vol 8, issue 4

Beam it down

12 April 2013
By Anne Harris
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A space solar power concept

Solar harvesting satellites would need to be extremely large

A space solar power concept

The cost of a space solar satellite such as this could be justified by it serving several ground stations

It sounds like science fiction, but beaming solar power from space is now technically feasible.

"I seem to spend half of my time convincing people that I am not mad," John C Mankins explains. He certainly fulfils some of the basic criteria: goatee beard, glasses and a penchant for outrageous ties. But that belies the 25-year Nasa veteran's pedigree and the genius of his ideas. For Mankins believes that we can solve the burgeoning energy crisis by beaming solar power from space.

Mankins, acknowledged as the leading expert on Space Solar Power, is president of the Space Power Association and Artemis Innovation – a developer of space solar systems. While at Nasa he led an $800m R&D programme and for many years was in charge of space solar studies at Nasa.

"There seems to always be two strongly polarised reactions to my concept," Mankins says. "Most people have never heard of it and so have no opinion. When they hear about it they say, 'well that makes sense, why aren't we doing that?'.

"Then there are other people who are familiar with it, but from the concepts of 30 years ago. Their reaction is, 'well this is completely infeasible and is so crazy we shouldn't even talk about it'. So there is really nobody in the middle."

It is easy to see the allure of space solar power. The Sun, with an expected life of five billion years, produces a trillion times the energy required by the entire planet. It has been a dream of scientists for over 50 years, but the reality is drawing closer.

The spiritual godfather of space solar power (SSP) is Peter Glasser, of Arthur D Little. His patent, filed in 1968, set the path for future endeavours. The basic concept of the solar power satellites (SPS) is quite elegant: a large platform positioned in space in a high Earth orbit continuously collects and converts solar energy into electricity.

This power is then used to drive a wireless power transmission (WPT) system that transmits the solar energy to receivers on Earth. Because of its immunity to night-time, to weather or to the changing seasons, the SPS concept has the potential to achieve much greater energy efficiency than ground-based solar power systems.

In the late 1970s there were several reports and, in the age of space discovery, funding flowed into numerous projects. By the 1980s, however, priorities had changed and after a less than complimentary report by the National Research Council funding ceased.

The baton was taken up around the globe in the 1990s, primarily in Japan, Canada and Europe. The US also returned to the fray with Nasa's 'Fresh Look' report. By the turn of the century it was generally recognised that SSP was technically feasible. The final piece in the jigsaw came with the International Society of Astronautics report in 2011, edited by Mankins, which concluded that the time was right for SSP.

"In its early days the programme structure was very much an Apollo type architecture," Mankins adds. "Let's go in guns blazing and dollars flying. And 20 years from now we would have spent a trillion dollars and solved the problem. That was so aggressive given the technical challenges and so wildly expensive that it was just unpalatable."

Mankins compares space solar to the development of fusion power, which has been researched since the 1950s. Billions and billions of dollars have been spent on research.

"Nobody says that even though it has been 50 years in the making, we shouldn't work on that," he continues. "It's just accepted. It's something that people don't know a lot about. They accept the idea because they heard it for a long time and are comfortable with it."

In Mankins' SPS Alpha concept, developed by Artemis Innovation, the incoming sunlight is intercepted by a large platform in a geostationary Earth orbit. It is converted on the platform from sunlight into electricity. The electricity is used to drive millions of solid state power amplifiers; similar to the amplifiers in phones. These operate in concert so that transmissions are in phase and produce a focused coherent beam of low-intensity microwave energy.

This collimated beam travels through space and the atmosphere to a target on the ground where there would be a large antenna, essentially a large mesh with a small antenna every three to five inches. The incoming beam of dilute electromagnetic energy would be absorbed by millions of individual antennas on this mesh.

The microwave rectifying antenna, for converting microwave energy back into electricity, was developed by Raytheon's Bill Brown back in the 1970s.

"It's already quite high efficiency," Mankins says. "It would collect the power and put it through transformers just like large photovoltaic array power plants."

The transmission is about 70 per cent efficient. Much improved from the 20 per cent achieved in the 1970s. But there is still the need for additional improvements.

Size matters

Solar energy is diffused, which means the satellite in space needs to be extremely large to collect about 2GW of power. There is about 1,400 watt per square metre in sunlight in the vicinity of the Earth. On a sunny day in the desert it reduced by a third by the time it hits the ground.

