In 2017, Japan tested a twin-turbine machine in the Kuroshio Current; the floating device later showed it could generate 100 kW from the Pacific’s “river”

In August 2017, engineers dropped a 330-ton machine into one of the ocean's fastest currents and waited to see if it would make it out alive. It held up better than that. It generated electricity. The twin-turbine device, named Kairyu, was confirmed capable of a rated 100 kilowatts of power during a towing test simulating the Kuroshio Current, and went on to generate power directly from the current itself once moored on-site off Kagoshima Prefecture, according to a joint announcement by Japan's New Energy and Industrial Technology Development Organization (NEDO) and IHI Corporation. That test was the basis for a technology that researchers continue to refine today.

The current that behaves like a river

The Kuroshio Current flows along the Pacific coast of Japan, transporting warm waters northwards with consistency. According to a study published in the IHI Engineering Review, this current is one of the strongest ocean currents and does not slow down during the day or throughout different seasons. While wind may be absent for several days, and the sun may disappear every night, this current keeps flowing as it carves its way through a largely calm ocean. It is precisely this constant nature that makes it interesting for energy planners.

A machine shaped like an underwater plane


IHI's prototype, Kairyu, looks a bit like an airplane without wings. The central pod houses the buoyancy and control systems and the two side pods each contain a turbine blade about 11 meters across. The two turbines spin in opposite directions, as the engineering team explains in a technical paper in the Pacific Northwest National Laboratory's Tethys Engineering database. This cancels out the turning force each blade creates. So the entire machine is steady in the current, rather than spinning out of control.

The 2017 test that made headlines

NEDO and IHI towed Kairyu to a site off of Kuchinoshima Island in Kagoshima Prefecture and lowered it into the current in August of that year. In a towing test that preceded the main demonstration, the machine confirmed it could produce a rated 100 kilowatts of power at a simulated current speed of 1.5 meters per second. Once actually suspended at a depth of 30-50 meters in the real current, Kairyu produced up to about 30 kilowatts, kept in position by an automatic control system. The test was the world's first verification of a floating turbine to directly draw power from the Kuroshio Sea, the two organizations said.

Second question: Will it last over time

A prototype that works once is one thing. But a machine that lasts for years is another. Building on that 2017 test, IHI carried out an extended testing campaign off Kagoshima from 2019 to 2021 to assess how the structure fared under real ocean stress and to see what it might cost at commercial scale. As the IHI Engineering Review paper puts it, no one had ever tried to spin an 11-meter turbine blade continuously in a natural ocean current, so the design team worked with a shipbuilding company and even aerospace engineers within IHI to reduce the risk of failure. The same study also found that the measured stress on the floating body was comfortably below the levels engineers designed for and estimated a capacity factor of around 43 percent for a hypothetical 10-megawatt installation designed to power the grid of an isolated island. The study doesn't specify whether that hypothetical installation means a single scaled-up turbine or a cluster of smaller units working together. Capacity factor just tells you what percent of the maximum possible output you get from a system over time.

Why should it matter to you

Much of the conversation about renewable energy focuses on solar panels and wind turbines. Ocean current power doesn't get the same hype even though it solves a problem those two don't: consistency. A generation growing up amid the climate crisis needs power that works quietly in the background, day and night, not just when the sun shines or the wind blows. Once installed, the machine sits roughly 50 meters below the surface, largely shielded from surface storms and typhoons, a quietly compelling idea for anyone ever worried about where clean energy is actually headed.

The catch: It isn't cheap yet

The IHI Engineering Review study estimated Kairyu’s power cost at about 56.5 yen per kilowatt-hour, likely higher than that of some other renewables such as solar power. About 75 percent of the cost is putting the equipment underwater and installing it, not from running it. Moorings, cables and offshore construction remain expensive and technically challenging. The researchers said those costs could come down over time, in part because similar construction techniques are already being developed in offshore wind projects taking place around Japan.

What’s next

Nobody involved in this project is saying ocean current turbines are going to replace solar or wind anytime soon. But the technology has gone from a concept on paper to a working prototype refined across an extended, multi-year test program spanning 2017 to 2021. IHI's engineers are now working to improve the accuracy of current fluctuation predictions, improve installation methods, and reduce costs before moving to a full commercial turbine. While the machine itself operates largely shielded from surface weather once installed, getting it into the water and servicing it is another matter; the same study lists rougher-water construction as the next hurdle. The aim is to install and service these turbines safely even when waves are 1.5 meters, a threshold researchers say would make offshore work possible on far more days of the year and bring commercial-scale, multi-turbine installations a step closer.