NREL: Code Fit for the Stars

A first-time paraglider would not—or, at least, should not—go grab any old equipment and jump off a cliff. That is not just risky; it is reckless.

The same advice applies to the blossoming wave energy industry, which is honing hundreds of prototypes to find the best, most cost-effective equipment to generate clean energy from ocean waves. Tossing an untested model into those waves, which can crash down with far more force than any wind blows, is often risky, expensive, and time consuming. Why go all the way to the ocean just to find out a design will not work?

But today, wave energy developers can bring the ocean to the computer thanks to a tool built by researchers at the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories. Called the Wave Energy Converter SIMulator (or WEC-Sim for short), this first-of-its-kind, open-source code allows developers to simulate how much electricity their theoretical device could produce and how well it could handle the open ocean. WEC-Sim, which just won a prestigious R&D 100 Award (also known as the “Oscars of Innovation”) might seem like a niche tool, but it can forecast outcomes for almost any water-bound machine, including how a NASA crew module bobs in the waves after making an oceanic landing.

“WEC-Sim can model a vast variety of floating-body systems,” said David Ogden, a mechanical engineer at NREL who helped develop the code. “And because it’s open-source, people can modify the code to fit even more unusual or uncommon design features.” In short, if it floats, WEC-Sim can probably simulate it.

And that is not easy.

Ocean waves move in six different directions: They heave up and down, rush forward and back, and swirl in elliptical orbits like satellites around the Earth. And there is far more than one way to harness this churning ocean energy. Wave energy converters—the machines that convert these ocean motions into electricity—come in a variety of shapes and sizes. Some are large paddles like beaver tails that pitch back and forth; others, like underwater cattails, sway in the surf; and some swim, snake-like, through the water or spin as waters rush in and out of a cylindrical turbine chamber.

To handle almost all these highly variable designs, developers can customize WEC-Sim. And they have been—in large numbers, too. To date, close to 100 wave energy researchers and developers have published papers crediting the tool for helping them analyze and fine-tune their machines. In 2021 alone, WEC-Sim was cited in a record 27 new papers. “But because developers don’t always publicly report their WEC-Sim use,” said Nathan Tom, another mechanical engineer at NREL who helped design the tool, “the number of developers who have relied on WEC-Sim’s simulations could be more than double that amount.”

Those hundreds of developers come from all over the world. WEC-Sim users are sprawled across almost every country. While most papers citing the software are from the United States, researchers in India, England, China, Spain, and Sweden are using WEC-Sim, too.

“WEC-Sim is great not only because it’s versatile,” said mechanical engineer Thanh Toan Tran, another member of NREL who helped build the code, “but also because it can reduce the uncertainty involved in building novel wave energy converters for grid-connected wave energy farms, platforms for recharging autonomous underwater vehicles, and much more.” Autonomous underwater vehicles are like sea robots, which drift, drive, or glide through the ocean often to collect data for marine research.

Increased certainty is perhaps WEC-Sim’s greatest gift to developers. The code provides precise data on how a wave energy device will work and also how each critical component will function in waves of various heights and forces. That includes the floating body (like the paddle or buoy), joints and constraints (that let it flex and stay anchored), the power take-off (which transforms ocean motions into usable electricity), and mooring systems (which keep the device tethered to one site).

All that knowledge can save developers a whole lot of time and money. It is painful enough to learn from a computer simulation that one joint might sabotage a device; it is far worse if that device is already built, shipped, and anchored far offshore.

So far, WEC-Sim has helped: optimize an award-winning self-charging data station (from CalWave Power Technologies Inc.), ensure NASA’s Orion crew module would stay upright in the ocean after landing (vital for a safe and quick recovery), and build an offshore jack-of-all-trades that could generate energy from wind, waves, and tides using just one floating platform.

And the software keeps getting better. As Ogden, Tom, Tran, and the rest of the national laboratory team, which includes researchers Kelley Ruehl, Carlos Michelen, Dominic Forbush, Adam Keester, and Jorge Leon at Sandia National Laboratories, collect more experimental data, they can use it to verify how well their theoretical models predict reality and then close any gaps. All this progress is, of course, also thanks to the hard work of former WEC-Sim team members, including NREL mechanical engineers Michael Lawson, Jennifer Van Rij, and Yi-Hsiang Yu.

Together, the two-laboratory team continues to add new and improved features to WEC-Sim to make it even more versatile. In October 2021, they released WEC-Sim version 4.4, which can now simulate how spherical joints (also known as ball-and-socket joints) and cable connections between two devices affect energy production. The team also added a wave visualization function that creates a video or GIF image that developers can watch to see how their design moves in smaller or bigger waves.

“Today, a lot of developers are going into the water to prove their technologies,” Tom said. “Their design could produce a lot of power, but it can also cost millions of dollars just to get it in the ocean. What’s exciting about WEC-Sim is that it can help reduce these costs by optimizing designs at home, which could make wave energy development faster, cheaper, and, well, smarter.”