Key Specifications of Battery Energy Storage System

Residential battery energy systems(BESS) have skyrocketed in popularity in European. Especially in Germany during the past few years for the reasons of energy crisis.

If you are wondering how to selecting the battery energy storage system, the following major technical parameters should be taken into consideration. Please keep reading.

System capacity of the battery energy storage system

It indicates the rated power storage capacity, usually in kilowatt-hours (kWh) or megawatt-hours (MWh). It is one of the important parameter index of the energy storage system. However, the real usable capacity is affected by the depth of charge(DOD) and the efficiency of the system.

BESS system capacity emphasizes the amount of energy that can be output or utilized, which is different from the definition of battery capacity. Battery capacity generally refers to the amount of charge that can be released by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.), in Ah (Ah), which represents the integral of current and time.

Energy loss and efficiency of the battery energy storage system

The efficiency of the energy storage system(BESS) reflects the energy loss of the system during charging and discharging process, which is the ratio of the energy released by the system to the charged energy. That is cycle efficiency.

The loss is not only related to the type of energy storage battery technology, but also depends on electrical links such as PCS.

The system efficiency in a narrow sense will mainly reflect the loss on the main circuit during the charge and discharge process, from the battery, DC bus, PCS and finally to the transformer (if it exists). However, in fact, in engineering applications, the power consumption of auxiliary equipment such as temperature control systems is often converted into the total loss, which affects efficiency.

In addition, the energy loss will also occur during the static process of the battery. The lead-acid battery is generally 1%~3%/month, while the lithium battery is less than 1%/month.

Maximum system power of the battery energy storage system

The maximum power of the system reflects the maximum charge and discharge capacity of the energy storage system, generally expressed in kilowatts (kW) or megawatts (MW).

This performance is determined by the design of the entire main circuit inside the battery, DC transmission circuit, PCS and AC access, and even through the loss under maximum power operation (the loss will be mainly converted into heat energy), which will affect the temperature control system and other auxiliary equipment the design of.

Battery energy storage systems with the same capacity have significant differences in function due to differences in maximum power; even for the same energy storage system, due to differences in operating power, there will be quadratic differences in efficiency.

When the power parameter is relatively large relative to the capacity parameter, such as 1MW/500kWh, it will be called a power-type energy storage system; otherwise, such as 500kW/1MWh, it will be called an energy-type energy storage system. So sometimes, the concept of time is also introduced, such as the former can be marked as 1MW/0.5h, while the latter can be marked as 500kW/2h.

Number of cycles of the battery energy storage system

Number of cycles in battery energy storage system is the lifespan of the storage battery. In BESS, the life of battery determines the life of the entire energy storage system because of its high costs.

The attenuation of the number of cycles will increase the internal resistance of the battery, and the loss and heat will also increase, which will further aggravate the attenuation process of the number of cycles.

 In addition, frequent overcharging and over-discharging will lead to the reciprocation of the dissolution and deposition of metal substances in the battery, which will also have a significant impact on the number of battery cycles and safety.

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