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The features of storage technologies must match application requirements
Unlike liquid or gaseous energy carriers, electrical energy is difficult to store and must usually be converted into another form of energy, incurring conversion losses. Nevertheless, many storage technologies have been developed in recent decades that rely on mechanical, electrochemical, thermal, electrical or chemical energy. Most of them are currently clustered in the investment “valley of death”, i.e. at the demonstration or early deployment phases, when capital requirements and risks are at their highest.
The applications electricity storage technologies are able to fulfill depend on their chemical and physical characteristics. Technologies must be assessed at the application level, taking into account power rating, storage duration, frequency of charge and discharge, efficiency and response time, and site constraints that determine power and energy density requirements.
In general, pumped hydro storage (PHS) and compressed air energy storage (CAES) are the most suitable for bulk storage applications. PHS uses the gravitational potential energy of two vertical reservoirs; water is pumped from a lower reservoir up to a higher reservoir during periods of off-peak demand, and the flow is reversed to drive a turbine during peak periods. CAES works by using electricity to compress air into a cavern or pressurized tank and later releasing the air to drive a turbine, which converts the energy back into electricity. However, both technologies face site availability issues.
Batteries are a major component of the storage landscape and can serve a wide range of applications with intermediate power and energy requirements. They differ according to their electrodes and electrolyte chemistries: sodium-sulfur (NaS) and lithium-ion (Li-ion) are the most suited for stationary storage thanks to their higher power and energy densities, and greater durability. Nevertheless, durability remains, together with costs and safety concerns, one of the biggest hurdles to commercial development. In addition to conventional batteries, research is being conducted into flow batteries, such as vanadium redox (VRB) or zinc-bromine (Zn/Br) batteries, which use the same reaction but with two separately stored electrolytes, allowing for power and energy decoupling. They are, for now, more costly due to their complex balance of system, and further development and demonstration efforts will be needed.
For applications where providing power in short bursts is the priority, flywheel, superconducting magnetic energy storage (SMES) and supercapacitors appear to be the most attractive, as a result of their high power density, high efficiency, high response time and long lifespan. However, costs are high and these technologies are currently at the demonstration phase.
Finally, despite its poor overall efficiency and high up-front capital costs, chemical storage seems to be the only way to provide the very large-scale and long-term storage requirements that could result from a power mix generated primarily by variable renewables.
Chemical storage consists of converting electricity into hydrogen by means of water electrolysis. It actually goes far beyond electricity storage since hydrogen can also be converted into synthetic natural gas or used directly as a fuel in the transportation sector or as feedstock in the industry. In contrast to other technologies, chemical storage is mainly driven by excess, rather than a shortage, of renewable energy. Thermal storage is also worth considering, but is mainly being developed as a means of electricity storage in association with concentrating solar power.