Environmental Engineering Reference
In-Depth Information
GEOTHERMAL ENERGY: THE BASICS
Geothermal energy processes utilize the natural heat of the Earth for beneficial pur-
poses when the heat is collected and transported to the surface. To gain a proper
understanding of geothermal energy as it is used at present, it is important to define
enthalpy
, the heat content of a substance per unit mass. Temperature alone is not suf-
ficient to define the useful energy content of a steam/water mixture. A mass of steam
at a given temperature and pressure can provide much more energy than the same
mass of water under the same conditions. Enthalpy is a function of
Pressure + Volume + Temperature = Enthalpy
Geothermal practitioners usually classify geothermal resources as high enthalpy
(water and steam at temperatures above about 180 to 200°C), medium enthalpy (about
100 to 180°C), and low enthalpy (<100°C). For the purpose of this text, though, it is
sufficient to think of temperature and enthalpy as the same.
e
arth
'
s
l
ayers
Earth is made up of three main compositional layers: crust, mantle, and core (see
Figure 6.2). The crust has variable thickness and composition; the continental crust
is 10 to 50 km thick, and the oceanic crust is 8 to 10 km thick. The elements silicon,
oxygen, and aluminum, among others, make up the Earth's crust. Like the shell of an
egg, the Earth's crust is brittle and can break. Based on seismic (earthquake) waves
that pass through the Earth, we know that below the crust is the mantle, a dense, hot
layer of semisolid rock approximately 2900 km thick. The mantle might be thought
of as the white of a boiled egg. It contains silicon, oxygen, aluminum, and more iron,
magnesium, and calcium than the crust. The mantle is hotter and more dense than
Mantle
Crust
Outer core
Inner
core
FIGURE 6.2
Layers of the Earth.
