This closing chapter offers no forecasts; there is no need to add to the large, and growing, volume of that highly perishable commodity. Reviews show that most long range (more than ten to fifteen years ahead) energy forecasts – whether at sectoral, national, or global level, and no matter if they were concerned with the progress of individual techniques, the efficiency gains of a particular process, overall energy demand and supply, or the price levels of key commodities – tend to fail in a matter of years, sometimes months. Given the post-World War II penchant for long-range forecasting, it is now possible to recite scores of such failures. Perhaps the most tiresomely notorious is the ever-elusive further fifty years that will be needed to achieve commercial nuclear fusion (generating electricity by fusing the nuclei of the lightest elements – the same kind of reactions that power the Sun). Common failures include forecasts of the imminent global peak oil production, and some of the most spectacular misses include the predictions of future crude oil prices (too high or too low, never able to catch the reality of highly erratic fluctuations).
Even if some individual numbers come very close to the actual performance, what is always missing is the entirely new context in which these quantities appear. Imagine that in 1985 (after the collapse of crude oil prices and a sharp drop in global oil production), you accurately forecast global oil production in 2015. Could anybody in 1985 have predicted the four events that changed the post-1990 world: the peaceful collapse of the U.S.S.R. (first leading to a rapid decline and then to an impressive resurgence of its oil output); the emergence of China as the world’s largest (in terms of purchasing power parity) economy (and also the world’s largest importer of oil); September 11, 2001 (with its manifold consequences and implications for the world in general, and the Middle East in particular); and the worst post-World War II global economic downturn in 2008-2009?
No forecasts then – only brief reviews of some key factors that will determine the world’s future quest for a reliable and affordable energy supply, and the major resource and technical options we can use during the next half-century. During that time, the basic nature of global energy supply will not drastically change, and the world will remain highly dependent on fossil fuels. At the same time, we know our fossil-fueled civilization to be a relatively short-lived phenomenon, and the next fifty years will see an appreciable shift toward non-fossil energy resources. At the beginning of the twentieth century, the world derived about sixty percent of its energy from coal, crude oil, and (a very little) natural gas. In 2015 eighty-two percent of the world’s total primary energy came from fossil fuels, with the rest supplied by primary electricity (hydro and nuclear) and phytomass fuels.
Even if the recoverable resources of fossil fuels (particularly those of crude oil and natural gas) were much larger than today’s best appraisals, it is clear they are not large enough to be the dominant suppliers of energy for an affluent civilization for more than a few centuries. Conversely, the combination of rapidly rising demand and the escalating costs of fuel extraction may limit the fossil fuel era to the past and present centuries – and the rapid progress of pronounced global warming, clearly tied to the combustion of fossil fuels, may force us to accelerate the transition to non-fossil energies. As already stressed in Chapter 1, the overall magnitude of renewable energy flows is not a constraint.
Biomass energies have been with us ever since we mastered the use of fire: wood, charcoal, crop residues, and dung are still used by hundreds of millions of peasants and poor urban residents in Asia, Latin America, and particularly throughout sub-Saharan Africa, mostly for cooking and heating. Our best estimates (there are no reliable statistics, as most of these fuels are collected or cut by the users themselves) put the worldwide energy content, at the beginning of the twenty-first century, of traditional biomass energies at about 40EJ, roughly eight percent of the world’s aggregate primary energy consumption. But the share is much lower when comparing useful energies, because most of the biomass is burned very inefficiently in primitive stoves. As already noted in Chapter 3, these wasteful uses also have considerable health costs, due to indoor air pollution, and there are also the serious environmental problems of deforestation and the reduced recycling of organic matter. Biomass energies could make a difference only when harnessed by modern, highly efficient techniques without serious environmental and social impacts: achieving this will be an enormous challenge.
Hydroenergy is the only kind of indirect solar energy flow extensively exploited by modern techniques, but, outside Europe, North America, and Australia, there is still considerable untapped potential – but also a growing opposition to environmental impacts of large hydro projects. As already emphasized, potentially the most rewarding, and by far the largest, renewable energy resource is the direct solar radiation that brings close to 170 W/m2 to the Earth. Post-2000 development of PV electricity generation has been growing exponentially, as has the installation of wind turbines. There is also the possibility of new designs of inherently safer and more economic nuclear electricity generation. I will review the advantages and drawbacks of all of these major non-fossil options. But before doing so I must stress the magnitude of future energy needs against the background of enormous consumption disparities and long-term energy transitions.
