Marine Renewable Energy

The vast global wave energy resource holds great promise as an abundant, carbon-neutral resource for electricity generation. For wave energy to make a significant and measurable role in reducing our carbon footprint, highly resolved and accurate assessments of the gross wave resource are a necessity. A comprehensive wave resource assessment provides a quantitative summary of the full directional wave spectra over a period of time and parameterizes the necessary data to mitigate uncertainty and risk. However, the reduction of detailed wave spectra into parametric representations inherently discards important details about the wave characteristics and introduces uncertainty. The goal of a resource assessment is to quantify the wave resource as completely as possible, through specific parameterizations, and minimize the associated uncertainty. This chapter provides an overview of in-situ and remote wave measurement data collection techniques, and an introduction to the dominant numerical wave propagation models used for wave resource assessments (WAM, WWIII, SWAN, TOMAWAC, and MIKE-21 SW). An explanation of standard oceanographic wave parameterizations and an in-depth review of dominant resource assessment methodologies provide a baseline assessment. Several higher fidelity assessment techniques, extreme value analyses, and additional environmental factors are subsequently presented, and the impacts on wave energy converter power production estimates are quantified. Finally, an introduction to marine spatial planning provides a framework within which to identify locations of interest for wave energy conversion. This chapter provides a detailed framework for baseline and higher-order resource assessment methodologies to provide policymakers with the necessary resource and uncertainty data to help nurture the nascent industry, provide developers with compulsory knowledge required to design wave energy converters, and allow utilities to design large-scale energy systems for grid integration of wave-generated energy.

Ocean wave energy has become the focus of governments and energy companies over the past decade. In spite of its unpredictability, this untapped source of energy appears to be a sustainable alternative to traditional sources of energy such as thermic and nuclear energies, or hydropower, all of which pose significant environmental and geopolitical problems. Open to the Atlantic Ocean at latitudes between 35°N and 65°N, the Atlantic Coast of Europe is blessed with one of the highest wave powers in the world—estimated to be between 33 and 76 kW/m wave crest. The European Commission has taken a proactive attitude towards encouraging and promoting the development of marine renewable energy during the near future. In this context, the European transnational project EnergyMare was commissioned to investigate the potential of marine renewable energy resources on the European Atlantic Coast as well as test innovative measurement techniques and promote the development of test sites. The targeted wave energy resources were assessed via a 10-year hindcast, using state-of-the-art spectral wave models WaveWatch III and SWAN set up on unstructured meshes or fine-resolution regular grids. The hindcasts were combined to simultaneously provide a holistic view of the wave energy distribution across the European continental shelf and fine-resolution maps of specific areas, in particular around archipelagos and complex coastlines, where wave characteristics can be affected by the presence of small islands, headlands, or irregular bathymetry, and at wave energy test sites. The domain size and timescale of the hindcasts enable a comprehensive description of the wave climate along the European Atlantic Coast, both in terms of its distribution and its seasonal and interannual variations. In particular, a comparison of wave activity at various coastal locations shows its dependence on latitude and arguably its more significant dependence on exposure to open Atlantic waters. Wave activity during the winter months is clearly predominant, but dominant peak activity was also occasionally observed during spring and autumn. In spite of increased winter wave activity over the past couple of years, data are insufficient to enable conclusions to be made about a persistent trend in the international wave climate. Continental-scale mapping of wave energy resources together with fine-resolution mapping of coastal areas provides an overview of the wave resources to help identify the best areas for energy or test sites. Such mapping also provides information about local wave characteristics and resources that can be used for diminishing installation risks or optimising a site by selecting the most appropriate devices or array configurations. In addition to evaluating wave resources, fine estimates of energy yield from a site may require a good understanding of the wave interaction in an array of converters where significant wave interference may be induced. Finally, long-term trend estimates or periodic re-evaluations of wave resources to address potential wave climate change will probably be necessary to achieve sustainable wave energy exploitation.

