Sun seekers

In July, a team of engineers from Europe and Japan will launch two small crawling robots into space. This humble experiment may come to represent a major step in the development of an emission-free energy source to meet all of Earth’s requirements.

The robots, one made in Europe and one expected to be from Japan, will be launched from Japan in a small test rocket. When the rocket reaches a height of 220km three daughter satellites will be separated from their mother satellite to stretch out a 40m x 40m Furoshiki net. A microwave antenna will then be switched on and begin transmitting signals to a receiver on the ground, and the robots will spend three minutes crawling across the net, under the eye of several cameras.

Later this month five robots, including two developed by Vienna University of Technology with funding from ESA, will undergo aircraft micro-gravity experiments in preparation for the rocket launch. The experiment could have huge implications for telecommunications, Earth observation and satellite navigation. If it is successful such robots could in future crawl along a large mesh of up to 1km x 1km in space, deploy an inflatable structure to create a rigid frame for the net, before assembling themselves into huge structures such as antennas.

But perhaps the most intriguing application for this concept is the part it could play in harvesting solar power from space and transmitting it to large receivers on Earth using microwave or laser beams. The huge structures needed to build solar power satellites would make construction difficult and expensive — a microwave transmitting antenna alone would be around 1km in diameter. So using robots to build the structures in space could cost less.

Robots are seen as a powerful potential tool for the set-up of a space solar power system, enabling space agencies to assemble huge structures in orbit.

The concept of solar power from space (SPS) was first proposed by Czech-US engineer Peter Glaser in 1968, but is now receiving renewed attention amid fears over climate change and the search for emission-free energy sources. Unlike terrestrial solar power plants, a space-based system could collect energy 24 hours a day, with no atmospheric conditions limiting its exposure to the sun. Plus solar flux, the energy or photon flow rate from the sun, is around eight times higher in space than it is on Earth.

According to ESA’s Advanced Concepts Team (ACT), if we could come close to the theoretical power transmission efficiencies via electromagnetic waves of 50 to 60 per cent, the electricity grid could be continuously supplied with 75–100W/m2 of space photovoltaic panel, about three or four times the amount from the same-sized terrestrial PV surfaces.

The technology could potentially generate up to several hundred GW of power, whereas a modern nuclear power plant generates around 1GW. Europe alone is expected to need around 500GW of electricity by 2020.

ACT is investigating solar power from space under its SPS Programme Plan. Its first phase, assessing the general viability of recent concepts for supplying power to Earth from space, was completed last year. Concepts include the European Sail Tower, which would consist of a 15km central tether with 120 power-generating solar sails attached in pairs down its length. The tower, which could generate 450MW (170W/m2) of power in space, resulting in 275MW (100W/m2) of electricity output on Earth, would be launched in individual sail modules into low Earth orbit, from which point electrical thrusters would propel each module to geostationary orbit, to be constructed into a tower robotically.

The second phase of ESA’s SPS programme, due to start in spring, will identify technology areas requiring further research before such systems become a possibility, and establish roadmaps for research priorities. The team will also investigate the integration of SPS concepts into future energy networks, in particular the use of space solar power for hydrogen generation.

The technologies needed to make SPS a reality are not yet mature enough for it to be seen as a serious option, said Dr Leopold Summerer, deputy head of ACT. Further development is needed in areas such as photovoltaic cell efficiency to ensure that the concept is ready to be taken seriously at the next big energy discussion after the current one, expected in 2020–25, he said. ‘SPS is too attractive not to do anything, but too immature to embark on anything really substantial for the moment.’

To ensure the concept is taken more seriously, ESA has brought together SPS experts and independent energy consultants under umbrella research groups. As a result of these discussions the SPS experts have convinced their terrestrial counterparts that there’s more to the concept than simply a way to build up the European launcher industry, said Summerer.

‘Using its own models, the terrestrial community has had to acknowledge that the energy payback time, or the time necessary to get as much energy out of the system as you put into it when you construct and maintain it, is lower for the space system than for the ground system, despite the launchers.’

This payback time, one of the fundamental parameters of any energy system, would range from four months to two years, according to the research.

