Power potential of a split tidal channel

The extraction of kinetic energy from tidal flows is an interest of the renewable energy industry with large scale assessments of the potential resource already conducted. These assessments however, use the natural kinetic energy flux as the primarily metric of the available resource. This approach has significant limitations when it is applied to tidal channels, particularly those tidal channels that branch into multiple sub-channels. Small amounts of energy extraction may not cause significant changes in the total flow through a channel, however the relative flows through the sub-channels can be drastically affected. It is this diversion of the flow that becomes the primary control on the extractable energy. As such, the relative resistance of the channels plays an important role.     

The tidal-stream energy resource in Passamaquoddy–Cobscook Bays: A fresh look at an old story

The Passamaquoddy–Cobscook Bay archipelago, located near the entrance to the Bay of Fundy where the mean tidal range is 5.7 m, has long been regarded a promising site for tidal-power development. Modern low-head turbines allow power extraction primarily from the kinetic energy of the tidal stream, rather than from the potential energy of an impounded basin, eliminating the need for expensive and elaborate systems of dams, locks and gates. Although the available power levels are much less in streaming applications, the resource may still be significant and accessible in areas with strong tidal flows, because the available power follows the cube of the current speed. The much lower costs per installed kilowatt and the relatively minor environmental impacts warrant a fresh look at the tidal-stream resource.Circulation models indicate that the peak power resource in narrow straits such as Letete Passage and Lubec Narrows exceeds 10 kW/m² of installed turbine aperture. In those locations an installation with the surface “footprint” of a typical aquaculture site could produce peak power levels of 1–2 MW under mean tide conditions, and perhaps twice that during spring tides, using modern turbines. Lower power levels are available in deeper, less restricted regions such as Western and Head Harbor Passages. Regions between islands and near headlands where flow speeds exceed about 2 m s⁻¹ (about 4 knots) offer a modest tidal-power potential that could be tapped at relatively low cost and with minimal impact on the environment, fisheries and navigation.     

Limits to tidal current power

Estimating the extractable power of tidal currents in channels is a practical question that has received attention recently. Analysis has clearly shown that the power potential is not given by the flux of kinetic energy, as has been commonly assumed. A general formula for the maximum available power is reviewed, along with assessments of the reduction if only partial fences are used, as would be required for navigational and ecological reasons. In typical situations, the maximum power obtainable may be achieved with a surprisingly small number of turbines, especially if allowance is made for the flow reduction caused by drag on the supporting structures of turbines which reduces the maximum power available. Finally, the flow through tidal turbines is compared with the cooling water demands of nuclear reactors generating the same power.     

The role of tidal asymmetry in characterizing the tidal energy resource of Orkney

When selecting sites for marine renewable energy projects, there are a wide range of economical and practical constraints to be considered, from the magnitude of the resource through to proximity of grid connections. One factor that is not routinely considered in tidal energy site selection, yet which has an important role in quantifying the resource, is tidal asymmetry, i.e. variations between the flood and ebb phases of the tidal cycle. Here, we present theory and develop a high-resolution three-dimensional ROMS tidal model of Orkney to examine net power output for a range of sites along an energetic channel with varying degrees of tidal asymmetry. Since power output is related to velocity cubed, even small asymmetries in velocity lead to substantial asymmetries in power output. We also use the 3D model to assess how tidal asymmetry changes with height above the bed, i.e. representing different device hub heights, how asymmetry affects turbulence properties, and how asymmetry is influenced by wind-driven currents. Finally, although there is minimal potential for tidal phasing over our study site, we demonstrate that regions of opposing flood- versus ebb-dominant asymmetry occurring over short spatial scales can be aggregated to provide balanced power generation over the tidal cycle.     

