Power in space and beaming it to Earth seemed the stuff of science fiction, the falling costs of solar panels and orbital systems mean it may soon be a commercial reality. Key to this will be designing and developing systems that can be economically constructed in orbit, and which will then produce power reliably with little or no maintenance.
The first serious design concepts, produced by NASA in the 1970’s had serious issues with regards to maintenance, with centralised power management systems and massive gimbals transferring gigawatts of power through slip rings hundreds of metres in diameter.
There are great advantages to be gained from space-based solar power. A geostationary satellite could receive continuous solar power at any time of day and beam it to Earth regardless of weather conditions using microwaves. Rather than thinking of this competing with conventional wind and solar, it makes more sense to consider it complementary.
Variable renewable sources will remain the cheapest way to generate power when they are available, but the need for supplementary dispatchable power to deal with prolonged periods of cold, dark and calm weather is a major challenge.
A single space power satellite could provide dispatchable power over a continental scale area, instantly switching power delivery to different receiving stations using phased array beam steering.
CHALLENGES
A major challenge for space-based solar power comes from the need for scale, which is a direct result of the inescapable physics of power beaming. This can be understood in terms of three interconnected considerations: atmospheric attenuation, orbital position, and beam diffraction.
Different frequencies of radiation are absorbed by the atmosphere at different rates, and there are actually only a few frequency bands that pass through the atmosphere relatively freely. The shortest wavelengths are around the visible light spectrum, which pass quite freely through the atmosphere on a clear day but are largely scattered and absorbed by humidity such as clouds and haze.
Microwaves are the shortest wavelengths which pass freely through the atmosphere regardless of weather conditions. The non-communications ISM frequencies of 2.45 GHz and 5.8 GHz (12cm and 5cm respectively) could be used to beam power from space.
Most satellites are in low earth orbits (LEO) between 300 km and 1,000 km above the ground and completing an orbit many times a day. A satellite in such an orbit spends most of its time over the oceans and, due to its relatively low altitude, the land is over the horizon and so it cannot beam power there. Additionally, it spends much of its time in Earth’s shadow and so is unable to provide power at night. Such as system does not provide the primary advantage of SSP over terrestrial solar power – it doesn’t provide continuous power.
John Mankins, who led many critical studies into space-based solar power while manager in the Office of Advanced Concepts and Technology at NASA through the 1990’s, and has worked on various projects, explains: “You can put them in low Earth orbit for demonstration purposes, but these satellites are not dense. They are light and fluffy; they are large and light.
And so, if it was in a low Earth orbit, not only would it be a horrible nightmare for orbital debris, but it would also tend to re-enter. It would re-enter really fast, because it’s like putting a feather outside the window of your car as you’re driving along at 80 km/h.”
In a geostationary orbit (GEO) 35,786 km high, a satellite maintains a fixed position over a point on the equator, orbiting once every 24 hours. From such a great height, power can be readily beamed to locations within a radius of thousands of miles. This means that just a few satellites could cover all the major urban centers on the planet.
Dr Mamatha Maheshwarappa, head of research and development at the UK Space Agency, explains: “Because we are looking at continuous power, to be able to beam it to Earth 24-7, and also to not have Eclipsing, it is better to have it as far as possible, that is in geosynchronous orbit. But going there directly would be really difficult because we need to test the technologies first. So, there is a step before that, demonstrations in low Earth orbit, highly elliptical orbits or in middle Earth orbit. But the ultimate goal is to go to geosynchronous orbit.”
All electromagnetic energy beams diverge eventually. At the short wavelengths of visible light, and coherent beams of lasers, this is minimised, but there is still divergence. For microwaves there is far more divergence. Divergence also reduces if the aperture is larger, which is why a lighthouse has a large reflector. The smallest possible spread, for a Gaussian coherent beam, is known as diffraction limited optics, governed by the equation: D_1 D_2=2.44 λ L.
Where D1 is the diameter of the transmitter, D2 is the diameter of the receiver, λ is the wavelength of the electromagnetic beam, and L is the distance from the transmitter to the receiver.
To beam energy from GEO, with a 5.8 GHz microwave and a transmitter aperture of 400m the beam on the ground would be 10km in diameter. A microwave receiving antennae (rectennae) consists of a mesh of wires, spaced at half the wavelength and suspended from pylons.
This would allow sunlight and rain to pass freely through it. This effectively gives the minimum size for a useful solar power satellite, any smaller and it can’t beam power to Earth. These beam divergence limitations make microwave energy beaming fundamentally safe, limiting the energy intensity. With a 2 GW capacity, the energy density at the surface of the earth would be comparable to sunlight.
Weaponisation would be impossible. While public access would probably be restricted under the rectenna, a person entering the site would suffer no immediate harm and some activities such as maintenance and agricultural may be carried out below it. While large, the receiving stations could be less intrusive than nuclear power stations.
POWER CONVERSION
At such a large scale, centralised power conversion equipment presents significant challenges for operations and maintenance in orbit. Additionally, the satellite must have a solar array which is always pointing towards the Sun, while its microwave antenna is always pointing towards the Earth.
