I’m thinking about an air motor driven by high pressure compressed air up to 4500 psi. This is about improving existing air motor configuration by pre-heating the inlet air, so the motor could be any kind of positive displacement motor like a piston motor or a rotary vane motor. Maybe this would apply to turbines also.
Highly compressed air is supposed to be inefficient in driving motors because the air cools significantly as it expands in the motor. Motors driven with such high pressure can ice up if the air supply is not extremely dry. I’m not sure exactly what the inefficiency is but I think it’s because the air does not expand to as great a volume as it could because it is denser at a lower temperature.
Question 1: What causes the inefficiency in expanding very high pressure air to drive a motor?
To preheat the inlet air I’m considering regeneration of heat from the exhaust through a passive heat exchanger and then additional heating by burning fuel. Imagine the heat exchanger as the inlet air passing through a coiled copper pipe inside the the exhaust pipe from the motor. The exhaust pipe will be fairly large so the motor doesn’t have to waster power pushing the exhaust air out of the motor, and it can get larger to slow down the passage of the exhaust air. In a passive heat exchanger the colder air will reach some temperature between it and the hotter air on the other side of the exchange. I don’t know how cold the exhaust will be, but the inlet air can be reduced in temperature by expanding it before it passes through the heat exchanger.
Question 2. In a such a heat exchanger is it better to maintain a high pressure in the inlet air to facilitate the transfer of heat, or to reduce the temperature of the inlet air to lower the midpoint temperature that will be reached? I’m sure that’s going to get complex. So using pretend numbers, if the exhaust air is 80 degrees will the heat exchange be more efficient if the inlet air is has higher temperature and pressure, or lower temperature and pressure? It seems that heat will transfer more readily to colder substances, but also transfers more readily to denser substances. I guess the question is more about how do you calculate the optimal temperature and pressure for a heat exchange, and I’ll bet I don’t even understand the answer.
So once the inlet air passes through the heat exchanger it needs to be heated more. My thought is to inject some propane and pass the mix over a preheated catalyst. This brings up the same questions about temperature and pressure as in the heat exchanger, but also something else.
Question 3a. Would catalytic burning of fuel in pressurized air result in a combustion?
Question 3b. Catalytic burning require an optimal ratio fuel/air ratio?
Question 3c. Is catalytic burning more efficient than a higher temperature flame as the makers of catalytic heaters claim?
Question 4. Does any of this make sense or sound insane (other than than why am I even thinking about this).
Question 1: What causes the inefficiency in expanding very high pressure air to drive a motor?
Question 1 : PV = nRT. When high pressure expands, the temperature drops. Ideal gas law. When you compressed the air to get the high pressure air, the air heated up - one proposal to fix this is to store the heat somewhere. There is a company attempting to develop this for commercial energy storage.
Question 2-4 : you’re now talking about a fuel burning engine. The “air motor” is either a turbine or a piston engine (or some slight variant thereof) and is no different now than the many thousands of variants of these engines developed over many decades. There are possible improvements to such an engine where the engine is significantly different, but these improvements are mainly possible if you plan to use some kind of hybrid electric propulsion and no longer have to worry about torque. (for example, “microturbines” are turbine engines driving electric generators, they are finally practical because torque isn’t important when driving a generator, only total power output. Torque doesn’t matter because you can control the torque required with the design of the generator)
Here is an implementation of what you want in questions 2-4.
I get a vibe of “perpetual motion, as long as you give it a push from time to time” - you’re trying to get a couple of basic physical effects to cancel out.
For one thing, it takes so much power to compress a gas to multi-k PSI that the efficiency of a motor using that source becomes almost irrelevant. Assuming you don’t care about the kWh going into the compression, I am not sure what that last percentage of efficiency in the motor gains you (over, say, the best theoretical design that does not try to overcome cooling losses, etc.)
