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A couple years ago I read a book[1] on space solar power. It was written before SpaceX really got going, and estimated a cost of 15 cents/kWh for a gigawatt-siz

A couple years ago I read a book[1] on space solar power. It was written before SpaceX really got going, and estimated a cost of 15 cents/kWh for a gigawatt-size solar power satellite in GEO. Old designs from the 1970s were monolithic beasts, and would be horribly expensive even if launch were free. The new designs use a large number of small identical components (of seven or eight types), which self-assemble in orbit. That way you can mass-produce them. I plugged SpaceX Starship launch costs into the book's estimate and got a total system cost of 5 cents/kWh, which is pretty good for steady clean power that doesn't need storage. [1] The Case for Space Solar Power by John Mankins

sandworm101 7y ago
For space-based solar to be efficient, ie for it to be better than just installing normal solar panels, the beamed energy would have to be at least as powerful as sunlight (1000watts per square meter). That is a very very high bar to overcome. Compared to the simplicity of solar+batteries, I don't see a future for beaming solar energy down from space. Maybe once every square meter of roof space has been dedicated to solar panels, once every city is essentially blanketed in panels, then it may be time to turn to space for more energy. One might say that space-based solar would operate 24/7 and so is better than normal solar panels. That's isn't reality. Low-orbit satellites spend half their day in darkness too. To hit a receiver on the night side of earth a satellite would have to be at a very high orbit, reducing beam efficiency and increasing launch costs.
- evgen 7y ago
  Low orbit sats spend far less than half the day in darkness; it is only when they are in the penumbra of the earth that they are in darkness. What makes space-based power interesting is that such sats have a line to sunlight (and therefore power) when a portion of the planet below them does not, enabling them to provide power for a period of time after sunset and also before sunrise.
  - deleted 7y ago
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  - adwn 7y ago
    > Low orbit sats spend far less than half the day in darkness; it is only when they are in the penumbra of the earth that they are in darkness. Yes, and over 24 hours, they are 12 hours in darkness: on a 90-minute orbit, 45 consecutive minutes are spent in Earth's shadow => half the day, they are in darkness. On the upside, they get full insolation during those 12 hours, unlike ground-based solar. On the downside, at night, the satellite will be in Earth's shadow whenever it's above the ground station, so LEO solar power satellites can't supply power at night (unless they carry batteries).
    - nfg 7y ago
      > Yes, and over 24 hours, they are 12 hours in darkness: on a 90-minute orbit, 45 consecutive minutes are spent in Earth's shadow => half the day, they are in darkness. FWIW I don’t think that’s correct. The effect isn’t significant in really low orbits but going by my hasty maths even at the ISS’ orbit of 350km a satellite spends - at worst - about 10 hours a day in darkness. I say at worst because the plane of the orbit also has an effect here. I make it that you can roughly calculate it as: 24 * ((asin (EarthRadius/OrbitRadius) *2) / (2 pi radians)).
- DennisP 7y ago
  The book's cost estimate is for a satellite in GEO. You get steady power for all but a few hours per year, and 5.4X as much energy from sunlight in 24 hours, compared to the same flat area of ground. According to the book you get about 50% power loss in transmission. The satellite design uses a large area of mylar (or similar) reflectors, and a smaller area of solar cells collecting concentrated sunlight. Concentrated solar is costlier on Earth because the mirrors need to be a lot sturdier. The ground collector covers a fairly large area but it's wire antenna and cheaper than solar panels. The Falcon Heavy payload to GEO is 41% of its payload to LEO. I assumed a similar cost ratio for my estimate. Starship prices at scale aren't precisely known yet, but low enough that launch would be a relatively small portion of project cost anyway. Large-scale grid storage is still quite expensive. Most "solar+storage" projects do not have enough storage to get through the night; if they did, it would cost significantly more than 5 cents/kWh.
  - sandworm101 7y ago
    Space mirrors beaming sunlight down to earth are far from a new idea. https://www.vice.com/en_us/article/9a3y8d/the-man-who-turned-night-into-day https://www.vice.com/en_us/article/9a3y8d/the-man-who-turned...
    - DennisP 7y ago
      I'm not saying they are, I'm just describing what makes this SPS design affordable. But unlike your link, the design is to concentrate sunlight to solar panels in orbit, then beam power to Earth via microwaves.
  - gnramires 7y ago
