In the OSTI Collections: Ocean Wave and Tidal Power

Dr. Watson computer sleuthing scientist.

Article Acknowledgement:

Dr. William N. Watson, Physicist

DOE Office of Scientific and Technical Information

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Hydroelectric power, produced by generators spun by rivers flowing through dams, is ultimately driven by gravity pulling the water downhill.  Electricity can also be generated by the movement of ocean water.  In this case two drivers other than downhill motion are at work. 

 

One driver is the wind that stirs up waves over the ocean’s surface.  The wave motion is a product of individual water molecules’ slowly drifting circulatory motion in a vertical plane while the wave itself—a pattern of motion—rapidly moves forward, transferring energy and momentum forward from one circulating water molecule to the next.[Wikipedia]  As a carrier of energy and momentum, the wave motion can be tapped to, e.g., drive generators to make circulating motions of their own.[Wikipedia]

 

The other driver has a different mechanism.  The ocean would have a more uniform level over the earth’s surface were it not for variations in the gravitational fields of other celestial bodies, particularly those of the moon and sun.  The part of the ocean closest to the moon is more strongly attracted to the moon than the slightly more distant ocean floor directly below it, and the part of the ocean floor furthest from the moon on the earth’s other side is more strongly attracted to the moon than the part of the ocean directly over it.[Wikipedia]  The entire ocean thus has an overall ellipsoidal shape instead of a spherical one, with a higher level along the ellipsoid’s long axis, roughly a line from the earth to the moon, and with a lower level along directions perpendicular to that axis.  To a smaller extent the sun also raises the ocean level along a second axis, but the effect is smaller because, although the sun’s overall gravitational strength is greater than the moon’s, the strength difference in the sun’s gravity from one side of the earth to the other is smaller than the strength difference in the moon’s gravity.  The effect of the sun is therefore to reinforce the moon’s effect on the ocean level when the sun, moon, and earth are mostly in line, and to reduce the moon’s effect when the sun-earth line and moon-earth line are more perpendicular.[Wikipedia]  Similar effects due to other celestial bodies are negligible. 

 

As the earth (and its overlying ocean) rotates beneath the sun and moon, the ocean doesn’t just fall and rise to its natural levels without anything else happening:  where the ocean level drops, some water has to move out to surrounding regions, and where the ocean level rises, water has to move in from elsewhere.  This motion constitutes the ebb and flow of the tides.  Because water has mass and viscosity, the ebb and flow don’t instantaneously follow the pull of the moon and sun; their timing is complicated further by the irregular shape of the ocean floor.[Wikipedia]  Even so, the result is that throughout the ocean, tides come in and go out about once or twice a day[Wikipedia]—in strong currents that can drive suitably-placed electric generators.[Wikipedia] 

 

Using ocean waves and tides for power generation has potential comparable to the harnessing of river flow.  According to one report (“Direct Drive Wave Energy Buoy”[SciTech Connect], p. 8), wave energy alone “has the potential to match or exceed that currently provided by conventional hydroelectricity”; “[a]ccording to the DOE, water power resource energy production can provide 15% of present U.S. electricity consumption by 2030”.  This is similar to an estimate by the Electric Power Research Institute which, according to another report (“Reedsport PB150 Deployment and Ocean Test Project”[SciTech Connect], p, 6), “conservatively estimated that 20% of all U.S. electricity could be generated by wave energy”, though this estimate wasn’t quoted with a date.  But using ocean waves and tides for power generation also presents opportunities and problems that differ from those of harnessing river flow.  Some of these are described in recent reports from investigators who have been addressing them. 

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Principles

 

Differences between using tides and river flows as power sources are discussed in the report “Power Generation for River and Tidal Generators”[SciTech Connect], along with the presentation “Turbine Control of a Tidal and River Power Generator”[SciTech Connect] written for the 2016 IEEE North American Power Symposium.  The reports describe how tidal power can be harnessed like river power, by damming up water as the tide comes in and releasing the water through generators as the tide goes out, and mention how both directions of tidal motion can be harnessed through other arrangements.  The reports also compare both tidal and hydroelectric power with solar and wind power, which have faster but less steady energy inputs.  Wind power systems, particularly, are similar enough to hydroelectric and tidal systems that the same equations (with different parameters) can be used to describe them. 

 

Figure 1.  Machines for harnessing tidal power may be similar to those used to harness wind power; the machines’ parameters will differ since they extract energy from fluids with different properties.  Left, a Gorlov-type wind turbine.  Right, a set of Gorlov-type tidal turbines.  (“Power Generation for River and Tidal Generators”[SciTech Connect], pp. 1, 3.)

