When the US Air Force's X-37B autonomous spaceplane launched into orbit for the sixth time this week, it was carrying an experiment to explore the potential for beaming solar energy from space to Earth. The Photovoltaic Radio-frequency Antenna Module (PRAM) was built by the US Naval Research Laboratory (NRL) to see how the basic technology works in actual space conditions.
Harnessing solar energy isn't a new idea for space applications. In fact, the idea dates back to the early 20th century and it was first applied practically on the first Vanguard satellite that launched on March 17, 1958. Since then, solar arrays have been common on most space missions. However, PRAM is designed for a much more ambitious application of the technology than just powering a spacecraft or instrument package.
Using solar panels on Earth has a number of disadvantages. Aside from the problems of nighttime, bad weather, and changing angles of sunlight, there's also the fundamental setback that sunlight passing through the atmosphere gets heavily filtered, especially in the blue portion of the visible spectrum.
Ideally, the best site of a solar power plant would be in orbit. There, the light is not filtered, shines 24 hours a day, can be at a constant angle, and the only limit on the output of the plant is the size of its array. Unfortunately, it would also mean finding an incredibly long extension cord to carry the electricity to Earth.
The alternative is to turn the energy from the solar array into microwaves and beam these to a collector on the Earth's surface to be converted into electricity. It's an idea that goes back decades and was a key part of Gerard O'Neil's space colony proposal, but until now, it's remained a laboratory curiosity.
"To our knowledge, this experiment is the first test in orbit of hardware designed specifically for solar power satellites, which could play a revolutionary role in our energy future," says Paul Jaffe, PRAM principal investigator.
PRAM consists of a 12-in (30-cm) square tile module with a solar panel and a microwave energy transmitter. Microwaves are used because the idea is to beam energy through Earth's atmosphere, though for other targets, like the Moon, lasers might be applied due to the lack of atmosphere. According to NRL, the goal of the experiment is to study the energy conversion process, thermal performance, and efficiency of the technology.
Based on the results of PRAM, the next step will be to build a fully-functional prototype system installed in a bespoke spacecraft to actually beam energy back to Earth. Though turning such technology into a large-scale commercial power source may be decades away, NRL says that near-term applications could be to beam energy to remote areas, such as forward military bases and disaster response areas.
Source: NRL