Solar Energy, Energy Storage Highlight AAAS Symposium

By Gabriel Popkin

A great and urgent need exists for major advances in energy efficiency and alternative energy technologies. This was the consensus of speakers at a symposium featuring several APS members that was held at this year’s American Association for the Advancement of Science (AAAS) Annual Meeting in Chicago.

Leaders in fields as varied as solar fuel generation, lithium-ion battery development, and superconductor research were unanimous in their calls for major increases in research programs to develop large-scale, cost-effective technologies that will provide energy security for the US and other countries, and reduce the levels of greenhouse gases in the atmosphere.

APS Fellow and Lawrence Berkeley Laboratory (LBL) Interim Director Paul Alivisatos spoke on “Nanoscale Materials for Solar Fuel Generation”. He said that the current state-of-the-art solar cells and those that will be developed in the near future are made of multiple materials with different band gaps, which allow them to optimally extract energy from incident light of different wavelengths. These cells can extract around 3 eV of energy from each incident photon and convert solar power to electricity at up to 50% efficiency. But the materials currently used to make them—crystalline silicon, and thin-film cadmium telluride or copper indium gallium selenide—are too rare and expensive to be scaled up to cover the US’s energy demands, much less the rest of the world’s. “The physics is there, but not the cost,” Alivisatos said. “The cost needs to come down by a factor of five.”

One way Alivisatos hopes to bring those costs down is by developing a new generation of photovoltaics based on nanocrystals. Because these nanocrystals can be smaller than typical electron wavelengths in the parent material, quantum tunneling of electrons between them could enable solar cells to provide more efficient charge separation and conduction using inexpensive materials such as iron pyrite or lead selenide. Nanocrystals themselves are also much cheaper to grow than the larger silicon crystals in current use, just as small diamonds are much more common, and thus cheaper, than large, defect-free diamonds.

But Alivisatos made no bones about the scale of the challenge. In order to generate the approximately 3.2 terawatts of power the US currently consumes, he estimates we would need to cover around 60 million acres (roughly a square 300 miles on a side) with solar panels converting power at 8% efficiency—not a likely scenario—and this demand is growing rapidly. Thus, Alivisatos said, “we are in a hurry” to develop cheap, efficient solar generating technology. He sees national research labs such as LBL serving as “anchor points” that can work with university and industry research groups to “re-connect basic and applied research in a ‘science-to-solutions’ approach” to this unprecedented energy challenge.

Another theme of the symposium was the need for advances in energy storage, where Alivisatos estimated we are a factor of two away from where we need them to be in parameters such as cost and energy density. One of the leading researchers working to change this is Yet-Ming Chiang, an APS member and professor of materials science and engineering at MIT. Chiang gave a talk on his work developing lithium iron phosphate as a replacement cathode material for lithium cobalt oxide, which is currently used in most lithium-ion batteries that power laptops and cell phones. Because iron is abundant and non-toxic, and iron phosphate is a safer and more stable structure than cobalt oxide, Chiang sees it as a promising material for the batteries in plug-in hybrid-electric and fully electric cars. But in Chiang’s vision, these batteries will do more than allow drivers to go 200 miles on a charge. He also believes that lithium-ion batteries, when scaled up, will be able to “hybridize the electric grid” by storing electrical energy and delivering it to the grid at times of high demand.

Chiang echoed Alivisatos in advocating for large increases in basic and applied research funding for energy storage technology, and argued that the battery industry’s current market share does not reflect its importance for the future. “Right now, if the automobile and electric industries are the size of wheels, the battery industry is a lug nut,” Chiang said.

Other talks were given by Nathan Lewis of the California Institute of Technology, APS Fellow John Sarrao of the Los Alamos National Laboratory, Vallampadugai Arunachalam of the Center for Study of Science, Technology, and Policy in Bangalore, India, and APS Fellow George Crabtree of Argonne National Laboratory. APS Fellow James Misewich of the Brookhaven National Laboratory co-organized and moderated the session.