Page 316 - Alternative Energy Systems in Building Design

290 OCEAN ENERGY TECHNOLOGIES

In large-capacity reservoirs, where the basins can be drained very quickly in every
cycle, power generation is at a very high rate but lasts only for short periods of time.
This makes it somewhat incompatible for grid interconnection. Fast pond water level
drainage also can have negative effects on the environment.

MODES OF OPERATION

Ebb generation In tidal ebbs, the basin is filled through the sluices when turbines
are set in freewheeling mode until high tide. On filling the reservoir, the sluice gates
and turbine gates are kept closed until the sea level falls to a low enough level to create
sufficient head across the barrage. When the sluice gates are lifted, the turbines
generate electric power until such time the head becomes low. This cycle is repeated
over and over again. This process is known as ebb generation or outflow generation
because power generation occurs as the tide ebbs.

Flood generation In this power-generation process, the basin is emptied through
the sluice gates, and turbines generate electricity at tide flood. This process is rela-
tively less efficient than ebb generation because the volume contained in the upper half
of the basin, where ebb-generation systems operate, is greater than the volume of the
lower half, where flood generation occurs.
Another factor that contributes to the lower efficiency is based on the fact that main
water flow to the basin is from a river and that as the tide rises, the difference between
the basin and the sea side of the barrage becomes less than would be desirable.

Bidirectional-flow generation Certain turbines, as discussed below, are designed
in a manner that makes them capable of power generation at tide ebbs and floods.

Pumping Turbines also can be powered in reverse by excess energy in the grid in
order to increase the water level in the basin at high tide for ebb generation and two-way
generation. The energy is returned to the grid during generation.

Two-basin schemes In this type of construction, tidal power generating plants
have two basins. One is filled at high tide, and the other is emptied at low tide. In this
type of operation, turbines are placed between the two basins. In general, two-basin
configurations have added advantages over the normal schemes just described because
power-generation time can be adjusted with certain flexibility, allowing for continuous
electric power production. Owing to larger construction and capital investment, two-
basin schemes are more expensive to build. However, there are geographic locations
for which this scheme would be well suited.

Bulb turbines In general, tidal turbines are constructed to meet the specific
requirements of a barrage design. One such turbine is seen at the La Rance tidal
plant, near St. Malo on the Brittany coast in France, which uses a bulb turbine