Tidal Stream Resource Research Articles

Assessment of the tidal stream power potential and impacts of tidal current turbines in the Strait of Larantuka, Indonesia

In this paper, results of a tidal stream resource assessment performed for the Strait of Larantuka, Indonesia are presented. The methodology followed is comprising of the estimates of power yield and evaluation of the effects of power extraction via high-resolution, three-dimensional flow models. Power densities exceeding 10kW/m2 are observed in the main channel. Current velocities, and wake characteristics of the turbines have been resolved reasonably well with low computational cost, considering different levels of energy dissipation. Although the results are promising, investigations using in-situ measurements alongside more advanced modelling techniques are still needed, and currently underway.

INVESTIGATION OF THE ENERGY POTENTIAL FROM TIDAL STREAM CURRENTS IN INDONESIA

In this paper, an advanced methodology developed for the assessment of tidal stream resources is applied to several straits between Indian Ocean and inner Indonesian seas. Due to the high current velocities up to 3-4 m/s, the straits are particularly promising for the efficient generation of electric power. Tidal stream power potentials are evaluated on the basis of calibrated and validated high-resolution, three-dimensional numerical models. It was found that the straits under investigation have tremendous potential for the development of renewable energy production. Suitable locations for the installation of the turbines are identified in all the straits, and sites have been ranked based on the level of power density. Maximum power densities are observed in the Bali Strait, exceeding around 10kw/m2. Horizontal axis tidal turbines with a cut-in velocity of 1m/s are considered in the estimations. The highest total extractable power resulted equal to about 1,260MW in the Strait of Alas. Preliminary assessments showed that the power production at the straits under investigation is likely to exceed previous predictions reaching around 5,000MW.

Tidal stream resource assessment uncertainty due to flow asymmetry and turbine yaw misalignment

The majority of tidal energy convertors (TECs) currently under development are of a non-yawing horizontal axis design. However, most energetic regions that have been identified as candidate sites for installation of TEC arrays exhibit some degree of directional and magnitude asymmetry between incident flood and ebb flow angles and velocities, particularly in nearshore environments where topographic, bathymetric and seabed frictional effects and interactions are significant. Understanding the contribution of directional and magnitude asymmetry to resource power density along with off axis rotor alignment to flow could influence site selection and help elucidate optimal turbine orientation. Here, 2D oceanographic model simulations and field data were analysed to investigate these effects at potential deployment locations in the Irish Sea; an energetic semi-enclosed shelf sea region. We find that observed sites exhibiting a high degree of asymmetry may be associated with a reduction of over 2% in annual energy yield when deployment design optimisation is ignored. However, at the majority of sites, even in the presence of significant asymmetry, the difference is <0.3%. Although the effects are shown to have less significance than other uncertainties in resource assessment, these impacts could be further investigated and quantified using CFD and 3D modelling.

Asymmetry of tidal currents off the W.Brittany coast and assessment of tidal energy resource around the Ushant Island

A method of tidal stream energy resource assessment around the Ushant Island in the Iroise Sea, using surface velocity time series from High Frequency radars (HFR) and ADCP measurements, is presented. Remotely sensed velocities provided by the radars allow to augment the industry standard approach of 3D numerical modeling and in-situ ADCP surveying to make a large-scale quantification of tidal stream resource. They capture the real ocean dynamics and thus provide context on the complex spatial variability of tidal currents that are so often feature at potential tidal energy sites. The observations show current velocities of 4 m/s northwest of the Island and in the Fromveur Strait, with 1 m/s value exceeded 60% and 70% of time respectively. Emphasis is given to the peculiarities in tidal flow asymmetry and to the study of the variation of vertical velocity profiles during different tidal stages. Radar derived velocities reveal a pronounced asymmetry between the flood and ebb flow around the Ushant Island, quantified by dimensionless number a - velocity asymmetry. The largest range of asymmetry variation, from 0.5 to 2.5, is observed in the Fromveur Strait. Harmonic analysis demonstrated that a joint variation of phase of the principal semi-diurnal (M2) and quarter-diurnal (M4) tidal velocity component accounts for flow asymmetry variation in the strait. Asymmetry in current direction is also quantified. ADCP measurements show that the vertical velocity profiles follow a 1/α power law with a power law exponent mostly depending on geographic location than on tidal stage. It was demonstrated that, in the Fromveur Strait, the 1/7 power law is appropriate to characterize the velocity profile. The combination of two sources of data enables characterization of the velocity variations in three spatial dimensions and in time thus increasing accuracy of the hydrokinetic resource assessment from HF radar observations. The estimation shows that the mean technical resource is 50% smaller in the lower half than in the upper half of the water column. The theoretical resource on average is three times higher than the technical resource and appears to be more sensitive to variations in the shape of the velocity profile in the lower layer.