    Those numbers look very attractive. What was the expected lifetime of the system? Did it take increased deterioration into account from radiation/increased sunpower? What about cooling? (or are they just pricing currently available satellite panel technology?) A big issue with a project like this are unforeseen costs (being a space-based project) and engineering costs (sometimes needs government stimulus). I'm also bothered sometimes those solutions are advertised as panaceas for climate problem; while currently we have mostly a policy problem (clean energy is almost on par with carbon energy, we're just missing key incentives for the new tech and phasing out carbon plants quicker). But indeed looks like a promising avenue of exploration.
- adrianN 7y ago
  Solar panels only recover around 20% of the sunlight that hits them under optimal conditions. Typical installations average to about 10-15% of their peak capacity, so they only harvest less than 4% of the ~1000W peak insolation if you average over a year. So if you manage to recover a larger fraction of the microwave beam you can get away with much less than 1kw/m^2. Plus you save on storage costs.
- unchocked 7y ago
  Not true. Photovoltaics are ~20% efficient, so you're looking at 200 W/m2 in full sunlight. Plus typical solar capacity factors are about 25% so you're down to 50 W/m2 average power generation, with a subjective penalty for intermittency. RF rectification is ~90% efficient, so that same 1000 W/m2 gives ~4.5x the power density of daylight PV, plus a capacity factor of nearly 100% gives 18x the energy density of terrestrial PV - and its baseload power. Mankins will pitch you 100 W/m2 as a useful RF energy density to be competitive with terrestrial solar. You may also be able to include photovoltaics in your receiving antenna and have it both ways.
  - deleted 7y ago
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  - extrapickles 7y ago
    If you replace microwaves with a laser, you can get much higher efficiencies out of a solar panel as solar panels really want to convert monochromatic light. What really kills microwaves for power transfer is the minimum sizes of the transmitter/receiver thanks to diffraction. If your going from GEO to ground, I believe you need on the order of square kilometer sized arrays on both ends. For ground to air applications, rectennas are too bulky and heavy to be practical. If you look at the last NASA power beaming challenge, all of the contestants went with optical power transfer because it was better in W/kg and in W/sq meter. Disclaimer: I’m involved with free-space optical power transfer.
    - DennisP 7y ago
      You're right about the size. The book I mentioned puts the cost of the ground antenna at 0.7 cents/kWh, so not terrible. Lasers have definitely been considered for SPS. The main challenge is that clouds get in the way. (And you don't want the power density so high that it works as a weapon, but I imagine you can avoid that.) Was the NASA challenge for SPS, or other applications?
      - sudosysgen 7y ago
        You mean 0.7 cents per kW, not kWh, right? Because hydroelectric energy costs 0.7 cents per kWh everything included, including profit.
- stcredzero 7y ago
  For space-based solar to be efficient, ie for it to be better than just installing normal solar panels, the beamed energy would have to be at least as powerful as sunlight 1) False. You just have to find contexts where installing solar panels is undesirable or impractical. A forward military camp in the hills of Afghanistan? 2) It's quite easy to exceed the energy density of solar panels with a microwave rectenna to receive power. In fact, a lot of the old designs were dedicated to reducing the power density for safety reasons. One might say that space-based solar would operate 24/7 and so is better than normal solar panels. That's isn't reality. Low-orbit satellites spend half their day in darkness too. Zero cloud cover. Zero dust buildup. In terms of access to solar flux, there are a lot of advantages to being in orbit. Also, there's a "simple" way to get around the tyranny of the rocket equation and get stuff to geosynchronous orbit cheaply: mine the moon, manufacture the silicon solar panels there, and use lunar electromagnetic mass drivers to deliver bulk cargo to geosynchronous orbit. So I would agree with a lot of what you're saying about impracticality with that caveat: "short of having lunar industrial infrastructure." But, given a major power that has the above, how is this so different from having fusion power? EDIT: But if you carry forward this thinking a few steps, you get to...Oh NO! So let's say that we don't get fusion power, but we do get to the point where lunar industrial infrastructure looks within reach. In that case, control over lunar resources will mean control over the most plentiful, clean, and convenient form of energy. Basically, more than half of the motivation behind major power wars in the last century and a half, has been control over resources, particularly energy. Having energy resources gives one military power which gives control over energy resources. This dynamic would seem to set up the next major power conflict past the Taiwan issue. Space could well wind up being the Caucasus Mountains/Persian Gulf of the early to mid 21st century. A major power conflict over energy resources which fundamentally involves the power densities implied by space travel just seems like BAD NEWS. Even worse. We first get the start of the above conflict. Then only afterwards does rapid wartime R&D finally yields military grade fusion power. Yup, we're living in our really nifty, really interesting Sci-fi future. "May you live in interesting times."
