The tide-driven mills of the past came into action only when the sea withdrew, in other words twice a day. However, for the sake of cost effectiveness, the Rance tidal power plant is capable of turbining both when the basin is filled and when it is emptied, at high tide and low tide.

To do so, Electricité de France developed a new type of turbine, bulb sets (also known as axial flow turbines), capable of operating in both directions. These sets are made up of a Kaplan turbine connected to a generator housed in a bulb-shaped casing.

Hermetically protected, the whole electromechanical set can be fully submerged in the water duct. The blades of the turbines can change directions according to the direction of the current. The 24 bulb sets in the Rance facility have impressive technical credentials: 5.3 metres in diameter, 470 tonnes in weight and a unit capacity of 10 MW.

To increase the power plant operating time further, the bulb sets were designed for use as pumps. Thus, when the sea has almost reached the reservoir level, the filling phase is accelerated by pumping. This supplement serves to increase the volume of water in the reservoir and thus, during the next flow, the turbines will be actuated earlier and for a longer time. This pumping-turbining system makes it possible to step up or anticipate generation depending on the electricity requirements of the network.