Should Britain bother with tidal lagoons?

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Bell Labs was perhaps the most important centre of technical innovation of the twentieth century. From transistors to solar panels, the list of world changing innovations that came out of the labs is incredible.

For years, Mervyn Kelly was the President of Bell Labs, and he had a simple criteria for judging the merit of a new innovation or invention, it had to be “better, or cheaper, or both”.

And when it comes to low carbon energy, we can probably rephrase it as follows:


These three criteria are critical for evaluating any low carbon source of energy.

Offshore wind is better than onshore wind in many key respects. Higher load factors and less visual impact may make it preferable despite being more expensive. At high latitudes, solar power is not scalable unless there are miraculous improvements in energy storage, something many solar advocates simply gloss over. Nuclear power plants provide reliable round the clock electricity irrespective of whether the sun has gone down or whether the wind is blowing. Biomass for heating is much less convenient than a gas furnace. And so on.

Which brings me to the announcement today that Britain might build the world’s first tidal lagoon.

So, let’s go through my three criteria, and see how tidal power in Britain stacks up.

Is it better?

This is a complex question. First, tidal lagoons provide reliable – depending on how you define reliable – electricity. Tides can be predicted with high accuracy years in advance. However, the power output will inevitably go close to zero on a regular basis. This can be predicted with much higher accuracy than either wind or solar.

The capacity factor of the first proposed lagoon in Swansea however is relatively low. A total capacity of 320 MW is to expected to have an annual output of 495,000 MWh. This equates to a capacity factor of only 18%, much lower than existing onshore wind (27% on average in Britain) and offshore wind (~35% in Britain).

How about power density? Wind farms take up a lot of space. How much does this tidal lagoon take up?

The tidal lagoon covers approximately 11 square kilometres. This gives us a power density of approximately 5 W/m2. Wind farms in Britain have power density of approximately 2.5 W/m2. In other words if you covered the lagoon in wind turbines you would generate half as much energy, assuming the area is typical of the rest of Britain.

So, the lagoon will take up less space than a wind farm. But it still takes up a lot of space. Power density of energy consumption in Britain is just over 1 W/m2. This lagoon is clearly not going to be a source of high concentration energy.

Unless you hate wind farms or nuclear power plants, it’s hard to see how this lagoon is better from a technical view point.

Is it cheaper?

The BBC reports that the first lagoon “wants £168 per MWh hour for electricity in Swansea, reducing to £90-£95 per MWh for power from a second, more efficient lagoon in Cardiff”.

In other words, the first lagoon will be more expensive than any existing source of low carbon electricity. The second one might offer up prices comparable to nuclear and onshore wind, but  will be cheaper than offshore wind. So, it looks as if we will have to build a bunch of them, and with luck the last couple will be cheaper than nuclear or onshore win.

It doesn’t sound like it ticks the cheaper box.

Is it scalable?

The answer here seems to be a clear no. On a global level, tidal potential is less than 1 TW. This compares with global energy energy consumption which is the equivalent of 15 TW, a figure that is certain to keep rising.

However, this 1 TW figure more or less assumes all available tidal energy is extracted. This clearly won’t happen. A more realistic estimate is less than 100 GW, a figure given in a PR leaflet by Alstom – you can rely on PR leaflets to give you credible upper estimates. And that’s 100 GW of capacity, which would equate to less than 50 GW of average output. This would further equate to a mere 0.5% of global primary energy demand.

This is nothing more than a rounding error in global energy consumption.

So it’s very difficult to see what merits there are in Britain investing in tidal power. If it was demonstrably cheaper than alternatives then things would be different. If it pays for itself then we don’t need to worry about it scaling up. But clearly tidal doesn’t pay for itself and it doesn’t scale up on a global level, and the global level is where it really matters.

Money that could be spent on tidal could be spent on offshore wind, next generation nuclear, or other low carbon technologies. The innovation that results from this could have further knock on effects by making the technologies cheaper for other countries, who will undoubtedly be interested in pursuing them. This cannot be said about tidal lagoons.

So why should we bother with them?

One final thought
Have a look at the image below, which is reproduced from the tidal lagoon PR. Can you really imagine this many people will be walking along the lagoon on a regular basis? Look how many people are walking on it. The whole thing is five miles long apparently. Who wants to walk along a five mile long tidal lagoon? Do they have such things anywhere?
So, what are the chances of it simply developing into an eyesore that no one walks along?
Quite high, I’d say.

5 thoughts on “Should Britain bother with tidal lagoons?

    Sam Gilman said:
    March 2, 2015 at 3:46 pm

    I absolutely appreciate why you did the calculation of potential output as a percentage of global demand to calm people who think this will power the world. However, how even is tidal like this as a resource? Would it be worthwhile for specific countries with higher availability? By analogy (I’m sure it’s not a perfect one, but still): the availability of geothermal as a percentage of global demand is pitiful, but for Iceland, it’s worthwhile, and potentially for Japan while it’s not great, it’s not to be sniffed at. (I’m increasingly struck by how geographically determined the solutions to decarbonisation might be, and how that cuts across people’s often ideological commitments to this source or that source.)


