Friday 5 December 2014

Black Friday SuperGen

The newspapers described Fri 28 November as 'black'. Not quite sure why; something to do with the embedded carbon in all the impulse purchases? Anyhow, it seemed an apt description to me, given the black week suffered by marine energy. The world leading wave developer, Pelamis, was in administration, and the world leading tidal developer, MCT, had been put up for sale by their owners, Siemens. Rumours of others following in their wake were confirmed this week (Aquamarine 'downsizing'). This was the run-up to the SuperGen Marine annual assembly, so it was bound to be interesting.

First off, I'd like to say that this was the nicest SuperGen assembly I've been to for ages. It was something akin to attending a wake of a dear aunt: there was a sense of solidarity and empathy and vulnerability. No mention was made of the situation during the talks, but this was the main subject during tea breaks.  I'd originally got the impression from optimistic news reports that key staff at Pelamis were being retained by a smooth transition to 'Wave Energy Scotland', but this seems not to be the case, even though the administrators are seeking a buyer. There was some call for hope that the MCT situation may be better than that suggested by Siemen's announcement.

The talks about the research in progress were also very engaging. I noted that there was some work on new device concepts. There was also mention of two of my favourite topics: availability and cost of energy.

Research on Device Concepts

Peter Stansby (Manchester) described the M4 Wave Power project (moored, multi-mode, multi-body). Their approach was interesting: the design came out of criteria they'd set themselves: cost competitive, one power take off (for simplicity), floating (for easy deployment), etc. Their solution is a system with 7 degrees of freedom. It operates along similar hydrodynamic principles to Dexawave and StingRAY but there is an awareness that the heave and surge forces on the buoy can be transferred to the pivot and that the position of the pivot point is worthy of investigation.

Lancaster University is also working on a similar idea (Daniel Richardson presented a poster on the MA-PAWEC concept).

Deborah Greaves (Plymouth) described experimental work on a wave energy converter with a flexible primary absorber. It consists of one fixed (rigid) volume, surrounded by a variable (flexible) surface-piercing volume. Immersed it looks like a hot air balloon. This design came out of an investigation by Adi Kurniawan which compared several configurations using compressible volumes.

Stuart Brown (Edinburgh/Flowave) reported experimental work on a half-ducted tidal turbine, the Subhub. He was tickled pink about the Flowave tank winning the green energy awards the previous day.

Stephanie Ordonez (Edinburgh) described work on a vertical axis tidal turbine that pitched its blades every rotation.

Improving availability

Tim O'Doherty (Cardiff) discussed condition monitoring in tidal turbines. They were able to detect that one blade was (deliberately) offset in pitch angle by the harmonic content of the power signal.

Peter Stansby discussed CFD and experimental modelling of a minke whale hitting a tidal turbine. He also reported on the 'New Wave' project, where an extreme wave is embedded in a representative spectrum. There will be a publication soon showing how the biggest forces do not occur at the peak of wave elevation.

Anna Young (Cambridge) presented work on modelling tidal turbines. Fatigue analysis indicated that the frequency of gusts determined the amplitude of the response. A lumped-mass drive train model showed how internal dynamics of drive trains can smooth response.

Improving cost of energy

Lars Johanning (Exeter) described a novel mooring system that was tested on Fred Olsen's Lifebuoy device. He noted that 'peak tension is the key to reducing costs'.

Shayan Sharifi (Strathclyde) reported on the ReC-ASM project: reducing the cost of marine renewables via advanced structural materials. The focus was on tidal turbines. Tests of carbon and glass reinforced epoxy resin blades were presented. He noted that cavitation was exacerbated by the presence of salt.

The PolyWEC project. was described David Forehand (Edinburgh): a dielectric elastomer for wave energy converters. The idea is to integrate the power take off (method of damping) into the primary hydrodynamic working surface.

Deborah Greaves noted that using a flexible body as the hydrodynamic working surface could reduce cost of energy, as the lower stiffness (compared to a similar rigid body) allowed for a higher natural period (without increasing the mass).

Kate Porter (Strathclyde) explained the importance of power regulation: maintenance costs of a tidal turbine can be reduced by keeping the design velocity below the peak velocity. The aim is to reduce thrust loading and hence associated structural costs. She described two methods of passive rotor control for tidal turbines. The first method couples the blade axial bending to blade feather: the fibre orientation ensures the blade twists as it bends. The second method is to let increased rotational velocity change the angle of attack and hence performance coefficient. An increase in tip speed ratio is preferable as this decreases the generator torque.

Matt Folley (Queens) noted that specialisation discouraged a holistic approach to design of wave energy converters. For example, reactive control has less potential for improving performance of devices with broad bandwidths. Heaving buoys have narrow bandwidths, and thus researchers working with heaving buoys are often interested in reactive control. He compared this to a single degree of freedom wave absorber with power take off in surge: small devices have an even narrower bandwidth than an absorber of the same size operating in heave. However, for larger devices, the bandwidth is much broader. Thus reactive control is less relevant.

Paul Taylor (Oxford) reported on a wave resource study of annual (rather than seasonal) variations. A single number was used to represent the annual wave power. The study showed that this number was highly correlated to the North Atlantic Oscillation, which is sensitive to climate change. Using North Atlantic Oscillation data, they showed that there was very high annual variability: the minimum and maximum power varied by a factor of three, and even year by year there was a variation of up to two. This study was for deep water waves; shallow water waves are less variable. Over a ten year sliding window, there is less variation in the '100 year wave' than the mean power.

Image credits:

'Driving on water not recommended' by Bad Jonni:

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