Wave Energy Sites Research Articles

Capture and simulation of the ocean environment for offshore renewable energy

The offshore renewable energy sector has challenging requirements related to the physical simulation of the ocean environment for the purpose of evaluating energy generating technologies. In this paper the demands of the wave and tidal energy sectors are considered, with measurement and characterisation of the environment explored and replication of these conditions described. This review examines the process of advanced ocean environment replication from the sea to the tank, and rather than an exhaustive overview of all approaches it follows the rationale behind projects led, or strongly connected to, the late Professor Ian Bryden. This gives an element of commonality to the motivations behind marine data acquisition programmes and the facilities constructed to take advantage of the resulting datasets and findings. This review presents a decade of flagship research, conducted in the United Kingdom, at the interfaces between physical oceanography, engineering simulation tools and industrial applications in the area of offshore renewable energy. Wave and tidal datasets are presented, with particular emphasis on the novel tidal measurement techniques developed for tidal energy characterisation in the Fall of Warness, Orkney, UK. Non-parametric wave spectra characterisation methodologies are applied to the European Marine Energy Centre's (EMEC) Billia Croo wave test site, giving complex and highly realistic site-specific directional inputs for simulation of wave energy sites and converters. Finally, the processes of recreating the resulting wave, tidal, and combined wave-current conditions in the FloWave Ocean Energy Research Facility are presented. The common motivations across measurement, characterisation, and test tank are discussed with conclusions drawn on the strengths, gaps and challenges associated with detailed site replication.

Wave energy along the Romanian Southern Black Sea coast

Wave energy is among the resources to provide the most significant contribution to the European energy system in the future. Detailed wave climate analysis is needed for the selection of accurate types of wave energy generation structure and feasibility of wave energy sites. There are current studies on spatial distribution of Black Sea wave energy, that use measured or modeled data. In this study we used measured data (significant wave height and period and wind speed) recorded in 4 locations on the Romanian Southern coast during 5 years (2013-2018) in order to estimate the available wave energy potential.

Modelling Offshore Wave farms for Coastal Process Impact Assessment: Waves, Beach Morphology, and Water Users

The emerging global wave energy industry has the potential to contribute to the world’s energy needs, but careful consideration of potential impacts to coastal processes in the form of an impact assessment is required for each new wave energy site. Methods for conducting a coastal processes impact assessment for wave energy arrays vary considerably in the scientific literature, particularly with respect to characterising the energy absorption of a wave energy converter (WEC) array in a wave model. In this paper, modelling methods used in the scientific literature to study wave farm impacts on coastal processes are reviewed, with the aim of determining modelling guidance for impact assessments. Effects on wave climate, beach morphology, and the surfing resource for coastal water users are considered. A novel parameterisation for the WEC array transmission coefficient is presented that, for the first time, uses the permitted power rating of the wave farm, which is usually well defined at the impact assessment stage, to estimate the maximum likely absorption of a permitted WEC array. A coastal processes impact assessment case study from a wave farm in south-west Ireland is used to illustrate the application of the reviewed methods, and demonstrates that using the new ‘rated power transmission coefficient’ rather than a WEC-derived transmission coefficient or complete energy absorption scenario can make the difference between significant and non-significant levels of coastal impacts being predicted.

Wave resource variability: Impacts on wave power supply over regional to international scales

The intermittent, irregular and variable nature of the wave energy resource has implications for the supply of wave-generated electricity into the grid; intermittency of renewable power may lead to frequency and voltage fluctuations in the transmission and distribution networks. This study analyses the wave resource over different spatial scales to investigate the potential impacts of the resource variability on the grid supply. It is found that the deployment of multiple wave energy sites results in a reduction in step changes in power, leading to an overall smoothing of the wave-generated electrical power.

Reliability and O&M sensitivity analysis as a consequence of site specific characteristics for wave energy converters

Estimates for lifetime costs of wave energy arrays are difficult to obtain due to the uncertainty surrounding weather windows and failure rates for wave energy converters (WECs). An operations & maintenance (O&M) simulation tool has been used to assess the sensitivity of WEC failure rates on the profitability of a wave farm, with the Pelamis P2 device used as a case study. Two offshore wave energy sites in the UK have been characterised and presented in terms of accessibility for marine operations and power generation. It was found that a wave farm at one site would incur higher costs due to vessels having to wait longer for suitable weather windows. This was balanced by higher generated revenue, showing how the tool can be used to support strategic planning and site selection. The results identify the sensitivity to failure rate estimations for different components, helping target future work to reduce uncertainties and costs. The results highlight the need for WEC developers to collaborate closely with component manufacturers in order to design the best device possible for the challenging marine environment. Collaboration enables more realistic failure rate estimates to be obtained, leading to better understanding of the operational costs for commercial wave energy farms.

