Tidal Energy Devices Research Articles

The value of delay in tidal energy development

Despite robust research, prototype development and demonstration of in-stream tidal energy devices, progress to the commercialization stage has been slow. Some of this can be attributed to a lack readiness or financing. However, when uncertainty is high, a developer may choose to delay a project until more is known. The option to delay has value for a company. This study applies the real option valuation model to an investment in a 10MW array of in-stream tidal energy conversion devices at the Fundy Ocean Research Centre for Energy (FORCE) in the Bay of Fundy, Nova Scotia, Canada. The values of investing and the option to delay are calculated. A sensitivity analysis of key drivers and scenarios with various input values to the option model are constructed to observe the impact on the 'invest versus delay' decision. The analysis suggests there is value in owning the option to develop, by leasing a FORCE berth, but waiting while uncertainty is resolved. Implications for policy-setting are discussed.

Wave–current interaction effects on tidal stream turbine performance and loading characteristics

The transient interaction between tidal currents and the rotation of a horizontal axis turbine rotor have the potential to induce high asymmetric loadings, which are subsequently transmitted to the drive shaft and potentially high speed drive train components. To mitigate the potential for early component failure, analysis of asymmetric loading on marine turbines is fundamental to the design process. To investigate these loads a turbine mounted on a circular stanchion has been used to highlight the effects of introducing more realistic boundary conditions. Depending on their wavelength, waves can also have a significant effect on the overall design decisions and placement of devices. Thrust loading and bending moments applied to the drive shaft can be of the order of hundreds of kN and kNm respectively.Knowledge of the flow regime can allow designers to evaluate material selection for components and incorporate some deformation capability of the turbine blades to increase the power output and potentially alleviate some of the stress distribution through key structural points. The resulting data can then be used to estimate component life via fatigue prediction.This paper includes a multi-physics approach to modelling tidal energy devices and the potential for modelling to inform device condition monitoring.

Tidal energy “Round Robin” tests comparisons between towing tank and circulating tank results

One key step of the industrial development of a tidal energy device is the testing of scale prototype devices within a controlled laboratory environment. At present, there is no available experimental protocol which addresses in a quantitative manner the differences which can be expected between results obtained from the different types of facilities currently employed for this type of testing. As a consequence, where differences between results are found it has been difficult to confirm the extent to which these differences relate to the device performance or to the test facility type.In the present study, a comparative “Round Robin” testing programme has been conducted as part of the EC FP VII MaRINET program in order to evaluate the impact of different experimental facilities on the test results. The aim of the trials was to test the same model tidal turbine in four different test facilities to explore the sensitivity of the results to the choice of facility. The facilities comprised two towing tanks, of very different size, and two circulating water channels.Performance assessments in terms of torque, drag and inflow speed showed very similar results in all facilities. However, expected differences between the different tank types (circulating and towing) were observed in the fluctuations of torque and drag measurements. The main facility parameters which can influence the behaviour of the turbine were identified; in particular the effect of blockage was shown to be significant in cases yielding for high thrust coefficients, even at relatively small blockage ratios.

A Self-Contained Subsea Platform for Acoustic Monitoring of the Environment Around Marine Renewable Energy Devices–Field Deployments at Wave and Tidal Energy Sites in Orkney, Scotland

The drive towards sustainable energy has seen rapid development of marine renewable energy devices (MREDs). The NERC/Defra collaboration FLOw, Water column and Benthic ECology 4-D (FLOWBEC-4D) is investigating the environmental and ecological effects of installing and operating wave and tidal energy devices. The FLOWBEC sonar platform combines several instruments to record information at a range of physical and multitrophic levels for durations of two weeks to capture an entire spring-neap tidal cycle. An upward-facing multifrequency Simrad EK60 echosounder is synchronized with an upward-facing Imagenex Delta T multibeam sonar. An acoustic Doppler velocimeter (ADV) provides local current measurements and a fluorometer measures chlorophyll (as a proxy for phytoplankton) and turbidity. The platform is self-contained, facilitating rapid deployment and recovery in high-energy sites and flexibility in gathering baseline data. Five 2-week deployments were completed in 2012 and 2013 at wave and tidal energy sites, both in the presence and absence of renewable energy structures at the European Marine Energy Centre (EMEC), Orkney, U.K. Algorithms for target tracking have been designed and compared with concurrent, shore-based seabird observations used to ground truth the acoustic data. The depth preference and interactions of birds, fish schools and marine mammals with MREDs can be tracked to assess whether individual animals face collision risks with tidal stream turbines, and how animals generally interact with MREDs. These results can be used to guide marine spatial planning, device design, licensing and operation, as different device types are tested, as individual devices are scaled up to arrays, and as new sites are considered.

