Power to gas (often abbreviated P2G) is a technology that converts electrical power to a gas fuel.[1] There are currently three methods in use; all use electricity to split water into hydrogen and oxygen by means of electrolysis.
In the first method, the resulting hydrogen is injected into the natural gas grid or is used in transport or industry.[2] The second method is to combine the hydrogen with carbon dioxide and convert the two gases to methane (see natural gas) using a methanation reaction such as the Sabatier reaction, or biological methanation resulting in an extra energy conversion loss of 8%. The methane may then be fed into the natural gas grid. The third method uses the output gas of a wood gas generator or a biogas plant, after the biogas upgrader is mixed with the produced hydrogen from the electrolyzer, to upgrade the quality of the biogas.
Impurities, such as carbon dioxide, water, hydrogen sulfide, and particulates, must be removed from the biogas if the gas is used for pipeline storage to prevent damage.[3]
Power-to-gas systems may be deployed as adjuncts to wind parks or solar-electric generation. The excess power or off-peak power generated by wind generators or solar arrays may then be used at a later time for load balancing in the energy grid. Before switching to natural gas, the German gas networks were operated using towngas, which for 50-60 % consisted of hydrogen. The storage capacity of the German natural gas network is more than 200,000 GW·h which is enough for several months of energy requirement. By comparison, the capacity of all German pumped storage power plants amounts to only about 40 GW·h. The storage requirement in Germany is estimated at 16GW in 2023, 80GW in 2033 and 130GW in 2050.[4] The transport of energy through a gas network is done with much less loss (<0.1%) than in a power network (8%). The storage costs per kilowatt hour are estimated at €0.10 for hydrogen and €0.15 for methane.[5] The use of the existing natural gas pipelines for hydrogen was studied by the EU NaturalHy project[6] and US DOE.[7] The blending technology is also used in HCNG.
Power-to-Gas (PtG) enables the natural gas pipeline network to be used for energy storage, resolving many of the integration issues that plague intermittent renewable energy sources such as wind and solar.
It is well known that finding a solution for scalable energy storage is critical in the pursuit of achieving a renewable energy future. While batteries, pumped-hydro, flywheels and other technologies have their merits, none are able to offer seasonal deep storage at the terawatt scale. Power-to-Gas is an elegant innovation that simply takes excess renewable electricity to create renewable hydrogen and methane for injection into natural gas pipelines or use in transportation. Existing gas pipelines can store hundreds of terawatt hours of carbon neutral methane for indefinite periods of time.
Germany has been pursuing the most aggressive renewable energy targets in the world under their Energiewende program. The Germans have been experiencing challenges in integrating large proportions of wind and solar power into the electric grid because peak power production periods do not correlate with peak demand, so there are sunny afternoons when solar PV is outproducing demand, and likewise windy nights when power production must either be curtailed or exported to neighboring countries at low prices. Adding to the technical challenge is the fact that wind and solar production can spike and drop off very quickly, with little warning, creating inefficiencies as grid managers scramble to match supply with demand.
Many technology pathways are being pursued towards the goal of broad-based energy storage to help meet the challenge of integrating renewables into the power grid. Batteries and flywheels are excellent for rapid discharge and frequency management but are not suitable for long-term storage. Pumped Hydro and Compress Air Energy Storage (CAES) offer longer-term storage but are fundamentally limited by the requirement of favorable geographies. Chemical conversion of electricity to gas allows the existing natural gas pipeline infrastructure to be leveraged for massive-volume, long-term, distributed storage that is cost competitive with other storage technologies. Additionally, the synthetic methane of hydrogen produced via PtG can be utilized as carbon-neutral transportation fuels or elsewhere in industry.
Germany has embraced PtG as a critical component in the Energiewende program. PtG enables German utility operators to manage the gas and power networks in tandem, shifting gas to power and power back to gas as needed throughout the day to match supply and demand. There are 30 PtG plants at various levels of commercial production throughout Germany and neighboring countries.
