Author Topic: Space solar power - for Mars  (Read 7150 times)

Offline alexterrell

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Space solar power - for Mars
« on: 02/13/2016 11:31 am »
I know we've had discussions on power sources for Mars base before, but I'd like to compare space solar power with ground solar power.

Advances in nuclear fission or fusion may make this irrelevant, but at current levels of "competitive advantage", a Mars base or colony will have a mix of both. But should the solar power be based on the ground or in a Mars synchronous orbit.

Solar panels in low latitude, dry locations can have a capacity factor of around 25%. (More if steerable, but that doesn't seem worthwhile). They also need a large amount of backup power - which on Mars would be a mix of electricity storage and demand shifting (running electrolysis when it's sunny).

If the panels are in orbit, their capacity factor is close to 100%. Is it possible to get a "grid-to-grid" efficiency of 25% by beaming the power?

That is clearly possible with microwave beaming, where efficiencies of >80% seem achievable. However, this only really works at Gigawatt scale (from a stationary orbit). So great for a later colony of 10s of thousands of people, but what about before?

Single wavelength solar panels can be 50% efficient. Is it possible to produce laser light at continuous MW scale at 50%? In which case, on an efficiency/utilisation basis, we're matching the ground based options.

Other Advantages of space based solar power.
- Reduced storage requirements by having 24/7 power.
- In the early days, the solar panels will be launched from Earth. It's easier to deploy them in Mars orbit than to land them on the surface. (Mars landing is very hard).
- It could be that electric propulsion is used to send payloads to Mars. In that case, after it's completed its primary mission, the electric propulsion unit attaches to the orbital array.
- The solar array could beam power to multiple locations, including larger mobile units (the technique should work from a receiver of about 20m x 20m).
- If in the early days, if there are missions to different locations in different launch windows, then a space based system could support all locations over the course of a decade or more.
- Can work at higher latitudes - where ground solar systems suffer from significant seasonal variations

Disadvantages
- Reliability? More things to go wrong and nothing is as reliable as your own solar panels on your own roof. (If they're designed for reliability).
- Weaponisation possibilities. we would look for a peak beam intensity of about 2KW/m2, with an average across the receiving array of 1KW/m2. Not enough to make a really dangerous weapon, unless several lasers were used together.
- Cooling requirements for the lasers. At 50% efficiency, there's a lot of heat to dump. How hot can lasers work at, as the hotter they are, the easier it is to radiate the heat.
- The system is unproven with low TRL.
- It still needs some solar cells on the ground - but these are less than 10% the size of what would be needed without the laser.

Sizes
1st system: 20m x 20m ground array receiving 400KW and outputting 200KWe. This would need about 1MW in orbit, from 1Ha (100mx100m) of solar panels. (How much would this mass)

Mid system: 100m x 100m receiving 10MW and outputting 5MWe. In orbit, 20MW from 20Ha (200m x 1000m) of panels.

Large scale system: This would use a microwave system at GW scale - greatly easing the radiator problem. The solar panels would be manufactured in orbit using aluminium, silicon, and silica from Phobos. Once GW scale solar power stations are being made, these could be equipped with ultra high Isp (30,000s) electric propulsion units and sent to Earth, in case we haven't sorted out nuclear power by then.


Is this something that should be considered, even for early bases? Even for NASA?



Offline guckyfan

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Re: Space solar power - for Mars
« Reply #1 on: 02/13/2016 12:06 pm »
You can pretty much rule out laser power transmission from orbit. It would not get through a dust storm. I don't know how dust affects microwave.

You can also rule out concentrating solar power on the surface. Again because of dust storms. They can obscure the sun enough that you don't see its locations in the sky. However scattered light gets throug quite well. Non concentrator solar arrays will still provide significant energy, even if reduced. Sun tracking solar arrays would lose their advantage over fixed installations during storms, so their output would be more reduced.
« Last Edit: 02/13/2016 12:06 pm by guckyfan »

Offline alexterrell

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Re: Space solar power - for Mars
« Reply #2 on: 02/13/2016 01:02 pm »
Quote
You can pretty much rule out laser power transmission from orbit. It would not get through a dust storm. I don't know how dust affects microwave.

