Typing “tidal turbine” into the world’s favourite search engine generates 500,000 hits in less than a quarter of a second and throws up a range of devices, some developed and proven, many no more than speculative artist’s impressions.
Using the same terms in www.ScienceDirect.com generates just over 1,000 possible scientific papers on the subject. Yet the number of devices that have actually been deployed in sea trials is less than 20.
This demonstrates that tidal energy is an attractive notion; all that is required is to take a marinised wind turbine, drop it in the sea at the right place and plug it in. So why aren’t our tidal current sites producing megawatt-hours on a daily basis? As ever with new technology, it’s not that simple.
The resource is reliable – very reliable. Using existing tidal height data, we can forecast the time-bound velocity of a tidal current at any chosen UK site for years ahead and quantify the energy output from a turbine of given characteristics placed in that current.
However, the assumption that tidal currents are well behaved streams that stop and start with the Admiralty charts is incorrect; many of these currents become shear flows at slack water, with an ebb current running at the surface and a flood current running on the seabed, or vice versa.
Some tidal currents rotate at slack water, and travelling vortices are also likely to be a major problem even when the flow is established. A travelling vortex can have its rotational axis at any orientation, depending on the structure that created it, and, along with shear flows, presents a significant challenge to turbine designers – that is, how to keep the blades on.
Tidal currents are already performing a useful function without our interference; they provide transport for larger organisms and distribution mechanisms for food particles and sediments.
These distribution mechanisms support the food chain all the way up to the diving birds that are known to fish at particular states of the tide and in particular fluid-flow regimes, often on the interface between fully turbulent and laminar flows.
The correlation with wind turbines doesn’t work very well, either. Whereas wind turbines are skimming energy from the atmospheric boundary layer, which is constantly re-energised from surrounding flow, tidal currents are bounded by geology on three sides and atmospheric pressure at the free surface.
Energy extracted from a tidal current in one place is not available downstream, and blockage effects created by too many turbines could lead to changes in the local flow patterns. This leads to the apparently paradoxical situation that deploying more tidal turbines in one site could lead to less energy being extracted.
The turbine itself is still open to question as to which embodiment is best for tidal installations.
The horizontal-axis three-bladed version is attractive because it has become the technology of choice for the wind industry.
There are difficulties, though, for horizontal-axis devices in tidal currents, partly caused by having to track the flow direction, but mainly due to depth.
Many of the initially attractive sites are relatively shallow (about 30m) and a 1MW rated turbine will need to be about 20m in diameter, which does not leave sufficient distance from the free surface to the blade tips for navigational or hydrodynamic reasons.
In the same location, a vertical-axis turbine can be 30-40m diameter with blades of 10m in length, generating more power and avoiding the need for alignment with the flow direction. But vertical-axis machines are less efficient and are poor self-starters.
In order to extract power for a moving flow, any device must be anchored, otherwise it just drifts with the flow.
Installation of tidal devices is a job that Aberdeen PLC should be very good at, but many companies with relevant experience are not coming forward to develop the necessary technologies and techniques.
The difficulty lies in the financial return for the work; tidal turbines will only ever generate kilowatt hours (kWh), not barrels oil equivalent, so installation of tidal devices has to be quick and cheap, meaning that the potentially appropriate technologies used in the North Sea are simply not attracted to tidal energy as an income generator.
Also, the North Sea, in many areas, has a thick sedimentary layer into which strong anchor systems can be deployed, but most tidal current sites are, by their very nature, scoured clean, leaving only bedrock or tight-packed cobbles as an installation medium.
Installing a steel pile into a drilled socket is probably the best absolute engineering solution, but it is just too expensive. Gravity anchors quickly become very large for a turbine of any significance and require a substantial boat to handle them – again, expensive. Potentially viable hydrodynamic installation devices have been proposed and tested, but still require development.
All tidal devices and their associated structures are susceptible to entrained debris within the flow, kelp being a particularly significant threat. By their nature, tidal devices are optimised for maximum lift and minimum drag; a few bits of stray kelp will upset many months of careful calculations, while a substantial build-up could endanger the entire machine.
So how do we address these significant difficulties facing the tidal energy “industry”?
No industry has ever started out big and perfect; Henry Ford could not have built an F1 car; Charles F. Brush managed to generate only 12kW from a 20m tall wind turbine in 1888, and 1970s mobile phones weighed 6kg and needed a suitcase to carry the batteries – how cool was that?
The tidal energy industry needs to get out of the lab and into the water, not with perfect 2MW devices, but with simple 500kW devices.
Small arrays of cheap, reliable tidal devices operating day in, day out to the point of tedium will do far more for technical development and investor confidence than the Saltire Prize will.
Don’t get me wrong, the Scottish Government’s support for marine renewables is most welcome, but a few calculations show that to achieve the prize with a tidal array requires an installed capacity of about 20MW to operate almost without failure for two years.
Depending on whose numbers are utilised, 20MW will cost about £60-100million installed. Anyone who has access to that sort of money does not need a prize.
Dr Alan Owen is director, Centre for Understanding Sustainable Practice (CUSP), at The Robert Gordon University, Aberdeen
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