Reverse flow turbines: pumped storage

Pumped storage is one of the most reliable energy storage technologies. New projects have been difficult to build because of the need for large water storage areas, usually lakes.

TVA reports that hydroelectric projects are easily equipped with reversible turbines. Then water below the dam can be pumped back above the dam. Hence, any hydro project can potentially be equipped for pumped storage. They can pair nicely with intermittent sources like wind and solar.

This technology has been available since the 1930s.


Interestingly, the round-trip efficiency of hydro pumped storage is about the same as that for electric batteries. Batteries might be slightly better.

From the link: According to data from the U.S. Energy Information Administration (EIA), in 2019, the U.S. utility-scale battery fleet operated with an average monthly round-trip efficiency of 82%, and pumped-storage facilities operated with an average monthly round-trip efficiency of 79%.

Using either hydro or electric batteries, you are still losing ~20% of the energy put into the storage system when you want to get that energy out.

  • Pete

That’s still way better than a combined cycle natural gas plant where you lose 40% of the energy. Or a coal plant that loses 60%.


I don’t think you can really compare these kind of percentages with any real meaning. In a thermal process, such as coal or NG your basis is the amount of potential thermal energy from a source where, essentially, you found the energy. So you dug up the coal or drilled for the NG which consumed, relatively, very little energy. So, a made up example might be that it cost 5 BTU to drill for the NG, 1 BTU to transport it and you got 100 BTU but were able to only recover 60 BTU in a CCGT plant. You ended up with 10x what you started with.

Another example is solar PV. Most panels are about 15% to 20% efficient. Way too low to bother with if you compare to most other sources. But, once you build the panel it is all “~free” energy after that. The efficiency number looks low only because the basis is the total solar input on a clear day with perfectly oriented panels. Panels that were only 10% efficient might be totally viable if they cost much much less or lasted longer or could be easily installed in more places, etc.

With pumped hydro (or a storage battery) the efficiency number does really matter because you are starting with electric energy that is already ready to be consumed right now and by storing it for later you are losing power you already had in a usable form. So, no matter what you lose energy, rather than gain it by looking for more found energy.
Of course I’m all in favor of batteries and pumped storage due to other benefits beyond the raw efficiency numbers.



Especially if you have power available now that can’t be used now. In a sense it is free and has no value–unless stored for later use.