This is the head which will be available to the turbine before considering head losses when the system is operating. The designer should always seek to optimise the value of gross head for the proposed scheme. This means to maximise the head but within reasonable costs as explained under scheme 'Types'. If different layout options are possible but provide different values of gross head, then these should be evaluated individually. Hydromatch will recommend the head value at Qmean for the gross head if flow and head data have been uploaded.
This is a key value for careful consideration and is selected by the designer by taking careful account of all the project design criteria and constraints. Several iterations of the scheme design with different design flow rates will be required before the value is finalised. This is one of the principle reasons to use dedicated software such as Hydromatch.
Flow availability which has been determined by site survey and catchment characterisation is the initial constraint. Typically a value of Qmean (i.e. the average annual flow rate) 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 turbine system is identified. Hydromatch will recommend Qmean as the design flow if flow data is uploaded.
Further factors which will affect the design flow rate include the following:
- Environmental constraints which will prescribe the 'hands off' flow and perhaps flow split requirements between the hydro power installation and the depleted river section.
- Grid connection constraints. e.g. what is the power export limit given the existing grid connection arrangements. What is the cost to upgrade the existing supply to increase the export capacity.
- Magnitude of user loads if an off grid scheme is proposed
- The scheme budget, finance availability and return on investment