Development of a hydraulic control mechanism for cyclic pitch marine current turbines

Tidal power generation by means of marine current farms is potentially a large renewable energy resource which could be harnessed in many coastal waters. Its availability is highly predictable in time, and the technology promises high energy conversion efficiency along with a relatively low impact on sea life due to its relatively small disturbance of natural tidal flows.A series of devices have so far been proposed and developed for the extraction and conversion of kinetic energy present in tidal flows into useful electrical power [1]. Designs include horizontal axis turbines, vertical axis turbines, and devices with oscillating lift surfaces. Up to date no technology has firmly established itself.This paper describes a novel hydraulic control mechanism designed for vertical-axis marine current turbines of the straight-bladed Darrieus type. It has been found to significantly improve turbine efficiency over conventional Darrieus turbines when operated at low blade tip-speed to tidal-flow-velocity ratios (TSR) and to give the turbine the ability to self-start reliably. The control mechanism enforces a cyclic pivoting motion on the turbine blades as they move around their circular flight-path. The movement of the pitch control is of sinusoidal shape and is continuously variable in amplitude. The blade actuation is powered by the turbine's own rotation and is implemented using a swash-plate mechanism in conjunction with a hydraulic circuit for every blade. For surface piercing turbines, this control mechanism may be remotely positioned in a dry nacelle above sea level. If the appropriate design is applied, this can offer access to the cyclic pitch control mechanism, gearbox and generator, even when the turbine is operational, promising lower maintenance and operating costs compared with submerged systems.     

A review of the potential water quality impacts of tidal renewable energy systems

The continued increase in the demand for energy, growing recognition of climate change impacts, high oil and gas prices and the rapid depletion of fossil fuel reserves have led to an increased interest in the mass generation of electricity from renewable sources. Traditionally, this has been pursed through riverine hydropower plants, with onshore wind systems growing steadily in popularity and importance over the years. Other renewable energy resources, which were previously not economically attractive or technically feasible for large scale exploitation, are now being considered to form a significant part of the energy mix. Amongst these, marine and in particular, tidal energy resource has become a serious candidate for undergoing mass exploitation in the near future, particularly in places with a tidal range of 4 m or more. Tidal renewable energy systems are designed to extract the kinetic or potential energy flow and convert it into electricity. This can be achieved by placing tidal stream turbines in the path of high speed tidal currents or through tidal range schemes, where low head turbines are encapsulated in impoundment structures, much like in low head riverine hydropower schemes. It is thought that these systems, when implemented at scales required to generate substantial amounts of electricity, have the potential to significantly alter the tidal flow characteristics, which could have knock-on impacts on the hydro-environment. This review gathers together knowledge from different research areas to facilitate an evaluation of the potential hydro-environmental impacts of tidal renewable energy systems, with a particular focus on water quality. It highlights the relevance of hydro-environmental modelling in assessing potential impacts of proposed schemes and identifies areas where further research is needed. A case study is presented of recent modelling studies undertaken for the Severn Estuary.     

Research priorities for assessing potential impacts of emerging marine renewable energy technologies: Insights from developments in Wales (UK)

The marine renewable energy industry is expanding globally in response to increased energy demands and the desire to curtail greenhouse gas emissions. Within the UK, Wales has the potential for the development of diverse marine renewable technologies, with a strong tidal range resource, areas of high tidal current energy, and a spatially limited wave energy resource. Targets have been set by the Welsh Government to increase the contribution of marine renewable energy to Wales' electricity generation, and the recent introduction of demonstration zones for tidal and wave energy aims to facilitate developers in device deployment. However, uncertainties remain about the potential impacts of devices, particularly for array scale deployments, planned at several sites, and for the extensive structures required to capture the tidal range resource. Here we review present knowledge of potential impacts, including physical, ecological and societal dimensions, and outline research priorities to provide a scientific basis on which to base decisions influencing the trajectory of Welsh marine renewable energy development.     

Tidal energy update 2009

Tidal energy has the potential to play a valuable part in a sustainable energy future. It is an extremely predictable energy source, depending only on the gravitational pull of the moon and the sun and the centrifugal forces created by the rotation of the earth–moon system. Tidal energy has been exploited on a significant scale since the construction of the La Rance tidal barrage in France in 1967. A tidal barrage utilises the potential energy of the tide and has proven to be very successful, despite opposition from environmental groups. Kinetic energy can also be harnessed from tidal currents to generate electricity and involves the use of a tidal current turbine. This is the more desired method of capturing the energy in the tides. However, tidal current turbine technology is currently not economically viable on a large scale, as it is still in an early stage of development. This paper provides an up-to-date review of the status of tidal energy technology and identifies some of the key barriers challenging the development of tidal energy. The future development of tidal current devices and tidal barrage systems is discussed as well as examining the importance of a supportive policy to assist development.     

