Energy Storage Technologies

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.

Safety

Survey of Technologies

Storage Technology	Efficiency Range	Cycle Life Range	Pros	Cons
Compressed Air Energy Storage (CAES)	40-55%	30 years	Mature bulk storage Low cost per kWh potential	Geographical limitations Requires fuel for heating producing CO2 emissions
Flow Batteries	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.
Flywheel Energy Storage	85-90%	>100,000 cycles	Fast response time High power capability	Low energy capacity High self discharge rates
Lithium Ion Batteries	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
Pumped Hydroelectric Energy Storage	70-85%	60-100 years¹	Ability to integrate inverter-based renewables Mature, flexible, bulk storage	Capital intensive Geographical limits Permitting (open-loop)
Thermal Energy Storage - End Use	Not applicable	10 - 15 years	Thermal demands are a large part of building and commercial loads	System engineering challenges to enable low cost installations
Thermal Energy Storage - Generation	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

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