Thermal Radiation Losses Research Articles

The Role of Two Models of Thermal Radiation and Variable Properties on Thermal Explosion Critical and Transition Conditions of the Optically Thin Droplets

ABSTRACT Determining the criticality and disappearance of optically thin droplets (gas phase) by thermal explosions is crucial for maintaining thermal stability in various industrial and nonindustrial applications. The governing nonlinear ordinary differential equations of this problem make use of two distinct approximation models for thermal radiation loss, which were completed by Cogley et al. and Sohrab et al. Two methods of solving the problem – the Semenov method and the inflection point method – are used to identify the circumstances of criticality and disappearance of criticality (transition conditions). The governing equations utilize the use of the temperature-dependent Arrhenius frequency factor of the reaction and the thermal conductivity of the gas. The analytical solution of the equations yields the transitional values of the distinctive parameters at various initial conditions. Numerical methods are used to determine the equation’s exact solution. At specific initial conditions, the impact of θo, θa, γSh,F, and αSh,F on the temperature-time histories of ignition (supercritical region), non-ignition (subcritical region), and the critical boundary between them is derived. Notable effects on the ignition, non-ignition, and critical temperatures and times include the changing frequency of the reaction, the gas thermal conductivity, and two approximation models of radiation heat loss. The two radiation heat loss approximation models of the analytical and numerical findings are compared.

Ideal Photothermal Materials Based on Ge Subwavelength Structure.

Photothermal materials often prioritize solar absorption while neglecting thermal radiation losses, which diminishes thermal radiation conversion efficiency. This study addresses this gap by introducing a germanium (Ge) subwavelength structure (SWS) designed to optimize both solar absorption and infrared emissivity. Using a self-masked reactive ion etching (RIE) technique, we achieved a peak absorption of 98.8% within the 300 nm to 1800 nm range, with an infrared emissivity as low as 0.32. Under solar illumination of 1000 W/m2, the structure's temperature increased by 50 °C, generating a heating power of 800 W/m2. Additionally, it demonstrated good mechanical and thermal stability at high temperatures and possessed a hydrophobic angle of 132°, ensuring effective self-cleaning. These characteristics make the Ge SWS suitable for application in solar panels, displays, sensors, and other optoelectronic devices.

Three-dimensional transient modeling and simulation of high-current multi-component vacuum arc under transverse magnetic field

This paper established a three-dimensional transient MHD model of a high-current multi-component vacuum arc under a transverse magnetic field (TMF) to simulate the dynamic characteristics of plasma parameters in the uniform motion mode of a contracted arc under lower TMF. The model considered the ionization–recombination process among the multi-components and used the P1 model to calculate the thermal radiation losses. The simulation results show that the ion number density of The TMF arc is on the order 1024 m−3, and when the plasma temperature reaches 4.3 eV, the effects of thermal radiation cannot be ignored. The majority of the plasma from the cathode and anode undergo acceleration and then deceleration before finally intersecting at the center of the arc, creating an extreme value of the density and the pressure, where a large amount of kinetic energy is converted into internal energy. The arc will be bent and deflected under the action of the ampere force, and the deflection of Cu3+ is especially obvious. The TMF affects the collision strength of the jet in the intersection area and, thus, affects the ion density, in which the change of Cu2+ is dominant. The ionization–recombination process of ions is mainly determined by the electron temperature, which is affected by the arc current. As the current decreases, Cu1+ increases and Cu2+ and Cu3+ decrease, and the change in ionization rate is the main reason for the change in the proportion of each ionic component; the heat flux density to the anode and cathode is on the order of 1010 W/m2, which heats the front of the arc and ensures the stable movement of the arc. Meanwhile, due to the high ion number density, the ion heat flux accounts for the main part of the heat flux, and the anode exhibits a higher proportion of ion heat flux.

