During peak load regulation of the power grid, it is necessary to shut down some thermal power units or reduce the output of thermal power units in the low peak period, resulting in the increase of unit coal consumption and unit start-up and shutdown costs. However, in the peak period, it is necessary to start fuel oil and gas units with high power generation costs, resulting in high marginal cost of power generation in the peak load period. In order to ensure the power balance at low load, most large thermal power units have to reduce to the minimum output, and small units need to be opened and stopped day and night, “two shift system” operation, which is very unfavorable to the safety and economy of unit operation. Taking 300MW in Shanghai as an example, when the unit output is 300MW, the coal consumption for power generation can be controlled at about 320G / (kW · h), while when the output is reduced to 120mW, the coal consumption rate is as high as about 400g / (kW · h), and the economy is greatly reduced. After the large-scale application of the energy storage system, the energy storage device can be started for energy storage under low load, and the unit can operate in a more economical output range, so as to obtain higher economic benefits. Once the energy storage system has formed a scale and improved the unit efficiency at low load, even considering the storage efficiency of the energy storage system, each ton of standard coal can generate 50~150kw · h more power. At the same time, the large-scale application of the energy storage system can not only improve the economic benefits of the power plant, but also promote its efficiency and emission reduction under the same power generation capacity, which is in line with the national energy policy and has obvious social effects.
Under the current on grid price mechanism in China, the on grid price does not set the peak valley price, but the sales price adopts the peak valley price mechanism, which reflects the difference of power generation cost to a great extent. In the case that the sales price adopts the peak and valley price and the on grid price is fixed, the energy storage devices installed by power grid enterprises charge when the load is low and the price is low, and discharge when the load is high and the price is high. In the process of buying at a low price and selling at a high price, they realize their explicit economic benefits, but the main body of the benefits is the whole society. When users install energy storage devices, they can reduce their own power purchase cost and capacity electricity price. The main body of this part of income is users. This report only analyzes the value analysis when the energy storage capacity accounts for a small proportion of the power grid capacity, and does not consider the impact of energy storage device peak shaving and valley filling on the peak valley electricity price difference.
- Reduce the conventional reserve capacity required for new energy power generation
Solar energy and geothermal power generation are the two main energy sources that can be used to generate electricity along with the tide. However, they are the two main challenges for the development and control of solar energy and wind power generation, which are the largest energy sources, Therefore, the system needs to be equipped with corresponding standby capacity to deal with its fluctuation under the condition of predicting its power output as accurately as possible.
Under the development trend of energy conservation and emission reduction, new energy power generation (mainly wind power and solar energy) has developed well in developed countries, and the development in Shanghai is also relatively rapid. As a coastal area, Shanghai’s main new energy power generation plan is wind power. Some research results point out that with the increase of the proportion of wind power capacity, the system needs a reserve capacity equivalent to the rated capacity of the wind farm; Moreover, the low utilization hours of wind power and its reverse peak shaving characteristics will reduce the peak shaving capacity of the power grid; When the proportion of wind power installed capacity is larger, the uncertainty of wind power generation power increases, the prediction error increases, and the required reserve capacity also increases.
As the randomness of new energy power generation will have an impact on the power grid, the power grid needs to be equipped with more standby capacity to deal with the fluctuation of new energy power generation. The battery energy storage device can quickly adjust its consumed / generated power and better replace the conventional power supply as the standby capacity of new energy power generation. Because the energy storage device is charged at the trough and discharged at the peak, the capacity that can be used for standby at a certain time changes, so we need to use the expected value of the remaining capacity of the energy storage device to evaluate the benefit of the energy storage device as the standby capacity. The energy storage device starts charging when the power is 0 at the beginning of the valley load period, and its power increases continuously. At the end of the valley load period, its power increases to the rated capacity, and starts discharging to 0 at the peak of the power grid. For the convenience of calculation, we idealize the energy storage device as uniform charge and discharge. Therefore, during the charge and discharge period, the probability distribution of the remaining electricity between 0 ~ PMT is uniform, and the expected value of the stored electricity is 0.5p T. The power that can be absorbed by the device or the power that can be generated by the system is the power that can be adjusted (p0.5t).
