Tidal Turbines (vaguely boaty)

[ParaHandy]

I don't have an accurate current wholesale figure for nuclear - perhaps Para could oblige? - but let's take £35/MWh

the nuclear generator, British Energy, received from the grid an average of £47.2/MWH for 6 months to 28 Sep 08. Previous 6 months it was £38.

Lord Adair justified this evening the additional cost of power in 2050 (?) of 25% to the greater cost of offshore wind farms. The FT 2 saturdays ago reported that the cost to Centrica, who are the UK's biggest investor in these things, of building an offshore farm now *exceeded* the cost of a nuclear plant. The figures Centrica quoted were, I think, from £2m to £3m per MW installed capacity with nuclear at £3m and a combined gas turbine plant at £2m per MW. These figures are installed capacity and not utilisation so the utilisation cost for an offshore farm will be significantly worse if the nuclear industry manages to run these plants better than they have in the past ...

The concern generally with Centrica and the gov's plans is that Shell and BP who were partners with Centrica have pulled out quoting the high costs as the reason.

[claymore]

I still think that cost aside - there is something rather comforting in the fact that should a turbine explode all you get is a bit of a splash - not so should Heysham, Windscale - I do apologise, Sellafield et al, go bang

[Olivepage]

True

But sadly a comforting feeling doesn't keep your feet warm.

Or cook the Sunday roast.

[Nick]

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Have you bothered to have a look at some of the new tidal technology under development, in particular the shrouded turbines?

If a decent wodge was put into accelerated R&D I see no reason why this technology couldn't rapidly become economically competitive. It is simpler than wind, operates in a more stable environment than wind and can provide base load. All the engineering is pretty basic stuff and this country has a fantastic sub-sea engineering base to deal with manufacturing and deployment.

[DaveS]

While, as I've said before, I support the harnessing of tidal stream energy, it is still early days, and we've still to see which of several technologies proves most financially efficient. I can see the benefits of using shrouding to increase velocity - my own outline design uses a variation of this - but ultimately it will be survival of the fittest in terms of kWh produced / £ spent rather than ultimate energy extraction efficiency. Think of wind turbines: some silent, and hence theoretically more efficient, designs are available, but the commercial industry has largely standardised on a simple three blade axial fan.

I also think it's important not to overstate the potential. Tidal generation will be predictable, giving a huge advantage over wind since predictable generation can be traded ahead. And it is true that a geographical spread of devices should give some useful averaging of the diurnal cycle (though this will be limited to some extent by the best sites being clustered). Nothing can be done, however, about the springs / neaps cycle. So if you want to regard tidal as true base load generation you have to take the aggregate output at the minimal point of the averaged diurnal cycle at neaps, which might be well under 10% of the aggregate installed capacity.

A more sensible view of succesful large scale tidal generation is that it should in the medium term be able to undercut, and hence displace, gas, oil and coal with obvious CO2 reduction benefits. Longer term, there will probably be a need for a nuclear design that can load follow if we are to achieve carbon free generation.

[Rowana]

The problem with tidal turbines is that nobody can see them under the water!

Our wonderful(??) politicians can point to a hillside full of wind turbines gently whirling away, or not as the case may be, and say, "Look at what a grand job we are doing for renewable energy"

Bit difficult to do that with a tidal turbine!


Cynical? Moi?

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[sarabande]

tidal energy production patterns do not always conform with the (normally) forecastable commercial usage patterns. Though if one could ring Scotland with tide turbines, it might be possible to utilise say, Craighouse and Loth, as they have high tides some 4h20 apart, to mitigate the lack of synchrony.

[Nick]

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I think you will find if you study the map below that there are high-energy tidal sites available in enough sites to provide constant input to the national grid.



The only factor that needs to be taken into account when calculating base load output is the difference in output from springs to neaps - which is of course totally predicatable.

The pessimist who said you could only count on 10% of potential output seems to have failed to recognise that there is a NATIONAL grid.

