How a hydropower dam makes electricity
Hydropower uses the energy of falling or flowing water. A dam stores water at height; releasing it through a turbine converts that stored energy into electricity.
In 60 seconds: how it works
A hydropower dam uses the energy of water falling from a height. Water released from the reservoir spins a turbine, and the turbine turns a generator.
- The dam holds back a reservoir, storing water high above the power station.
- Water is released into a large pipe called a penstock and rushes downhill.
- Adjustable gates control how much water reaches the turbine, which spins as the water hits it.
- The turbine shaft turns a generator, which produces electricity.
- The water leaves the turbine and flows back into the river below the dam.
- A transformer raises the voltage so the power can travel to the grid.
Want the engineering detail? Explore the 3D model, then read the engineering notes and the types and variations.
By Power Stations Worldwide · Related reading updated · Preparation and review scope. Educational illustrations; no independent engineering peer review is claimed.
Explore the equipment in 3D
Rotate equipment, follow energy flows and inspect each component. Read the steps below at any time.
Choose a step to read the process. Activate the 3D model or start a guided tour to use the animation controls.
Gravity-dam reservoir plant with two vertical Francis units and direct-grid synchronous generators. Powerhouse and turbine sections reveal linked pivoting gates, fixed casings, shafts, bearings and expanding draft tubes. Gross head, gate-flow law, efficiencies and 24-pole/50 Hz setting are illustrative; the steady-state model is not hydraulic, structural or dam-safety approval. Motion is slowed for inspection. Coloured overlays reveal flow through enclosed equipment; gold pulses show net electrical energy transfer, not individual electrons.
How it works
- Water at a higher level
- Controlled water flow
- Turbine rotation
- Generator electricity
- Transformer and grid
Hydropower turns the energy of water moving from a higher level to a lower one into electricity. An intake admits water and a channel or penstock carries it to the turbine. In the illustrated Francis machine, guide vanes control the flow entering the runner. The runner turns a shaft connected to a generator, and water returns downstream. Available power depends on both flow and hydraulic head: the energy available per unit weight of water, after losses in the water passages.
A reservoir can let operators choose when to release some of the water, within water-management and environmental limits. A run-of-river plant depends more directly on the river’s flow. Neither arrangement creates water, so seasonal inflow and drought affect production. Turbines are selected for the site’s head and flow rather than simply its installed megawatts. Fish passage, sediment movement and downstream water conditions also shape the design and operating rules.
Follow the water. Account for the power.
Two vertical Francis units, not Kaplan or Pelton machines. Each has its own water path and breaker; both feed one collector and step-up transformer. Main book passages were checked before this upgrade. Every numerical parameter is an illustrative teaching choice, not a real installation specification.
Selected: whole-system overview.
Simple explanation
Height supplies energy; gates regulate water. Gross head compares the reservoir and downstream water surfaces. The rack catches debris; the intake admits water and the penstock carries it to the spiral casing. Fixed stay vanes support/direct the passage. Linked wicket gates pivot to regulate incoming water; they are not the runner blades. [H1]
The runner drives a separate electrical machine. Water is directed inward and turns downward through the fixed-blade Francis runner. The runner and coaxial generator rotor rotate together; the casing, stator and bearing housings remain fixed. A connected synchronous unit runs at its grid-matching speed. More discharge supplies more torque and power, not a new steady RPM. [H1] [H2] [H4]
Water and electrical energy leave by different routes. The expanding draft tube reduces outlet velocity and recovers part of the velocity head as pressure, then leads to the tailrace. Three-phase alternating current (AC) goes through each unit breaker, the collector and transformer to the grid. Bypass releases avoid generation. The draft tube does not create extra energy. [H1]
Current operating balance
| Boundary | Power |
|---|---|
| Gross hydraulic input through both generating units | 12.210 |
| Intake / penstock loss | 0.581 |
| Turbine unconverted water / mechanical energy | 1.163 |
| Generator loss | 0.419 |
| Transformer / collection loss | 0.151 |
| Useful grid export | 9.896 |
Bypass release is accounted separately: 0.000 m³/s carries 0.000 MW of gross hydraulic potential, with zero attributed generator output. Total released hydraulic potential is 12.210 MW. This is not all heat and is not included in generating-path efficiency.
