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Reaching for the sky

Zinc auditions for a role in solar energy storage

The potential benefits of solar energy, says Sudhakar Neti, seem almost as endless as the clear Arizona sky when they are compared with the cost, the pollution and the politics of fossil fuels.

The shining sun radiates about 1,000 watts (1 kW) of energy per square meter of land, says Neti, a professor of mechanical engineering and mechanics. Every hour, the sun diffuses more energy onto the earth than the entire human population uses in a year. And sunlight is not confined by political boundaries or geography.

Harnessing energy from the sun can be cleaner than extracting energy from coal, oil or natural gas, Neti adds, and solar energy is versatile. Solar electric systems use photovoltaic cells to convert sunlight directly into electricity. Solar thermal systems employ panels of mirrors to concentrate sunlight and convert it into heat, which then is used to drive turbines or engines to generate electricity.

Neti and four other Lehigh researchers recently received a $1.5-million grant from the U.S. Department of Energy to tackle one of the biggest obstacles to the wider use of solar thermal technology – its storage.

The three-year award will enable the group to study two materials whose phase changes (from solid to liquid and vice versa) are optimal for the storage and release of energy generated by solar thermal systems.

The multidisciplinary makeup of Neti’s group reflects the variety of challenges posed by solar thermal storage technology.

Wojciech Misiolek, professor of materials science and engineering, is an expert in metals and metal processing. John Chen, professor emeritus of chemical engineering, is renowned for his work in heat transfer. Alparslan Oztekin, associate professor of mechanical engineering and mechanics, is a specialist in numerical calculations, and Kemal Tuzla, professor of practice in chemical engineering, is an expert in the packed heat bed transfer technology that the group will utilize in its storage system.

High expectations
The proponents of solar energy hardly suffer from a lack of enthusiasm. The magazine Scientific American predicted a year ago that solar power could provide the U.S. with 70 percent of its electricity and more than a third of its total energy needs by 2050. Other observers predict a more modest increase in the portion of energy demand that will be met by solar and other forms of renewable energy.

In any case, the future of solar energy will depend in large part on cost and availability, says Neti. And these will require novel heat-transfer methods as well as new materials that enable solar facilities to store energy long enough so power can be generated on cloudy days and at night.

“We do not yet have the means of storing energy to make solar energy viable on a large scale,” says Neti. “Even in places like Arizona where sunshine is abundant, we need storage for the night.”

Two storage technologies now used by solar power plants are the pumping of compressed air into underground caverns and the use of insulated tanks filled with molten salt. But these are not capable of storing solar energy for more than a day.

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Sudhakar Neti (front) and (l-r) Profs. Misiolek, Tuzla, Oztekin and Chen believe phase-change materials could optimize the storage of solar thermal energy.