Abstraction Licence

Authorisation granted by the Environment Agency to allow the removal of water from a source (permanently or temporarily)

Capacity factor

The ratio of energy output per year to the maximum output if the system runs at full rated capacity all year round.

Compensation Flow

The flow which must be left in the river at the point of abstraction, for ecological purposes.

Fish Ladder (or Fish Pass)

A structure consisting of a series of overflow weirs which are arranged in steps that rise about 30cms in 3 to 4m horizontally, and serve as a means for allowing migrant fish to travel upstream past a dam or weir.

Flow Duration Curve

A graph showing the percentage of time that the flow at a particular gauging station equals or exceeds certain values.

Forebay

An open tank for slowing down the incoming flow and settling out silt and gravel before the flow passes into the penstock.

Gauging Station

A site where the flow of a river is measured.

Gross Head

The difference between the upstream and downstream water levels.

Headrace

The channel that forms the inlet to a turbine.

Impoundment Licence

The authorisation granted by the Environment Agency to allow the obstruction or impeding the flow of water.

Installed Capacity

The total maximum output (kW) of the generating units in a hydropower plant.

Kilowatt (kW)

Unit of power, equal to 1000 watts

Kilowatt hour (kWh)

Unit of electrical energy, equal to the electricity supplied by 1 kW working for 1 hour.  1 kWh = 3,600,000 Joules

Leat or Lade

An open channel that conveys water at a shallow gradient from a river channel where sufficient head has been gained for a turbine to be installed. (Also sometimes called Goit or Contour Canal).

Net Head

The pressure head available to the turbine after friction losses through the intake and trash rack.

Output

The amount of power (or energy depending on definition) delivered from a piece of equipment, station or system.

Penstock

A pipe (usually steel, concrete or plastic) that conveys water under pressure from intake to turbine.

Sluice Gates

A vertical shaft slide gate, which can be operated either manually or by electric motors (there are other types).

Spillway

A controlled discharge of excess flow back into the river.

Tailrace

The channel that takes flow away from the turbine outlet

Trashrack

A protective screen that prevents large brances, tree trunks and other debris from entering and damaging the turbine.  It usually consists of vertical bars spaced between 30-100 mm apart.  The screen is typically cleaned by an automatic rake which removes the debris, either to a platform or to be flushed into the river.

Turbine

A machine converting the speed and/or pressure of flowing water into rotational energy.

Weir

A low dam which is designed to provide sufficient upstream depth for a water intake while allowing flow to pass over its crest.