From an article by Alexis Madrigal posted on WiredScience:
The nation's first commercial hydrokinetic turbine, which harnesses the power from moving water without the construction of a dam, has splashed into the waters of the Mississippi River near Hastings, Minnesota.
The 35-kilowatt turbine is positioned downstream from an existing hydroelectric-plant dam and — together with another turbine to be installed soon — will increase the capacity of the plant by more than 5 percent. The numbers aren't big, but the rig's installation could be the start of an important trend in green energy.
And that could mean more of these "wind turbines for the water" will be generating clean energy soon.
"We don't require that massive dam construction, we're just using the natural flow of the stream," said Mark Stover, a vice president at Hydro Green Energy, the Houston-based company leading the project. "It's underwater windpower if you will, but we have 840 or 850 times the energy density of wind."
Hydrokinetic turbines like those produced by Hydro Green and Verdant capture the mechanical energy of the water's flow and turn it into energy, without need for a dam. The problem for companies like Hydro Green is that their relatively low-impact turbines are forced into the same regulatory bucket as huge hydroelectric dams. The regulatory hurdles have made it difficult to actually get water flowing through projects.
The Federal Energy Regulatory Commission has oversight of all projects that involve making power from water, and the agency has recently shown signs of easing up on this new industry. In the meantime, the first places where hydrokinetic power makes in impact could be at existing dam sites where the regulatory red tape has already been cut.