ORNL technology could make solar energy more viable

December 15, 1999

OAK RIDGE, Tenn., Dec. 15, 1999 - Solar energy could get a mega-boost, effectively gaining a three-fold improvement over conventional technology, with a system being developed at the Department of Energy's Oak Ridge National Laboratory (ORNL).

The "full-spectrum solar energy system" harnesses the clean and abundant energy found in sunlight and uses that energy more efficiently than is possible with traditional solar energy systems. The technology departs from conventional approaches that focus on using solar energy for a single purpose, such as to generate power.

"Instead of inefficiently converting the visible light found in sunlight into electricity only to reconvert a sizeable portion back into interior light, it makes more sense to just collect and distribute the light directly," said Jeff Muhs, a researcher in ORNL's Engineering Technology Division.

"By using the visible portion of the light spectrum, we can reduce the amount of electricity we consume for lighting commercial buildings," Muhs said. "We can use the other portions of the spectrum to generate electricity."

Interior lighting is the single largest user of electrical power in commercial buildings, accounting for more than a third of all of the electricity consumed commercially in the United States.

The system being developed by Muhs and industry partners uses novel roof-mounted two-axis tracking concentrators that separate the visible and infrared portions of the sun's rays. Using large-diameter optical fibers, it distributes visible light to interiors of buildings. The system converts infrared, or non-visible, portions of the solar spectrum into electricity.

The most economical photovoltaic cells today use silicon-based materials, and the conversion efficiency of these devices is relatively low in most portions of the visible light spectrum.

"Our approach leverages the benefits of concentrating photovoltaic cells in the near-infrared portion of the spectrum, where they are generally more efficient," Muhs said.

Models developed at ORNL and independently verified by Antares Engineers & Economists suggest that the overall efficiency and affordability of solar energy can be improved by as much as threefold in commercial buildings when compared to conventional solar technologies.

To effectively use the visible light from the sun for interior lighting of commercial buildings, it must be combined with artificial light yet still maintain uniform lighting - even when the amount of sunlight changes. Muhs has dubbed this combination of natural and artificial light "hybrid lighting."

The benefits of hybrid lighting include the elimination of glare and variability, better utilization of direct sunlight, the reduction of architectural intrusion and the reduction of energy consumption during peak demand.

"Hybrid lighting is poised to compete with conventional day lighting and conventional solar technologies as the preferred use of solar energy in commercial buildings," Muhs said. "Integrated with full-spectrum solar energy systems, it offers even further advantages."

Industry partners agree that the potential for commercial application of a full-spectrum solar energy system is significant.

"Assuming our goal remains the same, and utility deregulation continues at its ever-increasing pace, full-spectrum approaches like this that take a holistic view of solar energy are the future of solar energy," Muhs said. "The ultimate goal of reducing greenhouse gas emissions using solar energy hasn't changed. We're just rethinking how best to do it in commercial buildings where lighting loads and energy costs are high during peak demand periods."

Other prospective partners, including Translight LLC of Pomfret, Conn., share Muhs' optimism.

"Our analyses indicate that there is a realistic opportunity to achieve simple paybacks of less than five years using this hybrid approach to solar energy," said Hanafi Fraval, director of Translight, a manufacturer of large-core optical fibers.

The ORNL-led initiative is also receiving support from the Solar Energy Industries Association in Washington, D.C., and Duke Solar in Raleigh, N.C.

ORNL is a DOE multiprogram research facility managed by Lockheed Martin Energy Research Corporation.
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