In his section, the category chemical energy storage addressed refers to energy that is stored in the form of chemical fuels to later be converted into mechanical, thermal or electrical energy for industrial and grid applications. Covering primarily power-to-gas technologies that use electricity to produce a gaseous fuel that stores energy. (Source: netl.doe.gov, sciencedirect.com)
Hydrogen energy storage (Power-to-hydrogen P2H2)
Hydrogen energy storage is a chemical energy storage technology consisting of converting surplus electricity generated by renewables during low energy demand periods into hydrogen through electrolysis. This energy can be stored and released again by using the hydrogen as a fuel for power generation in a fuel cell or in a combustion engine. (Source: fchea.org, bbc.co.uk)
Electrolysis is a chemical decomposition process in which an electrical current is passed through a chemical solution (that is water in the case of hydrogen production) in order to separate and extract certain elements. Main existing electrolysis technologies for hydrogen production are: Alkaline electrolysis, Proton Exchange Membrane (PEM) electrolysis and Solid Oxide Electrolysis (SOE). (Source: fchea.org, bbc.co.uk, renewableh2.org)
Alkaline electrolysis
Alkaline electrolysis is the most mature and simplest technology of the three, it also is the least expensive, most time-tested, and currently more efficient and most used in large-scale applications compared to the other commercial electrolysis technologies. Alkaline electrolyzers introduce an electric current through water containing an alkaline chemical catalyst such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) to produce hydrogen. (Source: renewableh2.org, sciencedirect.com)
Proton Exchange Membrane (PEM) electrolysis
PEM water electrolysis technology is based upon a solid polymer electrolyte membrane that can pass protons. When electrical current is applied to the electrolyzer, water fed to the anode (or oxygen electrode) is oxidized to oxygen and protons and electrons are generated. The protons (H+ ions) pass through the PEM to the cathode (or hydrogen electrode), where they meet electrons from the other side of the circuit, and are reduced to hydrogen gas. PEM electrolysis technology can be used in commercial, small- and medium-scale applications (< 300 kW) (Source: nelhydrogen.org, irena.org)
The conversion efficiency of both Alkaline and PEM electrolysis technologies is about 65%~70% (lower heating value). (Source: energystorage.org)
Solid Oxide Electrolysis (SOE)
Solid oxide electrolysis (SOE) technology is still in the research and development phase. In contrast to Alkaline and PEM electrolysis that use electric power as a source of energy to split water molecules, SOE relies on a combination of electric energy and heat, which makes it more advantageous as heat is generally less expensive to generate and store than electric power. (Source: renewableh2.org)
The following figure illustrates the functioning principle of each one of the previously described electrolysis technologies.
Hydrogen storage
Hydrogen can be stored physically, either as a gas or a liquid. When stored as a gas, high-pressure tanks of about 350–700 bar are required. Whereas when stored as a liquid, cryogenic temperatures are necessary since the boiling point of hydrogen at one atmosphere pressure is −252.8°C. Hydrogen can also be stored on the surfaces of solids (by adsorption) or within solids (by absorption). (Source: energy.gov)
Currently, many power-to-gas projects emerging in Germany and other European countries, most of which employ alkaline or PEM electrolyzers. Large scale hydrogen storage has already operated for several years in two locations: the UK and the USA, but a wider use in the context of fluctuating wind generation is still not realized. (Source: ease-storage.eu)
Power-to-Methane (P2M)
Power-to-Methane is a technology that converts energy, mainly electricity, from hydrogen produced through electrolysis and carbon dioxide that is captured from a flue gas via post-combustion capture, into methane through a chemical reaction called methanation, (as shown in the below figure). The resulting product is a type of synthetic natural gas or substitute natural gas (SNG), also known as e-gas. It shares the same properties as natural gas and can be used, stored and transported in the exact same way. If the electrolysis is powered by renewable energy and the methanation uses carbon capture, the resulting gas is a carbon-neutral energy source. (Source: man-es.com, ease-storage.eu)
Methane (CH4) is an interesting storage molecule since its volumetric energy density is about three times higher than the volumetric energy density of gaseous hydrogen at identical pressures. Methane production from syngas by hydrogenation of CO and CO2 using a nickel catalyst, reactions can be given as follows: (Source: backend.orbit.dtu.dk)
Power to methane concept is already implemented. It increases the e-gas contributions, enhances environmentally friendly long-distance mobility, fills CNG-Stations and offers a supply for the public electricity grid. (Source: ease-storage.eu)
Power-to-ammonia
Power to ammonia technology consists of converting both hydrogen that is produced by water electrolysis and nitrogen that is separated from air through an Air Separation Unit into ammonia (as shown in the following figure). The efficiency of the process can go up to 50-55%. (Source: ease-storage.eu)
Power-to-methanol
Power to ammonia technology consists of converting both hydrogen that is produced by water electrolysis and carbon dioxide that is captured from a flue gas via post-combustion capture into methanol by using a catalytic reactor. A distillation process located downstream from the methanol conversion process ensure the required product quality and an additional high temperature heat pump uses the waste heat generated from the electrolyzer to increase overall efficiency that can reach 50-55%. (Source: ease-storage.eu)
Power-to-gasoline
Power to gasoline technology consists of converting the methanol produced as described above into gasoline. (Source: ease-storage.eu)
The following figure illustrates power to methanol and power to gasoline production principle.
