Fossil fuels could last beyond the next century, but burning them for even a few decades risks global warming and loss of ecosystems such as coral reefs and arctic ice.
Nuclear power is an option, but supplying the trillions of watts that the world needs would require building a fission power plant every other day until 2050.
A growing number of scientists in labs from Berkeley to Boulder to Rochester are working to plug into a source that delivers more energy in an hour than the world's population uses in a year the sun.
For now, its energy is among the most expensive on the planet, more than four times the cost of electricity from coal, gas and nuclear sources.
But an eclectic bunch of physicists, biologists and materials scientists suggest they can deliver multiple breakthroughs in cost and technology for solar energy, if only the government would muster the will and the money to set them loose. This solar revolution, if it comes at all, would come from the microscopic world. Scientists say two advances nanotechnology and genetic modification make breakthroughs in harvesting energy from the sun likelier.
These scientists are tinkering with matter and living nature at the level of the invisibly small, in pursuit of solar technologies to spread over vast acreages of rooftops and land.
They are fashioning solar cells at the same atomic scale that light is converted to electricity. They also are studying how to breed plants and re-engineer microbes for more efficient conversion of sunlight into fuel.
It's a bit of a race between the biologists and the physicists to see who can deliver the sun's power inexpensively enough to compete with fossil fuels.
In time, researchers want to meld the two approaches into "bio-inspired" man-made devices or organisms that will crack water into hydrogen and turn carbon dioxide into natural gas.
"The drums are beating," said Caltech chemist Nathan Lewis, who headed a U.S. Department of Energy study of new solar research this year. "People who are living and breathing it are pretty invigorated. They say, 'If you let us go, we think we can get there.'"
Evangelists of the sun
Physicist Steven Chu walked away from $300 million worth of research at Stanford University to take charge of Lawrence Berkeley National Laboratory and tackle "the single most important problem that science and technology must solve in the coming decades."
As head of a national lab and a Nobel laureate, Chu is now the nation's highest-ranking evangelist on the threat of greenhouse warming and the promise of solar energy.
The PowerPoint slides that he carries to conferences blend an admiration for American ingenuity with warnings about the rise in temperatures and the loss of U.S. agricultural capacity. On a graph ranking nations by their energy consumption and productivity, the United States comes out "one of the piggiest but also the most productive," Chu said.
"It was able to develop an economy where the cost of energy did not matter," he said. "That's changed. And the sooner we change, the better. If we don't change this, our productivity is at risk."
In late October, Chu and corporate CEOs of Intel, Dupont and ExxonMobil made the rounds on Capitol Hill, arguing for more support for education and up to $1 billion a year for new, high-risk, high-payoff energy research, the chief recommendations of the National Academy of Sciences' Committee on Prospering in the Global Economy of the 21st Century.
Chu's latest idea is Helios, a center at the Berkeley lab that pulls biologists, materials scientists and chemists together for work on solar fuels.
"We're working towards delivering something that can create a transformative technology," he said.
The sun's allure is the clean, sustained intensity of fusion energy: From almost 100 million miles away, the sun delivers 120,000 terawatts to the surface of the Earth. Most energy sources fossil fuels, biomass, wind, waves and most foodstuffs originate as solar energy.
Physicist George Crabtree, head of materials science at Argonne National Laboratory on the outskirts of Chicago, calls the sun "the 800-pound gorilla" of potential energy sources.
"It's got the energy we need," said Crabtree, who co-chaired the DOE's solar technologies panel. "It's just the champion. We don't know how to get it out, that's the bottleneck."
Turning sunlight directly into electrical power and heat is today a negligible part of the global energy mix, less than 1 percent of generated power. The output of all solar or photovoltaic cells is less than one part of a million in global energy; it doesn't add up to a single gigawatt, the average power of a nuclear plant.
Typical solar cells are too inefficient and expensive for producing large amounts of energy, requiring enormous acreages to equal the output of coal-fired, natural gas or nuclear power plants. An area the size of the national highway system or about 10 times the rooftop space on all single-family homes would be required to meet U.S. domestic energy needs.
Making solar energy practical would require boosting efficiencies and cutting the cost of producing solar electricity at least 5 to 10 times over, to about two cents per kilowatt hour. Reckoning by past advances in solar cells and declines in the cost of solar electricity, experts in the field say they normally would expect to deliver 2-cent solar power in about 20 to 25 years.
Nanotech experts suggest they can cut that time in half.
"PV is ripe, it really is," said Ryne Rafaelle, director of the NanoPower Research Lab at Rochester Institute of Technology, referring to photovoltaics or solar cells. "PV is definitely it right now."
Paul Alivisatos, an associate director of the Berkeley lab and head of its Molecular Foundry, is a nanotech pioneer.
"There's a feeling," said Alivisatos, "that we could find a way to really use solar energy on a large scale within 10 to 15 years.
"The scientists are really jazzed up about this," he said. "It really does take a state-of-the-art science and apply it to a world problem that really matters. There's a lot of energy and idealism."
New breed of solar cell
The nature of their optimism, scientists say, lies in the newfound ability to make and manipulate matter hundreds of times smaller than bacteria, at the level of biological machinery and the level at which packets of light photons interact most efficiently with matter to produce electrical charge.
At the nanoscale, matter lies at the boundary between the Newtonian physics of the observed world and quantum mechanics of the subatomic realm. It acts differently.
Chop metals such as silver into small enough pieces, for example, and they create tiny electric fields that amplify light. Grow microscopic crystals of different shapes and sizes, and they can absorb different wavelengths of light.
Some of these properties have scientists eyeing solar-cell efficiencies five times greater than ordinary cells.
