[Bored chemist]

To be fair (and to stop myself looking silly for saying the pontoon is a lot like a ship) ships are actually quite strong. They need to be because, from time to time, they find themselves with the ends held up by waves and with nothing holding up their middles.
It might need to be a bit stronger than a ship but not much. Also, there would be several damn great eye bolts along the hull so the load would be distributed a bit.
It's still not going to work unless there's some magic change in the economics so I'd still like to see Mootle's answer to my question.
What can you change to make this idea work?

[Geezer]

To be fair (and to stop myself looking silly for saying the pontoon is a lot like a ship) ships are actually quite strong.

True, but they are not submarines. On the other hand, pontoons are sort of shallow depth submarines. They have to displace water with something a lot less dense (probably air) in order to exert a force on the cable. I'm not an expert by any means, but I seem to remember that large tankers rely on their contents to limit the stress on their hulls when they are submerged, and that won't work with a pontoon cos it would sink when the cable pulled on it []

(Obviously, I was being silly with the single eyebolt to make the point.)

[imatfaal]

to be fair both BC and Geezer are correct.  When a ship is correctly loaded within design parameters they are very tough and will ride out unbelievable seas in the North Atlantic and the Southern Oceans.  But - misloading or loading a cargo outside design parameters will rapidly cause the vessel to crack.

large ships draw a fair amount of water - but not enough I believe to make the structure overly weak when at full draft without cargo (ie forced further into the water without being filled with cargo). A vlcc will be around 15m deeper in the water when fully laden compared to a safe ballast and 18m deeper than being completely empty.  Whilst these ships do rely on the support of water to an extent - they are quite capable of being taken from the water and propped up on blocks for a few weeks every couple of years. 

[Mootle (OP)]

"The Pontoon will not bare a significant resemblance to the vessel design upon which a budget comparison is being drawn"
I rather suspect it will. It might be more square in plan and it won't need a pointy front end, but essentially a ship is a big box that floats and so is your pontoon.

The shipwrights have spent the whole of history learning how to do that. If you think you can do better I'm willing to bet you are mistaken.

I've tried to describe it to you but evidently without much success - it will all become clear once I've completed the scaled model if the house rules permit me to post it but as it stands it would be unfair for me to take your bet.

I asked earlier what you would like future development to improve on so this idea becomes viable.
I'm still waiting.
You can't change the tide and, as I have said, a rise in electricity cost won't help much.
All you can change are the construction materials and the method.
However those materials and methods have been with us for decades (at least) and millennia in some cases.
Do you really foresee an order of magnitude drop in the price of steel?
Do you think that the concrete that holds the pulleys down will become cheaper to make or pour?

Clearly, to make your idea work there has to be a major change somewhere and that change has to make your system a lot cheaper.

What do you think can make it work?

As I've said before there are many developmental steps required.

I agree, steel prices are only likely to go up with time (as energy costs soar). This is one reason for aiming to minimise the use of steel in the system design.

One of the biggest challenges will be the anchorage. I don't think the use of large quantities of concrete will be sustainable or cost effective. I'm working on a construction animation to show the sequences involved and would post this in due course - house rules permitting.

The system design will benefit from a flat pontoon structure. Since the hydraulic forces will be modest (compared with the Storage Vessel,) I'm researching the possibility of reinforced plastics for the main base. The challenge is to effectively spread the forces - time and a bit of FEA will tell.

[Geezer]

but not enough I believe to make the structure overly weak when at full draft without cargo (ie forced further into the water without being filled with cargo).

But how would you know? Have you ever seen a ship at full draft without any cargo (or ballast) []

That's really my point. Ships are not designed to be submarines.

[Geezer]

One of the biggest challenges will be the anchorage.

Friction could be just as bad. A 25:1 "block and tackle" has so much friction that it may offer no mechanical advantage over one with a much lower ratio. When you turn a 25:1 around and try to use it as a 1:25, there could be so much friction that the storage vessel won't budge and something will break instead.

Using gears instead of pulleys doesn't help much either.

[Bored chemist]

"I'm researching the possibility of reinforced plastics for the main base."
Do you think you will have some miraculous insight that has eluded the designers of things like bridge footings for millennia?

Seriously, if they still use concrete for building things like the Thames Barrier, why do you think plastic might be better?
Do you think they are idiots?

[Mootle (OP)]

"I'm researching the possibility of reinforced plastics for the main base."
Do you think you will have some miraculous insight that has eluded the designers of things like bridge footings for millennia?

For the main base of the Pontoon.

[Geezer]

"I'm researching the possibility of reinforced plastics for the main base."
Do you think you will have some miraculous insight that has eluded the designers of things like bridge footings for millennia?

For the main base of the Pontoon.

Yes, that's a big problem. Despite the statements of my learned colleagues Imatture and Bruised Chemist (who obviously couldn't engineer their way out of a paper bag) distributing the enormous force in the cable evenly over something with an enormous surface area like a pontoon (or a big honking boat) is a non trivial problem. And don't be fooled into thinking that distributing the pulleys will solve that problem either. The aforementioned friction will ensure that it won't.

