Radiative Cooling

• A process where a surface emits infrared radiation to cool itself.

• Can cool below ambient temperature if heat loss exceeds heat gain.

• Requires a clear view of the sky, especially effective at night.

• Governed by Planck’s law and Stefan–Boltzmann law.

Nighttime vs Daytime Radiative Cooling

• Nighttime: Only emits heat to space; no sunlight to counteract cooling.

• Daytime: Needs to reflect solar radiation while emitting mid-IR radiation.

• Daytime cooling is more challenging and requires advanced materials.

The Atmospheric Window

• Earth's atmosphere is transparent to infrared radiation in 8–13 µm range.

• Materials designed to emit strongly in this range can radiate heat into space.

• This transparency is key for effective radiative cooling to space.

Emissivity between skin temp and 4K

Materials for Radiative Cooling (high emissivity)

• Photonic crystals: Structurally engineered to control spectral emission.

• Polymers like PDMS: High mid-IR emissivity, transparent to visible light.

• Metal-dielectric composites: Combine reflection and emission tuning.

• Goal: Reflect sunlight, emit infrared in 8–13 µm band.

Q1 Where is the term of radiative cooling in the steady state model?

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Q2 If Ta>Ts?