<p>Radiative thermal management has become increasingly more relevant within the past few decades due to the avocation for higher efficiency buildings, increases in</p> <p>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< λ <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< λ <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 >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</p> <p>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</p> <p>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.</p> <p>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</p> <p>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</p> <p>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</p> <p>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</p> <p>pipe. Furthermore, by integrating the concentrated radiative cooling system as a preconditioner in an air conditioning system, we predict electricity savings of > 75% in Phoenix, AZ, and > 80% in Reno, NV, for a single-story commercial building. We further look into unique applications of radiative cooling for outdoor enclosures</p> <p>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</p> <p>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</p> <p>and a gain in the lifetime of the windings. The RC coating cooled a 25 kVA distribution transformer’s core by > 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 > 7% and > 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</p> <p>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</p> <p>(LSM) Barium Sulfate (BaSO4) coatings as efficient variable emissivity coatings (VECs). The photon transport through the micropatterned system is modeled using</p> <p>geometric optics and Monte Carlo coupled with geometric optics to obtain the coatings reflectivity, transmissivity, and emissivity to predict the ideal reflectivity and</p> <p>emissivity of the micropatterns. Then the micropatterned LSM coatings are experimentally fabricated using screen printing on a BaSO4 paint layer. The coatings are</p> <p>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.</p>

The greenhouses play a key role in food sustainable production, the purpose of the greenhouses is to make an artificial suitable environment to grow different kinds of plants. The cost of energy used in the greenhouses to ensure the optimum temperature, humidity, and CO2 concentration, makes up a large part of the final cost of food. Due to global warming, the successive energy crises, and the food crises, the need to make the greenhouses more energy efficient and to utilize renewable energy resources is rapidly increasing. The enclosed water curtain cooling system meets the special requirement of the greenhouse cooling system, and it has potential energy savings when it is integrated with other systems such as heat pumps, underground water sources, and surplus heat energy recovery. This system involves two special nylon foils, and a thin layer of water flows between the two foils, the two foils will be stuck to eachother by the cohesive force of the water-detergent mixture, the detergent was added to decrease the water surface tension and ensure the even distribution of the water-detergent mixture over the nylon foils. In this study, an experimental model of the enclosed water curtain was made and two sets of tests were conducted, the first set was at room temperature around 20°C, and the second test was at room temperature around 25.7 °C with an electrical heater, each set contains three tests to measure the cooling capacity of the curtain, and each test takes 2 minutes, the curtain dimensions were height and width of 1.04 m and 1.20 m respectively. By measuring the difference between the average inlet and outlet temperature of the water-detergent mixture before and after the curtain, and the mixture mass flow rate during the test period, the cooling capacityof the curtain was calculated using the energy balance equation.It was found that the curtain cooling capacity increases with the increase of ambient temperature, The large heat transfer area of the curtain which allows using higher water temperature for cooling, and the useful features of the water membrane like the high absorption of the wavelength of infrared and the high transparency of the wavelength of visible light, make this system meets the special requirements of the greenhouses cooling system. However, to be able to apply this system in real-life, the design of the curtain should be improved, and suitable materials should be chosen to make it more reliable. Also, All tests in this study were conducted in the workshop in the absence of solar radiation, therefore, the actual performance of the curtain needs to be evaluated with the presence of solar radiation, to be able to study the effects of the direct and diffuse solar radiation with various spectrum range.

<p dir="ltr">The first part of this work proposes a solution for the challenge discussed section 1.2.1 by designing and validating a radiative cooling paint that is durable when attached to a substrate, and biodegradable when dispersed into the environment. Said paint also features high solar spectrum reflectivity and high sky window (8-13 microns) emissivity which enables it to achieve sub-ambient surface temperature throughout the day, even under direct sunlight. Full-biodegradation is observed in lab-based biodegradability testing using a comparison test between a biodegradable sample and a non-biodegradable sample. </p><p dir="ltr">The second part of this work demonstrates an innovative dual-layer design featuring a thin layer of leuco dye based thermochromic paint applied on top of a thick layer of BaSO4-based ultra-white daytime radiative cooling paint. This design utilizes thermochromism, a temperature-activated reversible chemical reaction that drastically changes the absorptivity of the affected media. In this work, the leuco dye-based thermochromic top layer effectively works as an autonomous thermal switch that, when temperatures are high and cooling is required, it switches to a "colorless state," enabling the radiative cooling basecoat to reflect incoming sunlight and emit radiatively, effectively cooling the surface. Conversely, when temperatures are low and heating is needed, the thermochromic top layer activates and transforms into an absorbing surface. This activation blocks the reflective and emitting bottom layer from radiatively cooling the surface, and instead absorbs incoming radiation to heats up the surface.</p>

