The Cardiff Bay Tidal Lagoon – can it power 1.3 million Welsh homes?

By Roger Andrews | Energy Matters | October 10, 2017

“Cardiff Tidal Lagoon is now being developed as the first full-scale lagoon in our programme. With a potential installed capacity of around 3GW, this project could provide enough green, clean home-grown power for every home in Wales.” Mark Shorrock, Chief Executive, Tidal Lagoon Power.

In this post we investigate this claim. The results, as usual, are predictable.

The Cardiff Bay tidal project sneaked in under my radar. In fact I didn’t even know about it until I came across the article recently featured in Blowout Week 196. It has yet to get the go-ahead from the government (and may never get it), but planning is obviously well along, with the project reportedly in its “twelfth design iteration”. In addition, a lengthy environmental impact scoping report has been completed and the project has just received approval to connect to the national grid. According to the schedule the project will generate its first power in 2022.

And Cardiff Bay is big. It will have a nominal capacity of around 3GW – the official number is 3.24GW – and is estimated to cost around £8 billion. Production will be approximately 5.5TWh per year (giving a capacity factor of around 20%). The lagoon covers 70 sq km and is enclosed by a sea wall 20.5 kilometers long. In short, it’s Swansea Bay times ten. Figure 1 shows the project layout. The lagoon takes up half the width of the Bristol Channel:

Figure 1: Cardiff Bay lagoon showing sea wall location and turbine inlets/outlets (red). From Tidal Power’s environmental scoping report

I’m not going into technical details here because these have already been dissected by Euan Mearns and myself in previous posts on the Swansea Bay tidal lagoon project here and here. Instead I will concentrate on Mr. Shorrock’s claim that the project could “power … every home in Wales.”

The thing to remember about tidal power is that while it’s 100% predictable it’s also non-dispatchable, meaning that we can predict exactly when we won’t be able to dispatch it. And the reason it’s non-dispatchable is that the tide in the UK comes in and goes out twice a day and the lagoon generates power when the tide is ebbing or flowing, but no power at all when the tide turns. The result is four daily power spikes, separated by periods of zero generation, that bear no relation to fluctuations in demand. Figure 1 shows broadly what these spikes will look like. (No values are given on the Y-scale because the plot is purely illustrative. Values will come shortly):

Figure 2: Illustrative plot of daily tidal lagoon generation.

Another problem is the large difference in generation between spring and neap tides. Figure 3 shows Cardiff tide heights for October 2017. As discussed in the Swansea Bay post generation is a function of somewhere between the square and cube of the tide range, and as a result the Cardiff lagoon, were it in operation, would generate roughly ten times as much electricity per day during the spring tides around the 8th and 21st as it would during the neap tides around the 1st, 15th and 29th:

Figure 3: Cardiff tides, October 2017, data from Cardiff BSAC

Now we will turn to Welsh homes. According to the Census Bureau there are 1.3 million of them, and according to Energy UK the average UK household consumed 3,938 kWh of electricity in 2015, the last year for which I can find data. Assuming that Welsh households are average consumers then 1.3 million of them will consume 1.3 million times 3,938 kWh = 5.07Twh/year. This is less than the 5.5TWh Cardiff Bay is expected to generate. So far so good.

Now let us further assume that Cardiff Bay goes ahead and that its generation is evenly spread out between the 1.3 million Welsh households. Each household consumes 3,938kWh/year, representing an average load throughout the year of 0.45kW. But what does the household’s daily load curve look like? For want of better information I’ve assumed it’s the same as the total UK load curve, and after appropriate scaling I came up with the three load curves shown in Figure 4:

Figure 4: Daily load curves for the average Welsh household. Based on a graphic from energymag

Then I divided Cardiff tidal lagoon generation by 1.3 million households and superimposed it on the Figure 4 curves. The results for neap tides and spring tides are shown in Figures 5 and 6:

Figure 5: Comparison of Cardiff lagoon tidal generation (blue) with daily load curves for the average Welsh household, neap tides

Figure 6: Comparison of Cardiff lagoon tidal generation (blue) with daily load curves for the average Welsh household, spring tides. Tidal generation tops out around 2.3kW

What’s a Welsh homeowner to do about this? He or she has two options. Either fill a boxcar with storage batteries or believe Mr. Charles Hendry’s reassurance that National Grid can somehow smooth out these wild fluctuations:

There is an inevitable question about how the system could accommodate very significant volumes of power generation from tidal lagoons that may be predictable but not necessarily when demand is greatest. National Grid have been reassuring in their evidence to us that such power could be accommodated and managed, and as we move towards ‘smarter’ ways of managing energy demand, consumers will be more able to use power more cheaply when it is most plentiful.

Better get your washing done quickly, Mrs. Davies.