FACULTY OF SCIENCES

DEPARTMENT OF PHYSICS

CONTRIBUTION TO THE GEOTHERMAL ENERGY STUDY

IN RWANDA

Dissertation presented for the award

of a Bachelor of Sciences (BSc).

By: NDASHIMYE Maurice

Supervisor : Prof. MAREMBO KAREMERA Claver

Huye, March 2007

DEDICATION

To my mother,

I dedicate this work.

i

ACKNOWLEDGEMENT

Many people were involved with this study, We are grateful for the effort they made,

many helped us, many taught us or inspired us. We can’t name all to whom a debt of

gratitude is owed, but We will try.

First of all, let us express our deepest gratitude to our project supervisor

Pr Marembo Karemera who tirelessly went the extramile to improve the quality of this

work.

We would like to express our heart-felt gratitude to the late Hitimana Anastase

our beloved father who initiated our first steps to school and encouraged us to learn science

subjects from our tender age, even though he is physically gone, without his inspiration,

We would never have managed to realize this dream.

Furthermore, We express our sincere gratitude to our teachers from primary school

up to the National University of Rwanda for their intellectual guidance during our studies.

Particularly, We would like to express our special thanks to Pr Marembo Karemera,

Dr Safari Bonfils, Dr Ndahayo Fidele and Mr Mageza Celestin who made this all happen.

We’re highly grateful to the National University of Rwanda for providing financial

support to carry out this study, our thanks goes to the staff of the chemistry departement for

their help in our laboratory work, great appreciation to Mr Namugize J. Nepo, who spent so

many of his days guiding us in our laboratory work.

We also express our gratitude to our fellow students with who We share our student

life, for their cooperation and moral support. We appreciate the warmth and friendship that

existed between us and people We interacted with during our study at the National

University of Rwanda.

Finally, We’re especially indebted to our mother and our sisters who care and gave

this work a lot of guidance and encouragement.

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TABLE OF CONTENTS

DEDICATION..................................................................................................i

AKNOWLEDGEMENT..................................................................................ii

TABLE OF CONTENTS................................................................................iii

TABLE OF FIGURES....................................................................................vi

LIST OF TABLES..........................................................................................vi

SYMBOLS AND ABBREVIATIONS..........................................................vii

NOMENCLATURE FOR LANT FLOW DIAGRAMS................................vii

SOMMAIRE.................................................................................................viii

ABSTRACT....................................................................................................ix

CHAPTER I: INTRODUCTION....................................................................1

I.1. STUDY INTEREST...............................................................................................................................1

I.2. PROBLEMATICS...................................................................................................................................1

I.3. HYPOTHESIS.........................................................................................................................................1

I.4. OBJECTIVES.........................................................................................................................................1

I.5. METHODOLOGY..................................................................................................................................2

I.5.1. Documentary research..........................................................................................2

I.5.2. Samples and datas collection...............................................................................2

I.5.3. Research in laboratory.........................................................................................2

I.5.4. Learning about GIS technology...........................................................................2

I.6. WORK SUBDIVISION..........................................................................................................................2

CHAPTER II. GEOTHERMAL OVERVIEW................................................3

II.1. GEOLOGICAL BACKGROUND.........................................................................................................3

II.1.1. The Earth’s structure...........................................................................................3

II.1.2. The plate tectonic theory....................................................................................4

II.1.3. The Earth’s heat..................................................................................................6

II.1.3.1. Heat transfer within the Earth.......................................................................7

II.1.3.2. The Earth’s geothermal gradient and the thermal conductivity of rocks......9

iii

II.2. GEOTHERMAL RESOURCES..........................................................................................................10

II.2.1. General aspects.................................................................................................10

II.2.2. Hydrothermal systems......................................................................................11

II.2.3. Water-dominated fields....................................................................................12

II.2.4. Wet steam fields...............................................................................................13

II.2.5. Vapour-dominated fields..................................................................................13

II.2.6.Hot Dry Rock systems.......................................................................................14

II.2.7. Geopressured reservoirs....................................................................................15

II.3. UTILISATION OF GEOTHERMAL RESOURCES..........................................................................16

II.3.1. Electricity from geothermal fluids....................................................................16

II.3.2. Direct uses of geothermal energy.....................................................................17

CHAPTER III: GEOTHERMAL PROSPECTS AND ELECTRICITY

GENERATION..................................................................18

III.1. GEOTHERMAL RESERVOIR EXPLORATION.............................................................................18

III.1.1. Inventory and survey of surface manifestations..............................................18

III.1.2. Geological and hydrogeological surveys........................................................19

III.1.3. Geochemical surveys.......................................................................................19

III.1.4. Geothermometers............................................................................................20

III.1.5. Geophysical surveys........................................................................................21

III.1.5.1. Seismic surveys........................................................................................21

III.1.5.2. Gravity surveys.........................................................................................21

III.1.5.3. Magnetic surveys......................................................................................21

III.1.5.4. Electrical-resistivity surveys....................................................................22

III.1.5.5. Electromagnetic surveys...........................................................................22

III.1.5.6. Thermal-measurement surveys.................................................................23

III.1.6. Exploratory wells............................................................................................24

III.2. GEOTHERMAL ELECTRICITY GENERATION...........................................................................24

III.2.1. Thermodynamic background...........................................................................24

III.2.1.1. Thermodynamic laws...............................................................................24

III.2.1.2.Thermodynamic cycles..............................................................................29

A. The Carnot cycle.............................................................................................29

B. The Rankine cycle..........................................................................................31

C. The Kalina cycle.............................................................................................33

iv

III.2.2. Geothermal electricity generating cycles........................................................34

III.2.2.1.Direct intake cycles...................................................................................34

III.2.2.2.Flash steam cycles.....................................................................................35

III.2.2.3. Binary cycle..............................................................................................36

III.2.2.4.Combined or hybrid plants........................................................................37

III.2.2.5. Efficiency of generation...........................................................................38

CHAPTER IV: RWANDA GEOTHERMAL SYSTEM...............................39

IV.1. GEOTHERMAL IN AFRICA............................................................................................................39

IV.2. RWANDA GEOTHERMAL RESOURCES LOCATION................................................................40

IV.2.1. Southern region...............................................................................................40

IV.2.2. Western region................................................................................................40

IV.2.3. Northern region...............................................................................................40

IV.3. GEOTHERMAL FLUIDS ORIGIN...................................................................................................42

IV.3.1. Deep waters origin.........................................................................................42

IV.3.2. CO2 origin........................................................................................................42

IV.4.CALCULATION OF DEEP TEMPERATURES...............................................................................42

IV.4.1. Introduction on geothermometers...................................................................42

IV.4.1.1. Geothermometer with Silica...................................................................42

IV.4.1.2. Geothermometers with Na/K...................................................................43

IV.4.1.3. Geothermometer with Na/Li....................................................................44

IV.4.1.4. Adjustment considering Mg.....................................................................45

IV.4.2. Analysis of natural waters with Atomic Absorption Spectroscopy................45

IV.4.3. Deep temperatures estimated with geothermometers.....................................45

IV.4.3.1 Mashyuza prospects..................................................................................46

IV.4.3.2 Ruganji prospects.....................................................................................46

IV.4.3.3. Ntaresi prospect.......................................................................................47

IV.4.3.4. Amakera and Gihugu prospects...............................................................47

IV.4.3.5. Gishyita geothermal prospect..................................................................47

IV.5. CONCLUSION..................................................................................................................................47

IV.6. RECOMMENDATIONS...................................................................................................................48

REFERERENCES..........................................................................................49

APPENDIX....................................................................................................50

v

TABLE OF FIGURES

Figure II.1: The earth structure..............................................................................................3

Figure II.2: The basic concept of plate tectonics...................................................................5

Figure II.3: World pattern of plates.......................................................................................6

Figure II.4: Heat conduction.................................................................................................8

Figure II.5: An hydrothermal field......................................................................................11

Figure II.6: Schematic representation of a hot dry rock......................................................15

Figure III.1: A reversible cyclic process.............................................................................26

Figure III.2: Irreversible process.........................................................................................27

Figure III.3: T-S diagram for a Carnot heat engine.............................................................29

Figure III.4: T-S diagram for a simple Rankine cycle........................................................31

Figure III.5: Simplified Kalina cycle..................................................................................33

Figure III.6: Simplified flow diagram for a direct-steam geothermal power plant.............35

Figure III.7: Simplified flow diagram for a single-flash geothermal power plant..............36

Figure III.8: Simplified flow diagram for a basic binary geothermal power plant.............37

Figure III.9: Simplified flow diagram for a Kalina binary geothermal power plant...........37

Figure IV.1: East African rift system..................................................................................39

Figure IV.2: Rwanda Geothermal resources visited during our study................................41

Abacus I: Estimates of DMg...............................................................................................55

LIST OF TABLES

Table I : GPS data...............................................................................................................51

Table II : Chemical analyses results (NUR 2007)...............................................................51

Table III : Chemical analyses results (Chevron 2006)........................................................52

Table IV : Chemical analyses results (BRGM 1982)..........................................................52

Table V : Chemical geothermometry results ( NUR 2007).................................................53

Table VI : Chemical geothermometry results (Chevron 2006)...........................................53

Table VII : Chemical geothermometry results (BRGM 1982)............................................54

Table VIII: Estimated installation costs for a 10 MWe geothermal development..............54

Table IX : Estimated installation costs for a 50 MWe geothermal development................55

Standard atomic absorption conditions for Mg, Na, K and Ca......................................56-59

vi

SYMBOLS AND ABBREVIATIONS

HDR: Hot Dry Rock

HWR: Hot Wet Rock

GPS: Global Positioning System

GIS: Geographic Information System

GEA: Geothermal Energy Association

DC: Direct Current

Mg: Magnesium

Na: Sodium

Ca: Calcium

K: Potassium

MT: Magnetotelluric

NOMENCLATURE FOR PLANT FLOW DIAGRAMS

BCV: Ball Check Valve

C: Condenser

CP: Condensate Pump

CS: Cyclone Separator

CSV: Control and Stop Valves

CT: Cooling Tower

CW: Cooling Water

CWP: Cooling Water Pump

F: Flasher

FF: Final Filter

IP: Injection Pump

IW: Injection Wells

M: Make-up water

P: Well Pump

PH: Preheater

S: Silencer

SH: Superheater

SP: Steam piping

SR: Sand Remover

T/G: Turbine/Generator

Th: Thottle valve

WP: Water Piping

WH: Welhead valves

vii

SOMMAIRE

Le but principal de cet etude est de prouver qu’il est possible de produire de

l’électricite à partir des resources géothermales du Rwanda.

Pour réaliser notre travail, il était requis de se baser sur plusieurs paramètres de ces

resources, à savoir la temperature et le volume du réservoir geothermal, la composition

chimique du fluide geothermal, la pression de sortie du fluide geothermal, la géologie des

régions abritant ces ressources.

Etant donné que toutes ces données n’étaient pas disponibles, nous avons procédé à

un prélevement d’échantillons sur huit sources chaudes qui s’avèrent susceptibles de

produire de l’électricite, ces sources sont: Mashyuza I et II, Ntaresi, Gishyita, Cyabararika,

Gihugu et Ruganji I et II.

Une localisation de ces sources par GPS a été faite ainsi qu’une analyse au

laboratoire de ces échantillons par géothermometres chimiques, afin d’éstimer les

temperatures en profondeur de ces sources.

Après ces analyses, nous avons constaté que seules les sources de Mashyuza, Ntaresi et

Ruganji sont susceptibles de produire de l’électricite en utilisant la technologie binaire.

viii

ABSTRACT

With this study, we intend to proof that it is possible to produce electricity from

Rwanda geothermal resources.

In order to fulfill this aim, several parameters on these resources are required, among

them we can say deep temperatures and volume of the geothermal reservoirs, the geothermal

fluid chemical composition, geology of the reservoirs, the geothermal fluid up flow

pressure.