Energy needs: disparities, transitions, and constraints
The extent of future global energy needs cannot be understood without realizing the extent of existing consumption disparities. The per caput annual energy consumption in the U.S. and Canada is roughly twice as high as in Europe or Japan, more than three times as high as in China, more than fifteen times as high as in India, and more than thirty times as high as in the poorest countries of sub-Saharan Africa. Because of this highly skewed consumption pattern, the global annual average of about 75 GJ is largely irrelevant: global distribution of average consumption rates is bi-modal, with most of the rates for low-income countries below 30 GJ, and with high-income countries average above 150 GJ.
UNEQUAL ACCESS TO MODERN ENERGY
The enormous disparity in access to energy is most impressively conveyed by contrasting continental shares of the global population with their corresponding share of worldwide primary energy consumption: Africa, with about seventeen percent of the global population, consumed about four percent of all primary commercial energy in 2015; North America and Europe, with a nearly identical combined shares of global population, consumed almost forty percent. The most stunning contrast: the U.S. alone, with less than five percent of the world’s population, claims seventeen percent of the world’s primary commercial energy.
No indicator of high quality of life – very low infant mortality, long average life expectancy, plentiful food, good housing, or ready access to all levels of education – shows a substantial gain once the average per caput energy consumption goes above about 100 GJt/year. Consequently, it would be rational to conclude that the world’s affluent nations have no need to increase their already very high averages. At the same time, there are still hundreds of millions of people in the poorest countries who do not directly consume any fossil fuels and whose energy consumption must rise (Figure 29).
Because almost all the world’s population growth during the first half of the twenty-first century will take place in low- and medium-income countries (affluent populations, with the exception of the U.S., will either be stagnant or in decline), most future increases in fossil fuel and electricity consumption will be in Asia and in Africa. But there is no easy way to forecast this new demand, as it is a complex function of population growth and economic expansion, the changing composition of the primary energy supply and final energy uses (energy-intensive heavy industries compared to light manufacturing and service industries), and the adoption of new inventions and more efficient energy converters.
Figure 29 Where people need more energy: conditions in this Jakarta slum are replicated in scores of low-income countries (reproduced from Wikipedia)
To achieve a modicum of economic security, the average annual per caput consumption rates should more than triple in sub-Saharan Africa, and more than double in India. It is clear that future energy use in the world’s most populous and rapidly expanding economies will conform (with variations for national characteristics) to the general pattern of energy transitions that has taken place in affluent countries. Their two principal components were noted in Chapter 4: the declining share of coal in total primary consumption (although in China that share will remain relatively high, because of its extraordinary dependence on the fuel), and a steady rise of oil and natural gas consumption, leading to higher demand for imported liquids and gases.
Another key ingredient of the worldwide transition of commercial energy use has been the rising share of electricity in final consumption. In 1900, less than a generation after the beginning of electricity generation, little more than one percent of fossil fuel consumption was converted to electricity; by 1950 the global share rose to ten percent and by 2015 it reached thirty percent. Nearly everywhere, electricity use has been growing at a much faster rate than the consumption of fuels, because during the second half of the twentieth century fossil-fueled generation was extensively augmented by hydroenergy and nuclear fission.
The continued rapid growth of average per caput electricity consumption in low- and medium-income economies will be the only way to narrow the existing disparities. In 2015 the U.S. annual per caput average was more than 13 MWh, Japan’s was more than 8 MWh, and Europe averaged about 7 MWh. In contrast, China’s annual per caput mean was less than 4.5 MWh, India’s just above one MWh, and in sub-Saharan Africa (excepting South Africa) it remains well below one MWh. Despite decades of electrification programs, more than 1.2 billion people (mainly in India, Southeast Asia, and sub-Saharan Africa) still do not have access to electricity. Consequently, the global disparity in average per caput electricity use is greater than in the case of primary energy consumption, and the need for future production increases is more acute. This is perhaps most vividly portrayed by composite night-time satellite images, which starkly contrast brightly-lit affluent countries with the huge areas of darkness, or at best sparse light, over large parts of Asia, Africa, and Latin America.
