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 commercialise 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.
“We were confident it would work, but then we proved it,” says Chao. “Demonstrating the two SL1s is game-changing, because it’s 100 per cent 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 programme 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.
“Anything we put in the ocean is limited by its battery,” says Steve Jayne, a senior scientist at Woods Hole Oceanographic Institution, who not involved with Seatrec. “If you had unlimited energy available to you, you might be able to profile every day instead of every 10 days.”
Seatrec’s ocean generator doesn’t produce a lot of energy – each charging cycle tops it up with about half the energy of a single AA alkaline battery – but that’s more than enough to meet the needs of the low-powered sensors typically found on profiling floats. For applications that require more power, Chao says, it is possible to increase the size of the generator or simply daisy-chain smaller ones together.
The floats are also designed to work in any ocean environment, whether they are trapped among Arctic ice floes or diving among sharks in the tropics. All it takes to adapt them to different regions is tweaking the chemistry of their waxy guts so they solidify and melt at the correct temperatures.
Chao hopes that Seatrec’s ocean generator will deliver on a promise first conceptualised in the 1980s by the renowned oceanographers Douglas Webb and Henry Stommel. They envisioned a globe-spanning fleet of missile-shaped underwater research robots called Slocum gliders that would explore the oceans with the same dexterity, autonomy, and longevity that we’ve come to expect from the robots that NASA sends to explore other planets.
Like Seatrec’s SL1, these gliders would be powered by underwater temperature differences.
Although Webb, Stommel and their collaborators made progress toward bringing a global fleet of Slocums into existence, their vision is still a work in progress, says Matt Palanza, a programme engineer at the Woods Hole Oceanographic Institution’s Ocean Observatory Initiative who previously worked with Webb.
Palanza’s team at the Ocean Observatory oversees the largest civilian fleet of Slocum gliders in the world – 50 in total – and he says the reason there are not thousands patrolling the world’s oceans is simply a lack of funding. “The technology is there and continuously being developed,” he says.
Chao and the team at Seatrec believe that extending the vehicles’ life spans with limitless clean energy could drastically increase the size of the ocean research fleets. But the company is not the first to work on the technology.
In 2003, Webb built a prototype thermal glider that used temperature differences to control its ascent and descent in the ocean, but still relied on batteries for its electronics. In 2008 a team led by researchers at Woods Hole successfully deployed a different glider prototype in the Carribean that used ocean temperature differences to power an electric propulsion system.
The following year, Chao and a team of researchers from NASA and the Scripps Institution of Oceanography rolled out Solo-Trec, the world’s first profiling float powered completely by electricity generated from temperature differences.
“We’re standing on the shoulders of the giants in this community,” says Chao. “Webb solved half the problem early on to control buoyancy, but we wanted to solve the whole problem and be energy-neutral. Our innovation is to use mechanical expansion to generate electricity.”
Chao sees research applications for his energy generation technology as just the beginning. If he has his way, Seatrec will one day become the maritime equivalent of Tesla.
Seatrec is already working on a modified version of the SL1 power generation system that can be integrated with any underwater glider, which Chao says the company hopes to deploy for the first time in just a few months.
Later on, he says, the company plans to send the glider on an unprecedented solo trip across the Pacific Ocean powered only by Seatrec’s generator.
The company is also attracting attention from the defence sector. Last year, Seatrec partnered with Northrop Grumman to study the feasibility of using Seatrec’s generator to create a network of ocean-based charging and data relay stations for autonomous underwater vehicles.
- A Wired report