Geothermal Energy

Utilization and Technology

Mary H. Dickson,Mario Fanelli

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781134033973
• Edition: 1
• Topic: Ciencias biológicas
• Subtopic: Ecología

Geothermal energy refers to the heat contained within the Earth that generates geological phenomena on a planetary scale. Today, this term is often associated with man's efforts to tap into this vast energy source. Geothermal Energy: utilization and technology is a detailed reference text, describing the various methods and technologies used to exploit the earth's heat.

Beginning with an overview of geothermal energy and the state of the art, leading international experts in the field cover the main applications of geothermal energy, including:
electricity generation
space and district heating
space cooling
greenhouse heating
aquaculture
industrial applications

The final third of the book focuses upon environmental impact and economic, financial and legal considerations, providing a comprehensive review of these topics.

Each chapter is written by a different author, but to a set style, beginning with aims and objectives and ending with references, self-assessment questions and answers. Case studies are included throughout.

Whilst written primarily for professionals and students interested in learning more about geothermal energy, the book also offers those new to the field and the general geothermal community an opportunity to understand and review the potential of this exciting alternative energy source.

Published with UNESCO

CHAPTER 1

Geothermal Background

Mary H. Dickson and Mario Fanelli

Istituto di Geoscienze e Georisorse, CNR, Pisa, Italy

AIMS

1. To define what is meant by the term geothermal energy, and the relationship between geothermal energy and geological phenomena of a planetary scale; to evaluate how much of this energy could be recovered and exploited by humankind. To identify the main physical mechanisms occurring in the shallowest parts of the Earth’s crust, how these mechanisms can be harnessed to allow us to extract the Earth’s heat, and the main methods of research that can be used.

2. To show how geothermal energy can be utilized in numerous applications, from electricity generation to a wide range of direct heat uses, and the benefits that can be gained from these uses.

3. To emphasize the relatively minor impact of geothermal energy on the environment, without underestimating the risks that effectively do exist.

4. Finally, to demonstrate the potential benefits – to a community and/or to a nation – of exploiting indigenous geothermal resources, with emphasis on the importance of first making careful assessments of each specific situation, especially as regards quality of the resource and socio-economic conditions, and also of defining the programme of action.

OBJECTIVES

When you have completed this chapter you should be able to:

1. define the nature of the Earth’s heat, the phenomena related to the latter and how these phenomena influence the distribution of the geothermal areas in the world

2. define geothermal systems and explain how they function

3. define the main categories of geothermal energy

4. discuss the present status of the development of geothermal energy in the world

5. discuss the main research methods used in areas of potential geothermal interest

6. discuss the major forms of geothermal utilization

7. discuss the potential impact on the environment of geothermal energy.

1.1 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 is often used nowadays, however, to indicate that part of the Earth’s heat that can, or could, be recovered and exploited by humankind, and it is in this sense that we will use the term from now on.

1.1.1 Brief geothermal history

The presence of volcanoes, hot springs and other thermal phenomena must have led our ancestors 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 were excavated to a few hundred metres below ground level, that humankind deduced, from simple physical sensations, that the Earth’s temperature increased with depth.

Figure 1.1 The Earth’s crust, mantle and core. Top right: a section through the crust and the uppermost mantle

The first measurements by thermometer were probably performed in 1740, in a mine near Belfort, in France (Bullard, 1965). By 1870 modern scientific methods were being used to study the thermal regime of the Earth, 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 account the heat continually generated by the decay of the long-lived radioactive isotopes of uranium (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 on these models were not available until the 1980s, when it was demonstrated that there was no equilibrium between the radiogenic heat generated in the Earth’s interior and the heat dissipated into space from the Earth, and that our planet is slowly cooling down. To give some 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 42 × 10¹² W (conduction, convection and radiation). Of this figure, 8 ×10¹² W comes from the crust, which represents only 2 per cent of the total volume of the Earth but is rich in radioactive isotopes, 32.3 × 10¹² W comes from the mantle, which represents 82 per cent of the total volume of the Earth, and 1.7 × 10¹² W comes from the core, which accounts for 16 per cent of the total volume and contains no radioactive isotopes. (See Figure 1.1 for a sketch of the inner structure of the Earth). Since the radio-genic heat of the mantle is estimated at 22 × 10¹² W, the cooling rate of this part of the Earth is 10.3 × 10¹² W.

In more recent estimates, based on a greater number of data, the total flow of heat from the Earth is about 6 per cent higher than the figure utilized by Stacey and Loper (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 3 billion years, remaining at about 4,000°C at its base. It seems probable 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 × 10²⁴ MJ, and that of the crust is of the order of 5.4 × 10²¹ MJ (Armstead, 1983). The thermal energy of the Earth is therefore immense, but only a fraction could be utilized by humankind. 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 hot zones to or near the surface, thus giving rise to geothermal resources, but innovative techniques in the near future may offer new perspectives in this sector.

There are examples in many areas of life of practical applications preceding scientific research and technological developments, and the geothermal sector is no exception. In the early part of the nineteenth 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 hot waters emerging naturally or from specially drilled shallow boreholes. The boric acid was obtained by evaporating the hot fluids in iron boilers, using 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. 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 in reciprocating and centrifugal pumps and winches, all of which were connected with drilling activity or in the local boric acid industry. Between 1850 and 1875 the factory at Larderello held the monopoly in Europe for boric acid production. Between 1910 and 1940 the low-pressure steam in this area of Tuscany was brought into use to heat the industrial and residential buildings and greenhouses. In 1928 Iceland, another pioneer in the utilization of geothermal energy, also began exploiting its geothermal fluids (mainly hot waters) for domestic heating.

The first attempt at generating electricity from geothermal steam was made at Larderello in 1904. The success of this experiment indicated the industrial value of geothermal energy and marked the beginning of a form of exploitation that was to develop significantly from then on. Electricity generation at Larderello was a commercial success. By 1942 the installed geothermoelectric capacity had reached 127,650 kW_e. Several countries followed the example set by Italy. The first geothermal wells in Japan were drilled at Beppu in 1919 and in the United States at The Geysers, California, in 1921. In 1958 a small geothermal power plant began operating in New Zealand, in 1959 in Mexico, in 1960 in the United States, and in many other countries in the years to follow.

1.1.2 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 have to be imported, and in some cases it was the only energy source available locally. The countries that utilize geothermal energy to generate electricity are listed in Table 1.1, which also gives the installed geothermal electric capacity in 1995 (6,833 MW_e), in 2000 (7,974 MW_e), and the increase between 1995 and the year 2000 (Huttrer, 2001). The same table also reports the total installed capacity in 2004 (8,806.45 MW_e). The geothermal power installed in the developing countries in 1995 and 2000 represents 38 and 47 per cent 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 MW_e; by the end of the next five-year period (1984) this figure had reached 1495 MW_e, showing a rate of increase during these two periods of 500 per cent and 223 per cent, respectively (Dickson and Fanelli, 1988). In the next sixteen years, from 1984 to 2000, there was a further increase in the total of almost 150 per cent.

Geothermal power plays a fairly significant role in the energy balance of some areas, and of the developing countries in particular, as can be inferred from the data reported in Table 1.2, which shows the percentage of geothermal power with respect to total electric power installed in some of these countries, relative to 1998.

As regards non-electric applications of geothermal energy, Table 1.3 gives the installed capacity (15,145 MW_t) and energy use (190,699 TJ/yr) worldwide for the year 2000. There are now fifty-eight countries reporting direct ...