Power demand spikes in the early morning and later evening, when solar arrays can't help. But researchers in Scotland say orbital launch costs are getting so cheap that giant space-based reflectors could soon be a viable way to power these timeslots.
You could possibly look at this as a different way to do space solar – but instead of putting giant photovoltaic arrays in the sky and trying to beam the power down to Earth, you're just hanging big mirror structures in orbit, and using them to send more solar energy down to the surface for harvesting by terrestrial installations.
These orbital reflectors are a very old idea – indeed, Hermann Oberth proposed something very similar in his 1929 book Ways to Spaceflight, which he saw as a potential way of lighting cities, protecting crops from overnight frost, and maybe even keeping entire areas of the far North ice-free, making them more habitable through their gruelling dark winters.
Mind you, he also warned that such devices could be used to concentrate solar energy onto smaller areas for military purposes: "munitions factories can be exploded with it, tornadoes and thunderstorms produced, marching troops and their reserves destroyed, whole cities burned, and generally the greatest of damage done." But we'll leave that aside for the minute!
What's more, Russian scientists proved that the concept was possible back in 1993, when their 20-m (66-ft) Znamya-2 space mirror reflected a dim flash of light down to Earth that was visible in the night sky from parts of Europe.
A University of Glasgow team believes that now's the right time to give solar reflectors a serious crack, since SpaceX and other private companies promise they'll soon drive the per-kilogram cost of orbital launches down low enough that these reflectors could be commercially viable.
Here's the rough idea of what the researchers are calling the Solspace project: you'd send a number of reflector satellites into a high, sun-synchronous orbit designed to track over roughly the same ground path each day. This path would be chosen to make the reflectors visible from as many large terrestrial solar farms as possible.
Each satellite would deploy a hexagonal reflector made from aluminized Kapton, with each side measuring 250 m (820 ft) for a total area of 162,380 sq m (1.75 million sq ft).
These reflectors could be steered and aimed in orbit using four electrically powered control moment gyros in a pyramid configuration; by accelerating and decelerating these gyros, you could rotate the entire reflector in any direction.
With the reflectors sitting at an orbital altitude around 900 km (roughly twice as high as the ISS), the team says each mirror pass could illuminate a 10 sq km (3.9 sq mile) area on the surface for about 17 minutes, delivering somewhere around 34-36 MWh of bonus energy to the surface per pass. Thus, large solar farms such as the Sun Cable venture in northern Australia could capture that energy at whatever efficiency they can operate at, generating power during times of peak demand, when otherwise they'd be doing nothing.
The Solspace reflectors would focus on an array as long as they could deliver useful power, then rotate themselves away to deflect the energy off into space until they passed over the next large solar array, hopefully servicing many arrays every day, and potentially even becoming a consideration in the planning of new solar arrays. If a route could be planned to hit 13 large solar farms per day, with five orbital reflectors, the system could deliver 284 MWh of solar energy per day.
Would this cause a light pollution disturbance? No, says the Glasgow team. "Even at its brightest, we estimate that the illumination levels would last only a few minutes per reflector and not exceed an overcast day level," writes Solspace project team member Onur Çelik in The Conversation. "This means that, unless you are very close to the solar power farm, the illumination may not even be noticeable most of the time, especially at dawn/dusk times when the sky is already quite bright compared to nighttime."
In this way, orbital reflectors would offer a dawn/dusk power boost to compete with or augment the energy supplied by short-term grid-level energy storage solutions like lithium batteries. Batteries, for reference, currently have a levelized cost of storage (LCoS) somewhere around US$314 per MWh of energy stored and released.
The Solspace project is targeting a levelized cost of energy (LCoE) closer to US$70 for space-reflected solar generation, assuming an operational lifetime around 20 years and launch costs around US$232 per kilogram.
According to Georgetown Security Studies Review, SpaceX 's reusable Falcon 9 rockets have crashed the price of orbital launch down to around US$1,520/kg. The much larger Starship rocket will drop that to less than US$1,000/kg as soon as it begins commercial launch services, and Elon Musk has predicted that once multiple reusable Starships start operating at a high launch cadence, that cost too will drop by as much as 90% within 2-3 years.
Now, you might not want to set your watch by a Musk prediction, but SpaceX has certainly already achieved stunning results, opening up all sorts of orbital opportunities that might never have been possible under NASA's launch programs. So while the timeline is almost certainly very optimistic, it seems fair to expect that the US$232/kg launch costs Solspace needs will be realized in the not too distant future.
So these orbital solar reflectors could certainly become viable if all they were doing was feeding a number of large solar farms around the world. As for other uses, like beating back frost, giving crops a little more Sun, city lighting and whatnot... Well, who knows, those might yet be on the table once these things are deployed.
The study is open access in the journal Advances in Space Research.
Source: Solspace via The Conversation