While the overall efficiency of energy use in low- and medium-income countries is dismally low and should be greatly improved through technical innovation and better management, future higher energy needs cannot be met solely, or even mostly, through higher efficiency. Positive steps in this direction are essential. China’s post-1980 achievements, more than halving the energy use per unit of Gross Domestic Product (GDP) show their fundamental importance: without them, China would be now consuming more than twice as much energy for every unit of its economic product as it does, as well as burdening its already highly degraded environment with even more pollutants. High-efficiency conversions clearly benefit economies and the environment, but they reduce overall energy use only on an individual or household level, or for a single company, particular industrial process, or entire production sector.
On national and global levels, the record shows the very opposite; there is no doubt that higher efficiencies of energy conversions have led to steadily greater consumption of fuels and electricity. This paradox was noted for the first time by Stanley Jevons (1835–1882), a prominent English economist, in 1865. In his words: “It is wholly a confusion of ideas to suppose that the economical use of fuels is equivalent to a diminished consumption. The very contrary is the truth.” Jevons illustrated the phenomenon by contrasting the huge efficiency improvements of eighteenth-century steam engines (from Savery’s and Newcomen’s extremely wasteful machines to Watt’s improved design) with the large increases in British coal consumption during the same period.
Two examples illustrate this common phenomenon for modern energy-consuming activities. First, in the year 2000, the average American passenger vehicle (including SUVs) consumed nearly forty percent less fuel per kilometer than in 1960, but more widespread ownership of automobiles and the higher annual average distance driven (roughly 17,000 km, compared to 15,000 km in 1960) resulted in an increase of average per caput consumption. Second, during the twentieth century, the efficiency of British public street lighting rose about twenty-fold, but the intensity of this illumination (MWh per km of road) rose about twenty-five times, more than eliminating all efficiency gains. As a result, average per caput energy consumption continued to rise, albeit at a slower pace (as expected), even in mature, post-industrial economies.
During the 1990s, despite deep economic problems and the stagnation of its GDP, Japan’s average per caput energy consumption grew by fifteen percent; in the same period the already extraordinarily high U.S. and Canadian rates grew by about 2.5 percent, and France’s by nearly ten percent. But things have changed since the year 2000, and in 2015 the average per caput energy consumption in all major Western economies (U.S., Canada, Germany, France) and in Japan was either slightly below, unchanged or only marginally above the 1990 rates: affluent economies might be, finally, reaching their energy consumption plateaux. And affluent countries could reduce their energy use by a third and still remain well-off. That is not the case with lower income economies: since 1990 per caput energy consumption has soared in China and it has grown, at slower rates, in India as well as in the relatively best performing economies in Africa – and without further substantial gains those nations could never modernize. But they now face the entirely new constraint of rapid global warming, something that modernizing Europe, North America and Japan did not have to worry about in the past.
We have three choices if we wish to keep on increasing global energy consumption while minimizing the risks of anthropogenic climate change due to rising combustion of fossil fuels and keeping atmospheric levels of greenhouse gases from rising to as much as three times their pre-industrial level: we can continue burning fossil fuels but deploy effective methods of sequestering the generated CO2, we can revive the nuclear option, or we can turn increasingly to renewable energy. None of these options is yet ready to take over from fossil fuels on requisite scales, none could be the sole near-term solution, and all have their share of economic, social, and environmental problems.
