Net heat-flux measurement method for passive radiative cooling materials with ambient self-tracking

Passive Radiative Cooling (PRC) materials can efficiently dissipate thermal energy to outer space through the atmospheric window (8-14 um), while reflecting most incident solar radiation, enabling sub-ambient cooling under direct sunlight. This work presents RAFT (Radiative Ambient-referenced Flux Testbed), a compact device for accurate outdoor PRC materials characterization. RAFT employs a heat flux meter to directly measure net heat flux, with the PRC sample mounted on the sensing surface and thermally coupled to an ambient-tracked reservoir via a compact fan-based convective heat exchanger integrated with heat pipes. Outdoor experiments demonstrated reliable ambient temperature tracking and accurate net heat flux measurements, which were further verified using an openly-released numerical model. Compared with conventional resistive heater-based outdoor setups, RAFT overcomes key limitations: it eliminates the need for feedback-control, shortens the measurement chain, reduces measurement noise when sample temperature approaches ambient, and enables quantification of both cooling and heating net fluxes with a total measurement uncertainty of +/- 1.8 W m-2. Relative to liquid-flow-based systems, RAFT offers improved portability and avoids the complexities of fluid handling. The proposed setup provides a robust and practical framework for outdoor evaluation of PRC materials, addressing the current lack of standardized protocols for direct heat flux measurement.