Marine Renewable Energy

Effects of 3-D channel blockage and turbulent wake mixing on the limit of power extraction by tidal turbines

by Takafumi Nishino

T. Nishino and R.H.J. Willden
International Journal of Heat and Fluid Flow (accepted, May 2012)

Three-dimensional incompressible Reynolds-averaged Navier-Stokes (RANS) computations are performed for water flow past... more Three-dimensional incompressible Reynolds-averaged Navier-Stokes (RANS) computations are performed for water flow past an actuator disk model (representing a tidal turbine) placed in a rectangular channel of various blockages and aspect ratios. The study focuses on the effects of turbulent mixing behind the disk, as well as on the effects of channel blockage and aspect ratio on the prediction of the hydrodynamic limit of power extraction. To qualitatively account for the effect of turbulence generated by the turbine (rather than by the shear flow behind the turbine), we propose a new approach, called a blade-induced turbulence model, which does not use any additional model coefficients other than those used in the original RANS turbulence model. Results demonstrate that the power removed from the mean flow by the disk increases as the strength of turbulent mixing behind the disk increases, being consistent with the turbulent shear stress on the interface between the bypass and core flow passages acting in such a way as to decelerate the bypass flow and accelerate the core flow. The channel aspect ratio also affects the flow downstream of the disk but has less influence upstream of the disk; hence its effect on the limit of power extraction is relatively minor compared to that of the channel blockage, which is shown to be significant but satisfactorily estimated using one-dimensional inviscid theory previously reported in the literature.

Quantifying the Global Wave Power Resource

by Clym Stock-Williams

Published in Journal of Renewable Energy, 2012

Justifying continued development and large-scale deployment of Wave Energy Converters (WECs) requires quantification... more Justifying continued development and large-scale deployment of Wave Energy Converters (WECs) requires quantification of the potential resource. Currently, estimates are available for individual countries or, at low accuracy, for global resource. Additionally, existing estimates do not provide insight into potential future markets, i.e. the location of the resource.

Here, NOAA WaveWatch III data are analysed for a 6-year period to calculate wave energy potential. The global market is then quantified by calculating the energy flux across a line 30 nautical miles offshore. Results are presented by country, continent, hemisphere and for the globe.

Confidence values are also presented in the form of 95% confidence intervals. These limits provide insight into the uncertainty associated with the length of dataset used and the variability of the resource. This enables direct comparison with other resource assessment studies, whether using numerical model or measured data. An extensive survey of previous global and regional resource estimates is also conducted, in order to compare both results and methods.

Supplementing this, extractable resource is estimated by considering the deployment of an illustrative WEC (Pelamis P2). The global wave power resource is 2.11 ± 0.05 TW, of which 4.6% is extractable with the chosen WEC configuration.

Realizing the Potential of Tidal Currents and the Efficiency of Turbine Farms in a Channel

by Ross Vennell

published as : Realizing the Potential of Tidal Currents and the Efficiency of Turbine Farms in a Channel Renewable Energy, 2012, 47, 95-102

Tidal turbines in strong flows have the potential to produce significant power. However, not all of this potential can... more Tidal turbines in strong flows have the potential to produce significant power. However, not all of this potential can be realized when gaps between turbines are required to allow navigation along a channel. A review of recent works is used to estimate the scale of farm required to realize a significant fraction of a channel's potential. These works provide the first physically coherent approach to estimating the maximum power output from a given number of turbines in a channel. The fraction of the potential realisable from a number of turbines, a farm's fluid dynamic efficiency, is constrained by how much of the channel's cross-section the turbines are permitted to occupy and an environmentally acceptable flow speed reduction. Farm efficiency increases as optimally tuned turbines are added to its cross-section, while output per turbine increases in tidal straits and decreases in shallow channels. Adding rows of optimally tuned turbines also increases farm efficiency, but with a diminishing return on additional rows. The diminishing return and flow reduction are strongly influenced by how much of the cross-section can be occupied and the dynamical balance of the undisturbed channel. Estimates for two example channels show that realizing much of the MWatt potential of shallow channels may well be possible with existing turbines. However unless high blockage ratios are possible, it will be more difficult to realize the proportionately larger potential of tidal straits until larger turbines with a lower optimum operating velocity are developed.

