3. Tidal Stream

3.3.1. Technology Types (Level 2)

The physics of the conversion of energy from tidal currents is similar in principle to the conversion of kinetic energy in the wind. Many of the proposed devices have a superficial resemblance to wind turbines. There is no consensus on the form and geometry of the conversion technology itself. Wind systems are almost entirely horizontal-axis turbines and many developers favour this geometry for tidal conversion, but vertical-axis systems have not been rejected.

There are three basic steps involved in the energy transformation by a tidal stream energy converter:

• The turbine rotor (or any other type of prime mover that extracts the energy from the flow) is driven by the current. This converts the energy of the current into rotational energy of the shaft.
  
• The gearbox converts the low rotational speed of the turbine shaft to the desired speed of the generator shaft.
  
• The generator converts its shaft energy to electric energy which is transmitted to the shore via a cable on the sea bed.

Essentially, the energy converted into electricity by a tidal stream device is a function of the resource it is placed in (i.e. local tidal conditions), the device's prime mover, and the device's power take-off system (i.e. everything between the prime mover and the electrical terminals for connection to the grid). This is a dynamic system; changes to one aspect can have a significant effect on another.

Key factors influencing the performance of marine renewable devices

Although it is possible to make general observations about the performance characteristics of tidal stream devices and identify requirements for high performance that are common to many design variants, to understand performance characteristics in detail it is necessary to look closely at specific device designs. Because of the many ways that tidal stream devices can be configured, their performance characteristics vary widely.