  - loeg 7y ago
    Does the moon actually have any resources worth mining?
- deleted 7y ago
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- ChuckMcM 7y ago
  Microwave based power transmission systems do not rely on the photovoltaic effect for the conversion of the energy in the beam into electricity. Instead you use something called a "rectifying antenna" or rectenna which can be 85-90% efficient[1]. If you put 1000 watts of power down on a square meter of a rectenna you could pull 850 to 900 watts out of it. Most of the proposed space-based solar power systems (SSPS) posit a reasonably large rectennas (like maybe 10 - 20 acres or 4 - 8 hectares) sized fields. The challenge is to make the field low enough density such that things flying through it aren't harmed by it. Fortunately energy density falls off with the square of the area so if you have a enough open space you can make it arbitrarily low power per square foot or meter. [1] https://en.wikipedia.org/wiki/Rectenna https://en.wikipedia.org/wiki/Rectenna
- flipflipper 7y ago
  >Low-orbit satellites spend half their day in darkness too. To hit a receiver on the night side of earth a satellite would have to be at a very high orbit, reducing beam efficiency and increasing launch costs. Pedantic, but for LEO there are dawn-dusk SSO orbits that ride the terminator [1] so they get continuous sunlight, you could power some peoples evenings depending on how far the grid spans into the dark side. Not a solution for getting power at 2am though. Might be able to compete if the cost if a rectenna is cheaper/kwh than solar panels (and free as sunlight). Then you can use your same battery solution for night time. [1]: https://en.wikipedia.org/wiki/Sun-synchronous_orbit https://en.wikipedia.org/wiki/Sun-synchronous_orbit
marcusverus 7y ago
Wouldn't such a system still require storage? Their nights might be a little shorter by virtue of being in GEO, but they'd still spend a chunk of each day in Earth's shadow.
- DennisP 7y ago
  Less than you think, because of the planet's axial tilt. You put your satellite in orbit over a fixed spot on the equator: > satellites in geostationary orbit will spend some time in the shadow during what we call “eclipse seasons.” Each eclipse season lasts 44 days, during which the time that a satellite spends in eclipse (shadow) builds gradually from about a minute or two at the start of the season, to a maximum of 72 minutes at each equinox. It then gradually retreats over the next 22 days, at which point the solar arrays are again in the sunlight on a 24×7 basis. https://corpblog.viasat.com/how-satellites-are-affected-by-the-spring-and-autumn-equinoxes/ https://corpblog.viasat.com/how-satellites-are-affected-by-t... So you do have a little bit of downtime, but so do coal, gas, and nuclear plants. The few hours of shadow per year still leaves a capacity factor of over 99%.
T-A 7y ago
> The Case for Space Solar Power by John Mankins Wired ran a story featuring him last year: https://www.wired.com/story/how-to-get-solar-power-on-a-rainy-day-beam-it-from-space/ https://www.wired.com/story/how-to-get-solar-power-on-a-rain... The main news in it was this: https://www.robins.af.mil/News/Article-Display/Article/1998062/us-air-force-research-laboratory-developing-space-solar-power-beaming/ https://www.robins.af.mil/News/Article-Display/Article/19980...
ChuckMcM 7y ago
There are groups and conferences dedicated to Space-based solar power systems (SSPS)[1] That the Navy is going to fly this test vehicle is a pretty good endorsement of the concepts. [1] https://attend.ieee.org/wisee-2020/wp-content/uploads/sites/117/2020/01/IEEEWiSEE2020_SSP_Workshop_CFP.pdf https://attend.ieee.org/wisee-2020/wp-content/uploads/sites/...
ridgeguy 7y ago
Has anybody looked at station-keeping issues (briefly searched, found nothing)? A space based solar power array is a large solar sail. As it orbits, its angle of incidence to the solar wind will change, perturbing its orbit. Lightsail 2 [1] showed the solar wind can be used to raise orbit. I wonder if such effects would be critical for space-based power systems, or just another design constraint. [1] https://www.planetary.org/explore/projects/lightsail-solar-sailing/ https://www.planetary.org/explore/projects/lightsail-solar-s...

