• Energy Research

Abstract Radiative sky cooling has attracted worldwide attention due to its passive cooling potential with zero energy input. Daytime cooling materials with outstanding spectral properties are developing rapidly. Using these new cooling materials as roof coatings can become one of the most prominent radiative cooling methods integrated with buildings. Current existing models and numerical tools are not able to incorporate the spectral selectivity of radiative cooling materials and local precipitable water vapor, thus failing to accurately predict roof thermal performance. To this end, the paper newly develops a spectral selectivity-based passive radiative roof cooling model to integrate full spectral properties of emerging cooling materials and consider precipitable water vapor-induced atmospheric radiation for precise evaluation of roof cooling performance. A field experiment was carried out to validate the developed model. The mean bias errors (MBEs) of the model in the prediction of rooftop temperatures were found less than 4.6%, verifying the sufficient accuracy of the new model in characterizing roof temperature variations. After validation, the developed model was used to investigate the potential benefits of radiative cooling as a rooftop strategy in the hot and humid region of Hong Kong. The whole year on-site weather data were collected as the model input. Compared to the baseline coating, the reductions in monthly cumulative roof thermal transfer values by applying a porous polymer cooling coating are 18.5–22.3, 33.7–40.5 and 2.9–3.5 kWh/m2 for residential concrete-based, industrial galvanized steel-based, and standard-compliant commercial multilayered roofs, respectively. For the above three roofs, the radiative cooling coating is able to save the annual roof-induced cooling electricity of 54.7–76.6, 97.4–136.4, and 8.8–12.2 kWh/m2, corresponding to the cooling cost saving of 71.8–100.5, 127.8–179.0, 11.5–16.0 HK$/(m2·yr), respectively. The new spectral selectivity-based radiative roof cooling model can be widely employed to accurately evaluate the utilization of daytime radiative coolers as a rooftop strategy in improving building thermal and energy performance.

Abstract Radiative sky cooling has attracted worldwide attention due to its passive cooling potential with zero energy input. Daytime cooling materials with outstanding spectral properties are developing rapidly. Using these new cooling materials as roof coatings can become one of the most prominent radiative cooling methods integrated with buildings. Current existing models and numerical tools are not able to incorporate the spectral selectivity of radiative cooling materials and local precipitable water vapor, thus failing to accurately predict roof thermal performance. To this end, the paper newly develops a spectral selectivity-based passive radiative roof cooling model to integrate full spectral properties of emerging cooling materials and consider precipitable water vapor-induced atmospheric radiation for precise evaluation of roof cooling performance. A field experiment was carried out to validate the developed model. The mean bias errors (MBEs) of the model in the prediction of rooftop temperatures were found less than 4.6%, verifying the sufficient accuracy of the new model in characterizing roof temperature variations. After validation, the developed model was used to investigate the potential benefits of radiative cooling as a rooftop strategy in the hot and humid region of Hong Kong. The whole year on-site weather data were collected as the model input. Compared to the baseline coating, the reductions in monthly cumulative roof thermal transfer values by applying a porous polymer cooling coating are 18.5–22.3, 33.7–40.5 and 2.9–3.5 kWh/m2 for residential concrete-based, industrial galvanized steel-based, and standard-compliant commercial multilayered roofs, respectively. For the above three roofs, the radiative cooling coating is able to save the annual roof-induced cooling electricity of 54.7–76.6, 97.4–136.4, and 8.8–12.2 kWh/m2, corresponding to the cooling cost saving of 71.8–100.5, 127.8–179.0, 11.5–16.0 HK$/(m2·yr), respectively. The new spectral selectivity-based radiative roof cooling model can be widely employed to accurately evaluate the utilization of daytime radiative coolers as a rooftop strategy in improving building thermal and energy performance.

Abstract Radiative cooling is an appealing heat exchange form based on thermal radiation from terrestrial objects to outer space, which can be potentially applied to numerous cooling applications for system performance improvement without energy input. Due to the poor solar reflective ability of previous cooling materials, radiative cooling technology has been largely limited to nocturnal cooling for several decades. Thanks to the recent successful development of highly efficient selective and broadband thermal emitters either backed with high solar reflective metal films or intrinsically equipped with excellent solar reflectance properties, the daytime radiative cooling has been practically fulfilled, which is arousing worldwide research interests. Notably, the emerging nanophotonics or metamaterials-based fabrication approaches are widely reported with the prominent ability to tailor spectral properties for sub-ambient cooling enhancement. Porous polymer-based scalable paints with micro-and nano-pores are also developed with substantial daytime cooling capacities by considerably backscattering sunlight and emitting thermal radiation. This work comprehensively reviews the latest progress on radiative cooling regarding its theoretical fundamentals, material designs and a variety of novel applications with a special focus on building-integrated cooling performance improvement. Considering the preceding extensive research on cool roofs with remarkable potential for real-world implementations, radiative roof cooling has been discussed in terms of cool roof-based heat transfer models, thermal and energy performance, and economic and environmental benefits. Lastly, research prospects of the radiative cooling technology are envisaged to provide insight for further investigation.

Abstract Radiative cooling is an appealing heat exchange form based on thermal radiation from terrestrial objects to outer space, which can be potentially applied to numerous cooling applications for system performance improvement without energy input. Due to the poor solar reflective ability of previous cooling materials, radiative cooling technology has been largely limited to nocturnal cooling for several decades. Thanks to the recent successful development of highly efficient selective and broadband thermal emitters either backed with high solar reflective metal films or intrinsically equipped with excellent solar reflectance properties, the daytime radiative cooling has been practically fulfilled, which is arousing worldwide research interests. Notably, the emerging nanophotonics or metamaterials-based fabrication approaches are widely reported with the prominent ability to tailor spectral properties for sub-ambient cooling enhancement. Porous polymer-based scalable paints with micro-and nano-pores are also developed with substantial daytime cooling capacities by considerably backscattering sunlight and emitting thermal radiation. This work comprehensively reviews the latest progress on radiative cooling regarding its theoretical fundamentals, material designs and a variety of novel applications with a special focus on building-integrated cooling performance improvement. Considering the preceding extensive research on cool roofs with remarkable potential for real-world implementations, radiative roof cooling has been discussed in terms of cool roof-based heat transfer models, thermal and energy performance, and economic and environmental benefits. Lastly, research prospects of the radiative cooling technology are envisaged to provide insight for further investigation.