Emitter Temperature Research Articles

Fabrication of 0.6 eV Bandgap In0.69Ga0.31As Thermophotovoltaic Cells and Its System Demonstration

Thermophotovoltaic (TPV) is a promising energy conversion technology that can absorb the heat from a thermal radiator and transfer it into power. As the most significant energy converter, TPV cells need a narrower bandgap to realize a wider absorption spectrum range. In this work, the fabrication and characterization of single‐junction In0.69Ga0.31As TPV cells with a bandgap of 0.6 eV are presented. The main structure is grown on an InP substrate through metal‐organic chemical vapor deposition. Step‐graded InAsyP1−y buffer layers are used to mitigate the dislocations by relaxing the stress induced by lattice mismatch completely. Analysis of the composition, strain relaxation, layer tilt, and crystalline quality of each layer is demonstrated using triple‐axis X‐ray reciprocal space mapping and transmission electron microscopy. According to the tested results, each layer is found to be nearly fully relaxed and the InGaAs active layer grown on the buffer displays a high crystal quality. External quantum efficiency achieves 90% at 1100–1500 nm. Additionally, a TPV test platform is constructed to evaluate the cell performance. The maximum efficiency of the lattice‐mismatched TPV cell reaches 21.92% operating at a power density of 267.4 mW cm−2 and an emitter temperature of 1200 °C.

Multi-objective spectral range optimization for different heat source temperatures to maximize thermophotovoltaic performance using NSGA-II

The energy conversion efficiency (ECE) and output power density of thermophotovoltaic (TPV) cells are mutually constraining. To better utilize this relationship, a framework was proposed to integrate the internal quantum efficiency (IQE) of TPV cells with a multi-objective genetic algorithm (NSGA-II) to optimize both efficiency and power density across diverse operating conditions. A spectrally selective emitter with a temperature of 1000–3000 K was selected as the radiation source for the studied TPV devices, capable of emitting a spectrum between 0.4 and 2.0 μm. The gallium antimonide (GaSb) cell was selected as the exploration cell, operating within a temperature range of 0–200 °C. Through theoretical calculation, the changes in physical parameters and IQE curves of GaSb cells at various temperatures were determined. The optimal spectral range for varying cell and emitter temperatures was determined using the NSGA-II algorithm. It was found that the IQE curve decreases with increasing temperature. Due to the spectral mismatch, the TPV conversion efficiency is much less than 20 % when the spectral range of the cell is 0.4–2.0 μm at room temperature. By incorporating IQE into the multi-objective optimization, the efficiency and power density distribution curves can be divided into three regions. In practical applications, the corresponding regions can be selected based on different efficiency and output power requirements.

A coupled plasma-thermal model for hollow cathode power decomposition and parametric analysis

Abstract A 2D axisymmetric transient coupled plasma-thermal model is developed to simulate the plasma behavior during the self-sustained discharge of hollow cathodes, which presents a complete hollow cathode structure and energy transfer processes in multiphysics fields. The model has been validated by quantitative agreement between the simulation results and experimental data on the plasma and emitter temperature at the NSTAR cathode. The effects of thermal protection design, operating conditions, and geometric design on the cathode performance are analysed through electric and thermal power decomposition. The parametric analysis shows that the optimal thermal protection design is to use a 1/3 thickness cathode tube with 4 layers of radiation shielding close to the tube, which reduces 43.7% conductive and 61.1% radiative heat dissipation, respectively. Increasing the inlet flow rate counter-intuitively reduces the emitter temperature due to the potential reversal in the diffusion electric field dominated region, revealing that the flow rate can be traded for the dual optimisation of lifetime and power consumption. Under high current conditions, the IAT effect dominates the plume resistance to increase the discharge voltage after an inflexion point, which is the main factor limiting the cathode performance. A large internal radius gives a uniform low emission and helps to prolong life, while the orifice length should be avoided to be longer than 4 times the orifice radius due to the significantly enhanced Joule heating in the narrow orifice. The orifice radius determines the power deposition due to electron and ion bombardment through potential penetration. For high-current discharger cathodes dominated by electron bombardment, large or through-orifice designs are preferred, while for low-current neutralizer cathodes dominated by electron bombardment, small-orifice designs are recommended.

