Passive Adaptive Building Envelope to Minimize Heating and Cooling Loads

Xiao, Zhiying

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Passive Adaptive Building Envelope to Minimize Heating and Cooling Loads

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Building energy usage constitutes approximately 40% of total energy consumption in the U.S. and globally, a large portion of which is used to regulate temperatures within living and working spaces. More efficient and sustainable energy technologies, such as those enabling nearly or net-zero energy buildings (NZEBs), are critical for achieving long-term sustainability and climate neutrality. The development of passive solar enclosure technologies based on advanced materials is particularly attractive because it can offer cost-effective alternatives. This dissertation proposes viable strategies based on solid-solid phase change material (SS-PCM) for passive adaptive building envelopes aiming at decreasing energy consumption and emissions in building applications. Firstly, a series of numerical simulations were performed to investigate the optical and thermal processes of the SS-PCM based thermo-optically responsive enclosure system, as well as the synergies among different layers within the enclosure system, to help offset heat gains or losses in building enclosures. The impacts of the solar incoming angle and phase transition temperature on the absorptivity of the SS-PCM, which have a significant influence on the optical and thermal transfer processes, are explored. The feasibility and benefits of using the SS-PCM system in building enclosures under both warm and cold climates are investigated. Simulation results: (1) confirm the potential of the coatings to reduce undesirable heat exchange between building enclosures in all orientations and identify the roof as the preferred location of installing the SS-PCM system; (2) substantiate the thermal benefits of the system throughout the year and determine the optimal phase transition temperature of the SS-PCM with maximal energy saving; and (3) demonstrate more thermal benefits and energy saving of the SS-PCM coatings in warm climates compared to cold climates, which has been a challenge for most of existing passive solar facades. Secondly, an SS-PCM with leuco-dye coating system was developed to improve the thermal performance of the previous SS-PCM coating system. The thermal and optical properties of both SS-PCM coating systems have been experimentally determined; thereon, the thermal benefits of both systems have been investigated under summer-like as well as winter-like conditions. Comparisons were made between the thermal performance characteristics exhibited by the SS-PCM and SS-PCM-dye coating systems and those displayed by a benchmark surface covered in a standard black or white coating. Results validate the feasibility of both coating systems performing as adaptive building enclosure applications. The SS-PCM coating system reduced the temperature rise by 55.8% relative to that of a black reference in summer scenario, while the SS-PCM-dye coating system enhanced the temperature rise by 145% in winter scenario. The solar absorptivity and phase transition temperature are identified as the two critical parameters to design such coatings. The incorporation of leuco-dye into the SS-PCM coating increases the solar absorptivity especially in the visible wavelength range, thus resulting in a more pronounced heating effect during winter-like conditions compared to the dye-free SS-PCM coating. In contrast, the SS-PCM-dye coating reduced the cooling effect during the summer-like conditions compared to the dye-free coating. The SS-PCM and SS-PCM-dye coating systems are recommended to be employed in a cooling-dominant location like Houston and a heating-dominant location like Boston respectively to obtain optimal thermal benefits. Finally, an SS-PCM-PDMS coating system was designed to leverage passive radiative cooling effects into the system. The thermal and optical characteristics of the SS-PCM-PDMS coating system were modeled and refined to passively attain dynamic heating and radiative cooling effects per the surrounding temperature. The porous SS-PCM structures were explored to improve the radiative cooling effect, with a series of machine learning models developed to identify the impact factors on the optical properties of SS-PCM on amorphous and semi-crystalline phases, respectively. Then two optimized SS-PCM-PDMS coating systems with porosity were established based on the results of the machine learning models. The results (1) verify the feasibility of the SS-PCM-PDMS coating system, which exhibits switchable radiative cooling effect and solar heating effect in response to the ambient temperature; (2) indicate that machine learning models are able to capture the complex relationships between the variables of porous feature and optical properties of the SS-PCM systems; (3) establish two optimized SS-PCM-PDMS coating systems with porosity based on interpreting of the machine learning models. In summary, this dissertation develops and investigates several SS-PCM based coating systems for building envelopes to passively regulate heat transfer and maximize energy saving. Numerical simulation, physical experimentation, and data-driven techniques are utilized to design, analyze, and optimize these coating systems. The proposed strategies demonstrate significant energy-saving potential, providing viable pathways to enhancing building energy efficiency. Further research may focus on demonstrating these coating systems in real-world implementations.

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