Taking Nikola Tesla’s dream of transmitting electrical power wirelessly over long distances one step further, an engineering team at the California Institute of Technology has successfully transmitted microwave energy across open space between the experimental components of an orbiting satellite—including sending a portion of this power as a signal to a ground station on Earth. If the idea of space-based solar power stations can one day be made a reality, it could potentially provide uninterrupted sustainable solar energy for use here on the Earth’s surface, another tool in the fight against climate change.

Although solar power is a sustainable, low-cost alternative to producing electricity with fossil fuels, this method of power generation isn’t without its drawbacks, such as its large footprint in terms of the physical area the photovoltaic panels or focusing mirrors take up, and the method’s inability to operate at night. In space, however, there’s plenty of space, and if kept out of the Earth’s shadow a solar array could collect the Sun’s energy 24 hours a day, seven days a week.

Such a satellite would use microwaves to beam the collected energy to a receiving station on Earth: although the atmosphere readily absorbs many frequencies of long-wavelength EM radiation—a key component enabling the issue of global warming—there is a swath of frequencies in the microwave part of the spectrum that aren’t easily absorbed by the air that could allow for power transmission with little loss of energy to the atmosphere, and in turn minimizing the heating of the air along the path of the beam.

Although the logistics of getting a power station into orbit are just a matter of engineering problems that can be solved, there is the question of whether or not the concept of transferring power from one location to another over a long distance could be practical. To that end, Caltech’s Space Solar Power Project (SSPP) launched their Space Solar Power Demonstrator (SSPD-1) into a Sun-synchronous orbit on January 3 aboard a SpaceX Falcon-9 reusable rocket.

The SSPD-1 is comprised of three different modules: the Deployable on-Orbit ultraLight Composite Experiment (DOLCE) is a six-foot-square structure that demonstrates the packing scheme and deployment mechanisms of the satellite’s modular architecture; ALBA, a collection of 22 different types of photovoltaic cells that will be used to test the effectiveness of different types of space-based solar energy collection; and the Microwave Array for Power-transfer Low-orbit Experiment (MAPLE), the energy transmission experiment itself.

MAPLE uses the concept of interference to direct the microwave energy in a desired direction, rather than broadcasting the energy in all directions at once: like all electromagnetic radiation, microwaves travel through space as waves; if the trough of one wave happens to meet another wave that is cresting, the two waves, considered to be out of phase, cancel each other out; however, if the two waves are in phase, where they’re both either cresting or troughing at the same time, they amplify one-another.

By using an array of 32 discrete transmitters MAPLE can use those crests and troughs to direct the microwave beam in one direction, and by timing the phases across the array the system can change the direction of the beam without having to physically alter the orientation of the transmitter, eliminating the need for moving parts in the apparatus.

On March 3, MAPLE successfully transmitted microwave power through open space between the experiment’s transmitter array and a series of receivers on the spacecraft, situated roughly a foot away from the transmitters, activating a pair of light-emitting diodes with the power transmitted through open space.

To demonstrate that the system can also transmit power to a location on the ground, MAPLE successfully beamed a small portion of its energy to a receiver on Earth, received as a signal by the SSPP team’s receiver on the roof of Moore Laboratory.

“Through the experiments we have run so far, we received confirmation that MAPLE can transmit power successfully to receivers in space,” explained SSPP co-director Ali Hajimiri. “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.”

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

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