For this low head application domain, propeller turbines are
normally used, as can be seen in the typical turbine application
range diagram in Fig. 1. Propeller turbines are used at low head as their rotational speed is able to match grid frequency, but they
exhibit poor efficiency either side of the rated flow. At the larger
power scales, this is overcome by using variable inlet vanes and
blades, but at the pico hydropower scale this is not viable due to
increased cost, unit complexity and reduced system reliability. In
all the available reported work, impulse turbines, such as the Turgo
or Pelton turbine, are used in a medium- to high-head application
environment, as shown in Fig. 1. At low head, impulse turbines are
normally avoided as they have a slow rotational speed and are
large, but in theory their application at low head should be feasible
[6]. Impulse turbines are simple to construct, robust, inherently
modular and serviceable, and have good efficiency over a large variation
in flow rates. Low speed, high-pole number permanent-magnet
generators and variable frequency power electronic converters
can be used to interface with a 50/60 Hz grid, overcoming the slow
rotational speed issue. Using the criteria described above and
application domain, a Turgo turbine was shown in [5] to be a preferred
option.