Orbiting Thin Space Mirrors Could Boost Ground Based Solar Power

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A new research paper in the Acta Astronautica journal describes a constellation of mirror satellites to increase the power at the large solar farms on Earth.

Among all cases, the paper found SSO (sun synchronous orbit) and single reflector cases to be superior. A constellation with 20 reflectors could deliver a significant quantity of solar energy to existing solar power farm projects, which may be enhanced even further with strategically placed new solar power farms. Constellations of orbiting solar reflectors could then be seen as analogous to additional solar power farm in space, distributing solar energy globally rather than locally, enhancing the capacity of terrestrial solar energy.

The paper has only considered a single orbit altitude and a limited set of possible orbital configurations, which, nevertheless, has demonstrated the potential of solar reflector constellations to enhance terrestrial solar energy generation. Alternative constellation geometries could enhance this further for a truly global clean energy generation by orbiting solar reflector.

Nextbigfuture covered previous 2012 proposals to put lightweight mirrors up in space to direct sunlight onto large solar farms at night.

In 2018, China’s Chengdu was looking to launch satellites to light up the city at night. Lighting up a megacity at night is only step one. China has the most solar farms in the world. Large 5-gigawatt solar farms have the area of a megacity. China is preparing to triple the capital value of its solar farms. A constellation of $37 billion of solar mirror satellites will boost several large solar farms and would twice the power and efficiency of the Three Gorges Dam.

Orbiting solar reflectors (OSRs) are flat, thin and lightweight reflective structures (thin lightweight mirrors) that are proposed to enhance terrestrial solar energy generation by illuminating large terrestrial solar power plants locally around dawn/dusk and during night hours. The mirror would boost ground based solar farms only generating power during daylight. However, the quantity of solar energy delivered to the Earth’s surface remains low due to short duration of orbital passes and the low density of the reflected solar power due to large slant ranges. To compensate for these, the paper proposes a constellation of multiple reflectors in low-Earth orbit for scalable enhancement of the quantity of energy delivered. Circular near-polar orbits of 1000 km altitude in the terminator region are considered in a Walker-type constellation for a preliminary analysis. Starting from a simplified approach, the equations of Walker constellations describing the distribution of the reflectors are first modified by introducing a phasing parameter to ensure repeating pass-geometry over solar power farms. This approach allows for a single groundtrack optimisation to define the constellation, which was carried out by a genetic algorithm for single and two reflectors per orbit with an objective function defined as the total quantity of energy delivered per day, to existing and hypothetical solar power projects around the Earth. When full-scale constellations are considered with a number of reflectors, the quantity of solar energy delivered is substantial in the broader context of global terrestrial solar energy generation.

15 thoughts on “Orbiting Thin Space Mirrors Could Boost Ground Based Solar Power”

  2. And while we’re patting ourselves for having read essentially this every year-and-a-half for the last 15 years, let us also recall what vexes the proposal.

      irreducible reflected spot size

    Yep. Physics (or of you prefer the “optics” branch of physics) mathematically guarantees the the absolute smallest focal spot of light down here on Planet Dirt is

      minimum spot = 0.5334° / 360° • 2π • distance
      minimum spot = 0.00931 • distance

    This is because the roughly half degree optical width of The Sun cannot be focussed any smaller than the same 0.5334° from wherever it is reflected FROM, to where the spot is focussed TO.

    For example … a satellite 1,000 kilometers directly over your head, would focus a beautiful “picture of The Sun” on the land around you roughly 9.3 kilometers across. That’s one big picture of Sol! It also begs the question of how much actual light is focussed?

    If the big space mirror — from the observer’s perspective on Earth, when looking up at it — is the same extent (0.5334°) as Sol herself, then the amount of sunlight is AS BRIGHT as that coming from Sol. 1 Sol’s worth, in our 9 kilometer circle. (Likewise, the reflector also need be 9.3 km across, up there 1000 km in space.)

    One heck of a mirror!