Resonant-mode metasurface thermal super mirror by deep learning-assisted optimization algorithms

A “super-mirror” having ultrahigh infrared reflectance is achieved by an optimized photonic contrast grating metasurface. Finding ways to achieve this exceptional performance can be enabled by implementing global optimization and machine learning elements, such as Bayesian optimization and genetic algorithm. Here, we acquired an optimized grating design made of high-index germanium, which excites resonances that result in ultralow emittance at certain wavelengths. Our optimizations assisted in the discovery of hybridized coupling of Fabry-Pérot modes and guided modes in a monolithic microscale multilayered coating. We demonstrate constraints in the given geometric variable ranges improves the overall performance of algorithms. We also show the enhanced performance of a deep learning Feedforward Neural Network, which is implemented as the inverse design using the network trained with dataset obtained from Bayesian optimization and Genetic Algorithm approaches. The performance of the Feedforward Neural Network-assisted design produced normal emissivity difference by only +3.5 %, with lower sensitivity to grating dimensional parameter variations. The improvement is achieved by predicting and better understanding of the optical physics of resonant gratings. The proposed few-layer grating coating can be applied to space components, enclosures, and vessels to suppress thermal radiative heat loss.

Dual-Mode MXene-Based Phase-Change Composite Towards Enhanced Photothermal Utilization and Excellent Infrared Stealth.

Solar thermal collectors based on phase change materials (PCMs) are important to promote the civilian use of sustainable energy. However, simultaneously achieving high photothermal efficiency and rapid heat transfer of the PCM carrier typically involves a high proportion of functional materials, contradicting a satisfying energy storage density. In this work, a surface-engineered anisotropic MXene-based aerogel (LMXA) integrated with myristic acid (MA) to produce phase change composites (LMXA-MA) is reported, in which the laser-treated surface composed of the hierarchically-structured TiO2/carbon composites act as a light absorber to improve solar absorption (96.0%), while the vertical through-hole structure allows for fast thermal energy transportation from surface to the whole. As a result, LMXA-MA exhibits outstanding thermal energy storage (192.4 J·g-1) and high photothermal conversion efficiency (93.5%). Meanwhile, benefiting from the intrinsic low emissivity of MXene material, thermal radiation loss can be effectively suppressed by simply flipping LMXA-MA, enabling a long-term temperature control ability (605 s·g-1). The excellent heat storage property and switchable dual-mode also endow it with an infrared stealth function, which maintains camouflage for more than 240 s. This work provides a prospective solution for optimizing photothermal conversion efficiency and long-term thermal energy preservation from surface engineering and structural design.

Optically manipulated nitrate-salt-based direct absorption solar collectors for a photothermal energy harvesting system

This study introduces a nitrate-salt-based direct absorption solar collector (DASC) for a photothermal energy harvesting system with efficient solar energy harvesting and the reduction of thermal radiation loss. Investigations of the optical properties of snow-like solid-state binary nitrate salt in solar and thermal infrared (IR) spectral regions were first conducted in this study. Research revealed that the solid-state binary nitrate salt showcases high solar reflectivity (>90 %) and infrared emissivity (∼0.85), leading to a cooling power of 36.4 W m−2. This implies pure solid salt cannot gain energy from solar illumination. To tackle this, the proposed system incorporated nano graphite and a hot mirror. Consequently, the absorbed solar energy increased from 67.6 W m−2 to 898.7 W m−2, resulting in a heating power of 794.0 W m−2 under AM 1.5G solar irradiance. With the proposed approaches, melting the salt with lowest solar concentration ratio of 15 (∼12.5 kW m−2) was achieved.

Impact of Temperature and Optical Error on the Combined Optical and Thermal Efficiency of Solar Tower Systems for Industrial Process Heat

Concentrating solar thermal (CST) power towers can provide high flux concentrations at commercial scale. As a result, CST towers exhibit potential for high-temperature solar industrial process heat (SIPH) applications. However, at higher operating temperatures, thermal radiation losses can be significant. This study explores the trade-off between thermal and optical losses for SIPH applications using a collection of three case studies at operating temperatures that range from 900-1,550 °C. We assume blackbody radiation to represent the thermal losses at the receiver and we use ray tracing to estimate the optical losses. The results show the impact of process temperature on the maximum attainable system efficiency, as well as the higher flux concentration requirements as the temperature increases.

