Most of the microhydro turbines used in home-scale installations in the United States use either a Pelton- or Turgo-style runner coupled to an AC permanent-magnet alternator. These types of turbines are typically used at sites where the vertical drop is more than 15 feet. Water enters a screened intake at the top of a pipeline that runs downhill along the stream’s course. There is 1 psi of pressure for every 2.31 feet of drop in the penstock’s elevation. The resulting column of pressurized water above the turbine is routed through one or more nozzles inside the turbine’s housing, creating strong jets of water that are directed at the turbine’s runner to spin the alternator, which generates electricity.

In the Vietnamese low-head hydro unit that I purchased for Somsak’s installation, the physics is reversed. There is no penstock. Water is diverted from a stream and channeled through an open-top, elevated waterway that delivers water to the turbine. Instead of the penstock, a pipe called a draft tube is installed below the turbine. The draft tube sucks water through the propeller, which in turn spins the alternator. This particular turbine design can generate electricity at sites with as little as 5 feet of head so long as a sufficient flow rate is available. Because of this low head requirement, these turbines are frequently installed between terraces in the rice fields in Southeast Asia.

Unlike the output in many other systems, which either charge batteries or are grid-connected, the voltage of the Vietnamese turbines’ AC output is regulated and fed directly to AC loads, most commonly lighting. The inexpensive turbine’s AC output is not designed to be synchronized with the utility grid. While the power quality regulation is pretty sloppy—probably not something you’d want to subject your home entertainment system to—it’s definitely sufficient to power lighting and other simple appliances typically used in remote parts of Asia.