Could legions of massive, sunlight-harvesting satellites hovering in geosynchronous orbits where the sun almost always shines harvest and beam down to Earth all the energy people will ever need? Can they do so without emitting greenhouse gases into the bargain?
We're about to find out. The first hardware ever launched to test this ambitious concept, called space-based solar power (SSP), is now circling the globe in a 390-kilometer-high (242-mile-high) Earth orbit, awaiting a series of punishing tests in the harsh thermal and cosmic radiation environment of space.
Launched in mid-May, the pizza-box-sized SSP test system was carried into orbit on a U.S. Air Force X-37B spaceplane, which looks like a miniature version of the U.S. National Aeronautics and Space Administration (NASA)'s now-retired Space Shuttle, and which acts in part as an uncrewed orbital testbed for U.S. Department of Defense (DoD) space experiments. Called the Photovoltaic Radio-frequency Antenna Module, or PRAM, the slab of electronics is currently thought to be one of the most likely architectures for a viable SSP system.
"To our knowledge, this is very first instance of flight hardware that's intended for a solar power satellite being tested in orbit, so it is a pretty big deal from that standpoint," says Paul Jaffe, head of the spacecraft engineering team that developed the PRAM at the U.S. Naval Research Laboratory (NRL) in Washington, D.C.
China, Japan, South Korea, and India each are investigating SSP's supposed ability to supply clean, limitless energy to Earth, and even to human colonies on the Moon. Those nations are drawn to SSP because the basic ideas behind it are disarmingly simple, being predicated on the fact that more than enough solar energy powers past our planet, unexploited, all year round, to supply all Earth's energy needs.
If SSP satellites could capture, convert, and beam down enough solar energy, we could overcome the sparse output of terrestrial solar farms, which suffer from seasonal variations in daylight hours, the day/night cycle which leaves solar farms useless from dusk through daybreak, and the up to twenty-fold power loss as solar energy is absorbed and scattered by moisture-rich weather systems before it reaches a solar panel.
How might an SSP system work? Although there are many ways such a system could be put together, they would most likely all have to undertake the following key steps:
The satellites themselves can be augmented, in some designs, using free flying or built-in reflectors, which multiply the amount of solar energy captured by three to 15 times (provided they can withstand the accompanying heat). Situating them in geosynchronous orbit (35,800 kilometers, or more than 22,220 miles, above the Earth's surface) means the satellites will hover above a fixed point on the surface where a receiver antenna can be sited.
The idea first saw the light of day 79 years ago, as the plot backdrop to Isaac Asimov's short-but-engrossing 1941 story Reason (read it here). In this, an argumentative sentient robot (with a bit of a god complex) runs a solar energy collection station in deep space and must ensure a colossal "beam director" steers high-energy rays with great precision to receivers on Earth and Mars, so as not to cause "incandescent ruin" if their bad aim fries people and property on either side of the receiver.
Asimov's science fiction didn't start to approach science fact in any meaningful sense until 1968, when Peter Glaser, an aerospace engineer with Apollo program experience working for the Arthur D. Little consultancy in Cambridge, MA, published a paper suggesting ways to construct SSP stations with separate solar collecting and giant dish-based microwave transmitters.
However, based on the pre-carbon-fiber, heavy-metal aerospace technologies of the day, studies by Nasa and the U.S. Department of Energy determined that a single solar-receiving satellite would weigh in excess of 80,000 tons, putting launch costs per power station way beyond consideration.
More recently, as solar cell, antenna, and semiconductor amplifier materials efficiencies have improved, and advanced lightweight composite materials have become commonplace in demanding aerospace applications, engineers in Japan and the U.S. have hit on a smarter way to construct an SSP satellite. Instead of building them with separate, heavy, hard-to-assemble, expensive-to-launch solar arrays and separate microwave antenna systems, they have worked out how to integrate all the critical functions into a single flat "sandwich" module. Explains Jaffe, "The name comes from the fact that it's comprised of three different layers. It has a top layer of photovoltaics, a central layer of direct-current-to-radio-frequency conversion electronics, and then a bottom layer which, is the microwave antenna."
Kyoto University in Japan built the first such sandwich system in 2001, and in 2015 Northrop Grumman and Caltech began collaborating to do likewise, reporting their progress on the work in May of this year in the journal Acta Astronautica. Jaffe says Kyoto University's Spritz sandwich module "was quite large, but it was not designed to operate in space. The Caltech and Northrop Grumman one explored how to increase the amount of power a module can provide per unit of its mass, and it's fantastic, remarkable work. But again, that one was not designed to operate in space," says Jaffe.
