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

Technical Characteristics of Energy Storage

The specifics of a project's use case(s) will dictate the optimal 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, Energy, and Duration

Some of the most fundamental energy storage attributes are power (measured in Watts) and energy (measured in Watt-Hours). Energy storage power is usually provided in kilowatts (kW), megawatts (MW), or gigawatts (GW), while energy is the integral of power over time, so measured in kilowatt-hours (kWh), megawatts-hours (MWh), or gigawatts-hours (GWh), depending on the scale of the system. Sometimes, a duration (hours) is specified instead of energy, this is calculated by dividing the energy (watt-hours) by the power (watts) to return hours. This provides an estimate for the amount of time a system could operate at it's rated power, though in reality, the true time a system takes to charge or discharge is dependent on many factors or limitations.

In this analogy, 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.

Relationship Between Technologies and Their Ideal Applications

Different technologies have their own ideal energy-to-power ratio (i.e. duration) which makes them suitable for specific classes of use cases. The figure below breaks the duration domain (x-axis) into 4 different use case bins with common technologies added to the areas that they tend to perform best. Some energy storage technologies, like super-capacitors, are best at responding quickly and come in small modular form factors making them ideal for fast, "power" services like frequency response. Other technologies like pumped hydro are only feasible on a large scale, so are best suited for high "energy" services like energy time shifting.

Key Storage Technology Attributes for Project Evaluation

There are many technology attributes that are most important when performing site-specific project evaluations. So, what technology you ultimately decide to deploy will be a function of what is required for a specific use case. These attributes include: safety, expected operational life, deployment timeline, performance, technology maturity, siting considerations, lifecycle costs, and environmental or public health considerations.

Performance Attributes

This section will describe a few of the many energy storage performance attributes that should be considered. For a more exhaistive resource please visit the ESIC Energy Storage Test Manual, it is free to the public.

Efficiency: Ratio of the delivered discharge energy to the delivered charge energy, including facility parasitic loads. Many factors impact a system's measured efficiency:

• The specifics of the efficiency measurement and calculation
• Location of measurement (e.g. at the DC output of the battery or AC output of the inverters)
• Auxiliary or parasitic load losses
• Power level of a given cycle
• The weather / season

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

Response Time and Ramp Rate: Some energy storage technologies are able to respond quickly to dynamic control sugnals while others require more time to ramp up and respond with accurate output. 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 control commands.

Operational Life

The operational 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 safely and reliably performing its objectives. Different technologies will have drastically different degradation time frames and mechanisms, but most degradation impacts can be described by cycle life or calendar life

Cycle Life: Number of times the energy storage system can be charged and discharged before degrading beyond application requirement. Often, factors like each cycle's depth-of-discharge, temperature, and power level affect each cycle's contribution to lifetime degradation.

Cycle life is highly dependent on how the system is operated.

Calendar Life: Number of years the system operates before degrading beyond application requirement. Independent of cycle life.

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

Safety

Storing large amounts of energy in a confined space leads to a number of safety risks including fire, chemical, electrical, or physical hazards. Different technologies will have their own inherent safety risks or benefits depending on the mechanisms used to store energy. The safety of a system depends heavily upon proper planning and integration early on in a storage project. For help assessing fire risks, please refer to the ESIC Fire Hazard Mitigation Analysis, it is free to the public.

Survey of Technologies

Storage Technology	Technology Image	Efficiency Range	Cycle Life Range	Maturity Level	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