A smallsat 'hive' approach to space solar power
As commercial space transportation companies make their first tentative steps toward a low-Earth-orbit economy, looking for the killer app in microgravity that will make them rich, an almost limitless supply of wealth streams continuously through the Solar System, showing up at Cape Canaveral and Wallops Island, Va., and the wide open spaces of Mojave, Calif., every day at sunrise.
Advocates of space solar power (SSP) continue to refine their ideas for harnessing the Sun's energy, beaming it to Earth and plugging it into the power grid. The benefits are obvious—a clean source of energy that can power the planet's infrastructure without relying on the dwindling fossil reserves that drive the often-savage global economy we have today. Less obvious are the obstacles, but papers presented at the 63rd International Astronautical Congress in Naples, Italy, this month indicate some very good minds are at work on the problems, with some very interesting results.
The SSP idea has been around since 1968, when it was proposed by Peter Glaser of Arthur D. Little. It has been studied repeatedly, particularly in the U.S. and Japan. Technologists in both countries have run basic experiments in microwave power transmission that indicate SSP is feasible and could be developed fairly rapidly with a focused effort (AW&ST Sept. 22, 2008, p. 20). But the sheer scale of setting up a constellation of collectors in orbit able to handle the power loads that would be needed to realize the promise of SSP is daunting, and more expensive for now than deep-water drilling, mountaintop removal and fracking. Electric cars are creeping onto the highways, but the electricity for their batteries is generated the old-fashioned way.
“The major problem associated with [SSP] is to apply the technologies to the huge system at [gigawatt] level in power, [kilometer] level in size, and several tens [of] thousands of tons in weight,” writes Susumu Sasaki, of Japan's Institute of Science and Astronautical Science, in a technical paper presented in Naples. “Also it is [necessary] to make its power price be competitive with that of existing power-generation systems on the ground.”
Reviewing the technical readiness level of SSP components, Sasaki reports that in order to begin deploying commercial SSP spacecraft in the 2030s, “large advances” in power transmission will be needed in the next 10 years, followed by significant advances in large space structures in the 10 years after that, and a final five-year push in space transportation to kick off service. Photovoltaic cells, for example, need to move to 35-40% conversion efficiency from 15-30% in the Japanese model, with specific weight dropping to 1 gram per watt from 10-100 grams per watt and service life in space growing to 30-40 years from 10. Today's cost of $4-6 per watt needs to drop to $1-0.50.
To meet the transportation needs, Sasaki sees reusable launch vehicles (RLV) as the ultimate solution for SSP, and he finds encouraging developments in space tourism. “The technology gap from the suborbital flight to the orbital flight is considered very large, but the sub-orbital flight technologies could lead to breakthrough in the orbital RLV,” he writes.
John Mankins, a longtime U.S. SSP advocate, presented an update on an advanced concept under study withfunding known as Solar Power Satellite by means of Arbitrarily Large Phased Array (SPS-Alpha). The idea “represents a very different architecture for SPS, using a hyper-modular approach in which all platform elements can be mass-produced, and none are larger than a 'smallsat,'” he writes. “This could enable significantly lower development time/cost, much greater ease of manufacturing at lower cost, and significantly higher reliability.”
Basically, mass-produced “intelligent” spacecraft weighing 100-300 kg (220-660 lb.) would assemble themselves into a constellation shaped to collect, convert and transmit solar energy through the “hive” of other spacecraft to a transmitter array assembled in the same fashion (see illustration). Mankins says the idea is based on the behavior of bees and ants.
“SPS-Alpha incorporates the concept of the retrodirective phased array, which allows a large number of individual RF elements to be controlled and their transmissions made coherent through the use of a 'pilot signal' transmitted from the site of the planned receiver,” Mankins writes. “This technology (co-invented by Nobuyuki Kaya of Kobe University) allows the large microwave transmitter required for the concept to be assembled from modular elements via an RF version of adaptive optics.”
The SPS-Alpha study done foraims to establish an analytical proof of concept for its “technical and economic viability” and a near-term road map for development like Sasaki's for the more conventional approach he studied. Either one could be interesting reading for today's energy companies as they look for ways to invest some of the record profits they are pulling down, along with mountaintops.