Tidal currents can be used as a predictable source of sustainable energy, and have the potential to make a useful contribution to the energy needs of the UK and other countries with such a resource. One of the technologies which may be used to transform tidal power into mechanical power is a vertical axis turbine, the hydrodynamic analysis of which this thesis is concerned with. The aim of this analysis is to gain a better understanding of the power transformation process, from which position there is the possibility of improving the conversion efficiency. A second aim is to compare the results from different modelling approaches. Two types of mathematical modelling are used: a basic blade element momentum model and a more complex Reynolds-averaged Navier Stokes (RANS) model. The former model has been programmed in Matlab by the present author while the latter model uses a commercial computational fluid dynamics (CFD) code, ANSYS CFX. This RANS model uses the SST k-! turbulence model. The CFD analysis of hydrofoils (equally airfoils), for both fixed and oscillating pitch conditions, is a significant proportion of the present work. Such analysis is used as part of the verification and validation of the CFD model of the turbine. It is also used as input to the blade element momentum model, thereby permitting a novel comparison between the blade element momentum model and the CFD model of the turbine. Both types of turbine model were used to explore the variation in turbine efficiency (and other factors) with tip speed ratio and with and without an angle of attack limiting variable pitch strategy. It is shown that the use of such a variable pitch strategy both increases the peak efficiency and broadens the peak. The comparison of the results from the two different turbine modelling approaches shows that when the present CFD hydrofoil results are used as input to the blade element model, and when dynamic effects are small and the turbine induction factor is low, there is generally good agreement between the two models.

Transition towards a low carbon economy raises concerns of loss of security of supply with high penetrations of renewable generation displacing traditional fossil fuel based generation. While wind and wave resources are increasingly forecastable, they are stochastic in nature. The tidal current resource, although variable has the advantage of being deterministic and truly predictable. With the first Crown Estate leasing round complete for wave and tidal current energy, plans are in place to install 1000 MW of tidal capacity in the Pentland Firth and Orkney waters. The aim of the work presented in this thesis is to examine the role tidal current energy can realistically play in the future electricity mix. To achieve this objective it was first necessary to develop new methodologies to capture the temporal and spatial variability of tidal current dynamics over long timescales and identify metrics relevant in a tidal energy context. These methodologies were developed for project scale resource characterisation, and provided a basis for development of a national scale dataset. The creation of project and national scale tidal datasets capture spatial and temporal variability at a level beyond previous insight, as demonstrated in case studies of three important early stage tidal current energy development sites. The provision of a robust national scale dataset enabled the development of realistic scenarios for the growth of the tidal current energy sector in UK waters. Assessing the various scenarios proposed indicates that first-generation technology solutions have the potential to generate up to 31 TWh/yr (over 8% of 2009 UK electricity demand). However, only 14 TWh/yr can be sensibly generated after incorporating realistic economic and environmental limitations proposed in this study. The preceding development of methodologies, datasets and scenarios enabled statistical analysis of the matching characteristics of future tidal energy generation potential with the present UK electricity demand and trends of electricity usage. This analysis demonstrated that the UK tidal current energy resource is much more in phase than has previously been understood, highlighting the flaws in previous studies suggesting that a combined portfolio of sites around the UK can deliver firm power. As there is negligible firm production, base-load contribution is insignificant. However, the time-series generated from this analysis identifies the role tidal current energy can play in meeting future energy demand and offer significant benefit for the operation of the electricity system as part of an integrated portfolio.

The focus of this thesis is research on the performance of very low speed direct drive permanent magnet generators for energy conversion from marine and tidal currents. Various aspects involved in the design of these generators and their electromagnetic modelling using the finite element simulations are presented. For a detailed study, a 5 kW prototype generator has been designed and constructed based on finite element based simulations. Several experiments were conducted on the prototype generator. The experimental results were compared with the corresponding case simulations on the designed generator. The differences between the results predicted by the simulations and those predicted by the measurements were less than 10%. The part and overload performance of the generator has been investigated and it is found from both simulations and measurements that the generator is capable to efficiently operate at varying speeds. The tests on the experimental generator were made for speeds between 2 and 16 rpm and for load variations of 0.5 to 2 per unit. In this thesis it is shown that it is possible to design a very low speed direct drive generator for more or less any given marine current site and this is beneficial for projects aiming to develop a technical and economical viable marine current energy conversion system.