I understand that. The question is why does air that expands from a lower temperature at the same pressure less efficient than the same change in volume where the air starts out hotter? Is it because the cold air will expand to a lower volume? Is it the rate of expansion that makes the difference? Both? Or are there other factors I don’t know about?
I’m talking about heating the incoming air, it doesn’t have to be with fuel, but if it was there would still be far more air than fuel. Most of the energy to drive the motor would be from the highly compressed air. The engine would have very low emissions with a catalytic heater. (I am ignoring the energy used and possible emissions that result from compressing that air in the first place).
Not at all, just looking at the factors in improving efficiency and try to understand the principles involved.
As I noted above I’m ignoring the costs of compressing the air in the first place. And assuming an air motor of this nature is desired and already has problems with cooling loss then how to minimize those problems.
The same volume of air will exert higher pressure if at a higher temperature. Exactly what happens physically will depend on the design of your motor.
Well you wrote “To preheat the inlet air I’m considering regeneration of heat from the exhaust through a passive heat exchanger and then additional heating by burning fuel.” A compressed air motor doesn’t have hot exhaust.
Ok, so maybe it’s just that. Do you think the rate of expansion matters? There is a company making an air motor with a complex cycle that re-compresses some of the exhaust air to increase it’s temperature and then the high pressure inlet air mixes with that in the cylinder. They claim something-or-other about the effect of the inlet air hitting the hotter compressed air increasing efficiency, but it does require using some of the power produced in the engine to do that re-compression. Using fuel for additional heating is also being done.
I’m talking about regenerating the heat added to the incoming air. Some of that will be lost in the work but I doubt all of it. As I noted it may be colder than the incoming air is originally, so one of the things I’m trying to figure out is if the added heat can be effectively regenerated by lowering the temperature of the incoming air by expansion to below the outgoing air. Does that reduction in pressure and temperature outweigh any gains from regeneration?
I didn’t actually say this before, I’m thinking of air motors used for vehicles where the compressed air is stored in a lightweight tank. Tata is making one kind. Something called the MDI engine may be used in that one, it has the complex cycle to re-pressurize some of the exhaust air (come to think of it, it may just pressurize some of atmospheric air). These cars are marketed as zero or low emission vehicles although obviously emissions are going to occur from most power sources needed to compress the air in the first place. So I don’t think there’s practical application except in rare circumstances like vehicles that to run indoors but can tolerate the emissions from a small amount of fuel burned.
So what is the actual loss, and theoretical maximum gain? Is it enough to make an effort to fix it, or is it a huge amount of engineering to recover a small percentage in efficiency?
That would seem to be the starting point - be sure you’re actually trying to fix a problem worth fixing.
There’s an old parallel in programming, about spending many man-hours getting a piece of startup code to optimum efficiency… when even in the beginning, it executes once and takes 110 milliseconds. Shaving that to 50ms accomplishes… almost nothing.
Look. Take some gas at room temperature. Compress it down to a smaller space. It should be intuitively obvious that the temperature measured will rise - or use PV=nRT. Now, let that compressed gas cool to room temperature from the elevated temperature.
Now, release that compressed gas into some kind of compressed air engine, whether it be piston or turbine. It should be intuitively obvious that since you let the heat energy be lost from the compressed gas, you aren’t going to get as much energy out of your engine as you spent compressing the gas.
You could make up for the lost energy by burning fuel, sure…but then why bother with compressed air at all? The complexity of adding fuel tanks, burners, etc is a lot.
That applies to all “zero or low emission” vehicles. Batteries, hydrogen, compressed air, they’re all only as zero emission as their ultimate source of power. Do you think there’s not practical application for electric and hydrogen powered vehicles either?
One of the big issues with battery powered vehicles is the environmental cost of their construction. Fuel cells are still (I believe) vulnerable to much quicker decay than conventional engines and have lower efficiency than batteries. Compressed air would, I think, have less environmental costs for production, but low efficiency, and it’s the latter that makes them impractical, not that they require zero emission energy input to be overall zero emission.
