A pump-as-turbine (PAT) microhydro plant is just what it sounds like—the turbine is actually the impeller of a centrifugal pump “running backward” and the generator is simply the pump’s induction motor. PAT installations have been running reliably and efficiently for years. Utilities around the world also use the concept in massive pumped-storage installations. For village and household scales, the technology was pioneered by Arthur Williams in his book Pumps as Turbines: A User’s Guide, published by Intermediate Technology Development Group (ITDG, www.itdg.org). The Canadian hydro-power controls company Thompson & Howe uses PATs to power its factory. I’ve been working with the Border Green Energy Team (BGET) building PAT systems in Thailand for the past six years.
One key advantage of PATs is that centrifugal pumps are robust, mass-manufactured, more readily available, and less expensive than manufactured microhydro turbines. They’re also easier to fix, since it’s a lot easier to find a pump mechanic than a microhydro mechanic. A disadvantage is that unlike Peltons and turgos, a single installation is not efficient over a wide range of flows. This can be mitigated by having multiple PATs of different sizes each optimized for a different flow, and turning them on in combination to suit specific flow regimes. We generally design just for dry-season flow and use that year-round. Pump selection (head, flow, and mechanical characteristics) is key—if you’re serious about it, read Williams’s book.
A PAT system generally uses the pump’s induction motor as an AC generator. For grid-tied installations, induction motors are usually the easiest rotating generation to interconnect directly. For stand-alone installations, capacitors are required to provide reactive power that allows the pump’s induction motor to generate AC electricity. The process is not difficult, and is described in Nigel Smith’s Motors as Generators for Micro-Hydro Power, also published by ITDG press. You can find the essential equations from both Williams’s and Smith’s books in a PAT design spreadsheet at www.palangthai.org/docs/.