Harnessing the characteristic of compound parabolic concentrators for directional concentration of emitted thermal radiation and solar shielding in building-integrated radiative cooling

Abstract

Radiative sky cooling (RC) is a promising solution for meeting the growing cooling demands by passively dissipating waste heat into frigid outer space. However, current RC systems suffer from low cooling power density and limited installation flexibility, impeding their effective application as building cooling strategies. To overcome these challenges, a novel concentrated RC system coupled with a compound parabolic concentrator (CPC) is proposed and experimentally studied. The objective is to investigate the effectiveness of the CPC in enhancing the cooling capability of the RC system and the feasibility of achieving all-day RC in unfavorable working conditions when integrated with building roofs. During nighttime experiments in the humid Nottingham region, the CPC-RC system exhibited an average emitter temperature that was 5.83 °C lower than the ambient temperature, representing a 30 % and 13.6 % improvement in RC performance compared to the flat-RC system and trapezoidal-concentrated RC system, respectively. The Photopia optical software simulation indicates that when the modules are tilted to the north, the CPC functions as a solar shield, effectively limiting solar radiation reaching the emitter surface, which is advantageous for conducting daytime RC experiments. In the daytime experiment, the emitter temperature of the CPC-RC module in anti-sunward group was still 1.59 °C lower than the ambient temperature and 5.49 °C lower than that of flat-RC module. At night, the CPC-RC module of the three placement groups all showed the highest RC effect in the same group. The average emitter temperature of the CPC-RC module in the horizontal placement group is 0.6 °C lower than that of the flat RC module. This novel CPC-RC scheme presents a new energy-saving strategy for buildings and showcases its potential for achieving 24-hour RC when integrated into anti-sunward roofs.