Knowledge of the effects of wave energy converters (WECs) on the near and far wave fields is critical to the efficient and low-risk design of waveforms. Several computational wave models enable the evaluation of WEC array effects, but model validation has been limited. In this chapter, we validate two popular models with very different formulations: the phase-resolving model WAMIT and the phase-averaged Simulating WAves Nearshore (SWAN) model. The models are validated against wave data from an extensive set of WEC array laboratory experiments conducted by Oregon State University and Columbia Power Technologies, Inc (CPT). The experimental WECs were 1:33 scale versions of a commercial device (CPT “Manta”), and several different WEC array configurations were subjected to a range of regular waves and random sea states. The wave field in the lee of the WEC arrays was mapped, and the wave shadow was quantified for all sea states. In addition, the WEC power capture performance was measured independently via a motion-tracking system and compared to the observed wave energy deficit (i.e., the wave shadow). Overall, WAMIT displays skill in predicting the wave field both in offshore and in the lee of the WEC arrays. WAMIT simulations demonstrate partial standing wave patterns that are consistent with the observations. These patterns are related to wave scattering processes, and their presence increases the magnitude of the wave shadow in the lee of WECs. The pattern is less pronounced at longer wave periods where WECs behave more like wave followers. In these situations, the wave shadow is primarily controlled by the WEC energy capture and less so by scattering. The SWAN model accounts for the frequency-dependent energy capture of the devices and performs well for cases when the wave shadow is primarily controlled by the WEC energy capture. For regular wave cases, inclusion of the wave diffraction process is necessary, but SWAN simulations for wave fields with frequency and directional spreading capture the general character of the wave shadow even without diffraction. Finally, we suggest that WECs designed to operate such that the expected significant wave energy lies at periods near, or larger than, the period of peak energy extraction will minimize the wave shadow effect for a given gross extraction of wave energy, which leads to more efficient arrays with respect to environmental impact.

Hyrdokinetic tidal energy is the conversion of tidal current kinetic energy to another more useful form, frequently electricity. As with any other form of renewable energy, resource assessments are essential for the tidal energy project planning and design process. While tidal currents have significant spatial and temporal variability, the predictability of tidal flows makes deterministic modeling a suitable methodology for hydrokinetic tidal energy resource assessments. The scope (theoretical, technical, or practical resource) and scale (turbine, region, or project) of the assessment determine the basic concepts and methodology to be utilized and are described in this chapter. At the turbine scale, the technical resource is frequently quantified as the annual energy production (AEP) computed based on the velocity probability distribution for the specific location as well as the turbine properties. The uncertainty associated with the estimates of the AEP is highly dependent on the accuracy of the tidal constituent amplitudes and phases. Regional resource assessments are frequently used to determine the feasibility of tidal power at the scale of an estuary, using numerical models to predict the spatial distribution of the power density. In addition, simplified models or even analytical analysis can be done to produce an upper bound on the regional theoretical power, although with a high level of uncertainty due to the simplifications and assumptions. Resource assessments at the project scale provide both the theoretical and the technical energy as well as the practical energy accounting for many additional constraints, including social, economic, and environmental restrictions. The International Electrotechnical Commission technical specification for tidal energy resource assessments (IEC 2015) provides the essential guidelines for performing project-scale resource assessments. These guidelines include minimum grid resolution requirements as well as model calibration and validation procedures. In addition, larger projects will need to include the effect of energy extraction on the flow field to produce more accurate estimates of velocity probability distributions for computing the technical resource. An example case study demonstrating a regional feasibility and project-scale resource assessment is presented in this chapter.

When conducting tidal energy resource characterization and assessment, it is important to capture the strong variations of tidal currents in time and space. Field measurements can quantify many of these variations, which have both deterministic and stochastic components. The deterministic components occur on timescales of hours to years. As such, they are repeatable and well-suited to harmonic analyses associated with astronomical tidal forcing. The stochastic components are well-suited to statistical descriptions of fluid turbulence, from the short scales (milliseconds and millimeters), where dissipation occurs, to the long scales (seconds and meters), where large eddies occur. While the resolution of deterministic components may be adequate for characterizing annual energy production, both components need to be quantified to determine design loads on tidal energy conversion devices. In addition to the direct utility of field measurements to characterize and assess the tidal energy resource, field measurements are also essential to validate computational models used to assess the resource over large spatial domains.

