The theoretical physicist Freeman Dyson once imagined an alien civilization that was so advanced that it had surrounded its parent star with a giant, artificial shell. The inner surface of this “Dyson sphere” would capture solar radiation and transfer it towards collection points, where it would be converted into usable energy. Such a notion remains science fiction, but could a similar principle be used at a much smaller scale to harness the power of our own Sun?

After all, beyond the clouds, in the nightless blaze of near-Earth space, there is more uninterrupted solar power than humanity could realistically require for centuries to come. That’s why a group of scientists and engineers has, for more than 50 years, been dreaming up techniques to capture this energy in space and beam it back to ground.

“Space-based solar power”, as it’s known, has two huge benefits over traditional methods for tapping into the Sun and the wind. First, putting a sunlight-capturing satellite in space means we wouldn’t need to cover vast swathes of land on Earth with solar panels and wind farms. Second, we’d have an ample supply of energy even when, despite local weather conditions, it’s overcast or the wind has petered out.

And that’s the trouble with solar energy and wind power here on Earth: they can never meet our energy demands on a consistent basis, even if greatly expanded. Researchers at the University of Nottingham estimated last year that, if the UK were to rely totally on these renewable sources, the country would need to store more than 65 terawatt-hours of energy. That would cost over £170bn, more than twice that of the country’s forthcoming high-speed rail network (Energies 14 8524).

Most efforts to realize space-based solar power have, unfortunately, hit seemingly intractable technical and economic problems. But times are changing. Innovative satellite designs, as well as much lower launch costs, are suddenly making space-based solar power seem like a realistic solution. Japan has written it into law as a national goal, while the European Space Agency has put out a call for ideas. China and the US are both building test facilities.

Meanwhile, a consultation published by the UK government in 2021 concluded that space-based solar power is technically and economically feasible. Tantalizingly, it reckoned that this technological solution could be put into practice 10 years before the 2050 “net zero” goal of the Intergovernmental Panel on Climate Change. So is space-based solar power the answer to our climate’s woes? And if so, what’s preventing it from becoming a reality?

Space dreams

The original concept of solar power from space was dreamt up in 1968 by Peter Glaser, a US engineer at the consultancy Arthur D Little. He envisaged placing a huge disc-shaped satellite in geostationary orbit some 36,000 km above the Earth (Science 162 857). The satellite, roughly 6 km in diameter, would be made of photovoltaic panels to collect sunlight and convert it into electrical energy. This energy would then be turned into microwaves using a tube amplifier and beamed to Earth via a 2 km-diameter transmitter.

It’s the only form of green, renewable energy with the potential to provide continuous, baseline electrical power.

Chris Rodenbeck, US Naval Research Laboratory

The beauty of microwaves is they don’t get absorbed by clouds here on Earth and so would pass largely (though not totally) unhindered through our atmosphere. Glaser envisaged them being collected by a fixed antenna 3 km in diameter, where they would be converted into electricity for the grid. “Although the use of satellites for conversion of solar energy may be several decades away,” he wrote, “it is possible to explore several aspects of the required technology as a guide to future developments.”

The initial reaction was positive in at least some quarters, with NASA awarding Glaser’s company, Arthur D Little, a contract for further study. Over the years, however, the conclusions of subsequent studies into space-based solar power have ranged from cautiously positive to outwardly negative.

In 2015, for example, the technology received no more than a lukewarm verdict in a report from the Strategic Studies Institute (SSI) of the US Army War College, which cited “no compelling evidence” that space solar power could be economically competitive with terrestrial power generation. The SSI particularly criticized the “questionable assumptions” made by its proponents regarding getting such a huge orbiting structure into space. Simply put, the report stated that there aren’t enough launch vehicles, and those that are available are too expensive.

But the SSI’s less-than-glowing verdict came before private companies – especially SpaceX – began to transform the space industry. By combining reusable rocket systems with a trial-and-error attitude to research and development, the US firm has, over the last decade, slashed the cost of launch into near-Earth orbit by more than a factor of 10 (per kilo of payload), with plans to reduce it by an order of magnitude further. What the SSI considered a major limitation about launch costs is, in fact, no longer an issue.

Not that the cost of getting a satellite into space has been the only sticking point. Glaser’s original concept was deceptively simple, with many hidden challenges. For starters, as a satellite orbits the Earth, the angle between the Sun, the craft and the point on Earth to which the energy is sent is constantly changing. For example, if a geostationary satellite is trained on Earth, its photovoltaics will be facing the Sun at noon but have their backs to the Sun at midnight. In other words, the satellite would not generate electricity all the time.

The original solution to this problem was to continually rotate the photovoltaic panels relative to the microwave transmitters, which would stay fixed. The photovoltaic panels would then always point towards the Sun, while the transmitters would always face Earth. First put forward in 1979 by NASA as a development of Glaser’s ideas, the solution was extended further in a 2015 proposal by engineers at the China Academy of Space Technology in Beijing, who dubbed it Multi-Rotary Joints Solar Power Satellite, or MR-SPS (figure 1).

