Power-to-Gas Energy Storage is booming
Power-to-Gas is the process of converting surplus renewable energy into hydrogen gas by rapid response electrolysis and its subsequent injection into the gas distribution network. One of the most promising energy storage systems to storage renewables into the existing EU gas infrastructure.
The hydrogen produced is injected into the natural gas system to displace natural gas, so reducing greenhouse gas emissions and reliance upon fuel imports.
If the power is derived mainly from renewable power sources, only low-carbon hydrogen will be produced. Thereby the Power-to-Gas (P2G) approach can facilitate a transition from natural gas to a ‘green’ mixed gas by making use of both of the existing energy grids on a global scale.
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
The hydrogen compression and storage system stores up to 27 MWh of energy and increases the overall efficiency of the wind park by tapping into wind energy that otherwise would be wasted.
The second method is to combine the hydrogen with carbon dioxide and convert the two gases to methane using a methanation reaction (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.
- Methanation reaction
In 2014 the EU co-financed HELMETH (Integrated High-Temperature ELectrolysis and METHanation for Effective Power to Gas Conversion) research project started.The objective of the project is the proof of concept of a highly efficient Power-to-Gas technology by thermally integrating high temperature electrolysis with CO2-methanation. The process consists of a pressurized high-temperature steam electrolysis and a pressurized CO2-methanation module which are planned to be coupled in 2016. A methane output of approximately 30 kW (higher heating value) is targeted.
- Biological methanation
The biological methanation combines both processes, the electrolysis of water to form hydrogen and the subsequent CO2 reduction to methane using this hydrogen. During this process, methane forming microorganisms, release enzymes that reduce the overpotential of a non-catalytic electrode (the cathode) so that it can produce hydrogen.
This microbial power-to-gas reaction occurs at ambient conditions, i.e. room temperature and pH 7, at efficiencies that routinely reach 80-100%. However, methane is formed more slowly than in the Sabatier reaction due to the lower temperatures.
- Methanation reaction
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.
The impurities carbon dioxide, water, hydrogen sulfide, and particulates must be removed from the biogas if the gas is used for pipeline storage to prevent damage.
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 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.
The use of the existing natural gas pipelines for hydrogen was studied by the EU NaturalHy project and US DOE.
Reversible solid oxide electrochemical cells
There is potential to increase efficiency of power-to-gas storage. In 2015 a study published in Energy and Environmental Science found that by using reversible solid oxide electrochemical cells and recycling waste heat in the storage process a round-trip efficiency electricity to electricity of more than 70% can be reached at low cost.
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