Development of a new spectral selectivity-based passive radiative roof cooling model and its application in hot and humid region

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 vaporinduced 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.

Automated summary

The study developed a spectral selectivity-based passive radiative roof cooling model to improve building thermal performance. The model was validated through field experiments to accurately evaluate the benefits of using daytime radiative coolers as a rooftop strategy in reducing cooling electricity costs.