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Barriers to tidal energy are not necessarily mechanical

01 September 2014

Investment tidal power - one of the most reliable and predictable of renewable energy sources - continues despite some notable setbacks. Anthony George looks at the viability of tidal power generation schemes.

One thing is for sure: energy costs are rising inexorably and, as fossil fuel based resources become scarcer, renewable sources of energy will become increasingly important. But if renewables are to play a major part in the energy generation map of our medium-term future, then they have to be reliable. If they are not reliable, then costs go up and efficiency drops, which compounds the problems of viability and investment.

Tidal power sources look like a great alternative to vastly expensive nuclear power generation and (to some, it must be said) visually unappealing wind farms and fields of solar arrays. True, some significant players, apparently well advanced with the development of their prototype devices, have disappeared off the scene, but several major global engineering companies have stuck with it and are making significant investments in marine technologies - Kawasaki and Hyundai prominent among them.

The UK is clearly a leading centre for development, if the scale of recent acquisitions is anything to go by. Siemens bought UK based Marine Current Turbines (MCT) in 2012 and Alstom recently purchased Tidal Generation Limited from Rolls-Royce. Policy driven or not, they are certainly investing in the technology. Island nations with strong (and accessible) tidal currents clearly have a vested interest.

As the cost of mainstream energy rises, so tidal installations become more financially attractive, and while government policies are formulated to promote the installation of alternative power sources, either by raising feed-in tariffs or implementing schemes such as the Renewables Obligation, designed to encourage generation of electricity from eligible renewable sources in the United Kingdom, then there is an incentive, but still no great market pull. Progress is largely driven by technological advances made by the device developers themselves.

For the utility companies, this is a medium to long-term investment, so it falls to the engineering companies to lead the way; projects such as the offshore technology initiative in Orkney, Scotland, certainly help.

Reliability and robustness are critical and, despite the advanced site modelling techniques employed prior to installation, many companies are finding that conditions out in the open sea are harsher than predicted. Although there is no dominant turbine technology at this early stage, a number of the furthest developed designs on test are similar in principle to wind turbines, employing a traditional nacelle and horizontal-axis rotor arrangement.

Different configurations are being tested with twin or single rotors; some are fixed to the seabed others attached to floating rigs. Methods of power transfer include pumped hydraulics as well as on-board generators with the generation equipment and gearing contained within the nacelle.

Viability of an installation site is dependent on many factors and not limited to the suitability of the water flow rates; sea depth, proximity to population centres, infrastructure support and transport to site and, crucially, installation costs. All of these factors add to a Levelised Cost of Energy model (‘LCOE’ - the price at which electricity is generated taking into account many of the contributory cost variables) against which viability of different devices may be compared.

The basic principles of most undersea turbine designs are relatively familiar, based as they are on proven technologies from within the wind turbine industry. Factor in the operating environment and everything becomes an order of magnitude more complicated: installation requirements, accessibility, dynamic sealing, wet connection of power lines, not to mention the huge mechanical forces on the devices.

Many of the leading developers have experienced blade failures of some kind in the development and testing phase, so scale-up and full size testing are taking their time to progress into full commercial size installations.

The role of power transmission components
One company that has worked closely with tidal power generation system developers - Altra Industrial Motion - has collaborated on several turbine brake-system designs, ultimately destined for early arrays that are being funded by utility companies, though at present they remain beta sites rather than full-scale commercial farms.

The typical products Altra supplies to this industry, such as torque-limiting clutches and safety brakes, were designed-in with the primary requirements being to bring the turbine’s rotor to a controlled stop or to hold it during installation or maintenance operations.

Additionally, they perform an emergency stop function preventing over-speed of the rotor should flow velocity exceed safe levels, or in the event of grid loss (and a subsequent drop in generator load).

For the associated brake systems, Altra can supply a range of dry and ‘wet’ systems; Twiflex caliper disc brakes traditionally work in the dry environment of a sealed nacelle, but may be developed for use fully exposed to seawater. Wichita multi-plate clutch designs may be configured to act as a torque limiting device, protecting the drive train from torque spikes induced by extreme weather conditions, while similar ‘wet’ brake designs utilise oil-in-shear technology to develop torque. 

Apart from being inherently fully sealed, the great advantage afforded by these units is the avoidance of dry friction rubbing, so wear (and the consequent creation of dust and debris) is negligible. This is essential in order to achieve the 20 year or more operating life requirements that customers are requesting. Long service periods of five years or more are also typical, since the cost of physically visiting underwater installations (or retrieving them to a suitable dock) is substantial.

Floating devices have an advantage here, but still require the same technology to control peak loads and ensure maintenance is possible in controlled conditions. The brakes hold the rotor during installation or maintenance, or ‘park’ the turbine to prevent an over-speed situation in the event of predicted periods of high flow velocity. They are further rated to bring the rotor to a controlled stop in an emergency.

The brakes deliver a fail-to-safe operation that relies on a spring applied and, in most cases, hydraulically released principle; the electro- hydraulic power pack is tied into the system plc controller, if the unit loses grid connection or speed control (or even if the control system loses power) the brakes apply in a controlled many to safely stop the turbine without inducing excess loads in the drive.

Currently, the largest ‘wet’ brake systems are capable of providing dynamic braking torque up to 3 x 106Nm. As indications suggest turbines will need to be rated at 2-3 MW to become commercially competitive, it is inevitable sizes will increase and this technology - proven at the 1MW scale - will be there to address this future demand.

Anthony George is with the Heavy Duty Clutch and Brake Division of Altra Industrial Motion 

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