RENEWABLE ENERGY INDUSTRY FOCUS THE TIDE IS NIGH
The cost of energy generated by tidal power is still more expensive than offshore and onshore wind, and solar arrays. However, investment is still going into tidal power, one of the most reliable and predictable of renewable energy sources. Here, Anthony George of Altra Industrial Motion offers some analysis on the real world viability of these schemes
T
he cost of energy is going up as fossil fuels dwindle. Therefore, renewables
will become ever more important. If renewables are to play a major part in the energy generation map of our medium term future, then they have to be reliable. Tidal power sources look like a good alternative to the vast costs of nuclear power and the visual impact of large scale onshore wind and solar projects. While some significant players, apparently well advanced with the development of their prototype devices, have seemingly disappeared, several major global engineering companies have recently invested in marine technologies. The UK is clearly a leading centre for
development as Siemens bought UK- based Marine Current Turbines (MCT) in 2012 and Alstom recently purchased Tidal Generation from Rolls-Royce. Policy driven or not, they are certainly investing in the technology. Island nations with strong (and accessible) tidal currents obviously have a vested interest. As the cost of mainstream energy rises,
so tidal installations become more financially attractive, and while government policies are retained that promote the installation of alternative power sources, there is still no great market pull. Progress is largely driven by the technological improvements of the device developers.
ENGINEERING TO THE FORE This is a medium to long term investment for the utility companies, so it is falling to engineering companies to lead the way - projects such as the offshore technology initiative in Orkney, Scotland, certainly help. Altra has worked on several turbine brake system designs, ultimately destined for early arrays that are being funded by utility companies, but they remain beta sites rather than full scale commercial farms. 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
Left: Altra Industrial Motion has been involved in engineering a number of offshore energy projects
connection of power lines, and 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.
than predicted. 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 hold it during installation or for maintenance purposes. 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). Although there is no dominant turbine technology at this stage, a number of the furthest developed designs on test are similar 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 while others are attached to floating rigs. Methods of power transfer include pumped hydraulics as
well as onboard generators. Viability of an installation site is
dependent on many factors such as water flow rates, sea depth, proximity to population centres, infrastructure support and transport to site and installation costs. All of these factors add to a Levelised Cost of Energy model (LCOE - the price at which electricity is generated) against which viability of different devices may be compared. The basic principles of most undersea
turbine designs are relatively familiar. Factor in the operating environment and everything becomes more complicated, factoring in installation requirements, accessibility, dynamic sealing, wet
/AUTOMATION
TIDAL APPLICATIONS For the associated brake systems, Altra can supply a range of dry and ‘wet’ solutions. Twiflex caliper disc brakes traditionally work in the dry environment of a sealed nacelle, but may be developed for use fully exposed to sea water. 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-
Above: Altra can supply a range of dry and ‘wet’ solutions - Twiflex caliper disc brakes traditionally work in the dry environment of a sealed nacelle, but may be developed for use fully exposed to sea water
Left: 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
Right: the typical products Altra supplies to this industry include torque limiting clutches and safety brakes
Altra Industrial Motion
www.altramotion.com T: +1 815 389 6227
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. Currently, the largest ‘wet’ brake solutions are capable of providing dynamic braking torque up to 3MNm. As indications suggest, turbines will need to be rated at 2-3MW to become commercially competitive so it is inevitable sizes will increase and this technology - proven at the 1MW scale - will be there to address this future demand.
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