How Tidal Sail Generators Could Be More Efficient

If we want a future without fossil fuels, we can either build a ton of nuclear plants, hope for some as-yet undiscovered breakthrough, or increase our efforts to utilize renewable energy sources in every niche possible. That's one reason tidal generators are so interesting: Thanks to our orbiting Moon, tides offer a consistent source of energy for any seaside locale, regardless of if the Sun is shining or the winds are blowing.

The only problem, as compared to solar and wind power, is that tidal power generation remains difficult to produce cheaply and efficiently. But new research in the Journal of Renewable and Sustainable Energy may have found a step forward by calculating the most efficient arrangement for a tidal sail system.

The research focused on a system created by a Norwegian company by the name of Tidal Sails AS. As the firm's promo film embedded above shows, tidal sails are essentially conveyer belts with a number of flat "flags" that are powered by water flow. It's an interesting concept, and one immediately apparent advantage shown by the company's demo rig is that it appears they could be built modularly. That means that, potentially, tidal sail systems could be dropped in the nearest river for local power generation.

The problem lies in efficiency. While flowing water has plenty of power to propel sails forward, figuring out a way for them to return without too much drag is a harder task. The team developed a model for the energy contained in a turbulent marine system, and used computational fluid dynamics to calculate the most efficient design for a sail system based on lift and drag coefficients of various designs.

The result is a specific set of parameters for maximizing sail efficiency. If you're curious or perhaps building your own system, the paper states that the width of the sails should be the same as the distance of their gaps. The sails being propelled should be set at a 79 degree angle from the water's flow, and the ideal rotational speed is about one and a half times faster than that of the current.

(One advantage of modern sail designs, at least for sailing ships, is that they can move faster than the wind. A similar phenomenon should theoretically be possible underwater.)

For the non-physicists out there, the paper is interesting simply because it shows that tidal sail systems are valuable enough to be refined. Tidal Sails AS claims its pilot sail system shown in the video above produces 28 kilowatts, which would be about 20,000 kWh a month—enough for about 20 American homes, assuming load was steady. The company hopes to build plants that can produce 2-10 megawatts in the future, with a cheaper per-watt cost than comparable tidal installations.

Tidal sails alone aren't going to power the planet, nor will solar panels or wind turbines. But building a clean energy future is about maximizing potential wherever we can. That means developing a wide range of energy applications that can be distributed and installed where they're most needed and best fit. The environmental and cost questions about tidal generation still need to be answered—it's not like we can clog rivers with spinning slats without consequence, and the tech is not yet cheap enough to scale. Still, the interest in the "all of the above" approach is heartening.

@derektmead