Despite a great deal of theoretical research, and much interest shown by industries and governments, CO2 sequestration is only in early deployment stages. In 2015, fifteen sequestration projects were in operation and more than twenty under construction or in planning stages: if all of them were working by 2020 they would be removing an equivalent of a mere 0.2 percent of 2015 emissions. Even if that were followed by rapid construction of new projects, the near-term effect will be marginal and the eventual contribution of these techniques to the management of the global warming challenge remains uncertain. In contrast, we have more than half a century of experience of large-scale commercial generation of nuclear electricity, which has shown us what to avoid and what techniques to favor. The general expert consensus is that any development of the nuclear industry cannot be a replica of the first generation; there has been no shortage of new, ingenious designs aimed at minimizing or eliminating the concerns that contributed to the stagnation (and in some countries even retreat) of nuclear electricity generation. Several, so-called, inherently safe nuclear reactor designs provide passive guarantees of fail-proof performance: even operator error could not (as it did in Chernobyl) lead to a core meltdown. Large-scale adoption of nuclear reactors would be made easier by flexible sizing that would make it possible to deploy small modular units. But the future of the industry will not depend primarily either on better designs (they have been available since the mid-1980s), or on the fears of catastrophic accidents (risk that must be compared with hundreds of safely operating reactors and with risks of other energy conversions). What has to change is the public acceptance of this potentially risky but very rewarding form of electricity generation, and I have argued that there is little chance of any substantial worldwide return to nuclear generation unless it will be led by the world’s largest economy. But in 2017, U.S. nuclear plans seem no less confused and uncertain than they were ten or twenty years ago: there is no true strategic planning, and no sign that the public distrust of nuclear generation has eased. As the endless wrangling about the location and operation of the country’s permanent repository of high-level radioactive wastes shows, the combination of executive intents, legislative delays, and legal appeals makes for decades of irresolution and offers little hope for any determined state-sanctioned nuclear-based solution to the country’s future electricity needs.
All that may change, but not because the public finally appreciates the real relative risks of various electricity-generating options, as these have been known for decades, but because of the need for an accelerated decarbonization of the global energy supply required to deal with exceptionally rapid global warming. The nuclear option is not greenhouse gas free: we need coke to make the plant’s many steel components, and the cement for its massive concrete structures comes from fossil fuel-fired kilns. But in comparison with today’s dominant (coal-fired) mode of generation, nuclear plants produce at least ninety-five percent less CO2 per unit of electricity. If our civilization were to face a true global warming shock, this would be very appealing. Consequently, the most rational strategy of future energy supply would be to combine improvements in conversion efficiency (particularly in industrialized economies) with reduced rates of overall energy demand (especially in affluent countries), keep the nuclear option open during the development of innovative reactor prototypes, and increase the contributions of non-fossil sources as quickly as economically feasible and environmentally acceptable. Because capital investment considerations and infra-structural inertia mean that it takes several decades for any new energy source or conversion to claim a substantial share of the market, we should not waste any time in aggressively developing and commercializing suitable renewable options.
Renewable energies: biomass, water, wind, solar
Biomass energies could only become an important component of future energy supply after the development of large-scale, intensive production of selected crop and tree species convertible, by advanced techniques, into liquid or gaseous fuels or electricity. This strategy has three fundamental drawbacks. First, as explained in Chapter 2, photosynthesis operates with an inherently very low power density, and hence any large-scale biomass fuel production would claim extensive areas of farmland (and it would have to be farmland, rather than marginal land, to sustain high productivity). Second, humanity already claims a very high share (most likely close to one-fifth) of the biosphere’s net primary productivity (through harvests of food, feed, wood, grazing, and deliberately set grassland, and forest fires), and adding a further burden through massive fuel production would lead to a further loss of biodiversity and greater environmental degradation. Finally, the overall costs (economic, energetic, and environmental) of large-scale biomass energy production are very high.
Low power densities translate into very large land requirements (Figure 30). The U.S. has been diverting about forty percent of corn, its largest crop, to ethanol fermentation but the produced fuel is an equivalent of less than ten percent of the country’s gasoline consumption. Moreover, very few countries have enough farmland to even contemplate such mass-scale biofuel production. For example, replacing just a quarter of the world’s 2015 fossil fuel consumption with cultivated woody biomass would require (even with high average yields of 15 t/ha) tree plantations of more than 500 million hectares, roughly equal to the total combined forested land in Europe, the U.S. and Canada, clearly an impossible option. Devoting even limited areas to biomass crops would be irrational for the scores of densely populated countries that already have shortages of the arable land needed to secure their basic food supply and so are major food importers. Creating new biomass plantations would lead to further loss of natural grasslands, wetlands, and the lowland tropical forests.
