Low-emissivity Windows Research Articles

Evaluating UV-Protective Low-Emissivity Window Films: Implications for Thermal and Visual Comfort in Luminous Office Buildings

Evaluating UV-Protective Low-Emissivity Window Films: Implications for Thermal and Visual Comfort in Luminous Office Buildings

Extreme Dewetting Resistance and Improved Visible Transmission of Ag Layers Using Sub-Nanometer Ti Capping Layers.

As technology development drives the thickness of thin film depositions down into the nano regime, understanding and controlling the dewetting of thin films has become essential for many applications. The dewetting of ultra-thin Ag (9 nm) films with Ti (0.5 nm) adhesion and capping layers on glass substrates was investigated in this work. Various thin film stacks were created using magnetron sputtering and were analyzed using scanning electron microscopy/energy dispersive X-rays, Vis/IR spectrometry, and four four-point probe resistivity measurements. Upon annealing for 5 h in air at 250 °C, the addition of a 0.5 nm thick Ti capping layer reduced the dewet area by an order of magnitude. This is reflected in film resistivity, which remained 2 orders of magnitude lower than uncapped variants. This Ti/Ag/Ti structure was then deployed in a typical low-emissivity window coating structure with additional antireflective layers of AZO, resulting in a superior performance upon annealing. These results demonstrate an easy, manufacturable process that improves the longevity of devices and products containing thin Ag films.

Zwitterionic hydrogel smart windows: Radiative cooling, privacy protection and energy savings

Current thermochromic smart windows based on lower critical solution temperature (LCST) behavior can achieve solar regulation and energy saving by modulating solar transmission at the cost of visibility at higher temperature. To tackle this issue, a new concept of smart window based on the upper critical solution temperature (UCST) characteristic of zwitterionic hydrogel has been developed, giving daytime luminous transparency, all day radiative cooling and nighttime privacy protection. This design rule is suitable for energy saving smart windows for regions where cooling demand is dominant. The UCST smart window panel provides moderate luminous transmission (Tlum) of 37.1 % and low near-infrared transmission (TNIR) of 20.3 % during the daytime, all day near unity long-wave infrared emissivity (ƐLWIR) of 0.96 facing outdoor to promote radiative cooling with outer space, low ƐLWIR 0.19 facing indoor to suppress heat transfer, and low visible transmission (Tvis) of 1.4 % at night mode for privacy protection. Energy-saving simulations demonstrate that compared to commercial low-emissivity (low-E) windows in three climate zones (zone 0, 1, 2), the UCST smart window achieves an impressive annual energy-saving up to 30.7 %. This work introduces a novel conceptual design specifically tailored for climate zones where cooling demands prevail, offering a combination of moderate luminous transparency, suppressed NIR transmission, excellent radiative cooling with outdoor environment and reduced heat transfer with indoor combining with an added privacy protection feature.

Investigation of the Energy-Saving Potential of Buildings with Radiative Roofs and Low-E Windows in China

This study develops a model for buildings with a cooling roof, walls, and low-emissivity (Low-E) windows. This model is verified through experimental analysis. The cooling demands of standard buildings and cooling buildings are compared, and the energy-saving potentials of cooling buildings are analysed. It is found that compared to standard buildings, cooling buildings exhibit superior cooling performances attributable to the application of cooling materials. Considering Hong Kong’s weather data, the indoor temperature of cooling buildings can be sub-ambient. The cooling demands of cooling buildings are decreased from 75 W/m2 to 30 W/m2, indicating a 60% energy-saving potential. The nationwide cooling demand for a standard building across China is approximately 95.7 W/m2, whereas the nationwide summer average cooling demand for cooling buildings is 52.7 W/m2. Moreover, the cooling performance of a cooling roof is adversely affected by hot and humid weather conditions, resulting in lower temperature drops in southern regions compared to northern regions. However, the nationwide temperature drop across China can still be 1.6 °C, demonstrating promising cooling potentials. For the Low-E windows, the temperature can also be sub-ambient, with a nationwide average temperature drop of 1.7 °C. Therefore, the use of Low-E windows across China can also significantly contribute to energy savings for indoor cooling. Overall, the results of this study show that cooling buildings have high energy-saving potential under various climates. The proposed model can provide a reliable tool to facilitate relevant cooling evaluation by stakeholders, thereby benefiting the popularization of this technology.

