Monday, November 28, 2022

Will Space-Based Wireless Power Beams Solve the Energy Problem?

 

A recent article in a good-news website called The Brighter Side claimed that in a few years, we may be getting lots of energy from space-based wireless power stations.  A research group at Airbus, the European aerospace firm, has built a prototype and has high hopes for the technology's future.

 

Here's how it would work.  A solar panel about 2 km (more than a mile) across would be in geostationary orbit, probably an orbit with a tilt to it so that the satellite would be in continuous sunlight 24/7 rather than going through an Earth-caused eclipse once a day.  The amount of power thus generated—comparable to a standard fossil-fueled or nuclear plant—would then be converted into microwave energy, probably the same type of microwaves that heat your pizza.  The microwaves would be beamed through a large, sophisticated antenna array to suitable locations on Earth equipped with things called "rectennas"—antennas specially designed to receive microwaves and convert them efficiently into DC power.  The power would then be converted into standard AC for transmission on a grid, or conceivably used by mobile devices such as trucks and airplanes.

 

Sounds great, doesn't it?  Compared to earth-based solar panels, the ground-based technology is cheaper.  A rectenna is a lot easier to make than a solar panel.  As solar energy is more intense in space than on earth, it takes less solar-panel area in space to generate a kilowatt than it does on earth.  And there's the flexibility of beaming the power basically anywhere you want it, just as communications satellites beam signals to different locations.  What's in the way of our putting up lots of these and throwing fossil-fuel plants, and nuclear too for that matter, in the existential trash bin?

 

As a trained microwave engineer, I can answer that question.  Microwave power from solar-energy satellites is not a new idea.  Raytheon engineer William C. Brown conceived the idea in the early 1960s, and a key component was a crossed-field amplifier that he had invented earlier.  By 1976, Brown worked with NASA to transmit 30 kW of power over a distance of 1.5 km (almost a mile) using a 26-meter dish and a rectenna that was only 7.3 x 3.5 meters—about 10 by 25 feet.  I saw a video of that demonstration in which they used the power to light up an array of spotlights, and the lights gradually came on as the beam was directed at the rectenna.  The Airbus people have so far demonstrated a link only 36 meters long. 

 

If the idea has been around so long, why hasn't it been deployed commercially yet?  I can think of several reasons.

 

First, as even the optimistic Airbus researchers admit, the orbiting part of the system has to be really big to produce a useful amount of power.  Currently, it seems that the largest structure in orbit is the International Space Station, which is a little longer than a football field.  The solar array envisioned by the Airbus people would be twenty times longer and wider, if it was square.  At the current rate of space commercialization, however, such huge projects in space may become feasible.  But not yet.

 

Once you put the orbiting microwave power station in space, you have to be careful where you aim the beam.  The focusing ability of the station's antenna depends on how big the antenna is, and while it will be smaller than the solar array, you could easily imagine an antenna, say, 100 meters in diameter.  Some simple calculations I will spare the reader tell us that the beam on the ground from an antenna of that size in geostationary orbit, using the microwave-oven wavelength, will expose an area on the ground about 24 miles square to a microwave power density of more than a watt per square meter.  This level of energy would definitely be detrimental to human health and not real good for other vertebrates, either.  So a very large area on the ground, probably covering most of a good-size county, would have to be sequestered off and devoted to a giant rectenna farm.

 

The more speculative statements by the Airbus people of directly conveying microwave energy to mobile platforms such as airplanes are pretty much pipe dreams.  Not all the microwaves would be absorbed by the plane, and so people on the ground underneath would be at risk.  The tighter the beam you want, the larger the antenna has to be, so unless the designers want to make an antenna as big as the solar array, which brings up mechanical difficulties, I don't see how they can direct parts of the beam to highly specific locations on earth, although they could probably manage as many as a dozen or so without too much trouble.

 

The Airbus people are to be congratulated for dusting off an old idea that was clearly premature at the time it originated in the 1960s.  Back then it was prohibitively expensive to launch large structures into orbit.  We can count on both launch and space-based construction costs to decrease in real terms in the future.  So that is one big factor that makes reconsidering wireless power transfer from space a good thing. 

 

The drawbacks are substantial, however:  hazards to people and animals on the ground, the possibility of a space-based error that could send the beam skidding across the countryside into a major population center (there's a sci-fi scenario for you), and the difficulties of upkeep and maintenance—I suspect you'd have to have a few people permanently in space simply to keep the station running.  But it eliminates one of the big problems with most types of renewable energy these days: the fact that it's available only when the sun is out or the wind is blowing.  A properly designed geostationary-orbit power satellite would be available 24/7, through clouds, night and day. 

 

We'll know this technology's time has come when somebody like Elon Musk starts a company to do it.  In the meantime, though, it will remain what it has been for more than fifty years:  only an engineer's dream.

 

Sources:  I thank my wife for calling my attention to the article in The Brighter Side at https://www.thebrighterside.news/post/space-based-solar-power-beams-will-soon-be-powering-our-cities.  I also referred to the Wikipedia article "Wireless power transfer," and my 1975 ITT Reference Data for Radio Engineers handbook for antenna-beam calculations. 

No comments:

Post a Comment