Interesting Payload on Yesterday's Transporter 6 Mission

Among the satellites on the Transporter 6 mission yesterday, an interesting payload stood out to me.  CalTech launched a satellite to test out some ideas related to placing solar panels in space and beaming the energy back down to the power grid.  

I'm not sure when I first ran across this idea, but I'll SWAG in the late 1990s.  My reference point is that I was at a conference around '05 or '06 for Radio Frequency and Microwave design engineers that had a presentation on it and it was well known by then.  We referred to it as “The Microwave Engineer's Full-Employment Act.”  The questions swirl around how to implement such an idea.  All that said, this article points out there are known possible ways to attempt it but no hard data on the best ways to proceed.  Caltech is doing the experiments to try out different things. 

Maybe we should start with why and a rough overview of these systems.  Everyone that has used or experimented with solar panels knows their dirty little secret; as in they get dirty and their energy conversion drops.  Not just dirt and dust, but bird droppings, insect droppings, insect nests or anything else that can coat them (around here: salt mist from the Atlantic).  Clouds are no secret, I'd imagine everyone knows that on cloudy, rainy (or snowy) days, efficiency just goes into the toilet.  Space has two significant advantages as a place to put panels: first off, there's loss of some power in the atmosphere; so the same size panel could harvest more energy in space than in a desert somewhere.  Second, there's no dirt in space to degrade the panels.  There's space junk that could damage panels, but that hasn't been a big problem with satellites so probably won't be with solar panels. 

All the energy panels could capture in space won't do the power grid any good without being transmitted down to earth.  Much thought has been given to this and it seems the likely method would be beaming it down by radio.  Most likely would be by microwaves; the antennas will need to be specialized to have narrow beamwidth with (ideally) no power outside the antenna arrays themselves will receive the power, so that's considerably easier with microwaves than VHF or lower frequencies.  The problem will be all about getting every last bit of efficiency out of the transmitter and microwaves up to around 30 GHz are pretty mature in terms of this sort of tech.  

All of that out of the way, Caltech's approach is based on the simple economics that, at least for now (Starship has the potential to change this), the cost per pound to put stuff in orbit will be the hardest constraint. 

The design limits weight partly by minimizing the support structure for the functional hardware, including the wiring. It does so by making its "panels" self-contained, having their own structural support and power transmitter. These individual panels will be assembled like tiles to form a larger surface but will operate independently.

That design dictates what the Caltech team needs to test: a lightweight power transmitter, a thin membrane that can be deployed in space, and different photovoltaic materials that can be placed on the flexible membrane. And that's exactly what's now in space on their test hardware.

The hardware includes a unit called MAPLE (Microwave Array for Power-transfer Low-orbit Experiment), a set of lightweight, flexible microwave transmitters that are capable of the precise phasing relationships needed to form the transmit antenna beam.  In addition, MAPLE has two different test receivers on board so that the ability to beam the transmission in specific direction can be tested.  

The photovoltaic array being carried will be four square meters, and something that big has to be folded up to fit in pretty much any launch vehicle.  Their penchant for cute names for things for this mission calls this DOLCE. 

DOLCE is the Deployable on-Orbit ultraLight Composite Experiment, and it will extend once in orbit to cover a surface area of roughly four square meters. It's meant to test the framework used to extend and support the solar array in space.

The DOLCE hardware in its compact, flight-ready form. Caltech/Momentus photo.

There's yet another module on the mission called ALBA, but Caltech didn't say what that stands for.  It's important, though.  It will be a collection of 22 different photovoltaic materials and will be used to determine which of these holds up well to being in space.  

The commonly quoted value of solar power at the top of the atmosphere is 1367 W/m² (watts per square meter) and I believe this will be the case at whatever altitude the satellite is flying, so if that's really a four square meter solar array, it'll be getting 5468 Watts.  Of those 22 different photovoltaic materials they're testing we have to assume that the efficiency will vary. 

Tests of DOLCE, which largely consist of determining whether it successfully unfolds, should happen relatively quickly, with the results captured by onboard video cameras and streamed back to Earth. By contrast, they expect that tests of the photovoltaic materials will require about six months in orbit to produce clear results.

For the microwave bands that they expect to use, the current regulatory limits for exposure to radio radiation in the microwave bands we're talking about is 10 milliwatts per square cm; there are 10,000 sq.cm in one sq. meter, so 10 mW/cm² times 10,000 converts to 100 W/m².  A common design factor when putting up solar power is that 30% of that incoming 1367 W/m² is lost in the atmosphere, leaving 957 W/m² at the surface. I feel pretty certain that any designer putting together a system downloading millions or billions of watts to the surface will constrain their power density (W/m²) to meet whatever limit is allowed.  It strikes me as noteworthy that the radio power density at the surface will be close to 1/10 of the power of sunlight.  I'd be willing to bet you know more people afraid of being exposed to radio signals than sunlight.