Dynamic Building Envelope Materials for Enhanced Thermal Management and Energy Efficiency

Building envelopes, including opaque structures such as roofs and walls as well as transparent structures like windows, account for significant heat losses or heat gains that introduce huge heating and cooling loads, respectively, to the HVAC system. Considering the weather variation with seasons, adaptive building envelopes, through either passive or active approaches, are preferred to achieve high performance in the terms of both thermal management and energy efficiency.<br/>Aimed at enhancing heat gains in winter and heat sheds in summer, roofs and walls can be equipped with solar heating materials and radiatvie cooling materials that could be mechanically switched. Two recent examples are given with one controlled by the electrostatic force and the other fabricated from interwoven surfaces, which produced higher energy savings compared to the solely radiative cooling technology. However, perspectives on weather self-adaptive materials in building applications, especially for opaque structures, are offered to further discuss about the advanced materials with highly responsive feature and broader modulating band from solar to long wavelength infrared spectra.<br/>The thermo-responsive materials, W-doped VO₂ particles and hydrogels, have been utilized to develop dynamic windows in order to optimize the solar light transmission and long wavelength radiation triggered by the change of the ambient temperature. A scalable thermochromic smart window showed independent management over the visual, near-infrared (NIR), and long wavelength infrared (LWIR) radiation (De_LWIR=0.4) and presented dynamic heat dissipations while maintaining day lighting capability. When hydrogel material is applied to windows in a commercial building, the interactive effect of the low-e coating has been investigated and the design method has been provided. Numerical energy saving calculations to an as-synthesized hydrogel film combined with an IR-reflective coating have been conducted to evaluate the ultimate solar regulation performance and yielded annual space cooling energy savings of up to 30.6 kWh/yr.m² for Tucson, AZ, USA based on the area of window glass.<br/>To incorporate the numerical calculation for the spectra-selective radiative cooling material into the whole-building simulation tool, a systematic strategy of developing a new long-wavelength radiation spectrum integral module (LRSIM) suitable for the state-space model has demonstrated good accuracy (&lt; 3% deviation from measured data) with acceptable computational cost. However, although methods for evaluating the energy savings of the passively-driven dynamically-adaptive materials for building envelopes such as chromic materials have been studied and developed, there lacked of numerical algorithms and engineering-level control profiles to maximize the advantages of harvesting heat or cooling from natural resources (e.g., solar or outer space). Additional research work is needed to bridge this gap and therefore motivate the wide utilization of actively-enforced dynamic materials in buildings. As an example, a new electro-chromic window is used to present the different energy performances under the three pre-defined operation schedules, based on occupancy schedule, cooling/heating demand schedule, and the day lighting dominated schedule, for a single-room building in various climates.