-temperature sources can be used for space

heating and cooling. Cooling is accomplished with anabsorption type refrigeration system in four steps:

1

Generator: The generator

heats the refrigerant (lithiumbromide solution); this causesthe water in solution to boil,creating water vapor at highpressure.

2

Condenser: The high pressure

vapor is condensed to a liquidat higher pressure. Heat fromcompressing is removed.

3

Expansion valve: High pressureliquid water passes through anexpansion valve andbecomes a cold liquid vapormix. The pressure (and theboiling point) are reduced.

4

Evaporator and Absorber: Thecold water absorbs heat fromthe space to be cooled and isabsorbed by the LiBr to form aweaker solution. The solution isreturned to the generator.


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Snow melting is an application for low-temperature heat

in areas with ample geothermal sources like Iceland. Theidea is being tested also in other locations on bridgesand overpasses.

Tubing for snow melting is

installed before concreteis poured. This installationis in Klamath Falls, OR. Thecity has more than 50,000sidewalks and crosswalks

with district heating.

Source:NREL


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Greenhouses and fish farms are another application for

low-temperature geothermal heat. Farming operationsinclude aquatic plants and animals as well as certainfood drying applications (including onion, garlic, carrots,and celery dehydrators).

Alligators are raised inColorado at an elevation ofover 7500 ft (2300 m). Theseare part of the ColoradoGators Reptile Park, which

also has other reptiles andbirds on display. A geothermalwell keeps the water warmyear round.

Source:NREL


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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:


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A geothermal heat pump uses the same principle of

operation but absorbs heat from the ground into aglycol solution.


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The coefficient of performance (COP) is a measure of

efficiency for heat pumps and is the ratio of heat energydelivered 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 delivers40,000 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


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10-5 Environmental Impact

Geothermal plants use steam and need to recyclewater to maintain the source, so monitoring of thequality of injected water and flow is important. Plants

like the Geysers use treated wastewater for injection.

Water

Monitoring of ground water canbe done by injecting chemicaltracers. Tracer tests are used toevaluate the injected water flow

through the reservoir and theamount of injection-derived steamthat is produced.

Source:NREL


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10-5 Environmental Impact

Air pollution from geothermalplants is minor and is less that 1%of an equivalent fossil-fueledplant. Small amounts ofhydrogen sulfide (H2S) may be inthe steam; most can beremoved at the plant. Othergases include small amounts of

CO2and NOx.

Air

Source:NREL


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Selected Key Terms

Binary-cycle plant

Coefficient ofperformance

(COP)

Double-flashsteam plant

Dry-steam plant

A geothermal power plant that uses the brine

water or steam from the geothermal reservoir toheat and vaporize a secondary fluid with a lowerboiling point to drive the turbine and generator.

A measure of efficiency for a heat pump; it is theratio of the heat produced to the energy

consumed and it varies with the outsidetemperature.

A geothermal electrical plant that usessuperheated dry steam from a geothermalreservoir and routes it directly to a steam turbineand generator to produce electricity.

A geothermal plant with two pressure reducingstages to create high pressure and low pressuresteam. The high and low pressure steam is routed

to two different turbines, which turn a generator.


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Selected Key Terms

EnhancedGeothermal

System

Enthalpy

Solar module

Flash steamplant

A system in which a geothermal site that isdeficient in water or permeability is made to beproductive by artificial means.

The amount of energy in a system capable of

doing mechanical work; it is a function oftemperature, pressure, and volume.

A geothermal plant that creates steam from highpressure hot water (brine) using a special controlvalve or orifice plate to reduce the pressure and

cause some of the liquid to boil (or flash) intosteam, which is used to drive a steam turbine andgenerator to produce electricity.


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true/false quiz

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


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true/false quiz

2. Low-temperature geothermalsources are useful for generatingelectricity.


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true/false quiz

3. Hot water reservoirs are not found incold climates.


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true/false quiz

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


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true/false quiz

5. An advantage of a flash powerplant is that it isolates undergroundwater from the turbines.


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true/false quiz

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


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true/false quiz

7. Enhanced Geothermal Systems donot use artificial means to improveoutput.


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true/false quiz

8. Low-temperature geothermalsources can be used to coolbuildings.


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true/false quiz

9. Geothermal heat pumps tend to bemore efficient than standard heatpumps.


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true/false quiz

10. Tracers can track the flow of waterthrough a reservoir and the amountof steam that is produced.


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true/false quiz

Answers:1.T2.F3.F4.T5.F

6.F7.F8.T9.T10. T

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Geothermal Energy Lecture Notes.pdf