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4 Electricity Generation and Storage
It was the French physicist Charles Augustin Coulomb (1736–1806) who recognized the nature of the electrostatic force between charged particles and showed
that the force between them is proportional to the product of their charges (and
inversely proportional to the distance between them), as shown by Coulomb’s law
(Equation 1.4, where q designates charge in the units of coulombs, F is the force
in newtons, r is the distance between the two charges in meters, and k is a constant
equal to 8.99 × 109 N ⋅ m2/C2).
Just as a mass at some height has a potential energy due to gravitational forces, so
does a charged particle in an electromagnetic field: a charged particle (e.g., an electron)
will be forced to move by being either attracted or repulsed by the field, depending
upon the nature of the charges involved. This electrical potential energy (or electromotive force, EMF) has the units of volts, where a volt is equivalent to joule/coulomb.
Thus, when some material (e.g., a coil of wire in a generator) is moved through a magnetic field, the electrons in the material can be forced to move, generating electricity.
Obviously, moving the coil of wire through the magnetic field is work that requires
some source of energy, as in the raw energy of a fossil fuel. In a typical coal-fired
power plant, combustion of coal provides the thermal energy to generate high-pressure
steam, which turns a steam turbine coupled to a generator: voilá—electricity!
In the United States, the percentage use of natural gas for electrical generation is
approaching that of coal (see Figure 1.10), with hydroelectric and renewable electricity
The U.S. electrical generation by source, 2012
FIGURE 1.10 U.S. electrical generation by fuel type (%), 2011. (Adapted from U.S. Energy
Information Administration, 2013b. Net Generation by Energy Source: Total (All Sectors).
Retrieved 19 April 2013, from http://www.eia.gov/electricity/data.cfm#generation.)
Chemistry of Sustainable Energy
generation making up almost 12% of the total (U.S. Energy Information Administration
2013b). Many U.S. natural gas and coal-fired power plants have generating capacities
of well over 1000 MW, with operating efficiencies limited to 30–40% as a result of
the laws of thermodynamics, which we will turn to in Chapter 3. The largest nuclear
power plants in the United States are rated at a capacity of around 1300 MW (the net
total MWh used in the United States in 2011, for comparison, was 4,100,656 thousand
MWh (U.S. Energy Information Administration 2011)). There are other cleaner ways
of generating electricity: wind turbines (Chapter 4), fuel cells (Chapter 6), and solar
photovoltaics (Chapter 7). While these are very promising sources of electrical energy,
recall the terawatt level of consumption noted above. The comparisons listed below
demonstrate quite bluntly why the matter of scale will be a constant reminder as we
contemplate the chemistry of sustainable energy (Kirubakaran et al. 2009).
500 kW to
1 kW to 1 MW
10 kW to
200 kW to
The huge scale required for electricity generation and the variability of sustainable
sources of electricity such as solar and wind raise additional concerns. How useful is
wind energy when the wind is not blowing? How can photovoltaics provide power for
us when the sun is not shining? These concerns are the driving force for the development of a so-called smart grid and efforts to develop effective, large-scale modes of
storing electricity. While weaving together a smart grid is all about digital technology
(and thus not the purview of this text), electrical energy storage (EES) is strongly
embedded in the area of chemistry and materials science. Storage of electricity as
some other form of energy that can be converted back into electricity, possibly at a
moment’s notice, can even out the variability of a wind- or light-driven electricity
supply. There are several approaches to stationary EES systems, including supercapacitors that store electricity in electrical charges or flywheels that convert kinetic
energy back into electrical energy. Potential energy can be used to store electrical
energy, for example, by pumping large volumes of water into a raised reservoir where
it can reside until it is freed to pass through a hydroelectric generating plant. The EES
most relevant to this text is electrochemical energy storage, a.k.a. batteries. Because
the research progress associated with the development of large-scale EES is closely
related to that of fuel cells, this topic will be covered in some detail in Chapter 6.
