Integrated physical design, control design, and site selection for an underwater energy-harvesting kite system

This paper presents a co-design framework that optimizes the kite design, site, and controller of a kite-based marine hydrokinetic (MHK) energy-harvesting system. The formulation seeks to maximize a techno-economic metric, namely power-to-mass ratio, by simultaneously considering three key categories of decision variables while accounting for the coupling between the three. The simultaneous consideration presents computational challenges associated with optimizing a large number of decision variables, a subset of which (control variables) are time trajectories. The multi-fidelity co-design formulation presented in this work utilizes two techniques, namely nesting and layering, to solve the optimization problem in a computationally tractable manner without significantly compromising on accuracy. Specifically, nesting allows for efficient integration of the three optimization sub-modules into one integrated framework without accuracy losses, whereas layering allows for successive design space reduction as the overall optimization progresses from using a low-fidelity model to using a higher-fidelity model. The resulting integrated co-design tool was applied to a region of interest off the North Carolina coast to optimally choose a combination of deployment site, kite design, and control strategy. We show that the integrated co-design tool results in a two-fold performance improvement over benchmarks derived from sequential (or independent) optimization of the kite categories, thereby underscoring the need for co-design. Computational effectiveness is demonstrated by comparing the computational cost of the nested and layered approach against the estimated computational costs that would be required to perform a single high-fidelity integrated optimization over the entire design space.