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2 Energy, Technology, and Sustainability
without compromising the ability of future generations to meet their own needs,”
a decidedly vague definition with a clear human focus (U.N. World Commission
on Environment and Development 1987). The appropriate question may not be “is
this sustainable?” but “will this leave our planet in better or worse shape for future
generations?” As humans, of course, we are interested in sustaining our way of life,
but as we have come to increasingly dominate the planet, we have sustained our way
of life at the expense of others—the environment for future generations has been
irreversibly altered because of loss of habitat, decreased biodiversity, and irreparable
damage to the environment.
Not only is sustainability (and sustainable development) hard to define, it is
also relative. When it comes to consumption of energy, sustainability for an urban
household in the United States is immensely different from, say, sustainability for
an aboriginal clan in Australia. Support of the standard of living in North America
consumes drastically more energy than the rest of the world, such that if everyone
on the globe were to attain this same level of comfort, at least twice the Earth’s
natural resources would be required (Simms et al. 2010). Science and technology
have helped to create a wildly disparate distribution of wealth, health, and well-being
across the globe. As we look toward offering “sustainable energy solutions,” we must
consider the Earth and all its inhabitants.
Finally, we must ask the question “is this level of sustainability even achievable?”
An in-depth treatment of the planet’s “carrying capacity” is beyond the scope of this
text, but an oversimplified definition would be that carrying capacity is the maximum number of individuals (humans, for example) that can be sustained indefinitely
by an ecosystem without causing irreparable damage. One way to gauge Earth’s carrying capacity and our impact on our ecosystem is to look at our ecological footprint,
a measure of our demands on nature. These representations compare the resource
use (in this case, by humans) to the resource capacity (the planet Earth). As can be
seen from the graph in Figure 1.5, we have been overextending the carrying capacity
of our planet since 1976 (a value of 1 means that what the Earth can sustainably provide in 1 year was completely consumed). It is now estimated that we are consuming
the resources of over one and a half Earths (Roney 2010). Ultimately, “nature will
decide what is sustainable; it always has and always will” (Zencey May/June 2010).
1.2.2 Carbon Cycle
One of the biggest concerns associated with energy use is the increasing amount
of carbon in the atmosphere and its impact on global climate change. Clearly not
all energy is about carbon, but because of our reliance on fossil fuels, quite a lot
of it is. It is therefore important to review what we know about carbon’s fate in our
environment. The carbon cycle describes this exchange of carbon through four main
reservoirs: the atmosphere, the terrestrial biosphere, the oceans, and the sediments
(which include fossil fuels). Carbon in various forms moves through these sinks by
chemical, physical, biological, and geological processes in a cycle and is graphically depicted in Figure 1.6 (National Aeronautic and Space Administration 2013;
Olah et al. 2011). Carbon dioxide is the major source of carbon in our environment.
It is found in the atmosphere (0.825 × 1015 kg) and has appreciable solubility in the
Chemistry of Sustainable Energy
Human demands (1960–2007)
Number of the earths needed
FIGURE 1.5 Human demands on the carrying capacity of the Earth. (Reprinted with permission from Roney, J.M. 2010. Humanity’s Ecological Footprint, 1961–2007, edited by B.
Barbeau: Global Footprint Network, www.earth-policy.org.)
and waste products
CO2 ocean uptake
FIGURE 1.6 The carbon cycle. (Reprinted with permission from Olah, G. A. et al. 2011.
Anthropogenic chemical carbon cycle for a sustainable future. J. Am. Chem. Soc. 133:12881–
12898. Copyright 2011, American Chemical Society.)
6CO2 + 6H2O
hυ (solar energy)
6O2 + C6H12O6
FIGURE 1.7 Chemical conversions in photosynthesis.
oceans (45 × 1015 kg), but there is little exchange of CO2 from the oceans. Other
significant amounts of carbon are present in carbonate minerals (e.g., limestone)
and, obviously, in the dwindling supplies of hydrocarbon fossil fuels (10 × 1015 kg)
(Sørensen et al. 2008). Although it is hard to imagine, the carbon in your sandwich
(be it peanut butter or pastrami) may once have been the carbon in a coal seam, carbonate in the ocean, or CO2 in the atmosphere thanks to the carbon cycle. The processes in this cycle take anywhere from hours to millions of years and have occurred
many times over the course of Earth’s history (McElroy 2010).