They talk excitedly about "intermediate banding" and "multijunction semiconductors" and "hot carriers." What it boils down to is this: Ultra-small materials can take in more sunlight, move charges inside the cell more efficiently and generate more electricity.
Scientists have noticed that quantum dots bits of semiconductor crystal so small that they have almost no dimension can turn a single incoming photo into three pairs of charges and generate three times the electrical current.
Making quantum dots once was expensive and complex. Now it's a matter of simple chemistry in a vat.
"There are aspects of nanoscience where it becomes incredibly cheaper to make stuff," said Berkeley's Alivisatos.
To succeed, scientists would have to make solar cells almost as cheap as paint.
Still, the sun only shines by day, and humanity needs storable, portable energy.
"The problem is electricity is only 15 to 20 percent of human energy needs," said Arizona State University biochemist Tom Moore. "So you need a fuel."
Nature already turns sunlight into fuel. Purple and green bacteria break water into hydrogen. Plants and algae use solar energy to make sugars. Germs in soils, the guts of termites and cattle make those sugars by eating plant materials.
Plant photosynthesis works one molecule at a time, and generally 4 percent or less of incoming solar energy ends up as plant fuel. Sunlight is plentiful enough that plants haven't been under evolutionary pressure to convert more, but what they do already may be enough.
Midwest farmers now produce enough corn for digestion and fermentation into 4 billion gallons of ethanol. But a recent study by the Energy Department and the U.S. Department of Agriculture, titled the "Billion Ton Vision," found enough available farmland and forestland to produce tens of billions of gallons of biofuels, supplying a third or more of U.S. transportation fuel needs, without sacrificing food production.
"We're at the stage with this biomass technology that we have something that works. Can we make it work twice as efficiently? I think we could," said Chris Somerville, a Stanford biochemist who leads the Carnegie Institution Department of Plant Biology.
"I personally feel very optimistic," Somerville said.
Scientists think they can genetically modify both the plants and the microbes that digest them creating what Caltech's Lewis calls "Frankenplants and Frankengerms" to boost their energy conversion and storage rates.
For now, the economics of making ethanol out of corn is debatable. Switching to perennials such as switchgrasses and poplar trees drops the cost of biofuels.
The Berkeley lab's Chu envisions plants re-engineered to grow faster and fertilize themselves.
"You want to get a very rapidly growing plant and a very efficient way of getting that into a form of chemical energy," Chu said.
But scientists also are looking for enzymes to digest more of the plant's structural material into sugars and better forms of yeast or bacteria to ferment that into ethanol.
"Then you get about fourfold return in the ethanol over the energy that it costs the farmer to grow the plant," said ASU's Moore.
Chu envisions turning roughly a quarter of the nation's farmland over to energy crops. "Being pretty thrifty," he says, the resulting ethanol could replace "half to two-thirds of the transportation petroleum needs in the U.S.
"I could easily see us rechanneling agriculture subsidies into that," Chu said.
Plants and microbes also could be modified to produce human-usable fuels directly, such as hydrogen, methane or alcohols, scientists say.
"That's an enormous challenge, but it's cracking," said Crabtree.
The next step would be copying nature in completely manmade devices powered by the sun and producing fuels.
That, said ASU's Moore, "is a very tough thing to do.
"We need to find out how nature's catalysts work," he said. "We isolate their proteins and can get structure, but we haven't been able as chemists to synthesize the active parts of them."
Energy research lagging
For now, there isn't much federal money for solar energy research. In fact, policy analysts at Berkeley recently found that federal spending on energy research has dropped more than 50 percent since the 1980s.
This year's federal energy research budget is 11 percent lower than in 2004, according to physicist David Kammen, a professor in the University of California's Energy and Resources Group, and graduate student Gregory F. Nemet.
Kammen and Nemet found that private industry spending on energy research has declined even more steeply, so that drug companies now spend 10 times more than energy companies on research.
In both the federal and industry realms, Kammen and Nemet found, the spending cuts have been mirrored by declines in U.S. patents for energy technologies.
Patents for U.S. energy technologies soared in the late'70s and early'80s as scientists poured out ideas for energy conservation, fusion energy, more efficient gas engines and renewable energy. U.S. solar energy patents hit a peak of 225 a year, but have now fallen to about 50 solar patents a year, according to Kammen and Nemet.
At the same time, while the global market in solar cells has increased a hundredfold since the 1980s, the U.S. share of sales has been in decline since the late 1990s. Actual U.S. production of solar cells has been shrinking since 2002. Japan and Europe have more than double the market share and production.
After the Yom Kippur war and the Arab oil embargo sent fuel prices soaring and American motorists lining up at the pumps, President Carter ordered a massive scientific effort to wean the United States from foreign oil.
Between 1975 and 1982, the federal government doubled its spending on energy research under Project Independence, to a total of $50 billion in 2002 dollars. According to statistics kept by the National Science Foundation, Carter's energy program was double the national commitment of the Manhattan Project to build the atom bomb, at
$25 billion, but less than the Apollo program, President Kennedy's drive to put man on the moon, at nearly $185 billion, as well as President Reagan's defense buildup, at $445 billion, and the current war in Iraq, at more than $236 billion, in 2002 dollars.
New York University physicist Martin Hoffert has been calling since the late 1990s for an energy drive on the scale of the Manhattan Project. The Berkeley lab's Chu is the latest Nobel laureate to join the call for significantly greater energy research of all types.
"It's as though we have an asteroid barreling towards us. It's going to hit us in 50 years, and we're saying, 'Well, we have 50 years,'" Hoffert said. "It's going to take a while to get these solutions, and things are going to get worse before they get better. We really need them now."