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Before we can make any progress toward a sustainable energy future, we need to
learn from our past and present. Although proven reserves of fossil fuels may continue to meet our energy needs for some years to come and nonconventional sources
of fossil fuels are pushing back the “end” of oil, there will come an end, particularly
because our reliance on fossil fuels remains great (recall Figure 1.2). The story of
fossil fuels is a fascinating mix of history, politics, and science, with global security,
economics, and most certainly the environment demanding that we begin to phase
out our dependence on fossil fuels. The aim of this chapter is to understand what
fossil fuels are, where they came from, how we got to this point, and where we are
going with respect to finding fossil fuels for the foreseeable future. (N.B. Appendix
IV provides some additional information with respect to units and conversions peculiar to the oil and gas industry.)
2.1 FORMATION OF OIL AND GAS
Contrary to popular belief, deceased and decomposed dinosaurs are not the primary
source of fossil fuels that we use to power our planet. But before we can fully understand the source of petroleum (literally “rock oil”), we need to review a little geology.
Scientists have devised many cycles to explain the processes and conversions of our
planet, such as the carbon cycle to explain the chemical fate of carbon in the environment. The analogous rock cycle explains how rocks (far from sedentary objects) are
moved about by internal and external processes on and in our planet. In this cycle,
igneous, metamorphic, and sedimentary rocks are transformed and transported. We
are most concerned with sedimentary rocks for they are the source of oil and gas.
Produced from sediments deposited either on the land or at the bottom of a body of
water, sedimentary rocks represent only about 5% of Earth’s crust (Skinner and Porter
2000). Sediment—and hence, sedimentary rocks—comes from everywhere and
everything, including rock fragments, chemical precipitates (e.g., calcium carbonate
from bones and shells), long-dead marine organisms, and detritus in general. In the
sedimentation process, various kinds of materials are stratified such that sandy sediments (sandstone) overlay clay sediments (shales), with calcium carbonate-containing
sediments (limestones) at the bottom of the strata. Sediment is buried and compacted
to become sedimentary rock, typically at or near the site at which it was originally
deposited. As a result, oil (which ultimately comes from sedimentary rock, vide infra)
is not widely distributed on the planet, with obvious geopolitical consequences.
Given that sediment comes from everywhere and everything, the fact that a small
portion of sedimentary rocks consists of organic matter is self-evident. Roughly 18%
of the total amount of carbon in Earth’s crust is organic carbon in the form of oil,
gas, and coal (Schidlowski et al. 1974). Living things die and the carbon, hydrogen,
Chemistry of Sustainable Energy
nitrogen, and oxygen that were once a part of living biota become part of the landscape. In an aerobic environment, this organic matter is oxidized, but in an oxygenfree environment (as when trapped in a layer of sediment), oxidation is negligible.
Numerous types of organic compounds, then, are trapped in sedimentary rocks and
converted into crude petroleum and other carbon-rich materials under conditions of
heat and pressure over a geologic timescale.
Fossil fuels, then, are mixtures of hydrocarbons in a low oxidation state, formed
over a period of millions of years and trapped in sediment that makes them fairly inaccessible. What is the chemical composition of these mixtures and what is their origin? Given that ancient crude oil primarily consists of even-numbered carbon chains
and that the material is both optically active and levorotatory, it is widely accepted
that the vast majority of oil is biological in origin (Selley 1998). Decomposition of
this once-living matter into fossil fuels is a process whereby the highly oxygenated
organic matter is reduced to hydrocarbon species.
Lipids and lignins from the original plant or animal material are the primary
organic materials that decay to hydrocarbon fossil fuels. (Ultimately, of course, these
carbon atoms came from carbon dioxide in the process of photosynthesis.) Lignins,
found in all woody plant materials as a very complex polymeric mixture of phenols
and glycerol, are the source of solid fossil fuels such as peat and coal (Figure 2.1).
FIGURE 2.1 Lignin structure.