this may be an interesting adition to the thread...
http://news.bbc.co.uk/1/hi/england/cornwall/3477639.stm

On 8 Sep 2005 at 14:30, Gerhard Fiedler wrote:

> Olin Lathrop wrote:
> 
> > Gerhard Fiedler wrote:
> >> Not necessarily. It could be that the energy would get transformed from
> >> kinetic energy into other forms (heat?) anyway -- float or no float. Maybe
> >> the float just redirects a small part of the energy flow that goes from
> >> kinetic to thermal into electric instead; in that case, the float wouldn't
> >> introduce an additional slowing, but would cause a certain ever so slight
> >> cooling.
> > 
> > My point was where the energy was coming from in the first place.  I say it
> > comes from the kinetic energy stored in the rotation of the planet. You
> > seemed to disagree so I wanted to hear where you think it does come from. 
> 
> No, I didn't really disagree with that... Maybe Howard?
> 
> > But to address your comment above, you are saying that the energy in
> > moving the water up and down as tides gets dissipated anyway, therefore
> > directing it to make electricity doesn't remove any additional energy
> > from the source. I disagree with this also.  My main argument is that
> > much of the energy is not dissipated, but returns to the system.  It's
> > sortof like bouncing a mass at the end of a spring.  Once you get it
> > going, it take relatively little power to sustain a large oscillation.
> > That's because little is being dissipated.  The energy just gets sloshed
> > around between the spring and the mass.  Only the little bit lost to friction
> > and air resistance must be replaced by the driving mechanism each cycle. 
> 
> If this is true, then I agree with you.
> 
> > Certainly there is energy dissipated by the oceans as they rise and fall due
> > to tides, but most of it is stored and released as potential energy in the
> > height of the water column.  Think of a planet that has a uniformly thick
> > ocean layer over a perfectly smooth solid core.  The tides would be traveling
> > waves.  They would loose a little energy in the internal friction of the
> > water due to viscocity, and some more in the friction with the ground, but
> > most of the energy would be transferred around the globe in a big wave.  The
> > driving force is essentially in phase with the motion. 
> 
> Not sure how much the internal friction contributes. Any numbers?
> 
> > Now think of a large float used to extract this energy into up/down
> > motion of a fixed mechanical system.  If there is no force on the
> > mechanical system (the float just bobs in the water), then no energy is
> > extraced.  To extract energy, there has to be a force in the direction
> > of motion.  This means the float must be lagging the wave, causing a
> > phase shift in the wave itself. Once the wave is lagging its driving
> > force, the driving force must be imparting more energy to the wave.  The
> > extra energy has to come from somewhere, which is the kinetic energy stored
> > in the earth's rotation long ago. 
> 
> Yes, I agreed already to the source of this energy being the kinetic energy of
> the rotation. The question is really whether the energy would be conserved as
> kinetic energy if the float wasn't there, or if it would get lost (as kinetic
> energy) anyway -- maybe at the coast line. It seems to me that there's quite a
> bit of kinetic energy that gets lost (as kinetic energy) at the coast lines...
> 
> Gerhard
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