Nanophotonics-enabled smart windows, buildings and wearables

Abstract

Design and production of spectrally smart windows, walls, roofs and fabrics has a long history, which includes early examples of applied nanophotonics. Evolving nanoscience has a special role to play as it provides the means to improve the functionality of these everyday materials. Improvement in the quality of human experience in any location at any time of year is the goal. Energy savings, thermal and visual comfort indoors and outdoors, visual experience, air quality and better health are all made possible by materials, whose "smartness" is aimed at designed responses to environmental energy flows. The spectral and angle of incidence responses of these nanomaterials must thus take account of the spectral and directional aspects of solar energy and of atmospheric thermal radiation plus the visible and color sensitivity of the human eye. The structures required may use resonant absorption, multilayer stacks, optical anisotropy and scattering to achieve their functionality. These structures are, in turn, constructed out of particles, columns, ultrathin layers, voids, wires, pure and doped oxides, metals, polymers or transparent conductors (TCs). The need to cater for wavelengths stretching from 0.3 to 35 µm including ultraviolet-visible, near-infrared (IR) and thermal or Planck radiation, with a spectrally and directionally complex atmosphere, and both being dynamic, means that hierarchical and graded nanostructures often feature. Nature has evolved to deal with the same energy flows, so biomimicry is sometimes a useful guide.

Reflection of near-infrared light confers thermal protection in birds

“…Our results suggest that birds, and potentially other animals, may have solved the problem of competing selection for optical and thermal functions by modulating visible and NIR properties differently. Mimicking those solutions in developing artificial materials to enhance energy efficiency holds significant promise 27 , 28 . Our data also highlight that empirical measures of NIR reflectivity must be incorporated into mechanistic models predicting the effects of climate change, particularly extreme heat events, on individual fitness and species distributions.…”

An ode to polyethylene

“…Polyethylene films have long been used to provide such convection-blocking IR-transparent covers, allowing for efficient nighttime radiative cooling of buildings. [37][38][39] The daytime radiative cooling is more challenging, as the thermal radiation process has to compete with roof materials heating via sunlight absorption. Recently, this challenge has been met by the development of spectrally selective surfaces that efficiently reflect sunlight and simultaneously emit mid-infrared radiation.…”

Nanoporous fabrics could keep you cool

“…The same conceptual approach can help to improve thermal management of tents, buildings and vehicles (3,7,8), and will undoubtedly lead to many exciting advances of passive cooling technologies resulting in significant energy savings. …”