I had considered that, but it's not too much different for a ground based array. A ground based array would also need a larger effort to clean it. But dust storms are infrequent enough that you can turn down the main systems and live off the nuclear back up. They're not like cloud cover on Earth.

I'd think microwaves wouldn't be effected by a dust storm unless it has microwave absorbing particles. The amount of dust per square metre is pretty small.

Concentrating solar power needs expensive tracking systems, and has heat rejection which may be an issue on Mars (as it will for nuclear power and lasers).

Offline Solman

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Re: Space solar power - for Mars
« Reply #3 on: 02/14/2016 09:53 pm »
I disagree that space solar is less reliable. Multiple satellites in LMO would certainly have redundancy on their side. If it is assumed that significant cargo is brought from Earth and that this cargo is delivered by SEP, then there could be a large number of cargo craft to provide high total power for powering lasers or microwave beams.
LMO is more favorable for microwaves as well.
Great OP.
« Last Edit: 02/14/2016 09:56 pm by Solman »

Offline DAZ

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Re: Space solar power - for Mars
« Reply #4 on: 02/14/2016 10:23 pm »
Areostationary orbit is much closer on Mars then geosynchronous orbit on earth (about 11,000 miles as opposed to 22,000 miles) This is a big difference if youíre going to be beaming power.  The 2 moons might increase station keeping needs (especially Phobos) but itís not like you wonít have lots of solar power to compensate for that.  In addition you would get almost for free (relative to the mass of all the rest of it) a communications and high-speed data relay satellite.  It might even actually all be free if you just repurpose the SEP tugs as needed.

Offline alexterrell

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Re: Space solar power - for Mars
« Reply #5 on: 02/15/2016 11:34 am »
I hadn't thought of multiple satellites. That could be an option later, but the trouble with low orbits is that they have night time when the target has night.

Secondly, an electric thruster needs about 6km/s delta V to go from high earth orbit to high Mars orbit. It then needs about another 1km/s to get to Mars stationary orbit, and perhaps a further 3km/s to get to a low orbit (without looking up Mars orbital velocities).

With lasers, the space system is "down scalable". Every electric thrust mission that arrives could just park itself in Mars stationary orbit (the primary cargo would be released well before orbital insertion for direct entry), and operate independently. If after a few years there are five 200KW arrays, they can all point at the same surface array, providing excellent redundancy.

Offline Solman

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Re: Space solar power - for Mars
« Reply #6 on: 02/15/2016 07:52 pm »
I hadn't thought of multiple satellites. That could be an option later, but the trouble with low orbits is that they have night time when the target has night.

Secondly, an electric thruster needs about 6km/s delta V to go from high earth orbit to high Mars orbit. It then needs about another 1km/s to get to Mars stationary orbit, and perhaps a further 3km/s to get to a low orbit (without looking up Mars orbital velocities).

With lasers, the space system is "down scalable". Every electric thrust mission that arrives could just park itself in Mars stationary orbit (the primary cargo would be released well before orbital insertion for direct entry), and operate independently. If after a few years there are five 200KW arrays, they can all point at the same surface array, providing excellent redundancy.
If using SEP as the prime mover there would be many cargo vessels from the start seems to me and each of them by necessity would be in the 200KW class  wouldn't,t they?
I was assuming that cargo would be delivered by them to LMO where they would meet a reusable Lander but perhaps the cargo would go directly to EDL in which case the delta V to lower the vessel,s orbit would be extra. In that case the SEP craft could take their time and use aero-braking like the MRO did maybe.
Perhaps a medium orbit would be needed for lower latitude bases but higher latitude bases could be supplied by SEP craft in polar orbit that would be in sunlight. These higher latitudes also would have more craft in range at any given time just by the nature of polar orbits.

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