Optical wood with switchable solar transmittance for all-round thermal management

TL;DR

This paper introduces an optical wood with switchable solar transmittance that enables efficient all-season thermal management by combining passive cooling and heating functionalities in a single material.

Contribution

The study develops a novel optical wood with hierarchically porous structure and switchable transmittance, enabling both radiative cooling and daylight heating for energy-efficient building applications.

Findings

Achieves 94.9% solar reflectance and 0.93 infrared emissivity in the original state.

Can switch to a highly transparent state with 68.4% solar transmittance.

Provides cooling of 4.5°C below ambient in summer and heating of 5.6°C above in winter.

Abstract

Technologies enabling passive daytime radiative cooling and daylight harvesting are highly relevant for energy-efficient buildings. Despite recent progress demonstrated with passively cooling polymer coatings, however, it remains challenging to combine also a passive heat gain mechanism into a single substrate for all-round thermal management. Herein, we developed an optical wood (OW) with switchable transmittance of solar irradiation enabled by the hierarchically porous structure, ultralow absorption in solar spectrum and high infrared absorption of cellulose nanofibers. After delignification, the OW shows a high solar reflectance (94.9%) in the visible and high broadband emissivity (0.93) in the infrared region (2.5-25 $\mu$m). Owing to the exceptional mass transport of its aligned cellulose nanofibers, OW can quickly switch to a new highly transparent state following phenylethanol impregnation. The solar transmittance of optical wood (OW-II state) can reach 68.4% from 250 to 2500 nm. The switchable OW exhibits efficient radiative cooling to 4.5 {\deg}C below ambient temperature in summer (81.4 W m$^{-2}$ cooling power), and daylight heating to 5.6 {\deg}C above the temperature of natural wood in winter (heating power 229.5 W m$^{-2}$), suggesting its promising role as a low-cost and sustainable solution to all-season thermal management applications.