How can energy storage technology be applied to the power balance mechanism?

Balancing Responsible Parties (BRPs) exist in many power systems around the world. In the power system based on BRP, each participant ensures the safe operation of the power system through a balancing mechanism in the power market environment.

Any independent legal person, whether it has its own power generation equipment or not, and whether or not it has signed a power purchase/sale contract, can become a balancing responsible party BRP as long as it has signed a "BRP" balancing responsible party contract with the grid dispatcher (TSO).

Each BRP is associated with the same balance scale, and integrates the power injection, power consumption and power exchange contracts of each power entity unit under the power market. BRP is obliged to provide TSO with necessary forecast information so that the latter can better operate the grid; the BRP also needs to financially compensate the TSO under the Balance Scale to make up for the adjustment that the TSO is forced to make due to possible discrepancies between predicted and actual values.

The time interval (such as hourly) between the calculation of the above-mentioned power imbalance and the economic compensation method is different in different countries, but most of them are adjusted within the scope of the national level. In Europe, the cost of solving the imbalance is usually based on the increase (up regulation) or decrease (down regulation) of the genset power in order to achieve electricity balance under the balancing mechanism.

Using Figure 1 as an example, the fee calculation method used by RTE Transmission Systems (a French transmission system operator) to resolve imbalances is illustrated.

Figure 1 - The method of calculating the imbalance fee used by the French RTE company

Figure 1 - The method of calculating the imbalance fee used by the French RTE company

In Figure 1, PMPb is the average weighted cost of reducing power generation when there is excess power generation; PMPh is the average weighted cost of increasing power generation when power generation is insufficient; the factor K is the coefficient, which covers the common part of the increase and decrease of the generation cost and the capacity guarantee fee paid to the fast backup power source under the balancing mechanism; NB is the indicator sign. The average weighted cost (PMPb) of reducing generation in France when there is excess generation is 20% of the on-site transaction price. Likewise, the average weighted cost (PMPh) of additional generation when generation is insufficient is 120% of the on-site transaction price.

We note that the cost of compensating for the imbalance has enough regulatory efficacy to make BRP minimize its own imbalance as much as possible. This question is very important, and here we take Germany as an example to illustrate the high cost. The total cost under the German balancing mechanism in 2006 was about 800 million euros. To reduce this fee, BRP can take some of the following measures:

1) Improve the prediction accuracy of energy injection/consumption under the balance scale (whether using own technical means or using outsourced methods).

2) Enhance the reliability of power generation (if the unreliability of power generation capacity has a non-negligible impact on the imbalance of electric energy).

3) Improve the flexibility of its own power generation and consumption, or obtain more flexible power compensation from other power generators.

For this last measure, the BRP can use energy storage to reduce imbalances. To achieve this goal, it is necessary to know what the typical charge-discharge curve of energy storage looks like? And which energy storage technology is the most suitable?

An analysis of the generation/load balance curve of the French power system in 2008 shows that:

1) The longest duration of unbalance: 70h for insufficient power generation and 32h for excess power generation.

2) Average duration of unbalance: 6.4h for insufficient power generation and 5.3h for excess power generation.

3) The maximum power used: downward adjustment is 4500MW, upward adjustment is 5500MW.

Figure 2 shows the unbalance regulation curve of the French power system. As can be seen from the figure, the imbalance of electric energy supply and demand has an obvious daily cycle characteristic. Therefore, the ideal storage period of the most suitable energy storage technology is 2~15h.

Figure 2 - Balance regulation curve of power system

Figure 2 - Balance regulation curve of power system

Regardless of the maturity of energy storage technology, we predict the following energy storage technologies that may be used in this occasion:

1) Pumped storage (pumped storage power station, STEP).

2) Compressed Air Energy Storage (CAES, Adiabatic Cycle CAES).

3) Redox flow battery energy storage (vanadium flow battery).

4) Hydrogen storage.

5) Thermal storage (high temperature or low temperature energy storage).

In conclusion, the need to reduce the imbalance between electricity supply and demand has led to the application of BRP for energy storage, allowing it to perform daily charge-discharge cycles. In the power system, the benefits brought by the use of energy storage depend on the cost of its construction and use, as well as the level of expenses required to adjust the imbalance in the system.