ABSTRACT Due to the mismatch between solar spectrum and photovoltaic cells, solar thermophotovoltaic (TPV) technology attracts much interest. Near-field thermophotovoltaics (NFTPV) can further enhance radiation intensity and obtain optimized performance. This paper investigates the thermodynamic principles of solar NFTPV systems from the perspective of concentrating solar engineering. An NFTPV energy model under concentrated solar systems was established, which incorporated the non-radiative recombination losses in thermophotovoltaic cells to more accurately predict system efficiency. The parametric characteristics of solar NFTPV were analyzed from the perspective of energy balance, such as the effects of absorptance, concentration, and vacuum gap on the system performance. In addition, the shortcomings of NFTPV and the core of performance improvement were discussed; the limit efficiency of solar NFTPV was also investigated. It indicates that increasing the concentration ratio, reducing the vacuum gap, and reducing the absorber emittance can promote the system efficiency. The key to improving the system performance is to reduce the proportion of radiation loss. For micro/nano-film structure, where the vacuum gap is below 100 nm, the optimal concentration ratio was above 500, which is more suitable for dish-type concentrated application. The optimized solar NFTPV system achieved a maximum efficiency of 25.0%. Using micro/nano structure can reduce the need for small vacuum gap and the difficulty of NFTPV design in engineering. Different from previous research targeting a single NFTPV device with constant emitter temperature, this study focused on non-isothermal application scenarios of solar power generation, providing reference for NFTPV applications in solar engineering.

ABSTRACT In this era of ever reducing electronic component sizing, thermal management using Nano-encapsulated Phase Change Materials (nano-PCM) is gaining attention. This cooling technique utilizes latent heat absorption due to melting of nano-sized paraffin particles dispersed in a base-fluid. In the present study, a numerical model is developed based on Volume of Fluid multiphase model coupled with Enthalpy-Porosity model for analyzing forced convection cooling and melting characteristics of nano-PCM-based coolant in a Minichannel heat sink. Numerical simulations are performed to determine the effect of nano-PCM concentration and flow velocity on heat transfer and pressure drop in a rectangular minichannel heat sink of hydraulic diameter 2.383 mm with water as the base-fluid. The maximum improvement in heat transfer coefficient is found to be 36% (compared to pure water) by employing 1% concentration of nano-PCM in water. Although increasing the concentration of nano-PCM has been shown to enhance heat transfer performance, the corresponding rise in pumping power requirement due to the presence of nano-PCM particles is found to offset these benefits, particularly at higher concentrations. The study gives insight into the importance of optimizing flow velocity and particle concentration using the Figure of Merit parameter. It is evident from the simulations that at flow Reynolds number of 100 or above, the reduced residence time of particles inside the minichannel results in Figure of Merit dropping below that of water.

ABSTRACT Kinetic theory is a popular approach to model liquid-vapor phase change but accurate determination of evaporation and condensation coefficients remains a challenge. Reported values of coefficients vary by several orders of magnitude. For simplicity and convenience evaporation and condensation coefficients are assumed to be equal though there is little physical evidence to support this. This study presents a novel methodology to test this assumption using data from Constrained Vapor Bubble (CVB) experiments conducted on the International Space Station (ISS). The experiments consist of a quartz cuvette that is partially filled with n-pentane; heated and cooled at opposite ends to induce simultaneous evaporation and condensation around a central bubble. Data obtained from the NASA Physical Sciences Informatics (PSI) database enabled a three-dimensional reconstruction of the liquid–vapor interface. The net mass flux over the vapor bubble surface is zero at steady operation, providing a closure relationship for simultaneous and independent calculation of both evaporation and condensation coefficients. The resulting coefficient values are within 1% of each other but are not equal. The two coefficients are also within 2% of those predicted using transition state theory. When the evaporation and condensation coefficients are forced to be equal, the deviation from transition state theory is approximately 60%. This deviation monotonically increases with increasing rates of evaporation/condensation due to a systemic under-prediction of the bubble surface area. The agreement between derived coefficients and those predicted by transition state theory is maintained when the bubble surface area is corrected to account for Marangoni-induced interfacial instabilities.

