Page 19 - Geothermal Energy Renewable Energy and The Environment

1 Introduction

It is a well-known fact that the interior portions of the globe are very hot, the temperature ris-
ing, as observations show, with the approach to the center at the rate of approximately 1 degree
C. for every hundred feet of depth. The difficulties of sinking shafts and placing boilers at
depths of, say, twelve thousand feet, corresponding to an increase in temperature of about
120 degrees C., are not insuperable, and we could certainly avail ourselves in this way of the
internal heat of the globe. In fact, it would not be necessary to go to any depth at all in order
to derive energy from the stored terrestrial heat. The superficial layers of the earth … are at a
temperature sufficiently high to evaporate some extremely volatile substances, which we might
use in our boilers instead of water.

—Nikola Tesla, “The Problem of Increasing Human Energy With Special
References to the Harnessing of the Sun’s Energy,”
Century Illustrated Magazine, June 19.0.0.

As the above quote demonstrates, the vision of utilizing the Earth’s internal heat to benefit the world
is not new. It has long been recognized that energy, in the form of heat, is constantly radiating from
the Earth’s surface into space. Some of that energy is solar energy that has been absorbed by soil
and rock and reradiated as infrared radiation. But on average about 1% of that energy radiating into
space is from the Earth itself. Although it may seem insignificant, in fact the amount of heat energy
the Earth contains is staggering—there is enough heat energy in the subsurface to satisfy the energy
needs of every nation of the world many times over.
That heat energy is geothermal energy. It is remnant heat derived from the formation of the planet
four and a half billion years ago, as well as heat from the radioactive decay of naturally occurring
unstable isotopes. That heat is sufficient to power plate tectonics, which is the slow movement of
massive blocks of the Earth’s crust and upper mantle, and to drive mountain building processes that
occur when those blocks collide. It is also sufficient to melt rock, generate volcanoes, heat water to
form hot springs, and keep basements at a constant temperature. It is a perpetual and inexhaustible
energy resource.
With a few important exceptions, geothermal energy did not play a major role in the energy mix
associated with power generation or other applications until the latter half of the twentieth century.
At that time, growing interest in the environmental, economic, and social aspects of energy gen-
eration and use encouraged exploration of energy sources that would diminish reliance on fossil
fuels. This chapter will discuss the context of those changes and their implication for the develop-
ment of geothermal energy. The chapters that follow will consider specific topics that, if taken
together, provide a comprehensive body of knowledge for informed consideration of geothermal
energy use.

The Global enerGy landscape

The hisTorical role of fuel
One of the hallmarks of the human species is the creative use of energy. Over many centuries
humanity learned through experience, insight, and experimentation that fire could be controlled and
used to our mutual benefit. And, with that ability and skill, the quality of life has rapidly improved
for an ever-growing proportion of the planets’ people.

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