- Reduce the cost of power grid reliability
Urban power supply reliability refers to the ability of urban power supply system to continuously supply power to users. The interruption of urban power supply will not only cause huge economic losses, but also affect people’s life and social stability. With the rapid development of economy and the continuous improvement of people’s living standards, users have higher and higher requirements for the power supply reliability of urban distribution system. High power supply reliability of urban distribution system is not only the need of power users, but also the goal of the development of power supply enterprises. The development process of power supply reliability can be divided into three stages.
1) Low reliability level stage: the low reliability level stage is the primary stage of power supply reliability development. At this stage, the power supply reliability rate is generally below 99%, the corresponding average outage time of users is generally more than 87.6h, and the annual power supply reliability index fluctuates greatly.
2) Rapid development stage: the level of power supply reliability increases rapidly, the power supply reliability rate is generally more than 99%, and the corresponding outage time is generally less than 87.6h. The overall development trend of power supply reliability index is a spiral rise. The annual power supply reliability index fluctuates to a certain extent, but its fluctuation range is smaller than that in the first stage.
3) High reliability level stage: the power supply reliability level has increased to a high level, the power supply reliability index is generally more than 99.99%, and the corresponding average power failure time of users is generally less than 0.876h (about 53min). The annual power supply reliability index is relatively stable, with only small fluctuation, and its fluctuation range is smaller than that in stage 1 and stage 2.
At present, the reliability level of power supply in developed countries such as the United States, Britain, France and Japan is high, especially in Tokyo, Japan. After 1986, the power supply reliability rate of Tokyo Electric Power Company in Japan was more than 99.99%, and the corresponding average outage time of users was basically less than 0.876h (about 53min). In other words, the power supply reliability of Tokyo Electric Power Company of Japan was in the second stage of reliability development before 1986, and entered the third stage after 1986.
After years of development, the reliability level of urban power supply in China has gradually improved. Since 1992, the reliability rate of urban power supply in China has reached more than 99%. In other words, before 1992, China was in the first stage of reliability development. After 1992, it entered the second stage and is approaching the third stage. At present, the average power supply reliability rate of urban distribution network in China is below 99.9% (the corresponding average outage time of users is more than 8.76h), and that of a few cities is more than 99.9%.
At the same time, with the large-scale access of new energy such as wind power to the distribution network, it will greatly affect the power supply reliability of the system. Relevant studies show that with the increase of the capacity of the incorporated fan, the reliability parameters of the system increase in turn and the reliability of the system decreases. This is because the output of the wind turbine changes with the change of wind speed. Although the reduced conventional unit capacity has been replaced by wind power, the wind turbine cannot output at full load most of the time, so the reliability of the system continues to decline. Under the condition that the system reserve capacity is certain, wind turbines are used to replace some conventional generator sets.
From the above analysis, it can be seen that there is still a certain gap between the power supply reliability of Chinese cities and cities in developed countries. Especially under the current trend of large-scale development of new energy in China, taking various control means to improve the power supply reliability of power grid has also become an important factor in the birth of smart grid and an important task of power grid development.
Power grid reliability cost can be defined as the increased investment cost required by the power supply department to make the power grid reach a certain power supply reliability level, including operation cost and reliability benefit. It can be defined as the benefit obtained by users because the power grid reaches a certain power supply reliability level. Because it is difficult to estimate the social and economic benefits under a certain power supply reliability level, in the past, only indirect qualitative evaluation was made on the reliability benefits, which will be difficult to optimize the reliability. In order to facilitate measurement and calculation, the reliability benefit is expressed in terms of power shortage cost, that is, the economic loss caused by power shortage and power failure of users due to insufficient or interrupted power supply. Obviously, when the unit power shortage cost remains unchanged, the lower the power shortage cost, the higher the reliability benefit. In this way, reliability cost and reliability benefit can be measured in the economy of power grid. The application of energy storage devices in the distribution network can improve the power supply reliability of the power grid, and accordingly save the investment that the power grid needs to make to achieve the same power supply reliability. We can evaluate the benefits es of energy storage devices in reducing regional outage losses through the evaluation rate of power loss. Obviously, this can also indirectly reflect the power grid reliability cost saved by energy storage devices.