In hydroelectric power generation, I've got a pretty good understanding for how kinetic energy from falling water is transformed into electrical energy. But when looking at the entire system, what thing is losing energy such that work can be done by the electricity that is generated?

For example, when electricity is generated from wind power, there is actually a reduction in wind in the surrounding environment, meaning energy is taken out of the Earth's weather systems.

My understanding of hydroelectric power is that it is powered by water rising in the atmosphere (work is being done), condensing at a higher elevation, then flowing to a lower elevation. What is the difference in this whole picture where some of the falling kinetic energy is turned into electricity, vs. all of the water falling unobstructed? Does it move the earth (ever so slightly) because the water was moved upwards, and then has energy removed as it moves downwards?

Thanks in advance; I'm sure I didn't use the right terms for everything, but hopefully the meaning of the question makes sense!

  • $\begingroup$ Kinetic energy of "falling water" is not what is generating electricity. The gravitational potential energy of stored water generates electricity. Also, energy is not "lost" ... it is transformed from one form to another. And note - hydroelectric water loses pressure as it goes through a hydroturbine, and pressure is energy per unit volume. $\endgroup$ – David White Aug 14 '20 at 17:05

Let's take the case of a Francis turbine, which is just about the most efficient device available to convert the energy content of falling water into shaft work.

At the bottom-most depth of a dam, the water squirting out through a discharge pipe there has exactly the same discharge velocity as if it had fallen through the total water depth (under the influence of gravity) of the reservoir at that point.

The Francis turbine's inlet pipe accepts that flow of water, which is zooming into its rotor blades at that particular velocity. The flow of water out its discharge pipe is basically falling "limply" out of that pipe at nearly zero pressure, having had almost all of its kinetic energy content extracted as shaft work.

Continuity requires that the mass flow rate of the water into the turbine = mass flow rate of water out of the turbine; what is different is that the pressure of the inlet flow is almost entirely extracted, and the outlet flow is essentially at ambient pressure.

In this connection it is well to remember that fluid horsepower extracted by the turbine is equal to the mass flow rate times the pressure difference across it, minus the efficiency losses.


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