## 2 Battery and Hydrogen Energy Storage System
### Integration of BESSs and HESSs  
Combining hydrogen energy storage systems (HESSs) and battery energy storage systems (BESSs) is a smart move that will improve energy efficiency and sustainability in a number of industries. This integration capitalizes on the complementary strengths of both technologies to address individual limitations and optimize overall system performance [68; 69]. One conceptual approach involves using BESSs for immediate energy storage and rapid response needs. BESSs are effective in providing instant power support and stabilizing fluctuations in electricity supply and demand. In contrast, HESSs excel in storing larger quantities of energy over longer durations. Combining these capabilities creates hybrid storage systems that offer improved flexibility, resilience, and efficiency in managing renewable energy integration and grid stability [70; 71; 72; 73]. In a different framework (Table 7), excess renewable energy is used to concurrently charge a BESS and make hydrogen by HESS electrolysis when demand is low. After storage, the hydrogen can be used in fuel cells to produce electricity during periods of high demand or for other purposes, encouraging the effective use of renewable resources and aiding in the process of decarbonization [74; 75; 76; 77]. The feasibility and advantages of merging BESSs and HESSs are illustrated by a plethora of case studies and practical implementations. For example, hybrid systems have been \begin{table}
\begin{tabular}{c c c} \hline
**No** & **Application/Benefit** & **Details** \\ \hline \multirow{2}{*}{1} & \multirow{2}{*}{Energy Sector} & Reduces dependency on fossil fuels by using hydrogen to power fuel cells, which produce only water vapor as a byproduct while producing heat and electricity. \\ \multirow{2}{*}{2} & \multirow{2}{*}{Transportation} & In comparison to battery electric vehicles (BEVs), fuel cell electric vehicles (FCEVs) have longer driving ranges and require less time for refueling. \\ \multirow{2}{*}{3} & \multirow{2}{*}{Industry} & Uses hydrogen as a raw resource for several refining processes and for the production of compounds like ammonia. \\ \end{tabular}
\end{table} 
Table 6: Applications and Benefits. \begin{table}
\begin{tabular}{c c c} \hline
**No** & **Conceptual Framework** & **Details** \\ \hline \multirow{4}{*}{1} & \multirow{4}{*}{Hybrid Storage Systems} & Combines hydrogen energy storage systems (HESSs) for long-term storage with battery energy storage systems (BESSs) for short-term energy storage and quick reaction. \\  & & Provides improved resilience, efficiency, and flexibility in handling grid stability and the incorporation of renewable energy. \\ \multirow{4}{*}{2} & \multirow{4}{*}{Renewable Energy Utilization} & Utilizes surplus renewable energy to charge BESS and produce hydrogen through HESS via electrolysis during low-demand periods. Stored hydrogen is used in fuel cells for electricity generation during peak demand or other applications, optimizing renewable resource utilization and supporting decarbonization efforts.  
\end{table} 
Table 7: Conceptual Frameworks. implemented to stabilize grids and efficiently manage energy variations in areas where the use of intermittent renewable energy sources, such as solar and wind, is prevalent [78; 79; 80]. A pilot project integrates a BESS with HESS to optimize renewable energy utilization and grid stability. The BESS manages short-term fluctuations in renewable output, while excess renewable energy is stored as hydrogen through electrolysis. This stored hydrogen is later converted into electricity during peak demand or utilized as a fuel for hydrogen-powered vehicles, showcasing the versatility of integrated storage systems [81; 82; 83]. In industrial settings such as microgrids or remote applications, integrating BESSs and HESSs provides reliable energy solutions, reducing reliance on diesel generators or grid connections [84; 85; 86; 87]. Global research projects (Table 8) are still investigating and improving the combination of BESSs and HESSs.