[Mootle (OP)]

I don't think there is any doubt that some sort of pontoon arrangement can generate power from the tide. The version Mootle proposes is not likely to for a variety of reasons, but some very conventional hydraulics could easily overcome most of those problems.

But that's not the issue. The recovered energy is very small in relation to the size (and therefore cost) of the pontoons. That's not a problem that can be solved by any amount of engineering. It's simply a matter of basic physics. If seawater was ten times denser than it is, or if the tide rose ten time higher than it does, things might be different.

I think your conclusions are a little premature. But here are some video's that follow tidal and wave themes using hydraulics and compressed air that you've proposed.

I would maintain that the Buoyancy Engine has potential for large scale power generation but some of the other ideas may also have their place.

http://www.youtube.com/watch?v=av2Uf_AvDIA&feature=player_embedded

[Bored chemist]

Tidal power works.
There are several ways to implement it
http://en.wikipedia.org/wiki/Tidal_power
But I don't see Mootle's system ever being manufactured cheaply enough to be commercially viable.

[JP]

Interesting.  All the techniques basically involve building a dam or putting a generator in the water to harness the flow of water horizontally past it rather than the tidal rise. 

You could fill an inlet with pontoons to harness the energy, but you could get roughly the same amount of energy by damming the inlet off and harnessing the energy as the water flows into and out of the inlet due to the tides.  Obviously for a sizable inlet, its cheaper to build a dam than fill it entirely with pontoons. 

[Geezer]

Interesting.  All the techniques basically involve building a dam or putting a generator in the water to harness the flow of water horizontally past it rather than the tidal rise. 

You could fill an inlet with pontoons to harness the energy, but you could get roughly the same amount of energy by damming the inlet off and harnessing the energy as the water flows into and out of the inlet due to the tides.  Obviously for a sizable inlet, its cheaper to build a dam than fill it entirely with pontoons. 

Right - it's a shame really because tidal energy is very dependable, unlike wind and solar energy. Unfortunately, the energy density in the elevated seawater is very small, so you have to deal with gigantic quantities of the stuff to produce a decent amount of power, and that might have a serious impact on the environment.

Still, for some isolated locations where you need a limited amount of dependable power, a small-scale pontoon type generator might be the way to go.

[Mootle (OP)]

Interesting.  All the techniques basically involve building a dam or putting a generator in the water to harness the flow of water horizontally past it rather than the tidal rise. 

You could fill an inlet with pontoons to harness the energy, but you could get roughly the same amount of energy by damming the inlet off and harnessing the energy as the water flows into and out of the inlet due to the tides.  Obviously for a sizable inlet, its cheaper to build a dam than fill it entirely with pontoons. 

Right - it's a shame really because tidal energy is very dependable, unlike wind and solar energy. Unfortunately, the energy density in the elevated seawater is very small, so you have to deal with gigantic quantities of the stuff to produce a decent amount of power, and that might have a serious impact on the environment.

Still, for some isolated locations where you need a limited amount of dependable power, a small-scale pontoon type generator might be the way to go.

Actually, massive amounts of power can be generated (even more so with greater depth,) but with the Buoyancy Engine as the power is increased the generating period reduces.

[Bored chemist]

OK, but the average power is determined by the size of the floats and the tidal range.

[Geezer]

Actually, massive amounts of power can be generated (even more so with greater depth,) but with the Buoyancy Engine as the power is increased the generating period reduces.

Sure, as long as you are talking about instantaneous power. In terms of energy, the maximum energy output is limited by the displacement of the pontoon(s).

[Mootle (OP)]

Actually, massive amounts of power can be generated (even more so with greater depth,) but with the Buoyancy Engine as the power is increased the generating period reduces.

Sure, as long as you are talking about instantaneous power. In terms of energy, the maximum energy output is limited by the displacement of the pontoon(s).

I don't think average power or instantaneous power tell the full storey for power generation technologies such as this. It takes a wider view of the national grid and its frailties.

In terms of power generation there are various system arrangements that can be geared to certain applications, i.e., a few minutes of massive power output might be very useful for some scientific experiments or more typically a high power output for a few hours might be necessary to maintain services during peak demand.

For optimum ROI it is better to select a more modest power rating to meet a base load.

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

[Geezer]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

[Mootle (OP)]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

I disagree since this system involves a pulley system.

I would be interested to see a few examples of your energy density comparison based on sea water with a 50m head.

[damocles]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

I disagree since this system involves a pulley system.

I would be interested to see a few examples of your energy density comparison based on sea water with a 50m head.

