What is hydrogen?
On earth, most of the available hydrogen can be found in bonded form; it rarely occurs in elementary form as a pure gas. Water, methane, and mineral oil are the most well-known compounds in which hydrogen occurs. Alongside natural occurrences, hydrogen can also be produced. Today, two main ways of producing the element have been established: steam reforming and electrolysis.
Hydrogen in the energy context
Hydrogen as an element was discovered in the 18th century by English physicist and chemist Henry Cavendish. It is now used in a variety of sectors, for example refining and the chemical industry. However, it is only in recent years that its potential for the energy sector has been realized. Hydrogen is today considered a valuable energy carrier and should not be mistakenly referred to as an energy source. It is capable of delivering or storing an enormous amount of energy and can therefore be used to generate power with the right technologies, namely fuel cell technology. Fuel cells allow either the generation of electric power or the simultaneous generation of electricity and heat. To date, hydrogen has been used primarily in petroleum refining and fertilizer production, but transportation and power supply are also becoming increasingly stronger markets.
Generation of hydrogen
As described, hydrogen is found on earth mostly in solid compounds. Over time, however, mankind has found ways to use the element in its elementary form. Two methods of creating pure hydrogen have emerged:
Around 95% of the world’s hydrogen is produced in a steam methane reforming process from a methane source, such as natural gas. In this production process, methane reacts with high-temperature steam under pressure in the presence of a catalyst to produce hydrogen, carbon monoxide as well as a relatively small amount of carbon dioxide. As it is an endothermic process, heat must be added to the process for the reaction to occur. Steam reforming can also be used to produce hydrogen from other fuels such as ethanol, propane or even gasoline. Because carbon dioxide is also produced in this process, it is not the most environmentally-friendly option.
Electrolysis, or power-to-gas, is a procedure that generates hydrogen from water using electricity. The use of renewable energy sources enables carbon-free hydrogen production. The electrolysis reaction takes place in an electrolyzer. Like fuel cells, electrolyzers contain an anode and a cathode separated by an electrolyte, which are used to split water into hydrogen and oxygen. Electrolyzers differ according to the type of electrolyte substance used and the type of ions conducted. Three main types of electrolyzers exist: polymer electrolyte membrane electrolyzers, alkaline electrolyzers, and solid oxide electrolyzers.
Most hydrogen today is produced from fossil fuels, of which the most comes from natural gas. Although electricity is less commonplace, in the future, solar energy and biomass are expected to increase in popularity due to their cost-effectiveness.
Green hydrogen is produced through electrolysis that is powered by renewable energy sources and thereby emits no CO2. Since neither the production of green hydrogen nor the by- products hydrogen and oxygen are harmful to the environment, green hydrogen is climate-neutral.
The production of large quantities of green hydrogen requires significant power generation capacities from renewables. To make green hydrogen economically competitive, electricity prices must be as affordable as possible.
Turquoise hydrogen is produced through methane pyrolysis, a thermal process that splits natural gas into hydrogen and solid carbon. If the carbon remains permanently bound and is not burned during further processing, this can be considered climate-neutral. The reactors or blast furnaces used to split the methane would also need to be powered by renewable energy. However, since the extraction of the raw material natural gas, which serves as the basis for methane pyrolysis, often involves emissions, turquoise hydrogen is generally not completely climate-neutral.
Blue hydrogen is produced by the steam reduction of natural gas. In this process, natural gas is split into hydrogen and CO2. However, in this steam reforming process, the carbon dioxide is not released into the atmosphere, but stored or industrially processed. Carbon capture and storage (CSS) technology allows CO2 to be stored underground. This means that even with blue hydrogen, there are no CO2 emissions. However, the long-term effects of storage are uncertain, and leaks can still have a negative impact on the environment.
Gray hydrogen is the exact opposite of green hydrogen – it is not climate-neutral at all. Gray hydrogen is produced by steam reforming fossil fuels such as natural gas or coal. This process releases the waste product CO2 directly into the atmosphere. For every ton of hydrogen produced, ten tons of carbon dioxide are produced, which is what makes gray hydrogen very harmful to the environment. When electricity from fossil fuels and non-renewable energy sources is used instead of green electricity for the electrolysis of water, the hydrogen produced is also gray.
Sometimes other colors are used to describe the production of hydrogen. For red, pink and purple, an electrolyzer is used in the production process, which is powered by nuclear energy. Yellow hydrogen is produced using renewable energy sources such as solar power. White hydrogen is the type that is simply a waste product of other chemical processes. If coal is used to produce hydrogen, brown hydrogen is the result.
Use of hydrogen
Hydrogen is an universal talent and is already being used or tested in many fields:
- In industry: This is the dominant use of hydrogen today. Oil refining, ammonia, methanol, and steel production – all of the production processes behind these materials require hydrogen. Since almost all of the hydrogen needed comes from fossil fuel-powered generation, the potential for emissions reductions by switching to clean hydrogen in industry is significant.
- In transport: Vehicles powered with hydrogen and equipped with fuel cells are able to travel longer distances using less energy, which makes it especially interesting for heavy-duty transportation and public transport buses. However, sufficient refueling infrastructure is still not very advanced in most areas. In shipping and aviation, low-carbon fuel options are limited, increasing the prospects for hydrogen-based fuels in this sector.
- In the building sector: Hydrogen could be used as a replacement for natural gas. Many believe that its greatest potential lies in multi-family homes and commercial buildings, especially in densely populated cities.
- In energy storage: Hydrogen is one of the leading options for renewable energy storage. Hydrogen can be produced via power-to-hydrogen when renewables are abundant, then converted back into energy when energy demand is high and/or renewables are less productive. In addition, hydrogen and ammonia can be used in gas turbines to increase power system flexibility. Ammonia could also be used in coal-fired power plants, for example, to reduce emissions.
Given their zero- or near-zero-emission operation, hydrogen and fuel cells have the potential to significantly reduce greenhouse gas emissions in many use cases.
Challenges of hydrogen
Experts have long agreed that hydrogen can make a significant contribution to energy security in the future and play a viable role in the successful energy transition in some sectors. Numerous pilot projects around the world are already demonstrating this today. However, until this goes beyond the pilot stage, we still have a long way to go. Mankind has not yet achieved to develop production processes for hydrogen that are climate-neutral, energy-efficient and low-cost at once. Still today, the climate harmful gray hydrogen is the most common form in use. Hydrogen storage also remains a challenge: Because it's a highly flammable gas, hydrogen needs to be stored safely, making costly and extensive measures unavoidable – which doesn't make it any more attractive to companies and investors, really, and limits scaling opportunities considerably. Safety concerns seem also to impede or even hinder the mass introduction of transportation vehicles. Additionally, converting hydrogen back into electricity looks at a low energy efficiency. Given that energy is also lost in the production of hydrogen, discussions about energy efficiency and the practical use of hydrogen appear to be ongoing and challenging.
In all of this, the sluggish expansion of renewable energies must also be accelerated, unclear legal situations must be clarified with appropriate regulations, and the still high investment costs must inevitably be reduced. In order for companies to be able to scale up production, additionally, there must be certainty and improvement in the business environment. To achieve all of that, research is focused on reducing the overall cost of hydrogen by improving the efficiency and lifetime of hydrogen production technologies and reducing the cost of capital equipment, operation and maintenance.