What is Geothermal Energy?
Mary H. Dickson and Mario Fanelli
Istituto di Geoscienze e Georisorse, CNR , Pisa, Italy
Prepared on February 2004

INTRODUCTION
Heat is a form of energy and geothermal energy is, literally, the heat contained within the
Earth that generates geological phenomena on a planetary scale. ‘Geothermal energy’ isoften used nowadays, however, to indicate that part of the Earth's heat that can, or could,
be recovered and exploited by man, and it is in this sense that we will use the term fromnow on.
Brief geothermal history
The presence of volcanoes, hot springs, and other thermal phenomena must have led ourancestors to surmise that parts of the interior of the Earth were hot. However, it was not
until a period between the sixteenth and seventeenth century, when the first mines wereexcavated to a few hundred metres below ground level, that man deduced, from simple
physical sensations, that the Earth's temperature increased with depth.
The first measurements by thermometer were probably performed in 1740 by De
Gensanne, in a mine near Belfort, in France (Buffon, 1778). By 1870, modern scientificmethods were being used to study the thermal regime of the Earth (Bullard, 1965), but it
was not until the twentieth century, and the discovery of the role played by radiogenic
heat, that we could fully comprehend such phenomena as heat balance and the Earth's
thermal history. All modern thermal models of the Earth, in fact, must take into accountthe heat continually generated by the decay of the long-lived radioactive isotopes ofuranium (U238, U235), thorium (Th232) and potassium (K40), which are present in the Earth(Lubimova, 1968). Added to radiogenic heat, in uncertain proportions, are other potential
sources of heat such as the primordial energy of planetary accretion. Realistic theories onthese models were not available until the 1980s, when it was demonstrated that there wasno equilibrium between the radiogenic heat generated in the Earth's interior and the heatdissipated into space from the Earth, and that our planet is slowly cooling down. To givesome idea of the phenomenon involved and its scale, we will cite a heat balance from
Stacey and Loper (1988), in which the total flow of heat from the Earth is estimated at 42x 1012 W (conduction, convection and radiation). Of this figure, 8 x 1012 W come from thecrust, which represents only 2% of the total volume of the Earth but is rich in radioactive
isotopes, 32.3 x 1012 W come from the mantle, which represents 82% of the total volumeof the Earth, and 1.7 x 1012 W come from the core, which accounts for 16% of the total
volume and contains no radioactive isotopes. (See Figure 1 for a sketch of the inner
structure of the Earth). Since the radiogenic heat of the mantle is estimated at 22 x 1012 W,
the cooling rate of this part of the Earth is 10.3 x 1012 W.
In more recent estimates, based on a greater number of data, the total flow of heat from1the Earth is about 6 percent higher than the figure utilized by Stacey and Loper in 1988.
Even so, the cooling process is still very slow. The temperature of the mantle has
decreased no more than 300 to 350 °C in three billion years, remaining at about 4000 °C
at its base. It has been estimated that the total heat content of the Earth, reckoned above
an assumed average surface temperature of 15 °C, is of the order of 12.6 x 1024 MJ, andthat of the crust is of the order of 5.4 x 1021 MJ (Armstead, 1983). The thermal energy of
the Earth is therefore immense, but only a fraction could be utilized by mankind. So far
our utilization of this energy has been limited to areas in which geological conditions
permit a carrier (water in the liquid phase or steam) to ‘transfer’ the heat from deep hotzones to or near the surface, thus giving rise to geothermal resources; innovativetechniques in the near future, however, may offer new perspectives in this sector.

Figure 1
The Earth's crust, mantle, and core. Top right: a section through the crust and theuppermost mantle.
In many areas of life, practical applications precede scientific research and technologicaldevelopments, and the geothermal sector is a good example of this. In the early part of thenineteenth century the geothermal fluids were already being exploited for their energy
content. A chemical industry was set up in that period in Italy (in the zone now known as
Larderello), to extract boric acid from the boric hot waters emerging naturally or from2

specially drilled shallow boreholes. The boric acid was obtained by evaporating the boric
waters in iron boilers, using the wood from nearby forests as fuel. In 1827 Francesco
Larderel, founder of this industry, developed a system for utilizing the heat of the boric
fluids in the evaporation process, rather than burning wood from the rapidly depleting
forests (Figure 2).

Figure 2
The “covered lagoon” used in the first half of the 19th century in the Larderello area,
Italy, to collect the hot boric waters and extract the boric acid.
Exploitation of the natural steam for its mechanical energy began at much the same time.
The geothermal steam was used to raise liquids in primitive gas lifts and later inreciprocating and centrifugal pumps and winches, all of which were used in drilling or
the local boric acid industry. Between 1850 and 1875 the factory at Larderello held themonopoly in Europe for boric acid production. Between 1910 and 1940 the low pressure
steam in this part of Tuscany was brought into use to heat the industrial and residentialbuildings and greenhouses. Other countries also began developing their geothermal
resources on an industrial scale. In 1892 the first geothermal district heating systembegan operations in Boise, Idaho (USA). In 1928 Iceland, another pioneer in theutilization of geothermal energy, also began exploiting its geothermal fluids (mainly hotwaters) for domestic heating purposes.
By 1904 the first attempt was being made at generating electricity from geothermal
steam; again, it was to take place at Larderello (Figure 3).

3

Figure 3
The engine used at Larderello in 1904 in the first experiment in generating electricenergy from geothermal steam, along with its inventor, Prince Piero Ginori Conti.
The success of this experiment was a clear indication of the industrial value ofgeothermal energy and marked the beginning of a form of exploitation that was todevelop significantly from then on. Electricity generation at Larderello was a commercial
success. By 1942 the installed geothermoelectric capacity had reached 127,650 kWe.
Several countries were soon to follow the example set by Italy. In 1919 the firstgeothermal wells in Japan were drilled at Beppu, followed in 1921 by wells drilled at The
Geysers, California, USA. In 1958 a small geothermal power plant began operating in
New Zealand, in 1959 another began in Mexico, in 1960 in the USA, followed by manyother countries in the years to come.
Present status of geothermal utilization
After the Second World War many countries were attracted by geothermal energy,
considering it to be economically competitive with other forms of energy. It did not haveto be imported, and, in some cases, it was the only energy source available locally.
Table 1. Installed geothermal generating capacities world-wide from 1995 to 2000
(from Huttrer, 2001), and at the end of 2003.
1995-2000
(%)
1995
2000
2003
Country(increase inincrease
(MWe)
(MWe)
(MWe)
MWe )
(1995-2000)
Argentina
Australia
Austria
China
Costa Rica

0.67
0.15
28.78
55

0.15
29.17
142.5

0.39
87.5

4

1.35
159

0.15
1.25
28.18
162.5

El Salvador
Ethiopia
FranceGermany
GuatemalaIcelandIndonesia
Italy
Japan
Kenya
MexicoNew
Zealand
Nicaragua
Papua New
GuineaPhilippines
Portugal
RussiaThailandTurkey
USA
Total

105
4.2
50
309.75
631.7
413.7
45
753

161
7
4.2
33.4
170
589.5
785
546.9
45
755

56
7
33.4
120
279.75
153.3
133.2
2

53.3
240
90.3
24.3
32.2
0.3

161
7
15
0.23
29
200
807
790.5
560.9
121
953

286

437

151

52.8

421.3

70

70

-

-

77.5

-

-

-

-

6

1,227
5
11
0.3
20.4
2,816.7
6,833.35

1,909
16
23
0.3
20.4
2,228
7,972.5682
11
12
1,728.54

55.8
220
109
16.7

1,931
16
73
0.3
20.4
2,020
8,402.21

The countries that utilise geothermal energy to generate electricity are listed in Table1,
which also gives the installed geothermal electric capacity in 1995 (6,833 MWe), in 2000
(7,972 MWe) and the increase between 1995 and the year 2000 (Huttrer, 2001). The same
Table also reports the total installed capacity at the end of 2003 (8,402 MWe). The
geothermal power installed in the developing countries in 1995 and 2000 represents 38
and 47% of the world total, respectively.
The utilization of geothermal energy in developing countries has exhibited an interesting
trend over the years. In the five years between 1975 and 1979 the geothermal electric
capacity installed in these countries increased from 75 to 462 MWe; by the end of thenext five-year period (1984) this figure had reached 1495 MWe, showing a rate ofincrease during these two periods of 500% and 223%, respectively (Dickson and Fanelli,1988). In the next sixteen years, from 1984 to 2000, there was a further increase of almost
150%. Geothermal power plays a fairly significant role in the energy balance of someareas; for example, in 2001 the electric energy produced from geothermal resources
represented 27% of the total electricity generated in the Philippines, 12.4% in Kenya,
11.4% in Costa Rica, and 4.3% in El Salvador.
As regards non-electric applications of geothermal energy, Table 2 gives the installed5

capacity (15,145 MWt) and energy use (190,699 TJ/yr) world-wide for the year 2000.
During that year 58 countries reported direct uses, compared to 28 in 1995 and 24 in 1985.
The number of countries with direct uses has very likely increased since then, as well as
the total installed capacity and energy use.
The most common non-electric use world-wide (in terms of installed capacity) is heatpumps (34.80%), followed by bathing (26.20%), space-heating (21.62%), greenhouses(8.22%), aquaculture (3.93%), and industrial processes (3.13%) (Lund and Freeston,2001).
Table 2. Non-electric uses of geothermal energy in the world (2000): installed thermal
power (in MWt) and energy use (in TJ/yr). Taken from Lund and Freeston (2001).
Country

