As the world continues to guzzle energy, the race is on to create solutions that curb greenhouse gas emissions. Renewables are instrumental to the decarbonization of the electricity sector, although they contribute just 20 percent of U.S. electricity generation. Technologies such as wind and solar are intermittent in nature, and alone cannot fulfill global energy needs.

The world’s oceans may have the capacity to come to the rescue. There’s tidal power, a discontinuous energy source generated for 6- to 12-hour intervals, whose power generation technology works similarly to that of a wind turbine underwater. Salinity gradient power is another possible power source, where the salt concentration difference between freshwater and saltwater drives the transport of ions across a membrane—resulting in an electric potential that is converted to electricity.

Ocean thermal energy conversion produces electricity through temperature differences between surface seawater, and water at depths of 800–1,000 meters that can be as much as 20°C colder. The ocean’s warm upper layer produces a vapor that acts as a working fluid for driving turbines (often in conjunction with ammonia), while the cold layer condenses the vapor through a heat exchanger to ensure a vapor pressure difference for the turbines’ continued operation.

And then, there are the ocean’s ever-powerful waves. The vast potential of the ocean is clear to anyone who has gone sailing or surfing in rough waters. According to the Ocean Energy Council, 35,000 horsepower of energy is released as a wave breaks along a mile of coast. Since waves are a continuous energy source, they present a ripe opportunity for harvesting this relentless power in the form of hydrokinetic energy. In fact, the U.S. Energy Information Administration states that the annual energy potential of waves along U.S. coasts could be as much as 2.64 trillion kWh—enough to comprise 66 percent of today’s U.S. electricity mix.

Wave energy has the largest power potential of U.S. marine energy resources. (Image courtesy of National Renewable Energy Laboratory.)

The technological readiness of wave energy has historically been behind other renewable resources. However, Oakland, Calif.-based CalWave Power Technologies Inc. is trying to change that legacy with the design and development of its proprietary wave energy converter (WEC), called xWave.

“Wave power doesn’t have daily fluctuations because it’s a stored form of wind energy,” says Marcus Lehmann, CEO and co-founder of CalWave. “The day-night profile looks pretty much the same; there are some seasonal fluctuations, but they’re not as significant as for other renewables.”

Wave energy also has one of the lowest lifecycle emissions of current energy sources. (Image courtesy of Intergovernmental Panel on Climate Change.)

The Problem with Wave Energy

Wave power is the world’s largest untapped renewable resource due to various challenges in implementing the technology. Until recently, the overall budget for marine energy has been undersized compared to other renewable sources. The technology also suffers from the inability to easily test systems at scale or in controlled environments.

“We’ve done a lot of subscale testing—but fundamentally, every relevant system needs field testing,” explains Lehmann. “Testing a system offshore is very complex and expensive. You don’t have a location where you can just turn on and off ocean-sized waves. Getting a new architecture and system to work the first time in the field is extremely demanding. From a systems engineering perspective, you can do your failure-effect analysis, but you have so many compounding effects that are impossible to project altogether in the paper study.”

We have all seen our fair share of shipwreck movies, which highlight another obvious challenge that presents itself in the form of epic ocean storms. Marine systems need to be rugged in order to survive some of the harshest environments on the planet—not to mention the corrosiveness of the ocean’s salt water.

Finally, access and distribution can be tough. Full-scale WEC devices must be anchored miles offshore in deep water to harness maximum wave power, and export cables are necessary for connecting numerous WECs to local energy distribution systems.

How Does CalWave’s Technology Overcome Wave Energy’s Barriers?

Unlike conventional WECs that harvest energy on the ocean surface, CalWave has engineered the xWave to operate fully submerged at a range of depths. Once the system is deployed and anchored, it converts the relative motion of overpassing waves to power. Not only does this enable the device to maximize performance by capturing energy from multiple degrees of freedom, it also allows for the control of excessive structural loads when the ocean is particularly choppy.

Ocean waves are different from river and ocean currents, where particles continue to travel. With wave energy, particles stay local and move in orbitals. This orbital motion—along with the energy density and respective pressure field—goes down exponentially with water depth. CalWave has designed its submerged xWave system to operate in the most efficient orbital locations, while being able to avoid the largest pressure fields in case of extreme events such as storms. (GIF courtesy of CalWave.)

“Operating submerged allows the system to autonomously shut down during storms,” says Lehmann. “Using the water column to our advantage, we can avoid breaking waves, slamming loads, storm surges and also really large return waves.”

The xWave’s mechanisms for minimizing extreme wave loads are analogous to pitch and yaw control in wind turbines. Through a combination of structure, anchors and load-bearing parts, the complex system reduces peak loads that would otherwise have resulted in the xWave being overdesigned and costly.

“One of the drivers for our design was to be compliant with the 50-year return wave,” says Lehmann. “Because these projects are usually financed for 20 years and deployed for 20–30 years, every system has to be designed for the largest wave that happens in 50 years. Most of the time, you want to produce at the highest forces and power available—but that 50-year return wave is a pretty big cost driver, because the forces are so high.”

The xWave at sea. (Image courtesy of CalWave.)

In addition to the active load management features, the xWave’s architecture integrates a variable geometry control mechanism. Rather than the device geometry staying fixed once it is manufactured, the hydrodynamic behavior of the wave absorber body actively changes not only to operate efficiently in small waves, but also to optimize energy absorption in rough sea states with very large waves.

