How Do Hydroelectric Generators Work?

At its most basic, a dam is built on a large river where a significant drop in elevation is part of the topography.

You don't find hydroelectric generators located on flat coastal plains or in fields of grazing land where you see for miles. The elevation is crucial as gravity plays an important role in generating hydroelectric power.


The dam stores water behind it in a huge reservoir. Building the dams often required relocation of the population of small towns as the flooding of the manmade reservoir can cover thousands of acres.

The water intake is near the bottom of the wall and leads to a tunnel called a penstock. Gravity and water pressure push the water through the penstock and into a turbine that activates the hydroelectric generators.

You may think the water flowing over the top of the dam is used to product power but that is not the case. When water is cascading over the top it is overspill from the reservoir.

After flowing through the turbine the escaping water in the penflow is routed to the lower part of the river below the dam. This can create a strong current flowing down from the dam when power is generated.

The energy of flowing water is converted into mechanical energy by a turbine. The hydroelectric generator uses a magnet and a conductor that causes electricity to flow.

Magnetic Steel

The electromagnets in a large generator use loops of wire wound around magnetic steel laminations that are in stacks. These are named field poles and are mounted on the rotor of the turbine shaft.

The rotor turns at a fixed speed moving the field poles (the magnets) past conductors of the generator. This produces electricity and creates voltage.

One feature of hydroelectric generators is that demand for electricity is not constant throughout the day. There are peaks of demand that a power gird must be able to meet.

This might be during early morning when people are preparing for their day or just after the workday has ended and folks are coming home and turning up their furniture or lower air conditioning temperatures.

Peak Power Periods

Hydroelectric generators are much better at meeting the demands of peak power periods than power plants using fossil fuels.

Pumped storage is a method used by hydroelectric plants to keep enough water volume in reserve to meet varying power demands. Pumped storage allows water to flow through the turbines and then be routed to a storage pool.

This is done during hours when demand for power is at its lowest such as the middle of the night. It allows the plant to quickly adjust when demand rises and can provide additional power for an hour or more in high demand periods.

Pros and Cons

Once a dam is built the fuel required for hydroelectric generators is free. There is no pollution, greenhouse gas or waste produced in the process. The energy source is reliable. Unlike solar or wind power, the energy from water can be stored and maintained.

There are environmental concerns about hydroelectric power. The cost of building dams is huge and there can be negative impact on the environment around the dam and reservoir.

Bird and wildlife habitats may be destroyed and specific of fish endangered. There are few remaining sites that are conducive to building new dams and there is public sentiment against the expense and the potential danger associated with stability of large dams.

When hydroelectric generators were first used with dams in the U.S. environmental concerns were not considered.

When the effects of blocking water flow on salmon became known damns were amended to provide a way for salmon to bypass the dam as they moved to spawning grounds in their annual migrations.

It was also discovered that the quantity and quality of the water flowing below the dam's structure could be affected by changing a naturally flowing water source into a reservoir environment.

The biggest public fear expressed about hydroelectric power is the potential for devastating floods should a dam fail. Dams have proven to be very strong and dependable but there have been ruptures that have damaged farmland and cost lives of people and wildlife.

Finding ways to minimize damage to the environment and cost effect methods of producing power with water could be part of the renewable energy solutions in the future.

We know it is necessary to replace our use of fossil fuels as soon as possible and hydroelectric generators may well be part of that process.