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Plugging into the Sun
24 November 2007
From New Scientist Print Edition.
Dan Cho
David Cohen

Enlarge Beaming down

If it happens, it will be the space engineering feat that tops them all. Spanning several square kilometres, a space power station would be by far the largest orbiting structure ever built, dwarfing the International Space Station like a skyscraper towering over a tin shack. More importantly, it could be the answer to our energy woes.

While the engineering may be on a colossal scale, the idea behind space solar power is simple enough. Lob giant solar panels into geostationary orbit, then use the electricity they generate to send an intense beam of laser light or microwaves down to Earth where it will be converted back into electricity to be pumped into the grid. In one fell swoop we could slash CO2 emissions and reduce our reliance on oil. The beam could be used to deliver power to remote locations without the need for expensive transmission lines, and even provide instant on-demand electricity to soldiers in the field. One day the beams may even be used to power a new generation of spacecraft or help to control the weather (see "Power from the sky").

The dream of generating our electricity in space has been around for decades, but so far it has always proved too expensive to follow through. With a conference of space and energy specialists due to report in the next few weeks, that may be about to change. Energy prices are soaring and the security of fuel supplies is becoming a priority, so the conference, which took place in May at the Massachusetts Institute of Technology, was convened to work out whether space solar power was an idea worth reviving. Meanwhile, a study group put together by the Pentagon has been assessing its military benefits, and in Japan a $12 million 10-year programme to study space solar power has just been given an extra $2.3 million to fund experiments. So has the concept's time finally come, or will sky-high costs and safety concerns over powerful energy beams from space win out again?

It was in 1968 that Peter Glaser, an engineer at the consulting firm Arthur D. Little in Cambridge, Massachusetts, put forward the idea of harvesting energy from orbiting solar panels then converting it into microwaves that would be beamed back to Earth. Glaser envisioned gigantic arrays of solar panels with an area of some 50 square kilometres, each producing several gigawatts of power. They would be launched by reusable rockets lifting 250-tonne payloads into orbit - a tall order given that even today's space shuttles can lift no more than 50 tonnes. Hundreds of astronauts would have to live in space, toiling as construction workers to assemble a structure that would cover more than half the area of Manhattan. In his design, a 1-kilometre-long microwave antenna would beam power back to Earth while the solar panels were kept facing the sun. For this to work, the giant antenna would have to be mounted on gimbals to allow it to swivel independently of the rest of the array. The receiving antenna on the ground would be almost as impressive, and even more extensive, with an area of over 100 square kilometres. It was going to be a big job.

Giant generator

If the mammoth project could be made to work, the benefits are clear. Put a solar panel out beyond the Earth's atmosphere and it can generate almost 20 times as much electricity as it could on average at ground level, as it would not suffer losses due to atmospheric absorption, day-night cycles and cloud cover. Factor in the energy storage systems needed on Earth to cover for periods of darkness and that advantage could double (see Diagram). At the geostationary altitude of nearly 36,000 kilometres, every square metre of a satellite facing the sun would receive 1360 watts of solar energy almost continuously, even when the Earth below is blanketed by cloud.

According to Masahiro Mori, director of the Advanced Mission Research Centre at the Japan Aerospace Exploration Agency (JAXA) in Tsukuba, these efficiencies mean that even taking into account the energy needed for construction, a space-based solar power station would still produce around 6 times as much energy as a solar power station of the same surface area on Earth.

When NASA and the US Department of Energy took a look at space-based solar power in the 1970s, they concluded it was technically feasible, but the cost ruled it out. Just getting the first satellite up and running would cost the equivalent of $1 trillion at today's prices - and the scheme would require dozens like it. "The capital cost was so great it boggled the mind," says Martin Hoffert, emeritus professor of physics at New York University and a long-time supporter of space solar power.

