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'[EE] Power from tides'
2005\09\08@074833 by olin piclist

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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.

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.

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.

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.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\08@133056 by Gerhard Fiedler

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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?

{Quote hidden}

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

2005\09\08@135623 by Mike W

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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:

{Quote hidden}

> --

2005\09\08@142037 by olin piclist

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Howard Winter wrote:
> No!!!  While tidal effects caused by the Sun's gravity can be said to
> be because of the Earth's rotation (and thus the apparent movement of
> the Sun), the major cause of tides is the Moon,

Yes.  That was probably unfortunate wording on my part.  I wasn't trying to
distinguish between sun/moon causing tides, just that tides are caused
because the earth rotates thru an externally caused gravitational field.

> It comes from the potential energy of the water being pulled towards
> the various celestial bodies, so forming a higher column as water flows
> in from surrounding areas, and then lowering again as the external
> influence "moves away".  In one way it's similar to a "storm surge",
> where an area of low air pressure inside (say) a hurricane causes a
> hump in the water below it, but in this case the extra upwards force
> comes from added gravity above, rather than reduced air pressure.

Right, but you haven't said exactly where the energy to move the water comes
from other than "potential energy" of various celestial bodies.  So are you
saying that if we remove power from tides, the potential energy stored in
the earth-moon system will decrease?  In other words, the moon will move
farther closer?

>> By extracting the power from the float, you are making the planet slow
>> down ever so slightly.
>
> Well you are extracting the power from the water, which depending on
> the direction of flow towards your collection point may in fact cause
> pro-spin friction between water and Earth, so having the opposite
> effect!

But I'm only extracting power from vertical movement, not horizontal.

Here's a thought experiment:  Pretend the planet was rigid with no oceans.
There is nothing to deform, so there are no tides and no power lost in tidal
friction.  Now suppose you had two very large masses at opposite sides of
the equator.  If you hold them fixed, you can measure a slightly lower force
to hold them up (weight) when in line with the moon and a higher weight when
at right angles to the moon.  Now suppose you raised the masses when their
weight was lowest (moon overhead or 12 hours away), and lowered them when
their weight was highest (moon at horizon).  On the whole you would be
extracting energy from somewhere since you get back a little more energy
from lowering the mass than it takes to raise it.

Where does that energy come from?  Think of the gravitational forces on the
two masses.  The rest of the planet is rigid and the center of gravity
doesn't move since both masses are always moved in equal and opposite
directions together, so it cancels out.  What we are left with is the
gravitational forces between the masses and the moon.  Since gravity
decreases with distance, the mass closer to the moon will have a larger
effect, whatever that effect might be.  So let's consider the force on a
mass as the moon appears to rise and fall viewed from the mass over 1/2
rotation.  For the first 1/4 rotation the mass is low.  When the moon is
directly overhead, the mass is raised and then stays high for the next 1/4
rotation.  The mass will therefore be a little closer to the moon as it is
rotating away than it was when rotating towards it.  This means it will be
pulled forward a little less than pulled backward.  The net effect is a
torque on the planet opposite of its rotation, thereby slowing its rotation.
The difference in rotation speed represents a drop in the kinetic energy of
the planet, which is where the energy you got from raising/lowering the
masses came from.  If you raised/lowered the masses the other way, you would
be supplying energy, which would end up increasing the planet's rotation
speed.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\08@143632 by olin piclist

face picon face
Gerhard Fiedler wrote:
> 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...

Some of it of course does get dissipated, but I think a large chunk doesn't.
However I can't think of how to substantiate this one way or another.

In some places it is more clear than others.  The Bay of Fundy produces
large tides in part because it is about the right size for resonance at the
tide frequency.  The other contributing factor is that it gets narrower at
the end so that the height difference gets amplified as the resonant slosh
hits the tip.  The fact that resonance occurs at all proves that a decent
fraction of the energy is not dissipated each cycle.  Resonance requires
energy stored between cycles.

Clearly much of the slosh in the relatively small and narrow Bay of Fundy
does still allows significant parts of the tidal energy not to be dissipated
each cycle.  One can only imagine that the much smaller tidal slosh accross
large unobstructed oceans dissipates a lot less of its energy per cycle.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\08@152532 by Peter

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On Thu, 8 Sep 2005, 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.

I think that one must realise that the most people can do is *redirect*
some of the existing energy on earth. wave->mechanical (via heat
losses), chemical->thermodynamic->mechanical (via heat losses),
nuclear->heat->thermodynamics->mechanical->electrical etc. So it's a
scale thing. Should we ever get to do this on a real large scale it may
mean that we will be in a position to *control* climate on earth, at
least locally. *Eventually* all energy becomes heat (entropy) and some
of that is radiated out into space (blackbody radiation). So on the
larger scale of things it is not relevant if we burn down a forest or if
it decays by itself in 1000 years. However, on the personal, immediate,
human scale, it is very relevant that the forest not be burned down.

Whether the earth will slow down because of human activity, I strongly
doubt.

Peter

2005\09\08@155302 by Dave Tweed

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flavicon
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olin_piclist@embedinc.com (Olin Lathrop)
> Here's a thought experiment:  Pretend the planet was rigid with no oceans.

Unusual imagery, but equivalent to the usual explanation, which goes like
this: If the Earth deformed losslessly, the tidal bulges would always be
in line with the moon (ignoring solar tides for the moment). However, the
frictional losses cause the bulges to be dragged forward (in the direction
of the Earth's rotation) with respect to that line. This causes the gravity
vector that the moon sees to point slightly forward (in the direction of
the moon's orbital motion) of the center of the Earth. This actually
transfers energy from the Earth to the moon, raising its orbit and
increasing its orbital period.

