Energy Return on Investment (EROI).

This is the text of a book review that I wrote, and which has just been published online in the journal Science Progress.

Energy Return on Investment: A Unifying Principle for Biology, Economics and Sustainability. CHARLES A. S. HALL, Springer 2017 ISBN 9783319478203; xii + 174 pp; £37.99


The preeminent mathematical physicist, James Clerk Maxwell, famously described energy as being “the ‘go’ of things”. Thus, “energy” is the fundamental, underpinning driver and enabler of all processes in the universe. Since it takes energy to produce energy, in order to survive, animals must derive more of it from the food they stalk and hunt down than they expend in getting it, while to provide food and to serve all the other functions of a complex human society, it is necessary to recover very much more energy, overall, than is consumed in acquiring that energy. Such energy requirements may be gauged from Energy Return on Investment (EROI), the definition of which is deceptively simple: i.e. it is the amount of energy delivered to society, divided by the energy consumed in delivering it (and therefore not available to society for other purposes). As this ratio falls, fewer units of energy are made available for each unit of energy that is consumed in the production process. In the limit, for an EROI of 1:1, there is no net profit, since the amount of energy consumed is equal to that produced, thus rendering the exercise self-limiting and pointless (and for an EROI < 1:1, an energy sink is identified). EROI is a useful metric for determining the viability of an energy source, and we see that unconventional oil sources (e.g. oil sands and oil shale) tend to be more difficult to produce from than their conventional counterparts, and deliver fewer units of energy to society for each unit of energy that is consumed in the production process, i.e. a smaller energy return on investment (EROI). EROI is also related to the “net energy” yield; thus for an EROI of 5:1, 4 units of energy are delivered for each unit consumed in getting it (Einput), and so we can write:

Net Energy = (EROI - 1) x Einput.

The central EROI concept, and its broader ramifications for living systems, future energy production, and economics, are admirably collected together in this book; indeed, there is probably no one better qualified to accomplish such a task than Charles Hall, to whom we are indebted for coining the term “Energy Return on Investment”, although he credits others for the concept, particularly Leslie White, Frederick Cottrell, Nicolas Georgescu Roegan and Howard Odum (Hall’s doctoral advisor). In terms of the impact of EROI on society, Hall notes that the !Kung people, who live a hunter gatherer lifestyle in arid regions of western Africa, achieve an EROI of 10:1, while an EROI of 2.5:1 has been deduced for agriculture during the period 1300-1750. If such a decline in EROI occurred when agriculture was introduced, it is perhaps surprising that the global population more than doubled between 1 AD and the dawn of the industrial revolution in 1750 (i.e. before fossil fuels – initially, coal – were used significantly); however, this can be explained in terms of the large increase in the total amount of energy that could be, thus, captured. It has been estimated that former large scale forest management in Sweden could provide an EROI of 7:1, which was sufficient to support the production of metals, but such inchoate industrialisation was curtailed when the consumption of trees began to exhaust the forests. Indeed, this was the primary limitation for those societies that existed before the industrial revolution, in regard to their extent of development and population. Once a society hit the buffer, set by the energy available to it in the form of biomass, from forest and crops, created by photosynthesis, collapse occurred, and it could only begin to rise again once the natural environment had recuperated sufficiently.

The pivotal moment in social evolution came about with the introduction and exploitation of fossil fuels, which has driven/allowed the human population to more than quadruple during the past 100 years, to a current level of approaching 8 billion. Thus, prior to 1930, an EROI for oil production of, say, 50:1 could be obtained, and it was the ability to tap into such energy-rich sources, and on an enlarging scale, that drove an exponential growth not only in population, but the production/consumption of all other resources that have proved necessary to sustain it: rock phosphate, metals, concrete, water supplies, and energy itself (but, also in pollution and environmental degradation too). Social development has led to a rise in the number of consumers across the world, and so we see that a doubling of the number of humans during the past half century has been accompanied by a trebling in their overall global consumption of energy. EROI can be seen to be a critical factor for maintaining a given level of social development: thus, to merely extract oil needs 1.1:1, to also refine it takes 1.2:1, and to then distribute the fuel, 1.3:1. To run an entire road transportation system (building and maintaining roads, bridges and trucks etc.) needs 3:1; to also grow some grain, to put in the trucks, needs 5:1, and to support a family of workers, 7—8:1 is necessary. If an education system is also desired, this rises to 9—10:1, and if we introduce health care, on top, an EROI of 12:1 is necessary; to also have the arts, and other features of higher civilization, perhaps 14:1 is required (these numbers are approximate, but indicative).

