Structured transparent low emissivity coatings with high microwave transmission

Abstract In order to reduce the energy consumption of buildings, modern windows include metal-containing coatings. These coatings strongly attenuate the microwaves used for mobile communications. Here, we present a novel approach to improve radio signal transmission by structuring a low emissivity coating. Laser ablation is used to scribe a line pattern on the coating. The microwave attenuation of the initial coating ranges between -25 and -30 dB between 850 MHz and 3 GHz. The optimized patterning reduces it down to -1.2 ± 0.6 dB. The fraction of the ablated area is relatively low. Our experiment al results show that it is possible to reach a level of attenuation close to that of a glass substrate by removing less than 4% of the coating area. The ablated lines are thin enough to not be noticed in most common lighting situations. Therefore, we achieve a dual spectral selectivity: the coated glass is transparent in the visible range, reflective in the infrared and nearly as transparent as its glass substrate to microwaves. Additionally, numerical simul ations were performed and show that the attenuation at grazing incidences is dominated by the behaviour of the glass substrate

Solar heat gains through train windows: a non-negligible contribution to the energy balance

The sector of transportation accounts for about one third of the total energy consumption in Switzerland. A monitoring campaign of the energy consumption of a regional train revealed the critical energy-consuming systems. Heating, cooling and ventilation were identified as major consumers. Windows are a source of non-controlled heat transfer. In summer, it may result in overheating leading to larger cooling loads while in winter, it is an important source of thermal losses. Selective double glazing and solar protection coatings can reduce these effects. Angular-dependent optical properties of a selective double glazing have been measured, and the solar heat gain coefficient (g value) was determined. An estimation of the solar gains received by a panoramic waggon was performed using the monitored solar irradiation and the measured properties of the glazing. These data were compared to the heating and cooling energy consumption monitored in this waggon. Solar gains were found to be in the same order of magnitude that the heating energy during some sunny days. They were also compared to the estimated thermal losses through the glazing and the entire envelope. These results show that the solar gains play a non-negligible role in the energy balance of the waggon. Furthermore, thermal simulations were performed to evaluate the solar gains in different conditions. It showed that 7 to 13% of energy can be saved using the glazing adapted to the climatic conditions. In addition, improving the thermal insulation of the train envelope or equipping the train with an efficient heat recovery system can lead to significant energy savings

Color neutral nanocomposite nickel-tantalum oxide for electrochromic windows

Electrochromic windows can darken on demand to limit the solar gains entering a building to reduce the risk of overheating, while preserving the view towards the exterior. Yet, the switching speed and contrast as well as the durability of commercial products still need to be improved. Novel materials are investigated to address these shortcomings. Since gel or polymer electrolytes limit the durability of electrochromic glazing, all-solid state, inorganic devices are considered. The optical properties of doped nickel oxides were studied in order to obtain a color neutral anodic electrochromic oxide. The addition of tantalum was shown to increase the light transmittance and to provide a coating with better color neutrality compared to nickel vanadium oxides. The study of the crystalline structure by X-ray diffraction suggests that a nickel oxide-tantalum pentoxide nanocomposite is formed. These results are encouraging to use nickel tantalum oxide as an anodic electrochromic oxide

Experimental Determination of Optical and Thermal Properties of Semi-transparent Photovoltaic Modules Based on Dye-sensitized Solar Cells