sandworm101 7y ago

For space-based solar to be efficient, ie for it to be better than just installing normal solar panels, the beamed energy would have to be at least as powerful as sunlight (1000watts per square meter). That is a very very high bar to overcome. Compared to the simplicity of solar+batteries, I don't see a future for beaming solar energy down from space. Maybe once every square meter of roof space has been dedicated to solar panels, once every city is essentially blanketed in panels, then it may be time to turn to space for more energy. One might say that space-based solar would operate 24/7 and so is better than normal solar panels. That's isn't reality. Low-orbit satellites spend half their day in darkness too. To hit a receiver on the night side of earth a satellite would have to be at a very high orbit, reducing beam efficiency and increasing launch costs.

evgen 7y ago
Low orbit sats spend far less than half the day in darkness; it is only when they are in the penumbra of the earth that they are in darkness. What makes space-based power interesting is that such sats have a line to sunlight (and therefore power) when a portion of the planet below them does not, enabling them to provide power for a period of time after sunset and also before sunrise.
- deleted 7y ago
  [deleted]
- adwn 7y ago
  > Low orbit sats spend far less than half the day in darkness; it is only when they are in the penumbra of the earth that they are in darkness. Yes, and over 24 hours, they are 12 hours in darkness: on a 90-minute orbit, 45 consecutive minutes are spent in Earth's shadow => half the day, they are in darkness. On the upside, they get full insolation during those 12 hours, unlike ground-based solar. On the downside, at night, the satellite will be in Earth's shadow whenever it's above the ground station, so LEO solar power satellites can't supply power at night (unless they carry batteries).
  - nfg 7y ago
    > Yes, and over 24 hours, they are 12 hours in darkness: on a 90-minute orbit, 45 consecutive minutes are spent in Earth's shadow => half the day, they are in darkness. FWIW I don’t think that’s correct. The effect isn’t significant in really low orbits but going by my hasty maths even at the ISS’ orbit of 350km a satellite spends - at worst - about 10 hours a day in darkness. I say at worst because the plane of the orbit also has an effect here. I make it that you can roughly calculate it as: 24 * ((asin (EarthRadius/OrbitRadius) *2) / (2 pi radians)).
DennisP 7y ago
The book's cost estimate is for a satellite in GEO. You get steady power for all but a few hours per year, and 5.4X as much energy from sunlight in 24 hours, compared to the same flat area of ground. According to the book you get about 50% power loss in transmission. The satellite design uses a large area of mylar (or similar) reflectors, and a smaller area of solar cells collecting concentrated sunlight. Concentrated solar is costlier on Earth because the mirrors need to be a lot sturdier. The ground collector covers a fairly large area but it's wire antenna and cheaper than solar panels. The Falcon Heavy payload to GEO is 41% of its payload to LEO. I assumed a similar cost ratio for my estimate. Starship prices at scale aren't precisely known yet, but low enough that launch would be a relatively small portion of project cost anyway. Large-scale grid storage is still quite expensive. Most "solar+storage" projects do not have enough storage to get through the night; if they did, it would cost significantly more than 5 cents/kWh.
- sandworm101 7y ago
  Space mirrors beaming sunlight down to earth are far from a new idea. https://www.vice.com/en_us/article/9a3y8d/the-man-who-turned-night-into-day https://www.vice.com/en_us/article/9a3y8d/the-man-who-turned...
  - DennisP 7y ago
    I'm not saying they are, I'm just describing what makes this SPS design affordable. But unlike your link, the design is to concentrate sunlight to solar panels in orbit, then beam power to Earth via microwaves.
- gnramires 7y ago
  Those numbers look very attractive. What was the expected lifetime of the system? Did it take increased deterioration into account from radiation/increased sunpower? What about cooling? (or are they just pricing currently available satellite panel technology?) A big issue with a project like this are unforeseen costs (being a space-based project) and engineering costs (sometimes needs government stimulus). I'm also bothered sometimes those solutions are advertised as panaceas for climate problem; while currently we have mostly a policy problem (clean energy is almost on par with carbon energy, we're just missing key incentives for the new tech and phasing out carbon plants quicker). But indeed looks like a promising avenue of exploration.
adrianN 7y ago
Solar panels only recover around 20% of the sunlight that hits them under optimal conditions. Typical installations average to about 10-15% of their peak capacity, so they only harvest less than 4% of the ~1000W peak insolation if you average over a year. So if you manage to recover a larger fraction of the microwave beam you can get away with much less than 1kw/m^2. Plus you save on storage costs.
unchocked 7y ago
Not true. Photovoltaics are ~20% efficient, so you're looking at 200 W/m2 in full sunlight. Plus typical solar capacity factors are about 25% so you're down to 50 W/m2 average power generation, with a subjective penalty for intermittency. RF rectification is ~90% efficient, so that same 1000 W/m2 gives ~4.5x the power density of daylight PV, plus a capacity factor of nearly 100% gives 18x the energy density of terrestrial PV - and its baseload power. Mankins will pitch you 100 W/m2 as a useful RF energy density to be competitive with terrestrial solar. You may also be able to include photovoltaics in your receiving antenna and have it both ways.
