A complete Battery Energy Storage System (BESS) requires attention to two main performance indicators: one related to energy storage capacity and utilization, i.e., related to capacity; and the other related to the ability to replenish or release energy, i.e., related to power. The relationship between these two aspects is often used to distinguish whether the energy storage system is energy-oriented or power-oriented.
Battery energy storage system capacity
This metric represents the theoretical maximum energy capacity that a storage system can hold, typically expressed in kilowatt-hours (kWh) or megawatt-hours (MWh). This is one of the most important parameters of an energy storage system; however, its actual usable capacity is affected by the battery's depth of discharge (DOD) and system efficiency.
The capacity of a BESS (Battery Energy Storage System) 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 capacity of a battery under certain conditions (discharge rate, temperature, termination voltage, etc.), and is measured in ampere-hours (Ah), representing the integral of current over time.
Battery energy storage system：Maximum system power
The system's maximum power reflects the maximum charging and discharging capacity of the energy storage system, and is generally expressed in kilowatts (kW) or megawatts (MW). This performance indicator is determined by the design of the entire main circuit, including the battery, DC transmission circuit, PCS, and AC connection, and even the losses during operation at maximum power (these losses will mainly be converted into heat), which affects the design of the temperature control system and other auxiliary equipment. Energy storage systems with the same capacity can have significantly different functionalities due to differences in maximum power; even the same energy storage system will experience a squared difference in efficiency due to different operating power levels.
When the power parameter is relatively large compared to the capacity parameter, such as 1 MW/500 kWh, it is called a power-type energy storage system; conversely, if it is 500 kW/1 MWh, it is called an energy-type energy storage system. Therefore, the concept of time is sometimes introduced, such as the former being labeled as 1 MW/0.5 h, and the latter as 500 kW/2 h.
Battery energy storage system：Energy loss and efficiency
The efficiency of an energy storage system reflects the energy loss during the charging and discharging process. It can be understood as the ratio of the energy released by the system to the energy charged into it, also known as the cycle efficiency. This loss is not only related to the technical type of the energy storage battery but also depends on electrical components such as the power conversion system (PCS). In a narrow sense, system efficiency primarily reflects the losses in the main circuit during charging and discharging, from the battery, DC bus, PCS, and finally to the transformer (if present). However, in practical engineering applications, the power consumption of auxiliary equipment such as the temperature control system is often included in the total loss, thus affecting the overall efficiency.
Figure: BESS energy balance relationship
Battery energy storage system：Number of cycles
The number of charge-discharge cycles of a battery determines its lifespan. In an energy storage system, due to the high value of the battery, its lifespan also determines the lifespan of the entire system. The degradation of the cycle life leads to an increase in internal resistance, which in turn increases losses and heat generation, further accelerating the degradation process. Furthermore, frequent overcharging and over-discharging cause the repeated dissolution and deposition of metallic substances in the electrolyte, which also significantly impacts the battery's cycle life and safety.
For a certain type of lithium-ion battery, the number of cycles under 1C charging and 1C discharging conditions shows significant differences at different depths of discharge (DOD), as shown in the figure.
Battery energy storage system cost
The cost of energy storage systems is closely related to the system's capacity, power, and operating environment. Generally, in energy-type storage systems, the cost of batteries accounts for a relatively high proportion; while in power-type storage systems, the cost of batteries is relatively lower. However, in any case, the cost of the battery pack currently constitutes the main part of the overall BESS cost, and will also be the primary area for cost reduction in the future.
The unit of cost can be expressed as yuan/kWh or yuan/kW, but neither fully and accurately represents its meaning. Therefore, it is essential to specify both capacity and power simultaneously during discussions of specific projects.
Battery energy storage system：cost Response time
For BESS (Battery Energy Storage Systems), both power conversion and response time are in the millisecond range, which is sufficient for power system applications. This is where BESS excels compared to other physical energy storage methods such as flywheel energy storage and pumped-storage hydropower. However, due to limitations in voltage, installation methods, and battery cell capacity, the power and capacity of a single BESS unit are relatively limited. Therefore, in large-scale energy storage power plants, such as one composed of dozens of conventional low-voltage 5MW/2h energy storage systems connected in parallel, the bottleneck in response time will primarily be limited by the communication method and scheduling mechanism. It will also be affected by functions such as power coordination and circulating current suppression among the parallel devices. The final station-level response time may be in the hundreds of milliseconds or even seconds. Of course, a single 5MW/2h BESS unit is just a hypothetical example; the excessive parallel connection of batteries itself poses significant safety risks. Solving this problem requires changes in group control methods and breakthroughs and applications of new energy storage system technologies such as high-voltage direct connection.
Battery energy storage system：Other features
In other application scenarios or economic analyses, concepts such as specific energy (energy-to-mass ratio, Wh/kg), specific power (power-to-mass ratio, kW/kg), and energy density per unit area (energy-to-area ratio, Wh/m²) are also used. These concepts are relevant for calculating project transportation costs and land use requirements.