Commonly used terms

Gross head - The vertical drop between intake and discharge water level before any hydraulic losses are considered.

Head loss  - Hydraulic losses due to friction and turbulence. May require careful analysis to quantify exactly. A well-designed scheme should achieve total head loss of 10% or less. The extra cost of achieving a lower head loss may need to be considered.

Net head - Gross head with all head losses subtracted.

Flow rate – The volume of water passing a point in one second, measured in cubic metres per second or in litres per second if using metric units. One thousand litres per second is equal to one cubic metre per second therefore 50 l/s = 0.05 m3/s, for example.

Mean flow rate – The average flow at a particular site.

Design flow rate - This is selected by the designer by taking careful account of all project constraints. 

Flow availability which has been determined by site survey and catchment study is the initial constraint. Typically, a value of Qmean (i.e. average annual flow ) or lower is used as the design flow rate. As a first estimate of site viability, Qmean can be used if an accurate historical value can be established. If no turbines are available because Qmean is too high, then reduce the value of the design flow rate in 10% steps until a suitable system is identified. Further factors affecting the design flow rate include:

  • Environmental constraints (e.g. 'hands off' flow and/or flow split requirements) 
  • Grid connection constraints relating to the power export limit.
  • Magnitude of user loads ("off-grid" schemes)
  • Project finance and ROI requirements

'Hands off' flow rate – The flow rate designated to be left in the river at all times. This typically has a minimum value equal to the flow value at Q90 on the flow duration curve for the site.

Flow duration curve  - This is a method of characterising the flow variations at a particular site. A simple flow duration curve can be created by organising all measured flow values in order of magnitude with the largest first. The more data which is used, the more likely that the resulting graph will be indicative of future flow patterns. The Y axis is used to show flow rate and the X axis is divided into 100 percentage points. This is used to illustrate the probability of a particular flow rate being equalled or exceeded (PE or Probability Exceedence). For example, the flow rate occuring at the 50 per cent mark (referred to as Q50) would be equalled or exceeded for 50 per cent of the time, therefore a period of six months of the year if flow data in complete years is used. 

Head duration curve - Similar to flow duration curve, except expresses the change in head and the probability of a particular value being equalled or exceeded.  As the X axis is the same as for the flow duration curve, the two curves are often displayed on the same graph and therefore the relationship between head and flow can be illustrated. A head duration curve is important at some low head sites as the change in head levels will have a significant effect on energy capture.

Carbon dioxide saving – The quantity of CO2 emissions saved through the production of renewable electricity compared with grid electricity derived from fossil fuel sources. 

System efficiency – The combined efficiency of the whole scheme, sometimes referred to as the "water to wire" efficiency.

Energy capture – The total electricity generation calculated from the system efficiencies, flow and head duration curves (if applicable), usually over a time period of one year.

Turbine Performance - The turbine efficiency measures of how effectively the turbine converts hydropower into rotating mechancial power. The turbine efficiency varies according to changes in the flow and head levels away from the design conditions. Generally the efficiency decreases as the site conditions move further from the design conditions. The turbine performance curve shows how the turbine efficiency will change over the whole operating range and allows the energy capture estimates to be modified accordingly.

Site Adjustment Factor - This value allows the turbine performance to be adjusted to take into account site conditions which require a head an flow combination varying from the optimum turbine conditions.  For example, if a particular turbine could drive a generator directly (i.e. without the need for pulleys or a gearbox) then this could result in a cost saving and and improvement in overall efficiency. The site adjustment factor could be used to take into account any small reduction in turbine performance that may result from a direct drive combination. The default value of 1.0 should be used for inital energy assessment. On advice from turbine manufacturer or installer, this value may be altered as the scheme design progresses.