Power‐to‐gas is a storage technology aiming to convert surplus electricity from renewable energy sources like wind and solar power into gaseous fuels compatible with the current network infrastructure. Results of CO2 dissociation in a vortex‐stabilized microwave plasma reactor are presented. The microwave field, residence time, quenching, and vortex configuration were varied to investigate their influence on energy‐ and conversion efficiency of CO2 dissociation. Significant deterioration of the energy efficiency is observed at forward vortex plasmas upon increasing pressure in the range of 100 mbar towards atmospheric pressure, which is mitigated by using a reverse vortex flow configuration of the plasma reactor. Data from optical emission shows that under all conditions covered by the experiments the gas temperature is in excess of 4000 K, suggesting a predominant thermal dissociation. Different strategies are proposed to enhance energy and conversion efficiencies of plasma‐driven dissociation of CO2.

Abstract.A storage scheme for Renewable Energy (RE) based on the plasmolysis of CO 2 into CO and O 2 has been experimentally investigated, demonstrating high energy efficiency (>50%) combined with high energy density, rapid start-stop and no use of scarce materials. The key parameter controlling energy efficiency has been identified as the reduced electric field. Basic plasma parameters including density and temperature are derived from a simple particle and energy balance model, allowing parameter specification of an upscale 100 kW reactor. With RE powered plasmolysis as the critical element, a CO 2 neutral energy system becomes feasible when complemented by effective capture of CO 2 at the input and separation of CO from the output gas stream followed by downstream chemical processing into hydrocarbon fuels. Renewable energy and the need for storageWhile Renewable Energy (RE) sources including solar photo-voltaic, concentrated solar and wind make up an increasingly large fraction of the EU energy supply mix, the development of a suitable energy storage scheme has lagged behind. The need for storage is evident from the ill-matched supply and demand characteristics of RE both geographically and temporarily. This already causes problems in transport and uptake of RE, in particular the handling of excess wind power by the electricity grid [1]. In order to meet base load demand the presently installed wind power capacity is vastly under-employed whilst peak power handling requires over capacity on the grid, driving up total cost of the RE system. Storage capacity needed at EU level to bridge one day of electrical power demand is of the order of 10 TWh. By comparison, EU hydropower storage capacity is approximately 15 TWh. The only way to meet EU energy storage requirements over several days would be chemical storage [2].In order to meet the EU 2020 energy and climate change targets and to follow the EU 2050 Energy Roadmap, it is necessary to reduce CO 2 emission and to move away from fossil fuel altogether. However, a RE driven system could still employ non-fossil hydro-carbon based fuel, provided the CO 2 emitted into the atmosphere is recaptured and recycled. One such scheme is power to gas (P2G), in which excess electricity generated by wind or solar power is converted and stored into hydro-carbon-based (solar) fuels. Naturally, for being CO 2 neutral, the CO 2 emitted by burning these solar fuels must be a Corresponding author: a.p.h.goede@differ.nl This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.A storage scheme for Renewable Energy (RE) based on the plasmolysis of CO 2 into CO and O 2 has been experimentally investigated, demonstrating high energy efficiency (>50%) combined with high energy density, rapid start-stop and no use of scarce materials. The key parameter controlling energy efficiency has been identified as the reduced electric field. Basic plasma parameters including density and temperature are derived from a simple particle and energy balance model, allowing parameter specification of an upscale 100 kW reactor. With RE powered plasmolysis as the critical element, a CO 2 neutral energy system becomes feasible when complemented by effective capture of CO 2 at the input and separation of CO from the output gas stream followed by downstream chemical processing into hydrocarbon fuels. Renewable energy and the need for storageWhile Renewable Energy (RE) sources including solar photo-voltaic, concentrated solar and wind make up an increasingly large fraction of the EU energy supply mix, the development of a suitable energy storage scheme has lagged behind. The need for storage is evident from the ill-matched supply and demand characteristics of RE both geographically and temporarily. This already causes problems in transport and uptake of RE, in particular the handling of excess wind power by the electricity grid [1]. In order to meet base load demand the presently installed wind power capacity is vastly under-employed whilst peak power handling requires over capacity on the grid, driving up total cost of the RE system. Storage capacity needed at EU level to bridge one day of electrical power demand is of the order of 10 TWh. By comparison, EU hydropower storage capacity is approximately 15 TWh. The only way to meet EU energy storage requirements over several days would be chemical storage [2].In order to meet the EU 2020 energy and climate change targets and to follow the EU 2050 Energy Roadmap, it is necessary to reduce CO 2 emission and to move away from fossil fuel altogether. However, a RE driven system could still employ non-fossil hydro-carbon based fuel, provided the CO 2 emitted into the atmosphere is recaptured and recycled. One such scheme is power to gas (P2G), in which excess electricity generated by wind or solar power is converted and stored into hydro-carbon-based (solar) fuels. Naturally, for being CO 2 neutral, the CO 2 emitted by burning these solar fuels must be a Corresponding author: a.p.h.goede@differ.nl This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Microwave plasmas at atmospheric pressure are used for surface treatments like for example cleaning, sterilization or decontamination purposes, for a pre-treatment to increase the adhesion of lacquer, paint, or glue, and for the deposition of different kind of layers and coatings. Micro plasma jets can also be applied for biomedical applications and for treatment of small and complex geometries like for example the inside of capillaries. Larger plasma torches which exhibit higher gas temperatures can also be used for chemical syntheses like waste gas decomposition, methane pyrolysis, or carbon dioxide dissociation and for plasma spraying purposes. In the present publication an overview on the development and the investigation of the operating principle of two atmospheric pressure microwave plasma torches at frequencies of 2.45 GHz and 915 MHz will be presented. The plasma sources are based on a cylindrical resonator combined with coaxial structures. To explain how these plasma sources work, simulations of the electric field distribution will be discussed. Furthermore, some physical characteristics of an air and an Ar/H2 atmospheric plasma like gas temperatures, excitation temperatures and densities as well as the heating of the plasma by the microwave will be investigated.

There are very specific demands on the plasma processes used in various plasma technological applications. Microwave plasmas offer a wide range of applications for different pressures ranging from very low pressure (<0.1 Pa) over low pressure (0.1‐100 Pa) and medium pressure (103‐104 Pa) up to atmospheric pressure (105 Pa). This contribution is a short review on some microwave based plasma sources at different pressure ranges and a brief introduction into the plasma physics behind them (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)