Pint-size hydro power on tap

Rentricity, a start-up in New York City, has come up with a hydroelectric generator that lets municipal water facilities generate power. Pressurized water from the facility passes through a turbine, and the turbine produces electricity. The water subsequently comes out of your faucet.

The company doesn't like to use the term "hydroelectric power"--which conjures up images of large construction projects and regulatory tangles--but the principles are the same, Frank Zammataro, president of Rentricity, said during a meeting here at the Dow Jones Alternative Energy Innovations conference.

The system works because municipalities process millions of gallons of water a day and the water gets highly pressurized during the purification process. Some facilities process 9 million gallons of water a day and hold the water at 45 psi (pounds per square inch). If water came out of the faucet at that pressure, you'd have trouble washing your face without getting welts. Thus, water districts have to artificially bleed off the pressure.

But instead of doing that, the utility can make electricity. A single "Flow-to-Wire" micro-turbine generator from Rentricity can produce anywhere from 20 to 300 kilowatts of power, depending on the pressure and water flow. (A U.S. home solar system typically generates about 3 kilowatts.) Sensors and software from the company also monitor performance.

"It won't reduce the flow," Zammataro said. "We are taking off-the-shelf technology but configuring it in unusual ways."

At a minimum, the system needs to be put in a facility that processes a million gallons a day and holds the water at 35 psi. Potentially, there are 25,000 sites that could accommodate this equipment, according to the company.

Combined, these sites could produce a gigawatt of power, conceivably. While the generator alone isn't a solution to global warming, it can generate as much power as a big coal-driven power plant. Zammataro estimated that 1,000 of the systems could generate $30 million worth of electricity.

The company has already launched a successful pilot program with Aquarion Water in Connecticut and plans to bid on a project in California soon.

Rentricity doesn't sell the systems. Instead, it owns and operates them and then splits the revenue from electricity sales with the utility. Typically, the utility might get 30 percent of the revenue, Zammataro said. Payback should take about three years. Most of the time, a municipal water facility can accept a standard unit, although sometimes the company has to customize its equipment.

Rentricity's efforts touch on several trends. First, the company is making mammoth electricity plants small, similar to what Sopogy (which focuses on solar thermal power) and Puget Sound Tidal Power (tidal power) are doing. It is generating electricity from something that otherwise would not be exploited, like cellulosic ethanol companies. And Rentricity's revenue-sharing model is similar to what Microgy, the company that turns manure into methane, is doing in Texas. It's an alternative-energy cioppino.

Ultimately, Rentricity hopes to port its technology so that it can produce power with low-pressure, high-flow waste streams. Chemical and oil refineries consume massive amounts of water daily, but the pressure is relatively low.