Microwaves transmit solar power from space

Solar energy is in great demand these days. After all, together with wind power, it is touted as the most important form of energy in future greenhouse gas-neutral energy systems. Unfortunately, output depends on the time of day, season, and weather. And more than half of the incoming solar radiation is lost on its way through the Earth’s atmosphere due to reflection and absorption.

That is why scientists have been obsessed for decades by the vision of harvesting the sun’s energy in geostationary orbit—36,000 kilometers above our planet’s surface—and transmitting it to Earth by means of microwaves. The most comprehensive study to date on the feasibility of this futuristic scenario was undertaken by NASA and the United States Department of Energy (DoE) in the wake of the oil crisis from July 1977 onward. The sticking points at the time were technical and economic aspects as well as the enormous transport costs.

Other countries, such as China, Japan, South Korea, or Australia, did not start addressing the issue until much later. And the British government announced last year that it would invest 19 billion euros in it.

Europe has also recently begun giving serious thought to solar panels in space. The European Space Agency (ESA) expects it to be able to deliver a significant share of clean baseload energy starting from 2050. Studies by the consulting firms Frazer-Nash from the UK and Roland Berger from Germany from August 2022 recommend investing early on in space-based solar power technology in order to play a pioneering role. In Europe alone, the systems would be capable of generating 800 terawatts of electricity in the future, according to Frazer-Nash. That is equivalent to one-third of Europe’s electricity production in 2022.

Simple concept—highly complex implementation

As simple as the undertaking may seem in theory, the hurdles to implementing it are immense. The installation in space would gobble up hundreds of billions of euros. It would take years to put dozens of satellites, which would be ten times larger than the International Space Station (ISS), into orbit. And ESA estimates that the receiver stations on Earth would occupy areas of up to 70 square kilometers – or the size of 10,000 soccer pitches. Elon Musk—who actually stands to profit from such plans with his company SpaceX – called space solar power “the stupidest thing ever” a few years ago.

First successful test in space

Yet the conditions have changed: Transport costs have plummeted thanks to private space companies like SpaceX. Space hardware can now be manufactured much more cheaply on an industrial scale, and robots could assemble the huge systems in space in the future.

There are also highly promising developments. For instance, researchers at the California Institute of Technology (Caltech) have succeeded for the first time in the “Space Solar Power Project” (SSPP) in transmitting electrical energy produced from solar cells in space wirelessly to Earth. The project, generously funded to the tune of $100 million by US entrepreneur Donald Bren, launched a 50-kilogram test satellite at the beginning of the year to test solar panels that are suitable for space and power transmission to a receiver station on Earth via microwave.

The main role was played by MAPLE (Microwave Array for Power-transfer Low-orbit Experiment), an array of flexible, lightweight, high-power microwave transmitters made using low-cost silicon technology. The array relies on constructive and destructive interference between the individual transmission antennas to shift the focus and direction of the beamed energy without any moving parts.

And energy is delivered by solar tiles measuring ten square centimeters in size and weighing less than three grams. In space, they help create flat squares with an edge length of about 60 meters—with solar cells on the front and microwave transmitters on the rear. The power electronics are already integrated, too. These modules can ultimately form nine square kilometers of solar fields.

On Earth, the electricity is received by antenna fields that are square kilometers in size, where it is used directly or fed into the power grid.

The researchers have already been able to make LEDs on another satellite flash with their test setup. Moreover, energy reached the Caltech campus in Pasadena via microwave at the calculated time and predicted frequency.

The overall efficiency of such systems is still far too low and the costs are too high. Yet initial practical tests and the considerable financial resources allocated at least underscore the seriousness of this futuristic undertaking.