Building Optimisation Vis-À-Vis Solar Shading for Improved Comfort and Energy Efficiency in Classrooms

Excessive solar radiation negatively affects classroom occupants' perfor­mance and thermal comfort, especially in buildings with West and East-facing glazed openings. This study utilises fixed external shading devices and triple-glazed low-emissivity windows to optimise a classroom building in Nigeria. Employing hybrid ventilation mode in EnergyPlus simulations, the optimised model shows a 44% reduction in discomfort hours, a 23% decrease in cooling load, and a 16% drop in energy demand compared to the original design. Comparative analysis of the optimised model with the as-built and West-East oriented classroom reveals a 16% and 10% reduction in energy consumption per conditioned area, along with 56 KWh/m² and 32 KWh/m² savings in cooling demand, respectively. Despite the effectiveness of fixed shading in curbing solar gains, occasional glare persists. This research underscores that shading alone may not fully meet thermal comfort requirements, emphasising the importance of building fabrics, building orientation and climate-sensitive design.

Radiofrequency transmission through rectangular apertures in perimetrally uncoated low emissivity windows

Perimetrally uncoated windows can be a simple and low cost solution to the problem of radio frequency (RF) transmission in low-e glass when the requirements are not too demanding. However, this kind of non-uniform and relatively large structures are difficult to characterize experimentally. The aim of this work is to develop and experimentally verify a method for the simulation of the RF transmission of perimetrally uncoated windows. We propose to study these windows as a composition of four rectangular apertures in a metallic sheet. Using numerical simulations, we are able to obtain the transmission coefficient of these apertures. Then, a multilayer model based on transmission line theory is used to consider the effect of the different glazings and cameras which can be part of a window. Finally, the results of this study are compared to the measurements of complete windows in the 1000 MHz–6000 MHz frequency range.

Towards Sustainable Indoor Thermal Comfort in the Tropical Building Design: Comparative Performance Analysis of Fixed External Shading Elements and Differential Window Glass Thermal Properties

Tropical region is characterised by high outdoor solar radiation and temperature. These radiations penetrate into the interior particularly through the window glazing, thereby raising the indoor temperature with a resultant effect on high cooling energy demand (for comfortable indoor living condition). Of the identified three sustainable approaches towards inhibiting direct incoming indoor solar penetration in the tropical city of Ogbomoso, Nigeria, this study, through computer-based simulation technique, compared performance effectiveness of varying design interventions on the window composition/material adopted. Through multiple/iterative building performance simulation tool, DesignBuilder, the methodology involves virtual model of a three-bedroom residential building, typical of the prevailing building typology in the study area. This particular model served as the experimental control model. Three (3) variants of the control model, each with either externally mounted shading devices on the building envelope, or improved thermally resistant multilayer window glazing (i.e., double glazing) or low-emissivity coated window glazing material, were generated for individual performance effectiveness evaluation. All the four models were subjected to a twelve-month simulation cycle, to experience a year-round thermal conditions and evaluations individually. The results show that double-glazing window installation proved to be the most effective approach, with about 13.1 % improvement (i.e., solar radiation inhibition) on the indoor thermal gains. This is followed by the externally mounted shading devices (i.e., 11.1 % improvement) and the least inhibition (i.e., 5.0 %) was observed in the case of low-emissivity coated window glazing material. Adoption of double or more layers of glass panes in window fabrication for controlling indoor solar radiation penetration is therefore advised. Alternatively, integration of external window shading elements could be adopted. This study is directed towards reduction of cooling energy consumption in tropical buildings through efficient and sustainable indoor cooling mechanism capable of inhibiting solar gains into the building, with a focus on the performance roles and composition of windows in particular.

