geothermal introduction

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Geothermal Introduction

Department of Mechanical Engineering

[email protected]

[email protected]

The Course Outline

• Geothermal:

• Geophysics Regional Geothermal Potential;

• Geothermal energy applications; Harnessing

geothermal recourses; Dry Rock and Aquifer Analysis;

• Technology for Geothermal Resources; Applications:

low and high temperature applications;

• Electricity generation and thermal applications

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Geothermal background

• Aims– Define the term geothermal

– Application of geothermal energy

– Geothermal energy and environment

– Benefit of geothermal energy

• Objectives/learning out come– The nature of earth heat

– Geothermal systems and functions

– The status of geothermal in the world

– Impact on environment

– Major forms of geothermal Utilisation

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Background

• Geothermal energy is literally, heat contained with the Earth that generate geological phenomenon on a planetary scale

• It is the radiogenic heat: it is heat balance and the earth geothermal history

• The heat is generated by continually decay of the long lived isotopes of uranium (U238,U235) thorium (Th232 )and potassium(K40) which are present in the Earth

• The total heat flow from the Earth is estimated at 42x1012W. ( convection, conduction and radiation) from crust 2% by volume; 32x1012W from mantle,82%(300-350 0 C of the volume and 1.7 x1012W (4,000 0 C) from the core, 16% of the total volume, it does not contain any isotopes

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The Earth

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The earth is chemically differentiated into the crust, mantle and core of which the mantle makes up to 82% of the volume of the earth and 65% of its mass

The Earth

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Earth’s Structure

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Earth Structure

Internal structure of the Earth. Thickness of the crust and depths to various discontinuities from the Earth’s

surface are given in kilometers

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99 % of the earth‘s interior is hotter than 1000°C.

Global heat loss is 40 million MW.

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Background/Applications

• The utilisation of this form of energy is limited in the areas where geological conditions permit a carrier (water or steam) to transfer the heat from deep hot zones to near surface, thus giving rise to geothermal resources

• Utilisation

• Industrial heating

• Residential heating

• Greenhouse

• Electricity generation (1904)

• Now it is competitive source of energy for electricity generation( USA 2395 MW, Philipine 1931MW,Kenya 45 MW

• Non electric energy use USA 3,766 MW, Kenya 1.7 MW

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Heat Flow

The Temperature Gradient-Variations

Temperature Verses Depth

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Key Factors for Geothermal

Power

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Top fifteen countries utilising

geothermal energy in 2005

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Potential of renewable energy

sources (WEA 2000)

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Advantages of Geothermal

• Provides clean and safe energy for a little land, it can be build in many locations without having considerable impact.

• It is renewable and sustainable and avoids the use and importation of fossil fuel.

• It generates continuous and reliable base load

• Conserves fossil fuel and contributes to diversification of energy resources

• It has a near zero- GHG emission. The emissions apart from water vapour

• Very little space is required for unit power generated, typically 400m2.MW-1, for other forms of energy sources varies between 3,200 and 3,700m2.MW-1.

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Advantages cont.

• The average geothermal plant occupies only 400 m2 for the production of each gigawatt hour over 30 years.

• Geothermal energy use avoids the problems of acid rain,GHG

• Potentially hazardous elements produced in geothermal brines are removed from the fluid and injected back into the producing reservoir.

• Land use for geothermal well, pipelines, and power plants is small compared with land use for other extractive energy sources such as oil, gas, coal, and nuclear.

• Geothermal development projects often coexist with agricultural land uses, including crop production or grazing.

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The disadvantages of geothermal

• All known geothermal systems contain aqueous carbon dioxide species in solution, and when a steam phase separates from boiling water CO2 is the dominant (over 90% by weight) non-condensable gas.

• In most geothermal systems, non-condensable gases make up less than 5% by weight of the steam phase.

• Emissions of H2S

• It is not spread world wide, it is site specific

• Yield may decrease if a through study was not made.

• The corrosive nature of the salts may increase the maintenance costs.

