Should you install solar power plants in space to fight climate change?

The sun always shines in space. Hence the idea of ​​placing huge solar arrays in near-Earth orbit to supply humanity with electricity. There are no interfering clouds, no alternation of days and nights: in this way, intermittency, one of the main disadvantages of solar energy on Earth, is avoided.

An orbital solar power plant of this type was first proposed in 1941 by Isaac Asimov in his story cause. Since then, the idea has gained adherents and spread. It is so attractive that in August 2022 the director of the European Space Agency assured that it was being studied.

For its part, London says it wants to put 30 gigawatt solar panels into orbit by 2045, while Washington and Beijing have also announced they are working on it.

But is the idea of ​​sending photovoltaic power plants into space technologically feasible? May be. Although, as we will see, this does not allow us to answer the urgency of the climate challenge.

full sun

Solar energy is available in large quantities and distributed throughout the entire surface of the globe, although there are areas that receive more. Morocco has 3,000 hours of sunshine a year. Half in Norway.

In addition, this energy produces little waste, does not emit greenhouse gases during the power generation stage, and is low over the entire life cycle compared to fossil sources.

However, it also has disadvantages: solar cells require silicon and copper, and the Sun stops shining at night and in cloudy weather.

But there is neither night nor clouds in the orbital power plant. Solar panels will be in geostationary orbit at an altitude of 36,000 kilometers. They will spend less than 1% of their time in the Earth’s shadow. This is much better than in low orbit: the International Space Station, 450 kilometers high, regularly passes through the Earth’s shadow and loses about 30% of the sun’s energy.

How do we send energy to Earth?

For starters, let’s forget about cable streaming. A cable of this length, even if possible, would interfere with aircraft and satellites.

Although more attractive, let’s also forget about the laser. Even operating in the wavelength range that the atmosphere allows (“atmospheric window”), the interactions of the beam with air molecules (absorption and scattering) would greatly hinder the transfer of energy, especially at high humidity and cloudiness. .

The most popular option at the moment is the conversion of the collected light energy into electricity, which, in turn, is transformed into a microwave beam directed towards our planet. This beam will be captured by the vertical region of the Earth’s surface, where it will be converted back into electricity.

Airbus recently announced a successful ground test conducted in Munich with Emrod: a 2 meter diameter transmitting antenna that converted an initial power of 10 kilowatts into 5.8 gigahertz microwaves was able to transmit 2 kilowatts over a distance of more than 36 meters.

Will it produce more energy than a traditional power plant?

The very fact that companies are testing this process suggests that it can be cost-effective. But physics imposes some restrictions in terms of energy gain, space occupied and speed of execution.

The first advantage on paper is that a solar panel in geostationary orbit, always well oriented to the Sun, will, according to our calculations, produce about three times more energy than its counterpart in a high irradiance region such as the Sahara. This may seem like a lot, but it’s not enough.

Double conversion (from electricity to microwaves and back to electricity) necessarily leads to losses: now we are losing half the power. Therefore, the real gain, in comparison with a terrestrial plant, is not three, but only 1.5.

Can this figure offset the inconvenience (or even impossibility) of intervention to maintain it, as well as the cost of materials, energy, capital, and pollution that launching it into orbit entails?

How much space will it take on Earth?

The second advantage on paper: it is assumed that the orbital power plant will avoid the monopolization and artificiality of the earth’s surface, which can be used for many other purposes (to live, cultivate, preserve …).

In fact, capturing the energy sent by an orbital power plant, say, a few gigawatts, requires a very large area on Earth.

A microwave beam is not a thin straight line and a converging beam, as is sometimes imagined. This is a divergent cone: a thin tip at the beginning, a wide base at the end.

This phenomenon is called diffraction. A NASA study published in 1978 had already analyzed the case of an orbiting solar power plant capable of delivering 5 gigawatts of energy to Earth from 75 gigawatts of captured sunlight. A transmitting antenna with a diameter of 1 km was required, placed in orbit, and a receiving antenna on the ground with a size of 13 x 10 km (slightly larger than the surface of Paris), if the transmission of energy was carried out by a microwave beam with a frequency of 2.45. gigahertz.

The size of the antenna can be reduced using a higher frequency range and still be able to penetrate the atmosphere, at least as long as it is not too humid. A frequency of 100 GHz might be a good target: the orbital antenna would then be 30 meters in diameter and would be connected to a ground coverage area of ​​3.6 kilometers in diameter (112 times the diameter of the antenna), i.e. about 10 square kilometers.

Compare with the size of the most powerful land-based solar power plants: Bhadla in India with a diameter of 8 kilometers or Benban in Egypt with a diameter of 7 kilometers have an installed capacity of 2.2 and 1.7 gigawatts, respectively. In other words, the cosmic gain is disappointing: its footprint on Earth is of the same order as that of a ground facility of comparable power.

urgent action

Finally, let’s think about the race against climate change. We must shut down many thermal power plants as soon as possible. The few gigawatts put into orbit ten to twenty years from now pale in comparison to the 66 gigawatt panels installed on the ground in China alone in 2022.

And, above all, we must significantly reduce overall energy consumption in the face of a significant reduction in the current crisis of energy, resources and the environment. In fact, completely clean energy is one that is not consumed.

This article was prepared based on discussions with François Briens (Energy Systems Engineer and Economist), Jean-Manuel Trimont (Author and Lecturer), Aurélien Fico (Environmental Engineer and Instructor).

Emmanuel Rio, Enseignante-chercheuse, Paris-Saclay University; François Graner, Director of Research, CNRS, Université Paris Cité, and Roland Lehoek, Astrophysicist, CEA

This article was originally published on The Conversation. Read the original.

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