As the satellite orbits the Earth, in sync with the rotation of the Earth, the solar array and antenna must therefore rotate relative to each other. All of the power from this gigawatt scale power generator must also be transmitted through this rotating joint.
Because of the need for a single large transmitter, as explained above, this power transfer can’t be easily broken down into modules. The first concepts for solar power stations in the 1970’s therefore imagined massive slip rings 100’s of meters in diameter, within mechanical gimbals.
These power plants were expected to require thousands of people to operate and maintain them, living in large space stations. It is little wonder that little progress towards space-based solar power was made at that time.
Later concepts envisaged modular solar arrays, each with their own small gimbal, feeding power into a centralised power distribution system which was connected to the microwave antenna. These systems were often long towers that would be gravity stabilised so that their base would hang down to always face the Earth, with a single large antenna at the bottom.
Although this would avoid the need for a single massive gimbal with slip rings, with would remain mechanically complex, with lots of moving parts to potentially fail, as well as a centralised and very high-power distribution system. Such systems would still require considerable maintenance.
NEXT GENERATION
The latest generation of solar power concepts have solved these problems by creating architectures with no moving parts. For example, Space Solar’s CASSIOPeiA concept would spin about its axis in the opposite direction that it orbits the Earth, so that it is always orientated in the same direction relative to the Sun. It would have large mirrors at each end, constantly pointed at the sun, and concentrating sunlight onto a helical array of standardised modules, each including solar panels, power conversion and microwave antenna.
These modules would require no centralised power distribution but could produce a single focused beam as a phased array. This phased array could steer the microwave beam through 360 degrees about the axis of the helix, so that the beam could remain directed at a single point on Earth as the satellite rotates about its axis.
Coordination of the phased array could be achieved though synchronisation to a pilot beam pointing upwards from the receiving station, avoiding the need for any centralised control between modules on the satellite. This would also make it impossible for the beam to wander away from the target rectenna, providing a very high level of safety.
In order to be cost-competitive with existing decarbonised baseload power generation, such as nuclear power, the Levelised cost of electricity LCOE should ideally be below $100/MWh. With the current costs of commercial space solar cells and launches, even ignoring integration and finance costs, the LCOE would be about 30x higher than this.
However, a report on the CASSIOPeiA architecture produced by Frazer-Nash Consultancy for the UK Government concluded that a LCOE of $60/MWh is achievable with today’s technology. Recent analysis by researchers at Caltech have forecast similar costs for another modular architecture they have developed.
Dr Mamatha Maheshwarappa says: “I think two things are very important. One is being able to see the sun and Earth simultaneously. That gives you the maximum coverage and also to be able to provide power throughout the day. And the second thing is being safe. So whichever architecture ticks these two boxes would be OK… (Cassiopea) can look at the Earth and the Sun at the same time because the whole structure rotates in orbit.”
COST REDUCTIONS
To give some indication how these cost reductions might be achieved, the current list price for a Space-X Falcon 9 launch is $69,750,000, with a payload of 5,500kg to geosynchronous transfer orbit, in other words, $12,682/kg to GTO. The Falcon 9 uses kerosine rocket motors with a theoretical minimum propellant to payload ratio (kg fuel/kg payload), assuming zero mass for the rocket itself, of 117 to GEO.
With current kerosine and liquid oxygen prices, this means the propellent cost of getting to GEO is just $120/kg to GEO, less than 1% of the commercial cost. For a methane rocket, the propellant cost would be just $23/kg. If the empty mass of the spacecraft is actually assumed to be about 2x the payload, the fuel cost would be about 3x higher at $69/kg.
Currently, propellant is a small fraction of launch costs. Developing fully reusable launch systems with very low inspection and maintenance costs is therefore the first step in creating an affordable launch capability for space solar. Much of this cost is currently cleaning and inspecting kerosine rocket motors which have been heavily coked after use. With methane these costs will also be greatly reduced.
Similar gains are within reach as solar panels transition from silicon-based cells, which also require a significant layer of glass to shield them from high-energy radiation in space, to much lighter thin film panels.
One question people sometimes ask when they hear about space based solar power, is, “won’t beaming power to Earth cause global warming?” Well in theory, yes, but in practice, not noticeably. Unlike terrestrial solar, which simply converts energy already within the closed system into another form, space based solar does add energy to Earth’s atmosphere. But it’s a very small amount.
The total solar energy reaching the Earth’s upper atmosphere is approximately 1.5 billion TWh/year. Since 1800 global energy use has doubled every 40 years to reach 183,000 TWh in 2023, just 0.01% of the total energy from the sun, while greenhouse gas emissions have increased the energy balance by 0.8%. So, if we were to get all our energy from space, it would cause about 1% of the global warming currently caused by greenhouse gas emissions.
Space-based solar power has the potential to transform the decarbonisation of power grids, enabling higher levels of locally generated wind and solar power by providing low-cost dispatchable power that can be directed to any location almost instantly. Utilising designs that minimise in-orbit maintenance will be key to achieving the low costs required.