Letters

More Options Offered for Long-Term Energy Solutions

Before one adopts Albert Bartlett's thesis that population growth must be included in addressing issues of energy shortages and carbon dioxide emissions ("Thoughts on Long-Term Energy Supplies: Scientists and the Silent Lie," Physics Today, July 2004, page 53), it may be instructive to consider the other side of the energy coin. Perhaps more important than population growth is individual energy consumption. According to Paul Weisz's article in that same issue (page 47), an average American today consumes 10⁸ kcal of energy per year, about 10 times more than an individual from a developing nation; the same factor holds for CO₂ emission. It makes one ask, Is our energy consumption in the developed world, and especially in the US, really necessary to maintain our quality of life? Neither Bartlett nor Weisz addresses this question.

If all of Earth's population is to expect the same energy consumption as present-day America, clearly the world is in for real problems. In that case, there will always be wars and suffering as one country increases its population and its energy requirements at the expense of the nonrenewable resources of another nation. The recent war in Iraq is a case in point. Unfortunately, Earth's population is already being controlled by these energy-related forces. For example, Iraq's population is 2.5 million less than it would otherwise have been, largely due to a fivefold increase in child mortality there since the implementation of the United Nations–sponsored embargo.¹

I suggest that it is much more effective, and more just, to ask educated individuals in developed nations to give up their sport utility vehicles and turn off the lights when leaving a room than to ask that illiterate farmers in developing nations give up their natural desire for children—or even worse, to bomb them.

The solution to the short-term energy supply may not be as problematic as Bartlett implies if one considers improvements in efficiency and conservation. Worldwide, the present population growth rate is 1.2% per year and the rate is decreasing by 3% per year. If this continues, Earth's population will reach a stable maximum of about 8.9 × 10⁹ in about 250 years. If the developed world—say, one-sixth of the world's population—reduces its individual energy consumption by half and the developing world increases its individual consumption to one-fourth that of the US (Japan's individual consumption is presently half that of America's), so that on average each individual in the world consumes 0.225 × 10⁸ kcal/year, then the worldwide energy demand will not grow, and Weisz's graphs for the estimated reserves of oil, natural gas, and coal indicate that we could live happily for at least another 100 years before having to look for alternative energy resources.

Of course, satisfying this stable population in the long term would depend on our ability to eliminate our reliance on nonrenewable resources such as petroleum and uranium by improving our ability to harvest the Sun directly. I believe that physicists should turn their short-term efforts to improving energy efficiency and, like the farmer, their long-term efforts to making better use of solar energy. Let's leave population control to the social workers and politicians, who actually are doing a good job. A physicist who still feels the urge toward social action could preach energy conservation to the developed world.

On a final, perhaps philosophical note, the fundamental purpose of any life on Earth is to dissipate the free energy incident from the Sun. It is a thermodynamic requirement from which we cannot escape. Whether we do our share by increasing our population or by increasing our individual energy consumption is probably thermodynamically irrelevant, since we are still a long way away from dissipating the 10²² kcal per year Earth receives from the Sun. Perhaps the only real silent lie we physicists are perpetuating is to sometimes neglect this thermodynamic imperative. The inescapable good news, however, is that society will continue to invest at ever increasing rates in science and scientists toward the quickest possible dissipation of that free energy.

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The debate on energy resources and population clearly includes thoughtful and concerned people on all sides. Responses appear to be based on one of two different models.

In one model, population growth is slowing and will soon stabilize at a level at which both birth rate and death rate are low. Technological innovation and resource substitution will ensure that the Malthusian scenario of food and energy shortages, and their resultant malnutrition, disease, and wars need not occur.

The error in this model is that it only applies to countries where women have access to education and employment and need not depend on childbearing for status and security. But much of the developing world has a rapid population growth rate, due to the application of medical technology that limits infant mortality, without the application of contraceptive technology that limits the birth rate. Rapid population growth limits the use of resources to those needed for survival, with little left for education and job training, especially for women. They have no alternative to childbearing as a way of ensuring for themselves a respectable place in society.

In the other model, a high level of education and participation of women in the work force has stabilized the population in the developed countries. They have a low birth rate that balances a low death rate. But that is not happening in underdeveloped countries. In fact the frequent news reports of famines and local wars imply that unsustainable growth is occurring that will result in the Malthusian scenario in which population stability is achieved only with a high death rate to balance the high birth rate.