 

A turbine sped up too fast may be stressed to the point of damage.  In the symposium presentation, the authors describe methods adapted from wind-turbine technology for preventing this in water turbines as they operate at or near maximum efficiency when rotating at safe speeds.  The methods depend on how a turbine’s output power rises with increasing rotational speed up to a point of maximum efficiency, and falls with increasing speed beyond that point.  The turbine can thus operate efficiently at safe speeds but not run away if the water speed changes suddenly.  Since the water speed normally won’t change suddenly, the authors also showed how a turbine could be safely run even closer to its maximum theoretical efficiency if the water speed is known to have a specific maximum rate of increase.  The authors’ other report goes further to describe devices for converting turbines’ rotational energy into electric power.  The conversion begins with the use of strong rare-earth permanent magnets attached to the turbines to generate current with small generators, which in turn is run through electronics to efficiently transfer energy to an onshore power grid.  The authors made a detailed mathematical model to investigate the sizing of power converter components, the level of power quality, failures of electronic components and protection systems, power-plant and system dynamics, and turbine and power-plant interactions. 

 

Investigations of wave-power systems can also be aided by mathematical models.  Models constructed for that purpose are described in three presentations to the 35th International Conference on Ocean, Offshore and Arctic Engineering held at Busan, Korea in June 2016. 

 

Two of the presentations describe different tests of an open-source model devised by people at Sandia National Laboratories and the National Renewable Energy Laboratory to simulate how wave energy converters (WECs) function in ocean waves whose sizes are within and beyond the converters’ operating range.  “WEC-SIM Phase 1 Validation Testing—Numerical Modeling of Experiments”[SciTech Connect] tells how this mathematical model was checked against the behavior of a physical 1:33-scale model of a floating oscillating surge wave converter.  The scale model was put through heave, surge, and pitch experiments in which it was respectively lifted from the water to various heights, pulled sideways against resistance, or raised on one side (or had one flap raised) to different angles, and then released.  All these experiments were also simulated with the “WEC-Sim” mathematical model.  WEC-Sim’s calculations accurately matched some of the scale model’s behaviors but not others.  “Coupled Mooring Analyses for the WEC-Sim Wave Energy Converter Design Tool: Preprint”[SciTech Connect] describes how WEC-Sim was combined with a mathematical model of a wave converter’s mooring system called MoorDyn, and how this combined mathematical model was compared with a different commercial one, and with experiments on a 1:25-scale model of a floating power system connected to a three-point mooring.  Simulating the mooring’s behavior is difficult since fatigue forces and nonlinear hydrodynamics make a given mooring’s effects hard to determine, but doing so is important since the mooring design can not only affect how efficiently the wave converter functions, but whether it functions at all in certain circumstances.  The scale model was subjected to heave, surge, and pitch experiments in still water, regular waves, and irregular waves, and simulations of the experiments by both the combined WEC-Sim/MoorDyn model and the commercial model were found to compare well with them. 

 

Figure 2.  Design of 1:33-scale model of the floating oscillating surge wave energy converter discussed in “WEC-SIM Phase 1 Validation Testing—Numerical Modeling of Experiments”[SciTech Connect] (p. 3). 

 

The other conference presentation[SciTech Connect] was about a different simulation approach, focused on what is often the main contributor to the expense of a wave energy converter:  extreme loads on the converter induced by waves.  Extreme loads occur randomly when suitable waves encounter the converter in an appropriate position.   Predicting the size of those loads is critical for designing converters, and often requires experiment and computational analysis, but a computer search through all the wave environments that are likely to produce extreme loads takes a great deal of time.  The presentation describes how a less computer-intensive “most likely extreme response” method was used with WEC-Sim to see what five different extreme-wave conditions would do to a floating ellipsoid moored to the ocean floor.  Each condition is a possible “100-year” event as estimated from data taken near Humboldt Bay, California.  For this initial study, the most likely extreme response method was found to be a viable way to economically identify and analyze extreme loads, but the presentation notes that this type of method has been shown “less accurate when the load response depends heavily on the instantaneous wave train, vessel position, and its deformation, unless a random background wave was used” in the simulation.  Still unknown was the method’s accuracy for cases of significant complex nonlinear wave-object interactions. 

 

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Converters

 

Two other presentations to the same conference deal more with particular converter designs than with general design tools.  For both of these, earlier analyses of a wave converter’s performance were refined to describe its operating conditions more realistically. 