Algeria
Argentina
Armenia
Australia
Austria
Belgium
Bulgaria
CanadaCaribbean Islands
Chile
China
Colombia
Croatia
Czech Republic
Denmark
Egypt
FinlandFranceGeorgia
Germany
GreeceGuatemalaHonduras
Hungary
Iceland

Power
(MWt)

Energy(TJ/yr)

100
25.7
1
34.4
255.3
3.9
107.2
377.6
0.1
0.4
2,282
13.3
113.9
12.5
7.4
1
80.5
326
250
397
57.1
4.2
0.7
472.7
1,469

1,586
449
15
351
1,609
107
1,637
1,023
1
7
37,908
266
555
128
75
15
484
4,895
6,307
1,568
385
117
17
4,086
20,1706

IndiaIndonesia
IsraelItaly
Japan
Jordan
Kenya
Korea
Lithuania
Macedonia
Mexico
Nepal
Netherlands
New Zealand
NorwayPeru
Philippines
PolandPortugal
Romania
RussiaSerbiaSlovak Republic
Slovenia
SwedenSwitzerland
ThailandTunisia

80
2.3
63.3
325.8
1,167
153.3
1.3
35.8
21
81.2
164.2
1.1
10.8
307.9
6
2.4
1
68.5
5.5
152.4
308.2
80
132.3
42
377
547.3
0.7
23.1

2,517
43
1,713
3,774
26,933
1,540
10
753
599
510
3,919
22
57
7,081
32
49
25
275
35
2,871
6,144
2,375
2,118
705
4,128
2,386
15
201

TurkeyUnited Kingdom
USA*
Venezuela
Yemen

820
2.9
3,766
0.7
1

15,75621
20,302
14
15

Total

15,145

190,699* During 2003, these figures increased to 4,350 MWt and 22,250 TJ/yr (Lund, 2003)

7

NATURE OF GEOTHERMAL RESOURCES
The Earth's thermal engine
The geothermal gradient expresses the increase in temperature with depth in the Earth's
crust. Down to the depths accessible by drilling with modern technology, i.e. over 10,000
m, the average geothermal gradient is about 2.5-3 °C/100 m. For example, if thetemperature within the first few metres below ground-level, which on averagecorresponds to the mean annual temperature of the external air, is 15 °C, then we canreasonably assume that the temperature will be about 65°-75 °C at 2000 m depth, 90°105 °C at 3000 m and so on for a further few thousand metres. There are, however, vast
areas in which the geothermal gradient is far from the average value. In areas in which the
deep rock basement has undergone rapid sinking, and the basin is filled with geologically‘very young’ sediments, the geothermal gradient may be lower than 1 ° C/100 m. On theother hand, in some ‘geothermal areas’ the gradient is more than ten times the averagevalue.
The difference in temperature between deep hotter zones and shallow colder zones
generates a conductive flow of heat from the former towards the latter, with a tendency tocreate uniform conditions, although, as often happens with natural phenomena, thissituation is never actually attained. The mean terrestrial heat flow of continents andoceans is 65 and 101 mW/m2, respectively, which, when areally weighted, yield a global
mean of 87 mW/m2 (Pollack et al., 1993). These values are based on 24,774
measurements at 20,201 sites covering about 62% of the Earth's surface. Empiricalestimators, referenced to geological map units, enabled heat flow to be estimated in areas
without measurements. The heat flow analysis by Pollack et al. (1993) is the most recent
in print form. The University of North Dakota is currently providing access via internet toan updated heat flow database comprising data on oceanic and continental areas.
The temperature increase with depth, as well as volcanoes, geysers, hot springs, etc., arein a sense the visible or tangible expression of the heat in the interior of the Earth, but thisheat also engenders other phenomena that are less discernible by man, but of suchmagnitude that the Earth has been compared to an immense ‘thermal engine’. We will try
to describe these phenomena, referred to collectively as the plate tectonics theory, in
simple terms, and their relationship with geothermal resources.
Our planet consists of a crust, which reaches a thickness of about 20-65 km in continental
areas and about 5-6 km in oceanic areas, a mantle, which is roughly 2900 km thick, and acore, about 3470 km in radius (Figure 1). The physical and chemical characteristics of thecrust, mantle and core vary from the surface of the Earth to its centre. The outermost shellof the Earth, known as the lithosphere, is made up of the crust and the upper layer of the
mantle. Ranging in thickness from less than 80 km in oceanic zones to over 200 km incontinental areas, the lithosphere behaves as a rigid body. Below the lithosphere is thezone known as the asthenosphere, 200-300 km in thickness, and of a ‘less rigid’ or ‘more
plastic’ behaviour. In other words, on a geological scale in which time is measured inmillions of years, this part of the Earth behaves in much the same way as a fluid in certain
processes.

8

Because of the difference in temperature between the different parts of the asthenosphere,
convective movements and, possibly, convective cells were formed some tens of millionsof years ago. Their extremely slow movement (a few centimetres per year) is maintainedby the heat produced continually by the decay of the radioactive elements and the heatcoming from the deepest parts of the Earth. Immense volumes of deep hotter rocks, lessdense and lighter than the surrounding material, rise with these movements towards thesurface, while the colder, denser and heavier rocks near the surface tend to sink, re-heatand rise to the surface once again, very similar to what happens to water boiling in a pot
or kettle.
In zones where the lithosphere is thinner, and especially in oceanic areas, the lithosphereis pushed upwards and broken by the very hot, partly molten material ascending from theasthenosphere, in correspondence to the ascending branch of convective cells. It is thismechanism that created and still creates the spreading ridges that extend for more than
60,000 km beneath the oceans, emerging in some places (Azores, Iceland) and evencreeping between continents, as in the Red Sea. A relatively tiny fraction of the molten
rocks upwelling from the asthenosphere emerges from the crests of these ridges and, in
contact with the seawater, solidifies to form a new oceanic crust. Most of the material
rising from the asthenosphere, however, divides into two branches that flow in opposite
directions beneath the lithosphere. The continual generation of new crust and the pull ofthese two branches in opposite directions has caused the ocean beds on either side of theridges to drift apart at a rate of a few centimetres per year. Consequently, the area of theocean beds (the oceanic lithosphere) tends to increase. The ridges are cut perpendicularly
by enormous fractures, in some cases a few thousand kilometres in length, called
transform faults.
These phenomena lead to a simple observation: since there is apparently no increase in
the Earth's surface with time, the formation of new lithosphere along the ridges and the
spreading of the ocean beds must be accompanied by a comparable shrinkage of thelithosphere in other parts of the globe. This is indeed what happens in subduction zones,
the largest of which are indicated by huge ocean trenches, such as those extending along
the western margin of the Pacific Ocean and the western coast of South America. In the
subduction zones the lithosphere folds downwards, plunges under the adjacent
lithosphere and re-descends to the very hot deep zones, where it is "digested" by the
mantle and the cycle begins all over again. Part of the lithospheric material returns to amolten state and may rise to the surface again through fractures in the crust. As aconsequence, magmatic arcs with numerous volcanoes are formed parallel to thetrenches, on the opposite side to that of the ridges. Where the trenches are located in theocean, as in the Western Pacific, these magmatic arcs consist of chains of volcanicislands; where the trenches run along the margins of continents the arcs consist of chainsof mountains with numerous volcanoes, such as the Andes. Figure 4 illustrates thephenomena we have just described.
Spreading ridges, transform faults and subduction zones form a vast network that divides
our planet into six immense and several other smaller lithospheric areas or plates (Figure5). Because of the huge tensions generated by the Earth's thermal engine and theasymmetry of the zones producing and consuming lithospheric material, these plates drift
slowly up against one another, shifting position continually.