Figure 30 Comparison of power densities of energy consumption and renewable energy production
Moreover, modern liquid biofuels (required to displaced fuels refined from crude oil) have very low EROI (ranging from less than 1.5 to less than five) due to the combined energy cost of machinery, fertilizers, irrigation, and biomass conversion. But even the cultivation of those biomass crops that produce relatively high net energy gains would still have undesirable environmental impacts, above all increased soil erosion, soil compaction, and contamination of aquifers and surface waters by nitrogen and phosphorus lost from fertilizers, causing aquatic eutrophication (that is, the enhanced growth of algae, which disrupt the existing ecosystem).
Conversion of waste cellulosic biomass (logging and lumber mill residues, and crop residues not needed for protecting soils against excessive erosion and recycling nutrients) is the best choice, but scaling up this (just started) production to displace a significant fraction (say fifteen to twenty percent) of currently used fuels derived from crude oil will take time. Expanding fuelwood groves for household use and planting fast-growing species for commercial wood deliveries is desirable in areas with good growing conditions, or regions with plenty of available barren slope land, where afforestation may not only improve regional fuel supply but also reduce soil erosion. But any dreams of modern megacities fuelled by woody biomass should remain, for the sake of a reliable food supply and limited environmental impacts, just that.
Hydrolectricity is the largest modern non-fossil source of primary energy; the combination of relatively low cost, high suitability to cover peak demand, and the multi-purpose nature of most large reservoirs (they serve as sources of irrigation and drinking water, a protection against downstream flooding, recreation sites, and, increasingly, places for aquacultural production) should make it one of the most desirable choices in a world moving away from fossil fuels. This conclusion seems to be strengthened by the fact that on the global scale most of this clean renewable energy resource remains untapped: the International Commission on Large Dams put the global potential of economically feasible projects at just over 8 PWh, roughly twice the current rate of annual generation. As expected, the remaining potential is unevenly distributed. Europe, North America, Australia, and Japan have already developed as much of their large-scale hydrogenerating capacity as they can (there is always the potential for microstations), but Latin America has so far tapped less than a quarter, Asia less than a fifth, and Africa not even a twentieth, of their respective potentials.
This untapped potential would seem especially welcome, as it is precisely those continents where future demand will be highest, but it now appears that the development of hydrogeneration in those regions will not proceed either as rapidly or as exhaustively as was assumed during the closing decades of the twentieth century. In an important shift of perception, hydroenergy has changed from a clean, renewable and environmentally benign resource to a much more controversial cause of socially and environmentally disruptive, and economically questionable, developments. As a result, international and internal opposition to megaprojects (plants with multigigawatt capacities) has spread, and the willingness of governments and international lending agencies to finance such developments has declined. Sweden has banned further hydrostations on most of its rivers, Norway has set aside all existing plans, in the U.S., since 1998, the decommissioning rate for large dams has overtaken the construction rate, and many countries in Asia (most notably in India) and Latin America have seen vigorous public protests against new projects.
CONCERNS ABOUT LARGE DAMS
In 2000, the World Commission on Dams published a report which stressed that all future projects should consider social and environmental effects to be as important as the, traditionally dominant, economic benefits of electricity generation (or of irrigation or water supply). While some recent criticism has been ideologically motivated and clearly overwrought, there is no doubt that large hydroprojects bring a number of serious social and environmental changes. In Chapter 4 I noted the major concerns: the large numbers of people displaced by the creation of major reservoirs, the excessive silting of many storages, the aging of average river runoff and the fact that water reservoirs are (much like fossil fuels) sources of greenhouse gas emissions, as they release CO2 and CH4 from submerged and decaying trees and shrubs.
A new concern has emerged, as we see more indications of the inevitable deterioration of aging dams and contemplate the costs of their eventual decommissioning: these matters were given no, or insufficient, attention when they were built. We can only speculate about the ultimate life expectation of such massive structures, and have no good strategies to deal with the excessive silting and premature filling of reservoirs, which reduces their useful life span (in parts of monsoonal Asia affected by severe deforestation the process has already cut the expected duration of some reservoirs by as much as half). All this makes it much more unlikely that the remaining hydrogeneration potential will be developed as aggressively as it was in the twentieth century.
