In space, neither sunrise nor sunset, no day-night intermittence, no clouds, ideal for capturing solar energy… Research on solar orbital stations is not new and scientists are working there with optimism in order to meet the increase in our energy needs and the major challenges of the climate emergency. The concept is compelling…in theory only; because, despite the considerable technological progress, the solar power station in space would not be the miracle solution. Here’s why.
In space, the soleilsoleil always shines. What sparks the idea – crazy? – to deploy huge solar panels in orbitorbit of the Earth to supply humanity with electricity. No cloudsclouds intervening, no day-night alternation: this avoids “intermittence”, one of the major faults of theenergyenergy solar on Earth.
Such a solar power stationsolar power station orbital was first proposed in 1941 by Isaac AsimovIsaac Asimovin a short story titled Reason. Since then, the idea has gained supporters and is spreading – so attractive that in August 2022 we learned, through its director general, that the European Space Agency is considering it. London also claims to want to launch 30 gigawatts of solar panels into orbit from 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 available to us.
Is the idea of sending photovoltaic power plants into space technologically credible? Perhaps… 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 over the entire surface of the globe. Certainly more in Morocco, with its 3,000 hours of sunshine per year, than in Norway, half as lighted. In addition, this energy generates little waste, no issueissue of greenhouse gasgreenhouse gas during its electricity production phase, and little over its entire life cycle, compared to fossil sources. In short, among renewable energies, solar energy has a good press. Nothing being perfect, solar panels are greedy in siliconsilicon and in coppercopper. 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 geostationary orbitgeostationary orbit, at an altitude of 36,000 kilometers. They would pass through the Earth’s shadow less than 1% of the time. It is much better than in low orbit: indeed, the international space stationinternational space stationat an altitude of 450 kilometers, because of the regular passage in the shadow of the Earth, sees its solar panels lose approximately 30% of the power of sunshine.
Let’s start by forgetting about cable transmission, because a cable of this length, even if it were feasible, would give all planes and satellites a scare.
Although more attractive, let’s also forget the laserlaser. Even when operating in the range of wavelengthswavelengths what’atmosphereatmosphere lets through (“the atmospheric window”), the interactions of the beam with the moleculesmolecules of the’airair (absorptionabsorption et diffusiondiffusion) would singularly complicate the transmission of energy, and this all the more so as humidity and cloud cover are important. This would also raise some concerns about the military use of such a powerful device: we are talking here about transferring gigawatts, a thousand times more than a military laser capable of neutralizing an armored vehicle.
The option that currently has the ventvent at the stern consists of converting the light energy collected into electricity, which in turn is converted into a beam of microwaves sent downwards. This beam would be picked up by the vertical region of the earth’s surface, where it would be converted back into electricity.
The Airbus company recently announced the success of a ground test carried out in Munich with the Emrod company: a transmitting antenna 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 36 meters distance.
What energy gain compared to a ground-based plant?
The very fact that companies are testing the process suggests that it may be economically viable. But the physiquephysique imposes some limits, in terms of energy gain, space occupation and pace of installation.
First advantage on paper: a solar panel in geostationary orbit always well oriented towards the Sun, and not subject to the vagaries of clouds, provides according to our calculations approximately 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 back into electricity) necessarily causes losses: currently, we lose half the power. The real gain, compared to a plant on the ground, is therefore not three, but only 1.5.
Can it compensate for the inconvenience (or even the impossibility) of intervening for maintenance, and what putting it into orbit represents in terms of expenditure of materials, energy,argentargentand as pollution?
What floor area?
Second advantage on paper: the orbital power plant is supposed to avoid the grabbing and artificialization of the earth’s surface, which can be used for many other things (living, cultivating, preserving…).
In reality, capturing the energy sent by an orbital power plant, say a few gigawatts as one can imagine in the long term, requires a very large surface area on the ground. Indeed, a beam of microwaves is not a fine straight line, nor a fortiori a converging beam as a clever perspective or a really false illustration might make believe. It’s a conecone divergent: fine tip 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 5 gigawatts of power to the ground (from 75 gigawatts of lightlight captured solar). It required a transmitting antenna of 1 kilometer in diameter placed in orbit and an antenna of receptionreception on the ground 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 cross the atmosphere, in any case as long as the latter is not too humid. The frequency of 100 gigahertz could be a good compromise: the antenna in orbit would then be 30 meters in diameter, and would be associated with a collection surface on the ground 3.6 kilometers in diameter (one hundred and twelve times the diameter of the antenna ), i.e. a surface area of the order of 10 square kilometres.
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 turns out to be disappointing: the grip on the ground is of the same order as that of a terrestrial power plant of comparable power.
To do quickly
Finally, let’s think about the race of vitessevitesse against the climate changeclimate change. Many thermal power plants must be closed as soon as possible. A few gigawatts placed in orbit in ten or twenty years hardly weigh against the 66 gigawatts of panels installed on the ground in China alone in 2022. And especially in the face of the essential decrease in view of the current energy crisis, the mattermatter and the environment: we must now massively reduce our total energy consumption. Indeed, the only completely clean energy is that which is not consumed.