9

Figure 4
Schematic cross-section showing plate tectonic processes

Figure 5
World pattern of plates, oceanic ridges, oceanic trenches, subduction zones, andgeothermal fields. Arrows show the direction of movement of the plates towards the
subduction zones. (1) Geothermal fields producing electricity; (2) mid-oceanic ridges
crossed by transform faults (long transversal fractures); (3) subduction zones, where
the subducting plate bends downwards and melts in the asthenosphere.
The margins of the plates correspond to weak, densely fractured zones of the crust,characterised by an intense seismicity, by a large number of volcanoes and, because of
the ascent of very hot materials towards the surface, by a high terrestrial heat flow. As
shown in Figure 5, the most important geothermal areas are located around plate margins.

10Geothermal systems
Geothermal systems can therefore be found in regions with a normal or slightly above
normal geothermal gradient, and especially in regions around plate margins where thegeothermal gradients may be significantly higher than the average value. In the first casethe systems will be characterised by low temperatures, usually no higher than 100 °C at
economic depths; in the second case the temperatures could cover a wide range from low
to very high, and even above 400 °C.
What is a geothermal system and what happens in such a system? It can be described
schematically as ‘convecting water in the upper crust of the Earth, which, in a confined
space, transfers heat from a heat source to a heat sink, usually the free surface’(Hochstein, 1990). A geothermal system is made up of three main elements: a heatsource, a reservoir and a fluid, which is the carrier that transfers the heat. The heat source
can be either a very high temperature (> 600 °C) magmatic intrusion that has reachedrelatively shallow depths (5-10 km) or, as in certain low-temperature systems, the Earth's
normal temperature, which, as we explained earlier, increases with depth. The reservoir is
a volume of hot permeable rocks from which the circulating fluids extract heat. The
reservoir is generally overlain by a cover of impermeable rocks and connected to asurficial recharge area through which the meteoric waters can replace or partly replace
the fluids that escape from the reservoir through springs or are extracted by boreholes.
The geothermal fluid is water, in the majority of cases meteoric water, in the liquid or
vapour phase, depending on its temperature and pressure. This water often carries with itchemicals and gases such as CO2, H2S, etc. Figure 6 is a greatly simplified representationof an ideal geothermal system.

Figure 6
Schematic representation of an ideal geothermal system.

11

The mechanism underlying geothermal systems is by and large governed by fluid
convection. Figure 7 describes schematically the mechanism in the case of anintermediate-temperature hydrothermal system. Convection occurs because of theheating and consequent thermal expansion of fluids in a gravity field; heat, which issupplied at the base of the circulation system, is the energy that drives the system. Heatedfluid of lower density tends to rise and to be replaced by colder fluid of high density,
coming from the margins of the system.

Figure 7
Model of a geothermal system. Curve 1 is the reference curve for the boiling point ofpure water. Curve 2 shows the temperature profile along a typical circulation routefrom recharge at point A to discharge at point E (From White, 1973).
Convection, by its nature, tends to increase temperatures in the upper part of a system as
temperatures in the lower part decrease (White, 1973).
The phenomenon we have just described may seem quite a simple one but thereconstruction of a good model of a real geothermal system is by no means easy toachieve. It requires skill in many disciplines and a vast experience, especially when
dealing with high -temperature systems. Geothermal systems also occur in nature in avariety of combinations of geological, physical and chemical characteristics, thus giving
rise to several different types of system.
Of all the elements of a geothermal system, the heat source is the only one that need benatural. Providing conditions are favourable, the other two elements could be ‘artificial’.
For example, the geothermal fluids extracted from the reservoir to drive the turbine in ageothermal power-plant could, after their utilization, be injected back into the reservoir

12through specific injection wells. In this way the natural recharge of the reservoir is
integrated by an artificial recharge. For many years now re-injection has also been
adopted in various parts of the world as a means of drastically reducing the impact on theenvironment of geothermal plant operations.
Artificial recharge through injection wells can also help to replenish and maintain ‘old’ or‘exhausted’ geothermal fields. For example, in The Geysers field in California, USA, oneof the biggest geothermal fields in the world, production began to decline dramatically atthe end of the 1980s because of a lack of fluids. The first project of this type, theSoutheast Geysers Effluent Recycling Project, was launched in 1997, to transport treatedwastewater for 48 km to the geothermal field. This project has led to the reactivation of anumber of power plants that had been abandoned because of a lack of fluids. In thesecond system, the Santa Rosa Geysers Recharge Project, 41.5 million litres per day oftertiary treated waste-water will be pumped from the Santa Rosa regional sewage
treatment plant and other cities through a 66-km pipeline to The Geysers field, where it
will recharge the reservoir through specially drilled boreholes.

Figure 8
Schematic of a commercial-scale Hot Dry Rock.
In the so-called Hot Dry Rock (HDR) projects, which were experimented for the firsttime at Los Alamos, New Mexico, USA, in 1970, both the fluid and the reservoir are
13

artificial. High-pressure water is pumped through a specially drilled well into a deep
body of hot, compact rock, causing its hydraulic fracturing. The water permeates these
artificial fractures, extracting heat from the surrounding rock, which acts as a natural
reservoir. This 'reservoir' is later penetrated by a second well, which is used to extract theheated water. The system therefore consists of (i) the borehole used for hydraulicfracturing, through which cold water is injected into (ii) the artificial reservoir, and (iii)the borehole used to extract the hot water. The entire system, complete with surface
utilization plant, could form a closed loop (Garnish, 1987) (see Figure 8).
The Los Alamos project was the forerunner for other similar projects in Australia, France,
Germany, Japan and the UK. After a period of relative neglect, these projects have been
given renewed impulse from the discovery, firstly, that deep rocks have a certain degreeof natural fracturation, and further, that the methodologies and technologies adopted will
depend on local geologic conditions. The most advanced research in the HDR sector wasthat being conducted in Japan and under the European project at Alsace (France). Several
projects launched in Japan in the 1980s (at Hijiori, Ogachi and Yunomori), and heavily
financed by the Japanese government and industry, have produced interesting results
both from the scientific and industrial standpoints. The European HDR project, on theother hand, has been implemented over a number of phases, including the drilling of two
wells, one of which has reached bottom-hole at 5060 m. Very promising results havebeen obtained from their geophysical surveys and hydraulic tests, and the European
Project seems, for the moment, to be the most successful (Tenzer, 2001).

14

DEFINITION AND CLASSIFICATION OF GEOTHERMALRESOURCES
There is no standard international terminology in use throughout the geothermal
community, which is unfortunate, as this would facilitate mutual comprehension. The
following are some of the most common definitions and classifications in this iscipline.
According to Muffler and Cataldi (1978), when we speak generically about geothermal
resources, what we are usually referring to is what should more accurately be called theaccessible resource base; that is, all of the thermal energy stored between the Earth's
surface and a specified depth in the crust, beneath a specified area and measured fromlocal mean annual temperature. The accessible resource base includes the usefulaccessible resource base (= Resource ) — that part of the accessible resource base that
could be extracted economically and legally at some specified time in the future (lessthan a hundred years). This category includes the identified economic resource(=Reserve ) — that part of the resources of a given area that can be extracted legally at a
cost competitive with other commercial energy sources and that are known andcharacterised by drilling or by geochemical, geophysical and geological evidence.
Figure 9 illustrates in graphic form these and other terms that may be used by geothermal
specialists.
The most common criterion for classifying geothermal resources is, however, that basedon the enthalpy of the geothermal fluids that act as the carrier transporting heat from thedeep hot rocks to the surface. Enthalpy, which can be considered more or lessproportional to temperature, is used to express the heat (thermal energy) content of thefluids, and gives a rough idea of their 'value'. The resources are divided into low, medium
and high enthalpy (or temperature) resources, according to criteria that are generally
based on the energy content of the fluids and their potential forms of utilization. Table 3
reports the classifications proposed by a number of authors. A standard method ofclassification, as with terminology, would avoid confusion and ambiguity but, until sucha method exists, we must indicate the temperature values or ranges involved case by case,since terms such as low, intermediate and high are meaningless at best, and frequently
misleading.
Frequently a distinction is made between water- or liquid-dominated geothermal systems
and vapour-dominated (or dry steam) geothermal systems (White, 1973). In
water-dominated systems liquid water is the continuous, pressure-controlling fluid phase.
Some vapour may be present, generally as discrete bubbles. These geothermal systems,
whose temperatures may range from < 125 to > 225 °C, are the most widely distributed in
the world. Depending on temperature and pressure conditions, they can produce hotwater, water and steam mixtures, wet steam and, in some cases, dry steam. In
vapour-dominated systems liquid water and vapour normally co-exist in the reservoir,
with vapour as the continuous, pressure-controlling phase. Geothermal systems of thistype, the best-known of which are Larderello in Italy and The Geysers in California, aresomewhat rare, and are high-temperature systems. They normally produce dry-to-superheated steam.