Given that all of these datas were not available, samples were gathered from eight

hot springs judged to have a high potential. These springs are Mashyuza I and II, Ntaresi,

Cyabararika, Gihugu, Gishyita and Ruganji I and II.

A GPS location of these spring has been done and geothermometric analyses of the

gathered samples has been done in order to estimate deep temperatures of these springs.

From these analyses, we ended up with a conclusion that only Mashyuza, Ntaresi

and Ruganji can generate electricity by the use of binary technology.

ix

Chapter I: INTRODUCTION

I.1. STUDY INTEREST

Our study has been chosen in order to show that geothermal energy can be one of the

solutions to the energy crisis in our country.

In fact we have chosen geothermal energy because it is:

• Present in our country in a considerable quantity.

• Renewable

• Stable because it doesn’t depend on weathers or times .

I.2. PROBLEMATICS

Actually, Rwandan population is estimated at 8.2 million and the annual growth rate

at 2.5%. As consequence, we have an increasing energy consumption and if we consider the

economic activities development in our country, we realize that energy is really a big problem

for our country.

The current status is given by:

• A weak electrification (4% of the population).

• Most of energy comes from firewood which is dangerous for the ecosystem.

• A huge potential in solar energy, organic-gas and bio-gas but not used efficiently.

• Other forms of energy like Aeolian, geothermal… not exploited at all.

I.3. HYPOTHESIS

Can geothermal energy be economically exploited in Rwanda?

I.4. OBJECTIVES

Our study has three objectives:

• Location of all of the geothermal resources of Rwanda and establishment and

mapping the latter using GIS technology.

• Calculation of deep temperatures using chemical geothermometers

• Estimate the geothermal potential in Rwanda using calculations and simulations.

1

I.5. METHODOLOGY

I.5.1. Documentary research

In order to gather enough information on geothermal energy we have conducted

a documentary research in research centers, Infrastructure ministry and in different libraries.

We have also visited several websites.

I.5.2. Samples and data collection

We went in different regions where geothermal resources are located in order to collect

GPS data on these resources and take samples for laboratory analyses.

I.5.3. Research in laboratory

Our research has been conducted in laboratory whereby we intended to measure

different chemical elements concentrations from our samples.

I.5.4. Learning about GIS technology

We went in a GIS centre and learnt about this technology in order to establish a map of

Rwanda geothermal system.

I.6. WORK SUBDIVISION

This study is subdivided into four chapters:

I. INTRODUCTION

II. GEOTHERMAL OVERVIEW

III. GEOTHERMAL PROSPECTS AND ELECTRICITY GENERATION

IV. RWANDA GEOTHERMAL SYSTEM

2

Chapter II. GEOTHERMAL OVERVIEW

II.1. GEOLOGICAL BACKGROUND

II.1.1. The Earth’s structure

The Earth is formed by three concentric zones, crust, mantle and core (Fig. II.1).

The crust: The Earth’s crust is analogous to the skin of an apple. The thickness of the

crust (7 km on average under the ocean basins, 20–65 km under the continents) is insignificant

compared to the rest of the Earth which has an average radius of 6370 km.

Figure II.1: The Earth structure

Wells give us direct access only to the crust, and to depths not much beyond

10 km. Studies of seismic waves have shown that the Earth’s crust is thinner beneath the

oceans than beneath the continents, and that seismic waves travel faster in oceanic crust than in

continental crust. In part because of this difference in velocity, it is assumed that the two types

of crust are made up of different kinds of rock. The denser, oceanic crust is made of basalt,

whereas the continental crust is made of granite.

3

The mantle: The mantle lies closer to the Earth’s surface beneath the ocean (at a depth

of 7 km), than it does beneath the continents (20–65 km). It extends from the base of the crust

for about 2900 km. The most accepted hypothesis about the composition of the mantle is that it

consists of ultrabasic rock (very rich in Fe and Mg) such as peridotite, which is a heavy

igneous rock made up chiefly of ferromagnesian minerals. The Earth’s crust and uppermost

mantle together form the lithosphere, the outer shell of the Earth that is relatively rigid and

brittle (Fig. I.1). The lithosphere is split into a number of large blocks at continental scale or

more, which are called lithospheric plates in the plate tectonic theory.

The lithosphere (crust and upper mantle) is about 70 km thick beneath the oceans and

100–125 km thick beneath the continents. Its lower boundary inside the mantle is marked by a

particular layer, known as the low-velocity zone, in which seismic waves slow down. This

zone, extending to a depth of perhaps 200 km, or more, from the surface, is called the

asthenosphere.

Rocks in the asthenosphere may be closer to their melting point than rocks above or

below this zone. Mantle rocks in the asthenosphere are weaker than they are in the overlying

lithosphere, then the asthenosphere can deform easily by plastic flow, and convection can take

place within the asthenosphere as well as within the lower mantle.

The lithosphere seems to be in continual movement, probably as a result of the

underlying mantle convection, and plates of brittle lithosphere probably move easily over the

asthenosphere, which may act as a lubricating layer below.

The core: The Earth’s core extends from 2900 to 6370 km (the Earth’s centre): its

thickness, or radius, is 3470 km. The temperature in the core should be around 4000°C and the

pressure at the Earth’s centre is estimated at 3.6 million bar (360,000 MPa).

II.1.2. The plate tectonic theory

The plate tectonic theory, is a unifying theory that accounts for many apparently

unrelated geological phenomena. According to this theory the rigid outer shell of the Earth, or

lithosphere is divided into separate blocks or plates, termed lithospheric plates (Fig. II.3).

These plates move slowly across the Earth’s surface, at a speed of a few centimeters per year.

As the plates comprise continents and sea floors, the plate tectonics concept means that the

continents and sea floors are moving, sliding on top of the underlying plastic asthenosphere.

4

These plates either pull away from each other, slide past each other, or move towards

each other. The boundaries between plates are of three types (Figure II.2):

• Diverging plate boundaries (or spreading centres, or ocean ridges): These occur where two

plates are moving apart, thus permitting the upwelling of magma from the asthenosphere to

form new lithosphere. Most spreading centres coincide with the crest of submarine

mountain ranges, called mid-oceanic ridges which rarely rise above sea level . (e.g.

Iceland)

• Converging plate boundaries: These correspond to oceanic trenches, where two plates

converge and collide so that one plate slips and sinks below the other and is eventually

reabsorbed into the mantle and “destroyed” (for example, the Nazca plate in the eastern

Pacific Ocean). Convergence occurs when one plate is made of oceanic crust and the other

of continental crust. The less dense, more buoyant, continental plate will override the

denser oceanic plate. The oceanic plate sinks into the mantle beneath an overriding plate, it

becomes hotter as it descends deeper into the Earth’s interior and melts down.

• Conservative plate boundaries. These are faults where two plates slide past each other, so

that no lithosphere is either created or destroyed. In this case, the direction of relative

motion of the two plates is parallel to the fault. Conservative plate boundaries occur within

both the oceanic and continental lithosphere.

Figure II.2: The basic concept of plate tectonics.

5

The different types of plate boundaries were originally distinguished on the basis of

their seismicity. Earthquakes commonly occur at the boundaries of plates, and only

occasionally in the middle of a plate. There is a close correspondence between plate boundaries

and earthquake belts. We have already seen that plate tectonic processes are linked to mass

movement in the mantle. Because such movement can only occur within materials that have

the properties of fluids, this implies that the mantle shows some form of fluid behaviour.

Figure II.3: World pattern of plates. (Arrows show the direction of movement of the plates)

II.1.3. The Earth’s heat

The philosopher and mathematician Pitagoras (580-500 B.C.) declared that there is a

great fire in the Earth’s interior, and that this fire is manifest at the surface in the form of

volcanoes.

The first systematic measurements of temperature underground are accredited to the

British chemist Robert Boyle, well-known for his gas law. In 1671 Boyle wrote of the heat,

sometimes very strong, noted in the mines of Britain, and stated that temperature increases

with depth.

6

This phenomenon was reappraised in 1846 by young William Thomson, later Lord

Kelvin, one of the fathers of thermodynamics, who, in his PhD thesis at the University of

Glasgow, tried to estimate the age of the Earth from the “distribution and movement of heat

within it”.

However, it was not until 1882 that the values obtained for the geothermal gradient and

the thermal conductivity of rocks were combined by Lord Kelvin (their product gives the heat

flow).

The Earth’s heat flow is the amount of heat that is released into space from the interior

through a unit area in a unit of time. It is measured in milliwatt per square meter. It varies from

place to place on the surface, and it has varied with time at any particular place during the

history of our planet.

The Earth’s heat flow originates from the primordial heat, which is the heat generated

during the Earth’s formation, and from the heat generated since the Earth’s formation by the

decay of long-lived radioactive isotopes.

Although all radioactive isotopes generate heat as they decay, only isotopes that are

relatively abundant and have half-lives comparable to the age of the Earth (4.5 billion years)

have been significant heat producers throughout geological time and remain so at present. Four

long-lived radioactive isotopes are important heat producers:40K,232Th,235U and 238U.

The average heat flow from the continental crust (granite) is 57 mW/m2, and through

the oceanic crust (basalt) is 99 mW/m2. The Earth’s average heat flow is 82 mW/m2, and the

total global output is over 4x1013W, four times more than the present world energy

consumption which is 1013W. Continental heat flow appears to be derived from radiogenic

decay within the upper crust, together with the heat generated in the most recent magmatic

episode and the heat coming from the mantle. In the oceanic crust, the concentration of

radioactive isotopes is so low that radiogenic heating is negligible, and the heat flow largely

derives from heat flowing from the mantle below the lithosphere.

7

II.1.3.1. Heat transfer within the Earth

Two forms of heat transfer occur within the Earth: conduction and convection.

Conduction is a heat transfer process caused by a difference in temperatures between two areas

of a same medium or between two media in contact but without displacement of matter. This

process involves a transfer of random kinetic energy between molecules without the overall

transfer of material. Moving molecules strike neighbouring molecules, causing them to vibrate

faster and thus transfer heat energy. Conduction is the primary heat transfer mode in solids.

Metals are very good conductors of heat, whereas most rocks are relatively poor conductors.

This transfer of heat spontaneous from an area of high temperature towards an area of

lower temperature obeys the Fourier’s law : “The flux density of heat is proportional to the

variation in temperature.”

Let’s consider a quantity Q of heat s transferred from a point x with a temperature T to

a point x+dx of the same matter with a temperature T+dT

Figure II.4: Heat conduction

If we take dSx as a transversal isothermal surface, in the direction (0, X), we can

express the variation of the heat according to X:

dtdSx

TkdQ xxx δ

δ−= (II.1)

The thermal flux variation is then:

xxx dS

x

Tk

dt

dQd

δδ−==Φ (II.2)

The flux density is then:

x

Tk

dS

d

x δδϕ −=Φ= (II.3)

8

k is the thermal conductivity constant, it depends on the nature of the material and its

temperature.

Convection is the common heat transfer process in liquids or gases and consists of the

movement of hot fluid (that is, a liquid or a gas) from one place to another. Because motion of

material occurs, convection is a vastly more efficient process of heat transfer than conduction.

In fact if heat is generated in a layer of a liquid, it spreads in all the liquid by

conduction but if the variation in temperature exceeds a limit called “the adiabatic gradient”,

the liquid becomes unstable and the hot liquid goes up on the surface where it cools and returns

in the depths, this movement is called thermal convection. The adiabatic gradient is given by

the formula:

c

Tgαγ = (II.4)

Where α is the coefficient of expansion of the liquid, g the constant of gravity, T the

temperature and c the specific heat. The coefficient of expansion of a rock is lower in the

mantle than under the atmospheric pressure by experiments under high pressure, we estimate it

at 10-6/oC with this value we obtain γ=1 oC/Km.

