Mitigation of carbon dioxide (CO₂) concentration in the atmosphere can be accomplished by either removal of CO₂ from the atmosphere or avoiding emissions of CO₂ into it. Water electrolysis is a potential method for coping with the characteristic intermittency of renewable energy but it faces the fundamental problems of hydrogen storage and distribution. The use of zero-carbon electrolytic hydrogen may be facilitated by synthesising a hydrocarbon compound of higher energy density. A number of prospective processes were conceived and analysed in this study; at the heart of each was either a chemical or an electro-chemical reactor, in which zero-carbon Hydrogen could undergo a catalytic reaction with atmospheric CO₂ to form a desired hydrocarbon product.

After an initial screening of the prospective processes, a small-scale system was selected for detailed evaluation as a case study. The process was conceived for the production of methanol for use as a liquid fuel for one average vehicle (25MWh/year). It consisted of hydrogen production by PEM electrolyser using renewable electricity, atmospheric CO₂ capture by liquid solvent and the catalytic chemical synthesis of methanol.

It was estimated that the required production of 4,000kg/year of methanol would need a 6kW water electrolyser and a total energy input of 77.5MWh/year, which implies an overall energy efficiency of around 32% for the process. The operating costs analysis of the process showed a very strong dependence on the cost of electricity; an operating cost of 1.04US$/kg of methanol was found based on an electricity price of 0.07US$/kWh and 1.6US$/tonneH₂O.