Hydrofoil v. Buoy
The Hydrofoil offers the distinct advantage over the Buoy of increasing the lift at the top of the mooring system, by introducing dynamic lift to assist the static buoyancy. The additional lift results in an increased mooring angle (between the seabed and the mooring line), and therefore a reduced footprint. However, there are also a number of disadvantages of the Hydrofoil compared with the Buoy. The following provides a comparison of the two options.
There are a number of technical concerns with the Hydrofoil, including marine growth significantly affecting the lift and drag characteristics, as well as concerns regarding stability of the Hydrofoil and its dependency on the correct angle of attack.
In addition to these technical issues, there are also a number of economic issues. The Hydrofoil will have high manufacturing costs, due to the relatively complex structure and profile. Composite materials (rather than isotropic steel) may offer improved structural performance, but at further increased cost. An asymmetrical Hydrofoil profile would provide an improved lift to drag ratio compared with the symmetrical profile studied in this Project. However, this would further add to manufacturing costs. Marine growth has been identified as a significant technical issue for the Hydrofoil, which would also result in increased maintenance costs (e.g. regular application of anti-fouling).
The Buoy offers a simpler and more robust solution compared with the Hydrofoil, and has been chosen as the preferred solution for this Project. The Buoy will be a more cost effective solution compared with the Buoy, since sub-surface module buoyancy can be procured from suppliers to the offshore industry. Therefore the high costs of bespoke manufacture for the Hydrofoil would be avoided.
A number of mooring options were also investigated for the Buoy solution: Option 1 has a single mooring lines, Option 2 has multiple lines, and Option 3 has multiple mooring lines and a Spar buoy. The potential for increasing the energy yield by reducing the depth of the turbine with Options 2 and 3 was investigated, but was found not to significantly improve the financial income. However, Options 2 and 3 do still offer the advantage to allowing for more densely packed arrays. For constrained sites in narrow tidal channels, this increased array density may significantly improve the economic viability of developing the site.