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Modelling tidal stream power potential
Abstract
In recent years, there has been an increase in interest in renewable energy sources for a number of reasons. A particular interest in tidal energy has developed within the UK due to its numerous sites of high current velocity. In this article a development, based upon previous work, of an existing hydrodynamic computational model is shown which is used to study the potential generation and the physical impacts of tidal stream farms. An idealised geometry is used to study the impacts of installed capacity and general layout of tidal stream farms and a realistic UK west coast model is used to examine the potential of presently proposed in-stream farms.
... Recently, many researchers have made great efforts to identify the most appropriate locations for the placement of tidal turbines and to assess how much tidal current power (TCP) can be extracted from a specific site [5][6][7]. Tidal currents are highly dependent on the geographic characteristics. For instance, a channel with varying cross-sections linking with two large water bodies is usually rich in TCP [5][6][7][8][9]. ...
... for the placement of tidal turbines and to assess how much tidal current power (TCP) can be extracted from a specific site [5][6][7]. Tidal currents are highly dependent on the geographic characteristics. For instance, a channel with varying cross-sections linking with two large water bodies is usually rich in TCP [5][6][7][8][9]. ...
... Tidal currents are highly dependent on the geographic characteristics. For instance, a channel with varying cross-sections linking with two large water bodies is usually rich in TCP [5][6][7][8][9]. In addition, tidal currents can be accelerated in the area around a headland tip [10][11][12][13][14][15]. ...
The tidal current power (TCP) resource, the impact of TCP extraction on hydrodynamics and the influence of sea-level rise (SLR) on TCP output in the coastal waters of Kinmen Island (Taiwan) are investigated using a state-of-the-art unstructured-grid depth-integrated numerical model. The model was driven by eight tidal constituents extracted from a global tidal prediction model and verified with time series of measured data for tide level and depth-averaged current. The simulations showed reasonable agreement with the observations; the skill index was in the excellent (0.71–0.93) range with regard to simulating tide level and currents. Model predictions indicated that the channel between Kinmen and Lieyu serves as an appropriate site for deploying the tidal turbines because of its higher tidal current and deeper water depth. The bottom friction approach was utilized to compute the average TCP over a spring-neap cycle (i.e., 15 days). Mean TCP reached its maximum to 45.51 kW for a coverage area of 0.036 km2 when an additional turbine friction coefficient (Ct) increased to 0.08, and a cut-in speed of 0.5 m/s was used. The annual TCP output was estimated to be 1.08 MW. The impact of TCP extraction on the change in current is significant, with a maximum reduction rate of instant current exceeding 60%, and the extent of influence for the average current is 1.26 km in length and 0.30 km in width for the −0.05 m/s contour line. However, the impact of TCP extraction on the change in tide level is insignificant; the maximum change in amplitude is only 0.73 cm for the K2 tide. The influence of SLR on the TCP output in Kinmen waters was also estimated. Modeling assessments showed that due to SLR produces faster tidal current, the annual TCP output increased to 1.52 MW, 2.01 MW, 2.48 MW and 2.97 MW under the same cut-in speed and coverage area conditions when SLR 0.25 m, SLR 0.5 m, SLR 0.75 m and SLR 1.0 m were imposed on the model.
... This equation is widely used to assess the " theoretically available " tidal energy resource (e.g. Soerensen and Weinstein 2008; Walkington and Burrows, 2009; Carballo et al., 2009) without considering the reduction of the incoming flow rate due to energy extraction. However such an approach has been criticized by Garrett and Cummins (2008). ...
... One approach to take this feedback into account is to scale down the value given by Eq. (2) by multiplying its right hand side by a prescribed " efficiency coefficient " (Carballo et al., 2009 ). Another approach was employed in the modelling study of the near-field flow around the tidal turbine by Walkington and Burrows (2009). The reduction of vertically integrated flow speed was obtained by increasing the bottom drag coefficient within the energy farm .The drag coefficient was then determined from the prescribed rated power of the tidal device at a given inflow velocity. ...
... The recovery of kinetic energy due to increases in the potential energy has a much weaker effect in the open sea. Hence, the effect of reduction of maximum extractable power is stronger in the 3-D case considered here as compared to assessments done for 1-D geometry (Carballo et al., 2009; Walkington and Burrows, 2009). Walkington and Burrows (2009) also carried out 2-D modelling and noted a similar effect of current going around the farm to avoid the " blockage " so that power extraction is reduced. ...
This paper quantifies the backward effect on the ocean currents caused by a tidal stream farm located in the open shallow sea. Recent studies in channels with 1-D models have indicated that the power potential is not given purely by the flux of kinetic energy, as has been commonly assumed. In this study, a 3-D ocean circulation model is used to estimate (i) practically extractable energy resource at different levels of rated generation capacity of the farm, (ii) changes in the strength of currents due to energy extraction, and (iii) alterations in the pattern of residual currents and the pathways of passive tracers. As well as tidal streams, the model also takes into account the wind-driven and density-driven ocean currents. Numerical modelling has been carried out for a hypothetical tidal farm located in the Celtic Sea north of Cornwall, an area known for its high level of tidal energy. Modelling results clearly indicate that the extracted power does not grow linearly with the increase in the rated capacity of the farm. For the case study covered in this paper, a 100-fold increase in the rated generation capacity of the farm results in only 7-fold increase in extracted power. In the case of a high power farm, kinetic energy of currents is altered significantly as far as 10–20 km away from the farm. At high levels of extracted energy the currents tend to avoid flowing through the farm, an effect which is not captured with 1-D models. Residual currents are altered as far as a hundred kilometres away. The magnitude of changes in the dispersion of tracers is highly sensitive to the location. Some of the passive drifters analysed in this study experience significant variations in the end-to-start distance due to energy extraction ranging from 13% to 238% while others are practically unaffected. This study shows that both energy extraction estimates and effects on region wide circulation depend on a complex combination of factors, and the specific figures given in the paper should be generally considered as first estimates.
... Models of the effects of tidal energy extraction suggest changes in current flows and wave dissipation over regional scales [17]. The effects on sedimentary processes could be substantial [18][19][20]. For example, seabed height and sediment type may change several kilometres from a wave or tidal array [19][20][21]. ...
... The effects on sedimentary processes could be substantial [18][19][20]. For example, seabed height and sediment type may change several kilometres from a wave or tidal array [19][20][21]. It is also possible that the relocation/loss of sandbanks and associated communities may occur and altered wave dissipation towards shores could further effect coastal protection [18,22]. ...
Sedimentation in the sea occurs through natural processes, such as wave and tidal action, which can be exacerbated during storms and floods. Changes in terrestrial land use, marine aggregate extraction, dredging, drilling and mining are known to result in substantial sediment deposition. Research suggests that deposition will also occur due to the modern development of marine renewable energy. The response to individual burial under three depths of sediment, three sediment fractions and five burial durations was investigated in two mussel species, Modiolus modiolus and Mytilus edulis in specialist mesocosms. Both mussel species showed substantial mortality, which increased with duration of burial and burial by finer sediment fractions. M. modiolus was better able to survive short periods of burial than M. edulis, but at longer durations mortality was more pronounced. No mortality was observed in M. modiolus in burial durations of eight days or less but by 16 days of burial, over 50% cumulative mortality occurred. Under variable temperature regimes, M. edulis mortality increased from 20% at 8°C to over 60% at 14.5 and 20°C. Only M. edulis was able to emerge from burial, facilitated by increased byssus production, laid mostly on vertical surfaces but also on sediment particles. Emergence was higher from coarse sediment and shallow burials. Byssus production in M. edulis was not related to the condition index of the mussels. Results suggest that even marginal burial would result in mortality and be more pronounced in warm summer periods. Our results suggest that in the event of burial, adult M. modiolus would not be able to emerge from burial unless local hydrodynamics assist, whereas a small proportion of M. edulis may regain contact with the sediment water interface. The physiological stress resulting in mortality, contribution of local hydrodynamics to survival and other ecological pressures such as mussels existing in aggregations, are discussed.
... Carballo et al. [12] applied a two-dimensional horizontal finite element model to evaluate the tidal current energy resources in the Ria de Muros, which is the northwestern coast of Spain. Walkington and Burrows [13] implemented the two-dimensional integrated shallow water coastal circulation model ADCIRC to study the potential generation and the physical impacts of tidal current farms. An idealized geometry was used to study the impacts of installed capacity and the general layout of tidal current farms, and a realistic UK west coast model was used to examine the potential of the present proposal's in-stream farms. ...
... In the present study, the hydrodynamic model coupled with the transport model was used to compute not only the water surface elevation and velocity fields but also the water density (r). Several researchers [9,13,20] have used a constant value for the water density to assess tidal current energy resources, which may result in less accurate power estimates. ...
Recent developments in turbine technology allow the extraction of kinetic energy from tidal flows. This method of renewable energy generation is gaining popularity because of the high predictability of tides, the low environmental impact with no land occupation, and the lower investment needed for tidal turbines. In the present study, an existing three-dimensional finite element numerical model was refined to include an algorithm for computing the power density and mean power density around Kinmen Island of Taiwan. The refined model was validated against measured water levels and tidal currents at different gauging stations. The model results are in reasonable agreement with the measured data. The validated model was then used to assess the potential tidal current energy resources, including the distributions of power density at the mid-flood and mid-ebb of a mean spring tide and the mean power density. Three potential points at the east and west coasts of Kinmen Island were evaluated. The tidal current and corresponding power density at these points were computed with the model for a 15-day period to cover the spring-neap tide cycle. The annual energy output was obtained based on the power density curve to be numerically integrated. In light of the topographic considerations, the integration of the power supply, and the existing harbor/port facilities, the location near Suetau would be an appropriate site for establishing a tidal current power plant.
... Meeting this target will require substantial investment in new on-and offshore wind, as well as the UK's abundant wave and tidal energy resource potential (Iyer et al., 2013). Within the UK, various constraints, such as hydropower plant suitability and sunlight hours generally restrict renewable energy options to wind, wave and tidal power (Walkington and Burrows, 2009). Although these forms of technologies are crucial to meet future energy demands, certain technologies, such as wind and wave generation are variable since they are ultimately dependent on weather conditions (Denny, 2009). ...
