## 2 Hydrogen as a Natural Resource  
The hydrogen atom is the lightest and simplest of all elements [7]. The H\({}_{2}\) molecule is a colourless, non-toxic, highly combustible gas with no odours and with a low density, one-fourteenth of the density of air. To condensate gas hydrogen to liquid form at atmospheric pressure, the temperature has to be reduced to 20.3 K [7]. Released hydrogen disperses rapidly. Compared with other fuels, hydrogen has the highest combustion energy per unit mass [7]. Hydrogen is used in gaseous, liquid, or slush forms. Liquid hydrogen is transparent, with a subtle light blue tint. Slush hydrogen is a mixture of liquid and solid  
Figure 1: Metered cross-border electricity flow [5]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Primary Energy Resources** & **Share (\%)** \\ \hline Nuclear & 46.6 \\ \hline Hydro & 11.1 \\ \hline Fossil fuels & 21.5 \\ \hline Solar & 1.0 \\ \hline Other & 8.7 \\ \hline Import & 9.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Share of electricity generation on day of yearly peak [5]. hydrogen at triple-point temperature. The conditions to create triple-point hydrogen are a temperature of 13.8 K and pressure of 7200 Pa [8]. The highest possible temperature at which hydrogen vapour can be liquefied is 33.145 K, which is critical temperature. The corresponding critical pressure is 1300 kPa; the density at this critical point is 31.263 kg/m\({}^{3}\). Above this temperature, it is impossible to condense hydrogen into its liquid phase just by increasing the pressure [7]. At normal conditions, temperature, pressure, etc., hydrogen is not a very reactive substance; to accelerate the reaction, a catalyst or spark is needed, which causes a violent explosion [7]. Hydrogen is an extremely flammable gas. It burns in air and oxygen to produce water. In an air chlorine mixture, it can explode spontaneously just from exposure to a spark, heat, or even sunlight [8]. The flammability range of hydrogen is from 4% to 75% by volume in air at NTP (normal temperature and pressure). Hydrogen's flammability range expands with temperature; the flammability limit of hydrogen lowers to 3% at a temperature of 100 \({}^{\circ}\)C at NTP [6]. Hydrogen's adiabatic premixed flame temperature for stoichiometric mixture in air is higher compared to other fuels, at 2403 K. Stoichiometric hydrogen-oxygen is composed of 66.66% of hydrogen and 33.33% of oxygen volume. Hydrogen can be produced in several different ways. Thermochemical processes use thermal and chemical reactions to release hydrogen from organic materials such as fossil fuels and biomass, or from materials such as water. Microorganisms, such as bacteria and algae, can produce hydrogen through biological processes. Some thermal processes use energy from various sources such as natural gas, coal, or biomass that release hydrogen from their molecular structures [9]. In other processes, heat in combination with closed chemical cycles produces hydrogen from raw materials such as water. Natural gas reforming is an advanced production process, where hydrogen is produced by reforming natural gas in large chemical plants. Biomass gasification is a technological path that uses controlled processes involving heat, steam, and oxygen to convert biomass to hydrogen and other products, without combustion [9]. The production costs for various hydrogen production methods are shown in Table 4. The table shows methods that are currently available, with specified hydrogen production mode and efficiencies. All of these methods use electrolysis as the basic process of hydrogen production. These technologies are well-developed and commercially available, and systems are being developed that can efficiently use intermittent renewable energy. Electrolysis is a promising option for the production of carbon-free hydrogen from renewable and nuclear sources.