The viability of the space option also increases with the size of the plant. Space solar power is not competitive with terrestrial for relatively small plants. For plants of above 5GW launch costs of e600–700 per kg (£400–480 per kilo) for base-load power supply are needed for SPS to be competitive with terrestrial plants.

This will require a significant reduction in launch costs. But the increase in launch frequencies required to build an SPS system would go some way to reducing these costs, and this reduction could well open up new markets, further decreasing prices. Companies such as California-based SpaceX are already developing low-cost launch vehicles with the aim of making access to space more affordable. But with launch costs of $15.8m (£8.2m) for SpaceX’s 6,020kg payload Falcon V (£1,362 per kilo), there is still some way to go.

The concept of solar power-generating satellites is also being investigated as a means of transmitting power to bases on the Moon or Mars, where lunar eclipses and Martian dust storms would hamper the effectiveness of ground-based solar generators.

Beyond Europe and Japan, US researchers have also been looking at the concept. NASA first began studying SPS after the oil embargo of the mid-1970s. Over the years the agency has evaluated almost 30 systems. Chief among these is the Suntower concept. Similar in principle to the European Sail Tower, it consists of a constellation of tether-based solar satellites that would initially be deployed in low Earth orbit, then moved to an elliptical Earth orbit for operation.

While the status of the core technology meant that early concepts were prohibitively expensive, studies over the past 20 years have identified a steady improvement in many key technologies.

Promising research is being conducted on harnessing the sun’s power: space agencies are investigating the concept of harvesting energy from the sun for use as a power source on Earth.

John Mankins, manager of NASA’s Exploration Systems Research and Technology division and a key advocate of SPS, puts much of this progress down to advances in exploration technology. He said that while there’s currently no focused SPS programme at NASA, much of the core technology required to build an SPS system has advanced significantly in the past couple of years.

Mankins explained that important work has been done on the development of modular space structures that can be assembled and maintained in orbit by robots. The agency has been developing a range of walking and crawling robots since the late 1990s, including the anthropomorphic ‘robo-naut’, a highly flexible ‘snake’ robot, and the Skyworker mobile crane system concept.

Once an SPS system has been assembled it must still be moved into the optimum operational orbit, and Mankins said that work carried out on in-space transportation could be extremely important. ‘We have made substantial investment into advanced electromagnetic propulsion that is able to move large payloads cheaply out of low Earth orbit.’

But perhaps the most important strides have been made in the improvement in the conversion efficiency rate of solar cells. ‘We have developed new types of solar cell that are highly efficient and lightweight,’ he said.

Like their ESA counterparts, NASA’s researchers have also investigated a variety of approaches to wireless power transmission, including microwave phased arrays using magnetrons or solid state transmitters, as well as visible light transmission using solid state lasers. But Mankins said that beaming is one area in which NASA has made little progress.

The other key obstacle, he said, is the cost of access to space. ‘Large space solar power systems are going to weigh so much more than anything else we’re ever going to do that we’ve got to have really low-cost launches.’ While this may remain something of a dream one proposed method of keeping launch costs down for SPS is to develop smaller concepts that use solar mirrors to concentrate the sun’s rays.

Mankins said that while the technology exists to produce small-scale demonstration systems and put them into orbit with existing launchers, an economical system that sells power for profit is a couple of decades away. ‘If we make the right kind of progress, you could see SPS systems by 2030 — so many technologies are being driven by the needs of exploration that there’s a good foundation for it.’

But while Mankins believes that the SPS will be driven by exploration, others have claimed that the concept will be moved forward by more commercially minded industries.

Prof Marty Hoffert, a leading expert in climate change from New York University’s physics department, has suggested that, with co-operation from the communications and utility companies, it should be possible to piggyback space solar power systems on the ever-increasing number of low-Earth-orbiting (LEO) communications satellites.

Such a system would help share launch costs and provide access to an existing space-based infrastructure of sorts. Also, by using communications satellites in low Earth orbit, only a few hundred miles up, microwaves used to beam energy to Earth would disperse less than those beamed from geostationery orbit, enabling the construction of smaller ground-based receivers.

