Energy efficiency in buildings is a key research topic for maintaining energy sufficiency, lowering climate impact, and enabling cost savings. Heating, ventilation, and air conditioning (HVAC) systems can account for 25-30% of energy consumption in buildings. Especially in hot weather, active cooling methods such as fans and air conditioners are heavily used. It is estimated that air conditioning units worldwide will be accounting for 45% of the total global electricity consumption by 2050. An alternative method to complement active cooling is passive, or powerless, cooling. Passive cooling systems can be reflective, radiative, or a combination of both and can be applied to glass windows, roofs, etc. in buildings and vehicles.

In this work, we present passive cooling films and coatings based on plant-derived cellulose nanomaterial which is an abundant, renewable, environmental-friendly, and biodegradable material. We developed thermoresponsive nanocellulose films and coatings which change their transparency as a function of temperature. Aided by a thin reflective coating, the films act as a cooler in hot weather whereas they allow mild heating during cold weather, acting as a heat sink. The transition between the heating and cooling stage is automatic and rapid, and the thermal regulation temperature can be tuned. We have measured ~5 °C cooling potential with our films which can correspond to upto 20% energy savings in buildings. Potential applications for our solution are as a part of windows where they can offer adaptive passive cooling to spaces such as buildings and vehicles, thus increasing their energy efficiency.