... Although these forms of technologies are crucial to meet future energy demands, certain technologies, such as wind and wave generation are variable since they are ultimately dependent on weather conditions (Denny, 2009). These particular renewables also require the resource to be used as and when it is available (Walkington and Burrows, 2009), or stored with associated costs and losses. Although tides are intermittent they have the advantage over other forms of renewable energy of being predictable over long timescales (Cave and Evans, 1984). ...
Tidal energy is on the verge of commercial viability and full scale prototypes are meeting the challenges of the marine environment. The primary focus of the sector has concerned Horizontal Axis Tidal Turbines (HATTs); comprising a turbine supported by a tubular stanchion operating on a bi-directional, or yaw system. The direction of tidal flow, however, varies over the ebb or flood phases of a tidal cycle. This pa-per utilises tidal velocity data measured in Ramsey Sound (Pembrokeshire, Wales, UK), a macrotidal strait and proposed HATT installation site and combines with Computational Fluid Dynamics (CFD) to assess the impact of misalignment between a HATT and its surrounding free stream velocity. The majority of the veloci-ties within the northern area of Ramsey Sound tend to fall within a misalignment of ±20° for velocities greater than the economic viable threshold of 2 ms-1. However, bathymetry and coastline configuration influence both flow magnitude and direction. At the outer margins for the Sound, the velocities are acted upon by various promontories, reefs and shelving areas, which deflect and retard the flow, resulting in a flow direction greater than 20°, particularly towards the outer edges of the Sound. Utilising field data for numerical simulations will help inform the industry and increase investor confidence in this technology, whilst avoiding costly scaled ex-perimentation. It was found that an axial flow misalignment of ±10° results in approximately a 7% reduction in peak power, 3% in peak torque and 5% in peak thrust. The axial wake recovery length was shorter for the ±10° cases, recovering to 90% by 7D downstream, as opposed to 10D downstream for the aligned turbine.
... This numerical model allows prediction of the flow field changes due to tidal arrays , Walkington & Burrows, 2009, and enables us to evaluate the environmental impact of these arrays (Ahmadian & Falconer 2012, Xia et al. 2010. Since turbines will act as resistance to the flow, adding a turbine or turbines in an array will produce an effect on the flow. ...
As the world’s largest archipelagic country, Indonesia has enormous tidal resource potential. However, complexities that occur in this area complicate tidal resource assessment. The relative magnitude of different tidal components gives this area a variety of tidal types, from semidiurnal, as commonly found in the UK and Canada, to the diurnal type. There are not only daily and neap-spring cycle variations, but also in areas with significant diurnal components the power and thrust varies on an annual basis that follows the timing of the solstice-equinox cycle. These variations mean that an assessment in this region must be carefully planned in terms of the duration and the simulation start time. An assessment with a limited computational time should avoid the solstices and equinoxes in the simulation period.
The interaction between the diurnal and semidiurnal components also creates an asymmetric tidal flow. As the tidal stream moves bi-directionally, the asymmetry leads to uneven power production for flow in different directions. Therefore, turbine developers should consider this phenomenon in their turbine design, as the turbine might have more thrust in one direction. This asymmetry also tends to create a low Capacity Factor (CF).
Since the tidal stream varies daily, fortnightly and annually, it would not be practicable for a turbine developer to remove the maximum power from the flow. A capping strategy is necessary to address this issue. CF is a metric that is widely used to optimise turbine capacity. However, this thesis argues that CF might not be the only metric for decision making. The fraction of average power removed by the turbine and the thrust before and after the capping strategy is implemented are perhaps more important as decision-making tools in tidal energy exploitation.
The assessment of tidal energy resources in Indonesia shows that this country has a great opportunity for tidal energy exploitation. Five potential sites with different characteristics in terms of socio-economic background and environmental features are selected for assessment: Lombok Strait, Larantuka Strait, Sunda Strait, Lingga Regency and Sula Regency. A total of 5 GW average electricity production from just these five selected locations could be produced.
However, that number is based on analyses using a uniform blockage ratio. In reality, the deployment of turbines is limited by several factors, such as the use of straits for other purposes, the depth of the sites and environmental constraints. A study with more realistic turbines is essential for a proper assessment of tidal energy resources in an area. The assessments in this thesis also consider economic constraints. Based on realistic turbine deployment strategies, tidal energy exploitation in this area is economically viable. Apart from in Lingga Regency, the realistic assessments show that all other locations have a projected levelized cost of energy (LCOE) less than GBP 250/MW.
... It is this highest speed that has been used for design purposes in the present study. Given the required power output of 1.5 MW for the turbine system studied in this work and considering the main limitations on the size of tidal turbine blades [36] a blade with a length of 8 m was chosen. To identify the optimized rotating speed, the BEMT model was also used to calculate the variation of hydrodynamic efficiency as a function of the tip speed ratio (TSR) (Fig. 3). ...
Tidal turbine blades experience a significant bending moment in the root area during their long-term operation in a sea water environment. This necessitates using fibre reinforced polymer as the blade material to provide the required strength/stiffness. In this study, a design methodology based on hydrodynamic and Finite Element models with a view to examine the mechanical properties of composites was developed to evaluate the structural response of two commercial scale turbine blades (1.5 and 0.35 MW). Using the output from the hydrodynamic model, Finite Element stress analysis was conducted in order to determine the likelihood of blade failure. To incorporate appropriate failure modes with the blade model, failure analysis was conducted on the composite test coupons. The results of stress analysis show that the blade root area experiences high stress values with failure modes such as resin cracking, fibre/matrix interfacial de-bonding, delamination and fibre breakage. It is also predicted that the glass fibre composite blades require three times thicker laminates than a carbon fibre composite blade of similar design to maintain the same safety factor. Reasonable agreement between the numerically and experimentally determined strain fields on a small-scale blade indicates that the result of Finite Element stress analysis is valid.
... The models can be sensitive to the details of the flow geometry and bathymetry and must be carefully calibrated for the undisturbed flow conditions, so that we can have confidence in the results with inclusion of tidal stream turbines. Walkington and Burrows (2009) did an early study of implementing tidal stream technology into the ADCIRC 2D unstructured grid model. Li et al. (2017Li et al. ( , 2019 show results for tidal stream turbines in a coupled hydrodynamic-wave model (FVCOM). ...
Offshore Renewable Energy (ORE), comprising marine (wave and tidal energy), and offshore wind, has the potential to supply large amounts of ‘green’ sustainable energy, reducing CO2 emissions. The main obstacles to deployment so far are technical challenges and cost. However, there are also concerns about how harnessing offshore energy can affect the local habitats and marine life, as well as introducing far-field and long-term changes in the physical environment of the sea, which may combine with climate change in unforeseen ways to affect marine ecosystems. The precautionary principle, combined with the requirement for monitoring, introduces obstacles (and costs) which have so far prevented the deployment of offshore renewable energy on a large scale. Here we discuss the physical changes that may occur and the impacts these may have on habitats, species and ecosystems. We explore the possible environmental impacts of offshore wind and marine energy deployment and the options for mitigation of these. This information can assist planners, regulators and developers of offshore energy systems. Some examples of existing and proposed deployments are provided (mainly focusing on the UK), in order to illustrate discussion of the environmental issues. We identify the need for better understanding of the environmental impacts at a population and ecosystem level and identify a way forward to improve the environmental consenting process.
... Limited scenarios also exist for hydrokinetic energy extraction in some areas (e.g. Sutherland et al., 2007;Walkington & Burrows, 2009;Karsten et al., 2008). If we are to understand the systemic ecological consequences of extracting energy from waves and tides, we need to construct integrated hydrodynamic models with coupling between large-scale systemic processes and processes operating at the local scales at which energy extraction is likely to occur. ...
Water movements define some of the most important ecological factors determining the distribution of organisms in marine environments. This is true both at large spatial scales, where ecological connectivity and trophic coupling are defined by circulation patterns and vertical mixing structure, and at the much smaller scales at which individual organisms experience flow, turbulence and shear forces. Moving water possesses energy, and this is increasingly regarded as a resource for power generation, potentially meeting 15% of energy demands at a European level by the middle of this century. Conversion of hydrokinetic energy into other forms of energy that are useful for human purposes inevitably involves diversion of physical processes from their 'natural' pathways, with possible consequences also for ecological processes. In simple terms, extraction of energy from water flow involves reducing the average velocity of flow and hence changing the conditions experienced by an organism living in the flowing water. In reality, the hydrodynamic consequences of extracting energy are likely to be complex and site-specific, with changes in turbulence as well as both increases and decreases in local flow velocities. We use statistical models applied to incidence records for marine bryozoan species in Scottish waters to examine the extent to which their distribution may be governed by the same wave and tidal energy variables that influence the location of marine renewable energy developments, and address the question of whether it is possible to predict what might be the consequences of energy extraction for species distribution.
... Walkington and Burrows [68] used this code to evaluate the impact of tidal hydrokinetic turbine farms in the west coast of the UK. The turbines have been modeled considering an equivalent drag coefficient. ...
Tides can be a vast and predictable source of renewable energy. Due to the solar and lunar influx on our planet, they move large amounts of water periodically, and this energy can be harnessed using devices designed and positioned adequately, such as current turbines. However, the relation between the energy obtained with actual devices and the economic and environmental cost of their installation limits the practical application of these solutions. In order to optimize the design of this technology and achieve its successful installation and use, a detailed knowledge about the energy potential of tides at the specific location is necessary. This calculation is not easy and requires the use of specialized software tools. Currently, there is no specific software to evaluate the tidal currents energy potential, but there are more than a few codes able to calculate the hydraulic flow in rivers, estuaries and coastal regions. These programs are usually used for the calculation of pollutant dispersion and floods, but they can be adapted with more or less success. This paper reviews the available 1D, 2D, and 3D software tools with the aim of analyzing their functionality and their validity to evaluate the energy potential of tidal currents.