ABSTRACT Liquid alkali metals hold great potential in applications requiring efficient heat transfer, such as nuclear power plants, solar energy systems, and process technology. Their high latent heat of vaporization and exceptional thermal conductivity make them promising alternatives to conventional heat transfer fluids like air, water, and molten salts. Understanding the phase change characteristics of liquid alkali metals under non-equilibrium heating conditions is crucial for optimizing their performance in such applications. This study employs molecular dynamics simulations to investigate the nanoscale phase transition behavior of a thin liquid sodium layer, a representative alkali metal, in contact with a gold surface. After initial equilibration, the wall temperature is increased linearly to a target value over various heating periods. Temporal changes in temperature, pressure, center of mass, and wall heat flux are monitored for qualitative heat transfer analysis, while average heat flux and evaporative mass flux provide quantitative evaluation. The results reveal diverse phase change modes, including an intriguing phenomenon where the liquid sodium film undergoes explosive boiling, resembling the “Leidenfrost” effect, even at temperatures significantly below its critical point. The onset time and wall temperature for explosive boiling vary across heating conditions, offering essential insights for preventing the “boiling crisis” in sodium-based thermo-fluid systems. These findings advance the understanding of phase change phenomena in liquid alkali metals and their implications for high-performance heat transfer applications.

ABSTRACT Based on the study of two-phase flow boiling heat transfer characteristics in rectangular microchannels, this paper proposes a novel mechanistic model for accurately predicting the heat transfer coefficient (HTC) in microchannels. Addressing the limitations of existing models in describing the liquid film evaporation process, the proposed model incorporates a bubble growth mechanism and refines the slug flow regime by considering the capillary number, thereby accurately capturing the dynamic evolution of bubble morphology. Furthermore, the model integrates the shear effect during liquid film thinning into the heat transfer calculations, establishing a comprehensive theoretical framework that accounts for interfacial momentum transfer. This approach more accurately reflects the heat transfer characteristics at the gas-liquid interface. Experimental data obtained using the cooling fluid R134a were employed to validate the model, demonstrating that the prediction error of the new model for the HTC is within 15.0%, significantly improving accuracy compared to traditional models. Analysis using the new model reveals that the HTC initially increases and then decreases with increasing capillary number. This trend provides a theoretical foundation for optimizing the surface structure of microchannels and regulating flow parameters. The model is capable of accurately predicting the heat transfer characteristics and pressure drop variations of bubbles at any location within the channel, offering reliable design insights for the development of next-generation compact and high-efficiency heat exchangers.

ABSTRACT Molybdenum disulfide (MoS2), a promising two-dimensional material, has attracted significant attention due to its unique electronic and thermal properties, making it a potential thermoelectric material Extensive research is underway to improve the thermoelectric properties of MoS2 by enhancing the figure of merit (ZT). However, there is currently limited research on the effects of thickness and temperature on the thermoelectric properties of MoS2 films prepared by magnetron sputtering. In this paper, amorphous MoS2 films with different thicknesses were first deposited using magnetron sputtering, ranging from 43nm to 231nm. The Frequency Domain Thermoreflectance (FDTR) system was established to measure the thermal conductivity of MoS2 film. The experimental results indicate a thermal conductivity range of 0.49 to 0.53W/m·K, with thickness showing minimal impact. The thermal boundary conductance between the sputter-deposited MoS2 film and the SiO2 substrate is measured at 12±5MW/m2 ·K. The electrical conductivity and Seebeck coefficient were measured in the temperature range of 300 K to 450 K using LSR-3 equipment. The results indicate that the MoS2 films prepared in this study were all p-type semiconductors, with electrical properties increasing with temperature. Within the temperature range of 300–430 K, the power factor of all films increases with the rise in temperature, reaching a maximum value of 0.36 µW/cm·K2

ABSTRACT Heat storage for waste heat sources at 200–250 C, where most power is available, remains challenging due to the lack of suitable storage materials. Here, we explore thermochemical heat storage at these temperatures based on cobalt oxide/hydroxide chemistry under hydrothermal conditions that is uniquely closed to mass flows. The closed system removes the need to separate and store individual products and is therefore, expected to be simpler and more compact than existing chemistries that require separation and storage of the product gas. The hydrothermal dehydration of cobalt hydroxide is attractive among other metal hydroxides due to its relatively modest pressure requirement and a temperature that is well-matched to low-temperature waste heat recovery. We find the theoretical round-trip energetic and exergetic efficiencies to be ~60% and ~50% respectively. In cycling experiments of hydrothermal dehydration and hydration, characterized using TGA, XRD, and XPS, we find the dehydration kinetics to be reasonably fast but the hydration to be limited to 40% on repeated conversion. TGA and TEM analysis of the product further suggest that this limit arises from the diffusion resistance of water through the cobalt hydroxide layer on the surface of the reacting oxide. Doping with Mg can yield a higher conversion limit. This work yields fundamental insights into a chemistry for novel thermochemical heat storage useful for low-to-mid temperature waste heat recovery.

