Survival of the fittest in wave energy
Engineers at the University of Ghent, Belgium, are working on glass fibre/ polyester floating point absorbers for wave energy conversion. They are designed to stay at sea for 20 years, surviving extreme offshore weather conditions.
The research arose from a wider European project called Sustainable Economically Efficient Wave Energy Converter (SEEWEC).
It explored the feasibility of a wave energy converter conceived by Fred Olsen Ltd in Oslo, Norway. The design is of an oil rig-like platform that floats on 21 point absorbers mounted on vertical rods. The up and down motion of these ‘buoys’ in the waves is converted into electrical energy.
Fred Olsen stipulated that the point absorbers had to balance ‘survivability’ with efficient energy extraction, as well as allow for offshore maintenance and mass-production cost effectively and quickly.
Professor Wim Van Paepegem at the University of Ghent explains, ‘Most projects focus on optimum energy extraction from waves under normal operating conditions. [However], wave energy converters can only be economically efficient if they stay in the sea for 20 years or more, and all maintenance is done at sea. The survival of the platform and point absorbers under extreme states is a critical issue [that] is underestimated’.
Fred Olsen specified the use of fibre-reinforced composites at the start of the research. This was ‘based on cost calculations and maintenance requirements,’ notes Van Paepegem. He says that the reduced weight of the material, compared to corrosion-susceptible steel, means that structures
can be replaced more easily offshore.
The end result is a cone/cylinder/cone shaped point absorber that is made from a glass fibre/polyester composite and is about four metres in diameter and height. The design and geometry went through many iterations using finite element model simulations. These explored the impact of peak pressures on the structure, energy extraction, optimum local fibre orientations and winding angles, ply thicknesses, structural stiffness and local deformations. The exact angles and thicknesses are confidential, but Van Paepegem says that ‘glass fibre and a polyester matrix appear to give the best balance between cost and performance’.
Filament winding has been chosen as the preferred production method because of its suitability for axisymmetric structures. By working with Spiromatic, a company in Nazareth, Belgium, that specialises in producing composite silos using this technique, researchers found that the buoys could be cost effectively mass-produced. ‘Resin injection was investigated, but appeared too costly,’ says Van Paepegem.
He adds, however, ‘it is important to [realise] that the local fibre orientations in the filament wound composite change from place to place. So the complete winding pattern was first built up geometrically, and then the local fibre orientations were “mapped”. This is crucial, otherwise you can never reach a correct solution’.
In the field
Laboratory- and large-scale tests in a canal in Ghent were undertaken using two versions of the point absorber – a quasi-rigid and a deformable structure. Both buoys survived repeated lateral (breaking wave) and straight (where the bodies rise up and down) ‘slamming’ tests. These set-ups were based on wave pressures, calculated from Det Norske Veritas (DNV) rules, statistical storm forecasts and fluid-structure interaction simulations.
Wave slamming is characterised by high local peak pressures (about 10bar or more) with a duration of a few milliseconds.
The tests in the canal involved throwing the buoys from different heights of up to 7.2m. ‘The pressures are even more detrimental than real breaking wave conditions,’ says Van Paepegem. This is ‘because the wave contains air that causes some damping. Here, you are launching the buoys on a still water surface.’
Dr Fuat Kara of the Energy Technology Centre at Cranfield University, UK, sees potential for the work to create floating point absorbers that are cost effective and durable. He explains that the idea of moored buoys was developed in Norway in the 1970s and prototypes have been built, but ‘it is too expensive at the moment, because the technology is new’. He says the technique has, however, advantages over existing moored systems – the terminator and attenuator. ‘Terminators take the waves’ energy at 90º and attentuators at 180º. Point absorbers take the wave in any direction.’
Existing point absorber systems are, however, fixed to the sea bed and generate electricity using hydraulic means. They are therefore resricted to near shore shallow waters, with less wave energy.
Floating point absorbers could move offshore wave energy forward. Kara explains, that, as well as the structure itself, the mooring system plays a key role. ‘The mooring should not affect the motion of the point absorber.’
Although the four-year SEEWEC programme has now ended, Van Paepegem and his team are embarking on another collaborative project to explore deformable composite buoys further. While DNV standards stipulate the use of rigid bodies, the research at Ghent suggests that a non-sandwich structure could survive the conditions.
Van Paepegem adds, ‘The numerical tools and test set-ups could also be employed for other offshore composite structures’.