Operation and design of solar fuel plants involves a decision about the degree of coupling between the solar electricity profile and the plant. Full decoupling needs large scale battery storage to ensure power availability during the night while full coupling requires high conversion capacity during the day to realize the required average methanol production. An extended optimal control framework is presented that determines economic optimal operation. Extended indicates that operational and design degrees of freedom are considered simultaneously. Using a simplified dynamic model of the plant, the framework minimizes total fuel cost for an estimated cost structure by the year 2030. The results show that full coupling is economically preferred and that limited operational flexibility increases the manufacturing cost of methanol from approximately 1000 to 1200 USD/ton. Analysis of the results reveals the cost structure determines an Operational Tipping Point that marks a clear transition from coupled to decoupled operation.
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