Page 43 - Geothermal Energy Renewable Energy and The Environment

26 Geothermal Energy: Renewable Energy and the Environment

synopsIs
The Earth is a dynamic planet driven by heat. This heat, which can be accessed for power genera-
tion and other applications, comes from several sources. Formation of the planet involved a complex
series of events including impacts from objects in the early solar nebula, heat by radioactive decay
of short-lived isotopes, and the resultant melting of the planet. Density stratification then occurred,
with the release of gravitational potential energy as liquid iron settled to the interior to form the
core of the Earth. Additional heat is generated by the decay of longer lived isotopes, especially U,
Th, and K in the continental crust. The remnant heat from the early melting and slow cooling of
the Earth combined with the heat given off by radioactive elements has resulted in a heat budget
2
that provides an average rate of heat escaping from the surface of about 87 mW/m . Transfer of heat
in the Earth is dominated by conduction on local scales but by convection on global scales. The
convective processes, in particular, determine the distribution of heat sources in the near surface
through the processes that are manifest as plate tectonics. The largest heat sources are at spreading
centers and regions associated with subduction zone volcanoes. These various processes have lead
to a heterogeneous distribution of heat, with heat flow values on the continents ranging from less
2
2
than 30 mW/m to much more than 150 mW/m . Although power generation tends to occur in areas
2
with heat flow values greater than about 80 mW/m , virtually any region, regardless of heat flow,
can support the use of ground source heat pumps for heating and cooling of buildings. In addition,
so-called direct use applications in which warm waters are used for aquaculture, agriculture, and
other purposes can be developed anywhere in the vicinity of springs or warm subsurface waters that
can be found in a broad range of settings.

case sTUdIes

Examples of power generation from several of the key plate tectonic settings are described below.
Many of these sites are described in more detail in later chapters. An important consideration is that
plate boundaries are often the primary locations for developing geothermal power plants. However,
new developments and technological capabilities are making it possible to exploit geothermal power
in regions far from plate boundaries. This new approach reflects two important developments. One
is the ability to generate power from lower temperature resources than previously utilized, employ-
ing organic Rankine cycle technology, as described in Chapter 9. The other development is a new
concept for geothermal power generation, called enhanced geothermal systems (or EGS). The EGS
methodology is discussed in detail in Chapter 14. Applications of geothermal heat in nonpower set-
tings is discussed in detail in Chapters 10 and 11.

spreadinG cenTers

Salton Trough, California: The Gulf of California has its northern termination at the junction of
Baja California and mainland Mexico. Extending northward from this location is a broad, fault-
bounded valley that extends through that part of Mexico into Southern California. The Gulf of
California is a new ocean basin, formed by an extension of the main spreading center in the Pacific
Ocean, the East Pacific Rise. Spreading along that extension caused a sliver of Mexico (i.e., Baja
California) to split away from the mainland, beginning about 20 million years ago. The primary
motion direction of that sliver is to the northwest and is accommodated by slip along the San
Andreas Fault. In Southern California and Northern Mexico this spreading has resulted in the for-
mation of a major depression in the crust, which is the Salton Trough. The San Andreas Fault is a
part of this dynamic system.
Separation of the crust and the formation of the trough have resulted in the development of
an extensional environment that can accommodate the intrusion of hot magma that is associated
with the spreading system. This magma has risen to shallow levels in the Salton Trough, causing