      Robert Wilson said:
      March 2, 2015 at 4:43 pm

      Sam, I did say the following “If it was demonstrably cheaper than alternatives then things would be different. If it pays for itself then we don’t need to worry about it scaling up”. So, where geothermal, tidal etc. are relatively cheap then they are a reasonable idea.


    Chris Goodall said:
    March 2, 2015 at 5:26 pm

    Dear Robert,

    I start from the perspective that these lagoons are about the best thing that the UK could do to decarbonise. Here are the positive arguments.

    a) Energy density per unit area. 5 watts per sq m isn’t enough? Even though the lagoon is simply enclosing an area of sea with a breakwater? This is effectively costless surface area. We don’t lose farmland, very few people think it is ugly, it is politically uncontentious and it adds to local amenities by encouraging water sports and you criticise it for being only twice as good as onshore wind?

    b) Capacity factors. This is inevitable because the tide isn’t running strongly all the time. The sun doesn’t shine all the time either, nor the wind blow. At Swansea, the capacity factor will be about twice as much as solar would be. Other lagoons will be higher.

    c) Scale. Five lagoons give us – according to Tidal Lagoon Power – 8% of UK electricity for 120 years. This is not enough? There are many other UK locations where lagoons will work. Agreed that export opportunities are limited but there are points off the (long) Chinese coast where similar technology will work at scale. UK tidal can provide as much as new nuclear is ever likely to do, and far sooner. And tidal can happen without having to rely on importing Chinese cash with what look like increasingly onerous conditions.

    d) Cost. Perhaps Tidal Lagoon Power is exaggerating the financial attractiveness. But it is definitely saying that the second lagoon will be as cheap as nuclear. If they are correct, it won’t require ‘a bunch of them’ as you state. And the point you don’t make is crucial. The Lagoon gets the cash for 15 years, not the 35 demanded by EdF and others.

    e) You don’t mention the central advantage of tidal power. It is predictable for centuries ahead and, as important, the time of peak production varies across the UK. Although spring/neap variations will be the same across Britain, tidal power is effectively baseload. Offshore wind never will be.

    f) Local labour, mostly local materials, community shareholding, predictable costs once the first lagoon is built. Is nuclear really better than tidal as a financial proposition?

    Best wishes,



      Robert Wilson said:
      March 2, 2015 at 6:15 pm

      Thanks Chris,

      Let me address these points one by one.

      a) Whether this is politically contentious is debatable. There is a reason the PR for the lagoon focuses on comparing it with nuclear, it is to (cynically, I’m guessing) keep environmental groups on side. So far green groups are ambiguous on the issue, and they may quickly change making it a politically contentious issue. And if this project begins to look viable we can be confident the tidal barrage folks will do all they can to make it politically contentious.

      b) My simple point here is that the capacity factor makes it no better than wind or solar. Some people, including one Green MEP, seem to be under the impression that tidal offers baseload electricity, so highlighting the low capacity factor seems necessary.

      c) I think this comment proves my point. Global potential for tidal capacity is something like 100 GW, this would result in a power output of less than 50 GW. This is around one year of electricity demand growth in China. So your claim that it can work at scale in China seems totally out of proportion with the scale of energy consumption there.

      d) Perhaps you have misread what I wrote. I said the second one will be a comparable price with nuclear and onshore wind, and that after building some more it might be cheaper. But so might nuclear and onshore wind if you give them the money. Any technology can play the “give me more money and we’ll get cheaper” game.

      e) I don’t see how “tidal power is effectively” baseload. Baseload power plants are those that demonstrably are capable of providing reliable round the clock electricity, and run at 100% of rated capacity when needed. Tidal clearly cannot do this.

      f) I’m not entirely sure what your point is here. Nuclear power plants almost certainly beat a tidal lagoon in terms of local jobs. Once built, a tidal lagoon probably will employ very few people, unlike a nuclear power plant. And local jobs is probably a criteria where most low carbon energy does badly compared with fossil fuels. Big coal power plants employ far more people than wind farm.

      I think my general argument still stands up. With tidal, we are simply expanding what is likely to remain an essentially indigenous source of energy with little chance of export. As climate, energy or industrial policy the thing doesn’t add up to me.

      What’s the point of developing an industry to manufacture tidal turbines when we could develop one to build offshore wind turbines? And, say what you want about Hinkley C, but it has the Chinese interested. Can you imagine officials in the Communist Party in Beijing eyeing up a tidal lagoon in Swansea and thinking “We need a piece of that”? The whole thing seems peripheral to the real challenges of decarbonisation.


    refurbn16 said:
    March 5, 2015 at 7:13 am

    Robert I would be interested to hear your views on one component of Chris’s point d, namely that hinckley point will get its cfd subsidy for 35 years while the tidal lagoons will get it for 15 years. Surely that skews the actual levels of subsidy for each technology type. I assume it would require a bit of modelling to work out the extent of the skew as the level of subsidy will depend on future wholesale prices. A future electricity grid with high penetration wind, nukes and tidal lagoons would see pretty depressed wholesale prices on the whole so a 35 year cfd contract would be pretty attractive I would imagine. Has anyone done this work yet?


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