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Topic Title: Tidal Stream Turbine vs Tidal Stream Sail
Topic Summary: Would it be better to use Tidal Stream Sails?
Created On: 21 June 2013 09:49 AM
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 21 June 2013 09:49 AM
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jarathoon

Posts: 1041
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During an IET talk last night in the Cambridge Engineering Dept on Tidal Stream Turbines the question came to mind...

Would it be cheaper to use Tidal Stream Sails instead of Tidal Stream Turbines, to capture energy from our semi-diurnal tidal flows in the Severn Estuary etc?

A Tidal Stream Sail would be connected to a long cable on a winch drum that would rotate a generator as it was slowly pulled away from the generator by the tide. The generator and winch-like drum could be out the water in a wind turbine mast like structure; to ease installation and manitenance. Electricity grid connection would feed energy from an associated wind turbine as well as the tidal sail generator.

They seem to be trying Tidal Sails of complex form in Norway. http://tidalsails.com/

Good student project to think about the relative energy capture efficiencies of tidal stream turbines with the simplest types of underwater tidal steam sails e.g. circular disk. What material do you make the generator drum cable out of? How long will it last in a salt water environment? How strong does the cable and sail system need to be for a given size of sail?

James Arathoon


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James Arathoon
 21 June 2013 10:59 AM
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jarathoon

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For the adventrous student, what about a hybrid between a Tidal stream sail and a tidal stream turbine?

If you use a circular disc sail which forces water round its edges; What happens if turbine blades surround the circular disc sail?


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James Arathoon
 21 June 2013 01:19 PM
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ectophile

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It does sound rather complicated, and a potential hazard to shipping and other water users.

It also strikes me that the length of the cable will limit the duration of the energy generation. The tides are roughtly on a 12 hour cycle, which means it's going in for 6 hours and out for another 6. You could find yourself spooling out an awful lot of cable in 6 hours.

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S P Barker BSc PhD MIET
 21 June 2013 02:15 PM
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jarathoon

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You have to spool out cable slower than the tidal stream to generate energy. The slower the spool-out rate the greater the forces on the cable, generator, gearbox etc. The slower the spoolout rate the higher the gearbox ratio can be, as long as all the other kit is strong enough to take the loads.

I notice that new light weight ropes for elevators have just been developed which could allow skyscrapers to be built taller.

http://www.independent.co.uk/n...-in-size-8662443.html

Can we use this light weight rope in green energy production?

For students the question is; where does the sweet-spot lie between spool-out and the material reliability/strength limits (in a sea water environment) ? Can you put these generators in a line a set distance apart? What is the best separation?

Even if it does not work out as a technology its a tremendously thought provoking student project.

If you hybridise then you have to make the cable both strong and an energy conductor. The main problem to solve will be handing the rotation forces. Can you put ducts on the circular disc sail to counteract rotation forces?

If it doesn't work-out, think of something else!



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James Arathoon
 24 June 2013 05:34 PM
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acsinuk

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James
This is a great idea but what about this one? Imagine if we dammed the Severn under the M4 but left athe central span shipping lane only 300 metres wide open for ships and fish to pass unrestricted.
Then on each side erected a set of 2 stubby towers some 90 metres or so apart with a cavity along its center line to allow the drive belt of water fins or sails to return unhindered.
As the sea water washed along the outside incoming wall it would drive the belted fins along towards the gap tower which would then return inside the wall in an air/water gap. At the inner far end tower the liquid would drained down into the lower side at the end but the belt would go the other way around the inner tower seal chamber and re-emerge onto the higher tide side again.
To generate on an outgoing tide would require another set of belted fins which could also use the common gap inside the wall with the inner and outer drain sluice gate arrangement changed over.
Will try and draw a diagram and attach later.
CliveS
 24 June 2013 06:57 PM
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OMS

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1 square kilometre x 1m deep = 500KW

Given the engineering to create a wall with just a 300m gap, you'd need to be taking a hell of an energy density out with those fins I suspect.

Regards

OMS

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Failure is always an option
 28 June 2013 09:39 AM
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acsinuk

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Hi OMS,
Have a sketch to hand but how to attach is a mystery. To me the most important thing is to get the most out of our natural God given resources at a realistic cost.
You are correct in pointing out to squeeze the water through such a small gap would not be practical if up stream levels are to be maintained within a metre. The scouring would also be a nuisance.
But the point is we must keep on imagining new ways of getting at the gigaWatt power available in the Severn until we can harness it for the good of the economy.
CliveS
 28 June 2013 10:31 AM
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Avatar for OMS.
OMS

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. To me the most important thing is to get the most out of our natural God given resources at a realistic cost.


Easy - move a bit further west and slightly north, and dig 96% pure carbon biomass out of the ground at low cost and with huge bulk energy density -

Just don't use the "C" word

For me, having looked at several Seven projects over the years, the solution I think is tidal lagoons rather than obsesion with an "M4 barrage" - start off Swansea, on the right geology and hit every relevant point all the way back to Magor. Most of the infrastructure is already in place as a legacy of the steel industry

Regards

OMS

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Failure is always an option
 28 June 2013 01:04 PM
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jarathoon

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The Treasury's plan to build, repair and renew our key infrastructure, published yesterday.

https://www.gov.uk/government/publications/investing-in-britains-future

The Treasury are offering £305 per MWh for tidal electricity, plus loan guarantees.

So if you average 10 MW for 3 hours twice per day on tidal stream that gives a maximum yearly income of £6.6 million.

For a tidal stream sail with 1 km spool out, a continuous average pulling force of 3600 kN is required I think. Which is much too high.

