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 (H-2) via water electrolysis, with optional methane (CH4) synthesis from carbon dioxide (CO2) and H-2; 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 H-2 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 H-2 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 H-2 and CH4 produced via power-to-gas strongly depends on the electricity generation source. The assessed impacts of H-2 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 H-2. 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 H-2 and CH4 in the industry or the transport sector offers substantial reduction potential in GWP and primary energy demand. H-2 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 H-2 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.