| Unit | Shaft MW | Torque kN·m | Generator MWac | Frequency Hz |
|---|---|---|---|---|
| Francis 1 | 5.233 | 199.873 | 5.023 | 50 |
| Francis 2 | 5.233 | 199.873 | 5.023 | 50 |
Engineering details
Head and power. Hg is gross head (m), Q is unit discharge (m³/s), hc is intake/penstock head loss (m), and Hnet = Hg − hc is total energy head across the complete turbine. Head represents energy per unit weight; it includes elevation, pressure and velocity terms. Here both free-surface velocity heads are neglected. Net head is not the runner’s depth below the reservoir, and ρgHnet is not presented as a measured runner pressure. [H1]
Pe = ρ g Q Hnet ηt ηg · Pgrid = (Pe,1 + Pe,2) ηtx
Pe is generator AC output (W); ρ is water density (1,000 kg/m³); g is gravitational acceleration (9.80665 m/s²); ηt is aggregate turbine efficiency (0.90, including draft-tube, water and mechanical losses); ηg is generator efficiency (0.96); ηtx is transformer/collection efficiency (0.985). The efficiencies and density are assumed constants. Do not multiply in a second draft-tube recovery efficiency: its performance is already inside ηt. [H1]
Illustrative gate law, not a Francis hill chart. Let a be gate command / 100 for an admitted unit, or zero for an isolated unit regardless of its stored command. Qref = 10 m³/s at Href = 60 m net head, and k = 0.03 s²/m⁵. Assume Q = a Qref √(Hnet/Href) and hc = kQ². With A = a²Qref²/Href, algebra gives Q² = AHg/(1+kA). Each penstock is independent. These invented coefficients are not a friction factor, manufacturer gate curve, computed pipe hydraulics or tested efficiency map. Actual efficiency varies with head, discharge and speed; very small positive commands are conceptual examples, not recommended operation.
Synchronism and torque. n is mechanical speed in revolutions per minute (rpm), f is electrical frequency (Hz), p is total magnetic pole count, ω is angular speed (rad/s), and T is shaft torque (N·m). The checked book uses pole pairs; our p counts all poles. [H2]
n = 120 f / p = 120 × 50 / 24 = 250 rpm · ω = 2πn/60 · T = Pshaft/ω
The selected 24-pole wound-field generator connects directly to the selected 50 Hz grid. Constant speed is a steady synchronous assumption, not a governor simulation. Isolated/zero-flow units are shown parked with gates closed and breakers open; no synchronization, run-up, coasting, runaway or no-load motoring is animated. The animation uses 0.05 × physical rotation, then the display-speed multiplier.
Loss boundaries. Gross generating input ΣρgQHg equals conduit loss ΣρgQhc + turbine loss Σ(Pnet − Pshaft) + generator loss Σ(Pshaft − Pe) + transformer loss + grid export. A bypass release has its own ρgQspillHg term, where Qspill denotes this prescribed non-generating release, not a crest-spillway calculation. Generating-path efficiency is grid export divided by gross hydraulic input through generating units, undefined when that input is zero. No energy is counted twice.
Checked example · reset inputs
Set 62.5 m gross head, both gate commands 100%, both units admitted, and zero bypass flow. Each unit passes 9.960238 m³/s; conduit loss is 2.976190 m, leaving 59.523810 m net head. Each shaft delivers 5.233 MW at 250 rpm, giving 199.873 kN·m; each generator delivers 5.023 MWac.