... One common approach for modelling the power extraction by tidal stream turbines in depthaveraged ocean models, is to represent the drag imposed by the turbines as an increased bed friction, e.g. Walkington and Burrows (2009). However, it is difficult to associate this drag with a given arrangement of turbines (Adcock et al., 2015). ...
Co-location of offshore wind turbines at sites being developed for tidal stream arrays has been proposed as a method to increase capacity and potentially reduce the cost of electricity compared to operating either technology independently. This research evaluates the cost of energy based on capital expenditure and energy yield. It is found that, within the space required around a single 3 MW wind turbine, co-location provides a 10-16% cost saving compared to operating the same size tidal-only array without a wind turbine. Furthermore, for the same cost of electricity, a co-located farm could generate 20% more yield than a tidal-only array.
These results are based on analysis of a case-study site in the Pentland Firth. Wind energy is assessed using an eddy viscosity wake model in OpenWind, with a 3 MW rated power curve and thrust coefficient from a Vestas V90 turbine. Three years of wind resource data is from the UK Met Office UK Variable (UKV) 1.5 km numerical model and corrected against a 400 m Weather Research and Forecasting (WRF) model run over the site. Tidal stream energy is modelled using a semi-empirical superposition of self-similar plane wakes, with a generic 1 MW rated power curve and thrust based on a full-scale, fixed-pitch turbine. Coincident tidal resource data is from the Forecasting Ocean Assimilation Model (FOAM) at 7.5 km resolution and correlated with a 150 m ADvanced CIRCulation model (ADCIRC). Wave parameters are corrected from ERA-Interim data with six months of wave buoy data. Multiple tidal turbine array layouts are considered, with maximum tidal energy generated for a staggered array with spacing of 20 tidal turbine diameters, Dt , streamwise and 1.5Dt cross-stream. However, cheapest cost of electricity from the tidal-only array, was found for a single row of turbines, due to minimal wake effects.
Laboratory experiments were undertaken to validate the superposition wake model for use with large, shared support structures. Two rotors mounted either side of a central tower generate a peak wake velocity deficit 70% greater than predicted by superposition. This was due to high local blockage and a complex near-wake structure, with a corresponding increase in tower drag of 9%. Further experiments evaluated the impact of oblique inflow on turbines yawed at ±15°. These results validated a theoretical cosine correction for thrust coefficient and characterised the centreline wake drift with downstream distance.
Extreme environmental loads for a shared support structure, compared to structures for windonly and tidal-only, have also been modelled. A non-linear wave model was used to represent a single wave form with 1% occurrence for each hour of time-series data. Overturning
moment about the base of a shared support, with one wind and two tidal turbines, was found to be 4.5% larger than for a wind-only turbine in strong current and with turbines in different operational states. Peak loads across the tidal array were found to vary by 2.5% and so little load reduction benefit could be gained by locating a shared support in a more sheltered area of the array.
... MIKE 21 FM is a two-dimensional, depth-averaged flexible mesh model using a Reynolds averaged Navier-Stokes equation. For this study, an idealised, relatively shallow, model was used, which was modified from the benchmark test case domain developed in [31,32] with a tidal free surface forcing of a realistic amplitude from the Strangford Lough model [33] that was held uniform along the open boundary. The domain consisted of a high flow velocity channel between the shallow area of the open sea and an enclosed shoaling out basin with some deeper channels of up to 50 m with no river discharge (Table 1; Figure 1). ...
The effects of large scale tidal energy device (TED) arrays on phytoplankton processes owing to the changes in hydrodynamic flows are unknown. Coupled two-dimensional biogeochemical and hydrodynamic models offer the opportunity to predict potential effects of large scale TED arrays on the local and regional phytoplankton dynamics in coastal and inshore environments. Using MIKE 21 Software by DHI (https://www.dhigroup.com), coupled two-dimensional biogeochemical and hydrodynamic models were developed with simulations including no turbines or an array of 55 turbines with four solar radiation scenarios to assess the temporal and spatial changes of phytoplankton dynamics in an idealised domain. Results suggest that the effect of TEDs on phytoplankton dynamics accounted for up to 25% of the variability in phytoplankton concentrations, most likely associated with an increased residence time in an inshore basin. However, natural variation, such as the intensity of photosynthetically active radiation, had a larger effect on phytoplankton dynamics than an array of TEDs.
... In the Lewis et al. [13] model the resolution is 278 m at its finest meaning many of the islands and key bathymetric features are smoothed out as they smaller than the mesh elements. In Walkington & Burrows [14] the tidal turbines neglect the drag effect of the support structure. A specific model of Ramsey Sound was presented by Fairley et al. [15]. ...
A number of sites around the UK are being considered for development of tidal stream energy, one of which is Ramsey Sound off the coast of Pembrokeshire, South Wales. The Sound was used to test the prototype of the Delta Stream by Tidal Energy Ltd. After initial testing, a 10 MW tidal array was proposed at St David's Head. To investigate any possible environmental impacts of the array due to energy extraction, a case study of the Pembrokeshire coast was performed using a high-resolution depth averaged hydrodynamic model, Telemac2D, to investigate changes to hydrodynamics and morphodynamics. Results show that the proposed array of nine tidal energy converters will cause alterations to eddy propagation leading to changes in the velocity field up to 24 km from the tidal array. Changes in morphodynamics are predicted through alterations to the bed shear stress. Changes to the mean and maximum bed shear stress, over a 30-day period, are found to be more localised and extend 12 km from the array. These changes indicate that the proposed tidal array will lead to localised sediment accumulation and will act as a barrier to sediment transport, with potential consequences for the benthic ecology of the region.
... Walkington and Burrows [32] found that power extraction depends on the farm spatial layout and that this also affects erosion and sediment transport. Changes in the local flow features and water levels were also assessed by Ahmadian and Falconer [59] and Ahmadian et al. [60] in the Severn Estuary and Bristol Channel, south-west of England. ...
Before tidal stream energy is exploited, tidal power resource and environmental assessments must be undertaken. This thesis explores limits to power extraction for tidal sites defined by a strait between an island and landmass. Numerical simulations provided by Fluidity are used to analyse power extraction from locations in the strait and around the island for an idealised island-landmass domain and an actual coastal site. The numerical model is verified by comparing predictions with analytical solutions for inviscid flow past a circular cylinder located in the centre of a channel and in the vicinity of a wall. The model is then validated against laboratory measurements of flow patterns for impulsively-started flow past a submerged circular cylinder, and for flow past a surface-piercing circular cylinder in oscillatory laminar shallow flow. It is demonstrated that the numerical method captures satisfactorily the mechanisms of early wake formation, which indicates the model can be applied to assess tidal stream resource within the coastal geometries considered herein. Finally, the methodology to account for power extraction is satisfactorily verified for bounded and unbounded flows. Contrary to current practices, results from a parameter study for different idealised coastal sites reveal that the maximum power extracted in the strait is not well approximated by either the power extracted naturally at the seabed or the undisturbed kinetic power. Moreover, an analytical channel model underpredicts the maximum power extracted in the strait due to its inability to account for changes in the driving head resulting from power extraction and flow diversion offshore of the island. An exception is found for islands with large aspect ratios, with the larger dimension extending parallel to the landmass; i.e. the island-landmass geometry approaching that of a channel. In this case, the extracted power is satisfactorily approximated by the power naturally dissipated at the seabed and there is good agreement with the analytical model. The maximum power extracted in the strait is shown to decrease when water depths offshore are greater than in the strait, underlining the importance of fully understanding the wider bathymetry of a given site. A similar conclusion is reached when strait blockage is reduced. The power extraction in the strait is found to be sensitive to both viscosity and seabed friction, and these parameters need to be properly estimated during the setup and calibration of models in order to reduce uncertainty. Power extraction increases when turbines are sited simultaneously both in the strait and offshore. Tidal power assessment is performed for Rathlin Sound, off the coast of Northern Ireland. Again, no clear relationship is found between maximum power extracted in the strait and either the power dissipated naturally at the seabed or the undisturbed kinetic power. A similar ratio of power extracted to undisturbed kinetic power is obtained as for the equivalent idealised model. The analytical channel model underpredicts the maximum power extracted. The actual and idealised coastal site models indicate similar responses to changes in seabed friction, and similar reduction in power extraction with decreasing strait blockage.
... In the UK, a number of renewable energy sources, such as solar and hydro-power, are generally restricted by sunlight hours and plant suitability, respectively, which increases the attractiveness of other resources, such as wind, wave and tidal power [4]. However, by their very nature, wind and wave generation are variable and heavily dependent on weather conditions [5]. ...
Extensive Research and Development have helped drive the tidal energy industry towards commercial viability, with full-scale prototypes starting to meet marine environmental challenges. This paper utilises velocity data collected from a site off the welsh coast with Computational Fluid Dynamics (CFD) to determine the effects of misaligned flows on the performance of a tidal stream turbine. The field observations indicate that the majority of the currents tend to fall within ±20° of the principal flow direction for economically viable velocities. The CFD modelling suggests a reduction in the non-dimensional performance parameters as the angle of misalignment increases between the axis of rotation and the free stream velocity. The resultant magnitude of the bending moments acting on the rotor end of the driveshaft for the misaligned turbines was found to be up to ten times greater than the aligned turbine. The paper also shows that an accurate definition of the tolerance to axial flow misalignment between the free stream velocity and the axis of rotation of a turbine is required to prevent any negative effects on performance and loading.
... Two-and three-dimensional numerical simulations have been employed to study the effects of tidal energy extraction, with three dimensional simulations generally being used to study individual turbines or small groups of turbines, whereas two dimensional simulations are used to investigate large arrays of turbines through solution of the shallow water equations (see, for example: [6]- [8]). The grid resolution required for three dimensional turbine studies ensures that both the turbine and wake are resolved, which means it is possible to predict the power usefully extracted by the turbine, and, by taking a sufficiently large control volume, the power dissipated by mixing processes. ...
