Water currents are reliable, predictable, and powerful. Scientists estimate that Gulf stream ocean currents along the shore of the southeastern United States carry up to 23 gigawatts of raw power. Harnessing even a fraction of that could make ocean currents a big part of an overall renewable energy plan.
Converting water power to electricity is similar to harnessing wind power: use the current to spin a turbine. Air and water are fluids, and obey the same laws of fluid dynamics, so it’s not surprising that turbine designs for water currents are similar to wind turbines. Safrema Energy thinks that its Nexus vertical axis water turbine will leave other designs in its wake. Here's their description of the Nexus:
The Nexus turbine is designed to work with ocean currents, rivers, and tides. Unlike typical hydroelectric power, these water turbines don’t necessarily require a large head (vertical drop). Safrema Energy claims that the Nexus can produce power at 2.6 cents per kWh (with no subsidies) - cheaper than coal, gas, nuclear, photovoltaic, and wind. In fact, its cost per kWh is less than any other form of energy except “traditional” hydroelectric. The Nexus turbine can be placed into a river without having to build a dam, giving the Nexus a much smaller environmental impact.
A Nexus unit is 50 feet wide, forty feet long, and forty feet tall (30 of which is below the surface). Its patent-pending design is efficient and inexpensive to manufacture, install, and maintain. The unit includes two vertical axis turbines with optimized blades, which, according to Safrema’s tests, are 30% more efficient than existing blades. An augmenter increases the velocity of the water flowing through it by a factor of three. (See Hydropower Energy Harvesting for an explanation of this principle.) A front grate prevents aquatic animals and debris from entering the blades. Each turbine drives six electrical generators, which are designed to optimize weight, cost, reliability, and performance. My calculations tell me that each generator produces about 83 kW.
Each Nexus unit can generate over a megawatt of electricity - enough to power 800 houses. If located near shorelines, where most of the human population lives, water farms consisting of these turbines could meet a significant portion of our electrical needs. They also work in rivers where most of the non-coastal population resides. Safrema Energy envisions fleets of Nexus turbines floating on the surface and moored to the ocean floors near coastal regions. Their low profile doesn’t ruin the ocean view, and onboard monitoring and communication reduces the potential impact on maritime activities.
Safrema Energy includes a variety of specialists, including research scientists, practicing engineers, and energy industry executives. The company has been testing individual components and plans to build a one-third size prototype to test in the gulf stream off the coast of Florida. By January of 2015, Safrema hopes to begin a full-scale “turbine park.” I’m looking forward to seeing their results.