Caltech team achieve wireless energy transfer in space

Caltech’s space solar power demonstrator has demonstrated its ability to wirelessly transmit power in space and to beam detectable power to Earth for the first time.

The achievement was made by MAPLE (Microwave Array for Power-transfer Low-orbit Experiment), an array of flexible lightweight microwave power transmitters with precise timing control. 

MAPLE is one of the three experiments carried out by the California Institute of Technology (Caltech) as part of its Space Solar Power Project (SSPP), an ambitious plan to deploy a constellation of space solar panels that could harvest solar power in space and beam the energy back to Earth. 

“Through the experiments we have run so far, we received confirmation that MAPLE can transmit power successfully to receivers in space,” said Ali Hajimiri, Bren Professor of Electrical Engineering and Medical Engineering and co-director of SSPP.

“We have also been able to program the array to direct its energy toward Earth, which we detected here at Caltech. We had, of course, tested it on Earth, but now we know that it can survive the trip to space and operate there.”

Photo from space of the interior of MAPLE, with the transmission array to the right and the receivers to the left.

Photo from space of the interior of MAPLE, with the transmission array to the right and the receivers to the left. / SSPP

Image credit: SSPP

MAPLE uses constructive and destructive interference between individual transmitters, to shift the focus and direction of the energy it beams out — without any moving parts. It also relies on precise timing-control elements to enable the majority of the energy to be transmitted to the desired location and nowhere else.

MAPLE features two separate receiver arrays located about 30cm away from the transmitter. These arrays receive the energy, convert it to direct current (DC) electricity, and use it to light up a pair of LEDs to demonstrate the full sequence of wireless energy transmission at a distance in space.

The researchers tested this technology in space by lighting up each LED individually and shifting back and forth between them. 

The energy transmitted by MAPLE was detected by a receiver on the roof of the Gordon and Betty Moore Laboratory of Engineering on Caltech’s campus in Pasadena. The received signal appeared at the expected time and frequency and had the right frequency shift as predicted based on its travel from orbit.

“To the best of our knowledge, no one has ever demonstrated wireless energy transfer in space even with expensive rigid structures. We are doing it with flexible lightweight structures and with our own integrated circuits. This is a first,” Hajimiri said.

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Satellites in geosynchronous orbit receive sunlight for more than 99 per cent of the time – as they are not subjected to the cycles of a day and changes in weather – and at a much greater intensity than solar panels on Earth, making them a much more efficient source of energy. 

For this reason, space solar power could allow scientists to tap into a practically unlimited supply of solar energy.

When completed, SSPP could deploy a constellation of modular spacecraft that collect sunlight, transform it into electricity, and then convert it to microwaves that will be transmitted wirelessly over long distances to wherever it is needed – including locations that currently have no access to reliable power.

“The flexible power transmission arrays are essential to the current design of Caltech’s vision for a constellation of sail-like solar panels that unfurl once they reach orbit,” said Sergio Pellegrino, Joyce and Kent Kresa Professor of Aerospace and Civil Engineering and co-director of SSPP.

“In the same way that the internet democratised access to information, we hope that wireless energy transfer democratises access to energy,” Hajimiri said. “No energy transmission infrastructure will be needed on the ground to receive this power. That means we can send energy to remote regions and areas devastated by war or natural disaster.”

Detecting power from MAPLE on the roof of Moore Laboratory.

Detecting power from MAPLE on the roof of Moore Laboratory. / Ali Hajimiri

Image credit: Ali Hajimiri

The space solar power prototype, known as Space Solar Power Demonstrator (SSPD), was launched into orbit on the Transporter-6 mission, which lifted off from Cape Canaveral on Tuesday 3 January 2023. 

In addition to MAPLE, SSPC consists of two other experiments, the results of which are expected in the coming months. 

The first one is known as DOLCE (Deployable on-Orbit ultraLight Composite Experiment). It is a 1.8m-square structure that demonstrates the architecture, packaging scheme and deployment mechanisms of the modular space solar panels. 

The second experiment has been named ALBA, and it has been designed to collect 32 different types of photovoltaic (PV) cells, to enable an assessment of the types of cells that are the most effective in the punishing environment of space. 

The origins of SSPP date back to 2011, when philanthropist Donald Bren learned about the potential for space-based solar energy manufacturing in an article in the magazine Popular Science

Intrigued by the potential for space solar power, Bren approached Caltech’s then-president Jean-Lou Chameau to discuss the creation of a space-based solar power research project. In 2013, Bren and his wife, Brigitte Bren, a Caltech trustee, agreed to make a donation to fund the project. 

“The hard work and dedication of the brilliant scientists at Caltech have advanced our dream of providing the world with abundant, reliable and affordable power for the benefit of all humankind,” Bren said.

Caltech President Thomas F Rosenbaum added: “The transition to renewable energy, critical for the world’s future, is limited today by energy storage and transmission challenges. Beaming solar power from space is an elegant solution that has moved one step closer to realisation due to the generosity and foresight of the Brens.  

“Donald Bren has presented a formidable technical challenge that promises a remarkable payoff for humanity: a world powered by uninterruptible renewable energy.”

Last summer, China announced that the first launch for the construction of its solar power project in space had been scheduled for 2028 – two years earlier than originally planned – when a trial satellite orbiting at a distance of around 400km will test the technology used to transmit energy from the power plant to Earth.

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