- deleted 7y ago
  [deleted]
- extrapickles 7y ago
  If you replace microwaves with a laser, you can get much higher efficiencies out of a solar panel as solar panels really want to convert monochromatic light. What really kills microwaves for power transfer is the minimum sizes of the transmitter/receiver thanks to diffraction. If your going from GEO to ground, I believe you need on the order of square kilometer sized arrays on both ends. For ground to air applications, rectennas are too bulky and heavy to be practical. If you look at the last NASA power beaming challenge, all of the contestants went with optical power transfer because it was better in W/kg and in W/sq meter. Disclaimer: I’m involved with free-space optical power transfer.
  - DennisP 7y ago
    You're right about the size. The book I mentioned puts the cost of the ground antenna at 0.7 cents/kWh, so not terrible. Lasers have definitely been considered for SPS. The main challenge is that clouds get in the way. (And you don't want the power density so high that it works as a weapon, but I imagine you can avoid that.) Was the NASA challenge for SPS, or other applications?
    - sudosysgen 7y ago
      You mean 0.7 cents per kW, not kWh, right? Because hydroelectric energy costs 0.7 cents per kWh everything included, including profit.
stcredzero 7y ago
For space-based solar to be efficient, ie for it to be better than just installing normal solar panels, the beamed energy would have to be at least as powerful as sunlight 1) False. You just have to find contexts where installing solar panels is undesirable or impractical. A forward military camp in the hills of Afghanistan? 2) It's quite easy to exceed the energy density of solar panels with a microwave rectenna to receive power. In fact, a lot of the old designs were dedicated to reducing the power density for safety reasons. One might say that space-based solar would operate 24/7 and so is better than normal solar panels. That's isn't reality. Low-orbit satellites spend half their day in darkness too. Zero cloud cover. Zero dust buildup. In terms of access to solar flux, there are a lot of advantages to being in orbit. Also, there's a "simple" way to get around the tyranny of the rocket equation and get stuff to geosynchronous orbit cheaply: mine the moon, manufacture the silicon solar panels there, and use lunar electromagnetic mass drivers to deliver bulk cargo to geosynchronous orbit. So I would agree with a lot of what you're saying about impracticality with that caveat: "short of having lunar industrial infrastructure." But, given a major power that has the above, how is this so different from having fusion power? EDIT: But if you carry forward this thinking a few steps, you get to...Oh NO! So let's say that we don't get fusion power, but we do get to the point where lunar industrial infrastructure looks within reach. In that case, control over lunar resources will mean control over the most plentiful, clean, and convenient form of energy. Basically, more than half of the motivation behind major power wars in the last century and a half, has been control over resources, particularly energy. Having energy resources gives one military power which gives control over energy resources. This dynamic would seem to set up the next major power conflict past the Taiwan issue. Space could well wind up being the Caucasus Mountains/Persian Gulf of the early to mid 21st century. A major power conflict over energy resources which fundamentally involves the power densities implied by space travel just seems like BAD NEWS. Even worse. We first get the start of the above conflict. Then only afterwards does rapid wartime R&D finally yields military grade fusion power. Yup, we're living in our really nifty, really interesting Sci-fi future. "May you live in interesting times."
- loeg 7y ago
  Does the moon actually have any resources worth mining?
deleted 7y ago
[deleted]
ChuckMcM 7y ago
Microwave based power transmission systems do not rely on the photovoltaic effect for the conversion of the energy in the beam into electricity. Instead you use something called a "rectifying antenna" or rectenna which can be 85-90% efficient[1]. If you put 1000 watts of power down on a square meter of a rectenna you could pull 850 to 900 watts out of it. Most of the proposed space-based solar power systems (SSPS) posit a reasonably large rectennas (like maybe 10 - 20 acres or 4 - 8 hectares) sized fields. The challenge is to make the field low enough density such that things flying through it aren't harmed by it. Fortunately energy density falls off with the square of the area so if you have a enough open space you can make it arbitrarily low power per square foot or meter. [1] https://en.wikipedia.org/wiki/Rectenna https://en.wikipedia.org/wiki/Rectenna
flipflipper 7y ago
>Low-orbit satellites spend half their day in darkness too. To hit a receiver on the night side of earth a satellite would have to be at a very high orbit, reducing beam efficiency and increasing launch costs. Pedantic, but for LEO there are dawn-dusk SSO orbits that ride the terminator [1] so they get continuous sunlight, you could power some peoples evenings depending on how far the grid spans into the dark side. Not a solution for getting power at 2am though. Might be able to compete if the cost if a rectenna is cheaper/kwh than solar panels (and free as sunlight). Then you can use your same battery solution for night time. [1]: https://en.wikipedia.org/wiki/Sun-synchronous_orbit https://en.wikipedia.org/wiki/Sun-synchronous_orbit