A modelling study of the tidal stream resource of the Pentland Firth, Scotland

A high spatial resolution three-dimensional (3D) hydrodynamic ocean model of the Orkney Islands is used to investigate the tidal stream resource and physical environmental impact in the Pentland Firth, a high-resource area where the first arrays of tidal stream turbines are likely to be deployed in Scotland. Tidal stream turbines are represented in the model using a momentum sink in order to (1) find an upper bound estimate of the available resource, (2) explore alternative, more realistic scenarios, and (3) estimate their effect on the physical environment. The maximum extractable power scenario (M2 + S2 cycle mean of 5.3 GW) requires a high density of turbines deployed through the channels cross section and has a major impact on the physical environment. More realistic scenarios, with the placement of the turbines constrained by practical factors, resulted in considerably less extractable power but the physical environmental impact was disproportionately reduced. It was also found that a variable thrust coefficient can be used to optimise the performance of tidal arrays. Our work highlights the usefulness of 3D hydrodynamic models for tidal stream resource assessment and impact studies, and illustrates how the design of tidal stream arrays can affect the levels of sustainably harnessed tidal power.

Implementation and evaluation of the International Electrotechnical Commission specification for tidal stream energy resource assessment: A case study

Over the next decades, tidal stream energy aims to become a fully commercially viable energy source. For this purpose, complete knowledge regarding tidal stream resource assessment is essential. In this context, the International Electrotechnical Commission has developed a technical standard for the assessment of the tidal stream resource, “IEC 62600-201 TS: Marine energy - Wave, tidal and other water current converters - Part 201: Tidal energy resource assessment and characterisation”, offering a vast set of recommendations in the fields of data collection, numerical modelling, data analysis and reporting of the results with the purpose of standardising tidal stream resource characterisation. The standard divides resource assessments into two different classes: feasibility and layout design. The model setup procedure (mesh resolution, boundary conditions) and the computational effort required vary significantly from one class to another. For these reasons, the objective of the present work is to explore the proposed standard using the Orkney Region (N Scotland) as a case study. Overall, it was found that the standard works well, offering a detailed characterisation of the tidal resource; however, in order to improve its manageability, some aspects related to the grid resolution requirements and the approach to model a tidal energy converter could be revisited for future editions.

Tidal stream resource assessment in the Dover Strait (eastern English Channel)

