Page 28 - Geothermal Energy Renewable Energy and The Environment

2 Sources of Geothermal Heat:

Earth as a Heat Engine

The Earth gives the impression that it is dependably constant. Over the time scale of a human lifetime,
little seems to change, as John Burroughs noted when he wrote of “the unshaken permanence of
the hills” of Ireland (John Burroughs 1875, Winter Sunshine, vol. II). The reality, however, is quite
contrary to that experience. Every cubic centimeter of the Earth is in motion, and has been since
the Earth was formed four and a half billion years ago. Indeed, the Earth is a profoundly dynamic
entity. On the time scale of seconds earthquakes jar the Earth, over the time span of a few years
volcanoes appear and grow, over millenia landscapes slowly evolve, and over millions of years the
continents rearrange themselves on the planet’s surface.
The energy source to drive these processes is heat. Although the extrusion of molten rock at
volcanoes is perhaps the most dramatic evidence that heat energy exists in the Earth’s interior, there
is, in fact, a constant flux of heat from every square meter of the Earth’s surface. The average heat
flux for the Earth is 87 mW/m (Stein 1995; see sidebar for a discussion of the units used in this book
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and Thompson and Taylor, 2008). For a total global surface area of 5.1 × 10 km , this heat flux is
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equivalent to a total heat output of more than 4.4 × 10 W. For comparison, it is estimated that the
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total power consumed by all human activity in 2006 was approximately 1.57 × 10 W (U.S. Energy
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Information Agency 2008). Clearly, heat in the Earth has the potential to significantly contribute to
satisfying human energy needs.
This heat is the source of geothermal energy. The remainder of this chapter will consider the
origin, distribution, and properties of that heat.
orIGIn oF earTh’s heaT
In order to intelligently use the heat available in the Earth, it is important that the sources of that
heat be understood. Geothermal resources can heat homes, air condition greenhouses, dry spices
and vegetables, and generate electricity. Some of these applications can be pursued anywhere on the
planet, others require special circumstances. Using this resource in a way that is both economical
and environmentally sound requires that the characteristics of the resource be understood. This
chapter describes the origins of the Earth’s heat, the processes that determine how it is distributed
over the surface of the planet, and what determines its intensity.

heaT from formaTion of The core
Studies of meteorites and models of astronomical processes have posited that the Earth formed 4.56
billion years ago (Allégre, Manhés, and Göpel 1995; Göpel, Manhés, and Allégre 1994) by accretion
of material from the early solar nebula. Dust, sand-sized particles and other objects collided and
aggregated, forming terrestrial-sized planets within a few tens of millions years (Canup and Agnor
2000; Chambers 2001; Kleine et al. 2002; Kortenkamp, Wetherill, and Inaba 2001; Wetherill 1990;
Yin et al. 2002). The aggregating materials were composed of a variety of minerals, primarily
silicates similar to those that make up the rocks of the Earth, as well as native metals (primarily
iron) and frozen volatiles such as water and simple hydrocarbons.

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