Air pressure and Heat as an Energy Source?

Quoting Guest: "I was thinking the higher the pressure the higher the temperature goes. I was wondering if this was scaled up if it would produce enough heat to run a generator. I haven't heard of power being produced this way. Is this a sound idea?"

I guess if you need lots of compressed air and the heat generated is just a waste product, one could utilize it to drive a little steam power plant and so recover some % of the wasted energy.

However, as an idea to generate power as the prime objective, forget it - you need to input more power than what you can get out of it, so why bother?

What would you use as a prime mover to run the compressior? The only thing that I know that could compress a large enough volume of air to be useful is a Trompe. And the air is perfectly dry and cool. All the moisture and heat of compression is lost to the water in the compression process. All you would have to work with is the air pessure. But the air would be cool, dry and oil free...Might be useable as air chiller or to run a turbine/generator

http://www.animatedsoftware.com/pumpglos/glpulser.htm

Trompes date to 1060/70AD in Spain but might be older. There is one at (Ragged Chutes) Cobalt Ont. that has operated for 70 years without an overhaul. It was owned by Ontario Hydro and sill in operation last I heard.

You are right about higher pressure meaning higher temperature. But for any given compressor there is a max operating temp and pressure. It would be extremely unwise to exceed these limits.

It could cause serious damage to the compressor and motor, or the hot air could damage subsequent tools and equipment - at the best they become too hot to hold - but a lesser known hazard, if the air is being used to feed a facemask (used by a paint sprayer say) - the hot air could contain carbon monoxide that comes from burning compressor oil.

Larger industrial compressors have built-in controls to prevent over-pressure and over-temperature by shutting the compressor down. Which is not very helpful if the air is used in a critical production process - or for respiratory protection

Overheating is usually prevented by the use of air-cooled or water-cooled inter-coolers and after-coolers - and it is here where the surplus heat can be recovered (piped or ducted to somewhere useful).

But it is low grade heat. It is sometimes used to supplement heating and ventilation of factory air space. However, it is very inefficient to compress air anyway, but to a higher pressure just for the heat content makes no economic sense.

Your 80 gallon tank can at best warm the room up - but make sure the compressor is sited to suck in cold air.

I hope this helps.

Thanks for the responces. My thought wasent to use my compressor. My thought was to have high pressure tanks pressurized permanantly. The pressure is there just to make heat. Then use the heat for something useful.

I cannot give you the formulas off hand, but you seem to have missed some fundamentals in regards to heat. The premise that the stored pressurized air would stay hot would be great for your purposes, but is not correct.

The pressurized air in your tank is hotter than it's surroundings mostly because the low pressure gas molecules being supplied to the compressor (to pump into the tank) retained their molecular vibrations (heat) and when they are then confined to the smaller space in the tank, they have raised the amount of heat per cubic gallon of confined space. It is not the fact that the gas is at a high pressure that creates the heat. It is simply that you have confined the heat to a smaller space.

So if you pump the tank from empty to full at the recommended pressure and turn off your pump, you will note that the tank has gotten hot. Put a thermometer on your tank and wait a while. The tank will eventually cool to the temperature of the surroundings. This is because the tank radiates and conducts heat to the room.

If you now quickly empty the tank, you will find it has gotten colder than the surrounding air. This happens for the reverse reason. You have some air in the tank, but now the amount of heat per cubic gallon of that air is much smaller than it was when it was pressurized, and because of this it will make the tank colder than the surroundings. If you place your hand on the tank, your hand will feel cold as the tank now acts to exchange heat from its surroundings (your hand and the room air) to the air in the tank. (Actually the tank is now cold mostly because it has already released a lot of heat to the air that was sent out of the tank when you were releasing the pressure inside it.)

So, in short, the pressurized gas does not create a supply of heat for you to use. You will have to find a way of continually feeding heat into the tank if you want a continual source of heat. This is why someone suggested that if you were needing a lot of compressed air, you might find some way of using the wasted heat in a useful way.