15

Figure 9
Diagram showing the different categories of geothermal resources. (From Muffler andCataldi, 1978). The vertical axis is the degree of economic feasibility; the horizontal
axis is the degree of geological assurance.

Lowenthalpy
resources
Intermediate
enthalpy
resourcesHigh
enthalpy
resources

Table 3. Classification of geothermal resources (°C)
(a)
(b)
(c)
(d)

(e)

< 90

<125

<100

150

190

90-150125-225

100-200

-

-

>150

>225

>200

>150

>190

Source: (a) Muffler and Cataldi (1978).
(b) Hochstein (1990).
(c) Benderitter and Cormy (1990).
(d) Nicholson (1993).
(e) Axelsson and Gunnlaugsson (2000)
16

The terms wet, dry and superheated steam, which are used frequently by geothermists,need some explanation for readers who are not of an engineering background. To make it
as simple as possible, let us take the example of a pot filled with liquid water in whichpressure can be kept constant at 1 atm (101.3 kPa). If we then heat the water, it will begin
boiling once it reaches a temperature of 100 °C (boiling temperature at a pressure of 1atm) and will pass from the liquid to the gas (vapour) phase. After a certain time the pot
will contain both liquid and vapour. The vapour coexisting with the liquid, and inthermodynamic equilibrium with it, is wet steam. If we continue to heat the pot andmaintain the pressure at 1 atm, the liquid will evaporate entirely and the pot will containsteam only. This is what we call dry steam. Both wet and dry steam are called “saturated
steam”. Finally, increasing the temperature to, say, 120 °C, and keeping the pressure at 1
atm, we will obtain superheated steam with a superheating of 20 °C, i.e. 20 °C above thevaporisation temperature at that pressure. At other temperatures and pressures, of course,
these phenomena also take place in the underground, in what one author many years ago
called “nature's tea-kettle”.
Another division between geothermal systems is that based on the reservoir equilibrium
state (Nicholson, 1993), considering the circulation of the reservoir fluid and themechanism of heat transfer. In the dynamic systems the reservoir is continually recharged
by water that is heated and then discharged from the reservoir either to the surface or into
underground permeable formations. Heat is transferred through the system by convection
and circulation of the fluid. This category includes high-temperature (>150 °C) andlow-temperature (<150 °C) systems. In the static systems (also known as stagnant or
storage systems) there is only minor or no recharge to the reservoir and heat is transferred
only by conduction. This category includes low temperature and geopressured systems.
The geopressured systems are characteristically found in large sedimentary basins (e.g.
Gulf of Mexico, USA) at depths of 3 - 7 km. The geopressured reservoirs consist of
permeable sedimentary rocks, included within impermeable low-conductivity strata,
containing pressurized hot water that remained trapped at the moment of deposition of
the sediments. The hot water pressure approaches lithostatic pressure, greatly exceeding
the hydrostatic pressure. The geopressured reservoirs can also contain significant
amounts of methane. The geopressured systems could produce thermal and hydraulicenergy (pressurized hot water) and methane gas. These resources have been investigatedextensively, but so far there has been no industrial exploitation.
Geothermal field is a geographical definition, usually indicating an area of geothermal
activity at the earth’s surface. In cases without surface activity this term may be used toindicate the area at the surface corresponding to the geothermal reservoir below(Axelsson and Gunnlaugsson, 2000).
As geothermal energy is usually described as renewable and sustainable, it is important
to define these terms. Renewable describes a property of the energy source, whereas
sustainable describes how the resource is utilised.
The most critical factor for the classification of geothermal energy as a renewable energy
source is the rate of energy recharge. In the exploitation of natural geothermal systems,
energy recharge takes place by advection of thermal water on the same time scale as17

production from the resource. This justifies our classification of geothermal energy as arenewable energy resource. In the case of hot, dry rocks, and some of the hot wateraquifers in sedimentary basins, energy recharge is only by thermal conduction; due to theslow rate of the latter process, however, hot dry rocks and some sedimentary reservoirs
should be considered as finite energy resources (Stefansson, 2000).
The sustainability in consumption of a resource is dependent on its original quantity, its
rate of generation and its rate of consumption. Consumption can obviously be sustained
over any period of time in which a resource is being created faster than it is being
depleted. The term sustainable development is used by the World Commission on
Environment and Development to indicate development that ‘….meets the needs of thepresent generation without compromising the needs of future generations’. In thiscontext, sustainable development does not imply that any given energy resource needs to
be used in a totally sustainable fashion, but merely that a replacement for the resource canbe found that will allow future generations to provide for themselves despite the fact thatthe particular resource has been depleted. Thus, it may not be necessary that a specificgeothermal field be exploited in sustainable fashion. Perhaps we should direct ourgeothermal sustainability studies towards reaching and then sustaining a certain overalllevel of geothermal production at a national or regional level, both for electric power
generation and direct heat applications, for a certain period, say 300 years, by bringing
new geothermal systems on line as others are depleted (Wright, 1998).

18

EXPLORATION
Objectives of exploration
The objectives of geothermal exploration are (Lumb, 1981):
1.
2.
3.
4.
5.
6.
7.
8.
9.

To identify geothermal phenomena.
To ascertain that a useful geothermal production field exists.
To estimate the size of the resource.
To determine the type of geothermal field.
To locate productive zones.
To determine the heat content of the fluids that will be discharged by the wells in thegeothermal field.
To compile a body of basic data against which the results of future monitoring can beviewed.
To determine the pre-exploitation values of environmentally sensitive parameters.
To acquire knowledge of any characteristics that might cause problems during fielddevelopment.

The relative importance of each objective depends on a number of factors, most of which
are tied to the resource itself. These include anticipated utilization, technology available,
economics, as well as situation, location and time, all of which affect the exploration
programme. For example, the preliminary reconnaissance of geothermal manifestations
assumes much greater importance in a remote, unexplored area than in a well-known
area; estimating the size of the resource may be less important if it is to be used in asmall-scale application that obviously requires much less heat than is already discharging
naturally; if the energy is to be used for district-heating or some other application needing
low-grade heat, then a high-temperature fluid is no longer an important objective (Lumb,1981).
A large number of methods and technologies are available in order to reach theseobjectives. Many of these methods are in current use and have already been widely
experimented in other sectors of research. The techniques and methodologies that have
proved successful in mineral and oil or gas exploration will however not necessarily be
the best solution in geothermal exploration. Conversely, techniques of little use in oil
exploration could turn out to be ideal tools in the search for natural heat (Combs andMuffler, 1973).
Exploration methods
Geological and hydrogeological studies are the starting point of any exploration
Programme, and their basic function is that of identifying the location and extension of
the areas worth investigating in greater detail and of recommending the most suitable
exploration methods for these areas. Geological and hydrogeological studies have an
important role in all subsequent phases of geothermal research, right up to the siting of
exploratory and producing boreholes. They also provide the background information forinterpreting the data obtained with the other exploration methods and, finally, forconstructing a realistic model of the geothermal system and assessing the potential of19there source. The information obtained from the geological and hydrogeological studies
may also be used in the production phase, providing valuable information for the
reservoir and production engineers. The duration and cost of exploration can beappreciably reduced by a good exploration programme and an efficient coordination of
the research.
Geochemical surveys (including isotope geochemistry) are a useful means ofdetermining whether the geothermal system is water- or vapour-dominated, of estimating
the minimum temperature expected at depth, of estimating the homogeneity of the watersupply, of inferring the chemical characteristics of the deep fluid, and of determining thesource of recharge water. Valuable information can also be obtained on the type ofproblems that are likely to arise during the re-injection phase and plant utilization (e.g.
changes in fluid composition, corrosion and scaling on pipes and plant installations,environmental impact) and how to avoid or combat them. The geochemical survey
consists of sampling and chemical and/or isotope analyses of the water and gas fromgeothermal manifestations (hot springs, fumaroles, etc.) or wells in the study area. As thegeochemical survey provides useful data for planning exploration and its cost is
relatively low compared to other more sophisticated methods, such as the geophysicalsurveys, the geochemical techniques should be utilised as much as possible before
proceeding with other more expensive methodologies.
Geophysical surveys are directed at obtaining indirectly, from the surface or from depth
intervals close to the surface, the physical parameters of deep geological formations.
These physical parameters include:
1. temperature (thermal survey)
2. electrical conductivity (electrical and electromagnetic methods)
3. propagation velocity of elastic waves (seismic survey)
4. density (gravity survey)
5. magnetic susceptibility (magnetic survey).
Some of these techniques, such as seismics, gravity and magnetics, which are
traditionally adopted in oil research, can give valuable information on the shape, size,
depth and other important characteristics of the deep geological structures that could
constitute a geothermal reservoir, but they give little or no indication as to whether thesestructures actually contain the fluids that are the primary objective of research. Thesemethodologies are, therefore, more suited to defining details during the final stages of
exploration, before the exploratory wells are sited. Information on the existence ofgeothermal fluids in the geological structures can be obtained with the electrical andelectromagnetic prospectings, which are more sensitive than the other surveys to thepresence of these fluids and to variations in temperature; these two techniques have been
applied widely with satisfactory results. The magnetotelluric method, which exploits theelectromagnetic waves generated by solar storms, has been greatly improved over the last
few years, and now offers a vast spectrum of possible applications, despite the fact that it
requires sophisticated instrumentation and is sensitive to background noise in urbanised
areas. The main advantage of the magnetotelluric method is that it can be used to define
deeper structures than are attainable with the electric and the other electromagnetic
techniques. The Controlled Source Audiomagnetotelluric method (CSAMT), developed

