  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.