II.1.3.2. The Earth’s geothermal gradient and the thermal conductivity of rocks

Studies of the thermal behaviour of the Earth imply the determination of how

temperature varies with depth, and how such temperature variations may have changed

throughout geological time. However, studies of this kind are based entirely on measurements

made on, or within, a few km of the Earth’s surface.

The average gradient near the surface, say within a few km, is about 30°C/km,but

values as low as about 10°C/km are found in ancient continental crust and very high values

(100°C/km) are found in areas of active volcanism. Once the gradient has been measured, it

can be used to determine the rate at which heat is moving upwards through a particular part of

the Earth’s crust.

As the heat generally moves upwards through solid impermeable rock, the principal

mechanism of heat transfer must be conduction. The amount of heat flowing by conduction

through a unit area of 1m2 of solid rock in a given time, that is the rate of heat flow, is

proportional to the geothermal gradient and to a constant of proportionality which is known as

the thermal conductivity of rocks defined as the amount of heat conducted per second through

9

an area of 1m2, when the temperature gradient is 1°C/m perpendicular to that area. The unit of

thermal conductivity is the W/m K.

The gradient is measured in wells with electrical (platinum-resistance) thermometers.

Temperature logging is quick and relatively inexpensive.

The thermal conductivity of rock samples is best measured in laboratory, as there are no

reliable downhole methods. If the gradient is expressed in °C/km and conductivity in W/(m°C),

heat flow will be in mW/m2 (milliwatt per square metre).

II.2. GEOTHERMAL RESOURCES

II.2.1. General aspects

Geothermal resources are the thermal energy that could reasonably be extracted at costs

competitive with other forms of energy at some specified future time.

Geothermal resources are generally confined to areas of the Earth’s crust where heat

flow higher than in surrounding areas heats the water contained in permeable rocks (reservoirs)

at depth. The resources with the highest energy potential are mainly concentrated on the

boundaries between plates, where visible geothermal activity frequently exists. By geothermal

activity we mean hot springs, fumaroles, steam vents, and geysers. Active volcanoes are also a

kind of geothermal activity, on a particularly and more spectacular large scale.

Geothermal activity in an area is certainly the first significant indication that subsurface

rocks in the area are warmer than the norm. The local heat source could be a magma body at

600–1000°C, intruded within a few kilometers of the surface. However, geothermal fields can

also form in regions unaffected by recent shallow magmatic intrusions. The anomalous higher

heat flow may be due to particular tectonic situations, for example to thinning of the

continental crust, which implies the upwelling of the crust-mantle boundary and consequently

higher temperatures at shallower depths.

However, we need more than a thermal anomaly to have a productive geothermal

resource. We also need a reservoir, which is a sufficiently large body of permeable rocks at a

depth accessible by drilling. This body of rock must contain large amounts of fluids, water or

steam, which carry the heat to the surface.

The reservoir is bounded by cooler rocks hydraulically connected to the hot reservoir

by fractures and fissures, which provide channels for rainwater to penetrate underground.

10

These cooler rocks come out at the surface where they represent the so-called recharge areas of

the geothermal reservoir.

Fig. II.5. An hydrothermal field

Water moves inside the reservoir by convection, due to density variations caused by

temperature, transferring heat from the lowest parts of the reservoir to its upper parts. The

result of the convection process is that temperature in the upper parts of the reservoir is not

much lower that the temperature of its deeper parts, so that the lowest values of the geothermal

gradient are actually found inside the reservoir.

Convection, implying a real transfer of matter, is therefore a more efficient process of

heat transfer than conduction. Heat is transferred by conduction from the magma body towards

the permeable reservoir rocks, the reservoir, filled with fluids. Hot fluids often escape from the

reservoir and reach the surface, producing the visible geothermal activity described above.

II.2.2. Hydrothermal systems

The heat source, the reservoir, the recharge area and the connecting paths through

which cool superficial water penetrates the reservoir and, in most cases, escapes back to the

surface, compose the hydrothermal system.

The reservoir is the most important part of the system, from the point of view of energy

utilisation, and in fact we define the reservoir “as the hot part of the geothermal system that can

11

be exploited either by extracting the fluid contained (water, steam, or various gases), or using

anyhow its heat”.

The existence of a hydrothermal system will not necessarily ensure production at

industrial levels, Only a part of its rocks may be permeable, constituting a fluid reservoir, so

that the system will be able to produce industrially from that part only.

Four types of geothermal fluid reservoir have been identified:

• Hydrothermal

• hot dry rock

• geopressured

• magmatic

The reservoirs that can be exploited at present are the hydrothermal reservoirs. The

other three may be exploited industrially in the future after more technological development.

We have two kinds of hydrothermal reservoirs :

• water-dominated

• vapour-dominated

The latter having a higher energy content per unit fluid mass.

Water-dominated fields are further divided into hot water fields, producing hot water, and wet

steam fields producing mixtures of water and steam.

II.2.3. Water-dominated fields

They are capable of producing hot water at the surface at temperatures up to 100°C.

They are the geothermal fields with the lowest temperature, and the reservoir contains water in

liquid phase. The reservoir may not have a cover of impermeable rock acting as a lid, however

some of these thermal aquifers are overlain by confining layers that keep the hot water under

pressure. Temperatures in the reservoir remain below the boiling point of water at any pressure

because the heat source is not large enough. Surface temperature is not higher than boiling

temperature of water at atmospheric pressure.

These fields may also occur in areas with normal heat flow. On the surface there are

often thermal springs whose temperatures are, in some cases, near the boiling point of water. A

hot water field is of economic interest if the reservoir is found at a depth of less than 2 km, if

the salt content of the water is lower than 60 g/kg, and if the wells have high flow-rates (above

150 t/h).

12

II.2.4. Wet steam fields

They contain pressurised water at temperatures exceeding 100°C and small quantities

of steam in the shallower, lower pressure parts of the reservoir. The dominant phase in the

reservoir is the liquid one, and it is this phase that controls the pressure inside the reservoir.

Steam is not uniformly present, occurring in the form of bubbles surrounded by liquid water,

and does not noticeably affect fluid pressure.

An impermeable cap-rock generally exists to prevent the fluid from escaping to the

surface, thus keeping it under pressure. This is common, but not absolutely necessary. In

fact, at any depth below the water table, water bears its own hydrostatic pressure. When the

fluid is brought to the surface and its pressure decreases, a fraction of fluid is flashed into

steam, while the greater part remains as boiling water. Once a well penetrates a reservoir of

this type, the pressurised water rises into the well because pressure is lower there. The

consequence of the pressure drop is the vaporisation of part of the water, with the result that

the well eventually produces hot water and steam, with water as the predominant phase. The

water-steam ratio varies from field to field, and even from one well to the next within the same

field. As in many cases only steam is used to produce electrical energy, liquid water must be

removed at the surface in special separators.

The surface manifestations of these fields include boiling springs and geysers. The heat

source is large and generally of magmatic origin. The water produced often contains large

quantities of chemicals (from 1 to over 100 g/kg of fluid, in some fields up to 350 g/kg). These

chemicals may cause severe scaling problems to pipelines and plants. They are mainly

chlorides, bicarbonates, sulfates, borates, fluorides and silica. More than 90% of the

hydrothermal reservoirs exploited on an industrial scale are of the wet steam type. Electricity

generation is their optimal utilisation.

One typical exemple of wet steam fields producing electricity is Olkaria in Kenya.

II.2.5. Vapour-dominated fields

Vapour-dominated reservoirs (fields) produce dry saturated, or slightly superheated

steam at pressures above atmospheric. They are geologically similar to wet steam fields, but

the heat transfer from depth is certainly much higher. Research suggests that their permeability

13

is lower than in wet steam fields, and the presence of the caprock is of fundamental importance

here.

Water and steam co-exist, but steam is the continuous predominant phase, regulating

the pressure in the reservoir: the pressure is practically constant throughout the reservoir. These

fields are called dry or superheated fields. Produced steam is in fact generally superheated,

with small quantities of other gases, mainly CO2 and H2S.

The mechanism governing production in these fields is believed to be the following:

When a well penetrates the reservoir and production begins, a depressurized zone forms at

well-bottom.

This pressure drop produces boiling and vaporisation of the liquid water in the

surrounding rock mass. A dry area, i.e. without liquid water, forms near the well-bottom and

steam flows through this zone. Steam crossing the dry area starts to expand and cool, but the

addition of heat from the very hot surrounding rocks keeps steam temperature above the

vaporisation value for the pressure existing at that point. As a result, the well produces

superheated steam, for example with wellhead pressures of 5-10 bar (0.5–1 MPa) and a steam

outlet temperature of more than 200°C. Surface geothermal activity associated with vapour-

dominated fields, whether dry or superheated, is similar to the activity present in wet steam

fields.

About half of the geothermal electric energy generated in the world comes from six

vapour-dominated fields: Larderello (Italy) , Mt. Amiata, (Italy), The Geysers (California),

Matsukawa (Japan), Kamojang and Darajat (Indonesia) Of the approximately 100

hydrothermal systems that have been investigated, less than 10% are vapour-dominated, 60%

are wet steam fields (water-dominated), and 30% produce hot water.

II.2.6.Hot Dry Rock systems

Hot Dry Rock (HDR) geothermal reservoirs differ significantly from conventional

geothermal reservoirs, which probably exist only in the geologically favoured regions of the

world shown. In these regions, nature provides not only the hot rock, but also the hot water or

steam. HDR reservoirs are, instead, man-made reservoirs in rocks that are artificially fractured,

and thus any convenient volume of hot dry rock in the earth’s crust, at accessible depth, can

become an artificial reservoir. A pair of wells is drilled into the rock, terminating several

hundred meters apart.

14

Water is circulated down the injection well and through the HDR reservoir, which acts

as a heat exchanger. The fluid then returns to the surface through the production well, and thus

transfers the heat to the surface as steam or hot water.

In the other hand we have Hot Wet Rock systems (HWR) as enhancement of low-

permeability fracture systems in high temperature rocks exploiting the techniques developed in

the HDR experiments.

The most common stimulation techniques are hydraulic fracturing, chemical fracturing

and explosive fracturing.

Fig. II.6. Schematic representation of a hot dry rock (HDR) reservoir.

II.2.7. Geopressured reservoirs

Geopressured reservoirs are deep reservoirs (4–6 km) in large sedimentary basins

containing pressurised hot water that remained trapped at the time of deposition of the

sediment, and at pressures of up to 100% in excess of the hydrostatic pressure corresponding to

that depth, these fields could produce not only the thermal energy of the pressurized hot water,

but also hydraulic energy, by virtue of the very high pressure, and methane gas.

These three forms of energy can also be converted to higher value forms of energy

using available technologies.

Thermal energy can be converted to electricity in a geothermal turbine. Hydraulic

energy can be converted to electricity using a hydraulic turbine.

15

Dissolved methane gas can be separated and sold, burned, compressed, liquefied,

converted to methanol, or converted to electricity by fuelling a turbine.

II.3. UTILISATION OF GEOTHERMAL RESOURCES

Geothermal utilisation is divided into two categories, electric energy production and

direct uses. Conventional electric power production is limited to fluid temperatures above

150°C, but considerably lower temperatures can be used in binary cycle systems.

II.3.1. Electricity from geothermal fluids

According to the GEA report, the world geothermal electrical capacity installed in the

year 2006 was 8900MWe with the generation in that year of 56.5billion kWh.

The total electricity produced world-wide from all sources in that year was 14,855.8

billion kWh of which 3,282.5 billion kWh were generated by renewable sources (2,939.3

billions by hydropower alone, and 343.3 billions altogether by biomass, geothermal, wind,

solar and tidal). These values show that renewables with the exception of hydro play a very

minor role on the world energy scene.

The contribution of geothermal energy is entirely different if we distinguish between

industrialised and developing countries.

