Industry Insight
Evolution of tidal turbines imminent
21 December 2011
Tidal Today takes a snap shot of current tidal turbine designs and asks the experts how cost and funding impacts will change the shape not only of the industry, but the turbines themselves.
By Elisabeth Jeffries
While it is too early to talk about first and second generations, the tidal turbine is beginning to evolve. Sector leader Marine Current Turbines’ craft, Seagen, is likely to look rather different in a few years’ time.
Peter Fraenkel, the turbine’s designer and company Chief Technical Officer, envisages a third rotor within three years and 20 metre diameter rotors within four (compared to 16 metres now). This, he calculates, will raise the power produced by the turbine to 3.2 MW, an increase of over 125%.
Fraenkel’s reasoning is relatively simple; it is less about the rotor length in itself than the area swept by the rotors. “Twice the swept area [metres squared] literally yields twice the energy,” he says. Longer rotors, of course, create a wider swept area.
Size matters
But MCT’s horizontal axis turbine rotors are unlikely to exceed 20 metres, he indicates: “Rotors become disproportionately more costly to build when they exceed about 18 to 20m diameter.” The next step will be to add a rotor to the future turbine, known as Seagen U, which could be operating in 2015.
“Each and every installation needs to be as powerful as possible to overcome the high fixed overhead costs...a tidal turbine in a sensible velocity such as about 2.5 m/s needs 300 square metres of rotor per MW, so a device with a single rotor can barely produce 1 MW,” explains Fraenkel.
Hence, assuming project finance for an array is forthcoming, MCT will within a few years be building multi-rotored turbines.
MCT’s approach is quite different from that of Pulse Tidal, which has also considered the issue of increasing power generation from individual turbines, essential if the sector is to become commercially viable. The company’s turbine configuration, known as an oscillating hydrofoil, is suitable for shallower waters.
“The horizontal axis turbine is limited in scale in waters less than 50 metres deep, where you can’t get the rotor in the water,” argues Bob Smith, Pulse’s chief executive. This, he suggests, could restrict their development.
Design flexibility
Pulse’s turbine is a smaller device, which is also undergoing design changes following tests of the 100 KW prototype in the river Humber, Eastern England. The tests have indicated the need for a fully submerged turbine, which will entail several alterations to the design.
“The whole system will look different because of subsea deployment. We have spent a lot of time working on the power train, which is completed. The next machine will be about deployment, recovery systems and maintenance” says Smith.
Funding the next stage
If everything goes to plan, the company will install a 1.2 MW device in Scotland. It has already secured £7m from EU sources for this £20m project and is looking for further investment for the next stage. “We are deep in negotiations with a few big OEMs,” states Smith.
Irish company Open Hydro’s turbine is also going through a transition as there are plans for an array in Brittany, France. This is also a horizontal axis turbine. The company’s first turbine was deployed in the Bay of Fundy, Canada in 2009 and subsequently removed in 2010.
“The second, a 16m 2.2 MW rated Open-Centre Turbine was deployed in Paimpol, France in September 2011,” reports Sue Barr, Environment and External Affairs Manager.
The company’s plans indicate the turbine was due to be tested for a two month period over the autumn as part of a project funded by EDF before being retrieved. At the same time, ongoing testing of the turbines continues at EMEC.
Open Hydro’s design is quite different from MCT’s. “The Open-Centre Turbine is a bi-directional, permanent magnet generator designed from first principles for the tidal marine environment,” states Sue Barr. It has only one moving part, the rotor.
Environmental consideration
This, she explains: “reduces the likelihood of component failure in these challenging environments. The turbine is also unique in its design by having an open centre; this aids the turbine both hydrodynamically and reduces the likely impact on marine mammals and fish.” OpenHydro closed a significant funding round in 2011 and is not looking for seeking additional investment.
Verdant Power’s system is different again, though a horizontal axis turbine – but a smaller one than MCT’s and suitable for lower current velocities. “Our fifth generation turbine system will be submerged during Q2 of 2012 in New York’s East River.
It will be the beginning of our commercial build-out of our second grid-connected array of tidal turbines. This build-out will result from receiving a pilot commercial license from the US Federal Energy Regulatory Commission - the first in the world to be issued,” asserts Trey Taylor, Verdant Power’s co-founder and president.
Unlike MCT, the turbine’s rotor does not shift when the tide turns. “We use a fixed-pitch, constant speed rotor. It makes it easier and less expensive to scale the entire system and it has few parts to break or corrode as a result of intrusive salt water,” states Taylor.
Market evolution
The tidal industry will see its largest growth curve in 2013, according to industry executives and analysts. This curve will also prove as a crucial learning stage for those companies fortunate enough to have prototypes in the water and even more so for those that have commercial capabilities to provide power.
But trial and error is part of the process, therefore, those companies that possess flexibility and forward thinking within their designs, funding options and management will retain leadership positions and help develop the future shape of the tidal energy industry in a post-2013 marketplace.