A methodology of tidal flow resource assessment in the Dover Strait is presented. The resource assessment is performed using surface velocity time series recorded by Very High Frequency Radars (VHFR) and ADCP velocity measurements. Following the EMEC guideline, the major parameters of tidal flow conventionally used for tidal energy site screening are estimated and mapped. The combination of two sources of data allowed to characterize the current velocity variation in three spatial dimensions and in time, which increased confidence in hydrokinetic resource assessment from the radar data. Current velocities provided by the radars show strong spatial variation and fortnightly modulation. The most energetic area was found west of the Cape Gris Nez with the peak velocity of 2.5m/s, mean velocity of 1m/s, and spring tide average velocity of 1.4m/s. Velocities exceeding 1m/s are observed more than 50% of time there. Averaged velocity profiles derived from ADCP data were obtained for different stages of the tidal cycle and then approximated by a power law function. Using velocity time series provided by the radars and the power law velocity profiles, the power density time series in the surface and bottom layers were generated. The analysis of these data show that west of the Cape Gris Nez, the mean power density attains its maximum value 0.9kW/m2 in the surface layer and a peak value 5kW/m2. In the rest of the domain, the mean power density varies from 0.1 to 0.6kW/m2. The power density is found three times lower in the bottom layer. A three dimensional hydrodynamic model MARS-3D is used for comparison with experimental data. The model results are in good agreement with observations thus allowing the use of the model for assessing tidal stream resource in extended area.

Constraints on extractable power from energetic tidal straits

National efforts to reduce energy dependency on fossil fuels have prompted examination of macrotidal nearshore sites around the United Kingdom (UK) for potential tidal stream resource development. A number of prospective tidal energy sites have been identified, but the local hydrodynamics of these sites are often poorly understood. Tidal energy developers rely on detailed characterisation of tidal energy sites prior to device installation and field trials. Although first-order appraisals may make macrotidal tidal straits appear attractive for development, detailed, site-specific hydrodynamic and bathymetric surveys are important for determining site suitability for tidal stream turbine (TST) installation. Understanding the ways in which coastal features affect tidal velocities at potential TST development sites will improve identification and analysis of physical constraints on tidal energy development. This paper presents and examines tidal velocity data measured in Ramsey Sound (Pembrokeshire, Wales, UK), an energetic macrotidal strait, which will soon host Wales' first TST demonstration project. While maximum tidal velocities in the strait during peak spring flood exceed 3 m s−1, the northern portion of Ramsey Sound exhibits a marked flood-dominated tidal asymmetry. Furthermore, local bathymetric features affect flow fields that are spatially heterogeneous in three dimensions, patterns that depth-averaged velocity data (measured and modelled) tend to mask. Depth-averaging can therefore have a significant effect on power estimations. Analysis of physical and hydrodynamic characteristics in Ramsey Sound, including tidal velocities across the swept area of the pilot TST, variations in the stream flow with depth, estimated power output, water depth and bed slope, suggests that the spatial and temporal variability in the flow field may render much of Ramsey Sound unsuitable for tidal power extraction. Although the resource potential depends on velocity and bathymetric conditions that are fundamentally local, many prospective tidal energy sites are subject to similar physical and hydrodynamic constraints. Results of this study can help inform site selection in these complicated, highly dynamic macrotidal environments. In order to fully characterise the structure of the tidal currents, these data should be supplemented with 3-D modelling, particularly in areas subject to a highly irregular bathymetry and complicated tidal regime.

Effect of waves on the tidal energy resource at a planned tidal stream array

Wave–current interaction (WCI) processes can potentially alter tidal currents, and consequently affect the tidal stream resource at wave exposed sites. In this research, a high resolution coupled wave-tide model of a proposed tidal stream array has been developed. We investigated the effect of WCI processes on the tidal resource of the site for typical dominant wave scenarios of the region. We have implemented a simplified method to include the effect of waves on bottom friction. The results show that as a consequence of the combined effects of the wave radiation stresses and enhanced bottom friction, the tidal energy resource can be reduced by up to 20% and 15%, for extreme and mean winter wave scenarios, respectively. Whilst this study assessed the impact for a site relatively exposed to waves, the magnitude of this effect is variable depending on the wave climate of a region, and is expected to be different, particularly, in sites which are more exposed to waves. Such effects can be investigated in detail in future studies using a similar procedure to that presented here. It was also shown that the wind generated currents due to wind shear stress can alter the distribution of this effect.

