Cutler J. Cleveland . Energy Quality, Net Energy, and the Coming Energy Transition.  Department of Geography and Center for Energy and Environmental Studies, Boston University

The level of health, food security and especially material standard of living that exists today throughout the world is made possible by the expansive use of fossil fuels. While many take this affluence for granted, a long run view illustrates that the fossil fuel era is relatively new and will last for a relatively short period of time. For thousands of years prior to the Industrial Revolution, human societies were powered by the products of photosynthesis, principally fuel wood and charcoal. Widespread use of coal did not develop until the 18th century, oil and gas not until the late 19th century.

In 1800, the nation was fueled by animal feed, which powered the draft animals on farms, and wood — used for domestic heating and cooking and by early industry.

Wood and animal feed rapidly disappeared when coal became the dominant fuel, the latter due to the introduction of the first tractor in 1911.

The Industrial Revolution transformed the nation’s energy picture, substituting coal for wood on a massive scale.

By the time of World War I, coal accounted for nearly 75% of energy use. But coal’s place as the dominant fuel was fleeting as well.

Oil and natural gas quickly replaced coal, just as coal had replaced wood.

By the 1960s, oil and gas together accounted for more than 70% of total energy use; coal had dropped to less than 20%. Primary electricity has played a small but steadily growing role. Primary electricity refers to electricity generated by hydroelectric, nuclear, geothermal, solar, and other so-called “primary sources. The increase in the share of primary electricity towards the end of the period is due to the rise in nuclear generating capacity.

This long run view of energy raises an important question: what guided these transitions in the past, and to what extent can such information inform us about the impending transition from fossil to renewable fuels?

The transition from one major energy system to the next is driven by a combination of energetic, economic, technological and institutional factors. The energy-related forces stem from the tremendous economic and social opportunities that new fuels, and their associated energy converters, offered compared to earlier ones.

Energy plays a critical role in nature.

All organisms must use energy to perform a number of life-sustaining tasks such as growth, reproduction, and defense from predators. The most fundamental task of all is using energy to obtain more energy from the environment. When energy is used to do useful work, energy is degraded from a useful, high quality state to a less useful low quality state. This means that all systems must continuously replace that energy they use, and to do so takes energy.

This fundamental reality means that Energy Returned on Invested (EROI) and net energy are used to explain the foraging behavior of organisms, the distribution and abundance of organisms and the structure and functioning of ecosystems

For the overwhelming majority of their existence, humans obtained energy from the environment by hunting and gathering.

The EROI for food capture is the caloric value of the food capture to the expenditure of energy in the capture or gathering process.

Natural ecosystems produce enough edible food energy to support hunter-gatherers at densities no greater than one person per square kilometer. Traditional agricultural societies support hundreds of people square kilometer, enabling permanent settlements to grow in size and number. The greater surplus released labor from the land, creating the potential for people to move to urban areas and work in manufacturing and industry.

The economic usefulness of an energy converter is determined in part by its power, the rate at which it converts energy to do useful work.

Humans and draft animals convert energy to work at low power outputs. The energetic limits of people and draft animals set very definite economic and social limits.

The Industrial Revolution erased these limits with the introduction of the steam engine, which had a power output that dwarfed that of muscle power.

The higher power output of the steam engine enabled it to deliver a much large energy surplus than human labor or draft animals.

Given the economic advantage offered by heat engines powered by fossil fuels, it is no surprise that labor and draft animals we rapidly replaced by heat engines once they became available.

The United States’ economy illustrates this transition. In 1850, more than 90% of the work done in the economy was accomplished by human labor and draft animals.

Over the next half-century, engines powered by wood and then coal rapidly displaced the animate converters.

By the 1950s, labor and animals had almost been completely displaced. Of the economic changes driven by the new fuels and machines, one of the most dramatic was the effect on labor productivity. In agriculture, for example, the productivity of labor increased more than 100-fold relative to rates possible prior to the Industrial Revolution. This increase in labor productivity reduced the need for farm labor and workers moved to industrial jobs.

