The Ultimate Air Vehicle Idea

Stupid question time. Air expands when heated. For the same size of tank, you can store a much greater mass of air at ambient temperature than at high temperature as according to the ideal gas law: PV = nRT, where P is the air pressure, V = volume of gas, n = number of moles of gas, R = universal gas constant (8.314472 Jmol^-1K^-1) and T the gas temperature in Kelvins. How, then will storing heated air give you greater energy storage, and how do you propose to prevent the air from losing heat energy? With a heater? Wouldn't that consume energy itself? Even the most efficient insulation system will eventually allow heat loss.

Another thing; compressed air does mechanical work through kinetic energy, which is ½mv². How will storing less air give you greater potential energy then? Remember, when the air within cools down, the pressure within the tank is reduced.

Please explain how you intend to overcome all these obstacles. I know you have explained that you're not an expert, but the ideas you have proposed are very similar to the nonsense that all the "free energy" conspiracy theory scammers keep coming up with. I know you're probably not one of these jerks, so please be careful of what you read.

First of all, I am sorry that I didn't convey my idea very well in writing. I don't intend to store heated air.

The energy storage unit that I was suggesting would have two separate energy storage technologies packaged together. One energy store would be the traditional compressed air storage (maybe 5000 psi or something). The other storage unit would store heat (maybe in iron as some people do, or maybe in a phase change material). Phase change material uses the "latent heat of fusion" or the heat that it takes to melt a solid to store heat energy. This heat is returned when you want to use the heat as the substance freezes.

The exchangeable storage unit stores energy in two discrete ways: compressed air in a tank and heat in a thermal store.

When you "fill up" at the station, you exchange storage units.

The air from the tank goes first into a control unit in the vehicle that routes it accordingly. In most cases, it would go first to the thermal store (back to the energy unit) to be heated and gather more energy. Then this heated air would go to the engine for propulsion. Therefore, not as much of the compressed air will be used because more energy is being added to it (maybe making the range longer). After the engine is finished expanding the heated air, the air will be a little closer to ambient temperature instead of well below. When the vehicle slows down, the engine could compress air (heating it up) and put that air through the thermal store to loose some heat to it before it goes to the pressure tank. Presumably this air will go in the reverse direction through the thermal store so that it is cooled down significantly before it is stored in the pressure tank.

When the storage units are charged at the filling station, the compressor would compress air (which will heat up as it is being compressed). This will then first go through the thermal store to transfer the thermal energy there and allow for more molecules of air to be stored in the pressure tank. If the compressor runs off of a combustion process, then the exhaust of that combustion process can also be used to help heat the thermal store.

These energy storage units could be obtained for a significant deposit (probably a new vehicle would come with a set).

Because the units would be standardized, a smaller vehicle will take fewer units than a larger one. This would make the units interchangeable and a filling station would only have to carry one kind (maybe two kinds: one kind for cars and trucks and another kind for motorcycles and lawn mowers).

Just in case this still doesn't make sense, the filling station wouldn't fill the storage units while you are there. You will drop off your empty one and obtain a full one. The full one will have a hot thermal store and a full compressed air tank. The station will then fill the storage units (with air and heat) as they can. This will make it possible to use energy from a variety of sources to run the compressor to fill the storage units. If more heat is needed and not available (if an electric motor or wind turbine turns the compressor), then more air can be compressed than is necessary for filling the tank and the rest of the compressed air could be used for other purposes (such as producing dry ice or liquid nitrogen).

So, I am not proposing to get something from nothing. I am proposing to develop something that is easy to implement (as far as distributing energy goes) and is more efficient than just using compressed air by itself (when compressing the air the thermal energy usually is released into the atmosphere). I think that it is better to store the thermal energy and use it to add energy for re-expanding the gas than it is to try to re-capture it from the atmosphere when expanding the gas. Is it more efficient to have the exhaust of an air car really cool and be heated up by the atmosphere after it is exhausted, or is it better that that exhaust is closer to ambient temperature implying that that heat energy has already been added before it is exhausted?

I don't have an easy answer for heat being lost from the thermal store over time. Maybe a good thermal conduction insulator combined with a radiant barrier (reflective) would help.

