## 4 Analysis of Energy Storage System  
At present, most of the wind-solar hybrid power generation systems use secondary batteries that can be repeatedly charged and discharged as energy storage systems, and the electrical energy can be converted into chemical energy for storage. When using electrical energy, the stored chemical energy of batteries can be turned into electrical energy. When we choose a suitable energy storage device, the capacity of the energy storage device and its charge and discharge performance are mainly considered. The battery with high conversion efficiency and low loss is suitable for the design of the standalone renewable energy system. In addition, the maintenance cost and life of the battery should be also key factors in the design. Commonly used batteries include lead-acid batteries, nickel-hydrogen batteries, nickel-cadmium batteries, lithium-ion batteries and sodium-sulfur batteries [20]. The key performance comparisons of each battery are presented in Table 5.  
It can be seen from Table 5 that the operating temperature ranges of lead-acid battery and lithium-ion battery are more suitable than other batteries for the requirements of this system. Compared with lead-acid batteries, lithium-ion batteries have certain safety hazards and lithium-ion batteries cost more than lead-acid batteries. Thus, the lead-acid battery is selected to design energy storage system. A lead-acid battery consists of an electrolyte, positive and negative electrodes. \(P_{b}\) is used as the negative active material of lead-acid batteries, \(P_{b}O_{2}\) can be the positive active material, and the electrolyte is diluted \(H_{2}SO_{4}\). The energy conversion principle of lead-acid batteries can be expressed by the following chemical reaction equations. \[P_{b}O_{2}+H_{2}SO_{4}+P_{b}\overset{Disch\arg e}{\rightarrow}P_{b}SO_{4}+2H_ {2}O \tag{7}\]  
\[P_{b}SO_{4}+2H_{2}O\overset{Ch\arg e}{\rightarrow}P_{b}O_{2}+H_{2}SO_{4}+P_{b} \tag{8}\]  
Equations (7) and (8) can describe the discharge and charging process of the lead-acid battery, respectively. In the process of discharge, \(P_{b}\) on the negative electrode is oxidized to become \(P_{b}SO_{4}\), and \(P_{b}O_{2}\) on the positive electrode is reduced to form \(P_{b}SO_{4}\). Diluted \(H_{2}SO_{4}\) in the surrounding area as an electrolyte participates in chemical reactions, forming \(P_{b}SO_{4}\) while producing \(H_{2}O\). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Classification** & **Electrolyte** & **Principle** & **Operating** \\  & & & **Temperature (\({}^{\circ}\)C)** \\ \hline Lead–acid batteries & Dilute sulfuric acid & \begin{tabular}{c} Oxidation-reduction \\ reaction \\ \end{tabular} & \(-\)50 to 70 \\ \hline Lithium–ion batteries & \begin{tabular}{c} Organic lithium salt \\ electrolyte \\ \end{tabular} & Ion migration & \(-\)50 to 70 \\ \hline Nickel–cadmium batteries & \begin{tabular}{c} Potassium hydroxide \\ aqueous solution \\ \end{tabular} & \begin{tabular}{c} Oxidation-reduction \\ reaction \\ \end{tabular} & \(-\)20 to 45 \\ \hline Nickel–hydrogen batteries & \
\begin{tabular}{c} Potassium hydroxide \\ aqueous solution \\ \end{tabular} &
\begin{tabular}{c} Oxidation-reduction \\ reaction \\ \end{tabular} & \(-\)20 to 60 \\ \hline Sodium–sulfur batteries & Na-\(\beta\)-Al\({}_{2}\)O\({}_{3}\) & Chemical reaction & 300 to 350 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Key performance comparisons of each battery. In the process of charge, \(P_{b}SO_{4}\) on the negative electrode is reduced to form \(P_{b}\) and \(P_{b}SO_{4}\) on the positive electrode is oxidized to become \(P_{b}O_{2}\). The concentration of \(H_{2}SO_{4}\) in the surrounding area gradually recovers. The charging process of the lead-acid battery is not finished until \(P_{b}SO_{4}\) of the positive and negative electrodes is completely reduced to \(P_{b}\) and \(P_{b}O_{2}\). In the process of charge and discharge of lead-acid batteries, the terminal voltage can be expressed as the following equations.