Online ISSN
1752-1424
Print ISSN
1752-1416
IET Renewable Power Generation
Volume 4, Issue 6, November 2010
Volumes & issues:
Volume 4, Issue 6
November 2010
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- Author(s): M. Leijon
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 471 –472
- DOI: 10.1049/iet-rpg.2010.9119
- Type: Article
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- Author(s): J. Clarke ; G. Connor ; A. Grant ; C. Johnstone ; S. Ordonez-Sanchez
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 473 –487
- DOI: 10.1049/iet-rpg.2009.0209
- Type: Article
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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 (PMG). 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 250 kW 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 PMG. - Author(s): J. Cruz ; R. Sykes ; P. Siddorn ; R.E. Taylor
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 488 –497
- DOI: 10.1049/iet-rpg.2009.0174
- Type: Article
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Estimates regarding the assessment of the energy absorption characteristics of an array of wave energy converters (also referred to as a wave farm) are presented. Regular and irregular waves are used as input in a frequency-domain hydrodynamic model, which allows iterations in the array layout and farm control strategy. Under such an approach each array element can be controlled independently while keeping the design objective (maximisation of the wave farm energy yield). The distribution of power take-off (PTO) loading on the various array elements, as induced by the incoming sea, is also investigated. The approach is verified by comparing the estimates with results from a semi-analytic method developed at the University of Oxford. The overall objective of the study is to quantify the influence of the array layout and farm control in the performance of a wave farm under the action of irregular waves. The results show that the energy yield and the PTO loads are affected by such factors; hence these can be seen as key design drivers in order to reduce the uncertainty and thus the cost of energy when planning a wave farm. Further studies may address additional constraints, either technical or economical. This study is expected to contribute to the development of specific modules of a design optimisation tool for wave farms, and extends the findings originally presented at the Eighth European Wave and Tidal Energy Conference. - Author(s): F. Maganga ; G. Germain ; J. King ; G. Pinon ; E. Rivoalen
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 498 –509
- DOI: 10.1049/iet-rpg.2009.0205
- Type: Article
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Experimental results of tests carried out to investigate the hydrodynamics of marine current turbines are presented. The objective is to build an experimental database in order to validate the numerical developments conducted to characterise the flow perturbations induced by marine current turbines. For that purpose, we used a tri-bladed horizontal axis turbine. The work is dedicated to measuring the behaviour of the system and to characterising the wake generated by the turbine. The efficiency of the device is quantified by the measurement of the thrust and the amount of power generated by the rotor for various inflow conditions, whereas the wake is characterised by Laser Doppler Velocimetry. Particular attention is paid to the flow characteristic effects on the performance of a 0.70 m diameter turbine. The load predictions on the structure and the measured performance of the turbine over its working range of currents and rotational speeds are presented. The results showed that this kind of turbine is sensitive to the quality of the incoming flow. The turbulence intensity effects on turbine behaviour and on its wake are also characterised in order to study how the far wake decays downstream and to estimate the effect produced in downstream turbines. - Author(s): R.A. McAdam ; G.T. Houlsby ; M.L.G. Oldfield ; M.D. McCulloch
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 510 –518
- DOI: 10.1049/iet-rpg.2009.0194
- Type: Article
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This study outlines the procedures and results for a set of experiments on the transverse horizontal axis water turbine (THAWT), which is a variant of a Darrieus turbine. Testing of a 1/20th scale device was conducted in the combined wind, wave and current tank at Newcastle University. Flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of the THAWT design. Variants included a parallel-bladed device, a ‘truss’ device and a parallel-bladed device configured with blades pitched relative to the pitch circle tangent. Experiments were carried out using a speed controlled motor, allowing quasi-steady results to be taken over a range of tip speed ratios. The results demonstrate that, over a range of flow conditions, the device is capable of exceeding the Lanchester–Betz limit for kinetic efficiency. This is principally because of the relatively high blockage ratio that can be achieved with such a device. - Author(s): X. Garnaud and C.C. Mei
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 519 –530
- DOI: 10.1049/iet-rpg.2009.0166
- Type: Article
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Wave energy converters are usually designed to achieve maximum efficiency by impedance matching with the incoming waves. One of the simplest design is a buoy attached to an energy absorber (power-takeoff devise). For maximum efficiency, the buoy must be large enough in order to resonate at the peak frequency of the incident sea, and the extraction rate is the same as the rate of radiation damping. With one or few degrees of freedom the resonance peak(s) is(are) usually band-limited. The latter feature is common to all wave-power absorbers based on impedance matching. Inspired by the FO3 system being developed in Norway by Fred Olsen and ABB Associates, and the Manchester Bobber from UK, the authors examine theoretically power extraction by a compact array of small buoys, that do not resonate but possess many degrees of freedom. It is shown that such systems have certain advantages over a single large buoy. - Author(s): S.A. Mavrakos and G.M. Katsaounis
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 531 –544