[Bored chemist]

http://xkcd.com/969/
seems like an odd coincidence.
Anyway,
Whatever Mootle.
You can dream on. The laws of physics and economics are still going to be there when you wake up.
the pulleys are not just a source of friction, but pointless. If you have not worked out why then I don't think you are going to get much further with this.


Incidentally, Geezer, our views may differ, but the outcome is the same.
I think that he will need the pontoon to be built quite a lot like a boat because it's quite like a boat i.e. it is subject to huge stresses and it has to float.
You think it will be different from a boat because the forces will  be in essentially the opposite direction and point loads rather than distributed loads; which is fair enough.

When it comes down to it, what he will need will be a big strong floaty thing. He certainly won't get that any cheaper than a boat, so his price structure is dead in the water.
Perhaps he should fit it with some ropes and pulleys.

[Geezer]

When it comes down to it, what he will need will be a big strong floaty thing.

Actually, it really does not need to be all that strong. It could just be a biggish block of expanded polystyrene (like the one under the floor of my boat). The tricky bit is connecting it to the cable  []

[Geezer]

Of course, you could eliminate the need for a big strong floaty thing entirely if you used the variation in head to pump water up a hill insted. That would also eliminate the need for a mechanical gearing system because hydraulic systems let you swap head for volume and vice-versa.

Unfortunately, you might end up with something that looked a lot like this

 [ Invalid Attachment ]

[Bored chemist]

A big strong non-floaty thing.
But the fish are smiling so it must be OK.

[Mootle (OP)]

"I'm researching the possibility of reinforced plastics for the main base."
Do you think you will have some miraculous insight that has eluded the designers of things like bridge footings for millennia?

For the main base of the Pontoon.

Yes, that's a big problem. Despite the statements of my learned colleagues Imatture and Bruised Chemist (who obviously couldn't engineer their way out of a paper bag) distributing the enormous force in the cable evenly over something with an enormous surface area like a pontoon (or a big honking boat) is a non trivial problem. And don't be fooled into thinking that distributing the pulleys will solve that problem either. The aforementioned friction will ensure that it won't.

You are correct, to achieve an efficient 25:1 pulley for this application will be quite an engineering challenge. In this way, the pulley system would be another of those developmental areas.

Friction is a function of:

The quality of the pulley bearings.
Diameter of the pulley.
Slippage between the cable and the pulley.
Material selection for the cable.
Bend radius of the cable....

The strength of the cable would be a constraining factor, once the SWL is established for a suitable marine cable the number of pulleys would be determined from the total load (including dynamic loads,) and a sensible margin of safety (including redundancy).
 

[Mootle (OP)]

....You can dream on. The laws of physics and economics are still going to be there when you wake up.
the pulleys are not just a source of friction, but pointless. If you have not worked out why then I don't think you are going to get much further with this...

I thought we had got to the bottom of the 'laws of physics' - are you still questioning my claims?

We don't have the necessary information to answer the question of economics but basing this on the value of existing vessels is flawed. These vessels are designed with a particular function in mind. The pontoon has a very different set of constraints and this opens up a wide array of possibilities to value engineer a far cheaper solution.

Once the design has been developed it will be possible to assign some realistic costs. To draw conclusions at this stage is conjecture or at best an educated guess. I'm not saying that guess work doesn't have its place but when refining the accuracy of the costs it is always a good idea to acknowledge the deficiencies of the estimate. If once a developed design has been costed it does not provide a healthy RoI then I would simply leave it at that, either way I will have learned a lot from the journey. My research has so far lead to several interesting fields of design and research that I wouldn't have reached in my usual walk of life, i.e., artificial coral reef, submarine / ship, bridge, tunnelling, anchorages, tidal systems, wave energy / prediction, ocean currents, hydropower, ocean floor topography.

Inventors need wide horizons, it is only from a unique perspective that original ideas are born.

[imatfaal]

....You can dream on. The laws of physics and economics are still going to be there when you wake up.
the pulleys are not just a source of friction, but pointless. If you have not worked out why then I don't think you are going to get much further with this...

I thought we had got to the bottom of the 'laws of physics' - are you still questioning my claims?
..../snipped
Inventors need wide horizons, it is only from a unique perspective that original ideas are born.

Unfortunately Peter your idea has run into 3 forum posters - one a research scientist, one an engineer. and one a businessman; and all have criticised your idea from multiple angles and I believe concluded from their own knowledge and the information posted that your idea is fatally flawed.  Scientifically; the energy is limited, regardless of how much gearing or pulleys the energy available is limited by the volume of the pontoon and the size of the tide (a point I am not sure you grasp).  In Engineering terms this project is huge - the largest displacement of any floating object ever, dragging an object the size of a block of flats to the sea bed every 12 hours, as much wiring as the millennium dome, and all for 16MWh/day.  And Economically, with the best will in the world, with zero maintenance and running costs, and with unheard of efficiencies - you might start breaking even sometime in the 23rd century.  A hard-nosed analysis from three independent views cannot see the worth in this project

[Geezer]

A big strong non-floaty thing.
But the fish are smiling so it must be OK.

