5.5: Pumped Storage Hydroelectric Plants (PSHP)

One great advantage of hydropower technology is that it makes it possible to build plants in which large amount of energy can be stored and used later “on demand”. Such complexes are called “pumped storage plants”. In the area of energy storage, they are definitely the record-keepers. Energy can be stored in other ways, in electric batteries, or thermally in huge reservoirs of molten salts or as compressed air, (the Chapter 11 in this text is devoted specifically to energy storage methods). However, the largest existing hydroelectric storage complex (in the US, in Bath County, Virginia– and here is a 7-minute video) can store about 50 times more energy than the largest currently existing electric battery systems.

Figure \(\PageIndex{1}\): A general scheme of the Raccoon Mountain Pumped Storage Hydroelectric Plant. It uses dual-action Francis turbines. Details of the turbines and the motors/generators are not shown in the figure, we have to understand that they are all hidden in the unit marked as the “Powerplant Chamber” (source: Wikimedia Commons).

The idea of hydropower storage is very simple one needs two reservoirs, called the “lower” and the “upper”. When there is surplus of electric power (e.g., in the night hours), water is pumped from the lower pool to the upper one – this is the “storage mode”. Then, when the utility system uses maximum power (e.g., during the “peek hours”, the water from the upper pool is sent to turbines this part of the operation, called the “generating mode”, is exactly the same as in an “ordinary” hydropower plant.

There are some problems with this technology, though. If the complex is expected to store a large amount of energy, then the upper pool should be able to contain a huge amount of water, and it should be located considerably higher than the lower pool. The thing is that there are not too many places in the world that offer favorable conditions: first, there must be a mountain with a flat top and enough space in the summit region enabling one to build a sizable “upper pool”. Second, there must be a long steep slope, at the bottom of which the “lower pool” has to be installed. Third, there must be plenty of water available in the area. From the quiz about the Bath County storage complex, you will certainly conclude that finding other locations good for building complexes of similar magnitude may be a difficult task – and in many geographic areas, just a hopeless task!

There is a nice animation on Youtube showing the operation of a pumped storage system accompanied by an interesting comment.

There are several possible ways of building PSHP installations. One possible variant is to make the pumping unit and the electricity generating unit completely separate. It is how the first PSHPs were built. Yet, a smarter solution is to use the generator as an electric motor. It should be noted that electric power generators usually can work “the other way”, as motors. When the “input” to a generator is work, it converts it to an “output” in the form of electric power; and vice-versa, if the “in-put” is electric current sent to the generator, the “output is mechanical work. Accordingly, the next step was to build installations in which one generator/motor services both the turbine and the pump. And the newest simplification of the system is to use the Francis Turbine which is, as was mentioned earlier, a double-action device that can operate both ways: as a turbine extracting power from downhillflowing water, or as a pump sending water uphill. Essentially, all pumped storage installations built in the recent past use the Francis turbine/pump technology.

If you would like to find a more “in-depth” description of the Francis turbine technology, this article is certainly worth reading – because of excellent graphic material, beautiful photos and instructive technical drawings.

In Fig. \(\PageIndex{2}\) there is a graph showing a typical daily activity of a pumpedstorage plant: Pumping (mostly, during night hours), and Power Generation the power generated may sharply go up and down, depending on how much “backup” the nation’s power system needs at a given moment. Older installations are not capable of reacting so quickly to the demand – in the generation mode they “prefer” to deliver power at a constant rate. However, the ability of quickly changing the output power and to react promptly to changing power demand becomes crucial in times when the utility systems relay more and more renewable power sources. Windpower and solarpower, as we all know, may fluctuate quite rapidly, especially, when weather fronts are passing over a given region. Therefore, the energy storing facilities should be ready to smooth out the “ups and downs” in the power delivered by photovoltaic panels and wind turbines. Indeed, some new PSHPs are able to raise the output from zero to maximum power in about a minute.

Figure \(\PageIndex{2}\): An example of typical power distribution over a 24 hour period of a PSHP. Green color is the power used for pumping; red is the power delivered back to the grid (source: Wikimedia Commons).