 

One presentation addresses whether a particular method for controlling the output-power/hydrodynamic-load ratio of an oscillating-surge wave energy converter would maintain the same ratio in an environment of irregular ocean waves as it would in a more easily analyzed environment of regular waves.  “Balancing Power Absorption and Fatigue Loads in Irregular Waves for an Oscillating Surge Wave Energy Converter”[SciTech Connect] illustrates the converter (Figure 3) and outlines a detailed calculation to demonstrate that the control method would, in fact, still work.  The other presentation, though similarly titled (“Balancing Power Absorption and Structural Loading for an Assymmetric Heave Wave-Energy Converter in Regular Waves”[SciTech Connect]), is about a different calculational refinement for a quite different wave-conversion device called “the Berkeley Wedge” (Figure 4).  This converter, which only moves vertically, is designed to minimize viscosity effects on its motion and thus capture almost all the energy of incoming waves.  (The ocean beyond is thus made calm, so the wedge also functions as a breakwater.)  Previous attempts to calculate how the wedge’s controller could maximize its power output had not involved determining its structural loading, making it unclear whether maximizing the wedge’s power output would stress it excessively.  The presentation shows how calculations that include stress indicate a need for a different control strategy. 

 

 

Figure 3.  The device described in “Balancing Power Absorption and Fatigue Loads in Irregular Waves for an Oscillating Surge Wave Energy Converter”[SciTech Connect] (pp. 2, 3).  Left, fully open and fully closed configurations; right, coordinate system used in mathematical analysis of the converter’s behavior. 

Figure 4.  Two-dimensional shape of the vertically-moving “Berkeley Wedge” described in “Balancing Power Absorption and Structural Loading for an Assymmetric Heave Wave-Energy Converter in Regular Waves”[SciTech Connect] (p. 3). 

 

The two reports that were quoted above in the introduction describe different projects to develop buoys for converting wave power. 

 

“Reedsport PB150 Deployment and Ocean Test Project”[SciTech Connect] is about a partially successful project, conducted by Ocean Power Technologies, Inc. with Department of Energy sponsorship, to refine and field-test the company’s PowerBuoy® wave converter design.  This project ran into difficulties that made it untenable past a certain point.  Controlled descent of the wave system’s first anchor encountered complications that were compounded by problems with recovering the anchor, regulatory and financing issues, project delays, and the project’s power takeoff system being made obsolete by advances from other company projects.  So by mutual agreement between the company and the Energy Department, the project was terminated with only some of its objectives met.   Among those that were successfully met were the use of acoustic and electromagnetic-field data to determine (a) the baseline noise levels at the intended buoy deployment site—important for determining how much or little a deployment changes the noise level—and (b) the electromagnetic fields that would be generated by ocean currents and waves in the site’s local terrestrial magnetic field.  Another accomplishment was an analysis of different manufacturing options to determine how the converters could be better manufactured and transported while meeting their load-stress and fatiguing-stress requirements.  The report also discusses several lessons learned and says “[t]he most important ones are related to the challenges of deploying new technologies in the open ocean environment … such as complex logistics, regulatory maturity, qualified supply base and services providers, etc.”, even if the technologies have been used in other open-ocean industries. 

 

An earlier-stage project by Columbia Power Technologies—a demonstration of a utility-scale ocean wave energy converter connected to a power grid—is described in the report “Direct Drive Wave Energy Buoy”[SciTech Connect].  This work involved designing a full-scale converter with the aid of computer models and information from physical 1:33-scale model tests in a large Oregon State University wave tank, having the design reviewed and certified, and arranging for the manufacture, transport, and installation of one full-scale converter for field tests.  The project was completed in the face of its own difficulties.  Based on lessons learned from them, the report recommends facilitating future development of wave-energy converters by developing accurate, appropriately sized and rated power-takeoff systems along with programmable mooring controllers for scale-model tests, and making the certification process for developing technology clear and aligned with the expectations of the developers’ sponsors before development begins. 

 

 

Figure 5.  Wave-energy converter designed by Columbia Power Technologies.  (“Direct Drive Wave Energy Buoy”[SciTech Connect], pp. 9 and 10 of 37.) 

