Tidal power doesn’t offer an easy solution

WESTERN Mail reports and letters in the past week indicate that tidal power is back in the news.

Over the years, reports, articles and letters extol the tide’s predictability. May I add “predictably complex’’?

This little-discussed complexity makes harvesting tidal power in a way that is of use to customers at an affordable price so challenging.

In the first Swansea Bay Lagoon proposal the average power was 60MW from an installed turbine capacity of 320MW.

This power would be delivered in four three-hour 12-minute generating periods during the 24-hour 50-minute lunar day with no power in between.

At spring tide average power in these periods rises to 240MW, falling seven days five hours later to 10 MW at neap tide.

The grid can cope with this variability but its wholesale price of 15p/kWh was expensive compared to 5p/kWh grid cost in 2020.

Nor could there be local interest (the retail price of 19p/kWh is relevant here) so extreme is the variability.

DST Innovations lead the Eden Blue/Dragon project which is to be privately funded.

Power from the lagoon averaging 60MW and from a solar farm, averaging 13MW, is used locally for a data centre and housing.

The lunar day variability could be smoothed away even at spring tide by a 360MWh battery.

Is this what is planned? Batteries using sodium-hard carbon tech are now being developed by DST in West Virginia and Bridgend but it is not clear whether they would be ready for deployment in the Dragon.

This a local scheme with little impact beyond Swansea.

If the output from the proposed Colwyn Bay and Cardiff lagoons where the tides are three hours 30 minutes apart were combined, the 1.5GW output would be almost free of the lunar day variability. There would also be flood protection at the loss of power output.

Had the Severn Barrage been in operation today then its average power would be 2GW.

At spring tide the average power would be 8GW in the four generating periods of the lunar day, falling seven days five hours later to 1GW at neap tide.

Such variability is beyond the capability of the grid presently working at an average power of 35GW.

This is foreseen to rise to 89GW in 2050 and grid compatibility is still problematic.

The cost of barrage electricity at 20-30p/kWh would preclude its use to produce green hydrogen.

Some £300m has been spent on UK marine/tidal energy.

Tidal Stream turbines with outputs at the few MW level are to attract “contract for difference’’ at 21p/kWh from 2025.

It is very hard to judge proposals for arrays of such turbines since little operational data has been made public.

So in conclusion, tidal power is problematic for environmental issues, its costs, and its temporal characteristics.

There is stiff competition from offshore wind, floating or otherwise.

Professor Emeritus Tegid Wyn Jones, Physics and Astronomy

UCL