Super stable coal fly ash-based solid heat-collecting particles with excellent spectral selectivity

Solar thermal power generation has been widely employed as an effective method for efficiently harnessing solar energy, with solid heat-collecting particles serving as the medium for collecting and storing solar energy, thus holding significant developmental potential. However, challenges such as high thermal radiation losses and inadequate high-temperature stability have hindered the further advancement of solid heat-collecting particles. The present study addresses these challenges by utilizing solid waste fly ash to prepare solar-selective absorbing solid heat-collecting particles. Specifically, particles sintered at 1070℃ exhibit impressive characteristics, including an average solar absorptance of 93.26 % at room temperature, an emissivity in the mid-infrared range of 68.12 %, and a specific heat capacity of 0.82 J/(g·℃). Moreover, rigorous high-temperature stability tests demonstrate that these particles maintain a stable solar absorptance in the ultraviolet–visible-near-infrared range, consistently around 93.26 %. Additionally, the particles exhibit outstanding wear resistance and stability, further enhancing their suitability for practical applications. This research achieves the dual objectives of environmental sustainability and cost reduction by utilizing solid waste materials, while the preparation process remains simple, bolstering the feasibility of implementing these findings in real-world scenarios. In summary, this study effectively addresses the challenges of high thermal radiation losses and poor high-temperature stability in solid heat-collecting particles for solar thermal power generation.

Design and thermal test of high-vacuum insulator for heat delivery pipes

Thermal piping insulation of implants is crucial for heat delivery, production, collection, or storage at high temperature values. It is currently obtained by enveloping low thermal conductivity materials such as rockwool, fiberglass, polyurethane, polystyrene, and aerogel. However, better performances can be reached by adopting vacuum technology. In this case, conductive losses are annihilated, and the radiative heat transfer mechanism represents the only loss mechanism. Here, we compare a high vacuum-based novel solution and the traditional insulation for heat delivery applications. We propose a high vacuum- based solution consisting of an evacuated gap that surrounds the hot pipe coated by a thin aluminium foil. Experimental results using this novel solution show a fivefold reduction of the thermal radiation losses compared to the traditional solutions when in the temperature range between 100 °C and 250 °C.

Novel fabric-based 3D photothermal evaporator with advanced light-harvesting and thermal management design

Interfacial photothermal evaporation offers a promising and cost-effective method of obtaining freshwater. Traditional photothermal evaporators, when exposed to sunlight, typically have a heating region on their surface with temperature higher than room temperature. This can lead to thermal radiation loss and reduce evaporation efficiency. Therefore, this study presents an innovative design and fabrication of a fabric-based conical roll (FCR) evaporator, which enables low-temperature evaporation without a distinct heating region and facilitates the reversal of the thermal radiation path. In particular, the FCR-7.5-45 evaporator demonstrated remarkable performance even under 2.0 sun illumination, showcasing its advanced light-harvesting capability and excellent thermal management ability attributable to the conical spiral groove structure. The unique cold evaporation surface of the 3D evaporators, combined with this design, resulted in a high evaporation efficiency of 151.53 % and a rapid evaporation rate of 3.47 kg·m−2·h−1 under 1.0 sun illumination. Furthermore, the freshwater produced by the FCR evaporator met the drinking water standards set by international regulatory bodies. This indicates that the photothermal evaporator developed in this project holds great potential for widespread application in desalination and sewage treatment.

Biomineralization-Inspired Copper Sulfide Decorated Aramid Textiles via In Situ Anchoring toward Versatile Wearable Thermal Management.

Designing smart textiles for personal thermal management (PTM) is an effective strategy for thermoregulation and energy saving. However, the manufacture of versatile high-performance thermal management textiles for complex real-world environments remains a challenge due to the limitations of functional integration, material properties, and preparation procedures. In this study, an aramid fabric based on in situ anchored copper sulfide nanostructure is developed. The textile with excellent solar and Joule heating properties can effectively keep the body warm even at low energy inputs. Meanwhile, the reduced infrared emissivity of the textile decreases the thermal radiation losses and helps to maintain a constant body temperature. Impressively, the textile integrates superb electromagnetic shielding, near-complete UV protection properties, and ideal resistance to fire and bacteria. This work provides a simple strategy for fabricating multi-functional integrated wearable devices with flexibility and breathability, which is highly promising in versatile PTM applications.