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Energy Investment Cost for Electricity Generation

From Renewable (US Cents /kWh). Fridleifsson 2001

SourcesSources Current Current Energy CostEnergy Cost

Potential Potential Future costFuture cost

TurnkeyTurnkey

InvestmentInvestment

BiomassBiomass 55--1515 44--1010 900900--3,0003,000

GeothermalGeothermal 22--1010 11--88 800800--3,0003,000

WindWind 33--1515 33--1010 1,1001,100--1,7001,700

PhotovoltaicPhotovoltaic 2525--125125 55--2525 5,0005,000--10,00010,000

TidalTidal 88--1515 88--1515 1,7001,700--2,5002,500

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Data on earth and atmosphere

Region Distance from

surface km

Temperature OC Density (kg.m-3)

thermosphere 300 1125 3.6 x 10 -14

Mesosphere 85 -92 2 x 10 -8

Stratosphere 50 0 1xx 10 -6

troposphere 12 -60 3x x 10 -4

Surface 0 10-25 2 (continental)

3 (oceanic)

Crust 25 1100 3.3

Mantel 2900 3700-4500 5.7-10.2

Liquid (iron) core 5100 4300-6000 11.5

Solid Inner (iron) core 6350 4500-6600 11.5

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Nature of Geothermal resources

• The geothermal gradient : the increase in the temperature with the depth of the Earth 2.5 to 3.00C per 100m.

• There are some areas where the geothermal gradient is lower than the average it may be as low as 10C per 100m

• Some geothermal areas, the gradient can be more that ten times the average.

• That is where one finds geysers, hot springs, volcanic activities…

• The planet consist of crust 35-65km on continental areas and 5-6 km in ocean areas, mantle (2,900km) thick and core 2470 km in radius. .

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The Heat

The earth crust is composed of various types of rocks which contain some radioactive isotopes (U235,U238,Th232,K40)

The Heat passes through the crust by:

1. Natural cooling and the friction from the core

2. Radioactive decay of elements (uranium and thorium)

3. Chemical reactions

– The radioactive elements are concentrated in the crust

– If the heat flow is only conduction, the temperature gradient would have been constant.

– The temperature gradient is high in poorly conduction solids.

– And low in the region of increased heat transfer by convection usually by water.

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The Heat

• Natural hydrothermal circulation : Water percolates to deep aquifers to heated to dry steam result in geysers

• Hot igneous systems associated with the heat from semimolten magma that solidifies to lava.

• Dry rock fracturing : Poorly conducting dry rock –it stores heat for a number of years. Artificial fracturing from boreholes enables water to be pumped though the rock to extract heat.

• In practice geothermal energy plants are in Hyperthermal regions are associated with natural hydrothermal systems. In Semithermal regions both hydrothermal and hot rock extraction is developed.

• In the Normal Thermal gradient, there is not heat to make it commercially viable

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Geothermal resources

• A typical geothermal development project is locate

geothermal system which is viable

• There are five features for a geothermal resource

1. A large heat source

2. A permeable reservoir ( path of lower resistance)

3. A supply of water

4. an overlaying layer of impervious rock (retain geofluid)

5. Reliable recharge mechanism.

• If any of the above features is missing, the field may not be

worth exploiting

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Elements of geothermal system

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The physics of geothermal resources/

Characteristics of a geothermal site

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Geothermal resource

manifestation

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Hot Springs Bundubugyo

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Major plates of the world. Mid-oceanic ridges, transform faults, and trenches

and other subduction zones The seismic belts and the volcanoes are located

in the vicinity of the plate boundaries.

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Converging plates and the resultant

physiographic features

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Geographical Location of Resources

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Location of geothermal plants

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Geothermal Systems

• The geothermal systems are found in the regions with normal temperature gradient or higher that average temperature gradient areas.

• The temperature can be as low as 100 0C or 400 0C at the economic depth.

• The meteoric water replaces fluid that escapes or used. The water or steam contains chemicals and gases such as CO2 or H2S

• The mechanism underlying geothermal systems is by large convection by fluid convection

• Convection occurs because of the heating and consequent thermal expansion which is supplied at the base of the of the circulating systems.

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Geothermal Systems .

• Geothermal systems occur in nature in variety of combinations of geological, physically and chemical characteristics, thus giving rise to several types of systems.

• Of all the elements of geothermal systems it is the heat source which need to be natural.

• Other parts of the system can be artificial if the conditions are favourable.

• The injection well is where water is pumped back to restore replenish water reservoir

• Hot dry rock, both fluid and reservoir is artificial. Water is pumped under pressure through a drilled well. Second bore is bored to extract hot water or steam.

