Early last year, just a few weeks before the pandemic brought life in the United States to a standstill, Yi Chao and a small team of researchers dropped a slender metal tube into the Pacific Ocean off the Hawaiian coast. After nearly two decades as an oceanographer at NASA’s Jet Propulsion Laboratory, Chao had left the space agency to commercialize a seafaring generator that can harness the limitless thermal energy trapped in the world’s oceans. His company, Seatrec, is based just down the road from his old NASA stomping grounds in Pasadena, but Chao regularly travels to Hawaii to test hardware in the tranquil, cerulean waters around the Big Island. On this trip, Chao and his team planned to push their invention deeper than it had ever gone before. 

From the outside, Seatrec’s ocean thermal generator doesn’t look like much. The SL1 is about as tall as a person, 6 inches wide, and has a smooth, nearly featureless black and gray exterior. But it’s what’s inside that counts. The bottom of the cylinder is packed with a specially designed wax that changes its phase depending on the temperature. The wax solidifies when the SL1 lowers itself into the frigid depths of the deep ocean. Then, when the tube resurfaces, the relatively warm water causes its waxy innards to liquify. During the phase change from solid to liquid, the wax increases in volume and raises the pressure inside the tube, which forces a fluid through a generator and creates electricity. All the device has to do to recharge is descend into colder water to resolidify the wax; it can do this by releasing a tether or deflating an internal air bladder.

Over the course of three weeks last February, Chao and his team sent two of the company’s generators on several dives to 3,000 feet beneath the surface while attached to a profiling float. It was only the third time that an SL1 had ever been deployed in the ocean, and the submersible generators were going hundreds of feet deeper than they had on previous dives. Yet they still managed to generate enough energy to power many types of research tools deployed by oceanographers. It was an unambiguous success.

Two SL1 generators from Seatrec flank a profiling float during a test in Hawaii last year. 

Courtesy of Seatrec

“We were confident it would work, but then we proved it,” says Chao. “Demonstrating the two SL1s is game-changing, because it’s 100 percent energy-neutral. Essentially we solved the energy limitation of the underwater float.”

The SL1 is meant to be attached to submersible sensor-laden research robots known as profiling floats. These devices collect data during short trips as far as a mile beneath the surface. When they emerge from the depths, they beam that information to a satellite. Today, there are thousands of profiling floats drifting through Earth’s oceans as part of an international program called Argo. They remain the best tool scientists have for remotely studying the upper ocean, but their life span and data collection are severely limited by their power sources.

All the floats in the Argo fleet are powered by lithium-ion batteries, which are typically only good for about five years or a few hundred dives. Their battery reliance limits how often they can dive; a typical float only does it once every 10 days. And after its battery dies, a float is usually abandoned, because the cost of collecting it is higher than the cost of the device itself. Still, a float can cost as much as a new car, which makes them expensive pieces of jetsam.