    Surprisingly, such a mirror doesn’t need to be very perfect (compared to an astronomical telescope like Hubble). It could literally be made up from thousands of decently flat mirrors. Each flat remarkably lights up the 9 km focal spot on Earth with a nice round fuzzy image of Sol. Put a million of those images overlapped, and now you have 1 Sol, but coming from the mirror instead of our pretty unremarkable star.

    You want MORE than 1 sol’s worth solar energy boost? Simple math: make the apparent size of the space reflector — from the observer’s point of view — LARGER than Sol. More than 0.5334° across from down here looking up. Its all mathematically very linear. 1.0° is (1 ÷ 0.5334)² = 3.5× one Sol in brightness. Something like 3.5 kW per square meter. That’d be rather blisteringly hot!

    SOMETHING TO TRY YOURSELF … Take a small mirror, even a little 1 cm piece from a broken glass mirror outside some sunny day, and set it someplace to reflect a spot, oh, quite a bit down the street. Walk down there and measure how big the spot size is. Sure enough, it’ll be exactly 0.00931 times the distance. 100 meters? 0.9 meter spot.

    You don’t even need to walk 100 meters! Moreover, you’ll maybe be surprised to see on a piece of white paper the full-disk image of The Sun, too. 0.9 meter image of Sol is pretty big. You might see sunspots! Depends on how tiny of a mirror chip you used. Basically, the tradeoff is image brightness versus clarity. The tinier the mirror, the clearer the image, but the dimmer it gets.

    When viewing each of the last partial solar eclipses, I used this technique. Small chip of mirror resting in a tree, pointing (for a few minutes at least) into the window of my kitchen. Nice and dark inside with the curtains nearly closed. Whιtε walls. The picture of Sol was beautifully clear; we saw sunspots, and of course the big chunk of Sol blocked out by the passing Moon. Cool stuff. Easy physics. Just a bit of math.

    Anyway, no amount of perfecting “the curve” of the mirror will reduce the size of the image of Sol down here on Planet Dirt from the above equation prediction. Light just works that way.

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

    • Back in the 70’s we had some partial eclipses in Michigan, and the papers were all full of warnings about viewing it, so as a child I couldn’t go out. I punched a tiny hole in my window shade, placed mirrors around the room, and had a camera obscura that displayed the eclipse quite nicely.

      Also, it’s pretty neat seeing all the spots of light shining through the leaves turning into crescents…

      Yes, why does this proposal keep coming back? Fundamental optics makes it a non-starter unless you’re planning to light up a whole state. Didn’t we discuss this proposal here just a few years ago?

  3. Small Modular 200MW lead cooled reactors seem a lot easier to achieve than this.

    The lead corrosiveness problem was solved back in the 90s with aluminiumized steel. Perhaps someone just has to get permission and the government infastructure (ie HALEU) to build the damn things?

  4. Anyone who things SBSP can be used as a weapon will love this.
    The microwave solutions can’t be a weapon, by design. They’d be built to not be able to focus too much energy in one place.
    Solar concetrators focussing up to 20x solar energy on a location seems like a risky move. How do the ground-based panels reject all the extra heat? What happens to the ground around them? How can they be safely maintained without cooking the engineers? Can just switch off the mirror.
    So many questions…

    • I think it is more like an extra two hours at one Sol. One hour from one of the 20 in the constellation and another hour from another one on the edges of the light zones.
      the other 18 are focusing on 9 other locations around the equator.

      • Could be.

        20 mirrors, by necessity 9.3 km in diameter apiece, is a heck of a Space Engineering bill-of-materials! If one looks just at the proposition pictures, clearly one needs to have movable mirror elements. Thousands of them per satellite. This, just to keep the focussed image centered on the target solar panel array (or city, or farm in Greenland or …?)

        Note it was said somewhere, “nearly circular polar orbits” or something like that. Either (or both) nearly circular and nearly polar. OK, I bit, mathematically.

        I did the trigonometry, and worked out that if there’s a 120° illumination angle window above a particular PV ground array, at an altitude of 1000 km for the heliostat satellite, the as-viewed-from-center-of-Earth angle is 23° Thus it takes a bit more than 16 satellites in one coplanar orbital train to cover “a spot” underneath.

