The idea of sending a solar power plant to space and transmitting energy by microwave to the earth, which is not new, will not solve the problem of climate change.
In space, the sun is still shining. What arouses the idea – crazy? – to deploy huge solar panels in orbit from the earth to supply humanity with electricity. No clouds that intervene, no day-free alternation: we avoid “intermittent”, one of the big defects of solar energy on earth.
Such a orbital solar power plant was first proposed in 1941 by Isaac Asimov, in a new entitled Reason. Since then, the idea has won supporters and spread – so attractive that in August 2022, we learned, through its managing director, that the European space agency reflects it. London also claims to want to launch 30 gigawatts of solar panels in orbit from 2045, while Washington and Beijing also announced working on this path.
In fact, solar energy is one of the most acceptable energies we have.
Is the idea of sending photovoltaic power plants to space technologically credible? Perhaps … but, as we will see, it does not allow to respond to the urgency of the climate challenge.
Earth bathes in sunlight
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 less lit. In addition, this energy generates little waste, no greenhouse gas emissions during its electricity production phase, and little on its entire life cycle, compared to fossil sources. In short, among renewable energies, solar energy has a good press. Nothing being perfect, the solar panels are greedy in silicon and copper. Above all, sunshine stops night, and … when there are clouds.
But in an orbital power station, neither night nor clouds! The solar panels would be in geostationary orbit, at 36,000 km above sea level. They would pass in the shadow of the earth less than 1 % of the time. It is much better than low orbit: indeed, the international space station, at 450 km above sea level, because of the regular passage in the shadow of the earth, sees its solar panels lose around 30 % of the sunshine power.
How to bring this energy back to earth?
Let’s start by forgetting the cable transmission, because a cable of this length, even if it was achievable, would give scares to all planes and satellites.
Although more attractive, let’s also forget the laser. Even by operating in the wavelength range that the atmosphere lets pass (“the atmospheric window”), the beam interactions with air molecules (absorption and diffusion) would singularly complicate energy transmission, especially since humidity and cloud cover are significant. This would also arouse some concerns about the military use of such a powerful device: we are talking about transferring gigawatts here, 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 light energy collected into electricity, in turn converted into a bundle of microwaves sent downwards. This beam would be captured by the region of the earthly surface vertically, where it would be converted into electricity.
The Airbus company recently announced the success of a ground test made in Munich with the company EMROD: an issuing antenna of 2 meters in diameter converting an initial power of 10 kilowatts in microwaves of 5.8 Gigahertz made it possible to transfer 2 kilowatts 36 meters away.
What energy gain compared to a power plant on the ground?
The very fact that companies test the process suggests that it may be economically viable. But physics imposes some limits, in terms of energy gain, occupation of space and rhythm of implementation.
First advantage on paper: a solar panel in geostationary orbit always well oriented against the sun, and not subject to the vagaries of the 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. It may seem a lot, but it is not up to the issues. Indeed, the double conversion (of electricity to microwave, then again in electricity) necessarily causes losses: currently, we lose half of the power. The real gain, compared to a power plant on the ground, is therefore not three, but only 1.5.
Can he compensate for the drawback (or even impossibility) of intervening for maintenance, and what putting on orbit represents as an expense of materials, energy, silver, and as pollution?
The problem of the ground surface to occupy
Second advantage on paper: the orbital power plant is supposed to avoid grabbing and artificialization of the earth’s surface, usable 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 ground surface. Indeed, a bundle of microwave is not a thin straight line, nor a fortiori a convergent beam as could make it believe a skillful perspective or a really false illustration. It is a divergent cone: fine point at the start, wide base on arrival.
This phenomenon called “diffraction” is not anecdotal. A study by NASA published in 1978 discussed the case of an orbital solar power plant capable of delivering a power of 5 gigawatts on the ground (from 75 gigawatts of solar light captured). It required a 1 km diameter emission antenna placed in orbit and a 13 x 10 km ground reception antenna (a little more than the area of Paris), if the energy transmission was done with a microwave bundle whose frequency is 2.45 Gigahertz.
The dimension of the antenna can be reduced using a higher range of frequencies while remaining capable of crossing the atmosphere, at least as long as the latter is not too wet. 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 capting surface of 3.6 km in diameter (one hundred and twelve times the diameter of the antenna), an area on the ground of the order of 10 square kilometers.
Let us compare this at the size of the most powerful land power plants: Bhadla in India, 8 km in diameter, or Benban, Egypt, 7 km in diameter, have respective powers of 2.2 and 1.7 gigawatts. 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.
A false track
Finally, let’s think about speed racing against climate change. Many thermal power plants must be closed as quickly as possible. Some gigawatts placed in orbit in ten or twenty years hardly weigh in front of the 66 gigawatts of panels installed on the ground just in China in 2022. And especially in the face of the essential decrease in view of the current energy, matter and environment crisis: we have to reduce, now and massively, our total energy consumption. Indeed, the only completely clean energy is that which is not consumed.
This article benefited from discussions with François Briens (economist and engineer in energy systems), Jean-Manuel Traimond (author and speaker), Aurélien Ficot (trainer and engineer in environmental sciences).
Emmanuelle Rio, teacher-researcher, Paris-Saclay University; François Graner, CNRS research director, Paris Cité University and Roland Lehoucq, astrophysics researcher, Commission for atomic energy and alternative energies (CEA)
This article is republished from the conversation under the Creative Commons license. Read the original article.
All tech news in the blink of an eye
Add Numerama to your home screen and stay connected to the future!
