Electrochromic adaptive solar heater and sub-ambient passive daytime radiative cooler

Abstract

Heating, ventilation, and air-conditioning (HVAC) systems are major contributors to global energy consumption, underscoring the urgent need for energy-efficient building envelope technologies. Synergistic solar and radiative electrochromism offers a promising solution by leveraging the sun and outer space as sustainable thermodynamic resources. In this study, we demonstrate a dual-mode electrochromic device that enables reversible, non-volatile switching between solar heating and sub-ambient radiative cooling, achieving an annual energy savings of 73.7 MBtu and CO2 emission savings of 4063 kgCO2 in specific U.S. climate zones. The device can achieve true sub-ambient passive daytime radiative cooling by 1 °C in the cooling mode and raise it by 33 °C above ambient in the heating mode during daytime operation, controlled by electrical voltage. The device employs electrodeposited lossy amorphous Cu-Bi nanoclusters on defect-activated monolayer graphene, yielding high optical contrast between cooling (solar absorptance α = 7.99%, thermal emittance ε = 93.58%) and heating (α = 47.7%, ε = 20.14%) modes, with outstanding long-term durability. This work paves the way for scalable, durable, and dynamic thermal regulation, advancing sustainable building technologies.