12.209572 MW = 0.581408 conduit + 1.162816 turbine + 0.418614 generator + 0.150701 transformer + 9.896032 grid.
The generating-path efficiency is 81.05%. Values above are rounded for reading; all displayed current-state values and the original browser scene use the same unrounded calculation. This is a checked algebraic balance with invented parameters, not a real station rating or validated time-domain simulation.
What the cutaway does—and does not—show
- Regulating-ring links move the wicket gates; fixed runner blades do not pitch like a Kaplan’s. The original section exposes enclosed equipment without representing physical holes in an operating plant.
- Bearings guide the shaft and support thrust; foundations, penstock anchors and saddles support the machinery. Their simplified geometry does not validate loads, vibration, fatigue, foundations or dam stability.
- Water pressure/velocity change through a reaction turbine. Cavitation is associated with sufficiently low local pressure; water’s vapour pressure is an important reference, while nuclei and dissolved gas affect inception. A sudden change in penstock velocity can create a pressure wave called water hammer. [H1] No pressure field, cavitation margin or water-hammer transient is computed. Gate changes are instantaneous teaching states, not safe operating instructions.
- About 5 m per scene unit; distances are compressed, equipment details enlarged, water surfaces simplified. The numerical loss coefficient is not inferred from these dimensions. Particles show path/direction, not mass flow, fluid speed or electron motion.
- Hydropower depends on head and water availability. Reservoir operation also has environmental and water-management constraints; this model assumes sufficient inflow and does not predict reservoir depletion or ecological effects.
Download the editable original Blender scene. Generation and geometric-check scripts are included in the project source; this is educational visualization, not an approved installation design.
Main book references
- [H1] John S. Gulliver and Roger E. A. Arndt, editors. Hydropower Engineering Handbook. McGraw-Hill, 1991. ISBN 0-07-025193-2. Checked Arndt’s Chapter 4, printed 4.4–4.7, 4.15, 4.28–4.30 and 4.33–4.34, including equations (4.2), (4.9), (4.16a), in the University of Minnesota institutional scan. Also checked Chapter 5, §5.5, printed 5.55, for the qualitative water-hammer mechanism. These are selected passages, not a review of the entire book.
- [H2] Herbert H. Woodson and James R. Melcher. Electromechanical Dynamics, Part I. Wiley, 1968; lawful MIT OpenCourseWare reissue, 2009. Checked Chapter 4, §4.1.8, equation (4.1.95) and synchronous-speed examples in the MIT-hosted chapter. Our equation uses total poles instead of the book’s pole pairs.
Starting textbook · further reading only
[H3] Theodore Wildi. Electrical Machines, Drives and Power Systems, 6th edition, Pearson New International Edition, published 2013 / ©2014. Only the publisher metadata and contents were checked; technical passages were not accessible. H2 is the actually checked electrical-machine book foundation.
Supplementary technical documentation
[H4] US Department of Energy. Types of Hydropower Turbines, Francis section: fixed blades, scroll casing, wicket gates and draft tube. Supplementary identification, not evidence of our dimensions or efficiencies.
Upgrade sources checked . Original explanations and diagrams; no textbook illustration copied. Software, source and rigid-geometry checks do not establish independent engineering peer review, standards compliance, dam safety or design approval.
Inside the engineering
Follow the machinery, the working fluids and the electrical connection.
Mechanical
Separate intakes and supported penstocks feed each spiral casing. Fixed stay vanes precede linked wicket gates, which regulate discharge into fixed-blade Francis runners. Coaxial shafts drive the generator rotors; guide/thrust bearings and their housings stay fixed. Separate expanding draft tubes connect continuously to the tailraces.
Thermal & chemical
Gross head compares upstream and downstream surfaces; conduit losses reduce each unit’s available net head. The complete turbine converts hydraulic input to shaft power, including draft-tube recovery within its aggregate efficiency. Water returns downstream without being burned or boiled. The separate submerged low-level bypass adds no generation; the crest spillway is closed and unrated. Pressure fields, cavitation and water hammer are not simulated.
