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3 Energy Units, Terms, and Abbreviations
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).
Comparative Data for the Energy Density of Various
Source: Data from Sørensen, B. 2007. Renewable Energy Conversion,
Transmission & Storage. Burlington MA: Academic Press,
262; Staffell, I. 2011. The Energy and Fuel Data Sheet. http://
works.bepress.com (accessed 13 April 2013); Waldheim, L.
and T. Nilsson. 2001. Heating Value of Gases from Biomass
Gasification, Nyköping, Sweden: IEA Bioenergy Agreement
Subcommittee on Thermal Gasification of Biomass.
Energy density values (Table 1.5) can be used in certain situations to provide an
approximate comparison (Waldheim and Nilsson 2001; Sørensen 2007; Staffell 2011).
In general, the value reported as “energy density” is the same as the material’s LHV.
These data have been collected from a very wide variety of sources with the result
that there is almost invariably some disparity in reported values; therefore, the information presented in these tables is relative but provides some interesting insights. For
example, while H2 has the highest energy density by mass, its energy density by volume is (as expected) abysmal. Another application of energy density is in the realm of
energy storage (Section 6.10), where it refers to the ratio of the mass of energy stored
to the volume of the storage device.
Energy units and abbreviations. One of the more exasperating aspects of working in an energy-related field is the panoply of units, abbreviations, and equations.
The SI units we are familiar with (joules, watts, etc.) are not necessarily the units
used in industrial applications, and of course the use of SI units is, unfortunately,
not universal. What is worse is that many units are specific to a certain kind of
industry. Furthermore, a calorie is not necessarily a calorie: its measurement is temperature dependent and “1 calorie” may be 4.18674 J, 4.19002 J, or 4.18580 J depending upon what particular calorie one is referring to (the international steam table
calorie, the mean calorie, or the thermochemical calorie) or even the particular
country that is reporting the value. The familiar BTU is similarly afflicted. The
International Energy Agency maintains a unit converter (http://www.iea.org/stats/
unit.asp) as does the U.S. Energy Information Agency (http://www.eia.gov/energyexplained/index.cfm?page=about_energy_conversion_calculator) and Appendices
Chemistry of Sustainable Energy
I through IV provide most of the important conversion factors, SI prefixes, and
energy-related equations. Most global energy statistics are given in units of toe—
tonnes of oil equivalent. As one might expect, a tonne of oil equivalent is the amount
of thermal energy released when a metric tonne (1000 kg) of oil is combusted, a
value that is standardized at 41.868 GJ. The world consumption of final energy has
increased from roughly 4000 to over 12,000 Mtoe from 1971 to 2011—a truly staggering 5 × 108 terajoules (TJ) of energy (British Petroleum 2012; IEA/International
Energy Agency 2012).
1.4 ELECTRICITY GENERATION AND STORAGE
Energy use can be arbitrarily broken down into major uses: fuels for transportation,
energy for manufacturing, energy for heating/cooling systems, and finally energy for
our smartphones, computers, televisions, and so on. It is in the latter category that electricity plays a large role with at least 40% of the energy in the United States being used
to generate electricity (Girard 2010). Globally, about 40% of electricity use is industrial
(IEA/International Energy Agency 2012) with the world’s total final consumption of
electricity in 2010 reaching 1536 Mtoe—well over 17,800 terawatt-hours. Fossil fuels
figure prominently in the production of electricity, with coal alone providing about
40% of the world’s electricity (Figure 1.9) (Reisch 2012). As a result, roughly one-third
of global CO2 emissions comes just from electricity generation (Milne and Field 2013).
Because this most convenient of final energies plays such a large role in humanity’s
energy use, it is important to understand electricity and how it is made.
Electricity generation by fuel (world)
1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009
Biofuels and waste
FIGURE 1.9 World electricity generation by fuel (GWh). (Adapted from IEA/International
Energy Agency. 2012. 2012 Key World Energy Statistics. Paris, France, International Energy
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.)