marcusverus 7y ago

Wouldn't such a system still require storage? Their nights might be a little shorter by virtue of being in GEO, but they'd still spend a chunk of each day in Earth's shadow.

DennisP 7y ago
Less than you think, because of the planet's axial tilt. You put your satellite in orbit over a fixed spot on the equator: > satellites in geostationary orbit will spend some time in the shadow during what we call “eclipse seasons.” Each eclipse season lasts 44 days, during which the time that a satellite spends in eclipse (shadow) builds gradually from about a minute or two at the start of the season, to a maximum of 72 minutes at each equinox. It then gradually retreats over the next 22 days, at which point the solar arrays are again in the sunlight on a 24×7 basis. https://corpblog.viasat.com/how-satellites-are-affected-by-the-spring-and-autumn-equinoxes/ https://corpblog.viasat.com/how-satellites-are-affected-by-t... So you do have a little bit of downtime, but so do coal, gas, and nuclear plants. The few hours of shadow per year still leaves a capacity factor of over 99%.

T-A 7y ago

> The Case for Space Solar Power by John Mankins Wired ran a story featuring him last year: https://www.wired.com/story/how-to-get-solar-power-on-a-rainy-day-beam-it-from-space/ https://www.wired.com/story/how-to-get-solar-power-on-a-rain... The main news in it was this: https://www.robins.af.mil/News/Article-Display/Article/1998062/us-air-force-research-laboratory-developing-space-solar-power-beaming/ https://www.robins.af.mil/News/Article-Display/Article/19980...

ChuckMcM 7y ago

There are groups and conferences dedicated to Space-based solar power systems (SSPS)[1] That the Navy is going to fly this test vehicle is a pretty good endorsement of the concepts. [1] https://attend.ieee.org/wisee-2020/wp-content/uploads/sites/117/2020/01/IEEEWiSEE2020_SSP_Workshop_CFP.pdf https://attend.ieee.org/wisee-2020/wp-content/uploads/sites/...

ridgeguy 7y ago

Has anybody looked at station-keeping issues (briefly searched, found nothing)? A space based solar power array is a large solar sail. As it orbits, its angle of incidence to the solar wind will change, perturbing its orbit. Lightsail 2 [1] showed the solar wind can be used to raise orbit. I wonder if such effects would be critical for space-based power systems, or just another design constraint. [1] https://www.planetary.org/explore/projects/lightsail-solar-sailing/ https://www.planetary.org/explore/projects/lightsail-solar-s...