 

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Sites

 

The wave tank at Oregon State is part of the equipment of the Northwest National Marine Renewable Energy Center (NNMREC)[OSU, UW, UAF], a joint effort of Oregon State, the University of Washington, and the University of Alaska Fairbanks funded by the Department of Energy to facilitate marine-energy technology development, inform regulatory and policy decisions, and close key gaps in scientific understanding.  Many of the NNMREC’s goals and accomplishments are summarized in a 106-page report[SciTech Connect], some of which are specifically focused on in ten appendices that are available as a single additional file[SciTech Connect].  These include building and operating the test facility that includes the wave tank, advancing technologies for wave forecasting, increasing the reliability and survivability of marine energy systems, and investigating marine energy technologies’ compatibility with the environment, fisheries, and other marine resources. 

 

This last item, as dealt with by NNMREC, is described in one of the just-mentioned appendices (Appendix E).  NNMREC’s investigations included observations of spatial and temporal variations in habitat features and biological assemblages at one of their offshore test sites, mathematical modeling of converters’ immediate effects on the ocean-wave field and resulting secondary effects on the shoreline, assessment of how mooring equipment for wave-power facilities affects a site’s ambient noise, and both observation and mathematical modeling of noise from a functioning tidal turbine. 

 

Information like this matters, since a power facility’s environmental compatibility properly figures into whether to permit its construction.  But when this information is scarce, decisions to permit or not permit even an experimental facility have little basis.  Maximizing the usefulness of existing information about ocean-wave and tidal power converters was the goal of a series of workshops held in 2014 and 2015.   A summary of one of these workshops, “A Review of the Environmental Impacts for Marine and Hydrokinetic Projects to Inform Regulatory Permitting: Summary Findings from the 2015 Workshop on Marine and Hydrokinetic Technologies, Washington, D.C.”[SciTech Connect], was published by the National Renewable Energy Laboratory in July 2016.  “The general conclusion,” according to the report,

 

“was that early device deployment baseline and monitoring requirements were not necessarily commensurate with the uncertainties and potential risks of specific small-scale deployments and that in some instances, too much emphasis has been placed on studying potential effects of large-scale array deployment as compared to assessing observable and likely impacts given the current deployment of single devices and small-scale arrays.  At the same time, given the early stage of the technology’s development, the group discussed the need for continued research by government and academia to gain further understanding of potential project impacts at larger commercial scales.

 

“Given the current understanding of risk levels associated with small and temporary project development, assessments of perceived risk and monitoring needs, combined with actionable adaptive management strategies so that any issues that are identified can be rectified or mitigated, should be applied.  Without such an approach, the ability of [marine and hydrokinetic] technologies to develop through a process of single-device and small-scale serial deployment, leading to a low-cost and low-impact technology that can be deployed at commercial scale, will be very difficult.” 

 

An additional two-part report[SciTech Connect; SciTech Connect] from the National Renewable Energy Laboratory deals with deciding where to extract wave and tidal power next, based on market-opportunity studies of these power sources along United States coastlines.  According to both parts of the report, the costs of large-scale grid-connected projects to convert ocean-wave and tidal energy were poorly defined.  “Ideally,” say both parts’ summaries, “device designers would like to know the resource conditions at economical project sites so they can optimize device designs.  On the other hand, project developers need detailed device cost data to identify sites where projects are economical.  That is, device design and siting are, to some extent, a coupled problem.”  Accordingly, the report provides site data that seem likely to be important for all types of wave or tidal converters:  power density of the waves or tides, market size, energy price, power-transmission distance, shipping cost, and water depth.  The data were used to score each site’s likelihood for short-term and long-term development, with the scoring for long-term development leaving out energy price as a criterion.  Data for items other than the aforementioned ones were not included for either scoring because of their unavailability or their unequal relevance for different technologies.  Thus the site rankings are intended as a high-level guide that can be improved on as the ocean-power converter industry matures and converges around “a subset of device archetypes with well-defined costs”. 

 

The scoring for wave-energy sites led to the following conclusions: 

 

“In both [short- and long-term] scenarios, Hawaii and the Pacific Northwest (Northern California, Oregon, and Washington) rank at the top of the lists.  Hawaii ranks highest in the short-term scenario because it has high energy costs.  In the long-term scenario, Oregon ranks highest because it has a large market and an energetic resource.  Several East Coast states and Puerto Rico are also identified as potential wave energy deployment sites if technological innovations make it possible to efficiently generate electricity from the modest resource there.  There are also several small-market sites in Alaska and U.S. Pacific Islands that rank particularly well in the short-term analysis due to their high energy prices.  These locations may represent opportunities to demonstrate economical wave energy conversion as a stepping-stone to larger markets.” 

