Re: Unmanned sub crosses ocean

From: David Lee (davidlee_malvern_at_dont.use.this.bit.hotmail.com)
Date: 11/22/04 

Date: Mon, 22 Nov 2004 13:10:03 -0000

The comments quoted below basicaly are correct. However to a large extent
the motion does come "for free". This is nothing at all to do with
extracting "free energy" but merely because you don't have to supply any
energy to maintain motion in the absence of any force. Water is a
frictionless environment so if you can arrange your vehicle to have neutral
bouyancy (ie effectively zero gravity) then it should continue to move in
any direction that you choose to push it. In practice energy must be
supplied to overcome drag but, unlike friction, drag is proportional to a
power of velocity (at least the square) so that it becomes small at small
velocities.

The force required to drive the vehicle up and down is provided by varying
the bouyancy of the vehicle, which is controlled by changing its density,
accomplished by pumping a litre of oil into and out of an external bladder -
changing the vehicles volume by one litre. As Peter has already stated,
expansion must be carried out against the pressure at maximum depth and so
will require work to be done. However, if this done slowly (so that the
vehicle remains in thermal equilibrium with its surroundings) then this
process can be made as efficient as is thermodynamically possible (not 100%
since some energy will always be lost to entropy).

At a depth of 1km the pressure is about 10MPa (that's for fresh water - it
will be somewhat greater for brine). The energy required for an expansion
of 1Litre against this pressure is about 10kJoules. This looks like a big
number but a Joule is a small amount of energy (less than a quater of a
calorie - the energy required to raise the temperature of 1gram of water by
one degree Celsius). In fact 10kJoules is only the energy content of a
2400mAh NiMH AA rechargeable 1.2volt battery! For this you get a round trip
down to 1000m together with nearly 6km of horizontal motion. This is hugely
efficient in comparison with the conventional alternative using a research
vessel and a winched sounding probe.

David

Peter wrote...
> Alan Browne wrote:
> >> Hans-Georg Michna wrote...
> >
> >>> From that point of view I find a sailboat much more fascinating,
> >>> because it uses freely available energy rather than carrying a
> >>> battery to propel itself.
> >
> > A sailboat requires control of sails (via winches), rudder, etc. An
> > energy outlay similar to that required by the device described for the
> > ride it gets for free from gravity v. buoyancy.
>
> The difference is that you can actually extract energy from the wind
> while the gravitational field is conservative, meaning that no energy
> can be extracted by traveling in any path where the endpoints are at
> the same gravitational potential. So in going on a complete cycle of
> surface to maximum depth and back to surface there can be no net energy
> extraction from the gravitational field. In a stationary ocean (i.e.
> no currents), all the energy to drive the sub must come from the
> batteries - the gravity/bouyancy and wing-like fins are just means to
> transform the battery power into forward motion. In contrast, it's not
> hard to build a small sailboat where the power to operate the sails,
> rudder, and controller is also extracted from the wind and no external
> power source needs to be provided.
>
> > (It's just moving some
> > fluid a few cm every hour or so, the rest is indeed 'free').
>
> That fluid has to be pumped out of the hull when at its greatest
> depth so the pump needs to work against considerable ocean pressure.
> Work out the amount of energy that takes and you'll find that
> under ideal assumptions you'll still have to spend more energy pumping
> than you could possibly extract as propulsive energy. See my previous
> post for more details.
> The keys to the success of this sub have nothing to do with
> 'free energy' extraction, but rather with using a small, streamlined
> design that travels very slowly. Such a design requires very little
> energy to move long distances.
>
> Now of course if the sub used the techniques used by glider pilots, i.e.
> looking for rising thermal currents, floating with currents like the
> Gulf Stream, etc. that would improve the effectiveness. But an
> automated sailboat could also utilize such techniques by going with
> ocean currents and seeking favorable winds like the Trades.
>

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