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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Sure, much of the energy to a compressor is "lost" as heat, but when you expand the compressed air again, it draws most of the lost heat back from the surroundings and converts it to work.
So I prefer to think of that heat not as "lost" but as "borrowed", and the overall system efficiency is probably better than you think.
This also means that a compressed air car needs the opposite of a radiator (an interwarmer?) to take in heat. I remember that some older designs had trouble with the engine freezing up.
If you like to live dangerously, you could maybe solve the freezing problem and turn the car into a hybrid, by adding propane to the cylinders at higher throttle settings and igniting it with spark plugs.
Registered Member #2028
Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319
Sure, much of the energy to a compressor is "lost" as heat, but when you expand the compressed air again, it draws most of the lost heat back from the surroundings and converts it to work.
So I prefer to think of that heat not as "lost" but as "borrowed", and the overall system efficiency is probably better than you think.
I've never thought of it that way before. Is it really so that the thermal energy a pneumatic motor draws from its surroundings contributes in turning the shaft? I thought these "interwarmers" were there only to prevent the engine from freezing solid.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I've never thought of it that way before. Is it really so that the thermal energy a pneumatic motor draws from its surroundings contributes in turning the shaft? I thought these "interwarmers" were there only to prevent the engine from freezing solid.
Imagine that the cylinder is very cold, it will cause the pressure to be low. If you manage to keep the cylinder warm you will get more pressure and more work done. If the design is properly thought out you can recover most of the energy lost at the compressor.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Imagine that I run the compressor very slowly- so slowly that the air doesn't heat up much- it just loses the heat to the surroundings. Because I'm not working against the thermal expansion of the gas I don't need to use as much energy to compress the air. Then I use that compressed air to run this car on. Because I have an interwarmer on it it becomes more efficient. But you can get out just as much energy from the bottle of compressed air no matter how I filled it. If that's as much energy as the hot fast compressor used then we have solved the energy crisis.
I think the energy is irreversibly lost when the compressed air cools down in the bottle.
Registered Member #2028
Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319
Bjørn wrote ...
Imagine that the cylinder is very cold, it will cause the pressure to be low. If you manage to keep the cylinder warm you will get more pressure and more work done. If the design is properly thought out you can recover most of the energy lost at the compressor.
Ah i see, the gas laws cuts both ways. This makes sense, thanks for the enligthenment. And sorry for the rant.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
interesting discussions guys... I did recall reading somewhere about the heat issues, and in fact most commercial packs (esp Li-Ion) use an inert liquid to keep the cells cool.
The biggest problem with the cells used is that despite expensive control circuitry one bad cell can bring down the entire pack, and large format cells are horribly expensive.
Interestingly, there are old technologies such as first generation LiPo cells (using polymers that conduct best at >80C) which could potentially reduce costs.
-A
"Bother" said Pooh, as he dropped the nukes on Redmond...
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Ok, because I'm probably the only one here using battery powered vehicle everyday (it is a 1000W motor bicycle) I feel I can add few notes.
First of all forget about Li-ion. This technology is not popular anymore. Too dangerous, too fragile. Instead LiFePO4 is used and very promising. You can get very large industrial LiFePO4 batteries which I have not a very good experience (no internal electronics, no balancers despite they claim they have patented electrochemical balancing!) but they look pretty robust and it might be a problem of my charger which destroyed the battery relatively quickly. On the other hand multipack of LiFePO4 (model 26650) is absolutely great. After almost a year of everyday hard work the battery still holds its original capacity. I think the future is in LiFe.
Arguments about too high battery prices are very relative. It can be question of few more climate summits and the crazy world of petrol powered vehicles can extinct like dinosaurs
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I can see the slogan now... Choose life, choose LiFe!
Mates, can you post details of the charger that you used and how it destroyed your pack?
I believe Bored Chemist is right, the system would only be 100% efficient if the heat were rejected and regained reversibly. Reversibility implies zero temperature gradient, which means doing it infinitely slowly.
The more efficient the intercooling in the compressor, and the "interwarming" in the expander, the closer the system efficiency gets to 100%, but it can never get there. You would need an infinite number of compression and expansion stages, with perfect intercoolers and reheaters between each.
On the bright side, if everyone sat at home all day waiting on their infinitely slow compressor to fill up their car, it would save a lot of energy.
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Electric vehicles
