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Topic Title: Tidal Power.
Topic Summary: The misrepresentation of Tidal Power.
Created On: 28 May 2012 01:02 PM
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 28 May 2012 01:02 PM
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david911cockburn

Posts: 940
Joined: 13 April 2011

Hi All,

I am a little concerned that the Bristol channel 'Barrage scheme' is misrepresenting 'Tidal Power'.
Surely an attempt to capture water on the horizontal plane in this way is based on the principle of Hydro-electric power.

Please:
Imagine a ship moored just off of the coast.
Now imagine that that ship is connected to one end of a see-saw.
Now imagine that the central, pivot point of the see-saw is mounted on the sea shore.
Now imagine just how much force will be applied to the far end of the see-saw as the ship goes up and down with the tide.


Now exchange the ship for a pressurized vessel (something like a submarine) and bury the see-saw with a power station attached to the other end of the see-saw underground.

As long as the moon carries on revolving around the earth the tide will create 'motion'.
Power is defined as "heat, light or motion".
Build enough of these around the coast and we can supply all of our and future generations' needs!

Please discuss.
 29 May 2012 09:47 AM
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aroscoe

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David,

You need to think this through a bit more.. if you make something neutrally buoyant (e.g submarine) then how does it convert the changing sea level into energy? There will be no force ...

Also, if you do some simple back of the envelope calcs you'll find out that your scheme (if you used a buoyant structure and called it a "displacement" scheme) would only produce a tiny amount of energy, because your total distance travelled for every tidal cycle is only about 6-12m, so a total of 24 to 48m every day if you include bidirectional motion and 2 cycles. Even with a 20000 tonne displacement vessel (similar to the old HMS Invincible, 210m long), the energy is only 1.3 to 2.6 MWh per day, and it would be very difficult to capture because the speed of motion is only 4e-4m/s which is almost impossible to convert to electricity in practice, so this option is really only theoretical and not a practical solution.

Conversely, a single 1MW (peak) tidal turbine in a reasonable location has a rotor diameter of only about 20m with a rotation rate of about 5-15rpm (about the same as a 1MW wind turbine), and will give you on average about 0.37MW across the tidal cycle including neaps and springs. So, over 24 hours this will give you about 8.9 MWh, about 4-7 times more energy than the 20000 tonne solution but with a vastly smaller system. Fundamentally this is because you are extracting the energy from a nice fat 300m^2 cross-section of dense seawater as it moves past at a decent speed of up to about 3m/s (7500 times as your displacement solution).

This rotational speed is still quite awkwardly small, as 750-3000 rpm is a much more convenient speed for power generation at the 1MW scale, hence the ongoing work on different gearbox/drive solutions to optimise the energy extraction and reliability.

Andrew

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 29 May 2012 10:00 AM
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aroscoe

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Now I just read the original post again and feel bound to comment further...

I guess I'm also interested to know who (or what written reference) defines Power as "heat, light or motion", because these are all incorrect. Is this a tabloid or a reputable source?

I'm sure this was in the old 'O' level physics syllabus..

Energy has many forms (such as heat), and Power is the rate of Energy used/consumeed/transferred.

Power could be things like: heat transfer rate (kJ/s=kW), solar intensity (W/m^2) incident on a given surface area (m^2), or force times distance per second (Nm/s)

This last one is closest to your "motion" definition but to get any POWER you need a Force, over a Distance, and you need to do it as quickly as possible if you want more power. If the force is too low, the distance too short, of the time taken is too long, then you don't have much power!





-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 29 May 2012 02:21 PM
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david911cockburn

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Hi Dr.

Are you suggesting that a submarine stays still in the sea, relative to the sea bed as the tide rises and falls?

Yes you will need a large number of pressurized vessels, the idea being to surround the coast of every land mass and effectively create a 'waveform' of motion, causing very little environmental impact in the process.

The alternative method of harnessing tidal energy is to build a small number of immensely powerful stations (based on HEP) that have a massive impact on the environment, the Bristol channel proposal could be described as "going at it like a bull in a china shop" and is bound to scare people off of the idea and throw us back towards nuclear/fossil fueled stations!

If you have motion, heat or light any one of these can be converted to any one or all of the three. Therefore they are defined as forms of power, in simplistic terminology that we can all understand.

Regards,
Dave.
 29 May 2012 02:50 PM
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aroscoe

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Yep. If your submarine is neutrally buoyant, then the tide rising and falling won't cause any substantial force on it. Not the way you are thinking anyway, I dont think. The forces on it then due to tidal currents and turbulence will be far greater that any up or down forces due to tidal height itself.

Bizarrely, the effect might be opposite to what you are describing, but the forces will be small and the following is a theoretical explanation and I wouldn't propose it for power generation! It would depend upon the compressibility of the water versus your submarine.

If the water compresses a tiny bit due to the tide rising, and your submarine is utterly incompressible, then it might rise a bit until it again reaches a point of neutral buoyancy.