The error in the second model is that the present situation need not perpetuate itself. Provided globalization continues and productive capacity and resource control shift from the minority to the majority, the Malthusian scenario for the majority could be avoided. This option would also require that women worldwide be given access to family planning technology. It is also essential that women receive education and job training so that they have an attractive alternative in life to that of continual childbearing.

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Readers who responded in the November 2004 issue of Physics Today to the earlier articles on energy and population seem to fall into two main categories: those who believe the population problem is already solved through declining birth rates and those who believe the energy problem is already solved because we have nuclear power and continuing energy efficiency improvements. Both views are falsely optimistic and minimize the tremendous technology development problem we face: to provide sufficient energy for a prosperous world in the 21st century and beyond.

Even with the most dramatic conceivable drop in birth rate, the only way population will decrease sufficiently in coming decades is with a correspondingly dramatic increase in death rate. I am surprised so many physicists seem willing to accept that option.

Nuclear energy has four basic obstacles that may prevent it from ever being scaled up by the factor of 20 to 50 needed to address world energy needs: cost, incompetence, corruption, and waste. No breeder reactor, a technology necessary for nuclear fission to be a long-term solution, has ever been successful in the marketplace. Because each plant has such enormous energy content, staff incompetence, even at reactors billed as inherently safe, can lead to much more serious disasters than for other energy sources. A world filled with breeder reactors would necessarily include large-scale traffic in plutonium; just one criminal in the supply chain could trigger a nuclear catastrophe. And the long-lived accumulative character of nuclear waste justifiably frightens many educated members of the public. Billions of dollars have been spent on nuclear energy research, with little progress on resolving any of these issues at the scale that would be needed.

Energy efficiency improvements can only slightly mitigate the continued growth in world energy demand as developing countries advance. The energy problem we face is immense: In coming decades, trillions of dollars of energy infrastructure will need to be replaced with alternatives of some sort. All the renewable energy options face cost issues, both in production and in transmission and storage, that put them beyond large-scale deployment, at least until significant research investment brings those costs lower. The problem is on the scale of the cold war, but policymakers and the general public are not treating it as such. It is past time that the US Secretary of Energy should be given the same respect, and a comparable budget, as the Secretary of Defense, and be charged with resolving this critical problem for the nation.

Chemistry Nobel laureate Richard Smalley has been speaking on the energy problem around the country; I heard him recently at Brookhaven National Laboratory. His specific suggestion is a "nickel and dime" solution: a gasoline tax of $0.05/gallon and perhaps similar carbon taxes on other fossil fuels, to raise about $10 billion per year for alternative energy research. That's the scale we need, not the miserly $80 million solar energy gets in the current US budget. And physicists and engineers must energetically tackle the critical problems, just as they did 60 years ago for the Manhattan Project. Every year of delay in developing these alternatives further threatens the future well-being of humanity.

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Paul Weisz considers transforming America's energy economy using approximately 1000 square meters of solar cells per capita. That amounts to macroengineering; covering roughly 2.7% of the nation's area would alter radiative equilibrium and could effect climate change.

Earth's albedo is modest, but efficient solar cells reflect even less solar energy than the land they shade, a radiative forcing that can amount to hundreds of watts per square meter. Many cell designs also retard nighttime cooling. Weisz proposes 650 000 km² of photovoltaics in 11 nations alone. Add the rest, and the total is millions. Multiplying hundreds of watts per square meter over millions of square kilometers yields approximately 10¹⁴ W, rivaling the present anthropogenic CO₂ forcing.

This dark side of solar power competes with local efforts, like Los Angeles's Cool Cities Initiative, to limit the heat island effect of simmering expanses of asphalt by making sunlit surfaces lighter, not darker. Pale paving and roofing grow attractively cheaper as oil, electricity, and asphalt prices rise. Few Americans can swing a mortgage on 1000 m² of silicon, but whitewash is universally affordable. Even Senator John Kerry parks his sport utility vehicle on a brilliant white-shell Nantucket driveway, admirably offsetting the albedo deficit of the solar cells atop his yacht.

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In his comments, Paul Weisz concludes that our best hope is for solar cells and advanced nuclear energy. He dismisses wind energy with the assertion that "energy losses due to transmission, supply, and demand fluctuation or conversion to other energies will reduce the actual contribution" from his estimated maximum potential of 3–22 quads of energy per year, which is much less than the 100 Q required to sustain the US lifestyle. I find Weisz's statement illogical because the solar cells and nuclear sources will also require transmission, supply-to-load matching, storage, and conversion. More wind farms are being built than any other electricity-generation facilities because they are now the lowest-cost option. In its last quarterly report, Florida Power and Light noted that its profits from wind energy are enough to cover its losses from nuclear energy. Given that 2% of all solar energy reaching Earth is converted to wind energy,¹ the maximum potential at 30% conversion efficiency is 22 000 Q/yr.

