Hydraulic head
The vertical height difference between the water surface at a hydropower intake and the turbine outlet or tailwater, which together with flow determines available power.
Head is the potential energy per unit weight of water, expressed in metres. Hydropower designers distinguish:
- Gross head: the elevation difference between headwater and tailwater levels
- Net head: gross head minus friction and fitting losses in the intake, penstock and valves, typically 3–10% of gross head in a well-designed scheme
Because power is proportional to the product of flow and head, a high-head site needs much less water for the same output. A 1 MW plant needs about 0.12 m³/s at 1,000 m head but 12 m³/s at 10 m head, which drives turbine choice and civil works size.
Conventional head classes and matching turbines:
- Low head (under about 30 m): Kaplan, bulb, propeller and Archimedes screw units
- Medium head (30–300 m): Francis turbines
- High head (above about 250–300 m): Pelton and Turgo turbines; Bieudron in Switzerland operates at about 1,880 m
In practice selection uses the specific speed parameter, which combines head, flow and rotational speed. For pumped hydro storage, head sets energy density: 1 m³ of water falling 500 m stores about 1.36 kWh before losses, so high-head sites need far smaller reservoirs per GWh.
Related terms
All terms →- Pelton turbine
An impulse hydro turbine in which high-velocity water jets strike double-cupped buckets on a wheel; the standard choice for high heads above roughly 250 m.
- Francis turbine
A reaction-type hydro turbine with fixed blades in which water enters radially and leaves axially; the most widely installed design, used for medium heads of about 30 to 700 m.
- Kaplan turbine
A propeller-type reaction hydro turbine with adjustable runner blades and wicket gates, suited to low heads of about 2 to 40 m and large flows.
- Run-of-river hydro
Hydropower that diverts or uses the natural flow of a river with little or no reservoir storage, so output follows river flow rather than demand.
- Pumped hydro storage
Energy storage that pumps water to an upper reservoir when electricity is cheap or abundant and releases it through turbines when power is needed; the largest form of grid storage worldwide.