OT: Ramblings on Tidal Power (long)

I've been doing some calculations on tidal power. There are two locations of immediate interest ATM, Swansea Tidal Lagoon and the Pentland Firth Tidal Stream. The former envisages a horseshoe-shaped barrier built out from the shoreline at Swansea and enclosing a large body of water. As tides ebb and flow, turbines are driven by the flow of water out of and into the lagoon

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. The latter takes advantage of the very strong tidal streams in the Pentland Firth, between the north coast of Scotland and the Orkney Isles, using underwater turbines rather like wind turbines
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.

The Swansea scheme has a much-publicised maximum capacity of 320MW. It also claims to be able to supply power for 14 hours a day, and to be able to supply 155,000 homes. But what does that boil down to? The fourteen hours a day is the sum of four 3.5 hour periods of generation, interspersed with 2.5 hour periods of no generation at slack water while the tides turn. So very On-Off-On-Off. The Grid operators will love that! Although Swansea doesn't give the average power over a year, it's easy enough to calculate from the house numbers: it works out at about 58MW*. With an installed capacity of

320MW, that gives a capacity factor of about 18%. The cost of Swansea is going to be about £1bn, so about £17 million per MW of output, compared to Hinkley Point C at £6.2 million per MW
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. It would take about fifty schemes the size of Swansea to match the output of Hinkley C, at a cost of about £50bn. The Swansea scheme wants a strike price of at least £168/MWh, index linked for 35 years, nearly double the £92.5 strike price for Hinkley Point C but for the same length of time
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page5. It's only merit is that it's going to last a very long time, 120 years they claim, and because it isn't actually going to produce very much electricity, it won't put up the average price to the consumer by very much!

In the Pentland Firth, the complete Atlantis Resources/Meygen scheme proposes a total of 269 turbines, each of 1.5MW capacity, and having a total capacity of about 400MW, to be installed in the channel between John o' Groats and Stroma island. It too will be intermittent, just like Swansea: On-Off-On-Off. They claim it will power 175,000 homes. Using the same calculation as before, this equates to an output of about 66MW averaged over a year**, and a capacity factor of nearly

17%, not far removed from that of the Swansea scheme. To match Hinkley C, you'd need about 11,700 of such turbines (2880/66x269), assuming a capacity factor of 90% for Hinkley!

Oh, and I nearly forgot, the strike price for tidal stream electricity is a whopping £305/MWh!

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page7. I've not seen a figure for the estimated cost of the installation.

The take-away from all that is to be very wary of the headline publicity put about by the promoters of these and other renewable schemes. They quote maximum capacities and the number of homes supplied. Both are large numbers, purposely designed to impress and mislead the average Joe Public. Squillions of kilowatt-hours sounds much more impressive than a few terawatt-hours (even assuming JP understands either)! They deliberately avoid average figures, simply because the reality is unimpressive, and if this was realised by TPTB, it's much less likely that their schemes would get the go-ahead. IMO such presentations are immoral at best and plain dishonest at worst. The promoters are much more interested in getting on to the renewables gravy-train than they are in saving the planet, and they pull the wool over the eyes of gullible and easily persuaded environmentalist members of the public.

*Calculated from the number of homes they claim to be able to supply, and OFGEM's figure of 3,300 kWh for the average household annual electricity consumption.
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For example, for the Swansea Lagoon which claims to be able to supply 155,000 homes and has a maximum capacity of 320MW the calculation is as follows: 155,000 x 3300kWh/year = 511500000kWh/year 511500000 kWh/year = 511500000/365/24=58390kW, or 58.4MW. The capacity factor is given by 58.4/320x100 = 18.2%

**As the above calculation, but using 175,000 homes and with a maximum capacity of 398MW gives an actual output of 65.9MW and capacity factor of 16.6%.

Reply to
Chris Hogg
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That may be less of a problem than you make out. The tide times vary around the country, so combining a few schemes around the country would help to even out the supply.

Reply to
GB

That simply adds problems. Since the geogrpahical separation means you need long cables connecting them, and of course the BIG tides and te deep esutuaries are all on the Western Atlantic/Channel coast.

Reply to
The Natural Philosopher

That seems a good idea, actually. Let's build electrical cables connecting different parts of the country together. It needs a catchy name - how about National Grid?