Some regions of the world concurrently experience a high wave and a high tidal energy resource. These regions include the seas of the northwest European continental shelf, the Gulf of Alaska, New Zealand, northwest Australia, and the Atlantic seaboard of Argentina. Due to the interaction of waves and tides, special consideration needs to be given to resource characterization of marine renewable energy schemes developed in such regions. Waves have been shown to reduce the tidal current, which, because tidal-stream power is proportional to the cube of velocity, reduce the available energy resource. Further, waves can reduce the tidal-stream energy resource during extreme wave periods when ocean renewable devices may not operate. Waves should be also considered in the design and resilience of tidal-stream energy devices. Hence, waves can have a critical effect on the planning, operation, maintenance, and resource assessment of tidal energy sites. Conversely, tides can significantly alter wave properties through various wave-current interaction mechanisms. For example, tidal currents can alter wave steepness which is an important consideration in the design of marine energy mooring. Wave power, in general, is proportional to the wave group velocity and the wave height squared, both of which change in presence of tidal currents. Therefore, resource assessments of such regions should account for the way that one marine energy resource affects another at a variety of timescales from semidiurnal, spring-neap, to seasonal. Finally, wave-current interaction processes affect turbulence, and the dynamics of sediment transport; therefore, they should be considered when the impact of an energy device, or an array of such devices, on the environment is studied. This chapter introduces the basic concepts of wave-tide interaction in relation to the ocean renewable energy resource assessment. Various aspects of the marine renewable energy industry that are affected by wave-tide interactions, such as resource assessment and the influence of wave-tide interactions when characterizing the oceanographic site conditions, are discussed. Methods ranging from simplified analytical techniques to complex fully coupled wave-tide models are explained.

In this chapter, a general overview and characterization of ocean current resources for potential power generation in the global oceans are presented. They are based on analysis of two relatively long data sets of surface drifter-observed mixed-layer current and the satellite altimeter-derived surface geostrophic current. Spatial and temporal variations of the four most prominent western boundary currents—the Kuroshio, Mindanao, Gulf Stream, and Agulhas currents—and their available mean undisturbed ocean power densities are discussed. Several potential sites for ocean current power generation in the North Pacific, South China Sea, and Oceania are identified based on a criterion formulated by combining the frequency with which the ocean currents occur, the magnitudes of their speed, the water depths at which they occur, and their distances from the shore.

The potential for energy extraction from the fast-flowing Agulhas Current along South Africa’s East Coast is examined. Potentially suitable regions are evaluated using state-of-the-art satellite remote-sensing, predictive modelling, and in situ observation technologies. A mid-shelf location (91 m depth) and an offshore location (255 m depth) at approximately 32.51°S and 28.83°E are evaluated using these tools, and it is found that the current core borders on the mid-shelf location and passes over the offshore location with mean velocities of 1.34 m/s and 1.59 m/s, respectively, at the 30 m depth. Current velocity data derived from satellite remote-sensing and predictive models were compared to in situ current measurements from Acoustic Doppler Current Profilers to determine their ability to accurately capture current velocities for future use in the evaluation of energy extraction sites. Although the modelled data’s representation of the Agulhas Current’s velocities was a better comparison than the satellite product, the predictive model was less representative of the variability in the Agulhas Current. Further examination of the data showed that both the satellite and the predictive model are only able to accurately capture variability in the Agulhas Current on time scales longer than monthly. Despite this, the data provide useful insight into the unique challenges encountered when exploiting the Agulhas Current as a resource for energy generation; in particular, the irregular occurrence of large Agulhas Current meanders (known as Natal Pulses). The proposed energetic region is well positioned with respect to environmental, economic, and social aspects because the nearest medium voltage substation is 30 km from the point of contact at the coastline. The sites are not located within any existing or proposed marine protected areas or prime fishing grounds. If the mooring challenges in water depths of 250 m or greater are overcome, then such a turbine array can make a significant contribution to the South African electricity grid.

Ocean basin scale wind-driven currents provide a possible source of renewable energy using ocean turbine technology to convert kinetic energy of the flow to electricity. The Gulf Stream System in the North Atlantic Ocean is part of one of the largest subtropical gyres in the world. Within these gyres, the western intensification due to the Coriolis force produces some of the largest and most persistent ocean currents. This chapter discusses the potential for generating energy from the Gulf Stream System with a particular focus on the Florida Current portion. The overall characteristics related to the energy potential of the Gulf Stream are described based on 7 years of model simulations and 30 years of volume flux observations across the Florida Straits. Within the Florida Current portion of the Gulf Stream System, the mean kinetic power is found to be over 22 GW with a standard deviation near 6 GW. However, this variability was found to be contained within the top 100 m of the water column. Assessments based on the undisturbed flow indicate that deployment on the order of 5000 turbines could average over 5 GW of power. To quantify the effects of the energy extraction on the circulation to obtain a better estimate of the available power, idealized and realistic modeling of the ocean circulation are presented. The idealized model indicates that a mean of 5 GW of power could be dissipated within the Florida Straits, with much more power dissipated if broader regions are considered for energy extraction. However, the practical constraints on ocean current energy extraction, such as the acceptable range of impacts on the flow as seen in a realistic 3D ocean model simulation, lead to a reduction in the assessment of the power available.