Numerical modeling of the effect of tidal stream turbines on the hydrodynamics and the sediment transport – Application to the Alderney Race (Raz Blanchard), France

A regional 2DH hydrodynamic model is used to estimate the tidal stream resource of a site located in a macrotidal environment with extreme tidal velocities. The study site is the Alderney Race (Raz Blanchard in French) which is a straight located in the English Channel between the Alderney Island and La Hague cape (France). The estimation of the resource is used to build two realistic tidal energy extraction scenarios consisting in placing a 290 MW tidal turbine array in two different areas. Then, we analyze the impact of turbines on the hydrodynamics and the sediment transport. The hydrodynamic perturbation is restricted to the vicinity of the array where the mean current velocity reduction reaches 0.3 m/s locally (corresponding to 15% of the baseline velocity). Focusing on the variable driving the bedload (the critical erosion threshold exceedance), we confirm that tidal energy extraction tends to reduce the bedload rate and to deflect the sediment fluxes. Our simulations with a simple model for the suspended sediment transport suggest that tidal energy extraction has a significant effect on the area of deposition of the particles transiting through the tidal farm. For the baseline, the sediment particles transiting through the tidal farm deposit mostly in the eastern part of the English Channel. Depending on the location of the tidal farm, the sediment mass balance between the eastern and the western part of the English Channel changes drastically.

Realistic wave conditions and their influence on quantifying the tidal stream energy resource

When selecting suitable sites for tidal stream energy arrays a wide range of factors must be considered, from the magnitude of the tidal stream resource, to realistic oceanographic conditions. Previous computational and laboratory-scale investigations into the impact of waves upon tidal turbines (such as turbine blade loadings) and turbine arrays (such as array configuration) typically assume that waves propagate “inline” to the tidal current (waves following or waves opposing the tidal current with a 20° tolerance limit). We investigated the wave climate at typical tidal stream energy sites across the British Isles. The wave climate was simulated at 18 sites using a 7-year (2005–2011) SWAN wave model simulation of the northwest European shelf seas. The principal semi-diurnal lunar constituent (M2) was also estimated at these sites using the three-dimensional ROMS tidal model. A significant proportion of the wave climate (between 49% and 93% of the time), including extreme wave events (>10m wave heights), was found to be propagating in a direction which was “oblique” to the major axis of tidal flow (i.e. waves which propagate at an angle to the tidal current with a 20° tolerance limit) at all 18 selected sites. Furthermore, the average “inline” wave climate was 2.25m less in height and 2s less in wave period in comparison to the oblique wave climate. To understand the direct effect of waves upon the tidal stream resource, the dynamically wave-tide coupled COAWST modelling system was applied to an idealized headland case study, which represented the typical tide and wave conditions expected at first generation tidal stream energy sites. Waves were found to alter the simulated tidal velocity profile, which, because tidal stream power is proportional to velocity cubed, reduced the theoretical resource by 10% for every metre increase in wave height (R2 94% with 22 degrees of freedom) – depending upon wave period and direction. Our research indicates that wave angle should be considered when quantifying the impact of waves upon tidal turbines, such as computational fluid dynamic (CFD) studies, or laboratory-scale experiments of wake characteristics and turbine fatigue loading. Further, dynamically coupled tide-wave models may be necessary for a thorough resource assessment, since the complex wave-tide interaction affected the tidal resource; however, in situ observations of tidal velocity profiles during a range of wave events will be essential in validating such modelling approaches in the future.

Optimal phasing of the European tidal stream resource using the greedy algorithm with penalty function

The regular periodicity of astronomical tides allows their accurate prediction, and so it should be possible to determine how best to optimise the future distribution of arrays of tidal energy devices for any shelf sea region. By considering together the magnitude and phase of tidal currents over a shelf sea region, maximum aggregated power generation, with minimal periods of low generation, can be deduced. Here, we make use of the greedy algorithm to optimise future exploitation of the tidal stream resource over the northwest European shelf seas, a region which contains a world-leading tidal energy resource. We also apply a penalty function to the greedy algorithm, favouring the selection of future hypothetical sites where power generation would be out-of-phase with previously developed sites. Our results demonstrate that the Pentland Firth and Channel Islands would be optimal sites for parallel development for relatively low numbers of arrays, with important contributions from the Irish Sea for larger scale exploitation. Although there is minimal phase diversity between European tidal stream sites to deliver firm power generation, it is possible that the vertical tide could contribute to such baseload through the parallel development of lagoons or impoundments.

