• Energy Research

AbstractSimulating fully resolved Horizontal Axis Tidal Turbine (HATT) geometry for a time period great enough to resolve a fully developed wake, and accurately predict power and thrust characteristics, is computationally very expensive. The BEM-CFD method is an enhanced actuator disk and is able to reduce the computational cost by simulating a time averaged downstream velocity field. Current implementations fall short of accurately determining tip losses, which are a function of the hydrofoil geometry. This work proposes a method of addressing this shortfall by modifying the angle of attack to conform to the constraints outlined in Prandtl's lifting line theory, i.e. the zero lift angle of attack at the hydrofoil tip. The revised model is compared to existing BEM-CFD methods and validated against experimental data. The revised BEM-CFD method presented in this work shows a significant improvement over previous BEM-CFD methods when predicting power and thrust. The coefficient of power is reduced from 0.57 (approx. 30% above experiment) to 0.44 (approx. 3% above experiment). An increase in turbulence intensity in the rotor region, in particular at the wake boundary, improves the recovery of the wake without the addition of empirical turbulence source terms. Good correlation with experimental results for power, thrust and wake prediction, is observed. The model may also be applied to wind turbines.

A combined Blade Element Momentum - Computational Fluid Dynamics (BEM-CFD) model is applied to a 10 m diameter tidal stream turbine blade and the supporting nacelle and tower structure in a 700 m long rectangular channel. The modelling approach is computationally efficient and is suitable for capturing the time-averaged influence of the turbine on the flow. A range of simulations are conducted for the purpose of undertaking a comparative study of the influence of the turbine on mean flow characteristics. Variations in flow structure around the turbine for different flow conditions were evaluated. Simulations are conducted for a range free-stream velocities typical of potential tidal stream deployment sites, typically up to 3.0 m s-1. Velocity deficit profiles and wake dimensions are evaluated for each flow condition implemented. Downstream flow recovery is strongly linked to the flow velocity, and occurs over a longer distance with increasing velocity. For the range of velocities considered, some properties, such as wake length and the maximum wake length location increase linearly, or nearly-linearly with velocity. Other properties, such as the maximum wake width, and the recovery distance downstream demonstrate a tendency to converge towards a constant value. The key findings of this study highlight the significance of the free-stream velocity as an influence on the flow structure around and downstream of a tidal stream turbine.

A coupled blade element momentum – computational fluid dynamics (BEM–CFD) model is used to conduct simulations of groups of tidal stream turbines. Simulations of single, double and triple turbine arrangements are conducted first to evaluate the effects of turbine spacing and arrangement on flow dynamics and rotor performance. Wake recovery to free-stream conditions was independent of flow velocity. Trends identified include significant improvement of performance for the downstream rotor where longitudinal spacing between a longitudinally aligned pair is maximised, whereas maintaining a lateral spacing between two devices of two diameters or greater increases the potential of benefitting from flow acceleration between them. This could significantly improve the performance of a downstream device, particularly where the longitudinal spacing between the two rows is two diameters or less. Due to the computational efficiency of this modelling approach, particularly when compared to transient computational fluid dynamics simulations of rotating blades, the BEM–CFD model can simulate larger numbers of devices. An example of how an understanding of the hydrodynamics around devices is affected by rotor spacing can be used to optimise the performance of a 14 turbine array is presented. Compared to a regular staggered configuration, the total power output of the array was increased by over 10%.

Abstract Computational fluid dynamics (CFDs) simulation tools are developed to analyse the aerodynamic performance of a novel design vertical axis wind turbine (VAWT) and compared against careful data from a 1.6 m diameter device in a wind tunnel. The study investigates the extent to which a CFD model employing the simplest turbulence representation can provide a useful input to evaluate the impact of several key operational parameters: wind speed, rotor speed, yaw angle and blade pitch angle. The results show that simple turbulence modelling techniques are sufficient to evaluate the performance of the turbine in the designed operating conditions and can predict when the turbine will run outside each parameter’s operational range. However, such a simple turbulence representation may need further refinement to quantify more precisely the extent to which performance is affected when running some way outside this range.

Marine renewable energy (MRE) development will be crucial to achieve worldwide energy decarbonization. In Europe, 1 GW and 40 GW of ocean energy are set to be developed by 2030 and 2050, respectively. Support is essential if wave and tidal stream arrays are to become more economically viable than they currently are. Four recently developed open-access software tools are used in this study to investigate the critical and expensive elements of potential demonstration and commercial scale tidal projects. The tools have been designed and built to assist users with array configurations, foundation and mooring (F&M) design, operation and maintenance (O&M) strategies, and techno-economic analysis. Demonstration of their use is performed in this study to model scenarios for 2 MW, 10 MW, 40 MW, and 100 MW tidal energy projects employing typical 500 kW fixed and 2 MW floating turbines at the West Anglesey Tidal Demonstration Zone in the Irish Sea. The following metrics are examined: the power output and wake losses of staggered and line configurations; the design and costs of simple gravity-based foundations, gravity-based anchors and the four-chain catenary mooring system of a single turbine; the mean O&M costs and farm availability over the project life; and the breakdown of levelized cost of energy (LCoE) for all eight scenarios to ultimately reveal minimum values of 173 EUR/MWh and 147 EUR/MWh for fixed and floating tidal energy technologies, respectively. The thorough analysis facilitated within these four tools to forecast realistic situations in a specific location can help users design a tidal energy project for an area with considerable potential for commercial scale projects, and thus assist the ocean energy community in promoting and nurturing the sector in the years and decades ahead.

AbstractThe presence of Tidal Stream Turbines (TST) for tidal power production, leads to changes in the local physical environment that could affect fish. While other work has considered the implications with respect to conventional hydroelectric devices (i.e. hydroelectric dams), including studies such as physical impact with the rotors and pressure variation effects, this research considers the effects of sudden changes in pressure and turbulence on the hypothetical fish with respect to TSTs. Computational fluid dynamics (CFD) is used to investigate changes to the environment, and thus study the implications for fish. Two CFD methods are employed, an embedded Blade Element representation of the rotor in a RANS CFD model, and a blade resolved geometry using a moving reference frame. A new data interpretation approach is proposed as the primary source of environmental impact data; ‘rate of change of pressure’ with time along a streamtrace. This work also presents results for pressure, pressure gradients, shear rates and turbulence to draw conclusions about changes to the local physical environment. The assessment of the local impact is discussed in terms of the implications to individual fish passing a single or array of TST devices.

Abstract Driven by the need to verify a CFD model of a novel vertical axis wind turbine (VAWT) device, a 1.6 m diameter prototype was designed, built to a high specification and tolerance and then tested in the industry standard MIRA wind tunnel over the full wind range that might be expected in an urban environment. Although the original intention was simply to provide data to verify a CFD model of the specific device – documented separately – it was realised that given (a) the repeatability and quality of the experimental data, and (b) its utility for the authors in verifying their CFD simulations, the data itself might well form the basis for a requirement for a reliable benchmark for the wind engineering community in assessing CFD simulation technology against a specific VAWT device design with the complexity of a housing. The experiments cover a full range of operational conditions for a specific design including the variation of wind speed, wind direction, tip speed ratio and blade pitch angle, recorded in the experiments as well as the torque output and the pressure levels at a range of specific locations. Over 60 tests were carried out in such a way as to assess the repeatability of the measurements; the results showed an overall remarkable degree of consistency.