By the end of 2022, the cumulative installed capacity of power storage projects put into operation in China reached 59.4GW, an increase of 37% year-on-year. Among them, pumped storage accounts for the largest proportion, with a cumulative installed capacity of 46.1GW.
Hybrid energy storage system continued to maintain high growth, with cumulative installed capacity exceeding 10GW for the first time, more than doubling the same period in 2021, reaching 12.7GW.
1. Grid-side energy storage requirements
Among existing energy storage technologies, pumped storage and compressed air energy storage can realize high-power, large-capacity electric energy storage. However, the requirements for site selection are high, and the electrochemical energy storage technology that requires flexible site construction is generated in response to market demand.
At present, the top 10 energy storage lithium battery companies have different technical performances. And even the same energy storing technology has certain differences in different application fields.
In actual engineering, it is necessary to comprehensively compare and select appropriate energy storage technologies according to the requirements of different application scenarios. Large-scale energy storage on the grid side has its special requirements for energy storage systems:
● Long cycle life
The cycle life of energy storage batteries has an important impact on the reliable operation of the system. The long cycle life of energy storage batteries can reduce the replacement of batteries, which is of great significance for saving investment and construction costs and improving system economy.
● Fast response
The intermittent operation of the battery energy storage system requires the system to have good starting characteristics. Energy storage batteries switch frequently between charge and discharge states, which requires the battery to have a fast response speed.
● High charge and discharge efficiency
Improving the energy conversion rate of the energy storage system during storage will help improve the overall efficiency of the system, which in turn will affect the cost per watt-hour of the system.
● Good security
Due to the particularity of the power grid, operational safety is the primary issue that needs to be considered in the operation of the power grid. Devices that are not highly secure cannot enter the grid.
● Moderate cost
Cost is an important reference for the selection of batteries to store energy, and it is the determining factor for whether energy storage systems can be promoted and applied on a large scale. At this stage, there are three mainstream lithium battery energy storage technologies: lithium iron phosphate batteries, ternary lithium batteries and lithium titanate batteries.
Comprehensively weighing the service life, engineering performance, technological advancement and maturity, environmental protection and environmental friendliness, safety and other indicators of batteries for energy storing, lithium battery is recommended.
Supercapacitor is a relatively mature electromagnetic energy storage scheme, which has the advantages of high power, high cycle times, and high charge and discharge efficiency. The initial investment of the supercapacitor energy storage system is large, but the storage cost of kWh is low, only 0.1-0.2RMB/kWh.
The hybrid energy storage technology combining lithium batteries and supercapacitors can take into account the characteristics of the two, learn from each other's strengths, and provide a new idea for smoothing wind power fluctuations.
2. Advantages of hybrid energy storage system
The lithium battery-supercapacitor hybrid energy storage system has a high energy density and a long working time, which can well undertake the flattening work of the main low-frequency components in wind power fluctuations. The supercapacitor has high power density, fast charge and discharge response speed, and long cycle life.
The hybrid energy storage system can assist battery energy storage to smooth high-frequency components in wind power fluctuations, extend the service life of the battery, and give full play to the technical and economic advantages of both technologies of storing energy.
When dealing with fluctuating power, hybrid energy storage can provide higher peak power than a single battery. Better flattening is achieved, while the introduction of supercapacitors can reduce battery operating current, reduce battery internal resistance loss, and extend battery life. And the comprehensive electricity storage cost of hybrid energy storage system is significantly reduced.
3. Implementation scheme of hybrid energy storage system
The hybrid energy storage system structure is in the common AC bus, and the battery and supercapacitor are connected to the DC bus of the energy storage system through independent DC/DC converters. The DC bus is connected to the AC bus by an inverter and transformer.
The new energy power generation equipment is connected to the AC bus through inverters and transformers. This connection method can give full play to the advantages of large-scale energy storage and stabilize the grid-connected power of wind power plants as a whole.
The internal structure monitoring and management terminal of the hybrid energy storage system is connected to the booster station of the new energy power plant. It is implemented to accept power regulation instructions and realize information exchange. The charge and discharge system is connected to the 35kV AC bus via a circuit breaker and transformer.
Power distribution converters distribute power between two media. The power charge and discharge controller matches the voltage to control the charging and discharging status of the energy storage medium. The supercapacitor and lithium battery management system is responsible for equalizing the voltage difference between energy storage cells and improving the life of energy storing medium.
4. Economic analysis of hybrid energy storage system
In order to prove the economic feasibility of the hybrid energy storage system, data experiments were carried out using the second-level power generation data of a 2MW wind turbine to verify the suppression effect and calculate the cost of energy storing. Among them, the lithium battery capacity is 800 KWh.
With the increase of supercapacitor capacity, the processing power of supercapacitors increases, the processing power of lithium batteries decreases, the total number of seven-day cycles of lithium batteries decreases, the average storage cost decreases, and the initial investment of the system increases. It is more suitable to configure a 5 KWh supercapacitor for a 2MW wind turbine, and the cost of electricity consumption can be reduced by 1/3.
In line with the expected effect of the energy storage system. For the output power fluctuation suppression function of new energy, when the capacity ratio of lithium battery to supercapacitor is 160:1 (lithium battery 800 KWh and super capacitor 5 KWh), the relatively optimal solution of kWh storage cost and initial investment cost is achieved. When the supercapacitor reaches the end of its life, the lithium battery consumes 2.4 groups.
According to the energy ratio, if there is no supercapacitor, the lithium battery should consume 3.6 groups. Calculated according to the cost of lithium battery kWh 2000RMB and super capacitor kWh cost 100000RMB. The initial investment costs are RMB 4.34 million and RMB 5.76 million respectively. That is, the storage cost of electricity after adding supercapacitors is reduced by 24.6%, and the life of lithium batteries is extended by 1/6.
Calculating the initial investment cost according to the life cycle of a single lithium battery (at this time, the supercapacitor can still be used), the initial investment cost increases by 31.3%. If the capacity of the supercapacitor is sufficient to participate in frequency regulation and peak shaving and valley filling applications alone, the cost per cycle of electricity should be close to 0.1RMB, which has a large profit margin.
5. ConclusionCompared to single batteries, lithium-supercapacitor hybrid energy storage systems can better stabilize high-frequency components in wind power and achieve better leveling effects. Through a reasonable energy ratio, hybrid energy storage system can reduce the cost of system electricity storage, extend the life of lithium battery energy storage, and give full play to the technical and economic advantages of the two technologies of storing energy.