A technical, economic and environmental analysis of renewable gas produced from power to gas systems

Storing surplus or curtailed renewable electricity as a gaseous transport fuel is a suggested method of reducing greenhouse gas emissions, increasing the supply of indigenous energy, providing long-term energy storage, facilitating intermittent renewable electricity sources and providing an advanced source of renewable transport fuel. The technology involved is Power to Gas (P2G) which uses electricity to split water into hydrogen (H2) and oxygen (O2) through electrolysis (power to hydrogen). Subsequently power to methane involves combining the H2 with carbon dioxide (CO2) to produce methane (CH4) via the Sabatier reaction (4H2 + CO2 = CH4 + 2H2O). Storage of renewable electricity as CH4 allows for long term storage by utilising the existing natural gas grid infrastructure. The aim of this thesis is to evaluate the viability of renewable gas produced from P2G utilised as an upgrading system for a biogas plant by assessing the technical, economic, and environmental aspects of such a system. Various methods were applied in this thesis: a linear additive model was used for sustainability assessment; an Excel model and Superpro designer software were applied for technology and costs analysis; and GaBi software was used to assess environmental impact. The result of the sustainability assessments of large scale energy storage technologies (P2G, pumped hydroelectric storage (PHES), and compressed air energy storage (CAES)) indicated the benefits of P2G as a dynamic decentralised mechanism which facilitated long term storage in the natural gas grid and facilitated change in energy vector from electricity to gas and subsequent availability for renewable heat and transport. The potential resource of CO2 from biogas in Ireland was assessed at 430 Mm3 per annum if all potential feedstocks for anaerobic digestion are utilised; this required a resource of 7,654 GWhe to be consumed to produce H2 to react with the CO2 in the biogas. The production costs of renewable gas from several feedstocks were calculated. These varied (depending on system inputs such as cost of electricity and feedstocks) between €1 and €2.5/m3 of renewable methane. The recast Renewable Energy Directive (RED) requires transport biofuels to effect a 70% GHG saving on a whole life cycle analysis when compared to the fossil fuel displaced. Our work shows that for power to gas to be sustainable as an upgrading method the maximum GHG emission of electricity used to produce the hydrogen should be less than 25.7 CO2eq/MJ