Solar power from Earth orbit

As early as 1941, the writer Isaac Asimov described in his short story "Reason" how the earth could be supplied with energy from space. A space station in orbit around the earth collects sunlight and sends its energy to earth in concentrated form of radiation. Around 80 years later, science fiction is about to become reality.

Scientists at the California Institute of Technology have been working on the implementation of the old idea for years. Their "Space-based Solar Power Project" is about converting the sun's light into electrical energy with the help of solar cells, which is then sent as bundled high-frequency radiation to a receiving station on the ground. There, the microwaves are absorbed by special antennas and converted back into electricity.

A prototype is scheduled to be launched in December 2022 to demonstrate the feasibility of this technology. The cosmic solar power plant is made up of tiles measuring ten by ten centimeters, which convert sunlight into electricity on their surface. This is converted into microwaves by sophisticated electronics inside the tile, which are then radiated in the direction of the earth.

Hundreds of thousands of these tiles, each weighing less than three grams, will later be connected to form a large-scale system. To do this, the basic tiles are assembled into units measuring two by two meters, which in turn are assembled into aggregates measuring 60 by 60 meters. The fully developed facility is to have the shape of a hexagon – with an edge length of three kilometers. This corresponds to a solar cell area of ​​more than 20 square kilometers.

One of the brains behind the project is Ali Hajimiri, who received his bachelor's degree in electronics engineering from Tehran's Sharif University in Iran. He received his master's degree (1996) and doctorate (1998) from Stanford University. Together with colleagues, he developed the solar cell modules.

These have a sandwich-like structure: the solar cells on top, and a layer with computer technology underneath to control the entire system. The underside consists of chips arranged in a chessboard pattern, which can generate and emit microwaves. This combination of different technologies is unique. For this and other groundbreaking inventions, Hajimiri was inducted into the US National Academy of Inventors back in 2016.

"In the final stage, the solar power plant will give us an output of two gigawatts on earth," says Hajimiri. This roughly corresponds to the output of a nuclear power plant. However, the installation costs should be significantly lower for space technology. They are said to be less than $1.50 per watt.

However, this presupposes that the energy collected in the Earth's orbit can be bundled and transmitted to the Earth's surface in a sufficiently well-structured manner. The reverse is more common: a TV satellite transmits a signal that can then be received anywhere in Europe with a small parabolic antenna.

But how can microwaves be sent to earth in a focused manner? This requires a very large number of antennas that can be controlled individually by a computer. For each individual microwave antenna, this calculates the intensity and phase with which it must radiate so that the wave fields of all antennas overlap in such a way that the radiated energy ultimately only reaches a small area on the earth's surface.

But nobody need fear that such a system could be used as a beam weapon. The microwave radiation cannot be bundled that strongly. The receiving areas will be at least one square kilometer in size. Asimov's vision that an out-of-focus beam of energy from space would cause "glowing ruins" on Earth remains science fiction for now.

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