For much of human history, humans relied on their muscles to do work, whether it was gathering food, hunting animals, tilling a field, harvesting crops, hauling water, or simply getting from one place to another. Maximum sustained human exertion is in the range of 50–100 watts (W), with a reasonable average being about 75 W.1 The rate at which these tasks could be performed, i.e., the time it took to harvest a crop or move one kilometer was constrained by the limits of human exertion. Similarly, many tasks were simply impossible because the energetic demands were beyond human reach, whether it be objects that were too heavy to move or distances too great to travel.
The first significant expansion of energy available to humans was in the form of draft animals: animals domesticated to perform physical tasks, especially moving heavy loads. Horses, oxen, and water buffalo can sustain effort in the range of 400–800 W, with a reasonable average of about 600 W.1 The six- to eightfold advantage in power output of draft animals explains their transformative effect. It takes a fit and strong adult male about 400 hours to till one hectare of land. Give that person an oxen pair, and the time drops to about 65 hours.2 That represents about a sixfold increase in labor productivity in agriculture. The boost in productivity from draft animals came at a significant cost: the energy and land needed to sustain the animals. In the United States in 1910, about one-fifth of all farmland was devoted to feeding working animals.
The waterwheel, oarless sailing ship, and windmill were the first inanimate energy converters to replace the mechanical energy of humans and draft animals on a large scale. Their work capacities dwarfed people and animals and steadily improved over time. The machines had tremendous economic and social impacts:
- considerable labor savings
- large increases in the quantity of production
- viability of entirely new enterprises
- dramatic expansion of international travel (sailing ship)
The sailing ship, the waterwheel, and the windmill were important milestones in human energy history. But wind and water power faced immutable constraints imposed by the nature and distribution of kinetic energy in the wind and flowing water: variability, unpredictability, and sharp geographic gradients.
Many limits to wind water were largely erased by the steam engine. In stationary applications, reciprocating steam replaced waterwheels and windmills in textile mills, saws to cut timber and stone, winding engines used to hoist objects, iron furnaces, rolling mills, and early electric generators, among other applications. In many transportation applications, the steam engine moved people and goods faster, farther, more reliably, and, ultimately, more cheaply than the stagecoach and barge. In addition to productivity improvements and cost reductions, the steam engine allowed economic activity to be located according to criteria other than the availability of an energy source, i.e., the location of suitable sites for waterwheels or windmills.
Beginning in the late 19th century, a series of innovations propelled the capacity of prime members to new heights, and with it economic growth: the steam turbine (beginning 1880s), the internal combustion engine (ICE) (beginning 1860s), and the gas turbine (beginning 1930s). The ICE is a mainstay of ground transportation (people and goods), while the gas turbine made air transport ubiquitous. The steam turbine is the dominant machine in global electricity generation.
The expansion of the human population and economic output went hand-in-hand with changes in the power and number of machines that do useful work. Those same changes increased our capacity to transform the natural world in ways that now undermine our quality of life. The work that produces goods and services also transformed much of the planet’s land surface into human-dominated ecosystems, altered the climate, eliminated species, and produced scores of toxins that harm the health of people and natural systems.
Viewed through this historical lens, the transition to clean energy looks like this: shift existing devices (cars, furnaces) to low or GHG-free fuels where feasible (some sustainably sourced biofuels or gas); expanding the use of well-established power generation in sustainable ways (water turbine, steam turbine using nuclear energy), and deploying new devices (modern wind turbines, solar cells, batteries) at a large scale.
1 Smil, Vaclav, “Energy and Civilization: A History,” The MIT Press, 2018 Link
2 Pimentel, David and Marcia Pimentel, Eds., “Food, Energy, and Society,” CRC Press, 2007 Link