Running on Compressed Air?

Antius · 2008-03-18 07:06:10

I'm disappointed that the idea of mars cars powered by compressed CO2 is so quickly dismissed. Powering cars with a compressed version of the local atmosphere would be a simple and elegant method of propulsion; if it could be made to work. Is there anyone here that could look into the physics a little further?

This might work better as a stationary energy storage application, where power density and weight are not a problem. The energy required to boil the CO2 would be stored solar energy.

GCNRevenger · 2008-03-18 17:32:11

I'm disappointed that the idea of mars cars powered by compressed CO2 is so quickly dismissed. Powering cars with a compressed version of the local atmosphere would be a simple and elegant method of propulsion; if it could be made to work. Is there anyone here that could look into the physics a little further?

Well, the problem is pretty insurmountable: CO2 will turn to either a solid or liquid at any appreciable pressure, and such cryogens are useless to push vehicles with on the cold surface of Mars. So unless you have a huge gas tank for the stuff, you won't be able to carry enough to go very far.

For a stationary application, again, you are going to store solar power in a battery/heat sink to boil carbon dioxide to turn a turbine to turn a generator? Why not just store the solar energy directly in a battery/heat sink and use it directly, skipping the boiling CO2 bit.

Michael Bloxham · 2008-03-18 23:01:40

What about using compressed CO2 in combination with an RTG? The excess heat could be used to expand the CO2 for more efficiency and power, while cooling the RTG at the same time without the need for large radiators. The CO2 would be compressed with an electric compressor running off the RTG. Sounds pointless but the benefit would be a more compact RTG system and better power density.

GCNRevenger · 2008-03-19 07:07:03

This is not a great idea either, a compressor is a fairly inefficient machine so while the RTG's electrical output would increase, a lot of power would have to be syphoned off to run the compressor...

...but the real biggie is safety: the compressor would have to run all the time, because if it failed, then all the CO2 would boil/sublime in the tank, causing it to over-pressure and explode! Unless of course you dumped the CO2 overboard, in which case the RTG would overheat since it is wrapped inside a CO2 tank that would trap its thermal output.

Michael Bloxham · 2008-03-20 00:04:37

True. When you are dealing with a nuclear power system, it is probably not worth the risk to rely on an active cooling system.

SpaceNut · 2008-03-21 10:02:22

Was surprized by this reference on an old application of simular technology.
A Thermo-Pneumatic Tesla Turbine Locomotive

Pneumatic locomotives operating on compressed air were successfully used in coalmines over a period of several decades. The Porter Company (USA) built a model that used an 800-psi accumulator tank, an operating tank set at 280-psi and compound expansion piston engines. These original pneumatic locomotives operated successfully and safely over short distances (up to 60,000-ft) in tunnels, hauling ore cars out of mines. None of the locomotives that were used in both American and in Europe used an air heating system to increase power or raise operating efficiency.

GCNRevenger · 2008-03-22 18:44:30

I am all for compressed air for indoor use, its just not a good idea for the surface of Mars.

louis · 2008-03-28 18:41:58

One thing about compressed air: I've read it's probably one of the most efficient ways of storing energy - I think we are talking upper 90s as % efficiency. So it makes sense from that point of view.

noosfractal · 2008-03-28 19:29:57

One thing about compressed air: I've read it's probably one of the most efficient ways of storing energy - I think we are talking upper 90s as % efficiency.

While this is true for large electric power plants using abandoned salt mines to store energy with compressed air, using compressed air to store energy for vehicles (at say 300 atmospheres) is about 1/3rd as efficient as using Lithium Ion batteries (from a "generator to wheel" point of view). Air compressors are not terribly efficient.

louis · 2008-03-28 20:31:31

Thanks for that clarification Noos.

redhorizons · 2008-03-29 22:20:55

I am sure this is posted somewhere else. But since you say Lithium Ion batteries are more efficient then compressed air--would LI Batts work on Mars? I have read other debates that it is too cold for them.

noosfractal · 2008-03-30 00:00:00

I am sure this is posted somewhere else. But since you say Lithium Ion batteries are more efficient then compressed air--would LI Batts work on Mars? I have read other debates that it is too cold for them.