Let me know if it still seems that I am off the mark. It is correct that I am not an expert, but this all seems to make sense in my mind.

I am looking forward to your reply because I'd like for my idea to be clear to readers and I need feedback in order to determine if I am explaining myself properly.

IMPORTANT: The idea that I am proposing is not a "free energy" idea. I think that I should state what I intended to accomplish with this idea. I thought that the criticism that I was getting was to do with not understanding my idea, but I am now thinking that it has more to do with an assumption that I am suggesting a method of developing a propetual motion machine. Well, I'm not.

The intention of compressed air cars, I think, is to move the production of useable mobile energy from under the hood to a central place. A large amount of potential energy can be stored in a tank, and the using of that energy does not create any pollution. It is the creation of that potential energy that could cause pollution and use up finite resources, but there are more choices for energy sources when they are used in a central, stationary location. This, as far as I know, is the objective of the plug in hybrid as well. It is not the quest for free energy; it is the centralization of converting natural energy into useable energy.

Therefore, my objective is to make storing potential energy for mobile use more efficient; to make getting a "refill" easy and not time consuming, and to make a retrofit to the existing system of distribution easy.

My hope is that energy conversion to stored energy would be more efficient because it is taking into account storing the compressed air but is also saving the heat energy that is usually dissipated to the atmosphere.

I would hope that "refueling" would be simple because it would be the exchange of an energy storage unit (compressed air and heat rather than a chemical battery or something) that would happen at the refueling station rather than a recharging or refilling. This is an item of discussion that has come up before when talking about compressed air vehicles. People have been asking how long it would take to completely fill an air tank at a station.

I thought that it would be somewhat easy to modify a filling station because they could just burn the fuel that they are used to filling people up with in order to charge up the exchangeable storage units. (I.E., they can run an engine to run a compressor and use the heat from the compressed air and the engine exhaust to heat up the thermal store). This kind of thing has been done, so there is nothing new here. When a ground source heat pump is running, it is storing the heat from compressing the refrigerant in the ground when it is in air conditioning mode. This is a similar situation except that the heat is stored in a smaller, more compact thermal store than the ground.

At the same time, the refueling station can sell their traditional fuel to people that have traditional vehicles. After a while, other sources of energy can be used for turning compressors, such as wind, electricity from renewable sources, renewable fuelled engines, etc.

Maybe, as far as you are concerned, simply using an exchangeable air tank and neglecting the thermal energy is better. I do think, however, that significant advances have been made in storing thermal energy and I think that there is a significant amount available.

Again, I apologize for mis interpreting your reasons for criticism, please read through my material again with what I have said here in mind.

So you are proposing a heat storage system then? You'll still need an external heat source to heat it up first (which will require energy), a heat pump or some other similar device to transfer the heat to the air (which will again require energy), and a highly efficient insulation system to minimize heat loss.

You could use solar energy to create the heat source, but you'll still need some sort of compressor to fill the tanks. If you're thinking of using the heat produced from friction of the mechanical parts to keep the hot, it'll be too complex to be light and economical, and you'll still need the heat pump to tranfer the heat to the air.

Finally, the stored air will lose pressure as it is used up, resulting in power reduction, making the engine unreliable. All in all, it's just not worth doing.

I will try to keep my writing a little shorter. I think that you are missing things in my wordiness.

As far as I know, when you compress a gas, it heats up. Usually this heat, in most applications, is dissipated into the atmosphere and not used. This is how compression cycle refrigeration works. I say that we can run this hot air through a heat storage medium to cool it and store the heat that it removed from the compressed air. The air is cooler, so it takes up less space and the heat that was removed is stored. No heat pump would be required because the compressing of the air itself produces heat. Hopefully there will not be significant heat produced by friction like you suggested.

Now, when the air is used, it can come out of the tank at ambient temperature, be heated up via the thermal storage medium (flow through it in passages and pick up heat) and flow to the engine at the same pressure but with more heat added (less massive). This is already used in storage systems that use underground compressed air to run a turbine. In this case, the air is heated up with combustion instead of a heat store. I still am not sure why a heat pump would be necessary. Maybe you have to explain more things to me.

I'll deal with the rest of your concerns later. I think that one at a time is better because I think that you are getting tired by the end of my messages.