Inner-cascaded photovoltaic-thermionic-thermoradiative conversion for concentrated solar power

Photon enhanced thermionic emission (PETE) is a promising full-spectrum solar power technology capable of converting super-bandgap photons via photovoltaic and sub-bandgap photons through thermionic emission. However, PETE cathode with an infrared absorber, is usually accompanied with a high thermal radiation loss to the environment. This paper proposes an isothermal thermoradiative enhanced-PETE (iTR-PETE) configuration, where p-type cathode absorbs high-energy photons and n-p junction anode converts low-energy photons for thermoradiative purposes. A thermodynamic model is developed to assess the respective impacts of photovoltaic, thermionic and thermoradiative on solar iTR-PETE conversion. The positive and negative chemical potentials of the cathode and anode work together to increase thermionic power. The iTR-PETE converter achieves an excellent exergy efficiency of 55.43 % at a concentration ratio of 40 and an anode bandgap of 0.1 eV. Photovoltaic determines thermoradiative power, while thermoradiative increases waste heat to the secondary thermal cycle to improve the overall solar conversion efficiency. Along with an additional 4.24 % of power generated at anode, the thermoradiative effect reduces thermal radiation from the PETE converter by 28.54 %. The iTR-PETE combined system achieves a 92.3 % higher conversion efficiency than that of conventional PETE combined system.

Thermal Management of the Solar Evaporation Process.

Solar-driven interfacial evaporation has caught wide attention for water purification due to its green and environment-friendly properties. The key issue is how to effectively utilize solar radiation for evaporation. To fully understand the thermal management of the solar evaporation process, a multiphysics model has been built by the finite element method to clarify the heat transfer process for the improvement of solar evaporation. Simulation results indicate that the evaporation performance can be improved by tuning the thermal loss, local heating, convective mass transfer, and evaporation area. The thermal radiation loss of the evaporation interface and thermal convection loss to the bottom water should be avoided, and local heating is good for evaporation. Convection above the interface can improve evaporation performance, although it would enhance the thermal convective loss. In addition, evaporation also can be improved by increasing the evaporation area from 2D to 3D structures. Experimental results confirm that the solar evaporation ratio can be improved from 0.795 kg m-2 h-1 to 1.122 kg m-2 h-1 at 1 sun with a 3D interface and thermal insulation between the interface and bottom water. These results can provide a design principle for the solar evaporation system based on thermal management.

Highly efficient photothermal catalytic upcycling of polyethylene terephthalate via boosted localized heating

Photothermal catalysis driven by clean solar energy efficiently converts plastic waste into high-value-added products. The catalytic process involves the transformation of solar energy to chemical energy. However, designing photothermal catalysts with a high conversion efficiency and catalytic activity remains considerably challenging. In this study, a c-ZIF-8@SiO2 nanostructure is fabricated. It acts both as the photothermal reagent and the catalyst, displaying high photothermal conversion efficiency, catalytic activity, and stability in polyethylene terephthalate (PET) glycolysis. SiO2-coated c-ZIF-8 effectively reduces the thermal radiation loss of the carbon material, thus enhancing the local thermal effect of the catalytic system. Consequently, the conversion efficiency of PET achieved using photothermal catalysis is 3.4 times higher than that of thermal catalysis under the same conditions. An economic efficiency analysis proves that photothermal catalysis can save 6390000 kW·h of electricity and reduce up to 3089.59 tons of CO2 emissions for every 10000 tons of PET recycled. Therefore, the development of clean energy-driven photothermal catalysis technology could be a potential solution for the upcycling of waste plastics.