It was more for comparison than a recommendation to use Li-ion. Conventional Li-ion doesn't have a particularly great specific energy figure either. ( Now those Stanford nanowire batteries on the other hand ... )

You're right in that even Li-ion batteries designed for low temperatures stop working around -40 C, but if you choose to use Li-ion as energy storage for vehicles, I don't think that your main engineering challenge is going to be keeping the batteries warm - especially if the vehicle has a pressurized cabin. For example, the Mars Rovers use Li-ion for energy storage.

SpaceNut · 2008-03-30 11:34:14

Not to get to far off track but the rovers require a minimum of 100w to keep from freezing in that regards.

There is also an upper limit to the cell packs temperature as well in that they will and could short out, burn as well as explode from internal heat damge to excesive load currents.

There are also special storage and charging conditions as well that must be adhered to as well.

noosfractal · 2008-03-30 14:09:12

the rovers require a minimum of 100w to keep from freezing in that regards.

Note that the batteries aren't the only thing that needs warming - so there will probably be some such expenditure regardless. Particularly if your system needs to keep people warm.

Also, the rover engineers went to great lengths to avoid lubricants ('cause practical lubricants need heating above cryogenic temperatures), but can a larger transportation system really be built around lubricantless vehicles - even if they are powered by compressed gas? It seems like you're going to have to plan to keep a significant part of the vehicle "warm" (e.g., above -40 C). I think zhar should add aerogel to hir early industry diagram.

There is also an upper limit to the cell packs temperature as well in that they will and could short out, burn as well as explode from internal heat damage to excessive load currents.
There are also special storage and charging conditions as well that must be adhered to as well.

Every energy storage system has limits and limitations. It seems to me that for every "Li-ion batteries as dynamite only less stable" argument you can make, I can make a "compressed gas cylinders as fragmentation grenade only larger" counter.

Antius · 2012-08-17 07:28:26

Compressed or liquified air vehicles would have poor energy efficiency across the full thermodynamic cycle. Additionally the energy density would be poor, making for a short range.

Compressed CO2 might however be a useful energy supply for stationary energy storage where power density is far less important. One could imagine excess nuclear or solar energy being used to power a compressor, which would store CO2 within a reinforced concrete tank. On release, the CO2 could power machine tools directly. An air driven tool would be easier to manufacture than a direct electric powered tool, so this technology would favour insitu manufacturing technologies.

GW Johnson · 2012-08-20 10:51:39

A stationary compressor of some sort might be feasible, if it can spend a lot of time charging up the bottles. Most propulsion applications require high mass flow throughput rates to create thrust or power, certainly any kind of imaginable heat engine. Cycle pressures in such things are a few std atm. Compressing from 7 mbar to a few thousand mbar is a compression ratio in the thousands, not something that can be made mobile, or that can be made to provide high mass flow rate. We have things like that in submarines, but they are huge, massive, and produce a slow trickle, in proportion.

I would suggest compression by other means than pistons or turbine blading. 7 mbar is just too thin for that. CR 30 applied to near vacuum is still near vacuum, far at variance with what any imaginable heat engine would need.

There are things that burn with CO2, yes. Magnesium is one. I guess H2 might be another, although I am unfamiliar with doing that. Why not store really energetic reactants, like LOX and liquid methane, and using compressed atmospheric CO2 to dilute the reaction down to temperatures your engine could stand. Both ordinary piston and ordinary turbine engines become possible on Mars, done that way.

If you are doing ISRU creation of LOX and methane from local water and CO2, might as well compress some extra CO2 for the dilution gas.

There are some absorption-based methods of compression for CO2. Not enough mass flow for mobile use, but it's entirely feasible as the first stage in a practical system: something that gets you from 7 mbar to the 300-500 mbar that our ordinary machinery can handle. You have to put these tinkertoys together like that, in order to make this thing work.

