The Social Importance of Energy Return
Seasoned naturalists observe that animals are quite adept at deciding whether or not a particular food acquisition strategy is worthwhile for them. Over time and even over generations they’ve developed a sense for whether an opportunity to procure food will yield a positive energy return, offering them more Calories as nourishing food than they expend metabolically to acquire those Calories. This is a valuable skill to develop, one that has served living organisms of all kinds well.
Energy return isn’t just meaningful within the context of animal behavior; energy analysts have developed a similar framework to judge the value of fuels manufactured and used within human societies. The energy return of a fuel is calculated as a ratio, with the fuel’s heat output in the numerator and its required energy inputs in the denominator. Fuels with energy returns greater than one yield an energy profit and generate an energy surplus, while those that yield a return less than one are net energy losers.1
Modern industrial nations, with their complex economies, must retain access to fuels that deliver a high enough energy return that they yield substantial energy surpluses. Some of the fuel available in an economy must be diverted into its energy sector to manufacture tomorrow’s fuel, so only surplus fuel not needed by the energy sector is available to produce, transport and power other goods and services, enabling the transactions outside of the energy sector that make up the bulk of economic activity. Fuels that yield high energy returns and deliver substantial energy surpluses can support abundant economic activity, and societies with access to these fuels can build and maintain large, diverse, resilient economies. Societies forced to rely on lower yield fuels, on the other hand, can support little economic activity outside of their energy sectors and will face the trials and tribulations of energy poverty.
Estimates of energy return for individual fuels count two types of energy inputs: direct and indirect. Direct energy includes fuels consumed directly within the fuel’s production system, such as the electricity used to power the refinery that makes gasoline. Indirect energy includes fuels used to manufacture vehicles, machines and other infrastructure, including the energy needed to manufacture, maintain and repair the above refinery. Direct energy is relatively easy to account for, while estimating indirect energy is more tedious and less precise. In most production systems indirect energy use is larger in magnitude than direct energy use, so ignoring it paints an unrealistic portrait of a fuel’s real energetic cost.
Take for instance gasoline and diesel, two transport fuels important the world over. These fuels require about 0.2 units of direct and indirect energy inputs to deliver 1 unit of heat energy, yielding an energy return of 5.2 This means that for every unit of energy we invest to produce gasoline and diesel we get 5 units of energy back as fuel, for a net profit of 4 units of energy. That’s a return on investment of 400 percent, one you won’t likely achieve on the stock market. Given the comparatively high returns associated with most modern fuels, it’s no wonder they serve as the energetic foundation of industrial society.
The gasoline and diesel data presented above were gathered in the mid 1990s, and the energy costs associated with oil exploration, extraction, transport and refining have increased owing to exhaustion of accessible, higher quality resources and increased reliance on less-accessible, lower quality resources. As the energy costs associated with manufacturing gasoline and diesel rise, the energy return these fuels yield will decline. The declining energy return of gasoline and diesel is mirrored in most modern industrial fuels, including natural gas, coal, electricity and even uranium, and as the graphic above illustrates, once a fuel’s energy return declines below about 10, the surplus energy it offers falls precipitously.
What will be the social and economic consequences of this falling energy return? One possibility is that economic productivity outside the energy sector will contract over the coming decades to reflect the falling energy surpluses and the need for more economic transactions to revolve around the manufacture of tomorrow’s fuel. We might attempt to mitigate this by abandoning fuels with falling returns in favor of alternatives with higher and stable returns, but at the moment no such fuels can be adequately scaled up to meet this task. Perhaps people of the future will come to see the high return fuels upon which we built our industrial society as a progress trap, an energy subsidy best left unexploited.3 Or perhaps they’ll simply resent us for using them up. Regardless, the falling energy return of industrial fuels will change the face of modern economies, and the ways we attempt to adapt to those changes will define us as we progress through the 21st century.
- Energy return is also called energy return on invested or energy return on energy invested, and is often abbreviated with the acronyms EROI or EROEI. By convention the energy of sunlight is left out of these analyses, otherwise no fuel could yield an energy return > 1 by virtue of the second law of thermodynamics.
- Data from the USDA/USDOE joint report entitled An Overview of Biodiesel and Petroleum Diesel Life Cycles (Sheehan, et al.). The report focuses on diesel fuel, but gasoline’s energy inputs are similar.
- A progress trap is a condition wherein attempts at solving problems cause worse problems that require even greater effort to solve. The idea comes from Ronald Wright’s excellent book A Short History of Progress.
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