Compressed Air Car

S

SturdyBegger

10 mW
Joined
Aug 5, 2024
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Location
Kansas
Hello,

There is a compressed air car design that uses paraffin wax as a "phase change material." The paraffin wax is heated to liquid by an electric coil and when it re-solidifies the latent heat is transferred into the compressed air. I wonder, if rather than using an electric coil to heat the paraffin wax, if there could be a mini-split heat pump? Has anyone built one of these cars?

en.wikipedia.org

Compressed-air car - Wikipedia

en.wikipedia.org en.wikipedia.org

From,
Tyler
 
compressed air is extremely energy lossy, worse than hydrogen. The volumetric density of compressed air is also poor ( very limiting, range wise ) .there's a reason it didn't take off and all that everyone talks about is batteries 'n oil!
 
I think compressed air vehicles (small locomotives) used to work well in coal mines. But I'm not sure whether underground coal mines are still a thing. Above ground, steam reservoir locomotives worked where you couldn't have a fire, for example in ammunition plants. The video I linked to shows what is actually phase change energy storage on that locomotive: as the pressure drops more of the hot water boils and generates more steam.
 
neptronix said:
compressed air is extremely energy lossy, worse than hydrogen. The volumetric density of compressed air is also poor ( very limiting, range wise ) .there's a reason it didn't take off and all that everyone talks about is batteries 'n oil!
Click to expand...
True, the temperature changes soak up more energy than is available through the compression/decompression cycle. That's addressable, infinitely large heatsinks in both directions cancels it out, as does applying the heat generated during compression to the decompression part of the cycle (either full open of fully closed loops) and that seems to be what's going on here, heat being stored in a change state condition (latent heat of liquefaction) and fed back in on the opposite part of the cycle.

Cool if it works (no pun intended :) ) and can be done cost effectively, I'd be surprised if energy densities can be great enough for anything but niche use in mobile applications but compressed air storage is a bit of a holy grail for stationary applications.
 
neptronix said:
compressed air is extremely energy lossy, worse than hydrogen. The volumetric density of compressed air is also poor ( very limiting, range wise ) .there's a reason it didn't take off and all that everyone talks about is batteries 'n oil!
Click to expand...
I understand that the compression of the gas is lossy, of course thermal energy is lost. The idea that I want to research, though, is regenerating thermal energy from the atmosphere into the compressed gas using the refrigeration cycle and a phase change material. If you see the circuit in the wikipedia article that i included, I would make three of those circuits for redundancy, for continuous power, and also add three more circuits, three refrigeration cycles. The condensers would be on the PCM, the paraffin wax.

So, the vehicle uses electricity to regenerate thermal energy from the atmosphere into the compressed air. The compressed air is only a working fluid(there are two working fluids in this system.) This vehicle would be capable of "charging" while it is driving. So, on a 100°f day it would go farther or faster than it would on an 80°f day. It would still need to probably fill up with compressed air every time you park it, but you know what? Electricity is everywhere and it could be plugged in with an extension cord.
 
Look into the energy economics of it, all figures are awful. You need a massive tank, and your limit is PSI.

Improving the compression process from awful efficiency to allright efficiency will involve a bunch of weight and expense, and you still have very poor energy density. ( way worse than hydrogen, which is a commercial failure and money pit in transport R&D BTW )

I'd go and read research papers on what's been done before. Many have went down the path you're thinking about in the last 2 decades and hit a huge wall. Knowing about what those walls are will be informative.
 
stan.distortion said:
True, the temperature changes soak up more energy than is available through the compression/decompression cycle. That's addressable, infinitely large heatsinks in both directions cancels it out, as does applying the heat generated during compression to the decompression part of the cycle (either full open of fully closed loops) and that seems to be what's going on here, heat being stored in a change state condition (latent heat of liquefaction) and fed back in on the opposite part of the cycle.

Cool if it works (no pun intended :) ) and can be done cost effectively, I'd be surprised if energy densities can be great enough for anything but niche use in mobile applications but compressed air storage is a bit of a holy grail for stationary applications.
Click to expand...
This would be an awesome stationary generator. On the central air conditioner condenser, out side the house, if rather than blow the thermal energy off of the unit, there were this pneumatic circuit and a PCM, the thermal energy could be recaptured and discharged at will.
 
neptronix said:
Look into the energy economics of it, all figures are awful. You need a massive tank, and your limit is PSI.

Improving the compression process from awful efficiency to allright efficiency will involve a bunch of weight and expense, and you still have very poor energy density. ( way worse than hydrogen, which is a commercial failure and money pit in transport R&D BTW )

I'd go and read research papers on what's been done before. Many have went down the path you're thinking about in the last 2 decades and hit a huge wall. Knowing about what those walls are will be informative.
Click to expand...

