--- title: "Thermal versus hydro operational characteristics" subject: "Power Plant Design" chapter: 1 key: "semester_8/power_plant_design:thermal-vs-hydro-operational" source: https://www.bctengineer.com/q/semester_8/power_plant_design/thermal-vs-hydro-operational site: BCT Engineering --- # Thermal versus hydro operational characteristics Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation - Asked in: 2081 Chaitra, 8 marks; 2074 Magh, 8 marks; 2073 Bhadra, 6 marks (asked 3 times) - 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-operational - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1 ## Question Compare the operational characteristics of thermal and hydropower plant on the basis of reliability, grid connection, efficiency, operating range, economics and maintainability. ## Short answer Thermal plants offer firm power independent of weather but slow start, lower efficiency (30–40%), high running cost, complex maintenance, suited for base load. Hydropower is quick-starting, high efficiency (85–92%), low running cost, wide operating range, long life, ideal for peaking, but output depends on seasonal flow. ## Answer A **thermal power plant** burns fuel (coal, oil, gas) to raise steam that drives a turbine, while a **hydropower plant** converts the potential energy of water into electricity through a water turbine. Their operating behaviour is very different. #### Comparison | Basis | Thermal power plant | Hydropower plant | |---|---|---| | Energy source | Coal, oil, gas; must be bought and transported | Water; free, renewable, but seasonal | | Reliability | High and firm all year if fuel is available; independent of weather | Depends on river flow; run-of-river output falls in dry season; storage plants are firm | | Starting time | Slow, several hours for cold boiler | Fast, a few minutes | | Grid connection | Large synchronous units near load or coal mines; base-load role | Synchronous units, usually far from load in hills; needs long transmission lines | | Load following | Poor; boiler limits ramp rate | Excellent; quick ramp, suits peak load and frequency control | | Efficiency | About 30–40% (steam), 50–60% combined cycle | About 85–92% overall (water to wire) | | Operating range | Stable only above about 40–50% of rating; low-load operation inefficient | Wide range; Pelton and Kaplan good at part load, Francis about 40–100% | | Capital cost | Lower per kW, short construction time | High per kW, long construction time | | Running cost | High (fuel) | Very low (no fuel) | | Economics | Low capital, high running cost | High capital, low running cost, long life | | Maintainability | Complex (boiler, coal handling, ash, condenser, cooling) | Simple; main wear is turbine erosion by sediment | | Life | About 25–30 years | 50 years or more | | Staff required | Many | Few | | Environment | CO₂, SO₂, NOx, ash, thermal pollution | Clean in operation; land submergence, river ecology impact | #### Remarks - Thermal plants are best for **base load** where fuel is cheap; hydro storage plants are ideal for **peak load** and spinning reserve. - In Nepal, hydropower is the main source because of high head and many rivers, while thermal (diesel) plants such as Duhabi and Hetauda are only for emergency backup due to imported fuel cost. - A mix of both gives the best system: thermal for firm base load, hydro for peaking and fast regulation. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which pair of overall efficiencies is correct? A. Thermal 85-92%, hydro 30-40% B. Thermal 30-40% (steam), hydro 85-92% C. Both about 60% D. Thermal 50-60%, hydro 10-20% Answer: B. A steam plant is limited by heat rejection; hydro converts water to electricity with about 85-92% efficiency. (Key point: Efficiency comparison) 2. Which statement about starting and load following is correct? A. Thermal starts in minutes and follows load well B. Hydro starts in a few minutes and follows load excellently C. Both take several hours to start D. Thermal is better for frequency control than hydro Answer: B. Hydro ramps quickly and suits peak load; thermal boilers take hours and limit ramp rate. (Key point: Load following and starting) 3. What is the cost pattern of hydro compared with thermal? A. Low capital, high running cost B. High capital, very low running cost, long life C. Same capital, same running cost D. High capital and high fuel cost Answer: B. Hydro has no fuel cost but needs large civil works; thermal has lower capital but high fuel cost. (Key point: Economics) 4. What is the main wear problem in a hydropower plant? A. Ash handling B. Coal mill failure C. Condenser cooling D. Turbine erosion by sediment Answer: D. Hydro maintenance is simple, mainly sediment erosion of turbine parts, unlike the complex boiler, coal and ash systems of thermal plants. 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