But even without any obstacles to their construction, new hydrogeneration capacities could supply only a part of the expected demand, and then only by claiming large expanses of river valleys, forests, grasslands, settlements, and agricultural land. The average power density of existing hydrostations (actual generation rather than installed capacity: this adjustment is necessary because dry years curtail generation at many dams) equates to less than 3 W/m2 and they claim nearly 150,000 km2 of land. If all of the remaining potential were to be realized during the first half of the twenty-first century, new reservoirs would claim roughly 500,000 km2, an area as large as Spain. But hydroenergy can be also harnessed on a smaller scale, and many Asian, African, and Latin American countries have excellent potential for developing stations, with capacities less than 10–15 MW, which would not make much of a dent in a nationwide supply of a populous country, but could suffice to electrify a remote region or an island.
Wind energy, harnessed by large and efficient turbines, has emerged in the 1990s (less than a decade after a failed mini-boom during the 1980s) as the leading renewable energy choice, thanks largely to aggressive promotion and adoption in a number of Western European countries. Better designs, and larger sizes, of wind machines made a big difference: ratings rose from 40–50 kW during the early 1980s to 500–750 kW by the late 1990s, when the first turbines with capacities of more than 1MW went on line and by 2015 machines of 3-4 MW were common, with the largest offshore units at 8 MW. Danish designs (Vestas) have led the way, and the country also leads in per caput wind capacity. Worldwide totals of installed wind-generating capacity rose from 1 GW in 1985 to 432 GW in 2015, with China, U.S. and Germany in the lead. Average capacity factors have improved with better turbine designs to nearly twenty-five percent worldwide and to just over thirty percent in the U.S.. As already noted, in 2015 wind turbines generated just 3.5 percent of the world’s electricity, but exponential growth of new capacities is expected to continue.
To avoid conflict, much of future wind power will be in large offshore wind farms (a number of them already operate in Denmark, Sweden, the Netherlands, and the U.K.), or it will come from re-powering old sites with larger turbines. The ultimate extent will depend on the progress of integrating intermittent electricity sources into national and continental grids through high-voltage interconnections and large-scale electricity storage. Environmental concerns associated with large turbines range from the well-documented risks to migrating birds, to esthetic objections, both to turbines massed in large onshore wind farms and the size of the largest machines. Well-designed and well-maintained turbines should work for more than two decades but we have to accumulate operating experience with a very large number of offshore units to be able accurately to assess the long-term availability and reliability of these machines exposed to such hazards as hurricanes, heavy icing and air laden with salt aerosols.
Compared to wind-powered electricity, photovoltaics is still a much smaller contributor, accounting for just one percent of all electricity generation in 2015 – but the installations of new PV modules have been expanding faster than those of new wind turbines. China, U.S., Germany, Japan and Italy are the leaders but the power ratings of PV units are not directly comparable with other modes of electricity generation, because they are expressed in peak watts measured under high irradiance (1,000 W/m2, the equivalent of mid-day, clear-sky insolation) rather as an average performance.
Three fundamental reasons make the PV conversion of solar radiation into electricity the most appealing of all renewable sources: the unparalleled magnitude of the resource, its relatively high power density, and the inherent advantages of the conversion technique (no moving parts, silent operation at ambient temperature and pressure, and easy modularity of units), but the two key drawbacks are its still relatively low conversion efficiency and low capacity factors in less sunny regions. Efficiencies of PV cells have risen from less than five percent during the early 1960s, when the first modules were deployed on satellites, to twenty-five percent for high-purity silicon crystals in the laboratory, but the field efficiencies are around fifteen percent. PV films, made of amorphous silicon (or gallium arsenide, cadmium telluride, or copper indium diselenide), have reached as much as twenty-two percent in the laboratory, but deliver eleven to thirteen percent in field applications. Declining costs of PV cells have made them particularly competitive in sunny locations where their capacity factor can average twenty five percent (compared to just over ten percent in Germany).
