Transparent Heat Mirrors Research Articles

Design, fabrication and energy-saving evaluation of five-layer structure based transparent heat mirror coatings for windows application

Design, fabrication and energy-saving evaluation of five-layer structure based transparent heat mirror coatings for windows application

Experimental and theoretical investigation into effectiveness of ZnO based transparent heat mirror covers in mitigating thermal losses in volumetric absorption based solar thermal systems

The present study investigates efficacy of ZnO based transparent heat mirror covers as thermal loss mitigators in 'direct volumetrically absorbing' solar thermal platforms. Comprehensive experimental and theoretical modeling frameworks have been developed to understand and quantify the heat loss mechanisms. Detailed analysis reveals that performance characteristics are strong functions of the 'side' of the glass that has been coated (i.e. whether 'receiver facing' (RF) or 'sky facing' (SF) sides of the cover has been coated). Results show that employing ZnO based heat mirror as a cover significantly reduces the thermal losses relative to uncoated glass cover (27.72% and 21.44% reduction for RF and SF side coated heat mirrors respectively). Moreover, fundamental performance limits of ideal heat mirrors have also been determined for both RF and SF side coated cover configurations. Relative to the uncoated glass covers, ideal heat mirror covers (viz., RF and SF side coated) promise 47.42% and 36.07% thermal loss reduction respectively (@ 400 °C receiver surface temperature and 1.5 μm cut-off wavelength). Overall, the present work represents a significant step in improving the existing volumetric absorpion based solar thermal systems; particularly aiming at intermediate temperature applications (viz. industrial and domestical heating/cooling).

Intrinsic Properties and Future Perspective of HfO2/V2O5/HfO2 Multi-Layer Thin Films via E-Beam Evaporation as a Transparent Heat Mirror

HfO2 and V2O5 as multi-layer thin films are discussed for their potential use as transparent heat mirrors. Multi-layered HfO2/V2O5/HfO2 thin films with a thickness of 100/60/100 nm were prepared via e-beam evaporation on a soda–lime glass substrate. Rutherford backscattering confirmed the multi-layer structure with uniform surface. The as-deposited thin films were annealed at 300 °C and 400 °C, respectively, for 1 h in air. The transmittance of approximately 90% was obtained for all thin films. Due to the relatively low thickness and non-stoichiometry of HfO2, a band gap of approximately 3.25 eV was determined (instead of the theoretical 5.3–5.7 eV). The as-deposited thin films possessed conductivity of approximately 0.2 Ω−1cm−1 and increased to 1 Ω−1cm−1 and 2 Ω−1cm−1 for thin films annealed at 300 and 400 °C, respectively. Due to the unique intrinsic properties of HfO2/V2O5/HfO2 thin films, the results obtained are promising for application as a transparent heat mirror.

Experimental and Theoretical Investigation into Effectiveness of Zno Based Transparent Heat Mirror Covers in Mitigating Thermal Losses in Volumetric Absorption Based Solar Thermal Systems

Experimental and Theoretical Investigation into Effectiveness of Zno Based Transparent Heat Mirror Covers in Mitigating Thermal Losses in Volumetric Absorption Based Solar Thermal Systems

Charge Density-Versus Time-Controlled Pulse Anodization in the Production of PAA-Based DBRs for MIR Spectral Region

A robust and reliable method for fabricating porous anodic alumina (PAA)-based distributed Bragg reflectors (DBRs), operating in mid-infrared (MIR) spectral region, is presented. The method relies on application of high (UH) and low (UL) voltage pulse sequence repeated in cycles. PAA-based DBR consists of alternating high-(dH) and low-porosity (dL) layers translated directly into periodically varied refractive index. Two anodization modes were used: time- and charge density-controlled mode. The former generated dH + dL pairs with non-uniform thickness (∆d) and effective refractive index (∆neff). It is supposed, that owing to a compensation effect between the ∆d and ∆neff, the photonic stopbands (PSBs) were symmetrical and intensive (transmittance close to zero). Under the charge density-controlled mode dH + dL pairs of uniform thickness were formed. However, the remaining ∆neff provided an asymmetrical broadening of PSBs. Furthermore, it is demonstrated that the spectral position of the PSBs can be precisely tuned in the 3500–5500 nm range by changing duration of voltage pulses, the amount of charge passing under subsequent UH and UL pulses, and by pore broadening after the electrochemical synthesis. The material can be considered to be used as one-dimensional transparent photonic crystal heat mirrors for solar thermal applications.