Operational atmospheric and wave modelling in the California’s coastline and offshore area with applications to wave energy monitoring and assessment

ABSTRACTA new high-resolution operational atmospheric/wave forecasting system for the west coastline of the US, focusing especially to the California near and offshore area, is presented in this work. The new system is the result of the collaboration between two US and one European Universities. It consists of two state-of-the-art numerical prediction models (Regional Atmospheric Modeling System and WAve Model) supported by a new optimisation statistical module for the bias reduction and local adaptation of the results based on non-linear Kalman filtering and Bayesian statistics. The presented system has been evaluated against a wide number of National Data Buoy Center buoys records with very promising results. Moreover, applications to wave energy site assessment are presented revealing areas of the California coastline with increased power potential appropriate for wave power plants installation.

Wave energy extraction in Scotland through an improved nearshore wave atlas

Wave energy is expected to play an important role in the forthcoming years for the de-carbonisation of Scottish and British electricity production. This study underlines the importance of resource assessment and attempts to improve the quantifiable wave power resource, with use of a validated numerical model. While levels of wave flux are high for an area that may not always constitute the best option for wave energy applications. In this study, a long-term hindcast for the Scottish coastlines run from 2004 to 2014 (11years) improving the existing wave maps and resource estimations. Spatial and physical considerations of a third generation spectral model allow examination at locations of immediate interest for the ocean energy community. Utilising numerical wave models of finer resolution allows for the detailed coupling of potential wave energy converters (WECs) and site characterization. Such detail energy results allow for improved financial analysis that take into account the severity of local resource and its energy potential.

A simplified method to estimate tidal current effects on the ocean wave power resource

Although ocean wave power can be significantly modified by tidal currents, resource assessments at wave energy sites generally ignore this effect, mainly due to the difficulties and high computational cost of developing coupled wave-tide models. Furthermore, validating the prediction of wave-current interaction effects in a coupled model is a challenging task, due to the paucity of observational data. Here, as an alternative to fully coupled numerical models, we present a simplified analytical method, based on linear wave theory, to estimate the influence of tidal currents on the wave power resource. The method estimates the resulting increase (or decrease) in wave height and wavelength for opposing (or following) currents, as well as quantifying the change in wave power. The method is validated by applying it to two energetic locations around the UK shelf – Pentland Firth and Bristol Channel – where wave/current interactions are significant, and for which field data are available. Results demonstrate a good accuracy of the simplified analytical approach, which can thus be used as an efficient tool for making rapid estimates of tidal effects on the wave power resource. Additionally, the method can be used to help better interpret numerical model results, as well as observational data.

Assessment of wave characteristics and resource variability at a 1/4-scale wave energy test site in Galway Bay using waverider and high frequency radar (CODAR) data

This research presents an assessment of wave characteristics at the 1/4 scale wave energy test site in Galway Bay based on (1) data from a waverider buoy from 2009 to 2013 and (2) data from a high frequency radar system (CODAR) from 2011 to 2013. The main objective of this research is to provide an assessment of annual and seasonal wave characteristics and resource variability at a wave energy site. Such assessments are extremely important for wave energy test sites so as to inform the design, optimisation and maintenance of wave energy converters. An approach for classifying operational, high and extreme wave events is presented. The approach is based on percentage of occurrences of particular wave events and can be applied to any site and any wave parameter. In the present research it is separately applied to wave height and wave power. An additional objective is the validation of CODAR wave data for use in assessment of wave height characteristics; this was achieved by comparing CODAR data with waverider data. The research shows that the authors characterisation methodology is easy to apply and unambiguous to interpret. Due to the significant variation in wave parameters at the site from season to season and year to year, operational, high and extreme conditions are presented for the 5-year measurement period, individual years and individual seasons. The research also shows that wave heights determined from CODAR show good agreement with those from a waverider buoy and may be relied upon for accurate site characterisation.

