Tailoring Optical and Thermal Properties of Nanostructured Materials for Passive Radiative Cooling
Kim, Hannah
2021
Abstract
Passive radiative cooling has emerged as a promising way to reduce the amount of primary energy used for cooling. Specifically, cities could use passive radiative cooling to mitigate both urban heat island issues and electricity demand for air conditioning, which accounts for about 10% of electricity use in the U.S. according to the EIA. Decreasing energy consumption also plays an essential role in addressing global warming. For example, retrofitting 80% of commercial roofs in the U.S. could potentially reduce annual energy use by greater than 10 TWh and offset CO2 emissions by about 6 Mt. “Passive radiative” refers to the concept of selectively emitting thermal radiation to Space through the “atmospheric window” (i.e., 8 – 13 µm) without the input of energy. Low atmospheric absorption (high transmission) in this wavelength band allows objects to directly radiate heat to outer space. This effectively uses Space (~3 Kelvin) as a heat sink, which enables sub-ambient cooling. For example, nighttime cooling is a common phenomenon for high emitting materials. On the other hand, daytime cooling is particularly challenging because solar heating on Earth is ~10 times greater than the heat emitted to outer space, but by designing materials to reflect sunlight and emit in the infrared, sub-ambient cooling during peak solar hours is achievable. This dissertation includes a discussion on the optical and thermal properties needed for daytime cooling and demonstrates the cooling performance with outdoor measurements. In addition, background information, radiative cooling mechanisms, and past works are presented. This dissertation primarily focuses on materials with specific characteristic length scales that scatter solar radiation and enable emission in the infrared. First, the radiative and thermal transport of three candidate materials (BaF2, ZnS, and Polyethylene) with low absorption in the atmospheric window was modeled to predict the cooling performance of a nanoporous insulating layer. Physical morphology, intrinsic optical properties, and volume fraction are used as inputs to simultaneously solve the heat and radiative transfer equations and output the temperature profile of the nanoporous layer. This model offers a framework for radiative transport of nanoporous systems for potential design optimization. Polyacrylonitrile (PAN) nanofibers were fabricated and the scattering and transmission properties were investigated for electrospun fibers that feature spherical, ellipsoidal, and cylindrical morphologies. The nanofiber morphology was tailored by varying the polymer solution concentration used for electrospinning. The resulting PAN films (nanoPAN) with ellipsoidal morphologies achieve a solar reflectance ~95% while retaining >70% transmittance in the atmospheric window. These nanoPAN films can be paired with any emitting surface to promote radiative cooling, and outdoor measurements demonstrated a 50°C temperature reduction during the day when paired with a blackbody emitter compared to the blackbody control. The unique morphology and size distribution of PAN nanofibers can also be combined with existing radiative cooling emitter designs to further enhance the solar reflective properties. The addition of nanoPAN to a specularly reflective emitter enhanced the solar reflectance from 97% to 99% to more closely mimic nighttime radiative cooling conditions during the day. A ~5°C stagnation temperature and ~30 W/m2 cooling power enhancement were observed during peak solar hours. Overall, the work presented in this thesis demonstrates the ability to tailor the optical and thermal properties of nanostructured materials to achieve passive radiative cooling.Deep Blue DOI
Subjects
Passive Radiative Cooling Thermal Radiation Optical Materials
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