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Space Solar Power -- How Much More Power Can Be Collected?

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Skotty

Skotty

2014 S P85 | 2023 F-150L
Jun 27, 2013
2,686
2,269
Kansas City, MO
#1
I know Elon is very negative towards the idea of space solar power, and I have to mostly agree. However, I keep seeing information about how you get 2x the power from solar panels in space, and I find this suspect. Even a lot of the pro-space-solar camp have used this figure, but shouldn't it actually be quite a bit more than 2x?

If you look at the power collected by a solar panel over the day, it tends to follow a bell curve during daylight hours. Maybe if you have a motorized system that can follow the sun you might get a better return, but I would guess it's still somewhat of a bell curve. It's not like for half the day you get 0% and half the day you get 100%. It's more like half they day you get 0%, and the other half you get maybe an average of 50% to 60% if you were to average the bell curve. In space you should be able to get 100% all day long. I would guess this would make space solar panels able to collect at least 3x what they can on Earth. Factor in clouds, weather, and particulates taking away some of the potential, and it's probably more like 4x.

That said, I'm sure the economics of it still wouldn't work. But I'm just wondering if anyone has done any calculations or research to see what the real difference would be (not including conversion losses).
 
#2
Your basic idea is right, that there should be more hours of sunshine available in space.
But Space solar power is about a power plant in a low orbit around Earth, so it would be in Earth's shadow roughly 50%. With the exception of polar orbits that could face the sun 24h/day, but must be adjusted constantly by 1° per day as the Earth circles the sun.

So you just have increased performance due to lack of clouds and snow.

Then there are losses during energy transfer. Remember, you start with turning photons into electrons, then turn them back into photons (microwave beam) these get converted back into electrons. The last 2 steps are unique to space solar power. Efficiency rates in orbit will have to vastly excel terrestrial efficiency rate to make up for these losses. The net result still has to be vastly higher, since you have to compensate for cost of hauling all that stuff into orbit. Remember 1kg to orbit is roughly $1k, which will buy you 500-1000W terrestrial solar PV installed, grid-connected and running mostly maintenance-free for 20 years.
 
#4
@Skotty

Actually a trade-off between the greater efficiency of solar panels in space and the cost of the payload to be launched with a space rocket should be done.
I think that if Elon is negative about solar panels in space it means that he has already done this trade-off coming to the conclusion that placing solar panels in space is not worth of it.
 
#5
Zzzz... said:
There is no way at least right now to place 5 ton satellite on orbit for 5 mil. Or one ton for one mil.

Prices are usually higher then $10k per kg. Might be if SpaceX would succeed in making rockets reusable ....
Click to expand...

Thanks for the correction! BTW everytime I see a SpaceX video of a dragon mission to the ISS, I cringe on realizing that the dragon support module with these lovely solar panels is burning up in the atmosphere! :cursing:
wsjpoy201214.png
 
#6
VolkerP said:
But Space solar power is about a power plant in a low orbit around Earth, so it would be in Earth's shadow roughly 50%. With the exception of polar orbits that could face the sun 24h/day, but must be adjusted constantly by 1° per day as the Earth circles the sun.
Click to expand...

That "adjustment" comes free in a sun synchronous orbit; they use a slightly inclined orbit in conjunction with the oddball shape of the Earth (it's not quite round) to precess the orbit with no expenditure of fuel.

That said, I really think a solar power station would have to be in geosynchronous orbit; otherwise beaming the power to the ground would be even more problematic. It takes a lot more energy to get way up there.

The economics of this are crazy. You could build hundreds of times more solar on the ground for the same price. That's a whole other level of efficiency.
 
#8
Mind you, my question was merely academic. Theoretically, you could park a solar panel in permanent sunlight in space. I think the answer, as most of you kind of indicated, has more to due with compromises you have to make such that you could realistically beam the energy back to Earth. Such compromises would put the solar panels in Earth's shadow a certain percentage of the time.
 
#9
VolkerP said:
Thanks for the correction! BTW everytime I see a SpaceX video of a dragon mission to the ISS, I cringe on realizing that the dragon support module with these lovely solar panels is burning up in the atmosphere! :cursing:
wsjpoy201214.png
Click to expand...

What really looks like a waste - the Dragon itself while designed to be totally reusable by the contract with NASA is flying one mission and for a next one new Dragon should be produced. SpaceX was looking at launching couple Dragon based mission with clients other then NASA to utilize reusability of already produced spacecrafts... But not sure how they are doing on that front now.

- - - Updated - - -

ItsNotAboutTheMoney said:
Capabilities Services | SpaceX
$56.5M, 13, 150kg to LEO, 4, 850kg to GTO..
Click to expand...
I know. Way off $1k per kg. As for being cheaper then 10k per kg - SpaceX is pretty much an exception providing better then half price discount compared to most other launch systems. That is why I used "usually" word. I believe only Ukrainian Zenit 3 could offer cheaper then $10k per kg price point atm. But both of them, Zenit 3 and Falcon 9 fly relatively infrequently.

