Table of Contents
Introduction
Renewable hydrogen and Power to X describe a family of technologies that convert renewable electricity into hydrogen and then, if desired, into other useful energy carriers or products. They form an important bridge between variable renewable electricity and sectors that are difficult to electrify directly, such as heavy industry, long distance transport, and seasonal energy storage.
This chapter explains what renewable hydrogen is, how it is produced, and how Power to X pathways expand the use of renewable electricity beyond the power sector. It focuses on the basic ideas and main applications, without repeating broader topics that are addressed elsewhere in the course.
What Is Renewable Hydrogen
Hydrogen is a chemical element, and as a gas it is a colorless, energy rich fuel. On Earth, hydrogen rarely appears alone and is usually bound in molecules such as water or hydrocarbons. To obtain hydrogen gas, energy must be used to split these molecules. Hydrogen itself is therefore an energy carrier and not a primary energy source.
In energy discussions, hydrogen is often described by colors that indicate how it is produced. Renewable hydrogen usually corresponds to what is called green hydrogen. It is produced using electricity from renewable sources, such as wind, solar, or hydropower, to split water into hydrogen and oxygen. The key feature is that its production does not directly emit carbon dioxide if the electricity input is truly renewable and if no fossil based feedstocks are used.
Renewable hydrogen is important because it can store energy for long periods, can be transported, and can be used as a fuel or as a feedstock in chemical and industrial processes. It can replace hydrogen made from fossil fuels in existing uses and can also open new low carbon pathways in sectors that are hard to decarbonize.
Basics Of Hydrogen Production From Renewables
The main process for producing renewable hydrogen is water electrolysis. In electrolysis, direct current electricity is passed through water, which splits water molecules into hydrogen and oxygen. Pure water has low electrical conductivity, so electrolytes or special membranes are used to enable the process.
In simplified form, the core reaction can be written as:
$$
\text{Water} \rightarrow \text{Hydrogen} + \text{Oxygen}
$$
More precisely, for each molecule of water, the balanced overall reaction is:
$$
2\,H_2O(l) \rightarrow 2\,H_2(g) + O_2(g)
$$
An electrolyzer contains two electrodes, an anode and a cathode, separated by an electrolyte or membrane. At the cathode, hydrogen ions gain electrons to form hydrogen gas. At the anode, water loses electrons to form oxygen gas and positively charged hydrogen ions. The external power supply provides the electrical energy that drives this non spontaneous reaction.
Key idea: Electrolysis converts electrical energy into chemical energy stored in hydrogen, by splitting water according to
$$2H_2O \rightarrow 2H_2 + O_2$$
When the electricity comes from renewable sources, the resulting hydrogen can be considered renewable.
In practice, different electrolyzer technologies exist, such as alkaline electrolyzers, proton exchange membrane (PEM) electrolyzers, and high temperature solid oxide electrolyzers. They differ in operating temperature, materials, typical efficiency, and suitability for following variable renewable power. Although technical details vary, all share the basic principle of using electricity to split water.
The efficiency of electrolysis is the ratio of useful chemical energy stored in the produced hydrogen to the electrical energy consumed. Typical present day system efficiencies fall below 100 percent, which means that some energy is lost as heat. Improving efficiency and lowering costs are major goals in current research and deployment.
Hydrogen As An Energy Carrier
Once produced, hydrogen can be stored, transported, and converted back into useful forms of energy. As an energy carrier, hydrogen has several attractive features:
It has a high gravimetric energy density. Per unit mass, hydrogen contains more energy than many conventional fuels. However, as a gas at ambient conditions it has low volumetric energy density, which means that it takes a large volume to store a given amount of energy. This leads to the need for compression, liquefaction, or chemical storage in other materials.
Hydrogen can be burned in engines or turbines to produce heat and mechanical power. It can also be fed to fuel cells, which convert hydrogen and oxygen directly into electricity and water. Fuel cells enable more efficient and cleaner use of hydrogen compared to simple combustion.
