MAPLE, short for Microwave Array for Power-transfer Low-orbit Experiment, is one of the three key experiments within SSPD-1. It consists of an array of flexible lightweight microwave power transmitters driven by custom chips. For SSPP to be feasible, energy transmission arrays will need to be lightweight to minimize the amount of fuel needed to send them to space, flexible so they can fold up into a package that can be transported in a rocket, and a low-cost technology overall.
“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 at Caltech 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.”
Using constructive and destructive interference between individual transmitters, a bank of power transmitters is able to shift the focus and direction of the energy with beam steering. The transmitter array uses precise timing-control elements to dynamically focus the power selectively on the desired location using the coherent addition of electromagnetic waves. This enables the majority of the energy to be transmitted to the desired location and nowhere else.
MAPLE has two separate receiver arrays located about a foot away from the transmitter to receive the energy, convert it to 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 power transmission antennas are clustered in groups of 16, each group driven by one entirely custom flexible integrated circuit chip, and Hajimiri’s team now is assessing the performance of individual elements within the system by evaluating the interference patterns of smaller groups and measuring difference between various combinations.
MAPLE tested this in space by lighting up each LED individually and shifting back and forth between them. The experiment is not sealed, so it is subject to the harsh environment of space, including the wide temperature swings and solar radiation that will be faced one day by large-scale SSPP units.
“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,” says Hajimiri.
MAPLE also includes a small window through which the array can beam the energy. This transmitted energy 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.
Beyond a demonstration that the power transmitters could survive the launch on January 3, the experiment has provided useful feedback to SSPP engineers. The evaluation of the transmitter arrays will take up to six months to fully complete and will allow the team to sort out irregularities and trace them back to individual units, providing insight for the next generation of the system.
When fully built, SSPP will deploy a constellation of modular spacecraft that collect sunlight, transform it into electricity, 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.
Individual SSPP units will fold up into packages about 1 cubic meter in volume and then unfurl into flat squares about 50 meters per side, with solar cells on one side facing toward the sun and wireless power transmitters on the other side facing toward Earth.
“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,” says Sergio Pellegrino, Joyce and Kent Kresa Professor of Aerospace and Civil Engineering and co-director of SSPP.
“In the same way that the internet democratized access to information, we hope that wireless energy transfer democratizes access to energy,” Hajimiri says. “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.”