Wave energy resource assessment with AltiKa satellite altimetry: A case study at a wave energy site

AbstractA simple model to estimate wave power at a coastal site from satellite radar altimetry is proposed. We used data from the AltiKa altimeter on board the SARAL satellite because of its superior performance near the coast. The deep water approximation was applied to our 60 m deep site along with a simple empirical model to estimate wave period, T, from altimeter significant wave height, Hs, and backscattering coefficient, σ0. A known relation between zero‐crossing period, Tz, and wave energy period, TE, was used in combination with Hs to calculate wave power per meter wave crest (P). The annual and seasonal mean values of P using AltiKa parameters largely agreed with known ranges of P at the site. A comparison with shallower sites and sites closer to the coast revealed that for estimation of Tz from AltiKa the water depth could be taken into account and an empirical relation is given.

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.

A tool for combined WEC-site selection throughout a coastal region: Rias Baixas, NW Spain

The selection of the appropriate wave energy converter (WEC) and site is the basis for the installation of a wave farm in a region. For this purpose, the estimation of the energy that any WEC would produce at any location of interest is fundamental. Despite all its importance, this information or the elements required for obtaining it are currently available only at specific coastal locations or areas of interest. This work develops a tool for computing the energy that any WEC would generate at any coastal location within the Rias Baixas Region (NW Spain). With this aim, a methodology which allows the consideration of almost all the total energy available is used to characterize the coastal resource with a high spatial resolution. Then, a matlab-based application called WEDGE (Wave Energy Diagram GEnerator) is implemented for easy access to the stored data and automatic reconstruction of the resource at any coastal site in terms of a high-resolution characterization matrix (or energy diagram). As a result, the information required for accurate energy production computation throughout the region is available whereby a combined WEC-site selection can be conducted. Finally, the tool is used to compute the energy production of a total of 23 WEC-site combinations in an area within this region where a wave farm has been recently proposed. The results will underline the importance of a combined WEC-site selection for proper decision-making regarding wave energy exploitation.

Wave resource assessment along the Cornish coast (UK) from a 23-year hindcast dataset validated against buoy measurements

Long-term knowledge of the wave climate of a potential wave energy site is essential for project planning and design, not only for an understanding of the resource variability, but also for the prediction of design wave conditions. The southwest region of the UK is at the forefront of the country's wave energy development, with two operational test sites. However, no detailed long-term resource assessment has yet been performed. This paper presents a long-term wave hindcast for southwest England, performed using the numerical wave model SWAN, with a particular focus on two energy device test facilities: ‘Wave Hub’ on the energetic and exposed north Cornwall coast, and ‘FaB Test’ on the more sheltered south coast. A high-resolution wave model suite, aimed at establishing nearshore wave hindcasts, is described and evaluated. The suite is run for a 23-year period, starting in 1989 and continuing to 2011. The hindcast is compared with measurement data and the results are analysed for the two test sites. Special attention is given to the implications of present hindcast errors and how the hindcast errors can be minimized.

Spatial variability of ocean waves, from in-situ measurements

This paper describes the analysis of the spatial properties of ocean waves using measurements from an array of four directional wave buoys installed in a square formation, with side 500m, in the Celtic Sea, UK. Wave measurements in this area have been installed to support resource assessment and design for wave energy devices at the Wave Hub site off the North Cornwall coast. This unique deployment of multiple directional sensors provides high quality direct measurements of the spatial properties of the wave field. Spectral parameters measured simultaneously by all four buoys within the array are compared and it is demonstrated that wave conditions cannot be considered stationary across the measurement area. Differences in the measured wave fields were observed primarily in the low frequencies and are observed to be of a level sufficient to impact the assessment of site characteristics. Theoretical estimations of refraction and bottom friction indicate that these physical processes contribute to the observed measurements. The results demonstrate the potential effect of spatial variability in wave fields on the monitoring of wave energy sites, and highlight the requirement for accurate evaluation of physical processes.