There has been global interest in renewable energy for meeting energy demands, and as these demands increase, it will become of greater importance to utilize low-carbon energy sources to mitigate anthropogenic impact on the environment. Onshore hydropower is responsible for half of the electricity generated by a renewable source in the USA. In the ocean, marine hydrokinetic (MHK) energy in western boundary currents (WBCs) can be considered for electricity generation by submarine turbines. WBCs are a continuous and sustainable source of energy that could be transmitted to shore to support coastal communities in future years. The Gulf Stream is the WBC of the North Atlantic subtropical gyre, and it flows for part of its course along the upper continental slope off the southeastern USA. This large-scale current has maximum flow speeds exceeding 2 m s⁻¹, and this together with its proximity to the coastline distinguishes it as a potential source of MHK energy. Using current data from a moored acoustic Doppler current profiler (ADCP) and a regional ocean circulation model, MHK power densities offshore of North Carolina were found to average 798 W m⁻² for the ADCP and 641 W m⁻² for the model during a nine-month period at a potential turbine site, a difference of about 20%. The model was shown to have similar current speeds to the ADCP for slowly varying currents (fluctuations of weeks to months due to Gulf Stream path shifts), and lower speeds for higher frequency current variations (fluctuations of several days to a couple of weeks due to wavelike Gulf Stream meanders). This article considers the Gulf Stream as a prospective renewable energy source and assesses the power density of this WBC at multiple locations offshore of North Carolina. Understanding the Stream’s power density character, including its spatial and temporal variations along the North Carolina coast, is essential in considering the Gulf Stream as a future alternative energy resource.

Over the last decade, many studies have been conducted to estimate the upper limit of the theoretical resource of tidal stream energy and its associated influence on volume flux. However, studies aimed at evaluating the effects of tidal energy extraction on water exchange and transport timescale have been limited. This chapter provides a detailed review of different methods—from analytical methods to advanced three-dimensional numerical models—for quantifying the far-field environmental impacts of tidal stream energy extraction. Case studies from an idealized tidal channel–bay system and a realistic site in the Tacoma Narrows of Puget Sound, Washington State, USA, are given to illustrate the modeling approach for assessing the impacts of tidal stream energy extraction on flushing time using a three-dimensional numerical model. Model results indicated that the change in flushing time is approximately linearly proportional to the volume flux reduction when the relative change in volume flux is small. However, the rate of change in flushing time is several times greater than that of volume flux reduction. The present study demonstrates that flushing time can be used as a unique parameter for quantifying the environmental impacts of tidal stream energy extraction on water exchange in coastal waters.

The extraction of marine energy, through either tidal or wave array operation, will clearly influence the hydrodynamics of a region. Although the influence on tidal currents and wave properties is likely to be very small for most extraction scenarios, the influence on bed shear stress is likely to be greater, because bed shear stress is quadratically related to tidal currents and wave orbital velocities. Further, the transport of sediments is a function of tidal current and wave orbital velocity cubed. Therefore, even small modifications to the flow field through tidal or wave array operation could lead to significant impacts on regional sediment dynamics. In this chapter, after providing an introduction to sediment dynamics in the marine environment, we explore the impact of tidal energy devices/arrays on regional sediment dynamics, with a particular emphasis on offshore sand banks—important sedimentary systems that protect our coastlines from the full impact of storm waves. Next, we discuss how generating electricity from waves could influence nearshore sediment processes, such as beach erosion or replenishment, over a range of timescales. To assess the magnitude of impacts on sedimentary systems, it is essential to consider the scale of the impact in relation to the range of natural variability. We suggest ways in which impacts can be assessed using numerical models, tuned by in situ measurements, that quantify variability over a range of timescales from individual storm events and lunar cycles to seasonal and interannual periods. We also discuss the sedimentary processes associated with tidal lagoons, such as scour and sediment drift outside a lagoon and sediment accretion inside a lagoon.