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

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

In-situ ecological interactions with a deployed tidal energy device; an observational pilot study

At present, few studies exist that consider the relationship between species interactions and key environmental variables, with the added influence of offshore marine renewable energy technologies. Video footage and ADCP survey techniques were used, to examine the presence of fish and velocity flow rates within the vicinity of a deployed tidal energy device. Pilot trials were conducted across 15 day temporal periods, during the summer months of 2009 and 2010. Five random photographic stills were taken from the video footage at hourly intervals throughout each trial day to estimate species presence and abundance. Mean abundance then was compared between hour and day temporal scales and their relationship with velocity rate, for both years.Pollack, Pollachius pollachius was observed aggregating in shoals temporarily round the deployed device, with larger abundance observed in 2009 than 2010. No other species were identified from the surveys. Pollack abundance was significantly associated to velocity rate for both trial years. Increased abundance was related to a reduction in velocity rate for both years, with shoals potentially using the device for temporary protection or feeding strategies. Responses to tidal velocity also differed between years, with 2009 abundances ranging from 0 to 1.2 m/s and 2010 abundances between 0.5 and 1.7 m/s. Overall this preliminary study outlined a potentially useful approach to investigate species responses with new anthropogenic activities in the marine environment.

Spatial and temporal benthic species assemblage responses with a deployed marine tidal energy device: A small scaled study

The addition of man-made structures to the marine environment is known to increase the physical complexity of the seafloor, which can influence benthic species community patterns and habitat structure. However, knowledge of how deployed tidal energy device structures influence benthic communities is currently lacking. Here we examined species biodiversity, composition and habitat type surrounding a tidal energy device within the European Marine Energy Centre test site, Orkney. Commercial fishing and towed video camera techniques were used over three temporal periods, from 2009 to 2010.Our results showed increased species biodiversity and compositional differences within the device site, compared to a control site. Both sites largely comprised of crustacean species, omnivore or predatory feeding regimes and marine tide-swept EUNIS habitat types, which varied over the time. We conclude that the device could act as a localised artificial reef structure, but that further in-depth investigations are required.

Decision support tools for collaborative marine spatial planning: identifying potential sites for tidal energy devices around the Mull of Kintyre, Scotland

The expansion of offshore renewable energy production, such as wind, wave and tidal energy, is likely to lead to conflict between different users of the sea. Two types of spatial decision support tools were developed to support stakeholder workshops. A value mapping tool combines regional attributes with local knowledge. A negotiation support tool uses these value maps to support stakeholders in finding acceptable locations for tidal energy devices. Interactive value mapping proved useful to address deficiencies in data and to create credibility for these maps. The negotiation tool helped stakeholders in balancing objectives of the various stakeholders.

Modeling of in-stream tidal energy development and its potential effects in Tacoma Narrows, Washington, USA

Understanding and providing proactive information on the potential for tidal energy projects to cause changes to the physical system and to key water quality constituents in tidal waters is a necessary and cost-effective means to avoid costly regulatory involvement and late stage surprises in the permitting process. This paper presents a modeling study for evaluating the tidal energy extraction and its potential impacts on the marine environment in a real world site – Tacoma Narrows of Puget Sound, Washington State, USA. An unstructured-grid coastal ocean model, fitted with a module that simulates tidal energy devices, was applied to simulate the tidal energy extracted by different turbine array configurations and the potential effects of the extraction at local and system-wide scales in Tacoma Narrows and South Puget Sound. Model results demonstrated the advantage of an unstructured-grid model for simulating the far-field effects of tidal energy extraction in a large model domain, as well as assessing the near-field effect using a fine grid resolution near the tidal turbines. The outcome shows that a realistic near-term deployment scenario extracts a very small fraction of the total tidal energy in the system and that system wide environmental effects are not likely; however, there are near-field effects on the flow field and bed shear stress in the area of tidal turbine farm. Model results also indicate that from a practical standpoint, hydrodynamic or water quality effects are not likely to be the limiting factor for development of large commercial-scale tidal farms. Results indicate that very high numbers of turbines are required to significantly alter the tidal system; limitations on marine space or other environmental concerns are likely to be reached before reaching these deployment levels. These findings show that important information obtained from numerical modeling can be used to inform regulatory and policy processes for tidal energy development.

