11.4.2: Electricity → potential energy → back to electricity (gravitational energy storage and compressed air energy storage).

Harnessing the potential energy due to gravity. Known forms of mechanical potential energy are those associated with gravity, or those associated with elastic deformation of bodies. Both are used for energy storage, but the former on a much larger scale in pumped storage power plants. We’ve discussed these power plants extensively in Chapter 5. So let’s just briefly remind you that these power plants are the largest energy storage facilities in existence today. The largest in the USA (and in the world) in Bath County, Virginia can generate 2.4 GW of power for 10 hours, i.e. it can store 24 GWh of energy. The Raccoon Mountain power plant can generate 1.65 GW, but has an even larger capacity, 36 GWh. In 2018, as reported by the U.S. Energy Information Administration, the total capacity of pumped storage power plants in the USA was 22.9 GW. The total capacity is not given, but knowing that typically such power plants have a water supply for at least 10 hours it can be estimated that this capacity is at least 230 GWh. It is planned to build several new pumped storage power plants in the USA, including a 393 MW project in Oregon in the Klamath Falls area and the 1.2 GW Goldendale Energy Storage Project in Washington, not far from the John Day Dam at Columbia River.

Potential energy can be stored not only by raising water, one can do that as well by raising solid objects. Such a method has been used for centuries, e.g., for storing energy needed for the operation of clocks at church towers. Now in the Web one can find ideas of using the same method at a much larger scale. Suppose, for instance, that there is a shaft in a now defunct mine – say, 500 meters deep. It is conceivable to build a dedicated elevator and load it with 1000 tons of rocks, and use its potential energy to drive an electric power generator. The total energy released by lowering such weight would be:

\[ E = 10^{6}\;kg \times 500 \;m \times 9.81 \;m/s2 = 4905\;MJ = 1.363 \;MWh \]

Such a device, operating, say, for 4 hours, would deliver 341 kW of power – good for satisfying the evening-hours needs of a community of some 100 homes. Another idea – if there is a village not far from a hill with its summit 300 m above the village center (similar to the Dimple Hill or Vineyard Mountain in the Corvallis area), then it is possible to build a ramp with railway tracks and use two freight cars, each loades with 1000 tons of rocks and pulled uphill by cables, to generate a similar power as in the preceding example. One can find quite a few such ideas in the Web – but, so far, no reports of those ideas being implemented. But, who knows? – Maybe someday somewhere in the world the local community will wish to implement such a project?

Harnessing the potential energy of compressed bodies – of air, in particular. As mentioned above, potential mechanical energy is also contained in elastically deformed bodies, for example, helical or spiral springs. And indeed, until recently, they were very widely used for energy storage but only in small devices such as watches, turntables, portable film cameras or toy cars they all required “winding up” (which either stretches the spring, or – more often – compresses it). However, due to technological progress today we can find these things mainly in attics or in the museum.

It should be noted that springs are very efficient energy storing devices – those made of high quality steel can return over 99% of the input energy. This can be confirmed by a very simple demonstration – namely, the observation of oscillations of a weight suspended on a spring, as, e.g., in this YouTube clip. Such a weight can keep oscillating for long minutes, the energy dissipation is indeed very slow. Now, please take a look at the list of the largest energy storage installations in the world is given in this Wikipedia site. Note that if the installation capacity in GWh is taken into account, the devices using the CAES technology (Compressed Air Energy Storage) come only second after pumped storage power plants.

Compressed air behaves much like a compressed spring, which can be easily checked by squeezing a portion of air in a pump for inflating bicycle tires with the outlet blocked. Yet, the properties of a gas are not exactly the same as those of an elastic body. The point is that when gas is rapidly compressed, some of the input energy changes into thermal energy – in other words, it is used for heating up the gas. In theory, if the compressed gas were then kept in an adiabatic tank, i.e., a perfectly thermally insulated from the surroundings, it would be possible to recover 100% of the energy input. In theory, let us stress. But if the tank is not insulated and heat escapes from it, then even in theory much less energy is recovered than that inserted. A more detailed thermodynamic analysis of CAES can be found in this article extensively discussing various aspects of this technology.

In practice, devices for storing hundreds or even thousands of MWh are needed. In a tank withstanding the pressure of 100 bars, the energy stored per one cubic meter is 4 5 kWh. Then, for storing 1 GWh, reservoir volume as large as 200 000 m³ would be needed. Making such a sturdy structure of steel is conceivable – but, probably, the cost would be very high. A much better idea is to use existing underground reservoirs such as salt caverns left out in existing salt bed deposits after mining operations. The salt is mined by drilling a well down into the formation, and pumping water through the completed well to dissolve the salt which returns to the surface as brine. Many such currently existing caverns are used as reservoirs for storing natural gas. They can be used as well as tanks for large CAES installation. In fact, there are two such major objects currently operating. One, located in Huntorf, Germany, capable of storing 1.16 GWh, was commissioned in 1978 and it has been operating since then. It uses two caverns of a total volume of 310,000 m³. The other is the CAES installation in McIntosh, Alabama, USA, operating since 1991, with 560,000 m³ caverns and the storage capacity of 2.64 GWh.

In order to improve the recovery efficiency the Huntorf and the McIntosh facilities both use the so-called “diabatic” operating regime, in which the turbine operation is aided by the combustion of natural gas. The energy recovery efficiency is still not spectacular: 42% in Huntorf and 54% in McIntosh.

However, the operation of both of these installations is apparently costeffective since both have been kept in operation for years. One argument is that in such a “diabatic cycle” the cost of natural gas needed to obtain an energy unit is four times lower than for a gas-only power plant. It should also be taken into account that if the utility system (no matter whether it uses renewable energy or conventionally produced energy) is able to generate surplus energy but this capacity is not used because there are no surplus storage devices then the efficiency is ... zero percent! 42 % or 54 % are not actually spectacular figures, but they are always better than 0 %.

An interesting extensive discussion about the sense of using CAES and its future prospects can be found in this series of articles published by POWER magazine. Also very much worth recommended is this quite recent Web document devoted to these matters.

Geothermally Coupled Compressed Air Energy Storage. A very interesting concept seems to be the combination of CAES technology with the use of geothermal heat. If the underground caverns for storage will be located in geothermal bed rocks, then the problem of lowered storage efficiency due to heat loss at compression can be significantly eased, and if the rocks are hot enough, the storage efficiency may even exceed 100%. Particularly favorable conditions for the use of such hybrid technology exist in California and the Pacific Northwest (for example, by using hot basalts in the Columbia River Gorge area). To date, no such project has been implemented, but there are serious studies that can motivate for creating such hybrid storage reservoirs – interested readers may be advised to consult reports prepared by Pacific Northwest National Laboratory, by the U.S. Department of Energy, or Section 7 in this report compiled by Idaho National Laboratory.