Page 85 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Tidal Energy Chapter | 3 77

FIG. 3.28 Power coefficient and the Betz limit.

La Rance tidal power station in France (Fig. 3.29), with a peak output capacity
of 240 MW, is a good example and is one of the oldest operational tidal energy
projects [14].
As we know, water elevation changes due to astronomical tides. If we build
a barrier in a sea or an estuary, we can create an artificial basin and control the
inflow and outflow of water by gates (see Fig. 3.30). As a result of the barrier
and control of flow through gates we can impose a time lag between the water
elevation inside and outside of basin/lagoon. The difference of water elevation
gives rise to a potential energy that can be harvested.
Here, we explain and formulate the equations for an ebb-generation scenario.
The theory is quite similar for flood-generation or combined flood and ebb
generation scenario (double-effect; [15]). Referring to Fig. 3.31, let us assume
that the sluice gates are open during the flood tide; therefore, the water elevation
both inside and outside of a lagoon rise to the peak (Point 1) or high tide (filling
period; between Points 4 and 5). There would be small time lag between the
water level inside and outside of the lagoon because of flow constriction in
the gates. At high tide, the sluice gates would be closed, and consequently
water level inside the lagoon will initially remain constant. However, the water
elevation in the ocean falls, which will lead to a head on the turbines. The sluice
gates will remain closed until a suitable head for turbine operation is achieved.
This period is called standing (between Points 1 and 2), and during this time