Are underwater turbines the next big clean energy source?

The power of underwater currents is vast and untapped. But harnessing that power is no easy feat.

The US Department of Energy (DoE) is set to explore the potential of underwater turbines with the hopes of making them more efficient and cost-effective. If successful, the endeavour could lead to a significant shift in how we generate electricity and help move the world closer to a clean energy future. However, some challenges must be overcome before underwater turbines become a reality.

The design and deployment of most underwater turbines are still in their infancy and there is much to learn about how they interact with the underwater environment. The high cost of hydrokinetic turbines is another barrier to their widespread commercial adoption.

Yet DoE's Advanced Research Projects Agency-Energy (ARPA-E) is funding 11 projects with a $38 million bundle of grants made in November 2020 to develop cost-effective underwater turbine technologies. These projects are collectively called Submarine Hydrokinetic And Riverine Kilo-megawatt Systems—or SHARKS.

The SHARKS projects aim to develop marine technologies that can generate electricity at an end cost below $0.05 per kilowatt-hour. That's about one-sixth of the current cost of electricity from underwater turbines. Two examples of underwater turbines developed under SHARKS are Tidal Power Tug and Manta.

Invented by California-based energy developer Aquantis, the Tidal Power Tug turbine is a second-generation floating tidal energy converter. The turbine is equipped with a versatile spar-buoy platform that supports a 10-meter diameter, two-bladed variable-pitch rotor and a 160 kW drivetrain. Moreover, with a parallel-flow rotor, a vertical yawing spar buoy, and advanced analytical tools, the turbine is designed to achieve stable power generation in all sea conditions.

This underwater turbine is ideal for gulf stream currents along the US East Coast, which create multiple circular flows called gyres. The proximity of these energy-intensive gyres to major cities makes them a strategic energy resource.

Manta is another underwater turbine that aims to extract energy from moving water.

Designed by California-based SRI International research institute, Manta uses a lightweight foam and fiberglass kite with a small generator moored to the sea bed. When the turbine moves in the water currents, it reels out a spool of rope, spinning the generator and producing electricity in the process.

For protection against a collision with ships and storms, the kite-like turbine is equipped with a motor that can redirect and deflate the machine. Moreover, since Manta can generate power with minimal motion in slow currents, it is well suited to power small and isolated communities.

The SRI Institute received $4.2 million in funding from ARPA-E to test out Manta underwater turbine and prove its efficacy.

The National Renewable Energy Laboratory (NREL), the U.S. government's primary clean energy research facility, is working closely with several SHARK projects. To improve and analyze the efficiencies of these projects, NREL engineers are designing an open-source software that will combine technology design, control, and optimization of the new marine technologies.

Since underwater turbines work like wind turbines (both react to the fluid motion of water and air, respectively), NREL plans to predict the performance of SHARK projects using data from OpenFAST, an open-source wind turbine simulation tool. However, there's a minor glitch: water is much denser than air. NREL, however, is adapting OpenFAST to account for the physical differences.

Hydrokinetic energy has the potential to provide clean and renewable energy. Moreover, since water is constantly moving, underwater turbines can provide power around the clock, making it an even more reliable energy source than wind and solar.

With successful implementation, they can have the following positive impacts:

Underwater turbines can help reduce the dependence on a single energy source, providing much-needed diversity to national grids. They can also serve as a backup source of energy in case of power outages caused by severe weather conditions.

Since hydrokinetic turbines are a relatively new technology, they present an opportunity for countries to create new industries and generate employment. They also have the potential to reduce the cost of energy, thereby stimulating economic growth.

As hydrokinetic technologies capture energy from water systems, they have the potential to reduce greenhouse gas emissions. Moreover, since underwater turbines don't require the construction of dams or reservoirs, they have a minimal environmental footprint compared to other forms of renewable energy.

SHARKS underwater turbine technologies have the potential to lay the groundwork for a new renewable energy source—one that can be used to power small, isolated communities and big cities alike. Specifically for small communities that depend on expensive diesel generators, hydrokinetic turbines can provide a cheaper and cleaner energy source. Moreover, these underwater turbines don't have a visual or noise pollution footprint, making them an even more attractive proposition.

The biggest challenge for underwater turbines is demonstrating their technical and economic feasibility. But if they manage to do that, these turbines could become the next big renewable energy source.