During his recent visit to Swansea David Cameron was quoted as saying: “From the moment I heard about the (Swansea Bay Tidal Lagoon) project I have always been personally very keen on really examining it because it seems to me it has real transformational potential for Swansea — there’s obviously the energy side of it, the clean, green energy, but also the recreational transformation and economic transformation. I am excited by projects that can really transform.”

Tide power is a technology that Energy Matters hasn’t looked into in any detail, so here I will briefly review its potential as an energy source (I ignore its recreational benefits) using Swansea Bay and the “pipeline” of larger tidal lagoon projects that are scheduled to follow it as examples of the approach that tide power in the UK seems destined to follow. Is this approach really transformational? Or is it just another green pipe dream going nowhere?

Tidal Lagoon Swansea Bay (credit Renewable Energy World)

“Tidal Lagoon Swansea Bay plc is developing a 320MW tidal lagoon power project in Swansea Bay. The company aims to begin construction in the first half of 2015 with first power generation in the second half of 2018. The Swansea Bay project is the first of a pipeline of tidal lagoon power projects identified by the parent developer Tidal Lagoon Power plc (TLP), with five subsequent full-scale lagoons at various stages of development which could be operational by 2023. TLP anticipates that the total potential electricity generation from this pipeline could match or exceed 25TWh/year … equivalent to around 8% of UK electricity demand.”

(The five subsequent lagoons at various stages of development are Cardiff, Newport, West Cumbria, Colwyn Bay and Bridgwater Bay.)

What do we know about the Swansea Bay Tidal Lagoon project? One thing is that as a stand-alone project it’s neither efficient nor cost-competitive. With an installed capacity of 320MW and annual generation of 495GWh it has a load factor of only 18%, about half that of offshore wind. A report by Poyry estimates capital costs at £913 million (£2,853/kW), a levelized electricity cost of £150/MWh and a strike price of £168/MWh, much higher than the £92.50/MWh strike price at Hinkley Point. The strike price is, however, projected to decrease to parity with Hinkley for the much larger Lagoon 3, which is scheduled to be operational in 2023.

There are also questions as to whether these estimates are realistic. A recent back-of-the-envelope assessment by Thomas A. A. Atkins concludes among other things that mean power generation is overestimated by 25%. If so the load factor drops to 13.5% and the levelized cost and strike price increase substantially.

A more fundamental issue, however, is dispatchability. With solar and wind the world has an abundance of non-dispatchable renewable energy, but integrating large quantities of it with the grid poses serious problems. What the world needs is a source of dispatchable renewable energy that can be used as baseload or load-following power. Can tide power provide it?

A report entitled Tidal Lagoon Swansea Bay, Project Introduction says that it can. This report contains a map showing high tide times at the prime tide power sites Tidal Lagoon Power plc has identified around the UK coastline, part of which is reproduced below as Figure 1. The caption alongside the map makes the following claim: “Difference in high time tides around the UK creates potential to produce 24-hour base-load electricity from a network of lagoons.”

Figure 1: Prime UK tidal lagoon sites identified by Tidal Lagoon Power plc

But does it?

To evaluate this claim we must look first at how the Swansea Bay Tidal Lagoon project will generate power. Tides in UK are semidiurnal, meaning that there are two high tides and two low tides a day. Figure 2 shows tides at Swansea for a 24-hour period around March 20th, 2015 (data from Swansea tide times). The 10.4m tide range during this period is about as high as it gets at Swansea:

Figure 2: Swansea tides, 24-hour period around March 20th, 2015



Figure 3, also reproduced from the Project Introduction report, shows how energy will be generated. On every ebb tide water is stored in the lagoon and released when the head relative to sea level outside the lagoon reaches an optimum level, and on every flood tide the same thing happens in reverse (note how the claim regarding base-load power is repeated in the caption):

Figure 3: Swansea Bay 48-hour reservoir holding and power production sequence, reproduced from Tidal Lagoon Swansea Bay, Project Introduction.



The result can be considered as square-wave power output with an average of 3½ hours of generation followed by 2½ hours of no generation. This gives four bursts of tidal power a day with nothing in between, as shown diagrammatically in Figure 4:

Figure 4: Daily generation relative to tidal cycle, Swansea Bay, using March 20th 2015 tide data.



This on-off generation cycle can indeed be smoothed out by combining it with the output from a tidal plant of equal size where the tidal cycle is shifted by three hours relative to Swansea. Some fine tuning would be needed to avoid spikes but this shouldn’t create too many difficulties.

But here’s the problem. Figure 5 is a histogram showing high tide time differences for each of the 66 possible paired combinations of the 12 sites for which tide times are given in Figure 1. There are no two sites where the difference is three hours or even within an hour of three hours. The differences cluster around zero and six hours, meaning that combining output from any two sites or group of sites will tend to accentuate rather than smooth out the intermittent power delivery:

Figure 5: Histogram of differences in high tide times for all 66 possible paired combinations of the 12 potential tidal lagoon sites identified by Tidal Lagoon Power.

Of most interest is what the combined output from Swansea Bay and the five other proposed tidal lagoon projects will look like with all of them operating at full capacity (30TWh/year). Figure 6 shows the daily generation curve from the six projects. It’s about as far from baseload generation as it’s possible to get. (I estimated megawatts by assuming that the generation from each lagoon is proportional to the area of the lagoon and by factoring the results so that total generation equals the average daily generation (30TWh/365 = 82GWh)):

Figure 6: Combined daily generation from Swansea Bay and the five other proposed tidal lagoons operating at full capacity, using the March 20th 2015 tide data for Swansea



With a more judicious selection of lagoon sites it would of course be possible to combine output from the sites into something that does resemble baseload generation. But it can’t be done with the sites Tidal Lagoon Power plc has selected. The fact that Tidal Lagoon Power plc haven’t acknowledged this can only charitably be called an oversight.

And now it gets worse.

Figure 7 shows Swansea tides for the entire month of March 2015. The tidal range varies from 10.4m during spring tides to 3.5m during neap tides, but the concomitant variations in generation can’t be smoothed out by combining output from different installations because spring and neap tides are controlled by the orbits of the sun and the moon relative to the Earth and occur once every two weeks everywhere:

Figure 7: Swansea tides, March 2015



The variations in generation are also proportionately much larger than the variations in tidal range because the energy generated by the turbines is a function of some higher power of the flow rate. How high this higher power is depends on a number of factors specific to the operation, but it will probably be somewhere between the square and the cube of the tide range, so I have used these as upper and lower limiting cases. Applying them to the March 2015 tide ranges for Swansea gives the results shown in Figure 8, which displays the cube and the square of the tide range for each of the 119 ebb-and-flood cycles during March 2015: (Note that the scales are adjusted so that the averages plot at the same place on the Y scale and that the graph plots total generation during each ~6 hour ebb or flood tide cycle. Output during each of these cycles will actually consist of 3 ½ hours of generation followed by 2 ½ hours of no generation, as shown in Figure 4.)

Figure 8: Generation per tidal cycle analogued by tide range cubed (red) and tide range squared (blue), Swansea Bay using March 2015 tide data



Figure 9 converts the Figure 5 data into MWh so that the total generation during March matches the average monthly generation of 42,000MWh from Swansea Bay (495,000Mwh annual generation times 31 divided by 365). Note again that the generation totals are for 6-hour tidal cycles and do not show daily fluctuations:

Figure 9: Figure 9 Swansea Bay data converted to megawatts. Tide range cubed = red, tide range squared = blue



As discussed above we can’t smooth out these spring-neap fluctuations by combining output from different sites. We can do it only by storing the power for re-use. So how much storage do we need? To smooth out Swansea Bay generation to the point where it provides constant baseload power we would need about 7.5GWh for the tide range squared case and about 11.7GWh for the tide range cubed case (note that we need only consider the larger peak in the second half of the month). In short, we would need another Dinorwig -sized (9.1GWh) pumped hydro facility. And constructing another Dinorwig for a project that generates only 495 GWh/year is clearly not viable.

But that’s just Swansea Bay. What would combined generation from the full-scale 30GWh/year suite of tidal lagoons look like when the semidiurnal and spring-neap variations are included? Figure 10 gives my best assessment. To make it more readable the Figure covers only the period from March 19th through March 28th, 2015, i.e. the peak and trailing edge of the higher-amplitude spring/neap tide cycle shown in Figure 4, a cycle that repeats itself once every lunar month (29.5 days). Assumptions and procedures were:

Output from Swansea Bay and the five lagoons currently under consideration for development (Cardiff, Newport, West Cumbria, Colwyn Bay and Bridgwater Bay) is combined.

Spring-neap variations are taken from Figure 9.

The semidiurnal variations shown in Figure 6 are factored back in.

Megawatt output is adjusted to match total tide power generation over the ten-day period (240 hours/8760 hours times 30TWh annual generation = 0.82TWh).

The demand and wind power generation curves are from Gridwatch:

Figure 10: Combined generation from Swansea Bay and the five other proposed tidal lagoons operating at full capacity, using Swansea Bay tide data for March 19th through 28th, 2015. Wind generation is added for comparison purposes.



About all that can be said in favor of the tide power generation curve is that it’s predictable. As a source of baseload energy it makes wind look good.

Finally, how much storage would be needed to convert the tide power generated over this period into baseload generation so that it can compete head-to-head with nuclear, as some of its backers claim it can? It comes out to approximately 500GWh, over fifteen times current UK pumped hydro capacity, or if you like five million 100kWh utility-sized Tesla storage batteries. And even with this much storage tide power supplies only about 8% of total UK electricity demand.

We are now in a position to answer the question posed at the beginning of this post; is tide power really transformational or is it just another green pipe dream going nowhere? Clearly the latter, although this of course doesn’t mean that the politicians aren’t going to pursue it.