20

recently, uses artificially induced waves instead of natural electro-magnetic waves. The
penetration depth is shallower with this technique, but it is quicker, cheaper, and provides
far more detail than the classic MT method.
Thermal techniques (temperature measurements, determination of geothermal gradientand terrestrial heat flow) can often provide a good approximation of the temperature at
the top of the reservoir.
All geophysical techniques are expensive, although some more than others. Nor can theybe used indiscriminately in any situation or condition, as a method that produces
excellent results in a determinate geological environment may give very unsatisfactory
results in another. In order to reduce costs, it is therefore very important that thegeophysical method(s) be selected very carefully beforehand by geophysicists working inclose collaboration with geologists (Meidav, 1998).
Drilling of exploratory wells represents the final phase of any geothermal exploration
programme and is the only means of determining the real characteristics of thegeothermal reservoir and thus of assessing its potential (Combs and Muffler, 1973). The
data provided by exploratory wells should be capable of verifying all the hypotheses andmodels elaborated from the results of surface exploration and of confirming that thereservoir is productive and that it contains enough fluids of adequate characteristics forthe utilization for which it is intended. Siting of the exploratory wells is therefore a very
delicate operation.
Exploration programme
Before drawing up a geothermal exploration programme all existing geological,
geophysical and geochemical data must be collected and integrated with any data
available from previous studies on water, minerals and oil resources in the study area andadjacent areas. This information frequently plays an important role in defining theobjectives of the geothermal exploration programme and could lead to a significant
reduction in costs.
The exploration programme is usually developed on a step-by-step basis: reconnaissance,
pre-feasibility and feasibility. During each of these phases we gradually eliminate the lessinteresting areas and concentrate on the most promising ones. The methods used also
become progressively more sophisticated and more detailed as the programme develops.
The size and budget of the entire programme should be proportional to its objectives, tothe importance of the resources we expect to find, and to the planned forms of utilization.
The programme schedule should be flexible and reassessed as the results come in fromthe various surveys of each phase; similarly the geological-geothermal model should be
progressively updated and improved. These periodic re-assessments of the programmeshould ideally eliminate any operations that are no longer necessary and insert others,
according to the results attained at each stage. Clearly any reduction in the number andsize of the prospectings will lead to a decrease in costs, and also a corresponding increasein the risk of error or failure. Conversely, by decreasing the risk of error we increase theoverall cost. The economic success of a geothermal exploration programme hinges onfinding the proper balance between the two.

21

UTILIZATION OF GEOTHERMAL RESOURCES
Electricity generation is the most important form of utilization of high-temperaturegeothermal resources (> 150 °C). The medium-to-low temperature resources (< 150 °C)
are suited to many different types of application. The classical Lindal diagram (Lindal,
1973), which shows the possible uses of geothermal fluids at different temperatures, stilholds valid (Figure 10, derived from the original Lindal diagram, with the addition ofelectricity generation from binary cycles. Fluids at temperatures below 20 °C are rarely
used and in very particular conditions, or in heat pump applications. The Lindal diagramemphasises two important aspects of the utilization of geothermal resources(Gudmundsson, 1988): (a) with cascading and combined uses it is possible to enhance thefeasibility of geothermal projects and (b) the resource temperature may limit the possible
uses. Existing designs for thermal processes can, however, be modified for geothermal
fluid utilization in certain cases, thus widening its field of application.

Figure 10
Diagram showing the utilization of geothermal fluids (derived from Lindal, 1973)
Electricity generation
Electricity generation mainly takes place in conventional steam turbines and binaryplants, depending on the characteristics of the geothermal resource.
Conventional steam turbines require fluids at temperatures of at least 150 °C and areavailable with either atmospheric (back-pressure) or condensing exhausts. Atmosphericexhaust turbines are simpler and cheaper. The steam, direct from dry steam wells or, after
22

separation, from wet wells, is passed through a turbine and exhausted to the atmosphere(Figure 11).
With this type of unit, steam consumption (from the same inlet pressure) per
kilowatt-hour produced is almost double that of a condensing unit. However, theatmospheric exhaust turbines are extremely useful as pilot plants, stand-by plants, in thecase of small supplies from isolated wells, and for generating electricity from test wells
during field development. They are also used when the steam has a high noncondensable
gas content (>12% in weight). The atmospheric exhaust units can be constructed andinstalled very quickly and put into operation in little more than 13-14 months from theirorder date. This type of machine is usually available in small sizes (2.5 - 5 MWe).
The condensing units, having more auxiliary equipment, are more complex than theatmospheric exhaust units and the bigger sizes can take twice as long to construct andinstall. The specific steam consumption of the condensing units is, however, about halfthat of the atmospheric exhaust units. Condensing plants of 55 - 60 MWe capacity arevery common, but recently plants of 110 MWe have also been constructed and installed(Figure 12).
Generating electricity from low-to-medium temperature geothermal fluids and from thewaste hot waters coming from the separators in water - dominated geothermal fields has
made considerable progress since improvements were made in binary fluid technology.
The binary plants utilize a secondary working fluid, usually an organic fluid(typicallyn-pentane), that has a low boiling point and high vapour pressure at low temperatures
when compared to steam. The secondary fluid is operated through a conventional
Rankine cycle (ORC): the geothermal fluid yields heat to the secondary fluid throughheat exchangers, in which this fluid is heated and vaporises; the vapour produced drives anormal axial flow turbine, is then cooled and condensed, and the cycle begins again(Figure 13).

Figure 11
Sketch of an atmospheric exhaust geothermal power-plant. The flow of geothermal fluid
is indicated in red.

23

Figure 12
Sketch of a condensing geothermal power-plant. The flow of high-temperature fluid isindicated in red, and the cooling water in blue.