For a tidal stream sail with 5 km spool out, a continuous average pulling force of 720 kN is required.

Thus the requirement for new stronger and lighter materials than steel or nylon rope etc. The sail materials would have to handle very large loading forces. Could start off by trying to generate 100 KW and then work upwards.

If the tidal stream sail device shares the grid connections with 10MW wind turbines on top of two masts separated 5km apart; you get to experiment with tidal stream sail whilst at the same time generating an income from offshore wind assets.

Using a sail to accelerate water into a tidal stream turbine is worth thinking about as well, as this reduces the size of the turbine for a given output power. 3 generators potentially using the same mast and grid connection infrastructure would spread the costs of downtime.

If you mount the turbine on a rope in this way between two masts, then it is much easier to recover the turbine from the water for maintenance, than if a much larger and heavier turbine is mounted on the sea floor. It can also be mounted further from the sea floor, minimising erosion.

With lagoons you take away the considerable research and development element in terms of materials etc, but enormously increase the environmental impact, infrastructure and maintenance costs.

Are there any estimates out there for the cost per MW capital costs of building lagoons?

I agree rather than shutting coal stations prematurely (or indeed try to convert them to expensive and unsustainable imported biomass) to create a DECC fostered electricity generation crisis in 2015, we should dump the current Energy Bill, take any european fines due, and just use a carbon tax across the entire carbon market to spread the carbon reduction load across all sectors (not just the electricity sector).

James Arathoon



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James Arathoon
 28 June 2013 03:55 PM
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OMS

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With lagoons you take away the considerable research and development element in terms of materials etc, but enormously increase the environmental impact, infrastructure and maintenance costs.


Which allows a simple, quick and efficient build, with proven techniques and technology which reduces the maintenence burden - and other than tinkering,what infrastructure - the south wales coast is littered with grid connection points at sites of former power stations, steel works,refineries and the like.

Environmental impact - would lagoons create an significant environmental impact, personally I don't think so

As for flappy things in the water - this is the Severn estuary - it's like brown soup on a good day- an estimated 30 millions tons of "stuff" moves up and down it on a good tide - you'd need a whole host of maintenece on tidal sails - even if we had sutable materials available.

Why do it hard when you can do it easy - plenty of scope to deploy sails in the future - I would have thought the lagoon walls would be a good anchor point

Regards

OMS

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Failure is always an option
 28 June 2013 08:48 PM
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dlane

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I thought Swansea was getting a tidal lagoon if the consent goes through?

Swansea tidal lagoon

Kind regards

Donald Lane
 01 July 2013 12:08 PM
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acsinuk

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Yes, to tidal lagoon but it must be economical.
Anyway the way things are going we shall have 40 years of shale gas to keep us warm.
The government just needs to swallow its green pride and use the carbon tax money to pay off the EU; then encourage shale gas extraction in remote experimental MOD areas immediately.
CliveS
 03 July 2013 03:15 PM
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jarathoon

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Originally posted by: acsinuk

Yes, to tidal lagoon but it must be economical.

Anyway the way things are going we shall have 40 years of shale gas to keep us warm.

The government just needs to swallow its green pride and use the carbon tax money to pay off the EU; then encourage shale gas extraction in remote experimental MOD areas immediately.

CliveS


The Swansea tidal lagoon will apparently cost over £600 million.
If you could find ways of reducing the cost of this scheme to under £150 million, it might start to make some sense economically.

However they don't need to: with a proposed CfD tidal strike price of £305 per MWh the scheme would problably make money even if the costs were doubled to £1.2 billion.

They are estimating 400GWh of generation per year, which equates to a £122 million a year income at £305 per MWh

The current wholesale market rate for electricity is around £45 per MWh, which would bring in £18 million a year unsubsidised income.

Therefore they would be getting over £100 million a year in subsidies from bill payers and tax payers assuming loan guarantees are given to reduce loan interest rates.

To produce the same 400GWh electricity at a constant rate you would need to build fro example a 60 MW geothermal station in Bath say (at 80% utilisation).

This is why I absolutely loathe the "Contracts for Difference" system. It allows tax payer money to be wasted on gargantuan scales, without necessarily leading to technological break-thoughs which can slash costs long term.

The government should just help with research and early stage development and let the market determine when technologies are ready to roll-out as mainstream solutions.

Hopefully not too much damage is done before the 2015 election, after which DECC can be abolished and all the subsidy systems it created dismantled.

James Arathoon

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James Arathoon
 11 July 2013 07:59 AM
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rogerbryant

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A possibly more realistic analysis of a tidal flow system.

http://www.theengineer.co.uk/e...6666.article#comments

Best regards

Roger
 11 July 2013 11:28 AM
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jarathoon

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Originally posted by: rogerbryant

A possibly more realistic analysis of a tidal flow system.



http://www.theengineer.co.uk/e...rticle#comments



Best regards



Roger


That article relates to important work on estimating the energy extractable from up to 5 m/s flows in the best tidal stream in the UK, if not the world. For tidal stream turbines the extractable energy is proportional to the cube of the tidal stream velocity. It very would be possible to develop engineering solutions for the Pentland Firth that do not work elsewhere with slower tidal stream flows. It may not be wise to develop a bespoke solution here first in isolation; the market for products capturing energy from fast flows is limited.

I had in mind finding ways to get energy from tidal stream flows in the ball park of 1.5 to 2.5 m/s which is a larger resource.

No tidal stream technologies can be considered "realistic" until all the installation, operation and maintenance procedures and costs have been factored in. In terms of finding practical cost effective solutions moment we are at the students playing with ideas phase at the moment.

James Arathoon

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James Arathoon
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