--- title: "Pumped storage to reduce peak generation loss" subject: "Power Plant Equipment" chapter: 1 key: "semester_7/power_plant_equipment:pump-storage-peak-loss" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/pump-storage-peak-loss site: BCT Engineering --- # Pumped storage to reduce peak generation loss Power Plant Equipment · Chapter 1: Hydro Power Plant - Asked in: 2079 Baisakh, 8 marks; 2078 Bhadra, 5 marks; 2074 Chaitra, 10 marks (asked 3 times) - Video: [Hydropower Explained Inside a Pumped Storage Power Plant | How It Works](https://www.youtube.com/watch?v=E-m7Psbuup0) — saVRee - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/pump-storage-peak-loss - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-1 ## Question Explain the operation of pump storage hydropower plant with suitable scheme to reduce peak generation loss of steam power plant. ## Short answer A pumped storage plant uses off-peak surplus to pump water to an upper reservoir and releases it through a turbine during peak. To reduce steam plant peak loss, the steam plant runs at constant efficient load; off-peak surplus drives the pump, and peak generation from the hydro unit shaves the peak, avoiding part-load operation. ## Answer A **pumped storage plant** is a hydro plant with two reservoirs at different levels. It uses cheap surplus electricity in off-peak hours to pump water from the lower reservoir to the upper one, and releases the same water back through the turbine to generate power during peak hours. It works like a large rechargeable battery for the grid. ```text UPPER RESERVOIR (head pond) ~~~~~~~~~~~~~~~~~~~~~~~ | penstock / pressure shaft | (water flows down when | generating, up when pumping) v ^ +---------------------+ +-----------+ | reversible |------| motor- |---- grid | pump-turbine | | generator | (supply +---------------------+ +-----------+ or draw) | ^ v | tailrace ~~~~~~~~~~~~~~~~~~~~~~~ LOWER RESERVOIR (tail pond) ``` **Main parts:** upper reservoir, lower reservoir (or river/tail pond), intake, penstock or pressure shaft, power house with a reversible pump-turbine (usually Francis type) coupled to a reversible synchronous motor-generator, and tailrace. **Operation:** 1. **Generating mode (peak hours):** Water from the upper reservoir flows down the penstock, drives the machine as a turbine, and the machine works as a generator feeding the grid. The water is collected in the lower reservoir. 2. **Pumping mode (off-peak hours, e.g. night):** Surplus power from base-load plants (thermal, nuclear or run-of-river hydro) drives the machine as a motor. The machine rotates in the reverse direction and works as a pump, lifting water from the lower reservoir back to the upper reservoir. 3. The cycle repeats daily (or weekly/seasonally). **Energy relation:** Energy stored $E = \rho g V H$. Overall cycle efficiency $$ \eta_{cycle} = \frac{E_{generated}}{E_{pumping}} \approx 0.70 \text{ to } 0.80 $$ So about 1.3 kWh of off-peak energy is used to get 1 kWh at peak. It is still economic because peak energy is far more valuable than off-peak energy. #### Scheme to reduce peak generation loss of a steam plant A steam power plant is efficient only at a steady load near its rating. If it has to follow the daily load curve, it runs at part load at night (poor heat rate) and must be pushed above its economic loading, or backed up by costly peaking units, in the evening. Frequent load swings also cause thermal stress and higher fuel use per kWh. ```text Load (MW) peak met by pumped ^ storage (generating) | ______ | /######\ |--------/########\------ steam plant | ...... .... output kept | :pump: :pump: constant | :mode: :mode: +--------------------------> time (h) 0 12 18 24 .... = valley filled by pumping load ``` **Scheme (coordinated operation):** 1. The steam plant runs at a **constant, near-rated load** for all 24 hours, where its efficiency is highest. 2. During **off-peak hours** (night), the extra steam-plant output is absorbed by the pumped storage plant working in the pumping mode. This "fills the valley" of the load curve. 3. During **peak hours**, the pumped storage plant works as a turbine-generator and supplies the peak. This "shaves the peak". 4. The pumped storage plant also takes sudden load changes and acts as spinning reserve, so the steam units are not forced to ramp quickly. **Benefits:** - Higher average efficiency and lower fuel cost of the steam plant; fewer start-stops. - Load factor of the steam plant rises; the installed capacity needed for peaking falls. - Even with a cycle efficiency of 70–80 %, the saving is positive because cheap off-peak energy replaces expensive peak generation (gas turbines, diesel, or the steam plant at its worst heat rate). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In generating mode of a pumped storage plant: A. Water is lifted from the lower to the upper reservoir using surplus power B. The machine runs as a motor in reverse direction C. Upper reservoir water flows down the penstock, drives the turbine and the machine feeds the grid D. The plant only supplies reactive power Answer: C. At peak hours the reversible machine acts as turbine-generator with water flowing from upper to lower reservoir. (Key point: Two modes of operation) 2. In the scheme, how is the steam plant operated? A. At constant near-rated load for 24 hours where its efficiency is highest B. At part load at night and overloaded at peak C. Shut down at night and restarted for the peak D. Only at peak hours Answer: A. Keeping steam units steady near rating avoids poor part-load heat rate and thermal stress; storage handles the variation. (Key point: Steam plant at constant load) 3. What does the pumped storage plant do during off-peak hours in this scheme? A. It generates to help the steam plant B. It is idle C. It absorbs extra steam-plant output by pumping, filling the valley of the load curve D. It disconnects the steam plant Answer: C. Pumping adds load at night (valley filling), and generation at peak (peak shaving) keeps the steam output flat. (Key point: Valley filling and peak shaving) 4. Why is the scheme beneficial even though cycle efficiency is only 70-80 %? A. Because pumping energy is free B. Because cheap off-peak energy replaces expensive peak generation such as gas, diesel or steam at worst heat rate C. Because the cycle efficiency is really above 100 % D. Because steam plants cannot be run at constant load Answer: B. The value difference between peak and off-peak energy outweighs the 20-30 % cycle loss. (Key point: Why losses are acceptable) ## Related in Power Plant Equipment - [Operation of pumped storage hydro plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Apump-storage-operation) — Power Plant Equipment, chapter 1, asked 2× - [Pumped storage plant and feasibility in Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Apump-storage-nepal-feasibility) — Power Plant Equipment, chapter 1, asked 5× - [Francis turbine in pumped storage plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Afrancis-turbine-pumped-storage) — Power Plant Equipment, chapter 1 - [Steady state operation of hydropower plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Ahydropower-steady-state-operation) — Power Plant Equipment, chapter 1 - [When pumped storage is economically feasible](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Apumped-storage-economic-feasibility) — Power Plant Equipment, chapter 1 - [Steam plant improvement methods and reheating](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-methods-reheating) — Power Plant Equipment, chapter 7, asked 4× ## Related in other subjects - [Pumped storage plant: power, turbine, setting, surge](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apumped-storage-turbine-surge-numerical) — Hydropower Engineering, chapter 7 - [Pumped storage plant and benefit to Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apumped-storage-benefit-nepal) — Hydropower Engineering, chapter 2 - [Short note: Pumped storage hydropower](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Apumped-storage-hydropower) — Energy Environment and Society, chapter 5 - [Importance of storage plants; pumped storage layout](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astorage-importance-pump-storage-layout) — Hydropower Engineering, chapter 2