Power-to-Gas (PtG) is a technology designed to convert electrical energy, particularly surplus power generated from renewable sources, into a storable and transportable gaseous fuel. This process involves transforming electricity into chemical energy, primarily in the form of hydrogen or synthetic methane. PtG links the electricity grid and the gas network, a concept known as sector coupling. Its development supports the global shift toward cleaner energy systems and the stabilization of electrical grids experiencing a growing reliance on variable power sources.
Power-to-Gas as a Storage Mechanism
The shift to renewable energy sources like solar and wind presents a fundamental challenge: their power generation is intermittent, meaning electricity is often produced when it is not needed. This mismatch between supply and demand can lead to energy waste, a process known as curtailment, where generators must be shut down or excess electricity simply goes unused. Power-to-Gas technology provides a solution by converting this surplus electrical energy into a fuel that can be stored for long durations.
Traditional battery systems are well-suited for short-term energy storage, typically managing fluctuations over minutes or a few hours. However, they are not economically or physically practical for storing massive amounts of energy over weeks or months to manage seasonal variations in renewable output. Gas infrastructure, including salt caverns and depleted gas fields, offers capacity that can be hundreds of times greater than the electrical grid’s storage capacity. PtG leverages this existing large-scale infrastructure by converting the electrical energy into a form that can be stored and dispatched later.
The conversion of electricity into a chemical compound like hydrogen or methane allows for high-volume, long-duration storage that addresses the problem of inter-seasonal energy balancing. This chemical storage offers greater energy density and capacity than electrochemical batteries. By decoupling the time of generation from the time of consumption, PtG ensures that renewable energy can be utilized year-round, not just when the sun is shining or the wind is blowing.
The Core Energy Conversion Steps
The Power-to-Gas process chain begins with the conversion of water into its constituent elements using electricity in a device called an electrolyzer. This initial step, known as electrolysis, involves passing an electric current through water, which splits the molecule into hydrogen (\(\text{H}_2\)) and oxygen (\(\text{O}_2\)). The hydrogen gas produced is the primary energy carrier of the PtG system and is considered “green hydrogen” if the electricity source is renewable.
Several types of electrolyzers exist, including Alkaline, Polymer Electrolyte Membrane (PEM), and Solid Oxide Electrolysis Cells (SOEC), each offering different operational characteristics regarding temperature, pressure, and flexibility. PEM electrolyzers are highly responsive to the fluctuating power input from renewables, while SOEC systems operate at high temperatures, which can increase efficiency if waste heat from downstream processes is captured. The resulting hydrogen can be stored directly or used as a clean fuel in various applications.
In a second, optional step, the hydrogen is converted into synthetic natural gas (SNG), which is chemically identical to methane (\(\text{CH}_4\)). This process, called methanation, reacts the hydrogen with a carbon source, typically carbon dioxide (\(\text{CO}_2\)). The reaction, often utilizing the Sabatier process with a catalyst, transforms the gases into methane and water. This step releases heat, which can sometimes be captured to improve the system efficiency, with some advanced systems achieving efficiencies of up to 86%. The choice to perform methanation is driven by the desire to create a fuel that is fully compatible with the existing natural gas pipelines and end-use appliances.
Integrating Synthetic Gas into Existing Infrastructure
The gaseous products of the PtG process—hydrogen or synthetic natural gas—are designed for seamless integration into the energy network. Synthetic natural gas (SNG) is chemically indistinguishable from fossil natural gas and can be injected directly into the high-pressure gas transmission grid without modifications to the infrastructure. This allows the stored energy to be transported across vast distances and stored in existing underground facilities.
Direct injection of hydrogen is also possible, though it is typically blended with existing natural gas at lower concentrations. Natural gas infrastructure, particularly older steel pipelines, can suffer from hydrogen embrittlement, which restricts the blend ratio in the distribution network, often to a maximum of 5% to 20% by volume. However, the use of SNG bypasses these limitations, providing a drop-in renewable fuel solution.
Once in the gas network, the stored energy can be utilized in multiple sectors. It can be used for residential and industrial heating, replacing fossil fuels in furnaces and boilers. The gas can be used in the transport sector as a fuel for vehicles, or it can be converted back into electricity through a process known as “Gas-to-Power” using efficient gas turbines or fuel cells to meet peak electricity demand.