Anticipated milestone for tidal power
The ocean plays a key role in UK renewable energy strategy. 33 gigawatts of offshore wind capacity is targeted to be installed by 2020. Tidal power may soon become a significant part of the UK renewable energy portfolio. I'm guessing that both renewable energy advocates and the environmental community will be watching the tests of a new tidal technology prototype scheduled for the waters of Wales next summer with great interest. (GW)
Tidal turbines: The three-headed shape of things to come?
July 29, 2010
Plans are picking up pace at Tidal Energy Limited (TEL) to install an almost full-scale version of its DeltaStream tidal turbine in the fast-moving waters of Wales’ Ramsey Sound, with deployment of the three-headed device slated for late next summer.
Starting from a small-scale prototype first tested in 2002, the Cardiff-based company has managed to translate seed capital — secured in 2007 from renewable-energy development funds Eco2 and Carbon Connections — into a 1.2MW concept now ready for manufacture.
“Most of the last three years has been spent developing the technology to the point where we are content to commit to manufacture,” states TEL managing director Martin Murphy.
“We’ve taken a holistic approach to the design — it is not just the device, it’s the manufacture, the assembly process and the installation-deployment-recovery concept.”
TEL’s 300-tonne DeltaStream device is made up of a trio of tidal turbines mounted on the corners of a triangular gravity-base foundation.
At full scale it will use 15-metre-diameter fixed-pitch carbon-fibre-composite rotors to reach its nameplate generating capacity, but 12-metre-diameter models will first be installed at its year-long trials.
“Because there are so many unknowns in what is a harsh environment, we are taking the conservative approach of installing the smaller rotor to start,” states Murphy. “The engineering is rugged, it’s robust, it’s agricultural — it has to be.
“Even with a smaller-scale version, the output generated would leave us with too many uncertainties, so starting with a full-scale frame but a reduced diameter turbine — which has about 65% of the capacity of the full-rated turbine — will move us ahead and get us the answers we need for commercialisation.”
The 12-metre rotor will be swapped with a 15-metre one “if all goes smoothly”.
Marrying wind turbine and ship propeller technology, the machine’s geared 400kW turbines have been designed for rotational speeds of under 10rpm in a mean spring tide, with a blunt-blade design to shed excess power in order to ensure consistently high energy conversion.
In line with a philosophy of using off-the-shelf technology wherever possible, TEL opted for asynchronous gearbox-driven generation, with gear hubs and shafts filled with bio-degradable oil, and the flow of seawater through the nacelle working as a coolant.
The turbines are bi-directional, pivoting with the turning tide.
“The current speeds [of the tide] are around six knots [three metres per second] so it is not as fast-flowing as some of the areas up in the Pentland Firth,” acknowledges Murphy.
“But it is perfect for our device. Still, a turbine like this is more than just about extracting the laminar [turbulence-free] flow from the tidal stream; it has to be built to withstand the effects of the surface waves and the turbulence associated with high tidal flow too.”
The lightweight, tubular steel foundation is built around an equilateral triangle frame measuring 38 metres along each side.
This design gives the Delta-Stream a low centre of gravity, making it possible to deploy the machine without a positive anchoring system, while meeting structural stability requirements needed to avoid overturning and sliding in the current.
According to a case study from backer Carbon Connections, each machine could flow an average 5.5-7.7GWh over 20 years, power enough for 1,000 homes.
Ramsey Sound was selected as the pilot site after TEL was told that all the slots at the European Marine Energy Centre in Orkney, Scotland, were full.
But while the sound meets all of TEL’s criteria — including being free from trawling and commercial shipping — it has a delicate ecology that has prompted a far-reaching environmental impact assessment.
Consideration is being shown to local seal and porpoise populations, while the export cable route avoids the area’s Common Mussel beds.
“This is a 12-month trial. We want to learn how the device operates, what its performance characteristics and envelope are, and at the same time closely monitor what impact it has on the environment,” says Murphy.
“After the testing period, we are committed to returning the site to its ‘as before’ condition.
“The development of our site has been as important a part of our technology as the development of the technology proper.”
TEL’s environmental surveys are “continuing now even after we have put in our environmental statement and they will continue when we are in the water and when we come out”.
Though power generation during the upcoming testing phase is “not for commercial gain”, notes Murphy, the grid-connected device would “greatly contribute” to the energy demand of a community like St David’s — Britain’s smallest city, where the grid will take the generated power.
If all goes to plan, the flagship DeltaStream will be lowered by crane barge next summer through 31.5 metres of water onto its chosen spot on a shelf of flat bedrock, with two sinkers employed to aid in anchoring the structure’s lifting bridle and electricity cable. Power will flow ashore to grid connection via an 11kV subsea line.
Having been granted the acreage from the Crown Estate, TEL is now in the “final stages of determination” with the Department of Energy and Climate Change for an operating licence at the installation site.
The company’s project timeline calculates 15 months from getting the go-ahead from the UK authorities through to start-up of the first DeltaStream turbine.
“While the device is very simple, the execution of the wider project — including the dealing with the regulators — is necessarily quite complex,” stresses Murphy.
Some 13 contracts covering fabrication of the component parts, turbine assembly and system installation are waiting for TEL to push the button. The company expects to have all contractors signed up by the end of the year.
“We have a very clearly prioritised programme addressing what needs to come first — we have long-lead items such as the subsea cables, gearboxes and generators that we would like to get ordered,” says Murphy.
“This has become very much a Welsh project run by a Welsh company.
“As it has emerged, the technology is now looking like a pioneering concept.”
Darius Snieckus (firstname.lastname@example.org)