All of this energy -- both the energy transferred to the moon and the
energy that goes into the frictional losses -- comes from the rotational
energy of the Earth. The Earth is slowing down (which is why we need leap
seconds), and some day it will have one face locked facing the moon, just
like the moon has one face locked toward the Earth already.

When we extract energy from the tides, we're taking advantage of the
difference in the heights of the land tide and the water tide. Doing
so looks like additional frictional losses within the overall system.
In other words, we're transferring more of the tidal strain from the
relatively lossless water to the relatively lossy rock. The Earth
slows down slightly more than it otherwise would if we didn't do that.

-- Dave Tweed

2005\09\08@165221 by Gerhard Fiedler

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Olin Lathrop wrote:

> Gerhard Fiedler wrote:
>> 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...
>
> Some of it of course does get dissipated, but I think a large chunk doesn't.
> However I can't think of how to substantiate this one way or another.

Hm... Imagine the earth without continents. The tidal wave would move
around the globe, once in 24 h (not quite, but that's not the issue here).
Now you have a continent right there in its way.

This doesn't look like a good situation for ideal reflection -- which I
think would be necessary for not loosing substantial energy there.
Basically, what does not get reflected gets transformed in heat, I think.

Gerhard

2005\09\08@183039 by Bob Axtell

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Some time ago there was a large tide-power project in the Bay of
Fundy, Nova Scotia, Canada. My understanding was that it worked very
well, but was mostly a "capture the water at high tide, generate
electricity at
low tide (going out to sea) as well as high tide (coming in from sea)".

I don't know what happened to that project.

--Bob

Mike W wrote:

{Quote hidden}

>>--

2005\09\08@185229 by Howard Winter

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Bob,

On Thu, 08 Sep 2005 15:30:35 -0700, Bob Axtell wrote:

> Some time ago there was a large tide-power project in the Bay of
> Fundy, Nova Scotia, Canada. My understanding was that it worked very
> well, but was mostly a "capture the water at high tide, generate
> electricity at
> low tide (going out to sea) as well as high tide (coming in from sea)".
>
> I don't know what happened to that project.

There's a location where they do this in France, and depending on the timing they use surplus electricity to
pump water in and out at the high and low tides respectively, to increase the amount of stored energy for use
at peak times.

Cheers,


Howard Winter
St.Albans, England


2005\09\08@190250 by William Chops Westfield

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It occurs to me that extracting power from tides may not be such
a good idea if we really expect the power of oceanic storms to
increase in the future.  Talk about building infrastructure that's
particularly prone to storm damage!

BillW

2005\09\08@230417 by Dmitriy Kiryashov

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Questions out curiosity ( since we all into earth mechanics now :)

How much energy is stored in rotation of the earth itself ?
i.e. mass of earth versus the velocity of rotation will give
us what ?

What is the mass of world water ? let's estimate by mass of
world ocean and seas only, all the rivers and lakes etc are
excluded.

How much energy would be stored in world ocean waves ?
( water surface of earth is known ) For example if waves are
1 meter high with distance between waves lets say 30 meters.

Besides the earth rotation what else can be accounted for
creating a water waves ? i.e. moon and other planets gravity,
wind generated by thermal difference, something else, unknown ?



Gerhard Fiedler wrote:
{Quote hidden}

> -

2005\09\08@233952 by Dmitriy Kiryashov

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It is not likely a bouncing a mass at the spring.

Unless the coast line is properly "terminated" :)
there would be big loos of energy once wave is
hitting it. Reflection will be miserably low.


> 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.

2005\09\09@004314 by Jake Anderson

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i would think that the tide would build a potential (possibly) by rising but
theres no actual expendature of energy?
i havent quite worked out the mechanism but it feels like it should "give
back" the energy when the tide is falling.
energy would be expended where the tide fills and drains basins and lakes
etc.

> {Original Message removed}

2005\09\09@080806 by olin piclist

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Dmitriy Kiryashov wrote:
> It is not likely a bouncing a mass at the spring.
>
> Unless the coast line is properly "terminated" :)
> there would be big loos of energy once wave is
> hitting it. Reflection will be miserably low.

The tide rise and fall isn't the same as a propagating wave.  Such a wave
does dissipate most of its energy when it hits a beach.  You can think of
the tides more like a voltage slowly going up and down on a capacitor.
Think of tides in a closed body of water much much smaller than an ocean,
like a lake.  Of course the tidal effect is much smaller, but the water
alternately sloshes a little east, then a little west.  Each time it goes up
on the bank a little on one side and down a little on the other side.  The
only dissipation is the friction of the water as it moves this miniscule
amount back and forth.  The total enery lost each cycle is going to be much
much less than the amount of energy sloshed about as height difference.  The
same applies to the oceans, but it may be harder to visualize.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\09@142440 by Dmitriy Kiryashov

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Hi Olin.

If we have enclosed system ( earth ) how is that possible
to perform a work from inside of the system which will
change entire energy of the system ?

Regurdless of nature of the waves and media they distributed
I still think there are generic approaches about any waves
such as propagation, attenuation, superposition, reflection,
refraction, etc.

The major difference as I understood is length of the wave
right ? ( tidal waves are extremely long ) But that doesn't
neccessary remove their wave nature.

Question - if you allowing all that slow rising and falling
water to flow forth and back through the energy extracting
system what kind of loss do you discuss ? All the water
mass will finally in the position / levels it was supposed
to be. Gravitational pull doesnt change, all masses are the
same, what is variable here ?


Olin Lathrop wrote:
{Quote hidden}

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