This chapter describes the utilization of underwater acoustic models for the evaluation of marine-system noise impacts associated with the installation and operation of marine-hydrokinetic energy (MHK) devices, particularly in coastal oceans. Coastal environments are generally characterized by high spatial and temporal variabilities, which make them very complex acoustic environments. Underwater acoustic models serve as enabling tools for assessing noise impacts on marine systems by generating analytical metrics useful in managing coastal resources. This review is set in the context of an underwater soundscape, which is a combination of sounds that characterize, or arise from, an ocean environment. The study of a soundscape is sometimes referred to as acoustic ecology. Disruption of the natural acoustic environment results in noise pollution: The potential effects of anthropogenic sound sources on marine mammals and fish that could include auditory damage. Changes in the ocean soundscape have also been driven by natural factors arising from climate change, including ocean acidification. The field of underwater acoustics enables us to observe and predict the behavior of this soundscape and the response of the natural acoustic environment to noise pollution. Underwater acoustics entails the development and employment of acoustical methods to image underwater features, to communicate information via the oceanic waveguide, or to measure oceanic properties. Modeling tools traditionally used in underwater acoustics have undergone a necessary transformation to respond to the rapidly changing requirements imposed by this dynamic soundscape. Additional advances have been achieved using energy-flux techniques that can simplify the interpretation of sound-channel models. Nonintrusive measurement approaches include new acoustic-transmission options to minimize impacts on aquatic life. Applied underwater acoustic modeling technologies have further evolved over the past several years in response to new regulatory initiatives that have placed restrictions on the uses of sound in the ocean. The mitigation of marine-mammal endangerment is now an integral consideration in acoustic-system design, installation, and operation. Marine-mammal protection research has focused on simulating anthropogenic sound sources including seismic-exploration activity, merchant shipping traffic, and a new generation of multistatic naval sonar systems. Additional sources derive from ocean-renewable energy resources, including the deployment of wind farms, tidal turbines, and wave-energy devices. Many underwater acoustic models presently used in environmental impact assessments consider only the sound-pressure component of sound, which is the means by which marine mammals hear; however, the primary mechanism by which fish and invertebrate species detect sound is through the particle-motion component of sound. To assist practitioners in the proper usage of acoustic models for assessing the impacts of MHK device noise on marine systems, selection guidance is provided for the current inventory of underwater acoustic propagation and noise models.

The acoustic characteristics of marine energy converters are of interest to those attempting to quantify their environmental effects at larger scale. Such efforts are complicated by the time variation in marine energy converter sound caused by changes in the environmental forcing and converter operation, the difficulty of identifying marine energy converter sound amidst background noise from a range of sources, and the potential masking of marine energy converter sound by non-propagating flow-noise. This chapter discusses each of these challenges and proposes potential solutions to overcome them in a cost-effective manner.

Renewable ocean energy has huge potential to contribute to addressing both climate change and energy security concerns. To realise this potential, it is necessary to have planning and management frameworks that facilitate development of commercial-scale marine renewable energy farms, which harvest offshore wind, wave, and tidal energy. The primary focus of this chapter is ocean energy, namely wave and tidal sources. Currently, consenting and legal processes are often cited as a barrier to efficient and expedient deployment of devices in many locations internationally. This can create high levels of “regulatory risk” which can, in turn, have detrimental consequences for project development timelines and budgets as well as wider negative influence on project investors and financiers. Maritime Spatial Planning (MSP) is a relatively new approach to analysing and allocating parts of marine spaces for specific uses or objectives in order to achieve ecological, economic, and social objectives. MSP does not always result in ocean zoning but instead involves integrated approaches to prioritising uses and activities. As a process, MSP is ecosystem-based, integrated, adaptive, strategic, and participatory—stakeholders are actively engaged in the process. It does not replace single-sector planning or management, but it has a number of advantages that may benefit the development of the renewable ocean energy sector. It can provide greater certainty to the private sector in planning new investments and should reduce conflicts between incompatible users and activities. It should also promote more efficient use of marine resources and space, indicate opportunities for coexistence of activities, and facilitate the implementation of a streamlined permitting process for marine activities. This chapter outlines the planning and management frameworks in place for renewable ocean energy in countries that collaborate through the International Energy Agency’s Ocean Energy Systems Technology Collaboration Programme around the world. A particular emphasis is placed on MSP and how it influences the planning of energy activities currently or how it will influence future ocean energy activity. Implementation of MSP varies from jurisdiction to jurisdiction and can take many different forms. This chapter provides an overview of how the requirements of the ocean energy sector are taken into account when designing marine planning systems, how scientific information is reflected in the process, and the tools used to implement MSP. It also identifies how possible or currently experienced conflicts between different sectors or users are managed. The chapter concludes with a section on the key factors that limit implementation of MSP.