Hmmm.... That isn't how I remember it. Does not the temperature of the gas define the energy as evidenced by the average "speed" energy (both rotational, translational and vibrational) of the gas molecules? Is this not why diffusion is "faster" at higher temperatures? Are not the higher temperatures that result from compressing a gas due to the work that is done to compress the gas and increase the gas molecules average "speed" energy?

Ahh yes... I should have used the term movement or excitement to describe all moments of energy in the body of gas being compressed.

As to the compressor's work being the source of heat energy, the only way it heats up the gas is through the components of the compressor being heated through energy losses in the motor and friction in the mechanism. The heat from the motor and friction does indeed add conducted and radiated heat. At higher pressures especially, this is indeed significant, but not nessarily a major source of the added heat contained in the newly pressurized body of gas. (What work process is being reversed to cool the gas when it is released when the tank is emptied, and what is the formula for it?)

In regards to the comment: "As to the compressor's work being the source of heat energy, the only way it heats up the gas is through the components of the compressor being heated through energy losses in the motor and friction in the mechanism."

See http://en.wikipedia.org/wiki/Joule-Thomson_effect. The gas is heated (or cooled) when it is compressed due to the interaction between potential energy (distance between the molecules) and kinetic energy (temperature) of the gas regardless of exposure to any hot parts in the compressor. I don't think this temperature rise is negliable even at moderate pressure increases (atmospheric to 20 psig increases air about 40F).

"The pressure is there just to make heat. Then use the heat for something useful."

No Guest, you want to use the wrong resource for something useful! The heat is basically just wasted energy, because you cannot contain it! What you can contain is the pressure - so use that!

So where's the catch? You get less energy out of the pressurized system than what you had to put into it. Why? Because some energy has gone to waste as irrecoverable heat... Try it!

why not just feed that 90 psi compressed air through a vortex tube and get both heating and cooling flows from the same device?...george oerther oertg@aol.com

The fantasy of LASER Technology started with James Bond's 'Golden Finger'. That was years back. Louis Pasteur obtrusively emphasized that some unseen agents were responsible for sourness (fermentations), ropiness, bluish and fluorescent colourations and some inexplicable changes in food products. So every realized idea has root in fantasies. Mind hardly creates without imaginations! Who knows, we may one day land in the 6000°C-core of the sun or at least its 3000°C surface without insulation! Perpetum Mobil of First 1^st Order is trying hard not to be rubbished off or thrown into trash can! What I want to deduce from this razzmatazz is that your thinking is not off-course. It's idael and real!

The heat from your 80-gallon compressor accumulator can be used in regenerative form in a device called economizer. That is, if the recovery will off-set its cost. Otherwise, you can use the heat, which I'm sure is never close to boiling point (of H₂O, i.e. 100°C) in value, to dry non-corrosive things around. For the 90 psi-compression, adapting a controlled ejector or nozzle at the outlet of the accumulator will create a velocity in 3^rd power order. In simple mathematical term Energy, ξ is a function of the air velocity and other thermodynamic, scalar and vectoral parameters of the motive air i. e. the Energy or the Aero-motive force ξ = ƒ(µ, m, v³ , a, b.c,....etc). The (kinetic) energy so derived will now be directed to the tip (tangent) of turbine blade. This translates to rotational energy (torque) in the shaft. These motions ( combination of translational, rotational, etc.) propel the generator. In this case, the Aeromotive force, i.e. kinetic energy-ladden air stream whose Reynolds No is unequivocally more than 10⁶ ( Re >10⁶) is replacing the conventional Internal Combustion Engine Drive. The motion so produced is quintessential to Maxwell Induction in electo-magnetic field, which results in electricity generation. The current generated is further manipulated (transformed) for normal applications.

Question answered? Dig further!

Soji, Ibadan, Naija