Energy Limits to Growth Part III: Integrating Energy Sources

A process of elimination

A process for designing the energy system to meet society's future needs must start by recognizing the practical limits and potentials of the available energy sources. Since primary energy sources will be the most crucial ones for meeting those needs, it is important to identify those first, with the understanding that secondary sources will also play their roles, along with energy carriers (forms of energy that make energy from primary sources more readily useful—as electricity makes the energy from coal useful in millions of homes).

We can define a future primary energy source as one that meets, at a minimum, these make-or-break standards discussed above:
It must be capable of providing a substantial amount of energy—perhaps a quarter of all the energy currently used nationally;
It must have a net energy yield of 10:1 or more;
It cannot have unacceptable environmental impacts; and
It must be renewable.

The most cursory examination of our current energy mix yields the alarming realization that about 85 percent of our current energy is derived from three primary sources—oil, natural gas, and coal—that are non-renewable, whose price is likely to trend higher (and perhaps very steeply higher) in the years ahead, whose EROEI is declining, and whose environmental impacts are unacceptable. While these sources historically have had very high economic value, we cannot rely on them in the future; indeed, the longer the transition to alternative energy sources is delayed, the more difficult that transition will be unless alternatives can be identified that have superior economic and environmental characteristics.

Assuming therefore that oil, natural gas, and coal will have rapidly diminishing roles in our future energy mix, this leaves fourteen alternative energy sources with varying economic profiles and varying environmental impacts. Since even the more robust of these are currently only relatively minor contributors to our current energy mix, this means our energy future will look very different from our energy present. The only way to find out what it might look like is to continue our process of elimination.If we regard large contributions of climate-changing greenhouse gas emissions as a non-negotiable veto on future energy sources, that effectively removes tar sands and oil shale from the discussion. Efforts to capture and sequester carbon from these substances during processing would further reduce their already-low EROEI and raise their already-high production costs, so there is no path that is both economically realistic and environmentally responsible whereby these energy sources could be scaled up to become primary ones. That leaves twelve other candidates.

Biofuels (ethanol and biodiesel) must be excluded because of their low EROEI, and also by limits to land and water required for their production. (Remember: we are not suggesting that any energy source cannot play some future role; we are merely looking first for primary sources—ones that have the potential to take over the role of conventional fossil fuels.)That leaves ten possibilities: nuclear, hydro, wind, solar PV, concentrating solar thermal, passive solar, biomass, geothermal, wave, and tidal.

Of these, nuclear and hydro are currently producing the largest amounts of energy. Hydropower is not without problems, but in the best instances its EROEI is very high. However, its capacity for growth in the U.S. is limited. Nuclear power will be slow and expensive to grow. Moreover, there are near-term limits to uranium ores, and technological ways to bypass those limits (e.g., with thorium reactors) will require time-consuming and expensive research. In short, these two energy sources are unlikely candidates for rapid expansion to replace fossil fuels. Biomass energy production is likewise limited in scalability, in this case by available land and water, and by the efficiency of photosynthesis. America and the world could obtain more energy from biomass, and biochar production raises the possibility of a synergistic process that would yield energy while building topsoil and capturing atmospheric carbon. However, more research is needed before biochar production is ready for industrial-scale deployment. Competition with other uses of biomass for food and for low-energy input agriculture will also limit the amount of plant material available for energy production. Realistically, given the limits mentioned, biomass cannot be expected to sustainably produce energy on the scale of oil, gas, or coal.Passive solar is excellent for heating space, but cannot serve as a primary energy source to run transportation systems and other essential elements of an industrial society. It is best used for new buildings; the huge investment that would be required to retrofit or rebuild the existing building stock to maximize the use of passive solar is prohibitive.