Weisz also dismisses agricultural fuel production on the ground that agriculture currently provides barely more energy than it consumes. However, present agriculture is not trying to be sustainable but to maximize profits given cheap fossil fuel. Agriculture provided sustainable fuels—dung, oils, and wood—for millennia until the energy revolution. We should try to devise innovations to make biomass production sustainable again when fossil fuels are no longer cheap.

Consider urea as a fuel made from air, water, and electricity alone. Produced artificially on a scale of 120 million tonnes per year and also produced biologically, urea is noncorrosive, nonexplosive, essentially nontoxic, and almost nonflammable. Unfortunately, its energy per unit mass is not as great as some authorities require. The Bush administration's FreedomCAR Fuel Initiative set the target at 3.0 kWh/kg. One can do some engineering to recover waste heat from the fuel cell or the combustion engine to drive endothermic reactions needed to extract hydrogen from urea. Still, careful analysis shows that such recovery could provide only about 2.5 kWh/kg.

The key to sustainable energy is to develop a practical fuel system. I assert that the best sustainable fuel is guanidine, CN₃H₅, which provides 4 kWh/kg, or mixtures of guanidine and urea. I propose a combination wherein wind provides the majority of the energy and agriculture and aquaculture provide the carbon, hydrogen, nitrogen, and heat needed to package the energy as guanidine. If guanidine proves to be a practical fuel, then its relatively simple transportation would also solve the transmission, supply-to-load matching, and storage problems for any solar cell or advanced nuclear sources that do arise.

Although a molecule of guanidine contains only five hydrogen atoms, it can effectively store nine by extracting hydrogen from the water recovered from the exhaust. Guanidine is not as safe as urea because it easily reacts with water to form ammonia, but it ships in green containers, an indication that it is in the safest category for transportation. Economic processes for its mass production appear simple.²

To get the energy, consider wind. The best sites for wind farms are mostly over water and far from large consumption sites. However, for guanidine production, the wind farms can be located in the best sites.

The hydrogen for the fuel likely will come from water by electrolysis. This is efficient if the water is at high temperature and in the supercritical state. One will want to convert H₂ promptly to NH₃ via the high-temperature Haber process, then to urea and then to guanidine by moderate pressure–temperature processes.² Wind generators (and solar cells) do not produce much heat. The fuel-producing unit that converts electrical energy in excess of what can be sold immediately into guanidine will need hot N₂, hot CO₂, and hot H₂O.

A simple way to supply these hot gases is to burn organic material. It seems obvious that practical production of guanidine–urea fuels will find symbiosis with agricultural and aquacultural production of biomass fuel.

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Weisz replies: It is good to see Physics Today dedicating space to discussions about the role of fundamental physical laws in providing and constraining the growth of human civilization. The Letters clearly point to the importance of recognizing the vital dependence of human civilization on both the constraining physical laws of nature and the lifestyle choices of human individuals and populations. Broad discussion may be the only effective way to generate insights that can drive remedial actions in social behavior, technology development, sound policy, and honest politics.

My study translated customary numerical energy units into broadly understandable parameters—for example, human lifetimes. Karo Michaelian notes that the average American consumes 10⁸ kcal/yr of source energy, be it solar, fossil, or nuclear, in formal numerical units. As a similar illustration, that Figure is about 100 times a human's own source energy intake—as food at 2700–3000 kcal/day from solar energy—which is about 10⁶ kcal/yr. That person's added energy availability is therefore comparable to that of 100 human helpers. Since a person's biological ability to translate food-energy intake into work energy is 10–15% while external technological work efficiency may well average above 30%, current energy consumption may be considered to be equivalent to the use of some 200–300 "slaves" per capita.

Russell Seitz says that I propose "650 000 km² of photovoltaics in 11 nations alone." On the contrary, my table of data for 11 nations illustrates that the required photovoltaic-cell areas for many countries would be an impossibly large fraction of their total available land areas! Yet for other countries—the US and Australia, for example—the use of photovoltaics for major national energy use appears quite feasible, although it would constitute a large enterprise, or "macroengineering," as Seitz refers to it. Unfortunately, any methodology that will supply energy commensurate with current growth in population and per capita demand will involve macroefforts in technological innovation and social adaptation.