Using Hydroacoustics to Understand Fish Presence and Vertical Distribution in a Tidally Dynamic Region Targeted for Energy Extraction

The use of tidal currents by fishes for movements to and from onshore spawning, foraging, and nursery grounds is well documented. However, fishes’ use of the water column in tidal currents frequently exceeding 1.5 m · s-1 is largely unknown. With growing interest in extracting energy from the tides, understanding animal use of these dynamic environments has become essential to determining environmental effects of tidal energy devices. To assess the effects of a tidal energy device on fishes, we used down-looking single-beam hydroacoustic technology to collect pre-deployment data on the presence and vertical distribution of fishes at a pilot project site and a control site in Cobscook Bay, ME. Twenty-four-hour stationary surveys were conducted in each season of 2010 and 2011. Relative fish density and vertical distribution were analyzed for variation with respect to site, year, month, and diel and tidal cycles. A seasonal pattern in fish density was apparent in both years at both sites, with maxima in spring and late fall. Fish density was generally highest near the sea floor. Diel changes in vertical distribution were frequently observed, but changes in distribution related to tidal cycle were inconsistent. Results from the project and control sites were very similar, demonstrating that the control site provides a reference for quantifying changes in fish density and vertical distribution related to the tidal device. This approach and baseline dataset will be used to compare hydroacoustic data collected at the project and control sites after device deployment.

Fish Interactions with a Commercial-Scale Tidal Energy Device in the Natural Environment

Fish are a key part of the marine ecosystem likely to be affected by hydrokinetic tidal turbines, but little is known about their behavior around such obstacles in the natural environment. In September 2010, two DIDSON acoustic cameras were used to observe fish interactions with a commercial-scale turbine in Cobscook Bay, Maine. Twenty-two hours (nearly two tidal cycles) of footage were collected. Behaviors of individual fish and schools were classified (e.g., entering, avoiding, passing, or remaining in the wake of the turbine). We analyzed the effects of turbine motion (rotating or not rotating), diel condition (day or night), and fish size (small, ≤10 cm; large, >10 cm) on individual fish behaviors, and compared behaviors of individual fish to schools of fish. When the turbine was rotating, the probability of fish entering the turbine decreased by over 35 % from when it was not. The probability that fish would enter the turbine was higher at night than during the day, and this difference was greater for small fish than for large fish (probability of small fish entering = 0.147 day, 0.513 night; large fish = 0.043 day, 0.333 night). Fish were almost always present in the wake of the turbine. Schools of fish had a 56 % lower probability of entering the turbine than individual fish, and reacted at greater distances from the turbine (median distance of 2.5 m for schools, 1.7 m for individuals). This study indicates that fish behavior in response to tidal turbines appears to be similar to responses to obstacles such as trawls, and highlights the importance of environmental context in determining the effects of a tidal turbine on fish.

Tidal energy machines: A comparative life cycle assessment study

Marine energy in the United Kingdom is undergoing a period of growth in terms of development and implementation. The current installed tidal energy capacity is expected to rise to provide 20% of the United Kingdom’s electricity demand by 2050. This article used life cycle assessment to study four tidal energy devices, comparing their embodied energy and carbon dioxide emissions. The device designs studied included a multi-blade turbine, two three-blade horizontal axis turbine machines and an Archimedes’ screw device. These machines were chosen to represent a cross section of design for the device, foundation, installation and operation. Embodied energy was considered over the lifetime of each device. Energy use from fabrication, transport, installation, maintenance, decommissioning and recycling was all calculated and compared to the energy generated by each device. Finally, the embodied energy, CO2 intensity and energy payback periods were compared to those of conventional power generating systems and other renewable energy sources. Devices were studied based on a functional unit, defined as a 10 MW array installed for 100 years. Of the devices studied, the OpenHydro Open Centre turbine was found to have the best ratio of generated to embodied energy. All devices achieved CO2 and energy payback within 12 years and exhibited CO2 intensity between 18 and 35 gCO2/kW h. This compares favourably against current energy sources such as wind (8–12 gCO2/kW h), solar photovoltaic (~30 gCO2/kW h), nuclear (~70 gCO2/kW h) and coal (~1000 gCO2/kW h).

