Energy Storage 101/Technologies

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Defining Energy Storage

People often think of grid energy storage as electricity in / electricity out with some energy loss in between due to inefficiencies. A more inclusive "energy storage" definition should include technological nuances like supplemental energy sources (e.g. input fuels or heat injection). One must also consider that energy storage systems can output non-electrical energy in the form of heat, cooling, or fuel sources (e.g. hydrogen).

A generalized energy storage visual

Big Picture of Energy Storage Technology Deployment

Globally over 95% of installed storage is pumped hydropower, but battery storage is quickly growing. Approximately 98% of new ‘advanced’ storage projects are lithium ion battery systems.

A breakdown of global energy storage installations by technology. Data Source: CNESA "Global Energy Storage Market Analysis—2020.Q2 (Summary)"

Technical Characteristics of Energy Storage

The specific project use case(s) will dictate the desirable system attributes. Understanding these attributes and the trade-offs between them will help with the selection of a specific technology. For an exhaustive list of considerations, refer to the ESIC Technical Specification Template.

Power and Energy

Some of the most fundamental attributes to understand in energy storage are power (measured in Watts) and energy (measured in Watt-Hours).

Power (watts) is analogous to the keg’s flow rate (pints/hour) Energy (watt-hours) is analogous to the keg size (pints) Knowing the power (keg flow rate) and energy (amount of beer) we know how long we will have electricity (beer) for.

Useful Life

The useful life of an energy storage system is a tricky concept to define generally, but it typically refers to how long a system is able to operate before degradation prevents the system from performing its objective safely and reliably. Different technologies will have drastically different degradation time frames and mechanisms, but most degradation effects on useful life can be described by cycle life of calendar life

Cycle Life: Number of times the energy reservoir can be charged and discharged before degradation beyond application requirement. Depth of cycle impacts life expectancy.

Cycle life is highly dependent on how the system is operated

Calendar Life: Years until the storage system operates before degradation beyond application requirement. Independent of cycle life.

Systems degradation is also dependent on factors like average resting state-of-charge and average ambient temperature

Footprint

A system's footprint describes the amount of space that a technology, and its associated auxiliary components, will occupy. It is affected by many design characteristics including power density, energy density, and packaging choice.

A table showing the footprints associated with recent energy storage projects.

Developer, Site	Size	Footprint	Notes
Tesla, Ontario	20MW, 80MWh	~62,000 sq ft	Pre-packaged BESS enclosures
AltaGas/GreenSmith, Pomona	20MW, 80MWh	10,800 sq ft (Battery building)	Located at existing gas plant facility
AES, Escondidio	30MW, 120MWh	1 acre (43,560 sq ft)	24 x 640 sq ft trailers

Efficiency

Ratio of the delivered discharge energy to the delivered charge energy, including facility parasitic loads.

A breakdown of some of the energy inefficiencies that may affect a technology's overall efficiency

Response Time and Ramp Rate

Fast acting energy storage systems may perform dynamic grid services (like frequency regulation) better than conventional alternatives.

Due to their inherently low inertia, some inverter based energy storage technologies are able to react quickly to commands.

Survey of Technologies

Storage Technology	Technology Image	How it Works	Variations	Efficiency Range	Cycle Life Range	Maturity Level	Pros	Cons	Applications
Compressed Air Energy Storage (CAES)		Air is compressed (charging), stored, and expanded (discharging).		40-55%	30 years		Mature bulk storage Low cost per kWh potential	Geographical limitations Requires fuel for heating producing CO2 emissions	Bulk, long-duration services
Flow Batteries		Liquid anode (anolyte) and cathode (catholyte). Electrolytes flow through reaction cell and charge transfer occurs at a membrane. Vanadium-based chemistry is most mature, other chemistries being developed.	Vanadium Redox Zinc Bromine Coupled iron-chrome Zinc/Chlorine Organic	50-75%	20 years, >100,000 cycles (claimed)		Power (reactor size) decoupled from Energy (tank size) Limited impact of cycling on degradation Higher fire safety than lithium ion	Lower energy density Potential environmental spill risk OK to poor efficiency observed to-date Added system complexity with pumps etc.	Energy shifting for renewable integration, T&D deferral, potential for longer duration
Flywheel Energy Storage		Rotating mass stores rotational kinetic energy.		85-90%	>100,000 cycles		Fast response time High power capability	Low energy capacity High self discharge rates	Power quality, frequency regulation, wind generation stabilization; high energy flywheels are being developed for longer duration applications.
Lithium Ion Batteries		Shuttle lithium ions (Li+) between cathode (+) and anode (-). Fully charged when Lithium ions are fully intercalated in the anode.	Lithium Iron Phosphate (LFP), Nickel Manganese Cobalt (NMC), Nickel Cobalt Aluminum Oxide (NCA), Lithium Titanate Oxide (LTO)	80-92%	3,000 - 10,000 cycles 10 - 20 years		High power and energy density Low self-discharge rate High roundtrip efficiency Flexible configurations Leverage cost reductions from consumer electronics and electric vehicle markets	Cycle life limitations, especially with high depth of discharge Safety concerns around fire and explosion risk Supply chain constraints	Diverse applications from minutes to hours duration and from small scale residential to transmission connected.
Pumped Hydroelectric Energy Storage		Gravitational potential energy ↔ kinetic energy ↔ electricity Water is pumped up hill with excess electrical energy which is stored as gravitational potential energy. When energy is needed, water flows down through the generator to produce electricity.		70-85%	60-100 years1		Ability to integrate inverter-based renewables Mature, flexible, bulk storage	Capital intensive Geographical limits Permitting (open-loop)	Bulk, long-duration services, power regulation and load following
Thermal Energy Storage - End Use		Storing energy by heating or cooling a storage medium for end use in a thermal application.		Not applicable	10 - 15 years		Thermal demands are a large part of building and commercial loads	System engineering challenges to enable low cost installations	Ice, Chilled Water, Water Heater Used for building and district heating and cooling, cold food storage
Thermal Energy Storage - Generation		Storing energy by heating or cooling a storage medium and converting to electricity.		35-60%	20 - 30 years		Low cost incremental energy Non-toxic materials Can be integrated with existing power gen units	Low round trip efficiency  System engineering challenges to enable low cost installations	Molten Salt, Liquid Air, Concrete/Gravel/Sand Thermal, Electro-Thermal Energy Storage Used for bulk energy storage applications

Research and Resources at EPRI

Current Research Focus

• Long duration storage
• Non-lithium storage
• Lithium ion advancements

Resources and Engagement Opportunities

Resource	Access Level
Webcast Recording on Energy Storage Technology	Publicly Available
Emerging Energy Storage Technology Testing and Demonstration Supplemental Project	Supplemental Funders
Energy Storage Technology Database	Program 94: Energy Storage and Distributed Generation or Program 66: Advanced Generation and Bulk Energy Storage
Strategic Intelligence (SI) Articles	Program 94: Energy Storage and Distributed Generation
DER Forum: Technologies Discussion	Program 94: Energy Storage and Distributed Generation