# RE: Our Demon-Haunted World

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Sent: Friday, November 15, 2013 10:02 AM
Subject: Re: Our Demon-Haunted World

On 11/15/2013 12:15 AM, Chris de Morsella wrote:

So the energy would be stored in the potential energy of the heavy piston
and the water would just be a working medium?  Why not put the weight on a
cable and use a purely mechanical system?  I'd think that could be more
efficient that a water turbine.

Other engineering friends of mine have levelled the same criticism of that
idea. I guess the only counter argument could be that the very heavy
piston's weight would be born directly by the compressed water column,

>>Water doesn't compress.  You're just using it to move the weight up and
down.  As far as energetics go, it's no different than a cable hoist.

I realize that - what I meant to say is that the water column bears the
weight - hence no need for a cabling system. Water that is put under
pressure will occupy the same volume, but will have a potential energy (as
long as that pressure is maintained) that a similar water column not under
pressure will not have.

and that for very large masses the cable systems could present serious
problems with stresses from wear - have you ever seen when an elevator cable
is un-torqued; it literally whips around as the stresses are released.
Wondering about the stresses that repeated winding and unwinding of the
support cables would induce.

>>The main problem is just size.  I considered designing an energy storage
system to be used with home, rooftop PVs.  I looked around for something
heavy that could be lifted up to store energy.  My car.  It weighs about
2600lb.  Suppose I lifted it ten feet (high as the roof).  I've stored
26,000ft*lb = 10Wh  :-(

Friends of mine and I have kicked similar ideas - rail cars loaded with low
grade (but quite dense) iron ore that are cable hoisted up an ridiculously
steep grade and turn a generator when they slowly travel back down. Again -
we have been disappointed by the capacity.. All of this for that? SO I hear
you.

Clearly the mass that is moved between the high reservoir and the low
reservoir has to be quite immense - probably why only pumped storage makes
sense for this kind of kinetic potential energy storage. Certainly thought
the full reservoir behind a damn has a huge potential energy. Japan -
approximate figures from memory - has a pumped storage capacity built out
that is somewhere around 10% of its total capacity, which is a pretty
impressive figure.

Chris

Brent

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