# Real Prospects for Energy Efficiency in the United States(2010)

## Chapter: Appendix D: Definitions of Energy Efficiency

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Suggested Citation:"Appendix D: Definitions of Energy Efficiency." National Academy of Sciences, National Academy of Engineering, and National Research Council. 2010. Real Prospects for Energy Efficiency in the United States. Washington, DC: The National Academies Press. doi: 10.17226/12621.
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### DDefinitions of Energy Efficiency

The term energy efficiency is used in several ways. The definition perhaps most often used is based simply on how much of a given task or product (be it the heating of a building for a specified time, the miles driven by a car, or the tons of iron smelted) is achieved per unit of energy expended for that task or product. For example, the number of tons of iron, t, that can be recovered from ore per Btu of energy, E, used in the smelting process, t/E, is one possible measure of energy efficiency.

Another definition is based on the total energy, Etot, required to provide a product. According to this definition, the energy efficiency for making a ton of iron would be the tons of iron, t, per Btu of total energy required, including mining, transportation, smelting, and any other input, t/Etot.

Both of the measures of energy efficiency defined above would be termed first-law efficiency (derived from the first law of thermodynamics), being based simply on actual energy use and not taking into account such things as the excess entropy due to the irreversibility of real processes. Hence, in many situations, one may use a second-law efficiency (derived from the second law of thermodynamics), which, instead of energy, uses the free energy, usually the Gibbs free energy, G, where G = H – TS. H is the enthalpy, and H = E + pV, where p is pressure and V is the volume of the system—in this case the volume of the iron produced. T is the temperature and S is the entropy. Because most processes are carried out at constant pressure, enthalpy H is the most appropriate measure, and one uses H rather than energy E. If one wishes to use the second-law efficiency, one simply replaces E, the energy used, with G, the free energy, in the expressions for the first-law efficiency.

Page 316
Suggested Citation:"Appendix D: Definitions of Energy Efficiency." National Academy of Sciences, National Academy of Engineering, and National Research Council. 2010. Real Prospects for Energy Efficiency in the United States. Washington, DC: The National Academies Press. doi: 10.17226/12621.
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Additionally, one other kind of definition of energy efficiency is sometimes used, based on how much the actual process deviates from the thermodynamic limit. According to this definition, a perfect process would have a value of infinity for either its first-law or second-law efficiency; that is, the efficiency would be the tons of iron produced per amount of energy or free energy beyond the thermodynamic limit. Hence, for a perfect process, the denominators in these measures would be zero. No real process achieves the thermodynamic limit, of course, and so no real process has an infinite efficiency according to this last kind of definition.

It is also possible to use a more realistic counterpart of the (preceding) definition based on the comparison with the thermodynamic limit—namely, a comparison based on the most efficient possible process subject to a chosen time or rate constraint. This approach enables the user to compare, for example, the relative advantages and disadvantages (in energy efficiency terms) of higher-capacity but slow processes and lower-capacity but faster processes.

In practice, one very rarely encounters an explicitly stated definition of energy efficiency. Most commonly, people tend to use the very first definition, the amount of a task or product (the heating of a building for a specified time, the miles driven by a car, the tons of ore smelted, and so on) per direct unit of energy required for that task. When a different definition is being used, the user generally specifies which definition is being used. In this report, because the data have been taken from a very wide variety of sources, virtually none of which specified a definition, the panel assumed that the first and simplest definition was intended. This is not to imply that if the panel itself were to derive the efficiencies from primary data that it would use that same definition. The pragmatic course was taken here to allow the analysis to be carried out.

Page 315
Suggested Citation:"Appendix D: Definitions of Energy Efficiency." National Academy of Sciences, National Academy of Engineering, and National Research Council. 2010. Real Prospects for Energy Efficiency in the United States. Washington, DC: The National Academies Press. doi: 10.17226/12621.
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Page 316
Suggested Citation:"Appendix D: Definitions of Energy Efficiency." National Academy of Sciences, National Academy of Engineering, and National Research Council. 2010. Real Prospects for Energy Efficiency in the United States. Washington, DC: The National Academies Press. doi: 10.17226/12621.
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Next: Appendix E: Estimating the Net Costs and Benefits of Energy Savings »
Real Prospects for Energy Efficiency in the United States Get This Book
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America's economy and lifestyles have been shaped by the low prices and availability of energy. In the last decade, however, the prices of oil, natural gas, and coal have increased dramatically, leaving consumers and the industrial and service sectors looking for ways to reduce energy use. To achieve greater energy efficiency, we need technology, more informed consumers and producers, and investments in more energy-efficient industrial processes, businesses, residences, and transportation.

As part of the America's Energy Future project, Real Prospects for Energy Efficiency in the United States examines the potential for reducing energy demand through improving efficiency by using existing technologies, technologies developed but not yet utilized widely, and prospective technologies. The book evaluates technologies based on their estimated times to initial commercial deployment, and provides an analysis of costs, barriers, and research needs. This quantitative characterization of technologies will guide policy makers toward planning the future of energy use in America. This book will also have much to offer to industry leaders, investors, environmentalists, and others looking for a practical diagnosis of energy efficiency possibilities.

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