Has there ever been an endothermic vehicle? A cart that leaves a cold breeze behind it?
 
I think designs like this would not be practical because they would not be useful in all climates during all seasons. There must be an example of a vehicle that could regenerate thermal energy from the atmosphere when it is 100°f and the "exhaust" is air that is cooler.
 
If I had three heat pumps @ ~500w each, I am not putting in 1500w of power. I am putting in what 1500w of power can move through three redundant refrigeration cycles. So, on a hot day, I might go farther and/or further. So, yeah this would turn into a pretty massive vehicle and it would require lithium ion storage. I think that there would be a way of "balancing the budget" when you consider that the compressed air, the lithium, and the paraffin wax are all batteries. Would all of these things weigh more or less than the 3 banger motor that is in a Geo Metro?
 
It would be way too cold inside that Geo Metro w three redundant air conditioners. So, I would just heat pump the atmospheric gases that are rising off of the hot pavement.
 
Air motors only get to be like 1.7hp or something like that. I might have to use four of them and make the cart re-wheel drive.
 
SturdyBegger said:
I think designs like this would not be practical because they would not be useful in all climates during all seasons. There must be an example of a vehicle that could regenerate thermal energy from the atmosphere when it is 100°f and the "exhaust" is air that is cooler.
Click to expand...
Any thermal engine works by means of a gradient between its operating temperature and ambient temperature. Exhaust (matter and radiated heat) will always be in between. That's entropy.

Air tools and other compressed air engines get cold because the hot part was done previously. But the sum total is always a net release of heat. (They're not thermal engines, by the way, but elastic like windup toys. You can also observe the rubber band in a toy airplane getting warm when wound up and cold when released.)
 
I am sure these air motors would get hot. The air that is passing through them is carrying the latent heat from the paraffin wax after it solidifies. There might need to be three redundant motors for each of the two rear wheels of a cart.
 
SturdyBegger said:
I am sure these air motors would get hot.
Click to expand...

Then you don't understand adiabatic heating and cooling. The phase change hokey pokey is only about reducing the amount of heating on compression and chilling on decompression. It only reduces the thermal inefficiency, and only by adding manufacturing and materials inefficiency.
 
I don't understand a lot of things, but I think you are the person who is not really understanding me. Its okay if you don't want to discuss an air powered car, seriously, but please don't be so condescending. I have put a lot of thought into this and I could help by drawing some circuitry for the sake of clarity. It is not your fault if you don't understand my ramblings.

The Phase Change "Hokey Pokey" is about the discharge rate. The thermal energy that is in the liquid wax(latent heat) can be discharged instantaneously into the compressed air working fluid when the wax changes phases to solid. It is like a capacitor. Increasing the thermal energy in the compressed air will increase the work it can do in the air motor.

I think my design offers some key improvements to Reza Alizade Evrin and Dr. Ibrahim Dincer's air powered car which is highly documented and tested in the researchgate paper included here.


I want to make this circuit redundant. I was imagining this last night. If I make this circuit redundant, and I employ the power of three design principle, I can have continuous power. This is also an important fail safe. Each of these circuits would employ the condenser side of a refrigeration circuit, the PCM is part of the air over coolant heat exchanger. The coolant gathers thermal energy from atmospheric gases outside of the cart and compresses that energy into the condenser side, heating the wax into a liquid.

The addition of the refrigeration cycle to this design, improving efficiency, is like a "Heat Pump Water Heater" that is more efficient at heating water with electricity than an electric water heater. This DOE writeup says they are 2 to 3 times more efficient than an electric water heater, depending on the temperature of the closet you put them in. If your closet is 100°f it is probably 5x more efficient.

 
Last edited:
Sturdybegger, if you are serious about this design, then I would suggest giving us at least a few basic numbers, so we have something to discuss. Useful would be numbers like the target amount of energy stored in compressed air, and resulting storage volume and pressure, the target amount of energy stored as latent heat in wax and resulting mass of wax, a guess for the masses of the storage vessels, mass of the vehicle, etc.. From there one can then assess whether a vehicle with a useful speed and range can be built.
Without any numbers, it's just, excuse the pun, a bunch of hot air.

As I mentioned in my post, vehicles powered by stored gas expansion and latent heat do/did exist. I'm just not sure whether air plus phase change wax are viable storage media for a light personal vehicle, which I assume is what you are aiming for (as opposed to say a heavy locomotive).
 
I admit to being very naive about pneumatic tools (motors) and how they work but this thread raised my curiosity. What are the efficiencies of using the li-ion batteries to drive a brushless 1kw motor that drives a pneumatic driven wheel instead of direct torque to the wheel? I assume the transfer of enrgy to the pneumatic wheel would add more loss to the system. Here is a link that answered my question: Pneumatic vs. Electric Tool Calculations and Considerations
 
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