Energy storage has been a hot topic and track in the field of new energy in the past two years. Due to energy shortages, electricity prices, policy trends, and the international situation, the demand for household energy storage is mainly concentrated in overseas markets, especially in Europe, the United States, Japan, and Australia. The demand for energy storage is growing rapidly, and the world is ushering in a new stage of household energy storage explosion, and the penetration rate has room to increase tenfold. However, the development of home energy storage also faces some difficulties and challenges.
1. Energy storage system integration: complex installation
Household energy storage is a complex system that combines multiple energy sources and is oriented to ordinary households, which puts forward higher requirements for system installation. At present, the complex and time-consuming installation of household energy storage on the market has become the biggest problem for some users. At present, there are mainly two types of household energy storage systems on the market: low-voltage energy storage and high-voltage energy storage.
Home low voltage energy storage system
The home low voltage energy storage system refers to an energy storage system with a battery voltage range of 40-60V, which consists of several batteries connected in parallel and then connected to the inverter. Through the isolated DC-DC inside the inverter, it is coupled with the DC output of the photovoltaic MPPT at the bus bar. Finally, the inverter output is converted into AC power and connected to the grid, and some inverters have a backup output function.
The main problems of the low voltage energy storage system for household use are: the inverter and the battery are separated independently, the equipment is heavy, and the installation is difficult; the connecting wires of the inverter and the battery cannot be standardized and require on-site processing. As a result, the installation of the entire system takes a long time and increases costs.
Household high voltage energy storage system
The battery cluster of the household high-voltage energy storage system adopts a two-stage structure. Several battery modules are connected in series and output through the high-voltage control box. The voltage range is generally 85-600V. The output of the battery cluster is connected to the inverter, through the internal DC-DC unit of the inverter, and coupled with the DC output of the photovoltaic MPPT at the bus bar. Finally, the energy storage inverter output is converted into AC power and connected to the grid, and some inverters have a backup output function.
In order to avoid using different batches of battery modules directly in series, it is necessary to do a good job in strict batch management in all aspects of production, shipment, warehouse, and installation. This requires a lot of manpower and material resources, and the process will be very cumbersome and complicated, and it will also bring troubles to customers in stocking. In addition, the self-consumption and capacity attenuation of the battery will cause the difference between the modules to be enlarged, and the general system needs to be checked before installation. If the differences between the modules are large, manual power replenishment is required, which is time-consuming and labor-intensive.
2. Battery capacity mismatch: capacity loss due to module differences
Parallel mismatch of household low voltage energy storage system
Generally, the voltage range of the traditional household low-voltage energy storage system is 40-60V, and the expansion is realized by connecting multiple battery packs in parallel. Due to differences in batteries, modules, and wiring harnesses, the charging/discharging current of batteries with high internal resistance is small, and the charging/discharging current of batteries with low internal resistance is large. Some batteries cannot be fully charged/discharged for a long time, resulting in partial capacity loss of the battery system.
Series mismatch of household high voltage energy storage system
The general voltage range of the household high-voltage energy storage system is 85-600V, and the expansion is realized by connecting multiple battery modules in series. According to the characteristics of the series circuit, the charging and discharging current of each module is the same, but due to the difference in module capacity, the battery with a small capacity is charged/emptied first, resulting in some battery modules being unable to be fully charged/emptied for a long time, and part of the capacity of the battery cluster is lost.
3. Product operation and maintenance: technical and cost thresholds are too high
In order to ensure the reliable and safe operation of household energy storage systems, good operation and maintenance is one of the effective measures. However, due to the relatively complex structure of the household high-voltage energy storage system, which requires a high professional level of operation and maintenance personnel, in the actual use process, maintenance difficulties, time-consuming and labor-intensive problems often occur. The main reasons are as follows:
- For regular maintenance, it is necessary to perform SOC calibration, capacity calibration or main circuit inspection for the battery pack.
- When the battery module is abnormal, the conventional lithium battery does not have the automatic equalization function, and maintenance personnel need to go to the site to manually recharge the power, which cannot quickly respond to customer needs.
4. Mixing old and new batteries: accelerating the aging of new batteries and capacity mismatch
For household low-voltage energy storage systems where new and old lithium batteries are mixed, the internal resistance of the batteries varies greatly, which is likely to cause circulation, and the temperature of the battery will increase, which will accelerate the aging of the new battery. For household high-voltage energy storage systems, old and new battery modules are mixed in series.
Due to the barrel effect, the new battery module can only be used with the capacity of the old battery module, and the battery cluster will have a serious capacity mismatch. For example, the usable capacity of the new module is 100Ah, and the usable capacity of the old module is 90Ah. If they are mixed, the usable capacity of the battery cluster is only 90Ah. To sum up, it is generally not recommended to use old and new lithium batteries directly in series or in parallel.