The power potential of a fence of tidal turbines depends on the mass flux through the array and the variation of
the flow velocity within the array. The shallow water equations (SWE) are often used to analyse the power potential of a site, where the turbine array is modelled as a region of enhanced flow resistance. This method leads to errors in the power extracted by the turbine array as the velocity at the turbine plane, and therefore the power, is incorrect because the variation in flow speed across the array is not modelled by the shallow water model.
This paper develops a correction based on a Linear Momentum Actuator Disk Theory (LMADT) representation of the turbine array which equates the thrust between the simulated array and the analytically modelled turbine array. The power extracted by the turbine array can then be correctly evaluated, which is shown to be less than the erroneous power calculated from the simulated array flow velocity. The corrected shallow water flow model can be used to solve the more relevant reverse problem of the thrust required to deliver a given level of power generation. It is found that the thrust required, and thus overall energy removal (including wake mixing) from the flow is always higher for a given output power when the relationship between thrust and power is correctly modelled.
... Hashemi et al. (2015) investigated the influence of waves on tidal resource at Anglesey, showing that extreme wave-current interactions can reduce the tidal resource by 20%. Walkington & Burrows (2009) conducted an assessment of tidal stream power at multiple sites. However, the hydrodynamic effect of the tidal array at each of the four locations was considered in isolation. ...
A cumulative impact assessment of tidal stream developments in the Irish Sea has been conducted on a high-resolution depth-averaged hydrodynamic model, using Telemac2D. Eight sites were investigated, representing the proposed developments at the time of study. These included: Ramsey Sound, Anglesey, Strangford Loch, Mull of Kintyre, Torr Head, Fair Head, Sound of Islay and West of Islay. Only three projects showed array-array interaction: Fair Head, Torr Head and Mull of Kintyre. A smaller model domain was created for further analysis. Results showed Mull of Kintyre had little impact. Fair Head reduced the energy production at Torr Head by 17%, whereas, Fair Head only reduced by 2%. This was caused by the tidal asymmetry whereby the flood was stronger. When operated concurrently, the maximum power-output at Torr Head is 64.5MW, representing 31% reduction. If Torr Head can still operate commercially in the presence of Fair Head, then the additional environmental impact of Torr Head, such as the change in bed shear stress, is small. Within the Irish Sea, very few of the tidal projects investigated are geographically close to each other. As the industry develops, the risk of interaction to these sites will grow when more intermediary sites are developed.
... through full hydrodynamic oceanographic numerical modelling (Blunden and Bahaj, 2007;Walkington and Burrows, 2009;Hashemi et al. 2012;Serhadlıoğlu et al. 2013). ...
National efforts to reduce energy dependency on fossil fuels have prompted examination of macrotidal nearshore zones around the UK for potential tidal stream resource development. Although a number of prospective tidal energy sites have been identified, the local hydrodynamics of these sites are often poorly understood.
Tidal-energy developers rely on detailed characterisation of tidal energy sites prior to device field trials and installation. 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.
Ramsey Sound (Pembrokeshire, Wales, UK) will soon host Wales’ first TST demonstration project. However, the local hydrodynamics of the sound have been underexamined. Ramsey Sound experiences a marked tidal asymmetry, with local bathymetric features that affect flow fields which are spatially heterogeneous in three dimensions.
Using Ramsey Sound as a case study, this thesis has three objectives: (1) to examine the wake created by submerged objects through field- and laboratory-based measurements, (2) to experimentally investigate the effect of submergence on wake development and decay downstream of a conical island, and (3) to develop a TST suitability tool, which examines the effects of velocity, water depth and bed slope on power availability within a macrotidal coastal area.
Laboratory experiments have shown that submergence level is an important parameter controlling wake structure and extent, and that changes in submergence level affect both the 3-D flow structure in the near wake and the 2-D far wake of islands. Analysis of physical and hydrodynamic characteristics in Ramsey Sound, including tidal velocities across the swept area of the pilot TST, vertical shear in the stream flow, 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.
... Carballo et al. (2009) applied a two-dimensional horizontal finite element model to evaluate the tidal current energy resource in the Ria de Muros, which is the northwestern coast of Spain. Walkington and Burrows (2009) implemented the two-dimensional integrated shallow water coastal circulation model ADCIRC to study the potential generation and the physical impacts of tidal current farms. An idealized geometry was used to study the impacts of installed capacity and the general layout of tidal current farms. ...
FVCOM ocean model is applied to simulate the tidal wave system in the Qiongzhou Strait, and the error between investigation and simulation is acceptable. Based on that result, we assess the tidal energy of Qiongzhou Straits by the method of FLUX, and discuss the temporal and spatial distributions of tidal current energy resource in this area. An conclusion can be extracted like this: there is a higher power density of energy in the central area of this strait, relatively, lower on both sides; characteristic symbols of power density of energy such as most possibility speed, maximal density of.
... Tidal turbines share many similarities to wind turbines; however, due to the higher density of sea water they can produce 800-900 times more power when compared to an equivalent wind turbine of similar size operating at the same speed (Jahromi et al. 2011). A wide range of possible technologies have been proposed to determined the magnitude of the tidal current resource and to harness tidal currents (Walkington and Burrows 2009;Rourke et al. 2010), but as discussed below, these are currently in the demonstration phase. ...
Estimates of the recoverable energy resource from wind, waves, and tidal variations depend upon the precise assumptions employed. Nevertheless, it is clear that harnessing of these resources has the potential to supply a substantial fraction of the world's total electricity demand. Herein we have presented a brief summary of; the magnitude of the marine resource, the current status of efforts to harness these resources, and an analysis of the key parameters that dictate the magnitude and current ability to harness ocean energy and exhibit potential climate change sensitivity.
... Impacts of ORE devices on the other regulating services are equally poorly understood [87]. Modelling studies have been undertaken, which suggest, for example, that tidal streams arrays could have farfield effects on sedimentation patterns [161,162], which may impact erosion control and natural hazard protection, but at present little is known about these impacts. ...
... For example, some studies consider single channels that have turbines installed across their entire widths. c) In order to analyse the potential far-field effects of tidal turbine deployments, hydrodynamic models using shallow water equation solvers have been used [3]. These codes use different simplifications and assumptions than CFD models, running usually at lower spatial resolutions. ...
Available at http://www.masts.ac.uk/about/masts-publications/terawatt-publications/
... The Welsh Government has set targets to harness 4 GW of tidal and wave power by 2025 [14]. Meeting this target requires a better understanding of the tidal resource in Wales by constraining estimates of tidal power through site-specific velocity measurements [11e13] and through fully hydrodynamic oceanographic numerical modelling [10,15,16]. ...
... This kind of models is suitable for simulating the far-field current. These models have been applied to predict the tidal current distributions [5], to investigate tidal power generation potential [21][22][23], and to assess the environmental impact [7][8][9]. 3D SWE models have also been used to simulate the effect of turbines, in which a momentum sink is included in the vertical plane where the turbine is located [23][24][25]. ...
Energy sustainability has become one of the most concerned issues around the world, and tidal stream power is one of the promising types of renewable energy with merits of high predictability, relatively low cost, and low environmental impact. Numerical models based on the shallow water equations (SWE) have been applied to assess the energy distribution and environmental impact. In existing SWE models, the effect induced by tidal stream turbines on the fluid is usually represented by bed friction, which is only applicable for far-field studies. In the current study, a Blade Element Momentum (BEM) model has been developed and integrated into a three-dimensional (3D) SWE model to improve the accuracy for assessing tidal current energy distributions. Using the BEM approach, state variables can be obtained for a blade element of a turbine rotor. These state variables can be used to calculate the local thrust coefficient, which is introduced for representing the effect of rotors in the 3D SWE models. A 3D SWE model has been refined, by incorporating the proposed BEM model, to simulate the density distribution of a tidal turbine farm. The refined SWE model has been validated against measurements from a flume experiment of turbine arrays. The model performs generally well in predicting the distribution of velocity deficit and the recovery of wakes. In a field-scale application to a planned tidal power test site, the model was used to predicting the flow field for both a single turbine scenario and a turbine array scenario. Copyright © 2015 John Wiley & Sons, Ltd.
... Impacts of ORE devices on the other regulating services are equally poorly understood [87]. Modelling studies have been undertaken, which suggest, for example, that tidal streams arrays could have farfield effects on sedimentation patterns [161,162], which may impact erosion control and natural hazard protection, but at present little is known about these impacts. ...
... Sun et al. (2008) evaluated the local effects of tidal energy extraction on the flow field at a laboratory scale using a computational fluid dynamics code. Walkington and Burrows (2009) simulated the tidal stream power potential and impacts of installed tidal farm capacity on the west coast of the UK using a depthaveraged 2-D model. Draper et al. (2009) also used a 2-D model to investigate the effects of tidal energy extraction on tidal hydrodynamics in a simple tidal channel. ...
A 3-D coastal ocean model with a tidal turbine module was used in this paper to study the effects of tidal energy extraction on temperature and salinity stratification and density-driven two-layer estuarine circulation. Numerical experiments with various turbine array configurations were carried out to investigate the changes in tidally averaged temperature, salinity, and velocity profiles in an idealized stratified estuary that connects to coastal water through a narrow tidal channel. The model was driven by tides, river inflow, and sea surface heat flux. To represent the realistic size of commercial tidal farms, model simulations were conducted based on a small percentage (less than 10 %) of the total number of turbines that would generate the maximum extractable energy in the system. Model results show that extraction of tidal in-stream energy will increase the vertical mixing and decrease the stratification in the estuary. Installation of in-stream tidal farm will cause a phase lag in tidal wave, which leads to large differences in tidal currents between baseline and tidal farm conditions. Extraction of tidal energy in an estuarine system has stronger impact on the tidally averaged salinity, temperature, and velocity in the surface layer than the bottom layer even though the turbine hub height is close to the bottom. Finally, model results also indicate that extraction of tidal energy weakens the two-layer estuarine circulation, especially during neap tides when tidal mixing is weakest and energy extraction is smallest.
