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Compressed Air Car with Cogeneration

Compress the air for the car, use the heat for something else.
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Maybe this is like bringing two baked ideas together. (A lovely symmetry for my first half-bakery post)

Compressed Air cars--baked idea. They have some issues, but some of them work very well. They essentially take their energy from the heat in the ambient air to expand the compressed air inside a mechanical engine. Cogeneration--baked idea. In the context of this idea, using a single source of fuel to produce different kinds of power.

Idea: Compress the air for your car at home using an electrically powered compressor. When the air is compressed alot of heat is generated (pulled out of the air), use this heat to do something useful: heat your house, cool your food (with an abosorption cycle frig), heat water, even make electricity again, whatever. Meanwhile the compressed air is in your car is potential engergy in a way. When you drive, the compressed air expands by absorbing heat from the ambient air. (The same heat, fungibly speaking, that would have got put into the air by the compressor, except that you intercepted it for your hot water heater!). It's like you're getting your car propelled for free, by paying for your hot water. Or like you get free hot water by paying to propel your car. Or am I missing something?

JackyD, Jul 28 2009

One example of using waste heat http://www.kxma.com/news/409388.asp
[ldischler, Jul 29 2009]

Compressed air hybrid: http://www.futurepu...rchives/001552.html
[bs0u0155, Jul 27 2010]

[link]






       It sounds entirely bakable :)
goldbb, Jul 28 2009
  

       Somebody correct me if I'm wrong, but I think you're getting free hot water by paying - a lot - to propel your car. (I would suspect that compressing air wouldn't be a very efficient way to generate heat, compared with an electric element.) Compressed air is a very expensive, inefficient way to store energy.
neutrinos_shadow, Jul 28 2009
  

       [neutrino] I would think that compressed air is an /expensive way to store energy/ precisely because heat was created when it was compressed. If that heat is used, then the compressed air is cheap (virtually free). And I think that compressing a gas mechanically is a perfectly efficient way to produce heat: Compress, heat out. Expand, heat in. Perfectly thermodynamic.
JackyD, Jul 28 2009
  

       [neutrino] An electric element is pretty much the least efficient way of producing heat, as no use is made of the large entropy difference between electrical and heat at a temperature close to ambient.
spidermother, Jul 28 2009
  

       [JackyD], [spidermother]: Fair enough. Thermodynamics was never my strong point.
(Although, those Indian-made compressed air cars are typically advertised as 'no emissions' etc; they just move the emissions from the car to the local power plant. mumble... rant...)
neutrinos_shadow, Jul 28 2009
  

       one of the sites I visited for an aircar post specifically states that the compression process can be used for home heating though it didn't go into detail.
FlyingToaster, Jul 28 2009
  

       So somebody's onto the idea of using the waste heat [FlyingToaster]. It especially makes sense if the compressing operation is a larger centralized operation, like a filling station. The heat exchange equipment could be more easily justified. (And for sure this is not a no-emissions thing) But the big question remains: are you getting 2 for the price of 1 with this.
JackyD, Jul 29 2009
  

       well IIRC I already did the math for another post; your turn :D   

       [edit] looks like I didn't do the math... oh well, your turn anyways.
FlyingToaster, Jul 29 2009
  

       The idea is basically, if you're going to compress air for some purpose, why not use the waste heat? This might work on an industrial scale, where you have compressors in continuous use, but not in the home where you might fire up a compressor every few days. In industry, the combination is baked. See link.
ldischler, Jul 29 2009
  

       The reverse is also true, see Hilsch Vortex and its applications...   

