Space Based Solar Power For the Moon

[JohnFornaro]

My doctor had me cut back on supercritical CO2 last year, so I'm not up to date on the latest developments.

However, that brief news blurb from Sandia is promising.  Today, steam turbines would be available faster.  Of course, I have no idea if the tech is so simple that a development program for the supercritical lunar CO2 prime mover system could be successful within ten years, say.  Given the same amount of development dollars and time, I'd suggest, but wouldn't insist, that steam would win.

It would be a closed system, yes?  The heat source would be the mirror array, yes?  You'd have to trade carbon mass for turbine mass, among other things.

[Bill White]

It would be a closed system, yes?  The heat source would be the mirror array, yes?  You'd have to trade carbon mass for turbine mass, among other things.

Yes, but carbon mass could be scavenged from other applications, including the organic waste stream (including plastic containers) of the lunar base.

Incineration of the waste stream with lunar oxygen will generate CO2

Control the O2 levels and the same facility can make CO which can be used for carbonyl digestion of metallic asteroid fragments (Mond Process).

 

[Bill White]

The problem with both thermal solar and nuclear is cooling. There are rivers of running water you can use to cool down your steam. The only thing you could do, perhaps, is an underground system of piping that would cool things conductively, but now you're adding a whole other layer of mass and complexity.

What is needed is a heat sink.

Pump heat into the sink during the lunar day (have the sink be the "cold end" of a heat engine) then during the lunar night, reverse the process and extract heat from the sink with the sink being the "hot end" of the same heat engine.

Using supercritical CO2 (or helium or ? ? ?) as the working fluid rather than water avoids many of the drawbacks of steam including the availability of a wider band of useable temperatures.

= = =

Volatiles newly extracted from a cold trap can also be heated by this system, helping to cool the working fluid.

= = =

Whether a heat sink system would be cheaper or easier than space based solar power at an L point strikes me as a "trade study" question that cannot be answered without rigorous crunching of numbers.

Also remember, a solar event will radically degrade photovoltaic panels at an L point forcing their replacement. That is another factor to work into comprehensive cost estimates.

[JohnFornaro]

Forgot about the solar events and their effect on PV panels, which would also hold true on the lunar surface.  My preference, today, is for a reflective solar power concentrator, driving a steam turbine/generator combo.  But backing up to a carbon based Brayton cycle for a sec.  True that later on, there would be a "carbon cycle" at the base.  But that wouldn't be true at first.  The prime mover/generator thingy must be landed in fully operable condition, whether that is one nuclear battery "chunk", or a multiple series of landings of a complicated electric generation facility.

[A_M_Swallow]

Any steam based system for the Moon (or space) needs to be leak proof since it is difficult to replace water.

The current landers, launched on an EELV, can probably put 200 kg - 300 kg of cargo on the Moon.  That puts a limit on the mass of the generators, solar collectors and radiators.

[Warren Platts]

Any steam based system for the Moon (or space) needs to be leak proof since it is difficult to replace water.

The current landers, launched on an EELV, can probably put 200 kg - 300 kg of cargo on the Moon.  That puts a limit on the mass of the generators, solar collectors and radiators.

Current landers? There are no current landers, and 200 to 300 kg is certainly not the limit. You're only off by two orders of magnitude. Try reading the ULA lunar/depot-based architecture papers please.

[Proponent]

Volatiles newly extracted from a cold trap can also be heated by this system, helping to cool the working fluid.

Prompted by a discussion with MP99 several posts up the thread, I was thinking about the energy flow at each stage of the propellant-production process:  melting or sublimating ice, electrolyzing it and liquefying the resulting hydrogen and oxygen.  Electrolysis, which involves about 16 MJ/kg at 100% efficiency (50-ish percent seems more likely) dominates.  To an order of magnitude, the other two steps, each involving about 1 MJ/kg if the ice starts at a temperature of 30 K, cancel each other out.  If the feedstock is pure ice, then, the amount of waste heat associated with electrolysis (generating electricity and possibly heating the water) exceeds the heat needed to warm ice by an order of magnitude, and we can't dump much heat that way.

On the other hand, if the concentration of water ice is a just a few percent, then maybe it becomes feasible to dump heat by dumping warm slag some distance away.  This becomes easier if the the feedstock contains volatiles aside from water that you're not interested in trapping (although I suspect you'd probably want to keep most of the volatiles).

