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Economic viability of alternative horizontal axis tidal turbine concepts: operation and maintenance simplicity is the key?

A recent dti funded study[1] examined the difference in power capture between a variety of concept horizontal axis tidal turbines (HATT). The aim of the work was to examine the trade-off between design complexity and expected economic cost over the lifetime of tidal turbine farm. Two types of mechanical complexity were examined. The first assumed that the device was free to yaw as the tidal current changes direction. In this case the difference in power capture came from the use of either an optimum uni-directional blade or for a fixed device a bi-directional blade design developed at the University of Southampton. The second complexity was whether the blades would have a controllable pitch. For the basis of the comparison it was assumed that the basic turbine would have a fixed diameter of 20m and be sited in 40m water depth with a spring mean maximum tidal current of 2.5m/s. The methodology adopted was to analyse the hydrodynamic performance and tidal cycle energy capture using a blade element momentum code. In order to make a best-case comparison a blade shape design optimisation was carried out for both the uni and bi-directional blades by searching of the order of 50,000 combinations of chord and twist distributions. A range of alternative blade rpm control strategies were examined to see how this would influence the energy capture. A detailed mechanical system representation was developed so that the reliability and availability of each turbine within the farm could be assessed with a stochastic Monte-Carlo simulation applied to examine variability. The result of the work based on the establishment of a systematic framework, using realistic assumptions was the extent to which the loss in energy conversion efficiency of the simpler concepts was counterbalanced by a reduction in capital and O&M costs. It was concluded that such a simple system is technically feasible and is competitive on a life cycle cost basis and worthy of further consideration