Estimate of the tidal stream power resource of the Pentland Firth

The Pentland Firth is arguably the best-known candidate site for tidal stream power extraction worldwide. In this paper we estimate the maximum power that can be extracted by placing tidal stream power devices across the Pentland Firth and/or the individual sub-channels formed by the islands of Swona, Stroma and the Pentland Skerries. Using a depth-averaged numerical model, for the entire Firth we find that approximately 4.2 GW of power may be extracted, and this agrees reasonably well with predictions from an existing theoretical model. In contrast, for the sub-channels there is no single value to describe the power potential, but rather a range of power estimates because the extracted power from one sub-channel depends on the operation (or otherwise) of tidal devices placed in parallel sub-channels, or in series along the Firth. This range in output is of practical importance given present plans to lease separate sites within the Pentland to different device developers, and suggests that regulation of separate device developers will be crucial to achieve optimum performance across the entire Firth. Finally, we show that large scale energy extraction from the Pentland Firth does not lead to flow diversion around the Orkney Islands as a whole (as is sometimes assumed), however energy extraction in the Pentland Firth can augment the phase difference across smaller sub-channels in the Orkney Islands and this may increase their power potential.     

ME1—marine energy extraction: tidal resource analysis

This paper outlines some of the issues which need to be considered when analysing the extraction potential of a tidal current resource. Site selection is not a simple case of identifying an energetic site with an appropriately large peak tidal current. The characteristics of the current throughout the lunar tidal cycle must be considered. Furthermore, implicit in such an analysis is the assumption that the local tidal flow conditions will not be significantly altered by the energy extraction process itself. For high extraction rates, the general validity of this assumption is questionable. The influence of energy extraction upon the underlying hydraulic nature of the tidal environment must be considered. Analysis based upon open channel flow theory demonstrates that energy extraction in a simple channel driven by static head differences can have a significant upstream and downstream effect. This suggests that the environmental impact of energy extraction is not necessarily restricted to the immediate area around the extraction site. It also suggests that there is potential for the process of energy extraction to either diminish or even enhance the available resource at a particular site. Further research is required and is ongoing in this area. In the case examined, the limits to exploitation are shown to be inexact. However, a useful approximate guideline for resource analysis would be that 10% of the raw energy flux produced by the tide can be extracted without causing undue modification to the flow characteristics.     

Far-field dynamics of tidal energy extraction in channel networks

Tidal hydrokinetic power generation involves the conversion of the kinetic power in swiftly moving tidal currents to renewable electricity. Resource assessment is critical to understand the tidal hydrokinetic potential, but is complicated by a number of factors, including far-field effects. These are changes to the tidal regime caused by the increased resistance to flow as power is extracted from a channel network. This study addresses far-field effects in four prototypical channel networks: multiply-connected flow around an island, a branching network in which the flow bifurcates but does not converge downstream, and a network with multiple constrictions in series. These networks are modelled as one-dimensional channels with hydrokinetic power extraction in high current constrictions. Changes to tides, transport, frictional power dissipation, and kinetic power density are quantified for a range of extraction options. Depending on the type of network, the tidal regime may be either locally augmented or reduced by kinetic power extraction. The changes to kinetic power density throughout the network have important implications for resource assessment, particularly for networks with multiple extraction sites. Results suggest that existing analytical methods tend to over- or under-estimate the hydrokinetic resource because they do not allow for changes to the tidal forcing as a consequence of extraction. In general, site-specific numerical modelling is required to quantitatively predict far-field extraction effects and assess the hydrokinetic resource.     

Tidal energy leasing and tidal phasing

In addition to technical and economic constraints, tidal energy leasing is generally governed by demand for sites which contain the highest tidal streams, and does not take into account the phase relationship (i.e. the time lag) between sites. Here, the outputs of a three-dimensional tidal model are analysed to demonstrate that there is minimal phase diversity among the high tidal stream regions of the NW European shelf seas. It is therefore possible, under the current leasing system, that the electricity produced by the first generation of tidal stream arrays will similarly be in phase. Extending the analysis to lower tidal stream regions, we demonstrate that these lower energy sites offer more potential for phase diversity, with a mean phase difference of 1.25 h, compared to the phase of high energy sites, and hence more scope for supplying firm power to the electricity grid. We therefore suggest that a state-led leasing strategy, favouring the development of sites which are complementary in phase, and not simply sites which experience the highest current speeds, would encourage a sustainable tidal energy industry.     

