From the perspective of the entire power system, the application of energy storage can be divided into three major scenarios: generation-side energy storage, transmission and distribution-side energy storage and user-side energy storage. These three scenarios can be divided into energy-based demand and power-based demand from the perspective of the power grid. Energy-based demand generally requires a long discharge time (e.g., energy time-shift), and does not require a high response time. In contrast, power-based demands generally require a fast response capability, but generally do not have a long discharge time (e.g., system FM).
In practice, energy storage technologies need to be analyzed according to the demands in various scenarios in order to find the most suitable energy storage technology. This paper focuses on analyzing the three major application scenarios of energy storage.
Power Generation Side
From the perspective of the generation side, the demand endpoint for energy storage is the power plant. Due to the different impacts of different power sources on the grid, as well as the dynamic mismatch between power generation and power consumption caused by the unpredictability of the load side, the power generation side has more types of demand scenarios for energy storage, including six types of scenarios, such as energy time shift, capacity unit, load tracking, system frequency regulation, standby capacity, and grid integration of renewable energy.
Energy Time Shift
Energy time-shift is to realize peak and valley load shaving by means of energy storage, i.e., the power plant charges the battery during the low load hours and releases the stored power during the peak load hours. In addition, storing wind and solar energy from renewable energy sources and then moving them to other times for grid connection is also energy time-shifting.
Energy time-shifting is a typical energy-based application, which does not have strict requirements on charging and discharging time, and the power requirements for charging and discharging are relatively wide, but because of the user's electrical load and the characteristics of renewable energy generation, the frequency of the application of energy time-shifting is relatively high, with more than 300 times per year.
Capacity units
Due to the difference in electricity consumption load at different time periods, coal power units need to take on the peaking capacity, so they need to set aside a certain amount of generating capacity as the capacity of the corresponding peak load, which makes it impossible for thermal power units to reach the full generating state and affects the economy of unit operation. The use of energy storage can be charged at the time of low electricity loads and discharged at the time of electricity peaks to reduce load spikes.
The substitution effect of the energy storage system is utilized to release the capacity units of coal power, thus improving the utilization rate of thermal power units and increasing their economy. Capacity units belong to typical energy-based applications, which do not have strict requirements on the time of charging and discharging, and the power requirements for charging and discharging are relatively wide, but because of the user's electricity load and the characteristics of renewable energy generation lead to a relatively high frequency of the application of capacity time-shift, which is around 200 times per year.
Load Tracking
Load tracking is an auxiliary service for slow-varying continuously changing loads, which is dynamically adjusted to achieve real-time balance. Slowly changing continuous load can be subdivided into basic load and climbing load according to the actual situation of generator operation, and load tracking is mainly applied to climbing load, i.e., by adjusting the size of output, the climbing rate of traditional energy units can be minimized, so that they can smoothly transition to the level of scheduling instructions as far as possible. Compared with capacity units, load tracking has higher requirements for discharge response time, requiring the corresponding time to be at the minute level.
System Frequency Regulation
Frequency regulation is critical as changes in frequency affect the safe and efficient operation and life of power generation and power consuming equipment. In the traditional energy structure, the short-time energy imbalance of the grid is regulated by the traditional units (mainly thermal power and hydropower in China) by responding to the AGC signal. However, with the integration of new energy sources into the grid, the volatility and stochasticity of wind and light make the energy imbalance of the grid intensify in a short time,
and the traditional energy sources (especially thermal power) cannot meet the new demand due to the slow speed of FM, the lag in responding to the grid scheduling instructions, and the wrong actions such as reverse regulation sometimes occur. In contrast, energy storage (especially electrochemical energy storage) frequency regulation speed, the battery can be flexible in the charging and discharging state between the conversion, become a very good frequency regulation resources.
Standby Capacity
Standby capacity refers to the active power reserve reserved for guaranteeing power quality and safe and stable operation of the system in case of emergencies in addition to meeting the expected load demand. Generally, the standby capacity needs to be 15-20% of the normal power supply capacity of the system, and the minimum value should be equal to the capacity of the unit with the largest single-machine installed capacity in the system.
Since the standby capacity is for unexpected situations, the general annual operating frequency is low, if the battery is used to do standby capacity service alone, the economy can not be guaranteed, so it is necessary to compare it with the cost of the existing standby capacity to determine the actual substitution effect.
