(212d) A Generalized Framework for the Assessment of Solar Fuels Technologies

Reducing our greenhouse gas emissions, while improving the global standard of living, is one of the fundamental challenges of the 21^st century. One option to reduce greenhouse gas emissions is the conversion of carbon dioxide and water into fuels and chemicals in a solar refinery. Unfortunately, there are many technological challenges that must be overcome in terms of capturing and sourcing the feedstocks (namely CO₂, H₂O, and solar energy) and in catalytically converting CO₂ and H₂O to realize such a solar refinery.^1-5

To assess these routes and the technologies involved, we developed a general process model for a CO₂ conversion process, which consists of five subsystems: A) CO₂ capture from source (i.e. flue gas) and transportation to solar refinery, B) CO₂ conversion using solar energy, C) vapor and liquid separation system, D) vapor product purification system, and E) liquid product purification system.⁶ Several approaches are considered for the solar fuel production including solar driven electrochemical CO₂ reduction, photo-catalytic CO₂ reduction and CO₂ catalytic conversion using solar-derived hydrogen (e.g. methanol synthesis for CO₂ and H₂⁷).  Based on the material and energy balances, we developed an energy model to assess the overall energetic feasibility of the process, and we developed an economic model based on discounted cash flow analysis to calculate the minimum selling price of the product. These general models were employed to study a specific case where methanol is the final product of CO₂conversion. We performed sensitivity analysis on the specific case study to identify the key separation(s), catalysis, and solar energy utilization challenges for improving the overall economic feasibility of the solar to chemical energy process. From the analysis, we found that the reaction selectivity and conversion must be significantly improved to make the process feasible.

As there are a wide variety of nascent technologies for the conversion of CO₂ to fuels, our initial model of the CO₂ conversion subsystem was based on a few high-level metrics, including conversion and selectivity. However, as the CO₂ conversion subsystem was identified as one of the main process areas that requires improvement, we have adapted our general process model to consider several specific technologies, including photocatalysis and PV-electrolysis. Using these modified process models, we identify the trade-offs between these technologies and the key improvements necessary (e.g. catalyst activity, solar energy conversion efficiency) for each technology towards improving the economic feasibility of the solar refinery. 

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