Energy storage system composition and description of main parameters

Energy storage involves many technical parameters and concepts, which are particularly easy to confuse. I have compiled the collected information below for your reference.

1. Definition, function and composition of energy storage system

Energy storage technology refers to the use of chemical or physical methods to store electrical energy generated from primary energy sources and release it when needed. Including energy and material input and output, energy conversion and storage equipment. The Battery Energy Storage System (BESS) uses lithium/lead batteries as energy storage carriers. A system that stores electric energy within a certain period of time and supplies electric energy within a certain period of time, with functions such as smooth transition, peak shaving and valley filling, frequency regulation and voltage regulation. At present, energy storage systems mainly consist of energy storage units and monitoring and dispatching management units: energy storage units include energy storage battery packs (BA), battery management systems (BMS), energy storage converters (PCS), etc. The monitoring and dispatching management unit includes the central control system (MGCC), energy management system (EMS), etc.

2. Main concepts of components involved in energy storage technology

• Battery Energy Storage System (BESS)

• Battery Cell: a single battery, the smallest unit of a battery;

• Battery Module/Pack: a series of standard packages for individual batteries; Battery

• Battery Rack/Cluster: an energy storage unit composed of a series of battery modules;

• Battery Collection Panel (BCP): between the battery rack and the energy storage inverter, similar to the photovoltaic DC combiner box;

• Power Conversion System (PCS): bidirectional DC AC inverter;

• Battery management system (BMS): Intelligent management and maintenance of each battery unit, preventing overcharging and over-discharging of the battery, extending the service life of the battery, and monitoring the status of the battery.

• Energy management system (EMS)

• Heating, Ventilation and Air Conditioning System (HVAC): Usually used in battery containers to ensure battery ventilation, heat dissipation and warmth;

3. Multiple concepts of energy storage performance indicators

• Battery capacity: The charge Q that can be accommodated or released, that is, battery capacity (Ah) = current (A) x discharge time (h), the unit is generally Ah (ampere hour). For example, if the energy storage battery is marked 96Ah, then when the working current is 96A, it can theoretically be used for 1 hour.

• Battery energy: The energy stored in the battery, in Wh (watt hours), energy (Wh) = voltage (V) × battery capacity (Ah). For example, a 3.2 V/96Ah battery has an energy of 307.2Wh. If we connect 4 such batteries in series, it will form a battery pack with a voltage of 12.8V and a capacity of 96Ah. Although the battery capacity is not increased, the total energy is increased by 4 times.

• Charge and discharge rate (C-Rate): The discharge rate refers to the current value required to discharge its rated capacity within a specified time. It is numerically equal to a multiple of the battery's rated capacity. That is: charge and discharge current (A) / rated capacity (Ah), the unit is generally C (abbreviation for C-rate), such as 0.5C, 1C, etc.

• State of charge (SOC) state of charge, remaining battery capacity, range is 0~1, 1 means the battery is fully charged, 0 means the battery is completely discharged;

• Depth of discharge (DOD) represents a measure of the degree of battery discharge. It is the ratio of discharge capacity to rated capacity, in %. For example, 80% DOD means that 80% of the rated capacity is discharged during discharge.

• Battery remaining capacity (End of life, EOL), EOL 80% means that the overall battery capacity declines to 80% of the initial capacity after being used for a period of time, corresponding to BOL, Beginning of Life.

• Guaranteed power capacity (GPC): the minimum capacity released by the energy storage system within the specified use time.

• Cycle efficiency (Round-trip Efficiency, RTE): After considering the equipment loss and self-consumption during charging and discharging, the ratio of the total power released by the AC terminal when the battery is fully charged to the power required for the AC terminal to be fully charged.

• Cycle life: The life of a battery is divided into two parameters: cycle life and calendar life. Cycle life refers to the number of times a battery can be charged and discharged. That is, under ideal temperature and humidity, charge and discharge at the rated charge and discharge current, and calculate the number of cycles experienced when the battery capacity decays to 80%. Calendar life refers to the time span in which a battery reaches end-of-life conditions (capacity decays to 80%) under specific usage conditions under environmental conditions. Generally, owners may assess the smaller of the two

• Self-discharge: After the battery is fully charged, its capacitance will naturally decay when it is not in contact with external circuits and placed at room temperature. Usually, the ambient temperature has a greater impact on it, and excessive temperature will accelerate the self-discharge of the battery. The expression method and unit of battery capacity decay (self-discharge rate) is: %/month

Further reading: The global industrial and commercial energy storage market is expected to reach 24 billion yuan in 2025