However, as I told Beth Gardiner of the NYT, I’m not sure the current project design is the best possible one. I’d like to explain here a little more this concern or, rather, my hopes for a better design based on the alternative concept of « tidal gardens » as set forth by François Lempérière.
The present design for Swansea plant with one basin area of 11.5 km², an average of tidal range of 6,3 m foresees generation of 420 GWh with a 240 MW plant capacity. The corresponding cost per MWh is about 200 €. It’s hoped that future sites in U.K. with same technology will go down with a cost between 120 or 150 €/MWh. These studies are made with bulb units – the same that operate in any hydroelectric plant.
Replacing in Swansea the 240 MW of bulb units by 100 MW of in-stream turbines with the Tidal Gardens solution would reduce the cost per MWh by at least one third and possibly by half in future sites, and the production would be along much more hours, facilitating integration in electric systems.
Tidal Gardens reduce the costs with high tidal ranges and even more for low tidal ranges.
With the Tidal Garden solution, a different, new technology would be used, that of in-stream turbines (could we say « emerging » technology?). They would be installed in series, not parallel, in a lined channel of constant cross section about 20 m deep, few hundreds meter wide and up to about one km long. The channel may be closed by gates.
Adapting the opening time to the tidal range (spring tide or neap tide) and the number of operating turbines to the head between basin and sea, it is possible to keep the same water speed in the channel for instance 4m/s and as average turbines operate with full power half time, i.e. 4 000 hours per year to be compared with 2 000 hours in La Rance or Shiwah.
The rows of turbines may be much closer than in natural sites, in order to reduce the length and cost of channels. The installation of turbines may be in calm water, they be anchored to concrete bottom with short electric connection. The cost per kWh may thus be much lower than in natural conditions, with low and variable water speeds and difficult conditions for installation, connection and maintenance.
Various turbines designs may be used; orientable blades may not be necessary.