Hydropower Generation Mechanism

A hydropower plant generates electricity by using falling or flowing water to rotate a turbine connected to a generator. Its output depends chiefly on hydraulic head, the available hydraulic energy per unit weight of water, and discharge, the volume of water passing through the turbines each second.

Hydraulic head: energy available from each unit of water

In a conventional hydropower scheme, water moves from a higher upstream level to a lower downstream level. This elevation difference provides the energy that drives the turbine.

• Gross head is the difference between the upstream and downstream water-surface elevations.
• Net head is the head actually available to the turbine after hydraulic losses in the water-conveyance system are deducted.
• A greater net head provides more energy per unit volume of water, so it increases output when turbine discharge and efficiency remain unchanged.

Discharge: how much water supplies that energy

Discharge, denoted by Q, is measured in cubic metres per second. A larger discharge through the turbines means that more water transfers its energy each second.

• Turbine discharge is not necessarily equal to the river's total discharge: some water may bypass the turbines, be released for downstream needs, or spill when inflow exceeds plant capacity.
• A storage reservoir can shift the timing of water use, while a run-of-river plant generally depends more directly on available river flows.

How head and discharge combine

Electrical power is estimated using P = ρgQHη, where P is power in watts, ρ is water density, g is gravitational acceleration, Q is turbine discharge, H is net head in metres, and η is the combined turbine-generator efficiency.

• Power is proportional to Q × H: doubling either discharge or net head doubles output if the other variables remain unchanged.
• Efficiency accounts for the fact that turbines and generators cannot convert all available hydraulic power into electricity.
• For example, a net head of 100 metres, discharge of 50 cubic metres per second, and efficiency of 90% yield approximately 44 MW.

Why actual generation varies

Installed capacity limits the electricity a plant can deliver even when more water is available. Seasonal flows, reservoir levels and changing downstream water levels can alter discharge or net head.

• Power is the rate of electricity generation, whereas energy generated over a period depends on both output and operating duration.
• Consequently, high installed capacity alone does not guarantee high annual electricity generation.