Electronics Weekly spoke to Trident CEO Steve Packard to find out how these monsters of the sea work, and what they can do.
In principle, they are simple: the movement of a long cylindrical multi-pole magnet through a wound armature generates power every time one moves with respect to the other.
In use, the idea is that companies building wave-powered generators – known as ‘wave energy converters’ or WECs – will buy suitably-sized PowerPods to build onto the floats and other parts that assemble into their wave machines.
Why linear electromagnetic generators?
“Most of current early-stage wave energy demonstrations have used hydraulics. This is proven technology, and reasonably cheap, but putting hydraulics into a marine environment with its salt corrosion makes them prone to failure,” Steve Packard of Trident Energy told Electronics Weekly. “Trident has been working on a linear generator because it is simple and lower cost, with fewer moving parts.”
And how big?
Part of the design strategy behind PowerPods was to make them modular, from a kit of standard parts that can be assembled to make different sized generators for different wave environments.
In the North Sea, for example, which is a fairly benign wave environment, said Packard, a 1.5m stroke would be suitable, obtained by combining a 3m long magnet assembly with a 1.5m armature.
In general, four of these would be installed together, resulting in 6m of active armature. Another example Packard mentions involves a submerged absorber where pressure differential is used to increase available displacement (and therefore velocity) so that as much as 4m of movement is available to a suitably long PowerPod.
Either part can be the moving element – the heavier (two tonnes in this case) magnet stack or the cable-connected armature. Which bit moves depends on the WEC design. Extra moving mass can be an advantage in wave energy, said Packard – more of this later.
To keep it cost-effective, PowerPod is designed around ferrite, rather than rare-earth, magnets.
The design is tubular, and was recently improved through work with 42 Technology – both companies are based in Cambridgeshire.
As a result of practical design and theoretical work using Triden’s mathematical models, PowerPod II generates 50% more power that the original, is ‘fully marinised’ so that it can work at sea for years on end, and has a considerably stiffer magnetic stack so that it can work horizontally without the stack sagging and touching the armature.
“The design scales very nicely when we make longer or shorter versions,” said Packard. “We could re-design it get more kW/tonne, but manufacturing costs make it prohibitive.”
Getting some power from waves is relatively straight forward with such a generator, but getting as much power as possible requires more thought, which is what Trident has put into four-quadrant active control electronics which can both take power out and put power into a PowerPod.
The disadvantage is that this requires a local temporary energy store – through a DC bus, usually from local batteries.
The advantage is that the moving part, usually the magnetic stack, can be given arbitrary artificial spring and damping characteristics that suit individual waves.
“You don’t want the float to follow the wave’s motion. If you can provide a damper and spring action, it enables you to generate more power per metre of wave-front because you can put more force against wave which delivers more power per motion,” said Packard. “And you need to keep the float resonant with waves to capture high proportion of wave energy.”
He added that “hydraulics just don’t switch quickly enough to be able to do this”, and that to control it, “you need an algorithm that reacts in real-time because waves are not predictable, and that is where the clever bits come in”.
As part of a project funded by Highlands and Islands Enterprise through Wave Energy Scotland, the University of Warwick has been working on the four quadrant controller and the University of Victoria in Canada has been developing the control algorithm and modelling predictive control. “It’s going really well,” said Packard.
Trident’s concept is that machines are designed for the sea conditions present at the intended location.
The customer would provide what is known as an ‘occurrence matrix’, which describes what size waves are present at what times through a year, and the firm constructs the most cost-effective generator from its standard kit of parts.
“It comes down to sizing to give lowest levelised cost of energy for duration of operation including maintenance – for example, over 25 years – and not necessarily the highest efficiency or highest power output,” said Packard.
As an example application, Packard sees PowerPods providing energy during the construction of off-shore wind farms, and then continuing to produce power for years after while acting as an insurance policy should the wind turbine have to cold-start.
“Wind farms, during installation, need electricity for things like navigation lights, power tools, air conditioning and machine start, and they need it for about six months before the under-sea power cables arrive,” he said. “Currently power comes from diesel generators, but these are very expensive to fuel.”
Expense is not only the cost of fuel burned, he said, but, as it is delivered through umbilical pipes from boats, the cost of moving it, protecting the health and safety of the workers, and protecting the environment from spillage.
By installing power pods on the wind turbine base, he claims diesel use can be “reduced to 30, 20 or 10% of time”.
Pods would be connected to the UPS (un-interruptable power supply) that comes as part of a wind turbine package, and the UPS battery would provide the wave-by-wave temporary energy store needed by the four-quadrant controller.
The power system would also arbitrate between sub-sea cable, battery, wind turbine and PowerPod. “Wind farms consume power when there is no wind – the air conditioning uses power – and wind and waves are typically out of phase,” said Packard. “Wave power can keep the UPS constantly topped-up for situations when the grid is out and the wind is low, and it can move the turbine – you don’t want turbines stopped for extended periods.”
Trident sees its generators attached to the legs of the ‘jackets’ – four-legged frames derived from the oil and gas industry (see photo) – that are used to support larger wind turbines and those in deep water. “There is plenty of space inside footprint,” said Packard.
One 30kW WEG could displace diesel usage during construction, and one per leg could deliver an average of 40-70kW for years, he said.