- DOI: 10.1049/iet-rpg.2009.0191
- Type: Article
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This study investigates the effect that several different floaters’ geometries have on the performance characteristics of tightly moored vertical axisymmetric wave energy converters (WECs). A cylindrical buoy with and without vertical and horizontal skirts mounted at its bottom, a cone, as well as two piston-like arrangements, consisting of an internal floater (cone or cylinder) and an exterior torus, have been examined and comparatively assessed. The WEC's first-order hydrodynamic characteristics are evaluated using a linearised diffraction–radiation semi-analytical method. Axisymmetric eigenfunction expansions of the velocity potential are introduced into properly defined ring-shaped fluid regions surrounding the bodies and the potential solutions are matched at the boundaries of adjacent fluid regions by enforcing continuity of the hydrodynamic pressures and radial velocities. A dynamical model for the floaters’ performance in time domain is developed that properly accounts for the floater's hydrodynamic behaviour, the coupling terms between the different modes of motion and of the power take-off mechanism. Numerical results are presented and discussed, in terms of the expected power production. The results show parametrically the effect that the varying hydrodynamic characteristics of each particular floater's geometry have on the investigated WEC's performance characteristics. - Author(s): S. Draper ; G.T. Houlsby ; M.L.G. Oldfield ; A.G.L. Borthwick
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 545 –554
- DOI: 10.1049/iet-rpg.2009.0196
- Type: Article
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An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions. - Author(s): D.L. O'Sullivan ; G. Dalton ; A.W. Lewis
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 555 –567
- DOI: 10.1049/iet-rpg.2009.0187
- Type: Article
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In considering the grid connection of ocean wave energy devices, and in particular, the connection of large offshore arrays, there are a number of technical and economic challenges that need to addressed by wave energy device developers. As device developers approach pre-commercial operation, grid connection factors will become increasingly significant in both technical and financial terms. In this study, some generally applicable, non-device specific challenges are examined. The costing issues involved in procuring, building and maintaining a grid connection are reviewed. The technical impacts on device design and particularly on generator system selection are also examined in detail. A grid connection costing case study in the Irish context is performed as a practical indicator of these issues. - Author(s): M. Kawase and K.M. Thyng
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 568 –578
- DOI: 10.1049/iet-rpg.2009.0195
- Type: Article
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p.
568
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Inland coastal waters of the State of Washington, United States, are fjord-like in character and feature passages where tidal currents with speeds of 3 m/s or greater occur. Combined with close proximity to the major metropolitan area of Seattle and easy access to the power grid, the region is a prime candidate within the United States for tidal power generation. A new three-dimensional (3D) model of tidal circulation of these waters has been implemented. The model is based on Stanford University's SUNTANS code, and covers the eastern Strait of Juan de Fuca, Puget Sound, San Juan and Channel Islands, and the southern Georgia Basin with an unstructured triangular mesh of 250 m average resolution. It is forced with tidal currents from a regional tidal model along open boundaries. Barotropic tidal response is calibrated against compiled tidal data for the region; the system response is characterised in terms of sea surface height variability and energy dissipation. The model can simulate tides with quantitative accuracy within Puget Sound, but it over-predicts tidal range and under-predicts currents in the waterways leading up to the Main Basin of the Sound. Future plans for the model include extension of the model domain for the entire Georgia Basin, incorporation of partial bottom cells and baroclinic processes and improvements in bathymetry. - Author(s): G. De Backer ; M. Vantorre ; C. Beels ; J. De Rouck ; P. Frigaard
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 579 –591
- DOI: 10.1049/iet-rpg.2009.0188
- Type: Article
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The performance of an array of closely spaced point absorbers is numerically assessed in a frequency domain model. Each point absorber is restricted to the heave mode and is assumed to have its own linear power take-off (PTO) system. Unidirectional irregular incident waves are considered, representing the wave climate at Westhinder on the Belgian Continental Shelf. The impact of slamming, stroke and force restrictions on the power absorption is evaluated and optimal PTO parameters are determined. For multiple bodies optimal control parameters (CP) are not only dependent on the incoming waves, but also on the position and behaviour of the other buoys. Applying the optimal control values for a single buoy to multiple closely spaced buoys results in a suboptimal solution for the array. Other ways to determine the PTO parameters are diagonal optimisation (DO) and individual optimisation. These methods are applied to two array layouts consisting of 12 buoys in a staggered grid and 21 buoys in an aligned grid. Compared to DO, it was found that individually optimising the CP increased the energy absorption at Westhinder with about 16–18% for the two layouts, respectively. - Author(s): A. Babarit ; B. Borgarino ; P. Ferrant ; A. Clément
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 592 –601
- DOI: 10.1049/iet-rpg.2009.0190
- Type: Article