Hence, it is only due to the, relatively sudden, availability of cheap high-EROI energy that an increasingly sophisticated, and global, society emerged. It is, therefore, necessary to maintain EROI above a certain level, if the requirements of industrialised civilization are to be met; however, the EROI of fossil fuels has decreased from the golden days of perhaps 50:1, to less than 20:1, for conventional oil (11:1 in the US), indicating that technological advances in production are unable to keep pace with resource depletion, so that additional decreases in EROI can be expected. An increasing reliance on unconventional sources of “oil”, such as oil shale, oil (tar) sands, (ultra)deepwater drilling, gas/coal to liquids, biofuels, all typically with an EROI of < 10:1, to replace declining conventional oil production, can only further exacerbate the situation.

In regard to economics, Hall notes the existence of an inverse and exponential correlation between EROI and oil price, meaning that the harder (the more energy needs to be input) an oil source is to extract from (lower EROI), the more expensive it is: hence, the increasing necessity to extract from more challenging, unconventional, sources is likely to be reflected in higher oil prices. Furthermore, a lower EROI means that more of the economic activity undertaken by society is diverted to pay for the energy to run its overall economy; thus, there is less “net energy” available to meet the other needs/demands of society, meaning that “growth” becomes restricted, and maintaining its infrastructure, and level of function, more difficult. EROI may be seen to incorporate the counterweighing forces of technological advancement and depletion, and as the ratio falls, the “EROI cliff” is eventually reached, where reductions at values of say > 10:1 have a far less severe impact than those at lower values, where the net energy delivered rapidly plummets.

Much of current global discussion concerns whether prices alone are a sufficient deciding factor for energy policy, and if it is feasible to replace fossil fuels with “renewables”, i.e. solar, wind, and biofuels. Presently, most of the decisions over energy are based on economic analyses made by corporations, but prices are massively influenced by the kind of subsidies that may not exist in a future that is rather different from our present. Hall argues that EROI might provide a more reliable underpinning metric for devising future energy policy. For a society running on an overall EROI of 20:1 or more, only 5% or less, of its economic activity is consumed in obtaining the energy to run it; it is most probable that the reduced rates of economic growth, observed recently, are underpinned by falling EROI, as the energy returns of fossil fuels decrease (i.e. we have picked most of the low hanging fruits, and need to access more challenging and energy intensive fuel sources). We might, therefore, enquire as to what kind of society might be possible in the age beyond fossil fuels? The question is difficult to answer, since variable values of EROI have been estimated (mainly due to differences in the accounting of the input energy – hence my earlier remark that the definition of EROI is “deceptively simple”); however, solar photovoltaics come out at 6-12:1, wind energy, probably 18:1, nuclear power 5-15:1, and corn-based ethanol just about breaking even, at not much more than 1:1 (or even < 1:1, depending on the devil in the detail). On this basis, assuming that an industrial society needs an EROI of at least 10:1, if we are to achieve a low-carbon future, but one in which humanity continues to consume energy at its current rate of 18 TW (568 EJ used in 2017), a testing challenge is presented.

The book is engagingly written, in Professor Hall’s usual style, and presents a wealth of information, on topics such as thermodynamics and what energy really is in practical terms, biology, ecology (Hall’s original background), economics, and the likely prospects for attaining the kind of “sustainable” future that is spoken about widely, but often not given due arithmetical consideration. In conclusion, I thoroughly recommend it, as a source of reference and inspiration, and as a practical guide both to the world that is at hand, and the one we might wish for.

Chris Rhodes.