1. Why does the power system need peak shaving?
The power system is mainly composed of power generation side and power consumption side. China’s power generation side includes hydropower, thermal power, nuclear power generation, solar energy, wind power and other new energy power generation, and the power consumption side mainly includes factories, enterprises, shopping malls, families, etc. There are also some facilities that can both use electricity and generate electricity. This is the energy storage power station. The power generation side and the power consumption side are not necessarily balanced. The load of factories and enterprises is usually more power consumption in the daytime and less power consumption in the evening. The household load is generally less power consumption in the daytime and more power consumption in the evening. However, in general, the power consumption peak in the daytime and the power consumption trough in the evening. However, hydropower, thermal power and nuclear power generation are generally large power generation units. Once the equipment is started, it cannot be stopped casually. Solar energy, wind power and other new energy sources generate electricity according to the environment and climate. Power generation is unstable and changes at any time. Therefore, it is necessary to increase the output of generator at peak load; When the load is low, reduce the generator output and even shut down some units. Some generators in the power system are specially used for peak shaving, called peak-shaving units.
2. What are the peak shaving modes of the power system?
According to the requirements of the power system, the peak-shaving setting should be able to absorb the surplus electric energy of the grid when the load is low, and increase the electric energy supply when the load is high. The facilities should have the characteristics of flexibility and fast start. At present, the power sources available for peak-shaving of the power system are:
(1) Peak regulation of pumped storage units. The pumped-storage power station has two reservoirs with a certain height difference. When the power load is low, the water is pumped to the upstream reservoir, and when the power load is high, the water is discharged to the downstream reservoir for power generation. It is also called energy storage hydropower station. It can transform the surplus electric energy when the grid load is low into the high-value electric energy during the peak period of the grid. The advantages of pumped storage are that the technology is mature and reliable, the capacity is large, the peak can be eliminated and the valley can be filled, the equipment investment is small, and the efficiency is usually 70% – 85%. The disadvantages are that the location is relatively difficult and the floor area is large.
(2) Peak regulation of generator set. Including coal-fired thermal power units and gas turbine units, the load characteristics of the units can be adjusted to increase the output power at peak load and reduce the output power at low load. The advantages of generating unit peak shaving are small floor area, low initial investment and high efficiency. The disadvantages are that the thermal power plant is slow in response, the time from boiler start-up to turbine grid-connected power generation is long, and the grid power cannot be absorbed when the load is low.
(3) Peak regulation of energy storage power station. Power generation enterprises, power sales enterprises, power users, and power storage enterprises invest in the construction of power storage facilities, which can be built on the power generation side, or participate in auxiliary service market transactions as an independent entity; Or the electric energy storage facilities built on the user side can be regarded as distributed power supply sold to the power users nearby, as an independent market subject, for deep peak shaving. The energy storage power station has small area for peak regulation, obvious effect of peak elimination and valley filling, and fast response time. Its disadvantages are large initial investment and short battery life.
3. How do energy storage power stations participate in peak shaving?
The state encourages the allocation of electric energy storage facilities in centralized new energy power generation bases to participate in peak shaving auxiliary services. Electric energy storage facilities above 10MW can be subject to unified dispatching by power dispatching agencies. Energy storage facilities (power storage, electric heating and energy storage) built in power plants can participate in peak shaving jointly with power plants or independently. Among them, for the electric energy storage facilities built in the wind power station, after giving priority to the use of the wind power station, the rich capacity can participate in the auxiliary service market, and the user-side energy storage facilities (power storage, electric heating and energy storage) can only participate in deep peak shaving and start-stop peak shaving. For the energy storage facilities built in the power plant, the discharged electricity is settled according to the relevant contract electricity price of the power plant. The energy storage facilities on the user side can purchase electricity by themselves according to the market rules. When discharging, the electricity can be sold to the power users nearby to gain profits. The electric energy storage devices charged and discharged for more than 4 hours participate in the start-stop peak shaving of the power generation side, which is regarded as a thermal power unit with the lowest stable combustion power equivalent to the start-stop peak shaving.
4. Calculation of investment income of energy storage power stations participating in peak shaving
Whether the energy storage can ultimately be popularized and applied in the peak shaving auxiliary service market depends on its economy. The investment income of the energy storage power station comes from two sources, one is the income from the difference between peak and valley electricity prices, and the other is the income from peak shaving compensation. Take the Southern Power Grid as an example to calculate the investment income of an energy storage power station.
In January 2018, the Southern Regulatory Authority issued the Detailed Rules for the Implementation of Grid-connected Operation Management and Auxiliary Service Management of the Southern Regional Electrochemical Energy Storage Power Station (Trial), which is applicable to the energy storage power stations with a capacity of 2MW/0.5 hours or more that are directly dispatched by the southern regional municipal and above power dispatching agencies and have signed a grid-connected dispatching agreement with the power dispatching agencies. If the energy storage station enters the charging state according to the instructions of the power dispatching authority, it will compensate the charging power according to the statistics of the charging peak shaving service provided by it, and the specific compensation standard is 50000 yuan/MWh.
The peak and valley sections of Guangzhou Power Grid are divided as follows:
It is assumed that the energy storage system will be charged in the valley or flat section, and all the electricity will be discharged in the peak section, and the electricity sales income will be obtained in the peak discharge. The charging TV in the valley section and flat section will participate in the peak shaving of the auxiliary service market and obtain the peak shaving income. Then a set of energy storage system can be fully discharged twice a day under the above time division.
If a set of 20MW/5h energy storage system is arranged and the on-grid electricity price of wind power is assumed to be used for accounting when discharging, the total income of its participation in peak shaving is calculated as follows.
1) Daily peak shaving income
The power can be reduced by 40 MWh per day, and the daily compensation cost is calculated according to the specific compensation standard of 50000 yuan/MWh:
Deep peak shaving cost=40MWh * 500 yuan/MWh=20000 yuan
2) Daily electricity sales income
The peak discharge of energy storage and the charging in the flat valley are calculated according to the comprehensive price difference of 0.6 yuan/kWh. Assuming that the stored electricity can be sold at peak periods, and the charging and discharging efficiency is 80%, the daily electricity sales income is:
Revenue from electricity sales=40MWh * 1000 * 0.6 yuan/kWh * 0.8=19200 yuan
3) Annual income
In consideration of holidays, the whole year is calculated as 300 days, and the annual income is:
Annual income=(20000+19200) * 300=11.76 million yuan
4) Investment payback period
In the second half of 2018, the price of energy storage battery dropped significantly, and the cost of energy storage system decreased from 3000 yuan/kWh to about 2000 yuan/kWh. Considering other construction, labor and operation and maintenance costs, the total cost of energy storage system for 20MW/5h was 48 million yuan (2400 yuan/kWh × 2000kWh)。 The payback period of the whole system is 4.08 years (48 million yuan/11.76 million yuan).
Two cycles per day, 2448 cycles in total in 4.08 years (2 times × 300 days × 4.08 years), the cycle life of lithium ion battery, sodium sulfur battery, liquid flow battery and lead carbon battery can basically meet this requirement.
