  With the ever-growing need to reduce energy consumption, building materials
that passively heat or cool are gaining importance. However, many buildings
require both heating and cooling, even within the same day. To date, few
technologies can automatically switch between passive heating and cooling, and
those that can require a large temperature range to cycle states (>15o C),
making them ineffective for daily switching. We present a passively adaptive
radiative switch that leverages the expansion in phase-change energy storage
materials to actuate the motion of louvers and can cycle states in less than 3o
C. The black selective-absorber louvers induce high heat gain when closed, yet
when open, expose a white, emissive surface for low heat gain. During an
outdoor test in which temperature was held steady, our device reduced the
energetic cost of cooling by 3.1x and heating by 2.6x compared to non-switching
devices. Our concept opens the door for passively adaptive thermoregulating
building materials.