Relationships between water motion and size-specific survivorship and growth of the demosponge Amphimedon compressa

We examined the relationships between water motion and the distribution, abundance, and sizes (heights) of Amphimedon compressa, an abundant sponge in reefs of La Parguera, Puerto Rico. Amphimedon populations were surveyed at different depths (3, 5, 8 and 12 m) in 3 inner-shelf and 3 mid-shelf reefs, which are protected and fully exposed to wave action respectively. More detailed studies on sponge survival and growth were performed at Las Pelotas (LP), a low wave energy site, and Media Luna West (MLW), a fully exposed reef. The effects of Tropical Storm Dean on sponge survivorship were also evaluated at these reefs. With the exception of an inner-shelf site, Amphimedon was absent in the 3 m depth zone in all reefs. Otherwise, no consistent depth-related patterns in abundances were detected among reefs. Sponges' heights generally increased with depth with the tallest sponges occurring in the 12 m zone. Survivorships at LP before Dean were not size-specific with semiannual rates averaging 86.3%. Survivorships at MLW before Dean were size-specific with the lowest semiannual rates (69.4%) for large sponges (> 20 cm) and the highest (94.5%) for small sponges (< 10 cm). Tropical Storm Dean reduced survivorships to 41% and 30% at LP and MLW, respectively. At both sites, small and large sponges suffered significantly more losses compared to the mid-size sponges. Sponge growth rates were significantly higher at LP (4.5 cm yr − 1 ) compared to MLW (1.5 cm yr − 1 ). Growth rates were not size dependent at MLW while larger sponges grew faster at LP. The results of this study suggest that water motion may be an important factor controlling and structuring populations of A. compressa.

Coral reef encruster communities and carbonate production in cryptic and exposed coral reef habitats along a gradient of terrestrial disturbance

Encrusting calcareous organisms such as bryozoans, crustose coralline algae (CCA), foraminiferans, and serpulid worms are integral components of tropical framework-building reefs. They can contribute calcium carbonate to the reef framework, stabilise the substrate, and promote larval recruitment of other framework-building species (e.g. coral recruits). The percentage cover of encrusting organisms and their rates of carbonate production (g m−2 year−1) were assessed at four sites within a coastal embayment, along a gradient of riverine influence (high-low). As the orientation and type of substrate is thought to influence recruitment of encrusting organisms, organisms recruiting to both natural (the underside of platy corals) and experimental substrates were assessed. The effect of substrate exposure under different levels of riverine influence was assessed by orientating experimental substrates to mimic cryptic and exposed reef habitats (downwards-facing vs upwards-facing tiles) at each site. Cryptic experimental tiles supported similar encruster assemblages to those recruiting to the underneath (cryptic side) of platy corals, suggesting that tiles can be used as an experimental substrate to assess encruster recruitment in reef systems. Encruster cover, in particular CCA, and carbonate production was significantly higher at low-impact (clear water), high wave energy sites when compared to highly riverine impacted (turbid water), low wave energy sites. Cryptically orientated substrates supported a greater diversity of encrusting organisms, in particular serpulid worms and bryozoans. The inverse relationships observed between riverine inputs and encrusters (total encruster cover and carbonate production) have implications for both the current and future rates and styles of reefal framework production.

CONTRASTING PATTERNS OF ALLOMETRY AND REALIZED PLASTICITY IN THE SISTER SPECIES MAZZAELLA SPLENDENS AND MAZZAELLA LINEARIS (RHODOPOHYTA)1

Phenotypic differences between the low wave exposure Mazzaella splendens (Setchell et Gardner) Hommersand and the high exposure Mazzaella linearis (Setchell et Gardner) Fredericq could be due to plasticity or genetic differentiation. Common gardens were used to assess their levels of plasticity after describing allometric relationships. As thalli lengthened, stipe development for M. splendens almost ceased even though blades continued to expand, but M. linearis formed a larger stipe before developing a blade that continued to stay narrow at longer thallus lengths. Common gardens demonstrated that M. splendens regrown in the site of M. linearis produced a wider blade than M. splendens regrown in its natural low energy site and that M. linearis regrown in low wave energy either could not form a wider blade or became narrower than thalli from its high energy site. Tetrasporophytes of M. splendens produced a longer and thicker stipe in the high energy site, but the larger M. linearis stipe was not realized because its wider blades made it vulnerable to hydrodynamic removal. Mazzaella splendens therefore had low survivorship in the high wave energy site, and survivors were not long enough to reproduce. Survivorship and reproduction of M. linearis was similar in both environments. Some of the M. splendens and M. linearis characters are plastic, but this plasticity was insufficient for convergence of phenotypes, and blade width plasticity was maladaptive at least for M. splendens. Developmental systems producing the stipe and blade phenotypes of each species have undergone genetic differentiation.

