Understanding Heat Transport Mechanisms in Optically Transparent Thermal Loss Mitigators

Abstract

Optically transparent thermal loss mitigators have recently seen renewed research interests owing to their increasing relevance in the realms ranging from smart windows, efficient greenhouse designs and high-performance-low-cost solar thermal systems. In depth understanding of the heat transport mechanisms and their quantification is crucial for building efficient opto-thermal management strategies for optimization of the aforementioned systems. The present work serves to identify and quantify the key heat transfer mechanisms operative in a host of optically transparent thermal loss mitigators. In particular, comprehensive experimental modelling frameworks have been developed to investigate the efficacy of carbon dioxide gas (CO2), air, vacuum (0.07mbar), transparent heat mirrors (Indium tin oxide coated glass) and aerogels (silica-based) in mitigating thermal losses. Detailed and careful experimental modelling reveals that it is imperative to employ more than one thermal loss mitigator and choose correct absorber surface orientation (relative to the irradiation direction) to maximize thermal loss mitigation. Magnitude of absorber surface stagnation temperature has been employed as the figure of merit to quantitatively compare various optically transparent thermal loss mitigators. Under un-evacuated conditions, CO2 has emerged as potent alternative to more sophisticated optically transparent thermal loss mitigators like aerogels and transparent heat mirrors. Enhancements (relative to air) on the order of 2%-7%, 46%-84%, 57%-84% and 66%-86% are observed in case of CO2, vacuum, transparent heat mirrors (vacuum) and aerogel (vacuum) respectively.