Comparative study of numerical modelling techniques to estimate tidal turbine blade loads

This paper presents a method to obtain the pressure distribution across the surface of a tidal turbine blade, but without the extensive computational time that is required by 3D CFD modelling. The approach uses a combination of blade element momentum theory (BEMT) and 2D CFD modelling, where the inflow velocity vector for each blade element computed from the BEMT model is input to a 2D CFD model of each of the blade sections. To assess the validity of this approach, a comparison is made with both a BEMT and a 3D CFD model for three different blade profiles at full scale (NACA 63-8xx, NREL S814 and Wortmann FX 63-137). A comparison is also made of the NREL blade at smaller scale to investigate any Reynolds number effects on the model performance. The agreement is shown to be very reasonable between the three methods, although the forces are consistently slightly over-predicted by the BEMT method compared to the 2D-CFD-BEMT model, and the 2D-CFD-BEMT model over-predicts the pressure along the leading edge compared to the 3D CFD results. The proposed method is shown to be particularly useful when conducting initial blade structural analysis under dynamic loading

Towing tank and flume testing of passively adaptive composite tidal turbine blades

Composite tidal turbine blades with bend-twist (BT) coupled layups allow the blade to self-adapt to local site conditions by passively twisting. Passive feathering has the potential to increase annual energy production and shed thrust loads and power under extreme tidal flows. Decreased hydrodynamic thrust and power during extreme conditions means that the turbine support structure, generator, and other components can be sized more appropriately, resulting in a higher utilization factor and increased cost effectiveness

The development, design and characterisation of a scale model horizontal axis tidal turbine for dynamic load quantification

The paper describes the development and characterisation of three 0.9 m diameter lab-scale Horizontal Axis Tidal Turbines. The blade development process has been outlined and was used to generate a design specification. Each turbine houses instrumentation to measure rotor thrust, torque and blade root bending moments on each blade, in both `flapwise' and `edgewise' directions. A permanent magnet synchronous machine and encoder are integrated to allow for servo-control of the turbine as well as to provide position and rotational velocity measurements, resulting in three turbines that can be individually controlled using speed or torque control. Analogue signals are captured via a real-time operating system and field programmable gate array hardware architecture facilitating sample rates of up to 2 kHz. Results from testing the pilot turbine at three differing facilities during the development process are presented. Here good agreement, less than 7% variation, was found when comparing the testing undertaken at various flume and tow tank facilities. Lastly, the findings of a test campaign to characterise the performance of each of the three turbines are presented. Very good agreement in non-dimensional values for each of the three manufactured turbines was found

Blade element momentum theory to predict the effect of wave-current interactions on the performance of tidal stream turbines

The durability and reliability of tidal energy systems can be compromised by the harsh environments that the tidal stream turbines need to withstand. These loadings will increase substantially if the turbines are deployed in exposed sites where high magnitude waves will affect the turbine in combination with fast tidal currents. The loadings affecting the turbines can be modelled using various numerical or analytical methods; each of them have their own advantages and disadvantages. To understand the limitations arising with the use of numerical solutions, the outcomes can be verified with practical work. In this paper, a Blade Element Momentum coupled with wave solutions is used to predict the performance of a scaled turbine in a flume and a tow tank. The analytical and experimental work is analysed for combinations of flow speeds of 0.5 and 1.0 m/s, wave heights of 0.2 and 0.4 and wave periods of 1.5 and 1.7 s. It was found that good agreement between the model and the experimental work was observed when comparing the data sets at high flow conditions. However, even if the average values were similar, the model tend to under predict the maximum and minimum values obtained in the experiments. When looking at the results of low flow velocities, the agreement between the average and time series was poorer

Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms

Uptake of system L amino acid substrates into isolated placental plasma membrane vesicles in the absence of opposing side amino acid (zero-trans uptake) is incompatible with the concept of obligatory exchange, where influx of amino acid is coupled to efflux. We therefore hypothesized that system L amino acid exchange transporters are not fully obligatory and/or that amino acids are initially present inside the vesicles. To address this, we combined computational modeling with vesicle transport assays and transporter localization studies to investigate the mechanism(s) mediating [14C]L-serine (a system L substrate) transport into human placental microvillous plasma membrane (MVM) vesicles. The carrier model provided a quantitative framework to test the 2 hypotheses that L-serine transport occurs by either obligate exchange or nonobligate exchange coupled with facilitated transport (mixed transport model). The computational model could only account for experimental [14C]L-serine uptake data when the transporter was not exclusively in exchange mode, best described by the mixed transport model. MVM vesicle isolates contained endogenous amino acids allowing for potential contribution to zero-trans uptake. Both L-type amino acid transporter (LAT)1 and LAT2 subtypes of system L were distributed to MVM, with L-serine transport attributed to LAT2. These findings suggest that exchange transporters do not function exclusively as obligate exchangers.—Widdows, K. L., Panitchob, N., Crocker, I. P., Please, C. P., Hanson, M. A., Sibley, C. P., Johnstone, E. D., Sengers, B. G., Lewis, R. M., Glazier, J. D. Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms

Laboratory study of tidal turbine performance in irregular waves

Wave loading on tidal turbines is of key concern for determining blade and drive train design loads and the fatigue life of components. Furthermore, irregular waveforms are likely to add complexity to the loading patterns, and represent more realistic conditions. To investigate this issue, a set of laboratory tests was conducted in a large wave-tow facility at CNR-INSEAN, Rome. A 0.9 m diameter three bladed horizontal axis turbine model was fixed to the tow carriage and tested under tow, regular wave-tow and irregular-wave-tow conditions at a range of turbine rotational velocities. Thrust and torque on the blades and rotor were measured dynamically during testing using strain gauges. The control mode was switched between constant speed and constant torque to understand how this influenced turbine power capture and thrust loading, and assess the potential to use control methods to mitigate loading fluctuations. It was found that average power and thrust values were not affected by the control mode or the addition of regular or irregular waves. However, using torque control resulted in increased thrust fluctuations per wave period of the order of 40% of the mean thrust compared to under speed control. Therefore, the operational mode must be taken into consideration

Design process for a scale horizontal axis tidal turbine blade

If tidal energy extraction is to be maximised then emphasis needs to be placed on the design of the rotor geometry to optimise performance. The work documented in this paper describes the process used in the design and validation of a new blade based on the Wortmann FX63-137 aerofoil. BEMT was used as an initial tool to redesign the blade due to speed in which calculations can be completed. CFD models were produced after to incorporate the hydrodynamics and provide a 3D solution. The performance coefficients for CP and CT were calculated by each of the two computational methods for comparison with the experimental testing. The experimental testing was conducted at the INSEAN tow tank to provide validation for the computational models. The CFD model was found to closely predict the performance coefficients of the turbine at low TSR at and peak power. The BEMT model over predicted both the CP and the CT when compared to the experimental work, however was found to be good as an initial method for redesigning the blade

Effects of wave-current interactions on the performance of tidal stream turbines

The main objective of this paper is to analyse extreme cases of wave-current interactions on tidal stream energy converters. Experiments were undertaken in the INSEAN tow tank facility where flow velocities of 0.5 and 1m/s were used with and without waves. The wave variations studied in this testing campaign were between wave heights of 0.2 to 0.4m with a 2s wave period. These wave conditions were considered extreme cases considering the use of a turbine with a rotor diameter of 0.5m. The turbine was equipped with a torque transducer, an encoder and a strain gauge to measure power coefficients and forces on a single blade root. Therefore, the results of this experiment are used to improve the understanding of wave effects on tidal stream rotors by analysing not only the temporal variations of power and blade loading but also the peak variations of them