In a world where fossil fuel provides more than 80 percent of energy, what would it take to go completely green? Could the world switch over to power from only the wind, sun, waves, and heat from the Earth in only a few decades?
The question seems a fanciful one, when world leaders are stymied over proposals for far less dramatic cuts in the carbon dioxide emissions from global burning of coal, oil, and natural gas. But two U.S. researchers, a transportation expert and an atmospheric scientist, decided the time had come to apply blue-sky thinking to one of the world's greatest challenges.
"We wanted to show that wind, water, and solar power are available to meet demand, indefinitely," says study co-author Mark Delucchi, of the Institute for Transportation Studies at the University of California Davis. He and Mark Jacobson of the civil and environmental engineering department at Stanford University began to tally the build-out that would be needed to supply renewable energy for all the world's factories, homes, and offices, as well as all transport—cars, planes, and ships.
Their argument that such a revolution was both possible and affordable by 2030, first explored as a thought piece published in Scientific American before the 2009 Copenhagen climate talks, is detailed in a study published last month in the journal Energy Policy.
Steel, Concrete, and Minerals
Delucchi and Jacobson estimate that a drive for 100 percent renewable energy would require a massive building binge. For instance, the world would need nearly 4 million wind turbines, and they'd be big ones—rated at 5 megawatts (MW). That's two or three times the capacity of the majority of turbines on the market; 5 MW turbines were an innovation introduced offshore in Germany in 2006, and China just built its first 5 MW wind turbine last year.
The pair estimate that the world would need 90,000 large-scale solar plants, each with a capacity of about 300 MW—both those that rely on photovoltaic panels that make electricity directly, and concentrated solar power plants that focus the sun's rays to boil water to drive electric generators. At present, fewer than three dozen such utility-scale solar plants are in operation worldwide; most are far smaller.
And the big solar systems wouldn't displace the need for rooftop power; the researchers estimate a need for 1.7 billion 3-kilowatt solar PV systems as well. Think of that as one rooftop PV system for every four people on the planet.
Building all these new turbines, solar panels, and other infrastructure would eat up plenty of steel, concrete, and other resources. However, Jacobson and Delucchi concluded there are no significant economic or environmental constraints on the production of bulk materials such as concrete and steel, so they examined more closely the needs for less common materials.
The main bottleneck, they argue, could be the production of rare earth metals such as neodymium, which is often used in making magnets.
To build all the electric generators to go into the millions of wind turbines they envision, worldwide production of neodymium would have to more than quintuple. But there should be enough neodymium available, the study argues, since current world reserves of the element are about six times larger than needed.
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There are also ways around this bottleneck, Delucchi and Jacobson argue. Other types of magnets could be used in turbines, and rare earth metals could be recycled. No such recycling program exists today.
The researchers insist that none of the obstacles is great enough to block a path to fully renewable power by 2030. They do allow that it would be more feasible to stop building new power plants and vehicles that burn fossil fuels by 2030, and then replace the existing plants gradually to reach 100 percent green energy by 2050.
(Related: "Warming Solution: Just Stop Cold?")
"Technically you can do it," Jacobson says. "It really depends on will power."
Leaving Out Biofuel
In forging their road map for a fossil-free future, the researchers make their job all the more difficult by leaving out biomass, the renewable that currently owns the greatest share of the world energy mix. Due to the undesirable air pollution and land-use impacts of ethanol and biodiesel, they built their vision for a 100 percent renewable future without them. And due to concerns about waste disposal and proliferation, they also left out carbon-free electricity generation by nuclear power, which currently provides about 6 percent of world energy.
The world is far from on track to a biomass-free renewable future. Today, all renewables provide just 13 percent of world energy supply, and that share slips to 3 percent if biomass is left out, according to the International Energy Agency (IEA) 2010 World Energy Outlook.
If nations live up to the broad policy commitments they have made to reduce greenhouse gases—and that is by no means a given—the IEA projects those non-biomass renewables will rise to just 7 percent by 2035. With more aggressive action on climate change and promotion of renewables, the IEA projects that share would increase to just 11 percent. "Large-scale government support is needed to make renewables cost-competitive with other energy sources and technologies," the IEA concluded.
But Delucchi and Jacobson maintain that with the expected decline in renewable technology costs, the cost of what they call a "100 percent WWS" system—wind, water and solar—would be similar to that of the energy-delivery system today.
"The real challenge is matching supply with demand," Jacobson says. Heavy reliance on wind, which would provide half of world power in the researchers' scenario, and solar, which would contribute 40 percent, could risk reliability of the system, because of the variability of the winds and skies. But the authors say that can be largely addressed through interconnection of the system and by taking advantage of how the different renewables can work together.
"Wind and solar are very complementary," Jacobson says. "When the wind isn't blowing, you usually have a clear, sunny day. And vice versa—when there's less sunlight on a cloudy day, it's usually windy."
Geothermal systems that harness heat stored underground, and machines for harnessing energy in ocean waves and tides, would make a smaller contribution than wind and sun in the Delucchi-Jacobson scenario—about 6 percent of world energy. But because these forms are more consistent, they would help make the system more reliable.
(Related: "Can Geothermal Energy Pick Up Real Steam?"
Hydroelectric dams would also pitch in to provide about 4 percent of world energy—but because the authors believe that most of the best spots for dams are already taken, they don't envision anything nearly like the expansion they see for solar and wind.
Also aiding in the reliability of a system running completely on wind, water, and solar power, the authors say, is that it would need about a third less energy than a fossil-fired system. "It's mostly because of the conversion from combustion engines," like those in cars, Jacobson says, "to electric motors, which are much more efficient."
This study is far from the first to look at the tricky problem of integrating renewables. Sarah Barber, a mechanical engineer who specializes in wind turbines at the Swiss Federal Institute of Technology in Zurich, says that because of the variability of renewables, "the energy peaks need to be balanced out, requiring a more modern [electrical] grid."
She notes that Delucchi and Jacobson included in their estimates an updated grid and various forms of energy storage.
"It's relieving to see some serious studies being done on the actual feasibility of installing renewable energy systems," Barber says. "Studies such as this one should help clear up some question marks."
Still, the nitty gritty of making it work can be complicated. "Energy dips need to be quickly covered," Barber says, "such as with hydro pumps, as in Switzerland," which can store electricity by pumping it uphill, into reservoirs. Around the world, electric power systems that have added significant amounts of renewable energy to the grid require both costly and creative solutions. These run the gamut from batteries to far more complex electricity-management systems.
Replacing the internal combustion engine as rapidly as the authors envision would require a sea change, with all-electric cars just hitting the market now, and current projections that even by 2020 they will make up well under 10 percent of global auto sales.
(Related: "Rev Up Your Motors, Electric Cars Zip into View")
Daniel Kammen, the World Bank's chief technical specialist for renewable energy and energy efficiency, says that works like the Delucchi-Jacobson paper are useful because they add to the growing literature of low- and no-carbon scenarios.
"This paper is one such study that highlights the potential of renewables, without dealing with the details of a realistic energy generation and delivery systems," says Kammen, who is founding director of the Renewable and Appropriate Energy Laboratory at the University of California Berkeley. "As in many things, the devil is in the details. At present, we far from having 100 percent of energy from renewables—or even a majority of energy from renewables. This paper provides a useful accounting of the renewable energy resources without getting into the workings of a realistic energy systems."
The world is in need of a more detailed transition document that lays out "how we are going to make a zero-carbon world function," says Kammen, an adviser to National Geographic's Great Energy Challenge initiative. He says much such work is now under way around the world.