ABSTRACT Integrating thermoelectric generators (TEG) under photovoltaic (PV) panels in PV-TEG systems holds great potential for maximizing energy utilization by collecting waste heat from PV panels while increasing efficiency. However, the presence of air gaps at contact locations due to metal surface roughness presents a substantial issue, resulting in increased thermal contact resistance (RC) and decreased system efficiency. To overcome this barrier, thermal interface materials (TIM) are used to fill air gaps, effectively decreasing RC, boosting heat transmission, homogenizing surface temperatures, and eventually increasing system efficiency. Carbon-based TIMs, notably pyrolytic graphite sheet (PGS), have emerged as very efficient replacements for conventional heat dissipation materials in electronic applications. Using PGS as a TIM can improve the overall performance of PV-TEG systems. This study provides useful insights into the elements that influence solar cell and TEG efficiency while offering a comprehensive analysis of the potential of carbon-based TIMs in enhancing PV-TEG system performance.

ABSTRACT This paper investigates the magnetohydrodynamic (MHD) heat and mass transfer characteristics of an electrically conducting micropolar fluid over a vertical stretching and shrinking sheets in a porous medium, accounting for variable thermal conductivity and mass diffusivity. The Runge-Kutta-Fehlberg method, combined with the shooting technique, is used to solve the non-similar governing equations and analyzed the effects of important physical parameters. The Runge-Kutta-Fehlberg (RKF) method is a fourth-fifth order method that offers high accuracy and adaptive step sizing, making it well-suited for the non-linear differential equations governing the system. The RKF method iteratively adjusts the guess initial values until the solution meets the boundary conditions at infinity, which is crucial for accurate modeling in this type of flow. The novelty of this work lies in its detailed exploration of micropolar fluid behavior over stretching and shrinking sheets within a porous medium, incorporating the Darcy parameter, buoyancy effects, and variable thermal and mass diffusion properties. This study analyzes the combined influence of these parameters on microrotation, temperature, and concentration distributions on stretching and shrinking surfaces under radiative and porous conditions, which provides valuable implications for optimizing processes in industries like filtration, enhanced oil recovery, and thermal management. Results show that increasing the Prandtl number decreases the temperature distribution. The study also indicates that microrotation profiles decrease with buoyancy ratio parameter increase.Additionally, velocity and microrotation increase with the distance parameter ξ , while temperature and concentration profiles decrease with for both stretching and shrinking sheets. Furthermore, it is found that the Nusselt number increases by 62.66% when the Prandtl number is raised from 3 to 7, but only by 21.77% for an increase from 7 to 10, showing that the rate of increase in the Nusselt number diminishes as the Prandtl number rises.

ABSTRACT In this study, the evaporation of pure water and NaCl solution films under different electric fields was investigated using molecular dynamics, and the intrinsic mechanism of enhanced evaporation by electric fields was analyzed from the perspectives of forces and thermodynamics. The simulations showed that, apart from a slight inhibitory effect of a direct current electric field on the evaporation efficiency of pure water, square wave electric fields promoted the evaporation of both pure water and NaCl solution, as well as direct current electric fields on NaCl solution, with the effect increasing with the strength and frequency of the field. For example, under a 200 GHz square wave electric field, the evaporation rates of the pure water and NaCl solution films were 8.33 and 2.65 times higher than under the direct current electric field, respectively; under horizontal electric fields of 0.1 V/nm and 0.3 V/nm, the evaporation rates of the NaCl solution film were 1.13 and 1.66 times higher than natural evaporation, respectively. The rate of evaporation efficiency is reflected in the change of molecular kinetic energy. The reasons for these phenomena are closely related to microscopic parameters and properties such as intermolecular forces, the number of hydrogen bonds, and the coordination number.The application of different electric fields affects the microscopic properties between molecules, leading to changes in the evaporation rate.