Howard Winter wrote:
> Nobody has mentioned hydraulic storage - a hydraulic accumulator consists
> of a piston in a vertical cylinder on top of a column of
> hydraulic fluid, with a heavy weight resting on it.  Hydraulic pumps pump
> fluid into the system, and any energy that isn't used raises the
> weighted piston.  When the energy needs to be retrieved the piston
> descends, giving back the stored energy.  It's very efficient because
> unlike compressed air there's no heat-loss associated with it - only the
> friction of the piston and the fluid.  If there are no leaks, the
> stored energy doesn't degrade over time, like most other storage systems.

Howard, I don't know why I said "compressed air." Yes, the braking energy
was indeed stored using a hydraulic accumulator.The presenter's name is Gary
W. Rogers, he is the President and CEO of FEV Engine Technology, Inc. His
presentation was on "green" diesel engines, here is a link to the March 2005
newsletter that has his article:

http://www.saearizona.org/newsletter/march2005low.pdf

The hydraulic accumulator was mentioned only in passing, towards the end of
the presentation, as Mr. Rogers was quickly flipping through the final
slides. The article doesn't mention it at all. :(

 I don't recall exactly how it worked, but here's an article which mentions
a similar system:

http://www.evworld.com/view.cfm?section=communique&newsid=3365

Wikipedia has some interesting stuff on hydraulic accumulators:

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

William Chops Westfield wrote:
<snip>
> [...] battery charging is more than 20% efficient
> as well.  They might be comparing an overall cycle efficiency
> for electric to a hypothetical compressed gas motor efficiency,
> or they might just be wrong...

I believe he was talking about the overall cycle efficiency. The following
article quotes 36% efficiency for a full regenerative cycle:

http://www.hybridcars.com/flywheels.html

Which is higher than 20%, but there are many factors that affect efficiency,
including the rate of braking. The faster I break, the more energy is lost
(charging batteries is a relatively slow process). It seems to me that a
hydraulic accumulator would be more forgiving.

By the way, the main focus of the above article is on flywheels.

Best regards,

Vitaliy

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