Renewable Energy Grid Connection
Due to the wind power, photovoltaic power generation output randomness, intermittent characteristics, its power quality compared to traditional energy sources is poor, due to renewable energy power generation fluctuations (frequency fluctuations, output fluctuations, etc.) from a few seconds to a few hours between the power-type applications as well as energy-type applications, which can be generally categorized into three types of applications: renewable energy energy time-shift, renewable energy power generation capacity curing, and renewable energy output smoothing
The application can be generally categorized into three types of applications: renewable energy time shift, renewable energy capacity curing and renewable energy output smoothing. For example, for the problem of photovoltaic power generation abandoned light, it is necessary to store the remaining power generated during the day for discharge at night, which belongs to the energy time shift of renewable energy. As for wind power, the unpredictability of wind power leads to large fluctuations in wind power output, which needs to be smoothed, and thus is dominated by power-based applications.
Transmission and Distribution Side
The application of energy storage in the transmission and distribution side is mainly to alleviate transmission and distribution blockage, delay the expansion of transmission and distribution equipment and reactive power support three categories, compared with the application of the generation side, the transmission and distribution side of the application type is less, at the same time from the point of view of the effect is more of a substitution effect.
Alleviation of transmission and distribution blockage
Line obstruction refers to the line load exceeding the line capacity, the energy storage system is installed in the upstream of the line, when the line obstruction occurs, it can store the power that can not be delivered into the energy storage equipment, and when the line load is less than the line capacity, the energy storage system will discharge to the line. Generally for the energy storage system requirements discharge time in the hour level, the number of times in the operation of 50 ~ 100 times or so, belong to the energy-based applications, the response time has certain requirements, need to be in the minute level response.
Deferring the expansion of transmission and distribution equipment
Traditional grid planning or grid upgrades and expansions are expensive. In a transmission and distribution system where the load is close to the capacity of the equipment, if the load supply can be met most of the time in a year, and only at certain times of the year when its own capacity will be lower than the load in some of the peak hours, the energy storage system can be utilized to effectively improve the transmission and distribution capacity of the grid through a smaller installed capacity, thus deferring the cost of new transmission and distribution facilities and extending the service life of the original equipment.
Compared with relieving transmission and distribution blockage, deferring the expansion of transmission and distribution equipment works less frequently, and considering the aging of the battery, the actual variable cost is higher, so it puts forward higher requirements for the economy of the battery.
Reactive power support
Reactive power support is the regulation of transmission voltage by injecting or absorbing reactive power on transmission and distribution lines. Insufficient or excess reactive power can cause voltage fluctuations in the grid, affecting power quality and even depleting power-using equipment. Batteries can regulate the voltage of transmission and distribution lines by adjusting the magnitude of reactive power output from them with the assistance of dynamic inverters, communication and control equipment. Reactive power support is a typical power-based application with a relatively short discharge time but a high frequency of operation.
Electricity Consumption Side
The electricity consumption side is the end point of electricity use, and the user is the consumer and user of electricity. The costs and revenues of the power generation, transmission and distribution side are expressed in the form of tariffs, which are transformed into the costs of the user, and therefore the level of tariffs affects the user's demand.
Time-of-Use Tariff Management
The power sector divides 24 hours a day into peak, flat and low periods, and sets different tariffs for each of these periods, i.e. time-sharing tariffs. Time-of-consumption tariff management is similar to energy time-shifting, except that time-of-consumption tariff management is based on the time-of-consumption tariff system to regulate power loads, whereas energy time-shifting is based on the power load curve to regulate power generation.
Capacity tariff management
China has implemented a two-part tariff system for large industrial enterprises in the power supply sector: the volume tariff refers to the tariff billed according to the actual amount of electricity generated in transactions, while the capacity tariff depends mainly on the maximum value of the power used by the customer. Capacity charge management refers to the reduction of capacity charges by lowering the maximum power consumption without affecting normal production. Users can utilize the energy storage system to store energy in the low valley of electricity consumption and discharge negative loads in the high peak, thus reducing the overall load and achieving the purpose of reducing the capacity charge.
Improvement of power quality
Due to the existence of power system operation load nature of variable, equipment load nonlinearity and other problems, the user to obtain the power of voltage, current changes or frequency deviation and other problems, when the quality of power is poor. System frequency regulation and reactive power support are ways to improve power quality on the generation side and transmission and distribution side. On the user side, energy storage systems can also be used to smooth out voltage and frequency fluctuations,
for example, by using energy storage to solve problems such as voltage rise, dips, and flicker in distributed photovoltaic systems. Enhancing power quality is a typical power-based application, the specific discharge market and operating frequency vary according to the actual application scenarios, but generally require a response time of milliseconds.
Improve power supply reliability
Energy storage is used to improve the reliability of microgrid power supply, which means that in the event of a power failure, energy storage can supply the reserve energy to the end user, avoiding the interruption of power during the fault repair process, in order to ensure the reliability of power supply. The energy storage equipment in this application must have the requirements of high quality and high reliability, and the specific discharge length is mainly related to the installation location.
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