Isn't there a company that makes those lightweight vehicles that run on compressed air? For city driving, lower speeds, the "engine" is turned by compressed air. For highway driving, or when the air runs out, the same engine can run on gas.

Drawback? It is such a light vehicle, that a VW bug could take on 4 of them in a crash, and come out of it almost unscathed......

Also(not an endorsement), I believe he is suggesting that the heat CREATED by compressing the air be "stored", and used in vehicle as an additional source of power....

Me, if I was going to try to use the excess heat created, I would just use it to help defray the cost of making the air, rather than adding enough iron(heavy) to hold that kind of heat to a car that needs to be light in order to run on compressed air....but that still smacks a little to close to perpetual motion IMHO....

Hi there,

I am the "guest" that started this whole thing.

I like your kind of contribution a lot better, because, although you don't agree with my idea, at least you aren't just throwing out negative comments without knowing what you are commenting about.

I would, however, like to elaborate on some of my points that you indicated so that it might generate some more good discussion. You haven't convinced me yet that storing heat is a bad idea.

What I would like to see is an engine that can take the unregulated stored air and use it at full pressure (variable pressure depending on how full the tank is). Previous designs, as far as I know, re-heated the air after it was regulated by exchanging heat with ambient air. This replaces some of the heat that was lost by compressing the air in the first place, but the exchanger is likely to be large and cumbersome. It seems to me that it would also add unnecessary drag as well.

Also, I believe that heat storage is much more advanced now with phase change material than just using a chunk of porous iron. I used that example in the first post because it would be something that people could relate to. The method that people are using now is to melt and re-freeze a substance depending on whether heat is being added or depleted. The reason that this is smaller and lighter is because it takes much more heat energy to melt something than it does to raise its temperature after it is melted. I don't know the figures (and I am too lazy to look them up), but it takes more energy to melt ice than it takes to get the ice from below freezing to the freezing point. We learned this in high school physics.

Therefore, heat might not be too heavy to store.

By pre-heating the air before it is expanded or while it is being expanded, some energy would be available in the compressed air that would usually go into it after it is exhausted. I.E., it would normally be exhausted at a lower temperature than ambient and it would get heat from ambient air after it is exhausted (when it is no longer any good to the engine).

We know that people are already making compressed air propelled vehicles and we know that they generally use about 5000 PSI pressure. What about higher pressures?

I was also hoping to attract someone who was familiar with heat storage to indicate whether it is feasible or not.

There are also more advanced forms of insulation that haven't been commented on yet.

Let me know if you still think that I am missing the mark.

http://en.wikipedia.org/wiki/Phase_change_material

Should be able to develop something that would store heat for heating air.

As others have pointed out, it doesn't work that way. The heating of the air is of no practical value. As others with their compressed air car ideas have suggested, the service stations would sell/exchange cylinders of compressed air of standard sizes.

Next ask yourself how many cars would stop to refuel [re-air?] each day. That's a lot of cars and a lot of tanks. Then how do you think each tank will be removed and replaced? They will be somewhere within the car or under the car, not out exposed to crashes. Not as simple as sticking a hose down a filler tube. The car might have to be put on a lift and it could take an hour to have the new tanks put in, including the wait in line.

The service station would have to invest a lot of money in inventory even if it had a huge air compressor to recharge its own tanks and it would be constantly buying new tanks as some would always be going elsewhere. The compressor is also a big investment. Don't forget the labor costs of having service techs change the tanks. The more investment the fewer air stations get set up for business and it can be that one owner owns all the air stations in a town and can charge whatever he wants because of lack of competition. It happens with gasoline stations. So how do you save money then?

Then how many miles per tank and as the pressure drops, so does your speed, so consider too, how often the tank will have to be changed. Would you put up with that? I doubt that any but a few "perfect" environmental saints would.

The compressed air car has similar drawbacks to those of the EVs, range, speed, recharge time and inconvenience. Two good rules to keep in mind; 1. KISS [Keep It Simple Stupid], 2. Nothing is EVER simple.

Please help me evaluate this. Why is storing heat impractical? I'm not arguing that it is, but I'd like to know...

Is it because heat is too bulky to store even when using PCMs?

