Structurally, energy storage products can be categorized into containers or prefabricated cabins, and can be either outdoor or indoor cabinet types. Based on cooling methods, they can be either air-cooled or liquid-cooled. According to electrical structure, they can be either centralized or string-type. Based on the energy storage system, they can be either separate equipment and battery or integrated equipment and battery. Based on voltage levels, they can be either 1000V or 1500V systems. Based on the energy collection point, they can be either DC-coupled or AC-coupled.
Large/medium-sized energy storage products are currently mainly in the form of containers or prefabricated cabins. They are generally used on the power supply side and grid side, with a small number used on the user side. The cooling method is gradually transitioning from air cooling to liquid cooling. The electrical structure is mainly centralized, and string energy storage is gradually being added. The voltage is gradually becoming 1500V, and AC coupling is the main method.
The revenue model for commercial and industrial user-side energy storage is peak-valley arbitrage, involving full charging and discharging; the greater the charging and discharging volume, the greater the revenue; the higher the energy conversion efficiency and the lower the loss, the greater the revenue. Therefore, string-cluster controlled energy storage systems offer higher returns than centralized energy storage systems. String-cluster controlled energy storage systems have high energy utilization efficiency per cluster, eliminating the bottleneck effect; there is no parallel connection between clusters, no circulating current, and high energy conversion efficiency.
Distributed small-scale energy storage cabinets have higher maintenance and after-sales costs due to their dispersed layout; the small purchase volume also leads to higher unit prices for related components and the overall system. Currently, the estimated market price for small outdoor energy storage cabinets is approximately RMB 1.6/Wh. Centralized energy storage systems are priced at RMB 1.1-1.2/Wh (for purchases of tens of MWh).
All-in-one AC/DC integrated energy storage container system. Each battery cluster connects to a single PCS (Power Control System) for cluster-based management. High battery capacity utilization, no inter-cluster parallel circulating current, single-stage DC/AC converter unit, and high energy conversion efficiency. The string module does not contain internal DC switches, fuses, or AC circuit breakers. These are integrated externally, allowing for flexible configuration of these protection devices according to technical requirements. The PCS AC side is equipped with branch circuit breakers (optional) and a main circuit breaker (mandatory). Compared to centralized energy storage systems, the DC side eliminates the need for a DC combiner cabinet, DC-side switches and fuses on the PCS side, and a third-level BMS. The price difference between the two integration methods is narrowing. However, the returns of string energy storage systems are significantly higher than those of centralized systems. The effective capacity utilization (DOD) of centralized energy storage systems is 7.5% lower than that of string energy storage systems. The estimated cycle life is also 10% lower.
DC coupling
As shown in the diagram below, the direct current (DC) generated by the photovoltaic modules is stored in the battery bank through a controller. The power grid can also charge the battery bank through a bidirectional DC-AC converter. The energy collection point is at the DC battery terminal.
The working principle of DC-DC coupling: When the photovoltaic system is running, the MPPT controller charges the battery; when the electrical load demands power, the battery releases electricity, with the current determined by the load. The energy storage system is connected to the grid. If the load is small and the battery is fully charged, the photovoltaic system can supply power to the grid. When the load power exceeds the photovoltaic power generation capacity, both the grid and the photovoltaic system can supply power to the load simultaneously. Because neither photovoltaic power generation nor load power consumption is stable, the battery is needed to balance the system's energy.
AC coupling
The direct current (DC) generated by photovoltaic (PV) modules is converted into alternating current (AC) by an inverter, which can then be used to power loads or fed into the power grid. The power grid can also charge batteries via a bidirectional DC-AC converter. The energy collection point is at the AC end.
The working principle of AC coupling includes a photovoltaic power supply system and a battery power supply system. The photovoltaic system consists of a photovoltaic array and a grid-connected inverter; the battery system consists of a battery bank and a bidirectional inverter. These two systems can operate independently without interfering with each other, or they can be disconnected from the main power grid to form a microgrid system.
Based on current installed cases, modular, string, and AC-coupled solutions for user-side energy storage have become a trend, accounting for more than 80% of the market share. This solution is low-cost, flexible in configuration, and highly secure, making it suitable for industrial and commercial off-grid energy storage power stations. On the other hand, the DC-coupled centralized solution has simple wiring and stable system, making it suitable for small and medium-sized independent power stations.
