Ships, locks and potential energy

[jferstad]

Hello!

I have been pondering about potential energy and ship locks.

Big ships tend to have great mass but are also, hopefully, able to float.
So I am wondering if the water in a lock gets compressed by the ship's force of gravity?

If not, you could raise the ship by pumping water into the lock, hook the ship up to a crane,
and then let the water out and you would have increased the potential energy of the ship.

And since there's no such thing as free energy, there has to be a "catch" here...

Any explanation would be appreciated!

Thanks,
Johannes O Ferstad
Norway

 

[A.T.]

And since there's no such thing as free energy, there has to be a "catch" here...

... yeah what could it be. The energy used by the pump maybe?

 

[russ_watters]

There is no need for a pump. You just let water run into the lock from the higher surface.

Since the ship is displacing (replacing) water, it makes no difference to the water whether the ship is there or not.

 

[sophiecentaur]

In a lock, you don't so much 'pump' the water in as using the energy from the water cycle (ah, back to School!) to provide the upstream water's PE to lift the ship as it flows into the lock - transferring PE to it as it rises.
The ship's potential is the same, btw, whether it's floating in the water or hanging on a crane, if it's at the same height above the centre of the Earth.

 

[A.T.]

I know there is no pump in a normal lock, but the OP mentioned pumping water into the lock. In a normal lock you convert some potential energy of the water on the up side, into potential energy of the ship.

 

[jferstad]

Thanks for answering, but it didn't really answer my question.

You could theoretically (if there was such a thing as "no energy-loss") get the energy you used to pump the water back (using a turbine for example), and the boat would still be hanging in the crane..

sophiecentaur: do you mean that they use water from the "upper" level to fill the lock, thereby elevating the ship?

 

[sophiecentaur]

Usually, the upper section of canal is kept full of water by natural streams which are arranged to top it up. We're talking Heyday of Victorian Engineering. If the lock is into a Marina, then the water level will be topped up at high tide. I don't think anyone would pump water when not absolutely necessary.
In times of drought, they often 'close' the locks to conserve water and wait till they are full of boats before locking up or down. Each filling represents a lot of water - a significant percentage of the total volume for a short length of canal (e.g. where the canal ascends a hill in short stages)

And "no energy loss" is not a "theoretical" concept. It may be "hypothetical" to suggest a scenario in which a machine operates with no energy loss but, as we all know, you don't get perpetual motion under any circs.

 

[Dale]

You are certainly correct that the PE of the ship is increased by raising a ship in a lock. At the same time the PE of the water is decreased. This is no different in principle from hydroelectric power, or even old water wheels for mills.

 

[VooDooX]

I think what's he's saying is this. let's take the lock out of the picture, as well as the water cycle. let's imagine a rubber ducky that's maxed out in weight with lead pellets inside, so that it barley barely floats. then let's put it inside a small fish bowl, and use a small pump to put in water in. after that we hang the ducky from a string. then using a water wheel and a small opening at bottom, get all the energy from he water back. we still have a lead filled rubber ducky with potential energy there. but since the duck was put there, and the amount of energy used to pump the water was the same as the energy when the water left. (saying we have a perfect system with little resistance) we somehow got free energy?

 

[jferstad]

I guess I am still unable to get my question across.

Let me put it another way. Floating is a result of differences in density, correct?

So where does the energy that makes for example two liquids of difference density change place come from?

Or another scenario: if you have a floating device anchored to the bottom of an empty reservoir and fill it with water, the floating device will gain PE as the water gets deeper..

Can someone explain the source of this energy?

Thank you

 

[Dale]

I guess I am still unable to get my question across.

Let me put it another way. Floating is a result of differences in density, correct?

So where does the energy that makes for example two liquids of difference density change place come from?

Gravity.

Consider a system of two liquids, one of density 2 kg/l and the other of density 1 kg/l, with 1 l of each liquid in a cylinder of 2 l volume and 4 m height. Now, consider the gravitational PE with the dense liquid on top vs the gravitational PE with the dense liquid on the bottom. With the dense liquid on top the PE (mgh) is:
2 10 3 + 1 10 1 = 70 J

With the dense liquid on bottom the PE is:
2 10 1 + 1 10 3 = 50 J

So gravitational PE is lost as the liquids exchange places and work can be done.

Or another scenario: if you have a floating device anchored to the bottom of an empty reservoir and fill it with water, the floating device will gain PE as the water gets deeper..

Can someone explain the source of this energy?

What is the source of the energy for filling the reservoir? Whatever that source of energy it takes more of it to fill the reservoir with the same volume of liquid because the liquid must be raised further against gravity.

 

[sophiecentaur]

Here's another way of looking at the problem.
Imagine you had a very light, foam, boat. It will float almost entirely on top of the water. To make it go down to the depth of a 'normal' boat you would have to push down, displacing water and doing work (i.e. transferring energy) That is the amount of energy that is transferred when a boat sinks down or is buoyed up by rising water. It corresponds to lifting the volume displaced by a distance corresponding to the height of its centre of mass (or the centre of mass of the region of water displaced). I think that is the energy you are seeking to identify. It has to come from somewhere; it can come from someone operating a pump, when rain falls into a lake or for any of a thousand other possible mechanisms.
Talking in terms of of density differences will explain where the forces come from but, as you know, work (i.e. energy) is the product of a force times a distance and it is energy that you after.

 

[jferstad]

So you could harvest potential energy from the rain by for example anchoring floating devices on the bottom of water reservoirs used for hydropower?

By first emptying the reservoir (running it through the tunnel to the turbine) and transfort the original PE to electricity, and then anchoring the floating devices.

After a certain amount of rain has fallen the floating devices will have gained PE and you could use generators to transform it to electricity when you let the floating devices go to the too.

Would this make hydroenergy more effective or are we talking about very small amounts of energy?

 

[Dale]

It would be very small, but yes. Essentially what would happen is that you would keep the rain water at a slightly higher elevation so it would have a slightly greater energy by mgh.

 

[mgb_phys]

So you could harvest potential energy from the rain by for example anchoring floating devices on the bottom of water reservoirs used for hydropower?

You can do something similar with waves - called Salter's ducks

 

[VooDooX]

What abotu this a hydrogen Lighter then air blimp with wings you fill it with hydrogen let it get to 100,000 feet then use a pump onboard to reduce the hydrogen till the vehicle is heavier then air and glide with a 10-1 or even 20-1 glide ratio (due to buoyancy of the remaining hydrogen) and travel approx 360 miles or more i would say the energy gain is less then the energy used since moving the same object 363 miles at ground level would use far more power then the small electric pump used to reduce buoyancy so gravity would be a inexhaustible source of energy?

 

[jferstad]

Well if you are going to use the blimp to transport things it would be pretty heavy right?

Wouldn't that require a LOT of hydrogen? Which would require a LOT of energy to compress?

And an enormous hydrogenbaloon isn't too safe around heat? How about using helium?

 

[VooDooX]

lol it was just for principle fine use helium lol and you wouldn't need to compress it all just say make it 100lbs lighter then air wait for it to raise then make it 100 lbs heavier then air don't read into it to much just understand the basic concept of what I am trying to get across

 

[mgb_phys]

i would say the energy gain is less then the energy used since moving the same object 363 miles at ground level would use far more power then the small electric pump used to reduce buoyancy so gravity would be a inexhaustible source of energy?

So long as you want to go downwind, yes.
I think long distance wind powered transport at sea level has been done - with the advantage that you don't need to go downwind and can move quite large loads.

 

[VooDooX]

you can definantly go against the wind would just need to weight a little more meaning pump a little more helium out and you could even use the wind like a glider to gain altitude remember it turns from a blimp to a glider plus it still has the inertial weight let's say its a 20 ton blimp even tho its lighter then air it still cannot be nudged by wind just because it is lighter then the air the wind hitting it would deal with a 20 ton weight! so 100lbs less then boyant it will still have extreme PE even in a forward glide especially once it picks up speed it will be like a steam roller

 

[VooDooX]

ACTUALLY... CHECKMATE my idea is already in the planning phases damn idea stealers snatching stuff from my head as i sleep!
http://www.gizmag.com/go/3060/"
http://www.gizmag.com/pictures/hero/3060_01.jpg

The concept of sustained flight without the use of fuel seems far reaching even by today's advanced aviation standards. Even more so when you consider that aircrafts should ideally be able to carry heavy loads of passengers and cargo. However one innovator's astounding new designs could transform the air transport industry of the 21st Century if they turn out to be valid

Former nuclear designer, Robert D. Hunt of Hunt Aviation Corp has come up with a new "gravity powered aircraft technology" that he claims can accomplish sustained fuel-less flight. Hunt has designed a new hybrid aircraft: a "gravity-powered aircraft" which is a fixed wing, ridged skin airplane made of lightweight and modern composite materials. By October 2003, Hunt Aviation Corp had already begun the first phase of prototype construction, assembling a consortium of aviation manufacturers and suppliers that wish to support the revolutionary aircraft technology.

Interestingly, because this hybrid plane uses technology of gliding and aerostatic lift, the idea for sustained flight actually has more in common with the older technology of Leonardo Da Vinci's first primitive hang glider, than it does from the Wright Brother's engine powered airplane only a century ago.

The "Gravity-Plane", as Hunt Aviation likes to call it, uses gravity's dual properties - buoyancy which creates an upward motion in order to gain altitude, and gravity acceleration which creates a forward and downward gliding motion. The two motions combined form the heart of Hunt's new gravity powered technology, a technology that could make for a much healthier and cleaner environment.