Figure 13
Sketch of a geothermal binary power plant. The flow of geothermal fluid is in red, thesecondary fluid in green, and the cooling water in blue.
By selecting suitable secondary fluids, binary systems can be designed to utilize
geothermal fluids in the temperature range 85-170 °C. The upper limit depends on thethermal stability of the organic binary fluid, and the lower limit on technical-economic
factors: below this temperature the size of the heat exchangers required would render the
project uneconomical. Apart from low-to-medium temperature geothermal fluids andwaste fluids, binary systems can also be utilised where flashing of the geothermal fluids
should preferably be avoided (for example, to prevent well sealing). In this case,downhole pumps can be used to keep the fluids in a pressurised liquid state, and theenergy can be extracted from the circulating fluid by means of binary units.
24

Binary plants are usually constructed in small modular units of a few hundred kWe to afew MWe capacity. These units can then be linked up to create power-plants of a few tensof megawatts. Their cost depends on a number of factors, but particularly on thetemperature of the geothermal fluid produced, which influences the size of the turbine,heat exchangers and cooling system. The total size of the plant has little effect on thespecific cost, as a series of standard modular units is joined together to obtain larger
capacities.
Binary plant technology is a very cost-effective and reliable means of converting into
electricity the energy available from water-dominated geothermal fields (below 170 °C).
A new binary system, the Kalina cycle, which utilizes a water-ammonia mixture asworking fluid, was developed in the 1990s. The working fluid is expanded, insuperheated conditions, through the high-pressure turbine and then re-heated before
entering the low-pressure turbine. After the second expansion the saturated vapour moves
through a recuperative boiler before being condensed in a water-cooled condenser. The
Kalina cycle is more efficient than existing geothermal ORC binary power plants, but isof more complex design.
Small mobile plants, conventional or not, can not only reduce the risk inherent to drilling
new wells but, what is more important, they can help in meeting the energy requirementsof isolated areas. The standard of living of many communities could be considerably
improved were they able to draw on local sources of energy. Electricity could facilitate
many apparently banal, but extremely important operations, such as pumping water for
irrigation, freezing fruit and vegetables for longer conservation.
The convenience of the small mobile plants is most evident for areas without readyaccess to conventional fuels, and for communities that would be too expensive to connect
to the national electric grid, despite the presence of high voltage transmission lines in thevicinity. The expense involved in serving these small by-passed communities is
prohibitive, since the step-down transformers needed to tap electricity from high voltage
lines cost more than US$ 675,000 each, installed, and the simplest form of local
distribution of electricity, at 11 kV using wooden poles, costs a minimum of US$ 20,000
per kilometre (US$ 1994). By comparison, the capital cost (US$ 1998) of a binary unit isof the order of 1500-2500 US$/kW installed, excluding drilling costs. The demand forelectric capacity per person at off-grid sites will range from 0.2 kW in less-developed
areas to 1.0 kW or higher in developed areas. A 100 kWe plant could serve 100 to 500
people. A 1000 kWe plant would serve 1000 to 5000 people (Entingh et al., 1994).
Direct heat uses
Direct heat use is one of the oldest, most versatile and also the most common form ofutilization of geothermal energy (Table 2). Bathing, space and district heating,agricultural applications, aquaculture and some industrial uses are the best known forms
of utilization, but heat pumps are the most widespread (12.5% of the total energy use in2000). There are many other types of utilization, on a much smaller scale, some of which
are unusual.

25

Space and district heating have made great progress in Iceland, where the total capacityof the operating geothermal district heating system had risen to about 1200 MWt by theend of 1999 (Figure 14), but they are also widely distributed in the East European
countries, as well as in the United States, China, Japan, France, etc.

Figure 14
Simplified flow diagram of the geothermal district heating system of Reykjavik. (From
Gudmundsson, 1988)
Geothermal district heating systems are capital intensive. The main costs are initialinvestment costs, for production and injection wells, down-hole and transmission pumps,
pipelines and distribution networks, monitoring and control equipment, peaking stations
and storage tanks. Operating expenses, however, are comparatively lower than inconventional systems, and consist of pumping power, system maintenance, control andmanagement. A crucial factor in estimating the initial cost of the system is the thermalload density, or the heat demand divided by the ground area of the district. A high heatdensity determines the economic feasibility of a district heating project, since the
distribution network is expensive. Some economic benefit can be achieved by combining
heating and cooling in areas where the climate permits. The load factor in a system withcombined heating and cooling would be higher than the factor for heating alone, and theunit energy price would consequently improve (Gudmundsson, 1988).
Space cooling is a feasible option where absorption machines can be adapted togeothermal use. The technology of these machines is well known, and they are readily
available on the market. The absorption cycle is a process that utilises heat instead ofelectricity as the energy source. The refrigeration effect is obtained by utilising two
fluids: a refrigerant, which circulates, evaporates and condenses, and a secondary fluid or26

absorbent. For applications above 0 °C (primarily in space and process conditioning), thecycle uses lithium bromide as the absorbent and water as the refrigerant. For applications
below 0 °C an ammonia/water cycle is adopted, with ammonia as the refrigerant andwater as the absorbent. Geothermal fluids provide the thermal energy to drive thesemachines, although their efficiency decreases with temperatures lower than 105 °C.

Figure 15
Typical application of ground-coupled heat pump system (from Sanner et al., 2003).
Geothermal space conditioning (heating and cooling) has expanded considerably sincethe 1980s, following on the introduction and widespread use of heat pumps. The various
systems of heat pumps available permit us to economically extract and utilize the heat
content of low-temperature bodies, such as the ground and shallow aquifers, ponds, etc.
(Sanner, 2001) (see, for example, Figure 15).
As engineers already know, heat pumps are machines that move heat in a direction
opposite to that in which it would tend to go naturally, i.e. from a cold space or body to awarmer one. A heat pump is effectively nothing more than a refrigeration unit (Rafferty,1997). Any refrigeration device (window air conditioner, refrigerator, freezer, etc.)
moves heat from a space (to keep it cool) and discharges that heat at higher temperatures.
The only difference between a heat pump and a refrigeration unit is the desired effect,cooling for the refrigeration unit and heating for the heat pump. A second distinguishing
factor of many heat pumps is that they are reversible and can provide either heating or27

cooling in the space. The heat pumps, of course, need energy to operate, but in suitableclimatic conditions and with a good design, the energy balance
will be a positive one (Figure 16).
Ground-coupled and ground-water heat pump systems have now been installed in great
numbers in at least 30 countries, for a total thermal capacity of more than 9500 MWt (in2003). The majority of these installations are in the USA (500,000 installations for a totalof 3730 MWt), Sweden (200,000 installations totalling 2000 MWt), Germany (40,000
installations totalling 560 MWt), Canada (36,000 installations totalling 435 MWt),
Switzerland (25,000 installations totalling 440 MWt), and Austria (23,000 installations
totalling 275 MWt) (Lund et al., 2003). Aquifers and soils with temperatures in the 5 to
30 °C range are being used in these systems.

Figure 16
Sketch of a heat pump in heating mode (courtesy of the Geo-Heat Center, Klamath
Falls, Oregon, USA)The agricultural applications of geothermal fluids consist of open-field agriculture andgreenhouse heating. Thermal water can be used in open-field agriculture to irrigate
and/or heat the soil. The greatest drawback in irrigating with warm waters is that, toobtain any worthwhile variation in soil temperature, such large quantities of water are
required at temperatures low enough to prevent damage to the plants that the fields would
be flooded. One possible solution to this problem is to adopt a subsurface irrigation
system coupled to a buried-pipeline soil-heating device. Heating the soil in buried
pipelines without the irrigation system could decrease the heat conductivity of the soil,
because of the drop in humidity around the pipes, and consequent thermal insulation. The
best solution seems to be that of combining soil heating and irrigation. The chemical
28

composition of the geothermal waters used in irrigation must be monitored carefully to
avoid adverse effects on the plants. The main advantages of temperature control in
open-field agriculture are: (a) it prevents any damage ensuing from low environmental
temperatures, (b) it extends the growing season, increases plant growth, and boosts
production, and (c) it sterilises the soil (Barbier and Fanelli, 1977).
The most common application of geothermal energy in agriculture is, however, ingreenhouse heating, which has been developed on a large scale in many countries. The
cultivation of vegetables and flowers out-of-season, or in an unnatural climate, can now
draw on a widely experimented technology. Various solutions are available for achieving
optimum growth conditions, based on the optimum growth temperature of each plant(Figure 17), and on the quantity of light, on the CO2 concentration in the greenhouseenvironment, on the humidity of the soil and air, and on air movement.

Figure 17
Growth curves for some crops. (From Beall and Samuels, 1971)
The walls of the greenhouse can be made of glass, fibreglass, rigid plastic panels or
plastic film. Glass panels are more transparent than plastic and will let in far more light,
but will provide less thermal insulation, are less resistant to shocks, and are heavier andmore expensive than the plastic panels. The simplest greenhouses are made of single
plastic films, but recently some greenhouses have been constructed with a double layer offilm separated by an air space. This system reduces the heat loss through the walls by 30 40%, and thus greatly enhances the overall efficiency of the greenhouse. Greenhouseheating can be accomplished by forced circulation of air in heat exchangers, hot-water
circulating pipes or ducts located in or on the floor, finned units located along the walls
and under benches, or a combination of these methods (Figure 18).

