How Much Available Energy is in Compressed Air 2

[ChrisSwannie]

Hi,

Please excuse my lack of mechanical engineering knowledge - I am a computer engineer!!!

I would like to know how much energy is available (in KJ) is in 30m3 of air compressed to 2bar. I am trying to explain a concept to a group of kids and I cant make head or tail of the available energy formula for compressed air.

Help will be appreciated - from me and the kids!

 

[zdas04]

Let's assume that by "2 bar" you mean "2 bar above atmospheric pressure or 3 bar)". You can simply multiply volume times absolute pressure and convert units to get 9 MJ.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

The problem with stored energy like that is that you need to understand what happens to heat and temperature of the gas.

If you compress air into a fixed volume, the temperature of the air increases. What happens depends on whether that heat is removed as part of the compression, simply slowly leaks out of the tank or the tank is highly insulated and it stays at the same temperature.

However simply taken, as zdas04 says, multiply the absolute pressure in MPa - in this case 0.3 MPa(a) times volume in cubic metres and this turns in 9MJ of energy. Now if your exit is into the atmosphere and not the pure vacuum of space then the available energy is actually 6MJ as 3 will be left in the tank at atmospheric pressure. Of course getting that last MJ out may take some toime as the pressure dies in the tank.

Maybe a bit more complex than you imagined when talking to a bunch of kids?

Whether this energy is actually available is a matter of some doubt, but that wasn't your question...., nor whether you will get back the energy you put into compressing it (you won't).

What are trying to explain?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChrisSwannie]

Thanks Zdas04 and LittleInch. Appreciate the time you took - very helpful.

In terms of what I am trying to explain:

The kids (for a school science project - 13 yrs old) think that having cars drive over a flexible air-filled pipe will compress the air in the pipe which can be stored in tanks and then used to power a turbine of sorts. The pipe will refill using a industrial bellows-type device (think stomp rocket!). They think that the energy can then be used to power traffic lights. I had my doubts but offered to try and help with the math and got completely lost!

I am assuming that the weight of a car could pressurise a certain volume of air and that this could be stored in a pressure vessel and used to drive a turbine - but I am not sure how to go about working it out and then also explaining it to kids (confused people make horrible teachers!!!)

I love the fact that they want to work out the science and so they will learn from that, but I have my doubts about the economic or sustainability issues of the idea. But then again, them learning the science is the valuable bit so I am not to worried about the economic benefits. The 'compressor' in their use-case is free so maybe it is sustainable at some level?

Any thoughts?

 

[LittleInch]

I think you need to introduce them to the concept of work. To provide energy like this you need to do work on the air. That work is not free, but would be caused by the car having to squash something.

The concept is interesting, but flawed. The speed of the car is quite variable, from stationary to very fast. This makes it difficult to have something which squashes. The thing then needs to go back to its original shape so it can be squashed by the next car. This is probably best done with a spring and some non return valves so that new air is sucked into the device. It makes no sense to use energy to put air into the device to have something else squash it. might as well just use the original energy source....

The problem with storing this is that as the pressure increases in your storage device it becomes harder and harder for the car to squash the tube and it has to do more work.

Maybe work on a really low pressure accumulator which drives a turbine - a bit like a windmill.

What might work better is a longer section of hose placed where the wheels go so that as the car moves along, the tube is squashed axially and more air is ejected. As it passes the end of the tube the hose can then expand again sucking in more air form the other end.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChrisSwannie]

Thanks for the response LittleInch

I have introduced the kids to the concept of work as in 'there are no free lunches' but will put a more scientific spin on it in our next session

Must admit that I never thought about the fact that as the pressure rises you will need to put in more and more work to move the same volume of air in.

I like the squashing the tube axially idea, will give more volume. Spring bellow and one way valve is another good tip. Thanks.

Appreciate the advice and willingness to share knowledge.

 

[zdas04]

Another way is to use a lot of stages. Use a tube with a check valve on the end (suction valve), run it across the road and put another check valve (this one is the discharge of the first stage and the suction valve for the second stage), cross the road again and put another check valve. Continue back and forth as many times as you need to.

Calculate the volume of each segment while at rest. Calculate the volume of each segment with a car on it (make some basic assumptions about the size of the tires), the ideal gas law says the PV=mR_airT so at a constant temp and before the discharge check opens you know that:
P₁V₁=P₂V₂

The constant temperature is kind of a stretch, but your compression ratios will be so low that you won't get any measurable heat of compression. That will tell you how much compression you will get. With 10-12 loops you should get enough pressure to do some amount of work. The trick is having enough stages to get to a meaningful pressure with very low compression ratios.

Use something like 3/4 inch poly tubing that has enough resiliency to give you the spring effect that LittleInch was talking about.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Not a bad idea - the issue as noted above is that as you drive further along the bumps from the tubes will get harder and harder as the pressure increases....

Kind of like those rumble strips you get sometimes coming down to roundabouts on a fast road - at least we have them in the UK

I'd go for lots of strips, but aim to increase the total volume rather than the total pressure. If you spread them out enough you might get something close to continuous flow if the traffic level was high enough

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChrisSwannie]

Thanks o much gents. Appreciate the advice and info. Will let you know how it goes with the kids after our next session.

 

[zdas04]

I'm thinking you'll get about 1.026 compression ratios per stage, so it would take 27 loops to get to 2 bar(a), 44 loops to get to 3 bar(a) and 54 loops to get to 4 bar(a) (assuming 60 ft of 3/4 inch tubing between the check valves and two tires each 6-inches wide).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Compositepro]

Solar cells are used for powering traffic signals where grid power is not available. These are far simpler and cheaper than any sort of compressed air approach.

Also, the energy taken from the passing cars is not free. The cars will be slowed by these devices, causing the cars to use far more energy than is extracted from them. A logical question to ask of any power source is if the source is free, why not hook it up to the grid to compete with power plants. The answer it that they are so inefficient and costly that they come nowhere near the efficiency of a power plant. Even if cars were put on a tread mill to drive a generator, which would be about as efficient as you can be in getting power from a car, it would be very inefficient compared to a power plant.

This is a good time to teach children scientific thinking. The way you test ideas (hypotheses) is by challenging them and trying to think of why it won't work. It might sound negative, but it only takes one example to disprove a theory, whereas a hundred examples of why something might work proves nothing.

Of course, children still have a lot to learn about science to be able to do anything useful. But the principle of conservation of energy is probably one of the first they need to learn.

 

[ChrisSwannie]

Thanks again for the replies all and thanks Compositepro for joining the conversation!

The kids (and myself) have a lot to learn in this field. Whether it is a viable idea or not (and I suspect it isn't, as there are more efficient ways of producing electricity), I really want the kids to pursue it and draw their own conclusion.

With the knowledge you have shared with me I will try to help them build a scale model during which I would love them to learn:
1. Concept of Work
2. Conservation of Energy
3. Boyles Law (with a little bit of Thermodynamics thrown in - although not sure at our pressures will influence that much - but they need to be aware of it and the role it plays)
4. Efficiency
5. They will also learn some math principles in working with various formulas.

Anything else you Gents think may be helpful for them to learn on a project such as this? Remember they are 13 years old...

Just a side note - I am constantly amazed at the level of time that professional engineers will give to helping others learn, even for a topic that may seem mundane to them personally. It is really appreciated.