While there’s little government backing for such a system, researchers like Hoffert believe that private sector activity could help push the concept forward. One promising host for such a project would be the Iridium Satellite System, which uses a constellation of 66 low Earth-orbiting (LEO) satellites operated by Boeing to provide its customers, including the US Department of Defence, with complete coverage of the Earth. Satellite phone company Globalstar also operates a constellation of 48 LEO satellites, while Virginia-based global data service provider Orbcomm has 30 operational LEO satellites and a licence for 17 more.

Hoffert claimed that the future of SPS depends on the willingness of electrical and telecoms companies to get involved. He said that there is a general level of ignorance in the business community about the potential of SPS, and energy technology in general.

‘Engineers can solve the problem of transforming the world energy system away from fossil fuels, but it’s a major challenge, and we need to be open to new ideas like space solar power,’ he said.

Hoffert is one of an increasingly vocal group of engineers, physicists, atmospheric researchers and economists calling for a massive R&D programme in the US along the lines of the Manhattan & Apollo projects to develop a broad spectrum of alternative energy technologies. ‘Right now decisions on the global climate/energy problem are predominantly made by economists and politicians. Good guys, sometimes, but more people need to work on this who have the expertise and skills to make something happen.

Once innovative energy technologies are demonstrated convincingly, and the potential for cost-effectiveness shown, venture capitalists will pile on, as they did for computers, telecommunications, biotech and now nanotech.’

Could SPS be a compelling enough technology to make this happen? NASA’s John Mankins certainly thinks so. ‘The US currently generates something like 700 or 800GW, the world generates four times that. A hundred years from now it’s going to take thousands of gigawatts to satisfy the world’s needs. We will require a whole set of energy sources to do that and SPS could be one of the major ones.’

‘Sailing’ towards an energy solution?

With space exploration technology expected to be one of the driving factors in the development of space solar power, next month’s planned launch of Cosmos-1, the world’s first solar-sail powered spacecraft, will be watched with interest by the SPS community. The result of a collaboration between the US Planetary Society, the Russian Academy of Sciences and Moscow space industry designer Lavochkin, the 50kg probe will be launched from a submarine in Russia’s Barents Sea and carried into orbit onboard a converted intercontinental ballistic missile (ICBM).

The unmanned probe has eight 15m-long triangular solar sail blades made from wafer-thin aluminised reinforced mylar. When light from the sun is reflected from the surface of these sails, the energy and momentum of photons is transferred to them, effectively giving the spacecraft a push.

The craft will hopefully orbit Earth for around a month, with engineers controlling the sail to increase the spacecraft’s distance from the Earth.

Many scientists claim that spacecraft propelled using solar sails could one day make intergalactic space travel possible. With no need for fuel, solar sails provide a low but continuous thrust that could enable spacecraft on long missions to build up speeds far greater than those possible with chemical rockets. It’s estimated that in one year a solar-sail craft could reach 36,000mph, and in three years a speed of over 100,000mph. However, It would take about 1,000 years for a solar sail to reach one-tenth the speed of light.

Clearly, to travel such distances, a vehicle would also need light shining on it continuously, which could become a problem in the darkest reaches of space. Thus scientists have identified a future need for beamed lasers or microwaves than can operate over vast distances: sources similar to those required for SPS operation. In an interesting side project that will be watched keenly by those researching beaming technology, University of California astrophysicist Prof Gregory Benford plans to attempt to accelerate the Cosmos-1 probe by firing microwave beams at it from NASA’s 70m Goldstone satellite dish in the Mojave desert.

Dr Louis Friedman, Cosmos-1’s project director, said that because this mission will be the first flight ever of a solar sail spacecraft, space agencies are showing considerable interest. ‘NASA and the National Oceanic and Atmospheric Administration (NOAA) have asked to use flight data for their research programmes, and have agreed to help in mission operations.’ Last August NASA’s Solar Sail Propulsion Team at the Marshall space flight centre successfully deployed two 10m solar sails in a vacuum and this month plans a laboratory deployment of a sail more than 20m long.