Reply to
GB

The output of this system is low and the cost per MWh is high. What's not to piss on from a great height?

Reply to
Tim Streater

Once again you betray your total lack of numeracy with a qualitative statement about a quantitative problem. # The man who build a national grid out of 13A fuse wire...

Reply to
The Natural Philosopher

You mean that the rather small tidal generators would need a bigger grid than the one we have already? Pray explain why. :)

Reply to
GB

Well, in theory, yes. It was mentioned here in an earlier thread, but in reality how practical is it? Where exactly do you put them, and how many, and is the tidal range sufficient to make it worthwhile without even bigger subsidies/strike prices?

Tidal power stations perform best when built in places with a large tidal range. The Severn Estuary and Morecambe bay are among the best locations around the UK, with times of high and low water conveniently shifted so that the periods of slack water don't coincide. But there aren't too many other suitable places; possibly around The Wash and off the south-east coast, Folkstone/Hastings area, see

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(the location of the oldest and second largest tidal barrage in the world at La Rance, near St. Malo in Brittany, was chosen with good reason).

I assume that in areas with a lower tidal range, you have to enclose a larger area of water to get the same output as, for example, the Swansea scheme, or alternatively you just get less output in total. Either way your costs per MW of generation increase. Anyway, having to build extra tidal power stations to cover the gaps multiplies the cost of what is already an expensive system, £ per MW-wise. I certainly don't see where you're going to build fifty tidal lagoons of the capacity of Swansea to give an output equivalent to a nuke like Hinkley C.

And that's just tidal lagoons. Tidal flow turbines suffer the same four-times-a-day intermittency. While there are other places around the UK with reasonable tidal flows, none come close to those in the Pentland Firth, see

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(change scale with the slider; navigate to areas of interest using the box bottom-left on the map; choose what you want to see and colour code explanation by selecting from the options top LHS). See also David Mackay's map,
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and the table below it, where he gives estimates for six areas. Compare his estimate for the Pentland Firth with the remaining five. They are much less attractive. So it might be tricky getting an equivalent amount of power from somewhere other than the Pentland Firth, to offset the intermittency there.

And that's not even considering the variation of output between spring and neap tides!

It's so much simpler and cheaper to build more nukes. And they achieve the same goal as all this renewable stuff, i.e. reducing CO2 (assuming it's actually necessary, of course, which is in doubt, at best)!

Reply to
Chris Hogg

En el artículo , GB escribió:

:)

Reply to
Mike Tomlinson

Well, the nuclear power stations need fuel, which needs to be factored in. So, build cost is not everything.

More concerning, is how to store spent fuel so we don't poison future generations. I don't think the cost of that is included in the figures you gave.

Reply to
GB

Not that we have a satisfactory method, anyway.

Reply to
GB

you don't need to connect them, you just feed it into the grid

tim

Reply to
tim...

I think that's negligible in the scheme of things

there's likely to be more day to day operation costs in a nuke than a tidal barrier.

but it's still going to be negligible

tim

Reply to
tim...

Would be my take too

And I dare-say tidal lagoons will need dredging to stop them silting up.

Reply to
Andy Burns

En el artículo , GB escribió:

The current state of the art seems to be "let's chuck it all in a big hole in the ground and hope for the best".

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Reply to
Mike Tomlinson

En el artículo , Mike Tomlinson escribió:

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Reply to
Mike Tomlinson

There's information here

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The fourth bullet point in the masthead summary says "In assessing the economics of nuclear power, decommissioning and waste disposal costs are fully taken into account". I think they go into more detail further down.

Of course, seeing that it's written by The World Nuclear Association, you'll probably give the Mandy Rice-Davies reply. :-)

Reply to
Chris Hogg

This was not one of TNP's more insightful offerings, but I doubt he'll admit it.

He may be right that the best sites are all on the Western coast. However, comparing tide heights between say Plymouth and Blakeney (which is in Norfolk) the tide heights are rather similar. It may be that half a metre makes all the difference to the economics of tidal generation, though.

Reply to
GB

You forgot to include the cost of "balancing" their intermittent output... ;-)

Reply to
John Rumm

If they are "rather small" tidal generators then what the f*ck is the point of them?

Reply to
Tim Streater

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