(374f) A Comparison of Solar Methanol and Ammonia Production Explicitly Accounting for Intermittency

The combustion of fossil fuels in the transport sector contributes to anthropogenic CO₂ emissions and climate change. Sustainable fuel concepts are needed to replace established fuels and move toward a sustainable transport sector. Ways to decarbonise transport are electric vehicles (EVs), hydrogen fuel cell vehicles (HFCVs), biofuels, and carbon-neutral synthetic fuels (CNSF). While EVs are making strong advances in short-distance and personal transport, CNSFs may be needed for decarbonising long-distance and heavy goods vehicles. Among CNSFs, carbon-based fuels such as methanol have received considerable attention. However, in a post-fossil economy the prevalence of carbon is not a given. Nitrogen-based fuels like ammonia should be carefully examined. Not only is CO₂ more dilute in air, its reduction is more energetically expensive compared to that of N₂. Furthermore, sustainable fuels are required to be produced using renewable energy. When the energy source of the process is intermittent, as is the case with wind, solar, and tidal energy, intermittency needs to be taken into account in the design of the process. When the analysis of sustainable fuels only includes the synthesis plant assuming fixed cost and availability for the building blocks, the cost of intermittency is neglected and importance of flexibility in processes is ignored.

We developed a framework for the comparison of sustainable fuel production from air, water, and renewable energy. We perform simultaneous design and scheduling of sustainable fuel production including renewable energy source, air separation, electrolysers, and synthesis. All power requirements have to be met by the renewable energy source; heat requirements can be met via heat integration, residual heat requirements are met using hydrogen. In our work, we compare solar methanol and ammonia in London and Dubai with regard to process design and operation as well as cost.

Our results show that the optimal route depends on the location. In London, where the solar pattern exhibits high seasonality, the route requiring the least amount of gas storage is optimal. In Dubai, with lower seasonality, the most energy efficient route, requiring the least amount of PV and electrolyser capacity, is the optimal choice. Figure 2 shows the optimal routes for methanol.

We further determined that nitrogen-based fuels may be competitive with carbon-based fuels. When taking into account the energy needed to separate CO₂ or N₂ from air, it becomes apparent that the production of nitrogen-based fuels requires less energy and hence may be less costly. Our results support that carbon’s dominance in fuels may not prevail throughout the decarbonisation of transport.

We quantify the cost of intermittency to be as high as two thirds of the total cost. Figure 3 shows the cost of intermittency in London and Dubai for methanol and ammonia. This implicates that for CNSF, specifically solar fuels, the intermittency needs to be taken into account when making cost predictions. The value of flexibility of processes is accordingly high. Operating the synthesis plant flexibly reduces the seasonal storage needed and with it the overall cost. This implies that novel fuel production processes, such as one-step synthesis like photocatalytic ammonia production, or more flexible processes, may have advantages over established production routes even if they have higher individual CAPEX.