A contra-rotating marine current turbine on a flexible mooring : development of a scaled prototype

The contra-rotating marine current turbine concept developed by the Energy Systems Research Unit at the University of Strathclyde is aimed at extracting energy in a wide range of water depths by 'flying' a neutrally-buoyant device from a flexible, tensioned mooring. After successful proof of concept turbine trials, the development programme has moved on to investigate the performance of a scaled prototype of the complete system incorporating the turbine, submersible contra-rotating generator and mooring. The turbine/generator assembly has been tested in a towing tank, and the entire system is now undergoing sea trials. An investigation into turbine wake development (an area in which it is hoped that the contra-rotating turbine will have uniquely beneficial properties) has recently begun. Small single-rotor model turbines have been deployed in a flume. Trends observed so far are in accordance with those observed by other researchers

Contra-rotating marine current turbines : performance in field trials and power train developments

Development of a novel contra-rotating marine current turbine has been continuing at the University of Strathclyde. Continuous monitoring of blade bending loads during trials has enabled an investigation of blade-blade and blade-structure interactions. The former are a particular concern with a contra-rotating turbine, but there is now evidence to suggest that in normal operation these are relatively small. By contrast, blade-structure effects are clearly visible. A turbine complete with single-point mooring and submersible contra-rotating generator is presently being prepared for sea trials. Details of the machine and the test programme are described

Contra-rotating marine current turbines : single point tethered floating system - stabilty and performance

The Energy Systems Research Unit within the Department of Mechanical Engineering at the University of Strathclyde has developed a novel contra-rotating tidal turbine (CoRMaT). A series of tank and sea tests have led to the development and deployment of a small stand-alone next generation tidal turbine. Novel aspects of this turbine include its single point compliant mooring system, direct drive open to sea permanent magnet generator, and two contra-rotating sets of rotor blades. The sea testing of the turbine off the west coast of Scotland in the Sound of Islay is described; the resulting stability of a single-point tethered device and power quality from the direct drive generator is reported and evaluated. It is noted that reasonably good moored turbine stability within a real tidal stream can be achieved with careful design; however even quite small instabilities have an effect on the output electrical power quality. Finally, the power take-off and delivery options for a 250kW production prototype are described and assessed

Development of an experimental methodology for appraising the dynamic response of tethered tidal turbines

This thesis makes a comparison of different station keeping structures to support tidal energy converters. It was observed that the use of flexibly tethered turbines would be beneficial due to low material costs and the capability to permit the turbine’s self alignment to the flow regime. However, because of the uncertainties over their dynamic behaviour, it was considered that an analysis of response in a range of conditions was essential before they could be considered as practical station keeping system. Firstly, a static analysis was carried out for both rigid and flexible foundations. Thereafter, the thesis presents the development of an experimental methodology to study the dynamic response of tethered tidal energy converters. In this methodology, the alignment and oscillations of the three main rotational angles (i.e. roll, pitch and yaw), estimated over a period of time, were taken as the fundamental metrics of system behaviour. The analysis was extended into the frequency domain in order to estimate the intensity of the parameters that affect the turbine and its condition (e.g. blade failure, excessive backlash or misalignment, vortex shedding, etc.) Within the methodology development a series of steps were specified, based on established protocols related to similar concepts (for example EquiMar) where parameters such as the selection of test facilities, blockage ratio limits or safety factors in applied loads were discussed. Instruments to measure the dynamic motion of turbines were specified, along with other instruments to measure power, thrust, angular velocity and flow speed. The final steps in the methodology denoted methods to analyse the acquired signals. In order to verify the feasibility of the methodology, a series of experiments were carried out at various turbine models scales. Firstly, small turbine models were installed on a zero turbulence tow tank and a flume tank with significant levels of turbulence, under controlled conditions and at similar flow velocities. To compare the dynamic responses, studies were undertaken for a larger turbine deployed in the natural turbulence of an open tidal site and in a turbulent river stream affected by marine traffic. This thesis concludes that the methodology proposed is suitable to characterise the dynamic response of tethered devices at various model scales. The results presented showed the advantages and disadvantages of using various turbine configurations. Therefore, this methodology can be used develop and validate analytical models that predict the dynamic response of flexibly moored turbines

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

Design of a horizontal axis tidal turbine for less energetic current velocity profiles

Existing installations of tidal-stream turbines are undertaken in energetic sites with flow speeds greater than 2 m/s. Sites with lower velocities will produce far less power and may not be as economically viable when using “conventional” tidal turbine designs. However, designing turbines for these less energetic conditions may improve the global viability of tidal technology. Lower hydrodynamic loads are expected, allowing for cost reduction through downsizing and using cheaper materials. This work presents a design methodology for low-solidity high tip-speed ratio turbines aimed to operate at less energetic flows with velocities less than 1.5 m/s. Turbines operating under representative real-site conditions in Mexico and the Philippines are evaluated using a quasi-unsteady blade element momentum method. Blade geometry alterations are undertaken using a scaling factor applied to chord and twist distributions. A parametric filtering and multi-objective decision model is used to select the optimum design among the generated blade variations. It was found that the low-solidity high tip-speed ratio blades lead to a slight power drop of less than 8.5% when compared to the “conventional” blade geometries. Nonetheless, an increase in rotational speed, reaching a tip-speed ratio (TSR) of 7.75, combined with huge reduction in the torque requirement of as much as 30% paves the way for reduced costs from generator downsizing and simplified power take-off mechanism

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 effect of bathymetry interaction with waves and sea currents on the loading and thrust of a tidal turbine

This paper reports work undertaken to advance analytical methods used to evaluate the influence of bathymetry on wave- current interactions with tidal turbines. The model takes in to account the wave transformation due to a sudden depth change in the sea level. The functions developed provide solutions for wave transformation by changes in bathymetry to find how this change effects the torque and thrust exerted over a tidal turbine. Costal site data for the west coast of the US, from the US DoE, has been used to access the robustness of these analytical methods. The high resolution data sets used have monitored wave, sea and climatic conditions over a period of 8 years

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