Ultra-low frequency discharge instability of a barium tungsten hollow cathode at lower self-sustaining margin

Development of low-orbiting constellations stimulated the demand for low-power Hall thrusters, as well as their accompanying low-current hollow cathodes. In this paper, such cathodes were tested on quite low currents to search their self-sustaining limit, and were found to exhibit an ultra-low frequency instability at lower current limit. The instability would grow fast when the cathode current was below 0.3A, and oscillate on 0.1Hz to a few Hz with a visibly blinking plume. At the same time, the emitter temperature was fluctuating with a 15 °C peak-to-peak amplitude. The instability was confirmed to originate from the cathode interior, as a result of depleted plasma heating. Then the thermionic cathode had to rely mainly on neutral gas to heat the emitter. Because thermionic emission was in exponential relation with emitter temperature, the temperature fluctuation was then amplified to cause larger fluctuation in total current as well as in total discharge power, which led to deeper fluctuation in temperature. Regarding the heating of both neutral gas and emitter, an instability model showed that multiple unnoticed factors could determine the onset of instability, including the thermal inertia of neutral gas and emitter, the heat transfer from discharging plasma to neutral gas, the heat loss of cathode and the response time of power supply. As a result of this instability, the plume ion energy was elevated from <50eV to 30∼120eV due to the fluctuating density on cathode exit and fluctuating discharge voltage. It was estimated that the increased ion energy could accelerate the keeper erosion by 4∼22 times.

Near-field radiative thermal rectification assisted by Bi2Se3 sheet

The ability to control heat flux at the nanoscale opens up numerous exciting possibilities in modern electronics and the field of information processing. In this research, we propose a design with the focus on achieving efficient thermal rectification at moderate gap and relatively low temperature. This study centers on near-field thermal radiation between temperature dependent indium antimonide (InSb) and silicon carbide (3C-SiC) coated with bismuth selenide (Bi2Se3). Our investigation sheds light on the critical role played by the Bi2Se3 layer in enhancing various key parameters, including the net radiative flux, and thermal rectification efficiency (η). We achieved a substantial improvement in the η of a near-field radiative thermal rectifier (NFRTR) due to the presence of the Bi2Se3 sheet. This enhancement is contingent on factors such as the Fermi energy (Ef) of Bi2Se3, emitter temperature, and the vacuum gap (d). Our study culminated in the identification of an optimal design, achieving an impressive η of 75% at an emitter temperature (TH) of 350 K, with vacuum gap (d) set to 20 nm. Furthermore, increasing TH to 500 K resulted in even more promising outcomes, with the highest η reaching 93%. The need for operating the optimized device at moderate temperatures is to strike a balance between efficiency, safety, cost-effectiveness, and material compatibility. These findings represent a significant step forward in the development of efficient Bi2Se3-based NFRTRs, paving the way for future applications in thermal management, energy conversion systems, and thermal logic gates.

Graphene Thermal Infrared Emitters Integrated into Silicon Photonic Waveguides.

Cost-efficient and easily integrable broadband mid-infrared (mid-IR) sources would significantly enhance the application space of photonic integrated circuits (PICs). Thermal incandescent sources are superior to other common mid-IR emitters based on semiconductor materials in terms of PIC compatibility, manufacturing costs, and bandwidth. Ideal thermal emitters would radiate directly into the desired modes of the PIC waveguides via near-field coupling and would be stable at very high temperatures. Graphene is a semimetallic two-dimensional material with comparable emissivity to thin metallic thermal emitters. It allows maximum coupling into waveguides by placing it directly into their evanescent fields. Here, we demonstrate graphene mid-IR emitters integrated with photonic waveguides that couple directly into the fundamental mode of silicon waveguides designed to work in the so-called "fingerprint region" relevant for gas sensing. High broadband emission intensity is observed at the waveguide-integrated graphene emitter. The emission at the output grating couplers confirms successful coupling into the waveguide mode. Thermal simulations predict emitter temperatures up to 1000 °C, where the blackbody radiation covers the mid-IR region. A coupling efficiency η, defined as the light emitted into the waveguide divided by the total emission, of up to 68% is estimated, superior to data published for other waveguide-integrated emitters.

