Cellulose-Based Radiative Cooling and Solar Heating Powers Ionic Thermoelectrics

Abstract

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 °C temperature differences already at moderate solar irradiance of ≈400 W m^-2 .

Keywords: IR emissivity controlling; cellulose; ionic thermoelectrics; radiative cooling; solar heating.

Figure 1

Concept of ionic thermoelectrics driven by solar heating and radiative cooling. a) Optical properties of an ideal heater, together with normalized spectra of solar irradiance and atmospheric transmittance in grey. b) Schematic illustration of light interaction and heat flows for a lateral ionic thermoelectric device driven by a radiative cooler and solar heater. c) Optical properties of an ideal cooler, together with normalized spectra of solar irradiance and atmospheric transmittance in grey. Schematic illustration of light interaction and thermal flows for a lateral ionic thermoelectric device driven by a cooler and heater. d) Schematic of the basic structure of the heater material together with photograph and scanning electron microscopy image of a final heater film. e) Schematic of the basic structure of the cooling material together with photograph and scanning electron microscopy image of a final sample.

Figure 2

a) MIR reflectance of CA cooler and BC heaters with different thickness of ITO coating (from 0 to 900 nm). b) Schematic illustration of the sky simulator setup used to monitor radiative cooling performance of samples. c) Radiative cooling measurement using the sky simulator for CA cooler and BC heater materials with different thicknesses of ITO coating. d) Average ΔT extracted from (c) as a function of average reflectance in the wavelength range of 7–14 µm for the BC samples.

Figure 3

a) UV–vis‐near IR absorptance of a CA cooler (grey dashed line) and BC heaters with different thicknesses of ITO coating (colored solid lines). b) Schematic of the setup used to monitor the temperature of samples when exposed to the solar simulator. c) Solar heating measurement for CA cooler (grey dashed line) and BC heaters with different thicknesses of ITO coating (colored solid lines) using 1 SUN solar irradiance from the solar simulator (the illumination was turned off when samples were changed).

Figure 4

a,b) Optical properties of cooler (a) and optimized heater (b) in both the visible and MIR spectral regions. c) Schematic illustration of the outdoor setup used to monitor the temperature of a heater and a cooler when exposed to the sun and the sky. d–f) Continuous temperature measurement of cooler and optimized heater when exposed to the sun and the sky, separated into different panels for different solar irradiance ranges. The top part of each panel presents the measured solar irradiance. The middle part of each panel presents the temperatures and the bottom parts present differences in temperature between the heater and the outside (H—O), the cooler and the outside (C—O) and between the heater and the cooler (H—C). The shaded grey areas correspond to periods at which a shutter covered the setup.

Figure 5

a) Schematic of the lateral thermoelectric device. b) Schematic of outdoor setup used to measure the generated thermovoltage powered by optimized heater and cooler when exposed to the sun and the sky. c,d) Continuous thermovoltage measurement of the lateral device when exposed to the sun and the sky, separated into two panels for daytime and nighttime. The top part of each panel presents the measured solar irradiance. The middle part of each panel presents the temperatures and the bottom parts present accumulation of open circuit thermovoltage. The shaded grey areas correspond to periods at which a shutter covered the setup.