Tidal Current Turbines

Fig. 2: SeaGen tidal current turbine. Image used according to Creative Commons Attribution-Share Alike 3.0 Unported license.

Tidal current turbines operate on similar principles to that of wind turbines. Like wind turbines, tidal current turbines can either be horizontal axis turbines or vertical axis turbines. While the operating principles of tidal current turbines and wind turbines are very similar, there are several key differences related to the operating conditions. Under similar (ie ideal) operating conditions, water is much denser (~832x) than air and the flow speed is also much lower than air (O Rourke, et al., 2010). This increased density greatly increases the loading forces and moments exerted on tidal current turbines compared to wind turbines. Additionally, tidal current turbines must be able to generate electricity via both ebb and flood tides while also being able to withstand much higher structural loads when not generating (O Rourke, et al., 2010). A key advantage to tidal current turbines versus wind turbines is the greatly reduced turbine size required to produce the same amount of power, which can be seen in Figure 3.
Fig. 3: Size comparison of tidal turbine vs. wind turbine (Benelghali, et al., 2007).

Also like wind turbines, the kinetic power available is governed by the same equation: P=(1/2)ρCAv^3 (Benelghali, et al., 2007). C is the power coefficient which represents the fraction of power that can be extracted from the fluid stream. For wind turbines, this value is in the range of 0.25 - 0.3 whereas with tidal current turbines it is estimated to be in the range of 0.35 - 0.5 (Benelghali, et al., 2007). This indicates that tidal current turbines are more efficient on a per watt basis. The density of the fluid is represented by ρ, the cross-sectional area of the turbine is indicated by A, and the fluid velocity is represented by v.

Vertical axis turbines are the second type of tidal current turbine available. The vertical axis design is advantageous in that it allows the turbine to harness the tidal current from any direction, whereas the horizontal turbine design can only extract energy from two directions (Benelghali, et al., 2007).