Energy potential of a tidal fence deployed near a coastal headland

Enhanced tidal streams close to coastal headlands appear to present ideal locations for the deployment of tidal energy devices. In this paper, the power potential of tidal streams near an idealized coastal headland with a sloping seabed is investigated using a near-field approximation to represent a tidal fence, i.e. a row of tidal devices, in a two-dimensional depth-averaged numerical model. Simulations indicate that the power extracted by the tidal fence is limited because the flow will bypass the fence, predominantly on the ocean side, as the thrust applied by the devices increases. For the dynamic conditions, fence placements and headland aspect ratios considered, the maximum power extracted at the fence is not related in any obvious way to the local undisturbed kinetic flux or the natural rate of energy dissipation due to bed friction (although both of these have been used in the past to predict the amount of power that may be extracted). The available power (equal to the extracted power net of vertical mixing losses in the immediate wake of devices) is optimized for devices with large area and small centre-to-centre spacing within the fence. The influence of energy extraction on the natural flow field is assessed relative to changes in the M2 component of elevation and velocity, and residual bed shear stress and tidal dispersion.

Modelling of unsteady effects for a direct drive tidal energy converter

In this study, unsteady effects on an oscillating profile type tidal energy device are being investigated. A numerical method has been developed, based on vortex shedding from the trailing edge, in order to perform the investigations. The method is applied to a test case and the influence of the reversal period ratio, angle of attack and the velocity ratio on the unsteady effects has been systematically investigated. It has been concluded that for the moderate angle of attack the efficiency of the device will not be significantly affected while changes in the reversal period ratio and the velocity ratio play an important role in the efficiency of the device.

Assessing the sensitivity of seabird populations to adverse effects from tidal stream turbines and wave energy devices

AbstractFurness, R. W., Wade, H. M., Robbins, A. M. C., and Masden, E. A. 2012. Assessing the sensitivity of seabird populations to adverse effects from tidal stream turbines and wave energy devices. – ICES Journal of Marine Science, 69: . Tidal turbines and wave energy devices may affect seabird populations through collision mortality, disturbance and habitat loss. Given the pressures to harness tidal and wave energy, especially in Scottish waters, there is an urgent need to assess population-level impacts on seabird species. With a lack of deployed devices to monitor in areas of importance for seabirds, our approach uses data from scientific literature on seabird ecology and conservation importance likely to influence population vulnerability to “wet renewables” in Scottish waters. At this stage however, we can only infer likely interactions with tidal and wave devices. We identify black guillemot, razorbill, European shag, common guillemot, great cormorant, divers and Atlantic puffin as the species most vulnerable to adverse effects from tidal turbines in Scottish waters. We identify divers as the species most vulnerable to adverse effects from wave energy devices in Scottish waters. Wave energy devices seem likely to represent a lesser hazard to seabirds than tidal turbines, and both forms of energy capture seem likely to represent a lower hazard to seabirds than offshore wind farms (wind-power plants). The indices developed here for Scottish seabird populations could be applied to populations elsewhere. This approach will help in identifying likely impacts of tidal and wave energy deployments on seabirds, and in optimizing deployment of resources for compulsory environmental monitoring.

Life cycle comparison of a wave and tidal energy device

Tidal and wave energy devices are often discussed as a future contributor to the UK’s energy mix. Indeed, marine energy resources are said to have the potential to supply up to 20 per cent of the nation’s electricity demand. However, these technologies are currently at the development stage and make no meaningful contribution to the national grid. A number of devices have been developed, but no single method has emerged as the leading technology. This paper aims to compare two promising devices, one wave device and one tidal device, and assess the life cycle properties of each. A life cycle assessment of the Oyster wave energy device was conducted as part of this study, and a comparison of this and the SeaGen marine current turbine was undertaken. In both cases a ‘cradle-to-grave’ assessment was carried out, calculating emissions from materials, fabrication, transport, installation, lifetime maintenance, and decommissioning (including recycling). The SeaGen tidal device was calculated to have an energy payback period of 14 months, and a CO2 payback period of 8 months. The equivalent figures for the Oyster device were 12 and 8 months, respectively. The respective energy and carbon intensities for the two devices were 214 kJ/kWh and 15 gCO2/kWh for the SeaGen and 236 kJ/kWh and 25 gCO2/kWh for the Oyster. The calculated intensities and payback periods are close to those of established wind turbine technologies, and low relative to the 400–1000 g CO2/kWh of typical fossil fuel generation. With further developments in construction and deployment efficiency these intensities are expected to fall, so the devices appear to have the potential to offer a viable contribution to the UK’s future energy mix.

Model testing of suction caissons in clay subjected to vertical loading

A wide range of new offshore applications are emerging in the energy sector. The oil and gas industry is targeting minimum facility applications, whilst the renewable energy sector is developing offshore wind turbines, as well as a number of wave and tidal energy devices. The design and installation of the foundations are key considerations in the financial viability of such offshore engineering projects. Suction caisson foundations are a potential solution for these new developments, but design guidance is relatively sparse. This paper considers the vertical loading response of a caisson foundation in clay, during installation and under both monotonic and cyclic vertical loading. The main contribution is the presentation and interpretation of high quality experimental data. Vertical loading is critical for the design of a multi-footing structure of the type that might be used for large offshore wind turbines. We first consider the installation behaviour and compare data from pushed installations and a suction installation with results from a theoretical calculation. We then consider cyclic vertical loading tests, focussing on cyclic amplitudes that take the foundation into tension. Detailed displacement data and pore pressure data are presented.

Modelling tidal energy extraction in a depth-averaged coastal domain

An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions.

Integrating Computational Fluid Dynamic and prototyping technologies in the investigation of multi-element profiles for a high-lift variable pitch vertical-axis tidal power generator

Multi-element profiles have been used for decades to assist aircraft during landing and take-off by significantly increasing lift during these vital stages of operation. This paper presents the preliminary Computational Fluid Dynamic results used to validate and optimise the design of a multi-element profile for a vertical-axis tidal stream energy device, where the relative velocities, fluid type and Reynolds number differ significantly from that found in the aerospace sector. The resulting designs and optimisation have allowed for the manufacture of stereolithography prototypes for in situ testing in the 'wet' facilities at Lancaster University. A vertical-axis device has been selected as it offers significant advantages in shallower tidal water, often found near shore, thus reducing the required infrastructure and set up costs associated with the deployment of new technologies. The UK is in a prime position to exploit energy capture from tidal stream with the advantage over other renewable sources in that it is completely predictable. Such exploitation requires the quick turnaround offered through design, analysis and product development technologies.

On Assessing the Reliability and Availability of Marine Energy Converters: The Problems of a New Technology

There is now considerable interest in marine renewable energy both in the UK and elsewhere in the world with the growing recognition that fossil fuel supplies are finite and further increases in carbon gas emissions will hasten climate change. A major factor in the viability of any potential wave or tidal energy device (marine energy converter (MEC)) is its reliability and availability. However, as for any new technology, this is difficult to predict in advance with confidence. This paper describes the issues that must be addressed to assess the reliability and availability of MECs. It starts with a brief overview of the characteristics of the principal types of wave and tidal energy converters and a review of earlier work on estimating reliability for wave energy converters. It is shown that MECs have many subsystems and components in common with one another and with other existing land-based and marine systems. The ways in which experience with existing systems may be adapted to predict reliability and availability for MECs are discussed and a new embryonic database for MECs is outlined. The choice of distribution for the time to failure of MEC components is discussed and an argument is advanced for the use of the log-normal distribution. The availability of MECs is affected very much by the environment in which they operate and the maintenance strategy adopted. The effects of environment and location upon maintainability and hence availability are described and how availability may be modelled is discussed. Finally the need for a framework for assessing the reliability and availability of MECs and other related issues are discussed.