John Mankins, a former NASA research manager who worked on space solar power, says a lot has changed since then. Mankins now spends his days as a cheerleader for space solar power through his company Managed Energy Technologies, based in Ashburn, Virginia. He points to three key developments that could bring down the size and cost of a solar power satellite to realistic levels. First, solar cells are now four times as efficient at converting solar energy to electricity as they were in the 1970s, and improving, so the area of solar arrays required can be cut. Beaming technology has improved too. Solid-state devices can now be used to point microwave beams electronically rather than relying on a swivelling antenna, so small, easily assembled modular antennas could be used in place of the kilometre-high monolith originally called for. Finally, robots are now capable enough to do much of the construction work.

Some of these advances were noted in 1995, in a feasibility study called "Fresh Look" commissioned by NASA. Though the study found the prospects favourable, NASA did not pursue the project and cancelled its funding in 2001. According to Mankins, the decision was influenced by officials' view that it was not part of NASA's job to develop new energy sources. Similarly, the US Department of Energy has shown little enthusiasm for space technology. No single agency is willing to take overall responsibility, Mankins complains.

In Japan it has been a different story. Research into the idea began in several universities in the 1980s. The country's ongoing $12 million project, led by Mori and funded jointly by JAXA and the Ministry of Economy, Trade and Industry, is about to complete its first stage after more than a decade of work. Mori says space solar power is crucial to the country's future. "Japan is a country with few natural resources and depends on a vast amount of foreign energy," he says. This, combined with the need to cut CO2 emissions, means that space solar power systems are "the only option for future large-scale, stable, national and clean energy requirements in our country". Mori's goal is to create a design that will cost under $1 billion to build and produce 1 gigawatt of power at a cost of 7 to 8 cents per kilowatt-hour. By comparison, electricity produced by coal-fired power stations costs around 5 ¢/kWh, and nuclear about 10 ¢/kWh. His design comprises two concentrator mirrors measuring 2.5 by 3.5 kilometres that focus sunlight onto two solar panels each 1.25 kilometres across. These connect to a microwave transmitter 1.8 kilometres across that beams the power to a terrestrial receiver station (see Diagram). He says it could be built by 2030.

Groups at Kobe University and the Institute for Unmanned Space Experiment Free Flyer (USEF) in Tokyo have pursued rival designs. They are focused on cutting weight to reduce launch costs, improving solar cell efficiency and exploring ways of hardening the cells to radiation. In their design, 100-square-metre combined solar-cell and microwave transmitter units can be built up, checkerboard-like, into a flat mesh that would be anchored to a control satellite. The design means the solar panels cannot be reoriented to face the sun, so output is cut by around one-third compared with Mori's design. The team says this is a price worth paying: their design is cheaper and can begin to transmit power at a much earlier stage in its construction, while its modular nature means it can be expanded later.

Last year, another team from Kobe University and the University of Tokyo put a similar concept to the test. They sent a rocket up to a suborbital altitude of 210 kilometres to deploy a 130-square-metre net - essentially a spiderweb made of wire - connecting a microwave transmitter to two robots that could autonomously move around the web. The idea was that eventually hundreds of these robots would be deployed, equipped with solar panels, to self-assemble and beam power back to Earth. The apparatus had just 4 minutes of microgravity in which to configure itself and transmit a signal. The experiment was a success, but full results have yet to be published.

A recent study led by Yutaro Kobayashi at USEF estimated that the CO2 footprint of a space solar power plant over its lifetime would be about the same per kilowatt-hour of electricity generated as that of a nuclear plant. With climate change rising up the political agenda, it is this sort of environmental advantage, together with concerns for energy security and rising oil prices, that is helping to spur a resurgence of space solar power.

For the US National Security Space Office (NSSO), it is a different aspect of space solar power that is the attraction: its possible military applications. This year, the office, which is part of the Pentagon, charged air force lieutenant colonel Michael "Coyote" Smith with the task of investigating the possibilities of space solar power. Though Smith had no funding for a formal investigation, he was able to recruit a group of volunteer specialists from academia and industry to participate in an online discussion through blogs and forums. This culminated in a conference in Colorado in September and a report recommending that the US government spend $10 billion over the next 10 years to develop a small satellite capable of beaming 10 megawatts of energy to Earth. If successful, this would then entice private industry to become involved and develop the technology, the report suggests.