It doubles as an artificial reef, and, as you can see, fish like them.

Actually, it does not need to be very strong at all. It's pressurised so that it only needs to handle the variation in air pressure caused by the variation in head, which is not much at all (unfortunately). It could even be a flexible air bag of sorts. One big problem is securing the thing to the seabed.

I have to agree that it's probably not much use for a large power generation scheme, but it might be interesting for small scale generation on remote islands or similar where the cost of bringing in power is already prohibitive.

[Bored chemist]

"I thought we had got to the bottom of the 'laws of physics' - are you still questioning my claims?"
There are two ways your idea could be made to work.
Make it vastly cheaper or get much more energy from it.
I was covering both bases.

[Geezer]

Mootle,

I agree with BC and Matt about the viability and practicality of your approach. From a practicality standpoint alone, even if you could overcome the many issues that have been raised, keeping all that "kit" functioning in a highly corrosive submerged environment would be horribly expensive.

On the other hand, it is an intersting problem to try to solve. Here's another suggestion. It may not address viability, but it might address some of the practicalities. I'm sure BC and Matt will be more than delighted to point out any flaws it might have.


 [ Invalid Attachment ]

Basically, it consists of a pontoon attached to a (an?) hydraulic ram. The piston in the ram is attached to the seabed by a substantial cable. A high pressure hose connects the ram to a hydraulic motor (not shown) that could be used to do a lot of different things. The motor could sit on top of the pontoon, or on land, whichever is more convenient. The ram is filled with some sort of environmentally friendly liquid lubricant - vegetable oil perhaps?

As the tide rises, the force on the piston increases, and this drives the motor. (The fluid could also be prevented from escaping so that the energy can be tapped at a later point during the incoming tide.) When the tide falls again, the ram is refilled with liquid so that it's ready for the next cycle. The refilling does not consume much energy because it only has to lift the mass of the piston and cable against gravity.

To size the thing, we'd decide on the maximum hydraulic pressure in the hose between the ram and the motor first. If we decided that was 3000 psia, it tells us (I think!) that the volumetric ratio between the pontoon and the ram is around 3000/15, or 200:1 (that should be the equivalent of the pulley ratio in a mechanical version).

Although 3000 psia is highish, it's not all that high either. We might go up to around 12000 psia if it was worth it (the motor technology might get a bit exotic, but it would get us to an 800:1 ratio, which ain't too shabby).

That ratio would mean that every 1000 litres of pontoon displacement would require a whopping 1.25 litres of ram displacement.

However, even if this were to work in practice, it can't do anything to alter the fact that the available energy is determined by the displacement of the pontoon and tidal variation. Consequently, even if you can reduce the cost of the energy extraction system to zero, the cost of the pontoon is going to determine the viability of the system as a whole.

EDIT: Astute members of the forum will have already noted that the maximum possible travel of the piston in the ram should, ideally, be at least as great as the tidal variation.

EDIT2: I wonder how this stacks up against wind power? I do know that the economics of wind power are (to put it politely) a wee bit dodgy, and the aesthetic impact of wind power can be really horrible. The last time I was in Scotland, I was pretty disgusted by the desecration I observed. 

[Mootle (OP)]

Unfortunately Peter your idea has run into 3 forum posters - one a research scientist, one an engineer. and one a businessman; and all have criticised your idea from multiple angles and I believe concluded from their own knowledge and the information posted that your idea is fatally flawed.

Interesting. The question posted was will this buoyancy engine work. In summary and disregarding areas of speculation the response was that the principle is sound and that the claimed energy balance and potential revenue stream is theoretically possible.

Scientifically; the energy is limited, regardless of how much gearing or pulleys the energy available is limited by the volume of the pontoon and the size of the tide (a point I am not sure you grasp).

Agreed, the tidal range and pontoon are the main constraints of the energy balance. I have no idea what gave you the impression that I do not understand this basic premise. 

In Engineering terms this project is huge - the largest displacement of any floating object ever, dragging an object the size of a block of flats to the sea bed every 12 hours, as much wiring as the millennium dome, and all for 16MWh/day.

Agreed, it is a big project, much bigger than you have indicated once it is scaled up. There are no easy rides with renewables, nothing comes for free.

And Economically, with the best will in the world, with zero maintenance and running costs, and with unheard of efficiencies - you might start breaking even sometime in the 23rd century.  A hard-nosed analysis from three independent views cannot see the worth in this project

It is too soon for me to draw any conclusions as to the economic viability. That can only come once there is more data to put into the business case. The schematic animation is only intended to get the principle across. Unfortunately, I'm not ready to answer the question on financial viability until the design has been sufficiently developed.

Once again, all input is appreciated - many thanks.