In any event, in most of the high-energy sites there is a high percentage of total flow for a high percentage of the time. To give a local example, in Cuan Sound and the Corryvreckan peak flow ocurs not long after the tide turns and is maintained for most of the cycle. The period of reduced flow is a small percentage of the total cycle.

[DaveS]

Nick, I'm not being pessimistic. Perhaps it's a matter of definitions: base load generation can deliver its stated output continuously for the best part of a year at a time: some nukes have managed 2 years at almost full load. This is very different from a generation source which, by its nature, has cyclic output capability. If you want to compare apples with apples you have to take the guaranteed minimum of the cycle if you want to claim Tidal generation as base load. The rest of the generation (i.e. most of it) is predictable, but intermittent. This does not in any way detract from its worth in displacing CO2 producers.

I repeat, this is potentially a very valuable energy source, but it will not benefit from being over-hyped. (Then again, given Wee Eck's heroic renewables ambitions and anti-nuke blind spot, maybe it will...)

[Nick]

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This is very different from a generation source which, by its nature, has cyclic output capability.

I fail to see what your quibble is Dave. Yes, there is a cyclic output, but it is known and maybe varies by 50%. Base load capability for sensibly installed tidal generation is what can be deliverd by the average tidal flow at neaps, a known amount.

Diurnal variation is taken out of the equation by having stations all round the coast so that when it is slack water in (say) the Pentland Firth it is full flow in the Menai Straits. Sites are chosen for their overall flow rate rather than peak flow - i.e. sites where the springs/neaps difference is not too great, where slack water time is at a minimum etc etc.

After doing the sums you put in enough turbines to proivide the base load you need. Scaled up to mass production our existing North Sea developed subsea engineering industry would produce and deploy these like sweeties.

If the USA can put a man on the moon in ten years when they have no idea how then surely we can produce huge amounts of leccy in a similar time from existing essentially simple technology - if there is a will. As for whether or not is is strictly competitive in current (pun) economic terms - well that is really not the issue IMO . . . it is whether it is viable and economic 20 years down the line that is important.

[DaveS]

I'm beginning to see why Para was loosing his rag...

Let me try to explain what you choose to describe as my "quibble".

First, let's assume that you're right, and that with suitably sited generators it is possible to entirely even out the diurnal generating pattern of individual generators. That would give us a steady output equal to maybe half of the aggregate of the individual generators' peak outputs on that particular day. If it happened to be a spring tide day we could expect an aggregate steady output of about 50% of the aggregate installed generator capacity. That's pretty good.

Then, sadly, it starts to go downhill a bit. You say

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sites where the springs/neaps difference is not too great...

I would like some examples of such sites. I know of none. For example, grabbing the first chart to hand (2326), I see one tidal diamond in the south end of the Sound of Luing which shows flood: 2.1 Sp, 0.9 Np, ebb: 2.3 Sp, 0.9 Np. That's in line with my experience: about a 2:1 velocity ratio between springs and neaps.

So, with a 2:1 velocity ratio, at neaps do we get half of the generated output at springs? Sadly, no. Assuming no other limiting factors, the output is proportional to the cube of the stream velocity. (This also applies to wind turbines.) So at neaps we get one eighth of the springs output and, combining this with the averaging factor above, gives us about 6% of the aggregate installed capacity.

Hence my initial, back of the envelope, figure of "under 10%". No, it isn't rocket science - just arithmetic.

So tidal generation can not realistically be regarded as base load plant. That does not mean it is worthless - far from it, for all the reasons already given. It has sufficient merits IMHO to not require hype.

[Nick]

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Assuming no other limiting factors, the output is proportional to the cube of the stream velocity.

I had of course overlooked this eensy weensie little law of physics, which was tucked away somewhere but failed to emerge in time for me to make sense of your seeming pessimism . . .

So, the amount of power available is not as great as at first it may appear. In fact it is as you point out MUCH smaller at neaps. So - we can only rely on the neap output as a contributor to base load. Fair enough, but surely it is still a reliable contribution to base load, just a much smaller one. Turbines can be designed to operate efficiently at the lower speed limit, provided some way is found of coping with the higher speeds that will pertain for 90% of the 14 day Springs/Neaps cycle - by feathering the turbine blades perhaps.