How strong is the connection between energy use and economic growth?

One hypothesis is that the link is weak.  This is because it’s assumed that as fossil fuels become scarcer, their price will rise, which in turn will trigger technological changes and substitutions that improve energy efficiency. Indeed, many believe that the price shocks in 1973-74 and 1979-80 led to the adoption of many new energy efficient technologies. Second, the shift to a service-oriented, dot-com economy will de-couple energy use from economic activity. A dollar’s worth of steel requires 93,000 Btu to produce in the United States; a dollar’s worth of financial services uses 9,500 Btu. Thus, it stands to reason that a shift towards less-energy intensive activities will reduce the need for energy.

A second hypothesis is that the connection between energy use and economic output is strong.   The heat equivalent of a fuel is just one of the attributes of the fuel and ignores the context in which the fuel is used, and thus cannot explain, for example, why a thermal equivalent of oil is more useful in many tasks than is a heat equivalent of coal.

Because of the variation in attributes among energy types, the various fuels and electricity are less than perfectly substitutable in production or consumption. For example, a Btu of coal is not perfectly substitutable with a Btu of electricity; since the electricity is cleaner, lighter, and of higher quality, most people are willing to pay a premium price per Btu of electricity.

Consider incoming solar energy. The land area of the lower 48 United States intercepts 500 times of the nation’s annual energy use. But that energy is spread over nearly 3 million square miles of land, so that the energy absorbed per unit area is very small. Plants, on average, capture only about 0.1% of the solar energy reaching the Earth. This means that the actual plant biomass production in the United States is very small (compared to the overall incoming solar energy).

Power density combines two attributes of energy sources: the rate at which energy can be produced from the source and the geographic area covered by the source. A coal mine in China, for example, can produce upwards of 10,000 watts per square meter of the mine. As the above examples indicate most solar technologies have low power densities compared to fossil fuels.

A low energy and power density means that large amounts of capital, labor, energy and materials must be used to collect, concentrate and deliver solar energy to users.

This makes them more expensive than fossil fuels. The difference between solar and fossil energy is best represented but their energy return on investment (EROI). The EROI for fossil fuels tends to be large while that for solar tends to be low. This is the principal reason that humans aggressively developed fossil fuels in the first place. Fossil fuels have allowed us develop lifestyles that also are very energy intensive. The places that we live, work and shop have very high power densities. Supermarkets, office buildings and private residences in industrial nations demand huge amounts of energy. This very energy-intensive way of living, working, and playing have been made possible by fossil fuels sources that are equally as concentrated. Another quality difference between renewable fuels and fossil fuels is their energy density: the quantity of energy contained per unit mass of a fuel. For example, wood contains 15 Mj per kilogram; oil contains up to 44 Mj per kilogram.

Conclusion

Among the countless technologies humans have developed, only two have increased our power over the environment in an essential way.

Georgescu-Roegen called these Promethean technologies. Promethean I was fire, unique because it was a qualitative conversion of energy (chemical to thermal) and because it generates a chain reaction that sustains so long as sufficient fuel is forthcoming. The mastery of fire enabled man not only to keep warm and cook the food, but, above all to smelt and forge metals, and to bake bricks, ceramics, and lime. No wonder that the ancient Greeks attributed to Prometheus (a demigod, not a mortal) the bringing of fire to us.

Promethean II was the heat engine. Like fire, heat engines achieve a qualitative conversion of energy (heat into mechanical work), and they sustain a chain reaction process by supplying surplus energy. Surplus energy or (net energy) is the gross energy extracted less the energy used in the extraction process itself. The Promethean nature of fossil fuels is due to the much larger surplus they deliver compared to muscle energy from draft animals or human labor.

The energy surplus delivered by fossil fuel technologies is the energetic basis of the Industrial Revolution.