Research Reports: Marine and Hydrokinetic Energy

What research is ongoing in the marine and hydrokinetic energy industry? Read on to get some highlights.

A variety of research results pertinent to the marine and hydrokinetic energy market were presented during Waterpower Week in Washington’s Marine Energy Technology Symposium, May 2018 in Washington, D.C., U.S. Below are excerpts from some of the top papers presented at METS, with content covering siting, standards, wave quantification, heave plates, composite materials and more.

For more learning opportunities available in the field of MHK, HydroVision International 2018 features a six-session conference track over two days, June 27 and 28, in Charlotte, N.C., U.S.

Siting performance metrics

Trent Dillon with the Northwest National Marine Renewable Energy Center says that when it comes to micro-siting (the prioritized placement of turbines within a general deployment site), three metrics can be used: power output, foundation weight and overhead clearance limits.

In a case study on siting a pair of small cross-flow turbines at the inlet of Sequim Bay, Wash., U.S., metrics and underlying trends were uncovered that are useful to inform deployments at other sites and scenarios of interest.

Power output of a single turbine was estimated over the two-month period using a turbine dynamics model. Mean power, maximum power, and the ratio between max and mean power were calculated. Foundation weight needed to be sufficient to resist overturning, and the minimum foundation mass required was computed. Overhead clearance was evaluated based on the allowable distance between the top of the turbine and the mean lower low water at the location.

Results indicated that power output varies substantially throughout the channel, the required foundation mass is generally within the limits of the deployment vessel, and overhead clearance substantially limits the candidate deployment locations.

The author says that next steps include further study of the prospective deployment sites, design of the turbine lander, and refinement of the expected power and foundation weight calculations.

Standards and certification

Jonathan Colby with Verdant Power says consensus-based standards and certification (including testing) play a critical role for the commercial success of the marine energy industry.

Early guides for marine energy were published by the European Marine Energy Centre in 2009 and EquiMar in 2011. The International Electrotechnical Commission’s Technical Committee 114 (marine energy — wave, tidal and other water current converters) was formed in 2007 and its first technical specification (TS) was published in 2011. International certification activity began in 2011, and the Marine Energy Operational Management Committee of the IEC System for Certification to Standards Relating to Equipment for use in Renewable Energy Applications was formed in 2014.

In addition to the terminology TS, published in 2011, TS’s for the power performance assessment of electricity producing converters were published in 2012 for wave and 2013 for tidal. These are key to the marine energy industry as they enable a common comparison between various technologies at a given wave or tidal energy site.

In 2015, TS’s for wave energy and tidal energy resource assessment and characterization were published. And in 2016 the Design requirements for marine energy systems was published. This fundamental document began the process of codifying the best practices necessary to design, operate and maintain a marine energy converter.

The IEC TC 114 is engaged in writing new TS’s, with five pending publication in 2018. Additionally, three project teams are writing new TS’s that will be published in 2019 and 2020. And several of the published TS’s will be updated to their second editions in the next two to three years. As new standards are developed and existing standards refined, and as the certification products are accepted globally, the reliability, performance and safety of marine energy converters should increase significantly. In addition, the cost to enter global markets should be reduced.

Studying composite materials

Alexander J. Gonzalez with Florida Atlantic University discussed the results of a study of sandwich composites, which was conducted to help further understand how material selection and composite manufacturing choices affect ocean current turbine rotor blade strength.

These materials are becoming a suitable candidate for ocean current turbine rotor blades, but their dominant failure mode is face/core delamination. Composites containing carbon/epoxy face sheets and syntactic and polyurethane foam cores were fabricated, and their debond toughness was quantified. Additionally, the use of a chopped strand mat at the face/core interface was tested.

Several material pairings were chosen and manufactured at Florida Atlantic University. Material pairings with a higher debond toughness coincide with a longer lifespan during sea operation, reducing maintenance requirements. Results indicate that the addition of a chopped strand mat at the face/core interface increased average debond toughness by about 14%.

Extreme wave heights

Ning Li with the University of Hawaii at Manoa discussed the complex wave climate in Hawaii, with extratropical storms generating swells from the northwest to north during the boreal winter, the Southern Hemisphere Westerlies and mid-latitude cyclones bringing modest swells to south-facing shores in the summer, trade winds generate wave from the northeast to east year-round, and subtropical cyclones and passing cold fronts generating wind swells from all directions during the winter.

Devices deployed at the Wave Energy Test Site require survival analysis to deal with various wave conditions. Two waverider buoys were deployed, one in 2012 and one in 2016, to record the seasonal variations of wave conditions at WETS. However, the five-year duration of record is insufficient for quantification of extreme events.

Numerical modeling was used to develop a detailed description of the complex wave climate and extreme events in support of operations and survivability analysis at WETS. A global and Hawaii regional hindcast dataset from February 1979 to May 2013 was produced and validated with buoy and satellite observations. This hindcast provides a basis for extreme value analyses of commonly occurring wave conditions at WETS.

Heave plate hydrodynamics

Curtis J. Rusch with the University of Washington presented an analysis of how various parameters of heave plates – which provide the necessary reaction force for some point-absorbing wave energy converters – affect hydrodynamics. The heave plate parameters studied included porosity, hole size and perimeter.

Previously, the Keulegan-Carpenter, or KC, number was identified and is a parameter that describes the ratio of hydrodynamic forces from water inertia and viscous effects of oscillating plates across scales.

Thirteen different heave plate designs were studied in three geometric groups: squares, rectangles and Newman shapes. The geometry of a Newman shape is based on a weighted sum of sines and cosines. Both solid and perforated heave plates were included. The area enclosed by the perimeter of each heave plate is identical. The perforated plates have a constant solidity of 90% but varied hole sizes.

The heave places were tested using a vertically oriented linear actuator placed at the edge of a dock. An expected KC number was below one. It was shown that, at low KC numbers, added mass is the dominant component of the total hydrodynamic force, but it is significantly reduced when porosity is introduced. Drag shows little change between all of the plates tested. This motivates the need to better understand added mass.

The author says that this information will aid in the design of future heave plates, allowing for a more targeted approach to their design and selection.

Feedback control for wave converter

Giorgio Bacelli with Sandia National Laboratories covered the design and implementation of a simple, stable and causal feedback resonating control for a heaving point absorber wave energy converter. This is needed because one of the assumptions in the derivation of the complex conjugate controller is that it allows the deice to resonate and absorb the maximum amount of power at all frequencies. However, in practice, the large majority of the power transported by waves is concentrated in a limited frequency band and the tuning of the optimal controller for all frequencies is unnecessary.

The feedback resonating controller approximates the response of the complex conjugate controller in a limited frequcy band. Thus, the feedback resonating controller is able to absorb more than 95% of the power absorbed by the complex conjugate controller in the desired interval without requiring information about incoming waves. The author says the only required measurement is the velocity of the buoy.

Elizabeth Ingram is managing editor of Hydro Review and facilitator for the Marine and Hydrokinetic Energy track at HydroVision International.