Reducing our greenhouse gas emissions, while improving the global standard of living, is one of the fundamental challenges of the 21st 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.
1. N. S. Lewis and D. G. Nocera, Proceedings of the National Academy of Sciences, 2006, 103, 15729-15735.
2. G. Centi, E. A. Quadrelli and S. Perathoner, Energy & Environmental Science, 2013, 6, 1711-1731.
3. A. Steinfeld, Solar Energy, 2005, 78, 603-615.
4. B. A. Pinaud, J. D. Benck, L. C. Seitz, A. J. Forman, Z. B. Chen, T. G. Deutsch, B. D. James, K. N. Baum, G. N. Baum, S. Ardo, H. L. Wang, E. Miller and T. F. Jaramillo, Energy & Environmental Science, 2013, 6, 1983-2002.
5. S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano, 2010, 4, 1259-1278.
6. J. A. Herron, J. Kim, A. A. Upadhye, G. W. Huber and C. T. Maravelias, Energy & Environmental Science, 2015, 8, 126-157.
7. O. S. Joo, K. D. Jung, I. Moon, A. Y. Rozovskii, G. I. Lin, S. H. Han and S. J. Uhm, Industrial & Engineering Chemistry Research, 1999, 38, 1808-1812.