Hydrokinetic energy conversion systems and assessment of horizontal and vertical axis turbines for river and tidal applications: A technology status review

The energy in flowing river streams, tidal currents or other artificial water channels is being considered as viable source of renewable power. Hydrokinetic conversion systems, albeit mostly at its early stage of development, may appear suitable in harnessing energy from such renewable resources. A number of resource quantization and demonstrations have been conducted throughout the world and it is believed that both in-land water resources and offshore ocean energy sector will benefit from this technology. In this paper, starting with a set of basic definitions pertaining to this technology, a review of the existing and upcoming conversion schemes, and their fields of applications are outlined. Based on a comprehensive survey of various hydrokinetic systems reported to date, general trends in system design, duct augmentation, and placement methods are deduced. A detailed assessment of various turbine systems (horizontal and vertical axis), along with their classification and qualitative comparison, is presented. In addition, the progression of technological advancements tracing several decades of R&D efforts are highlighted.     

Status and potentials of tidal in-stream energy resources in the southern coasts of Iran: A case study

Interest in renewable energy in Iran has increased continually over the past decade. Iran has an excellent hydro power energy resource and the use of this resource will assist in the development of a sustainable energy future. Iran – with its many narrow channels and significant tidal range – might be expected to have considerable potential for tidal current power generation. The Khowr-e Musa Bay is a large coastal embayment on the south-western coast of Iran in which the peak tidal currents exceed 2 m/s. It is therefore a promising site for tidal stream power. The assessment employed a statistical method, for estimating tidal current energy resource at the selected site, during one lunar month (since 6 November 1996 to 7 December 1996). With the introduction of constraints and limitations, the technical, practical, accessible and viable tidal current energy resources were obtained.     

Impact of different operating modes for a Severn Barrage on the tidal power and flood inundation in the Severn Estuary,UK

The Severn Estuary has a spring tidal range approaching 14 m and is regarded as having one of the highest tidal ranges in the world. Various proposals have been made regarding the construction of a tidal barrage across the estuary to enable tidal energy to be extracted. The barrage scheme originally proposed by the Severn Tidal Power Group (STPG) would be the largest project for tidal power generation in the world if built as proposed. Therefore, it is important to study the impact of different operating modes for this barrage on the tidal power output and flood inundation extent in the estuary. In this paper, an existing two-dimensional hydrodynamic model based on an unstructured triangular mesh has been integrated with a new algorithm developed for the estimation of tidal power output, which can account for three barrage operating modes, including ebb generation, flood generation, and two-way generation. The refined model was then used to investigate the impact of different barrage operating modes on the tidal power output and the associated extent of flood inundation along the Severn Estuary. Predicted results indicate that the mode of flood generation would produce the least electrical energy and cause a larger reduction in the maximum water levels upstream of the barrage. Two-way generation would provide an improvement to these conditions, and produce an equivalent amount of electricity to that from ebb generation, with a low installed capacity and a small loss of intertidal zones. Therefore, the mode of ebb generation or two-way generation would appear to be a preferred option for power generation, because both would offer benefits of acceptable electrical energy and reduced flood risk.     

Forty candles for the Rance River TPP tides provide renewable and sustainable power generation

Prices of oil and other fossil fuels have proven a powerful incentive for the alternative energy hunters. The alternatives include the various forms of ocean energy that, often considered uneconomical for electricity generation, have become attractive and competitive. Many sites throughout the world have been considered, at one time or another, suitable for implantation of a tidal power station, but very few have witnessed implementation of often ambitious plans. The Rance River tidal power plant, near St Malo in Brittany (France) is an exception. It is celebrating in 2006, 40 years of durable, loyal and productive service.     

