Actual or total catchment area (km²)

The "actual or total catchment area" of a water-course at a give point is equal to the algebraic sum of the natural catchment area at that point and the diverted catchment areas which modify its inflows. By extension, the actual catchment are of a head installation is defined as the sum of the actual catchment areas of each of its intakes. (UNIPEDE)

Cavitation

Noise or vibration causing damage to the turbine blades as a results of bubbles that form in the water as it goes through the turbine which causes a loss in capacity, head loss, efficiency loss, and the cavity or bubble collapses when they pass into higher regions of pressure.

Characteristic generation (kWh)

This concept of "characteristic generation", which is specified to the hydro field, represents what could have been generated by a head installation during a given period. It is obtained by adding the energy losses to the net generation. All energy losses except those when an actual flow exceeds the maximum usable flow of a head installation. By extension, a mean characteristic generation of a head installation can be calculated over a long period. This concept is different from energy capability because it does not take into account the variability of storage in its reservoirs. It should be noted that the generation in question is a net generation. The latter is generally calculated on the basis of the generation at the machine terminals less the transformer losses, as well as the consumption of all the electrical auxiliaries (auxiliary hydro sets supplied by another head installation, auxiliary services supplied by the distribution network, etc…). (UNIPEDE)

Classification of hydro-electric head installations

Hydro-electric head installations are classified according to the use that can be made of the cumulative flow that they receive, depending on whether the cumulative flow must be used within a short period of time or whether it can be retained for a certain period. This criteria is based on the reservoir filling period "D" calculated using the annual characteristic mean flow. Run-of-river head installations: These head installations normally operate on base load and use the cumulative flow continuously or receive (or use) environmental flow releases. Storage head installations: Hydro-electric head installations storing their cumulative flows wholly or partly in their retaining works in order to generate during hours of higher demand. According to the filling period of a reservoir it can be defined as follows : Pondage 2 hours < D < 400 hours Reservoir D = 400 hours These head installations are normally operated in such a way as to allow load following. By extension, when the operation of a head installation is directly related to a that of a reservoir upstream and the intermediate inflows are negligible, these head installations must be considered to belong to the same category as the one which governs them. Different types of head installations with pumping: Pumped storage head installations or head installations with pumping are those in which water can be raised by means of pumps and stored, to be used later for the generation of electrical energy. The basic difference between the various types of pumped storage head installation arise from the way in which the turbines and pumps are arranged in the hydraulic circuits. In all types of pumped storage stations, the pumps and turbines are connected to one or several interconnected upper reservoir(s). When the pumps and turbines are connected to the same lower reservoir or to reservoirs which are themselves connected, the pumping cycle can be repeated many times. A distinction must be made according to whether the upper reservoir is fed by significant natural flows. If, on the other hand, the pumps and turbines are connected to physically separate lower reservoirs without a hydraulic connection between them except by way of an upper reservoir, there can be no pump/turbine cycle as such and the pumps have only the role of pumping into the upper reservoir the contribution of water acquired at their level. A station having this latter configuration is known by the term "station with contributory pumping". These head installations are classified in the categories referred to in definitions above, according to the filling period of the reservoir "D". Pure pumped storage head installation: A "pure pumped storage head installation" is one without significant natural cumulative flows into the upper reservoir. Note : The natural cumulative flow into the upper reservoir, in an average year, permits a utilisation period of the maximum electrical capacity in the turbine mode less than or equal to 250 hours (average value in France and Italy). Mixed pumped storage head installation: A "mixed pumped storage head installation" is one with significant natural cumulative flow into the upper reservoir.

Diverted catchment area (km²)

The "diverted catchment area" is a catchment area, the inflows to which are artificially diverted (by gravity or pumping) from their natural course to another water course for energy purposes or some other purpose. The characteristics of the deviation works may result in a limitation of the deviated flows, and so not all the inflows to that catchment area may be diverted. A diverted catchment area may itself be natural or actual. (UNIPEDE)

Energy head height (m)

The "energy head height" of a head installation for a given flow in the intake works, is the difference in altitude between the cross section of the charging point of the intake works and, depending on the case, either the tail-race cross section, or the Pelton reference level. Two characteristic values are defined: Maximum energy head height (m) - The "maximum energy head height" is defined as the difference between the maximum normal operating level and the maximum tail race level or the Pelton reference level. It is measured at the maximum usable flow of all the generator sets of the head installation. Normally it corresponds to the maximum electrical capacity of the head installation. Mean energy head height (m) - The "mean energy head height" is the difference, for the average usable flow in operation, between the mean operating level and the mean tailrace level in operation or the Pelton reference level.

Gross maximum head height (m)

The "gross maximum head height" of a head installation is the difference in level between the maximum normal operating level in the retaining and the minimum tail-race level works. When the tail-race level is affected by the head height of another installation, the tail-race level corresponds to the maximum normal operating level of the downstream reservoir. It is defined assuming the following conditions: maximum normal operating level, zero flow through turbines,zero guaranteed flow.

Guarantee of Origin

An electronic certificate representing 1 MWh of electricity production used for the purpose of proving to final customers the share or quantity of renewable energy that was supplied to them. (Directive 2009/28/ec)

Intermediate catchment area (km²)

The "intermediate catchment area" of a water-course at a given point is equal to the difference between its own actual catchment area and those of the works situated immediately upstream to it. In the case of a head installation the inflow from the intermediate catchment area represents the theoretical "inevitable run of the river" when all the upstream head installations are shut down, without overflow, and when all independent storage works are closed. (UNIPEDE)

Maximum usable flow or plant capacity flow (m³/s)

The "maximum usable flow" of a head installation is the maximum flow corresponding, under continuous operating conditions, to the complete opening of the turbines at maximum normal operating level of the reservoir and the minimum flow of the receiving water course or possibly the normal reservoir level of the downstream reservoir. It is not necessarily the same as the flow which corresponds to the maximum electrical capacity output.

Natural catchment area (km²)

The "natural catchment area" of a water course at a given point consists of all the areas from which rainfall would flow by surface run-off into the bed of the watercourse at the point in question in the absence of any upstream division. By extension, the natural catchment area of a head installation is defined as the sum of all the natural catchment areas of its intakes. (UNIPEDE)

Net head height (m)

The "net head height of a head installation", under specified conditions of inflow and operation, is the head height actually used by its turbines, i.e. the difference between the level corresponding to the manometric height at the turbine inlet, taking into account the equivalent velocity head at this point, and, o in the case of reaction turbines, the tail-race level increased by the equivalent velocity head at that point o in the case of impulse turbines, the average level of injection (Pelton). In practice, the net head height is the gross maximum head height less the sum of the pressure losses in the intake works for the given flow.

Tail-race level (m)

The "tail-race level" is the level of the water which has passed through the turbines, measured at the end of the turbine discharge works. It is a function of the flow which passes through the turbine and of either: the level of a still at the end of the discharge works, or the flow in the turbine outlet reception bay, or the level of the downstream reservoir, if it has an influence. (UNIPEDE)