That leaves six sources: Wind, solar PV, concentrating solar thermal, geothermal, wave, and tidal—which together currently produce only a tiny fraction of total world energy. And each of these still has its own challenges—like intermittency or limited growth potential.Tidal, wave power and geothermal electricity generation are unlikely to be scalable; although geothermal heat pumps can be used almost anywhere, they cannot produce primary power for transport or electricity grids.Solar photovoltaic power is still expensive. While cheaper PV materials are now beginning to reach the market, these generally rely on rare substances whose depletion could limit deployment of the technology. Concentrating PV (which uses lenses to gather light onto small, highly efficient silicon wafers) is achieving ever-lower costs and ever-higher efficiencies, and could be competitive with coal, nuclear, and natural gas power generation on an installed per-watt capacity basis within just a few years, however the problem of intermittency would remain.With good geographical placement, wind and concentrating solar thermal have good net energy characteristics and are already capable of producing power at affordable prices. These may be the best candidates for non-fossil primary energy sources of the future—yet once again they suffer from intermittency.

Thus there is no single "silver bullet" energy source capable of replacing conventional fossil fuels directly—at least until the problem of intermittency can be overcome—though several of the sources discussed already serve, or are capable of serving, as secondary energy sources.This means that as fossil fuels deplete, and as society reduces reliance on them in order to avert catastrophic climate impacts, we will have to use every available alternative energy source strategically. Instead of a silver bullet, we have in our arsenal only BBs, each with a unique profile of strengths and weaknesses that must be taken into account.

But since these alternative energy sources are so diverse, and our ways of using energy are also diverse, we will have to find ways to connect source, delivery, storage, and consumption into a coherent system by way of common energy carriers.

A common carrier

While society uses oil and gas in more or less natural states (in the case of oil, we refine it into gasoline or distil it into diesel before putting it into our fuel tanks), we are accustomed to transforming other forms of energy (such as coal, hydro, and nuclear) into electricity—which is energy in a form that is easy and convenient to use, transportable by wires, and that operates motors and a host of other devices with great efficiency. With a wider diversity of sources entering the overall energy system, the choice of an energy carrier, and its further integration with transportation and space heating (which currently primarily rely on fossil fuels directly), become significant issues.

For the past decade or so there has been some dispute as to whether the best energy carrier for a post-fossil fuel energy regime would be electricity or hydrogen.1 The argument for hydrogen runs as follows: Our current transportation system (comprised of cars, trucks, ships, and aircraft) uses liquid fuels almost exclusively. A transition to electrification would take time, retooling, and investment, and would face difficulties with electricity storage (discussed in more detail below); physical limits to the energy density by weight of electric batteries would mean that ships, trucks, and aircraft could probably never be electrified in large numbers. The problem is so basic that it would remain even if batteries were substantially improved. Hydrogen could more effectively be stored in some situations, and thus might seem to be a better choice as a transport energy carrier.

Moreover, hydrogen could be generated and stored at home for heating and electricity generation, as well as for fueling the family car. However, because hydrogen has a low energy density per unit of volume, storage is a problem in this case as well: hydrogen-powered airplanes would need enormous tanks representing a substantial proportion of the size of the aircraft. Moreover, several technological hurdles must be overcome before fuel cells—which would be the ideal means to convert the energy of hydrogen into usable electricity—can be widely affordable. And since conversion of energy is never 100 percent efficient, converting energy from electricity (from solar or wind, for example) to hydrogen for storage before converting it back to electricity for final use will inevitably entail inefficiencies.

The problems with hydrogen are so substantial that many energy analysts have by now concluded that its role in the energy future will be limited (we are likely never to see a "hydrogen economy"), though for some applications it may indeed make sense. Industrial societies already have an infrastructure for the delivery of electricity. Moreover, electricity enjoys some inherent advantages over fossil fuels: it can be converted into mechanical work at higher efficiencies than can gasoline burned in internal combustion engines, and it can be transported long distances much easier than oil (this is why high-speed trains in Europe and Japan run on electricity rather than diesel).

And so the primary problems with further electrifying transport using renewable energy sources such as wind, solar, geothermal, and tidal power remain: how to overcome the low energy density of electric batteries, and how to efficiently move electricity from remote places of production to distant population centers.2

Energy storage and transmission

The energy densities by weight of oil (42 megajoules per kilogram), natural gas (55 MJ/kg), and coal (20-35 MJ/kg) are far higher than those of any electricity storage medium currently available. For example, a typical lead-acid battery can store about 0.1 MJ/kg, about one-fifth of one percent of the energy-per-pound of natural gas. Potential improvements to lead-acid batteries are limited by chemistry and thermodynamics, with an upper bound of less than 0.7 MJ/kg.

