Hydroelectric Power in Water Pipes

Municipal water systems move large quantities of water through underground pipes. Why not use that flow to generate electricity? It seems obvious, but in most cases it’s not practical.

That’s because many water systems use electric pumps to move water from an aquifer or reservoir to its customers. Putting a hydroelectric generator in the pipes would take energy away from the flow, so you’d need a more powerful pump to move the water. Thanks to the third law of thermodynamics, you’d lose more electricity with the larger pump than you would generate with the turbines. It’s a net-loss. Unless...

Gravity Is the Key

There is an exception, however. If the water flows naturally from a higher elevation, and the water wasn’t pumped to that elevation (as in a water tower), then the force that moves the water is just gravity. No man-made energy is involved, so it’s possible to extract some of the energy from the flowing water and convert it to electricity. And that’s exactly what the city of Portland Oregon is doing in conjunction with Lucid Energy. Here’s the CEO of Lucid Energy, Gregg Semler, discussing the concept:

Semler said two things rather quickly, but they are crucial to implementing this successfully: 1) the source of water must be free-flowing (gravity fed), not pumped; 2) there must be extra pressure available, because the pressure after the turbines will be lower than it was before. You don’t want your water customers to have unacceptable water pressure just so you can generate clean electricity.

In Portland’s case, the water source where this is being implemented is indeed gravity fed. The Bull Run Watershed collects water from rain and snow melt. It includes two reservoirs, both which are at a higher elevation than the city of Portland, so no added energy is needed to move the water. In fact, the two reservoirs have dams with hydroelectric generators that produce nearly 85 GWh of electricity each year. That’s a little less than 5 MW per generator.

Pressure Is the Other Key

The Portland Water Bureau guarantees at least 20 PSI of pressure to all of its customers, and claims that most homes receive two to four times that amount. That’s more than enough excess to drive the turbines, which reduce pressure by up to six PSI, without adversely affecting Portland’s water customers. Each turbine is capable of generating 50 kW, and the system in Portland includes four series turbines in a 42 inch diameter pipe.

Pilot Project

For now, one section of pipe is in place and being tested. When fully operational in March of 2015, it should produce over 1100 MWh each year, enough to power more than 150 homes. (That’s a realistic estimate; if you assume peak power and constant production, neither of which is likely, the system could potentially generate 1750 MWh/year.) If the pilot is successful, the city will install more, obviously keeping a watchful eye on pressure losses. The project is financed by private investors who will sell power to Portland through a twenty year Power Purchase Agreement (PPA). At the end of the contract, Portland can purchase the system or enter into a new PPA.

According to the US Geological Survey, 66% of public water comes from surface water. Presumably some of that is pumped at some point, but even if only half of that were gravity fed, the potential for electricity production from municipal water pipes is significant. Will Portland become a trailblazer in renewable energy or is this just a pipe dream?