Power generation consists of electricity production from power plants and real-time sales of electricity, both wholesale (such as purchased by electricity suppliers) and retail (directly supplied to end users of electricity).
Electricity is less regulated in France, so generators compete freely in the electricity market. Pure generators sell all the electricity they produce on the wholesale market. Co-generators, which both generate and sell electricity, use all or part of the electricity they generate to meet the needs of large consumers, which are their main source of income.
Regardless of how a generator views the electricity market, the amount of electricity it generates and the amount of revenue it generates will be affected by several different factors:
1) The amount of electricity sold by a generator at any time depends on the installed capacity of the generator set, the power demand of the load, and the competitiveness of the power generation cost.
2) The price of electricity sold, whether it is sold locally or delivered to other places through the grid, is related to the operation of the electricity and power system.
Faced with these uncertainties, the key for power generators is how to optimize and ensure safe power production and ensure operational profitability.
Electricity prices fluctuate with hourly, weekly, and seasonal changes in load demand. In order to maximize power generation revenue, it is very important to maximize electricity sales when electricity prices are the highest.
By managing the energy storage system, electricity can be stored when electricity prices are low and then sold when electricity prices are high. Therefore, the energy storage system can be used as an adjustment lever to improve the power generation revenue of generators.
In order to be able to obtain more benefits from energy storage, the capacity of the energy storage system should be large enough to carry out tens of hours of charge-discharge cycles. In addition, the power of the energy storage system should also be large enough (in the order of hundreds of megawatts) such that the energy storage system can be charged at night and discharged during the day, or charged on weekends and discharged during weekdays. In general, the more mature energy storage technologies positioned for the above purposes are large-scale energy storage systems, such as pumped storage power stations (STEP) and compressed air energy storage systems (CAES).
For large-scale pumped storage power stations, the use of seasonal energy conversion between summer and winter (which can be recycled for hundreds of hours) can also obtain greater benefits, such as storing water in the rainy season and releasing water to generate electricity in the early season. It is worth noting, however, that the most suitable locations for such energy storage systems, such as valleys, are largely used up in France.
In short, according to the power consumption characteristics of the load (summer-winter, weekend-weekday, day-night), the energy storage system can pass through the peak load transfer or smooth the load curve to maximize the income of the generator. In addition, energy storage can also enable generators to gain new benefits in other areas, mainly in the following aspects:
1) Reduce fuel costs: Use the cheapest fuel to generate electricity at off-peak times, and shift it through peak-to-valley, thereby reducing the amount of expensive fossil fuels that need to be used during peak loads.
2) Optimize the electricity sales strategy in the market: when electricity prices are more favorable, more electricity is sold.
3) Alleviate the limitations of various dynamic constraints that affect the operation and management of the generator set: optimize the operation of the generator set by smoothing the load curve (for example, reduce the number of start and stop of some units with higher start and stop costs).
4) Reduce the level of CO2 emissions and correspondingly reduce the cost of purchasing carbon emission permits: especially in the case of low-carbon hydropower or nuclear power bearing the base load, using the energy storage system as a supplement to power generation can reduce the fossil fuel power generation used to meet the peak load demand, thereby reducing CO2 emissions.
2. High-power energy storage can reduce the operational and operational risks of the power generation system
The application of energy storage technology can reduce the possible adverse effects of generator sets during operation, such as the lack of available capacity for conventional power generation, cold fronts, and lack of wind. Energy storage systems are an effective solution to mitigate the above-mentioned adverse effects, and can avoid reliance on other solutions, such as purchasing energy from the futures market, or investing in the construction of backup power generation capacity.
Energy storage technology can also smooth out extreme peak loads in the load curve that may only occur for a few hours a year, thereby reducing investment in additional generation capacity for such extreme peak loads. Therefore, energy storage can delay or even reduce this new generation capacity, which is also a cost saving for generators.
In addition, energy storage can also limit the financial risks in the electricity market due to fluctuations in electricity prices by providing the required electricity in a timely manner, thereby reducing the dependence of generators on other solutions, which is another manifestation of the value of energy storage.
