Tidal is the new wave

The 2015 SuperGen annual assembly was forward-looking and upbeat. The recent closure of Abengoa and Aquamarine did not send out the same shockwaves as the demise of Pelamis and MCT a year earlier. Prof Robin Wallace warned that the biggest threat to marine energy was pessimism. So it was good to detect what can be best described as a determination to weather the present storm. There was a particularly high level of enthusiasm and ‘sexy science’ coming from tidal energy research, the highlights of which are worthy of sharing.

 

Tidal energy research

Tim O’Doherty: Waves reduce tidal turbine wakes. Steady state flow is not enough to represent flow when yawed. Assymmetric bending on blades led to rapidly changing bending moments. Fault simulation investigated whether torque measurements used to used to identify a pitch offset in one blade.

Paul Taylor: The impact of ‘shear’ (vertically degradation of current speed due to zero speed at bottom, and shear forces) on loading was simulated. Variation in flow profile leads to a change in angle of attack of 2°, resulting in fatigue, as there is a large flux and torque variation. A possible solution is to pitch by ±1° during each cycle. I asked whether cyclical pitch control could be applied to the problems of bending fluctuation due to yaw, to which the answer was ‘Yes, come back next year’.

Ian Masters: Array modelling for tidal was discuseed. There is no need to mesh the blade for array modelling – one can get basic results with an actuator disk. Coastal array models indicated curved wakes if a tidal stream is curving around a headland. Power shedding (control of the torque coefficient) changes the wake.

Curved wakes in horizontal flow fields around a headland 'fence' for (a) a BEM-CFD model and (b) a coastal area model, from Masters et. al. (2015)

Duncan Sutherland: The momentum reversal lift turbine was described. This is a horizontal axis turbine with three blades that change pitch with each cycle. At any time, one is capturing power in lift, one is capturing power in drag, and a third is load-shedding.

Jon Side: The impact of arrays on biological and physical processes was assessed. Over the tidal cycle, bottom sand position changes over a metre. Power extraction changes this by ~3cm. Max Carcas asked 'is the regulator happy with the environmental impact?' Jon answered that changes were not significant due to present planned development, and that the method and understanding are most important research outcomes.

Louise Kregting: Numerical methods could be used to bring down the time and cost of an environmental impact assessment, a requirement to obtain a licence to disturb wildlife. Fluidity (Imperial college open source code) and MIKE were compared. Results were linked to particle, nutrient, plankton and detritus models. Dispersal of coral larvae (reef connectivity) was studied. There were only minor effects for a 800MW array but up to 10% change for a 8GW array. An array of that scale would impact tidal elevation along large parts of the UK coastline.

Angus Creech: Some amazing graphics showed simulations of tidal flow in the Sound of Islay. Scottish Power ADCP data was used. Significant changes were made to Fluidity, that will be released back to the community.

Daniel Parsons: The effects of the distance between the turbine and the seabed were modelled. A turbine located close to the seabed had a shorter, more asymmetric wake, and a greater impact on the current speed near the seabed.

Anna Young: A new way of controlling tidal turbines was described. Instead of pitching the whole blade at the hub, only the tip of blade is pitched, which allows a much smaller controller.

Wave energy research

Johannes Spinneken: Non-linear crest amplifications were described, i.e. larger crests than you’d expect in large Hs. In bottom hinged flaps, slam loads were correlated to flap velocity not Hs. Compressible solvers are not able to capture slamming events. Ben Yeats asked for Johannes’s views on scaling laws. He answered there was a difference between fresh and salt water in terms of air entrainment.

Matt Folley: Work at Queens on wave energy arrays over the last decade was described. One study considered 24 heaving buoys: the performance was shown to be inconsistent (assymetric). The reasons for this were listed as model reproducibility, manufacturing tolerance, and variations in the wave field in the tank. OWCs were found to be more consistent. Radiation patterns were mapped in a tank for terminator/attenuator/diagonal and chevron arrays. 10 years of EMEC data were used to simulate ideally controlled attenuators.

David Forehand: A doubly fed induction generator was added to the wave to wire model developed by David and his colleagues. They investigated the effects of generator inertia and WEC spacing on power smoothing. Progress on DTOcean was described: a techno-economic analysis was conducted into connection options considering distance from shore and reactive power compensation. Progress on PolyWEC was described: wave to wire efficiencies were around 20%. They investigated using the membrane directly as the wave activated body: water on one side, air on the other. They encountered difficulty in irregular seas knowing when to charge and discharge. Max Carcas asked whether they’d looked at the economics of PolyWEC. David answered that they had, and it had compared favourably to Pico. However they were redoing it as they had used data when Pico was not performing well.

Peter Stansby gave updates on StepWEC; Lars Johanning described the use of hydrophone recordings to monitor moorings; and Adi Kurniawan gave updates on the experimental testing of several devices with deformable primary oscillators. Adi described a compensation system to overcome the difficulty in Froude scaling both pressure and volume in systems with air transmission systems.

Policy

Henry Jeffrey described SuperGen's strategic and policy work. Ocean Energy Europe is a network for promoting ocean energy. One of its initiatives is TP Ocean. TP Ocean advises on priorities for the H2020 programme, and runs the Ocean Energy Forum , which is working on a strategic roadmap. Henry is coordinating the European Energy Research Alliance’s (EERA) joint programme on Ocean Energy. This is something like virtual research institute, which aims to coordinate European ocean energy research along agreed priority themes, and to facilitate collaborations, via an annual event and themed workshops. The EERA works closely with OCEANERA-NET, which funds collaborative research on ocean energy. Henry also highlighted a new report on levelised cost of energy.

Henry Jeffrey also presented for WES. He described WES’s aim to bring cold industrial logic to the innovation process. Plans for calls on control and station-keeping have been put on hold until advice is sought for these calls. Henry said that within a few weeks there would be a closed (invitation only) call to tender for landscaping reports on control. He said it was up to the wave energy community to convince WES that stationkeeping research was a priority.




Image credits


Anas versicolor by Wikipedia: https://en.wikipedia.org/wiki/File:Anas_versicolor_versicolor%29-_Macho.jpg

'Curved wakes in horizontal flow fields' is reproduced with kind permission, being Fig 10 in Masters et al, 'A Comparison of Numerical Modelling Techniques for Tidal Stream Turbine Analysis', Energies, 2015, 8(8)