Passive radiative cooling favors the transfer of energy to the deep space (2.7K) by emitting in the transparent atmosphere region (8-13m) and reflecting incoming solar irradiation. To achieve desired daytime or night time cooling performance, scientists have explored various fine-tuned photonic material combinations and layering techniques. However, the high cost, UV absorption or telecommunication interferences due to the metallic material used. Scalable and low-cost nonmetal materials have been studied, but the absorption in the UV range still remains a limitation. Single crystal CaCO_3was found to be highly reflective in the UV range, but it has not been explored for radiative cooling applications yet. In this work we first studied the reflectance in the solar range of seashells of multi-millimeters thick, and found over 70% reflectance. Inspired by this promising result, we fabricated a bio-inspired material — CaCO_3 acrylic nanocomposite, and optimized the nanoparticle size to most strongly reflect the sunlight. We analyzed its performance using Mie Theory and Monte Carlo Simulation for multiple size distribution with dependent scattering correction. The results are in excellent agreement with the experimental data. With 60% volume concentration, the simulation results showed that the total solar reflectance of CaCO_3 can achieve up to 97% . Insights obtained from this work will aid researchers in selecting economical, scalable, and manufacturable materials for radiative cooling applications. <br>

The coldness of outer space is a huge thermodynamic resource that can be utilized as an infinite heat sink that helps in cooling terrestrial objects without the need for electrical energy through a phenomenon known as radiative sky cooling. In the last decade, radiative cooling has seen an increasing attention as a sustainable and clean cooling method and many researchers made smart use of it as a thermal management method. One example in the literature is the radiative cooling of solar cells. Like solar cells, Light Emitting Diodes (LEDs) are semiconductor devices that deteriorate because of high temperatures. Specifically, the high temperature in LEDs lowers their efficiency and lifetime. Therefore, reducing the temperature by increasing heat dissipation can help in optimizing the efficiency of the LED. In this work, I investigate a novel low-cost solution that can help in reducing the temperature of outdoor LEDs through radiative cooling. The suggested solution utilizes the coldness of outer space to radiatively cool the LED by using a layer of a visible-reflective-infrared-transparent material, nanoporous polyethylene (nanoPE), as a cover to reflect the visible light back to earth while transmitting infrared radiation to outer space. I theoretically discuss the potential cooling performance of LEDs in the suggested design and estimate a cooling power enhancement by 128 W/m2 in ideal conditions compared to current designs. In addition, I study the fabrication and characteristics of nanoPE and show how it can be used as a reflective/diffusive cover for LEDs. Lastly, I experimentally demonstrate the use of nanoPE as a cover for LEDs and show an LED temperature reduction of 15 ⁰C in the laboratory environment and 4 ⁰C outdoor and calculate a relative LED efficiency increase of 28% in the indoor scenario and 4% in the outdoor scenario. This efficiency increase can result in an energy saving of 2.2 TWh in the United States corresponding to at least 0.44 MMT CO2 emission reduction making this cooling solution attractive due to its low cost and high impact.

The contributions to knowledge documented in this doctoral thesis are two-fold. The first contribution is in the application of a new biomimetic design tool called BioTRIZ. Its creators claim it can be used to facilitate the transfer of biological principles to solve engineering problems. The core case-study of this thesis documents how this tool was used to frame and systematically explore low-energy solutions to a key technical problem in the underdeveloped field of radiative cooling. Radiative cooling is a passive mechanism through which heat from a building can be rejected to the sky – an abundant but underused natural heat sink. Published in the Journal of Bionic Engineering, the study was the first independent application of BioTRIZ in the academic literature. The second contribution to knowledge is in the design, development and testing of the most promising biomimetic concept to come out of the BioTRIZ radiative cooling study. ‘Heat-selective’ insulation gives a roof mass a cool view of the sky because integrated pathways focus and channel longwave thermal radiation through it. It is biomimetic because it achieves infrared transparency by adding structural hierarchy to the component, rather than manipulating the properties of the material itself. Test panels on a rooftop in central London cooled to between 6 and 13 degrees below ambient temperature on May and April nights. Radiative cooling powers of between 25 and 70 W/m2 were measured when plates were at ambient temperature. Daytime radiative cooling below ambient temperature occurred when clouds blocked direct sunlight. Radiative cooling power was increased by 37% using reflective ‘funnels’. Two additional BioTRIZ analyses are presented as minor case studies. They each attend to a key technical problem in the field of passive thermal energy storage in buildings. They serve to illustrate the type of results that can be expected from using BioTRIZ during low-energy building design.