29

Figure 18
Heating systems in geothermal greenhouses. Heating installations with natural air
movement ((natural convection): (a) aerial pipe heating; (b) bench heating; (c) low
position heating pipes for aerial heating. (d) Soil heating. Heating installations withforced air movement (forced convection): (e) lateral position;(f) aerial fan; (g) highposition ducts; (h) low-position ducts. (From von Zabeltitz, 1986.)
Exploitation of geothermal heat in greenhouse heating can considerably reduce theiroperating costs, which in some cases account for 35% of the product costs (vegetables,
flowers, house-plants and tree seedlings).
Farm animals and aquatic species, as well as vegetables and plants, can benefit in quality
and quantity from optimum conditioning of their environmental temperature (Figure 19).
In many cases geothermal waters could be used profitably in a combination of animalhusbandry and geothermal greenhouses. The energy required to heat a breeding
installation is about 50% of that required for a greenhouse of the same surface area, so a
cascade utilization could be adopted. Breeding in a temperature-controlled environment
improves animal health, and the hot fluids can also be utilised to clean, sanitise and dry
the animal shelters and waste products (Barbier and Fanelli, 1977).

30

Figure 19
Effect of temperature on growth or production of food animals. (From Beall andSamuels, 1971).
Aquaculture , which is the controlled breeding of aquatic forms of life, is gaining
world-wide importance nowadays, due to an increasing market demand. Control of thebreeding temperatures for aquatic species is of much greater importance than for land
species, as can be seen in Figure 19, which shows that the growth curve trend of aquatic
species is very different from that of land species. By maintaining an optimum
temperature artificially we can breed more exotic species, improve production and even,in some cases, double the reproductive cycle (Barbier and Fanelli, 1977). The species thatare typically raised are carp, catfish, bass, tilapia, mullet, eels, salmon, sturgeon, shrimp,
lobster, crayfish, crabs, oysters, clams, scallops, mussels and abalone.
Aquaculture also includes alligator and crocodile breeding, as tourist attractions and fortheir skins, which could prove a lucrative activity. Past experience in the United States
has shown that, by maintaining its growth temperature at about 30 °C, an alligator can begrown to a length of about 2 m in 3 years, whereas alligators bred under natural
conditions will reach a length of only 1.2 m over the same period. These reptiles havebeen bred on farms in Colorado and Idaho for some years now, and the Icelanders areplanning something similar.
The temperatures required for aquatic species are generally in the 20 - 30 °C range. The
size of the installation will depend on the temperature of the geothermal source, thetemperature required in the fish ponds and the heat losses from the latter.
The cultivation of Spirulina can also be considered a form of aquaculture. This
single-celled, spiral-shaped, blue-green micro-algae is frequently called ‘super-food’because of its nutrient density; it has also been proposed to solve the problem of faminein the poorest countries of the world, although at the moment it is being marketed as a31

nutritional food supplement. Spirulina is now being farmed in a number of tropical andsub-tropical countries, in lakes or artificial basins, where conditions are ideal for its fast
and widespread growth (a hot, alkaline environment rich in CO2). Geothermal energy has
already been used successfully to provide the heat needed to grow Spirulina throughout
the year in temperate countries.
The entire temperature range of geothermal fluids, whether steam or water, can be
exploited in industrial applications, as shown in the Lindal diagram (Figure 10). The
different possible forms of utilization include process heating, evaporation, drying,
distillation, sterilisation, washing, de-icing, and salt extraction. Industrial process heat
has applications in 19 countries (Lund and Freeston, 2001), where the installations tend
to be large and energy consumption high. Examples also include concrete curing, bottlingof water and carbonated drinks, paper and vehicle parts production, oil recovery, milk
pasteurisation, leather industry, chemical extraction, CO2 extraction, laundry use,
diatomaceous earth drying, pulp and paper processing, and borate and boric acidproduction. There are also plans to utilise low-temperature geothermal fluids to deicerunaways and disperse fog in some airports. A cottage industry has developed in Japanthat utilises the bleaching properties of the H2S in geothermal waters to produce
innovative and much admired textiles for ladies' clothing. In Japan they have also
experimented a technique for manufacturing a lightweight 'geothermal wood' that isparticularly suited to certain types of constructions. During treatment in the hot springwater the polysaccharides in the original wood hydrolise, rendering the material moreporous and thus lighter.
Economic considerations
The elements that have to be considered in any cost estimate, whether assigned to plant oroperating costs, and the price of the 'products' of geothermal energy, are all more
numerous and more complicated than in other forms of energy. All these elements mustnevertheless be carefully evaluated before launching a geothermal project. We can onlyoffer a few indications of a more general character, which, together with information on
local conditions and on the value of the geothermal fluids available, should help the
potential investor to reach a decision.
A resource-plant system (geothermal power facility) consists of the geothermal wells, thepipelines carrying the geothermal fluids, the utilization plant and, frequently, a
re-injection system as well. The interaction of all these elements bears heavily on
investment costs, and must therefore be subjected to careful analysis. To give an example,
in the generation of electricity, a discharge-to-the-atmosphere plant is the simplest
solution, and is therefore cheaper than a condensing plant of the same capacity. It will,
however, require almost twice as much steam as the condensing plant to operate, and,
consequently, twice as many wells to feed it. Since wells are very expensive, acondensing power plant is effectively a cheaper option than the discharge-to-atmosphere
plant. The latter is, in fact, usually chosen for reasons other than economy.
Geothermal fluids can be transported over fairly long distances in thermally insulated
pipelines. In ideal conditions, the pipelines can be as long as 60 km. However, thepipelines, the ancillary equipment needed (pumps, valves, etc.), and their maintenance,32

are all quite expensive, and could weigh heavily on the capital cost and operating costs ofa geothermal plant. The distance between the resource and the utilization site should
therefore be kept as small as possible.

The capital cost of a geothermal plant is usually higher, and sometimes much higher, thanthat of a similar plant run on a conventional fuel. Conversely, the energy driving a
geothermal plant costs far less than conventional fuel, and corresponds to the cost of
maintaining the geothermal elements of the plant (pipelines, valves, pumps, heatexchangers, etc.). The higher capital outlay should be recovered by the savings in energy
costs. The resource-plant system should therefore be designed to last long enough toamortise the initial investment and, wherever possible, even longer.
Appreciable savings can be achieved by adopting integrated systems that offer a higher
utilization factor (for example, combining space-heating and cooling) or cascade systems,
where the plants are connected in series, each utilising the waste water from thepreceding plant (for example, electricity generation + greenhouse heating + animalhusbandry) (Figure 20).

Figure 20
Cascade uses of geothermal energy (courtesy of the Geo-Heat Center, Klamath Falls,
Oregon, USA)
In order to reduce maintenance costs and shut-downs, the technical complexity of theplant should be on a level that is accessible to local technical personnel or to experts who
are readily available. Highly specialised technicians or the manufacturers should ideallybe needed only for large-scale maintenance operations
or major breakdowns.
33

Finally, if the geothermal plant is to produce consumer products, a careful market survey
must be carried out beforehand to guarantee an outlet for these products. The necessary
infrastructures for the economic transport of the end-product from the production site to
the consumer should already exist, or be included in the initial project.
Table 4. Energy and investment costs for electric energy production from renewable(from Fridleifsson, 2001)
Current energy
Potential future
Turnkeycostenergy costinvestment cost
US cent/kWh
US cent/kWh
US$/kW
Biomass
5 - 154 - 10900 - 3000
Geothermal
2 - 101-8
800 - 3000
Wind
5 - 13
3 - 101100 - 1700
Solar (photovoltaic)
25 - 125
5 - 255000 - 10 000
Solar (thermal
12 - 18
4 - 103000 - 4000
electricity)Tidal
8 – 158 – 151700 - 2500

The foregoing observations can be applied to any form of utilization of geothermalenergy and any local conditions and are therefore of a purely qualitative nature. For aquantitative idea of investments and costs we recommend the World Energy ssessment
Report, prepared by UNDP, UN-DESA and the World Energy Council, and published in
2000. The WEA data are given in Tables 4 and 5, which also compare geothermal energy
with other renewable forms of energy (Fridleifsson, 2001).
Table 5. Energy and investment costs for direct heat from renewables (from Fridleifsson,
2001)
Current
Potential future
Turnkeyenergy costenergy costinvestment cost
US cent/kWh
US cent/kWh
US$/kW
Biomass (including
1-5
1-5
250 - 750
ethanol)
Geothermal
0.5 - 5
0.5 - 5200 - 2000
Wind
5 - 13
3 - 101100 - 1700
Solar heat low
3 - 202 - 10500 - 1700
temperature

