Durability issues and service lifetime prediction of electrochromic windows for buildings applications

In general, the purposes of this paper are to elucidate the crucial importance of durability and service lifetime prediction (SLP) for electrochromic windows (ECWs) and to present an outline for developing a SLP methodology for ECWs. The specific objectives are (a) to illustrate the generic nature of SLP for several types of solar energy conversion or energy conservation devices, (b) to summarize the major durability issues associated with ECWs, (c) to justify using SLP in the triad of cost, performance, and durability rather than just durability, (d) to define and explain the seven major elements that constitute a generic SLP methodology, (e) to provide background for implementing the SLP methodology for ECWs, including the complexity of the potential degradation mechanisms, and (f) to provide an outline of studies using ECWs for improving the durability of ECW materials and predicting a service lifetime for ECWs using the SLP methodology outlined in objective (d). Our major conclusions are that substantial R&Dis necessary to understand the factors that limit ECW durability, and that it is possible to predict the service lifetime of ECWs.     

Environmental assessment of electrochromic glazing production

The life cycle analysis method was used to determine the environmental impacts associated with the production of an electrochromic (EC) glazing (called ECD). This paper describes the inventory analysis for all the basic materials used during the manufacture of the ECD, i.e. K-Glass, tungsten oxide (WO₃), poly-methyl methacrylate (PMMA), propylene carbonate (PC), lithium perchlorate (LiClO₄) and acetic silicone sealant. K-Glass, PC and PMMA account for the 98% of the total device mass and the CO₂ emissions during their production processes are 810 g. The total embodied energy was estimated to be 49 MJ/ECD, with 32.1 MJ/unit of them derived from the K-Glass. The comparison of the total embodied energies of the ECD and various insulating glass units concluded that mass-produced EC glazings could easily compete with them in terms of environmental performance, anticipating cost attenuation and overall thermal and optical behavior. The above analysis could be implemented for the reduction of the embodied energy of the ECD life cycle, since it is proposed as an energy saving device.     

Durability evaluation of electrochromic devices – an industry perspective

Several issues regarding the working environment and the stability of prototype electrochromic (EC) windows are discussed. In this study, we focus on issues to attain confidence in the durability of EC devices for energy efficient architectural glazing. The environmental conditions that EC glazing are subjected to, are detailed and discussed. Comparisons are made to actual prototype, real time EC window exposure testing. Testing of EC mirrors and liquid crystal glazing is compared to EC glazing. During testing of glazing in Arizona, surface temperatures of 56–65°C (uncolored-colored) were measured on EC glazing panels, with ambient air temperature of 40°C. The usual panel heating rate ranged from 14.3°C/h to 21°C/h. The steepest swing occurred during a thunderstorm with a 40°C rise in 15 min. In EC testing studies in Yokohama, it was determined that indoor testing and outdoor testing did not achieve the same results, with outdoor testing being more severe. It was also determined that the critical test parameters were temperature, solar intensity (especially UV), depth of coloration, charge capacity and change in transmittance. As a result of this study, we recommend a regimen of testing covering thermal cycling, UV stability, thermal storage and thermal shock.     

Thermal performance of an electrochromic vacuum glazing

Using a three-dimensional finite volume model, the thermal performance of an electrochromic vacuum glazing was simulated for insolation intensities between 0 and 1200 W m⁻². The electrochromic evacuated glazing simulated consisted of three glass panes 0.5 m by 0.5 m with a 0.12 mm wide evacuated space between two 4 mm thick panes supported by 0.32 mm diameter pillars spaced on a 25 mm square grid contiguously sealed by a 6 mm wide metal edge seal. The third glass pane on which the electrochromic layer was deposited was assumed to be sealed to the evacuated glass unit. The simulations indicate that when facing the indoor environment, the temperature of the glass pane with the electrochromic layer can reach 129.5 °C for an incident insolation of 600 W m⁻². At such temperatures unacceptable occupant comfort would ensue and the durability of the electrochromic glazing would be compromised. The glass pane with the electrochromic layer must therefore face the outdoor environment.     

