Emergy and Power

Emergy and Power

By Elliott Campbell

Power is often referred to as the ability to exert influence and cause impact, in politics, sport, or the economy. In science, power, or work per time is the rate of flow of useful energy. However, power’s physical definition is what creates power as we think of it colloquially. A stronger person will generate more power than a weak one, often winning a sporting contest, a military with more people and machines of war will be able to exert more power in a conflict, and an economy with a larger workforce and industrial capacity will have greater power, driving an economy. While not always immediately obvious, physical power is at the root of productivity both in society and the environment.


The Fourth Law of Thermodynamics - Maximum Power

The laws of thermodynamics explain the immediate behavior of energy in a system; energy is always conserved, energy degrades in any reaction, and entropy is zero at absolute zero temperature. In 1922, Alfred Lotka put forth a proposed fourth law of thermodynamics to further explain energetic behavior in the longer term and at all scales, the maximum power principle (MPP). This proposed law states that a system self organizes to maximize the throughput of available energy, i.e. power.  H.T. Odum (1996) modified the MPP to account for the quality of the available energy flows. The maximum empower principle (MEmPP) explains success in evolutionary competition as the result of the maximization of emergy flow or empower within a system.  As with the other energy laws, the MPP and MEmPP apply to all systems— ecosystems, geologic, human, cosmic, et al.


Net Emergy of Sources and Processes

While power is fundamental to all emergy analyses of the energy flows in a system, for example, the calculation of the transformity of a flow or storage, we often associate the term “net emergy” with energy production processes. This has been a rich area of study in the emergy field, necessary to answer the question of which energy production processes are viable in supporting a complex society. Emergy is a tool particularly suited to the analysis of energy production processes, because it considers the past use of available energy required for the materials, labor and the environmental inputs required for the production of a primary energy source, inputs that are missing from an energy return on investment analysis. In addition, it looks at the net energetic contribution of the energy source, which is missing from economic analyses. H.T. Odum’s seminal works (summarized in Odum 1996 and Odum 2007) started to answer the question stated above. His work in the 1980’s indicated that ethanol produced from corn in the United States has an emergy yield ratio (the principle index of the net contribution of an energy source to the larger system) close to 1, meaning that it has little net benefit to society. If lawmakers had considered these findings, the payment of billions of dollars in wasteful ethanol subsidies could have been avoided.

A recent study (Campbell 2015) compared Odum’s work evaluating the emergy yield of fossil fuels done in the 1980’s and early 1990’s to the yield of fossil fuels produced from oil sands and hydraulic fracturing, finding that the yield ratio has decreased significantly for oil production.  Campbell found that the range of the Emergy Yield Ratio (EYR) for oil had dropped from 8-11:1  to  3-5:1, but the EYR of natural gas had dropped less, going from 6-10:1 to 4-8:1, depending on the nature of the resource and mode of production.

The observed decrease in the EYR indicates that emerging methods of fossil fuel production are providing a smaller net benefit to society, possibly opening the door for renewable energy alternatives to be competitive with fossil fuels. Many emergy studies have been done of renewable energy, starting with the work that H.T. Odum did in the 1980’s such as the aforementioned ethanol work and an early evaluation of solar energy. In his initial study Odum found that energy produced from photovoltaic (PV) cells had a EYR of 0.5, indicating that solar power resulted in a net energy loss to society at that time. PV technology has evolved rapidly and studies that have analyzed these new technologies, have found improved EYR’s (Raugei et al. 2007, Paoli et al. 2008, Brown et al. 2012); however, solar technologies still provide a smaller net benefit than that derived from fossil fuel production. Some renewable energies, e.g. wind, hydroelectric, geothermal, have the potential for a high net yield to society, but these energy sources are spatially concentrated in certain regions and some are intermittant. This indicates that renewable energy sources may not be able to sustain the same macro-scale societal complexity as that which has been possible using fossil energy sources.



Emergy Analysis has the unique ability to holistically evaluate energy production alternatives, guiding society towards methods of production that yield the greatest net benefit to society. In H.T. and Betty Odum’s 2001 book, A Prosperous Way Down, they provide evidence for the inability of renewable alternatives to fully replace the current energy basis, i.e. fossil fuels, of our complex society and suggest policies that will help society transition from a high energy, growth-based economic system to a more sustainable future.



Brown, M.T., Raugei, Marco, Ulgiati, Sergio. 2012. On boundaries and ‘investments’ in Emergy Synthesis and LCA: A case study on thermal vs. photovoltaic electricity. Ecological Indicators, Volume 15, Issue 1, Pages 227–235

Campbell, Elliott. 2015. Emergy analysis of emerging methods of fossil fuel production. Ecological Modelling, Volume 315, Pages 57–68

Odum, H.T. 1996. Environmental Accounting: Emergy and Environmental Decision Making. Wiley. 384 pages

Odum, H.T. and Elisabeth Odum. 2002. A Prosperous Way Down: Principles and Policies. University Press of Colorado. 344 pages

Odum, H.T. 2007. Environment, Power, and Society for the Twenty-First Century: The Hierarchy of Energy. Columbia University Press. 432 p.

Paoli, C, Vassallo , P., Fabiano, M. 2008. Solar power: An approach to transformity evaluation. Ecological Engineering. Volume 34, Issue 3, Pages 191–206

Raugei, Marco, Bargigli, Silvia, Ulgiati Sergio. 2007. Life cycle assessment and energy pay-back time of advanced photovoltaic modules: CdTe and CIS compared to poly-Si. Energy, Volume 32, Issue 8, August 2007, Pages 1310–1318