... Thus, harnessing tidal energy from strong tidal current regions using in-stream (hydrokinetic) tidal devices has quickly gained more attention. A number of studies have been conducted using either analytical or numerical models to evaluate the maximum amount of tidal energy that can be extracted from a tidal system Cummins 2005, 2008;Bryden and Couch 2007;Sutherland et al. 2007;Blanchfield et al. 2008;Draper et al. 2009;Walkington and Burrows 2009;Atwater and Lawrence 2010;Yang et al. 2013). However, these physically based calculations did not consider associated environmental effects, and the calculated energy removal potential is unlikely to be achieved in real-world estuaries and tidal basins. ...
To assess the effects of tidal energy extraction on water quality in a simplified estuarine system, which consists of a tidal bay connected to the coastal ocean through a narrow channel where energy is extracted using in-stream tidal turbines, a three-dimensional coastal ocean model with built-in tidal turbine and water quality modules was applied. The effects of tidal energy extraction on water quality were examined for two energy extraction scenarios as compared with the baseline condition. It was found, in general, that the environmental impacts associated with energy extraction depend highly on the amount of power extracted from the system. Model results indicate that, as a result of energy extraction from the channel, the competition between decreased flushing rates in the bay and increased vertical mixing in the channel directly affects water quality responses in the bay. The decreased flushing rates tend to cause a stronger but negative impact on water quality. On the other hand, the increased vertical mixing could lead to higher bottom dissolved oxygen at times. As the first modeling effort directly aimed at examining the impacts of tidal energy extraction on estuarine water quality, this study demonstrates that numerical models can serve as a very useful tool for this purpose. However, more careful efforts are warranted to address system-specific environmental issues in real-world, complex estuarine systems.
... Refs. [31,12]), these studies have focused mainly on tides or sediment transport [25]. Wave characteristics at potential tidal stream sites should be considered in several respects, such as wave induced hydrodynamic loading, operation and maintenance, wave-tide interactions, and sediment transport. ...
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.
... Impact studies using numerical models and field observation have indicated that natural levels of energy extraction by underwater turbine installations may result in maximum changes of order centimeters in water level, tens of cm/s in tidal current magnitude, and centimeters in bed level (Neill et al., 2009(Neill et al., , 2012. In addition, changes in residual currents (e.g., Walkington & Burrows, 2009) and Lagrangian particle excursions (e.g., Shapiro, 2011) have also been demonstrated in modeling studies. Key metrics from a field environmental monitoring program and select numerical model studies are summarized in Table 1. ...
The Finite-Volume Community Ocean Model (FVCOM) is configured to evaluate the potential impact of the proposed Muskeget Tidal Energy Project on circulation and sediment transport in the surrounding region. The extraction of tidal kinetic energy from the water column is modeled by augmenting the momentum equations with additional drag terms parameterized using local flow velocities and parameters specific to the installed turbine farm. Model-computed power output compares well with estimates based on velocities derived from a shipboard acoustic Doppler current profiler (ADCP). Total extracted power from the proposed installations during a spring ebb tide represents roughly 9% of the natural power in the deep section of the channel and 30% of the natural tidal dissipation in the turbine installation region. Due to this low level of extraction, turbine installations at the proposed transects result in relatively minor differences in the tidal current magnitude (2.5%), water level (0.8%), sediment flux (0.6%), and bed level (9%). Computations also indicate that the proposed installation generates minimal impacts to the tidal harmonics (3.3% change in amplitude and 1-min delay in phase) and tide-induced depthaveraged residual currents (2.8%). Model-computed extraction at increased levels is associated with greater perturbations to the natural conditions.
... Previous studies have shown that the power production from a tidal farm that spans the whole width of the channel is considerably greater than that only spans a portion of the channel with the same number of turbines for most installed capacities (Garrett and Cummins, 2008;Walkington and Burrows, 2009). In additional to downstream merging of the wakes, another primary reason is the flow pattern change, i.e. high velocity is located in areas without turbines. ...
This paper presents a three -dimensional modeling study for simulating tidal current energy extraction in large areas, with a momentum sink term being added into the momentum equations. Due to the limits of computational capacity, the grid size of the numerical model is generally much larger than the turbine rotor diameter. Two models, i.e. local grid refinement model and coarse grid model, are employed and an idealized estuary is set up. The local grid refinement model is constructed to simulate the power generation of isolated turbine and the impacts on hydrodynamics, which is used to define the deployment of turbine farm and quantify a combined thrust coefficient for multiple turbines located in a grid element of coarse grid model. The model results indicate that the performance of power extraction is affected by array deployment, with more power generation from outer rows than inner rows due to velocity deficit influence of upstream turbines. Model results also demonstrate that the large-scale turbine farm has significant effects on the hydrodynamics. The tidal currents are attenuated within the turbine swept area, and both upstream and downstream of the array. While the currents are accelerated above and below turbines, which is contribute to speeding up the wake mixing behind arrays. Water levels are heightened in both low and high water levels as the turbine array spanning the full width of estuary. The magnitude of the observed impacts is found to increase with the array expansion, especially for the low water levels.
... Exploratory 2-D modelling has demonstrated the tendency for water flows to divert around a tidal-stream farm, as a consequence of the blockage, which is likely to reduce the fraction of incident energy which is extractable, a factor that has not been properly accounted for in a number of the earlier assessments of tidal stream resources. ( Walkington and Burrows, 2010). ...
The paper presents findings from a recent two-year Joule Centre (JC) study, 'Tapping the Tidal Power Potential of the Eastern Irish Sea' funded by the Northwest Development Agency (NWDA), as well as subsequent developments. This evaluated the scope for reliable electricity generation from a combination of estuary barrages/lagoons and tidal-stream energy devices using 0-D and 2-D computer modeling. The emphasis was towards conjunctive operation incorporating allowances for other schemes outside the region, including a 'Severn' barrage, an integral part of the study being potential impacts of the energy removal on the overall tidal dynamics of the Irish Sea and environmental issues arising. Estimates arising suggest that the estuarial waters of the North West stretching from the Dee & Mersey, to Morecambe Bay and the Solway Firth are capable of meeting about 5% of national electricity demand. This places it on a par with the 'Severn' estuary, as a potential major contributor of renewable energy in the UK. INTRODUCTION The geographical location of the United Kingdom and the seas that surround it provide internationally enviable renewable resources. Technologies for wind power extraction are now mature and an increasing role for the opportunistic capture of this intermittent energy source for the electricity grid is firmly established. Marine wave energy offers even greater scope for the future. Even more exclusive, however, is the potential for tidal energy extraction from around the UK coastline, it being estimated that the UK holds almost half the European resource at 50 TWh/year. The most attractive locations for harnessing tidal power are estuaries with a high tidal range for barrages, and other areas with strong tidal currents (e.g. straits and headlands) for free-standing tidal stream devices. Barrage schemes, drawing on established low-head hydropower technology, are fully proven. The La Rance plant in France has now passed its 40 th year of operation.
... While modeling of waterbodies for tidal energy extraction has been aimed largely at examining the potential to maximize energy harvest (Karsten et al., 2008;Sutherland et al., 2007;Walkington and Burrows, 2009;Draper et al., 2009;Polagye et al., 2009;, efforts have also been made to examine the effects that tidal energy extraction might have on the marine environment (Shapiro, 2010;Defne et al., 2011;Hasegawa et al., 2011;Neill et al., 2012;Wang et al., 2014). ...
... Much resource assessment has been conducted using the renewables energy atlas [1]. However, it is recognised that in coastal regions and channels, the resolution of this atlas means resource may be underestimated [8] and therefore it must be supplemented by more detailed models [9] or measurements [10]. Once total resource is calculated, estimation of power output for a given area can be achieved in a variety of ways. ...
... Hydrodynamic models of bays and estuaries have traditionally been used for coastal engineering applications including the design of development projects such as breakwaters and harbours, sediment transport processes and water quality. More recently their application has been utilized by the marine energy industry to determine optimal sites for exploitation of marine energy converters (MEC) [1], as well as in the development of tools to enable MEC farm layouts to be optimised for maximum energy extraction [2,3]. Hydrodynamic models are also increasingly being used to couple hydrodynamics with ecological processes such as larval dispersal [4][5][6] and to provide closer insight into conservation and water quality concerns [7]. ...
Hydrodynamic models are a powerful tool that can be used by a wide range of end users to assist in predicting the effects of both physical and biological processes on local environmental conditions. This paper describes the development of a tidal model for Strangford Lough, Northern Ireland, a body of water renowned for the location of the first grid-connected tidal turbine, SeaGen, as well as the UK's third Marine Nature Reserve. Using MIKE 21 modelling software, the development, calibration and performance of the model are described in detail. Strangford Lough has a complex flow pattern with high flows through the Narrows (~3.5 m/s) linking the main body of the Lough to the Irish Sea and intricate flow patterns around the numerous islands. With the aid of good quality tidal and current data obtained throughout the Lough during the model development, the surface elevation and current magnitude between the observed and numerical model were almost identical with model skill >0.98 and >0.84 respectively. The applicability of the model is such that it can be used as an important tool for the prediction of important ecological processes as well as engineering applications within Strangford Lough.
... Thus, harnessing tidal energy from strong tidal current regions using in-stream (hydrokinetic) tidal devices has quickly gained more attention. A number of studies have been conducted using either analytical or numerical models to evaluate the maximum amount of tidal energy that can be extracted from a tidal system Cummins 2005, 2008;Bryden and Couch 2007;Sutherland et al. 2007;Blanchfield et al. 2008;Draper et al. 2009;Walkington and Burrows 2009;Atwater and Lawrence 2010;Yang et al. 2013). However, these physically based calculations did not consider associated environmental effects, and the calculated energy removal potential is unlikely to be achieved in real-world estuaries and tidal basins. ...
