Tidal Stream Power Research Articles

The impact of spring-neap tidal-stream cycles in tidal energy assessments in the Chilean Inland Sea

The Chilean Inland Sea (CIS) has natural conditions for marine tidal energy development. Recent studies show that lunar variability in the tides should be considered, because the practical power generation varies considerably when large differences exist between spring and neap tides. Using a high resolution (∼1 km) 2D depth-averaged numerical model, we investigate the effect of lunar cycle variability in the CIS and this impact on tidal energy evaluation by validating a 30 days simulation. There are two highly energetic areas where currents exceed 2 m/s 50% of the time: the Chacao and Desertores Channels. Both zones are dominated by semi-diurnal tides, and tidal current amplitude shows large changes during spring and neap tides. Due to these changes power density can vary considerably in a period of 2 weeks. In the Chacao Channel, tidal-stream power is four times as large during spring tide (40 kW/m2) compared to neap tide (10 kW/m2). Tidal-stream power is only significant during spring tide in the Desertores Channel (8 kW/m2). This work is a contribution to understanding the tidally driven flows in an inland sea and the importance of considering the variations between spring and neap tides in tidal-stream energy assessment.

Wave and tidal power measurement using HF radar

The use of HF radar systems located on the coast to measure ocean surface waves and currents is reviewed. Examples are given showing temporal, spatial and directional variability in the wave and tidal power resource. HF radars are unique in being able to provide such data continuously and over wide areas of sea where marine renewable installations are being considered or already in place. Two projects just getting underway at the time of writing are described. One will use HF radar to provide current and wave data for a French tidal stream power application and the other is aimed at supporting the Welsh Wave Hub site whilst also exploring methods to improve the robustness of the HF radar wave measurements.

Characterising the tidal stream power resource around France using a high-resolution harmonic database

Abstract Although tidal stream energy is highly predictable, the distribution of the resource varies over small spatial scales and over tidal-to-decadal time scales, requiring detailed models and accurate analysis techniques. The present study investigates the spatial and temporal variability of the tidal stream energy resource around France, using a tidal current harmonic database. The 250 m resolution tidal database covers western Brittany and the western English Channel, two regions that have strong potential for tidal array development. As well as generating a refined resource assessment for the region, a series of metrics are considered to assess resource variability, both spatially and temporally. Particular attention is dedicated to variability over spring-neap time scales (resulting from M 2 and S 2 compound tides) and current asymmetry (governed by M 2 and M 4 velocities). A clear contrast in the nature of the resource is found between sites located off the Cotentin Peninsula, which exhibit low spring-neap variability and tidal asymmetry, leading to a more continuous and therefore attractive energy conversion, and sites in western Brittany, with greater variabilities over semi-diurnal and fortnightly time scales.

Scale effect and undersea noise of counter-rotating propellers installed in tidal stream power unit

More attentions are paid to the renewable energies in face of global warming and depletion of fossil fuel. Tidal energy as one of the renewable energies has fruitful advantages as predictable and sustainable resources. The authors have proposed a counter-rotating type tidal stream power unit and investigated preliminarily the effect of the Reynolds number on the output, namely scale effect, and undersea noise of the counter-rotating propellers. The hydraulic loss increases at the low Reynolds number with accompanying the boundary layer separation. The noise has the dominant frequencies corresponding to the blade passing frequencies in counter-rotating propellers.

Tidal stream resource characterisation in progressive versus standing wave systems

Characterisations of the tidal stream resource and its variability over various timescales are crucial for the development of the tidal stream energy industry. To date, no research has compared resource sensitivity in standing wave (when peak currents occur midway between high and low water) and progressive wave (where peak currents occur at high and low water) tidal systems. Here, we compare the flow regimes of standing wave versus progressive wave systems and the associated variations in tidal stream power with applications to device deployment options (floating-platform turbines versus bottom-mounted turbines). We use a validated 3D numerical model (ROMS) of a globally-significant tidal energy shelf sea region (Irish Sea), to test the hypotheses that the influence on potential extractable energy, and suitability for different devices, may be markedly different between these contrasting systems. Power density was also calculated and compared for floating versus bottom-mounted devices using in-situ current data (ADCPs) obtained from a standing wave site and a progressive wave site. We show that progressive wave systems are characterised by velocity-asymmetry over a tidal cycle (i.e. stronger peak flows at high water than at low water), leading to power-asymmetry. Such power asymmetry was shown to have more of an effect on floating device technology, where an assumed turbine depth tracks the sea surface, in contrast to bottom-mounted technology, where the hub height is fixed at a certain position above the sea bed. Shallow, high-flow regions where tidal range is large contained up to 2.5% more power density from bottom-mounted compared with floating turbines; however, there were areas where floating devices were exposed to higher mean currents over a tidal cycle. Standing wave systems, where flow asymmetry is minimised, did not particularly favour either technology. The results highlight the requirement for detailed resource assessments to consider the vertical plane, and are applicable to all potential tidal stream energy sites.

