*3.1. Hydrogen—Energy Vector within a Sustainable Energy System*

Today hydrogen is recognized as a non-polluting energy carrier because it does not contribute to global warming if it is produced from renewable sources [26]. In addition, hydrogen is the only secondary energy carrier that is suitable for a wide range of applications in the market [27]. At the center of attention is the fact that hydrogen can be obtained from a wide range of primary energies [28]. It can be used advantageously for a wide range of applications, ranging from transport and portable to stationary use [29]. In addition, hydrogen can also be used in decentralized systems without emitting carbon dioxide [30]. Hydrogen is already a part of today's chemical industry, but as a source of energy its rare benefits can only be achieved with technologies such as fuel cells [31].

Since hydrogen can be produced from a wide range of primary energies and can be consumed in a larger number of applications, it will become an energy center, just as electricity today is [32]. The advantage of hydrogen over electricity is that it can be stored in the medium and long term [33]. As a consequence, an energy carrier helps to increase the stabilization of energy security and price, giving rise to competition between different energy sources [34].

Veziroglu [35,36], editor of the journal specialized in hydrogen technology and energy, the *International Journal of Hydrogen Energy* summarizes several features that recommend the use of hydrogen as a secondary energy vector produced using unconventional technologies:


Hydrogen is considered by more and more specialists to be a true fuel of the future. Hydrogen is condensed to −252.77 ◦C, and the specific weight of liquefied hydrogen is 71 g/L, which gives it the highest energy density per unit of mass between all fuels and energy carriers: 1 kg of hydrogen contains as much energy as 2.1 kg of natural gas or 2.8 kg of oil. This characteristic of it, made of hydrogen the fuel used in the propulsion and energy supply of the spaceships. Unlike other fuels, such as oil, natural gas and coal, hydrogen is renewable and non-toxic when used in fuel cells. Hydrogen has a very high potential as environmentally friendly fuel and in reducing the import of energy resources [37–42]. Even if the use of hydrogen at present seems unprofitable, due to the improvement of the technologies, we could assist in the not too distant future in the development of a hydrogen-based economy [43].

Etymologically, the word hydrogen is a combination of two Greek words, meaning "to make water" [44]. Produced from non-fossil sources and raw materials, using different forms of alternative energy (solar, wind, hydroelectric, geothermal, biomass, etc.), hydrogen is considered to be a prime fuel in the supply of so-called "green energy" [45]. Thus, hydrogen-fueled systems can be considered as the best solution for accelerating and ensuring global energy stability [43]. Hydrogen is expected to play an important role in the future energy scenarios of the world, the most important factor that will determine the specific role of hydrogen will probably be the demand for clean growing energy [35]. At the same time, hydrogen can, to some extent, replace fossil fuels and become the preferred clean, non-toxic energy carrier in the near future [36]. The main characteristics of hydrogen [35,37], presented in Table 1, recommend it as an alternative fuel to the classic ones.



In order to highlight the advantages that hydrogen has, compared to other fuels, the main properties of the various fuels currently used are presented in Table 2.



Analyzing the table information it can be concluded that the main arguments in favor of the use of hydrogen as synthetic fuel, obtained from renewable sources, are the following: it has the highest energy/mass unit of all the fuel types; it is ecologically friendly, as from its combustion produces only water vapor, indicating that for hydrogen the amount of carbon emissions is zero; it has the highest energy reserve factor, and the largest conversion factor into electricity, respectively, and for this reason it is considered the best of the fuels presented, and the energy efficiency is very high. Hydrogen is expected to play an important role in future energy scenarios globally [46,47]. The advantages that promote it as an energy vector in relation to other forms of energy are:


Barriers to be overcome refer to issues regarding:


Schematically the pro/against hydrogen arguments are shown in Figure 3.

**Figure 3.** Advantages and barriers regarding the use of hydrogen as energy vector [40].

In view of the research and development programs supported in this field, the technical problems regarding the production, storage and distribution of hydrogen, together with the reduction of costs and the increase of the life of the equipment used in the generation of energy based on hydrogen, will be solved shortly time, and hydrogen will become a possible solution for providing fuels, at the same time being an alternative energy resource to the traditional ones.