The growing interest in harnessing tidal energy has raised concerns
about the impact of energy extraction on water circulation, and the
implication those changes can have on water quality and the marine food
web. There are few direct observations of the effect of energy
extraction on ecosystems; however our understanding of the magnitude and
importance of these effects can be enhanced through numerical analysis
at the appropriate temporal and spatial scales This paper presents a
numerical modeling study to simulate in-stream tidal energy extraction
and assess its effect on the circulation and mixing in a tide-dominated
estuary using a three-dimensional (3D) unstructured grid finite volume
coastal ocean model. A tidal turbine module is incorporated into the
hydrodynamic model using a momentum source/sink approach. The tidal
turbine module is applied to simulate the tidal energy extraction in an
idealized tidal system. A series of numerical experiments are carried
out to assess the effect of tidal energy extraction on volume flux,
vertical velocity structure, and flushing time within the system. The
implication of changes in physical processes due to tidal energy
extraction on water quality is also discussed, including changes in
dissolved oxygen, nutrients and chlorophyll.
Tidal stream energy has the potential to contribute to a diverse future energy mix. As the industry moves towards commercialisation and array scale deployment, there is an opportunity to better understand the uncertainties around energy yield assessments. Energy yield assessments are used widely in the wind industry to evaluate the potential energy production from a prospective project. One of the key challenges is to quantify and reduce uncertainty in energy yield assessment. This thesis investigates ways to achieve this through utilising lessons learnt from the established wind industry. An evaluation of both the wind and tidal energy yield assessment process is conducted, highlighting where synergies can be used to increase understanding of uncertainty for the nascent tidal industry. The processes are comparable starting with a campaign to collect site data to characterise the resource at the measurement location. The next stage is to evaluate the long term variations, however this is where the two methods differ. Analysis of long term wind effects requires correlations to be made between short term site data and long term reference data from alternative sources. An assessment of tidal variations over longer periods utilises harmonic analysis, which is capable of deconstructing the individual astronomical variations of the tide and reconstructing them to predict future variations. Despite harmonic analysis being able to determine the astronomical effects of the tide, there are uncertainties in the measurements of tidal flow which are associated with non-astronomical effects. Effects such as turbulence introduce uncertainty when evaluating measured tidal data. This is one area which is investigated further in the thesis. Methods to evaluate the turbulence intensity from real ADCP data are investigated. The next stages require creating a numerical model of the site to extrapolate the data spatially to other areas of interest (such as a turbine location). Energy yield predictions for both wind and tidal are made by combining a power curve with the long term resource. The energy yield outputs are then adjusted to account for energy losses and uncertainties are applied to produce final energy yield values with the attributed probability values associated. Statistical methods are applied to harmonic analysis to assess the level of uncertainty in long term predictions of tidal variations. A method using spectral analysis is applied to evaluate the residuals between measured and modelled data and proves to be accurate at determining missing tidal constituents from the analysis. A method for evaluating the turbulence intensity of the flow is shown, to better understand the stochastic nature of the tidal signal. An investigation is conducted to assess the propagation of bed friction uncertainty, in hydrodynamic modelling, and the resulting impact on the predicted power output from a theoretical fence of tidal turbines spanning a tidal channel. The methodology is based on first conducting sensitivity studies by varying a parameter in the model and calculating the power. Then using a mean and standard deviation for the input parameter, the impact of the uncertainty can be transferred to the estimate of power. The results show that a larger uncertainty associated with the bed roughness tends to over predict the estimation of power. This work aims to inform the standardisation of practices and guidelines in tidal resource assessment and to support developers, consultants and financiers in future tidal energy yield assessments. The final chapter includes procedural recommendations for future tidal energy projects, summarising methods to calculate uncertainty and recommendations to reduce them.
Based on the finite-volume coastal ocean model (FVCOM), a three-dimensional numerical model FVCOM was built to simulate the ocean dynamics in pre-dam and post-dam conditions in Bachimen (BCM). The domain decomposition method, which is effective in describing the conservation of volume and non-conservation of mechanical energy in the utilization of tidal energy, was employed to estimate the theoretical tidal energy resources and developable energy resources, and to analyze the hydrodynamic effect of the tidal power station. This innovative approach has the advantage of linking physical oceanography with engineering problems. The results indicate that the theoretical annual tidal energy resources is about 2×10 8 kWh under the influence of tidal power station; Optimized power installation is confirmed according to power generation curve from numerical analysis; the developable resources is about 38.2% of theoretical tidal energy resources with the employment of one-way electricity generation. The electricity generation time and power are 3479 hours and 2.55×10 4 KW, respectively. The power station has no effect on the tide pattern which is semi-diurnal tide in both two conditions, but the amplitudes of main constituents apparently decrease in the area near the dam, with the M 2 decreasing the most, about 62.92 cm. The tidal prism shrinks to 2.28×10 7 m 3 , but can still meet the flow requirement for tidal power generation. The existence of station increases the flow rate along the waterway and enhances the residual current. There are two opposite vortexes formed on the east side beside the dam of the station, which leads to pollutants gathering.
This report provides an overview of the state of affairs (1) with regards to the deployment of wet renewables and (2) marine energy storage systems; (3) how they affect abiotic and biotic compo-nents of the marine ecosystem and (4) developments and concepts on cumulative impact assess-ments related to marine renewable energy devices and (5) future perspectives.
This report provides the scientific basis to address the OSPAR request for advice on the current state and knowledge of studies into the deployment and environmental impacts of the following wet renewable energies and marine energy storage (floating, coastal infrastructure), tidal stream (screws, kites), tidal flow (barrage, lagoon) and others. Advice should cover the status of wet renewable developments in the OSPAR region, future prospects, potential environmental prob-lems (sea bed habitat loss/disturbance, fish, marine mammals, birds, seascape/ public perception, and cumulative impacts), potential benefits, next steps and conclusions”. The request was di-rected towards the Working Group on Marine Benthal Energy Developments (WGMBRED) and the Working Group on Marine Renewable Energy (WGMRE).
A pre-meeting chaired by Jan Vanaverbeke, Belgium (WKWET, 15–16 January 2019) at ICES Headquarters, was attended by 11 participants from 4 countries, including members of WGMBRED and WGMRE and additional experts. The group analysed the OSPAR request, agreed on a structure for the report, and certain experts volunteered to conduct a literature re-view and provide the necessary knowledge base for the report.
WGMBRED met from 12–15 February 2019 in Brussels, Belgium. The input from WKWET par-ticipants was compiled, quality checked and adapted where needed; when relevant expertise was represented in the group. WKWET experts, not present at WGMBRED, reviewed text, where needed, and a first version of this report was delivered to WGMRE.
WGMRE met in Oostende (Belgium) from 26–28 February 2019. Participants reviewed the WKWET report following input from WGMBRED, quality checked, and adapted where neces-sary. Relevant experts contributed additional text and data to tables on MRE developments in ICES areas, and provided text on public perceptions and future prospects of MRE.
This report presents an overview of the currently known “wet renewables” (all marine renewa-ble energy devices, excluding offshore wind devices) and how their deployment will likely change in the future. It further provides an overview of the concepts and techniques of related to marine energy storage devices. Given the conceptual and experimental stage of marine energy storage devices, and the absence of data on how these devices affect the marine environment, the report is limited to a description of these marine energy storage devices.
This report provides a receptor-based summary of how the wet renewables can affect the marine environment. Receptors are either abiotic (hydrodynamics, physical seabed and sediment transport) or biotic (benthos, fish, marine mammals, birds, sea turtles, otters and polar bears). To avoid repetition, effects on these receptors were grouped according to pressure-inducing com-ponents (static component of the device, dynamic component of the device, cables) of wet re-newables or consequences of their presence.
The report further discusses the developments on cumulative impacts assessments associated with wet renewables deployment in addition to many other human activities, and the need to move away from “data rich – information poor” monitoring of structural aspects of the marine ecosystem to hypothesis-driven functional research at the relevant spatial and temporal scales. This will require cross-border coordination in data collection, data storage and exchange and the development of a joint research agenda.
This report provides an overview of the state of affairs (1) with regards to the deployment of wet renewables and (2) marine energy storage systems; (3) how they affect abiotic and biotic compo-nents of the marine ecosystem and (4) developments and concepts on cumulative impact assess-ments related to marine renewable energy devices and (5) future perspectives.
This report provides the scientific basis to address the OSPAR request for advice on the current state and knowledge of studies into the deployment and environmental impacts of the following wet renewable energies and marine energy storage (floating, coastal infrastructure), tidal stream (screws, kites), tidal flow (barrage, lagoon) and others. Advice should cover the status of wet renewable developments in the OSPAR region, future prospects, potential environmental prob-lems (sea bed habitat loss/disturbance, fish, marine mammals, birds, seascape/ public perception, and cumulative impacts), potential benefits, next steps and conclusions”. The request was di-rected towards the Working Group on Marine Benthal Energy Developments (WGMBRED) and the Working Group on Marine Renewable Energy (WGMRE).
A pre-meeting chaired by Jan Vanaverbeke, Belgium (WKWET, 15–16 January 2019) at ICES Headquarters, was attended by 11 participants from 4 countries, including members of WGMBRED and WGMRE and additional experts. The group analysed the OSPAR request, agreed on a structure for the report, and certain experts volunteered to conduct a literature re-view and provide the necessary knowledge base for the report.
WGMBRED met from 12–15 February 2019 in Brussels, Belgium. The input from WKWET par-ticipants was compiled, quality checked and adapted where needed; when relevant expertise was represented in the group. WKWET experts, not present at WGMBRED, reviewed text, where needed, and a first version of this report was delivered to WGMRE.
WGMRE met in Oostende (Belgium) from 26–28 February 2019. Participants reviewed the WKWET report following input from WGMBRED, quality checked, and adapted where neces-sary. Relevant experts contributed additional text and data to tables on MRE developments in ICES areas, and provided text on public perceptions and future prospects of MRE.
This report presents an overview of the currently known “wet renewables” (all marine renewa-ble energy devices, excluding offshore wind devices) and how their deployment will likely change in the future. It further provides an overview of the concepts and techniques of related to marine energy storage devices. Given the conceptual and experimental stage of marine energy storage devices, and the absence of data on how these devices affect the marine environment, the report is limited to a description of these marine energy storage devices.
