Batteries are the most visible storage technologies due to their ubiquity in portable electronics, but energy can be stored using many other mechanisms like supercapacitors.  Though alternative means of energy storage tend to have a lower energy density than batteries and therefore are not as easily portable, they have great utility in stationary applications like our electricity grid.  Some stationary storage devices are useful for peak shaving: relieving congestion on hot days when transmission lines are straining at maximum capacity. Other, larger-scale stationary storage devices have the potential to offset the intermittency of renewable energy generation. In this fourth and final post on energy storage, we will discuss the storage mechanisms, applications, and limitations of a few of lesser known, exciting energy storage technologies.

 Pumped Hydro

The most common type of energy storage in the electric grid is pumped hydro, which by nature is large and stationary.  The mechanism for pumped hydro storage is pretty simple – water is pumped up a hill. That’s it. Once it’s up the hill, the water is stored as potential energy in a reservoir at the top, and released through a turbine to generate electricity when demand requires it.  It is relatively cheap and easy to co-locate with existing hydropower, but cannot be used in geographically flat regions.  Switzerland has employed pumped hydro for over a century.

 Flywheels

Flywheels store energy in rapidly spinning disks or columns. The energy is stored as kinetic (in this case rotational) energy, which can be rapidly converted to electricity. Flywheels don’t store a lot of energy per unit volume, but they can deliver that energy very quickly, so they are very good for high-power applications. The Princeton Plasma Physics Laboratory, for example, uses flywheels to deliver the power for its nuclear fusion experiments because the local grid cannot accommodate such sudden peaks in power demand.  Flywheels do have standby leakage losses, however, so they can’t be used for very long-term storage.  They are usually kept in chambers underground because their failure mechanism is explosion due to rapid crack propagation along the surface. Not ideal.

 Superconducting magnetic energy storage (SMES)

SMES stores energy in the magnetic field created when current is sent through a superconducting donut. SMES is very efficient, but it is extremely expensive because the superconductors must be kept close to absolute zero. It also requires a huge amount of underground space in order to be deployed on a large scale. In other words, very cool, but not well-applied yet.

 Flow Batteries

Flow batteries are the only technology in this post that rely on electrochemical energy storage.  They behave somewhat like regular batteries with the exception that the active material is a liquid. Traditional batteries are limited by the amount of material in the electrodes, but in flow batteries the active material can be replenished with new material from storage tanks, while the used material can be stored and regenerated at a later point.  Flow batteries are currently used in a number of small grid applications.

 Compressed Air Energy Storage (CAES)

In typical CAES systems, air is compressed in underground aquifers and this potential energy is harnessed when the air is released to run a turbine. The air is frequently used to co-fire a natural gas generator; the generator requires compressed air to function.  CAES is most appropriate for large-scale grid energy storage, and suitable geology can be found in many locations where pumped hydro is infeasible. In the US, many potential sites are located near areas with significant wind energy potential, so CAES could be used to smooth the highly variable output from wind farms and allow for greater penetration of renewables in the grid.  A couple of CAES systems have been in use for decades, including the 110 MW facility in McIntosh, Alabama, which has been storing compressed air in a salt dome formation since 1991.  There have been proposals for new CAES facilities but most have been held up due to financial concerns.

Thermal energy storage

Thermal energy storage can refer to any energy storage technique where energy is stored by changing the temperature of some bulk material.  Waste heat from electricity generation, for example, can be used to heat water; this water can be stored in a tank and used later to heat buildings or provide warm water for showers. Water can also be chilled into ice during off-peak hours, stored, and used during peak demand for cooling.  Heat may also be stored in other materials, like molten salts.  In solar thermal electricity generation, sunlight is focused onto molten salts, heating them to very high temperatures.  Energy is stored by maintaining these salts at high temperature, and this heat is later used to heat water and run a steam turbine to produce electricity.

Conclusion

As we have seen, energy storage impacts portable electronics, electric vehicles, peak energy in our transmission lines, and large-scale storage for the grid.  No single technology is ideal for all applications, and the most appropriate technology in one case may not be of any use in another. Energy density, power density, mechanism, cost, safety, and many other features all contribute to each technology’s applicability. Maximizing the optimal characteristics of each allows them all to play an important role in increasing the efficiency of the electric grid and vehicles. Collectively, they are creating the capacity for our transition to a carbon neutral world.