The demand for energy efficiency of buildings and on-site electricity production is rising. Building integrated photovoltaic can provide a part of the electricity demand. Many studies are related to the module efficiency. However, architectural integration, optical and thermal properties also require attention. Semi-transparent modules are especially interesting for architectural integration in the glazed part of the façade. Dye sensitized solar cells, offering color and semi-transparency, are in the process of market introduction. However, dye-sensitized solar cells are fragile and there are not many examples of architectural integration due to the technical challenge of introducing these cells in a glazing. A glazing containing colored photovoltaic modules could be used to design an active façade. In order to determine the thermal behaviour of the building, the precise optical and thermal properties of the used materials need to be known. Performances of semi-transparent photovoltaic modules based on dye-sensitized solar cells were investigated. These modules come from the same manufacturer, using the same technology. However, they differ in terms of shades and nuances. Common practice is to indicate optical properties at normal angle of incidence. Yet, for most latitudes, the properties for a large range of angles of incidence are more relevant. Therefore, the spectral transmittance and the reflectance were measured at 12 angles of incidence ranging from 0° to 75°. From these data, the solar direct transmittance τe, the solar direct reflectance ρe and the visible transmittance τv and selectivity were calculated. The solar gain factor was determined on a prototype double glazing under illumination with a solar simulator by measuring the temperatures of the external and inner surface of the product. Combined with the values of absorptance obtained from the transmittance and reflectance values, this measurement allows us to determine the internal heat transfer coefficient qi and thus the solar gain factor of the double glazing (also called total energetic transmittance or g-value). Final performance of a façade containing these modules will depend on the composition of the double glazing in which they will be laminated. The performance of the module itself will help to determine the best composition for each climate. For instance, a solar protection coating may be needed. The modules can be laminated to a glass pane and then be assembled in a double glazing. Therefore the architectural integration is facilitated and compatible with existing façade systems. In highly glazed building, a part of the façade could then be a photovoltaic façade and deliver a fraction of the energy demand while providing colourful options to enhance the aesthetic of the building

Development and characterization of electrochromic oxide and ion conductor deposited by reactive magnetron sputtering

In order to modulate the solar gains of a building, electrochromic windows can be used. To improve their durability, inorganic solid state electrolytes are considered. This work reports on the investigation of tungsten trioxide (WO3) and lithium phosphorous oxynitride (LiPON) used as electrochromic oxide and solid state ion conductor respectively. The impacts of porosity on the optical and the electrochemical properties as well as the determination of ionic conductivity of LiPON are studied. Electro-optical performances were studied by developing an in-situ experimental set-up allowing the measurement of the spectral transmittance during the electrochemical characterizations. The techniques of chrono-amperometry and simultaneous spectrophotometry were used to determine the coloration efficiency as well as the response times for coloration and bleaching of the WO3 films. Electrochemical impedance spectroscopy was performed to investigate the electrochromic and electrolyte materials. The ionic conductivity was measured for LiPON sandwiched in metal-insulator-metal (MIM) cells

Study of Si doped VO2 thin films for solar thermal applications

A new generation of smart solar thermal collectors is based on thermochromic thin film technology. Thermochromic absorber coatings change their optical properties with temperature, particularly in the infrared range. In order to prevent overheating, the collector selectivity is deliberately hindered, through an increase of the thermal emittance above a critical temperature. The transition temperature of pure thermochromic VO2 (68°C) needs to be adjusted for solar thermal applications and to do that Si doping is attempted. Thus, pure and Si doped vanadium dioxide thin films are deposited by reactive magnetron sputtering and the films are characterized by four point probe and in-situ X-ray and UV photoelectron spectroscopic measurements. While a modest increase in the transition temperature with doping is reached (up to a critical Si concentration when the transition is lost), the insulating character of the films is also enhanced (decreased density of states at the Fermi level)

In situ core-level and valence-band photoelectron spectroscopy of reactively sputtered tungsten oxide films

In this paper, we study tungsten oxides deposited by reactive magnetron sputtering. The total working pressure during deposition was varied in order to obtain different morphology. The effects on the electronic properties and chemical composition were studied by XPS and UPS. It was observed that, for the same argon to oxygen ratio in the gas feed, the decrease in total working pressure implies a decrease of the oxygen content in the film. In the nearly stoichiometric WO3 films, W 4f5/2 and W 4f7/2 form a distinct doublet peak. In sub-stoichiometric films, the films do not exhibit a well-resolved doublet and suggest multiple oxidation states of tungsten.The valence-band spectra of the sub-stoichiometric samples present an additional feature below the Fermi edge (~0.5 eV). This peak is assigned to W 5d1 because of the presence of W5+. It is consistent with the changes on the core-level spectra. XPS results, UPS features, and visual aspect are in agreement and suggest that the total working pressure has a strong influence on the oxygen content and therefore on the oxidation state of tungsten in the films