More efficient photovoltaic cells would be most welcome because of their relatively high power densities: efficiencies close to twenty percent would translate to electricity generation rates between 20–40 W/m2, two orders of magnitude better than biomass conversion, and one better than most hydro and wind projects. The future rapid growth of PV installations is assured but, as in the case of wind turbines, their eventual share of electricity generation will depend on their successful integration of this intermittent energy source in large grids, and hence on the development of new high voltage interconnections and new (slowly improving) storage capabilities.
Innovations and inventions: impossible forecasts
The most welcome advance would be a large-scale affordable means of electricity storage: with it a combination of wind-driven and PV electricity generation could provide a significant share of electricity supply. Capacities of large storage batteries have gone up but by 2015 the best ratings were in tens of MW (of massed lithium-ion batteries) dischargeable over several hours. What is needed, particularly for the world’s growing megacities, are storages on GW scale — and pumped storage still remains the only effective way of doing that. This uses two water reservoirs at least several hundred meters apart in height; electricity not needed by the grid is used to pump water from the lower to the upper storage, where it is kept until released for generation during periods of peak demand. The worldwide capacity of pumped storage is close to 130 GW; the largest unit (Bath County in Virginia) rates 3 GW. But pumped storages are expensive, and the requirement for reservoirs in high relative elevations makes them inconceivable in densely populated lowlands. There is another option that would obviate electricity storage: inexpensive intermittently generated electricity could be used to electrolyze water and variable flows of renewable energies would produce hydrogen as a major energy carrier.
HYDROGEN AS ENERGY CARRIER
Hydrogen cannot, contrary to what so many popular writings repeatedly imply, be a significant source of energy. Unlike methane, it is not present in huge reservoirs in the Earth’s crust, and energy is needed to produce it, from either methane or water. But some of its properties make it an outstanding energy carrier. Its key advantages are superior energy density (liquid hydrogen contains 120 MJ/kg compared to 44.5 MJ/kg for gasoline), a combustion that yields only water, and the possibility of using it in fuel cells.
The key advantages of fuels cells (electrochemical devices that combine hydrogen and oxygen to produce electricity) are the absence of moving parts, a quiet and highly efficient (commonly in excess of sixty percent) operation, and their modularity (they can be made small enough to power a laptop or large enough to generate electricity in multi-megawatt plants). An enormous amount of research interest in fuel cells has led to exaggerated expectations of their early commercialization, but their cost (except for a few relatively small niche markets) is still high, only a few models are available and refueling infrastructure is extremely sparse — and unless it will be in place, carmakers will be reluctant to mass-produce hydrogen-powered fuel cell cars. The transition to hydrogen-powered vehicles would also be complicated by the need for energy-dense storage and safe handling. Uncompressed hydrogen occupies 11,250 l/kg; pressurizing it into a high pressure steel tank reduces this to 56 l/kg, but this is equivalent to less than three liters of gasoline, or enough fuel to move an efficient compact car less than fifty kilometers. Liquefied hydrogen occupies only 14.1 l/kg but needs to be kept below –241°C, an immense engineering challenge in a small vehicle. Adsorption on special solids with large surface areas, or absorption by metal hydrides seem to be the most promising options.
The safety of hydrogen distribution is no smaller challenge. While the highly buoyant gas leaks quickly and it is non-toxic (making its spills more tolerable than those of gasoline) its ignition energy is only one-tenth that of gasoline, its limit of flammability is lower, and its range of flammability much higher. These will mean much stricter precautions at hydrogen stations than those now in place at gasoline filling stations.
Moving toward a system dominated by hydrogen is clearly consistent with the long-term decarbonization of the modern energy supply, but the progress will be gradual and we should not expect any large-scale transition to a hydrogen economy during the coming generation (Figure 31). The hydrogen:carbon (H:C) ratio of dominant fuels has moved from around 0.5 in wood (for actually oxidized H atoms), to about 1.0 in coal, and 1.8 for liquid hydrocarbons. The continuation of this trend points first to the emergence of natural gas (with H:C of 4) as the leading fossil fuel, and eventually (but almost certainly not during the first half of the twenty-first century) perhaps to a hydrogen-dominated world. But trends can be derailed or accelerated by social or political upheavals, or enter frustrating culs-de-sac, and only those that are strongly entrenched and rely on mature techniques have a high probability of continued adoption, accompanied by further innovation. Neither hydrogen nor a strong revival of the nuclear option belong to this category, and hence any forecasts of future milestones or diffusion rates of these techniques are just guesses.