Meanwhile, John Mankins, a former NASA engineer, invented a rival solution in 2012. Dubbed SPS Alpha, his idea was to keep the solar panels and transmitter fixed, but install numerous mirrors surrounding the panels (figure 2). Known as heliostats, these mirrors would be able to rotate, continuously redirecting sunlight onto the solar panels and thereby allowing the satellite to supply power to the Earth without a break.

Neither MR-SPS nor SPS Alpha, however, is satisfactory, according to Ian Cash, director and chief engineer at International Electric Company Limited in Oxfordshire, UK. A former designer of electronic systems in the automotive, aerospace and energy sectors, Cash turned his mind a decade ago to the private development of clean, large-scale sources of energy. Initially lured by the potential of nuclear fusion, he was put off by its “really difficult” problems and quickly alighted on space-based solar power as the most practical option.

For Cash, the problem with both MR-SPS and SPS Alpha is that they have to rotate some parts of the satellite relative to others. Every part would therefore have to be physically connected to another and need an articulated joint that moves. Trouble is, when used on satellites like the International Space Station, such joints can fail due to wear and tear. Omitting articulated joints would make a solar-power satellite more reliable, Cash concluded. “I wanted to find out what it would take to have a solid-state solution that always sees the Sun and Earth,” he says.

By 2017 Cash had figured it out, or so he claims. His CASSIOPeiA concept is a satellite that essentially looks like a spiral staircase, with the photovoltaic panels being the “treads” and the microwave transmitters – rod-shaped dipoles – being the “risers”. Its clever helical geometry means that CASSIOPeiA can receive and transmit solar energy 24 hours a day, with no moving parts (figure 3).

Cash, who intends to profit from CASSIOPeiA by licensing the related intellectual property, claims many other benefits to his concept. His proposed satellite can be built of hundreds (and possibly thousands) of smaller modules linked together, with each module capturing solar energy, converting it electronically to microwaves and then transmitting them to Earth. The beauty of this approach is that if any one module were struck by cosmic rays or space debris, its failure wouldn’t knock out the entire system.

Another advantage of CASSIOPeiA is that the non-photovoltaic components are permanently in shadow, which minimizes heat dissipation – something that’s a problem in the convectionless vacuum of space. Finally, as the satellite is always oriented towards the Sun it can occupy more types of orbit, including those that are highly elliptical. It then would be, at times, closer to Earth than if it were geostationary, which makes it cheaper as you don’t need to scale the design on the basis of such a huge transmitter.

Perhaps unsurprisingly, Cash’s competitors do not agree with his assessment. Mankins, who is now based at Artemis Innovation Management Solutions in California, US, disputes that the articulated heliostats in his SPS-Alpha concept are a problem. Instead, he claims they are “a simple extension of [a] very mature technology” that is already used to concentrate sunlight to heat fluids and drive turbines in “solar towers” here on Earth. He also believes that the dual mirrors required by CASSIOPeiA could be a problem as they must be very precisely built.

“I have high regard for Ian and his work; his more recent CASSIOPeiA concept is one of several that are very similar in character, including SPS-Alpha,” says Mankins. “However, I don’t agree with his expectation that CASSIOPeiA will prove to be superior to SPS-Alpha.” For Mankins, the best approach to space-based solar power will ultimately depend on the results of development projects, with the actual cost per kilowatt-hour of electricity here on Earth being the crucial factor.

Scalable and striking

Interest in space solar power has received an added boost in the wake of the UK government’s 2021 report into the technology, which could scarcely have been more positive about the concept. It was drawn up by engineers at the UK-based consultancy Frazer-Nash, who corresponded with a number of space-engineering and energy experts – including the inventors of SPS Alpha, MR-SPS and CASSIOPeiA.

The report concluded that a 1.7 km-wide CASSIOPeiA satellite in geostationary orbit transmitting solar radiation to a 100 km² array of microwave receivers (or “rectenna”) located here on Earth would generate 2 GW of continuous power. That’s equivalent to the output from a large conventional power station. It’s also far better than, say, the existing London Array wind farm in the Thames estuary, which is about 25% larger but generates an average power of barely 190 MW.

More striking, however, was the report’s economic analysis. Based on an estimate that a full-sized system would cost £16.3bn to develop and launch, and allowing for a minimum rate of return on investment of 20% year-on-year, it concluded that a space-based solar-power system could, over its roughly 100-year lifetime, generate energy at £50 per MWh.

Frazer-Nash says that’s 14–52% more expensive than current terrestrial wind and solar energy. But, critically, it’s 39–49% cheaper than biomass, nuclear or the most efficient gas energy sources, which are the only ones currently able to offer uninterrupted “base load” power. The report’s authors also said that their conservative estimate for costings “would be expected to reduce as development proceeds”.

“It’s incredibly scalable,” says Martin Soltau of Frazer-Nash, one of the authors. And with the level of sunlight in the space around Earth being far brighter than down below, he reckons every solar module would collect 10 times as much as it would if installed on the ground. The report reckons that the UK would need a total of 15 satellites – each with its own rectenna – to provide a quarter of the country’s energy needs by 2050. Each rectenna could be located alongside or even within an existing wind farm.

