- Dr Stallard joined the University of Manchester in October 2006 after two years of interdisciplinary research concern... moreDr Stallard joined the University of Manchester in October 2006 after two years of interdisciplinary research concerning the performance and economics of marine energy converters. This followed completion of a DPhil in the Ocean Engineering Dynamics group at Oxford University where he developed an interest in both vortex- and wave-induced forces. Since 2006 research activities have focused on arrays of wave and tidal stream devices.
Staff webpage: http://www.mace.manchester.ac.uk/people/staff/academic-staff/profile/?staffId=463
Recent and ongoing projects include:
ALL-TT (2014-15): Arrays of Long Life Tidal Turbines (Lead). EPSRC Newton UK-China Marine Energy development feasibility project with Northeast Normal University, Jilin Province, China
StepWEC (2013-16): Step Change for Wave Energy Conversion (Co-I). EPSRC Challenge research project led by Manchester University with University of Oxford, University of Bath.
X-MED (2012-15): Extreme Loading of Marine Energy Devices (Co-I). EPSRC Challenge research project led by Manchester University with University of Plymouth, University of Edinburgh, Scottish Assoc. Marine Sci.
WEC Wakes (2012-13): Experimental study of wave farms at DHI, Denmark (Co-investigator). EU FP7 Hydralab IV project led by University Ghent with 6 EU academic partners
ReDAPT (2010-14): Reliable Data Acquisition Platform Tidal: Energy Technologies Institute (ETI) industry-academia project led by Alstom Ocean. Co-investigator of Manchester contribution on behalf of EdF. Contributed to development and evaluation of massively parallel CFD methods for prediction of unsteady loads on a full-scale tidal stream turbine.
PerAWaT (2009–13): Performance of Wave and Tidal Array Systems. Energy Technologies Institute (ETI) industry-academia project led by GL-Garrad Hassan. Lead of work package: Designed, conducted and evaluated Turbine Array Experiments and supported development of commercial software for farm energy yield predictions.
EquiMAR (2008-11): Equitable Appraisal of Marine Energy: EU FP7 Collaboration project led by University of Edinburgh with 22 partners. Lead of work package: Developed and published procedures for assessing the economic viability of commercial scale wave- and tidal-stream projects. (www.equimar.org)
Tidal Stream Turbulence Research (2007–08): (Lead) Funded by NWDA Joule Centre.
Supergen Marine Phase I (2004-06): Economic Assessment of Large Scale Marine Energy Project, University of Lancaster.edit
Commercially available wind yield assessment models rely on superposition of wakes calculated for isolated single turbines. These methods of wake simulation fail to account for emergent flow physics that may affect the behaviour of... more
Commercially available wind yield assessment models rely on superposition of wakes calculated for isolated single turbines. These methods of wake simulation fail to account for emergent flow physics that may affect the behaviour of multiple turbines and their wakes and therefore wind farm yield predictions. In this paper wake-wake interaction is modelled computationally (CFD) and physically (in a hydraulic flume) to investigate physical causes of discrepancies between analytical modelling and simulations or measurements. Three effects, currently neglected in commercial models, are identified as being of importance: 1) when turbines are directly aligned, the combined wake is shortened relative to the single turbine wake; 2) when wakes are adjacent, each will be lengthened due to reduced mixing; and 3) the pressure field of downstream turbines can move and modify wakes flowing close to them.
Research Interests:
Page 1. An Experimental Study of Closely Spaced Point Absorber Arrays Tim Stallard, Peter K. Stansby and Alan J. Williamson School of Mechanical, Aerospace and Civil Engineering, University of Manchester. Manchester, UK. ABSTRACT ...
Deployment of co-located wind and tidal stream turbines is proposed as a method for reducing cost of electricity generation from either technology individually. Energy yield for wind turbines is modelled using an eddy viscosity wake model... more
Deployment of co-located wind and tidal stream turbines is proposed as a method for reducing cost of electricity generation from either technology individually. Energy yield for wind turbines is modelled using an eddy viscosity wake model and for tidal turbines using a method of self-similar superposition of wake deficits. Yaw strategy is considered for the tidal turbines, finding that although a continuous yaw strategy generates highest yield, a slack-tide strategy offers a suitable compromise with mechanical complexity. A case-study of the MeyGen site in the Pentland Firth is considered for co-location. The addition of 12MW of wind capacity to a 20MW tidal array results in a twofold increase in annual energy yield, compared to operating the tidal turbines alone. Phasing of the tidal cycle means that during a neap tide, the combined system may be entirely dependent on wind generation, but during a spring tide there is a regular tidal supply. Steady state loads for wind and current ...