Impact of human interventions on tidal stream power: The case of Cádiz Bay

This work quantifies how human interventions can modify the tidal energy potential in a mesotidal estuary and if these variations can be used to increase the production of renewable energy at a highly altered bay. The analysis is applied to Cádiz Bay (Southern Spain), where important human interventions are developing, specifically a new port terminal and a 5 km long bridge. Due to these interventions, significant impacts on the bay hydrodynamics are expected. To assess the variations on the tidal energy resource after interventions, a three dimensional numerical model is used to obtain the bay hydrodynamics (Delft3D model). It was calibrated and validated against water levels and currents at tidal and sub-tidal scales using the results of two extensive field surveys. Two scenarios are defined, corresponding to the pre and post interventions situations. Results show that before interventions, only one area has the potential to locate a Tidal Energy Converter (TEC) array. However, after interventions a new area arise due to the increase of flow velocities induced by human modifications of the bay geometry. The energy potential is assessed for these two locations, and their efficiencies (potential energy per device) are obtained, indicating that after interventions the new TEC array is even more efficient than that of the pre interventions scenario, and an increase of 20% in the energy that can be potentially produced is obtained.

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.

Tidal stream energy resource assessment in the Qiantang River Estuary, China

ABSTRACTA tidal stream energy resource assessment can be achieved through direct measurements of tidal elevations or flow velocities, theoretical formulas, and numerical models. This paper first described the development of renewable energy in China. Then, the tidal stream energy resource in the Qiantang River Estuary was assessed. The present work established a 2D numerical model for calculating the power density in the estuary and validated it with in situ water level, tidal current, and suspended sediment concentration measurements. Three possible sites found near the Hangzhou Bay Bridge were selected for detailed resource assessment. The results indicated that if only 50 turbines are deployed to each site, the tidal stream energy output by all turbines would be 7.5 GWh/year. Finally, building a tidal stream power generation device into an existing bridge structure was proposed at the Qiantang River Estuary.

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.

Assessing the impact of tidal stream energy extraction on the Lagrangian circulation

Whereas numerous numerical modelling have been conducted to gain further insights about the hydrodynamic characteristics of tidal stream energy sites, the long-term transport of water particles has been routinely ignored. However, a refined evaluation of circulation pathways is fundamental to assess water-quality changes induced by tidal energy converters. The present study investigates the tide-induced Lagrangian circulation in north-western coastal waters of Brittany (France, western Europe), including numerous islands, shoals and straits with strong potential for turbine farm implementation. Particular attention is dedicated to the hydrodynamic impact of tidal stream power extraction from a series of horizontal-axis turbines. An original approach, based on the coupling of the Lagrangian barycentric method with predictions from a model at high spatial resolution, is considered to assess circulation pathways in a single synoptic view. Spatial results reveal a complex Lagrangian circulation highly controlled by bathymetry and islands geometry. The residual flow in the Fromveur Strait, separating Ushant island from Molène archipelago, displays thus a strong asymmetry, between a prominent north-eastern pathway with residual currents up to 0.45ms-1 and a southward circulation. The positions of cyclonic and anti-cyclonic recirculations, both upstream and downstream the Strait, are furthermore precisely defined. The north eddy is found closely located to the sand bank of the Four. The extraction of tidal stream power impacts mainly the residual Lagrangian circulation along the current stream emerging from the Strait, with a tendency for surrounding eddies to get closer to the tidal stream energy site. The displacements of the south and north recirculations may thus exceed 1.5km with possible implications on the evolution of surrounding seabed features.

A comparison of numerical and analytical predictions of the tidal stream power resource of Massachusetts, USA

The coastal waters of Massachusetts, USA encompass tidal phenomena that generate flows of sufficient magnitude for commercially viable power extraction. We examine the tidal power resource of the Massachusetts coastal region with two high-resolution hydrodynamic tidal models: a regional model encompassing the coastal waters of southeastern New England and a local domain model of Cape Cod Canal. Both models have been subject to comprehensive skill assessment using available surface elevation and ADCP measurements. Based on the model results, we identify five high-energy sites (Cape Cod Canal, Muskeget Channel, Quicks Hole, Robinson Hole and Woods Hole) for evaluation of the maximum extractable tidal power. The power extraction at these sites is modeled using linear momentum actuator disk theory applied to a cross-channel array of turbines. Of the sites evaluated, Muskeget Channel has the greatest resource, with an estimated maximum extractable power of 24 MW. The estimated total power available from all five sites is 44 MW. These estimates agree within 21% with predictions from analytical approaches at all sites. Potential applications for the models include: providing developers with an initial assessment of the resource, guiding observation programs for further study of the resource, and facilitating optimization of turbine array design.