If the scheme were scaled up further, it could in principle deliver over 150% of all global electricity demand (although a resilient energy supply would usually dictate a broad mix of sources). Space-based solar power, Soltau adds, would also have a much lower impact on the environment than Earth-based renewable energy sources. The carbon footprint would be small, there would be few demands on rare-earth minerals, and there would, unlike wind turbines, be no noise or tall visible structures.

If that all sounds too good to be true, it might well be. The Frazer-Nash report admits to several “development issues”, notably finding ways to make wireless energy transfer more efficient. Chris Rodenbeck, an electrical engineer from the US Naval Research Laboratory in Washington DC, says that large-scale demonstrations of the technology are hard to achieve. They require sustained investments and targeted advances in electronic components, such as high-power rectifier diodes, which are not readily available.

Fortunately, wireless energy transmission has been advancing for decades. In 2021 Rodenbeck’s team sent 1.6 kW of electrical power over a distance of 1 km, with a microwave-to-electricity conversion efficiency of 73%. On the face of it, that’s less impressive than the most powerful demonstration of wireless energy to date, which took place in 1975 when staff at NASA’s Goldstone lab in California converted 10 GHz microwaves to electricity at an efficiency of above 80%. Crucially, however, Rodenbeck used lower-frequency 2.4 GHz microwaves, which would suffer much less atmospheric loss in space.

To counteract the higher diffraction (beam spreading) that naturally occurs at lower frequencies, the researchers exploited the surrounding terrain to “bounce” the microwaves towards the receiver array, thereby improving power density by 70% (IEEE J. Microw. 2 28). “We did [the test] fairly quickly and cheaply during the global pandemic,” says Rodenbeck. “We could have achieved more.”

Initial construction will require a 24/7 factory in space, with an assembly line like a car factory on Earth.

Yang Gao, University of Surrey

Rodenbeck is optimistic about the prospects of space-based solar power. Whereas nuclear fusion is, he claims, “running up against basic problems of physics”, space-based solar power – and wireless power transfer – is merely “running up against dollars”. “[It’s] the only form of green, renewable energy with the potential to provide continuous, baseline electrical power,” Rodenbeck claims. “Barring a technical breakthrough [in] controlled nuclear fusion, it seems highly likely that humanity will harness space solar power for future energy needs.”

A note of caution, though, comes from Yang Gao, a space engineer at the University of Surrey in the UK, who admits that “the sheer scale” of the proposed space system “is quite mind-blowing”. She believes the initial construction might well require “a 24/7 factory in space, with an assembly line like a car factory on Earth”, probably using autonomous robots. As for maintaining the facility, once built, Gao says that would be “demanding”.

For Cash, what’s crucial is the orbit that a space-power satellite would occupy. A geostationary solar-power satellite would be so far from Earth that it would require huge and expensive transmitters and rectennas to transmit energy efficiently. But by taking advantage of multiple satellites on shorter, highly elliptical orbits, says Cash, investors could realize smaller working systems on the CASSIOPeiA concept with a fraction of the capital. SPS Alpha and MR-SPS, in contrast, would have to be full sized from day one.

Is there enough will?

And yet the biggest challenge for space-based solar power may not be economic or technical, but political. In a world where substantial numbers of people believe in conspiracy theories surrounding 5G mobile technology, beaming gigawatts of microwave power from space to Earth could prove a tough sell – despite the maximum beam intensity being barely 250 W/m², less than a quarter of the maximum solar intensity at the equator.

In fact, the UK report admits that its proponents need to test the public appetite, and to “curate a conversation” around the key ideas. But there are real technical and societal considerations, too. Where will the rectennas be sited? How will the satellites be decommissioned at their end of life without adding to space junk? Will there be space in the microwave spectrum left for anything else? And will the system be vulnerable to attack?

In the wake of its report, the UK government unveiled a £3m fund to help industries develop some of the key technologies, with former business secretary Kwasi Kwarteng saying that space-based solar power “could provide an affordable, clean and reliable source of energy for the whole world”. That pot of cash is unlikely to go far towards an undertaking of this scale, which is why Soltau has helped to set up a business called Space Solar, which hopes to raise an initial £200m from private investors.

Meanwhile, what he calls a “collaboration of the willing”, the Space Energy Initiative, has gathered scientists, engineers and civil servants from over 50 academic institutions, companies and government bodies, who are working pro bono to help bring a working system to fruition. SpaceX is not yet on the list, but Soltau claims to have caught the US company’s attention. “They’re very interested,” he says.

Cash does not doubt that investment will be found. Terrestrial renewables can’t deliver uninterrupted, base-load power without enormously costly battery infrastructure, while nuclear always faces stiff opposition. Space-based solar power, Cash believes, is a vital part of the mix if we’re to hit net-zero, and simply asking people to use less energy is a “dangerous idea”. Most wars have been fought over a perceived lack of resources,” he says. “If we don’t look at how to keep civilization moving forward, the alternative is very scary.”