In the third century BCE, King Hiero II of Syracuse asked Archimedes to devise a number of death traps to thwart Roman invaders. Among the many designs the great inventor drew up was a solar death ray. The basic idea was to build an array of mirrors that could reflect rays of light into a central blast, causing Roman ships to burst into flame. It's unlikely the weapon ever made it past the blueprint stage, but it became an incredibly influential model nonetheless. Archimedes was perhaps the first solar power convert, searching for a way to take advantage of the inconceivable amount of energy our friendly neighborhood star barfs up every second.
The only thing that would make Archimedes' solar death ray more fascinating is if it was technically feasible, socially benevolent, and in space. That's where John Mankins comes in. A NASA veteran, aerospace entrepreneur, and space-based solar power (SBSP) expert, Mankins designed the world's first practical orbital solar plant. It's called the Solar Power Satellite via Arbitrarily Large PHased Array, or SPS-ALPHA for short. If all goes to plan, it could be launched as early as 2025, which is sooner than it sounds when it comes to space-based solar power timelines.
Scientists have been aware of the edge the “space-down” approach holds over terrestrial panels for decades. An orbiting plant would be unaffected by weather, atmospheric filtering of light, and the sun's inconvenient habit of setting every evening. SBSP also has the potential to dramatically increase the availability of renewable energy.
The SPS-ALPHA could revolutionize disaster relief, give developing countries access to reliable power, and provide the planet with an affordable green energy option.
I recently caught up with Mankins to discuss the SPS-ALPHA's progress and potential. “Because the power plant is in orbit, it can deliver power to any place on the ground that it can see,” he said. “A single solar power satellite would deliver power to on the order of a third of humanity—not all at the same time, but any of that market could, in principle, be addressed.” The SPS-ALPHA could revolutionize disaster relief, give developing countries access to reliable power, and provide the planet with an affordable green energy option. Plus, it's shaped like a margarita glass. What's not to love?
Most aerospace professionals would tell you there is, in fact, a lot not to love. When space solar was first suggested in the 1970s, the projected costs were gargantuan, giving the concept something of a quixotic reputation that holds strong today. “Most people in the aerospace industry learned, when they were coming up as new engineers, that solar power satellites are impossible, wildly expensive, and that anybody who works on them is a nut,” said Mankins. And it's no wonder. The old-school vision of such a satellite would weigh about 50,000 tons, cover an area of 5 x 10 km, and require a budget of at least half a trillion dollars. That exiled it firmly into the land of aggressively wishful thinking, where O'Neill cylinders and Martian terraforming hang out.
But times have changed. Solar cells have increased their efficiency from 10 percent to as high as 30 percent. Amplifiers have shrunk, and swarm technology has ushered in new possibilities for SBSP. The SPS-ALPHA is a reflection of these advances: a 21st century satellite with a dramatically lower price tag. Its key innovation is that it's an elegant, biomimetic flock of smaller modules, rather than the integrated hulk of yesteryear. “It intends to imitate how semi-autonomous insects operate, like hives of bees or colonies of ants,” said Mankins. “Everything is done on ID tags or barcodes. Every piece knows who the other piece is, and how it's doing, and if it wants to be repaired, or if it wants to be left alone.”
The swarm concept is not the only size/cost reduction. The SPS-ALPHA will be primarily made up of thin-film mirrors, instead of the chunkier photovoltaic cells of ground-based solar. These mirrors reflect and concentrate sunlight, and then direct that energy to a central photovoltaic on the back of the satellite's array. Over on the other side of the array, which faces Earth, microwave-power transmission panels will beam the energy down in the form of radio waves.
Mankins differs from some other SBSP scientists when it comes to his preference for the low-frequency chunk of the spectrum. The idea of using lasers is popular with many, because higher frequencies would reduce the size of the satellite's transmitters and the receiver on Earth. When it comes to spacecraft, smaller is usually better, but Mankins draws the line at shooting lasers at the planet. High frequency blasts can damage retinas, destroy electronics, and potentially ignite fires or explosions. “Think about the Death Star,” he warned. The risk factor outweighs the seductive, compact grace offered by lasers. After all, nobody wants Earth to go the way of Alderaan.
Since Mankins is dead-set on low-intensity microwave transmitters, the receiver on Earth will be large—about 6 to 8 km in diameter, positioned 5 to 10 meters above the ground. It will be constructed from millions of rectifier diodes—true quantum devices—wired together. When assembled, the receiver will look like a huge mesh, or a fishing net. The diodes are impressively efficient, and will utilize 80 to 90 percent of the energy beamed down from the satellite. And even though it covers a lot of ground, the receiver's environmental impact will be negligible. It could even be hung over farmland—like the Arecibo Telescope—without impinging much on the ecology of the area.
According to Mankins, the biggest obstacle that the development of the satellite faces is the widespread perception that all SBSP is inherently impractical and expensive. “There really is no significant technical difficulty in building the first prototype and flying it,” he said. But there are improvements that need to be made before the SPS-ALPHA can become not only a functional orbiting solar power plant, but a commercially viable energy source.
The initial goal is to get the cost down to 10 cents per kilowatt hour—about two cents less than the average American currently pays. In order to hit that target, the problem of waste heat has to be addressed. There is no air in space, and thus no way to cool down the modules. They have to be able to manage heat efficiently, or they'll fry. Mankins has outlined four ways to address this issue. The first is to make the modules lighter, perhaps by building them from carbon composite instead of aluminum. The second and third are about amping up the efficiency of the solar cells and the solid-state amplifiers, respectively. The last is to develop materials that would make the solar cells and electronics cope at higher temperatures. Solving any two out of four, and you've got yourself a cost-efficient orbiting solar plant. It's only a matter of time.
Mankins is a fan of rapid prototyping, and wants to develop the SPS-ALPHA in three year increments. That way, it will be at least a fourth generation model when it's finally ready to deliver power from space. The year 2025 may seem a long way away, but if there's anything that's worth the wait, it's a satellite capable of plugging the Earth into the sun.