Fisheries, the environment and offshore wind farms: Location, location, location.

by Magnus Johnson

Co authored with Dale Rodmell

Windfarms and fishers are conflicting for space.  Developers have money and infrastructure to make their case... more Windfarms and fishers are conflicting for space.  Developers have money and infrastructure to make their case while fishers are often marginalised.

Estimating the power potential of tidal currents and the impact of power extraction on flow speeds

by Ross Vennell

Renewable Energy, Volume 36, Issue 12, December 2011, Pages 3558-3565
http://dx.doi.org/10.1016/j.renene.2011.05.011

A simple method for estimating the potential of currents in tidal channels to produce power is presented. The method... more A simple method for estimating the potential of currents in tidal channels to produce power is presented. The method only requires measurement of the peak tidal volume transport through the channel without turbines, along with a bottom drag coefficient and the channel's dimensions. A recent existing method for estimating potential requires measurements of the undisturbed transport as well as water levels at both ends of the channel to give the head loss. The adaptation of the existing method presented here exploits analytic solutions for the transport and optimal farm drag coefficient and does not require measurement of the head loss. The equations presented allow both the channel's potential and the flow reduction due to power extraction to be estimated using a calculator. Thus the presented method has much of the ease of use of the older KE flux method, but is more reliable as it includes retardation of the flow by the turbines. The presented method can be used for the initial assessment of channels to determine if the additional measurements required to use the existing method are warranted. It can also be used where the headloss in the channel is too small to measure reliably. The presented equations enable the maximum power available to be simply traded off against environmentally acceptable flow speed reduction. The presented method is applied to two example channels. Cook Strait NZ has an estimated potential of 15. GW, while the entrance channel to Kaipara Harbour has a potential between 110. MW and 240. MW

Tuning turbines in a tidal channel

by Ross Vennell

Journal of Fluid Mechanics, Volume 663, 25 November 2010, Pages 253-267
http://dx.doi.org/10.1017/S0022112010003502

As tidal turbine farms grow they interact with the larger scale flow along a channel by increasing the channel's drag... more As tidal turbine farms grow they interact with the larger scale flow along a channel by increasing the channel's drag coefficient. This interaction limits a channel's potential to produce power. A 1D model for a tidal channel is combined with a theory for turbines in a channel to show that the tuning of the flow through the turbines and the density of turbines in a channel's cross-section also interact with the larger scale flow, via the drag coefficient, to determine the power available for production. To maximise turbine efficiency, i.e. the power available per turbine, farms must occupy the largest fraction of a channel's cross-section permitted by navigational and environmental constraints. Maximising of power available with these necessarily densely packed farms requires turbines to be tuned for a particular channel and turbine density. The optimal through-flow tuning fraction varies from near 1/3 for small farms occupying a small fraction of the cross-section, to near 1 for large farms occupying most of the cross-section. Consequently, tunings are higher than the optimal through-flow tuning of 1/3 for an isolated turbine from the classic turbine theory. Large optimally tuned farms can realise most of a channel's potential. Optimal tunings are dependent on the number of turbines per row, the number of rows, as well as the channel geometry, the background bottom friction coefficient and the tidal forcing.

Tuning tidal turbines in-concert to maximise farm efficiency

by Ross Vennell

Journal of Fluid Mechanics, Volume 671, 25 March 2011, Pages 587-604
http://dx.doi.org/10.1017/S0022112010006191