In the industrialised countries, where the installed electrical capacity are very high (tens

or even hundreds of thousands GWe), geothermal energy represents one percent, at most, of

the total.

In developing countries, with a limited electrical consumption but good geothermal

prospects, electrical energy of geothermal origin could, on the contrary, make quite a

significant contribution to the total: at the moment, for instance, 21% of the electricity in the

Philippines comes from geothermal sources, 20% in El Salvador, 17% in Nicaragua, 10% in

Costa Rica and 8% in Kenya.

16

II.3.2. Direct uses of geothermal energy

The utilisation of natural steam for electricity generation is not the only possible

application of geothermal energy. Hot waters, that appear to be present in large parts of all the

continents can also be exploited and offer interesting prospects, especially in space heating,

agriculture and industrial processes.According to the GEA, the thermal power installed for

non-electrical uses of geothermal energy worldwide in the year 2005 has been estimated at

16,000 MWt, distributed among 72 countries.

17

Chapter III: GEOTHERMAL PROSPECTS AND ELECTRICITY

GENERATION

III.1. GEOTHERMAL RESERVOIR EXPLORATION

Present technology and economic factors restrict extraction of geothermal energy to the

upper few kilometres of the Earth’s crust. Geothermal wells, to date, are drilled to less than 5

km depth.

As in the search for any natural resource, a strategy for geothermal energy exploration

must be defined and followed. Once a geothermal region has been identified, the next step is to

use various exploration techniques to locate the most interesting geothermal areas and identify

suitable targets for fluid production.

It is necessary to estimate temperature, reservoir volume and permeability at depth, as

well as to predict whether wells will produce steam or just hot water. Ideally weshould also

estimate the chemical composition of the fluid to be produced. To obtain this varied

information, there are a number of exploration techniques available:

• Inventory and survey of surface manifestations,

• Geological and hydrogeological surveys,

• Geochemical surveys,

• Geophysical surveys, and finally

• Exploratory wells.

To reduce the cost of exploration, it is normally approached in a prescribed sequence of

steps, altering the order from time to time depending on our prior knowledge of the area in

question. In some cases, high costs will lead to the elimination of some steps in the sequence.

III.1.1. Inventory and survey of surface manifestations

The knowledge of surface thermal manifestations (hot springs, steam vents, fumaroles,

etc.) and their physical and chemical characteristics is of fundamental importance.

This information, which can usually be obtained simply and at relatively low cost, is

extremely useful for subsequent planning of exploration.

18

The surface survey is conducted in two consecutive phases:

• The collation, processing and standardisation of published and recorded data relative to

local manifestations (chemistry, temperature, flow-rates, etc.),

• The collection of new data, water samples, gas samples, temperature measurements, etc.

III.1.2. Geological and hydrogeological surveys

These are not limited to studies of groundwaters, but also include geological surveys

that provide information on the stratigraphic and structural framework of the area. Geothermal

reservoirs are often associated with volcanoes and volcanic regions and therefore volcanology

also offers many examples of how geological field data give evidence of the location, nature

and size of a geothermal resource.

An hydrogeological surveys permit us to correlate the hydrothermal manifestations

with faults, fractures and other tectonic features. These surveys are aimed at identifying the

distribution of confined and unconfined aquifers that will permit us to reconstruct the

underground pattern of water circulation.

Fluid inclusions may give information on the temperature of deposition of inclusions

and therefore determine the temperature and salinity of geothermal fluids. Fluid inclusions are

defects in crystals, formed during or after deposition. All crystals have inclusions. Some

inclusions are solid, others empty, a few contain fluids. Inclusions need to be multi-phase

(liquid and vapour) to be most useful.

III.1.3. Geochemical surveys

Geochemical exploration can start simultaneously with geologic and hydrogeologic

reconnaissance, provided that springs and other geothermal manifestations are available for

fluid sampling. Geochemical studies of geothermal fluids involve three main steps:

• Sample collection,

• Chemical analysis

• Data interpretation.

The types of samples collected are water samples from hot springs, steam samples from

fumaroles, gas samples from hot pools.

Geothermometers enable to estimate the temperature of deep reservoirs by analyzing

hot fluids samples and calculating the ratios of certain chemical elements.

19

The content of tritium and 14C radioisotopes permit us to evaluate the age of the

geothermal fluids, that is, the time lapsed since their infiltration into the ground.

Geochemical surveys with the use of tracers can also offer information on the direction of

movement of subsurface groundwater and of reinjected fluids, and also the type of corrosion

and scaling problems that could be encountered during the operation of wells and a plant.

Hydrogen and oxygen isotopes can be used to identify the recharge areas of the

geothermal reservoirs, in fact the isotopic composition of a rain precipitation is dependent on

its formation temperature .

III.1.4. Geothermometers

Geothermometers enable to estimate the temperature of deep reservoirs by analyzing

hot fluids samples and calculating the ratios of certain chemical elements (i.e., Na, K, Mg, Ca,

etc.), and making some adjustment for the degree of mixing of the hot geothermal reservoir

water with cooler groundwater in the shallow part of the hydrothermal system.

In fact, at particular temperatures, common assemblages of minerals will tend towards

equilibrium with a given water chemistry. It has been noted that for certain parameters or ratios

of parameters, the relationship between temperature and chemical composition will be stable

and predictable.

These parameters or ratios of parameters are known as geothermometers. In order for

these to work, we have to assume that effects of dilution are insignificant and that

thermodynamic equilibrium has been attained.

In general geothermometers based on ratios will be more resistant to dilution effects

than those based on absolute concentrations.

In addition, it should be realised that the temperature indicated by the geothermometer

is not necessarily the maximum temperature of the water, but the temperature at which mineral

and water phases were last in equilibrium with respect to the phases in question.

The three principal indicators of deep reservoir temperatures, to be sought in hot spring

chemistry, are silica, magnesium and sodium/potassium ratios.

Silica concentrations are more reliable for hot springs of high discharge than those of

low discharge. Magnesium is of limited value as a temperature indicator, but its total absence

could be suggestive of economically useful temperatures (at least 200°C), as magnesium is

retained in clay rocks which are stable at high temperatures.

20

III.1.5. Geophysical surveys

Classical geophysical techniques, namely, seismic, gravity and magnetic surveys as

applied to geothermal research, can be defined as indirect methods. These methods, in fact, are

not directly associated with the properties of the hot fluids that are being sought. Rather, they

yield information about the attitude and nature of the host rocks. However, there are other

geophysical methods that may directly reveal variations in the physical properties of the rocks

caused by the presence of hot and saline fluids. These techniques include electrical-resistivity,

electromagnetic and thermal-measurement methods.

III.1.5.1. Seismic surveys

Elastic waves are transmitted through rocks, and their velocities can be used to help

determine the structure and properties of rock bodies. Seismic waves are introduced into the

Earth by detonating an explosive charge in a shallow borehole or by using a large mass to

thump the surface. Returns of seismic waves are measured at the surface.

Seismic waves also originate naturally from earthquakes and microearthquakes, and

these waves can also be detected at the surface. Interpretation of the seismic information can

provide data on the location of active faults that can channel hot fluids towards the surface.

III.1.5.2. Gravity surveys

Variations in the Earth’s gravity field are caused by changes in the density of

subsurface rocks. Gravity surveys are rather simple and inexpensive. Gravity anomalies alone

are not necessarily indicative of a geothermal region, but they do give valuable information on

the type of rocks at depth and their distribution and geometric characteristics.

III.1.5.3. Magnetic surveys

The Earth has a primary magnetic field which induces a magnetic response in certain

minerals at and near the Earth’s surface. By detecting spatial changes of the magnetic field, the

variations in distribution of magnetic minerals may be deduced and related to geologic

structure.

21

However, each magnetic mineral has a Curie temperature, above which it loses its

magnetic properties. For iron, the Curie temperature is 760°C.

Aeromagnetic surveys are much more commonly used in geothermal exploration than

ground based surveys. The basic principle is to detect zones which are magnetically

featureless, due to destruction of magnetite in near-surface rocks by hydrothermal alteration.

III.1.5.4. Electrical-resistivity surveys

Most electrical methods are based on measurement of the electrical resistivity of the

subsurface. Resistivity in the Earth is often largely affected by electrical conduction within

waters occupying the pore spaces in the rock. Consequently, resistivity varies considerably

with porosity. Temperature and salinity of interstitial fluids tend to be higher in geothermal

reservoirs than in the surrounding rocks. Consequently, the resistivity of geothermal reservoirs

is generally relatively low. It is this contrast in resistivity between hot water-saturated rocks

and the surrounding colder rocks that is used in resistivity surveys.

These techniques are based on injection of current into the ground and measurement of

voltage differences produced as a consequence at the ground surface.

One of the major drawbacks with electrical methods is the shallow depth of penetration.

III.1.5.5. Electromagnetic surveys

Induction or electromagnetic methods are a tool for determining the electrical resistivity

distribution in the Earth by means of surface measurements of transient electric and magnetic

fields. These fields can be naturally or artificially generated.

These methods are more suitable for measuring the low resistivities of geothermal

reservoirs than the above-mentioned electrical-resistivity methods. Furthermore, in geothermal

areas the surface resistivity is sometimes so high as to prevent current from entering the

ground, and the electromagnetic methods, with a much deeper penetration, help eliminate the

screening effect of very resistive surface rocks.

Currents of varying frequency (generally from a few to several tens of thousands of Hz)

are transmitted into the ground, either via the electrodes as in the electrical methods, or by

induction loops. Mobile stations measure, at several points, the electrical and magnetic fields

created by this transmission.

22

Comparison between these fields enable the resistivities of the underlying formations to

be obtained, as a function of the frequency used, that is as a function of the depth, as in the

magnetotelluric soundings (MT).

Magnetotelluric soundings use natural oscillations of the Earth’s electromagnetic field

to determine the resistivity structure of the sub-surface. The electromagnetic waves are

assumed to be planar and nearly vertically incident on the surface of the Earth where they are

detected. The lower the frequency, the deeper the penetration is taken to be, but the longer it

takes to collect a signal with a satisfactory signal to noise ratio. The depth of penetration of MT

surveys is also much greater than of during current (DC) resistivity measurements, and it is

often possible to achieve a penetration as great as 3–5 km with a reasonable degree of precision

and in a reasonable period of time, this can be compared to a maximum depth of penetration

from most DC methods of less than 2 km. MT does not require a current source.

III.1.5.6. Thermal-measurement surveys

In geothermal research the traditional geophysical methods mentioned above, which

originally had been developed for the oil industry, are used side-by-side with more specific

techniques.

Geothermal prospecting provides information on the thermal conditions of the

subsurface, the area distribution of the Earth’s heat flow, and the location and intensity of

thermal anomalies. To be more specific, geothermal prospecting allows us:

• To verify the existence of high-temperature fluids in areas without surface manifestations,

but in which the geostructural and hydrogeological situation is favourable to hydrothermal

circulation;

• To more precisely site deep drilling in areas that are considered potentially productive;

• To delineate the boundaries of geothermal fields that have been identified, so as to avoid

drilling of dry holes in non-productive marginal areas;

• To acquire data for evaluation of the geothermal potential of the field.

Heat flow measurements are made by drilling small diameter (10 cm), shallow wells

(generally <300 m), the number of which depends on local conditions and on the results one

wants to achieve. The geothermal gradient is obtained from temperatures measured with

electric thermometers at various depths along a well. Temperature logging is quick and

relatively inexpensive.

23

Thermal conductivity of the rocks in the interval in which the gradient has been

measured is usually determined by laboratory measurements on core samples. The product of

the gradient and conductivity gives the heat flow.

Sometimes gradient values alone are sufficient to give the information required.

However, this is possible only if the survey is carried out in areas that are lithologically

homogeneous at depth, in which the thermal conductivity can be considered constant.

In geothermal areas the heat flow is higher than the general background level, so that high heat

flow values are a good indicator of underlying geothermal resources.

