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2 Energy, Technology, and Sustainability

2 Energy, Technology, and Sustainability

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Energy Basics



7



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



8



Chemistry of Sustainable Energy

Human demands (1960–2007)



1.6



Number of the earths needed



1.5

1.4

1.3

1.2

1.1

1

0.9

0.8

0.7

0.6

1960



1965



1970



1975



1980



1985

Year



1990



1995



2000



2005



2010



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.)



Sunlight

Atmospheric CO2



Photosynthesis



Organism

decay



Human and

animal

Organic respiration

carbon



Dead organisms

and waste products



Vegetation

plant

respiration



Root

respiration



Industrial

commercial

residential

automotive

emissions



Human uses



CO2 ocean uptake

Fossil fuels



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.)



9



Energy Basics



6CO2 + 6H2O



hυ (solar energy)



(CH2O)n



6O2 + C6H12O6



Carbohydrates

(chemical energy)



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



(1.2)



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



(1.3)



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



10



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

highly questionable.

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



(ppm)



103



S

Cu



102

10



Sn



1

10–1

10–2

10–3



Zn



Ga



Bi



Cd



In



Se

Te



102

103



($ ton–1)



104

105

106



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

Royal Society.)



Energy Basics



11



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

culture.



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



12



Chemistry of Sustainable Energy



TABLE 1.3

Heat of Combustion Data for Various Substances



Substance



Heat of Combustion

(kJ/mol)



Ethanol

Ethane

1-Butanol

n-Butane

n-Hexane

n-Heptane

n-Octane



–1368

–1560 (gas phase)

–2670

–2876 (gas phase)

–4163

–4817

–5430



Heat of Combustion

(kg ∙ cal/g)

(Temperature, °C)

326.7 (25)

372.8 (25)

639.5 (25)



Density

(g/mL, 20°C)

0.7893



0.8098



0.6603

0.6838

0.7025



995.0 (25)

1150.0 (20)

1302.7 (20)



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.



TABLE 1.4

Comparison of Higher (HHV) and Lower Heating Values (LHV)

HHV



LHV



HHV



MJ/kg

Natural gas (32°F/1 atm)

Propane

Gasoline

Hydrogen (70 MPa)

Crude oil

Ethanol



52.2

50.2

46.5

142.2

45.5

29.8



LHV

MJ/L



47.1

46.3

43.4

120.2

42.7

27.0



39.0

99.8

34.8

5.63

38.8

23.5



35.2

91.9

32.7

4.76

36.8

21.3



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).



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