34

ENVIRONMENTAL IMPACT
During the 1960s, when our environment was healthier than it is nowadays and we wereless aware of any threat to the earth, geothermal energy was still considered a 'clean
energy'. There is actually no way of producing or transforming energy into a form thatcan be utilised by man without making some direct or indirect impact on the environment.
Even the oldest and simplest form of producing thermal energy, i.e. burning wood, has adetrimental effect, and deforestation, one of the major problems in recent years, firstbegan when our ancestors cut down trees to cook their food and heat their houses.
Exploitation of geothermal energy also has an impact on the environment but there is no
doubt that it is one of the least polluting forms of energy.
Sources of pollution
In most cases the degree to which geothermal exploitation affects the environment is
proportional to the scale of its exploitation (Lunis and Breckenridge,1991). Table 6
summarises the probability and relative severity of the effects on the environment ofdeveloping geothermal direct-use projects. Electricity generation in binary cycle plants
will affect the environment in the same way as direct heat uses. The effects are potentiallygreater in the case of conventional back-pressure or condensing powerplants, especiallyas regards air quality, but can be kept within acceptable limits.
Table 6. Probability and severity of potential environmental impact of direct-use projects
ImpactProbability of occurring
Severity of consequences
Air quality pollution
L
M
Surface water pollution
M
M
Underground pollution
L
M
Land subsidence
L
L to M
High noise levels
H
L to M
Well blow-outs
L
L to M
Conflicts with cultural and
L to M
M to Harchaeological features
Social-economic problems
L
L
Chemical or thermalL
M to Hpollution
Solid waste disposal
M
M to H
L = Low; M = Moderate; H= High
Source: Lunis and Breckenridge (1991)
Any modification to our environment must be evaluated carefully, in deference to therelevant laws and regulations (which in some countries are very severe), but also because
an apparently insignificant modification could trigger a chain of events whose impact isdifficult to fully assess beforehand. For example, a mere 2-3 °C increase in thetemperature of a body of water as a result of discharging the waste water from autilization plant could damage its ecosystem. The plant and animal organisms that are
most sensitive to temperature variations could gradually disappear, leaving a fish species
35without its food source. An increase in water temperature could impair development of
the eggs of other fish species. If these fish are edible and provide the necessary support
for a community of fishermen, then their disappearance could be critical for thecommunity at large.
The first perceptible effect on the environment is that of drilling, whether the boreholes
are shallow ones for measuring the geothermal gradient in the study phase, or
exploratory/producing wells. Installation of a drilling rig and all the accessory equipment
entails the construction of access roads and a drilling pad. The latter will cover an area
ranging from 300—500 m2 for a small truck-mounted rig (max. depth 300 —700 m) to1200—1500 m2 for a small-to-medium rig (max. depth of 2000 m). These operations will
modify the surface morphology of the area and could damage local plants and wildlife.
Blow-outs can pollute surface water; blow-out preventers should be installed whendrilling geothermal wells where high temperatures and pressures are anticipated (Lunis
and Breckenridge, 1991). During drilling or flow-tests undesirable gases may bedischarged into the atmosphere. The impact on the environment caused by drilling mostly
ends once drilling is completed.
The next stage, installation of the pipelines that will transport the geothermal fluids, andconstruction of the utilization plants, will also affect animal and plant life and the surface
morphology. The scenic view will be modified, although in some areas such as
Larderello, Italy, the network of pipelines criss-crossing the countryside and the replantcooling towers have become an integral part of the panorama and are indeed a famous
tourist attraction.
Environmental problems also arise during plant operation. Geothermal fluids (steam or
hot water) usually contain gases such as carbon dioxide (CO2), hydrogen sulphide (H2S),
ammonia (NH3), methane (CH4), and trace amounts of other gases, as well as dissolved
chemicals whose concentrations usually increase with temperature. For example, sodium
chloride (NaCl), boron (B), arsenic (As) and mercury (Hg) are a source of pollution if
discharged into the environment. Some geothermal fluids, such as those utilised for
district-heating in Iceland, are freshwaters, but this is very rare. The waste waters fromgeothermal plants also have a higher temperature than the environment and therefore
constitute a potential thermal pollutant.
Air pollution may become a problem when generating electricity in conventional
power-plants. Hydrogen sulphide is one of the main pollutants. The odour threshold forhydrogen sulphide in air is about 5 parts per billion by volume and subtle physiologicaleffects can be detected at slightly higher concentrations (Weres, 1984). Various
processes, however, can be adopted to reduce emissions of this gas. Carbon dioxide isalso present in the fluids used in the geothermal power plants, although much less CO2
is discharged from these plants than from fossil-fuelled power stations: 13 – 380 g forevery kWh of electricity produced in the geothermal plants, compared to the 1042 g/kWhof the coal-fired plants, 906 g/kWh of oil-fired plants, and 453 g/kWh of natural gas-fired
plants (Fridleifsson, 2001). Binary cycle plants for electricity generation anddistrict-heating plants may also cause minor problems, which can virtually be overcome
simply by adopting closed-loop systems that prevent gaseous emissions.

36Discharge of waste waters is also a potential source of chemical pollution. Spentgeothermal fluids with high concentrations of chemicals such as boron, fluoride orarsenic should be treated, re-injected into the reservoir, or both. However, thelow-to-moderate temperature geothermal fluids used in most direct-use applications
generally contain low levels of chemicals and the discharge of spent geothermal fluids isseldom a major problem. Some of these fluids can often be discharged into surface waters
after cooling (Lunis and Breckenridge, 1991). The waters can be cooled in special storage
ponds or tanks to avoid modifying the ecosystem of natural bodies of waters (rivers, lakes
and even the sea).
Extraction of large quantities of fluids from geothermal reservoirs may give rise to
subsidence phenomena, i.e. a gradual sinking of the land surface. This is an irreversible
phenomenon, but by no means catastrophic, as it is a slow process distributed over vast
areas. Over a number of years the lowering of the land surface could reach detectablelevels, in some cases of the order of a few tens of centimetres and even metres, andshould be monitored systematically, as it could damage the stability of the geothermalbuildings and any private homes in the neighbourhood. In many cases subsidence can beprevented or reduced by re-injecting the geothermal waste waters.
The withdrawal and/or re-injection of geothermal fluids may trigger or increase thefrequency of seismic events in certain areas. However these are microseismic events
that can only be detected by means of instrumentation. Exploitation of geothermal
resources is unlikely to trigger major seismic events, and so far has never been known to
do so.
The noise associated with operating geothermal plants could be a problem where theplant in question generates electricity. During the production phase there is the higher
pitched noise of steam travelling through pipelines and the occasional vent discharge.
These are normally acceptable. At the power plant the main noise pollution comes fromthe cooling tower fans, the steam ejector, and the turbine 'hum' (Brown, 2000). The
noise generated in direct heat applications is usually negligible.

37

PRESENT AND FUTURE
The thermal energy present in the underground is enormous. A group of experts has
estimated (Table 7) the geothermal potential of each continent in terms of high- andlow-temperature resources (International Geothermal Association, 2001).
Table 7. Geothermal potential world-wide (IGA, 2001).
High-temperature resources suitable forelectricity generation
Low-temperature
resources suitable
Conventional andConventional
for direct use inbinarytechnology inmillion TJ/yr oftechnology in
TWh/yr ofheat(lower limit)
TWh/yr ofelectricity
electricity
EuropeAsia
AfricaNorth America
Latin America
Oceania
World potential

1,830

3,700

> 370

2,970

5,900

> 320

1,220

2,400

> 240

1,330

2,700

> 120

2,800

5,600

> 240

1,050

2,100

> 110

11,20022,400> 1,400

If exploited correctly, geothermal energy could certainly assume an important role in
the energy balance of some countries. In certain circumstances even small-scale
geothermal resources are capable of solving numerous local problems and of raising theliving standards of small isolated communities.
The data reported by Fridleifson (2003) give some idea of the role played by geothermalenergy with respect to other renewable energy sources: of the total electricity produced
from renewables in 1998, i.e. 2,826 TWh, 92% came from hydro power, 5.5% frombiomass, 1.6% from geothermal, 0.6% from wind, 0.05% from solar, and 0.02% fromtidal. Biomass constitutes 93% of the total direct heat production from renewables,
geothermal represents 5% and solar heating 2%.

38

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