       Therefore, baked, possibly not widely known to exist
4whom, Jul 29 2009
  

       Maybe the idea becomes more interesting if you first think "I need some heat" rather than "I need some compressed air". How will I make the heat? I'll use an air compressor. It's a very efficient way to make heat. So I got my heat, and I used it for its purpose. But now, voila!, I have a bunch of compressed air that I can also use to do work. Like a wound clock, or a compressed spring. 2 for the price of 1, right?
JackyD, Jul 30 2009
  

       I wanna see some math for "It's like you're getting your car propelled for free"
FlyingToaster, Jul 30 2009
  

       Well, I'm not sure how to approach it mathmatically, but. If we follow the energy... Some, perhaps most, of the heat resulting from the initial compression of the air, is not created by the electrical energy input, it is harvested from the air itself. Compressing the air removes its heat. That means that in the first stage of this cycle (compressing the air) we're reusing all of the electrical energy from the compressor (friction, electrical resistance) and we're extracting heat from the air and putting ALL of it to useful work. Say, heating our water. When the compressed air is forced into a cylinder in the car, it expands, thus absorbing more heat from the ambient air. This is the energy source for the expanding air, the heat of the ambient air. Without ambient heat the air would not expand at all. The amount of heat absorbed from the air surrounding the car should be the exact amount of heat that was "squeezed" out of the air in the compression phase. We used the heat in the first instance to warm our water. We used the heat in the second instance (in effect, again) to propel the car. Of course there is no double creation of energy here, I never meant to imply that, but this process allows us to use energy contained in the ambient air for productive work TWICE in the cycle.
JackyD, Aug 02 2009
  

       And use the air expansion in the car to keep your beers cool.
BunsenHoneydew, Aug 04 2009
  

       great idea.... I actually had this idea last night and was going to post.... aw well. However, this can be improved. The car in question should be some form of compressed air hybrid. It's fairly simple to get energy regeneration from braking to compress air, and a well designed electronic engine management could quite easily syphon off engine power at high-efficiency opportune moments (during overrun/engine braking etc). In addition, the heat needed to expand the air could be drawn from the otherwise waste exhaust/radiator heat. How the compressed air could be added is interesting also, a dedicated automatically clutched cylinder would work. But I wonder if an existing IC engine could have additional compressed air injectors?   

       The great thing about using air as both a working fluid and storage medium is that every piece of the tech is mature tech. There's no waiting around for 16hrs to charge an electric car off household electricity. Just let the compressor run whenever electricity is cheap rate, then top up whenever. Out and about? petrol/gas stations could easily offer an economical top up.   

       In this system, in a roundabout way, you'd be using your car's waste heat to warm your house.... neat.
bs0u0155, Jul 25 2010
  

       I dislike compressed air cars because they assume that air weighs nothing. The maximum energy density using current technology (carbon fibre / kevlar) is under 200kJ/kg. Bread has 10,000kJ/kg.   

       Also, not all waste heat can be used efficiently. Compressors lose efficiency as they get warmer, so they are most efficient when kept cool. If you extract work from the heat rather than removing it as quickly as possible, you lose efficiency. So, you could probably use it to melt the snow on your drive, but not heat the hot water tank.
marklar, Jul 25 2010
  

       If you put a couple gallons of water in the tank, it can soak up the heat, then release it gradually... insulated tank required.
FlyingToaster, Jul 25 2010
  

       FlyingToaster: that energy denisty is actually pretty good. Certainly better than Lead-acid cells (146kJ) NiCd (140kJ) and not too far off LiIon (400kJ). Factor in the cost of batteries, both financial and environmental, life time (Prius batteries drop cells all the time) performance over time (batteries degrade) charging time and complexity. Compressed air wins on nearly all. That and nanotube materials are a handful of years away, so no reason why the energy density can't increase... actually I've had an idea...
bs0u0155, Jul 27 2010
  

       actually there are some pretty good reasons why the energy density cannot increase. density for one. The math on compressed air hybrids is really really harsh. Although the energy density may compare with batteries the energy/volume does not and no solution currently exists for the explosion hazard involved in using the new high pressure tanks.
WcW, Jul 28 2010
  

       pneumatic brakes are baked, so you could ditch the tedious hydraulic system, and a huge compressed air tank is the starting point for a truly whizz-bang horn. Cars needn't be limited to fog 'lights' any more.
bs0u0155, Dec 11 2012
  

