Bio-inspired daytime passive radiative cooler and its application in building HVAC systems

Passive radiative cooling dissipates heat from sky-exposed surfaces to the cold universe by strongly emitting thermal radiation within the 8-13 μm wavelength range of the electromagnetic spectrum and reflecting radiations elsewhere, resulting in the surface temperature being perpetually lower than ambient. It is a potential scheme for dramatically conserving energy for refrigeration and space cooling in buildings. After the discovery of daytime radiative cooling, a wide range of materials have been found promising for an 8-13 μm wavelength range emissive solar reflective cooling device. Here, we consider the thermal selective emitter inspired by the thermoregulatory effect of triangular hairs on Sahara silver ants. FDTD simulations were utilized to optimize the biomimetic surface made of 8-13 μm wavelength range emissive materials such as SiO2 and polydimethylsiloxane (PDMS), and bottom solar mirror, revealing improved emissivity in the 8-13 μm wavelength range by patterning triangular arrays on the surface. With the optimized model, we have successfully fabricated bio-inspired daytime passive radiative cooler and this cooler has a potential to provide 172 W/m2 net cooling power, more than 40 % increase compared to that reported previously, and 11 °C temperature reduction below the ambient air temperature under the direct sunlight. Furthermore, we developed an energy balance mathematical model to study the potential application of passive radiative coolers in HVAC systems of buildings. Fluid flows in an isolated loop such that the coolant can be chilled by the radiative cooler and transported to the demand side for spacing cooling. This leads to the partial replacement of conventional vapor compression refrigeration by the radiative cooling panel. The cooling performance and indoor air temperature are evaluated by numerical analysis. The simulation results show that the proposed passive radiative cooling system with 100 m2 can retain an indoor temperature maximum to 10 °C below ambient and produce nearly 1600 W cooling power.