This report provides a receptor-based summary of how the wet renewables can affect the marine environment. Receptors are either abiotic (hydrodynamics, physical seabed and sediment transport) or biotic (benthos, fish, marine mammals, birds, sea turtles, otters and polar bears). To avoid repetition, effects on these receptors were grouped according to pressure-inducing com-ponents (static component of the device, dynamic component of the device, cables) of wet re-newables or consequences of their presence.
The report further discusses the developments on cumulative impacts assessments associated with wet renewables deployment in addition to many other human activities, and the need to move away from “data rich – information poor” monitoring of structural aspects of the marine ecosystem to hypothesis-driven functional research at the relevant spatial and temporal scales. This will require cross-border coordination in data collection, data storage and exchange and the development of a joint research agenda.
A tidal turbine simulation system is developed based on a three-dimensional oceanographic numerical model. Both the current and turbulent controlling equations are modified to account for impact of tidal turbines on water velocity and turbulence generation and dissipation. High resolution mesh size at the turbine location is assigned in order to capture the details of hydrodynamics due to the turbine operation. The system is tested against comprehensive measurements in a water flume experiment and results of Computational Fluid Dynamics (CFD) simulations. The validation results suggest that the new modelling system is proven to be able to accurately simulate hydrodynamics with the presence of turbines. The developed turbine simulation system is then applied to a series of test cases in which a standalone turbine is deployed. Here, complete velocity profiles and mixing are realized that could not have been produced in a standard two-dimensional treatment. Of particular interest in these cases is an observed accelerated flow near the bed in the wake of the turbine, leading to enhanced bottom shear stress (∼2 N/m² corresponding to the critical stress of a range of fine gravel and finer sediment particles).
The influence of sea level rise on tidal power output and tidal energy dissipation is investigated by means of numerical simulations. The hydrodynamics in the Taiwan Strait were simulated using a new unstructured-grid, depth-averaged numerical model. Eight tidal constituents (M2, S2, N2, K2, K1, O1, P1, and Q1) were used to specify the open boundary conditions to drive the model. The observed data, including the time-series water level and tidal current, were used to validate the numerical model. The model results were in reasonable agreement with the measured data. The values of root mean square error (RMSE) for water level and tidal current are in the ranges of 0.06–0.19 m and 0.12–0.20 m/s, respectively. Moreover, the modeling amplitudes for eight tidal constituents determined using the present model were also similar to those determined using the regional inverse tidal model. The model predicted results indicated that the Penghu Channel is an appropriate location for deploying a tidal power plant because of its deep water (>100 m) and fast tidal current (>1.5 m/s). Furthermore, four sea level rise (SLR) scenarios were adopted to investigate the influence of SLR on tidal energy dissipation and tidal power output in the Penghu Channel. The simulation results showed that the total tidal energy dissipation was 11.23 GW for the baseline condition and increased corresponding to different SLR scenarios. The mean tidal power output was 42.15 MW when the additional turbine friction coefficient was set to 0.225. The extractable tidal energy increased by 1.62 MW, 2.09 MW, 2.63 MW, and 3.52 MW for SLR values of 0.87 m, 1.11 m, 1.40 m, and 1.90 m, respectively. We found that sea level rise increased tidal energy dissipation and energy output in the Penghu Channel.
Korea has developed various new and renewable energy resources since 2000 due to increasing demand for Green Energy around the world. Because the west coast of Korea has an extreme tidal range, many research projects aimed at harnessing tidal energy have been conducted there successfully. Though the study of Dynamic Tidal Power (DTP) was started as a new tidal energy source about 20 years ago, its characteristics are not yet fully understood. DTP has the potential to have less environmental impact than does a conventional barrage type tidal power generator. Its characteristics were analyzed in many test cases using a numerical model. The theoretical maximum tidal power of DTP became the same as the maximum tidal range by controlling the phase difference. This showed that DTP has great potential for a very successful future in the Yellow Sea. Moreover, DTP could provide an attractive energy source even in areas of lower tidal range than indicated in previous studies.
Based on the finite-volume coastal ocean model (FVCOM), a three-dimensional numerical model FVCOM was built to simulate the ocean dynamics in pre-dam and post-dam conditions in Bachimen (BCM). The domain decomposition method, which is effective in describing the conservation of volume and non-conservation of mechanical energy in the utilization of tidal energy, was employed to estimate the theoretical tidal energy resources and developable energy resources, and to analyze the hydrodynamic effect of the tidal power station. This innovative approach has the advantage of linking physical oceanography with engineering problems. The results indicate that the theoretical annual tidal energy resources is about 2×108 kWh under the influence of tidal power station; Optimized power installation is confirmed according to power generation curve from numerical analysis; the developable resources is about 38.2% of theoretical tidal energy resources with the employment of one-way electricity generation. The electricity generation time and power are 3479 hours and 2.55×104 KW, respectively. The power station has no effect on the tide pattern which is semi-diurnal tide in both two conditions, but the amplitudes of main constituents apparently decrease in the area near the dam, with the M2 decreasing the most, about 62.92 cm. The tidal prism shrinks to 2.28×107 m3, but can still meet the flow requirement for tidal power generation. The existence of station increases the flow rate along the waterway and enhances the residual current. There are two opposite vortexes formed on the east side beside the dam of the station, which leads to pollutants gathering.
In the present study, an existing three-dimensional finite volume computational ocean model (FVCOM) was refined and configured including an algorithm for computing the power density and mean power density at Qiongzhou Strait of China. The refined model was validated with the measured tidal levels and tidal currents at different gauging stations. The model results are in reasonable agreement with the measured data. Based on the modeling results, we assess the resource of the tidal stream energy in the Qiongzhou Strait and discuss the temporal and the spatial distribution of the tidal current energy there. The conclusion is extracted: the higher power density occurs in the middle area of the strait, and lower at both sides. Characteristics of power density such as the maximum possibility speed, maximum power density during the spring tide period and the neap tide period, have the similar distribution. The southeast part and central area of the strait are of rich tidal current energy, where the maximum possibility speed can reach to 4.6 m/s, and the maximum power density of the spring tide period and the neap tide period can reach 5 996 and 467 W/m2 separately in the surface layer The annual mean power density can reach 819 W/m2. Statistical length of accumulative time of the velocity exceeding 0.7 m/s is about 4 717 h at local point during a year. The total theoretical tidal current energy resource is approximately 189.55 MW and the available exploited energy on present technology condition is 249, 20.2 and 263 GW/a separately by using the methods FLUX, FARM and GC in the Qiongzhou Strait. © 2016, The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg.
In the present work, the tidal stream energy in surrounding coastal zones of ZTD is calculated. The tidal current velocity is gotten by three dimensional numerical modeling. The tidal current model is validated by measurement of tidal current observed in 4 points surrounding ZTD. The numerical results given by the tidal current model already shows that: the tidal current velocities given by the model agree with the measured velocities generally. The characteristics of tidal currents around ZTD are analyzed and the following tidal stream energy density is calculated. The maximum tidal stream energy flux of unit width occurs around the middle locations of ZTD southern areas.
Increasing concerns over global climate change and sustainable fuel procurement are driving the search for new ways to derive energy from the seas. Globally, the offshore wind energy sector has progressed rapidly, and wave and tidal-current energy converters are now approaching deployment at commercial scales. To date, most studies of the ecological effects of marine renewable energy development have concentrated on birds and marine mammals. Here, we focus on the consequences for benthic flora and fauna, and for benthic habitats across a variety of scales. We use a “Benthic Footprint” concept to discuss the potential for species-specific environmental responses, and to consider the poorly understood cumulative effects of wind, wave, and tidal-current energy operations on marine ecosystems. Collaborations between ecologists, industry specialists, and government bodies, as well as better designs for devices, arrays, and developments consisting of multiple arrays, can contribute to the goal of reducing the Benthic Footprint of marine renewable energy, thereby facilitating large-scale implementation of these technologies.
In the present work, the tidal stream energy resource in Langyatai (LYT) strait was calculated based on tidal current velocity. To obtain the tidal current velocity, a tidal current numerical model was setup and validated by measurements of tidal current in 2 points around LYT strait with observing period of 25 hours in spring tide. The calculated tidal current velocities agree well with the measured velocities, which shows the reliability of the model. The characteristics of tidal currents around LYT were analyzed and the following tidal stream energy density was calculated: The velocity category interval containing maximum energy potential for LYT strait is 1.5~1.6 m/s and the power for the entire section of the strait is 1.927 MW.
An analysis of transient momentum balances is carried out to elucidate circulation, dynamics, and exchange mechanisms at shallow barotropic tidal inlets. Circulation is computed using a depth-integrated, fully nonlinear, time-stepping, finite-element model with variably spaced grids having horizontal resolution down to 50 m. Velocity and elevation fields from the model are used to directly evaluate the contribution of each term in the momentum equations to the overall momentum balance. A transformation of the x-y momentum terms into an s-n coordinate system is used to simplify the interpretation of the dynamics and provide vivid illustrations of the forces and resulting accelerations in the flow. The analysis is conducted for an idealized inlet and contrasted with a highly detailed model of Beaufort Inlet, North Carolina. Results show that momentum balances in the immediate vicinity of these inlets vary significantly in time and space and oscillate between two dynamical states. Near maximum ebb or flood, the alongstream momentum balances are dominated by advective acceleration, pressure gradient, and bottom friction. Cross-stream balances are dominated by centrifugal acceleration and pressure gradients. Near slack, balances more closely follow linear wave dynamics, with local accelerations balancing pressure gradients, and (to a lesser degree) Coriolis. Comparisons between the idealized inlet and Beaufort Inlet show broad similarities in these momentum balances. However, natural inlet geometry and bottom topography, as well as the tidal transmission characteristics of the sounds behind Beaufort Inlet produce strong asymmetries. Moreover, momentum balances are highly localized, often with subkilometer length scales. The dynamics are used to explain the physical mechanisms for inlet exchange. In particular, the results indicate that the cross-stream dynamics generate a ''wall'' along the length of an inlet during the stronger phases of the tide. The wall is established by opposing cross-inlet pressure gradients and centrifugal forces, and it poses a significant barrier to cross-inlet exchange during the stronger phases of the tide but is absent near slack.
