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

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

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

Towing tank testing of passively adaptive composite tidal turbine blades and comparison to design tool

Passively adaptive bend-twist (BT) tidal turbine blades made of non-homogeneous composite materials have the potential to reduce the structural loads on turbines so that smaller more cost effective components can be used. Using BT blades can also moderate the demands on the turbine generator above design conditions. This paper presents experimental towing tank test results for an 828 mm diameter turbine with composite BT blades compared to a turbine with geometrically equivalent rigid aluminum blades. The BT blades were constructed of a graphite-epoxy unidirectional composite material with ply angles of 26.8° to induce BT coupling, and an epoxy foam core. For steady flow conditions the BT blades were found to have up to 11% lower thrust loads compared to rigid blades, with the load reductions varying as a function of flow speed and rotational speed. A coupled finite element model-blade element momentum theory design tool was developed to iterate between the structural (deformation and stresses) and hydrodynamic (power and thrust loads) responses of these adaptive composite blades. When compared to the experimental test results, the design tool predictions were within at least 8% of the experimental results for tip-speed ratios greater than 2.5

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.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

Contra rotating marine turbines tank tests to analyse system dynamic response

Paper describes contra rotating marine turbines tank tests which aim to analyse system dynamic response

Tow tank tests of tethered marine turbines to determine system dynamic response

Describes tow tank tests of tethered marine turbines to determine system dynamic response

Experimental tank testing of contra rotating marine turbines to quantify the parameters influencing system dynamic response

Paper describes experimental tank testing of contra rotating marine turbines to quantify the parameters influencing system dynamic response

Analysis of a single point tensioned mooring system for station keeping of a contra-rotating marine current turbine

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). Novel aspects of this turbine include two contra-rotating sets of rotor blades directly driving an open-to-sea permanent magnet generator. The balancing of reactive forces by the use of contra-rotation enables the use of a single point compliant mooring system for station keeping. A series of tank and sea tests have led to the deployment and demonstration of a small stand-alone next generation tidal turbine. The stability of a single-point mooring system is examined and power quality from the direct drive generator is evaluated. It is noted that good stability from a single point mooring can be achieved within a real tidal stream; however from sea testing of the turbine off the west coast of Scotland in the Sound of Islay, it is shown that some instabilities in device station keeping may have an effect on the output electrical power quality. Finally, the scaling up of the power take-off and delivery options for a 250kW production prototype are described and assessed. It was concluded that the most promising option was an enlarged version of the system already tested, namely a direct-drive contra-rotating permanent magnet generator

Numerical modelling techniques to predict rotor imbalance problems in tidal stream turbines

Load fluctuations caused by the unsteady nature of tidal streams can have severe impacts on turbine components. As seen in the wind industry, turbine blades can become misaligned due to a fault in the pitch mechanism or blade deformations arising over time. These misalignments will represent a loss of power capture and perhaps even premature failure of the components if not detected in time. Computational fluid dynamic (CFD) techniques can be used to predict the performance of a turbine with a misaligned blade. However, these numerical modelling techniques quickly become computationally expensive when modelling realistic, time-varying conditions. Blade Element Momentum Theory (BEMT) offers a quicker and simpler approach, although with several limitations. In this paper BEMT is adapted to predict the performance of a three bladed tidal turbine with one or two blades offset from the optimum pitch setting. This approach is compared with a CFD model to study the effectiveness of both methods to predict power and thrust when a rotor blade has an offset. The simulations were undertaken at three flow speeds (0.9, 1.0 and 1.1 m/s). Both numerical models are compared to experimental data that was obtained at a flume tank in similar flow conditions. The results showed that both BEMT and CFD are able to predict power coefficients when there is a small offset of one rotor blade. However, the predictions were poorer when two blades had two different offsets at the same time