Metrological setup for accurate determination of passive radiative cooling power

Passive radiative cooling (PRC) enables heat rejection to the cold sky through the atmospheric transparency window and has attracted increasing attention as a power-free cooling strategy. However, reported cooling performances remain difficult to compare and often unreliable, primarily due to the lack of metrologically sound measurement methods and quantitative uncertainty assessment. In this work, a fluid-based calorimetric apparatus is designed according to a traceable metrological framework for the accurate determination of PRC cooling power. The system offers a compact panel-like geometry with uniform liquid flow distribution and employs high-sensitivity, SI-traceable sensors. In particular, the temperature difference between inlet and outlet water is measured directly using a calibrated multi junction differential thermocouple (thermopile) achieving an expanded uncertainty of just 12.4 mK, allowing to reliably measure also small temperature differences between the panel outlet and inlet ports. Together with a calibrated flow-rate measurement and a full uncertainty budget, the proposed design allows relative uncertainties consistently below 10% over almost the complete range of power densities relevant to PRC materials. Even at moderate fluxes, and in the presence of non-radiative heat transfer contributions, the setup achieves a significant improvement over the measurement uncertainties reported in the literature for comparable setups. These results and comparative analysis demonstrate that metrology-driven design is essential for reliable PRC performance assessment and provide a robust benchmark for future material development and system-level studies.