Figure 31 Decarbonization of the world’s energy supply (calculated from data in UN and BP statistics)
In contrast, there is no doubt that the combustion of fossil fuels – gradually becoming more efficient, cleaner, and less carbon-intensive – will dominate the global energy supply during the next two generations. As electricity will be supplying a steadily higher share of the world’s final use of energy, its already generally highly efficient conversions will become even better. The greatest room for improvement is in lighting, and light emitting diodes are a most promising innovation. They have been around for many years as the little red or green indicator lights in electronic devices, and (although you may think you have a light bulb there) then became common in car brake lights, tail-lights, and turn signals, and also in traffic lights. But since 2012 they have started, finally, to make their greatest impact as long-lasting, highly efficient (and also full-spectrum) replacements of conventional incandescent light bulbs. So our grandchildren will use lights that may be, on average, at least fifty percent more efficient than ours. There is also little doubt that our continued reliance on fossil fuels will be first augmented and then progressively supplanted by renewable energies: major hydroenergy projects in Asia and Africa and by wind-powered electricity generation and PV conversions on all continents (Figure 32).
Figure 32 Brazos wind farm in West Texas with 160 one-megawatt turbines (reproduced from Wikipedia)
Inexpensive intermittently generated electricity integrated into large grids and relying on considerable storage capacity can take over large shares of markets now served by fossil fuels (including, eventually, passenger cars) but there are no near-term prospects that it could displace high energy density fossil fuels in such major sectors as intercontinental flight and shipping (both for container and bulk cargo vessels). And we have no readily deployable commercial techniques to produce such fundamental materials as pig iron, cement or ammonia on required scales without fossil fuels. But history makes it clear that the train of human ingenuity is not about to stop. Although major inventions tend to come in irregularly spaced clusters rather than an orderly progression, half a century is long enough to see the emergence, and even substantial diffusion, of several new inventions whose universal adoption could transform the energy foundations of late twenty-first-century civilization. Such developments are highly probable, but their nature and their timing are entirely unpredictable. Remember the two major late twentieth-century examples: neither the emergence of mass air travel (thanks to the invention of the gas turbine and its much improved turbofan designs) nor the invention of solid state electronic devices (transistors, integrated circuits, and microprocessors) were predicted to bring major technical and economic advances and social changes even just a few years before their commercial debuts!
The transition to an energy system based predominantly on non-fossil resources is in only its earliest phase. In some ways this appears to be a greater technical and social challenge than the last epochal shift (from animate energies and biomass fuels to coal, hydrocarbons, engines, and electricity). Given the knowledge and resources at our command, this challenge should be manageable but the shift will not be (it cannot be) extraordinarily rapid. We now have much more powerful scientific and technical means to come up with new solutions, and we also have the benefits of unprecedented information sharing and international co-operation, and can take advantage of various administrative, economic, and legal tools aimed at promoting the necessary adjustments, from more realistic pricing to the sensible subsidies required to accelerate the diffusion new and promising techniques or help them to achieve a critical market mass more quickly. But the task ahead is daunting, because the expectations for energy futures are high. They combine the anticipation of continued supply improvements (in access, reliability, and affordability) in already affluent (or at least fairly well-off) countries (whose populations total about one billion), not only with the necessity of substantial increases in average per caput energy consumption among the world’s (now more than five billion) less fortunate people, but also with the need to harness, distribute, and convert these massive energy flows in ways compatible with long-term maintenance (and in many cases major enhancement) of local and global environmental quality. Such challenging, fundamental transformations offer the best opportunities for creative solutions and effective adaptations. The evolutionary and historical evidence shows that humans are uniquely adapted to deal with change. While our past record of ingenuity, invention, and innovation is no guarantee that another fairly smooth epochal energy transition will take place during the next few generations – it is a good foundation for betting that our chances are far better than even.