<p>Polymer nanocomposites are a constantly evolving material category due to the ability to engineer the mechanical, thermal, and optical properties to enhance the efficiency of a variety of systems. While a vast amount of research has focused on the physical phenomena of nanoparticles and their contribution to the improvement of such properties, the ability to implement these materials into existing commercial or newly emerging processing methods has been studied much less extensively. The primary characteristic that determines which processing technique is the most viable is the rheology or viscosity of the material. In this work, we investigate the processing methods and properties of nanocomposites for thermal interface and radiative cooling applications. The first polymer nanocomposite examined here is a two-component PDMS with graphene filler for 3D printing via a direct ink writing approach. The composite acts as a thermal interface material which can enhance cooling between a microprocessor and a heat sink by increasing the thermal conductivity of the gap. Direct ink writing requires a shear thinning ink with specific viscoelastic properties that allow for the material to yield through a nozzle as well as retain its shape without a mold following deposition. No predictive models of viscosity for nanocomposites exist; therefore, several prominent models from literature are fit with experimental data to describe the change in viscosity with the addition of filler for several different PDMS ratios. The result is an understanding of the relationship between the PDMS component ratio and graphene filler concentration with respect to viscosity, with the goal of remaining within the acceptable limits for printing via direct ink writing. The second nanocomposite system whose processability is determined is paint consisting of acrylic filled with reflective nanoparticles for radiative cooling paint applications. The paint is tested with both inkjet and screen-printing procedures with the goal of producing a thermally invisible ink. Radiative cooling paint is successfully printed for the first time with solvent modification. This work evaluates the processability of polymer nanocomposites through rheological tailoring. </p><br>

<p>Part I of this thesis is a theoretical background to parts II and III.</p><p>Part II treats the phenomenon of decreased visibility through a glazing due to external water condensation, dew, on the external surface. Some simulations are presented where it is shown that under certain circumstances condensation can be expected. A combination of coatings on the external surface is suggested to overcome the problem of external condesation. It consists of both a coating which decreases the emissivity of the surface and a hydrophilic coating which reduces the detrimental effects to the view through the window.</p><p>Fresnel calculations of the optical properties are used to discuss the feasibility of using different coatings. A new test box was used to verify that the proposed window coatings perform as expected.</p><p>Part III is a study on the angular dependence of solar absorptance in windows. Optical properties vary with the angle of incidence of the incoming light. The variation is different from one window pane to another. </p><p>A model is proposed to approximate the angular variation of the solar absorptance in window panes. The model is semi-empirical and involves dividing the wide range of windows into nine groups. To which group a window belongs, depends on how many panes it has and on the features of the outer pane. The strength of the model is that it can be used without knowing the exact optical properties of each pane of the window. This makes it useful in the many cases when these data are not given by the manufacturer and Fresnel calculations to get the optical properties of the window are not feasible. The model is simple and can be added as an appendix to existing standards for measuring optical properties of windows.</p>

Part I of this thesis is a theoretical background to parts II and III. Part II treats the phenomenon of decreased visibility through a glazing due to external water condensation, dew, on the external surface. Some simulations are presented where it is shown that under certain circumstances condensation can be expected. A combination of coatings on the external surface is suggested to overcome the problem of external condesation. It consists of both a coating which decreases the emissivity of the surface and a hydrophilic coating which reduces the detrimental effects to the view through the window. Fresnel calculations of the optical properties are used to discuss the feasibility of using different coatings. A new test box was used to verify that the proposed window coatings perform as expected. Part III is a study on the angular dependence of solar absorptance in windows. Optical properties vary with the angle of incidence of the incoming light. The variation is different from one window pane to another. A model is proposed to approximate the angular variation of the solar absorptance in window panes. The model is semi-empirical and involves dividing the wide range of windows into nine groups. To which group a window belongs, depends on how many panes it has and on the features of the outer pane. The strength of the model is that it can be used without knowing the exact optical properties of each pane of the window. This makes it useful in the many cases when these data are not given by the manufacturer and Fresnel calculations to get the optical properties of the window are not feasible. The model is simple and can be added as an appendix to existing standards for measuring optical properties of windows.

Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua-ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water-cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua-ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water-cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

No description available.

<p>The thesis covers the following topics:</p> <p>1. aggregation and size effect on metal-polymer nanocomposite thermal interface materials</p> <p>2. diffusion limited cluster aggregation lattice simulation on thermal conductivty</p> <p>3. thermal interfacial resistance reduction between metal and dielectric materials by inserting an intermediate metal layer</p> <p>4. absence of coupled thermal interfaces in al2o3/ni/al2o3 sandwich structure</p> <p>5. ultra-efficient low-cost radiative cooling paints</p>