# Hydrogen Production from Offshore Wind Parks: Current Situation and Future Perspectives
## 2 System Components
### Hydrogen Storage  
Storage of hydrogen is similar to natural gas, with a few key differences: mainly, when some metals come in contact with hydrogen they can suffer hydrogen embrittlement, which leads to increased degradation and chance of material failure [15]. Another difference to consider is increased leakage, especially in underground natural structures such as aquifers, but also in links between pipeline sections or links in valves, due to the small size of the hydrogen molecule [15]. The bacterial reaction also constitutes a problem, since some bacteria decompose hydrogen, leading to what can be considered losses, as the purity of stored hydrogen decreases [2]. The main approaches in storing hydrogen are gaseous storage and liquid storage; other approaches like chemical storage exist, but only on a much smaller scale, so they won't be considered. Gaseous storage can be divided into two methods: fabricated tanks (usually metal) and storage in natural underground structures like aquifers and salt caverns. Hydrogen density has a nearly linear relation with pressure [16], so a greater storage pressure leads to a smaller volume needed to store a certain amount of hydrogen gas. However, due to material properties and operational costs, hydrogen is not stored at pressures higher than 100 bar [15], which corresponds to a density of roughly 7.8 kg/m\({}^{3}\)[15]. Figure 1: Efficiency curve of a PEMEL. Adapted from IRENA [11]. A simple model can be developed to compute the rough dimensions of the storage facilities needed. The flow rate of a typical 25 MW electrolyzer is \(Q_{n}=5000\) Nm\({}^{3}\)/h [17]. To convert the hydrogen flow rate (\(Q_{n}\)) from Nm\({}^{3}\)/h to mass (\(M\)) in ton/day, Equation (1) is used. \[M=\frac{24Q_{n}}{11.1}\times 10^{-3} \tag{1}\]  
In Equation (1), the factor 24 converts hours into days, the factor 11.1 comes from knowing that 1 kg of hydrogen equals 11.1 Nm\({}^{3}\) of hydrogen (Normal conditions are 0 \({}^{\circ}\)C temperature and 1 atm pressure), and the factor 10\({}^{-3}\) is to convert kg to ton. The corresponding hydrogen volume \(V\) in m\({}^{3}\)/day is  
\[V=\frac{M}{7.8\times 10^{-3}} \tag{2}\]  
where \(7.8\times 10^{-3}\) ton/m\({}^{3}\) is the hydrogen density for a pressure of 100 bar at a temperature of 20 \({}^{\circ}\)C.  
A linear variation of the hydrogen flow rate with the electrolyzer power was considered. This allowed us to obtain the flow rates for the electrolyzer power ranging from 25 MW to 1000 MW. Table 1 contains the electrolyzer power, flow rate, mass (Equation (1)) and volume (Equation (2)) of hydrogen produced per day if the electrolyzers are run at full power. The linear approximation used was based on a similar commercial 17.5 MW PEMEL that produces 340 kg/h [18]. When linearly scaled to 25 MW, a production rate of 11.6 tons/day is achieved, a similar value to the one presented in Table 1 (which is 10.8 tons/day). To estimate the dimensions of the cylindrical storage tanks, the equation for a cylinder's volume (Equation (3)) was used, where \(V\) is the storage tank volume, \(h\) is the height, and \(r\) is the radius. \[V=h\cdot\pi\cdot r^{2} \tag{3}\]  
Table 2 contains the possible approximate radius and height for cylindrical tanks to be able to store all the hydrogen produced during 24 h with the electrolyzers at full power. Table 2 shows that even for large electrolyzer projects (1000 MW, for instance) the required storage facilities are feasible, and the corresponding dimensions are reasonable and therefore practicable. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Power (MW)**} & \multirow{2}{*}{**Flow Rate (Nm\({}^{3}\)/h)**} & **Hydrogen Mass** & **Hydrogen Volume** \\  & & **(ton/day)** & **(m\({}^{3}\)/day)** \\ \hline
25 & 5000 & 10.8 & 1386 \\
100 & 20,000 & 43.2 & 5544 \\
500 & 100,000 & 216 & 27,720 \\
1000 & 200,000 & 432 & 55,440 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrolyzer power, flow rate, and mass and volume of hydrogen produced.