1.3 Energy storage metrics and performance indicators

Energy storage metrics are crucial for comparing and evaluating different technologies. They help us understand how much energy can be stored, how quickly it can be delivered, and how long the system will last.

Key metrics include energy density, power density, efficiency, and cost. These factors determine which storage solutions are best for specific applications, from electric vehicles to grid-scale systems. Understanding these metrics is essential for optimizing energy storage design and implementation.

Energy Capacity and Power

Energy and Power Density

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• Energy density quantifies the amount of energy stored per unit volume or mass of an energy storage system (Wh/L or Wh/kg)
  • Higher energy density allows for more compact storage systems
  • Important for applications with space or weight constraints (electric vehicles, portable electronics)
• Power density measures the rate at which energy can be delivered per unit volume or mass (W/L or W/kg)
  • Higher power density enables faster charging and discharging
  • Crucial for applications requiring high power output (grid frequency regulation, power tools)

Depth of Discharge (DoD)

• Depth of Discharge (DoD) refers to the percentage of the total energy capacity that is discharged from a storage system
  • 100% DoD means the system is fully discharged, while 0% DoD indicates a fully charged state
• DoD impacts the cycle life and performance of storage systems
  • Deeper discharges generally lead to shorter cycle life due to increased stress on the system
  • Shallow discharges (low DoD) can extend the system's lifespan but may limit the usable energy capacity
• Optimal DoD varies depending on the storage technology and application requirements
  • Lead-acid batteries typically have a recommended DoD of 50-80%
  • Lithium-ion batteries can often be discharged up to 80-90% DoD without significant degradation

Efficiency and Cost

Energy Storage Efficiency

• Efficiency measures the ratio of energy output to energy input in a storage system
  • Expressed as a percentage, with higher values indicating less energy loss during the storage process
• Round-trip efficiency considers the energy losses during both charging and discharging cycles
  • Calculated as the ratio of energy discharged to energy charged
  • Includes losses due to conversion, self-discharge, and other inefficiencies
• Efficiency is a key factor in determining the overall performance and cost-effectiveness of energy storage systems
  • Higher efficiency means more usable energy and potentially lower operating costs
  • Pumped hydro storage typically achieves round-trip efficiencies of 70-85%
  • Lithium-ion batteries can reach round-trip efficiencies of 85-95%

Levelized Cost of Storage (LCOS)

• LCOS represents the average cost per unit of energy stored and discharged over the lifetime of a storage system
  • Expressed in units of currency per kilowatt-hour ($/kWh)
  • Considers capital costs, operating costs, and the total energy throughput
• LCOS allows for a fair comparison of different storage technologies and configurations
  • Lower LCOS indicates more cost-effective energy storage
  • Factors influencing LCOS include system efficiency, cycle life, depth of discharge, and initial investment
• LCOS is an important metric for evaluating the economic viability of energy storage projects
  • Helps in determining the optimal storage technology and sizing for a given application
  • LCOS for lithium-ion batteries has decreased significantly in recent years, making them more competitive with other storage options

Durability and Performance

Cycle Life and Longevity

• Cycle life refers to the number of complete charge-discharge cycles a storage system can undergo before its capacity or performance degrades to a specified level
  • Typically defined as the number of cycles until the capacity reaches 80% of its initial value
  • Longer cycle life means the system can be used for more cycles, extending its operational lifetime
• Factors affecting cycle life include depth of discharge, operating temperature, and charge/discharge rates
  • Shallow discharges and moderate temperatures generally lead to longer cycle life
  • Fast charging or discharging can accelerate degradation and reduce cycle life
• Cycle life is a critical consideration for the long-term reliability and cost-effectiveness of energy storage systems
  • Lithium-ion batteries can typically achieve several thousand cycles (2,000-5,000) depending on the specific chemistry and operating conditions
  • Flow batteries and pumped hydro storage can have cycle lives exceeding 10,000 cycles

Response Time and Self-Discharge

• Response time measures how quickly an energy storage system can respond to changes in demand or supply
  • Faster response times are essential for applications requiring immediate power adjustment (frequency regulation, voltage support)
  • Electrochemical storage technologies like batteries and supercapacitors have fast response times in the range of milliseconds to seconds
• Self-discharge rate indicates the rate at which a storage system loses stored energy over time without being used
  • Expressed as a percentage of the total stored energy lost per unit time (e.g., 1% per day)
  • Lower self-discharge rates are desirable to minimize energy losses during periods of inactivity
• Response time and self-discharge rate impact the suitability of storage technologies for different applications
  • Pumped hydro storage has relatively slow response times (minutes to hours) but very low self-discharge rates
  • Supercapacitors have extremely fast response times (milliseconds) but higher self-discharge rates compared to batteries