Rapid tumor detection via a fibroblast activation protein-alpha activated fluorogenic probe

Fibroblast activation protein-alpha (FAPα) is an extensively known serine protease that participates in various important physiological processes and found to be highly related to tumor progression and invasion, and its high expression level in various human epithelial carcinomas makes it a potential cancer biomarker. However, as a tumor-specific expression enzyme, only a few fluorescent probes targeting FAPα have been reported, and most of these probes possess either a low emission window or a short Stokes shift, which severely obstructs their application in tissue imaging. Thus, it is an urgent need to acquire more suitable FAPα-targeted fluorescent probes for cancer identification.Herein, by incorporating a previously reported fluorophore that possesses a large Stokes shift (>190 nm) and long emission window (650–700 nm) with a specific recognition unit (N-acylated-glycine-proline, Ac-GP) for FAPα, we acquired a “turn-on” biosensor named TMN-AcGP for FAPα activity sensing. Owing to the high sensitivity and improved optical properties of the newly developed fluorescent probe, in vitro cultured cancer cells and ex vivo dissected tumor tissues were successfully detected and distinguished from normal cells and tissues with a high tumor-to-normal ratio rapidly and accurately.

Microstructure FSS patterning to improve 5G microwave signals through low-e plastic windows

Low emissivity (low-e) windows are widely used in the architectural sector to block the infrared (IR) radiation from the Sun. The windows contain a multilayer nanoscale coating of metallic and dielectric layers. The metallic layer, which is responsible for the IR reflection, also attenuates the radio and microwave frequencies used for modern-day technologies such as Fifth Generation (5G) communications. As there is an ever-increasing demand for a reliable interior-to-exterior signal coverage, low-e windows should be transparent to such signals. A class of surface modification — Frequency Selective Surface (FSS) is the technique of choice to be applied on the thin metallic coating to transmit the wireless signal.In this work, a thin silver (Ag) film — 10 nm thick was deposited by electron beam evaporation on polycarbonate substrates as an example low-e coating. Despite excellent IR blocking (64 %) and visible light transmittance (60 %), it also presented a high attenuation of 20 dB at 5G signal bands (72–82 GHz).​ FSS patterns of various geometries and sizes were applied via laser ablation and evaluated to provide the lowest attenuation. We demonstrated that the application of the hexagonal pattern provided largest improvement reducing the 5G attenuation value from 20 dB to 1 dB, without compromising the visible transmittance and having only a minor effect on IR reflection.

Analysis of the energy and visual performance of a building with photochromic windows for a location in southern Italy

Windows have a considerable influence on the thermal balance of buildings. The aim of the present work is to evaluate the performance of a photochromic window (PCW) in terms of building's energy demands and natural lighting. In these windows, a fundamental role is played by the solar transmission coefficient, which can change depending on the incident solar radiation. A comparison is made with the case of using clear double glazing and low emissivity (Low-E) windows. The analysis is performed using EnergyPlus, where a laboratory of DIMEG, University of Calabria (Rende, Italy) is modelled. The model is calibrated to be representative of reality and can be used to compare different types of windows under the same external conditions. Results are obtained for a south-facing window, varying its size. The results show, for this location in southern Italy, that the PCW allows a reduction in annual energy consumption compared to the clear window (up to 9.3%) and compared to the Low-E window (up to 4.1%). The daylight provided by the PCW is greater than the other two solutions and, in some cases, also reduces the risk of glare.

Metal Mesh Design for Dynamic Windows Based on Reversible Metal Electrodeposition