Numerical Weather Prediction (NWP) models such as Weather Research and Forecasting (WRF) are widely used for prediction of wind resource at potential wind farm sites and, increasingly, for energy yield prediction. Such models solve a... more
Numerical Weather Prediction (NWP) models such as Weather Research and Forecasting (WRF) are widely used for prediction of wind resource at potential wind farm sites and, increasingly, for energy yield prediction. Such models solve a reduced form of the Navier-Stokes equations with typical resolution of 20-1000 m in the vertical axis and 1-2 km in the horizontal axes. Sub-grid models have previously been developed to represent wind farms including by modification of momentum sink and turbulence kinetic energy source terms within cells occupied by turbines. Here, semi-empirical wake models are employed to assess the extent of losses between turbines within a small group such as within a single WRF. Variation of thrust and power with wind speed and direction were obtained using the modified PARK and Eddy Viscosity methods in OpenWind. The influence of wake-losses on yield was evaluated through WRF simulations of resource only and with standard and modified turbine parameterizations. A...
Numerical studies have shown that the power capture and displacement amplitude of a body located within an array can be significantly greater than that of the same body in isolation. Linear models of array interactions, such as those... more
Numerical studies have shown that the power capture and displacement amplitude of a body located within an array can be significantly greater than that of the same body in isolation. Linear models of array interactions, such as those compared by Mavrakos and McIver (1997) [1] have received only limited comparison to experimental measurements and may be inadequate for predicting the response of even an isolated device, particularly if the float is of shallow draft (Vantorre et al., 2005 [2]). This paper compares numerical predictions of the response of an array of shallow draft heaving floats to experimental measurements. Numerical simulations are based on hydrodynamic forcing obtained from analysis of the full array using WAMIT. Comparisons are drawn between the displacement amplitude of an isolated device and devices within linear arrays of varying orientation. The main focus is on a linear array of 5 hemispherical floats similar to the arrangement studied in [1] and [3]. Response ...
Many studies have been published concerning the influence of the immersed shape (in still water) of a floating body on its response and power capture from ocean waves. With a few notable exceptions, much of this analysis has assumed small... more
Many studies have been published concerning the influence of the immersed shape (in still water) of a floating body on its response and power capture from ocean waves. With a few notable exceptions, much of this analysis has assumed small amplitude motion and linear models have been employed to predict response. The form of the upper surface of such a body has received little attention. Here, it is shown that the upper (top) surface of a floating body can be designed to ensure that the response amplitude of the body is within a specified value. This is of considerable importance to the survivability of wave energy devices. The approach used is to affect a large increase of both natural period and hydrodynamic damping for only a small change of float mass. These two factors impose a hydrodynamic limit on the displacement which may be exploited to avoid the 'end-stop' problem often encountered in wave device design. To demonstrate the change of response, experimental measureme...
Research Interests:
Many studies have been published concerning numerical predictions of wave energy device and array responses in both idealised and realistic wave-fields. However, there are few comparisons of predicted array response to experimental... more
Many studies have been published concerning numerical predictions of wave energy device and array responses in both idealised and realistic wave-fields. However, there are few comparisons of predicted array response to experimental measurements, particularly for irregular waves. Although time domain models are widely used for modelling the behaviour of isolated devices, the response of arrays has typically been predicted by assuming linear superposition of steady-state response to regular waves. In this experimental study, the validity of approaches based on linear theory is investigated for an isolated device and array of two devices at 4-and 3-radius centre-to-centre spacing. A range of regular wave frequencies over which response is linear with wave height is identified and these response measurements are used to predict time-varying response to an irregular wave. In irregular waves, the influence of previous wave cycles, the time required for the device to reach a steady respons...
Experiments have been performed in the DHI Shallow Water Wave Basin (Denmark), on large arrays of up to 25 heaving point absorber Wave Energy Converters (WECs), for a range of geometric layout configurations and wave conditions. WEC... more
Experiments have been performed in the DHI Shallow Water Wave Basin (Denmark), on large arrays of up to 25 heaving point absorber Wave Energy Converters (WECs), for a range of geometric layout configurations and wave conditions. WEC response, surge forces on the WECs and modification of the wave field are measured to provide data for the understanding of WEC array interactions/effects. Wave conditions studied, include regular, polychromatic, long- and short-crested irregular waves. The experimental arrangement and the obtained database are presented. For irregular long-crested waves, up to 18.1% attenuation of significant wave height is observed downwave a rectilinear array of 25 heaving WECs.