III.1.6. Exploratory wells

The final stage of an exploration survey is exploratory well drilling. Usually the final

diameters of these wells are on the order of 20 cm or less, allowing the insertion of special

logging tools to measure various parameters from the surface to total depth, and sometimes to

carry out fluid production tests.

Since most geothermal reservoirs are made up of fluid-filled fractures, it is essential

that an exploratory well intersects as many fractures as possible. In some cases it may be

necessary to redrill the well at an angle in order to intersect the natural fracture pattern. Since

natural fractures are related to tectonic activity (folding and faulting), the siting of exploratory

wells is greatly dependent on our geologic interpretation of the local structural conditions.

III.2. GEOTHERMAL ELECTRICITY GENERATION

III.2.1. Thermodynamic background

III.2.1.1. Thermodynamic laws

The first law states that energy cannot be crated or destroyed, it can only change forms.

This law cannot be proved mathematically, it is based on experimental observations and

nothing has yet been found in nature that violates this law. The law can be written for a closed

system in an equation form in the following manner:

irs EEE =− (V.1)

Where Es is the energy supplied to the system, Er energy removed from the system and

Ei increase in the energy level of the system.

For a constant mass system, the only forms of energy that can be supplied or removed

from a system are heat energy and work energy.

24

Adopting a sign convention such as heat energy entering a system is positive and the

work energy leaving the system is also positive. We can now write the first principle for a

constant mass system as:

12 EEEWQ −=∆=− (V.2)

Where Q is the energy of the system before the transformation, W the work done by the

system and the energy variation.

The enthalpy is a thermodynamic property of substance and is defined as the sum of its

internal energy and the product of its pressure and volume. The enthalpy is given by:

pVUH += (V.3)

For a body undergoing a constant pressure process, its work is given by

( )∫ −==2

1 12 VVppdVW (V.4)

Application of the first law gives

( ) 1212 UUVVpQ −=−− (V.5)

Thus

( ) ( )1212 VVpUUQ −+−= ( ) ( )1122 pVUpVU +−+= 12 HH −=

Then we have:

Q H∆=

Thus, for a constant pressure process, the amount of heat supplied to a body equals the

change in its enthalpy.

The first law of thermodynamics is the principle of conservation of energy. It forbids

creation or destruction of energy, but permits transformation of one form of energy into

another, it doesn’t put any restriction on the direction in which the transformation of energy

may occur, that is why we need another law which governs the direction of transformations.

The second law of thermodynamics specifies in what direction a process may proceed.

There are two classical statements called second law of thermodynamics. One is due to

Clausius and the other one to Kelvin.

• The Clausius statement: It is impossible to construct a device that executes a

thermodynamic cycle so that the sole effect is to produce a transfer of heat energy from a

body at a law temperature to a body at a high temperature.

25

• The Kelvin-Planck statement: It is impossible to construct a device that executes a

thermodynamic cycle, exchanges heat energy with a single reservoir, and produces an

equivalent amount of work.

There are two important corollaries of the second law.

1. Any heat engine operating between two constant temperature reservoirs cannot have an

efficiency that is greater than the efficiency of a reversible engine operating between the

same two reservoirs.

2. All reversible heat engines operating between two constant temperature reservoirs have the

same efficiency regardless of the operating media used by these engines. The efficiency is

solely a function of temperature of the reservoirs.

Let’s consider a system undergoing a reversible cyclic process, path 1-2-3-4-1 and

interacting with a number of energy reservoirs, Inasmuch as the cyclic process is reversible, the

system and the reservoirs have essentially identical temperatures at every point in the cycle.

Figure III.1: A reversible cyclic process

If we consider the Clausius statement i.e. that heat transfer always takes place from

high temperatures to low temperatures we can introduce the Clausius inequality which states

that the algebraic sum of the ratios of heat energy extracted to temperature for each of the

various reservoirs exchanging heat energy is less than or equal to zero.

We can then write the Clausius inequality for this system as:

∫ ≤−−−− 014321 T

Qδ (V.9)

26

Now let the system reverse the cycle, that is, follow the path 1-4-3-2-1, for this path we can

write:

∫ ≤−−−− 012341 T

Qδ (V.10)

The only way to satisfy both these equations is to have

∫ ∫∫ =+= −−−−−−−− 014332114321 T

Q

T

Q

T

Q δδδ (V.12)

Then: ∫ ∫ =− −−−− 0341321 T

Q

T

Q δδ or ∫ ∫ −−−− =

T

Q

T

Q δδ341321 (V.13)

This result means that if we have a reversible process taking a system from point 1 to

point 3, then the integral of the quantity, δQ/T, is the same regardless of the path chosen from

point 1 to point 3. If a function when integrated, has the same value regardless of the path of

integration so long as the two end points are the same, this function is a point function, the

integrand in such case is an exact differential. We represent the quantity δQ/T by an exact

differential dS of a thermodynamic property, called entropy S. Which is a function of the

thermodynamic state of a system and it is defined only for equilibrium states. Only changes in

entropy can be calculated. Then we have:

reversibleT

QdS

= δ

and ∫=−

2

112

reversibleT

QSS

δ (V.14)

Now suppose that the system is brought back from state 3 to 1 by some irreversible

process. This is represented by dotted line 3-α-1 in figure III.2.

27

Figure III.2: Reversible and Irreversible processes

The Clausius inequality for the irreversible cycle 1-2-3- α-1 becomes

∫ <−−−− 01321reeservoirT

Qδα Or ∫ ∫ <+ −−−− 013321

reservoirT

Q

T

Q δδα (V.17)

If we subtract the equation (V.15) from the above inequality we obtain:

∫ ∫ <− −−−− 014313 T

Q

T

Q

reservoir

δδα (V.18)

The second integral in the above inequality is for reversible process 3-4-1 and therefore

can be replaced by ∆S or S1-S3 note that the starting point is 3 and the end point 1. Thus we

obtain the inequality :

∫ <−−−− 0)( 3113 SST

Q

reservoir

δα or ∫ −−>−

reservoirT

QSS

δα 1331 )( (V.19)

This result means that in any irreversible process, the change in entropy is always

greater than the integral of the quantity δQ/T .

Thus for an irreversible process we have T

QdS

δ> (V.20)

For a system undergoing an irreversible process from state 1 to state 2,

∫>−2

112

leirreversibT

QSS

δ (V.21)

If we consider an isolated system i.e. which doesn’t interact with its surrounding in any

manner; that is, δQ=0.

Combining these results we can write, for an isolated system:

( ) 012 ≥− SS or ( ) 0≥∆systemisolatedS (V.22)

Which is another expression of the second law of thermodynamics which states that:

The entropy of an isolated system not at equilibrium will tend to increase over time,

approaching a maximum value.

A mathematical statement of the Second Law is:

28

0≥dt

dS (V.23)

Where S is the entropy and t the time.

III.2.1.2.Thermodynamic cycles

A thermodynamic cycle is a combination of processes taking a system through a

succession of states and returning the system to its initial state.

A. The Carnot cycle

The carnot cycle was historically the first to be used for thermodynamic analysis. It is a

cycle that exchanges heat energy with only two reservoirs. It receives heat energy from a high-

temperature reservoir and rejects heat energy to a low temperature reservoir in a reversible

quasistatic manner. We are going to discuss the Carnot cycle operated as a heat engine but it

can also be reversed and function as a refrigerator or as a heat pump.

The carnot cycle consists of two reversible constant temperature processes and two

reversible adiabatic, or isentropic, processes. The T-s diagram for the Carnot cycle is shown in

figure III.2.

Consider a piston cylinder system containing a gas for execution of the Carnot heat

engine. The thermodynamic state of this gas is represented by point 1 on the T-s diagram. Let

an energy reservoir at temperature Th supply heat to the gas reversibly so that the gas has now

a new state, state 2. Then allow the gas to undergo an isentropic expansion (Q=0) to state 3.

Now let the gas reject heat to another reservoir at temperature Tl in a reversible constant

temperature process until state 4 is reached . Finally, we return the gas to its initial state 1 by

subjecting the gas to isentropic compression (Q=0).

29

Figure III.3: T-s diagram for a Carnot heat engine

The thermal efficiency of power cycle is the ratio of the net work of the cycle to the

heat energy supplied to the cycle. In view of the first law, the net work output of a cycle equals

the net heat input to the cycle.

outincycle QQW −=

Where Qin and Qout are the magnitudes of the heat energy supplied to and rejected by the cycle,

respectively.

Equation reversibleT

QdS

= δ

is written on a unit of mass as Tdsq =δ

And considering the figure III.2 we have:

( )1221 SSTABAreaQ hin −=−−−= (V.24)

( )4333 SSTABAreaQ lout −=−−−= (V.25)

Since process 2-3 and 4-1 are isentropic, we can write S3 = S2 and S4 = S1 so that:

Qout = Tl ( S2 – S1 ) (V.26)

30

The net work of the cycle is given by

outincycle QQW −=

( ) ( )1212 SSTSST lh −−−=

( )( )12 SSTT lh −−= (V.27)

The thermal efficiency of the cycle, ηt, is

( ) ( )

( )12

12

sup SST

SSTT

Q

W

plyheatgross

outputworknet

h

lh

in

cyclet −

−−===η (V.28)

Or

h

l

h

lhcarnot T

T

T

TT−=

−= 1η (V.29)

According to the second law there cannot be a device which converts thermal energy into work

with 100% efficiency. For a carnot cycle, this means

11 <−=−

=h

l

h

lhcarnot T

T

T

TTη (V.30)

Or 0>lowT (V.31)

In other words it is impossible to have an energy reservoir with its temperature equal to

absolute zero. The unattainability of absolute zero is sometimes called the third law of

thermodynamics. Then, since the Carnot cycle is reversible, the two corollaries of the second

law stated earlier mean that it is impossible to have an engine operating between two energy

reservoirs whose thermal efficiency is greater than that of a carnot engine operating between

the same two reservoirs.

B. The Rankine cycle

31

Figure III.4: T-s diagram for a simple Rankine cycle.

We begin the cycle with the saturated liquid state of the working medium, denoted by

point 1 in figure V.3. Process 1-2 is the isentropic compression of the working fluid from a

saturated liquid state to a compressed liquid state. This process is carried out in a feed water

pump. Usually, the temperature rise in this process is very small. Process 2-α-3 involves

constant pressure heating from a compressed liquid state to a saturated vapor state.This

operation takes place in a steam generator or in a boiler. In process 2- α the temperature

increases and the process requires heat transfer from a high temperature heat energy source,

while in the process α -3 there is no temperature change as the process involves a change of

phase. High-pressure saturated steam is admitted into the turbine at point 3, and it is expanded

isentropically, delivering work. Finally the wet steam at state 4 is condensed to a saturated

liquid state, state 1, by using a cooling agent as water.

In the analysis of the simple Rankine cycle, it is assumed that the changes in the kinetic

energy and the potential energy of the working medium are negligible. This assumption is quite

realistic. Another assumption made is that all processes are reversible, and that there are no

pressure drops or heat losses in the pipes. In reality there are irreversibilities and pressure

drops. For the reversible Rankine cycle, area 6-2-α-3-4-5-6 represents the heat energy supplied

to a unit mass of the working medium, and area 4-1-6-5-4 represents the heat energy removed

to the unit of mass.

Consequently, the difference in the two areas, namely area 1-2- α -3-4 represents the

net work obtained from the cycle.