Lithium-ion batteries have improved upon the energy density of lead-acid batteries by a factor of about 6, achieving around 0.5 MJ/kg; but their theoretical energy density limit is roughly about 2 MJ/kg, or perhaps 3 MJ/kg if research on the substitution of silicon for carbon in the anodes is realized in a practical way. Could other elements achieve higher energy storage by weight? In principle, compounds of hydrogen-scandium, if they could be made into a battery, could achieve a theoretical limit of about 5 MJ/kg. Thus the best existing batteries get about 10 percent of what is physically possible and 25 percent of the demonstrated upper bound.It is possible to store energy in electric fields (via capacitors) or magnetic fields (with superconductors). While the best capacitors today store one-twentieth the energy of an equal mass of lithium-ion batteries, a new company called EEstor claims a ceramic capacitor capable of 1 MJ/kg. Existing magnetic energy storage systems store around 0.01 MJ/kg, about equal to existing capacitors, though electromagnets made of high-temperature superconductors could in theory store about 4 MJ per liter, which is similar to the performance of the best imaginable batteries. Chemical potential energy can be stored as fuel that is oxidized by atmospheric oxygen. Zinc air batteries, which involve the oxidation of zinc metal to zinc hydroxide, could achieve about 1.3 MJ/kg, but zinc oxide could theoretically beat the best imagined batteries at about 5.3 MJ/kg. A more promising choice is to use hydrogen for storage.

Research is moving forward on building-scale systems that will use solar cells to split water into hydrogen and oxygen by day and use a fuel cell to convert the gases to electricity at night.3 However, as discussed above, this technology is not yet economical.4 Better storage of electricity will be needed at several points within the overall energy system if fossil fuels are to be eliminated from it: not only will vehicles need efficient batteries, but grid operators relying increasingly on intermittent sources like wind and solar will need ways to store excess electricity at moments of over-abundance for times of peak usage or scarcity.

Energy storage on a large scale is already accomplished at hydroelectric dams by pumping water uphill into reservoirs at night when there is a surplus of electricity: energy is lost in the process, but a net economic benefit is realized in any case. This practice could be expanded, but it is limited by the number and size of existing dams, pumps, and reservoirs. Large-scale energy storage by way of giant flywheels is being studied, but such devices are likely to be costly. The situation with transmission is also daunting. If large amounts of wind and solar energy are to be sourced from relatively remote areas and integrated into national and global grid systems, new high-capacity transmission lines will be needed, along with robust two-way communications, advanced sensors, and distributed computers to improve the efficiency, reliability, and safety of power delivery and use.

For the U.S. alone, the cost of such a grid upgrade would be $100 billion at a minimum, according to one recent study.5 The proposed new system that was the basis of the study would include 15,000 circuit miles of extremely high voltage lines, laid alongside the existing electric grid infrastructure, starting in the Great Plains and Midwest (where the bulk of the nation's wind resources are located) and terminating in the major cities of the East Coast. The cost of building the wind turbines to generate the amount of power assumed in the study would add another $720 billion, spent over a 15-year period and financed primarily by utilities and investors. However, this hypothetical project would enable the nation to obtain only 20 percent of its electricity from wind by 2024. If a more rapid and complete transition away from fossil fuels is needed or desired, the costs would presumably be much higher.

However, many energy analysts insist that long high-capacity power lines would not be needed for a renewable energy grid system—such a system would best take advantage of regional sources — offshore wind in the the US Northeast, active solar thermal in the desert Southwest, hydropower in the Northwest, and biomass in the forested Southeast. Such a decentralized or "distributed" system would dispense not only with the need for costly high-capacity power line construction but would also avoid fractional power losses associated with long-distance transmission.6

Yet problems remain: one of the advantages of a continent-scale grid system for renewables would be its ability to compensate for the intermittency of energy sources like wind and solar — if skies are overcast in one region, it is likely that the sun will still be shining or winds blowing elsewhere on the continent. Without a long-distance transmission system, there must be some local solution to the conundrum of electricity storage.

Transition plans

As noted above, there is an existing literature of plans for transitioning U.S. or world energy systems away from fossil fuels. It would be impossible to discuss those plans here in any detail. They include plans that include nuclear power7 as well as those that exclude it8. Some see a relatively easy transition to solar and wind9, while others do not10.