Can they be improved to the point where the low efficiency in use compensates for a cleaner production? I don’t know, but I notice in the linked article that tata’s car is a somewhat lightweight people transport box.
Yes, I’m getting that it’s just less energy in the lower temperature air. I’m not bothering with compressed air, it’s just a hypothetical to better understand the principles involved. But you make a good point about the complexity, air motors have had limited usage in specialized areas, and light weight and low cost have been a feature of them, so any improvement in efficiency has to overcome the added costs. That is the kind of thing **Amateur Barbarian **mentioned above.
If in the future filling stations maintain large tanks of pressurized air then air cars could be recharged in a couple of minutes. So maybe one day large tanks of compressed air will be filled at the power source, maybe a windmill farm out on the prairie, then transported by an air powered truck to a service station where you can refill your air car. Then you have near zero emissions, and possibly reasonable cost. However, to get there air cars have to show a reasonable level of efficiency using more costly high emissions air compressed by common power sources. The Tata has a 50 mile range and 50MPH top speed (figures vary) and after that takes many hours to re-pressurize it’s air tanks (the motor is reversible becoming a compressor). That doesn’t sound like it’s very practical right now, and because of the losses in the compression process that car will produce more emissions per mile traveled than a 1959 Bulgemobile.
The whole point of these machines is NOT reduced emissions or improved efficiency versus current state of the art conventional powerplants when viewed in totality from end to end. So let’s stop quibbling about that.
The proposed benefit is either in reduced operating costs, since some emissions cost more dollars than others do. e.g. electric cars & overnight-priced electricity are cheaper to run versus the current price of gasoline.
Or it’s about portability, moving the heavy polluting part of the energy cycle off the vehicle and onto fixed infrastructure, while leaving the light weight, small, and low-emissions part connected to the vehicle. Underground mining equipment is an example where they’ll tolerate horrible end-to-end efficiency & increased total environmental impact in exchange for zero emissions at the point of vehicle use.
These are completely reasonable engineering goals.
I don’t have much to add to Tripolar and the others’ musings on the details of how it might work.
At some level you have to assume the folks doing the engineering know what they’re doing. So your job becomes to decide if the company making this product is only salesmen or schlocksters, or whether there’s really a clue-equipped engineering staff buried in there somewhere.
A couple years practical operating experience with the product will determine if the science is sound and if the extra benefit of this heat regeneration system is worth the extra complexity. Even in niche markets, crappy products don’t tend to survive that long.
I realized from the start that this thread was about an isolated technical question and not a proposal for an alternative energy system, but… between hypercompressed air and H2, I’d go with H2. Both have tremendous “creation” losses, but H2 seems to have more flexible application and more power density. Long ago, a proponent put H2 in perspective for me: don’t think of it as a fuel, because it becomes nonsensical against other fuel choices because of the enormous inefficiency of creating and compressing it. It’s better thought of as a pipeline… much as compressed air is. Making the compressed fluid an active one compounds the energy storage, though. Some H2 designs even use the high-pressure fuel to drive an auxiliary turbine on its way to fuel-cell or combustion pressures.
Back to the temperature/pressure efficiency discussion, though. I am curious to find out if there is any improvement to be had by raising the gas temperature.
Apparently the process from energy input to compression to mechanical energy output is horribly inefficient. I really don’t understand why one would intentionally cool and reheat the compressed air unless this is to get more air into the tank. That requires more “wasted” energy. Why not insulate the tank and store hot air? But once you have air energy stored in the tank the conversion efficiency would seem to depend on motor design. Some energy is lost to the creation of entropy in the air expansion.
Anyway, I found the following links; the first is exactly what you’re describing but the gas state descriptions leave out steps that they assume you know. The second describes storing compressed air energy in abandoned gas wells and assumes the earth absorbs the heat and that the government will finance the system so cost and efficiency do not matter and the available storage volume is so great that there will be a net benefit.