## 2 Necessity and Principles of ESC
### Necessity  
The core feature of EI is transformation and interconnection of multiple energy sources, and its energy network has expanded from power grids to diversified networks including transportation networks, oil networks, gas networks, water networks and so on. As a result, a variety of ES methods such as hydrogen storage, thermal storage and coal storage etc. have emerged. Various ESs of EI are shown in Figure 1. Electricity, coal, oil, gas, etc. can be converted to each other, and the unused energy is transmitted through different energy networks and then stored. Finally, user demand data, energy supply data and ES data are analyzed by an ES system and different energies are distributed rationally. In order to ensure the stable and efficient operation of EI, various forms of ESs should be matched and coordinated properly, i.e., ESC. The necessity of ESC is analyzed from five aspects. (1) Reduce ES redundancy of EI system  
Different forms of ESs have their corresponding and changing needs in ESC. Nevertheless, the maximum demand for each kind of ES does not occur at the same time. Thus, converting a small-demand ES to a large-demand one can effectively utilize the remaining energy of the ES with small demand, which helps to reduce ES redundancy. (2) Make full use of the remaining capacity of large ES equipment  
For some large-scale ES equipment, such as pumped storage (PS), compressed air energy storage (CAES), etc., their capacity is large, and the power grade is high, so the remaining capacity can be applied to other places while meeting the needs of EI. Therefore, in the information sharing environment, ESC takes full advantage of the remaining capacity of large ES equipment. (3) Improve EE of EI system  
Transportation costs caused by ES can be reduced through different energy conversion. Moreover, storage costs generated by energy redundancy can be reduced owing to alternative energy as well. In addition, ESC can decrease the cost of equipment renewal brought by energy storage. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Types** & **Power Grade** & **Continuous Discharge Time** & **Time of Stored Energy** & **Cost/(8/(4/W-hi) & Lifetime/(Vears) & **Conversion Efficiency** \\ \hline PS & 100 MW–5000 MW & More than 1–24 h & hour–month & 5–100 & 40-60 & 70–85\% \\ CAES & 5 MW–300 MW & More than 1–24 h & hour–month & 2–50 & 20–40 & 60–70\% \\ SMES & 100 KW–10 MW & 1 ms–8 s & minute–hour & 100–10000 & \textgreater{}20 & \textgreater{}96\% \\ FES & 0–250 KW & 1 ms–15 min & second-minute & 1000–5000 & \textgreater{}15 & 2\textgreater{}90\% \\ SCES & 0–300 KW & 1 ms–60 min & second-hour & 300–2000 & \textgreater{}20 & \textgreater{}95\% \\ LTIFES & 100 KW–300 MW & 1–8 h & minute–day & 3–30 & 20–40 & \textless{}60\% \\ HTIFES & 0–60 MW & More than 1–24 h & minute–month & 30–60 & 5-15 & \textless{}60\% \\ LAB & 0–20 MW & Second-hour & minute–day & 200–400 & 5-15 & \textless{}80\% \\ LB & 0–100 KW & Minute–hour & minute–day & 1200–4000 & 5-15 & 80–85\% \\ RFB & 100 KW–5 MW & Second–hour & minute–day & 250–300 & 10–20 & 70–80\% \\ \hline \hline \multicolumn{7}{c}{PS: Pumped Storage; CAES: Compressed Air Energy Storage; SMES: Superconducting Magnetic Energy Storage; FES: Flywheel Energy Storage; SCES: Super Capacitor Energy Storage; LITES: Low Temperature Thermal Energy Storage; HTIFES: High Temperature Thermal Energy Storage; LAB: Lead-Acid Battery; LB: Lithium Battery; RFB: Redox Flow Battery.} \\ \end{tabular}
\end{table} 
Table 1: Technical parameters of ESs [48; 49; 50]. * Improve EUE of EI system  
Through conversion and substitution of energy, all kinds of ESs can be reasonably collocated. It is preferred to make full use of ES with higher conversion efficiency. At the same time, the storage time of some ESs with a higher self-discharge rate can be reduced, thereby reducing energy attenuation so as to improve EUE. * Guarantee security of EI system  
All kinds of ESs are mutually reserved and supported in safety.