Reduced order 1 + 3D numerical model for evaluating the performance of solar borehole thermal energy storage systems

This study presents a computationally efficient numerical model for solving the fluid flow and heat transfer phenomena in co-axial boreholes heat exchangers arranged on a rectangular grid (N × N) drill hole pattern. Such systems of boreholes are an integral component of a Solar-Borehole Thermal Energy Storage System for seasonal thermal storage application. The numerical model couples a one-dimensional fluid flow and convective heat transfer model in the heat exchanger with a three-dimensional model of conductive heat transfer in the strata. In addition to solving mass and heat transfer in borehole systems, it integrates a solar thermal collector system and building dynamic thermal load to design a Solar- Borehole Thermal Energy Storage system for a 182-unit residential apartment building. Simulation also considers and evaluates the conductive, convective, and radiative thermal losses of the system. The model is validated using experimental data from a coaxial borehole heat exchanger and compared to the results of a commercial finite volume solver for multiple co-axial boreholes. The computational cost of developed numerical model is reduced by a factor of 194 compared to a commercial finite volume solver. Solar- Borehole Thermal Energy Storage system simulation is performed for a period of five years considering hourly fluctuations in solar irradiance and building dynamic thermal energy demand. Parametric study is performed on depth and spacing of boreholes, and mass flowrate in the solar thermal collectors. It is found that a system of 100 boreholes, each 100 m deep, linked to 674 solar thermal collectors can provide the heating for the building with a total thermal demand of 5600 GJ per year.

Thermodynamic and Magnetic Topology Evolution of the X1.0 Flare on 2021 October 28 Simulated by a Data-driven Radiative Magnetohydrodynamic Model

Solar filament eruptions, flares, and coronal mass ejections (CMEs) are manifestations of drastic releases of energy in the magnetic field, which are related to many eruptive phenomena, from the Earth’s magnetosphere to black hole accretion disks. With the availability of high-resolution magnetograms on the solar surface, observational data-based modeling is a promising way to quantitatively study the underlying physical mechanisms behind observations. By incorporating thermal conduction and radiation losses in the energy equation, we develop a new data-driven radiative magnetohydrodynamic model, which has the capability of capturing the thermodynamic evolution compared to our previous zero-β model. Our numerical results reproduce the major observational characteristics of the X1.0 flare on 2021 October 28 in NOAA active region 12887, including the morphology of the eruption, the kinematics of the flare ribbons, extreme ultraviolet (EUV) radiations, and the two components of the EUV waves predicted by the magnetic stretching model, i.e., a fast-mode shock wave and a slower apparent wave, due to successive stretching of the magnetic field lines. Moreover, some intriguing phenomena are revealed in the simulation. We find that flare ribbons separate initially and ultimately stop at the outer stationary quasi-separatrix layers (QSLs). Such outer QSLs correspond to the border of the filament channel and determine the final positions of flare ribbons, which can be used to predict the size and the lifetime of a flare before it occurs. In addition, the side views of the synthesized EUV and white-light images exhibit typical three-part structures of CMEs, where the bright leading front is roughly cospatial with the nonwave component of the EUV wave, reinforcing the use of the magnetic stretching model for the slow component of EUV waves.

Effect of low temperature on thermal runaway and fire behaviors of 18650 lithium-ion battery: A comprehensive experimental study

To investigate the effect of operating temperature on the thermal runaway (TR) and combustion behavior of lithium-ion batteries(LIBs), a series of LIB thermal abuse experiments were performed after cycling at different temperatures and cycle rates. Local heating of the electrothermal film was used to simulate the abuse process triggered by the TR of adjacent LIB in the module. Combustion behavior, cell surface and flame temperatures, thermal radiation and mass loss were measured. For the cell cycling at 0 °C, the amount of gas in the exhaust phase was obviously reduced, and the intensity of flame combustion was reduced. For the battery after cycling at − 10 °C, deflagration occurred within 2 s after the safety valve cracked. The experimental results show that the onset temperature of TR of LIB decreased significantly with the operating temperature decreasing from 25 °C to − 10 °C. Low-temperature cycling causes safety valve cracking and TR to occur earlier. This work presents in detail the characteristics and mechanism of TR behavior changes of LIBs at low temperatures, which provides guidance for process safety assurance and fire protection design for the practical engineering applications of battery at low temperatures.

MXene-Decorated Smart Textiles with the Desired Mid-Infrared Emissivity for Passive Personal Thermal Management.