Evaluation of tidal stream resource in a potential array area via direct measurements

ADCP transects have used to characterise tidal stream resources in the Ramsey Sound area of Pembrokeshire, UK. Previous resource assessments have previously suggested that this area is one of the most promising for tidal stream deployments in the UK and this contribution confirms the commercial viability of the area. In this study three channels were considered: Ramsey Sound itself and two channels to the west formed by small offshore islands. Current velocities were used to compute the tidal energy flux through the channels. Maximum instantaneous peak flux through the three channels ranges from 180 MW to 70 MW. Flux cross-sections are presented and the impact of meso-scale bathymetric features on flux and on the cross-transect variation of maximum flux over the tidal cycle is described and discussed. Theoretical values of extractable power potential are calculated and range between 7.2 MW and 21.8 MW. These values are approximately ¼ of the average flux through the measured cross-section. One channel is identified as being preferable for the first stage of array deployments given greatest homogeneity of flux through the channel cross-section and it having the highest power potential.

Impact of tidal energy converter (TEC) arrays on the dynamics of headland sand banks

Previous results from one-dimensional model studies have demonstrated that large-scale exploitation of the tidal stream resource could have a significant impact on large-scale sediment dynamics. In this research, we model the impact that such exploitation would have on the dynamics of headland sand banks. Such sand banks, formed by the large eddies generated by strong tidal flow past headlands, have an important role in natural coastal protection, since they cause waves to refract and dissipate energy. Therefore, a tidal energy converter (TEC) array developed in the vicinity of a headland could alter this natural form of coastal protection. Here, we investigate the impact of TEC array operation on idealised headland sand bank formation, followed by a case study, the Alderney Race: a strait of water between the island of Alderney (Channel Islands) and Cap de la Hague (France). This is achieved through the application of a morphological model that incorporates TEC device operation as an additional bed friction source term in the three-dimensional hydrodynamic module. Through a series of model experiments, we demonstrate the impact that a full-scale (300 MW) TEC array would have on sediment dynamics when sited near a headland. This modelling study demonstrates that a 300 MW TEC array located in the vicinity of a headland could lead to a considerable change in the maintenance of headland sand banks over a spring-neap cycle. If the scale of this change is demonstrated to be significant compared to the natural range of inter-annual and inter-seasonal sand bank variability, then developers of TEC arrays would be advised to examine ways in which they could reduce the environmental impacts of TEC arrays sited near headlands. The most obvious of these is to limit the scale of the array, but if we assume that developers wish to exploit the tidal energy resource to its maximum, the alternative is to site the array strategically (within the bounds of economic feasibility) such that it will not interfere with the natural morphodynamics of the headland system.

조류자원의 평가에 관한 이론적 연구

This paper outlines extraction potential of tidal stream resources from the simplified channel in which flow is driven by a head difference between inlet and outlet. Energy extraction alters the flow within a simple channel, and extraction of 10% energy flux in a natural channel would give rise to a flow speed reduction of about 5.7%. If 20% of the undisturbed energy flux is extracted, the flow speed is reduced by 11.3%. The simple channel also suggests that extractable energy might be higher if flow speed reductions are considered acceptable.

Briefing: 10 MW Triton – a breakthrough in tidal stream power

This article sets the new Triton platform concept in the context of the tidal stream resource, the Cinderella of renewable energy. Triton makes possible new levels of generating capacity, ease of installation and simplicity of maintenance. The article is written from the perspective of an engineer who pioneered UK wind power technology in the 1980s and 1990s with the Wind Energy Group, and now sees some of these principles applicable to tidal flows.