ABSTRACT A moored multi-body line absorber is an attractive option for offshore wave energy conversion. Laboratory studies have been undertaken to determine capture width with multi-mode excitation and heave resonance for the three-float... more
ABSTRACT A moored multi-body line absorber is an attractive option for offshore wave energy conversion. Laboratory studies have been undertaken to determine capture width with multi-mode excitation and heave resonance for the three-float system M4 where the adjacent float spacing is about half a typical wavelength giving anti-phase forcing. The floats increase in diameter and draft from bow to stern and the bow and mid float are rigidly connected by a beam. A hinge with a damper above the mid float absorbs power from the relative rotation between the bow/mid float and the stern float. The resonant heave frequency for each float is different. Anti-phase surge forcing between mid and stern floats is substantial, while there is no hydrostatic stiffness producing resonance. This represents a hydrodynamically complex system and the laboratory experiments indicate high overall capture widths in irregular waves across a range of peak periods without damping optimisation. With different spectral peakedness and directional spread, the capture width is greater than 20 % of a wavelength (based on the energy period) across a range of peak periods typical of an offshore site for floats with a rounded base. The maximum capture width was about 37 % of a wavelength with rounded base floats; having rounded rather than flat bases increased energy capture by up to 60 % by reducing energy losses due to drag. For floats with flat bases comparisons with a geometrically scaled device five times larger and with similar magnitudes of equivalent damping showed similar capture widths as a proportion of wavelength.
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ABSTRACT A line absorber consisting of three cylindrical floats is shown to have high crest capture widths for wave energy conversion across a broad band of frequencies. The bow, mid and stern floats are small, medium and large... more
ABSTRACT A line absorber consisting of three cylindrical floats is shown to have high crest capture widths for wave energy conversion across a broad band of frequencies. The bow, mid and stern floats are small, medium and large respectively; the floats are spaced about half a wavelength apart so that forces and motion of adjacent floats are substantially in anti-phase. The bow and mid float are rigidly connected by a beam and a beam from the stern float is connected to a hinge above the mid float for power take off. The draft of the stern float enables heave resonance at a prominent wave frequency and the smaller draft of the mid float provides resonance at a somewhat lower frequency. Experimental results at about 1:8 scale show capture widths greater than 25% of a wavelength in regular waves and greater than 20% of a wavelength in irregular waves across a broad range of wave periods. A time-stepping model for regular waves with coefficients from linear diffraction theory showed similar power prediction with a generic drag coefficient of 1.8. The model shows the importance of surge forcing and heave resonance. The model also shows that reducing drag coefficient will increase capture width.
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A brief review of the literature is provided on the characteristics of marine currents and the approaches used for simulating tidal turbines. The feasibility of using CFD models to simulate time-dependent turbulent flow around a tidal... more
A brief review of the literature is provided on the characteristics of marine currents and the approaches used for simulating tidal turbines. The feasibility of using CFD models to simulate time-dependent turbulent flow around a tidal turbine is then explored. Two different approaches for ...
Research Interests:
There are a great many studies that investigate the force on bodies in periodic oscillatory motions, but almost no studies that focus on the kinds of fluid loading that are of great relevance to offshore structure designers; namely, the... more
There are a great many studies that investigate the force on bodies in periodic oscillatory motions, but almost no studies that focus on the kinds of fluid loading that are of great relevance to offshore structure designers; namely, the problem of a cylinder subjected to the forces from ocean wave groups, especially those of large amplitude. In this study, we move a vertical circular cylinder in non-periodic horizontal orbital motion through stationary fluid, in a towing tank at Re≲1.1510^4. The motion is chosen to represent the relative fluid motion incident upon a horizontal cylinder with its axis aligned parallel to the crest of a large ocean wave group, as defined by the NewWave formulation. The vector form of the well-known Morison equation provides a good representation of the measured forces. By measuring the force components in the radial and azimuthal directions, we clearly demonstrate that the presence of vigorous force fluctuations at a higher frequency than the orbital motion are associated with vortex shedding that is otherwise masked by the choice of coordinates. We find vortex frequencies comparable with those for flow past fixed bodies and a transverse force magnitude similar to the fixed flow case at the same average speed. Finally, we show that by retaining only an azimuthal (constant) drag coefficient term to represent the fluid loading throughout the wave group orbits, the resolved X- and Y-force fluctuations agree well with measured forces. This demonstrates that we can obtain a reasonable estimate of time-varying forces using a single term. It is expected that such a simple force representation will become less effective in shallower fluids and for smaller wave amplitudes relative to body size.