More likely is that your vessel is a bit more compressible than the seawater (water is pretty incompressible). If this is the case then as the tide rises, your vessel gets may get compressed a bit and then is less buoyant and actually sinks, rather than rising, with the tide. This is the classic childrens "Diver in a bottle" experiment.

All in all, making something neutrally buoyant but submerged is actually pretty hard to balance accurately, and results in very small forces.

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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 29 May 2012 03:01 PM
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david911cockburn

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Hi Doc,

Experimentation required then?
 29 May 2012 03:17 PM
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david911cockburn

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Doc,

I'm not suggesting that we actually use old submarines, I am just suggesting that we use something that can go up and down with the tide, or even in and out with the tide, that is out of sight and mind.

Also I am suggesting that a large number of what ever we use each producing moderate amounts of power, is likely to be more environmentally friendly (and therefore more likely to get passed the Green party) than massive eyesores like a barrage right across the Bristol channel.
 29 May 2012 05:38 PM
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ArthurHall

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There has been an experimental generator running on Islay which uses a similar method. Basicaly a pipe is laid out to sea, The sea end is open, the land end is sealed. The motion of the sea and waves create changes in the presure in the pipe, this presure is used to drive a generator. This is a bit more elegent than Davids sugestion and is fairly imune to damage from storms. As far as I know the system worked, but not well enough to develop.
 30 May 2012 09:21 AM
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aroscoe

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Arthur,

Yes, this is the Wavegen/Voith oscillating water column device. Its capturing wave energy (not tidal), so the technical idea and physics is quite different to the post that originally started this thread. There are problems with such devices: reliability and survivability being the main ones, and some other smaller issues such as flicker due to the fluctuating power flows which can be resolved using parallel short-term energy storage or reactive power modulation. However, to my knowledge this particular device is still actively being developed and has been generating electricity on a reasonably consistent basis over the last couple of years.

http://www.wavegen.co.uk/what_we_offer_limpet.htm

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 30 May 2012 10:21 AM
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david911cockburn

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Hi All,

You seem to be concentrating on pressure.

The picture in my minds eye is more along the lines of filling large plastic barrels with sea water and allowing them to go up and down with the tide.
Or are they likely to sink as the tide rises?

Another gentleman has already been kind enough to inform me that solar power stations don't work in the desert!
 01 June 2012 01:10 PM
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david911cockburn

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Dr. Roscoe,

You seem to have gone very quite, is there a problem?
 01 June 2012 02:26 PM
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aroscoe

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The only problem is with the barrel idea! You need to work out your forces, speeds, and the number of kW you can produce per million barrels, and a way of extracting the energy good luck

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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 01 June 2012 03:04 PM
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david911cockburn

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Hi Again Doctor,

You do not seem (a) very enthusiastic or (b) very friendly!
Is this a Scottish thing?
Why would you again be regarding what I say as literal?
Why choose millions of 45 gallon drums?

Firstly the vessel chosen would need to be aerodynamic (or what ever passes for being aerodynamic under water) from all directions.
Second it would make a lot more sense if it was larger than 45 gallons.
Third it would also be nice if it blended in with its' surroundings.

But filling it with sea water as opposed to air would make it rise and fall with the tide, would you at least agree with that my irritable friend?
 05 June 2012 11:14 PM
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dlane

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If you fill a vessel with seawater and place it in the sea it will then sink. I don't think you will get much vertical movement from it with the tide going up and down. You will also have the weight of the seesaw counteracting any movement created.

Even if you do get the movement, as Dr. Roscoe has pointed out it will be very slow and limited and that will need to be converted to movement to generate electricity which will not be easy. I guess you could look into DC generation and storage in batteries for conversion as an alternative to AC generation.

There are however, other issues that I can forsee. the length of the seesaw will affect the performance. The length inland from the pivot point will need to be at least the same as the length going out to sea otherwise motion will be lost and that may produce a powerstation inland larger than you are currently thinking of.

We also then need to look at the environmental issues. Sinking a powerstation into the ground may look environmentally friendly because it is unseen but the construction works would be large and expensive and create their own environmental impact.

You also speculate of many small installations of this nature having less environmental impact than one large one and I am not convinced by this. Environmental impact from an installation is generally high in its local area and diminishes the further away you go from the installation. Our shorelines contain some of the most sensitive and rare ecological systems and many small installations along a coast line would introduce many areas of high environmental impact that in total would probably add up to a higher impact than one large installation created in a specific area. The more installations you go for, then the less selective you have to become about the areas where you put them, whereas you can be more picky when choosing areas for fewer larger installations even though there may be greater impact to the local area.

You also have to look at economies of scale. The electricity generation industry is mostly privately financed and many small installations offer lower returns than fewer larger installations. You also have issues surrounding the National Grid that isn't currently designed to cater for many small generation schemes based around the coast line. On top of that the protection systems employed across the Grid are also designed to operate with generators with a high level of inertia within them to generate large fault levels, again something that many small installations struggle to create. These issues aren't insumountable but they do need consideration.