Numerical evaluation of one-dimensional transparent photonic crystal heat mirror coatings for parabolic dish concentrator receivers

High temperature solar receivers can convert concentrated solar power to heat to drive chemical production or mechanical cycles to produce electricity. Transparent windows with selective coatings that reflect thermal radiation can be used to cover receivers to improve their efficiency. However, in parabolic dish concentrators (PDC) these windows can reflect or absorb incoming solar radiation, and may actually reduce the efficiency and power output from PDC receivers. In this work numerical analysis shows that one-dimensional transparent photonic crystal heat mirrors (TPCHMs), which have the form of a modified dielectric mirror, can be designed to be highly transmissive to solar radiation but highly reflective towards thermal radiation. Results show that TPCHMs can be designed to provide significant enhancements to PDC receiver efficiencies operating at lower solar concentration ratios. Specifically, numerical analysis shows efficiency improvements of at least 62%, and 193% compared to either open-face receivers or the case when the receiver window is coated with a transparent conducting oxide film at operating temperatures of 1000 K and 1500 K, respectively. These results suggest that TPCHM covers have potential for enhancing the performance of smaller PDC systems operating at relatively low solar concentration ratios.

Flexible Transparent Heat Mirror for Thermal Applications.

Transparent heat mirrors have been attracting a great deal of interest in the last few decades due to their broad applications, which range from solar thermal convection to energy-saving. Here, we present a flexible Polyethylene terephthalate/Ag-doped Indium tin oxide/Polydimethylsiloxane (PAIP) thin film that exhibits high transmittance in visible range and low emissivity in the thermal infrared region. Experimental results show that the temperature of the sample can be as high as 108 °C, which is ~23 °C higher than that of a blackbody control sample under the same solar radiation. Without solar radiation, the temperature of the PAIP thin film is ~6 °C higher than that of ordinary fabric. The versatility of the large-area, low-radiation-loss, highly-transparent and flexible hydrophobic PAIP thin film suggest great potential for practical applications in thermal energy harvesting and manipulation.

Transparent Photonic Crystal Heat Mirrors for Solar Thermal Applications

Numerical calculations are performed to determine the potential of using one-dimensional transparent photonic crystal heat mirrors (TPCHMs) as transparent coatings for solar receivers. At relatively low operating temperatures of 500 K, the TPCHMs investigated herein do not provide a significant advantage over conventional transparent heat mirrors that are made using transparent conducting oxide films. However, the results show that TPCHMs can enhance the performance of transparent solar receiver covers at higher operating temperatures. At 1000 K, the amount of radiation reflected by a transparent cover back to the receiver can be increased from 40.4% to 60.0%, without compromising the transmittance of solar radiation through the cover, by using a TPCHM in the place of a conventional transparent mirror with a In2O3:Sn film. For a receiver operating temperature of 1500 K, the amount of radiation reflected back to the receiver can be increased from 25.7% for a cover that is coated with a In2O3:Sn film to 57.6% for a cover with a TPCHM. The TPCHM that is presented in this work might be useful for high-temperature applications where high-performance is required over a relatively small area, such as the cover for evacuated receivers or volumetric receivers in Sterling engines.

Opto-structural and surface properties of silkworm-like nickel oxide thin films

Nickel oxide (NiO) is one of the most promising transparent conducting oxides. The optoelectronic and electrochemical properties of these materials depend on their morphology and affected by the preparative parameters. In this study, the effects of annealing temperature (AT) on the surface morphology and opto-structural properties of a sol-gel prepared NiO thin films are reported. Raman spectra showed two main bands at 560 and 1079 cm−1, arising from O–O planar and Ni–O stretching vibrations, respectively. Atomic force microscopy (AFM) analysis illustrated that AT enhances the silkworm-like morphology and the films’ roughness. XRD and Raman analysis, as well as AFM investigation confirmed the crystallinity improvement after annealing. UV-vis-NIR spectroscopy showed that the films are highly transparent (40%–97%) and their bandgap decreased from 4.05 to 3.88 eV. The refractive index, porosity, packing density, and the dispersion parameters found to be sensitive to AT. Water contact angle measurements showed that the un-annealed film is hydrophilic. The films’ wettability decreased significantly with AT. Based on the obtained results, the films are a candidate for electrochemical, transparent heat mirrors and optical windows applications.

