Underwater turbines could harness a massive amount of energy—but could cause problems for boat navigation.

Free Flow Power, a startup company based in Gloucester, Massachusetts, USA, has received preliminary permits from the U.S. Federal Energy Regulatory Commission (FERC) granting it the right to explore energy production at dozens of sites along the lower Mississippi over the next three years.

The company’s plan is ultimately to install tens of thousands of turbines anchored to the bottom of the Mississippi River to provide more than a gigawatt of renewable energy.

The ambitious Mississippi project, however, relies on relatively unproven technology. The only commercial hydrokinetic river-power system operating in the US is a single turbine deployed by Hydro Green Energy close to a conventional hydropower dam on the Mississippi River in Hastings, Minnesota.

Free Flow hopes initially to deploy hydrokinetic power on an unprecedented scale: hundreds of 40 kW turbines, attached to pylons jutting out from the riverbed at 88 locations along the Mississippi.

Free Flow’s chief financial officer, Henry Dormitzer, argues that river power has distinct advantages. “The water flows in one direction, it doesn’t have salt in it, and, in the case of the Mississippi, people have spent 100 years tracking water flows and velocities,” he says. A 2007 study by the Electric Power Research Institute in Palo Alto, California, predicted that the U.S. could develop three gigawatts of hydrokinetic power from rivers by 2025. “There is no question the potential for hydrokinetic river power is huge, but this industry is so young, it’s very hard to say how economically viable it will be,” says Andrea Copping, a senior program manager at Pacific Northwest National Laboratory in Sequim, Washington.

But the Mississippi is also one of the world’s busiest waterways, and the company will have to demonstrate that its turbines will not interfere with commercial shipping, and that it will have no negative impact on the river’s wildlife.

In July 2009, Free Flow began a six-month test of a one third scale pilot turbine in the Mississippi, and the company is now testing a commercial-scale prototype in the laboratory. Free Flow has also received $7.4 million in funding from investors and from the US Department of Energy that will allow it to test its most recent prototype in the Mississippi next year. The company is currently seeking additional funding to test four turbines, each attached to a separate pylon, over a 12-month period, as required by FERC as part of the licensing process.

Free Flow uses a shrouded turbine design that channels water through the turbine’s blades and over a rotor with seven blades that are designed for a slow spin rate to minimise fish strikes. The turbines will be sited 10 or more feet off the riverbed. At this depth, water moves, on average, at one to three m/s.