Storage of electricity is the holy grail of renewables. Without the ability to store electricity, renewables will remain uneconomic.
Electricity must be generated as it is consumed, unless there is a way to store excess electricity for use at a later time – referred to as time-shifting renewables.
The storage issue isn’t a one size fits all issue.
Renewables require the ability to store very large quantities of electricity.
Uninterruptable power supplies, on the other hand, only require storing enough electricity to provide a buffer until back-up supplies can be brought on-line. The nature of the buffer depends on the required quality of the electricity supplied by the storage medium, in terms of voltage stability, interruptible cycles and frequency.
There are storage devices covering the spectrum between these two extremes, for doing such tasks as smoothing transmission and distribution to provide better quality in terms of voltage regulation, power factor correction and integrating distributed generation.
The real storage issue relates to renewables.
Electricity can be stored as chemical, thermal or mechanical energy.
Batteries utilize chemical storage. These include lead-acid, Lithium-ion, Sodium-Sulphur and flow batteries.
Salt pits at concentrating solar power (CSP) plants are a form of thermal storage. Using ice in cooling systems is another method of thermal storage.
Pumped storage, where water is pumped uphill to a reservoir, is a form of mechanical storage. Compressed air is another form of mechanical storage. Flywheels, such as those produced by Beacon Power Corp., which recently went bankrupt, is another method for mechanical storage.
Another approach is to use hydrogen as a storage medium.
Storage becomes a critical element of grid operation as the impact of renewable portfolio standards (RPS) or renewable energy standards (RES) become more pronounced. Specifically, as the percentage of electricity from renewables grows from a few percent to 20% or more.
In California, where RPS has been taken to an extreme, regulators are in the process of requiring utilities to equip their systems with increasing amounts of storage. This, of course, will increase costs and the prices paid by consumers for electricity. (Assembly Bill 2514 directs the California Public Utilities Commission to determine energy storage procurement targets.)
In attempting to explain the value of energy storage, the CPUC avoids the financial cost of storage by including societal benefits in the cost equation, such as avoiding use of fossil fuels and greenhouse gas (GHG) emissions.
All of the storage mediums described above, have limitations.
Pumped storage, which is a proven technology, requires building expensive dams and reservoirs, while incurring objections from environmental organizations.
Most batteries have limited storage capacities. Those that could store large amounts of electricity, such as Sodium-Sulphur, are very large and expensive.
Compressed air storage requires a large space, with sufficient volume in which to store the compressed air.
In summary, we currently lack the ability to store large quantities of electricity at low cost.
Until this changes, renewables will remain a very costly and inefficient method for generating electricity.
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