In this paper we present a novel system for power electronics cooling consisting of a thermosyphon loop composed of an evaporator, to which the power modules are attached, and of a condenser, formed by a set of vertically mounted aluminum panels. To facilitate the optimization of the design of the condenser, which is critical for the whole system performance, we developed a simulation tool based on a multiscale mathematical model valid in the natural convection regime. The mathematical model and the numerical method adopted to solve it have been validated by comparing the simulation results with experimental measurements