Enhancement of plasmonic transmittance of porous gold thin films via gold/metal oxide bi-layers for solar energy-saving applications

Thin films of noble metals, such as gold, have high visible transparency and infrared reflectance. The former is due to the presence of surface plasmons, and the latter is due to the large density of free electrons. Gold thin films have a transmittance peak at 520 nm, which is closely matched to the maximum emission of solar radiation. These properties have been utilized in several applications, including energy-saving transparent heat mirrors that consist of a bi-layer of a thin gold film over-coated with a dielectric layer. These coatings transmit light and reflect heat. For such an application, the transmittance of the thin gold film needs to be enhanced; this is achieved by the dielectric layer that functions as an antireflective coating. In this work, the dielectrics were metal oxides that are transparent in the visible range. The selection criteria based on the band gap, and refractive index were first imposed to identify potential metal oxides. This resulted in the choice of ten metal oxides (CeO2, HfO2, MoO3, Nb2O5, NiO, SnO2, Ta2O5, TiO2, WO3, and ZrO2). The bi-layer performance was then optimized theoretically. The gold-metal oxide layers were subsequently fabricated, and their optical spectra were measured over the ultraviolet–visible-near infrared ranges. The performance of the heat mirrors was assessed in terms of their visible and solar transmittance and reflectance. A figure of merit was defined to evaluate the suitability of the oxides. The optical properties suggest that tungsten oxide (WO3) is the best oxide; however, hafnium oxide (HfO2) has the best performance for solar energy applications.

Applicability of Heat Mirrors in Reducing Thermal Losses in Concentrating Solar Collectors

Abstract In the present work, a novel parabolic trough receiver design has been proposed. The proposed design is similar to the conventional receiver design except for the envelope and the annulus part. Here, a certain portion of the conventional glass envelope is coated with Sn-In2O3 and also Sn-In2O3 coated glass baffles are provided in the annulus part to reduce the radiative losses. The optical properties of the coated glass are such that it allows most of the solar irradiance to pass through, but reflects the emitted long wavelength radiations back to the absorber tube. Sn-In2O3 coated glass is referred to as “transparent heat mirror.” Thus, effectively reducing the heat loss area and improving the thermal efficiency of the solar collector. A detailed one-dimensional steady-state heat transfer model has been developed to predict the performance of the proposed receiver design. It was observed that while maintaining the same external conditions (such as ambient/initial temperatures, wind speed, solar insolation, flow rate, and concentration ratio), the heat mirror-based parabolic trough receiver design has about 3–5% higher thermal efficiency as compared to the conventional receiver design. Furthermore, the heat transfer analysis reveals that depending on the spatial incident solar flux distribution, there is an optimum circumferential angle (θ = θoptimum, where θ is the heat mirror circumferential angle) up to which the glass envelope should be coated with Sn-In2O3. For angles higher than the optimum angle, the collector efficiency tends to decrease owing to increase in optical losses.

Indium free electrode, highly flexible, transparent and conductive for optoelectronic devices

WO3/Ag/WO3 multilayer structures were used as ITO free transparent electrode, transparent heat mirrors and transparent heaters. WO3/Ag/WO3 stacked layers were deposited by sequential sublimation, evaporation under vacuum. After optimization of Ag thickness (16 nm), they exhibit low sheet resistance (8 Ω/sq), high transmittance in the visible (TMax = 91.5%, averaged T400-700 = 80.6%) and high reflection in the near infrared and infrared regions. These values are optimal when it is used as transparent electrode but, as transparent heat mirrors 18 nm are better due to higher reflection in the NIR and IR. All these properties made possible to use them in different devices. When used as transparent anode in organic photovoltaic cells, they allow achieving performance similar to those obtained with ITO. Their transmission and reflection spectra show that they can also be employed as transparent heat mirrors. Similarly, studies dedicated to heating properties of the WO3/Ag/WO3 multilayer structures show that their performance are comparable to those obtained with another possible substituent to ITO, silver nanowires thin films.

