**3. Hydrogen energy**

Hydrogen is an excellent choice as an energy source for heat and power, among many other uses, due to its many positive qualities, including its overall storage capacity, efficiency, renewability, cleanliness, massive distribution, high conversion, zero emissions, sources, versatility, and quick recovery [12]. It is, therefore, recognized as the most promising and environmentally beneficial energy source of the twenty-first century. It is essential to industrial processes like the generation of ammonia, oil refinement, and water-gas switch reactions [13].

The demand for hydrogen has recently increased dramatically in industrialization's classic oil upgrading industries, including hydro-desulfurization, hydrogenation, and ammonia processing. Due to its affordability and easy access to hydrogen, the majority of the liquid-compressed hydrogen gas used in industrial processing is created commercially using the compression method [14]. Solar power can also be used to produce hydrogen from renewable sources like lignocellulosic biomass or water purification.

#### **3.1 Hydrogen production**

Numerous methods exist for obtaining hydrogen from various sources, including electrolysis of water, biofuels, petroleum-based liquids, microbes, and biofuels [14]. Hydrogen production methods and classifications are briefly schematized in **Figure 1**. As seen in the Figure, classification of hydrogen according to the production

**Figure 1.**

*Hydrogen production methods and hydrogen classification.*

*Introductory Chapter: Hydrogen Energy DOI: http://dx.doi.org/10.5772/intechopen.108635*

method is made with color codes. Greenhouse gas emissions that may occur during the production process are also considered in color coding. For example, hydrogen produced through the electrolysis of water using electricity from renewable sources is coded green and produces near-zero greenhouse gas emissions. Hydrogen, coded in blue, is produced through carbon capture and storage and steam methane reforming. Greenhouse gas emissions from the production of blue hydrogen are characterized as low. Greenhouse gas emissions are high in the gray hydrogen production, produced through steam methane reforming using natural gas [15].

#### **3.2 Hydrogen storage and transportation**

There are various methods for storing and transporting hydrogen after its production. The transportation process is done by compressing the hydrogen in gas form or converting it into liquid form in a pressurized environment and then loading it into tankers. However, due to the increasing need for hydrogen in the coming years, it is possible to transport hydrogen through existing natural gas pipelines. For storage, the priority is concentrated on methods that allow transportation. Methods that prioritize transportation for the storage of hydrogen, liquid hydrogen, gaseous hydrogen, metal hydride, and chemical storage. The storage methods of hydrogen are presented in **Figure 2**.

**Figure 2.** *Hydrogen storage methods (adopted from [16]).*

## **3.3 Hydrogen applications**

Hydrogen is an energy storage medium, as well as an energy carrier. It has multiple fuels uses—hydrogen vehicles, stationary power sources, building heating, industrial feedstock, and industrial energy [17] (**Figure 3**). Thus, hydrogen is attracting the

**Figure 3.** *Production and applications of hydrogen [18].*

keen interest of several groups from governments to companies as it offers ways to decarbonize a lot of sectors, especially those that have been proven to be difficult to reduce emissions meaningfully. Moreover, in the industry, hydrogen has had an increasingly important role along the production chain in the last few years [19].