EDIT:  "Orders of magnitude" -> "an order of magnitude."

[A_M_Swallow]

Any steam based system for the Moon (or space) needs to be leak proof since it is difficult to replace water.

The current landers, launched on an EELV, can probably put 200 kg - 300 kg of cargo on the Moon.  That puts a limit on the mass of the generators, solar collectors and radiators.

Current landers? There are no current landers, and 200 to 300 kg is certainly not the limit. You're only off by two orders of magnitude. Try reading the ULA lunar/depot-based architecture papers please.

I am not talking about the ULA proposals, they are many years in the future.  I am talking about the Lunar Lander test bed NASA is actually test flying.
http://morpheuslander.jsc.nasa.gov

[Andrew_W]

Well, this discussion has affirmed my belief that planetary surfaces are lousy places to build high energy demand industries.

[Warren Platts]

Wait first of all lets talk about the power required.  Where do you get the megawatt figure.  As with most things it would be wise to start out small than grow.  An operation with a few tens of kilowatts is more than enough IMHO to get things started.

Prompted by a discussion with MP99 several posts up the thread, I was thinking about the energy flow at each stage of the propellant-production process:  melting or sublimating ice, electrolyzing it and liquefying the resulting hydrogen and oxygen.  Electrolysis, which involves about 16 MJ/kg at 100% efficiency (50-ish percent seems more likely) dominates.  To an order of magnitude, the other two steps, each involving about 1 MJ/kg if the ice starts at a temperature of 30 K, cancel each other out.  If the feedstock is pure ice, then, the amount of waste heat associated with electrolysis (generating electricity and possibly heating the water) exceeds the heat needed to warm ice by an order of magnitude, and we can't dump much heat that way.

On the other hand, if the concentration of water ice is a just a few percent, then maybe it becomes feasible to dump heat by dumping warm slag some distance away.  This becomes easier if the the feedstock contains volatiles aside from water that you're not interested in trapping (although I suspect you'd probably want to keep most of the volatiles).

EDIT:  "Orders of magnitude" -> "an order of magnitude."

The 16 MJ/kg is what I got based on the standard enthalpy of formation of water. I've heard that 50% efficiency is about what can be expected, so that's 32 MJ/kg.

I agree that electrolysis is the big consideration, although Jim here has called the liquification process "an energy hog". From what I understand, it's not a simple matter of running it through a refrigerator and radiating the heat: it requires several steps of compression, and compressing air takes a lot of energy. But your main point stands: we can figure out a rough order of magnitude energy requirement by focusing on the electrolysis step.

Now let's consider what order of magnitude production would be required. What we want is an ISRU station that can make a difference. It should be more than a mere demo. It should be more than a self-licking ice cream cone. It should not slow down the Mars project; rather, it should speed it up, or at least make it more viable. It should not merely duplicate what a Mars Direct architecture can do; it should do it better, safer, faster, cheaper.

So what we really want is enough extra propellant to enable a completely reusable Mars architecture.

Keep in mind that to stock an L2 depot, 60% of the LH2/LO2 is spent just lifting it to the depot (leaving enough left over so that the reusable ACES-71 propellant tender can make it back to the Lunar station). So it takes 10 kg of production to deliver 4 kg to L2.

Production Levels:

10 mT/year ---> will lift a couple of ascender modules per year
100 mT/year ---> will make a dent in the cost of running the Lunar station
1000 mT/year ---> will render the base self-sufficient for propellant
10000 mT/year ---> will enable a reusable Mars architecture

So total energy required to electrolyse 10000 mT = 3.2 X 10¹⁴. Let's round that up to 5 X 10¹⁴ to account for the melting and liquification processes plus miscellany energy expenditures.

Half a quad/year is the basic energy requirement.

Thus the minimum power running full blast 24/7 = 5 X 10¹⁴ J/yr / pi X 10⁷ sec/yr = 16 MW

So round that up to 20 MW. That's how I got that figure. So if significant down time due to Lunar night is a factor, then the power would have to upped even more.

