Although the US has plenty of terrain that's well placed for producing solar power, the intermittent nature of that power and the distances of these sites from major population centers on the east coast puts severe constraints on what we can do with it. Space-based solar power, which can gather energy around the clock and transmit it to most of the populated areas of the planet, provides a way around these limits, one that was already being contemplated before the energy shocks of the 1970s.

Unfortunately, the prohibitive launch costs and challenges of sending the energy back to earth have left matters at the contemplation stage, but with the current focus on renewable energy, several companies are now betting that we'll see hardware in space well before the next decade is over.

Going commercial

To understand what's changed, we talked with William Maness and Philip Owen of PowerSat, one of a handful of companies planning to commercialize space-based power. Maness emphasized that the systems won't require any new technologies, saying, "We aren't doing science fiction, although it sounds like it; we're looking for a steep decline in the cost curve, but no breakthroughs are required."

PowerSat hopes to combine several recent developments, most of which serve to cut down on the weight of what's put into orbit. At least one of these has found applications on earth: thin-film solar cells (PowerSat is looking at CIGS-based cells—Copper, Indium, Gallium, and Selenium—on a titanium or aluminum substrate). These can reach high efficiencies despite being only microns thick.

To deploy these in space without heavy equipment, PowerSat is looking to leverage some of the testing NASA has done on the use of inflatable structures in space. All of that means that far more energy-producing hardware can be put in space as a single unit. Maness says that the company's models predict that it can get about 17MW out of a single 10-ton unit.

Most proposals for space-based solar have involved a constellation of satellites, each transferring those 17MW to a central unit for transmission back to earth. This adds to the complexity of the system and means at least one satellite has to integrate a very large amount of power. PowerSat hopes to avoid all that. The satellites will receive a pilot signal from the ground and use that to coordinate their energy-carrying return signal to the ground-based receiver. "The satellites act as a radio frequency cloud to create a phase array of phased arrays," Maness says.

When the microwave signal hits the ground, the transmission from each satellite should be additive—all of which dramatically cuts down the weight and complexity of the hardware that has to be put into orbit.

The other place that PowerSat plans to save weight is in propulsion. "Traditionally, powersets are in geosynchronoous orbit and relied on a chemical tug to pull them out to geo," Maness said. But NASA's work with ion engines, which are lightweight but require an electric power source, have provided an alternate method—clearly, once the solar arrays unfurl, there will be no shortage of power.

When asked about the risk of opening up something that large amidst the junk in low earth orbit, Maness said that "a collision would have very little impact outside some small, critical areas." In most cases, space junk would rip straight through the thin film material.

This setup will allow PowerSat to "start in all sorts of initial orbits" and take six to eight months to transfer to a geosynchronous orbit. "We can focus on the lowest cost ride," Maness said. All told, these differences should mean that launch weight is cut by two-thirds compared to more traditional schemes—the 17MW satellite will come in at about 10 tons. And, since launch weight means dollars, this represents a huge cost savings.

Grounding the power

On the ground, the microwave power sent by the satellites is received by a structure that's about a mile wide and between one and two miles long, depending on how far north of the equator it's based. Maness says that there are chunks of unallocated microwave frequency that can easily handle the 230 watts per square meter that's allowed by the EPA. Despite the size, the cost of the ground stations are only a small fraction of the total expense; PowerSat estimates it at $100 million or so. That's largely because the hardware is very diffuse. Rain and sun can pass right through it, and Maness suggested the ideal location might be over an orchard or corn field, where the added heat could be advantageous.

And there is some added heat; models suggest about two degrees Fahrenheit for every 10 minutes in the core of the signal—about a quarter of what you'd get on a sunny day at the beach, Maness said. He indicated that the EPA had done extensive testing, and birds and other animals that might be bothered by the microwaves simply leave the area. It's not completely without effect, though: "Your cell phone will definitely stop working," Maness said.

That last item is probably the biggest technological limit to deployment. Since the receiving hardware is relatively cheap and can be built just about anywhere, it would be very tempting to site it where it could be easily connected to the grid near major population centers. Unfortunately, the interference with communications is likely to severely limit the number of locations where people will accept this sort of facility. There's also the prospect of public uneasiness with being targeted by microwaves from space, although the comfort level with microwave appliances has increased dramatically in the last few decades.

In the end, Maness can build a convincing case that the primary limit for satellite-based power will be financial, not technological. We'll take a closer look at the money trail in a later post.