Mootle that would not be a fair comparison because it would assume a 100% energy conversion in your yet-to-be-designed "pulley system". I have no engineering background, but previous posts in this thread suggest that the energy conversion in any pulley system with a 25:1 upgearing would be lucky to reach 5%. The fair comparison would be water with a 2.5 m head perhaps?

[Bored chemist]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

I disagree since this system involves a pulley system.

I would be interested to see a few examples of your energy density comparison based on sea water with a 50m head.

OK,
if the depth is 50M the pressure is about 5bar or 500,000 Pa
Each cubic metre of stored"space" at that depth represents 500KJ of energy.
A common way to store energy is to use a flywheel so lets use that as a comparator.
A disk made from steel 1 metre in diameter and 14 cm or so thick would have a mass of a tonne- the same as a cubic metre of water (near enough).
That gives a moment of inertia (I) of 0.5*1000*.5*.5 =125 (I think the units are kg m^2)

The stored energy would  be 1/2 I (omega)^2
500,000=62.5 (omega) ^2
So, to store the same energy as a cubic metre of tank i.e. 500 KJ, the angular velocity would have to be 89 radians per second.
If I have the maths right it only needs to do about 850 RPM to store the same energy and it doesn't need a set of pulleys and ropes.
Flywheels used for energy storage are generally spun a lot faster than that.
So, compared to a simple flywheel, your system isn't very good.

Actually, it might be easy to make it a lot better.
Any generator that is expected to deliver very high peak power will have a lot of thick wires and a lot of iron in the rotor. All that metal will have a lot of mass, and it will be rotating.
So, rather than messing about with pontoons and tanks, you might be able to use the generator itself as a flywheel (it's a fairly common technique for getting high peak powers) and use much cheaper electricity from the mains to spin it up (many generators can be run "in reverse" as motors.

[Geezer]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

I disagree since this system involves a pulley system.

I would be interested to see a few examples of your energy density comparison based on sea water with a 50m head.

I hope I don't have to refer you to Homer Simpson again!

Forget the gears, pulleys and all other paraphernalia. We are talking about the energy density of the seawater which is the only source of energy input to the system.

Let's say the tide rises 2m every tide. That means the potential energy of each kg of water elevated by the tide has increased by

1 x 9.81 x 2 = 19.62kJ

There are two tides in 24 hours, so the potential energy per kilogram of water has increased by a whopping 39.24kJ in 24 hours.

By comparison, 1kg of gasoline has an energy density of 44.4MJ. That's only a bit more that 1000 times greater.

You can mess around with gears, pulleys, cranks, hydraulics and levers till the cows come home, but you can never alter the fact that the energy density of the water elevated by the tide is very small (unless you can make tides rise and fall a lot further, or significantly alter the density of seawater.)

 

[Bored chemist]

I don't think he's trying to rewrite thermodynamics, energy is conserved, but power isn't and you could store the tidal energy harvested and then let it out in a rush to produce a high peak power.
It's possible, but pointless because there are better ways to do this(not to mention that the efficiency will drop due to bigger viscous losses in the pipes.

[Geezer]

I don't think he's trying to rewrite thermodynamics, energy is conserved, but power isn't and you could store the tidal energy harvested and then let it out in a rush to produce a high peak power.
It's possible, but pointless because there are better ways to do this(not to mention that the efficiency will drop due to bigger viscous losses in the pipes.

Yes you could do that, but the energy density of seawater elevated by the tide is still very small which is why tidal systems need to harness very large volumes of seawater to produce much useful energy.

[Geezer]

It's possible, but pointless because there are better ways to do this(not to mention that the efficiency will drop due to bigger viscous losses in the pipes.

I'm more concerned it won't even get that far. When the tide rises there is a distinct possibility that the pontoon won't even budge because of the friction in the pulley system. 

[Mootle (OP)]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

I disagree since this system involves a pulley system.

I would be interested to see a few examples of your energy density comparison based on sea water with a 50m head.

I hope I don't have to refer you to Homer Simpson again!

Forget the gears, pulleys and all other paraphernalia. We are talking about the energy density of the seawater which is the only source of energy input to the system.

Let's say the tide rises 2m every tide. That means the potential energy of each kg of water elevated by the tide has increased by

1 x 9.81 x 2 = 19.62kJ

There are two tides in 24 hours, so the potential energy per kilogram of water has increased by a whopping 39.24kJ in 24 hours.

By comparison, 1kg of gasoline has an energy density of 44.4MJ. That's only a bit more that 1000 times greater.

You can mess around with gears, pulleys, cranks, hydraulics and levers till the cows come home, but you can never alter the fact that the energy density of the water elevated by the tide is very small (unless you can make tides rise and fall a lot further, or significantly alter the density of seawater.)