Is it because heating up air doesn't help it to expand?

Is it because people are still thinking that my idea is to store the heat and the air in the same container or vessel?

Is it that compressing air doesn't make it heat up?

Is it because you don't think that there exists good enough insulation?

I'm not sure where the idea of storing heat looses its practicality. Would it be useful in the compressed air hybrids that some people are working on? Is it just that my idea is a proposed "refill" idea instead of a range extender one?

Please fill me in, I'm a bit slow.

As far as storing the heat, it is not going to help increase the "power" of the air. Also, in an air powered, or an EV, it is most useful to keep the weight as low as possible. By storing the heat, anything you use is going to increase the weight, and you will also immediately hit a point of diminishing return.

Utilising the excess heat from compressing the air isn't such a bad idea, it is just how you want to do it.

If it were me, I would use the heat to help provide enegy for compressing air....that way, it does not add to the weight of the vehicle, and still does not waste that potential energy...

Thanks for your explanation. I accept now that storing heat for mobile use is not practical. I still, however, disagree with your statement that heat doesn't add value when expanding gas. Otherwise, we wouldn't be using heat to expand gas in internal combustion engines and turbines, etc. What about Iowa's stored energy project?

http://www.isepa.com/about_isep.asp

Would it be better for them to just use the compressed air to turn their turbines rather than heating it with natural gas first?

Thanks for your patience in helping me to understand this.

BTW, my intention was not to promote that compressed air vehicles were the way of the future, I just wanted to think of a way to make the proposed compressed air vehicles a little more practical and energy efficient. I saw somewhere that the energy return from compressing and expanding gas is less efficient than just thermally expanding the gas in the first place. I.E., a compressed air vehicle is less efficient than an ICE powered vehicle.

Later.

OK, this involves some guess work from me.

It sounds like they are mixing gas with the air first,

"During peak power demands, the stored air will be released, mixed with a fuel and used to power combustion turbines"

Also, it would appear that is the compressed air that is assisting in the generation of electric by natural gas, not natural gas assisting compressed air generation. Thus, there MAY be a need to heat the mixture to achieve efficiency. The website is kind of unclear about this.

Remember, a compressed gas cools as it expands.

I was referencing an earlier post as well, when I answered you, at least in my mind, he-he, but I guess that you could say heating it up on board a vehicle would probably not give enough increase in power output to overcome the additional weight added by the heating mechanism.

Hope that clears it up a bit more...

Thanks, this is good. I've come to the conclusion also that storing heat on a moving vehicle would take up too much space and weight even with advanced heat storing methods.

In the underground compressed air storage situation, the compressed air helps because it takes the place of having to compress the air with a real time compressor driven by the gas turbine. Nevertheless, the gas turbine works because it heats the compressed air so that it will be more efficient as it expands and cools. If the exhaust air is cooler than ambient temperature, then efficiency is lost. The higher the change in heat, the better.

I do believe, however, that a heat storage device would be helpful in a vehicle when storing compressed air via decelerating.

Compressing the air will produce heat which can be stored and then re-used again when the vehicle is accelerating. Even if the heat is stored in the expansion/compression device.

Thanks for sticking with me.

Those who criticize the compressed air car are behind reality. Such cars are in series production already by Tata in India.

With respect to the heat storage there is a problem: heat flow requires a temperature difference between the source and the to be heated fluid. This leads to a limitation when the compression temperature is stored and also when the stored heat is used for compressed air heating. The energy losses are such that the additional weight will not allow any positive result on the contrary the range will decrease in comparison with the car without heat accumulator.

The exchange of compressed air bottles can be very fast so that this limitation can be neglected. The electric car has same problem to solve and already analysed solutions allow the exchange of a battery pack in a few minutes. Same type can be adapted to "air batteries".

How personal injuries due to the presence of compressed air reservoirs in case of accident can be discussed but different solutions are possible and any way in case of accident no car is a totally safe place to be.

The braking energy recovering can not be total since braking has to be progressive and controlled. It will not allow (as mechanical direct recovering only) a high degree of saving.

As a general principle the idea is correct but according to several laws of physics it is not quantitatively feasible.

This is the difference between qualitative thinking and quantitative realisation.