- If the idea of orbital photovoltaic plants is technologically credible, it does not respond to the urgency of the climate challenge, according to our partner The Conversation.
- London, however, says it wants to launch 30 gigawatts of solar panels into orbit as early as 2045, while Washington and Beijing have also announced that they are working in this direction.
- This analysis was conducted by Emmanuelle Rio, professor and researcher in physics at Université Paris-Saclay, François Graner, CNRS research director at Université Paris Cité, and Roland Lehoucq, researcher in astrophysics at the French Alternative Energies and Atomic Energy Commission (CEA).
In space, the sun always shines. What sparks the idea – crazy? – to deploy huge solar panels in orbit of the Earth to supply humanity with electricity. No clouds interposing, no alternation of day and night: we avoid "intermittency", one of the major defects of solar energy on Earth.
Such an orbital solar power plant was first proposed in 1941 by Isaac Asimov, in a short story entitled Reason. Since then, the idea has gained supporters and is spreading. It is so attractive that we learned, in August 2022 and through its Director General, that the European Space Agency is thinking about it.
London also says it wants to launch 30 gigawatts of solar panels into orbit as early as 2045, while Washington and Beijing have also announced that they are working in this direction.
In fact, solar energy is one of the most acceptable energies we have.
Is the idea of sending photovoltaic power plants into space technologically credible? Maybe... But, as we will see, it does not make it possible to respond to the urgency of the climate challenge.
Under the sun
Solar energy is available in large quantities and distributed throughout the globe. Certainly more in Morocco, with its 3000 hours of sunshine per year, than in Norway, half as bright. In addition, this energy generates little waste, no greenhouse gas emissions during its electricity production phase, and little over its entire life cycle, compared to fossil sources.
In short, among renewable energies, solar energy has good press. Nothing being perfect, solar panels are greedy in silicon and copper. Above all, the sunshine stops at night, and... when there are clouds.
But in an orbital power plant, neither night nor clouds! The solar panels would be in geostationary orbit, at an altitude of 36,000 kilometers. They would pass in the shadow of the Earth less than 1% of the time.
This is much better than in low orbit: indeed, the International Space Station, at an altitude of 450 kilometers, because of the regular passage in the shadow of the Earth, sees its solar panels lose about 30% of the power of sunshine.
How to bring energy back to Earth?
Let's start by forgetting cable transmission, because a cable of this length, even if it were feasible, would give scares to all planes and satellites.
Although more attractive, let's also forget about the laser. Even operating in the wavelength range that the atmosphere allows to pass ("the atmospheric window"), the interactions of the beam with air molecules (absorption and diffusion) would significantly complicate the transmission of energy, especially since humidity and cloud cover are important.
It would also raise some concerns about the military use of such a powerful device: we are talking about transferring gigawatts, a thousand times more than a military laser capable of neutralizing an armored vehicle.
The option that is currently on the rise is to convert the collected light energy into electricity, which in turn is converted into a microwave beam sent downwards. This beam would be captured by the vertical region of the Earth's surface, where it would be converted back into electricity.
Airbus recently announced the success of a ground test carried out in Munich with the company Emrod: a transmitting antenna of 2 meters in diameter converting an initial power of 10 kilowatts into microwaves of 5.8 gigahertz made it possible to transfer 2 kilowatts at a distance of 36 meters.
What energy gain compared to a ground-based power plant?
The very fact that companies are testing the process suggests that it may be economically viable. But physics imposes some limits, in terms of energy gain, space occupation and pace of implementation.
First advantage on paper: a solar panel in geostationary orbit always well oriented facing the Sun, and not subject to the vagaries of clouds, provides according to our calculations about three times more energy than its counterpart in a well-exposed region, such as the Sahara for example.
That may sound like a lot, but it's not up to the challenge. Indeed, the double conversion (of electricity into microwaves, then again into electricity) necessarily causes losses: currently, half of the power is lost. The real gain, compared to a ground-based power plant, is therefore not three, but only 1.5.
Can it compensate for the inconvenience (or even impossibility) of intervening for maintenance, and what putting into orbit represents as an expense of materials, energy, money, and pollution?
What floor area?
Second advantage on paper: the orbital power plant is supposed to avoid the monopolization and artificialization of the earth's surface, usable for many other things (living, cultivating, preserving ...)
In reality, capturing the energy sent by an orbital plant, say a few gigawatts as we can imagine in the long term, requires a very large surface area on the ground. Indeed, a microwave beam is not a thin straight line, nor a fortiori a converging beam as a clever perspective or a really false illustration could lead us to believe. It is a divergent cone: fine point at the start, wide base at the finish.
This phenomenon called "diffraction" is not anecdotal. A NASA study published in 1978 discussed the case of an orbital solar power plant capable of delivering to the ground a power of 5 gigawatts (from 75 gigawatts of sunlight captured).
It required a transmitting antenna 1 kilometer in diameter placed in orbit and a ground receiving antenna of 13 x 10 kilometers (a little more than the area of Paris), if the transmission of energy was done with a microwave beam whose frequency is 2.45 gigahertz.
The size of the antenna can be reduced by using a higher frequency range while remaining able to pass through the atmosphere, at least as long as the atmosphere is not too humid.
The frequency of 100 gigahertz could be a good compromise: the antenna in orbit would then have 30 meters in diameter, and would be associated with a ground catchment area of 3.6 kilometers in diameter (one hundred and twelve times the diameter of the antenna), or a ground area of about 10 square kilometers.
Compare this to the size of the most powerful onshore solar power plants: Bhadla in India, 8 kilometers in diameter, or Benban, in Egypt, 7 kilometers in diameter, have installed capacities of 2.2 and 1.7 gigawatts respectively.
In other words, the expected gain by going into space is disappointing: the footprint is of the same order as that of a terrestrial power plant of comparable power.
Fast...
Finally, think about the speed race against climate change. Many thermal power stations need to be shut down as soon as possible. A few gigawatts placed in orbit in ten or twenty years weigh little compared to the 66 gigawatts of panels installed on the ground in China alone in 2022.
OUR "SPACE" DOSSIER
And especially in the face of the necessary degrowth in view of the current crisis of energy, materials and the environment: we must reduce, now and massively, our total energy consumption.
Indeed, the only completely clean energy is the one that is not consumed.
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This article is produced by The Conversation and hosted by 20 Minutes.
- Sciences
- The Conversation
- Sun
- Space
- Climate change
- Solar energy
- Solar panels
- Photovoltaic