The present analysis, which takes into account EROEI and other limits to available energy sources, suggests first that the transition is inevitable and necessary (due to the fact that fossil fuels are depleting and have declining EROEI) and that the transition will be neither easy nor cheap. Further, it is reasonable to conclude from what we have seen that a full replacement of energy currently derived from fossil fuels with energy from alternative sources is probably impossible over the short term, and may be unrealistic to expect even over longer time frames. It is not within the purpose of this study to design yet another detailed transition plan. Such exercises are useful but inevitably depend on somewhat arbitrary assumptions: decisions about how much of a hypothetical energy mix should come from each of the potential sources (wind, solar, geothermal, etc.) depend on projections regarding technological developments and economic trends.

The final plan may consist of a complex set of scenarios, with increasing levels of detail adding to the document's value as an analytical tool; yet all too often real-world political and economic events turn such scenarios into forgotten pipe-dreams.

The actual usefulness of energy transition plans is more to show what is possible than to forecast events. For this purpose even very simple exercises can sometimes be helpful in pointing out problems of scale. For example, the following three scenarios for world energy, which assume only a single alternative energy source using extremely optimistic assumptions, put humanity's future energy needs into a simple but helpful cost perspective.11

Scenario 1: The World at American Standards: If the world's population were to stabilize at 9 billion by 2050, bringing the entire world up to U.S. energy consumption (100 quadrillion Btu annually) would require 6000 quads per year. This is more than twelve times current total world energy production. If we assume that the cost of solar panels can be brought down to 50 cents per watt installed (one tenth the current cost and less than the current cost of coal), an investment of $500 trillion would be required for the transition, not counting grid construction and other ancillary costs—an almost unimaginably large sum. This scenario is therefore extremely unlikely to be realized.

Scenario 2: The World at European Standards: Since Europeans already live quite well using only half as much energy as Americans do, it is evident that a U.S. standard of living is an unnecessarily high goal for the world as a whole. Suppose we aim for a global per-capita consumption rate 70 percent lower than that in the U.S. Achieving this standard, again assuming a population of 9 billion, would require total energy production of 1800 quads per year, still over three times today's level. Cheap solar panels to provide this much energy would cost $150 trillion, a number over double the current world annual GDP. This scenario is conceivable, but highly unlikely.

Scenario 3: Current per-Capita Energy Usage: Assume now that current world energy usage is maintained on a per-capita basis (if people in less-industrialized nations are to consume more, this must be compensated for by reduced consumption in industrial nations), again with the world's population stabilizing at 9 billion. In this case the world would consume 700 quads of energy per year. This level of energy usage, if it were all to come from cheap solar panels, would require $60 trillion in investment—still an enormous figure, but one that might be achievable over time.

Of course, as noted above, all three scenarios are extremely simplistic: on one hand, they do not take into account amounts of energy already coming from hydro, biomass, etc., which could presumably be maintained, and it would not be necessary to produce all needed energy from new sources. But on the other hand, costs for grid construction and electrification of transport are not included. Thus on balance, the costs cited in the three scenarios are if anything probably understated.

The conclusion from these scenarios seems inescapable: unless energy prices drop in an unprecedented and unforeseeable manner, as fossil fuels deplete and are phased out for environmental reasons the world's economy is likely to become increasingly energy-constrained. It is highly unlikely that the entire world will ever reach an American or even a European level of energy consumption, and even the maintenance of current energy consumption levels will require a massive level of investment.

The case for conservation

The problem of how to continue supplying energy in a world where resources are limited becomes much easier to solve if we find ways to proactively reduce energy demand. And that project in turn becomes easier if there are fewer of us wanting to use energy (that is, if population shrinks rather than continuing to increase).