<p>Radiative cooling is an approach that utilizes the material reflectance in solar spectrum to reflect solar irradiation and emit the energy to deep space (2.7K) through the transparent portion in atmosphere (8-13μm). Therefore, radiative cooling is a passive cooling method that can generate a large reduction in energy consumption in the cooling sector. Scientists have been researching on the best solution for passive radiative cooling, including the utilization of multi-layer techniques with a metallic base layer. However, the current solutions are usually not cost effective and thus limited in the commercial applications. We initially started with the experiment on single-layer cooling paints embedded with TiO₂nanoparticles, and we were able to achieve a partial daytime radiative cooling effect of 60Wm^-2 Built upon our lab’s success of full-daytime sub-ambient cooling based on BaSO₄-acrylic paints, we experiment with colored cooling paints based on BaSO₄ nanoparticles instead of TiO₂ nanoparticles. Our results show much enhanced solar reflectance while matching the color, indicating the potential for colored cooling paints, although outdoor tests have not shown significant temperature drop compared to commercial colored paints yet. At the same time, we also explore creatures with shells in nature for possible solutions. Seashells are collected and the microstructures and radiative properties are characterized. The results provide insights into bio-inspired radiative cooling solutions.</p>

Small scale renewable energy harvesting is an attractive solution to the growing need for power in remote technological applications. For this purpose, localized thermal gradients on-chip—created via radiative cooling—could be exploited to produce microscale renewable heat engines running on environmental heat. This could allow self-powering in small scale portable applications, thus reducing the need for non-renewable sources of electricity and hazardous batteries. In this work, we demonstrate the creation of a local thermal gradient on-chip by radiative cooling of a 90 nm thick freestanding silicon nitride nanomechanical resonator integrated on a silicon substrate at ambient temperature. The reduction in temperature of the thin film is inferred by tracking its mechanical resonance frequency, under high vacuum, using an optical fiber interferometer. Experiments were conducted on 15 different days during fall and summer months, resulting in successful radiative cooling in each case. Maximum temperature drops of 9.3 K and 7.1 K are demonstrated during the day and night, respectively, in close correspondence with our heat transfer model. Future improvements to the experimental setup could enhance the temperature reduction to 48 K for the same membrane, while emissivity engineering potentially yields a maximum theoretical cooling of 67 K with an ideal emitter. This thesis first elaborates a literature review on the field of radiative cooling, along with a theoretical review of relevant thermal radiation concepts. Then, a heat transfer model of the radiative cooling experiment is detailed, followed by the experimental method, apparatus, and procedures. Finally, the experimental and theoretical results are presented, along with future work and concluding remarks.

Low grade heat sources have a large amount of thermal energy content. Due to low temperature, the conventional power generation technologies result in lower efficiency and hence cannot be used. In order to efficiently generate power, alternate methods need to be used. In this study, a supercritical organic Rankine cycle was used for heat source temperatures varying from 125°C to 200°C. Organic refrigerants with zero ozone depletion potential and their mixtures were selected as working fluid for this study while the cooling water temperature was changed from 10-25°C. Operating pressure of the cycle has been optimized for each fluid at every heat source temperature to obtain the highest thermal efficiency. Energy and exergy efficiencies of the thermodynamic cycle have been obtained as a function of heat source temperature. Efficiency of a thermodynamic cycle depends significantly on the sink temperature. At areas where water cooling is not available and ambient air temperature is high, efficient power generation from low grade heat sources may be a challenge. Use of passive cooling systems coupled with the condenser was studied, so that lower sink temperatures could be obtained. Underground tunnels, buried at a depth of few meters, were used as earth-air-heat-exchanger (EAHE) through which hot ambient air was passed. It was observed that the air temperature could be lowered by 5-10°C in the EAHE. Vertical pipes were used to lower the temperature of water by 5°C by passing it underground. Nocturnal cooling of stored water has been studied that can be used to cool the working fluid in the thermodynamic cycle. It was observed that the water temperature can be lowered by 10-20°C during the night when it is allowed to cool. The amount of water lost was calculated and was found to be approximately 0.1% over 10 days. The different passive cooling systems were studied separately and their effects on the efficiency of the thermodynamic cycle were investigated. They were then combined into a novel condenser design that uses passive cooling technology to cool the working fluid that was selected in the first part of the study. It was observed that the efficiency of the cycle improved by 2-2.5% when passive cooling system was used.