High-resolution and reduced-order multi-physics simulation on thermal–hydraulic and thermoelectric characteristics of the thermionic space reactor core

The simulation of a thermionic space reactor core necessitates a comprehensive consideration of the intricate interplay between fluid flow, heat transfer, and thermionic energy conversion, constituting a multi-physics coupling challenge. Given the absence of a thermionic conversion model in contemporary CFD (Computational Fluid Dynamics) codes, accurately simulating the thermal–hydraulic and thermo-electric behavior of thermionic reactors in existing CFD software is particularly challenging. To analyze the multi-physics coupling phenomena in thermionic reactors, this paper revises the conjugate heat transfer solver in the proprietary 3D (three-dimensional) finite-volume CFD code, mFVM (modular Finite-Volume Method Code), to enable bidirectional coupling with the thermionic conversion model. This enhancement aims to predict the high-resolution three-dimensional temperature field and thermoelectric characteristics of the thermionic reactor core. Furthermore, a reduced-order multi-physics model of the thermionic reactor core is established using RESYS (Reactor System Simulation Code). The numerical simulation outcomes for both the high-resolution and reduced-order multi-physics models, pertaining to electrically heated and fission-heated TFEs (Thermionic Fuel Elements), are validated against experimental data. The results indicate that the emitter electron cooling effects caused by thermionic emission reduced the maximum temperature of emitter by about 200 K for both electrically heated and fission-heated TFE. Therefore, a bidirectional coupling between the thermal–hydraulic model and the thermionic conversion model is essential for precisely calculating the temperature field within the TFE and the thermionic reactor core. Thereafter, both the high-resolution model and the reduced-order model are utilized to investigate the thermal–hydraulic and thermoelectric characteristics of the TOPAZ-II reactor core. The calculated electrical power of the TOPAZ-II reactor is 5.39 kWe using the mFVM model and 5.47 kWe using the RESYS model. Finally, an analysis of the thermoelectric conversion characteristics based on the simulation results of the reduced-order model reveals that the TOPAZ-II reactor system is capable of load-following only when the load resistance exceeds 0.06 Ω under nominal full power operation conditions.

Surface activation of n-type AlGaN with cesium and oxygen to enhance thermionic emission

The aim of this study is to enhance the characteristics of thermionic emission of AlGaN surface through surface control employing cesium (Cs) and oxygen. Cs-deposited AlGaN has significant applications in thermionic energy converters. However, as the emitter temperature increases, the thermal desorption of Cs from AlGaN surface increases, resulting in a decrease in the thermionic emission current. Therefore, focusing on the high affinity between Cs and oxygen, we investigated the possibility of suppressing thermal desorption by depositing Cs and oxygen on AlGaN surface. The thermionic emission current measured when Cs and oxygen were alternately deposited on AlGaN surface was 1.9 × 10−3 A cm−2 at 500 °C. The thermionic emission current was significantly higher than that obtained with Cs-only deposition (2.0 × 10−5 A cm−2). In addition, we attempted to reproduce the effect of dynamic surface changes on thermionic emission employing a new thermionic emission model (modified Richardson–Dushman model) that considers the correlation between a specific surface reconstruction phase and its thermionic emission component. The results suggest that the adsorbed component of Cs-deposited AlGaN exhibits three Cs adsorption sites with different desorption energies, while the adsorbed component of Cs/O2 co-deposited AlGaN exhibits at least four Cs adsorption sites with different desorption energies. It is suggested that the increase in adsorption components with higher desorption energies, caused by the deposition of oxygen, may have reduced the thermal desorption and improved Cs coverage and stability.

Broadband mirrors for thermophotovoltaics.

We present an innovative solution to improve the efficiency of thermophotovoltaic (TPV) devices by tackling the problem of sub-bandgap photon losses. We propose an optimized design for thin-film mirrors using inverse electromagnetic design principles, thereby enhancing the average reflectivity and photon re-use. Our method surpasses the traditional Bragg mirror by employing a gradient-descent based optimization over Bragg mirror geometrical parameters, leveraging the transfer matrix method for derivative calculations. The optimized structure, based on continuously chirped distributed Bragg reflectors proposed herein demonstrates a remarkable increase in reflectivity beyond 98%, over an almost three-octaves bandwidth (0.1eV-0.74eV). We show that the incident power loss in InGaAs TPV cells at an emitter temperature of 1200°C is significantly reduced. While our work shows considerable promise, further exploration is needed to ascertain the practicability and robustness of these designs under various operational conditions. This study thus provides a major step forward in TPV technology, highlighting a new route towards more effective energy conversion systems.

