Krish Thiagarajan Sharman, the Endowed Chair in Renewable Energy and a professor in the Mechanical and Industrial Engineering Department, will be working with the National Renewable Energy Laboratory (NREL) on a one-year, $211,386 grant supported by the Department of Energy (DOE) to create variable geometry wave energy converters. Such devices “can provide a paradigm shift in the offshore renewable community that will push the industry towards commercialization of electricity generation from ocean waves,” as Sharman and his colleagues say.
Dr. Nathan Tom of the NREL and Sharman are the two lead researchers on the project. They conclude that their DOE research could have a transformative potential for significant cost reduction related to wave energy conversion. Possibly the most important impact of their research into variable geometry load control is the ability to greatly reduce the cyclic fatigue loads on all of the system components of wave energy converters, or WECs.
According to the DOE proposal, the U.S. and many other countries possess significant wave energy resources close to human population centers, but these resources have traditionally been too costly to exploit commercially. According to the Electric Power Research Institute, the technical resource potential from ocean waves is between 898 and 1,229 terawatt-hours/year, which could meet the electricity demand of close to 90 million U.S. homes.
However, to date there are no commercial WECs providing power to the U. S. electricity grid, and the technology is considered to be in early stage development.
As Tom and Sharman hypothesize in their DOE proposal, “Following the technological readiness trajectory of the wind energy industry, we theorize that significant cost reductions and improved system survivability of WECs will not be obtained until a greater load-shedding capability is considered in the design of the WEC hull.”
The researchers add that the wind energy industry overcame a similar challenge with the development of wind turbine blades that could pitch about their aerodynamic center to reduce system loads at higher wind speeds. Wind technology developers now have two “control knobs” to optimize system performance and significantly reduce structural costs—the generator, or power take-off, and the turbine blade pitch angle. But most WEC developers are currently limiting themselves to optimizing the power take-off only.
Sharman and Tom believe that variable-geometry WECs, equipped with two “control knobs” similar to those in wind turbines, can provide a revolutionary shift in the offshore renewable community that will drive the industry to create and commercialize electricity generation from ocean waves.
The control success provided by pitching wind turbine blades has prompted NREL to investigate the load shedding capabilities of WECs with variable geometry modules. These control modules add a second “control knob” for WEC developers and enable significant load-shedding capabilities in larger wave environments, resulting in a more tunable load profile, more intelligent power take-off, and a substantial decrease in levelized cost of energy.
“While this approach is applicable to many WEC device types,” say the researchers, “we propose a bottom-fixed variable-geometry oscillating surge WEC. Controlling the position and location of the variable-geometry moduels allows the frontal surface of the device to be altered, enabling an increase or decrease in the wave-excitation forces/torques.”
Tom and Sharman theorize that the most important effect of variable geometry load control is the ability to greatly reduce the cyclic fatigue loads on all of the system components.
In this proposed effort, NREL will be partnering with UMass to demonstrate the cost saving benefits of their devices by developing state-of-the-art hydrodynamic models, multivariable load shedding control strategies, and validate the approach through a scaled experiment.
A model of the WEC will be built at UMass Amherst and tested in a wave-current laboratory, currently under construction in Gunness Hall. This is a 10-meter long facility, with a width of 1.2 meters and an operating water depth of 1 meter. Waves will be generated by a plunging wedge on one end and will be absorbed by an efficient parabolic beach at the other end. Glass walls on either side will allow for high-speed photography and visualization. Purpose-built instrumentation for measuring incident wave power and absorbed power will be used.
This design has potential to not only reduce capital costs through reduction of materials and load on WECs, but it can also be adapted for a variety of different types of wave energy devices.
The grant by the DOE was announced by its Office of Technology Transitions in the latest round of the Technology Commercialization Fund — a program that transitions research and development funding to applied energy programs to advance promising technologies with the potential for impact across industry. (September 2019)