Since it is the sea water at depth which would act upon turbine / generator set it is more in keeping with convention to refer to this as the working fluid. You must recalculate based on this in order to perform a fair comparison.
 

[Mootle (OP)]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

I disagree since this system involves a pulley system.

I would be interested to see a few examples of your energy density comparison based on sea water with a 50m head.

Mootle that would not be a fair comparison because it would assume a 100% energy conversion in your yet-to-be-designed "pulley system". I have no engineering background, but previous posts in this thread suggest that the energy conversion in any pulley system with a 25:1 upgearing would be lucky to reach 5%. The fair comparison would be water with a 2.5 m head perhaps?

A well engineered 25:1 pulley system could achieve high efficiency although it is appreciated that it is easier to achieve high efficiency with lower pulley gearing ratio's. Value engineering and consultation with experts in that field would inform the built solution.

In any case the energy density of the working fluid is unaffected by the pulley ratio. For instance a 5:1 ratio would simply take 5 tidal cycles to achieve the target depth instead of 1 based on 25:1. Thus, the gearing ratio is only considered when calculating the energy availability. Many surface tidal energy systems do suffer from low energy density and this would constrain their eligibility for large scale power generation but this system does not fall into that category.

[Mootle (OP)]

Interesting.  All the techniques basically involve building a dam or putting a generator in the water to harness the flow of water horizontally past it rather than the tidal rise. 

You could fill an inlet with pontoons to harness the energy, but you could get roughly the same amount of energy by damming the inlet off and harnessing the energy as the water flows into and out of the inlet due to the tides.  Obviously for a sizable inlet, its cheaper to build a dam than fill it entirely with pontoons. 

Dams are actually quite costly as you can see from a quick Google. I would estimate building a large pontoon is much cheaper. Of course that is only one part of the equation. Most things have a cost and storing energy is no exception. Most viable locations in the UK have been taken and countries with more viable locations tend to make full use of them. This is because hydropower is very useful.

Think of the Buoyancy Engine as a portable dam.

[Mootle (OP)]

It is true that the buoyancy of the Pontoon is one of constraints but I thought your comment on energy density was also a little misleading. The energy is effectively stored in the Storage Vessel (SV) and once the SV has reached the desired depth the energy density can be considerable.

There was nothing misleading about my statement. The source of the energy is the potential energy increase in the mass of water, and that is simply a function of the change in height and the mass. The energy density is very small.

The energy can be recovered in different ways, but you can never overcome the limitation imposed by the low energy density of the elevated water, and that fundamental limitation applies to all tidal energy systems.

I disagree since this system involves a pulley system.

I would be interested to see a few examples of your energy density comparison based on sea water with a 50m head.

OK,
if the depth is 50M the pressure is about 5bar or 500,000 Pa
Each cubic metre of stored"space" at that depth represents 500KJ of energy.
A common way to store energy is to use a flywheel so lets use that as a comparator.
A disk made from steel 1 metre in diameter and 14 cm or so thick would have a mass of a tonne- the same as a cubic metre of water (near enough).
That gives a moment of inertia (I) of 0.5*1000*.5*.5 =125 (I think the units are kg m^2)

The stored energy would  be 1/2 I (omega)^2
500,000=62.5 (omega) ^2
So, to store the same energy as a cubic metre of tank i.e. 500 KJ, the angular velocity would have to be 89 radians per second.
If I have the maths right it only needs to do about 850 RPM to store the same energy and it doesn't need a set of pulleys and ropes.
Flywheels used for energy storage are generally spun a lot faster than that.
So, compared to a simple flywheel, your system isn't very good.

Actually, it might be easy to make it a lot better.
Any generator that is expected to deliver very high peak power will have a lot of thick wires and a lot of iron in the rotor. All that metal will have a lot of mass, and it will be rotating.
So, rather than messing about with pontoons and tanks, you might be able to use the generator itself as a flywheel (it's a fairly common technique for getting high peak powers) and use much cheaper electricity from the mains to spin it up (many generators can be run "in reverse" as motors.

I really don't think you understand how the renewable energy sector works but let's indulge your notion and ignore the finite nature of the primary energy sources used for a typical grid power generation.

As stated, more than once, I haven't indicated a cost yet because the design is under development but let's consider the cost of the main alternatives:

A nuclear power station requires a Uranium mining site, Uranium refining plant, nuclear power plant,  containment, disposal etc... 

or perhaps you prefer a fossil fuel alternative so we would have...

The fact is energy generation isn't cheap, none of the methods work without taking a long term investment approach even before we get into renewable technologies. Feel free to develop your flywheel idea but if it's all the same to you I'll persue my idea for the time being.