Adaptive Variable Emissivity Reflector for Seasonal and Daily Thermal Regulation in Regions with Significant Temperature Variations

Abstract Temperature‐adaptive variable emissivity reflectors offer an electricity‐free and environmentally friendly cooling strategy, holding substantial potential to improve the global energy landscape. However, challenges associated with unoptimized solar absorptance can lead to overcooling or overheating when the modulation of infrared emittance is held constant, thereby increasing extra energy consumption. Herein, a temperature‐adaptive variable emissivity reflector based on W‐Mg co‐doped VO₂ (W‐Mg‐VER) is proposed, engineered to optimize solar absorptance (α_L = 0.40) while maintaining effective infrared emittance modulation (Δɛ = 0.69) near ambient temperature. To maximize year‐round energy savings in regions experiencing significant daily temperature fluctuations, W‐Mg co‐doped VO₂ is employed to reduce solar absorptance by modifying electron occupancy in the V 3d orbital and expanding the optical bandgap. Outdoor experiments have validated the remarkable temperature management and energy‐saving capabilities of the W‐Mg‐VER, facilitated by its seamless transition between radiative cooling and heat‐retaining modes. Numerical simulation indicates that a W‐Mg‐VER roof covering 100 m² would save 152.9 GJ of energy annually in locations with significant daily temperature variations. Moreover, W‐Mg‐VER demonstrates robust performance, exhibiting less than 1% degradation in emittance tunability and solar absorptance after 10 000 cycles. This approach provides valuable insights and practical guidance for significantly enhancing global energy savings.