Passive Radiative Cooling
Passive radiative cooling is a technique that dissipates heat from an object to the cold of outer space through the atmospheric window (8–13 μm infrared spectrum) without requiring external energy input. This process allows surfaces to reach temperatures below ambient air temperature, even under direct sunlight, by balancing solar reflection and thermal emission.
Mechanisms & Principles
- Solar Reflectance: High albedo materials reflect incoming solar radiation (>90%) to minimize heat absorption.
- Thermal Emissivity: High emissivity in the mid-infrared range facilitates efficient radiative heat loss through the atmospheric transparency window.
- Energy Balance: Achieves sub-ambient temperatures when radiative cooling power exceeds absorbed solar and convective heating.
Materials & Applications
- Meta-surfaces: Engineered photonic structures (e.g., multilayer dielectrics) designed for optimal spectral control.
- Polymer Films: Lightweight, flexible coatings based on polytetrafluoroethylene (PTFE) or barium sulfate ().
- Architectural Integration: Roof coatings and building envelopes to reduce air conditioning loads.
- Textiles: Cryo-Paint: Passive Radiative Cooling for Sub-Ambient Temperature Control demonstrates application in self-cooling clothes, maintaining lower temperatures than surrounding air in direct sunlight without power.
Key Metrics
- Cooling Power: Typically 50–100 W/m² under standard conditions.
- Temperature Delta: Capable of achieving of 3–8°C below ambient depending on humidity and wind speed.
See Also
- Atmospheric Window
- Stefan-Boltzmann Law
- Thermal Management