Global warming potential of hydrogen and methane production from renewable electricity via power-to-gas technology

Power-to-gas technology enables storage of surplus electricity from fluctuating renewable sources such as wind power or photovoltaics, by generating hydrogen (H₂) via water electrolysis, with optional methane (CH₄) synthesis from carbon dioxide (CO₂) and H₂; the advantage of the latter is that CH₄ can be fed into existing gas infrastructure. This paper presents a life cycle assessment (LCA) of this technological concept, evaluating the main parameters influencing global warming potential (GWP) and primary energy demand.

The conducted LCA of power-to-gas systems includes the production of H₂ or CH₄ from cradle to gate. Product utilization was not evaluated but considered qualitatively during interpretation. Material and energy balances were modeled using the LCA software GaBi 5 (PE International). The assessed impacts of H₂ and CH₄ from power-to-gas were compared to those of reference processes, such as steam reforming of natural gas and crude oil as well as natural gas extraction. Sensitivity analysis was used to evaluate the influence of the type of electricity source, the efficiency of the electrolyzer, and the type of CO₂ source used for methanation.

The ecological performance of both H₂ and CH₄ produced via power-to-gas strongly depends on the electricity generation source. The assessed impacts of H₂ production are only improved if GWP of the utilized electricity does not exceed 190 g CO₂ per kWh. Due to reduced efficiency, the assessed impacts of CH₄ are higher than that of H₂. Thus, the environmental break-even point for CH₄ production is 113 g CO₂ per kWh if utilized CO₂ is treated as a waste product, and 73 g CO₂ per kWh if the CO₂ separation effort is included. Electricity mix of EU-27 countries is therefore not at all suitable as an input. Utilization of renewable H₂ and CH₄ in the industry or the transport sector offers substantial reduction potential in GWP and primary energy demand.

H₂ and CH₄ production through power-to-gas with electricity from renewable sources, such as wind power or photovoltaics, offers substantial potential to reduce GWP and primary energy demand. However, the input of electricity predominately generated from fossil resources leads to a higher environmental impact of H₂ and CH₄ compared to fossil reference processes and is not recommended. As previously bound CO₂ is re-emitted when CH₄ is utilized for instance in vehicles, the type of CO₂ source and the allocation method have a significant influence on overall ecological performance.