Storage Technologies

Summary for Decision Makers

The storage technologies covered in this primer range from well-established and commercialized technologies such as pumped storage hydropower (PSH) and lithium-ion battery energy storage to more novel technologies under research and development (R&D). These technologies vary considerably in their operational characteristics and technology maturity, which will have an important impact on the roles they play in the grid. Figure 1 provides an overview of energy storage technologies and the services they can provide to the power system.

Several key operational characteristics and additional terms for understanding energy storage technologies and their role on the power system are defined in the Glossary. Table 1 provides several high-level comparisons between these technologies. Many of these characteristics are expected to change as R&D for the technologies progress. Some technology categories, such as lithium-ion or lead-acid batteries, comprise multiple subtypes that each feature unique operational characteristics; comparisons of subtypes within technologies are considered in their respective sections.

Table: Qualitative Comparison of Energy Storage Technologies

Electrochemical Energy Storage Technologies

Lithium-ion Battery Energy Storage

Lithium-ion is a mature energy storage technology with established global manufacturing capacity driven in part by its use in electric vehicle applications. In the utility-scale power sector, lithium-ion is used for short-duration, high-cycling services. such as frequency regulation, and increasingly to provide peaking capacity and energy arbitrage services. Lithium-ion has a typical duration in the 2- to 4-hour range, with price competitiveness decreasing at longer durations. Despite the technology’s propensity to suffer thermal runaway leading to fire concerns, recent battery pack technology and software innovations are addressing these safety concerns.

Flow Battery Energy Storage

Flow battery technology is relatively nascent when compared to lithium-ion but offers long duration, the ability to deeply discharge its stored energy without damaging the storage system, and exceedingly long life cycles. This uniquely positions flow batteries for longer duration services such as load following or peaking capacity. While flow batteries have higher upfront costs than lithium-ion, their longer life cycle can lead to significantly lower lifetime costs. Flow batteries are also typically safer and are less reliant on rare materials, depending on the specific chemistry. Given flow batteries’ low energy and power density, these systems tend to be larger than other equivalent storage technologies.

Lead-Acid Battery Energy Storage

Lead-acid energy storage is a mature and widely commercialized technology like lithium-ion, but several characteristics, such as its short cycle life and its inability to remain uncharged for long periods or to be deeply discharged without permanent damage, have limited its applications in utility-scale power system applications. Ancillary services that require frequent, shallow charging and discharging like frequency regulation may be better suited for lead-acid, compared to less frequent, deeper discharge applications like peak demand reduction.

Sodium-Sulfur Battery

Sodium-sulfur storage technology is in the initial commercialization phase. Its high energy density, low levels of self-discharge (which correspond to higher efficiencies), and relatively long cycle life make it well suited for longer duration services such as peaking capacity and energy arbitrage. These systems are also lower costs relative to other storage technologies due to its reliance on common, abundant, and cheap materials. However, sodium-sulfur requires high temperatures to operate (300°–350°C) making it difficult to deploy.

Mechanical Energy Storage Technologies

Pumped Storage Hydropower (PSH)

PSH is the most mature energy storage technology, with wide commercialization globally. PSH systems are large facilities comprising reservoirs of different elevations. Electricity is generated when water passes through turbines when moving from the upper to lower reservoir. The technology’s large capacities and long durations that make it well-suited for services such as load following or energy arbitrage, charging during times of cheap power and meeting demand during system peaks. Drawbacks of PSH include its geographic requirements and high upfront capital cost.

Flywheel Energy Storage

Flywheels are an established, widely commercialized mechanical energy storage technology that utilizes a rotor and low-friction bearings to convert electricity to and from rotational kinetic energy. Rapid charging and discharging capabilities, relatively little maintenance, long lifetimes, and short discharge durations, make flywheels practical for maintaining power quality in uninterruptible power supply applications and for short duration services like grid frequency regulation. However, high energy costs limit this technology’s deployment in other areas.

Compressed Air Energy Storage (CAES)

CAES is a form of mechanical energy storage that uses electricity to compress and store ambient air for later use. When needed, this compressed air is withdrawn from the storage medium, expanded, and passed through a turbine to generate electricity. The high energy capacity, long duration times of the technology, and slower response times make CAES more suitable for providing peak capacity, secondary and tertiary operating reserves, and energy arbitrage. While CAES energy costs is lower than other technologies, its deployment is limited by its unique geological requirements (air is typically stored in underground salt caverns or other hard and porous rock geologic formation). More advanced variations of CAES such as adiabatic compressed air energy storage (A-CAES) and liquid air energy storage (LAES) are still nascent and in pilot-testing phases.

Gravity Energy Storage (GES)

GES is an immature technology that uses established mechanical bulk storage principles, using the potential energy of a mass at a given height. PSH is based on these principles, utilizing water as the elevated mass. GES can provide long-term energy storage making it useful for slower, longer-duration services such as peaking capacity, load following, and energy arbitrage. Emerging GES technologies typically use a low-cost and abundant medium such as sand, concrete, gravel, or rock.

Other Energy Storage Technologies

Hydrogen Energy Storage Systems

Hydrogen energy storage systems for electricity rely on the production, storage, and eventual reconversion of the hydrogen into electricity (either through the combustion of hydrogen gas, or the direct conversion of hydrogen and oxygen in a fuel cell). Despite its capability of providing short-term services like frequency regulation, hydrogen is currently unable to compete with electrochemical energy storage like lithium-ion batteries for shorter duration services on a cost-basis. However, hydrogen energy storage is suited for long-duration storage useful for shifting surpluses of renewable energy in the spring to deficits in the winter or summer. In addition to the power sector, hydrogen storage has potential applications in transportation and industrial processes as those sectors electrify.

Thermal Energy Storage

TES is an established technology that relies on storing energy as heat and extracting the heat at a later period, either to meet heating demands directly or to generate electricity. TES is marked by long durations of several hours and is therefore a good fit for peaking capacity needs and is often combined with concentrated solar power (CSP). In a CSP plant, TES is used to shift energy generated during high solar hours to evening or nighttime periods. In district heating applications, thermal energy storage enables flexible operations of combined heat and power (CHP) plants.

Supercapacitors

A supercapacitor, or ultracapacitor, stores energy by static charge. These systems have high power and low energy capacities. Supercapacitors are useful for power quality applications, as they can frequently charge and discharge at high currents for short durations. Supercapacitors sustain power gaps for up to 60 seconds with quick recharging capabilities. When paired with electrochemical devices, they have been shown to improve the efficiency and lifetime of the battery components.

Superconducting Magnetic Energy Storage (SMES)

SMES systems store energy in the electrical charge of a coil of superconducting material, exhibiting zero resistance below certain temperatures, requiring external cooling infrastructure. SMES devices enable near-instantaneous absorption or injection of high levels of power over short time frames, such as in power quality applications. SMES systems are marked by high power densities, low energy densities, very fast reaction times, and long cycle lives.