Gross and net potential between two sections

Gross potential is the theoretical power of the river between two sections, using all the flow and the full head difference with 100% efficiency. Net potential is the power that can actually be developed after deducting head loss, efficiency and the part of the flow that cannot be used.

Gross potential

Divide the reach into small sub-reaches where the flow is nearly constant (the flow increases downstream with tributaries and catchment area). For each sub-reach:

$$P_{gross} = \sum \rho g Q_i \Delta H_i = \sum 9.81\, Q_i\, \Delta H_i \ \ (\text{kW})$$

where $Q_i$ is the mean flow (m³/s) and $\Delta H_i$ is the drop in elevation (m) of the sub-reach. Annual energy = $P \times 8760$ kWh. Flow comes from the flow series at the two sections (area-ratio method for ungauged sites), and elevations from topographic maps or survey.

Net (technical/economic) potential

• Choose the design discharge $Q_d$ from the flow duration curve (e.g. $Q_{40}$ for RoR), after leaving the environmental flow.
• Net head $H_n = H_{gross} - h_f$, where $h_f$ includes the intake, desander, waterway and penstock losses (about 5-10% of gross head).
• Overall efficiency $\eta$ of the turbine, generator and transformer (about 80-90%).

$$P_{net} = 9.81\, \eta\, Q_d\, H_n \ \ (\text{kW})$$

• Annual energy is found from the monthly flows up to $Q_d$. Economic potential is the part for which benefit-cost ratio is greater than one.