Illustration courtesy JAXA
Published December 5, 2011
In an aircraft hangar in Germany, scientists are firing lasers across the room in the hopes of bringing science fiction to life: beaming solar power directly from space.
According to a new report [PDF] by the International Academy of Astronautics (IAA), space-based solar technologies now in development in the lab will be technically feasible and ready for practical demonstration within the next decade or two.
What's more, based on existing technologies, space-based solar could be an economically viable alternative to today's commercial energy sources within the next 30 years, concludes the report published last month.
In fact, the Japan Aerospace Exploration Agency, or JAXA, is "going forward on their [space-based solar] demo plan, with satellites scheduled to be up by the end of the next decade and a full pilot system by 2030," said Frank E. Little, of the Space Engineering Research Center at Texas A&M University.
According to the IAA report, such flight demonstrations "that validate [solar power satellite, or SPS] systems concepts to a high level of maturity . . . appear to be essential in order to build confidence among engineers, policy makers, and the public and allow space solar power technology maturation and SPS deployment to proceed."
For now, though, the most frequently cited barrier to deploying—or even testing—many of the existing space-based solar platforms is the cost of launching the necessary equipment into orbit.
"There's a cost of building equipment, but also the cost associated with designing an experiment that's man-rated," Little said.
"If I put a device on the International Space Station [for example], because there are people on the ISS, the level of safety required increases the cost of the project. We looked at doing such a project and it's not cheap.
"You also have to pay for a ride [to get the experiment] from Earth to the station, and if you account for all costs, it's millions of dollars."
That's why some experts in the field argue that, when considering a return on investment, launching large-scale space solar operations might not be the best place to start.
"Some of the initial aims of this work haven't been to look at replacing all existing energy supplies," said Stephen Sweeney of the University of Surrey in the United Kingdom, who's working on the laser experiments in Germany.
"It's a case of how to deliver energy somewhere where you most need it, such as disaster areas and military sites."
Energy in a Vacuum
The core idea of space-based solar has been in development since the 1970s: Place solar panels on a satellite, beam the collected energy to a receiver on Earth, and convert the beam to electricity.
Collecting sunlight in the vacuum of space means that the solar panels can harvest our star's intense energy without losses due to atmospheric absorption.
A satellite in geosynchronous orbit can be exposed to sunlight around the clock with no interruptions due to cloud cover. And from orbital heights, the power stream can be redirected quickly to places with the greatest need.
With funding from the European aerospace group EADS-Astrium, Sweeney's team has been studying the best way to beam power from a solar-collecting satellite to the ground. So far, the group favors a narrowly focused laser in infrared wavelengths.
"It's still an ongoing experiment," Sweeney said. "We're using an aircraft hangar where we can fire the laser across the building and put photovoltaics [PV] on the other side, then we can look at things that influence power transfer over that kind of a distance."
Similar experiments have been done before with microwave transmission, including a 2008 experiment that successfully beamed 20 watts of solar energy from a mountaintop in Maui to receivers on the Big Island of Hawaii—92 miles (148 kilometers) away.
Microwaves at frequencies up to about 10 gigahertz can move through Earth's thick atmosphere with little absorption even when it's raining, allowing most of the power to travel from the solar collector to the receiver.
But microwaves tend to spread out as they travel, so the farther the waves go, the larger the receiver must be to capture the energy being beamed.
A Foldable System?
Most space-based solar projects call for satellites in orbit about 21,750 miles (35,000 kilometers) above Earth's surface. At that height, a microwave receiver on Earth would need to cover hundreds of square miles.
Instead, Sweeney's team decided to "pick a wavelength that we know will make it through the atmosphere without being absorbed . . . [but that's also] a narrow beam, which means you have a particular point on Earth where you can target energy delivery."
Hence their use of an infrared laser in a wavelength that's eye-safe and that wouldn't cause skin damage, because the rays wouldn't be absorbed by epidermis.
With the narrow laser beam, the receiver on the ground would need to be only about 80 or 90 feet (up to 30 meters) across, Sweeney said.
A receiver could then be made with PV cells imprinted on thin films to create small foldable or rollable systems that could be deployed easily even in hard-to-reach locations such as military outposts and disaster zones.
"They have these blankets that can lay over a house or string between trees that have PV cells woven in. It's sufficiently light that it could be transported and sturdy enough that it could be stretched out" to become a receiver for power beams from space.