Plants and animals play a central role in the carbon cycle. Atmospheric CO2 is
taken up by plants, which utilize photosynthesis (Figure 1.7) to generate glucose
plus oxygen. Carbon dioxide, in turn, is produced from the metabolism of glucose
in cellular respiration (Equation 1.2). Thus, cellular respiration is the reverse of photosynthesis in terms of the overall production of CO2, although the actual chemical
mechanisms of the two processes are very different.
(CH 2 O)n + O2 → 6H 2 O + 6CO2 + energy
A close look at Figure 1.6 clearly illustrates the problem of fossil fuels in the
overall balance of CO2 in the carbon cycle. A huge amount of carbon was safely
ensconced in the Earth in the form of coal, oil, and natural gas. By combusting this
sequestered carbon pool in our vehicles and power plants, the balance of carbon in
the environment has been significantly altered, producing increasing levels of CO2 in
the atmosphere (from 270 ppm in late 1800 to over 400 ppm today) and contributing
to global climate change (Equation 1.3, Tans and Keeling 2013).
Cn H 2n +2 + O2 → CO2 + H 2 O + energy
Atmospheric levels of CO2 continue to climb with little effective effort by humanity
to reduce or reverse this disturbing trend.
1.2.3 Resource Availability
As we further consider the sustainability of our energy solutions, resource availability is a concern that is amplified by the problem of scale. Several ostensibly
renewable, clean, or sustainable energy solutions rely upon materials whose longterm existence is uncertain. For example, the scarce rare earth elements neodymium
and dysprosium are used in the generators in wind turbines. Lanthanum, another
Chemistry of Sustainable Energy
rare earth, is found in the catalysts used in cracking hydrocarbons to make fuels
from crude oil, and all of these rare earth metals are supplied by only one country—
China (Knowledge Transfer Network 2010). These elements are considered “critical” or “near-critical” in terms of supply risk (Bauer et al. 2011). We will see that the
platinum group metals (Ni, Pd, Pt) are exceedingly important metals in the catalytic
conversion of all kinds of materials in energy production; tons of ore are required
to make just one troy ounce of platinum, palladium, or rhodium. Furthermore,
almost all of the world’s supply of platinum is from two mines in South Africa.
Many other transition metals (ruthenium, osmium, iridium, silver, etc.) are considered “endangered elements” in waning supply due to rapidly increasing use (Stanier
and Hutchinson 2011). Several materials used in solar photovoltaics, too, are of special concern. Figure 1.8 presents some of the elements that are of particular value
in the manufacture of photovoltaics (Chapter 7). The occurrence in Earth’s crust is
shown in ppm in the upper half of the graph; the cost below. Clearly, indium (used
extensively in photovoltaics and electronics) and tellurium (a particularly effective
material in thin-film photovoltaics) are especially scarce; their sustainable use is
But rare earth and transition metals are not the only elements at risk: with increasing population comes the need for additional biomass (for food and, potentially, for
energy production), and phosphorus is essential for biomass production. The current
consumption of phosphorus is not sustainable: like fossil fuels, phosphorus-containing
FIGURE 1.8 (See color insert.) Cost versus availability of materials important in the solar
photovoltaic industry. (Peter, L. M. 2011. Towards Sustainable Photovoltaics: The Search for
New Materials. Philos. Trans. R. Soc. Lond. A, 369(1942): 1840–1856. By permission of the
rock is mined from nonrenewable deposits and used to manufacture, among other
things, fertilizer. As with the other elements mentioned above, as the supplies become
more limited, the cost of extracting these materials increases dramatically. The global
production of phosphorus has been predicted to peak as early as 2035 (Schröder et al.
