In pumped storage

Question

In pumped storage

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Option C: Downstream water is pumped up-stream during off load periods

Explanation & Concepts:
A pumped storage plant functions as an energy storage system that converts off-peak electrical energy into potential energy by pumping water from a lower reservoir to an upper reservoir. During peak load hours, this stored water is released through turbines to generate electricity to meet the grid demand.

Working Principle: During low demand (off-load) periods, surplus electricity from the grid is used to drive motor-pump sets, lifting water to the upper reservoir. When peak demand occurs, the process is reversed: water flows back to the lower reservoir through the penstock, spinning a turbine-generator set to supply power to the grid.

Key Formulas: $P = \eta ho g Q H$ — where $P$ is power, $\eta$ is efficiency, $ho$ is density, $g$ is gravity, $Q$ is flow rate, and $H$ is head.

Key Points: Used for grid stabilization and peak load shaving.; The round-trip efficiency of modern pumped storage plants is typically between 70% and 85%.; Acts as a large-scale energy storage mechanism.; Often uses reversible pump-turbines (Francis type) to save costs.

Advantages: Rapid response to load changes; High reliability for grid frequency regulation; Long operational life

Disadvantages: High initial capital investment; Requires specific geographical conditions; Energy losses during pumping/generating cycles

Applications: Grid peak load management; Frequency stabilization of the national grid; Black start capability for power systems

Additional Info: The storage capacity is directly proportional to the volume of water stored and the height (head) of the upper reservoir.. Option A is incorrect because pumps do not produce power; they consume it. Option B is incorrect as water is stored by elevation (potential energy), not high pressure. Option D is partially true but incomplete; the distinct characteristic is the bi-directional pump-turbine operation between reservoirs.

Answer: C is correct — Downstream water is pumped up-stream during off-load periods to store potential energy for later peak-hour generation.

Exam Tip: Always remember that pumped storage is not a primary energy source, but an energy storage device; the system efficiency $\eta_{total} = \eta_{pump} 	imes \eta_{turbine} 	imes \eta_{transmission}$ is always less than 1.

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Option A: Power is produced by means of pumps

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Option B: Water is stored by pumping to high pressures

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Option D: Water is re circulated through turbine

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