A UK-developed tidal turbine that works at maximum efficiency at both high and low tide could be used to generate power from tidal rivers and inland waterways, says Cornwall-based FreeFlow 69. However, development could switch to the US if support from the UK is not forthcoming, the company warned.
The turbine, called the Osprey, is a vertical axis design with its gearbox and generator above the water level. ’It’s equally efficient at all stages of the tide or river height, so it’s at maximum efficiency at low and high water,’ said FreeFlow 69 director Pat Cooke.
Although the design is confidential, Cooke said the length of the rotor is crucial, rising from the sea or riverbed to the water height at the top of the tide. ’It’s long enough that it’s always in the full depth of water. When the tide goes out, it leaves the top of the turbine turning in air, which doesn’t affect its efficiency.’
The Osprey turns in the same direction whether the tide is going in or out, allowing it to generate power 24 hours a day. In contrast, horizontal-axis turbines, which resemble propellers, only turn in one direction so if they are to generate on flood and ebb tides, they have to be swivelled around or the blades have to move along their axis. ’That’s why we think it’ll be more efficient than horizontal axis turbines,’ said Cooke.
This is not Cooke’s first entry into the tidal energy sector — previous concepts have included the Ocean Hydro Electricity Generator (OHEG), designed to use both the kinetic energy of the tidal water and the potential energy of the change in water depth as the tide comes in.
’With the OHEG, at the time, we were going to use propeller-type tidal turbines, because we thought they were fully developed and viable, but it turned out they weren’t,’ said Cooke. ’So we designed our own vertical axis turbine.’
Osprey is the first of the company’s concepts to make it to prototype stage. Derbyshire-based stainless steel fabricator Able Engineering — of which Cooke is also chairman — has built parts for the prototype system, which was then assembled for preliminary testing at FreeFlow 69’s base in Fowey. ’Able Engineering has been established since 1979 and we’re designing and building things, mainly stainless steel pressure vessels, every day,’ said Cooke.
The companies are now at work building a prototype to be tested in open water. ’We’re looking at the minimum size for the device to be around 1kW generating capacity, for a small river, and the prototypes we’re designing and building at the moment are going to be at least that size,’ said Cooke. The test prototype will consist of twin contra-rotating turbines mounted between the hulls of an aluminium catamaran. ’It will be lowered into the water and tested at various speeds. If everything goes according to plan, it will be validated later this year.’
The design is easily scalable, however. Cooke said the maximum size would be a 5MW generator, which would probably use multiple rotors. ’And because it’s modular, you can add on and on as long as you want, just building up the capacity,’ he said.
Cooke’s plan is to build two large units with 10MW capacity, one for the Bristol Channel and one for the Straits of San Francisco. ’They’re the obvious places to start. The Bristol Channel is the second most powerful place on the planet for this type of energy, but the UK government seems a bit slow to get things moving, so we’re looking at the States as well. The DTI turned us down twice,’ he said.
Cooke admitted tidal energy is still at an early stage, with a large number of diverse, horizontal and vertical axis rotor designs. ’It will be some years before it pans out which are the most efficient, then the industry will standardise on those; the others will die out,’ he predicted. And if the Osprey turns out to be the winner? ’It’ll be copied soon enough.’