The figure below is a generic figure showing parts of a typical “run-of-river” hydroelectric power scheme. Not all the components shown are needed in all systems, but it does illustrate the major elements that might be present.
A “run-of-river” scheme is one where the flow available for the hydropower system is related to the immediate flow in the river. There is no reservoir so power varies with flow. An example of a non-run-of-river scheme might be Hoover Dam or any pumped storage scheme.
Dam or Weir
In some schemes, a low head weir might be used to help fill a channel feeding the hydropower system, but this is not required in all instances. The hydropower system may be downstream of the weir or it might be located on or adjacent to the weir structure.
Leat (or Leet)
This is typically an open, nearly level channel for moving the water downstream to gain head.
Some systems might contain an overspill to control the water level within the leat system.
Mill or Settling Pond
Some systems contain a pond, typically so debris can settle out of the water stream before entering the turbine. This is not necessary for an Archimedes screw system.
For turbines other than Archimedes Screw based systems, a pipe decends steeply to the hydropower system/power house. In some instances the turbine might be located part way down the head drop with a “siphon” pipe continuing from the turbine to the lower water level.
Turbine System (power house)
The hydropower turbine system might be located either up at the weir or at the end of the leat/penstock system. The system consists of the turbine, gearbox, and generator and might also include the controls and power conditioning electronics.
The control system and power conditioning equipment can be remotely located (not to far) if needed.
In the illustration, there is very little of a tailrace, but on some systems there may be some distance for the water to travel from the hydropower system back into the river.
On low head sites, the water levels can vary considerably (especially the tail water level) as a function of river flow (a greater % change compared to high head systems) and understanding this variation as a function of flow is absolutely critical to accurately predicting the energy generating performance of a system.
In a typical small hydropower scheme it is important to have access to the consumer of the electricity and/or the GRID. When investigating a potential site, it is even better if there is a transformer near the proposed location since most small hydropower schemes need to have their voltage levels raised for transmission for any great distances.
The depleted reach (of the river) is the length of river between intake to the leat and where the water re-enters the river. It is not pointed out on the diagram, but this distance is a critical parameter is determining what an appropriate “hands-off flow” and “design flow” for the system might be. Longer depleted reaches usually mean that the design flow needs to be less and the hands off flow more.