3.3.1. Hydrogen (H2)

An electrolyzer is a type of electrochemical converter that splits water into hydrogen and oxygen using electricity. It is an endothermic reaction, which indicates that heat is required throughout the process. Hydrogen may be stored under pressure in gas bottles or tanks for nearly indefinite periods of time. Electrolysis releases oxygen into the environment rather than retaining it, and oxygen from the air is utilized to create electricity [26].

## 3.3.2. Synthetic Natural Gas (SNG)

Methane (synthetic natural gas or SNG) may be synthesized to store energy. SNG can be stored in pressure tanks, underground, or fed directly into the gas infrastructure. To prevent energy losses, CO2 and H2 transport to the methanation plant should be avoided. The fundamental drawback of SNG is its low efficiency as a result of conversion losses in electrolysis, methanation, storage, transport, and power production [27]. The overall AC-AC efficiency of 35% is significantly lower than that of hydrogen [13].

#### *3.4. Electrical Storage Systems*

The classifications of EES are as follows:
