Fuel production from CO2 using solar-thermal energy: system level analysis

Sunshine to Petrol (S2P) is a technology framework using a concentrated solar energy source and energy depleted CO2 and water feedstocks for producing liquid hydrocarbon fuels as sustainable alternatives to vulnerable and limited supplies of conventional petroleum. S2P encompasses numerous design configurations that integrate several unit operations to thermochemically convert CO2 and water to a final energized marketable product. In an earlier paper, hereafter referred to as Paper I, we established both a baseline system design and a methodology for evaluating system efficiencies, economics, and lifecycle impacts. Therein we demonstrated that design details of the balance of system following the initial solar to chemical conversion could have significant impact on full system efficiencies, which largely determine both economics and the lifecycle. Here we assess and compare results from three types of choices in the system configuration: the initial solar to chemical conversion, separations, and the final product. Each design option begins with CO2 capture. Options A-C differ in the initial solar splitting: (A) splitting CO2, (B) splitting H2O and (C) splitting both CO2 and H2O. Significantly, we find that splitting both has notable advantages over splitting just one, in efficiency and consequently in derived minimum selling price (MSP) of a methanol product. Option D splits both but replaces the methanol end-product with likely higher value Fischer Tropsch (FT) liquids. The production of the FT end product comes with a small decrease in solar to fuel energy efficiency (similar to 3.5% relative decrease from option C) and a small relative increase in the energy equivalent MSP (similar to 5%). Importantly, we find that in all options, the primary contributor to MSP is the cost of capital for the solar thermochemical sub-system (including the solar collectors) and not in the balance of system components or operating costs. The advantages of options C and D, over the baseline A, stem primarily from the decrease in CO2 to recover and recycle, motivating changing the separation component and replacing conventional and mature MEA-based CO2 separations with a technology to recover the minor component, CO. Of significance, we find that the choice of separations can yield considerable system benefits; for example in option F, splitting both CO2 and H2O, and separating CO from the produced CO2/CO mix, the system efficiency increases by 10% relative to option C for a resource-efficient full system solar to liquid fuel energy efficiency of 12.9%, and the MSP decreased by 18%. Motivated to determine if attractive economics are plausible and to identify the largest opportunities to reduce cost, we show results of a sensitivity analysis for key system and economic parameters. Finally, we construct alternate scenarios that consist of reductions in the most sensitive parameters, which include: solar utility prices, the solar dish-CR5 price, and the interest rate. The most optimistic but plausible parameter set yields an encouraging MSP for methanol of USD 4.24 per GGE (gallon of gasoline energy equivalent). The promising configuration splits both CO2 and H2O, separates the minority component CO, reduces solar derived utility costs an anticipated 60%, achieves 20% reductions in estimated manufacturing cost of the dish/CR5 component, and obtains a favorable 6.4% interest rate.