Lighter-than-air (Aerostatic) lift may be explained by the principal of buoyancy, also known as the Archimedes Principal. Gravity exerts a greater pull on more dense materials than on less dense materials, which causes buoyancy. For example, a bubble rises in water and helium rises in air because they are less dense than the surrounding "lifting" fluid.

In the Hunt Aviation's "Gravity-Plane", buoyancy is created by gas bags filled with helium within two large rigid pontoon shaped lifting bodies. This buoyancy lifts the "Gravity-Plane" to high altitudes to create lighter-than-air lift.

Despite being a better "lifting gas" than Helium, Hydrogen is generally not used in this way because it is combustible. Inert Helium, widely used in lighter-than-air airships, can now be used to attain altitudes of over 100,000 feet and may be built very large to carry heavy loads of passengers and cargo approaching 1,000 tons according to Hunt. By comparison, a U. S. military C-17 heavy lifter only carries 70 tons.

Even better than Helium , according to Hunt, is the idea to use a vacuum-lift system in the hybrid aircraft. During normal operation of the aircraft, lift is provided by the vacuum contained within rigid cells. As a precautionary measure, the new hybrid aircraft will use a Dual-Aerostatic-Lift system that will include the use of vacuum-lift and the use of a lifting gas. The lifting gas is expanded into collapsible gas bags, in the event of rupture of the vacuum-lift cell wall.

Obvious benefits of the technology are that the aircraft does not require fuel, which is aviation's main cost. This also makes the aircraft safer in terms of fuel burning or exploding. Furthermore, having no waste emissions or noise, the aircraft is extremely environmentally friendly. "Hunt's invention is the first practical use of gravity to provide a motive force by forming a continuous cycle out of two forces of gravity with the result being, for the first time ever, self-sustained fuel-less flight and this is a tremendous and historic accomplishment", stated Gene Cox, President of Hunt Aviation Corp.

 

[sophiecentaur]

I tend to be suspicious of anyone who claims "gravity powered flight". Gravity can't "power" anything as it is a conservative field. It takes as much energy getting up there as you can get back coming down. If this were not the case, the orbits of the Planets would no be elliptical for a start.
It all smacks too much of perpetual motion and I don't think this Forum would have time for it.

 

[VooDooX]

so your saying my post of an actual company being run by a retired nuclear designer is a perpetual motion gimic? it completely explains that its using gravity both ways first by making it lighter then the surrounding air so gravity makes heavier air fall below it thus lifting the object then once at altitude making it heavier then air ... it sounds like pretty sound reasoning to me and where does it say perpetual motion.. the ships batterys will need to be recharged I am sure but will use no fuel other then onboard electronics to run stuff here's some more light reading

http://www.fuellessflight.com/index.htm"

 

[mgb_phys]

Air weighs 1.2kg/m^3 at sea level, at 100,000 ft is almost nothing.
To have a lifting capacity of 1000t at 100,000ft the airship is going to have to be roughly the size of Belgium.

There are no winds at 100,000ft but to get there you have to go through the 100-200mph jetstream.

 

[VooDooX]

Its a lot lighter then even helium so your wrong the ship will have Vaccum bags with a complete vacuum inside instead of helium and incase you missed the linkhttp://www.gizmag.com/go/3060/"

 

[Dale]

The problem with vacuum bags is that they collapse. In order to keep them from collapsing you have to make them out of rigid materials, which tend to be rather dense. That essentially negates any advantage you would get by using vacuum instead of helium.

 

[VooDooX]

hey i didnt design it you would have to goto http://www.fuellessflight.com/index.htm and read up then if you still see a problem THEN post about it they also talked about heated helium just like hot air is more buoyant heated helium is the same way also the prototype is being built by the us marine corps so i doubt you will find a design flaw that says" hey look at me i can't be made to work in real life"

 

[sophiecentaur]

so your saying my post of an actual company being run by a retired nuclear designer is a perpetual motion gimic? it completely explains that its using gravity both ways first by making it lighter then the surrounding air so gravity makes heavier air fall below it thus lifting the object then once at altitude making it heavier then air ... it sounds like pretty sound reasoning to me and where does it say perpetual motion.. the ships batterys will need to be recharged I am sure but will use no fuel other then onboard electronics to run stuff here's some more light reading

http://www.fuellessflight.com/index.htm"

I'm saying that it is no more "gravity powered" than you are when you ride your bike up hill and then roll down the other side. Gravity is not a Source of Energy. One way or another, you are giving the craft some gravitational potential energy by doing work on it, some of which you can later get back.
As long as we bear that in mind we only need to consider the efficiency of such a system. We don't actually get anything 'for nothing' in any real system.

It is just possible (it wouldn't be the first time) that it is the reporter in that extract who has got hold of the wrong end of the stick about the proposal. However, merely being described as a "former nuclear designer" does not instantly make all your (commercial) proposals watertight. Ad Hominem doesn't necessarily work when applied in either direction.