Solar-power satellites could bathe a narrow region in microwaves, allowing targeted transmission to military bases anywhere within line of sight of the satellite. One possibility is that soldiers in the field could wear a receiver antenna to absorb the microwaves and charge up the batteries for their electronic equipment. Even if this application never materialised, beaming down power could become a cost-effective alternative to generating electricity on some far-flung military bases, where it can cost more than $1 per kilowatt-hour - about 10 times the price domestic consumers pay. "That's the first realistic application I've heard," says Olivier de Weck, an astronautics systems engineer at MIT.

The officers involved in drafting the NSSO report have repeatedly said that the military wants to be a "customer" for space solar power, rather than develop it itself. The report did raise the possibility of a new government agency to oversee such a project, but not how it might be funded.

With enthusiasm for space solar power in limited supply in government circles, what can we expect from the upcoming MIT conference report? "The meeting was trying to work out whether there's something to pursue here," says Geoff Landis of NASA Glenn Research in Cleveland, Ohio, one of the participants. "Overall the conclusion was 'It's doable if we make a number of breakthroughs'." Two areas are key, Landis says: upping the capacity of a solar cell from the 400 watts per kilogram available now to 1000 watts per kilogram, and reducing space launch costs from today's $2000-plus per kilogram to $500 per kilogram.

Others are reluctant even to go this far. "You've got to think what you gain and what you lose by going to space. It eliminates storage and transmission problems, but at a pretty enormous price," says Roger Angel, a solar power enthusiast at the University of Arizona, who was also at the MIT meeting. "We can make solar power on the ground cheaper than you ever could from space. I can think realistically about 2500 square km in Arizona. Think about that in space. That's a lot of stuff in space."

De Weck is concerned by the safety issues of using concentrated beams of energy. "It could be dangerous. We don't know what the effects would be on airplanes accidentally flying through such beams. And what if a malfunction caused a beam to misalign and illuminate a populated area? That's a big deal-breaker in my opinion."

The recommendations of the MIT report are likely to be influential. If they are positive, space solar power could be back on the agenda. If not, the 40-year-old idea is likely to stay on the shelf for many more decades.

Dan Cho is a writer based in New York City

From issue 2631 of New Scientist magazine, 24 November 2007, page 42-45

Power from the sky INTO SPACE ON A LIGHT BEAM Resembling a giant bicycle wheel orbiting at an altitude of 480 kilometres, a giant space solar power station, dubbed Powersat, could revolutionise space launches, according to Leik Myrabo of Lightcraft Technologies in Bennington, Vermont. Myrabo envisages an orbiting network of Powersats, each of them 1 kilometre across and using the latest film photovoltaic cells to generate 20 gigawatts of electricity. These satellites would beam microwave power towards a new generation of "lightcraft" carrying six to 12 passengers, which would use the microwave energy to propel themselves into orbit. By feeding some of the electricity they generate to superconducting magnetic storage systems, the Powersats would keep energy in reserve that could be delivered even when they are in darkness. Eventually, he says, these stations could be used to provide power for a whole new generation of space launch vehicles. Myrabo, who presented his idea at a conference on beamed power in Nara, Japan, says the system could be in place by 2025. TWEAKING A TWISTER Can you divert the course of a tornado? Bernard Eastlund and Lyle Jenkins of Eastlund Scientific Enterprises in Houston, Texas, think you can. They have modelled the effects of using microwaves to raise the temperature of up to 100 cubic kilometres of the airmass in which a rainstorm might develop. They found that microwave heating from a source of around 1 gigawatt could help to suppress tornado formation, and suggest that space-based solar power systems might one day be used to control the development of tornadoes.

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