Harnessing tidal power around the UK's coast has so far been limited by the cost of the large dams and barrages required and unpredictable results.
A British company, in conjunction with Oxford University researchers, believes it has devised a way to overcome this obstacle by creating a new type of horizontal axis turbine that can be used underwater at depths of up to 30 meters, at an economical cost.
Conventional propeller-type turbines are like underwater wind turbines and the number of suitable sites for them are vastly reduced by the size of their large blades, limiting their use to waters at least 30 meters deep. The THAWT (Transverse Horizontal Axis Water Turbine) technology, by contrast, is designed for deployment in shallower, lower velocity, tidal waters.
Developed by Oxford University's Department of Engineering Science in conjunction with Kepler Energy, THAWT uses a stressed truss configuration with carbon composite hydrofoil blades.
Put simply, as the water flows past the fence a head of water is produced that increases the turbine's efficiency. The phenomenon is called a 'blockage' of the turbines and gets larger in proportion to the length of the fence.
Guy Houlsby, professor of civil engineering at Oxford University, says their design is an improvement on the vertical Darrieus wind turbine used in some turbine systems.
"The original Darrieus turbine has blades that are parallel to the axis of rotation, and that means that the loads in the blades are carried entirely by bending of the blades. That results in very high stresses," said Houlsby. "The re-design that we've done changes the blades so that they form this triangulated structure, and that's a very stiff and very strong structural form. And that means that the loads in the blades are principally carried by axial forces and that means that the stresses are much lower."
Kepler says their design has minimal moving parts in the water, while its generator and other electrical equipment are installed in dry columns, increasing their reliability, efficiency, and shelf life. The generating units consist of two sets of blades sitting on three columns with a single generator in between.
"The water flows at right angles to the axis of the turbine so, as the turbine turns, lift is generated by these blades," explained Houlsby.
Peter Dixon, chairman of Kepler Energy, says the patented turbine is the most efficient yet designed. According to Dixon, "the rotor is suited to lower velocity, shallower waters, which are areas where you can't put conventional axial flow turbines, because to make them powerful enough you need to make them very big in diameter and if you make them very big in diameter they're going to stick out of the water. So this turbine goes places other turbines cannot and generates electricity at an economical cost."
A one kilometer (0.6 mile) long tidal energy fence, capable of creating 30 megawatts at peak performance, has been proposed for installation in the Bristol Channel, a major inlet and river estuary between England and Wales. The project would cost an estimated £143 million ($224 million) and could be operational by 2021. Some experts believe that if the tides flowing in and out of the channel are correctly harnessed, they could supply up to five percent of the energy requirements of the UK.
"The design we have at the moment and the proposition we have at the moment is to put a tidal fence, which is a chain of these turbines in the Bristol Channel, and if we can build up to say ten kilometers worth, which is a very extended fence, you're looking at power outputs of five or six hundred megawatts and just to visualize that that's like one small nuclear reactor's worth of electricity being generated from the tides in the Bristol Channel," said Dixon.
Dixon says that THAWT has a series of other advantages. It's hardy, with each rotor having a 25 year design life and the columns and electricity connectors 100 years. It could also have positive knock-on effects for Britain's carbon fiber manufacturing industry. He says that THAWT's electrical output would be equal to that of a nuclear power station, without any of the risk, and because the blades move at a relatively slow speed there is no danger to fish swimming through the fence.
In addition, it could be used in conjunction with a separate, less productive, tidal lagoon system, consisting of circular retaining walls embedded with turbines which capture the tide's energy.
"A lagoon generates maximum at the turn of the tide when a tidal turbine like ours is actually static, not turning, so together they constructively interfere, as the scientists would say. Very effective....it's very advantageous to have both," said Dixon.
Dixon says production costs will be between £100 and £130 ($157 USD to $203 USD) per MWh for the 10 kilometer fence proposed for the Bristol Channel in the future, markedly cheaper than lagoons.
A scale prototype of THAWT has been stress tested successfully twice at Newcastle University.
The developers say the system could be used in waters off France and many Asian countries, such as Japan, China, the Koreas, Indonesia, India, and the Philippines.