To calculate the area of the SBSP array, we can use the following formula:

A = P / C / eff_{L2 array} / eff_{Laser conv} / eff_{Lunar rec}

where A is the area of the L2 SBSP array and C is the solar constant (1366 W/m²)

Conservatively, if we assume the efficiency of the PV arrays (including the receiver on the Lunar surface is 35% and the efficiency of converting electricity to laser light is only 10%, then for 20 megawatts at Lunar surface:

20 MJ/sec / 1.366 KJ/sec/m² / 0.35 / 0.10 / 0.35 = 120 hectares = 1.2 km²

Optimistically: 50% efficiency for the PV arrays and 65% for laser conversion (e.g., with free electron lasers), then we get 9 hectares. So the size of the solar array can vary by an order of magnitude depending on the efficiency. (With 35% PV efficiency the area = 18 hectares.)

For comparison, if the solar panels were emplaced on the Lunar surface, then (assuming 40 MW for peak power), the area would be 8.4 hectares (35% PV efficiency).

[Bill White]

If vapor phase pyrolysis of lunar regolith can allow the extraction of LOX, then concentrated passive solar can provide much of the energy input needed without the need for a 20 MW power plant.

http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources08.pdf

Advantages of "brute force" thermal pyrolysis:

(1) Flexibility in primary energy sources

(2) No need to import chemical reagants

(3) Metallic by-products

[Hop_David]

If vapor phase pyrolysis of lunar regolith can allow the extraction of LOX, then concentrated passive solar can provide much of the energy input needed without the need for a 20 MW power plant.

http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources08.pdf

Advantages of "brute force" thermal pyrolysis:

(1) Flexibility in primary energy sources

(2) No need to import chemical reagants

(3) Metallic by-products

Do you know many joules it'd take to make a tonne of oxygen? Glancing at the pdf, I see big temperatures. So I'm guessing getting oxygen from lunar regolith via pyrolysis would take more joules than electrolyzing water.

There are schemes to provide joules in thermal form by concentrated sunlight. But I haven't seen figures on thermal watts per kilogram for mirrors.

[A_M_Swallow]

{snip}


There are schemes to provide joules in thermal form by concentrated sunlight. But I haven't seen figures on thermal watts per kilogram for mirrors.

From the Wikipedia page on solar sails "5 micrometre thick Mylar sail material mass 7 g/mē".  That includes the reflective surface.
http://en.wikipedia.org/wiki/Solar_sail

The supports for the mirror will increase the mass.  A deployment mechanism may also be needed.

[Warren Platts]

Well, this discussion has affirmed my belief that planetary surfaces are lousy places to build high energy demand industries.

Floating in zero g is even worse. Maybe we should just give up on space industries....

If vapor phase pyrolysis of lunar regolith can allow the extraction of LOX, then concentrated passive solar can provide much of the energy input needed without the need for a 20 MW power plant.

http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources08.pdf

Advantages of "brute force" thermal pyrolysis:

(1) Flexibility in primary energy sources

(2) No need to import chemical reagants

(3) Metallic by-products

It's not that easy. The way it works if first you have to find ilmenite enriched regolith. Even then you'll only get 4 or 5 percent O2 at best. Then you have treat it with imported hydrogen (since you don't want to go for the ice). This makes water. So then you're back to square 1 and have to electrolize the water anyways. Sorry. No dice.

[Andrew_W]

Well, this discussion has affirmed my belief that planetary surfaces are lousy places to build high energy demand industries.

Floating in zero g is even worse. Maybe we should just give up on space industries....

Right, you asked for it...

[Bill White]

It's not that easy. The way it works if first you have to find ilmenite enriched regolith. Even then you'll only get 4 or 5 percent O2 at best. Then you have treat it with imported hydrogen (since you don't want to go for the ice). This makes water. So then you're back to square 1 and have to electrolize the water anyways. Sorry. No dice.

Nope, vacuum pyrolysis of lunar oxygen does not require hydrogen and it can be done entirely with concentrated solar thermal energy Also, the end product is oxygen not water and therefore no electrolysis is required.

Vacuum pyrolysis can be done without catalysts or consumed reagents and without a multi-mega watt space solar power beaming facility. 

But this sub-thread is off thread so I started a new one . . .

[Proponent]

I agree that electrolysis is the big consideration, although Jim here has called the liquification process "an energy hog".