 

[mgb_phys]

The helium balloon for the highest parachute jump (100,000ft) was 85,000m^3 and carried a payload of 750kg (man + capsule)
So to lift a 1000ton cargo (assuming no increase in the mass of the structure to hold this) would take a volume of 115million m^3, or 1000x bigger than a Zeppelin.

 

[VooDooX]

again i didnt design it lol read the explanations on the companys site it goes into ALOT of detail click technical and it explains it all there then tell me why it won't work he said POTENTIALLY up to 1000tons probably given a ships maximum conceivable size not will carry 1000tons read the site before you say it won't work because everythign your arguing is covered there I am sure liek i said being developed by us marine corps so i don't think its bs

 

[russ_watters]

so your saying my post of an actual company being run by a retired nuclear designer is a perpetual motion gimic?

Yes, that sounds about right to me.

At best, it's a crappy glideer, but I doubt it is even that good. But the language used in the claims is so glowing, it implies much more.

hey i didnt design it...

What a coincidence, neither did he! A comic book illustration is not a design.

then tell me why it won't work

It's not so much that you can't lift a glider with a balloon, it's more a matter of there not being any good reason to. It just isn't a good idea. But by making it sound like a good idea, he has to exaggerate the claims so far as to make them laughable. And you've already had a couple of people point out some obvious flaws with his reasoning. I'd add another: balloons and gliders are both slow. And one more: The term "vacuum bag" is a self-contradiction.

 

[sophiecentaur]

It would be conceivable to make a helium balloon which carried a compressor to put the gas back 'in the bottle'. The energy for that would be what was effectively driving the balloon down again, once it had reached it operating height. (Very much like blowing and filling flotation tanks on a submarine).

I'm not sure that, on a total cost basis, it would be better value than just dumping the Helium when you wanted to descend. The gas cylinders and compressor would be a large extra payload. This proposal is, in effect, doing the same thing by doing work on a mass of gas.

If you want to use the Potential Energy due to your altitude to provide forward motion through the air, you would need to compress / expand more gas. The PE lost would need to be supplied by 'subtracting the bouyancy' which actually involves work equal to 'pressure times volume change'. This needs to be equal to the losses due to drag (at whatever speed you could actually go) over the length of the journey. The energy to achieve this speed with this amount of drag is the same as if you used a propellor / jet engine or tethered ducks. You are getting nothing for nothing. Certainly you aren't getting anything 'from gravity'. If you fully analyse the energy transfers in the situation there can be no inherent advantage - except that which any lighter than air machine can have (requiring no energy to keep it actually afloat).
Build yourself a Graaf Zepplin and do it better than they did nearly a hundred years ago and you could be on to something but don't go near "powered by gravity". Readers of these Forums are far too canny to take the bait, I'm sure. They aren't falling over each other to buy shares!

 

[mgb_phys]

I think the reason for recompressing the helium is more operational convenience.
If you want to operate like a Zeppelin between fixed airports then it's easy to just dump ballast to climb and dump helium to sink - then refill at the end of the trip.

If you want to use blimps as construction cranes, battlefield, or remote site delivery systems then having them entirely self contained might be a good idea. After all if you can truck in 10,000m^3 of helium to refill the craft for it's return trip you might as well truck in the cargo.

Composite gas tanks and solar powered pumps might make it practical, especially if you only need to adjust the pressure by a few % as a trim tank.

 

[sophiecentaur]

I think the reason for recompressing the helium is more operational convenience.
If you want to operate like a Zeppelin between fixed airports then it's easy to just dump ballast to climb and dump helium to sink - then refill at the end of the trip.

If you want to use blimps as construction cranes, battlefield, or remote site delivery systems then having them entirely self contained might be a good idea. After all if you can truck in 10,000m^3 of helium to refill the craft for it's return trip you might as well truck in the cargo.

Composite gas tanks and solar powered pumps might make it practical, especially if you only need to adjust the pressure by a few % as a trim tank.

But that's just not true in the quoted context. It may apply if you are just out for a pleasure trip but, if you want to lift a large payload and place it somewhere else, then you need to be changing a huge proportion of the total displacement.
But the original system proposes transport over large distances - implying the need for lots of energy - all of which would have to come from climbing then gliding. But, as I said before, doing it that way is not inherently better than towing it with a skein of ducks; same amount of energy needed.
It strikes me as an idea looking for an application. It's all down to the energy budget, in the end and shouldn't be led by the subjective appeal of a newly invented idea.

IFF we could get an extremely large, rigid structure that would stand large pressure differences and which had a negligible mass, then it would make a good air displacement vessel (lighter than air). You'd still have to get it from A to B, which is what is implied by the Buck Rogers craft in the picture.