       //use this heat to do something useful// Almost every energy exchange involves massive loses of heat. If you bake this point, and turn waste heat in something useful (a energy form other than heat), all the energy problems of humanity are solved :-)
piluso, Dec 11 2012
  

       Hey I have an idea why don't we make traditional incandescent bulbs that are inefficient and produce a byproduct of heat. This can be used to slightly heat a house effectively making the light bulbs both a light and a heater. Shall I start investing in this idea? Who is up for it. We can mass produce these light bulbs from government grants and get councils to give them away using posters of piles of coal and explain that this is how much they save each year.
PainOCommonSense, Dec 11 2012
  

       There is actually room for a bit of "free energy" in there somewhere.   

       In the tank the air stores energy equal to the effort of pumping minus the cogenerated heat, at ambient temperature. That's based on pressure x mass.   

       Now say your tank pressurizes to 4k psi, but the engine only uses 1k psi. Venting the tank into a regulated tube at 800psi, then letting the tube warm up to produce 1k psi at ambient before use, is a gain in energy.
FlyingToaster, Dec 11 2012
  

       But regulating from 4,000 psi to 800 psi wastes 4/5 of the energy!
spidermother, Dec 11 2012
  

       [Spider] I'm choosing to believe that was sarcastic, since pressure regulators simply pass air at a reduced pressure, not vent the excess or something like that, so no pressure is wasted.
MechE, Dec 11 2012
  

       It wasn't intended as sarcastic. I thought that ordinary pressure regulators did simply waste the potential energy difference as heat. The only way to avoid that would be to harness the energy to compress external air - converting high pressure, low flow into low pressure, high flow; but if you could do that, you'd simply send the power to the wheels.
spidermother, Dec 11 2012
  

       To use an electrical analogy, a simple 3-pin voltage regulator, or a diode, reduces the voltage without changing the current; it therefore wastes power as heat inside the device. A more sophisticated device, such as a switch-mode regulator, can reduce voltage while increasing current, and therefore wastes less energy. An ordinary pressure regulator is like the former, not the latter.
spidermother, Dec 11 2012
  

       Typical pressure regulators use a spring and diaphragm set-up such that air on the downstream side can't be at any higher pressure than the set point. Thus they only let through the portion of the air that is needed to reach that point. I suppose I wasn't completely fair, as they do waste a tiny bit of energy flexing the spring and such, but it's minimal. Most will, however, vent higher pressures found on the downstream side, as a safety precaution.   

       Thus in [FT]s approach, you would either need a non-venting regulator (rare) or a check valve immediately after the regulator.   

       That being said, there is no real gain in energy, it's more of limiting losses. Air is at pressure and ambient. You expel an amount of air that would (at ambient) produce 1ksi into a forward tank. In doing so, however, it is cooled such that it only produces 800psi. All letting it re-warm does is regain energy that would otherwise be lost if it was vented still cold.
MechE, Dec 11 2012
  

       Anyway, it would have been facetious, not sarcastic, as I have no reason to attack [FlyingToaster] (beyond ordinary target practice).
spidermother, Dec 11 2012
  

       I don't see how that can be the case. Any device that reduces the pressure in a constant mass flow must reduce the energy in that flow proportionally - whether or not useful work is harvested from the pressure difference. It's the same concept as throttle loss. My electrical analogy was apt. Apt!
spidermother, Dec 11 2012
  

       Lay on, MacSpidey: I just fell prey to the same thing that haunts people who correct other people's typos.
FlyingToaster, Dec 11 2012
  

       Anyway, if I had wished to be facetious, I would instead have picked on //That's based on pressure x mass.// and advised that you find an excuse to avoid that side of the family this season.
spidermother, Dec 11 2012
  
      
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