What is String Energy Storage Solution?
Under the background of carbon neutrality, building a new power system dominated by new energy has become a global consensus, and energy storage, as its core link, has become the key to promoting the transformation of energy structure. Energy storage technologies of different routes affect the safety, efficiency, economy and other dimensions of energy storage power stations, and have gradually become one of the focuses of industry attention.
This article will focus on analyzing the two mainstream technical paths in the current energy storage field - centralized and string energy storage, and compare the two solutions from multiple dimensions:
System Structure
Centralized energy storage usually has a large capacity and volume per device, and is assembled in a containerized manner. Its system structure is that Pack batteries are connected in series to form a battery rack, and multiple battery racks are connected in parallel on the DC side, and then fed into an energy storage inverter to convert into AC power, which is then stepped up by a transformer and connected to the grid.
String energy storage generally adopts modular design, and the equipment uses intelligent string energy storage cabinets. The single cabinet equipment is small in size, and multiple energy storage cabinets form an energy storage subsystem, and multiple subsystems form a string energy storage system. Its system structure is that the Pack batteries are connected in series to form a battery rack. A single battery rack (that is, a battery in an energy storage cabinet) is connected to an energy storage converter (integrated in the energy storage cabinet) to convert it into AC power. After multiple energy storage cabinets are connected in parallel and AC is converged, they are connected to the transformer for boosting and connected to the grid.
Efficiency
When multiple racks are connected in parallel on the DC side of a centralized energy storage system, differences in internal resistance between racks will become increasingly apparent over long-term operation, leading to inter-rack circulation, heat generation, and accelerated battery aging. This is when the “barrel effect” becomes apparent. Inconsistency between racks causes the overall system performance to be constrained by the rack with the lowest performance. The capacity attenuation of a single rack affects the performance of the entire cabinet, and becomes increasingly apparent over time, reducing the cycle life of the system, greatly affecting the charge and discharge volume and revenue over the entire life cycle.
There is no parallel connection on the DC side of the string energy storage system, and the PCS manages each rack separately, so there is no problem of cell consistency mismatch caused by circulating current. In addition, the system has no "barrel effect" between racks, and the capacity decay of a single rack only affects the performance of a certain rack, which has little impact on the charging and discharging of the system. The system cycle life is longer, the charging and discharging volume over the entire life cycle is higher, and the rate of return is higher.
Safety
The single cell capacity of the centralized energy storage system is relatively large, generally 3MWh~5MWh, and is composed of 8~12 battery racks connected in series and parallel. Once a single rack has thermal runaway, it is very easy to spread to the entire cabinet, and the design of multiple battery racks sharing a fire protection system also has certain safety hazards. Once an accident occurs, it may cause a large-scale fire.
The string energy storage system adopts single-rack thermal management to ensure better temperature uniformity between each Pack, which can not only extend the service life of the battery, but also improve the safety of the system; the fire protection configuration uses a single rack as the minimum control and isolation unit. When an accident occurs, the fault area is quickly isolated and will not affect other energy storage cabinets, which minimizes the scope of the accident and ensures the safety of personnel and system.
Flexible
Centralized energy storage systems have the characteristics of large capacity and volume of a single device, high difficulty in transportation, and high requirements for the installation site, so their application scenarios are limited. Due to the multi-rack management design, it is difficult to mix old and new batteries, and capacity expansion or power replenishment must be done in units of cabins, which has poor flexibility.
The battery cabinet of the string energy storage system adopts a modular design, with a minimum power of 50kW and can be arbitrarily matched to form systems of various power and capacity; a single cabinet is small in size, easy to transport and install, and suitable for a variety of application scenarios such as industrial and commercial users, shared energy storage, new energy distribution storage, and photovoltaic storage and diesel microgrids; the system supports the mixed use of new and old batteries, and can flexibly expand or supplement power according to actual needs, greatly improving the flexibility and maintainability of the system.
Stable
In a centralized energy storage system, one container is maintained by a thermal management system, which has poor temperature uniformity and will have a certain impact on the stability of the system. In addition, when a single pack or rack has a problem, the entire cabinet must be shut down for inspection, which takes a lot of time and manpower to operate and maintain, and will also greatly affect the discharge benefits.
The string energy storage system adopts an efficient thermal management system with one cluster and one management, which has good temperature uniformity, long battery life, and relatively stable system operation. When a single point failure occurs, it will only affect the operation of the faulty cabinet, and other fault-free cabinets can operate normally, which has little impact on the operation of the entire system.
Operation and maintenance
The battery compartment of the centralized energy storage system is large, the on-site debugging of the current compartment is complex, and the debugging cycle is long; the internal space of the container is crowded, lacks operation and maintenance channels, and maintenance is difficult; when the system fails, the manufacturer is generally required to rush to the site for maintenance, the system downtime is long, and the operation and maintenance cost is high.
The string energy storage system manages one cluster at a time, and the single rack is operated and maintained as a whole; when the system fails, it can be accurately located to a single rack, without affecting the operation of other cabinets; once the device in the system, such as PCS, fails, the operation and maintenance personnel can replace the standby machine in time, improve the operation and maintenance efficiency, and save the operation and maintenance cost.