A Numerical Prediction of Tip Vortices from Tandem Propellers in the Counter-Rotating Type Tidal Stream Power Unit

Ocean energy has a potential of providing a large amount of renewable energy around the world. One of the forms of ocean energy, tidal stream power is widely recognized as the continuous, predictable and eco-friendly ocean energy source. Unique tandem propellers that can counter-rotate have been designed to generate electric power effectively from a tidal stream. This type of power unit has several advantages compare to the conventional unit with a single propeller. At the design of the tidal stream power unit, it is important to investigate the structure of the tip vortex tubes shedding to predict the load of the propeller. In this research, we investigated the tip vortex shedding using the CFD method for the conventional single propeller and counter-rotating type tandem propellers and estimated the performance efficiency using RANS (Reynolds Averaged Navier-Stokes) model and we confirmed the limitation of RANS model on the calculation of the tip vortex stretching.

Counter-Rotating Type Horizontal-Axis Bidirectional Propellers for Tidal Stream Power Unit

Tidal stream power units with horizontal-axis propellers are one of promising technologies for generating the renewable green energy. The ebb and flow require that the power unit must operate in bidirectional tidal streams. Hence a tidal stream power unit with counter-rotating type horizontal-axis bidirectional propellers is proposed in this paper. The blades with fully-symmetrical hydrofoils were optimized numerically. The output and flow conditions predicted by the computational fluid dynamics simulations are compared with the results of the wind tunnel experiments at the higher tip speed ratios, which are of expected usual operating conditions of this unit. The numerical and experimental results show good agreements. It is also confirmed that the flow discharged from the counter-rotating type propellers has no swirling component, though the single propeller generates the unacceptable swirling component.

Flow field estimation analysis based on the Kalman filter FEM for selection of tidal stream power generator locations

In this paper, we present a shallow water flow field estimation analysis, based on the Kalman filter FEM, for locating the optimal positions for tidal stream power generation systems. In our flow field analysis, we adopt the shallow water equation as the governing equation. The Galerkin and the selective lumping methods are employed as discretization techniques in space and time, respectively. The Kalman filter theory is applied to estimate the flow field. As a numerical example, we carry out a flow estimation analysis for Tokyo Bay. The high estimation accuracy of the flow field estimation based on the Kalman filter FEM is confirmed by comparison with conventional FEM. The electric power generation potential is also computed using the estimated flow field.

東京湾における潮流発電ポテンシャル算定に対するカルマンフィルタFEMの適用

In recent years, tidal stream power generation is attracting attention as renewable energy. When a turbine submerged in the sea is rotated by a tidal current, kinetic energy is transformed to electric energy in this power generation method. In this paper, we present a flow field estimation analysis based on the Kalman filter FEM for locating the optimal positions for tidal stream power generation systems. We employed the shallow water equation as the governing equation. The Galerkin and the selective lumping methods are used as discretization techniques in space and time, respectively. The Kalman filter theory is applied to estimate the flow field. As a numerical example, we carried out a flow estimation analysis for Tokyo Bay and the electric power generation potential is calculated using the estimated flow field. It was confirmed that the estimation accuracy of the flow field estimation analysis by the Kalman filter FEM is higher than that by the conventional FEM.

The parameterisation of turbulence in the marine environment

ABSTRACTThere are many problems in the fields of flow modelling around structures and tidal stream energy yield analysis which require a thorough understanding of the turbulent and time-averaged flow speeds in marine environments. In this paper we examine the relationship between the turbulence intensity and mean tidal flow speed at a potential tidal stream power site. We report data from the Humber Estuary wherein an Acoustic Doppler Current Profiler were used to capture vertical profiles of the high frequency and mean tidal flow speeds throughout Spring and Neap, Flood and Ebb cycles. We show not only that our results extend earlier work but also suggest that the turbulence intensity, IT, can be described parametrically in terms of the mean flow, U, by an inverse power function where the coefficient appears to be dependent upon the anisotropic nature of the turbulence. For the data reported here, the coefficient has value of about 17–18 and the exponent lies between −0.6 and −1.0. Confirmation of this relationship should not only improve engineering design work and energy yield analyses in turbulent tidal flows but also be applicable to other problems such as the prediction of sediment mass transport and pollution dispersal in estuarine management studies.