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Single-flash Steam Power Plants

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Single-flash Steam Power Plants

• Typical steam conditions:

155-165°C and 0.5-0.6 MPa

• Design conditions:

Currently it is required about 8 kg steam per saleable kWh

• Waste brine (unflashed) can be up to 80% of the fluid

produced

• The waste brine is reinjected unless there is a direct heating

application

• Reinjection wells must be available for fluid disposal

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Double-flash Steam Power Plants

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Double flash System

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Double-flash Steam Power Plants

• Scaling and non-condensable problems

are minimum

• Raises the efficiency up to 20-25% and the

plant cost only by 5% .

Extremely large volumes of geothermal fluid are

required -> sometimes can be as much as 5

times more fluid than for a dry steam plant with

the same power output.

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High Pressure/intermediate /Low SteamTurbines

.

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Dry Steam Power Plants

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Schematic diagram of a direct dry-

steam plant

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• Efficiency is strongly affected by non-condensable gases (CO2, H2S, etc)

• The gases cause higher residual pressures at the back end of the turbine

• They reduce the suction efficiency -> direct economical impact

• To avoid the presence of gases the plants are equipped with ejectors which have an impact on efficiency (steam supply or electrical power is required for their operation)

• Non-condensable gases cannot longer be released to the atmosphere so they must be trapped chemically or reinjected with the waste water

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• Low environmental impact

• Noncondensable gases in the steam are isolated in the

condenser and removed by means of vacuum pumps or

steam-jet ejectors

• The sulfur from certain types of abatement systems is in pure

form and may be sold commercially or disposed of in an

appropriate.

• The excess condensate from the cooling tower is reinjected as

is any liquid trapped from the steam transmission pipelines.

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Binary Cycle Power Plants

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Binary system

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• The geofluid is compressed and passed through the heat

exchagers and finally disposed in the injection wells still in

liquid phase

• Binary plants constitute 33% of all geothermal units in

operation but generate only 3% of the total power

• Typical geofluid conditions: 150°C

• η ranges between 10% and 13%

• (ηcarnot =26% for Τ = 150°C )

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Binary system

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Turbine Generator

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Flash Steam Power Plants

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Production and reinjection well

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Out door Units

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Indoor Units Hydraulic Conductivity

• Hydraulic Conductivity Kw is a measure of permeability of a

rock. Darcy Law states that the speed v of a fluid moving

through a porous medium is proportional to the pressure

gradient causing the flow .

• Where H is the effective head of water and L length along

the direction of the flow.

• The volume of water (Q) flowing in a unit time though a

cross section area A (m2)

L

HK w=υ

L

HAKQ w=

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Hydraulic Conductivity of Materials

Material Porosity (%) Hydraulic Conductivitym.day-1

Silt

Clay

40-60

50-60

10-2

10-2

Sand Stone 5-30 10-2 - 1

Slate 0.001-1 10-4 – 10

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Dry Rock and Aquifer Analysis

• Area A Surface temperature To

Depth•Overlaying materials

T1minimum useful temperature

Hot dry rock

T2 temperature at maximum depth

z1

d1

z2

Profile of hot dry rock for calculating the heat content density p, specific heat capacity c, Temperature gradient dT/dz=G 60

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Consider a large mass of dry material . The rock has a density ρr, and specific heat capacity crand cross section area A. The heat transfer within the mass is largely by conduction. There is linear increase of temperature with the depth z. The reference point is the surface of the earth where z = 0.

Let the minimum useful temperature be T1 at depth z1. The above expression

can be modified to :

The total heat content of the rock from near surface z1 to depth inner depth z2

The useful heat content δE, at temperature T greater than T1 in an element of thickness δz at depth z is:

GzTzdz

dTTT o +=+= 0

1101 GzTzdz

dTTT o +=+=

G

TTz 01

1

−=

)()()()( 11 zzGczATTczAE rrrr −=−= δρδρδ

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The total heat content of the rock from near surface z1 to depth inner depth z2

dzzzGAcEz

z rr )( 10

2

1

−= ∫ ρ

2

1

)2 1

2

0

z

z

rr zzz

AGcE

−= ρ

2

)( 212 zz

AGcrr

−= ρ

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Geothermal Analysis

• Let the average temperature greater than T1 be θ

)(2/)( 1212 zzGTT −=−=θ

)( 12 zzAcC rrr −= ρ

θro CE =

2/)( 221 zzACE rro −= ρ

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Geothermal analysis

• Assume that heat is extracted from a rock uniformly is

propositional to the temperature greater that T1 by a flow of

water : volume V, density ρ , heat capacity Cw;. The water will

be heated to temperature of θ in a near perfect heat

exchanger. τ is the time constant.

dt

dccV rww

θθρ −==.

