We propose an optimization-based framework for the synthesis of systems that are subject to variability in two frequencies. We use “scenarios” and “modes” to represent seasonal and daily variability respectively. We construct a multimode superstructure to consider different flowsheets alternatives for each mode. We formulate the optimization problem as a two-stage multimode stochastic programming approach with embedded models in each scenario. We illustrate the applicability of our framework through the design of concentrating solar power (CSP) plants.