GW

elderflower · 2017-02-01 11:40:18

We can produce Mars type vacuum on earth , and we do this routinely. We have pretty good vacuum pumps that will do the first couple of compression stages for us. After that we are just using common compressor technology to go the rest of the way to Earth atmospheric pressure. The real trouble is that the density of Mars atmosphere is so low that, to get a useful mass flow through the compression system the machines will need to be enormous, even at the bottom of Hellas basin.
There might be a thermal adsorbtion-desorbtion or chemical system which will enable us to concentrate the CO2,but that too is likely to be very large and power hungry.
Neither option will be mobile, so we will have to use a heat engine of some sort or a source of electric power if we want a long range. If short range is acceptable, muscle power might be enough!

GW Johnson · 2017-02-01 19:06:02

If you pick up solid CO2 and put it in a closed bucket, then heat the bucket, the vaporizing CO2 inside will pressurize the bucket far more effectively than any other scheme I ever heard of. Once bucket pressures are in the range of 1 atm, then conventional compression equipment can take it to any pressure you desire with reasonable efficiency and weight, just like here.

GW

SpaceNut · 2017-02-01 20:39:36

https://en.wikipedia.org/wiki/Climate_of_Mars

What is the Temperature of Mars?

Mars can't retain any heat energy. On average, the temperature on Mars is about minus 80 degrees F (minus 60 degrees C). In winter, near the poles temperatures can get down to minus 195 degrees F (minus 125 degrees C). A summer day on Mars may get up to 70 degrees F (20 degrees C) near the equator, but at night the temperature can plummet to about minus 100 degrees F (minus 73 C). Frost forms on the rocks at night, but as dawn approaches and the air gets warmer, the frost turns to vapor, and there is 100 percent humidity until it evaporates.

https://wattsupwiththat.com/2011/04/21/ … ic-shifts/

http://www.airproducts.com/~/media/File … ram-18.pdf

SpaceNut · 2017-10-17 16:23:09

Oh wow another topic to fix...

Add content discusion that is a mix of this with a twist...

GW Johnson wrote:

Sorry. I put pencil to paper again. That’s what you have to do to determine feasibilities and thus the reality of the nifty ideas we generate and debate. Again, my intention is not to rain on any parades, just to figure out what might actually be more effective to attempt.
Off the Caterpillar website: front end loader 914M: 12,677 lb, 73.0 HP. Bucket capacities 1-1.6 cu.yd. In metric, bucket capacities .75-1.2 cu.m, weight 5749 kg, power 54.4 KW. This is a Diesel engine device. It would have to have all-electric propulsion and hydraulic power sources, and batteries that must be recharged, to operate on Mars. The rubber tires might work if not too cold-soaked, but tire heat requires power. The operator would need an MCP suit and Antarctic-type exterior clothing over it.
As a wild guess, 1 cu/m nominal bucket capacity, 6 metric tons, 60 KW max power rating running at duty cycle 35%. Wild-guess that it can operate for about 2 hr before requiring recharge, which is 0.35 * 60 KW * 2 hr = 42 KW-hr battery capacity. Lithium-ion batteries, charge energy = 1.5 * operating energy, as another wild guess, for 63 KW-hr to charge. (That factor is 2 for lead-acid batteries.)
To get one day’s 8 hr shift of working, will require 4 such machines, used one after the other, and recharged either during the off-shift, or while not working during the shift. Assuming you can dig up 1 bucket’s worth up and drive over to the point where it will be processed in only 5 minutes, then another 5 min back to the mine point, that means you get 1 cu.m of damp regolith to the processing machinery every 10 minutes. That’s 6 cu.m per hour, or 48 cu.m for the shift.
You get that 48 cu.m of regolith per sol using 24 metric tons of machinery, and with 252 KW-hr worth of battery-charging electricity. If you let it take all night to recharge (16 hr worth of off-shift time), that’s 15.75 KW worth of recharge power all night long. It’s dark at night, solar simply cannot do that.
If you recharge during the day (during the work shift), then you have the 6 shift hrs the machine is not working. That’s 42 KW to recharge. If the peak solar panel production is 0.5 KW/sq.m (at Mars with dimmer sun), and you use the typical 0.30 factor to account for low sun angles early and late in the sol, you will need >280 sq.m of absolutely-clean panels just to charge the batteries on the diggers.
The regolith is nominally 5% water by mass. Using a nominal specific gravity of 2 for loose particulate matter, 1 cu.m is 2 metric tons, 2000 kg. You get 48 of those bucketfuls per shift. 5% of that is water, so you got 4800 kg (about 4800 L) of water for your investment: 24 metric tons of diggers, half-a-hundred KW of electricity to keep their batteries charged, and either a SAFE-400 or >~300 sq.m of solar panel to get that electricity, for only 4800 L of water after an 8-hour shift. The crew needs to drink some of that, so not all of it could go toward propellant manufacture, or greenhouse experiments, etc. To get more water requires more machinery and more power, in proportion.
Now, think melting ice with hot water’s heat. In US customary, latent heat of fusion is 540 BTU/lbm. Specific heat of the liquid is 1 BTU/LBM-F. The temperature range I have to deal with is 212-32 =180 F. I need 3 lb of cooling water for every lb of ice I melt, theoretically. Call it 5:1 for real-world inefficiencies of heat transfer. Now I get to use hot water ice melting around the clock, not during a work shift.
Let’s arbitrarily pick 1 kg/s hot water flow rate as “typical” of what I can tap off the reactor coolant flow (probably not a SAFE-400, a real pressurized water design, but maybe there’s some sort of heat exchanger a SAFE-400 could use instead of the radiator it was designed for, I dunno!).
Using an Earth day of 86,400 sec to approximate a Martian sol, I have some 86,400 kg of hot water, which at 5:1 should melt some 17,280 kg of ice. So this kind of heat source should produce something like 17,000 L per sol! If instead I can safely tap off 10 kg/s hot coolant flow, the yield is closer to 170,000 L per sol! Something between those two figures should be easily feasible, though.
Does everyone now understand why I want to send hot water (could be reactor cooling water!!!) down a pipe into a buried glacier to melt ice in mass quantities (10’s-to-100’s of cu.m’s not just 100’s of L’s per day), and just pump it back up that pipe for a pump power likely well under a KW? The yield appears to be 2 to 3 orders of magnitude higher, while the effort and equipment required is far lower.
The same effort disparity is why our surface transportation here at home runs on oil, not coal, even though coal is where we started. Why go to all the effort and bother to run a strip mine for coal, when all you need is an oil well? Once the well is done, almost no effort goes into the continuing recovery.
The vehicle with the drill rig is likely smaller, lighter, and lower-power than one of these diggers. I need that drill rig, a few tons of drill pipe and well casing materials, and a lousy little water pump with a couple of spares. I only need it when drilling the well, although I may to drill another well after a while. Once the well is in place, if I use reactor coolant as my hot water source, the only power draw is the pump. That delivers tons and tons of water per sol to the inlets of the water-processing plants.
So, now judge: which is the better resource to exploit for water? Damp regolith, or buried ice? I vote for the buried glacier. Why? Less machinery to transport, less power draw while operating, and much more water per day available, to support all the various uses.
Damp regolith is mostly everywhere, but not entirely everywhere! Massive buried ice deposits are only in certain locations, which we think (!!) we have identified with remote sensing. There always has been, and always will be, a disparity between remote sensing and ground truth. That disparity can easily kill a crew equipped incorrectly for what the site actually offers, even if it is not a very great disparity at all. THAT is why the pathfinder mission(s) with some sort of deep-drill rig is so crucially important.
There is still simply no substitute for ground truth, especially subsurface ground truth. There is also simply no comparison between the concentrated resource versus the thinly-spread one.
GW

SpaceNut · 2017-10-17 16:35:38

Antius wrote:

Good post from GW. It is always useful when someone puts extra time in to do arithmetic on these concepts.
A small mobile PWR reactor would work well for water mining. Something the size of a road tanker, that can be towed to different locations between uses. The heat exchanger can be a single loop design with the boiler on its side and vertical steam separator. Decay heat removal can be accomplished by radiating through the skin of the towed reactor. If the vehicle is 3m wide and say 10m long and radiates at 200C, then using steffan-boltzmann with an emissivity of 0.9, gives a maximum decay heat dumping capability of 177kW. If sustained decay heat levels are about 1% of operating power, that would mean a mobile reactor of this type would have maximum operating power of 17.7MW.
That is about enough to melt and warm up about 30 litres of water per second. The reactor would be partially shielded by a water blanket, which would also serve as a reflector. It would generate relatively high levels of external radiation while active, so should only be activated remotely. Since refuelling would be extremely difficult on Mars and fuel is only a modest part of system mass, it would make sense to design a system that uses an elongated core with a movable reflector, such that core life can be decades.
If the base is relatively close to the glacier, we could pump the water back using polypropylene/polyethylene pipes that can be rolled up and dragged to new drilling points.
A stored heat engine is another option that might beat Li-ion battery power in terms of performance. Molten silicon can store up to 1MWh of thermal energy in 1m3. Charging would involve the use of heating elements and is not rate limited in the way that batteries are. As a working fluid, we could use liquid CO2, compressed to at least 5.1 bar. With a conversion efficiency of 30%, 300kWh of mechanical power would keep your digger going for 6 hours before recharging.
The enthalpy change of boiling of CO2 is relatively poor, so you would need to keep a bowser of compressed liquid CO2 close to your digger and refill regularly. You would need more liquid CO2 than the equivalent volume of diesel here on Earth, because heat transfer rates will limit the peak temperature of the CO2 in the engine. But again, this is not rate limited in the way that recharging batteries is. Liquid CO2 can be produced by compressing and refrigerating Martian air and stored in carbon steel tanks indefinitely at ambient Martian temperatures. So this is a good way of storing intermittent energy that can then be used to power diggers and air tools when needed. Not suitable for powering transportation due to the relatively low energy density, but it would work for applications close to a base.
If a rover could carry a flexible solar array and a compressor, it would be interesting to compare the performance of a stored heat engine to Li-Ion batteries. An RTG might be useful in this way. During the night, the RTG could power a small heat engine that could compress CO2 into a cylinder where it would liquefy. During the day, the vehicle would use direct heat from the RTG to boil the CO2 in a heat engine, generating propulsive power.
Last edited by Antius (Today 06:45:33)

SpaceNut · 2017-10-17 16:38:13

I like the approach to use the RTG to store the CO2 in the evening and even the heat to cause the engine to work with the added boost from solar during the day.

earlier pages now fixed

Last edited by SpaceNut (2017-10-17 20:04:38)

SpaceNut · 2017-12-09 17:21:51

On mars we will not be breaking any land speed records but we can definitely look for distance on a charge...

Compressed Air Motorcycle Breaks 80 MPH With Ease

o2-pursuit-compressed-air-motorcycle-e1352133891646.jpg

Sure change it to 4 wheels for safety and we have a light mass transport for mars.

The Yamaha WR250R frame was fitted with a compressed-air engine, and a standard scuba diving tank, which substitutes nicely for the gas tank. Opened up all the way, the O2 Pursuit can travel over 60 miles on a single tank, and up to 87 mph.

http://www.earthtechling.com/2009/12/on … d-air-car/

louis · 2017-12-09 18:18:06

Looks like compressed air vehicles have come on a lot. In terms of Mars exploration, they have advantages: you could create a fuel trail, with compressed gas tanks being placed every 50 miles or so. No need to charge up as with an electric vehicle. Certainly needs looking into, although I would still favour electric vehicles with some onboard PV panel input.

SpaceNut wrote:

On mars we will not be breaking any land speed records but we can definitely look for distance on a charge...
Compressed Air Motorcycle Breaks 80 MPH With Ease
http://earthtechling.com/wp-content/upl … 891646.jpg
Sure change it to 4 wheels for safety and we have a light mass transport for mars.
The Yamaha WR250R frame was fitted with a compressed-air engine, and a standard scuba diving tank, which substitutes nicely for the gas tank. Opened up all the way, the O2 Pursuit can travel over 60 miles on a single tank, and up to 87 mph.
http://www.earthtechling.com/wp-content … 684328.jpg
http://www.earthtechling.com/2009/12/on … d-air-car/

GW Johnson · 2017-12-09 19:24:02

Compressing from 1013 mbar is relatively efficient (around 65% energy efficiency). Compressing from 6 mbar is not. Unless you harvest dry ice and do much of the compression by phase change, to near 1000 mbar.

GW

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