        The confounding problem of course is that the Earth isn’t sitting still. It too is rotating. So… while a string-of-pearls heliostatic satellites might be able to light up a strip ±11.5° wide continuously, the earth moves under the strip. So… you need MORE satellites. Roughly ½ of the square of them. ½16² = 128 satellites.

        Maybe fewer are really required, with fancy-schmancy not-so-circular orbits and not-so-exactly-polar inclinations. Call it about half, or 60? Sure. I’ll give the people much smarter than me, that.

        So, 9.3 kilometer in diameter (or ever bigger if elongation-ellipses are taken into consideration), fast-moving, ground-target tracking, thousands-of-elements mirrors … times 60.

        Color me purple and call me an eggplant. It feels like Science Fiction.
        ________________________________________

        Interestingly, there’s another mathematical reality worth remembering. The total number of ground-spots illuminated of course depends on the “dwell time” over each spot required to achieve mission objectives. If for example, we want OUR spot to be illuminated 5 hours a day at full brightness, well … then that leaves the other 19 hours for illuminating other ground spots. Obvious, right?

        What’s not obvious is that some of the satellites — depending on season — will ALWAYS be shadowed by Earth itself.

        No output.
        No gig.

        This cuts total available illumination to downside Earth. The other — as I’ve alluded to before — is that The Good Earth is vexingly covered (71%!) with oceans. Oceans without islands or other obvious targets for the loitering the artificial sun-sats pointing arrays.

        Basically, without a gargantuan scheme of satellite-to-satellite reflectors to “beam around” the captured sunlight, it goes to waste. Which is a waste to account for in figuring program viability.

        My guess is that at any time, of the 60 birds in our example constellation, at any time, only 15 are someplace coincident with having sunlight, and being within view of a thirsty ground station. The more ground stations, “the better” of course, but up to the limit where there are too many at peak to actually bathe them all in sunlight for their contracted duty-cycle illumination.

        Bottom line. This is an absolutely fascinating science / engineering / physics / trigonometry exercise, but I think the “minimum possible useful scale” of it isn’t something humankind will be investing in any time soon.
        ________________________________________

        PS … and then there’s the waste heat.

        While 60 9.3 km diameter heliostat satellites potentially is bringing a lot of useful solar power down to Earth, this is additional heating, more than from Old Sol herself. And PV patches — 9.3 km across, 70 km² or 7,000 hectares apiece, and they aren’t bright titanium white reflectors. They’re almost pitch black absorbers! I think the Global Warming crowd ought to be up-in-arms about this proposal.

        ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
        ⋅-=≡ GoatGuy ✓ ≡=-⋅

        • Using dichroic-type mirror materials, the reflected light can be restricted to the frequencies of light a given solar panel array can convert, while letting IR pass through.

          A cold mirror.

          Keeping on-station a miles-wide mirror that is, in effect, a solar sail adds another layer of complexity.

        • At an altitude of 1000 km, the mirrors would be shadowing the Earth for most of the time they were in sunlight, wouldn’t they? So even if they did divert some light Earthward that would otherwise miss at dusk and dawn, the added heat influx might be minimal.

          • Yes. You’re right about that.

            In fact it ought to be almost entirely symmetric: the satellites are IN Earth’s shadow just as long as they’re shadowing Earth on the Sun-side.

            And with the very real long term photon-sail bits working against stable orbits, I’m pretty sure all those sunside transits would be utilized to send the reflected beam off in directions that correct the orbital perturbations.

            Win-win.
            Win.

            ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
            ⋅-=≡ GoatGuy ✓ ≡=-⋅

    • It’s not going to especially heat the atmosphere, but it is kind of amusing that PV power is promoted as a cure to global warming, and now people want to divert onto the Earth extra sunlight that would otherwise miss it, just to get a few extra hours out of the panels.

      Thus forgetting why you wanted to build them in the first place…

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