 

Because tidal energy density is more spatially localized than wave energy, sites favored for tidal power development were more specific:  

 

“The majority of the U.S. tidal energy resource is found in the following regions:  1) the southern coastline of Alaska; 2) sections of the northeastern coast, especially Maine and Massachusetts; 3) Puget Sound, Washington.  Top sites in these regions—the Western Passage in Maine, Tacoma Narrows in Washington, and Cook Inlet in Alaska—rank consistently in the top five for both the short-term and long-term scenarios.  Outside of these regions, there are several sites—including San Francisco Bay, the Florida Keys, and the estuaries of many U.S. rivers—that may become economical in the long-term, especially if technology emerges that is capable of efficiently harnessing energy from low-velocity sites.  In the short-term scenario, sites surrounding Nantucket Sound, Massachusetts (i.e., south of Cape Cod) are also particularly attractive.” 

 

Thus these may be the next sites where Americans will try to produce power from the ocean’s waves and tides.

 

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References

 

Data collected using funds from the Energy Department’s Wind and Water Power Programs can be found in the Marine and Hydrokinetic Data Repository (MHKDR), and through searching the DoE Data Explorer for the Repository’s name or acronym

 

Wikipedia

 

Wind wave: Physics of waves

Wave power

Tide: Forces

Tide: Range variation:  springs and neaps

Tide: Timing

Tide: Characteristics

Tidal power

 

Reports available from OSTI’s SciTech Connect

 

“Direct Drive Wave Energy Buoy” [Metadata]

2016-08-22

Columbia Power Technologies, Inc.

 

“Reedsport PB150 Deployment and Ocean Test Project” [Metadata] (Large File)

2016-06-03

Ocean Power Technologies Inc.

 

“Power Generation for River and Tidal Generators” [Metadata]

2016-06-01

National Renewable Energy Laboratory

Ocean Renewable Power Company

 

“Turbine Control of a Tidal and River Power Generator: Preprint” [Metadata]

2016-08-01

National Renewable Energy Laboratory

Ocean Renewable Power Company

 

“WEC-SIM Phase 1 Validation Testing -- Numerical Modeling of Experiments: Preprint” [Metadata]

2016-08-01

Presented at the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2016), 19-24 June 2016, Busan, South Korea

Sandia National Laboratories

Oregon State University

National Renewable Energy Laboratory

 

“Coupled Mooring Analyses for the WEC-Sim Wave Energy Converter Design Tool: Preprint” [Metadata]

Presented at the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2016), 19-24 June 2016, Busan, South Korea

2016-07-01

National Renewable Energy Laboratory

University of Maine

Oregon State University

 

“Application of the Most Likely Extreme Response Method for Wave Energy Converters: Preprint” [Metadata]

Presented at the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2016), 19-24 June 2016, Busan, South Korea

2016-07-01

National Renewable Energy Laboratory

 

“Balancing Power Absorption and Fatigue Loads in Irregular Waves for an Oscillating Surge Wave Energy Converter: Preprint” [Metadata]

Presented at the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2016),19-24 June 2016, Busan, South Korea

2016-06-01

National Renewable Energy Laboratory

 

“Balancing Power Absorption and Structural Loading for an Assymmetric Heave Wave-Energy Converter in Regular Waves: Preprint” [Metadata]

Presented at the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2016),19-24 June 2016, Busan, South Korea

2016-07-01

National Renewable Energy Laboratory

University of California, Berkeley

 

“Northwest National Marine Renewable Energy Center” [Metadata] (Large File)

2016-06-30

Oregon State University

University of Washington

National Renewable Energy Laboratory

 

“Northwest National Marine Renewable Energy Center”, Appendices [Metadata] (Large File)

2016-06-30

Oregon State University

University of Washington

 

“A Review of the Environmental Impacts for Marine and Hydrokinetic Projects to Inform Regulatory Permitting: Summary Findings from the 2015 Workshop on Marine and Hydrokinetic Technologies, Washington, D.C.” [Metadata]

2016-07-01

National Renewable Energy Laboratory

H.T. Harvey & Associates

Kearns & West, Inc.

 

“Marine Hydrokinetic Energy Site Identification and Ranking Methodology Part I: Wave Energy” [Metadata]

2016-10-01

National Renewable Energy Laboratory

 

“Marine Hydrokinetic Energy Site Identification and Ranking Methodology Part II: Tidal Energy” [Metadata]

2016-10-01

National Renewable Energy Laboratory

 

Additional references

 

Electric Power Research Institute (EPRI)

American Society of Mechanical Engineers (ASME)

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