Hydrogen is already widely used in industrial processes such as ammonia production and oil refining. Today, most hydrogen used in these sectors comes from fossil fuels. Replacing this with renewable hydrogen can significantly reduce emissions in existing hydrogen demand. Beyond these current uses, hydrogen can provide a link between the power sector and other sectors where direct electrification is difficult.
What Is Power To X
Power to X is a broad concept that refers to converting electrical power, usually from renewable sources, into other energy carriers, fuels, or chemical products. The X stands for different possible outputs, such as gas, liquid fuels, heat, or materials. The common feature is that electricity acts as the starting point for producing something that can be stored, transported, or used in a different sector.
Renewable hydrogen is central to many Power to X pathways, because hydrogen often serves as an intermediate product. First, renewable electricity is used to produce hydrogen via electrolysis. Then, hydrogen can be used directly or converted into other molecules by combining it with carbon or nitrogen that come from sustainable or recycled sources.
Power to X is particularly attractive when there is excess renewable electricity that would otherwise be curtailed. By converting this surplus electricity into other forms, Power to X can increase the overall utilization of renewable resources and help balance the power system.
Power To Gas And Synthetic Methane
One important branch of Power to X is Power to Gas. In this pathway, renewable electricity is used to produce hydrogen. This hydrogen can be injected into existing gas networks in limited proportions or further processed to create gases that are compatible with current gas infrastructures.
A frequent target product is synthetic methane. Methane is the main component of natural gas. To produce synthetic methane, hydrogen is combined with carbon dioxide in a process called methanation. The general chemical reaction for methanation can be written as:
$$
CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O
$$
Here, carbon dioxide and hydrogen react to form methane and water. If the hydrogen is renewable and the carbon dioxide is captured from biogenic sources or from the atmosphere, the resulting methane can be considered a low carbon or nearly carbon neutral gas.
Key relation: In Power to Gas methanation, renewable hydrogen and carbon dioxide can be combined according to
$$CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O$$
This allows production of synthetic methane that can use existing gas pipelines and storage.
Synthetic methane can be injected into existing gas grids, stored in underground facilities, and used in gas fired power plants or heating systems. This can extend the value of gas infrastructure into a low carbon future. However, each step conversion from electricity to hydrogen, then to methane, and later back to heat or electricity involves efficiency losses. Managing these losses and choosing appropriate uses is an important part of Power to Gas planning.
Power To Liquids And E Fuels
Power to Liquids refers to pathways where renewable electricity ultimately yields liquid fuels. These fuels are often called synthetic fuels, electrofuels, or e fuels. They can be used in existing engines, vehicles, ships, and aircraft with little or no modification, which makes them attractive for sectors where liquid fuels are hard to replace.
The general concept of Power to Liquids involves producing renewable hydrogen first, then reacting it with carbon containing feedstocks to form liquid hydrocarbons or alcohols. Several routes exist. One example is to produce a synthesis gas mixture, which contains hydrogen and carbon monoxide, and then convert it into longer chain hydrocarbons using catalytic processes.
Although specific reaction chains vary, the overall idea is that hydrogen provides energy and reducing power, while carbon from sustainable sources gives the structure of the fuel molecules. If the carbon is captured from the atmosphere or from biogenic sources, the resulting fuel can be close to carbon neutral when considering the full cycle of emissions and uptake.
Power to Liquids fuels can include synthetic diesel, gasoline like substances, kerosene for aviation, or methanol. These e fuels are particularly relevant for long distance aviation and shipping, where high energy density liquids are still the most practical option. Their production is more complex and energy intensive than direct electrification, but they provide flexibility where other solutions are not yet feasible.
Power To Chemicals And Materials
Beyond fuels, Power to X can supply basic chemicals and materials using renewable electricity. This is often called Power to Chemicals. Many chemicals that underpin modern industry, such as ammonia, methanol, and synthetic hydrocarbons, currently rely on fossil fuels both as a source of energy and as a carbon feedstock.