Tuning is essential to maximise the output of turbines extracting power from tidal currents. To realise a large... more Tuning is essential to maximise the output of turbines extracting power from tidal currents. To realise a large fraction of a narrow channel's potential, rows of turbines not only have to be tuned for a particular tidal channel, they must also be tuned in the presence of all the other rows, i.e. tuned in-concert. The necessity for in-concert tuning to maximise farm efficiency occurs because the tuning of any one row affects a channel's total drag coefficient and hence the flow through all other rows. Surprisingly, in several circumstances the optimal in-concert tunings are the same or almost the same for all rows. Firstly, in both constricted and unconstricted channels, rows with the same turbine density have the same optimal tuning. Secondly, turbine rows in channels with a quasi-steady dynamical balance typically have almost the same optimal in-concert tunings, irrespective of their turbine density or any channel constrictions. Channel constrictions, occupying a large fraction of the cross-section or adding more rows of turbines, also make optimal tunings more uniform between rows. Adding turbines to a cross-section increases a farm's efficiency. However, in a law of diminishing returns for quasi-steady channels, turbine efficiency (the output per turbine) decreases as turbines are added to a cross-section. In contrast, for inertial channels with only moderate constrictions, turbine efficiency increases as turbines are added to a cross-section.

The Development of Marine Based Wind Energy Generation and Inshore Fisheries In UK Waters: Are They Compatible?

by Magnus Johnson

This paper deleted itself recently - I have replaced it. The first author is Dale Rodmell, now at the NFFO, dale.rodmell@nffo.org.uk

Offshore wind energy is set to make an increasing presence in a number of European countries in the coming years.... more Offshore wind energy is set to make an increasing presence in a number of European countries in the coming years. Developments potentially add to increasing pressures upon
inshore fisheries, which raise questions about the compatibility of the two types of economic activity. We examine the potential ecological effects of the development of offshore windfarms upon fishery resources. We also study the direct effects upon fishing activity and techniques, by focussing on those effects occurring at each stage of a windfarm’s development. Consideration is then given to how fisheries issues and industry concerns are represented and accounted for in national planning system decision making, drawing on early
experience in the UK. In order to improve impact prediction, there is a need for further research to develop understanding of environmental and ecological affects upon fisheries, principally noise and vibration and electromagnetic field effects. However, of particular importance for the fishing industry is better understanding of the effects of installations on the range of fishing techniques undertaken. This would feed into judgements on restricting fishing activity for safety reasons, and assist in determining the socio-economic impact of
developments on fishing communities so that the location of developments can be planned appropriately to maximise their compatibility with fisheries. Fishing groups should be
involved in pre-development decision making as much as possible in order to build trust and maximise the scope for reconciling conflicting issues to the benefit of both wind energy
developers and the fishing industry.

Grid Integration of Wave Energy in Ireland: A Case Study

by Anne Blavette

Proceedings of the International Conference on Ocean Energy, Bilbao, Spain (2010)

Integrating Wave and Tidal Current Power: Case Studies Through Modelling and Simulation

by Anne Blavette

report of the International Energy Agency - Ocean Energy Systems

Dynamic Characteristics of Wave and Tidal Energy Converters & a Recommended Structure for Development of a Generic Model for Grid Connection

by Anne Blavette

report of the International Energy Agency - Ocean Energy Systems

Grid Integration of Wave and Tidal Energy

by Anne Blavette

Proceedings of the 30th International Conference on Ocean, Offshore and Artic Engineering, OMAE2011 (2011)

"Determination of metrics for the evaluation of environmental disturbance from wave energy arrays." Int.Conference on Ocean Energy 2010

by Robert Beharie

Co-authored with Professor Jonathan Side. Presented to ICOE2010.

An economical shore-based method for quantitative measurement of littoral wave action or ‘exposure’ is presented. The... more An economical shore-based method for quantitative measurement of littoral wave action or ‘exposure’ is presented. The ‘Terobuoy’ consists of a robust design that utilises a sacrificial material to deduce measurements of wave energy and direction over a sustained period. Further, the methodology can cheaply provide simultaneous measurements of wave action over integrated time periods in any number of locations. The method may not only provide a way of assessing the possible ecological and physiological effects that wave energy converter arrays could have on shoreline species but also give an impartial quantifying method for environmental impact assessments and environmental monitoring This would reduce uncertainties and financial outlay for stakeholders and investors in marine renewable energy technologies. Initial results from installations in Orkney indicate that it can provide both a measurement of accumulated energy and direction, for a given period, due to hydrodynamic forces on the shoreline. Research linked to this work could, for the first time, allow a direct quantitative measurement of littoral wave action that can be understood in terms of habitat and biotope classification systems, such as the European Nature Information System (EUNIS), the database for NATURA2000 protected areas.