Solar Selective Volumetric Receivers for Harnessing Solar Thermal Energy

Given the largely untapped solar energy resource, there has been an ongoing international effort to engineer improved solar-harvesting technologies. Toward this, the possibility of engineering a solar selective volumetric receiver (SSVR) has been explored in the present study. Common heat transfer liquids (HTLs) typically have high transmissivity in the visible-near infrared (VIS-NIR) region and high emission in the midinfrared region, due to the presence of intramolecular vibration bands. This precludes them from being solar absorbers. In fact, they have nearly the opposite properties from selective surfaces such as cermet, TiNOX, and black chrome. However, liquid receivers which approach the radiative properties of selective surfaces can be realized through a combination of anisotropic geometries of metal nanoparticles (or broad band absorption multiwalled carbon nanotubes (MWCNTs)) and transparent heat mirrors. SSVRs represent a paradigm shift in the manner in which solar thermal energy is harnessed and promise higher thermal efficiencies (and lower material requirements) than their surface absorption-based counterparts. In the present work, the “effective” solar absorption to infrared emission ratio has been evaluated for a representative SSVR employing copper nanospheroids/MWCNTs and Sn-In2O3 based heat mirrors. It has been found that a solar selectivity comparable to (or even higher than) cermet-based Schott receiver is achievable through control of the cut-off solar selective wavelength. Theoretical calculations show that the thermal efficiency of Sn-In2O3 based SSVR is 6–7% higher than the cermet-based Schott receiver. Furthermore, stagnation temperature experiments have been conducted on a laboratory-scale SSVR to validate the theoretical results. It has been found that higher stagnation temperatures (and hence higher thermal efficiencies) compared to conventional surface absorption-based collectors are achievable through proper control of nanoparticle concentration.

Self-Cleaning Transparent Heat Mirror with a Plasma Polymer Fluorocarbon Thin Film Fabricated by a Continuous Roll-to-Roll Sputtering Process.

This paper proposes a novel self-cleaning transparent heat mirror (SC-THM) produced by depositing a plasma polymer fluorocarbon thin film on a silver-and-SiN x multilayer structure fabricated by continuous roll-to-roll sputtering. The optimal structure and the thickness of each thin film of three-layer and five-layer SC-THMs were derived from optical simulation. In the five-layer SC-THM, the visible light transmittance was 60.67% at a wavelength of 406 nm and the infrared (IR) transmittance was 6.86% at a 1000 nm wavelength and 2.50% at a 1500 nm wavelength. The value of the performance parameter Tvis/ Tsol was 1.70. The SC-THM exhibited self-cleaning with a very good water repellency of more than 111°, achieved by applying a low-surface-energy fluorocarbon thin film to the top layer. This study successfully demonstrated the IR blocking properties of an SC-THM through IR reflection and IR irradiation experiments.

Bi-layered energy efficient coatings as transparent heat mirrors based on vanadium oxide thin films

Energy-efficient coatings based on two-layer-structured transparent heat mirrors were fabricated by depositing a vanadium oxide thin film on a silver thin film by thermal evaporation. Transparent heat mirrors are energy efficient coatings designed to save energy in hot climates by allowing only the visible part of solar radiation to pass through them and reflecting the infrared heat. These transparent heat mirrors can be used as energy-saving windows in energy-efficient buildings and green house agriculture. First, the optical, structural, and chemical properties of individual vanadium oxide thin films were investigated related to their application in transparent heat mirrors. Then, in order to realize their potential as energy efficient coating, optical characterization was performed on a two-layer structure. The performance of these coatings was calculated through integrated spectral transmittance and reflectance in the range of visible and infrared radiations. In addition, chemical depth profiling was performed to probe the diffusion of elements along the deposited layers. The fabricated coatings were found to show the expected behavior as transparent heat mirrors of high visible transparency and high infrared reflection.

Application of nickel oxide thin films in NiO/Ag multilayer energy-efficient coatings

Energy-efficient coatings based on transparent heat mirrors are reported. These coatings are spectrally selective in that they transmit light and reflect heat in the form of infrared radiation, and thus save energy in buildings. In this work, these coatings were achieved by depositing a two-layer structure consisting of nickel oxide deposited on silver by thermal evaporation. First, the structural, chemical, and optical properties of nickel oxide thin films, that are relevant to their use in transparent heat mirrors, were investigated. This was achieved through x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, and spectrophotometric characterization. Then, the optical properties of the two-layer structure were determined. Their performance was evaluated through integrated spectral quantities over the solar and visible ranges. Moreover, chemical depth profiling was carried out to investigate the spatial distribution of the elements among the layers. The deposited heat mirrors were found to exhibit the desired behavior of high visible transparency and high infrared reflection.

