Cellulose-Based Radiative Cooling and Solar Heating Powers Ionic Thermoelectrics

Cellulose opens for sustainable materials suitable for radiative cooling thanks to inherent high thermal emissivity combined with low solar absorptance. When desired, solar absorptance can be introduced by additives such as carbon black. However, such materials still shows high thermal emissivity and therefore performs radiative cooling that counteracts the heating process if exposed to the sky. Here, this is addressed by a cellulose-carbon black composite with low mid-infrared (MIR) emissivity and corresponding suppressed radiative cooling thanks to a transparent IR-reflecting indium tin oxide coating. The resulting solar heater provides opposite optical properties in both the solar and thermal ranges compared to the cooler material in the form of solar-reflecting electrospun cellulose. Owing to these differences, exposing the two materials to the sky generated spontaneous temperature differences, as used to power an ionic thermoelectric device in both daytime and nighttime. The study characterizes these effects in detail using solar and sky simulators and through outdoor measurements. Using the concept to power ionic thermoelectric devices shows thermovoltages of >60 mV and 10 degrees C temperature differences already at moderate solar irradiance of approximate to 400 W m(-2).

Automated summary

Cellulose is a sustainable material with high thermal emissivity and low solar absorptance. By introducing additives such as carbon black, solar absorptance can be achieved. However, the high thermal emissivity of such materials leads to counteractive radiative cooling when exposed to the sky. To address this issue, a cellulose-carbon black composite with low mid-infrared (MIR) emissivity and a transparent IR-reflecting indium tin oxide coating is used. This composite material provides opposite optical properties compared to solar-reflecting electrospun cellulose, resulting in spontaneous temperature differences when exposed to the sky.