Harnessing the characteristic of compound parabolic concentrators for directional concentration of emitted thermal radiation and solar shielding in building-integrated radiative cooling

Radiative sky cooling (RC) is a promising solution for meeting the growing cooling demands by passively dissipating waste heat into frigid outer space. However, current RC systems suffer from low cooling power density and limited installation flexibility, impeding their effective application as building cooling strategies. To overcome these challenges, a novel concentrated RC system coupled with a compound parabolic concentrator (CPC) is proposed and experimentally studied. The objective is to investigate the effectiveness of the CPC in enhancing the cooling capability of the RC system and the feasibility of achieving all-day RC in unfavorable working conditions when integrated with building roofs. During nighttime experiments in the humid Nottingham region, the CPC-RC system exhibited an average emitter temperature that was 5.83 °C lower than the ambient temperature, representing a 30 % and 13.6 % improvement in RC performance compared to the flat-RC system and trapezoidal-concentrated RC system, respectively. The Photopia optical software simulation indicates that when the modules are tilted to the north, the CPC functions as a solar shield, effectively limiting solar radiation reaching the emitter surface, which is advantageous for conducting daytime RC experiments. In the daytime experiment, the emitter temperature of the CPC-RC module in anti-sunward group was still 1.59 °C lower than the ambient temperature and 5.49 °C lower than that of flat-RC module. At night, the CPC-RC module of the three placement groups all showed the highest RC effect in the same group. The average emitter temperature of the CPC-RC module in the horizontal placement group is 0.6 °C lower than that of the flat RC module. This novel CPC-RC scheme presents a new energy-saving strategy for buildings and showcases its potential for achieving 24-hour RC when integrated into anti-sunward roofs.

Screening and optimization of metal-insulator-metal selective emitter in thermophotovoltaic system

In thermophotovoltaic (TPV) systems, it is crucial that the selective emitters can tailor emission spectrum to match the bandgap of photovoltaic (PV) cells and largely enhance the system efficiency. In this work, a metamaterial emitter based on the metal-insulator-metal (MIM) structure is proposed to obtain the high energy conversion efficiencies. The geometric parameters of MIM emitter have been investigated to obtain an excellent radiation spectrum of emitter composed of W and HfO2. The excellent emission performance of MIM emitter is attributed to the excitation of surface plasmon polariton (SPP) and cavity resonance, and the structure of MIM emitter is insensitive for different polarization modes. The 21 material combinations of MIM emitters have been screened to obtain the optimal emitter matching GaSb and InGaAsSb cells. This work identifies the crucial role of structure and materials into the emitter of a TPV system. In the evaluation of MIM emitter and TPV System, when the operating temperature of emitter increases from 1400[Formula: see text]K to 2000[Formula: see text]K, the system efficiency of optimal W/HfO2/W MIM emitter increases from 20.26% to 30.41%, while the output electric power increases from 3.59[Formula: see text]kW/m2 to 42.48[Formula: see text]kW/m2. The phenomenon indicates that the MIM emitter with the optimal material combinations and geometric parameters can significantly improve the matching degree with GaSb and InGaAsSb cells. Our results will be helpful to expand the optimization scope of metamaterial emitters in TPV systems.

Effectiveness of multi-junction cells in near-field thermophotovoltaic devices considering additional losses

Abstract Thermophotovoltaic (TPV) energy converters hold substantial potential in converting thermal radiation from high-temperature emitters into electrical energy through photovoltaic (PV) cells, offering applications ranging from solar energy harvesting to waste heat recovery. Near-field TPV (NF-TPV) devices, focused on enhancing power output density (POD), exhibit unique potential by harnessing photon tunneling. However, this potential can be mitigated by additional losses arising from high photocurrent densities and corresponding scalability issues. This study comprehensively investigates the effectiveness of multi-junction-based NF-TPV devices, accounting for additional losses. We propose two approximative expressions to quantify the impact of additional losses and characterize current density-voltage curves. Verification against rigorously optimized results establishes a criterion for effective performance. Our method provides precise POD estimations even for devices with 10 or more subcells, facilitating performance analysis across parameters like vacuum gap distance, cell width, emitter temperature, and the number of subcells compared to far-field counterparts. This research outlines a roadmap for the scalable design of NF-TPV devices, emphasizing the role of multi-junction PV cells. The analytical framework we developed will provide vital insights for future high-performance TPV devices.