The satellite could start beaming power as soon as the receiver is in place—likely a much faster process than sending generators, fuel, and other heavy cargo over large distances into areas with limited transportation infrastructure.
Sky Power for Troops
The oft-cited drawback to space-based power has been the cost of setting up the infrastructure. According to John Strickland of the National Space Society, current government-sponsored launch costs to orbit are about $10,000 a pound and have not changed much for 40 years.
But cheaper commercially developed rockets, such as the SpaceX Falcon Heavy, are now in the running to serve as the U.S. successors to the retired space shuttle fleet for delivering cargo—and someday people—to low-Earth orbit. The Falcon Heavy, for example, will be able to launch payloads for as little as $686 per pound in less than two years, Strickland said. A reusable version of the Falcon rocket family is also under development, which could further reduce launch costs.
Also, space-based solar may seem a more practical solution if it is first targeted toward some of Earth's most costly energy problems. Texas A&M's Little thinks military or disaster relief efforts would be good starting points for space-based solar.
"If you look at the cost to the military of supplying energy to forward-operations bases-in addition to the personnel cost of transporting energy such as diesel-the cost of a barrel of oil equivalent is very high—much higher, in terms of the kilowatt cost of electricity, than the normal utility cost of a few cents per kilowatt-hour," he said.
The price for space-based solar power may appear high next to the price of ordinary grid electricity, which is heavily dependent on cheap coal. But tactical military installations and temporary emergency relief stations typically rely on generators that run on expensive petroleum fuel. One comparison: U.S. households in 2011 were paying on average about 11 cents per kilowatthour for electricity, but in war-torn Afghanistan some diesel-powered projects sponsored by the U.S. Agency for International Development are delivering energy at about 40 cents per kilowatthour.
In its November report, IAA said its baseline for an ideal space-solar operation would be a full-scale commercial satellite that could deliver electricity to commercial markets on Earth at a price between 10 cents and 50 cents per kilowatthour.
A portable space-based solar system would produce "probably a few to tens of megawatts, so you're looking at really not trying to supply an entire city but just the critical infrastructure [such as hospitals], in particular in the early days before you can start rebuilding," Little said.
Little's approach for designing a working system is slightly different: a microwave-laser hybrid.
In a paper presented in August at the International Union of Radio Science 30th General Assembly, Little outlined a design for a space-based solar platform that first beams a laser from a solar-collecting satellite to an airship flying about 12 miles (20 kilometers) above Earth's surface.
The airship-which could be a high-altitude balloon or an unmanned, lightweight craft—would be equipped to transform the laser to microwaves and beam that energy to a ground receiver.
"The difficulty with just lasers is that, in almost all instances, clouds will significantly scatter a laser beam. If you have clouds come in over the area you want to target, you have no electricity," Little said.
A hybrid system would cover most of the distance with lasers and then use microwaves-in a frequency that won't be interrupted by clouds-to move energy through the bulk of Earth's atmosphere.
The shorter distance between the airship and the ground would mean the microwave receiver could be just 130 feet (40 meters) wide, Little said.
In addition, use of an airship could give rescue workers access to more than just power, OU's Flournoy said.
"When a disaster destroys infrastructure, it destroys cell towers as well as the electric grid. . . . An airship would be able to direct communications as well as beam energy down to a location," he said.
The airship could even carry sensors, akin to remote sensing devices on Earth-watching satellites, "that would allow persons conducting the rescue to scan debris and locate people trapped in the aftermath of a disaster," Flournoy said.
"That may sound like science fiction, but most things in science do when you haven't seen them for yourself," Flournoy said.
Keeping Energy on Track
While testing a full space-to-ground system would be a major step in advancing the idea of space-based solar, Little said, there's still a lot that can be done on Earth, such as testing a retrodirective targeting system.
Retrodirective antennas transmit signals back in the same direction from which they came, ensuring, for instance, that the power beam won't wander as it goes between a satellite and an airship. Such a system could be tested by firing a laser from the ground to a moving aircraft.
And the University of Surrey's Sweeney is hopeful that as both spaceflight technology and solar efficiency advance, costs will decrease, ultimately making space-based solar widely attractive as an energy source.
"In a space-based approach, the cost is mainly all up front. Once it's up there, it's up there, and ideally the equipment shouldn't require much in the way of maintenance," Sweeney said. And the fuel itself would be free, of course, since no one owns the sun's energy.
Farther down the line, he added, "Once there's a commercial, recyclable method of getting things into orbit at low cost, that's when we can scale it up."
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