Economists are fond of saying that “there is no such thing as a free lunch”. This adage applies to the harnessing or extraction of energy resources, whether they are nonrenewable – such as fossil fuels – or renewable sources like solar and wind.

Put simply, it takes a certain amount of energy input to produce a flow of energy output. For example, energy is consumed in the process of exploring and drilling for oil, just as it is consumed in the process of manufacturing and erecting wind turbines that produce electrical energy. The net energy gain – or energy surplus – is the amount of energy output minus the amount of energy input used directly or indirectly in the production process (including energy embedded in machinery). A closely related concept is the energy return on investment (EROI), which is the ratio of energy output to energy input. The larger the ratio, the more bang one gets for one’s buck.

The EROI and net energy are, argu- ably, among the most important variables underlying economic performance and societal complexity.

This pair of variables is influenced by a variety of factors across both time and space. One determinant is the quality or level of concentration of the energy resource. For instance, it takes less energy to produce oil from conventional onshore fields than from deep-water offshore wells, tar sands or oil shale. Another factor is the sophistication of the technology used in the production process, which may improve over time. An example are improvements in photovoltaic solar cell design, which raise the efficiency with which sunlight is converted into electricity.

The main reason fossil fuels – coal, oil and natural gas – have been dominant over other energy sources is that they have his- torically delivered a relatively high EROI and a massive energy surplus. But depletion gradually erodes the EROI of these finite resources, since the largest and most accessible oil and natural-gas fields were typically discovered and exploited earlier.

New York University’s Professor Charles Hall and his colleagues have estimated the EROI for a wide variety of energy sources.

Calculations show that the EROI for oil in the US declined from 100:1 in the 1930s to 30:1 in the 1970s and around 15:1 in 2010. The EROI for global oil and gas is also on a declining trend and currently stands at about 18:1.

Unconventional oil resources like tar sands and oil shale may be huge, but the EROI is estimated at less than 5:1, which partly explains their comparatively high production costs.

Estimates of the EROI for nuclear electricity are highly variable, depending on which steps in the production chain are included. Hall and his colleagues regard a realistic range as being between 5:1 and 15:1.

The EROI for hydroelectricity can be over 100:1 in favourable locations, but its geographical extent is often limited, which caps the amount of surplus energy available. Wind power has a very competitive EROI, averaging 18:1, as a result of improvements in the efficiency of turbines. Tidal range energy, wave power and ocean current power technologies are still in their infancy and, thus, robust EROI estimates are not available.

Solar photovoltaic electricity has an EROI of about 6.8:1, while that for concentrating solar power may be even lower. However, a big advantage of solar energy is the massive resource base and potential energy surplus. With continuing technological developments, the prospects are steadily improving for solar power.

The EROI for biofuels depends on several factors, including the type of feedstock, the farming methods used and the favourability of soil and climatic conditions for plant growth. Maize-based ethanol in the US has a ratio that is very close to 1:1 and the industry has survived on subsidies. In contrast, ethanol derived from sugar cane in Brazil has an EROI in the region of 8:1. Biodiesel typically has a ratio somewhere between these values.

We can summarise with two points. First, the net energy yield of fossil fuels has historically been much higher than that of most other energy sources, including nuclear power and many renewables. Second, the EROI and energy surplus yielded by finite fossil fuels and uranium-based nuclear power has been declining over time as a result of resource depletion, while the EROI for many renewable energy sources is rising as technology improves.

The race between resource depletion and technological progress is on. The future path and complexity of human civilisation depends on the force that wins.

Our society faces the colossal challenge of rapidly developing alternative energy sources that generate sufficient surplus energy to replace fossil fuels. Otherwise, material standards of living will decline – beginning with those of poorer people – as ever more resources have to be devoted to generating useful energy rather than to producing other goods and services.

EROI figures indicate that the future lies in renewables like wind and solar, not unconventional hydrocarbons.

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