Application issues for large-area electrochromic windows in commercial buildings

Projections of performance from small-area devices to large-area windows and enterprise marketing have created high expectations for electrochromic glazings. As a result, this paper seeks to precipitate an objective dialog between material scientists and building-application scientists to determine whether actual large-area electrochromic devices will result in significant performance benefits and what material improvements are needed, if any, to make electrochromics more practical for commercial building applications.Few in situ tests have been conducted with large-area electrochromic windows applied in buildings. This study presents monitored results from a full-scale field test of large-area electrochromic windows to illustrate how this technology will perform in commercial buildings. The visible transmittance (T_v) of the installed electrochromic ranged from 0.11 to 0.38. The data are limited to the winter period for a south-east-facing window. The effect of actual device performance on lighting energy use, direct sun control, discomfort glare, and interior illumination is discussed. No mechanical system loads were monitored. These data demonstrate the use of electrochromics in a moderate climate and focus on the most restrictive visual task: computer use in offices.Through this small demonstration, we were able to determine that electrochromic windows can indeed provide unmitigated transparent views and a level of dynamic illumination control never before seen in architectural glazing materials. Daily lighting energy use was 6–24% less compared to the 11%-glazing, with improved interior brightness levels. Daily lighting energy use was 3% less to 13% more compared to the 38%-glazing, with improved window brightness control. The electrochromic window may not be able to fulfill both energy-efficiency and visual comfort objectives when low winter direct sun is present, particularly for computer tasks using cathode-ray tube (CRT) displays. However, window and architectural design as well as electrochromic control options are suggested as methods to broaden the applicability of electrochromics for commercial buildings. Without further modification, its applicability is expected to be limited during cold winter periods due to its slow switching speed.     

Daylighting performance of an electrochromic window in a small scale test-cell

In this paper, the results of an experimental investigation aimed at assessing the performance of electrochromic (EC) windows with respect to daylighting control in buildings are presented. The research is performed under real weather conditions by a small scale test-cell equipped with a small area double glazing unit (DGU) where one pane consists of an EC device with visible transmittance τ_v ranging from 6.2 to 68.1% and the other of an ordinary clear float glass (τ_v ≈90%). Experimental tests are carried out as a function of time, weather conditions, test-cell orientation and switching strategies. These data are integrated with spectrophotometric measurements. Results show that the angle selectivity of the glazing combined with its active switching effect allows a wide range of selectable transmission states to suit the latitude and orientation of a building in relation to the local climatic conditions. For south facing windows and under the involved climatic conditions EC glazing driven by a dynamic control strategy can be very effective in reducing discomfort glare caused by high window brightness. Glare reduction can be realized contemporarily maintaining the work plane illuminance to adequate level for computer based office tasks so without compromising much of the available daylight. Furthermore, since EC glazing is never switched to heavily darkened states (τ_v >20%), colour rendering of inside objects should be always acceptable, although internal illuminance level could be slightly lower than to what users prefer in relation to the correlated colour temperature of the incoming light. These results change when considering west orientation for which high-luminance direct sunlight patches are registered on the work-plane even for EC glazing switched to its lowest transmitting state letting suppose that EC windows cannot provide full control of uncomfortable direct sunlight effects without integration of additional shading devices.     

Large-area electrochromic glazing with ion-conducting PVB interlayer and two complementary electrodeposited electrochromic layers

A new laminated large-area electrochromic glass consisting of two FTO-coated glass panes coated with complementary electrochromic thin films by electrodeposition and laminated together by the use of an ion-conducting PVB sheet is presented. The visible light transmittance can be changed between 77% and 8% and the solar transmittance between 56% and 6%.     

