¹ Electric&Energy Department, Usak University, Usak, 64200, Türkiye.

* Correspondence: canan.kandilli@usak.edu.tr

In this study, the structure-dependent thermal behavior of natural zeolite–based back layers is experimentally investigated as a stable alternative to conventional phase change materials. Graphite-modified and unmodified natural zeolite materials were implemented in consolidated plate and loose powder configurations and tested under identical real meteorological conditions using two geometrically and hydraulically equivalent PVT systems. Thermal performance was evaluated based on the inlet–outlet temperature difference of the working fluid and the corresponding thermal efficiency. The results demonstrate that material architecture plays a decisive role in governing thermal behavior. Plate-form configurations exhibit higher and more stable temperature differences together with smoother efficiency profiles than powder-form systems, indicating enhanced thermal inertia and reduced internal thermal resistance. Graphite modification significantly improves heat spreading and effective thermal conductivity, with the most stable and pronounced performance observed for graphite-modified zeolite plates. Thermal efficiency values exceeding unity occur during low-irradiance periods due to sensible heat storage and delayed heat release, confirming the functional role of zeolite back layers as active thermal buffers rather than passive elements. From an ovonic-inspired perspective, this behavior represents a structure-driven, history-dependent thermal response governed by material architecture rather than phase change phenomena, establishing graphite-modified natural zeolite plates as robust functional back layers for next-generation PVT systems.