Appling the laws of thermodynamics, for the various processes in the simple Rankine

cycle, we obtain the following for a unit mass flow rate:

Process 1-2: Isentropic compression; hense

021 =−q and 1221, hhws −=− − (V.32)

Process 2- α-3: Constant pressure heating and

032, =−sw and 2332 hhqq in −==− (V.33)

Process 3-4: Isentropic expansion; hence

32

043 =−q and 4343, hhws −=− (V.34)

Process 4-1: Constant pressure heat removal and

014, =−sw and 1414 hhq −=− − (V.35)

The net work of the cycle is

( ) ( )124321,43, hhhhwww sscycle −−−=+= −− (V.36)

The thermal efficiency of the Rankine cycle is

( ) ( )23

1243

hh

hhhh

q

w

in

cycleRankine −

−−−==η (V.37)

With the same upper and lower limits of the temperature for the Carnot and the Rankine

cycles, the Rankine cycle efficiency is less than the Carnot cycle efficiency. We may reason

that the average temperature at which heat is supplied in a Rankine cycle is less than the

temperature at which heat is supplied in a Carnot cycle, thus leading to the lower efficiency of

the Rankine cycle.

C. The Kalina cycle

The use of mixtures as working fluids has opened new possibilities to improve the

efficiency of power cycles with less costly equipment.

The Kalina cycle, which use a ammonia-water mixture, may show 10 to 20% higher

efficiency than the conventional Rankine cycle. The ammonia-water mixture boils at a variable

temperature unlike pure water which boils at a constant temperature.

Variable temperature boiling permits the working fluid to maintain a temperature closer

to that of the hot combustion gases in the boiler, thus, improving the exergy efficiency, a fact

which has been well known among scientist and engineers. But there was no practical, efficient

way to condense the mixture back to a fluid for recycling until the Kalina cycle was

introduced.

33

Figure III.5. Simplified Kalina cycle

By circulating the mixture at different compositions in different parts of the cycle,

condensation (absorption) can be done at slightly above atmospheric pressure with a low

concentration of ammonia, while heat input is at a higher concentration for optimum cycle

performance.

Figure III.5 shows the simplified Kalina cycle assumed in this study. This is a

bottoming cycle feed by exhaust gases (1, 2) to the boiler. Superheated ammonia-water vapor

(3) is expanded in a turbine to generate work (4).

The turbine exhaust (5) is cooled (6, 7, 8), diluted with ammonia-poor liquid (9, 10) and

condensed (11) in the absorber by cooling water (12, 13). The saturated liquid leaving the

absorber is compressed (14) to an intermediate pressure and heated (15, 16, 17, 18). The

saturated mixture is separated into an ammonia-poor liquid (19) which is cooled (20, 21) and

depressurized in a throttle and ammonia-rich vapor (22) is cooled (23) and some of the original

condensate (24) is added to the nearly pure ammonia vapor to obtain an ammonia

concentration of about 70% in the working fluid (25). The mixture is then cooled (26),

condensed (27) by cooling water (28, 29), compressed (30), and sent to the boiler via

regenerative feedwater heater (31).

The mass flow circulating between the separator and the absorber is about 4 times that

of the turbine, thus, causing some additional condensate pump work. However, this loop makes

possible the changes in composition between initial condensation in the absorber and heat

addition in the boiler. By changing the dew point of the mixture, the waste heat from the

34

turbine exhaust, which is lost in a Rankine cycle, can be used to dilute the ammoniawater

vapor with a stream of water, thus, producing a mixture with a substantially lower

concentration of ammonia which allows condensation at a much higher temperature.

Usually thermodynamic properties of pure fluids and information of the departure from

ideal-solution is sufficient to derive mixture properties. Stability, secondary reactions, safety,

etc must, of course, also be considered.

III.2.2. Geothermal electricity generating cycles

III.2.2.1.Direct intake cycles

The simplest and cheapest of the geothermal cycle used to generate electricity is the

direct-intake non-condensing cycle. Steam from the geothermal well is simply passed through

a turbine and exhausted to the atmosphere: there are no condensers at the outlet of the turbine

(Figure V.5). Such cycles consume about 15–25 kg of steam per kWh generated.

Non-condensing systems must be used if the content of noncondensible gases (CO2,

H2S, NH3, CH4, N2 and H2) in the steam is very high (greater than 50% in weight), and will

generally be used in preference to the condensing cycles for gas contents exceeding 15%,

because of the high power that would be required to extract these gases from the condenser.

Direct intake condensing plants, with condensers at the outlet of the turbine and

conventional cooling towers, show a much lower consumption, only 6–10 kg of steam per kWh

generated, but the gas content of the steam must be less than 15%. The specific consumption of

steam of these units is greatly influenced by the turbine inlet pressure.

For pressures ranging from 15 to 20 bar (1.5–2.0 MPa), the consumption is close to 6

kg/kWh. For pressures ranging from 5 to 15 bar (0.5–1.5 MPa) the consumption is from 9 to 7

kg/kWh, and it becomes much greater for even lower pressures.

35

Fig….: Simplified flow diagram for a direct-steam geothermal power plant.

In power plants where electricity is produced from dry or superheated steam (vapour-

dominated reservoirs), steam is piped directly from the wells to the steam turbine. But vapour-

dominated systems are less common in the world, steam from these fields has the highest

enthalpy (energy content), generally close to 670 kcal/kg (2800 kJ/kg), at present these systems

have been found only in Indonesia, Italy, Japan and the USA. These fields produce about half

of the geothermal electrical energy of the world.

Flash steam plants are used to produce energy from these fields that are not hot enough

to flash a large proportion of the water to steam in surface equipment, either at one or two

pressure stages.

III.2.2.2.Flash steam cycles

Hydrothermal fluids above 150°C can be used in flash plants to make electricity.

Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the

fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator.

If any liquid remains in the tank, it can be flashed again in a second tank to extract even more

energy.

Figure III.7: Simplified flow diagram for a single-flash geothermal power plant

III.2.2.3. Binary cycle

If the geothermal well produces hot water instead of steam, electricity can still be

generated, provided the water temperature is above 85°C, by means of binary cycle plants.

These plants operate with a secondary, low boiling-point working fluid (freon,

isobutane, ammonia, etc.) in a thermodynamic cycle. The working fluid is vaporised by the

36

geothermal heat in the vaporiser. The vapour expands as it passes through the organic vapour

turbine, which is coupled to the generator. The exhaust vapour is subsequently condensed in a

watercooled condenser or air cooler and is recycled to the vaporiser by the motive fluid cycle

pump (Figure III.7) The efficiency of these cycles is even lower: between 2.8 and 12%.

However, the binary power plant technology has emerged as the most cost-effective

and reliable way to convert large amounts of low temperature geothermal resources into

electricity, and it is now well known that large low-temperature reservoirs exist at accessible

depths almost anywhere in the world.

The most used binary cycle in geothermal power generation is the Rankine cycle which

uses organic working fluid. The Kalina cycle is being developed as a competitive improvement

of the Rankine steam cycle. By using an ammonia-water mixture as the working fluid and a

condensing system based on absorption refrigeration principles.

Figure III.8 : Simplified flow diagram for a Rankine binary geothermal power plant

Figure III.9: Simplified diagram for a Kalina binary geothermal power plant

37

III.2.2.4.Combined or hybrid plants

Since geothermal fluids are found with a wide range of physical and chemical

properties such as temperature, pression, noncondensable gases, scaling and corrosion

potential,…

A variety of energy conversion systems have been developed to suit any set of conditions. The

basic systems described in the earlier sections can be combined to achieve more effective

systems for particular applications. Thus, the following hybrid plants can be designed: Direct –

steam/Binary plant; Single flash/Binary plant; Integrated single and double flash plant.

Properly designed hybrid systems have a higher overall efficiency compared with using the

two systems.

III.2.2.5. Efficiency of generation

The efficiency of generation ηshows how well a plant converts the exergy of the

resource into useful output.

For a geothermal plant, it is found as follows: η=W/me

Where W is the net electricity delivered to the grid, m the required total geofluid mass flow

rate, and e is the specific energy of the geofluid under reservoir conditions. The latter is given

by: e=h(Pl-Tl)-h(Po-To)-To[s(Pl-Tl)-s(Po-To)]

The specific enthalpy h, entropy s, are evaluated at reservoir conditions, P l and Tl and at the so

called dead-state, Po and To which correspond to the local ambient conditions.

38

Chapter IV: RWANDA GEOTHERMAL SYSTEM

IV.1. GEOTHERMAL IN AFRICA

The geothermal potential of Africa is mostly centered within the East African Rift,

which provides both the extensional rifting environment associated with forced convective

systems and the volcanic environment that favors the development of large, high temperature

geothermal systems. The East African Rift System is a major geologic structure that extends

from Mozambique north to the Red Sea rift (Figure IV.1). The rift consists of two main

braches.

• The eastern branch, which extends from Mozambique north through Tanzania into Kenya,

Ethiopia, and Djibouti, has the best geologic expression.

39

• The western branch, which extends along the western borders of Tanzania, Burundi,

Rwanda and Uganda, is less developed and less active seismically and volcanically.

Active volcanoes are associated with the rift, two active volcanoes occur along the

Western Branch in Democratic Republic of the Congo, Nyiragongo and Nyamuragira. The

highest volcano, Mt. Kalisimbi, which is considered dormant, is in Rwanda.

These volcanoes are all part of the Virunga Volcanoes.

Despite the widely recognized geothermal potential of the East African Rift, the region

remains largely undeveloped. Except for one field in Kenya and another in Ethiopia,

geothermal exploration has not progressed beyond the reconnaissance exploration stage, even

though the geothermal potential of most East African countries was inventoried in the 1970’s

and 1980’s. To date, only one high temperature geothermal system associated with a volcanic

center has been developed. This system in Kenya, known as Olkaria and hosts a total of 129

MWe of installed generating capacity.

Figure IV.1: East African rift system

IV.2. RWANDA GEOTHERMAL RESOURCES LOCATION

Geothermal resources that we are going to discuss in this part are geothermal resources

with surface manifestations, generally we observe a temperature rise as we approach the axis of

the rift located at the west of the country and from an assessment made in 1982 we have the

following geothermal resources countrywide.

IV.2.1. Southern region

• Ntaresi geothermal resources : A few of hot spring emergences, in the left side of the

Lukarara valley(North of Nyamagabe). With a low emergence temperature, that is, around

20.9oC.

40

• Akanyaru geothermal resources : Tepid water emerges with a low flow (≈ 1l/s) and some

gaseous emissions.

IV.2.2. Western region

• Mashyuza geothemal resources : Many hot springs flow up from the Mashyuza graben

bordering faults. Their emergence temperature is about 54.2oC.

• Gishyita geothermal resources :Located in the south-western part of Karongi, we have

an emergence of hot water at about 31.7 oC, with a little gaseous emission.

• Ruganji geothermal resources : We have two hot springs located at 500m from

BRALIRWA on the peninsula of Kivaga in the lake Kivu. They are the hottest sources of

the country 73.5oC, the liquid flow is about 1l/s for each source and there is a weak gaseous

.

IV.2.3. Northern region

Amakera geothermal resources : Located in the East of Musanze, this source is cold 18.7 oC, presents a strong gaseous emission and deposits of Iron hydroxide.

Gihugu and Buseruka geothermal resources : Located in the west of Ntaruka electric power

plant. Characteristics similar to the source of Amakera.

Gitagata geothermal resources : Two emergences on left side bank of a small valley located

in the North-West of Base, the flow is of approximately 1l/s at a temperature of 37 oC.

41

Figure IV.2: Rwanda Geothermal resources visited during our study

IV.3. GEOTHERMAL FLUIDS ORIGIN

IV.3.1. Deep waters origin

The water molecule is made up of two types of atoms, Hydrogen and Oxygen, these

atoms can have different masses, the ratio between Oxygen atoms of mass 18 and those of

mass 16 as well as the number of the Hydrogen atoms of mass 1 compared to those of mass 2

will allow to characterize and distinguish water from different origins.

An analysis made on geothermal waters of Rwanda showed that all of them have a meteoric

origin.

42

IV.3.2. CO2 origin

The isotopic ratio of Carbon 13 allow to know the origin of CO2 dissolved in water, the

atmospheric CO2 has a Carbon 13 ratio ranging between 0.6% and 0.8% .

