Sustainability assessment of renewable energy-based hydrogen and ammonia pathways

Although renewable energy resources are more commonly used in the near future, there arises a requirement of integrating alternative storage options. Storing energy in terms of chemicals presents a promising approach for several reasons such as transportability, minimal losses and storage practices. In this aspect, hydrogen and ammonia are two of the carbon-free energy carriers, which can be used for energy storage purposes. Currently, hydrogen, and ammonia are dominantly produced by steam reforming of natural gas in the world. Hydrogen can also be produced through electrolysis or electrochemical processes. The Haber–Bosch process is the most common method to produce ammonia, whereas there are recently new alternatives to reduce the required temperature and pressure of the reaction through electrochemical cells. Ammonia as a sustainable fuel can be used in several kinds of combustion engines, gas turbines, burners with only small modifications and directly in fuel cells which is a very significant advantage compared to other types of fuels. In this work emerging hydrogen and ammonia production pathways, namely electrolysis and photoelectrochemical routes, are assessed in terms of sustainability by considering their (1) efficiencies, (2) environmental impact and (3) cost. Electrolysis is performed using either hydropower or wind power routes, whereas solar energy is used for photoelectrochemical hydrogen and ammonia production. In addition, the obtained results are compared with the conventional steam methane reforming method to signify the importance of emerging options.