- - - Updated - - -

Skotty said:
Mind you, my question was merely academic. Theoretically, you could park a solar panel in permanent sunlight in space. I think the answer, as most of you kind of indicated, has more to due with compromises you have to make such that you could realistically beam the energy back to Earth. Such compromises would put the solar panels in Earth's shadow a certain percentage of the time.
Click to expand...

Geostationary orbit is almost always under sunshine, few hours a year it is in the shadow IIRC. And you most likely want satellite to be in the same position toward Earth based collecting station. Otherwise complexity of the system would grow enormously.

But I'm in no way suggesting that idea of space based solar is a viable one. I agree with Musk in this regard, it would be too expansive and economically non viable project.
 
#11
#12
The other problem is how are you going to get the power down to earth? What are the efficiency losses? And how hard is it to turn the power transmissions mechanism (and therefore the solar energy collector satellite) into a space-to-ground weapon?

I think I prefer my solar panels on the ground for now.
 
#13
rcc said:
The other problem is how are you going to get the power down to earth? What are the efficiency losses? And how hard is it to turn the power transmissions mechanism (and therefore the solar energy collector satellite) into a space-to-ground weapon?

I think I prefer my solar panels on the ground for now.
Click to expand...
You just use a long extension cord! ;)

Ever play sim-city? one of the power plants was exactly this, they called it a microwave power plant, idea was that it was solar panels in space and focused the energy in to a microwave beam to a ground station on earth.... One of the "disasters" that could befall your city was the microwave beam "missing" the receiving station and destroying a few square blocks of your city...
 
#15
Shawn Snider said:
Unless I've missed it among the post's, nobody has mentioned Earth's massive debris field of man-made space-garbage. There's also the random particles/debris from interaction's within the solar system.
Click to expand...


There isn't a massive debris field of man-made space garbage. The total amount of stuff in space is really very tiny. It doesn't take a huge amount to be a problem as far as collisions are concerned, because a single impact can destroy an entire satellite, but an object passing between a solar collector and the sun once, or even several times, will not make a big impact on the energy collected.
 
#17
Back to the main topic, solar panels in geosynchronous orbit would get a lot more sunlight.
  • Its orbit would have a radius of 42,164 km, compared to the earth's radius of 6,371 km. So instead of spending 12 hours (on average) in the earth's shadow that a terrestrial solar panel inevitably has, it would spend 69 minutes of each day in shadow. This effect alone gives almost a 2:1 advantage to orbital solar.
  • The earth's atmosphere absorbs about 30% of the sunlight before it can hit terrestrial PVs, even at high noon.
  • At times other than noon, the longer path through the atmosphere absorbs yet more of the available sunlight, creating a (non-Gaussian) bell curve distribution of available energy. Let's call this another 30% derating:
    CLOUD.GIF
  • Clouds reduce available energy even more. How much depends on the location (which is why the Atacama Desert in Chile is being considered for big solar farms). In a more typical but still great location, perhaps we'd lose 10% to clouds
Taking all these factors into account, orbital solar has about 95% insolation (relative to a notional satellite at, say, Lagrangian Point 4 or 5), while terrestrial has about 22% (.5*.7*.7*.9). So orbital solar has about 4.3x the available energy.

Of course, not all is wine and roses for the orbital solar platform:

  • The transmission losses will be horrific. Instead of using copper wires, the power is transmitted by focused microwave beams (paper on this from Georgia Tech is HERE). Claimed efficiencies are around 86% in the transmission, plus another 5% or so in the DC-AC conversion loss, plus another 5% or so transmission losses from the (remote!) antenna to load. That's about 78% total efficiency.
  • Panel heating could be an issue. Although space is very, very cold, hot things only become cold in space through black-body cooling (radiating energy), so the panels could get very hot indeed. Hot panels have low efficiency. Clearly this issue has been addressed by NASA et al., but I don't know the net effect on efficiency.
  • Repairs don't happen. You'd probably design redundancy in all your circuits, because there's no way it would be cost effective to have manned missions to repair the satellite or components that fail. So you're probably looking at costs for the PV systems at some large multiple of a terrestrial system, and a faster degradation-over-time curve.
  • The solar satellite will have to have thrusters, communications, on-board computers, etc. not needed on land, adding cost.
  • Fuel eventually will run out, shortening life.
  • Operating costs will be huge. Having to continually track the satellite, etc. paying round-the-clock crews, paying for communications kit, etc.
  • And you have to pay SpaceX huge fee to get the satellite into orbit.

Bottom line: while there's a theoretical upside in using space-based solar, it's hard to see how the business case really stacks up.
 
#18
It may become more feasible in the future, if one of those thrusters that don't require fuel turns out to actually work, it becomes much cheaper to get things from LEO up to higher orbits. If they can be made small enough, they can also provide attitude control indefinitely.

It then becomes possible to build massive arrays, and get them in place for relatively low cost. But for now, the energy needed to get things to orbit is too much for the panels to economically make up for.
 
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