GaAs solar cells prove more efficient than silicon
By John McHale
LOS ANGELES - Engineers at Hughes Space and Communications International Inc. in Los Angeles are building a HS601 HP satellite, ground station support, and launch services for Orion Asia Pacific Corp. in Rockville, Md. using gallium arsenide (GaAs) solar cells, because GaAs is more efficient at turning photons into electricity than traditional silicon.
Space industry electronics designers are using GaAs solar cells because they can generate the same power as silicon solar cells, yet in smaller packages, says Terry Cavicci, manager of advanced systems at Spectrolab in Sylmar, Calif. For silicon cells to produce the same power as GaAs, their sizes would have to be increased, making it difficult to fold up the cells in the nose of a rocket, he adds.
When engineers began using GaAs in the early 1980s, they grew it on a GaAs substrate; now it is grown on a germanium substrate. Germanium is less fragile and more capable of holding together in space than GaAs, Cavicci says.
The average cell efficiency at the beginning of life for a GaAs cell is 19 percent compared to 12 percent for silicon, Cavicci explains. Engineers measure efficiency by dividing the power given out by the power taken in.
"We prefer to look at the end-of-life performance, when a satellite has been in space for 10 years, he explains. The end-of-life efficiency for silicon is 10 percent and 14.5 percent for GaAs.
The new satellite being developed at Hughes, Orion 3, will expand Orion`s current fleet and is the company`s first satellite to serve the Asia Pacific region. Orion 3 will provide business communications in Korea, China, India, Japan, Australia, Southeast Asia, Oceana, and Hawaii.
Orion 3 is a high-power satellite able to carry payloads twice as powerful as those on a standard HS 601 satellite through such innovations as advanced battery technology and a xenon ion propulsion system.
The Orion 3 payload consists of 10 C-band transponders delivering a minimum 33 to 37 dBW and 33 Ku-band transponders delivering 44 to 53 dBW.
The C-band transponders will provide broad distribution services, particularly to television and other program distributors. The Ku-brand transponders are primarily for private business networks and direct-to-home video services. The satellite has a life expectancy of at least 15 years.
Orion 3 will launch on a Delta 3 rocket and will orbit above the Pacific Ocean.
Also being refitted with GaAs solar cells is the Hubble Space Telescope, which will have them installed during its 1999 servicing mission, says David Scheve observatory development manager at the NASA Goddard Space Flight Center in Greenbelt, Md.
While GaAs solar cells offer vastly enhanced space-based power generation today, future generations of solar cells will be even more efficient, says Bruce Anspaugh, an engineer at the NASA Jet Propulsion Laboratory in Pasadena, Calif.
By the turn of the century, designers will build solar cells in three layers, one of GaAs, one of gallium indium phosphate, and one of germanium substrate.
Multiple-junction cells use more of the sunlight`s energy than cells do today, Cavicci says. Today`s solar cells use sunlight wavelengths no longer than 880 nanometers. But multiple junction cells can generate electricity with wavelengths longer than 880 nanometers.