The conclusion that we will probably have less energy to use, though not yet supported by official projections from the International Energy Agency, seems well supported by the analysis here. Fossil fuel supplies will probably decline faster than alternatives can be developed to replace them. New sources of energy will in many cases have lower EROEI profiles than conventional fossil fuels have historically had, and will require expensive new infrastructure to overcome problems of intermittency. Moreover, the current trends for energy demand reduction and for falling investment in new energy supplies (especially from fossil fuels, but other energy sources as well), resulting from the ongoing global economic crisis, are likely to continue for several years. How far will supplies fall, and how fast? Taking into account depletion-led declines in oil and natural gas production, a leveling off of energy from coal, and the recent shrinkage of investment in the energy sector, it may be reasonable to expect a reduction in global energy availability of 25 percent or more during the course of the next 25 years. Factoring in expected population growth, this implies substantial per-capita reductions in available energy. These declines are unlikely to be evenly distributed among nations, with oil and gas importers being hardest hit.

Thus the question the world faces is not whether to reduce energy consumption, but how. Policy makers could choose to manage energy unintelligently (maintaining fossil fuel dependency as long as possible while making both poor choices of alternatives and insufficient investments in them), in which case results will be catastrophic. Transport systems will wither, global trade will contract dramatically, and energy-dependent food systems will falter, leading to very high long-term unemployment and perhaps even famine. However, if policy makers manage the energy downturn intelligently, an acceptable quality of life could be maintained in both industrialized and less-industrialized nations; at the same time, greenhouse gas emissions could be reduced dramatically. This would require:
the re-localization of much economic activity (especially the production and distribution of low-value, bulky items and materials) in order to lessen the need for transport energy12;
the construction of highly efficient rail-based transit systems and the redesign of cities to reduce the need for human transport13;
the retrofit of building stock for maximum energy efficiency (energy demand for space heating can be dramatically reduced through super-insulation of structures and by designing to maximize solar gain)14;
the redesign of food systems to substantially reduce energy inputs15;
a reduction of the need for energy in water pumping and processing through intensive water conservation programs (considerable energy is currently used in moving water, which is essential to both agriculture and human health).16

The goal of all these efforts must be the realization of a steady-state economy, rather than a growth-based economy. This is because energy and economic activity are closely tied: without growth in available energy, economies cannot expand. It is true that improvements in efficiency, the introduction of new technologies, and the shifting of emphasis from basic production to provision of services can enable some economic growth to occur without an increase in energy consumption, but such trends have inherent limits. Over the long run, static or falling energy supplies must be reflected in economic stasis or contraction. However, with proper planning, there is no reason why, under such circumstances, an acceptable quality of life could not be maintained. For the world as a whole, this might entail partial redistribution of energy consumption, with industrial nations reducing consumption substantially, and less-industrial nations increasing their consumption somewhat in order to foster global equity.Societal adaptation to basic limits inevitably raises the question of population. When population grows but the economy remains the same size, there are fewer economic goods per person. If energy constraints effectively impose a limit to economic growth, then the only way to avert continuing declines in per-capita access to economic goods is to limit population by, for example, providing economic incentives for smaller families, access to birth control, and support for poor women to obtain higher levels of education.

Policy makers must begin to see population shrinkage as a goal, rather than an impediment to growth that must be overcome through encouragement of either fertility or immigration.Energy conservation can take two fundamental forms: curtailment and efficiency. Curtailment describes situations where uses of energy are simply discontinued (for example, we can turn out the lights in rooms as we vacate them). Efficiency describes situations where less energy is used to provide an equivalent benefit (an equivalent example would be the replacement of incandescent bulbs with compact fluorescents or LEDs). Efficiency is typically preferred, since few people want to give up tangible benefits, but efficiency gains are subject to the law of diminishing returns (the first 10 percent gain may be cheap and easy, the next 10 percent will be somewhat more costly, and so on), and there are always ultimate limits to possible efficiency gains (it is impossible to light homes at night or to transport goods with zero energy expenditure). Nevertheless, much could be achieved over the short term in energy efficiency across all sectors of the economy. The transition to a steady-state economy will require a revision of economic theories and a redesign of financial and currency systems.17 These efforts will almost certainly be required in any case if the world is to recover from the current economic crisis. Realistic energy descent planning must begin at all levels of society. We must identify essential economic goods (food, water, shelter, education, health care, and so on) and decouple these from meaningless consumption that in recent decades has been encouraged merely to stoke economic growth. Our energy future will be defined by limits, and by the way we respond to those limits. Human beings can live within limits: the vast majority of human history played out under conditions of relative stasis in energy consumption and economic activity; it is only in the past two centuries that we have seen spectacular rates of growth in economic activity, energy and resource consumption, and human population. Thus a deliberate embrace of limits does not amount to the end of the world, but merely a return to the normal pattern of human existence.