Multifunctional and long-term stable wearable heating systems have attracted extensive attention from experts, yet smart textiles that only rely on harvesting the body's heat without additional energy still face huge challenges in practical applications. Herein, we rationally prepared the monolayer MXene Ti3C2Tx nanosheets via an in situ hydrofluoric acid generation method, which was further employed to construct a wearable heating system of MXene @ polyester polyurethane blend fabrics (MP textile) for the passive personal thermal management through a simple spraying process. Owing to the unique two-dimensional (2D) structure, the MP textile presents the desired mid-infrared emissivity, which could efficiently suppress the thermal radiation loss from the human body. Notably, the MP textile with an MXene concentration of 28 mg/mL exhibits a low mid-infrared emissivity of 19.53% at 7-14 μm. Significantly, these prepared MP textiles demonstrate an enhanced temperature of more than 6.83 °C compared with those of favorably traditional fabrics, involving the black polyester fabric, pristine polyester polyurethane blend fabric (PU/PET), and cotton, suggesting a charming indoor passive radiative heating performance. The temperature of real human skin covered by MP textile is 2.68 °C higher than that covered by cotton fabric. Impressively, these prepared MP textiles simultaneously possess attractive breathability, moisture permeability, mechanical strength, and washability, which provide new insight into human body temperature regulation and physical health.

Internally shielded receivers for parabolic trough solar concentrators operating with supercritical carbon dioxide: Analytical assessment

An urgent need to enable the operation of parabolic trough concentrators at higher temperatures for higher efficiency of the coupled power cycles is manifested in the state-of-art literature. Supercritical carbon dioxide is a promising fluid for the next generation of these concentrators. However, the inflated radiative thermal losses at elevated temperatures are the main challenge, and the relatively large absorber diameters are questionable for supercritical pressures. This study proposes a novel 1-D analytical model, coupled with Monte Carlo ray tracing, for evaluating internally shielded receivers based on the absorber tube’s diameter and eccentricity, the heat shield’s opening angle, as well as the operating conditions. The results suggest that increasing the tube diameter reduces the performance dependency on tube eccentricity, but also reduces the optical performance. Shielding the receiver is favorable for opening angles up to 160°. The impact of operating pressure is marginal, compared to flow rate and temperature. A shielded receiver with an opening angle of 160°, eccentricity of −10 mm, and tube diameter of 50 mm maximizes the exergy efficiency up to 45.9%, whereas a receiver with corresponding values of 80°, −15 mm, and 80 mm, respectively, maximizes the energy efficiency up to 77.2%. Shielded receivers also boost the efficiency by up to 10.6%, compared to conventional 70 mm receivers. These performance enhancements are expected to elevate the efficiency of the integrated systems with minimal economic and environmental impacts due to the simplicity of the proposed solution.

Thermoradiative induced absorber-emitter to enhance solar thermoelectric conversion

Solar thermoelectric power, is promising for use in small-scale CSP system due to its flexible architecture and silent operation. However, the conversion performance of standalone solar thermoelectrics is still far from ideal at lower concentration ratio. This paper tends to introduce a self-powered thermoradiative absorber-emitter at the top to enhance the overall solar thermoelectric power generation. Thermodynamic model based on energy balance has been established, and the implications of critical factors on the conversion performance of the thermoradiative-thermoelectric (TR-TE) combined system have been thoroughly examined. The findings show that an optimal ratio of absorber/emitter surface area of 1 is favored due to a tradeoff between thermal radiation loss from the absorber to the ambient and thermoradiative emission dissipation, as well as the thermoradiatives with a low bandgap and a high output voltage. The overall conversion efficiency of the solar TR-TE combined system approaches to 27.86 % at a cold junction temperature of 300 K and a concentration ratio of 50. Accordingly, the maximum efficiency of the combined system is 6.51 times that of independent solar thermoelectrics and is ∼13.67 % higher than independent solar thermoradiatives. In the TR-TE combined system, thermal radiation from the absorber to the environment accounts for 3.83 % of the solar radiation input as compared to 2.47 % in the independent solar TE generator. While the heat dissipated at the cold junction of a solar TR-TE system is 43.66 %, considerably less than that of 84.6 % for independent solar thermoelectrics.