Lenses Boost Solar Cells' Power

Richard Gaughan

Two solar concentrator arrays will supply Deep Space 1's power-hungry propulsion system with more than 2 kW at the beginning of the mission.

The solar concentrator arrays with refractive linear element technology allow a relatively small area of semiconductor photovoltaic cells to provide the power of a much larger array. Courtesy of Jet Propulsion Laboratory/NASA.

In keeping with the goals of NASA's New Millennium program, the arrays not only will provide power, but also will demonstrate new technology. Both the material and the array design represent advanced technology. Cylindrical Fresnel lenses provide a 7:1 concentration, allowing a single multijunction GaInP₂/GaAs/Ge cell to collect solar energy equivalent to that gathered by seven cells.

The solar concentrator arrays with refractive linear element technology were developed for the Ballistic Missile Defense Organization by AEC-Able Engineering in Goleta, Calif. A launch failure prevented its earlier space validation, but interest in the technology has remained high, primarily because of the cost savings it represents.

Concentrated light

"Solar cells are the most expensive part of arrays, and arrays are often a very expensive part of spacecraft," said David Murphy, AEC-Able's technical director for this project. "This technology allows us to cut the cost significantly."

The concentration is produced by a 3.3-in.-wide Fresnel lens, manufactured by Entech of Keller, Texas, from an aerospace adhesive with more than 30 years of spaceflight heritage. The lenses are produced in rolls hundreds of feet long. The prismatic side faces the solar cells, and the sunward side of the lenses is curved into the shape of an arch. "That arch is critical," said Mark O'Neill, president of Entech, "because it keeps us tolerant of slope and shape errors, which allows us to have a relatively large acceptance angle of two degrees."

To protect the molded lens from ultraviolet degradation, Entech laminates a 0.003-in. ceria-doped glass arch to the curved surface of the 0.01-in.-thick lenses. The lens length is limited only by practical handling concerns. The 720 Fresnel lenses on Deep Space 1's arrays are 8.4 in. long and are held 3.62 in. from the cells.

The multijunction cells are more than 22 percent efficient in concentrated sunlight. But the real advantage comes with the reduced cell area, which makes it practical to protect the cells with thicker-than-normal cover glass. The thick glass increases the radiation resistance of the photovoltaic cells, which opens up a range of space applications that can best be met in high-radiation orbits. AEC-Able already has been approached about commercial satellite applications in low-, mid- and geosynchronous Earth orbits, according to Murphy.