If the energy transition is wisely managed, it will almost certainly be possible to maintain, within this steady-state context, many of the benefits that our species has come to enjoy over the past decades—better public health, better knowledge of ourselves and our world, and wider access to information and cultural goods such as music and art.As society adopts alternative energy sources, it will at the same time adopt new attitudes toward consumption, mobility, and population. One way or another, the transition away from fossil fuels will mark a turning point in history as momentous as the Agricultural Revolution or the Industrial Revolution.

1. Jeremy Rifkin, The Hydrogen Economy, (New York: Tarcher, 2002). Joseph Romm, TheHype about Hydrogen: Fact and Fiction in the Race to Save the Climate, (Island Press, 2005).

2. Some Thoughts on the Obama Energy Agenda from the Perspective of Net Energy, The Oil Drum, (2009), http://netenergy.theoildrum.com/node/5073 more

3. Anne Trafton, "'Major discovery' from MIT primed to unleash solar revolution", MIT News, Massachusetts institute of technology (2008), http://web.mit.edu/newsoffice/2008/oxygen0731.html

4. Kurt Zenz House and Alex Johnson, "The Limits of Energy Storage Technology", Bulletin of the Atomic Scientists (January 2009), http://thebulletin.org/web-edition/columnists/kurt-zenz-house/the-limits-of-energy-storage-technology

5. Rebecca Smith, "New Grid for Renewable Energy Could Be Costly", The Wall Street Journal (February 9, 2009), http://online.wsj.com/article/SB123414242155761829.html

6. Ian Bowles, "Homegrown Power" New York Times, March 6, 2009, www.nytimes.com/2009/03/07/opinion/07bowles.html?em

7. Intergovernmental Panel on Climate Change, Climate Change 2007— Mitigation of Climate Change, Working Group III Fourth Assessment Report, http://www.ipcc.ch/ipccreports/ar4-wg3.htm

8. Arjun Makhijani, "Carbon-Free and Nuclear-Free: A Roadmap for U.S. Energy Policy", Science for Democratic Action, Institute for Energy and Environmental Research (August 2007), http://www.ieer.org/sdafiles/15-1.pdf

9. Rocky Mountain Institute http://ert.rmi.org/research

10. Ted Trainer, Renewable Energy Can Not Sustain a Consumer Society, (Dordrecht NL: Springer, 2007).

11. Praveen Ghanta, "How Much Energy Do Need", post to True Cost, February 19, 2009, http://truecost.wordpress.com/2009/02/19/how-much-energy-do-we-need/

12. Jason Bradford, "Relocalization: A Strategic Response to Climate Change and Peak Oil", post to The Oil Drum, June 6, 2007, http://www.theoildrum.com/node/2598

13. Richard Gilbert, Transport Revolutions: Moving People and Freight Without Oil Earthscan, 2008).

14. Passive House Institute, "Definition of Passive Houses", http://www.passivhaustagung.de/Passive_House_E/passivehouse_definition.html

15. PostCarbon Institute and The Soil Association, The Food and Agriculture Transition (to be published 2009), Richard Heinberg, "The Food and Farming Transition", MuseLetter #199, Global Public Media, http://globalpublicmedia.com/museletter_199_the_food_and_farming_transition

16. Ben Block, "Water Efficiency Key to Saving Energy, Expert Says," Worldwatch Institute website, February 11, 2009, http://www.worldwatch.org/node/6007. California Energy Commission, California's WaterEnergy Relationship, CEC7002005011SF (November 2005), http://www.energy.ca.gov/2005publications/CEC-700-2005-011/CEC-700-2005-011-SF.PDF

17. Herman Daly and Josh Farley, Ecological Economics: Principles and Applications, (Island Press, 2003) chapter 14.

De Richard Heinberg, autor de vários livros como Peak Everything: Waking Up to the Century Of Declines, The Oil Depletion Protocol: A Plan to Avert Oil Wars, Terrorism and Economic Collapse e Powerdown: Options and Actions for a Post-Carbon World.