3. Ancillary services of energy storage
By configuring energy storage, power generators can improve the functions of generator sets in the power system, such as energy storage participating in system frequency regulation or grid failure recovery. Of course, this depends on the scale and technical characteristics of the energy storage system. A brief analysis of these functions is given below.
Since different countries have very different frequency regulation characteristics, there are also many differences in frequency regulation requirements. For convenience, in this chapter only for France, we divide the frequency modulation into three types: sub-, secondary, and tertiary.
1) One FM. The purpose of primary frequency regulation is to maintain the real-time power generation-power consumption balance of the power generation system through direct and automatic power generation control of the participating frequency regulation generator sets. Especially in the interconnected power grid, due to the frequency change due to failure, a frequency modulation can keep the frequency stable for several seconds.
The primary frequency regulation can be realized because some of the grid-connected generator sets participating in the frequency regulation have active backup (primary backup), and these devices usually operate in a low power state. Figure 1 depicts the active power/frequency characteristics of an FM generator set in steady state operation.
Figure 1 - Active power/frequency characteristics of FM generator set in steady state operation
For the regional power grid within the scope of UCTE (Union for Coordination of European Electricity Transmission), the required primary backup capacity must be able to compensate for the power loss when the reference fault occurs, that is, it must be able to compensate for the sudden power generation loss of 3000MW. In addition, in order to reduce the frequency deviation in steady state, the minimum value of the ratio that the units participating in frequency regulation need to meet. Here, ΔP represents the power variation of the frequency-modulating units, and Δf represents the frequency variation in the steady state. This ratio is the energy required for system frequency modulation. For the first synchronization of UCTE (Western Europe region), the required energy is at least 18,000 MW/Hz, and for the second synchronization region (Balkan region), at least 3,000 MW/Hz. Therefore, the primary reserve capacity can be easily allocated to multiple generator sets in the system to ensure the regulated energy required for the stable operation of the system.
In order to achieve the frequency stability of the system, there are certain requirements for the response time of the reserve capacity. Generally, when the required compensation capacity is less than or equal to 1500MW, it is required to be released within 15s; when the required compensation capacity reaches 3000MW (the power loss when the reference fault occurs), it is required to be released within 30s.
2) Secondary frequency modulation. Secondary frequency regulation is a centralized automatic adjustment of the generator sets participating in frequency regulation, so that the system frequency and the power exchange with the adjacent power grid can reach the predetermined target value. Compared with primary frequency regulation, secondary frequency regulation is not realized by using local information, but requires a command to be sent to the generator set. This command is calculated by the control center of the grid dispatch (TSO). Due to the existence of active standby (secondary standby) in the generator set, the realization of secondary frequency regulation is guaranteed. Only generators with a certain capacity (above 120MW in France) can participate in secondary frequency regulation.
3) Tertiary frequency modulation. The tertiary frequency regulation is a manual frequency regulation, mainly used to: ① release the primary and secondary reserve capacity, and adjust the frequency to a predetermined value when the secondary frequency regulation fails to achieve the adjustment target (maybe due to the lack of secondary reserve capacity); ②when the imbalance of power supply and demand increases slowly, the system is restored to a balanced state.
Tertiary frequency modulation is also used to solve the problem of transmission grid congestion, of course, this depends on the policies of different countries.
The tertiary frequency modulation can have different time scales for the scheduling of the tertiary active power reserve capacity. In France, the dispatch of these spare capacities is communicated to generators by telephone from the TSO control centre.
Tertiary frequency regulation is often associated with the balancing mechanism of grid dispatch (TSO), which is similar to a fixed bid, where the party responsible for balancing submits a adjustment plan, such as the generator providing the precise amount by which the generator set is adjusted up or down, and then TSO selects a scheme suitable for the system to ensure the balance of power generation and electricity consumption and the safety of the system.
(2) Power grid restoration
When part or all of the power grid collapses, the goal of fault recovery is to restore power to the grid as soon as possible. First, ensure that the power supply of high-priority loads is restored, and then gradually restore to full load until the entire grid operates normally.
Fault recovery of the grid consists of a series of steps and relies on generator sets. The energy storage system can participate in the failure recovery of the power grid similar to other generator sets, but it depends on the capacity and technical characteristics of the energy storage system. In France, only generators above 40MW are allowed to participate in this mode of operation.