---
title: "When pumped storage is economically feasible"
subject: "Power Plant Equipment"
chapter: 1
key: "semester_7/power_plant_equipment:pumped-storage-economic-feasibility"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/pumped-storage-economic-feasibility
site: BCT Engineering
---

# When pumped storage is economically feasible

Power Plant Equipment · Chapter 1: Hydro Power Plant

- Asked in: 2080 Baisakh, 5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/pumped-storage-economic-feasibility
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-1

## Question

What is pumped storage power plant? In which situation it will be economically feasible?

## Short answer

A pumped storage plant uses surplus off-peak power to pump water to an upper reservoir and generates during peak hours with a reversible pump-turbine. It is economically feasible when peak energy price exceeds off-peak price divided by cycle efficiency, cheap off-peak surplus is available, and a suitable high-head site exists.

## Answer

A **pumped storage plant** is a hydro plant with an upper and a lower reservoir. In off-peak hours it uses surplus grid power to pump water up; in peak hours the water flows back through the turbine to generate power. The same reversible pump-turbine and motor-generator usually do both jobs.

```text
 Upper reservoir
     |  penstock
     v ^
 [pump-turbine]--[motor-generator]--grid
     | ^
 Lower reservoir
```

Cycle efficiency is about 70–80 %, so it is a net consumer of energy. It is **economically feasible** when:

1. **Large price difference between peak and off-peak energy.** The value of peak energy must exceed the cost of pumping energy divided by the cycle efficiency: $C_{peak} > C_{off\text{-}peak}/\eta_{cycle}$.
2. **Cheap surplus energy is available** in off-peak hours from base-load plants (thermal, nuclear, run-of-river hydro) or from solar and wind that would otherwise be spilled.
3. **Base-load plants must run steadily**, e.g. a system with many steam plants, where peak-following causes poor efficiency and wear.
4. **Good site:** two reservoirs close together with a high head (short penstock per metre of head), water available, firm foundation and low sediment.
5. **Need for fast reserve and regulation:** grids that need spinning reserve, frequency control and black-start capability value the plant beyond its energy.
6. **Peak load lasts only a few hours**, so a modest storage volume is enough.
7. Existing reservoirs can be used (lower cost civil works).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What price condition makes pumped storage economic?
   A. C_peak < C_offpeak
   B. C_peak > C_offpeak / eta_cycle
   C. C_peak = 0
   D. C_offpeak > C_peak x eta_cycle
   Answer: B. Peak energy value must exceed pumping energy cost divided by cycle efficiency to cover the cycle loss. (Key point: Price condition)

2. Which source suits pumping energy?
   A. Cheap surplus from base-load plants, run-of-river hydro, or otherwise spilled solar/wind
   B. Expensive peak-hour energy
   C. Diesel generators only
   D. Energy bought at peak tariff
   Answer: A. The scheme needs cheap off-peak surplus to be profitable. (Key point: Source of pumping energy)

3. Which site feature favours a pumped storage plant?
   A. Two reservoirs far apart with low head
   B. Two nearby reservoirs with high head, firm foundation and low sediment
   C. A single flat lake
   D. A site with scarce water
   Answer: B. High head close together means a short penstock per metre of head and smaller storage volume. (Key point: Site requirements)

## Related in Power Plant Equipment

- [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
- [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×
- [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×
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## Related in other subjects

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- [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