Energy Storage 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.

Safety

Survey of Technologies

Storage TechnologyTechnology ImageEfficiency RangeCycle Life RangeMaturity LevelProsCons
Compressed Air Energy Storage (CAES)CAES.png40-55%30 years
  • Mature bulk storage
  • Low cost per kWh potential
  • Geographical limitations
  • Requires fuel for heating producing CO2 emissions
Flow BatteriesFlowBattery.PNG50-75%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 StorageFlyWheel.PNG85-90%>100,000 cycles
  • Fast response time
  • High power capability
  • Low energy capacity
  • High self discharge rates
Lithium Ion BatteriesLithiumIonImage.PNG80-92%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 StoragePumpedHydro.PNG70-85%60-100 years1
  • Ability to integrate inverter-based renewables
  • Mature, flexible, bulk storage
  • Capital intensive
  • Geographical limits
  • Permitting (open-loop)
Thermal Energy Storage - End UseThermal EndUse.pngNot applicable10 - 15 years
  • Thermal demands are a large part of building and commercial loads
  • System engineering challenges to enable low cost installations
Thermal Energy Storage - GenerationThermal-Generation.png35-60%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