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

Tidal stream turbines are exploited in regions of high tidal currents. Such energy extraction will alter the hydrodynamics of a tidal region, analogous to increasing the bed friction in the region of extraction. In addition, this study demonstrates that energy extracted with respect to tidal asymmetries due to interactions between quarter (M₄) and semi-diurnal (M₂) currents will have important implications for large-scale sediment dynamics. Model simulations show that energy extracted from regions of strong tidal asymmetry will have a much more pronounced effect on sediment dynamics than energy extracted from regions of tidal symmetry. The results show that energy extracted from regions of strong tidal asymmetry led to a 20% increase in the magnitude of bed level change averaged over the length of a large estuarine system, compared with energy extracted from regions of tidal symmetry. However, regardless of the location of a tidal stream farm within a tidal system, energy extraction reduces the overall magnitude of bed level change in comparison with non-extraction cases. This has practical application to many areas surrounding the UK, including the Irish Sea and the Bristol Channel, that exhibit strong tidal currents suitable for exploitation of the tidal stream resource, but where large variations in tidal asymmetry occur.     

Experimental verifications of numerical predictions for the hydrodynamic performance of horizontal axis marine current turbines

The conversion of the kinetic energy presented by ocean or marine currents offers an exciting proposition as it can provide regular and predictable energy resource. The majority of the proposed designs for converting this type of kinetic energy are based on the concept of the horizontal axis turbines, which has common characteristics to those being used in wind energy. Although a lot can be learnt and transferred from wind turbine technology, there are significant differences. These include the effects of the free surface and the occurrence of cavitation. Consequently, any developed numerical methods need to be verified. This study reports on the development and verification of simulation tools based on blade element momentum theory—a commercial code (GH-Tidal Bladed) and an academic in-house code (SERG-Tidal). Validation is derived from experimental measurements conducted on a model 800 mm diameter turbine in a cavitation tunnel and a towing tank. The experimental data includes measurements of shaft power and thrust generated by the turbine for a series of blade pitch settings and speeds. The results derived from the two codes are compared. These indicate that the two developed codes demonstrate similar trends in the results and provide a satisfactory representation of the experimental turbine performance. Such results give the necessary confidence in the developed codes resulting in appropriate tools that can to be utilised by developers of marine current turbines.     

Variability and phasing of tidal current energy around the United Kingdom

Tidal energy has the potential to play a key role in meeting renewable energy targets set out by the United Kingdom (UK) government and devolved administrations. Attention has been drawn to this resource as a number of locations with high tidal current velocity have recently been leased by the Crown Estate for commercial development. Although tides are periodic and predictable, there are times when the current velocity is too low for any power generation. However, it has been proposed that a portfolio of diverse sites located around the UK will deliver a firm aggregate output due to the relative phasing of the tidal signal around the coast. This paper analyses whether firm tidal power is feasible with ‘first generation’ tidal current generators suitable for relatively shallow water, high velocity sites. This is achieved through development of realistic scenarios of tidal current energy industry development. These scenarios incorporate constraints relating to assessment of the economically harvestable resource, tidal technology potential and the practical limits to energy extraction dictated by environmental response and spatial availability of resource. The final scenario is capable of generating 17 TWh/year with an effective installed capacity of 7.8 GW, at an average capacity factor of 29.9% from 7 major locations. However, it is concluded that there is insufficient diversity between sites suitable for first generation tidal current energy schemes for a portfolio approach to deliver firm power generation.     

On the definition of the power coefficient of tidal current turbines and efficiency of tidal current turbine farms

During the last decade, the development of tidal current industries has experienced a rapid growth. Many devices are being prototyped. For various purposes, investors, industries, government and academics are looking to identify the best device in terms of of cost of energy and performance. However, it is difficult to compare the cost of energy of new devices directly because of uncertainties in the operational and capital costs. It may however be possible to compare the power output of different devices by standardizing the definition of power coefficients. In this paper, we derive a formula to quantify the power coefficient of different devices. Specifically, this formula covers ducted devices, and it suggests that the duct shape should be considered. We also propose a procedure to quantify the efficiency of a tidal current turbine farm by using the power output of the farm where no hydrodynamic interaction exists between turbines, which normalizes a given farm's power output. We also show that the maximum efficiency of a farm can be obtained when the hydrodynamic interaction exists.