http://dx.doi.org/10.1098/rspa.2008.0345
http://dx.doi.org/10.1098/rspa.2008.0345
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Measurements of breaking wave forces on vertical circular columns have been re-analysed, where substantial magnification over forces because of non-breaking non-linear waves may occur, by a factor of up to 2.8. The analysis shows that... more
Measurements of breaking wave forces on vertical circular columns have been re-analysed, where substantial magnification over forces because of non-breaking non-linear waves may occur, by a factor of up to 2.8. The analysis shows that this factor increases as the depth parameter kd decreases, being close to unity for kd > 1.5 (k is the wave number and d is depth). Non-breaking wave forces are predicted reasonably by non-linear stream function wave theory using Morisons equation with empirical drag and inertia coefficients. A study was then made of the magnitude of wave overturning moment in relation to hub moment due to wind on standard 2 and 5 MW turbines with a 6 m diameter column. This showed that the wave moment in extreme conditions is greater than wind hub moment for depths greater than about 7 and 13 m for the 2 and 5 MW turbines, respectively. Monopiles are normally used in depths below about 30 m and extreme moments due to waves occur when waves are depth-limited and defined by the Miche criterion, well below the limiting value of kd for the onset of depth-induced breaking. The maximum moment for these depths is expected to be predicted reasonably.
http://dx.doi.org/10.1049/iet-rpg.2012.0205
http://dx.doi.org/10.1049/iet-rpg.2012.0205
Research Interests:
Many studies have been published concerning the influence of the immersed shape (in still water) of a floating body on its response and power capture from ocean waves. With a few notable exceptions, much of this analysis has assumed small... more
Many studies have been published concerning the influence of the immersed shape (in still water) of a floating body on its response and power capture from ocean waves. With a few notable exceptions, much of this analysis has assumed small amplitude motion and linear models have been employed to predict response. The form of the upper surface of such a body has received little attention. Here, we show how the shape of the upper (top) surface of a floating body can be designed to ensure that the response amplitude of the body is within a specified value. This is of considerable importance to the survivability of wave energy devices. The approach used is to achieve a large increase of both natural period and hydrodynamic damping for only a small change of float mass. These two factors impose a hydrodynamic limit on the displacement which may be exploited to avoid the `end-stop' problem often encountered in wave device design. To demonstrate the change of response, experimental measurements are presented of the response of an axisymmetric float with rounded base and conical upper surface with rounded perimeter due to a range of regular, irregular and focused wave conditions. Power extraction is not considered since the mechanically undamped response represents the worst case. In contrast to a simple, straight-sided axisymmetric float, a smaller change of mass is required to satisfy a particular response amplitude limit. Although a significant reduction is not expected, hydrodynamic damping may reduce with increasing physical scale, and this remains to be quantified.
http://dx.doi.org/10.1016/j.apor.2009.08.001
http://dx.doi.org/10.1016/j.apor.2009.08.001
Research Interests:
Numerical models which account for the multiple response modes of floating wave energy converters (WECs) in operating conditions require experimental data for validation. Measurement and observation of complex hydrodynamic mechanisms are... more
Numerical models which account for the multiple response modes of floating wave energy converters (WECs) in operating conditions require experimental data for validation. Measurement and observation of complex hydrodynamic mechanisms are also required to inform the development of modelling tools suitable for the simulation of response to extreme waves. Experimental measurements are reported of the motion of an axisymmetric float to regular and near-focused waves. The mechanical system, incident wave conditions and response in a 2D vertical plane are detailed to facilitate comparison to numerical simulations. The system comprises a heaving float connected to a counterweight by an inextensible cable over two pulleys to provide a simplified representation of the slowly varying surge constraint of a mooring system. Translation of the float is measured using an optical encoder. Motion in heave, surge and pitch are also determined by a position identification method based on analysis of video footage. For low frequency regular waves, the float prescribes an elliptical trajectory and the variation of response amplitude with wave amplitude is linear. At higher frequencies, drift of up to one-third of the float radius is observed and the float oscillates along an arc. More complex motions are observed due to the three large amplitude waves of a near-focused wave group. During these waves the upper surfaces of the float are partly immersed and motion occurs in heave, surge and pitch.
http://dx.doi.org/10.1016/j.apor.2012.10.008
http://dx.doi.org/10.1016/j.apor.2012.10.008
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It is well known that a wake will develop downstream of a tidal stream turbine owing to extraction of axial momentum across the rotor plane. To select a suitable layout for an array of horizontal axis tidal stream turbines, it is... more
It is well known that a wake will develop downstream of a tidal stream turbine owing to extraction of axial momentum across the rotor plane. To select a suitable layout for an array of horizontal axis tidal stream turbines, it is important to understand the extent and structure of the wakes of each turbine. Studies of wind turbines and isolated tidal stream turbines have shown that the velocity reduction in the wake of a single device is a function of the rotor operating state (specifically thrust), and that the rate of recovery of wake velocity is dependent on mixing between the wake and the surrounding flow. For an unbounded flow, the velocity of the surrounding flow is similar to that of the incident flow. However, the velocity of the surrounding flow will be increased by the presence of bounding surfaces formed by the bed and free surface, and by the wake of adjacent devices. This paper presents the results of an experimental study investigating the influence of such bounding surfaces on the structure of the wake of tidal stream turbines.
http://dx.doi.org/10.1098/rsta.2012.0159
http://dx.doi.org/10.1098/rsta.2012.0159
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Time-varying thrust has been measured on a rotor in shallow turbulent flow at laboratory scale. The onset flow has a turbulence intensity of 12% at mid depth and a longitudinal turbulence length scale of half the depth, about 5 times the... more
Time-varying thrust has been measured on a rotor in shallow turbulent flow at laboratory scale. The onset flow has a turbulence intensity of 12% at mid depth and a longitudinal turbulence length scale of half the depth, about 5 times the vertical scale, typical of shallow flows. The rotor is designed to have thrust and power coefficient variations with tip speed ratio close to that of a full-scale turbine. Three extreme probability distributions give similar thrust exceedance values with the Type 1 Pareto in mid range which gives 1:100, 1:1000 and 1:10000 exceedance thrust forces of 1.38, 1.5 and 1.59 times the mean value. With opposing waves superimposed the extreme thrust distribution has a very similar distribution to the turbulent flow only. Exceedance forces are predicted by superposition of a drag force with drag coefficient of 2.0 based on the wave particle velocity only and with an unchanged mean thrust coefficient of 0.89. These values are relevant for the design of support structures for marine turbines.
http://dx.doi.org/10.1016/j.jfluidstructs.2014.09.012
http://dx.doi.org/10.1016/j.jfluidstructs.2014.09.012
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An effective way of using computational fluid dynamics (CFD) to simulate flow about a rotating devicefor example, a wind or marine turbineis to embed a rotating region of cells inside a larger, stationary domain, with a sliding interface... more
An effective way of using computational fluid dynamics (CFD) to simulate flow about a rotating devicefor example, a wind or marine turbineis to embed a rotating region of cells inside a larger, stationary domain, with a sliding interface between. This paper describes a simple but effective method for implementing this as an internal Dirichlet boundary condition, with interfacial values obtained by interpolation from halo nodes. The method is tested in two finite-volume codes: one using block-structured meshes and the other unstructured meshes. Validation is performed for flow around simple, isolated, rotating shapes (cylinder, sphere and cube), comparing, where possible, with experiment and the alternative CFD approach of fixed grid with moving walls. Flow variables are shown to vary smoothly across the sliding interface. Simulations of a tidal-stream turbine, including both rotor and support, are then performed and compared with towing-tank experiments. Comparison between CFD and experiment is made for thrust and power coefficients as a function of tip-speed ratio (TSR) using Reynolds-averaged NavierStokes turbulence models and large-eddy simulation (LES). Performance of most models is good near the optimal TSR, but simulations underestimate mean thrust and power coefficients in off-design conditions, with the standard k ϵ turbulence model performing noticeably worse than shear stress transport k ω and Reynolds-stress-transport closures. LES gave good predictions of mean load coefficients and vital information about wake structures but at substantial computational cost. Grid-sensitivity studies suggest that Reynolds-averaged NavierStokes models give acceptable predictions of mean power and thrust coefficients on a single device using a mesh of about 4 million cells.