An analysis made on Nyamyumba and Cyabararika gave a ratio of 0.47% and 0.2% for

Mashyuza. This means that CO2 from these sources has a deep origin.

IV.4.CALCULATION OF DEEP TEMPERATURES

IV.4.1. Introduction on geothermometers

Concentrations of certain chemical elements dissolved in geothermal waters are controlled by

their solubilities and the latter are function of the temperature.

The relation Solubility-temperature allowed the establishment of equations that make possible

the calculation of the temperature in geothermal reservoirs.

These equations are called geothermometers, some of the are based on absolute concentrations

and others on concentrations ratios.

IV.4.1.1. Geothermometer with Silica

Silica is one of the major chemical constituent of the Earth crust. The various allotropic

varieties like Quartz, Chalcedony and amorphous Silica can be dissolved in water, this reaction

of solubilization is mainly function of the temperature and doesn’t depend on the pH or the

ionic force of the solution.

In a laboratory analysis, the dissolved Silica is obtained by precipitation of an allotropic variety

of Silica, this latter depend on the temperature of the solubilization.

If the temperature is higher than 160oC the precipitated form is quartz.

Between 120 oC and160oC we have quartz and Chalcedony.

Under 120 oC, only Chalcedony.

Amorphous Silica is obtained for low temperatures.

Based on the solubility curves of these different allotropic varieties, the following equations

has been established:

43

For Quartz: 15.273log19.5

1309

2

0 −−

=SiO

CT

For Chalcedony: 15.273log69.4

1032

2

0 −−

=SiO

CT

For amorphous Silica: 15.273log26.0

731

2

0 −+

=SiO

CT

Concentrations are expressed in mg/l

All that is valid only if the concentration of the dissolved Silica were not modified during the

transfer from the reservoir to the surface.

The modification can be consequence of:

A mixture with surface water.

Boiling or evaporation .

To avoid errors due to the modifications of concentrations, we use geothermometers based on

concentration ratios.

IV.4.1.2. Geothermometers with Na/K

From the reaction 8383 OSiAlNaNaOSiAlK ⇔+ +

We have two non-miscible feldspars, moreover, this reaction depends only on the pressure and

the temperature but in our case the pressure influence is weak compared to that of the

temperature.

Thus the Na/K ratio depends only on the temperature, and the corresponding equation is:

15.273log993.0

9330 −−

=KNa

CT

Concentrations in mg/l

Based on a concentrations ratio, the process of dilution particularly with surface waters

doesn’t affect this geothermometer.

The most serious disadvantage of this geothermometer is the supposition of the

equilibrium of the reaction that governs this geothermometer, it is difficult to know if this

equilibrium had been really attained.

Most of the time, the ration Na/K ranges between 3 and 30 thus, giving temperatures

between 150oC and 500oC.

44

To overcome these difficulties, a geothermometer Na-K-Ca has been proposed as follows:

15.273loglog24.2

16470 −+−

=NaCaKNa

CTβ

Concentrations in mol/l

We first try β=4/3, if we obtain a temperature higher than 100oC, we change for β=1/3.

If not β=4/3 gives a good estimate. This geothermal is like a limitation, if we take β=4/3 and

we obtain a temperature lower than 100oC, better to reject the temperature calculated by Na/K,

but the reverse is not true.

IV.4.1.3. Geothermometer with Na/Li

This geothermometer is empirical, there is no scientific proof to its operation, it has been

established from a statistical study.

For waters with a moderate salinity (Cl<10g/l) we have the following equation:

15.273log38.0

10000 −−

=LiNa

CT

For brines (Cl>10g/l) :

15.27313.0log

11950 −−

=LiNa

CT

Concentrations are expressed in mol/l

This geothermometer has an advantage of being obeyed by cold surface water, this is not the

case for Silica nor Na/K geothermometers.

IV.4.1.4. Adjustment considering Mg

• If T(Ca,Na,K) < 70oC there is no adjustement.

• If this temperature exceeds 70oC, we calculate the expression:

100xKCaMg

MgR

++=

Concentrations expressed in equivalents/l

45

• If R>50, We are in presence of a shallow acquifer with a temperature close to that of

the spring. In this case there is no need to consider any geothermometer.

• If T(Ca,Na,K)>70 and 1.5<R<5, we use the abacus I (Appendix) to estimate DMg,

which is the temperature correction to take from T(Ca,Na,K).

IV.4.2. Analysis of natural waters with Atomic Absorption Spectroscopy

This method describes the determination of Calcium,Magnesium, Potassium and

Sodium in natural waters and may be applicable to other elements.

Prepare all standard solutions except Ca and Mg by suitable dilutions of the stock

solutions described under the Standard Conditions for each element presented in the appendix.

For Ca and Mg, dilute the the stock solutions with 5% Lanthanum solution and HCl to give

dilute standards wich contain 0.25% La and 5% HCl.

Filter each sample through a 0.45 micron micropore membrane filter, if necessary to

avoid clogging of the burner capillary. Aspirate each sample directly, except for Ca and Mg.

For Ca and Mg you have to dilute with La and HCl. Read the concentration of the element of

interest directly against the appropriate standards given in the appendix.

Ca and Mg results should be corrected by using a reagent blank.

IV.4.3. Deep temperatures estimated with geothermometers

Hot springs had been sampled in 1982. The chemical analyses from these samples are

reported table IV in the appendix, because the sampling conditions and preservation techniques

were not well understood for the samples collected in 1982, in order to verify we visited eight

most interesting geothermal springs namely Mashyuza I and II, Ntaresi, Gishyita, Cyabararika,

Gihugu and Ruganji I and II, we took new samples from these hot springs for analyses.

(table II)

We also have located these hot springs with a GPS, geographic coordinates of these

sources (table I )and a map from these GPS data is provided as figure IV.2.

From our Atomic Absorption analyses (Table II) and geothermometric calculations

(table VI) we concluded that the AAS machine we used was not well calibrated, but

considering previous prospects we ended up with the following estimates for the deep

temperatures.

46

IV.4.3.1 Mashyuza prospects

The calculated temperatures are very similar for the two points of emergence.

The cations geothermometers indicate a high temperatures (200-250oC), let us note that the

temperature calculated by the Na/Li ratio geothermometer is 150oC.

This geothermometer often gives lower values than others and thus makes it possible to

fix the minimal value of the estimate.

The temperature calculated by the silica concentration geothermometer is low. Indeed,

this geothermometer is directly influenced by dilution with surface cold water. In spite of the

heterogeneity of the computation results, one can retain that the temperature is higher than

150oC, and could approach 200oC. We have also a Cesium concentration, which is in favour of

a high temperature of the reservoir.

We note that there exists on the south-eastern edge of the graben (in Burundi), a thermal

spring, with chemical characteristics very similar to that of Mashyuza. This shows the

homogeneity and the possible extension of the reservoir which feeds these sources and then,

the existence of a geothermal field in this region.

IV.4.3.2 Ruganji prospects

The two springs provide very similar results. The cations geothermometers give values

around 180oC and low values for Na/Li (approximately 60oC).

The temperature calculated with the silica concentration to in equilibrium with quartz is

of approximately 140oC. In addition, these sources have relatively high Cs and Li

concentrations compared to the other sources and a week Mg concentration.

We concluded that this water reaches a deep temperature of approximately 160oC.

IV.4.3.3. Ntaresi prospect

The results obtained with the various geothermometers are homogeneous, which

increases the reliability of the results. The Li content is higher, the content in As and Cs

relatively high and the Mg is present in small quantity. Count held of these various results, The

depth temperature is of approximately 180oC.

IV.4.3.4. Amakera and Gihugu prospects

47

For these sources, the Cs, As and Li concentrations are very weak whereas the Mg has a

very high concentration. It appears that these sources are probably originating from a cold

surface reservoir.

IV.4.3.5. Gishyita geothermal prospect

The results from the cations geothermometers are heterogeneous, The Cs, As and Li

concentrations are relatively low as that of Mg. But with only one source we cannot have a

reliable estimate of depth temperature. However the results are not in favor of high

temperatures.

IV.5. CONCLUSION

From our study, we realized that Mashyuza, Ruganji and Ntaresi are geothermal fields

that can be economically exploited by the use of binary technology, estimates for 50MWe and

10MWe are reported in tables IX and X.

The most likely geologic model for Ruganji geothermal field is that the reservoir waters

are rising vertically along a normal fault near the eastern boundary of the East African Rift.

The fluids could be mixing along their flow path from the reservoir with low temperature

ground water or water from Lake Kivu. The reservoir waters may also be degassing during

their ascent as suggested by the relatively small amount of gas being vented from the hot

springs. An alternative and less likely model is that the hot springs represent a distal outflow of

fluids from a high temperature reservoir associated with the Virunga Volcanoes to the north.

For Mashyuza geothermal field, the most likely geologic model for this prospect is that

warm waters are rising vertically along the fault forming the western margin of the graben. The

upside potential is limited.

However, additional geologic mapping could locate extensions of the fault, geologist

from the cement factory noted that travertine deposits also occur in Burundi to the south and

that they may occur along the same geologic structure.

Tertiary volcanics, primarily basaltic lavas, are noted both south and west of the hot

spring area. Although no age dates are available for these lavas, they appear to be too old to

indicate that a viable magmatic heat source underlies the geothermal system.

For Ntaresi, there is no available geologic model, but all of the geothermometers

48

showed that probably there is a geothermal field in that region further research have to be

conducted in order to locate and evaluate the potential of this field.

IV.6. RECOMMENDATIONS

Our recommendations for the three regions suspected to be geothermal fields are

provided as it follows:

Additionnal geothermometric analyses should be done in order to verify and complete

available datas on these fields, if possible, a complete set of gas samples should be obtained

from the hot springs. At the very least, samples for Helium isotope analysis. The He isotopes

should help reveal if the heat source contains a component of magmatic gas, or whether the

fluids are being conductively heated while circulating in Precambrian metamorphic rocks.

A detailed geologic map should be created through additional field work. The

distribution of hydrothermal alteration, hot spring deposits, and thermal springs should be

mapped within a five to ten kilometer radius of the hot springs. Aerial photography and remote

sensing images would assist with the field mapping and the location of geologic structures.

Geophysical surveys, such as magnetotelluric, electromagnetic and regional gravity surveys

should be conducted to define the extensions of the prospective area. Consideration should also

be given to conducting resistivity surveys, although these may be cost prohibitive.

Ultimately, a well will need to be drilled in each region in order to assess the temperature of the

resources. The surface location for the first well would probably be located near the hot spring.