This paper describes the potential environmental changes caused by tidal power installations with illustration for schemes in the eastern Irish Sea, focusing mainly on major estuarine barrages. The generic impacts in the near-field and far-field are discussed. Results from a zerodimensional and a two-dimensional model are presented: the former allows rapid calculations to be made for a large range of options while the latter allows the full effect on two-dimensional hydrodynamics to be investigated. It is shown that there may be a significant change in tidal amplitude at the coast of Northern Ireland. The bed stress in the Bristol Channel will be significantly reduced if a Severn barrage is constructed. Some effects on the tidal mixing are expected although the location of tidal fronts in the Irish and Celtic Seas will not be changed significantly. The largest environmental impact is expected to be on the amount of inter-tidal area retained after construction of an estuarine barrage. It is shown that the loss of mudflats can be substantially reduced by using a dual-mode (ebb and flood generation) scheme with an increased number of turbines over the lowestcost option.
The tidal power available for electricity generation from in-stream turbines placed in the Minas Passage of the Bay of Fundy is examined. A previously derived theory is adapted to model the effect of turbine drag on the flow through the Minas Passage and the tidal amplitude in the Minas Basin. The theoretical maximum power production over a tidal cycle is determined by the product of the amplitude of the forcing tide in the Bay of Fundy and the undisturbed volumetric flowrate through the Minas Passage. Although the extraction of the maximum power will reduce the flowrate through the Minas Passage and the tides in the Minas Basin by over 30 per cent, a significant portion of the maximum power can be extracted with little change in tidal amplitude as the initial power generation causes only an increase in the phase lag of the basin tides. Two-dimensional, finite-element, numerical simulations of the Bay of Fundy–Gulf of Maine system agree remarkably well with the theory. The simulations suggest that a maximum of 7 GW of power can be extracted by turbines. They also show that any power extraction in the Minas Passage pushes the Bay of Fundy–Gulf of Maine system closer to resonance with the forcing tides, resulting in increased tidal amplitudes throughout the Gulf of Maine. Although extracting the maximum power produces significant changes, 2.5 GW of power can be extracted with a maximum 5 per cent change in the tidal amplitude at any location. Finally, the simulations suggest that a single turbine fence across the Minas Passage can extract the same power as turbines throughout the passage but that partial turbine fences are less efficient.
The maximum tidal power potential of Johnstone Strait, BC, Canada is evaluated using a two-dimensional finite element model (TIDE2D) with turbines simulated in certain regions by increasing the drag. Initially, side channels are closed off so that the flow is forced through one channel to test the validity of a general analytic theory [1] with numerical results. In this case, the modelled power potential of 886 MW agrees reasonably well with the analytic estimate of 826 MW. In reality, two main channels, Discovery Passage and Cordero Channel, connect the Pacific Ocean to the Strait of Georgia. Turbines are simulated in Johnstone Strait, northwest of the two main channels, and separately for Discovery Passage and Cordero Channel. Northwestern Johnstone Strait is similar to the one channel case as the flow must go through this channel, but Discovery Passage and Cordero Channel are different as the flow can be diverted away from the channel with the turbines and into the other channel. The maximum extractable power in northwestern Johnstone Strait is found to be 1335 MW, which agrees well with the theoretical estimate of 1320 MW. In Discovery Passage and Cordero Channel, the maximum extractable power is modelled to be 401 and 277 MW, respectively, due to the flow being partly diverted into the other channel. In all cases, the current is reduced to between 57 and 58 per cent of the undisturbed flow, close to the 56 per cent predicted by the analytic theory. All power calculations are for the M2 constituent alone, as this is the largest current in the region. The total power from the eight major constituents (M2, S2, N2, K2, K1, 01, P1, and Q1) can be obtained by multiplying the power estimates for M2 by 1.12.
This presentation will discuss the background to the development of Marine Current Turbines Ltd’s pioneering technology for the exploitation of tidal stream energy. This will include details of: • the 1994, 15kW Loch Linnhe “Proof of Concept” Project, and earlier work, • the 300kW 2003 Seaflow Project, • the present 1.2MW Seagen Commercial Demonstrator Project, • future plans for scaled up “Second Generation” technology to follow Seagen. The paper will then give the rationale and an outline of the essential features of MCT’s technology and an overview of the predicted performance and economics of the planned commercial technology together with an indication of MCT’s time-line for its development. The discussion will cover nontechnical issues such as consent requirements (and environmental impact issues) and an outline of MCT’s Business Plan and how it is to be implemented.
We present an assessment of the uncertainties present in a high-resolution (˜7 km horizontal resolution and 20 s-levels in the vertical) coupled hydrodynamic ecosystem model of the northwest European continental shelf. The models in question are a three-dimensional baroclinic circulation model developed for massively parallel computers (the Proudman Oceanographic Laboratory Coastal Ocean Modelling System) and a sophisticated ecosystem model, representing the functioning of the ecosystem with 52 state variables (The European Regional Seas Ecosystem Model). In this paper and part 2, which focuses on the ecosystem model, we simulate the period of the North Sea Project (August 1988 to October 1989) and attempt as comprehensive an examination of model uncertainties as the available data permits. We are able to make some degree of assessment for all aspects of the physics model, apart from the turbulence closure scheme, and use a simple cost function to systematically compare the accuracy of different variables. We find some aspects (notably tidal currents and elevation, and sea surface temperature) are well modelled, with RMS errors less than 0.4 standard deviations of the data, whereas some aspects (e.g. residual current speed and salinity) have RMS errors similar to the standard deviation of the data. Improving the residual currents and salinity comparison requires improving several aspects of both the model (forcing and formulation) and observational data (quantity and quality). In addition to directing improvements in the model, this exercise also suggests where data assimilation effort might prove most fruitful. We also demonstrate that this resolution is sufficient to model complex biophysical interaction in both the horizontal (e.g. enhanced production at fronts) and vertical (e.g. mid-water production modulated by the spring neap cycle).
With concerns mounting over the UK’s energy future and the effects of climate change, it will soon become paramount that all viable sources of renewable energy are fully exploited. This study has examined the scope for reliable and fully predictable tidal electricity generation from the conjunctive operation of 5 major estuary barrages on the West Coast of the UK in an attempt to establish the potential scale of the extractable resources. Two levels of investigation have been undertaken: simple 0-D (‘two-tank’) modelling of barrage energy generation under different operational modes, using the hydraulic characteristics of turbine performance; and 2-D modelling of tidal hydrodynamics over a wide sea area in a computational grid incorporating the barrages with turbines and sluices. It has been demonstrated that more than 33TWh per year of electricity should be attainable, from 22GW of installed capacity, this representing close to 10% of present UK demand.
This work presents a case study for the power potential of a tidal stream connecting a bay to the open ocean. The extractable power, averaged over the tidal cycle, from Masset Sound, located in Haida Gwaii, Canada, is estimated as 79 MW when only the dominant M2 tidal constituent is included in the analysis. The value increases to 87 MW when the three dominant constituents are included. It is shown that extracting the maximum power from Masset Sound will decrease both the maximum water surface elevation within the bay and the maximum volume flowrate through the channel to approximately 58 per cent of their undisturbed values.
An open and important question in tidal in-stream energy conversion is the level of kinetic power extraction that is possible without unacceptable environmental degradation. In general, the effects of large-scale kinetic power extraction on estuary-scale fluid mechanics are not well understood. In this paper, these effects are quantified for an idealized estuary using a one-dimensional, time-dependent numerical model. The numerical domain consists of a long, wide inlet and basin connected by a constricted channel. Kinetic power densities within this constriction are suitable for in-stream energy conversion. Modelling shows that the extraction of kinetic power has a number of effects, including: (a) reduction of the volume of water exchanged through the estuary over the tidal cycle; (b) reduction of the tidal range landward of the array; and (c) reduction of the kinetic power density in the tidal channel. These impacts are strongly dependent on the magnitude of kinetic power extraction, estuary geometry, tidal regime, and non-linear turbine dynamics. It is shown that it may be misleading to relate these impacts to the fraction of kinetic energy extracted from the system. Results highlight the importance of time-dependent modelling and the incorporation of non-linear turbine dynamics.
There is an upper bound to the amount of power that can be generated by turbines in tidal channels as too many turbines merely block the flow. One condition for achievement of the upper bound is that the turbines are deployed uniformly across the channel, with all the flow through them, but this may interfere with other uses of the channel. An isolated turbine is more effective in a channel than in an unbounded flow, but the current downstream is non-uniform between the wake of the turbines and the free stream. Hence some energy is lost when these streams merge, as may occur in a long channel. We show here, for ideal turbine models, that the fractional power loss increases from 1/3 to 2/3 as the fraction of the channel cross-section spanned by the turbines increases from 0 to close to 1. In another scenario, possibly appropriate for a short channel, the speed of the free stream outside the turbine wake is controlled by separation at the channel exit. In this case, the maximum power obtainable is slightly less than proportional to the fraction of the channel cross-section occupied by turbines.
This paper outlines present thinking on the determination of accessible tidal current resources within channels and other potentially exploitable locations. The fundamental principles behind tides and tidal currents are briefly discussed and the implications of temporal and spatial variations on the evaluation of the resources considered in the context of artificial energy exploitation. The thinking behind the flux approach to resource estimation is presented and an example based on the Pentland Firth is considered. The impact of energy extraction on the flow patterns is considered in both one and two dimensions and the principles required for three-dimensional analyses are presented in a generic form.
In some few special areas of the world, the range of the variation
of the sea level due to the tide can be impressive. For centuries man
has been harnessing this energy with the operation of tidal mills. At
present, the design of tidal power does not present new scientific
issues. Nevertheless, additional research and development should be
carried out in three fields: interface of the tidal power station output
with National Grids and above all a sound assessment of its economic
interest; design and implementation of work according to the site; and
environmental effects. On November 26, 1966, the La Rance tidal power
station was inaugurated after 25 years of design in various fields, and
25 years afterwards it is still the only industrial prototype of a large
size tidal power station. Currently several important schemes are also
under study throughout the world
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