Thursday, January 12, 2017

Swansea Bay - greenish light at the end of the tunnel

Exciting news as it appears that the Government is leaning towards the Swansea Bay tidal lagoon.

Rated at 320MW, the lagoon isn't a silver bullet for the UK's energy mix and carbon targets but it does offer a means of demonstrating the (already well-established) technology in the UK.

There's talk of a strike price for a CfD of around £90/MWh, which makes the power cheaper than Hinckley point, and the technology is expected to operate for 120 years (and leave no nuclear waste!).

The company website states that the project will generate >530 GWh/yr and has installed capacity of 320 MW.  This indicates a capacity factor of 18.9% (see for the company's data).  As a capacity factor, this isn't high: onshore wind aims for 30%+ in England and up to 50% in Scotland, but ultimately it's the cost per MWh that counts.

The technology has been well proven in any number of dams but also at La Rance tidal power station in France.  La Rance, according to Wikipedia, has been in operation since 1966 - 50 years - and produces an average 57MW against installed capacity of 240 MW - 24% capacity factor).

One concern we've heard voiced about tidal schemes in the Bristol Channel is that they could interfere with the resonant nature of the tidal movement in the area.  Think of the water in the Bristol Channel as a tuning fork, but vibrating back and forth every tidal cycle and with the rotation of the earth providing a regular push to keep the system vibrating.  By changing the volume of water, the resonance of the system could change and if the resonant frequency moved far enough from the frequency of the push, the whole system could break down.

In the case of the Swansea Bay project, this seems unlikely, as the proportion of the volume of water in the lagoon compared to the Bristol Channel upstream of the site is very small.  We guestimate the surface area of the lagoon to be around 2 square miles and the area of the Bristol Channel upstream to be approximately 600 square miles.  So even as an area proportion, the lagoon is 0.3% of the total.  In terms of volume, it will be even lower as the average depth of the Bristol Channel is greater than the average depth in the lagoon.  If the average depth is three times as much, the volume of water in the lagoon is 1/1000 or 0.1% of the total.  It's hard to see that affecting the resonant frequency very much.  Imagine trimming 1/10 of a millimetre off the tines of a tuning fork - the frequency it generates isn't going to change much.

It's a different story for a tidal barrage, of course, where much greater volumes are affected, and even the Cardiff project, which has an area of 70 km2 might be getting large enough to have an effect.  We'll try to look at this more in the coming weeks.

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