Thermal performance analysis of an electrochromic vacuum glazing with low emittance coatings

Thermal performance of an electrochromic (EC) vacuum glazing (VG) was modelled under ASTM standard winter conditions. The EC VG comprised three 0.5 m by 0.5 m glass panes with a 0.12 mm wide evacuated space between two 4 mm thick panes sealed contiguously by a 6 mm wide indium based edge seal with either one or two low-emittance (low-e) coatings supported by a 0.32 mm diameter square pillar grid spaced at 25 mm. The third glass pane on which the 0.1 mm thick EC layer was deposited was sealed to the evacuated glass unit. The whole unit was rebated by 10 mm within a solid wood frame. The low-e coating absorbed 10% of solar energy incident on it. With the EC VG installed with the EC component facing the outdoor environment, for an incident solar radiation of 300 W m⁻², simulations demonstrated that when the EC layer is opaque for winter conditions, the temperature of the inside glass pane is higher than the indoor air temperature, due to solar radiation absorbed by the low-e coatings and the EC layer, the EC VG is a heat source with heat transferred from the glazing to the interior environment. When the emittance was lower to 0.02, the outdoor and indoor glass pane temperatures of the glazing with single and two low-e coatings are very close to each other. For an insolation of 1000 W m⁻², the outdoor glass pane temperature exceeds the indoor glass pane temperature, consequentially the outdoor glass pane transfers heat to the indoor glass pane.     

PCM glazing systems

This paper presents a different approach for thermal effective windows, i.e. windows that reduce the energy transmitted into or out of a room. The idea is to use a double sealed glass filled with a phase change medium (PCM) whose fusion temperature is determined by solar–thermal calculations. The PCM used is polypropylene glycol. The investigation includes modelling of the heat and radiation transfer through a composite window and optical investigation of conventional and PCM filled windows, testing of the window and comparison with numerical simulations. A one-dimensional model for the composite glass window is developed to predict the thermal performance as a function of the geometrical parameters of the panel and the PCM used. Optical measurements were realized using photo-spectrometry to determine the transmittance, reflectance and absorptance. The specimens used include single glass of different thicknesses, double glass of different gap spacing and thicknesses filled with air or PCM, and finally coloured PCM. The results indicate big reductions in the energy transmitted, specially in the infra-red and ultraviolet regions, while maintaining a good visibility. © 1997 by John Wiley & Sons, Ltd.     

Evaluation of electrochromic windows impact in the energy performance of buildings in Mediterranean climates

Old buildings refurbishment is essential for the global improvement of building energy indicators. Within this context, the paper focuses on the energy savings that may occur when using electrochromic (EC) windows, an interesting emerging technology alternative to shading devices to control solar gain in buildings located in Mediterranean climates. The EC windows technology is briefly presented and the optical properties adjustments of the glasses are discussed according to the operated range. The EC window dynamic behavior and the different control strategies are modeled and implemented in the ESP-r building simulation program. The EC window impact in the energy needs for heating and cooling is studied, considering different ambient parameters (exterior dry bulb temperature, interior dry bulb temperature and incident radiation) and set points for the EC control. A comparison of several windows solutions (single, double-glazing and EC windows), the type of building, internal gains from occupancy, lighting and equipment and the orientation of windows are considered for discussion through the analysis of the energy needs for heating and cooling. It is concluded that for this climate the best positive results are obtained when the EC are used in the west façade. For the south façade the results show no significant advantages in using EC windows.     

Optical properties of electrochromic all-solid-state devices

We have investigated the optical properties of an all-thin-film electrochromic device, with a thin film of ZrO₂ acting as an ion conductor. The device also employed electrochromic layers of amorphous or crystalline WO₃ and NiV_xO_yH_z. Transmission (T) and reflection (R) spectra were recorded in the wavelength range 300–2500 nm at different intercalation levels, both for single films and complete devices. The results show that T decreases significantly upon intercalation in the WO₃ thin films as well as in the devices. The reflectance only shows minor changes.     

Status of reversible electrodeposition electrochromic devices

The status of electrochromic devices based upon the reversible electrodeposition of thin bismuth–copper films is reported. The electrochemistry and interface chemistry of the system relevant to information display applications are briefly described. Low information content displays are currently being produced in small quantities with saturated black and white contrast ratio of 25 : 1 and lifetimes of greater than ten million cycles. Electrochemical scanning tunneling microscopy (ECSTM) studies on highly ordered pyrolytic graphite (HOPG) substrates were undertaken as a first step in the unraveling of the detailed nucleation and growth behavior of the Bi–Cu system at the atomic level. The results obtained support previous studies using scanning electron microscopy and surface spectroscopy. Preliminary results on the plating current efficiency of the system using a quartz crystal microbalance (QCM) are reported.     