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In this study, an array of two interacting wave energy converters with hydraulic power take off (PTOs) is considered. A time domain model is derived by using classical linear potential theory. Numerical simulations and parametric studies are performed in order to determine the influence of the distance on the capture width of each system in the array, in both regular and irregular waves. It is shown that when the systems are close, the front system (which meets the waves first) is more affected by wave interaction than the rear system. But the wave interactions decrease faster with the distance for the front system than for the rear system. For this latter system, it seems that the effect of wave interactions remains noticeable even far (14 diameters) from the first system. - Author(s): M. Rahm ; C. Boström ; O. Svensson ; M. Grabbe ; F. Bülow ; M. Leijon
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 602 –612
- DOI: 10.1049/iet-rpg.2009.0180
- Type: Article
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In this study, the design, construction, deployment and operation of an offshore underwater substation is discussed. The seabed placed substation interconnects three linear generator wave energy converters (WECs) at the Swedish Lysekil wave energy research site. The power from the WECs fluctuates because of their direct-driven topology. The generator voltage has varying electrical frequency and amplitude. To reduce the fluctuations, the individual voltages of the WECs are rectified and the power is added on a common DC-bus in the substation. The voltage is inverted, transformed and power is transmitted to an on-shore resistive load. The substation was retrieved on two occasions since the deployment in the spring of 2009. The functionality of the substation is validated by comparing voltage and current wave forms from Simulink with measured results from laboratory experiments. In addition, a sample of results from real offshore operation is illustrated and discussed. With a proportional-integral-derivative (PID)-regulator in the inverter control, the small fluctuations in the DC-bus voltage could be minimised. However, this would reduce the energy storage capability of the DC-link smoothing capacitors. - Author(s): M.E. Harrison ; W.M.J. Batten ; L.E. Myers ; A.S. Bahaj
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 613 –627
- DOI: 10.1049/iet-rpg.2009.0193
- Type: Article
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The actuator disc is a useful method for parameterising a tidal stream turbine in a solution of the Reynolds-averaged Navier–Stokes equations. An actuator disc is a region where similar forces are applied to a flow as would be imposed by a turbine. It is useful where large-scale flow characteristics are of interest, such as the far wake, free surface effects, or installation of multi-turbine arrays. This study compares the characteristics of the wake of an actuator disc, modelled using a steady solution to the Reynolds-averaged Navier–Stokes (RANS) simulated equations, with the k–ω shear stress transport (SST) turbulence model, to experimental data measured behind discs of various porosities. The results show that the wake of the experimental and modelled discs has similar characteristics; in both model and experiment, velocity in the near wake decreased as thrust coefficient increased. However, the near wake region in the experiment was shorter than simulated in the model because of near wake turbulence. This, combined with lower ambient turbulence levels in the model, meant that the far wake recovered further downstream, while showing similar overall trends in velocity and turbulence intensity. - Author(s): S.D. Weller ; T.J. Stallard ; P.K. Stansby
- Source: IET Renewable Power Generation, Volume 4, Issue 6, p. 628 –637
- DOI: 10.1049/iet-rpg.2009.0192
- Type: Article
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Much of the published work concerning the response and power output of closely spaced arrays of heaving wave energy devices concerns behaviour in regular waves only and is based on numerical analysis. To date, limited experimental work has been published and it remains unclear how device interactions predicted in idealised models relate to the response of proposed devices in realistic irregular wave-fields. Experimental measurements of the power absorbed by a small two-dimensional array of heaving devices in both regular and irregular waves in a wide flume are reported. In regular wave conditions, positive interactions (where the average power output of the array exceeds the same number of isolated devices) are measured. These tests indicate that the occurrence of positive interactions is largely dependent on the incident wave period and the performance of adjacent devices. Preliminary tests indicate that float responses tend to be smaller when subjected to short period irregular waves of matching peak frequency and standard deviation of surface elevation. The data presented provide an insight into interactions within irregular wave conditions and forms a basis for evaluating numerical models.
Editorial: Selected, expanded papers from EWTEC 2009
Analysis of a single point tensioned mooring system for station keeping of a contra-rotating marine current turbine
Estimating the loads and energy yield of arrays of wave energy converters under realistic seas
Experimental characterisation of flow effects on marine current turbine behaviour and on its wake properties
Experimental testing of the transverse horizontal axis water turbine
Comparison of wave power extraction by a compact array of small buoys and by a large buoy
Effects of floaters’ hydrodynamics on the performance of tightly moored wave energy converters
Modelling tidal energy extraction in a depth-averaged coastal domain
Regulatory, technical and financial challenges in the grid connection of wave energy devices
Three-dimensional hydrodynamic modelling of inland marine waters of Washington State, United States, for tidal resource and environmental impact assessment
Power absorption by closely spaced point absorbers in constrained conditions
Assessment of the influence of the distance between two wave energy converters on energy production
Offshore underwater substation for wave energy converter arrays
Comparison between CFD simulations and experiments for predicting the far wake of horizontal axis tidal turbines
Experimental measurements of irregular wave interaction factors in closely spaced arrays
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