The question describes a specific type of hydroelectric power plant operation where water is moved between two reservoirs to store energy. During periods of low electricity demand, water is pumped from a lower reservoir to an upper (main) reservoir. When demand for electricity is high (peak loads), the water is released from the upper reservoir back down to the lower reservoir, passing through turbines to generate electricity.

Understanding Pumped Storage Hydroelectric Plants

This method of using excess off-peak power to pump water to a higher reservoir for later use during peak demand is the defining characteristic of a pumped storage hydroelectric power plant.

• These plants act like a large battery, storing potential energy in the form of elevated water.
• They provide grid stability by absorbing surplus energy during low demand and supplying quick-response power during high demand.
• The process involves two reservoirs at different elevations.

Why Other Options Don't Fit

Let's look at the other options:

• Surge tank power plant: A surge tank is a protective vessel connected to the penstock (water conduit) to absorb pressure surges when the flow of water is rapidly changed, such as during turbine shutdown or startup. It is a component of some hydroelectric plants, not a type defined by energy storage through pumping.
• Run-of-river plant: This type of plant generates electricity using the natural flow and gradient of a river. It typically has limited or no water storage capacity. Power generation depends directly on the river's flow rate and cannot store energy by pumping water back upstream.
• Conventional power plant: This is a very broad term and usually refers to thermal power plants (coal, gas, nuclear) or traditional hydroelectric plants with significant upstream storage reservoirs (but not necessarily designed for pumping water back up). While hydroelectric plants are conventional, this option doesn't specifically highlight the pumping aspect for storage described in the question.

Based on the description of pumping water to an upper reservoir during low demand to meet peak loads, the only type of plant that fits this operation is a pumped storage plant.