On Sun, 6 Aug 2006 14:18:38 -0700, William "Chops" Westfield wrote:

> 
> On Aug 6, 2006, at 2:44 AM, Vitaliy wrote:
> 
> > I remember a while back one of the presenters at the local SAE chapter 
> > meeting mentioned that storing the braking energy as
> > compressed air achieves up to 70% efficiency, compared to
> > only 20% for batteries.
> 
> I find both figures hard to believe.  70% is about the number
> I see quoted for high efficiency gas turbines (which do NOT
> operate on compressed gas; efficiency is dependent on gas input
> temperature.)  I'm pretty sure compression is nowhere near that
> efficiency.  And 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...

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.

Back when there was a "London Hydraulic Power Company" there were hydraulic accumulator towers in a number of places on the system, and at 
least one still exists (at Limehouse, although it is no longer functional - they've built a staircase through the weight-case so visitors 
can climb to the top!).  Tower Bridge had six of them, four of which were housed in the rounded brickwork bases of the towers.

I don't know how efficient the whole hydraulic pump/accumulator/motor system would be, but it could be worth considering.

Cheers,


Howard Winter
St.Albans, England


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