Page 29 - Geothermal Energy Renewable Energy and The Environment

12 Geothermal Energy: Renewable Energy and the Environment

Table 2.1
heat Generation of the primary heat producing elements
material k U Th
Heat production (W/kg of element) 3.5 × 10 –9 96.7 × 10 –6 26.3 × 10 –6

Source: Beardsmore, G. R. and Cull J. P., Crustal Heat Flow, Cambridge, UK: Cambridge
University Press, 2001.

As the planet accreted material, the kinetic energy of incoming bodies was transformed, in
part, to heat when the bodies impacted the planetary surface. That process lead to an increase in
temperature of the growing planet. In addition, as the planet grew in size, pressures in the interior
increased, compressing the silicate minerals and other materials, ultimately contributing to elevating
the internal temperature of the planet.
The early solar nebula also had a significant abundance of radioactive elements with short half-lives
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(e.g., Al, t = 740,000 years; Hf, t = 9 million years; Mn t = 3.7 million years. The t isthe
1/2
1/2

1/2
1/2
half-life, which is the time required for one half of a given amount of a radioactive element to decay).
Radioactive decay occurs via one of several mechanisms: α-decay, which is emission of a helium
nucleus; β-decay, which occurs when a neutron becomes a proton with the emission of an antineutrino
and an electron; β′-decay, which occurs when a proton becomes a neutron with the emission of a neutrino
and a positron; γ-decay, which occurs when a gamma ray is emitted; and electron capture, when an
inner electron is captured by a nucleus. Although neutrinos do not significantly interact with matter, the
other products of radioactive decay, as well as the recoil of the decaying nucleus heat the immediately
surrounding environment when the particles collide with the atoms surrounding the decaying isotopes.
The kinetic energy of the particles is transformed to heat that results in raising the local temperature.
Table 2.1 summarizes the heat production for the primary radioactive elements that currently provide
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heat to the Earth. Although Al is no longer present on the planet, it was abundant during the early
formation of the Earth and contributed substantially to raising its internal temperature.
In combination, these processes resulted in the temperature of the Earth’s interior exceeding
that of the melting point of iron. Due to its high density relative to that of the silicates with which it
was in contact and because of its mobility in the liquid state, the liquid iron migrated to the center
of the Earth forming a liquid core. That migration process also contributed to heating the Earth,
since movement of the iron to a position of lower gravitational potential resulted in the release of
gravitational potential energy.
In combination, these processes lead to the formation of a differentiated planet with a hot, liquid
metal core within less than 30 million years of the planet’s formation (Kleine et al. 2002; Yin et
al. 2002). Since that time, the core has been slowly cooling, resulting in the growth of a solid inner
core and diminution in size of the liquid outer core. Today, the solid inner core has a radius of
approximately 1221 kilometers (Figure 2.1). The liquid outer core extends to about 3,480 kilometers
from the center of the Earth, making it about 2200 kilometers thick. Although difficult to establish
and subject to considerable debate, the temperature at the boundary between the inner and outer
core is probably between 5400 and 5700 K. The temperature at the outer edge of the liquid core is
about 4000 K (Alfé, Gillian, and Price 2007).
Some of the heat (approximately 40%; Stein 1995) used in geothermal applications ultimately
derives from this remnant heat from the early formation of the Earth’s core. The remaining 60% is
derived from the decay of long-lived radioactive isotopes.

heaT from radioacTive decay of lonG-lived isoTopes
The formation of the core was a fundamental event in the history of the Earth. The redistribution
of metal was accompanied by density stratification within the remaining planetary body. Although