"Exploring Links Between Wave Regime Changes and Biotic Assemblages." SuperGen Marine Assembly 2009 - Poster

by Robert Beharie

Understanding the ecological effects from the anthropogenic reduction of wave energy impacting upon the coastline has... more Understanding the ecological effects from the anthropogenic reduction of wave energy impacting upon the coastline has become increasingly important due to environmental protection legislation. Developers are now required by governments to provide an environmental impact assessment (EIA) prior to any installations, yet consultants producing EIA’s face the problem of having no prior data from the installation of large scale wave energy converter (WEC) arrays. They also have no quantitative method of determining to what extent the shoreline ecology could be affected through any changes to the wave energy regime.

"The environmental effects of wave energy converter arrays upon the intertidal zone." Young Coastal Scientist and Engineers Conference 2009

by Robert Beharie

Prestented at the 'Young Coastal Scientist and Engineers Conference 2009'

The environmental effect of the reduction of wave energy impacting upon the coastline is one that is not well... more The environmental effect of the reduction of wave energy impacting upon the coastline is one that is not well understood, not just because of the highly dynamic environment, but also due to the difficulty and cost of measuring a quantitative value of the wave energy regime. There are many variables which ultimately determine the wave action on a particular point on the shoreline and many more which directly affect the organisms in that environment; wave action is just one of 44 main variables that need to be taken into consideration.

Eddy Formation Behind a Coastal Headland

by Roger Falconer - Cardiff University

Modeling Solute Transport Using QUICK Scheme

by Roger Falconer - Cardiff University

Water Quality Simulation Study of a Natural Harbor

by Roger Falconer - Cardiff University

Paper Published in: Falconer, R. A. 1986. A water quality simulation study of a natural harbour. Journal of Waterway, Port, Coastal and Ocean Engineering, ASCE. 112(1), 15-34.Paper Published in:

Developing Procedures for the Sustainable Use of Mangrove Systems

by Roger Falconer - Cardiff University

Paper 50: Franks, T. and Falconer, R. A. 1999. Developing procedures for the sustainable use of mangrove systems. Agricultural Water Management. 40(1), 59-64.

Abstract
The concept of integrated water management forms the background to a research project carried out in... more Abstract
The concept of integrated water management forms the background to a research project carried out in India and Malaysia. The project involves the development of numerical models to improve understanding of processes in mangrove systems, and related socio-economic investigations to analyse the interests and decision-making of stakeholders in mangroves systems. The paper summarises some of the conclusions arising from the research, including the continuing need for improved technical understanding of mangroves and its incorporation in participatory decision-making.

Longitudinal Dispersion Coefficients In Natural Channels

by Roger Falconer - Cardiff University

Paper 63: Kashefipour, S. M. and Falconer, R. A. 2002. Longitudinal dispersion coefficients in natural channels. Water Research, 36(6), 1596-1608.

Abstract
Details are given herein, of the development of an equation for predicting the longitudinal dispersion... more Abstract
Details are given herein, of the development of an equation for predicting the longitudinal dispersion coefficient in riverine flows, based on 81 sets of measured data, and obtained from 30 rivers in the USA. This equation relates the dispersion coefficient to the hydraulic and geometric parameters of the flow and has been derived using dimensional and regression analysis, with a high correlation coefficient (i.e. R2=0.84). The formulation has been compared with many other existing empirical equations, frequently used to predict the longitudinal dispersion coefficient in riverine flows, with the comparisons based on four different statistical methods. These statistical comparisons have shown that the new equation appears to be more accurate than the other equations considered. The new dispersion equation was then linearly combined with a similar equation recently proposed by Seo and Cheong (J. Hydraul. Eng. ASCE 124 (1998) 25) and this combined equation was then also analysed using statistical methods. The existing empirical equations used to estimate the longitudinal dispersion coefficient and the new equations proposed in this study were included in the advective dispersion equation to predict the suspended sediment concentrations at three sites in the Humber Estuary sited along the northeast coast of England. The average percentage errors between the predicted- and measured-field data for the proposed new dispersion equations were less than those obtained using the previously documented equations.