The space transmitter is expected to be 1,000m in diameter and fed by solar collectors, which would be three to five times larger. The receiver on the ground would be the size of a small lake – about six miles across.

"In the concept, the interception of the sunlight is done with large thin film mirrors, like sails," Mankins says. "Then the energy is reflected to where you need it on the platform. In some of the old concepts from the 1970s, it was done with large conventional PV arrays which is one of the technical problems that those concepts had."

A high proportion of the cost would be in the space-based system. To dilute this cost it would theoretically be possible for one space system to service several ground stations as it passed over them.

Another huge advance since space solar's inception is the robotics required to assemble the space platform. "Robotics from 1975 were extremely primitive, very simple automated arms with no flexibility," says Mankins. "Today you can give your kid a robot for Christmas that is more powerful. Similarly, the concept with SSP Alpha and its modular approaches is that these bits are launched in many packages ready to go when they get into space, it is all assembled by robots working in the colony."

The first module would weigh around 300lb. "The first step is to build one and test it on Earth," Mankins explains. "Then build half a dozen and test them in orbit. It's like any kind of production curve problem. You want to get to the point where you can make hundreds of thousands of these things as though they were PCs or automobiles. But that takes time to build up to that point.

"At each stage we need to increase the scale and the fidelity of the demonstration. First on Earth, then in low Earth orbit. Then in high Earth orbit, first with one, then with ten, and then with hundreds."

The beauty of Mankins' concept is that it is perfectly scalable. "It's like having a PC with a USB port and hooking it up to a wireless router," he says. "You can change your hard drive. You can upgrade your operating systems. You can do all of those things but it is still the same PC. With this modular architecture, you can establish the base line and later on you can slot on the robotics. You can put in higher efficiency antennas and evolve the whole thing bit by bit.

"We need to make it gravity gradient stable," Mankins continues. "The same forces that cause tides. It would need propulsion to stay where you put it. It would constantly be tugged by the gravity of the Moon and Sun. Even a small geostationary communications satellite has to have North/South station keeping because they get pulled gravity and need propulsion to keep them where you want them to be."

Beam me down

One of the main areas of concern in matters of safety is the microwave beam itself. Mankins concedes that it is a legitimate worry but there are no real concerns.

The concept would be to keep the intensity of the beam in the vicinity of the Earth below 10 or 20 per cent of summer sunlight. The peak would be 200 watts per square metre. "If you think about being out on the street on a cloudy day, then the cloud moves away and you feel the warmth," Makins adds. "That would be the difference from going outside of the beam to inside of the beam. You keep the beam intensity low to keep it safe. Then you don't let anybody go into it. You have to have a no fly zone above it. This is just like they do with nuclear power plants; aircraft don't fly above them, they re-route.

More efficiency is always good, but the biggest challenge that solar power has is heat management. It has got all the components of a PC and all the associated heat concerns. The PC uses a fan's convection and air to cool itself. In space there is no air. "The satellite is sitting in a vacuum," Mankins says. "The biggest challenge is to come up with a design that optimises the rejection of the radiation away from the satellite of the waste heat.

"We've made progress in some technologies. Higher efficiency solar cells. Higher efficiency electronics. Electronics that are made in new materials that can stand higher temperatures."

Future

When it comes to a timescale for the roll-out of SSP, Mankins explains that it is all funding driven. The first real ground demo with realistic pieces could be done in less than three years, but will cost in the order of $6m. The next step would be a ground-based prototype. This would take another three years to make the first prototype and then ten years to build a big one, again depending on the funding.

"If you look at the logistics curve, the S-curve of all these industries and the way they step up, the front end of the S-curve for space solar power could be the order of a decade," he says.

"I will agree with you that in the concepts of the 1970s you really had to have billions of dollars to make any progress," Mankins continues. "With the modular architecture it's just not true anymore. You don't have to have a billion dollars to build the first prototype. You do need about hundred million dollars, but it's not a billion, it's not tens of billions and it's not a trillion, because you can build the prototype from lots of small identical pieces."

Despite its attractiveness – no need for energy storage because in space the Sun shines continually – space solar may flounder on the rocks of fiscal strictures. "You have to have money," Mankins concedes. And quite where that money will come from is, at the moment, the biggest hurdle to the technology's deployment."

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Space solar power

In 2011 the International Academy of Astronautics published the first international assessment of space solar power. Here are ten of the main findings.