I understand your thoughts on the Severn Barrage and public perception of the environmental impact, but just by putting something out of site doesn't mean that it won't have an environmental impact that people will not pick up on.

Kind regards

Donald Lane
 06 June 2012 10:02 AM
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williamjohn

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1000 ton vessel rising and falling 10 feet every 6 hours on the tide could produce
1000 x 2240 x 10 / [ 6 x 60 x 33000] = 1.9 horse power = 1.4 kW
Could be a lot cheaper to use pv panels.
Is my calculation right?
John

Edited: 06 June 2012 at 10:14 AM by williamjohn
 06 June 2012 10:12 AM
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david911cockburn

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Originally posted by: dlane

If you fill a vessel with seawater and place it in the sea it will then sink. I don't think you will get much vertical movement from it with the tide going up and down. You will also have the weight of the seesaw counteracting any movement created.



Even if you do get the movement, as Dr. Roscoe has pointed out it will be very slow and limited and that will need to be converted to movement to generate electricity which will not be easy. I guess you could look into DC generation and storage in batteries for conversion as an alternative to AC generation.



There are however, other issues that I can forsee. the length of the seesaw will affect the performance. The length inland from the pivot point will need to be at least the same as the length going out to sea otherwise motion will be lost and that may produce a powerstation inland larger than you are currently thinking of.



We also then need to look at the environmental issues. Sinking a powerstation into the ground may look environmentally friendly because it is unseen but the construction works would be large and expensive and create their own environmental impact.



You also speculate of many small installations of this nature having less environmental impact than one large one and I am not convinced by this. Environmental impact from an installation is generally high in its local area and diminishes the further away you go from the installation. Our shorelines contain some of the most sensitive and rare ecological systems and many small installations along a coast line would introduce many areas of high environmental impact that in total would probably add up to a higher impact than one large installation created in a specific area. The more installations you go for, then the less selective you have to become about the areas where you put them, whereas you can be more picky when choosing areas for fewer larger installations even though there may be greater impact to the local area.



You also have to look at economies of scale. The electricity generation industry is mostly privately financed and many small installations offer lower returns than fewer larger installations. You also have issues surrounding the National Grid that isn't currently designed to cater for many small generation schemes based around the coast line. On top of that the protection systems employed across the Grid are also designed to operate with generators with a high level of inertia within them to generate large fault levels, again something that many small installations struggle to create. These issues aren't insumountable but they do need consideration.



I understand your thoughts on the Severn Barrage and public perception of the environmental impact, but just by putting something out of site doesn't mean that it won't have an environmental impact that people will not pick up on.



Kind regards



Donald Lane


Hi Donald,

Sea water sinks in sea water?
 06 June 2012 10:22 AM
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aroscoe

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William,

Not sure exactly what your figures and conversions/units are in your equation for HP but I get the same average power answer in the end for a floating vessel which displaces 1000 tonnes:

1000 tonne x 1000kg/tonne x 9.81N/kg x 3m = 29.4MJ
spread over 6 hours i.e. 6 x 3600s = 21600s

i.e an average power of about 29.4MJ/21600s=1400W

It will be much cheaper to build a static solar array to produce this much power, or, alternatively, a tidal microturbine of diameter ~1.2 metres will also produce an average power of about 1.4kW in a stream with a peak spring speed of about 2.7m/s. However, as with wind power and the comments by dlane, economies of scale and the difficulties/costs of electrical connection and power offtake mean that you'd be much better off building a smaller number of much larger ~1MW (peak) tidal turbines with diameters of ~20m to give average powers of about 350kW each, than thousands of 1.4kW microturbines.



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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 06 June 2012 10:34 AM
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david911cockburn

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Hi All,

Forgive me but you seem to be comparing costs of reliable versus unreliable, these are incomparable!

The moon will not stop orbiting the earth, or if it does we won't be able to live on the earth!

You people are obviously very intelligent, of this there can be no argument; can any of you understand the concept of 'functional earthing'?
If you can I could use your help.
 06 June 2012 10:46 AM
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david911cockburn

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Hi All,

Functional Earthing.

Within the pages of the 16th Edition of the wiring regulations known as BS 7671 there is a concept known as 'functional earthing', it was designed to reduce the 'interference/impedance' caused by earth leakage current emanating from the cooling fans of computers.

The electricity supply industry appears to have tried to ignore/destroy the concept of 'functional earthing', even going so far as to call into question the very 'safety' of 'safety earthing', in an effort to maintain its' turnover/profits!

Now can any of you understand this concept?
 06 June 2012 11:00 AM
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david911cockburn

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Originally posted by: williamjohn

1000 ton vessel rising and falling 10 feet every 6 hours on the tide could produce

1000 x 2240 x 10 / [ 6 x 60 x 33000] = 1.9 horse power = 1.4 kW

Could be a lot cheaper to use pv panels.

Is my calculation right?

John


A thousand tonne vessel rising ten feet, that is located one hundred feet from the pivot point of a thousand foot long see-saw would cause the other end of the see-saw to fall by?
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