Energy-saving spectrally-selective coatings based on MoO3/Ag thin films

Transparent heat mirrors are spectrally selective coatings that transmit light and reflect heat in the form of infrared radiation. In this work, these coatings were achieved by depositing a two-layer structure consisting of molybdenum oxide thin films deposited on silver thin films by thermal evaporation. First, the structural, chemical, and optical properties of molybdenum oxide thin films, that are relevant to their use in transparent heat mirrors, were investigated. Then, the optical properties of the two-layer structure were determined. Moreover, chemical depth profiling was carried out to investigate the spatial distribution of the elements among the layers. The deposited heat mirrors were found to exhibit the desired behavior of high visible transparency and high infrared reflection.

TiOx/Ag/TiOx multilayer for application as a transparent conductive electrode and heat mirror

Amorphous TiOx films and Ag layer were deposited by electron-beam evaporation on soda-lime glass at room temperature. The details regarding the structure, surface morphology, and optical properties of the as-prepared TiOx films were examined by X-ray diffraction, scanning electron microscopy, and ultra-violet (UV) -visible-near-infrared (NIR) spectrometry. The TiOx films exhibit amorphous phase with an optical band gap of 3.35 eV. The polygrains oriented along the (111) and (200) directions in the Ag films were adopted to supply carriers into the TiOx film and lower the sheet resistance of the stacked layer. The multilayer exhibited a sufficiently large Ag thickness (>15 nm), low resistance, high UV transmittance, visible transmittance, and high NIR reflection. Dependence of Ag thickness, TiOx bottom-layer, and TiOx overlayer on the optical and electrical properties of TiOx/Ag/TiOx were explored. A figure of merit (FOM) was used to find an optimal structure for a multilayer with superior conductivity and visible transparency. An FOM of 9.8 × 10−2 (Ω−1) at the visible wavelength of 550 nm for a TiOx/Ag/TiOx stacked layer with an 18-nm-thick Ag and a 20-nm-thick TiOx was achieved. The TiOx/Ag/TiOx sample annealed at 500 °C 10 min also shows a good thermal stability.

Energy-saving transparent heat mirrors based on tungsten oxide–gold WO3/Au/WO3 multilayer structures

Transparent heat mirrors are multilayer structures that transmit visible light while reflecting infrared heat. Heat mirrors based on tungsten oxide and gold multilayers WO3/Au/WO3 were fabricated by thermal evaporation, and their performance was investigated as a function of the thickness of the gold layer. First, the properties of individual layers were investigated. Atomic force microscopy revealed that all layers possessed smooth surfaces that were suitable for optical applications. The transmittance of the gold layers was found to decrease as the thickness is increased, with an opposite trend followed by infrared reflectance. In the multilayers, the thickness of the WO3 was fixed at 34nm, whereas the thickness of the gold layers was varied in the range 20–44nm. X-ray photoelectron spectroscopy was used to investigate the elemental diffusion among the various layers, and it revealed the presence of inter-diffusion of elements throughout the layers. The performance of the heat mirrors was evaluated on the basis of their optical behavior. The optimum thickness of the gold layer was found to be 36nm, with a peak spectral transmittance of 84%.

Effect of Sintering Agents on the Sintering and Physical Properties of ATO (Antimony Doped Tin Oxide)

Antimony doped tin oxide (ATO) is a typical p-type semiconductor and widely used due to its good properties such as electrical conductivity and transparency in visible range, while reflects infrared light. These features allow ATO to be used as transparent electrodes, heat mirrors and energy storage device, display panel. However, the use of tin oxide ceramics is limited by the low densification during sintering due to the dominance of non-densification mechanism such as surface diffusion or evaporation condensation. So, sintering agents such as Cobalt (II) oxide (CoO), zinc oxide (ZnO), manganese dioxide (MnO2) used to improve the densification of SnO2 by forming lattice solid solution or liquid phase. In this study, we studied the effect of the sintering agents. The antimony-doped tin oxide nanoparticles were synthesized by a sol-gel method. After sintering ATO with sintering agents such as MnO2 and CoO, sintered body characteristics were investigated by XRD, SEM, thermal conductivity, resistivity and interesting characteristic.