Hydrogen is the most common element in the universe. The first chemical element in the periodic table, hydrogen is the initial "fuel" of stars. In the universe, 92% of what is known is made up of hydrogen. It is the stuff that stars are made of and a fundamental building block of all life on Earth. Its name means "water generator" and was coined in the late eighteenth century. 70% of the Earth's surface is covered with water (H 2 O), a chemical compound made up of hydrogen (H) and oxygen (O). Hydrocarbons, such as methane (CH 4 ) and crude oil, are also important hydrogen-containing compounds. Most hydrogen is therefore found in chemical compounds: on its own it is an invisible, odorless and non-toxic hydrogen gas, lighter than air and which only liquefies at a temperature of -252°C. 

It can be used to produce other compounds or as a fuel to produce energy. Is possible to produce, accumulate, move and use energy in different ways thanks to this very versatile element. It can be produced using renewable energy, nuclear, natural gas, coal and oil. The emission-free properties make green hydrogen a promising candidate for the transition to a new energy carrier. With green hydrogen, the goal of a low-carbon economy becomes achievable. 

Three main types of hydrogen are "grey" hydrogen, "blue" hydrogen and "green" hydrogen.

Hydrogen produced using energy from hydrocarbons, particularly natural gas, is known as “ grey ” hydrogen.

Hydrogen produced from natural gas and later decarbonised  using carbon capture and storage is sometimes called blue or decarbonised hydrogen. 

Hydrogen produced via electrolysis, by splitting water into oxygen and hydrogen using electricity from renewable sources is sometimes called green or renewable hydrogen.

Yes, hydrogen is safe if handled correctly. Hydrogen is a non-toxic gas at room temperature and atmospheric pressure. Hydrogen is explosive under certain conditions when mixed with air – similar to natural gas. When handled responsibly, green hydrogen is less dangerous than other flammable fuels. Therefore, tried and tested international standards exist on how to design and monitor hydrogen installations.

Hydrogen is an energy carrier, not an energy source and can deliver or store a tremendous amount of energy. Hydrogen can be used in fuel cells to generate electricity, or power and heat. 

Hydrogen has long been used in the chemical industry for many manufacturing processes. The interest in hydrogen for storage and generation of electrical energy has only been focused on more strongly in recent years. Above all, the very volatile electricity generated from photovoltaic and wind power plants strengthens the need for flexible energy storage. There are a multitude of possible applications for hydrogen technologies:

• Production of electricity, heat and water for various end uses.
• Power generation and energy storage.
• Combined heat and power generation.
• Energy storage of renewable energies.
• Hydrogen generation and Storage.
• Industrial processes.
• Transportation and mobility.
• Residential and commercial gas utility systems.
• Export market.

As an energy carrier, hydrogen can be converted to electricity, heat or kinetic energy and can be used:

• For stationary applications, through the production of electricity and/or heat in buildings (principle of combined heat and power production)
• For industrial applications by using hydrogen as a chemical compound
• For mobile applications by using hydrogen as a driving force

Hydrogen technologies are mechanical or chemical applications in which hydrogen is used. A distinction can be made between five types of hydrogen technologies:

1. Technologies for hydrogen production.
2. Technologies for hydrogen use.
3. Technologies for hydrogen storage.
4. Technologies for hydrogen distribution. 
5. Technologies for infrastructure. 

The hydrogen supply systems investigated in this work are concerned with hydrogen that is produced from water electrolysis. 

The three hydrogen supply systems investigated are as follows:

• (1) Decentralized-Standalone (Dec-Sa). In this system, hydrogen is produced at the refueling station, with a standalone system using dedicated wind power plants and/or solar PV that are located in the vicinity of the refueling station and provide the electricity for the electrolyzer. Hydrogen storage tanks are used to store the hydrogen between production and demand at the refueling station.
• (2) Decentralized-Grid-connected (Dec-Gc). This is similar to the Dec-Sa system but uses electricity supplied from the local electricity grid to power the electrolyzer. Current and possible future electricity price scenarios are used.
• (3) Centralized-Grid-connected (Cen-Gc). This is a large-scale, centralized hydrogen production system using electrolysis, from which the hydrogen is distributed to several refueling stations via trucks. Large-scale, lined rock cavern (LRC) centralized storage is used to smoothen the seasonal variations, while a storage system similar to that used in the decentralized systems (Dec-Sa and Dec-Gc) is used to store the hydrogen at a higher pressure at the refueling station. Current and possible future electricity price scenarios are used.

Hydrogen is the most common chemical element in the universe. It can be stored as a gas or liquid, or made part of other molecules, and has many uses such as fuel for transport or heating, a way to store electricity, or a raw material in industrial processes.

Hydrogen can be produced through electrolysis, which uses electricity to split water molecules into hydrogen and oxygen, with the hydrogen being captured and used as a fuel. 

Hydrogen energy can be stored as a gas and even delivered through existing natural gas pipelines. 

When it is produced using renewable energy or processes, hydrogen is an emissions free fuel and becomes a way of storing renewable energy for use when it is needed.