Toxicity Evaluation with the Microtox ® Test to Assess the Impact of In Situ Oiled Shoreline Treatment Options: Natural Attenuation and Sediment Relocation

Changes in the toxicity levels of beach sediment, nearshore water, and bottom sediment samples were monitored with the Microtox ® Test to evaluate the two in situ oil spill treatment options of natural attenuation (natural recovery––no treatment) and sediment relocation (surf washing). During a series of field trials, IF-30 fuel oil was intentionally sprayed onto the surface of three mixed sediment (pebble and sand) beaches on the island of Spitsbergen, Svalbard, Norway (78°56 ′ N, 16°45 ′ E). At a low wave-energy site (Site 1 with a 3-km wind fetch), where oil was stranded within the zone of normal wave action, residual oil concentrations and beach sediment toxicity levels were significantly reduced by both options in less than five days. At Site 3, a higher wave-energy site with a 40-km wind fetch, oil was intentionally stranded on the beach face in the upper intertidal/supratidal zones, above the level of normal wave activity. At this site under these experimental conditions, sediment relocation was effective in accelerating the removal of the oil from the sediments and reducing the Microtox ® Test toxicity response to background levels. In the untreated (natural attenuation) plot at this site, the fraction of residual oil remaining within the beach sediments after one year (70%) continued to generate a toxic response. Chemical and toxicological analyses of nearshore sediment and sediment-trap samples at both sites confirmed that oil and suspended mineral fines were effectively dispersed into the surrounding environment by the in situ treatments. In terms of secondary potential detrimental effects from the release of stranded oil from the beaches, the toxicity level (Microtox ® Test) of adjacent nearshore sediment samples did not exceed the Canadian regulatory limit for dredged spoils destined for ocean disposal.

Influence of Spacing on Mechanically Transplanted Seagrass Survival in a High Wave Energy Regime

Abstract The wave‐exposed nature of much of the southwestern Australian coastline considerably reduces the protective influence of seagrasses, and sediment movement appears to be relatively unaffected by their presence. Present seagrass restoration efforts focus on the deployment of large mechanically transplanted “sods” of seagrass as a means of combating the negative effects of water motion on transplant survival. The aim of this study was to investigate the combined role of wave energy and transplant spacing on sediment movement and transplant survival to provide guidance for seagrass transplantation in areas of high wave energy. One hundred sixty sods (0.25 m2) of seagrass were mechanically extracted from a mixed meadow consisting of Amphibolis griffithii (Cymodoceaceae) and Posidonia coriacea (Posidoniaceae) and planted in a high wave energy site with the treatments configured as three replicates of 16 sods placed in 4 × 4–meter squares at distances of 0.5, 1.0, and 2.0 meters apart. An additional 16 single sods were planted randomly throughout the site. Monitoring was conducted at two monthly intervals and consisted of counting the number of sods surviving and measuring the shoot density of seagrass species within each surviving sod. Sediment height was monitored using a series of sediment plates and an electronic sediment level sensor. Sod spacing had no significant effect upon transplant survival, which remained above 90% for 4 months after transplantation and then declined with the onset of winter (June to August). After 14 months individual sod survival was between 9% and 40%. Initial shoot densities were 200 to 500 shoots/m2 and declined to less than 50 shoots/m2. Sediment fluctuations up to 35 cm were noted, occasionally taking place over a matter of hours, and storms during winter caused significantly increased sediment movement. This probably curtailed rhizome extension and prevented the expansion of the transplants. This study indicates that the ability of seagrasses to influence sediment would appear to vary with the prevailing hydrodynamic regime and that a reappraisal of the notion that all seagrass communities trap sediment is necessary.

Taphonomic Alteration of Reef Corals: Effects of Reef Environment and Coral Growth Form. I. The Great Barrier Reef

Taphonomic alteration in coral death assemblages showed high variability with respect to reef environment and growth form at Orpheus Island on the Great Barrier Reef Australia. A greater degree of physical and biological alteration occurred in the lower-energy leeward Pioneer Bay site relative to the higher-energy windward Iris Point site. Greater degrees of taphonomic alteration also occurred at 6-7 m than 2-3 m water depth within each site. Clear gradients in the degree of taphonomic alteration of reef corals with reef environment indicate the utility of corals as taphonomic indicators in ancient reef settings. Because greater taphonomic differences between depths occurred at the high wave-energy site relative to the lour wave-energy site, differentiation of fossil reef taphofacies might be greatest in high energy reef settings. Interpretation of ancient reef sedimentary environments may be aided by analysis of taphonomic alteration. of reef corals.