Search

A linear potential flow model of a rotating lift-based wave energy converter is developed by assuming that the lift is generated by a pair of equal and opposite circulations and that the amplitude of motion is small. The linearisation of the hydrodynamics means that the forces can decomposed and expressions for the wave excitation force and radiation damping force are derived independently and shown to be related to each other through the Haskind Relations. The expressions for the forces are used to show that there is an optimum phase and product of circulation and radius of rotation to maximise the wave power extracted, which is equivalent to the optimum phase and amplitude of motion from ‘conventional’ wave energy converter theory. It is also shown that at this optimum condition 100% of the incident wave energy can be extracted. It is shown that the forces are directly proportional to the velocities due to the motion of the vortices, the water particle velocities due to the incident wave, and the water particle velocities induced by the vortices. The effect of the vortex-induced water particle velocities is considered and the importance of including these velocities on the passive generation of circulation, e.g. by hydrofoils, is highlighted. The impact of a sub-optimum product of circulation and radius of rotation is also investigated and shown that the power capture is not highly sensitive to the optimal conditions in the same way as ‘conventional’ wave energy converters

Transverse Axis Crossflow Turbines (TACTs) are a niche subset of tidal turbines. TACTs are not as well understood as the more traditional horizontal axis turbine and associated flow theory which leans heavily on advances in wind energy and marine propulsion. This paper reviews laboratory and field based experimental fluid dynamics work from the perspective of turbine performance. The available literature deviates significantly in perspective and scope since it is found that not all papers declare a full complement of parameters, thereby making it difficult to check or validate the respective results. Therefore, identifying trends amongst the variable and sparse datasets is difficult. None of the papers reviewed cite adherence to the recommended tank testing guidelines. The work reviewed analyses aspects such as mounting supports, solidity, blockage and blade support locations in isolation, but the cumulative impact of these variables is unknown. Arising from the analyses carried out as part of this review, blade loading, solidity and blockage were identified as key parameters and are the subject of planned research. Trends between solidity and blockage with tip speed ratio were identified. This paper contributes to the understanding of TACT performance through the tidal turbine performance curve and highlights the need for comprehensive physical testing and model validation.

Tidal resource assessment for the characterisation of turbine performance or Annual Energy Prediction currently uses the method of bins as recommended by international standards. An alternative method is proposed in this paper and applied to the Sustainable Marine Energy PLAT-I deployment in Connel Sound, Scotland. This method may be suitable for tidal turbines which operate from the surface. Three instrumentation types are used in this work, a bed-mounted Acoustic Doppler Profiler (ADP), and platform-mounted Acoustic Doppler Velocimeter (ADV) and Electromagnetic Current Meter (ECM). By comparing the resource characteristics from these three sources, a comparison of their velocity magnitudes and turbulence characteristics is made, demonstrating the difference between methodologies. It was found that the ADP evaluated using the method of bins produced a more conservative velocity distribution, in comparison to the ADV and ECM. Consequently, a representative AEP showed a difference of 3.8kWh (50% of ADP total) for the month of data collected. When comparing the Turbulence Intensity between devices, the ADP and ECM had similar metrics whilst the ADV had up to 14% higher values. The significance of these differences requires further work comparing them to the SME PLAT-I turbines power output to ascertain which best represents the onset flow experienced by the turbine and if there is a correlation between power performance and turbulence intensity.

Experimental testing of physical turbines, often at a smaller scale, is an essential tool for engineers to investigate fundamental design parameters such as power output and efficiency. Despite issues with scaling and blockage which are caused by limitations in size and flow velocity of the test facilities, experimental tank testing in laboratory environments is often perceived as offering more control and thus trustworthier results than field testing. This paper presents field tests of a tidal turbine, performed using a self-propelled barge in real tidal flow and still water conditions, that are compared to a towing tank test. Factors influencing the performance characteristics, such as the choice of velocity sensor, vessel handling and data processing techniques are investigated in this paper. Direct comparison with test results of the exact same turbine obtained in an experimental test facility further confirms that field testing with robust data analysis capabilities is a viable, time and cost efficient alternative to characterise tidal turbines.

Representing the velocity at a tidal energy extraction site in a standardized manner is essential for the development of the tidal energy sector. The International Electrotechnical Commission (IEC) have issued a technical specification to determine the mean representative current velocity (MRV) relative to a tidal energy converter (TEC). This method determines the MRV using an acoustic Doppler profiler (ADP). This is used to evaluate the power performance of a TEC and so can have a significant influence on the power performance uncertainties, due to the cubed relationship of the MRV and power. This paper investigates these uncertainties in the MRV by considering four sources of uncertainty in the ADP. These are turbulence intensity, tilt, Doppler noise and beam misalignment of an ADP. A synthetically generated dataset is used to define current velocity and profile using the characteristics of currents and ADP. The generated synthetic dataset is processed to calculate the MRV based on the standard IEC method. An alternative method to calculate the MRV, called the temporal-spatial method (TSM), is proposed and the two methodologies are used to assess the error of the MRV. A sensitivity analysis shows that the errors of the MRV based on the standard IEC method are always higher than the TSM. The biases in the MRV based on the different combinations of the uncertainty are investigated and it is found that the bias for the MRVs based on the TSM method is generally less than that of the IEC method.

The sustainable deployment of Horizontal Axis Tidal Turbines will require effective management and maintenance functions. In part, these can be supported by the engineering of suitable condition monitoring systems. The development of such a system is inevitably challenging, particularly given the present limited level of operational data associated with installed turbines during fault onset. To mitigate this limitation, a computational fluid dynamics model is used to simulate the operational response of a turbine under a known set of fault conditions. Turbine rotor imbalance faults were simulated by the introduction of increasing levels of pitch angle offset for a single turbine blade. The effects of these fault cases upon cyclic variations in the torque developed by the turbine rotor were then used to aid creation of a condition monitoring approach. A parametric tidal turbine rotor model was developed based on the outputs of the computational fluid dynamics models. The model was used to facilitate testing of the condition monitoring approach under a variety of more realistic conditions. The condition monitoring approach showed good performance in fault detection and diagnosis for simulations relating to turbulence intensities of up to 2 %. Finally, the condition monitoring approach was applied to simulations of 10 % turbulence intensity. Under the 10 % turbulence intensity case the rotor monitoring approach was successfully demonstrated in its use for fault detection. The paper closes with discussion of the effectiveness of using computational fluid dynamics simulations extended by parametric models to develop condition monitoring systems for horizontal axis tidal turbine applications.

The first aim of the research presented here is to examine the effect of turbine control by comparing a passive open-loop control strategy with a constant rotational speed proportional–integral–derivative (PID) feedback loop control applied to the same experimental turbine. The second aim is to evaluate the effect of unsteady inflow on turbine performance by comparing results from a towing-tank, in the absence of turbulence, with results from the identical machine in a tidal test site. The results will also inform the reader of: (i) the challenges of testing tidal turbines in unsteady tidal flow conditions in comparison to the controlled laboratory environment; (ii) calibration of acoustic Doppler flow measurement instruments; (iii) characterising the inflow to a turbine and identifying the uncertainties from unsteady inflow conditions by adaptation of the International Electrotechnical Commission technical specification (IEC TS): 62600-200. The research shows that maintaining a constant rotational speed with a control strategy yields a 13.7% higher peak power performance curve in the unsteady flow environment, in comparison to an open-loop control strategy. The research also shows an 8.0% higher peak power performance in the lab compared to the field, demonstrating the effect of unsteady flow conditions on power performance. The research highlights the importance of a tidal turbines control strategy when designing experiments.