Adaptive Composite Blades for Horizontal Axis Tidal Turbines R.F. Nicholls-Lee, S.R. Turnock and S.W. Boyd

1. Fluid Structure Interactions Research Group Adaptive Composite Blades for Horizontal Axis Tidal Turbines R.F. Nicholls-Lee, S.R. Turnock and S.W. Boyd Fluid Structure Interactions Research Group, Faculty of Engineering and the Environment rnl@soton.ac.uk Background Recently the world has seen a resurgence in the development of the renewables industry, essentially to reduce the environmental impact of energy generation such as that produced from fossil fuels, and create energy security for countries using renewable energy, decreasing reliance on energy imports. The oceans are an untapped resource, capable of making a major contribution to our future energy needs. They have the potential to equal or exceed wind energy in the make-up of our future electricity demands. In the search for a non polluting renewable energy source, there is a push to find an economical way to harness energy from the ocean. There are several different forms of ocean energy that are being investigated as potential sources for power generation. These include thermal energy, wave energy, offshore wind energy, tidal energy and ocean current energy, but these can only be applied if the technology can be successfully developed to exploit such resources reliably and cost effectively. Project Breakdown Aim This work aims to increase the efficiency of horizontal axis tidal turbines through optimisation of the blade design • Composite Blades • An adaptive textile composite = a structure tailored to exhibit desirable elastic deformation behaviour which is not necessarily proportional to the imposed load. E.g. bend-twist coupled beam (mirror layup, Figure 4). • Preliminary analysis indicated potential improvements of: • 2.5% increase in annual energy capture • 10% decrease in thrust loading • Half as likely to cavitate • Blade Design • Considerations: Cavitation, Yaw, Structural loading, Efficiency, Cost, Maintenance • Design tools – BEMT, CFD, FEA, FSI • Variable pitch blades vs wholly bi-directional blades. Figure 3 illustrates gains in power capture from differing blades Figure 2: CFD of a horizontal axis tidal turbine Figure 1: Tidal stream energy density around the UK[1] • Experimental and Numerical Structural Analysis • Basis central blade spar - carbon, bend-twist coupled, uniform planform, double box beam • Laser based experimental setup – Figure 5 • Finite Element Analysis • Results: • - Generally good correlation between experiments and FEA, especially at low loads (Figure 6) • - FEA sensitive to manufacturing accuracy • - FEA does not model progressive plywise failure or matrix cracking observed in experiments Figure 3: Fixed pitch, variable pitch and bi-directional blade comparison Figure 4: Preliminary analysis of adaptive blades • Adaptive Blade Design Tool • Blade input geometry developed and manipulated • CFD run – pressure loading and performance data obtained • Pressures applied to geometry in FEA – structural deflections noted • Deformed structure remodelled in CFD • Iterated until max bend is <10mm • Results concur with preliminary analysis: • - >5% increase in Cpow – Figure 7 • - >12% decrease in CT – Figure 8 Figure 6: Comparison of FEA and Experimental bend Figure 5: Experimental setup Conclusions and Further Work • Tidal energy is an immature industry with great potential, but to further it requires efficient and economic devices to be developed - blade design of HATTs is a key area for optimisation. • Different blade types have been compared, and it was found that adaptive composite turbine blades could improve device performance with preliminary results suggesting that a 10% reduction in thrust loading and 2.5% increase in annual energy capture is achievable with bend-twist coupled blades. • A design tool has been created for the development of passively adaptive bend-twist coupled HATT blades and further validation will be carried out Figure 7: Comparison of fixed pitch, preliminary adaptive and design tool adaptive blades - Cpow Figure 8: Comparison of fixed pitch, preliminary adaptive and design tool adaptive blades - CT References: [1] BERR, Atals of UK Marine Renewable Energy Resources: A Strategic Environmental Assessment Report, BERR, Editor. 2008. FSI Away Day 2012