The Naked Scientists

The Naked Scientists Forum

Author Topic: Are two accumulators better than one for pneumatic hybrid vehicles?  (Read 7641 times)

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
This:



must be heavier than a single (lower pressure) accumulator using the same energy in compression. So, where does the efficiency gain come in?

Is less work lost as heat (isothermal) in this configuration?
« Last Edit: 03/06/2010 00:29:51 by chris »


 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
At first sight it looks like it may be using compound engine techniques.  The first compound engines that I'm aware of were steam engines, and they worked on the principle that after the steam had been used in the high pressure cylinders it still had enough energy to be usefully passed through a low pressure cylinder before being vented or recycled back to the water reservoir.  This meant that more work was done by each 'unit' of steam.

See: http://en.wikipedia.org/wiki/Compound_engine
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
I don't think it's possible to say without more information. As Lee says, it could be compounding, but that would require multiple expanders which is not indicated in the diagram. Come to think of it, the diagram does not reveal too much at all  :D
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
« Last Edit: 01/06/2010 19:04:26 by peppercorn »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Thanks! I get it now. It's a(n?) hydraulic system. The low pressure reservoir does not store energy at all. It's just a place to put the hydraulic fluid after it has done work in the motor - it's probably vented to the atmosphere. Essentially, it's an oil tank.

The energy is stored in the compressed gas in the high pressure accumulator during braking by pumping fluid from the tank to the accumulator. During acceleration, the flow is reversed and the pump acts as a motor.
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
From my article:
"Fluid is stored in a low-pressure reservoir. A pump moves the fluid from the reservoir to a high-pressure accumulator."

Oh. I wasn't expecting that at all (!?).
That's the trouble with glimpsing an idea on the web & then (later) looking for another article to illustrate your point/question, I guess  ???

Saying that I'm (almost) sure the first article I read was purely pneumatic, with HP & LP cylinders acting as the differential-pressure energy storage - as opposed to medium pressure and outside air pressure.  Would this make sense or am I delusional?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
From my article:
"Fluid is stored in a low-pressure reservoir. A pump moves the fluid from the reservoir to a high-pressure accumulator."

Oh. I wasn't expecting that at all (!?).
That's the trouble with glimpsing an idea on the web & then (later) looking for another article to illustrate your point/question, I guess  ???

Saying that I'm (almost) sure the first article I read was purely pneumatic, with HP & LP cylinders acting as the differential-pressure energy storage - as opposed to medium pressure and outside air pressure.  Would this make sense or am I delusional?

I'm not familiar with differential-pressure energy storage. I think the only way it could produce a greater pressure than high pressure v ambient would be to use high pressure v vacuum, but the increase in total presssure would hardly seem worth the aggravation.

Can you dig up a link to the first article?

I do like the UPS hybrid system though. I should do some calculations, but it seems that the gas compression  method will not generate a lot of heat, so the compression efficiency should be high. Similarly, the gas expansion method does not absorb a lot of heat, so it too should be quite efficient.

I suppose the big issue is how much energy can be stored in the high pressure accumulator. 
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
I suppose the big issue is how much energy can be stored in the high pressure accumulator. 
Here:
http://en.wikipedia.org/wiki/Energy_density
indicates 4MJ/kg for 300bar

...compared with 0.72MJ/kg for Li-ion batteries.


Still looking for that original article. But I have a feeling there might be some sense to running from a, say 100 bar tank to a 400 bar tank over 1 bar to 300 (if only for stationary apps).  ???
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
I suppose the big issue is how much energy can be stored in the high pressure accumulator. 
Here:
http://en.wikipedia.org/wiki/Energy_density
indicates 4MJ/kg for 300bar

...compared with 0.72MJ/kg for Li-ion batteries.


Still looking for that original article. But I have a feeling there might be some sense to running from a, say 100 bar tank to a 400 bar tank over 1 bar to 300 (if only for stationary apps).  ???

If I understand the tables correctly, the 4MJ/kg for compressed air does not include the mass of the storage tank.

What would be the advantage in pressurizing both tanks? Would that not just mean there was some stored energy that could not do work?
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
If I understand the tables correctly, the 4MJ/kg for compressed air does not include the mass of the storage tank.

I expect if the tank is made from carbon fibre, then the weight increase would still give better MJ/kg than batteries.

What would be the advantage in pressurizing both tanks? Would that not just mean there was some stored energy that could not do work?

To explain:
System A consists of a high pressure (30MPa) tank driving a (ideal) pneumatic motor expelling spent air to an ambient pressure reservoir (ie. the outside air).
System B consists of a higher pressure (40MPa) tank driving the same motor expelling to an intermediate pressure (10MPa) reservoir.
Both systems could then be recharged by running in reverse.

I would think that the work available in both systems is equivalent...  Is this true?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
To explain:
System A consists of a high pressure (30MPa) tank driving a (ideal) pneumatic motor expelling spent air to an ambient pressure reservoir (ie. the outside air).
System B consists of a higher pressure (40MPa) tank driving the same motor expelling to an intermediate pressure (10MPa) reservoir.
Both systems could then be recharged by running in reverse.

I would think that the work available in both systems is equivalent...  Is this true?

I think the work would be equivalent, but I'm not sure what advantage B would have over A. Why go to the trouble of having two tanks when one would do?
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
I think the work would be equivalent, but I'm not sure what advantage B would have over A. Why go to the trouble of having two tanks when one would do?

Neither do I and having been unable to locate the original article, it must have come from a (boring) dream  :D
Just a thought: Is the heating/cooling of gases in A and B also equivalent?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
I think the work would be equivalent, but I'm not sure what advantage B would have over A. Why go to the trouble of having two tanks when one would do?

Neither do I and having been unable to locate the original article, it must have come from a (boring) dream  :D
Just a thought: Is the heating/cooling of gases in A and B also equivalent?

I'm hesitant to say without running the numbers. Most of my intuitive guesses about thermodynamics turn out to be wrong  :D

I use a tool at www.thermofluids.net  It's worth checking out.
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
I use a tool at www.thermofluids.net  It's worth checking out.

Geezer - many moons ago your good self recommended that this tool would be helpful in my investigations.

It took a bit of getting my head around the way it worked, but I can indeed see it could prove very useful.  However the trail version has now run out and I am not at this time inclined to buy the full version.

... And anyway I like being lazy and getting other (clever-er-er) peeps to do my thinking for me  ;D
 

Offline marketing

  • First timers
  • *
  • Posts: 1
    • View Profile
That was really worth reading and the explanation was really awesome. I got so much of information about accumulator. I don't know that from where the energy in compression is gain but all had explain it very well, good job done by you.
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
That was really worth reading and the explanation was really awesome. I got so much of information about accumulator. I don't know that from where the energy in compression is gain but all had explain it very well, good job done by you.

Glad we could help... not quite sure if there is a complete explanation of accumulators, gas-laws, etc in the posts above, but if you have any further ideas or questions I'd be happy to try to field them.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums