Aug 29 2022

ESA Considers Space-Based Solar

The big science and technology news today is the Artemis I launch, an uncrewed test flight that will orbit the Moon on a six week flight. I thought I would be writing about that today, but prior to the launch I actually don’t have much to add to the extensive reporting. I’ll probably have something to say after the launch. But there is other space news, this one from the European Space Agency (ESA). The ESA is considering a proposal for space-based solar power, which also makes a nice follow up to my previous post updating solar technology.

This is one of those ideas that, when I first heard it, I thought it was a great idea. There are some obvious benefits to placing solar panels in orbit, then beaming the energy down to Earth. Above the atmosphere in geostationary orbit, the solar panels would receive sunlight 24 hours a day and without any concern for clouds or weather. Panels can also be optimally oriented to the sun without having to worry about the Earth’s rotation. How much of a factor is this? Depends on location of ground-based solar, but for example Arizona can expect to have 7.5 peak solar hours per day, while New Jersey has 4 hours. There is still some off-peak energy production, but less. Overall light exposure efficiency can vary from 20-40%. In space we can get 100% light exposure efficiency.¬†Being based in orbit also solves the intermittency problem. Solar can become a reliable baseload source of energy.

The obvious downside is that it’s expensive to get stuff into orbit, but until you run¬† the numbers it may not be obvious if space-based solar can be cost effective, compared to other options like nuclear or geothermal. This is the primary reason for the ESA feasibility study. Unfortunately, when you start to run the numbers, and consider all aspects of logistics, space-based solar looks worse and worse.

First, the solar arrays will need to be in geostationary orbit, 35,785 km (22,236 miles) above the ground. It takes a lot of energy to get stuff into such a high orbit, and therefore heavy lift rockets would be required. The ESA proposal, to cover one third of their current energy supply, would require dozens of installations with 10 times the mass of the ISS. This would require thousands of heavy lift launches.

We have satellites in geostationary orbit, but this would be different. These solar arrays would need to be built, like the ISS. They would also need to be maintained. People cannot stay for any length of time in such a high orbit due to radiation exposure. Let’s say we build a station to house workers, with adequate shielding for solar radiation, or that we have shuttle-like ships to take them there and live in during their mission. We have to factor all that into the cost. Ideally we would use some combination of self-assembling arrays with some robotic assistance, but that technology would need to be developed.

And all this is the least of the problems. The primary issue, of course, is cost. Here is one analysis that concludes space-based solar costs about three orders of magnitude more for the energy produced than ground-based solar. Even if you quibble about the details, it doesn’t matter – three orders of magnitude is a deal-killer. There also is no advantage in land use because of the needed microwave receiver. The idea is to convert electricity produced by the solar panels into microwaves which are beamed down to Earth and then converted again into electricity on the ground. That’s three energy conversions instead of one, with a great deal of energy loss along the way, eating significantly into that space-based solar advantage. But also, the microwave receiver on the ground would have to be huge, about the size of a solar array with a similar energy flux. You see the problem here – why not just fill that space with solar panels? Even if it produces less overall energy and is intermittent, the cost is 1/1000th. You could build a bigger solar array and battery backup for much less money.

Another aspect of the financial issue is that a solar-based installation in geostationary orbit is a huge upfront fixed cost. The idea is that it would slowly pay for itself over time. However, what else will be happening over that time? Let’s say that we install silicon-based solar panels with a long lifespan, 50 years. Keep in mind that the 24 hours a day of sun cuts both ways – it produces more energy, but shortens the lifespan. So these would be panels that would last for 100 years or more on Earth. But let’s say we can do that (not a stretch). What is the energy landscape going to look like over those 50 years of payback for the space-based solar installation? How cheap will ground-based energy production become? The cheaper it becomes, the less valuable that space-based solar is, but there is nothing we can do about it, the money has been spent. Also, how much more efficient will solar panels be in 20 years, or 30?

Space-based solar is simply not going to happen anytime soon. The costs and logistics are simply prohibitive. It’s a cool sounding idea, and at first the advantages seem extreme, but any realistic analysis kills the idea. Further, we simply need to be taking a low-hanging fruit approach to getting our energy infrastructure to zero carbon. There are so many places to put solar on the ground, such as rooftops, that we need to put our efforts there first. Do this, upgrade the grid, keep developing battery technology as other approaches to grid storage, and that will be a far better investment that putting massive projects into high orbit. This might be an idea for next century, or perhaps would better serve a moon settlement, but is simply not practical for now.

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