2010). Recycling or recovery of waste phosphorus is currently limited at best, and large
amounts of phosphorus are lost in runoff from agricultural fields, contributing to eutrification in reservoirs and the infamous “dead zone” of the Gulf of Mexico (Massefski
and Capelli 2012). Given the importance of phosphorus in agriculture, it is imperative
that sustainable phosphorus production and use be achieved (Schröder et al. 2010).
Of course (as we will see in Chapter 2), a huge resource that is of limited and
dwindling availability is fossil fuel. While new methods for the extraction of natural gas and petroleum are unlocking previously inaccessible reserves, fossil fuels
are, ultimately, a nonrenewable resource and their continued use contributes to
global climate change. Furthermore, all of these materials pose a dilemma when
considering their implementation on the global scale that is required to meet our
current energy requirements. However, one substance perhaps more than any other is
especially important when it comes to sustainable development of energy solutions:
water. Water is used in enormous volumes in manufacturing, agriculture, and energy
production. A tempting source of clean energy is hydrogen gas, which can be produced from the “splitting” of water (Chapter 5). However, as the planet warms, the
population grows and water becomes increasingly scarce; there is real concern that
water will become the “new oil,” with ample strife already resulting from competition for this vital resource in arid areas across the globe (Wachman 2007).
In the end analysis, our place in this closed system that is the Earth requires
that we not only attempt to find replacements for at-risk elements and materials but
also reduce our use and develop methods to recycle what we do use. The value of
resources in our waste stream is often higher than what it costs to obtain them in the
first place. But in order to efficiently recycle valuable materials, products must be
designed with recovery in mind, a feature that is not prevalent in today’s disposable
1.3 ENERGY UNITS, TERMS, AND ABBREVIATIONS
Energy value. The amount of energy that can be delivered per unit mass or volume is
an important facet of fuels. Fuels deliver their energy specifically by combustion, in
contrast to other energy sources such as batteries or solar cells. It is relatively straightforward to directly compare the amount of energy contained, say, in a ton of corn cobs
to that in a barrel of oil if we focus on their heat of combustion. As Table 1.3 illustrates, the heat of combustion per gram or mole of substance increases with increasing
molecular weight (c.f. butane, hexane, heptane, and octane), and hydrocarbons release
much more heat per gram or milliliter than alcohols (c.f. ethane/ethanol or butane/
butanol). Thus, octane (a good approximation for gasoline) has a higher heat of combustion—and more energy per gram or milliliter—than ethanol or butanol.
When the energy content of a fuel is reported on a per mass or per volume basis,
it is often referred to as the material’s energy value. Energy value comes in two
forms: the lower heating value (LHV; also known as net calorific value) and higher
Chemistry of Sustainable Energy
Heat of Combustion Data for Various Substances
Heat of Combustion
–1560 (gas phase)
–2876 (gas phase)
Heat of Combustion
(kg ∙ cal/g)
Source: Data from Weast, R., Ed. 1974. CRC Handbook of Chemistry and Physics.
Cleveland, OH: CRC Press, D243–248.
heating value (HHV or gross calorific value). HHV is defined as the amount of heat
released by combusting a specific quantity (in mass or volume) at an initial temperature of 25°C until it is completely combusted and the products have returned to a
temperature of 25°C. The HHV includes the heat released in bringing the vaporized
water back to the liquid state. LHV, on the other hand, is defined as the amount of
heat released (as above) when the products cool to only 150°C. In this case, there is
no heat of vaporization to capture; hence a smaller heating value is obtained. This is
the case, for example, when running a system at high temperatures, as in a boiler. A
comparison of HHV and LHV for various fuels is presented in Table 1.4.
Comparison of Higher (HHV) and Lower Heating Values (LHV)
Natural gas (32°F/1 atm)
Hydrogen (70 MPa)
Source: Data from Boundy, B. et al. 2011. Biomass Energy Data Book. U.S.
Department of Energy/Office of the Biomass Program/Energy Efficiency and
Renewable Energy; Staffell, I. 2011. The Energy and Fuel Data Sheet. http://
works.bepress.com (accessed 13 April 2013).