Electrical
Each 24-pole synchronous generator runs at 250 rpm in the selected 50 Hz generating state. Gate changes alter torque and output, not synchronous speed. Per-unit breakers feed a three-phase collector, transformer and grid. Zero-flow or isolated units are parked with open breakers; synchronization, auxiliaries, reactive power and protection are omitted.
Main types and variations
The model shows a conventional plant with a Francis turbine. The first three variations describe how the plant manages water; the last three describe turbine designs. These are complementary choices: a reservoir or run-of-river plant still needs a turbine suited to its site.
Reservoir or impoundment plant
A dam holds water in a reservoir above the powerhouse. Operators release water through turbine passages, using the difference between upstream and downstream water levels. Storage can shift generation to a different time from the arrival of river inflow, subject to reservoir limits and other water uses.
What changes in practice
Storage adds operating flexibility, but electricity production must share the water with needs such as flood management, irrigation, ecosystems and water supply.
Run-of-river or diversion plant
A diversion channels part of a river through a waterway and turbine before returning it downstream. It uses the river’s natural fall and may have a weir or limited pondage rather than a large storage reservoir. Generation therefore follows available river flow more closely.
What changes in practice
The name describes the water arrangement; it does not mean every project is dam-free or has no effect on the river.
Pumped-storage hydropower
Electricity drives pumps that move water from a lower reservoir to an upper one. Later, water flows back through turbines to generate. Some plants use reversible pump-turbines and motor-generators; others separate the pumping and generating equipment. The stored energy is associated with the elevated water.
What changes in practice
This is energy storage: it returns less electricity than pumping consumed. It shifts energy through time rather than supplying a new primary energy source.
Francis reaction turbine
A spiral casing distributes pressurised water around the runner. Adjustable guide vanes direct it inward through curved runner blades, after which it leaves along the shaft direction through a draft tube. Pressure and velocity changes both contribute to the torque transmitted to the generator.
What changes in practice
Francis turbines suit a broad range of medium and higher heads. The runner blades are normally fixed; guide vanes regulate the incoming water.
Kaplan and propeller turbines
Water passes mainly along the axis of a propeller-shaped runner. A Kaplan turbine adjusts both runner-blade pitch and guide-vane position to accommodate changing conditions. A fixed-blade propeller turbine uses the same general flow direction but has a narrower efficient operating range.
What changes in practice
These reaction turbines are useful where head is relatively low and flow is high. Adjustable blades help a Kaplan machine respond to changing water conditions.
Pelton impulse turbine
Nozzles turn high-pressure water into fast jets. Each jet strikes split buckets around the runner’s rim and changes direction, transferring momentum to the shaft. The runner operates in an air-filled housing rather than a fully pressurised water passage, and the spent water drains away.
What changes in practice
Pelton machines suit high-head sites with relatively modest flow. Their jets and buckets require a different powerhouse arrangement from a submerged reaction runner.
Worth knowing
- Run-of-river plants use the river’s flow with little or no storage.
- Pumped-storage plants pump water uphill when electricity is plentiful and release it later.
- Output depends on rainfall and snowmelt, so it changes from year to year.
See real examples
The atlas includes 9,244 hydro records worldwide.
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Textbook references
Suggested technical background for this topic. These books are further reading, not proof that every explanation was checked against every edition. See the full bibliography and review scope.
- Hydropower Engineering HandbookTurbine types, penstocks, spillways and hydropower plant layout (free to download).
- Renewable and Efficient Electric Power SystemsHead, flow and the power equation for hydroelectric plants.
Sources
- EIA: Hydropower explained ↗
- US DOE: Hydropower turbines ↗
- US DOE: Head, tailwater and hydropower terminology ↗
Cutaways reveal enclosed equipment. Layouts, dimensions, materials and animation speeds are illustrative; this is an educational model, not a plant design or a thermodynamic simulation.