Optimization of a Weyl-semimetal-based near-field heat transfer system

Near-field radiative heat transfer (NFRHT) is inherently limited by Lorentz reciprocity, which can be partially mitigated through the application of an external magnetic field. However, recent studies have discovered that Weyl semimetals (WSMs) possess the ability to achieve nonreciprocal surface plasmon polariton modes without relying on an external magnetic field. Building upon the nonreciprocal nature of WSMs, this study investigates NFRHT between two WSM slabs and compares different WSM materials. The primary focus of the comparison lies in analyzing the heat flux under varying emitter and receiver temperatures. Three of the selected WSM materials exhibit similar performance in NFRHT, showing sensitivity to a decrease in the receiver temperature due to their low dielectric tensor components. In contrast, Eu2IrO7 displays distinctive characteristics, being sensitive to changes in the emitter temperature as a result of its higher dielectric tensor components. Additionally, a reassessment of the nonreciprocity in Co2MnGa, Eu2IrO7 and Co3Sn2S2 is conducted. Furthermore, Bayesian optimization algorithms are employed to optimize parameters and address the challenge of low thermal radiation observed in existing WSM NFRHT systems. The results highlight the achievement of an optimal WSM material that achieves an impressive heat flux of 5.29 × 108 (W/m2) at an emitter temperature of 310 K and a receiver temperature of 290 K. Notably, this heat flux surpasses that of Co2MnGa by 2.5 times in the context of NFRHT. The discoveries presented in this study promises to offer practical implications for the development of nonreciprocal thermal energy management systems.

Nanophotonic broadband infrared antireflection coatings based on dielectric Si3N4 nano-pillar arrays

Dielectric Si3N4 nano-pillar arrays (NPA) are designed for the broad infrared waveband antireflection applications. The Wood-Rayleigh anomaly diffraction and Mie-type scattering (M − S) resonance characteristics of the NPA nanophotonic structures are analyzed, which show the excellent optical performances. M − S resonance can be broadened by high-index semiconductor substrate and the interactions between nanopillars, which resulting the expected ultra-low light reflection at the broad infrared waveband. The orderly and uniformly large area Si3N4 NPAs are successfully fabricated by low-cost nanosphere lithography, and the measured infrared light reflection spectrums are fit well with the theoretical results. For the GaSb TPV cell with the emitter temperature of 1200 °C, the ultralow flux-spectrum-weighted reflection of ∼1.72 % is obtained with the fabricated Si3N4 NPA, which demonstrate great potential for infrared optoelectronic applications.

High vacuum flat plate photovoltaic-thermal (HV PV-T) collectors: Efficiency analysis

Through a numerical model developed in MATLAB, we investigate the performance of a novel hybrid flat plate photovoltaic-thermal collector under high-vacuum (HV PV-T) to optimize the solar-to-thermal energy conversion and efficiently meet the thermal loads of industrial processes up to 150 °C along with additional production of electrical energy. In the proposed design, the photovoltaic (PV) cell is positioned directly above the Selective Solar Absorber (SSA) in a multi-layered PV-SSA structure.The performance analysis of the system has been first carried out by considering the theoretical Shockley-Queisser limit of the electrical efficiency, different values of energy bandgap (0.66 eV ≤Eg≤3.00eV), emittance (0.1≤ɛ≤1), and working temperature (25°C ≤TPV≤175°C) of the PV layer, and secondly by focusing on a wide variety of actual semiconductive materials. We analyzed the specific case of high bandgap materials, such as CdTe, CdS, and GaAs reported in previous publications. The analysis performed shows that full exploitation of the incident solar radiation with HV-PVT collectors can produce about 12.2% of electrical efficiency while 76.4% of the incident power remains available for thermal conversion at 100 °C. Moreover, obtaining the same annual energy using stand-alone solutions (i.e., a combination of the best PV and solar thermal collectors commercially available) would require up to 50% of the collector area more than the proposed HV PV-T collector.

A novel design of a retarding field electron energy analyzer with a cavity electrode providing extremely high energy resolution

An electron energy analyzer based on the retarding electrostatic field generated by a cavity electrode was prototyped, where the designed energy resolution is smaller than 10 meV at acceleration voltages Vo less than 5 kV and a half angle spread θo less than 20 mrad for the incident electron beams with the scanning voltage range of 3 V. Precise energy resolution, however, has not been recognized experimentally because of the difficulties of equipping such a narrow energy spread electron source. This paper describes the principle of the newly developed energy analyzer and shows the obtained energy spectra from a Schottky emitter ZrO/W(100) with a tip of the curvature radius of 1.5 μm, which could highly reproduce the theoretically expected spectra at various emitter temperatures because of its predominantly thermal emission.

