Reversible Solar Heating and Radiative Cooling Devices via Mechanically Guided Assembly of 3D Macro/Microstructures.

Solar heating and radiative cooling are promising solutions for decreasing global energy consumption because these strategies use the Sun (≈5800 K) as a heating source and outer space (≈3 K) as a cooling source. Although high-performance thermal management can be achieved using these eco-friendly methods, they are limited by daily temperature fluctuations and seasonal changes because of single-mode actuation. Herein, reversible solar heating and radiative cooling devices formed via the mechanically guided assembly of 3D architectures are demonstrated. The fabricated devices exhibit the following properties: i) The devices reversibly change between solar heating and radiative cooling under uniaxial strain, called dual-mode actuation. ii) The 3D platforms in the devices can use rigid/soft materials for functional layers owing to the optimized designs. iii) The devices can be used for dual-mode thermal management on a macro/microscale. The devices use black paint-coated polyimide (PI) films as solar absorbers with multilayered films comprising thin layers of polydimethylsiloxane/silver/PI, achieving heating and cooling temperatures of 59.5 and -11.9 °C, respectively. Moreover, mode changes according to the angle of the 3D structures are demonstrated and the heating/cooling performance with skin, glass, steel, aluminum, copper, and PI substrates is investigated.

Bidirectional thermo-regulating hydrogel composite for autonomic thermal homeostasis.

Thermal homeostasis is an essential physiological function for preserving the optimal state of complex organs within the human body. Inspired by this function, here, we introduce an autonomous thermal homeostatic hydrogel that includes infrared wave reflecting and absorbing materials for improved heat trapping at low temperatures, and a porous structure for enhanced evaporative cooling at high temperatures. Moreover, an optimized auxetic pattern was designed as a heat valve to further amplify heat release at high temperatures. This homeostatic hydrogel provides effective bidirectional thermoregulation with deviations of 5.04 °C ± 0.55 °C and 5.85 °C ± 0.46 °C from the normal body temperature of 36.5 °C, when the external temperatures are 5 °C and 50 °C, respectively. The autonomous thermoregulatory characteristics of our hydrogel may provide a simple solution to people suffering from autonomic nervous system disorders and soft robotics that are susceptible to sudden temperature fluctuations.

Tailoring the Spectral Absorption Coefficient of aBlended Plasmonic Nanofluid Using a CustomizedGenetic Algorithm.

Recently, plasmonic nanofluids (i.e., a suspension of plasmonic nanoparticles in a base fluid) have been widely employed in direct-absorption solar collectors because the localized surface plasmon supported by plasmonic nanoparticles can greatly improve the direct solar thermal conversion performance. Considering that the surface plasmon resonance frequency of metallic nanoparticles, such as gold, silver, and aluminum, is usually located in the ultraviolet to visible range, the absorption coefficient of a plasmonic nanofluid must be spectrally tuned for full utilization of the solar radiation in a broad spectrum. In the present study, a modern design process in the form of a genetic algorithm (GA) is applied to the tailoring of the spectral absorption coefficient of a plasmonic nanofluid. To do this, the major components of a conventional GA, such as the gene description, fitness function for the evaluation, crossover, and mutation function, are modified to be suitable for the inverse problem of tailoring the spectral absorption coefficient of a plasmonic nanofluid. By applying the customized GA, we obtained an optimal combination for a blended nanofluid with the desired spectral distribution of the absorption coefficient, specifically a uniform distribution, solar-spectrum-like distribution, and a step-function-like distribution. The resulting absorption coefficient of the designed plasmonic nanofluid is in good agreement with the prescribed spectral distribution within about 10% to 20% of error when six types of nanoparticles are blended. Finally, we also investigate how the inhomogeneous broadening effect caused by the fabrication uncertainty of the nanoparticles changes their optimal combination.

Absorption characteristics of a metal-insulator-metal nanodisk for solar thermal applications.

Due to their ability to confine light in a sub-wavelength scale and achieve coherent absorption, plasmonic nanostructures have been intensively studied for solar energy harvesting. Although nanoparticles generating localized surface plasmon resonance (LSPR) have been thoroughly studied for application in a direct absorption solar collector (DASC), nanoparticles exciting magnetic polaritons (MP) for use in a DASC have not drawn much attention. In this work, we report a metal-insulator-metal (MIM) nanodisk that can excite MP peaks apart from the LSPR in the solar spectrum. It was found that the MIM nanodisk generates a broader and relatively more uniform absorption band compared to a pure metallic nanodisk. The MP peaks were also found to cause less significant scattering compared to those associated with the LSPR. We finally showed that the peaks induced by the MIM nanodisk are highly tunable by varying the particle dimensions, making the proposed MIM nanodisk a potential candidate for solar thermal applications.

Spectrally Selective Inorganic-Based Multilayer Emitter for Daytime Radiative Cooling.

Daytime radiative coolers are used to pump excess heat from a target object into a cold exterior space without energy consumption. Radiative coolers have become attractive cooling options. In this study, a daytime radiative cooler was designed to have a selective emissive property of electromagnetic waves in the atmospheric transparency window of 8-13 µm and preserve low solar absorption for enhancing radiative cooling performance. The proposed daytime radiative cooler has a simple multilayer structure of inorganic materials, namely, Al2O3, Si3N4, and SiO2, and exhibits high emission in the 8-13 µm region. Through a particle swarm optimization method, which is based on an evolutionary algorithm, the stacking sequence and thickness of each layer were optimized to maximize emissions in the 8-13 µm region and minimize the cooling temperature. The average value of emissivity of the fabricated inorganic radiative cooler in the 8-13 µm range was 87%, and its average absorptivity in the solar spectral region (0.3-2.5 µm) was 5.2%. The fabricated inorganic radiative cooler was experimentally applied for daytime radiative cooling. The inorganic radiative cooler can reduce the temperature by up to 8.2 °C compared to the inner ambient temperature during the daytime under direct sunlight.

Design of a Broadband Solar Thermal Absorber Using a Deep Neural Network and Experimental Demonstration of Its Performance.

In using nanostructures to design solar thermal absorbers, computational methods, such as rigorous coupled-wave analysis and the finite-difference time-domain method, are often employed to simulate light-structure interactions in the solar spectrum. However, those methods require heavy computational resources and CPU time. In this study, using a state-of-the-art modeling technique, i.e., deep learning, we demonstrate significant reduction of computational costs during the optimization processes. To minimize the number of samples obtained by actual simulation, only regulated amounts are prepared and used as a data set to train the deep neural network (DNN) model. Convergence of the constructed DNN model is carefully examined. Moreover, several analyses utilizing an evolutionary algorithm, which require a remarkable number of performance calculations, are performed using the trained DNN model. We show that deep learning effectively reduces the actual simulation counts compared to the case of a design process without a neural network model. Finally, the proposed solar thermal absorber is fabricated and its absorption performance is characterized.