Energy Storage: Definitions and Technical Routes (Ultra-Detailed)

Energy Storage: Definitions and Technical Routes (Ultra-Detailed)

(1) Definition of Energy Storage

  Energy storage refers to the process of storing energy through media or equipment and releasing it when needed. Through flexible charging and discharging control, it realizes the matching of energy production and consumption in time and space, serving as the foundation for system flexibility.

  Energy storage is a vital technology and basic equipment underpinning the new power system. It can provide multiple services for grid operation including peak shaving, frequency regulation, standby power supply, black start, and demand response support, acting as a crucial measure to improve the flexibility, economy and safety of the traditional power system. It can significantly boost the absorption capacity of renewable energy such as wind and solar power, support distributed power generation and microgrids, and function as a key technology driving the shift of primary energy from fossil fuels to renewable energy. Moreover, energy storage facilitates the open sharing and flexible trading of energy production and consumption as well as multi-energy coordination. It constitutes the core foundation for building the Energy Internet, advancing power system reform and fostering new forms of energy industry.

(II) Technical Routes of Energy Storage

  According to the way energy is stored, energy storage can be divided into five categories: mechanical energy storage, electromagnetic energy storage, electrochemical energy storage, thermal energy storage and hydrogen energy storage. Among them, mechanical energy storage mainly includes pumped hydro storage, compressed air energy storage, flywheel energy storage and so on; electromagnetic energy storage mainly includes supercapacitors and superconducting magnetic energy storage; electrochemical energy storage mainly includes lithium-ion batteries, lead-acid batteries, flow batteries, sodium-sulfur batteries and so on.

  It is divided into four major categories according to functional requirements of application scenarios: capacity-oriented energy storage technology, energy-oriented energy storage technology, power-oriented energy storage technology and backup energy storage technology. Generally, the four categories are capacity-oriented (≥4h), energy-oriented (approximately 1–2h), power-oriented (≤30min) and backup (≥15min).

(III) Technical Indicators of Energy Storage

  Key technical indicators of energy storage technology mainly include energy density, power density, charge-discharge rate, energy storage efficiency, cycle life, response time and so on.
  Energy density (Wh/kg): It refers to the energy stored by a battery per unit mass, and 1 Wh equals 3600 Joules (J) of energy. Energy density is determined by the material properties of batteries; for example, the energy density of ordinary lead-acid batteries is approximately 40 Wh/kg.
  Power density (W/kg): It refers to the rate at which a battery per unit mass can output energy during discharge. Power density is also determined by material properties, and there is no direct correlation between power density and energy density—higher energy density does not mean higher power density. In fact, power density describes the rate capability of a battery, namely the maximum current at which a battery can discharge. Power density is critical for the research and development of batteries and electric vehicles. A high power density enables electric vehicles to accelerate rapidly. The power density of conventional lead-acid batteries generally ranges from tens to hundreds of watts per kilogram, indicating their poor high-rate discharge performance, while lithium-ion batteries can currently achieve a power density of several kilowatts per kilogram.
  Charge-discharge rate: Charge-discharge rate = charge-discharge current / rated capacity. The symbol "C" is used to denote the charge and discharge rate capability of a battery, where 1C represents the current required to fully discharge a battery within one hour.
  Energy storage efficiency: It refers to the ratio of the electric quantity stored by an energy storage device to the input energy.