Innovating building energy regulations enabled by radiative sky cooling: Enhanced code of practice for overall thermal transfer value (OTTV) of super-cool roofs

Abstract

Radiative sky cooling, which utilizes outer space as the cooling source for heat dissipation, is attracting global interests for its ability to cool passively without using any energy. Considering the swift advancements in the production of cost-effective radiative sky cooling materials on a large scale, it is of practical significance to incorporate radiative sky cooling technology into buildings to improve energy efficiency, necessitating amendment of the corresponding regulations. With this objective, the present research particularly focuses on the impact of radiative sky cooling-based super-cool roof strategies on the code of practice for Overall Thermal Transfer Value (OTTV), which is a prescriptive building regulation introduced by the Hong Kong government for building design, aimed at providing correction factors for local typical roofs integrated with advanced radiative sky cooling materials. To achieve the purpose, a new model for predicting roof heat transfer based on radiative sky cooling was established. Scaled-down experiments were conducted to fully validate the developed model with sufficient accuracy. The findings indicate that the implementation of super-cool roof strategies through radiative cooling-based techniques can substantially decrease roof heat gains, with reductions ranging from 73.9 % to 90.7 %, yield considerable energy savings, averaging between 3.5 and 45.0 kWh/m2, as well as significant cost savings on cooling expenses, averaging from 5.8 to 74.2 HKD/m2. Moreover, the adoption of such strategies has the potential to reduce carbon emissions by 6.3 to 81.2 kg/m2 during the cooling season. These outcomes vary depending on factors like the coefficient of performance (COP), radiative cooling materials, and specific roof types. Correlations between roof heat gains and OTTVs are established with a high accuracy of fit, achieving a coefficient of determination as high as 0.978. The average correction factors of five cutting-edge radiative cooling materials range from −0.00563 to −0.01655 in response to different typical roof types. Utilizing these correction factors, incorporating radiative sky cooling materials results in the OTTV of super-cool roofs varying from −0.7216 to −0.239 W/m2, which are significantly below the prevailing OTTV limit of 21 W/m2 enforced for Hong Kong’s commercial buildings. The proposed approach in this study can provide a straightforward and practical approach to assess the OTTV of typical building roofs when integrated with cutting-edge radiative sky cooling materials. This research offers significant perspectives into evolution and innovation of building energy policies incorporating radiative sky cooling technology.