Resource characterization of sites in the vicinity of an island near a landmass

Renewable energy technologies are undergoing rapid development, the global aim being to achieve energy security and lower carbon emissions. Of marine renewable energy sources, tidal power has inherent predictability and large theoretical potential, estimated to exceed 8000 (TW h)a−1 in coastal basins. Coastal sites in the vicinity of an island near a landmass are prime candidates for tidal stream power exploitation by arrays of turbines. This paper characterizes numerically the upper limit to power extraction of turbines installed at such sites. It is demonstrated that the maximum power extracted from the strait is generally not well approximated by either the power dissipated naturally at the seabed or the undisturbed kinetic power of flow in the strait. An analytical channel model [C. Garrett and P. Cummins, “The power potential of tidal currents in channels,” Proc. R. Soc. A Math. Phys. Eng. Sci., vol. 461, no. 2060, pp. 2563–2572, Aug. 2005] provides lower predictions than the present numerical model of available power in the strait due to the analytical model not accounting for changes to the driving head resulting from power extraction and flow diversion offshore of the island. For geometrically long islands extending parallel to the landmass, the numerically predicted extracted power is satisfactorily approximated by the power naturally dissipated at the seabed, and there is reasonable agreement with the estimate by the channel analytical model. It is found that the results are sensitive to choice of boundary conditions used for the coastlines, the eddy viscosity, and bed friction. Increased offshore depth and lower blockage both reduce the maximum power extracted from the strait. The results indicate that power extracted from the site can be maximum if extraction is implemented both in the strait and offshore of the island. Presence of the landmass and increasing island dimensions both enhance power extraction.

The influence of waves on the tidal kinetic energy resource at a tidal stream energy site

Successful deployment of tidal energy converters relies on access to accurate and high resolution numerical assessments of available tidal stream power. However, since suitable tidal stream sites are located in relatively shallow waters of the continental shelf where tidal currents are enhanced, tidal energy converters may experience effects of wind-generated surface-gravity waves. Waves may thus influence tidal currents, and associated kinetic energy, through two non-linear processes: the interaction of wave and current bottom boundary layers, and the generation of wave-induced currents. Here, we develop a three-dimensional tidal circulation model coupled with a phase-averaged wave model to quantify the impact of the waves on the tidal kinetic energy resource of the Fromveur Strait (western Brittany) - a region that has been identified with strong potential for tidal array development. Numerical results are compared with in situ observations of wave parameters (significant wave height, peak period and mean wave direction) and current amplitude and direction 10m above the seabed (the assumed technology hub height for this region). The introduction of waves is found to improve predictions of tidal stream power at 10m above the seabed at the measurement site in the Strait, reducing kinetic energy by up to 9% during storm conditions. Synoptic effects of wave radiation stresses and enhanced bottom friction are more specifically identified at the scale of the Strait. Waves contribute to a slight increase in the spatial gradient of available mean tidal stream potential between the north-western area and the south-eastern part of the Strait. At the scale of the region within the Strait that has been identified for tidal stream array development, the available mean spring tidal stream potential is furthermore reduced by 12% during extreme waves conditions. Isolated effects of wave radiation stresses and enhanced bottom friction lead to a reduction in spring tidal potential of 7.8% and 5.3%, respectively. It is therefore suggested that models used for tidal resource assessment consider the effect of waves in appropriately wave-exposed regions.

The impact of seabed rock roughness on tidal stream power extraction

Numerical assessments of environmental disturbances induced by a tidal farm project rely usually on local modifications of the friction coefficient over the area covered by a proposed array. Nevertheless, no study has investigated the sensitivity of predictions to surrounding seabed friction. The present investigation focuses on impacts of roughness parameterisation of rock outcrops, a typical seabed of tidal stream sites. A high-resolution depth-averaged circulation model is implemented in the Fromveur Strait off western Brittany, a region with strong potential for array development, integrating the heterogeneity of sediment bottom types. Rock roughness strongly influences initial predictions of tidal current and kinetic energy in the Strait with variations of available power up to 30 %. Tidal energy extraction induces noticeable reductions of tidal currents and bottom shear stresses up to 15 km from the array considered till surrounding sandbanks. Rock roughness impacts farm-induced modifications of tidal currents, bottom shear stresses and stream powers till north-eastern and southward edges of the Strait with major absolute differences identified in its central part. Surrounding sandbanks are finally suggested to variations of shear stresses from 9 to 17 % over the Bank of the Four with possible implications on local sediment deposition.

Performance Research of Counter-rotating Tidal Stream Power Unit

Performance Research of Counter-rotating Tidal Stream Power Unit