τθρ

θθ dt

dtc

cd

r

ww −=−=

τ

τ

τ

θθθ

t

r

t

eE

dt

dE

cE

e

−

−

−=

==

0

0

ww

rr

ww

r

cV

zzAC

cV

C

ρ

ρ

ρτ .

12.

)( −==

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Geothermal Energy

In hot aquifer, the heat source lies within a layer of water deep

beneath the ground surface. Assuming that the thickness of

the aquifer (h) is much less than the depth z2 below the

ground level and all the water is at temperature T2.

The fraction aquifer containing water, the porosity is p’. The

remaining space of the rock of density ρ.

The maximum useful temperature is T1. Determine the

characteristics of the rock :

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Geothermal Hot Aquifer

• Area A Surface temperature T0

Material above

z2 aquifers

Depth T2 Hot water aquifer

h at Temperature T2

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Geothermal Hot bed rock

.

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Example

(a) Calculate the useful heat content per square kilometre of dry

rock granite to the depth of 7 km. Take geothermal temperature

gradient at 400C km-1, the minimum useful temperature as 140K

above the surface temperature, ρr = 2700 kg.m-3, cr = 820 J kg-1K-

1.

(b) What is the time constant for the useful heat extraction using

the flow rate of 1m3s-1km-2?

(c) That will be the useful heat extraction rate after 10 years

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Solution

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Assignment

(a) Calculate the useful heat content per square kilometre of dry

rock granite to the depth of 7km. Take geothermal

temperature gradient at 400C km-1, the minimum useful

temperature as 140K above the surface temperature, ρr =

2700 kg.m3, cr = 820 J. kg-1K-1.

(b) What is the time constant for the useful heat extraction with

a pumped extraction of 100 litres per second per square

kilometres.

(c) That will bed the useful heat extraction rate after 10 years

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Other Environmental Impacts

Source: (Lunis and Breckeridge 1991)

Impact Probability of Occu Severity Conceq.

Air Quality L M

Surface H2O Pol M M

Underground H2O L M

Landscape L L to M

High Noise Level H H to M

Well Blowout L L to M

Conflict, Caltural L to M M to H

Secio-economic L L

Chem-therm Pol L M to H

Solid Waste Disp M M to H

Toxicity and environmental effects

• A deterioration in surface water

o Aquatic life

o Stock if water is used for stocking

o Crops if water is used for irrigation

o Human and plants if water is used for drinking

• Lithium, boric acid and high salinity

o Changes the structure and properties of soil,

o Boron weight loss in animals

o Highly saline not palatable and can effectively affect the

aquatic life

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Toxicity and environmental effects

• The relative toxicity of arsenic in organic form

• High concentration lead to chronic acute poisoning in human and animal

• aquatic plants can absorb inorganic arsenic if eaten can cause problems

• Arsenic is also acrogenic

• Skin and other forms of cancer has been observed in population taking high concentration of arsenic

Mercury

• Bio- accumulation in aquatic plants and animals in form of methyl-mercury – it toxic form affects the central nervous system. The human and animals they are at greater risk. The mercury is mainly thru food chain.

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Toxic (H2S) and Environmental Effects

• Hydrogen sulfide is detectable at low concentration( 0.3 mg/kg)

• Odour complaints from those living windward

• As the concentration increases it becomes sweeter and finally disappears at 150 mg/kg

• At higher levels it is very dangerous to human health

• Headache, leg pain, irritation of respiratory track (10-500 mg/kg)

• Loss of consciousness (500-700 mg/kg)

• (>700 mg/kg) laboured breathing

• (>1,500 mg/kg) Death

• The gas can only leave the body through urine, intestines. Otherwise its effect to environment is limited.

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geothermal introduction

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