Marine renewable energy development – research, design, install

The UK government has committed to a reduction of carbon dioxide emissions for England and Wales by 20% below 1990 levels by 2010 and to increase the amount of electrical energy generated by renewable energy sources to 10% by 2010 and 15% by 2015. Wave and tidal energy are likely to contribute significantly to achieving these targets as important renewable energy technologies for the UK. The south-west region in particular has the potential to generate substantial amounts of renewable energy from its wave and tidal stream resources. The South West of England Regional Development Agency (SWRDA) recognises the potential of the marine energy industry for the region and is supporting demonstration projects in marine energy through the Wave Hub project and the Peninsular Research Institute in Marine Renewable Energy (Primare). The Wave Hub project provides a unique opportunity for interdisciplinary research to aid developments in marine renewable energy. In this paper, a description of some of the new ongoing research being undertaken jointly by researchers at the Universities of Exeter and Plymouth within Primare is presented.

The impact of tidal stream turbines on large-scale sediment dynamics

Tidal stream turbines are exploited in regions of high tidal currents. Such energy extraction will alter the hydrodynamics of a tidal region, analogous to increasing the bed friction in the region of extraction. In addition, this study demonstrates that energy extracted with respect to tidal asymmetries due to interactions between quarter ( M 4) and semi-diurnal ( M 2) currents will have important implications for large-scale sediment dynamics. Model simulations show that energy extracted from regions of strong tidal asymmetry will have a much more pronounced effect on sediment dynamics than energy extracted from regions of tidal symmetry. The results show that energy extracted from regions of strong tidal asymmetry led to a 20% increase in the magnitude of bed level change averaged over the length of a large estuarine system, compared with energy extracted from regions of tidal symmetry. However, regardless of the location of a tidal stream farm within a tidal system, energy extraction reduces the overall magnitude of bed level change in comparison with non-extraction cases. This has practical application to many areas surrounding the UK, including the Irish Sea and the Bristol Channel, that exhibit strong tidal currents suitable for exploitation of the tidal stream resource, but where large variations in tidal asymmetry occur.

Marine current turbines: Pioneering the development of marine kinetic energy converters

This paper gives the rationale and background to an already advanced research and development (R&amp;D) programme aimed at developing technology for the commercial exploitation of kinetic energy from marine currents. This is followed by a brief overview of the characteristics of the tidal stream resource, the technical principles by which it may be exploited, and the key technical challenges that need to be overcome. The paper includes a description of the pioneering ‘Seaflow’ Project involving the installation and testing, since May 2003, of a prototype 300 kW tidal turbine 3 km off Lynmouth. The next stage of Marine Current Turbines Ltd's R&amp;D programme is then described: this involves the development of a 1 MW twin axial-flow rotor system, called ‘Seagen’ which is planned for installation early in 2007. The installation and testing of ‘Seagen’ will mark a landmark stage in the R&amp;D programme because it will form the basis for the commercial technology to follow. A brief outline of future plans beyond ‘Seagen’ is also given.

Vertical-axis tidal-current generators and the Pentland Firth

This paper extends ideas presented to the World Renewable Energy Conference [1, 2]. One idea involves the impedance of flow channels and its relevance to the maximum tidal-stream resource. Estimates of the inertial and damping terms of the impedance of the Pentland Firth suggest a much higher resource size than studies based purely on the kinetic flux, because adding extra turbines will have less effect on flow velocities than in a low impedance channel. This very large resource has pushed the design of the turbine towards the stream velocities, depth, and seabed geology of this site. A second idea is an algorithm to control the pitch of close-packed vertical-axis generators to give an evenly distributed head. Finally, there are suggestions for a seabed attachment aimed specifically for conditions in the Pentland Firth and intended to allow rapid installation of a self-propelled tidal-stream generator.