http://dx.doi.org/10.1002/fld.3849
http://dx.doi.org/10.1002/fld.3849
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This paper presents results from numerical simulations of a 3-bladed horizontal axis tidal stream turbine. Initially, Reynolds Averaged Navier Stokes (RANS) kω Shear Stress Transport eddy-viscosity and Launder Reece Rodi models were used... more
This paper presents results from numerical simulations of a 3-bladed horizontal axis tidal stream turbine. Initially, Reynolds Averaged Navier Stokes (RANS) kω Shear Stress Transport eddy-viscosity and Launder Reece Rodi models were used for code validation and testing of a newly implemented sliding mesh technique for an unstructured finite volume code. Wall- and blade-resolved large-eddy simulations (LES) were then performed to study the complete geometry at various tip speed ratios (TSR). Thrust and power coefficients were compared to published experimental measurements obtained from a towing tank for a range of TSR (4, 5, 6, 7, 8, 9 and 10) at a fixed hub pitch angle. A strong meandering is observed downstream of the supporting tower due to interaction between the detached tip vortices and vortex shedding from the support structure. The wake profiles and rate of recovery of velocity deficit show high sensitivity to the upstream turbulence intensities. However, the mean thrust and power coefficients were found to be less sensitive to the upstream turbulence. Comparisons between RANS and LES are also presented for the mean sectional blade pressures and mean wake velocity profiles. The paper also presents an overview of modelling and numerical issues relating to simulations for such rotating geometries.
http://dx.doi.org/10.1016/j.ijheatfluidflow
http://dx.doi.org/10.1016/j.ijheatfluidflow
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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... more
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.
http://dx.doi.org/10.1049/iet-rpg.2009.0192
interactions within irregular wave conditions and forms a basis for evaluating numerical models.
http://dx.doi.org/10.1049/iet-rpg.2009.0192
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Experiments have been performed in the Shallow Water Wave Basin of DHI (Hrsholm, Denmark), on large arrays of up to 25 heaving point absorber type Wave Energy Converters (WECs), for a range of geometric layout configurations and wave... more
Experiments have been performed in the Shallow Water Wave Basin of DHI (Hrsholm, Denmark), on large arrays of up to 25 heaving point absorber type Wave Energy Converters (WECs), for a range of geometric layout configurations and wave conditions. WEC response and modifications of the wave field are measured to provide data for understanding WEC array interactions and to evaluate array interaction numerical models. Each WEC consists of a buoy with a diameter of 0.315 m and power take-off (PTO) is modeled by realizing friction based energy dissipation through damping of the WECs motion. Wave gauges are located within and around the WEC array. Wave conditions studied include regular, polychromatic, long- and short-crested irregular waves. A rectilinear arrangement of WEC support structures is employed such that several array configurations can be studied. In this paper, the experimental arrangement and the obtained database are presented. Also, results for wave height attenuation downwave a rectilinear array of 25 heaving WECs are presented, for the case of irregular waves. Up to 16.3% and 18.1% (long-crested) and 11.2% and 18.1% (short-crested waves) reduction in significant wave height is observed downwave the WEC array, for the radiated wave field only and for the combination of incident-diffracted-radiated (perturbed) wave field, respectively. Using spectra at different locations within and around the array, the wave field modifications are presented and discussed.
http://dx.doi.org/10.3390/en7020701
http://dx.doi.org/10.3390/en7020701
Research Interests:
Many devices have been proposed for generating electricity from the oscillatory motion of a floating body in waves which are generally irregular. This study undertakes numerical modelling and small-scale experimental testing of a power... more
Many devices have been proposed for generating electricity from the oscillatory motion of a floating body in waves which are generally irregular. This study undertakes numerical modelling and small-scale experimental testing of a power take-off system for a heaving float. A power take off system is employed to provide high speed rotational input to a standard induction generator. A numerical model of the coupled hydrodynamic and electrical system is described with particular focus on the effect of generator control strategy on the time-varying response and power output of the system. The numerical model with three empirical hydrodynamic coefficients is calibrated against experimental measurements in regular waves. The control method includes a static characteristic and a proportional integral (PI) controller to maximise average power output whilst reducing the peak rate of change of torque in the driveshaft compared to a system with no control applied. The control strategy is implemented within a model drive-train with a geometric scale of 1:67. Experimental tests are reported and model predictions of time-varying response have a form similar to the measured response. Average power output from irregular waves is predicted within 11% for frequencies less than 1.3 Hz (periods greater than 6.3 s full scale) and wave heights greater than 30 mm (2 m full scale).
