--- title: "Thermal versus hydro: load curve and stability" subject: "Power Plant Design" chapter: 1 key: "semester_8/power_plant_design:thermal-vs-hydro-load-curve" source: https://www.bctengineer.com/q/semester_8/power_plant_design/thermal-vs-hydro-load-curve site: BCT Engineering --- # Thermal versus hydro: load curve and stability Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation - Asked in: 2074 Bhadra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/thermal-vs-hydro-load-curve - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1 ## Question 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. ## 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. ## Answer 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 ```text 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 | Aspect | Thermal plant | Hydropower plant | |---|---|---| | Position on load curve | Base load | Peak load (storage), base in wet season (RoR) | | Starting time | Hours (cold start 6–8 h) | Minutes | | Ramp rate | Slow (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 control | Limited by boiler | Excellent; used for AGC and spinning reserve | | Voltage support | Good; located near loads | Good; but long lines can limit stability | | Grid connection | Near load or coal mines; short lines | Remote sites; long transmission lines | | Black start | Needs external supply | Can black-start the grid | | Efficiency | 30–40% | 85–92% | | Operating range | Above about 40–50% load | Wide; Pelton/Kaplan good at part load | | Running cost | High (fuel) | Very low | | Capital cost | Lower | Higher | | Reliability | Firm if fuel available | Depends on river flow | | Maintenance | Complex, many auxiliaries | Simple; sediment erosion | | Environment | Emissions, ash | Clean; 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where does thermal normally operate on the daily load curve? A. Peak B. Base load C. Only at night D. Not at all Answer: B. Thermal is best at constant output and supplies base load; storage hydro takes peaks. (Key point: Plant placement on load curve) 2. Which is correct? A. Thermal starts in minutes, hydro in hours B. Hydro starts in minutes and ramps fast; thermal cold start takes hours C. Both start in seconds D. Both take 8 hours Answer: B. Hydro suits peaks and frequency control; thermal cold start is 6-8 hours. (Key point: Starting and ramp rates) 3. Which plant can black-start the grid? A. Thermal B. Hydro C. Neither D. Both without supply Answer: B. Thermal needs an external supply, while hydro can start without it. (Key point: Black start) 4. Typical grid connection difference: A. Hydro near load with short lines; thermal remote B. Thermal near load or coal mine with short lines; hydro remote needing long lines C. Both remote D. Both have no lines Answer: B. Hydro sites are fixed by the river and often remote. (Key point: Grid connection) 5. Which is better for frequency control (AGC, spinning reserve)? A. Hydro B. Thermal because of the boiler C. Neither D. Both equal Answer: A. Thermal is limited by the boiler; hydro responds quickly. (Key point: Frequency control) ## Related in Power Plant Design - [Thermal versus hydro operational characteristics](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-vs-hydro-operational) — Power Plant Design, chapter 1, asked 8× - [Solar versus hydro operational characteristics](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asolar-vs-hydro-economics) — Power Plant Design, chapter 1, asked 5× - [Tidal versus wind power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Atidal-vs-wind) — Power Plant Design, chapter 1 - [Thermal versus nuclear power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-vs-nuclear) — Power Plant Design, chapter 1, asked 2× - [Hydro versus thermal: techno-economic and environmental](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydro-vs-thermal-techno-economic) — Power Plant Design, chapter 1 - [Thermal, hydro and solar PV plants compared](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-hydro-solar-pv) — Power Plant Design, chapter 1, asked 8× ## Related in other subjects - [Power sources and hydro versus thermal](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apower-sources-hydro-vs-thermal) — Hydro Power, chapter 3 - [Load duration curve; diesel plant; ROR flow duration](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aload-duration-diesel-import-fdc) — Hydropower Engineering, chapter 3 - [Hydro and thermal power sources](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ahydro-thermal-sources) — Power System Analysis I, chapter 1 - [Firm power and plant load factors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afirm-power-load-factors) — Hydro Power, chapter 3