A comparison of capacity factors for different energy sources. Capacity factor is the ratio of the actual energy produced by an energy-generating system versus the energy that it can produce at maximum output over a given period of time. (Image courtesy of U.S. Energy Information Administration.)

According to CalWave, the xWave achieves a minimum capacity factor of 40 percent on its own. Lehmann asserts that if the device is co-located and combined with offshore wind, a joint capacity factor of over 80 percent can be reached by both systems. This presents opportunities to combine wind and wave farms while sharing the same electrical export infrastructure for year-round renewable energy production. As it turns out, the peak production profiles for both energy sources are counter-cyclical; wind produces the largest amount of energy in the summer months, and waves are strongest in the winter months.

As for the corrosion problem, CalWave employs the same materials and techniques as other offshore technologies while ensuring that the chemical makeup of all coatings adheres to local regulations. The team collaborates with their naval architecture partners to specify corrosion-resistant marine paint. Like ship hulls, CalWave places sacrificial anodes along the WEC’s body to create a galvanic cell—where active metals with a more negative electrochemical potential (e.g., zinc) are consumed via oxidation reactions, in favor of the WEC body itself (i.e., the cathode). To combat biofouling, the xWave device is protected against corrosion by an epoxy base coat topped by a layer of paint that resists organic growth.

While obstacles to testing in the open ocean include difficulty obtaining permits and a scarcity of available test sites, U.S. wave energy infrastructure is beginning to gain momentum. CalWave will soon be completing its first six-month at-sea pilot off the coast of San Diego using a scaled version of its xWave unit. The objectives of the pilot include testing and validating installation procedures, operations, autonomous controls, performance, reliability and survivability of the xWave system in open ocean.

“Our tidal adjustment, performance optimization, and shutdown mechanisms have shown to be reliable and robust, operating fully autonomous since November 2021,” said Thomas Boerner, CalWave’s Chief Technology Officer, in a follow-up interview this month. “The device has survived two 10-year storm events since deployment, representative of 50-foot waves at full scale.”

Following its pilot, CalWave will deploy its 100kW xWave unit during the first round of open-water testing at PacWave in 2023. PacWave—a 20MW test site currently being constructed off the Oregon coast by the U.S. Department of Energy (DoE) and Oregon State University—is the first federally-approved, commercial-scale, pre-permitted wave energy test facility in the U.S.

PacWave is the nation’s first accredited, grid-connected, open-water test site. (Image courtesy of Oregon State University.)

CalWave’s Origin Story

Lehmann, who holds a master’s degree in mechanical engineering and a PhD in systems engineering, conceived CalWave’s submerged pressure differential device in collaboration with University of California Berkeley in 2012. The design was inspired by the ability of a muddy seafloor to effectively absorb the energy of ocean waves that pass overhead—much like a spring damper model.

“I was fascinated by an idea I read in the MIT Technology Review, and built the first version of the unit with my undergrads during Thanksgiving 2012,” says Lehmann. “We knew the mathematical model and the general configuration. We decided to see how far we could get with rapid prototyping and bolted something together with plywood, spare bike tires and a pump we borrowed from another professor in the lab.”

Lehmann’s team built upon their understanding of wave fluid dynamics by experimenting with prototypes in the wave tank at UC Berkeley. This led to Lehmann’s first publication and patents, which UC Berkeley filed. Lehmann was accepted into the first cohort of Lawrence Berkeley National Laboratory’s Cyclotron Road, where he formed his founding team and created CalWave. His work as co-inventor for several patents eventually earned him a spot in Forbes magazine’s “30 under 30” list for Energy in 2016. CalWave went on to win second place out of 92 teams in the U.S. DoE’s Wave Energy Prize, after demonstrating more than a three-fold improvement in energy capture.

CalWave is supported by Autodesk’s Technology Impact Program, which donates software to enterprises doing environmental or social good. In its early days, the wave energy company leveraged Autodesk’s CAD and CFD software during the engineering phase of its WEC system, where structural analysis tools assisted with iterations on parameters such as the size of the absorber body and strength of the device’s drivetrain.

Autodesk Technology Centers—which offer access to fabrication equipment, design technology, technical experts and a global community for innovators looking to better the world—further provided CalWave with access to a wide range of advanced manufacturing equipment. This enabled CalWave to build the first parts of the WEC’s drivetrain and carry out scaled-down bench testing. The team used a simulation model to dynamically test their technology in parallel with their hardware and bench setup. This ended up being critical for testing parts without the complexities of going out in the open ocean.

In January 2022, CalWave was awarded $7.5 million by the U.S. DoE to propel its wave energy technology toward commercial use. Down the line, CalWave hopes to bring its scalable WEC devices to small island developing states (SIDS)—whose coastal communities currently have no option but to generate power through imported fossil fuels.

Areas around the globe where wave power is strongest. (Image courtesy of Intergovernmental Panel on Climate Change.)

CalWave has demonstrated how engineers can come up with new and better ways to address problems as gargantuan as tapping into the planet’s oceans. Judging from CalWave’s journey, prospects look favorable for wave energy to create ripples toward a fully renewable energy grid.