# Innovative Strategies for Combining Solar and Wind Energy with Green Hydrogen Systems
## 3 Innovative Strategies
### Optimized Energy Conversion and Storage
#### 3.1.2 Advanced Storage Solutions for Hydrogen  
Storage is a crucial factor of the green hydrogen ecosystem, facilitating the management of hydrogen produced during phases of high renewable energy generation. Advanced storage technologies are essential to ensure that hydrogen can be stored and retrieved safely and cost-effectively. Various hydrogen storage methods include compressed gas tanks, hydrogen tanks, cryogenic liquid, physical storage metal hydride storage, and underground storage [43]. This section provides an in-depth analysis of three of these storage solutions. A comparative analysis of these hydrogen storage techniques is provided in Table 3.  
Compressed Hydrogen Storage: Compressed hydrogen storage is the most common method used for both stationary and mobile applications. This well-established technology enables rapid hydrogen filling and release without requiring additional energy for release. However, compressing hydrogen to high pressures consumes approximately 13-18% of its lower heating value, impacting the overall cost-effectiveness of the process. Increasing pressure results in only a marginal increase in the power requirements for compression. Typically, hydrogen is stored in cylindrical vessels, because spherical vessels are less practical for mobile applications. These vessels must be lightweight, durable, and capable of withstanding high pressures while resisting hydrogen diffusion and potential embrittlement. The four types of compressed hydrogen storage vessels--Type I, Type II, Type III, and Type IV--are detailed in [8]. Underground geological caverns also offer a long-term storage solution, though concerns about hydrogen purity due to potential contaminants in the caverns may arise. Liquid Hydrogen Storage: Storing hydrogen in its liquid form (LH\({}_{2}\)) provides a much higher density compared to gaseous storage, enhancing the volumetric energy density. At \(-253\)\({}^{\circ}\)C, the density of liquid hydrogen is about 71 g/L, making it more energy-dense than compressed hydrogen. Liquid hydrogen storage is a well-established technology that allows for high rates of hydrogen release and low adiabatic expansion energy under cryogenic conditions. In its liquid state, hydrogen is non-corrosive, and storage generally involves stainless steel and aluminum alloy vessels with adequate insulation. However, the cost and energy demands for liquefaction are significant [43]. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Storage Method** & **Hydrogen Content (wt.\% H\({}_{2}\))** & **Volumetric Density (g/L)** &
\begin{tabular}{l} **Volumetric Energy** \\ **Density (MJ/L)** \\ \end{tabular} \\ \hline Compression & & & \\
1 bar, RT & 100 & 0.0814 & 0.01 \\
350 bar, RT & 100 & 24.5 & 2.94 \\
700 bar, RT & 100 & 41.4 & 4.97 \\
700 bar, RT, (inlc. Type IV tank) & 5.7 & 40.8 & 4.9 \\ Liquid Hydrogen & & & \\
1 bar, \(-253\)\({}^{\circ}\)C & 100 & 70.8 & 8.5 \\
1 bar, \(-253\)\({}^{\circ}\)C (inlc. tank) & 14 & 51 & 6.12 \\ Metal Hydrides & & & \\ MgH\({}_{2}\) & 7.6 & 110 & 13.2 \\ FeIH\({}_{2}\) & 1.89 & 114 & 13.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of hydrogen storage methods in Ref [43] with significant modifications. Solid-State Storage: Solid-state hydrogen storage involves hydrogen chemically reacting with metals or metal alloys to form metal hydrides. In this process, hydrogen molecules dissociate into atomic hydrogen on the metal surface, diffuse into the bulk material, and become chemisorbed within the metal or alloy structure. This interaction can cause the metal lattice to expand by approximately 20-30% of its initial volume. Metal hydrides are formed either through direct reactions between hydrogen and the metal or via the electrochemical dissociation of water molecules.