06-03 PRAM space-based solar power

[Image above] The Photovoltaic Radio-frequency Antenna Module (PRAM). The hardware is the first orbital experiment designed to convert sunlight into radio frequency microwaves for transmission back to Earth. Credit: U.S. Naval Research Laboratory

Though the popularity of solar energy as a renewable energy source continues to grow, the photovoltaic (PV) systems used to harvest that energy have not.

Many communities view PV infrastructure with a “not in my backyard” mentality—they support expanding use of renewable energy but do not want to see the actual facilities near their homes. The reasons for community resistance are numerous, but the outcome is that many solar companies are left looking for places to install their arrays.

In their search for places to install PV infrastructure, solar companies have taken several creative approaches, including floating solar panels on reservoirs and co-locating them with crops. And near the end of last month, scientists took a big step toward opening another place for installation—outer space.

Space-based solar power—from science fiction to reality

The idea of harvesting solar energy in outer space and beaming it back to Earth is attributed to American science fiction writer and professor of biochemistry Isaac Asimov. In 1941, Asimov published the sci-fi short story “Reason,” in which a space station transmits energy collected from the Sun to various planets using microwave beams.

The first real science description of a satellite solar-power system came almost 30 years later, in a Science article published in 1968. The author of the paper was Peter E. Glaser, a Czechoslovakian-born American scientist and aerospace engineer.

Glaser was granted a U.S. patent for his concept in 1973, and in 1974, NASA signed a contract with international management consulting firm Arthur D. Little, Inc., of which Glaser was vice president, to lead four other companies in a broader study on the technology.

Though the group’s feasibility report found the concept had several major problems, they concluded it showed enough promise to merit further investigation. NASA and the Department of Energy jointly investigated the concept between 1978–1986 before the project was discontinued. However, interest in the concept reemerged in 1997 with the publication of NASA’s “Fresh Look” study, which concluded an “absolute requirement” for achieving space-based solar power was investing in technology to drive down the costs of earth-to-orbit transportation.

Since that study, the attractiveness of space-based solar power dimmed somewhat as developments in solar panels caused the price of land-based PV arrays to decrease considerably, making space-based arrays less economically worthwhile. However, research on space-based panels did continue, and this May, researchers from the U.S. Naval Research Laboratory achieved a new milestone in such studies—they launched the first orbital experiment with space-based solar power.

Testing space-based solar power in space

On May 17, the U.S. Air Force launched its secret X-37B space plane for a long-duration mission in low Earth orbit. However, for this sixth launch of the plane, the Air Force gave some rare details about its cargo—it included a solar panel developed by the Naval Research Laboratory that would be used to conduct the first orbital experiment with space-based solar power.

In a press release about the experiment, the Naval Research scientists explain the PV device, a 12-inch square panel dubbed the Photovoltaic Radio-frequency Antenna Module (PRAM), converts sunlight into radio frequency microwaves.

“We could’ve also converted for optical power transmission,” Chris Depuma, PRAM program manager, says in the release. “Converting to optical might make more sense for lunar applications because there’s no atmosphere on the Moon. The disadvantage of optical is you could lose a lot of energy through clouds and atmosphere.”

Technical details are scarce, but the press release does note that the solar cells were manufactured with an additional layer to capture blue light.

“There’s more blue in the spectrum in space, allowing you to add another layer to solar cells to take advantage of that,” Paul Jaffe, PRAM principal investigator, explains in the release. “This is one reason why the power per unit area of a solar panel in space is greater than on the ground.”

In the video below, you can catch some tantalizing glimpses of PRAM being built. Though it may be a while before we learn results of the experiment, knowing we’ve reached this next level in space-based solar power research is exciting indeed!

Credit: USNRL, YouTube