A Battery Energy Storage System (BESS) primarily consists of batteries (for energy storage), power conversion systems (PCS or DC/DC converters), a local controller, a power distribution unit, a prefabricated enclosure, and other auxiliary equipment such as temperature and fire protection systems. Under the unified management of the local controller, the BESS operates independently or receives instructions from an external energy management system (EMS) to perform energy scheduling and power control, ensuring safe and efficient operation. The BESS architecture is shown in the figure.
A battery is a device that stores energy using chemical reactions. It converts chemical energy into electrical energy through electrode oxidation-reduction reactions between active materials within the battery casing, and outputs electricity to an external circuit in the form of voltage/current. Compared to non-rechargeable primary batteries, secondary batteries used in energy storage can be repeatedly charged and discharged. These mainly include lead-acid batteries, lithium-ion batteries, vanadium batteries, and sodium-sulfur batteries. Because the charging and discharging process of a battery is essentially an electrochemical reaction, it is often accompanied by phenomena such as heat generation, crystallization, and gas evolution, which affect the battery pack's lifespan, efficiency, and safety. Furthermore, to increase the capacity and voltage level of energy storage systems, BESS batteries consist of several parallel or series-connected battery cells. Therefore, from a safety perspective, especially for lithium-ion batteries, which pose an explosion risk under severe overcharging or extreme high-temperature conditions, a Battery Management System (BMS) becomes a very important component of the BESS. The BMS effectively monitors, protects, balances energy, and provides fault alarms for the battery cells and battery clusters, improving the overall efficiency and lifespan of the energy storage battery.
A PCS (Power Conversion System) is the power conversion and electrical interface between the battery and the power grid or electrical load. Although the cost of PCS has been continuously decreasing with the development and application of power electronic devices, it largely determines the output power quality and characteristics of the entire energy storage system. In conjunction with the BMS (Battery Management System), it also affects the battery's lifespan and safety.
The local controller, through communication, sensor detection, and node monitoring, achieves awareness of the entire energy storage system's status, logical control, coordinated operation of main and auxiliary equipment, and fault handling, thereby improving the efficiency and availability of the BESS. The functions of the local controller are quite flexible and its scope may expand depending on the project. For example, in a simple and small-scale microgrid system, the local controller may also extend its control to photovoltaic equipment, diesel generator sets, and AC distribution switches. In a larger power plant containing multiple energy storage systems, multiple local controllers may work in a cascaded manner to complete more complex task assignments. The upper-level local controller handles the start-stop coordination and power distribution between energy storage systems, while the lower-level local controllers primarily handle the relevant control work within their respective energy storage systems.
Prefabricated modules, serving as the carrier and platform for energy storage systems, ensure the adaptability of the energy storage system to various complex environments, possessing functions such as waterproofing, thermal insulation, fire resistance, vibration resistance, and electromagnetic shielding. In terms of structural form, the choice often depends on the natural conditions and labor costs of the project location, with options including fixed buildings, containers, or outdoor cabinets. Fixed buildings have a longer construction period and higher costs; while containers and outdoor cabinets have certain advantages in terms of manufacturing and transportation costs. Therefore, in most current projects, small and medium-sized (below 1 MWh) energy storage systems, which require a more aesthetically pleasing appearance, often utilize outdoor cabinets;
While large systems, which require higher protection levels and structural strength, mostly use containers. Taking containers as an example, as shown in the figure, the container body needs to be designed with sufficient strength based on dynamic and static load stress analysis, and the container can be modified with additional reinforcing beams if necessary. At the same time, auxiliary components such as escape signs and escape locks need to be installed on the container in accordance with relevant standards.
Other auxiliary equipment includes battery busbar and protection cabinets (BCP), control and distribution cabinets, and local monitoring cabinets (optional). In some microgrid projects, customized equipment such as switchgear may also be installed at the AC interface and managed by a local controller, thus becoming part of the BESS.