Finite-stage nonreciprocal thermal circulators for radiative energy harvesting: Benchmarks for isolation, termination, and etendue.

Reaching the Landsberg limit for radiative energy conversion requires controlling not only spectra but also the direction in which radiative entropy is transported. We develop a finite-stage nonreciprocal thermal-circulator framework for positive illumination (solar conversion) and negative illumination (radiative-cooling work extraction). The ideal theory is expressed in terms of open-system exergy and a minimal port-network picture: a three-port circulator with a cold termination acts as a two-port thermal isolator, and cascaded isolators enforce directed adjacency between effective photon reservoirs. In response to the central practical limitation of nonreciprocal radiative cooling, emphasized recently by Liu et al. we recast the problem as a benchmark map rather than a claim of immediate device-level cooling enhancement. The reported work output values (in unit of W m-2) are hemispherical-equivalent upper envelopes; experimentally relevant output is reduced by captured free-space etendue, coupling efficiency, finite isolation ratio, finite termination temperature, atmospheric transmission, and out-of-window loss. We therefore introduce explicit sensitivity factors for isolation, termination, and captured etendue and ask how good a nonreciprocal system must be before the finite-stage advantage remains measurable. Finite-stage calculations give temperature and wavelength targets for 3-5 stages, while the practical benchmark identifies the isolation, termination, and coupling requirements needed to make those targets consequential. This formulation links ideal Landsberg-type limits to testable design requirements for nonreciprocal thermal photonics.