Refererences

1. Paul Kruger and Caren Otte, Geothermal energy:Resources, Production, Simulation,

Stanton University, June 1973

2. H. Christopher H. Armstead, Geothermal energy: Review of research and development,

Unesco 1974

3. John W. Twidell & Anthony D. Weir, Renewable energy resources, Spon 1986

49

4. R. Fabriol & P. Verzier, Reconnaissance geothermique de la republique du Rwanda,

BRGM 1983

5. Enrico Barbier, Geothermal energy technology and current status, an overview,

Pergamon 2002

6. Cornel Otieno Ofwana, Olkaria east geothermal system, Kenya, The United Nations

University, September 2002

7. Ronald DiPippo, Small Geothermal Power Plants: Design, Perfomance and Economics,

GHC Bulletin, June 1999

8. Bureau de recherches géologiques et minières, Ministere de l’Economie, des Finances

et de l’Industrie (France), Les géothermomètres chimiques, Note technique nº 13,

Février 1999

9. Francis F. Huang, Engineering thermodynamics: Fundamentals and applications,

Collier Macmillan, 1976

10. Mac Naughton, Edgar, Elementary steam power engineering, Wiley 1966

11. http://macservcart.uncc.edu/faculty/haas/geol13190/termpap/harris/ visited on August

10, 2006

12. http://www.cc.utah.edu/~ptt25660/geo.html visited on Dec 18, 2006

13. http://www.geothermalenergyassociation.org Visited on Feb 20, 2007

50

APPENDIX

Table I: GPS data

Location Latitude Longitude ElevationMashyuza 1 S 02º 35.102' E 029º 01.191' 1136mMashyuza 2 S 02º 34.956' E 029º 00.938' 1173m

NtaresiCyabararika S 01º 30.369' E 029º 39.270' 1833m

Gihugu S 01º 30.837' E 029º 38.223' 1801m

51

Ruganji 1 S 01º 44.396' E 029º 16.439' 1470mRuganji 2 S 01º 44.322' E 029º 16.421' 1470mGishyita 1 S 02º 10.058' E 029º 18.362' 1683mGishyita 2 S 02º 04.626' E 029º 22.939' 1638m

Table II: Chemical analyses results (NUR 2007)

Site/Parameter K Na Ca MgWavelength 766.49 589.00 422.67 285.21Mashyuza 1 66.7 171.5 14.3 273.5Mashyuza 2 97.8 164.9 2.5 11.5

Ntaresi 108.1 173.5 6.4 189.5Cyabararika 204.3 172.9 6.7 266.5

Gihugu 220.3 164.4 29.7 341.5Ruganji 1 70.7 167 14.8 112Ruganji 2 75.8 164 11.6 118.5

Concentrations in mg/l

Table III: Chemical analyses results (Chevron 2006)

Site/Parameter ToC pH Na K Ca MgMashyuza 1 33 7.0 291.2 45.3 89.5 51.8Mashyuza 2 47 6.5 307.8 48 76 55Ruganji 1 70.6 7 518.8 39.8 36.4 11.1Lake Kivu - 8 110.9 87.5 8.1 80.1

Site/Parameter B SiO2 Cl SO4 HCO3

Mashyuza 1 1.18 50.2 141 50.5 1115.8Mashyuza 2 1.07 48.3 137.9 55.3 1122.7Ruganji 1 0.55 58.5 236.8 62.1 1137.3Lake Kivu 0.10 7.9 32 22.3 796.1

52

Concentrations in mg/l

Table IV: Chemical analysis results (BRGM 1982)

Concentrations in mg/l

Table IV: Chemical analyses results (BRGM 1982)

Site/Paramete

r

Cl K Li SiO2 HCO3 SO4

Mashyuza 1 120.89 45.36 0.9 84.72 1049.5 48.03Mashyuza 2 127.98 47.31 0.95 75.1 1061.7 46.01

Ntaresi 155.99 26.59 1.66 186.86 1690.2 99.9Gishyita 48.92 24.91 2.8 155.02 1037.2 27.9

Cyabararika 209.88 179.87 0.05 156.22 2038 42Gihugu 180.1 179.87 0.05 153.21 2123.4 50

Ruganji 1 233.99 41.06 0.41 109.95 1122.7 54Ruganji 2 233.99 40.67 0.41 105.75 1122.7 44

Concentrations in mg/l

Site/Parameter ToC pH Cond. Ca Na MgMashyuza 1 41.8 6.45 2.9 76.95 287.4 51.77Mashyuza 2 54.2 6.26 3.5 72.94 298.8 53.96

Ntaresi 20.9 6.28 2.8 220.03 321.8 96Gishyita 31.7 6.09 2.1 149.89 224 17.6

Cyabararika 18.7 6.22 2.5 121.04 174 205.86Gihugu 72.4 6.2 2.7 121.84 174 226.03

Ruganji 1 70.6 6.69 5.2 37.99 531 11.39Ruganji 2 31.7 6.47 5.2 37.79 528.7 11.12

53

Table V: Chemical geothermometry results (NUR 2007)

Prospect R Na-K Na-K-Ca(β=4/3) Na-K-Ca(β=1/3)Mashyuza 1 90.3 1327.6 57.9 400.960302Mashyuza 2 26.5 944.7 233.5 493.0808995

Ntaresi 83.49 911.6 134.9 424.1406831Cyabararika 79.78 602.5 103.4 347.2217986

Gihugu 79.8 559.8 -44.8 210.5012566Ruganji 1 78.35 1232.4 45.8 380.985298Ruganji 2 79.48 1145.2 66.2 400.960302

Table VI: Chemical geothermometry results (Chevron 2006)

Prospect Spring ToC Quartz Na-K Na-K-Ca Na-K-Ca-MgGisenyi 75 110 161 to 212 181 72

Mashyuza 54 101 to 102 241 to 272 204 to 205 21 to 27

Table VII: Chemical geothermometry results (BRGM 1982)

Prospect Spring ToC Quartz Chalc. Amorphous SiO2

Mashyuza 1 42 128 99 9Mashyuza 2 54 122 92 4

Ntaresi 21 176 153 52Gisenyi 1 72 143 116 22Gisenyi 2 71 141 113 20

Prospect Na-K Na-Li Na-K-CaMashyuza 1 259 151 204Mashyuza 2 259 151 205

Ntaresi 201 193 92Gisenyi 1 195 63 181Gisenyi 2 195 64 181

Table VIII: Estimated installation costs for a 10MWe geothermal development

Item Cost in US$ (Million) Cost in Rwf(Billion)

54

2 production wells 7 3.851 Injection well 3.5 1.925

4 Km of 36" brine pipeline 7.1 3.90510 MW power plant 12 6.6Office and Salaries 2 1.1

Total 31.6 17.38

Table IX: Estimated installation costs for a 50MWe geothermal development

Item Cost in US$ (Million) Cost in Rwf(Billion)7 production wells 24.5 13.475

4 Injection well 14 7.75Km of 36" steam pipeline 8.8 4.845Km of 36" brine pipeline 10.6 5.83

2Km of condensate pipeline 0.16 0.08850 MW power plant 50 27.5Office and Salaries 6 3.3

Total 114.06 62.733

55

Abacus I: Estimates for DMg

Standard Atomic Absorption Conditions for Mg (12)

Wavelength

(nm)

Slit

(nm)

RelativeNoise

CharacteristicConcentration

(mg/l)

CharacteristicConcentrationCheck (mg/l)

LinearRange(mg/l)

285.2

202.6

0.7

0.7

1.0

3.9

0.0078

0.19

0.30

9.0

0.50

10.0

1. Recommended Flame: air-acetylene, oxidizing (lean, blue)

2. Data obtained with a standard nebulizer and flow spoiler. Operation with a High

Sensitivity nebulizer or impact bead will typically provide a 2-3 × sensitivity

improvement.

3. Characteristic Concentration with a N2O-C2H2 flame at 285.2 nm: 0.036 mg/L

Standard Flame Emission Conditions for Mg

56

Wavelength

(nm)

Slit

(nm)

Flame

285.2 0.2 Nitrous oxide-acetylene

Stock standard solution: MAGNESIUM, 1000 mg/L. CAUTIOUSLY dissolve 1.000

g of magnesium ribbon in a minimum volume of (1+1) HCl. Dilute to 1 liter with 1% (v/v)

HCl.

Inteferences: Aluminum, silicon, titanium, and phosphorus depress the magnesium

signal. This effect can be controlled by the addition of lanthanum

(0.1% as chloride) to samples and standards.

The use of the nitrous oxideacetylene flame will also overcome the effect, but

ionization should be controlled by the addition of an alkali salt (0.1% or more potassium as

chloride) to samples and standards.

Standard Atomic Absorption Conditions for Na (11)

Wavelength

(nm)

Slit

(nm)

RelativeNoise

CharacteristicConcentration

(mg/l)

CharacteristicConcentrationCheck (mg/l)

LinearRange(mg/l)

589.0

330.2

0.2/0.4

0.7

1.0

0.63

0.012

1.7

0.50

80.0

1.0

---

1. Recommended Flame: air-acetylene, oxidizing (lean, blue)

2. Data obtained with a standard nebulizer and flow spoiler. Operation with a High Sensitivity

nebulizer or impact bead will typically provide a 2-3 × sensitivity improvement.

3. Data collected with an alkali salt (0.1% or more) added to control ionization.

Standard Flame Emission Conditions for Na

57

Wavelength

(nm)

Slit

(nm)

Flame

285.2 0.2 Nitrous oxide-acetylene

Stock standard solution: SODIUM, 1000 mg/L. Dissolve 2.542 g of sodium chloride,

NaCl, in deionized water and dilute to 1 liter with deionized water. Preparation of

uncontaminated standards for this element is difficult.

Inteferenes: Ionization should be controlled by the addition of an alkali salt (0.1% or

more potassium or cesium as chloride) to samples and standards. In the

presence of high concentrations of mineral acids, the sodium signal is reduced.

Doublets: The 589.0 nm and the 303.2 nm sodium lines are actually doublets

(589.0 nm/589.6 nm, 303.2 nm/303.3 nm)

Standard Atomic Absorption Conditions for K (19)

Wavelength

(nm)

Slit

(nm)

RelativeNoise

CharacteristicConcentration

(mg/l)

CharacteristicConcentrationCheck (mg/l)

LinearRange(mg/l)

776.5

769.9

404.4

0.7/1.4

0.7/1.4

0.7

1.0

1.4

1.9

0.043

0.083

7.8

2.0

4.0

350.0

2.0

20.0

600.0

1. Recommended Flame: air-acetylene, oxidizing (lean, blue)

2. Data obtained with a standard nebulizer and flow spoiler. Operation with a High

Sensitivity nebulizer or impact bead will typically provide a 2-3 × sensitivity

improvement.

3. Data collected with an alkali salt (0.1% or more) added to control ionization.

4. A red filter which absorbs radiation below 650 nm should be used.

Standard Flame Emission Conditions for K

58

Wavelength

(nm)

Slit

(nm)

Flame

766.5 0.2/04 Air-acetylene

Stock standard solution: POTASSIUM, 1000 mg/L. Dissolve 1.907 g of potassium

chloride, KCl, in deionized water and dilute to 1 liter with deionized water.

Interferences: Ionization can be controlled by the addition of an alkali salt (0.1% or

more cesium or lanthanum as chloride) to samples and standards. Strong concentrations of

mineral acids may cause the potassium signal to be depressed.

Doublets: The 404.4 nm potassium line is actually a doublet (404.41nm/404.72 nm)

Standard Atomic Absorption Conditions for Ca (20)

Wavelength

(nm)

Slit

(nm)

RelativeNoise

CharacteristicConcentration

(mg/l)

CharacteristicConcentrationCheck (mg/l)

LinearRange(mg/l)

422.7

239.9

0.7

0.7

1.0

14.0

0.092

13.0

4.0

600.0

5.0

800.0

1. Recommended Flame: air-acetylene, oxidizing (lean, blue)

2. Data obtained with a standard nebulizer and flow spoiler. Operation with a High

Sensitivity nebulizer or impact bead will typically provide a 2-3 × sensitivity

improvement.

3. Characteristic Concentration with a N2O-C2H2 flame at 422.7 nm: 0.048 mg/L

Standard Flame Emission Conditions for Ca

Wavelength Slit Flame

59

(nm) (nm)766.5 0.2/04 Air-acetylene

Stock standard solution:CALCIUM, 500 mg/L. To 1.249 g of calcium carbonate,

CaCO3, add 50 mL of deionized water. Dissolve by adding dropwise 10 mL of HCl. Dilute to

1 liter with deionized water.

Flame Adjustement: The absorption of calcium is dependent on the fuel/air ratio and

the height of the light beam above the burner. Although maximum sensitivity is obtained with

a reducing (fuel-rich) flame, an oxidizing(fuel-lean) flame is recommended for optimum

precision.

Other Flames: Calcium determination appears to be free from chemical interferences

in the nitrous oxide-acetylene flame. Ionization interferences should be

controlled by the addition of alkali salt (0.1% or more K as KCl).

60