Good point.  If you have a heat sink that's colder than the boiling point of the substance to be liquefied, then pumping isn't necessarily needed for liquefaction.  But even if we dump heat into feedstock at 30 K, I can see that we are likely to need some pumping to liquefy hydrogen.  Perhaps in principal one could construct radiators that would see only the very low temperature of the sky, but that's gotta a pretty big undertaking.

All of this reinforces the conclusion that dumping heat into the slag isn't likely to help a lot in eliminating waste heat.

Now let's consider what order of magnitude production would be required. What we want is an ISRU station that can make a difference. It should be more than a mere demo. It should be more than a self-licking ice cream cone.

I can see that at the scale needed to support a Mars transportation system, lots of electricity will be needed.  I guess for starters, I'm thinking more modestly, along the lines of Spudis & Lavoie, who initially enable merely lunar exploration.  Doesn't it make sense to start on this scale, iron the wrinkles out, and then move on to large-scale production?

[Warren Platts]

I agree that electrolysis is the big consideration, although Jim here has called the liquification process "an energy hog".

Good point.  If you have a heat sink that's colder than the boiling point of the substance to be liquefied, then pumping isn't necessarily needed for liquefaction.  But even if we dump heat into feedstock at 30 K, I can see that we are likely to need some pumping to liquefy hydrogen.  Perhaps in principal one could construct radiators that would see only the very low temperature of the sky, but that's gotta a pretty big undertaking.

All of this reinforces the conclusion that dumping heat into the slag isn't likely to help a lot in eliminating waste heat.

Dissipating the heat leftover from the liquification process is something I have glossed over so far, I must admit. Need to do some more research and figure out a rough estimate of the total heat energy that must be dissipated.

Now let's consider what order of magnitude production would be required. What we want is an ISRU station that can make a difference. It should be more than a mere demo. It should be more than a self-licking ice cream cone.

I can see that at the scale needed to support a Mars transportation system, lots of electricity will be needed.  I guess for starters, I'm thinking more modestly, along the lines of Spudis & Lavoie, who initially enable merely lunar exploration.  Doesn't it make sense to start on this scale, iron the wrinkles out, and then move on to large-scale production?

It depends strongly on the landers you use. The Spudis and Lavoie proposal assume landers that are underpowered and a lot more expensive to operate than the DTAL landers that ULA proposes. As a result, they have to put ISRU propellant production on the critical path for human landings. As such, their plan represents a worse-case scenario. It only gets better from theirs as launch costs go down and better landers are evolved.

Me personally, my thinking is that time = money, so it's best to get to full production as soon as possible, leapfrogging incremental steps wherever possible. The thing isn't going to happen overnight in any case--we're talking 10 or 20, perhaps 30 years, so there will be plenty of time to evolve designs as we go along.

I'm just thinking we need to integrate our Lunar and Mars architectures as much as possible. If there was a clear commitment by NASA to get enough ISRU Lunar propellant to enable a reusable Mars architecture, I believe that politically, this would unify the space community by getting more Mars Society types to support a Lunar ISRU effort.

[Solman]

 I was wondering if a Kraft Ehricke "Solletta" type solution might be worthwhile - a series of large thin mirrors in polar orbit reflecting onto large ground based mirror or PV arrays at the poles to increase insolation and thereby make the ground based solar more effective. This would also allow regions of the Moon away from the poles to be lit up at night.
 The mirrors would have to be able to change orientation as they orbited and this would be a challenge for large mirrors but I wonder if a distributed combination of thrusters and mechanical actuators could do the job.
 Making the entire Moon, not just the poles, much more open to settlement and industrialization would be the payoff.

Sol

[Proponent]

I confess I'm unfamiliar with Ehricke's Solletta proposal, but I worry about mirrors in orbit reflecting sunlight to the ground, simply because the size of the illuminated spot on the ground is at least 0.01 times the distance from the mirror to the ground, 0.01 being the apparent diameter of the sun in radians.  If you manage to place the mirror in a lunar orbit at an altitude of 10 km -- and it would be tough to keep it there for very long, given lunar mascons -- then the spot size is a minimum of 100 m if the mirror is passing directly overhead.  And most of the time, it's not going to be even above the horizon, much less overhead.