IDENTIFICATION OF FUTURE EMERGING TECHNOLOGIES IN THE OCEAN ENERGY SECTOR
PART 6 - Materials
The final part of our breakdown of the European Commission's report on the workshop to identify future emerging technologies in the ocean energy sector (link) looks at materials. The materials used in ocean energy devices face the fundamental challenges of the marine environment and have a varied set of additional requirements depending on the application. An idealised material for ocean energy applications would be light, strong, resistant to biofouling and corrosion, have an ability to be formed into many complex geometries, be durable over a 20 year lifetime and have a low cost. A bit more that your traditional trilemma; if this material exists please let us know!
It is typically pretty easy to tick a few of the boxes listed above, but generally that comes at a cost. To date the majority of solutions have gone for more conventional structural steel constructions that have been painted and had anodes applied to mitigate the effects of seawater. It's certainly a viable solution, but not always appropriate or the best choice for all parts of a device.
For example, most tidal turbine blades are composite structures (for example Atlantis & Orbital Marine turbines); ticking the lightweight and strong boxes, but typically at a considerable cost. These also require coating to minimise biofouling as the external shape is important to hydrodynamic performance.
Concrete is often used in gravity foundations, but is also being considered in WEC design. The material cost is attractive and it can be formed into complex shapes, but the structural performance is generally only good in compression, unless pre- or post-tension is applied to the concrete.
Sealing of devices can be a challenge especially as you go further subsea. Conventional ship propeller seal designs can be implemented at a variety of sizes in shallower water depths, but have not been verified or designed for greater water depths.
Highly flexible materials are of interest to wave energy devices, as we mentioned in the PTO blog (link) with dielectric polymers, but also with inflatable structures. These inflatables can offer reduced weight and size onshore, while providing a hydrodynamic shape and potential storm protection modes offshore - they are at a low TRL level at present though.
Aside from flexible materials and composites, plastic mouldings are being considered for offshore applications. The benefits include in-built corrosion and fatigue resistance. These polymers can be moulded with other materials set into the structure in the moulding process to provide mounting points, ballast or additional strength. It is thought that this technique in rotational moulding could be highly effective for WECs, particularly point absorbers (CorPower for example). However, polymers typically have a low yield strength and a much higher ultimate strength - to mkae best use of this the design must allow for varying (yielding) geometry, which is a significant challenge.