Dynamic windows, which allow electronic control over visible light and solar irradiation, enable responsive environments free of glare and unwanted heat. Electronic control over visible light transmittance and solar heat gain coefficients will allow dynamic windows to be integrated with algorithms to optimize for energy efficiency and comfort. The implementation of this technology increases building energy efficiency by up to 20% compared to traditional, static low-emissivity windows.[ 1] Furthermore, worker productivity can be increased by 2% due to reductions in eye strain, headaches, and drowsiness.[ 2] Dynamic windows based on reversible metal electrodeposition (RME) represent an exciting, new class of electrochromic devices. These RME windows function like a battery by electrochemical movement of metallic ions dissolved in a nearly colorless aqueous electrolyte between a transparent conducting working electrode and a metal mesh or ion-intercalation counter electrode (see Figure). Metals are ideal light-modulators for dynamic windows because they can be color neutral, inert, photostable, and opaque at 20-30 nanometer thicknesses. Our team has most recently demonstrated metal-based dynamic windows that boast a dark state below 0.001% visible light transmittance in less than 10 minutes, an ultrawide range for optical and solar modulation (ΔTvis=0.76 and ΔSHGC=0.56) with uniform and color neutral (C*<5) tinting in prototypes >900 sq. cm.[ 3] To achieve a dark state transmission <0.1%, which has been defined as privacy state, ~150 mC/sq. cm of charge must be shuttled between the working electrode and counter electrode. We first outline the counter electrode design parameters for these high-contrast RME dynamic windows: high transparency, capacity, and surface area while maintaining low haze, sheet resistance, and cost. We show that metal meshes are a leading candidate for the counter electrode material based on these design parameters. Metal meshes also keep the design simple by having the same reversible electrochemical reaction on both electrodes (in opposite directions).We then analyze the material composition of the metal mesh to ensure cycling durability in a Cu-Bi electrolyte. This analysis shows that previously used Cu mesh in as the counter electrode material increases [Cu2+] over cycling (~200 privacy cycles) which results in degradation optically and electrochemically. We then develop a transparent mesh using an inert core with a thin metal coating that enables a high capacity (1.5 C/sq. cm) Cu-Bi layer for a metal mesh counter electrode. Finally, we incorporate this custom designed mesh in an RME dynamic window and demonstrate 250 privacy cycles.Developing a strong understanding of reversibly electroplating metals using multivalent metallic ions with variable sizes yields valuable insight to batteries where reversible plating is critical for operation and durability, particularly with deep cycling.

Low-Emissivity Window Films as an Energy Retrofit Option for a Historical Stone Building in Cold Climate

Low-emissivity (low-E) window films are designed to improve the thermal comfort and energy performance of buildings. These films can be applied to different glazing systems without having to change the whole window. This makes it possible to apply films to windows in old and historical buildings for which preservation regulations often require that windows should remain unchanged. This research aims to investigate the impacts of low-E window films on the energy performance and thermal comfort of a three-story historical stone building in the cold climate of Sweden using the simulation software “IDA ICE”. On-site measurements were taken to acquire thermal and optical properties of the windows. This research shows that the application of the low-emissivity window film on the outward-facing surface of the inner pane of the double-glazed windows helped to reduce heat loss through the windows in winter and unwanted heat gains in summer by almost 36% and 35%, respectively. This resulted in a 6% reduction in the building’s annual energy consumption for heating purposes and a reduction in the percentage of total occupant hours with thermal dissatisfaction from 14% (without the film) to 11% (with the film). However, the relatively high price of the films and low price of district heating results in a rather long payback period of around 30 years. Thus, the films seem scarcely attractive from a purely economic viewpoint, but may be warranted for energy/environmental and thermal comfort reasons.

Ultrathin Metals on a Transparent Seed and Application to Infrared Reflectors.

Ultrathin metal films (UTMFs) are widely used in optoelectronic applications, from transparent conductors to photovoltaic cells, low emissivity windows, and plasmonic metasurfaces. During the initial deposition phase, many metals tend to form islands on the receiving substrate rather than a physically connected (percolated) network, which eventually evolves into continuous films as the thickness increases. For example, in the case of Ag and Au on dielectric surfaces, percolation begins when the thickness of the metal film is at least about 5 nm. It is known that the type of growth can be changed when a proper seed layer is used. Here, we show that a CuO layer directly deposited on a substrate can dramatically influence surface wetting and promote early percolation of polycrystalline Ag and Au UTMFs. We demonstrate that the proposed seed is effective even when its thickness is sub-nanometric, in this way maintaining the full transparency of the receiving substrate. The Ag and Au films seeded with CuO showed a percolation thickness close to 1 nm and were morphologically and optically characterized from an ultraviolet (λ = 300 nm) to a midinfrared (λ = 15 μm) wavelength. Infrared reflectors, a mirror and a resonant plasmonic structure, were also demonstrated and uniquely tuned by electrical gating, this being possible owing to the small thickness of the constituting Au UTMF.

