Water is often stored high above the city that consumes it. To reach its destination, water travels through a system of gradually shrinking pipes until it comes out the faucet. As the water travels, excess pressure gathers in the pipes, which is dissipated by pressure valves. All that built-up energy leaves the system, wasted.
This type of energy waste is hard to design away. Some water systems are so old they still have hollowed-out cedar logs as pipes. And huge numbers of people depend on the systems continuing to function without a break. So rather than figuring out ways not to waste energy, it's easier to harness and repurpose it. Rentricity, an energy company based in New York City, is doing exactly that by creating electricity from excess pressure in water pipes.
Frank Zammataro, Rentricity’s president, learned about the inefficiencies of municipal water systems after he spent too many hours thinking about a New York City water tower. In 2001, he and his colleagues looked down from their 40th floor Midtown office on the building below and joked about how every time someone flushed on the first floor, water had to travel from the tower on top of the building all the way down. Someone, they thought, ought to put in a wheel in the pipe to capture that squandered energy.
Zammataro took the idea seriously enough that he scheduled a visit to Rensselaer Polytechnic Institute in upstate New York to investigate further. The professors told him that a single building didn’t have enough sustainable flow or pressure to make harvesting the energy practical. But one suggested he take a look at municipal water systems and the points where they regulate pressure in the pipelines.
Rentricity takes the energy from those points in a water system and uses it to turn a turbine, creating electricity. The system itself didn’t require any breakthrough technology: the company’s primary innovation was recognizing that a valuable resource was being thrown away. “We applied the technology in a unique way and in a unique place,” says Zammataro. “We're not innovating at a turbine level.”
The company installed its a pilot project in Connecticut in 2007 and has now completed two successful commercial installations, with three more in the works. One of its first projects, a dual-turbine system in Keene, New Hampshire, generates 62 kilowatts of energy when it’s operating at peak capacity. That’s not a huge amount, but it covers the system’s needs with enough leftover to sell power back to the grid. Zammataro puts the total potential power generation from water systems “in the 100s of megawatts.” For comparison, a single gas-fired power plant can generate more than 2 gigawatts.
But an innovation like Rentricity’s doesn’t need to take over from coal, oil, and gas on its own. Most visions of a clean energy future rely on an increasingly diverse range of power sources, and hydrokinetic energy like this has an advantage over solar and wind because it’s reliable and predictable. Rentricity’s technology depends on the same basic idea that a dam does: Store water up high and capture the energy created as it falls down. Although they’re old technology, dams still account for 35 percent of America’s renewable energy—a larger chunk than any other source. The Global Cleantech Cluster Association recently named Rentricity one of the ten “best in class” clean tech companies in the world.
Municipal water systems aren’t the only area where Rentricity concept could be applied, either. Two of the company's current projects are testing out the idea on wastewater systems. Industrial facilities, whether they pipe in paper pulp or Campbell’s soup, encounter the same issues as water systems, Zammataro says: The contents of the pipes move quickly down relatively steep slopes. Rentricity is also looking at designing smaller turbines to harvest energy from smaller pipelines.
“This kind of energy recovery needs to be made non-discretionary,” Zammataro says. He points out that infrastructure of water systems is due for an upgrade. “If we’re going to rebuild water pipelines, when you reinstall, you should have to determine if you can install a generator instead of a pressure regulating valve.”