Expanding Solar Energy Globally

"The U.S. imports 30% of its energy--about half in the form of petroleum and natural gas. Overall, 85% of our energy nowadays comes from fossil fuels. And the situation is expected to get worse. Experts predict that within 20 years, two-thirds of our petroleum and natural gas will be imported, and more than 85% of our energy will come from fossil fuels."

With those ominous words, Patricia M. Dehmer kicked off a Department of Energy workshop last month in Bethesda, Md., at which a group of internationally recognized scientists was charged with a demanding mission: to identify the key basic research needs essential for utilizing solar energy effectively.

Organizing workshops with the goal of enumerating and prioritizing research topics isn't new to DOE or to Dehmer, who is associate director of DOE's Office of Basic Energy Sciences (BES). In 2002, DOE brought together experts in various scientific disciplines to tackle tough questions regarding future energy security in the U.S. One of the products of the gathering was a strongly worded recommendation calling for a national energy research initiative "with the intensity and commitment of the Manhattan Project." The authors cautioned that anything short of a national-scale program "would likely be too little, too late."

DOE followed up in 2003 with a workshop focusing on the hydrogen economy, a vision in which hydrogen, ultimately obtained from renewable sources, replaces petroleum as the principal fuel and energy carrier (C&EN, June 9, 2003, page 35).

The report prepared by the hydrogen workshop committee, Dehmer pointed out, "remarkably" changed President George W. Bush's hydrogen fuel initiative by including a new component of basic research. In particular, the hydrogen program allocates $20 million in new funding this year for fundamental research in hydrogen production, storage, and usage.

In addition, scientific societies followed DOE's lead and began organizing sessions at their conferences that focused on specific research issues related to a future hydrogen economy. "The workshop stimulated the scientific community to think seriously about the underlying scientific problems," Dehmer remarked. 

MOTIVATED BY the success of the hydrogen workshop, BES convened last month's meeting, according to Dehmer, in the hope that it will "revitalize and redefine" a solar energy research program as an essential component of a broad strategy for establishing energy security.

Before breaking up into topical panels, plenary speakers set the stage for the discussions with overview presentations. For example, Nathan S. Lewis gave a global perspective on the role of solar power as a primary source of energy. Lewis, a chemistry professor at California Institute of Technology, served as workshop chairman. George W. Crabtree, a senior scientist at Argonne National Laboratory, cochaired the event.

Global consumption of energy is on the order of 13 terawatts (13 trillion W or 13 trillion J per second), of which the U.S. consumes one-fourth, Lewis stated. The figure includes energy derived from oil, coal, and gas; hydroelectric and nuclear power; and various types of renewable sources. Currently, solar photovoltaic--meaning electricity derived from sunlight using solar cells--contributes only one part in a million to the total energy supply, he added. Photovoltaic power is five to 10 times more expensive on a kilowatt-hour basis than electricity derived from burning petroleum resources.

On the basis of those and other statistics, Lewis argued that, given the size of proven reserves of oil and gas and industry's ability to convert the abundant supply of coal into liquid fuel, economic factors alone most likely will not drive renewable sources such as solar energy to play a central role in primary power generation.

Other factors, though, such as greenhouse gas buildup in the atmosphere and its environmental consequences, may stimulate a shift in the mix of energy sources used around the globe. On the basis of models that project population and economic growth and their relation to energy consumption, Lewis cautioned that atmospheric carbon dioxide levels will increase within 50 years to two to three times the preanthropogenic level of about 270 ppm unless a substantial fraction of the planet's energy needs can be provided by carbon-free sources.

As an example of the effect that power-generation methods have on atmospheric CO₂ levels, Lewis cited predictions that, to maintain CO₂ levels at 350 ppm (about 25 ppm lower than today's value), all carbon-emitting energy sources would need to be abolished within 45 years.

The Caltech researcher went on to compare geothermal, hydroelectric, nuclear, wind, and other alternative power sources. He argued that "the energy option we should pursue is dictated by the physics of the planet--and it comes from the sun." He observed that "more energy in the form of sunlight hits Earth every hour than all the energy derived from all the fossil fuels consumed on the planet in an entire year." Lewis concluded by stressing that success with solar energy on a global scale requires inexpensive and efficient systems for converting sunlight to energy and effective strategies for storing it.

For the better part of two days, engineers, chemists, physicists, and other scientists pored over technological challenges to effective use of solar power worldwide and prepared preliminary lists of priority research directions. The experts were divided into three panels: solar electric, which discussed direct conversion of sunlight to electricity in photovoltaic and related devices; solar fuels, which covered conversion of light energy to chemical fuels; and a cross-cutting panel that dealt with fundamental issues common to all solar energy technology as well as topics in solar thermal energy.

For photovoltaic electricity to play a dominant role in energy supply, the solar electric panel concluded, a key requirement is reducing the cost of producing solar electricity from today's 25-30 cents per kWh to roughly 2 cents per kWh. The panel was led by Arthur J. Nozik, a senior research fellow at the National Renewable Energy Laboratory, Golden, Colo.

One strategy for achieving this goal is designing new low-cost organic and inorganic materials and photoelectrochemical systems with customized properties for use in multijunction solar cells. In these devices, distinct semiconductor layers are sandwiched together to harvest a larger fraction of the solar spectrum than conventional cells. Other approaches include designing cells in which one photon generates multiple electron-hole pairs (leading to higher current) and high-efficiency cells that can convert energetic (or hot) electrons into increased photovoltage and photocurrent.

Northwestern University chemistry professor Michael R. Wasielewski headed the solar fuels panel, which discussed several topics, including unresolved issues in natural photosynthesis and bioinspired systems that use light to make fuel. A key recommendation of that group is to focus on developing assembly techniques for controlling the spatial arrangement--and hence the effectiveness--of the light-harvesting, photoredox, and catalytic components used for splitting water into hydrogen and oxygen. One of the many challenges in that area is understanding how excitation energy and charge flow from one component to the next in an integrated system.

The panel also stressed the need for developing new theoretical tools capable of probing structure-function relations in complex systems. The group emphasized the importance of devising strategies for protecting components of artificial photosynthetic systems and methods for repairing or replacing the components if they malfunction. Other high-priority topics include elucidating catalytic reaction mechanisms, developing noble-metal-free catalysts for solar production of fuels, and harnessing primary reactions in natural photosynthesis to produce useful chemical products and fuels. 

NOVEL MATERIALS with tailored properties is an issue that came up repeatedly in the discussions held by the cross-cutting panel, which was led by A. Paul Alivisatos, a chemistry professor at the University of California, Berkeley. For example, panelists called for studies to find improved materials for concentrating solar energy and other solar thermal technologies.

Another materials issue focused on transparent conducting materials. Most of today's photovoltaic cells are prepared on a layer of indium tin oxide (ITO), a transparent material that admits sunlight to the cell and also serves as an electrode. Despite its widespread use, this material is far from ideal because the global supply of indium is limited and ITO isn't an especially good conductor. Nanotube-based materials and new types of polymers are being studied as possible replacements for ITO.

The cross-cutting panel also recommended focusing on basic interface phenomena. An advanced understanding of charge transport and other processes that occur at the boundary between dissimilar materials--for example, a metal contact and a nonmetal circuit element--will benefit solar technology.

Recent advances in controlling biomolecular systems and patterning material at the nanoscale are likely to be a boon to solar technology, Alivisatos remarked, because nearly every microscopic step in photovoltaic cells occurs on the nanometer-length scale. "It's a good time scientifically to take on the solar energy challenge," he added.

The full report is scheduled to be delivered to DOE in July and disseminated to the public in August.