Passive radiative cooling below ambient air temperature under direct sunlight.

Area of Science:

• Materials Science
• Thermodynamics
• Energy Science

Background:

• Cooling is a major global energy consumer and drives peak electricity demand, particularly in buildings.
• Passive cooling strategies offer significant energy savings by reducing electricity input.
• Radiative cooling, leveraging Earth's atmospheric transparency window, has been effective at night but not during daytime.

Purpose of the Study:

• To demonstrate daytime radiative cooling below ambient air temperature under direct sunlight.
• To develop and test a photonic device capable of passive cooling during peak demand hours.
• To explore the use of the universe's coldness as a renewable thermodynamic resource for cooling.

Main Methods:

• Experimental demonstration of a passive cooling device under direct sunlight.
• Utilizing a thermal photonic approach with an integrated photonic solar reflector and thermal emitter.
• Fabricating a seven-layer HfO2 and SiO2 device designed to reflect solar radiation and emit thermal radiation within the atmospheric transparency window.

Main Results:

• Achieved radiative cooling nearly 5°C below ambient air temperature under direct sunlight.
• The photonic radiative cooler reflected 97% of incident sunlight.
• Demonstrated a cooling power of 40.1 W/m² at ambient temperature under intense solar radiation (>850 W/m²).

Conclusions:

• A tailored photonic approach can enable passive cooling technologies for enhanced energy efficiency.
• Daytime radiative cooling below ambient temperature is experimentally feasible.
• The cold of space can be harnessed as a renewable thermodynamic resource for cooling, even during daytime.