1 Solar Power Satellites (SPS) appear to be technically feasible within 20 years using technologies existing now in the laboratory that could be developed and demonstrated.

2 The mature technologies and systems required to deploy economically viable SPS immediately do not currently exist; however, no fundamental breakthroughs appear necessary and the degree of difficulty in projected R&D appears tractable.

3 Very low-cost Earth-to-orbit (ETO) transportation is a critically needed supporting infrastructure in which new technologies and systems must be developed to establish economic viability for commercial markets.

4 The potential economic viability of SPS has substantially improved during the past decade as a result of the emergence both of government incentives for green energy systems, and of premium niche markets.

5 Establishing the economic viability of SPS will require a step-wise approach, rather than being achieved all at once. In particular SPS platform economics, space transportation economics, in-space operations economics and integration into energy markets will likely require iterative improvements to build confidence and secure funding for further developments.

6 Low-cost ETO transportation is an enabling capability to the economic viability of space solar power for commercial baseload power markets. These appear to be technically feasible during the next 20 years using technologies existing in the laboratory. However, the technologies required for this future space capability are not sufficiently mature for system development to begin at present.

7 Acceptable ETO systems for future SPS must be environmentally benign space transportation infrastructures to launch the satellites cannot result in harmful pollution of the atmosphere.

8 Technology Flight Experiments (TFEs) to test critical technology elements and Technology Flight Demonstrations (TFDs) that validate SPS systems concepts to a high level of maturity appear to be essential in order to build confidence among engineers, policy makers, and the public and allow space solar power technology maturation and SPS deployment to proceed.

9 The International Space Station (ISS) appears to represent a highly attractive potential platform at which various SSP and related TFEs could be performed.

10 Spectrum management is an issue of particular importance that must be addressed early due to the time-consuming international processes that are in place vis-à-vis use of the electromagnetic spectrum and orbital slot allocations.

What other options are available?

Updated 1979 SPS Reference

This approach is epitomised by the architecture used in the 1979 SPS Reference System Concept. It involves one or more large, Sun-pointed solar collection systems and an Earth-pointed WPT system that involves the use of microwave radio frequency for power transmission.

The architecture is an extremely large three-axis stabilised platform. Connecting Sun-pointing and Earth-pointing elements is a large scale power management and distribution system (either high-voltage or superconducting), including a live rotating coupler. This architecture includes large-scale ground-based rectifying antennas as receivers for the transmitted power, as well as appropriate safety assurance systems. The receivers might be positioned within 100km or less of markets to be served.

SPS Electric Laser Concepts

Electric laser SPS concepts can be electric-laser based or solar-pumped laser. Electric lasers appear to be the most feasible in the foreseeable future. Within the area of laser SPS, there are several alternative systems approaches, involving either integrated platforms comprising multiple individual laser systems or constellations of free-flying laser platforms.

The concept chosen for characterisation by the IAA study was that of an integrated platform, comprising multiple largely independent solar power generation and laser power transmission elements. The receiver is assumed to be band gap tailored PV arrays; these might be placed within 100km of markets to be served, but must comply with safety constraints.

SPS Sandwich and Related Concepts

The Type III SPS option is the SPS Sandwich and related concepts, implemented with a highly modular architecture. This involves a light-redirection-based approach to energy distribution on the SPS platform. It also depends upon the successful local integration of solar power generation, PMAD and WPT systems in extremely large numbers of individual modular space systems.

This architecture option includes large-scale ground-based retenna systems as receivers for the microwave power, as well as appropriate operational safety assurance systems. The receivers might be positioned within 100km or less of markets.

Further options

There are a diverse number of other concepts for SSP, including alternatives types of SPS platforms and alternative deployment locations. Many of these options were identified in the 1995-1997 Nasa SSP Fresh Look Study, the purpose of which was to determine whether new technologies (emerging since the 1970s) might make possible new, more affordable SPS systems concepts. Several of the more well-known and interesting alternative space solar power concepts include:

– The SunTower SPS (LEO or GEO),

– The Integrated Symmetrical Concentrator (ISC) SPS (GEO)

– Lunar surface-based Lunar Solar Power (LSP),

– GEO-based Solar-Pumped Laser SPS,

– Earth-Sun L-2 Libration Point SPS

– Earth Orbiting Reflectors (Sunlight reflected to earth).

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