Parametric study of solid-solid translucent phase change materials in building windows

Thermal energy storage and solar radiation management are crucial to improve the sustainability and energy efficiency of buildings. Compared with the implementation of phase change materials (PCMs) in opaque components, the energy saving potential of incorporating PCMs in transparent glazing windows is much less studied and not well understood. Here we present a comprehensive parametric study of novel PCM windows for building energy saving with a focus on optimizing and quantitatively distinguishing the contributions from the optical and thermal properties of the PCM, which is particularly useful for the design of solid-solid PCM windows. We investigate a reference commercial office building using EnergyPlus by developing an equivalent model of our PCM window that is compatible with EnergyPlus’s modeling capabilities. Compared with a clear-clear double-pane window, the integration of 3 mm solid-solid PCMs with optimal properties in warm, mixed, and cold climates can respectively save up to 17.2%, 14.0%, and 5.8% energy for the HVAC (heating, ventilation, and air conditioning) system, and 9.4%, 6.7%, and 3.2% energy for the whole building. We also demonstrate that these energy savings are most sensitive to the solar absorptance of PCMs for all three climates. The optimal transition temperature varies with climate and is related to the climate and solar radiation heat gain. Other issues are also briefly discussed, such as hysteresis, window orientations, and the effect of interior lighting. Although the optimal PCM windows show energy saving performance comparable with low-emissivity windows, the PCM windows provide a unique advantage in terms of shifting HVAC loads which can provide benefits to the electrical grid.

Improvement on Window Glassing for an Enhanced Indoor Thermal Environment of Tropical Office Buildings

The intense solar radiation absorbed through the window glassing and transmitted into the indoor environment constitute thermal discomfort and increased cooling energy demand for buildings in the tropical climatic zone. This is because tropical climates are generally characterised by hot thermal distress almost all the year round. The need for improvement on window glassing of office buildings arise as a result of the global consensus on reducing energy demand of building sector. In office buildings, higher level of thermal and visual comfort is required for productive work performance within the indoor environment. This place office buildings in a position to enhance the realisation of a sustainable environment in the tropical climatic zone. This paper focus on effect of two types of window glassing on indoor air temperature and achieving improvement on single pane ordinary clear glassing window through the use of low-solar gain low-emissivity windows of two existing office buildings in the warm-humid zone of the tropical climate of Nigeria. An experimental investigation was scheduled during rainy and dry seasons of the year to collect outdoor and indoor microclimatic data of air temperatures in the west-facing natural ventilated office spaces within the existing office buildings through the use of Tiny Tag Ultra 2 Dual Channel Temperature/Relative Humidity (TGU-4500) Data loggers. Data collected were sorted and analysed through descriptive statistics. Improvement on single glassing window of existing office building was obtained through calculation of heat gain characteristics and comparison with that of low-solar gain low-emissivity glass window heat gain characteristics. Results showed that single pane ordinary clear glassing window exhibits higher indoor temperatures in the monitored office space. 89.54% improvement was recorded by using low-solar gain low-emissivity glass window over single pane clear glass window in the investigated office buildings. The study therefore recommends the use of low-solar gain glass low-emissivity window in office buildings within the warm-humid zone of tropical climate of Nigeria.

Transmission Enhancement Methods for Low-Emissivity Glass at 5G mmWave Band

This letter presents transmission enhancement methods for low-emissivity (low-E) windows at millimeter wavelength (mmWave) frequencies. These methods are required for installation of effective indoor customer premises equipment for mmWave fixed wireless access (FWA) services. Low-pass filtering frequency selective surfaces are patterned with low-E coatings by laser, and the effects are verified theoretically and experimentally. Antireflection (AR) dielectrics are additionally utilized and found to reduce transmission loss in various types of window regardless of frequency and incident angle. The results of the current study achieve lower losses for various types of window and incident angles. A transmission loss improvement is also achieved in a real FWA scenario, which can be applied for the coverage extension of FWA services.