Analysis of Near-Field Thermophotovoltaic Devices Using Graphene–Germanium Schottky Cell

In this work, we investigate the performance of graphene-based Schottky junction thermophotovoltaic (TPV) devices in near-field conditions. Despite the low cost and excellent photoelectric properties of graphene, earlier studies have focused primarily on the contribution of the graphene layer to the photocurrent, assuming an internal quantum efficiency (IQE) of 100%. Our numerical model of a graphene/germanium Schottky junction TPV device reveals that the semiconductor layer predominates in the generation photocurrent, with an IQE of graphene less than 40%. We also evaluate the photocurrent densities generated by the semiconductor and graphene at an emitter temperature of 1000 K and a vacuum gap of 100 nm. Results show that using an indium tin oxide (ITO)-covered tungsten (W) emitter can increase photocurrents by a factor of around 10 and 11 for the semiconductor and graphene, respectively. Additionally, using a hyperbolic metamaterial (HMM) emitter can enhance photocurrents by around 4.7 and 5.2 times for the semiconductor and graphene, respectively. However, this comes at the cost of higher heat flux from the HMM emitter. Our findings will provide valuable insights for the design and optimization of TPV devices to improve their photocurrent and efficiency.

GEOMETRIC OPTIMIZATION OF A NANOSTRUCTURED W-SiO2-W SELECTIVE EMITTER WITH TEMPERATURE DEPENDENT EMISSIVITY FOR THERMOPHOTOVOLTAIC APPLICATIONS

Metal-Insulator-Metal (MIM) nanostructures provide tunable multiple absorption/emission peaks desirable for spectroscopy, light sensing and thermophotovoltaic (TPV) applications. The efficiency of TPV systems can be improved by employing MIM emitters with resonators that allow high emission above PV cell bandgap and low emission elsewhere. Although there have been attempts to design MIM emitters for TPV systems, a comprehensive study that investigates and optimizes different resonator shapes is lacking. In this study, broadband TPV emitters with W-SiO2-W nanostructures are optimized for pairing with GaSb PV cells. A numerical approach is followed utilizing finite-difference time-domain method and particle swarm optimization scheme, MIM emitters with four resonator shapes: disk, square, pyramid, and cone are dimensionally optimized to attain an emissivity spectrum that overlaps with high quantum efficiency region of the GaSb cell. The optimized emitters are compared for efficiency, power output, material consumption, as well as their optical response to temperature and angular effects. At an emitter temperature of 1600 K, electrical power outputs of 2.335-2.418 W·cm-2 and spectral efficiencies of 56.2-57.6% are obtained. It is found that flat resonators tend to achieve similar performance to that of pointy resonators with shorter heights. Among the considered shapes, disk emitter demonstrates the highest efficiency with minimum material consumption. Compared to a plain W emitter at the same temperature, the disk MIM emitter exhibits significantly higher spectral efficiency and electrical power output (34% and 215% respectively). The results demonstrate the successful use of nano-elements in TPV systems, and the potential for fabricating and realizing such structures.

Limits to the Energy-Conversion Efficiency of Air-Bridge Thermophotovoltaics

As the energy economy becomes increasingly decarbonized, low-cost energy storage grows ever more important. Thermal batteries in combination with thermophotovoltaic (TPV) cells are one major source of storage. The lowest-loss TPV cells utilize an air bridge (AB) with a gold back reflector. In this work, the authors determine a 55.5% thermodynamic efficiency limit for AB-TPVs at an emitter temperature of 1400 K, based on detailed balance. Including losses from nonradiative recombination, finite resistance, and free-carrier absorption, the practical efficiency limit is 48.6%. This work provides a road map for evaluating and limiting losses, leading to even higher AB-TPV efficiencies.

Polarization-mediated multi-state infrared system for fine temperature regulation

Passive radiative cooling has been spotlighted as a promising energy-saving cooling technology owing to its energy-free and zero-carbon emission for addressing global energy and climate crises. Although radiative cooling can significantly save cooling energy in hot weather, it inevitably accompanies undesirable cooling in cold weather resulting from a single-state of strong thermal emission. Dual-state emitters have recently been developed for self-adaptive thermoregulation, but they still exhibit energy loss in moderate weather. Herein, we report a “continuous” temperature-regulation system by introducing an infrared (IR) polarization valve as the energy-balancing channel. The proposed scheme controls the emitter temperature simply by the in-plane rotation of the IR polarizer as if closing and opening the valve, which presents heating/cooling capabilities of −17 to 51 W/m2 and an energy-saving of &amp;gt;20 GJ/year compared with the conventional emitters in all climate zones. Outdoor experiments demonstrate the precise temperature regulation with the range of ΔTcool &amp;gt;2 °C. This proof-of-concept demonstration in the outdoors verifies our approach’s reliability, suggesting its applicability in residential buildings, farms, and electronic devices.