abstract: With the desire of high standards of comfort, huge amount of energy is being consumed to maintain the indoor environment. In US building consumes 40% of the total primary energy while residential buildings consume about 21%. A large proportion of this consumption is due to cooling of buildings. Deteriorating environmental conditions due to excessive energy use suggest that we should look at passive designs and renewable energy opportunities to supply the required comfort. Phoenix gets about 300 days of clear sky every year. It also witnesses large temperature variations from night and day. The humidity ratio almost always stays below the 50% mark. With more than six months having outside temperatures more than 75 oF, night sky radiative cooling promise to be an attractive means to cool the buildings during summer. This technique can be useful for small commercial facilities or residential buildings. The roof ponds can be made more effective by covering them with Band Filters. These band filters block the solar heat gain and allow the water to cool down to lower temperatures. It also reduces the convection heat gain. This helps rood ponds maintain lower temperatures and provide more cooling then an exposed pond. 50 &#956;m Polyethylene band filter is used in this study. Using this band filter, roof ponds can be made up to 10% more effective. About 45% of the energy required to cool a typical residential building in summer can be saved. / Dissertation/Thesis / M.S. Architecture 2013

abstract: This research focuses mainly on employing tunable materials to achieve dynamic radiative properties for spacecraft and building thermal management. A secondary objective is to investigate tunable materials for optical propulsion applications. The primary material investigated is vanadium dioxide (VO2), which is a thermochromic material with an insulator-to-metal phase transition. VO2 typically undergoes a dramatic shift in optical properties at T = 341 K, which can be reduced through a variety of techniques to a temperature more suitable for thermal control applications. A VO2-based Fabry-Perot variable emitter is designed, fabricated, characterized, and experimentally demonstrated. The designed emitter has high emissivity when the radiating surface temperature is above 345 K and low emissivity when the temperature is less than 341 K. A uniaxial transfer matrix method and Bruggeman effective medium theory are both introduced to model the anisotropic properties of the VO2 to facilitate the design of multilayer VO2-based devices. A new furnace oxidation process is developed for fabricating high quality VO2 and the resulting thin films undergo comprehensive material and optical characterizations. The corresponding measurement platform is developed to measure the temperature-dependent transmittance and reflectance of the fabricated Fabry-Perot samples. The variable heat rejection of the fabricated samples is demonstrated via bell jar and cryothermal vacuum calorimetry measurements. Thermal modeling of a spacecraft equipped with variable emittance radiators is also conducted to elucidate the requirements and the impact for thermochromic variable emittance technology. The potential of VO2 to be used as an optical force modulating device is also investigated for spacecraft micropropulsion. The preliminary design considers a Fabry-Perot cavity with an anti-reflection coating which switches between an absorptive “off” state (for insulating VO2) and a reflective “on” state (for metallic VO2), thereby modulating the incident solar radiation pressure. The visible and near-infrared optical properties of the fabricated vanadium dioxide are examined to determine if there is a sufficient optical property shift in those regimes for a tunable device. / Dissertation/Thesis / Doctoral Dissertation Aerospace Engineering 2020

<p>Photovoltaic (PV) cells have become an increasingly ubiquitous technology; however, concentrating photovoltaics (CPV), despite their higher theoretical efficiencies and lower costs, have seen much more limited adoption. Recent literature indicates that thermal management is a key challenge in CPV systems. If not addressed, it can negatively impact efficiency and reliability (lifetime). Traditional cooling methods for CPV use heat sinks, forced air convection or liquid cooling, which can induce an extremely large convection area, or parasite electric consumption. In addition, the moving parts in cooling system usually result in a shorter life time and higher expense for maintenance. Therefore, there is a need for an improved cooling technology that enables significant improvement in CPV systems. As a passive and compact cooling mechanism, radiative cooling utilizes the transparency window of the atmosphere in the long wavelength infrared. It enables direct heat exchange between objects on earth’s surface with outer space. Since radiated power is proportional to the difference of the fourth powers of the temperatures of PV and ambient, significantly greater cooling powers can be realized at high temperatures, compared with convection and conduction. These qualities make radiative cooling a promising method for thermal management of CPV. In this work, experiments show that a temperature drop of 36 degree C have been achieved by radiative cooling, which results in an increase of 0.8 V for open-circuit voltage of GaSb solar cell. The corresponding simulations also reveal the physics behind radiative cooling and give a thorough analysis of the cooling performance.</p>

No description available.