NRL's PRAM sandwich module is the only one designed for spaceflight to date, and will test the ability of the three-layer approach to withstand the extremes of cosmic heat stress. "One of the most challenging, and indeed underappreciated, things about spacecraft is how challenging it is to control thermal issues," says Jaffe. Heat cannot be lost through convection in the vacuum of space, so it must be conducted away from components where it poses a danger and then radiated into space.
He says the International Space Station is equipped with "a set of zig-zag-type structures that look like they could be solar arrays, but which are in fact radiators, and those are a critical part of the space station, helping maintain its thermal balance. But with a space solar power satellite, we need to avoid using such big, deployable radiators because that adds mass and complexity: you actually have to move the heat to those radiators, which is not a trivial task," says Jaffe.
For SSP satellites to cope with temperatures that vary from a torrid 300°C in direct sunlight to a frigid –170°C for the few days each year that geosynchronous orbits are eclipsed by the Earth's shadow, intricate design of the sandwich structure was necessary. Jaffe and his team have a trick up their sleeve here, and it's one they have patented, which involves use of stepped and conical internal structures in the sandwich that operate as high surface area radiators, in addition to using the Earth-facing antenna array as a radiator, too.
Yet only when the X-37B spaceplane leans towards the Sun and exposes the PRAM to it will they learn whether, and to what extent, they may have succeeded. Because it's a classified mission, says an NRL spokesman, we cannot know how, or when, the experiment will take place. Also, because the spaceplane contains other experiments that may be interfered with by the transmission of microwaves, the PRAM's antenna layer will not transmit microwaves to Earth, but instead will dump them into a dummy radio-frequency load, where they will be measured.
If the orbital test works, it will lend NRL some bragging rights, but there's still a very long way to go. If sandwich structures are indeed the way to go, researchers need to work out how kilometer-scale SSP satellites can (a) best be launched and (b) be assembled in space by armies of robots, at such a high altitude that it's no job for astronauts.
SSP satellites also face a host of issues related to beaming microwaves to Earth. Although aiming the space-to-Earth beam to strike a target 10km wide means it will be weak enough for even an aircraft to fly through without harm to its systems, there is huge competition for frequencies on the RF spectrum, so it is not out of the question that SSP could be forced to use millimeter waves, or even lasers, to beam power down from space.
Even if the technical specifications can be agreed upon, no one should expect any short-term benefits, says Jaffe. After all, he points out, in hydroelectric power generation technology, we didn't start by building the Hoover Dam; the technology developed over many years before it scaled and became cost-competitive. "I think space solar power has the opportunity to be cost-competitive, but it depends on a lot of things that just haven't happened yet, and which may or may not happen," he says.
The low-cost, reusable rockets made by Space Exploration Technologies (SpaceX) of Hawthorne, CA, likely will be vital to fueling the economics of SSP, by providing affordable launch capacity. Yet one of the loudest voices opposing the idea of SSP has been SpaceX CEO Elon Musk, who has gone on record as saying he thinks there are too many energy conversion steps (solar to electricity to microwave in the sandwich, and then back to electricity on Earth), making the inevitable losses at each conversion stage untenable.
NRL is sticking to its guns, however, and Jaffe points out that the global positioning system (GPS), which the lab also had a major hand in developing, also sounded like an unlikely notion before it changed everybody's lives with its hyper-accurate, space-based positioning, navigation, and timing capabilities.
We'll know in a matter of months how the experiment fared in orbit. "While we haven't received a formal data delivery from PRAM yet," says NRL's Victor Chen, "we can confirm that the experiment turned on and appears to be operating normally. We hope to have enough data in the coming weeks in order to publish some results this Fall."
When that happens, we'll then be able to see if SSP looks like it's headed for something like GPS-style glory, or if it gets kicked permanently into the long grass, like that other infamous space-age dead-end, the space elevator.
Paul Marks is a technology journalist, writer, and editor based in London, U.K.
One factor not taken into account in this article on SSP is the cost and efficiency loss associated with delivering the power to where it's actually needed (densely populated areas). The receiving antenna array on Earth, which converts the incoming microwave beam from a geostationary satellite back into electricity, is described as being sited in 'unpopulated areas'. In other words, the electricity generated in this way is far from its ultimate destination. The additional transmission network required to get this electricity to customers needs to be factored into the cost-benefit equation. I'm actually with Elon Musk on this one. It's actually more economic and efficient to keep the solar panels on Earth.
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