With renewable hydrogen, ammonia can be produced without fossil fuels as the hydrogen source. Ammonia synthesis combines nitrogen, usually taken from the air, with hydrogen. Traditionally, hydrogen for ammonia production comes from natural gas. By switching to hydrogen from electrolysis, the process can be largely decarbonized.
Another example is methanol, which can be synthesized from hydrogen and carbon containing feedstocks. Renewable pathways for methanol can supply a building block for many chemicals and also serve as an energy carrier or fuel.
Power to Chemicals can, in principle, be extended to many carbon based products. The central idea is that renewable electricity replaces fossil energy in the production process, and renewable hydrogen plus captured carbon replace fossil feedstocks. This has the potential to significantly reduce emissions across the chemical industry, which is one of the most energy and carbon intensive sectors.
Sector Coupling And System Integration
Renewable hydrogen and Power to X are important tools for coupling different parts of the energy system. Electricity, gas, heat, transport, and industry are often considered separate sectors, but Power to X technologies provide physical links that allow energy and carbon to flow between them.
For example, surplus wind or solar power can be used to produce hydrogen. That hydrogen can support industrial processes, fuel fuel cell vehicles, or be converted into synthetic gas that helps balance seasonal heating demand. In emergencies or peak demand situations, stored hydrogen or derived fuels can be used to generate electricity again, contributing to flexibility and resilience.
Sector coupling through Power to X can help integrate a high share of variable renewable electricity into the overall energy system. It broadens the range of uses for renewables beyond the power sector and provides additional options for decarbonization in transport, heating, and industry. However, it brings the challenge of coordinating infrastructure, ensuring that hydrogen production, storage, transport, and use develop in a coherent way.
Efficiency And Energy Losses In Power To X Pathways
Every time energy is converted from one form to another, some portion is lost as waste heat or in other losses. Power to X pathways often involve several conversion steps, for example from renewable electricity to hydrogen, then from hydrogen to another fuel, and finally from that fuel to useful work or heat.
Electrolysis itself has an efficiency below 100 percent. Additional steps such as compression, liquefaction, or chemical synthesis also consume energy. When hydrogen or synthetic fuels are later used in engines or turbines, further losses occur. Fuel cells can provide relatively high conversion efficiency back to electricity, but overall chains still involve cumulative losses.
For these reasons, Power to X is usually most valuable in applications where direct use of electricity is difficult or impossible. Whenever a process can be electrified directly with high efficiency, such as electric motors for many vehicles or heat pumps for buildings, this often uses less total energy than creating and using hydrogen based fuels for the same purpose.
Guiding principle: Because each energy conversion step reduces total efficiency, Power to X is best reserved for sectors and applications that are hard to electrify directly, while direct electrification should be preferred where it is practical.
Understanding these efficiency aspects helps in making informed choices about where to apply Power to X solutions in a sustainable and economical way.
Potential Benefits And Key Challenges
Renewable hydrogen and Power to X offer several potential benefits for a sustainable energy system. They can provide low carbon fuels for difficult sectors like heavy industry, shipping, and aviation. They can help store renewable energy over long periods, stabilize the grid by absorbing excess electricity, and use existing infrastructure in new low carbon ways.
They also open pathways to decarbonize current uses of hydrogen and chemical production that now rely on fossil fuels. By enabling sector coupling, they contribute to a more integrated and flexible energy system, which is important as the share of variable renewables increases.
At the same time, key challenges remain. Costs of electrolyzers, renewable electricity, and associated infrastructure need to fall further to make renewable hydrogen widely competitive. Building new pipelines, storage facilities, or conversion plants, or adapting existing ones, requires investment and careful planning. Ensuring that electricity used for hydrogen is truly renewable is essential, otherwise total emissions can remain high.
There are also questions about where to source the carbon for synthetic fuels, how to avoid locking in inefficient or high emission practices, and how to align Power to X development with broader climate and sustainability goals. Addressing these challenges will shape how renewable hydrogen and Power to X contribute to the future energy system.