An array of small hydro turbines at Saint Anthony Falls in Minneapolis. (Photo courtesy Brookfield Renewable Energy Group)

A U.S. Department of Energy report last month sought to draw attention to an underutilized national resource: dams.

The nation’s waterways are broken up by more than 80,000 dams, but only about 2,500 of them are used for generating power.

The Midwest could add more than 5,000 megawatts of generating capacity by incorporating turbines into existing dams on the Ohio and Mississippi rivers, the report says.

A newly operating small hydro plant in Minneapolis offers a potential model for developers elsewhere in the region.

The 9.2-megawatt Lower Saint Anthony Falls hydroelectric project was built into an Army Corps of Engineers’ lock and dam on the Mississippi River. It started generating in December.

It’s the first U.S. facility to use a technology called StrafloMatrix — a modular system in which turbines and generators sit in window-fan-like boxes that can be stacked and assembled to fit unique spaces.

The Lower Saint Anthony Falls project has 16 turbines stacked two deep and eight across. The turbines are made by an Austrian company called Andritz Hydro.

The equipment is designed to be compact enough to fit inside existing dams. It also allows for individual sections to be pulled out of the water for maintenance without disturbing the rest of the system.

“It’s an interesting concept,” said Peter Rodrigue, a senior consultant with Hatch Energy in Amherst, N.Y.

In general, it’s more economical to have fewer, larger turbines, but a matrix system may have an advantage for certain existing facilities, Rodrigue said.

Hatch Energy consulted on a proposed project on the Ohio River in the 1990s that would have used the technology and concluded they weren’t economical at that time.

But hydropower is a very site-specific business, more so than wind power, and so it’s hard to generalize about how widespread a turbine could be used, Rodrigue said.

Brookfield Renewable Energy Group and Nelson Energy, which own the facility, applied for their federal permit in 2004 and started construction in 2009. The $38 million project was paid for in part with a grant from Xcel Energy’s Renewable Development Fund.

Glenn Cada, a fish biologist with the Oak Ridge National Laboratory who studies the impact of hydropower, said he’s unaware of any conclusive studies on how the small, matrix turbines might affect aquatic life.

There are three main ways in which fish are harmed by hydroelectric facilities. One is being struck by turbine blades. Others are the extreme pressure changes and water velocities.

“It’s like a fish traveling through the jet of a fire hose,” Cada said. “There’s turbulence and sheer stresses that can remove scales or even tear gill cover.”

The smaller blades leave less space between them for fish to swim through, Cada said, but on the pressure change and water velocities in matrix turbines are likely less intense and therefore more fish-friendly.

As a run-of-the-river hydro facility, it doesn’t involve creating any new reservoir, avoiding the carbon-sink penalty we reported on Wednesday.

“[M]any of the monetary costs and environmental impacts of dam construction have already been incurred,” the DOE report says, “so adding power to the existing dam structure can often be achieved at lower cost, with less risk, and in a shorter timeframe than development requiring new dam construction.”