Performance level criteria for semi-transparent photovoltaic windows based on dye-sensitized solar cells

Since the discovery of the photovoltaic (PV) effect in the 19th century, various PV windows have been developed to improve the energy performance of a building and expand the supply of renewable energy. Dye-sensitized solar cells (DSSCs) have attracted attention as promising alternatives to conventional silicon solar cells because of their transparency and aesthetic value. This study aims to analyze the effect of DSSC windows on the building environment and energy performance and build a database to help architects select appropriate DSSC windows for future zero-energy buildings (ZEBs). We use DesignBuilder, a building energy analysis program, to evaluate indoor illuminance, indoor temperature, cooling and heating energy, lighting energy, and power generation. Our prototype DSSC windows exhibit an improved heat transmission rate (i.e., U-value) but lower visible-light transmission (VLT) than low-emissivity glazing windows; thus, they decrease the heating energy and increase the cooling and lighting energy. We predict that DSSC windows meeting the criteria of 13% power conversion efficiency (PCE) and 30% VLT will achieve energy savings of 4861.44 kWh/yr. PV windows with over 50% VLT with any PCE can be beneficial to net zero-energy and net energy-plus buildings.

Energy efficient solutions for retrofitting a residential multi-storey building with vacuum insulation panels and low-E windows in two European climates

Almost the 40% of Europe’s energy consumption is used for heating and cooling buildings. Thus, the reduction of greenhouse gas emissions and mitigation of the climate change require the renovation and efficient use of energy of its building stock. Vacuum Insulation Panels represent a very attractive alternative to traditional insulation materials because high thermal performance can be achieved with a smaller thickness of insulation. The article reports a study of heating savings by thermally refurbishing the envelope of a multi-storey residential block with vacuum insulation panels on opaque portions of its east, south, and west elevations. Without the provision of cooling, however, the cladding may cause overheating in some climate conditions. A set of double- and triple-glazed low-emissivity windows with different glazing systems and frames has been designed for use in conjunction with wall cladding and the heating and cooling demands have been analysed for a series of cases. Heat transfer simulations using Trnsys, Window, and Optics have been conducted on the building, located in the cool climate of Berlin and the warm climate of Bilbao, to determine the best combination of insulation and type of window.

Outdoor-to-Indoor Channel Measurement and Coverage Analysis for 5G Typical Spectrums

The fifth-generation (5G) mobile communications system will adopt the millimeter wave (mmWave) band for outdoor-to-indoor (O2I) coverage to achieve ultrahigh data rate. However, it is a challenging task because of the large path loss and almost total blocking by building walls. In this work, we performed extensive measurements on the O2I propagation at 3.5, 4.9, and 28 GHz simultaneously by using a multiband channel sounder. We captured the path loss distribution and angular power arrival profiles. We also measured the penetration loss at 28 GHz through different kinds of glass windows. The widely adopted ordinary glass windows introduce the penetration loss of 3 to 12 dB that is acceptable and makes mmWave O2I coverage feasible. But the low-emissivity (low-E) windows that will be more popular in the future introduce 10 dB higher loss. The measurement results in this work can help analyse and anticipate the O2I coverage by mmWave, which is important for the design and deployment of the 5G network.

Impact of Low-E Window Films on Energy Consumption and CO2 Emissions of an Existing UK Hotel Building

In order to fulfil the UK government’s ambitious goal of 80% reductions in greenhouse gas emissions by 2050 compared to the levels of 1990s, unprecedented measures for improving the energy efficiency of buildings are needed. This study investigates the impact of a specific type of Low-emissivity (Low-E) window film—Thinsulate Climate Control 75—on the holistic energy consumption of an existing United Kingdom (UK) hotel building. Building modelling and energy simulation software EDSL TAS is used to conduct the study. The result of the simulations demonstrates that by applying Thinsulate films, savings in heating, cooling, and total energy consumptions are achieved by 3%, 20%, and 2.7%, respectively. Also 4.1% and 5.1% savings are achieved in annual CO2 emissions and total energy costs, respectively, while the initial costs may be an issue. This study found that application of Low-E window films results in slightly better energy performance of the hotel regarding its heating-dominant climate. The study also recommends using average annual actual energy consumption data for a time range, instead of picking a single year’s data for validating purposes.