A vast quantity of energy is present in the world’s oceans, driving marine currents around the globe. The development of tidal current turbines (TCTs) is an exciting field of research which will allow us to recover some of this energy. It is estimated that up to 34% of the UK’s electricity demand could be extracted from tidal currents.
As this is an immature technology, still in the development stages, associated costs are high. With this in mind it was thought that, in order to minimise the payback period of any capital investment, power output must be maximised in order to recover maximum revenue. In order to do this, both design and contextual parameters and their effect on power output needed investigating.
While there are a number of methods of tidal power extraction, it is thought that axial turbines will play a large part in the future of this technology and for this reason it was decided to focus upon the design of axial turbine rotors. The investigation into design parameters was manifested in the form of a parametric study of various aspects of blade geometry and a brief exploration of the effect of utilising different aerofoil profiles. The contextual portion of the study dealt with the effects of operating in the harsh marine environment: erosion; corrosion; and marine growth; i.e. surface degradation.
The performance of different rotor designs was evaluated using an advanced rotor design and analysis tool which utilised blade element momentum (BEM) theory. In order to complete the study it was necessary to add additional options and functionality to this programme. The main additions were some additional aerofoil profile options and the ability to use varying profiles along the length of the blade.
In order to add additional aerofoils to the code, lift and drag characteristics were required. Where possible, this data would come from experiment, but where experimental data was not available, an established aerofoil analysis programme (XFOIL/RFOIL) was used to generate the necessary information.
Following the analysis of power outputs, the impact any variation would have on revenues was then investigated. This was particularly important for the blade degradation analysis as preventative measures could then be looked at, costed, and their overall economic value evaluated.
Results indicate that degraded turbine blade surfaces result in significant reduction in power output and associated revenue. So much so, that even using the least durable preventative blade coating at the price of the most expensive coating would result in increased revenue.
Various aspects of blade design, e.g. via the use of blade twist, pitch, taper or by employing different aerofoil profiles can provide incremental increases in power output of up to 5%. A combination of these could be used to optimise performance.
It was also noted that, while there are many benefits to an axial turbine with two co-axial, contra-rotating rotors, increase in power output is marginal. However, results suggest that the power output of such a device may be less susceptible to the effects of blade degradation.
Finally, this group believes that while tidal current turbines hold great potential, they still require significant investment into research and development before large scale exploitation becomes commercially viable.
Future work within research and development will need to focus on the temporal aspect of blade surface degradation and how it impacts revenue recovery of tidal current turbines.