I do not dismiss wind energy as James Van Vechten says. On the contrary, I point out that "wind energy provides a significant potential resource contribution." Numerical estimates of the contribution of wind farms must be adjusted for energy losses in transmission, storage, or conversion technologies. Added energy cost accommodations are necessary because of the very large diurnal and weather-related swings in any solar-derived energy productivity. This is noted and illustrated for solar cells by the shift of the blue area (the nominal productivity at the generation site) to the yellow area (the net contribution) quantified in Figure 5 of the original article (Physics Today, July 2004, page 47).

Van Vechten is clearly searching for a fuel that may be easier and safer to handle than hydrogen. In the context of examining the serious problem of source-energy supply and demand for society, we must recognize that the multiple processes in generating guanidine and its starting materials will consume still greater amounts of existing source energies than will hydrogen production. Many ideas concerning energy technologies are interesting but need a net energy analysis that embraces the positive and negative contributions of all steps of the new system.

Many good ideas regarding social behavior, such as conservation, population control, and peaceful cooperation between providers and consumers, must be appraised in terms of what is accomplishable in a time frame shorter than the rate of source-energy starvation.

All of the Letters and comments appear to agree on the magnitude of the evolving energy supply problem and on the mutual involvement of basic scientific arithmetic and human behavior. The challenge is comparable to the Manhattan Project, as noted by Arthur Smith, but is even larger in magnitude, the requirement of broad scientific understanding and social skills, and the necessity for international participation. Such effort must be guided and sustained by longer-range wisdom, policy, and activity than characterize the lifetime of political appointments. The major task lies in the arena of public understanding: basic education.

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Bartlett replies: Physicists acknowledge that population growth is a major cause of our energy problems. Why then do they offer all manner of diversionary suggestions but avoid addressing population growth limitation as a solution? Karo Michaelian suggests that "perhaps more important than population growth is individual energy consumption." Reducing per capita annual consumption of energy is an important initial step. Reducing it in the US by 1% each year would be a real achievement. But US population growth is about 1.2% per year, so the achievement would not lower total consumption. Our national goal must be to reduce the total annual consumption of nonrenewable energy for many coming years.¹

Michaelian points out that annual per capita energy consumption in the US is 10 times that in developing nations. That fact emphasizes the importance of stopping US population growth, a course of action Michaelian seeks to avoid. How can we ask other countries to stop their population growth unless we are willing to set an example and stop our own?

It would be wrong to ask that "illiterate farmers in developing nations give up their natural desire for children." In accord with Brian Tinsley's call that "women receive education and job training so that they have an attractive alternative" to bearing children, I think we in the US should increase our support for domestic and foreign aid programs in education, economic opportunity, family planning, and maternal health, with the global goal that every child is a wanted child. That aid would cost much less than a war.

The population division of the United Nations, in a report released 24 February 2005, states that "by 2050 the world population is expected to reach 9.1 billion . . . and would still be adding 34 million persons annually." So it is difficult to imagine that the solution suggested by Michaelian would be a happy one with the population growing for another 250 years to almost 9 billion people and with individuals in the developed world consuming energy at twice the rate of those in the developing world.

Social workers and politicians have mostly failed to address the population problem, so it follows that we scientists have the professional obligation to call attention to the fact that population growth is the most important problem humans face. By failing to do this, we are propagating a silent lie.

Michaelian observes that we are a long way from dissipating the energy that Earth receives from the Sun. That is probably less important than a comment I recently received from David Pimentel, a global agricul-tural scientist at Cornell University. Pimentel said that we humans currently appropriate for our own use about half of Earth's net primary production of biomass.

Arthur Smith suggests that "the only way population will decrease sufficiently in coming decades is with a . . . dramatic increase in death rate." There is evidence to the contrary. Fertility rates have dropped dramatically in many parts of the world, and much of Europe is at or near zero population growth.

Smith identifies the problems that must be addressed if nuclear power is to be expanded. In addition there is a political problem, if the citizens of all 50 states vote to prohibit the storage of nuclear waste in their respective states.

I agree with Smith that taxes on energy are needed to reduce consumption and to fund the needed large increases in research on renewable energy. A good first step would be to change the gasoline tax to a sales tax so that tax revenues would rise as gasoline prices rise.

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