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Thermal versus hydro: load curve and stability

Short answer

Thermal plants take base load with slow start and ramp, while hydro takes peak load with fast start and excellent frequency control. Hydro has higher efficiency, lower running cost, black-start capability, but depends on river flow; thermal is firm if fuel available but polluting. In mixed systems thermal base, hydro peaks.

Compare the operational characteristics of the thermal power plant and hydropower plant regarding daily load curve, stability, grid connection and operational characteristics and so on.

A thermal power plant converts the chemical energy of fuel into electricity through steam, while a hydropower plant converts the potential energy of water. Their ability to follow load and support the grid is very different.

Daily load curve and plant placement

 Load
  |            peak (hydro storage, gas)
  |        ___/^^^\___        /^^\
  |  _____/            \_____/    \__
  | |   intermediate (hydro / CCGT)  |
  | |--------------------------------|
  | |   base load (thermal, RoR hydro, nuclear)
  +----------------------------------------> hour
  0          6         12        18     24
  • Thermal: best at constant output; takes the base load portion of the curve.
  • Hydro: run-of-river plants take base load during high flow; storage and peaking RoR plants take the peak.

Comparison

AspectThermal plantHydropower plant
Position on load curveBase loadPeak load (storage), base in wet season (RoR)
Starting timeHours (cold start 6–8 h)Minutes
Ramp rateSlow (a few %/min)Fast (up to 100%/min)
Stability (transient)Large inertia, steam turbine high speed (3000 rpm, 2 pole)Lower inertia per MW, slow speed salient pole; water inertia causes non-minimum-phase governor response
Frequency controlLimited by boilerExcellent; used for AGC and spinning reserve
Voltage supportGood; located near loadsGood; but long lines can limit stability
Grid connectionNear load or coal mines; short linesRemote sites; long transmission lines
Black startNeeds external supplyCan black-start the grid
Efficiency30–40%85–92%
Operating rangeAbove about 40–50% loadWide; Pelton/Kaplan good at part load
Running costHigh (fuel)Very low
Capital costLowerHigher
ReliabilityFirm if fuel availableDepends on river flow
MaintenanceComplex, many auxiliariesSimple; sediment erosion
EnvironmentEmissions, ashClean; land and ecology impact

Remarks

  • In a mixed system, thermal plants supply base load efficiently and hydro plants follow peaks and regulate frequency.
  • In Nepal, the load curve has a sharp evening peak; storage (Kulekhani) and peaking run-of-river plants meet it, while thermal plants are kept only for emergency.

✓ Answer checked by Claude Opus 5.5 (high effort)

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