http://dx.doi.org/10.1016/j.ijome.2014.09.001
http://dx.doi.org/10.1016/j.ijome.2014.09.001
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A method is presented of estimating the responses of axisymmetric bodies floating in spread irregular seas, using a Laplace transfer-function formulation of a floating body time-domain model. A general-case spread-wave model is... more
A method is presented of estimating the responses of axisymmetric bodies floating in spread irregular seas, using a Laplace transfer-function formulation of a floating body time-domain model. A general-case spread-wave model is formulated, using separate wave excitation transfer functions, and a simplification of this approach is proposed, reducing both model complexity and computation time. Responses are computed using both approaches and a comparison made to assess the circumstances in which the simplified approach may be used effectively. The results are also interpreted to highlight the implications of using an equivalent unidirectional wave as an approximation to a spread wave.
http://dx.doi.org/10.1016/j.oceaneng.2006.10.004
http://dx.doi.org/10.1016/j.oceaneng.2006.10.004
Research Interests:
Among the assumptions upon which linear time-invariant models of floating bodies are based is that the body motions are so small that any change in the bodys angular position can be disregarded. However, it is often a major design... more
Among the assumptions upon which linear time-invariant models of floating bodies are based is that the body motions are so small that any change in the bodys angular position can be disregarded. However, it is often a major design requirement of a wave energy conversion device that the response amplitude is large, thereby invalidating one of the assumptions of the linear model. In particular, the immersed geometry of a body undergoes considerable variation when it is moved in pitch. With regard to this we investigate the difference in performance between a quasi-linear model in which the change of immersed surface is modelled by time-varying parameters and a basic linear model in which the immersed surface is time-invariant. The time-varying parameter model is realized by interpolation between the appropriate parameter values of a set of linear time-invariant (LTI) models derived for the different immersed surfaces that occur at discrete body displacements. It is shown that the responses predicted using the time-varying parameter model are closer to those measured experimentally than those of a standard frequency-domain model. Particular improvement occurs when the responses are large, such as at or near the resonance frequency. A problem which may limit the general use of the model is also discussed.
http://dx.doi.org/10.1016/j.apor.2007.05.001
http://dx.doi.org/10.1016/j.apor.2007.05.001
Research Interests:
The paper presents a time domain model of a heaving buoy wave-energy converter and investigates the tuning problem in irregular seas. The tuning issue is addressed by employing both fixed (passive) and adaptive (active) power-take-off... more
The paper presents a time domain model of a heaving buoy wave-energy converter and investigates the tuning problem in irregular seas. The tuning issue is addressed by employing both fixed (passive) and adaptive (active) power-take-off settings. The fixed power-take-off tuning approach includes models based on tuning the device natural frequency to either the energy frequency or peak frequency of the sea-state or a weighted average of several peak frequencies. The adaptive tuning approaches employ a sliding discrete Fourier transform frequency analysis, or a time-series analysis of the measured wave elevation and device velocity to estimate a localized dominant wave frequency and hence calculate power-take-off settings. The paper presents details of these tuning techniques by discussing issues related to the modelling, simulation, and predicted power captures for each method. A comparative study of each method along with practical implications of the results and recommendations are also presented.
http://dx.doi.org/10.1243/09576509JPE291
http://dx.doi.org/10.1243/09576509JPE291
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The paper aims at introducing practical methods for power capture performance enhancement of a heaving wave energy converter in irregular seas. The optimum control solution requires tuning to wave frequency based on wave force... more
The paper aims at introducing practical methods for power capture performance enhancement of a heaving wave energy converter in irregular seas. The optimum control solution requires tuning to wave frequency based on wave force information. However, identification of the wave frequency in irregular seas is considered to be a complex and difficult task. This is partly due to technical difficulties in determination of the wave force. Besides, there are no clear guidelines for identification of wave frequency from an irregular sea state based wave force information. In a typical application, one of the available sources of information about the wave properties is the wave elevation record. The proposed approach presents a method for estimation of the wave frequency information from the wave elevation data by using signal processing and filtering techniques. The proposed method uses filters to generate an estimation of wave force information, which is used to identify the local wave frequency by method of a time-series analysis of the data. This wave frequency information is then used in tuning the device. The details of the proposed techniques, the model of the wave energy converter, the simulated sea states and the related simulation results are also presented.