SO, now that I agree with you and admit that I made a basic arithmetical error, a couple of further observations / questions for you:

So due to the cube of velocity constant you would might therefore have to install eight times more turbines to generate a reasonable percentage of base load . . . apart from financial considerations, would the generation of 'excess' power at springs be a problem? Assuming that battery storage is not an option, can you conceive of a mechanism to disconnect a precentage of the turbines at any one installation, or think of some other way to dump excess generating capacity? Would it be necessary? Would eg feathering the blades work?

Or - more efficiently but more complicated mechanically - perhaps you could have an expandable venturi/shroud, increasing the cross-sectional area across which the current was funnelled into the turbine so that the volume of water entering was adjusted to compensate for the speed of the current. I can see some engineering challenges with this approach and we are getting away from the charming simplicity of a standard shrouded turbine, but it might be possible.

Thre's a lot of energy in the tidal system and plenty of sites where units could be deployed. Surely if turbines become cheap enough it doesn't really matter how 'inefficient' the energy capture by percentage is, it still provides a percentage of our base load that is a know factor, in a way that wind patently does not. I think my point is that it is all too easy to write off new technologies by seeing difficulties rather than opportunities for solutions . . .

At least you are a bit more positive than Para and are talking engineering and physics, not accountancy - which is a somewhat discredited discipline in my book. Hope you are feeling calmer now . . .

[ParaHandy]

So tidal generation can not realistically be regarded as base load plant. That does not mean it is worthless - far from it, for all the reasons already given. It has sufficient merits IMHO to not require hype.

we're going to need something and fairly quickly. the grid peaked yesterday at 60GW and whereas before christmas the grid pulled in the full 2GW from France, it's taking 0.7GW now which I presume is the consequence of the £-Euro and might also mean that we're using up more quickly the availabe o/p of the power stations which opted out? Hopefully its windy where you are - none down here ...

[Nick]

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we're going to need something and fairly quickly.

I guess big Winter high pressure systems show up the futility of dependence on large scale wind generation like nothing else. 'Fairly quickly' isn't going to happen whether its tidal, nuclear or whatever, is it? What time scale are you looking at?

Happy New Year Para.

[DaveS]

Guid New Year! (Wonder if we'll be wishing this thread the same next year? )

Many of us are of the belief that real trouble this winter (first proper cold weather in years) has only been averted by the start of the recession, confirming that the government either has access to a very effective good luck charm or is in league with Auld Nick.

For those who take an interest in electrical demands etc., this site: http://www.bmreports.com/bsp/bsp_home.htm contains much interesting information on a pretty up to date basis. You can see how the current rise in demand is being met by burning more coal: I noticed that all units at Cockenzie apppeared to be running when I drove past at about noon today.

Unfortunately, apart from the CO2 issue, and small inconveniences like the European Major Combustion Plant Directive which will soon make their operation illegal, the problem is that these faithful old coal stations are just that - very old - nearer 50 than 40 now, and well past their design life as front line plant. In the traditional method of operating a power industry they would by now be used only for peaking duty with the newer, more efficient, stations taking on the base load.

Unfortunately, there are no newer, more efficient, base load power stations, since we no longer have a traditionally organised power industry. Instead we have a world-first model of unrestrained competition in generation that, apart from the "dash for gas" (which with the decline in North Sea production has now created its own problem of dependency on Russia), has failed to deliver - or even start building - any new base load power stations in the 18 years of its existance. This is hardly surprising, since a power station is expensive, takes a long time to build and, under the current rules, having built it there is no guarantee of selling its output. Meanwhile, a shortage of generation keeps the electricity price high...

Before condemning the thoughtlessness and short-termism of those who fail to make the generation investment decisions, consider this: if you were a competitive Generator, with a responsibility for your shareholders' profits, but no overall responsibility for keeping the lights on (that's the Government's job!), what would you do?