Advantages and Disadvantages of Tidal Current Technology



Advantages


A major advantage of harnessing tidal currents for the generation of electricity is the predictability and reliability of the energy source. Tidal currents can be accurately predicted to within 98% accuracy decades into the future (Benelghali, et al., 2007). Figure 10 shows the tidal velocity in the Raz de Sein for the year 2007.

Tidal_Period.jpg
Fig. 10: Tidal current speed as a function of time (Benelghali, et al., 2007).

From an engineering standpoint, tidal current turbines are fairly straightforward to design as they are built on the same principles as wind turbines. Due to the reliability and predictability of tidal currents, extreme force loadings are typically no more than 100% of normal operating loads (P. L. Fraenkel, 2006). From an environmental impact perspective, tidal current turbines are relatively benign (the same cannot necessarily be said for tidal barrages). Their impacts on marine life is minimal as the rotational speed of the turbine blades are very slow compared to wind turbines. Also, because of the modular nature of the technology, effects on current flow and sediment transport can be minimized (P. L. Fraenkel, 2006). Tidal turbines also have to potential to completely replace dependence on fossil fuels for certain regions where this technology is favorable. This could greatly reduce harmful emissions into the atmosphere. The energy return on investment for tidal turbines is also predicted to be much higher than most other energy technologies (P. L. Fraenkel, 2006). Over a projected 20+ year lifetime of a tidal current turbine, it is estimated that the investment return in terms of energy payback will be greater than 40x the energy required to build, install, and maintain the system (P. L. Fraenkel, 2006).

Disadvantages


Tidal turbines are very site-specific. For tidal barrages to operate effectively, a minimum tidal range of 5 m is necessary in order for the system to be economically viable (O Rourke, et al., 2010). For tidal current turbines to be a viable means of generating electricity, a predictable current of at least 2 m/s (3.9 knots) is required (O Rourke, et al., 2010). A major disadvantage of tidal barrages is their environmental impact and large capital costs. The biggest environmental impact is due to the fact that a dam is essentially being placed across the entire mouth of a bay. This will undoubtedly impact marine life, tidal flow, and sediment transport. Additionally, tidal barrages may affect marine traffic as well. This impact is lessened by using an ebb generating system as opposed to a flood generating system because the water level within the bay is maintained at a much higher level (O Rourke, et al., 2010). While there are few tidal barrages available in order to do a detailed analysis of their environmental impact, the impact of the La Rance tidal barrage is well documented. The entire estuary was completely closed off from the ocean for 2-3 years during construction and there was a long recovery period before a new equilibrium was achieved (Pelc & Fujita, 2002). Additionally, there was a reduction of inertial area, slower currents, reduced salinity ranges, and changes in the characteristics of the bottom water, which all greatly affected the marine life (Pelc & Fujita, 2002). Because of the potential for major environmental impact, it appears that tidal current turbine farms would be the best option (on a regional scale) as an alternative or replacement for fossil fuels in the generation of electricity.