http://dx.doi.org/10.1016/j.oceaneng.2010.12.016
http://dx.doi.org/10.1016/j.oceaneng.2010.12.016
Research Interests:
We examine the economic and environmental impact that the installation of 3 GW of marine energy capacity would have on Scotland. This is not a forecast, but a projection of the likely effects of meeting the Scottish Government's targets... more
We examine the economic and environmental impact that the installation of 3 GW of marine energy capacity would have on Scotland. This is not a forecast, but a projection of the likely effects of meeting the Scottish Government's targets for renewable energy through the development of a marine energy sector. Energy, with a particular focus on renewables, is seen by the Scottish Government as a key sector, with high growth potential and the capacity to boost productivity (Scottish Government, 2007a. The Government Economic Strategy. The Scottish Government, Edinburgh). The key nature of this sector has been identified through targets being set for renewable energy to achieve environmental and economic benefits. Using a regional computable general equilibrium (CGE) model of Scotland we show that the development of a marine energy sector can have substantial and beneficial impacts on GDP, employment and the environment over the lifetime of the devices, given the encouragement of strong indigenous inter-industry linkages. Furthermore, there are also substantial legacy effects that persist well beyond the design life of the devices.
http://dx.doi.org/10.1016/j.enpol.2008.02.020
http://dx.doi.org/10.1016/j.enpol.2008.02.020
Research Interests:
Conversion of marine energy sources, including ocean waves and tidal currents, into electricity is a rapidly developing industry. Although many technologies have been proposed and some have generated electricity at full scale, it is... more
Conversion of marine energy sources, including ocean waves and tidal currents, into electricity is a rapidly developing industry. Although many technologies have been proposed and some have generated electricity at full scale, it is difficult to predict which technology will be economic at large scales of installation. Several studies have been conducted which estimate the cost of electricity on the basis of schematic designs. However, each study represents a best estimate of the future cost based on current design details and direct comparison between the results of these studies is not straightforward. A methodology for directly comparing different wave energy concepts and potential locations would be beneficial to aid investment decisions. In this study we describe how the established data envelopment analysis technique could be employed for this purpose. The developed model is employed to rank the efficacy with which several types of conceptual and prototype wave energy conversion technologies generate electricity from the wave energy resource available at UK and US sites.
http://dx.doi.org/10.1016/j.ejor.2007.01.021
http://dx.doi.org/10.1016/j.ejor.2007.01.021
Research Interests:
This paper offers a holistic approach to the evaluation of an ocean renewable energy (ORE) technology type or specific project in order to provide a comprehensive assessment of both narrow economic and broader socioeconomic performance.... more
This paper offers a holistic approach to the evaluation of an ocean renewable energy (ORE) technology type or specific project in order to provide a comprehensive assessment of both narrow economic and broader socioeconomic performance. This assessment incorporates methods from three pillars areas: Economic - financial returns and efficient use of resources, Social -employment, social and community cohesion and identity, and Environmental - including the physical environment and pollution. These three pillars are then considered in the broader context of governance. In order to structure this evaluation, a novel parameter space model was created, defined by the three pillars and by the scale of the system under assessment. The scale of the system ranged from individual components of an ORE project; to projects comprising of a number of devices; through to a geographic regions in which multiple farms may be deployed. The parameter space consists of an inner circle representing the boundary of interest for a private investor, or a firm, developing an ORE project. The outer circle is characterised by assessment tools typically employed at the broader stakeholder level including economic, social, and environmental methods that can be employed at local, regional or national scale and which are typically employed to inform policy and decision making regarding ORE. Governance sets the stage within which management occurs. Wider impacts to the firm undertaking the project will take into account “externalities” of the project across the three fields. In this model, key methods identified are mapped onto this parameter space and the connectivity explored. The paper demonstrates that the three pillars are inter-connected and each must be considered in any meaningful assessment of ORE sustainability. An integrated assessment approach has the ability to address both the private and the public aspects of an ORE development. This analysis provides insights on existing best practice, but also reveals the potential for disconnect between an ORE project’s commercial viability and its contribution to environmental and social goals.