Engineering Spectrally Selective and Dynamic Coatings for Radiative Thermal Management
Creators: name: Peoples, Joseph Arthur
Publisher: name: Purdue University Graduate School
Year: 2022
Subjects: Energy generation, conversion and storage (excl. chemical and electrical)

description: Radiative thermal management has become increasingly more relevant within the past few decades due to the avocation for higher efficiency buildings, increases in power densities with decreases in form factors, and cutting-edge technologies for space exploration. This research focuses on engineering coatings with spectrally selective optical properties to achieve ultra-efficient thermal management via passive radiative cooling of both terrestrial and extraterrestrial applications. Terrestrial radiative cooling is a phenomenon of passively cooling exterior surfaces below ambient temperatures by engineering coatings to exhibit low absorptance in the solar spectrum (0.25 μm&lt; λ &lt;2.5 μm), such that a minimal amount of solar irradiation is absorbed, and high emittance in the transmissive portion of the atmosphere (8 μm&lt; λ &lt;13μm), i.e. the sky window, to lose heat to deep-space for a net cooling effect. Deep-space is considered to be an infinite heat sink at 3 K. Extraterrestrial radiative cooling requires the same criteria as terrestrial radiative cooling, however, there is no atmosphere to block a portion of the solar irradiation or the emission from the surface. A key requirement for achieving passive radiative cooling for an ideal emitter during daytime is a total solar reflection &gt;85%, and every 1% above this threshold results in ≈10 W/m2 gain in cooling power. Here, recognizing the broadband nature of solar irradiation, we propose and test a new concept of enhancing solar reflection at a given particle volume concentration by using hierarchical particle sizes, which we hypothesize to scatter each band of the solar spectrum, i.e. VIS, NIR and UV effectively. The hypothesis is tested using a TiO2 nanoparticle-acrylic system. Using the Mie Theory, the scattering and absorption efficiencies and asymmetric parameter of nanoparticles with different sizes and combinations are calculated, then the Monte Carlo Method is used to solve the Radiative Transfer Equation. An overall total solar reflection of ≈91%, which is higher than the ≈78% and ≈88% for 100 nm and 400 nm single particle sizes, respectively was achieved from our hypothesis. With increasingly better RC materials being demonstrated in literature, there is a growing need to understand the real-world utility and benefit of RC with regards to energy savings. A fundamental limit of current radiative cooling systems is that only the top surface facing deep-space can provide the radiative cooling effect, while the bottom surface cannot. Here, we propose and experimentally demonstrate a concept of “concentrated radiative cooling” by nesting a radiative cooling system in a mid-infrared reflective trough, so that the lower surface, which does not contribute to radiative cooling in previous systems, can radiate heat to deep-space via the reflective trough. Field experiments show that the temperature drop of a radiative cooling pipe with the trough is more than double that of the standalone radiative cooling pipe. Furthermore, by integrating the concentrated radiative cooling system as a preconditioner in an air conditioning system, we predict electricity savings of &gt; 75% in Phoenix, AZ, and &gt; 80% in Reno, NV, for a single-story commercial building. We further look into unique applications of radiative cooling for outdoor enclosures of electrical equipment, as demonstrated with a case study of coating pole-type distribution transformers. Utilizing RC paint on the exterior of the case would allow further dissipation of heat to deep-space, as well as, increase the solar reflectance to lessen the heat load on the case. A single 25 kVA pole-type transformer is modeled via CFD with two different exterior case coatings, the standard grey coatings commonly utilized and an RC coating, BaSO4 paint, is analyzed under different operating loads. The RC coating demonstrates great benefits from a thermal management perspective and a gain in the lifetime of the windings. The RC coating cooled a 25 kVA distribution transformer’s core by &gt; 11oC when compared to the standard case and even shows below ambient cooling of the case under minimal heat generations. The lifetime of the distribution transformers was increased by a minimum of 55% when comparing the standard case to the case with a radiative cooling paint based on the Aging Acceleration Factor. A more traditional application of radiative cooling paints is to utilize them on the exterior of buildings to offset the cooling energy demand for air conditioning. This work develops a high-fidelity RC model which accounts for pertinent weather factors including precipitable water, sky clearness, and dynamic convective heat transfer coefficients based on wind speed to further understand the energy savings. We implement our RC model on a single-story residential building to study the impact of RC in every unique ASHRAE climate zone in the United States using the 16 DOE recommended representative cities. Our results show &gt; 7% and &gt; 12% cooling energy savings across the United States for NREL’s building and typical buildings, respectively. Furthermore, warm climates yield the greatest cooling energy savings of up to 22% and 46% for the NREL and the Typical building, respectively. Extraterrestrial radiative thermal management is becoming increasingly pertinent with the development of new space technologies and the need to discover what is beyond our world. Space presents extreme thermal environments for radiative transfer, from a total eclipse case where the body radiates to deep space at 3 K to a full solar load where 1400 W/m2 is radiated onto the surface and a hybrid of both situations. The goal of this work is to engineer micropatterned Lanthanum Strontium Manganite (LSM) Barium Sulfate (BaSO4) coatings as efficient variable emissivity coatings (VECs). The photon transport through the micropatterned system is modeled using geometric optics and Monte Carlo coupled with geometric optics to obtain the coatings reflectivity, transmissivity, and emissivity to predict the ideal reflectivity and emissivity of the micropatterns. Then the micropatterned LSM coatings are experimentally fabricated using screen printing on a BaSO4 paint layer. The coatings are characterized by their temperature-dependent variable emissivity and solar absorptivity from the dual-layer micropatterned coatings. Furthermore, a computational model for a body-mounted cylindrical radiator was developed to investigate the real implications a VEC can have on crewed space vehicles, as well as define some target guidelines for VEC’s to achieve in future technologies.