Optical indices of lithiated electrochromic oxides

Optical indices have been determined for thin films of several electrochromic oxide materials. One of the most important materials in electrochromic devices, WO₃, was thoroughly characterized for a range of electrochromic states by sequential injection of Li ions. Another promising material, Li_0.5Ni_0.5O, was also studied in detail. Less detailed results are presented for three other common lithium-intercalating electrochromic electrode materials: V₂O₅, LiCoO₂, and CeO₂–TiO₂. The films were grown by sputtering, pulsed laser deposition (PLD) and sol–gel techniques. Measurements were made using a combination of variable-angle spectroscopic ellipsometry and spectroradiometry. The optical constants were then extracted using physical and spectral models appropriate to each material. Optical indices of the underlying transparent conductors, determined in separate studies, were fixed in the models of this work. The optical models frequently agree well with independent physical measurements of film structure, particularly surface roughness by atomic force microscopy. Inhomogeneity due to surface roughness, gradient composition, and phase separation are common in both the transparent conductors and electrochromics, resulting sometimes in particularly complex models for these materials. Complete sets of data are presented over the entire solar spectrum for a range of colored states. These data are suitable for prediction of additional optical properties such as oblique transmittance and design of complete electrochromic devices.     

A reversible inorganic electrochromic solution system

Electrochromic devices that respond to varied levels of applied electrical potential by changing light opacity are presently prepared from either insertion compounds or reversible electrodeposition of metals from solution. We report here another approach to electrochromics based on the reversible deposition/dissolution of silver oxide onto FTO-coated glass from an aqueous solution containing silver (I)-ammonia complexes. The basic redox chemistry underlying the functioning of this electrochromic system has been identified using traditional techniques such as X-ray diffraction, scanning electron microscopy, and cyclic voltammetry, as well as a quartz nanobalance for measuring the working electrode weight at different potentials. The AgNH₃⁺↔AgO reaction is responsible for the coloration/decoloration of the working electrode, which can be repeatedly cycled between various states of visual opacity. Optical measurements reveal that the transmittance of the working electrode in the visible region drastically drops from 80% without silver oxide deposition to 4% when it is coated with a silver oxide film.     

Printed, flexible electrochromic displays using interdigitated electrodes

We describe herein some of our initial studies in pursuit of a simple, economical method of mass producing electrochromic displays. The approach we have taken is to print the display on polymer film utilizing commercially available conductive inks in an interdigitated electrode structure with a conductive metal oxide powder, dispersed in a polymer binder, as the electrode surface. A range of electrochromic materials suitable for use with an aqueous gel electrolyte have been explored and examples presented.     

Structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process

By virtue of gemini surfactant template, nanostructured tungsten oxides thin films were prepared from the modified tungsten hexachloride sol-gel techniques. Temperature was varied as it is an important factor for crystallization, surface morphology and microstructure of tungsten oxides, from the studies of X-ray diffractions, scanning electron microscopy and transmission electron microscopy. The mesoporous sample calcined at 300 °C has tri-dimensional vermicular mesopores with nanocrystallites embedded in the pore wall, while such uniform structure would be destroyed by higher calcination temperature of about 400 °C. X-ray photoelectron spectroscopy was used for analyzing the surface-binding states and the stoichiometry for the oxides. Electrochromic characterization was implemented by simultaneous voltametric and spectrophotometric measurements of tungsten oxides/indium tin oxide (ITO) electrodes. The investigation results showed that organized pore-wall nanostructure has strong effects on the electrochemical and chromogenic properties depending on the specific surface area and the impacts from the evolved crystallization.     

Electro-optical analysis of PEDOT symmetrical electrochromic devices

New symmetrical electrochromic devices were constructed, with PEDOT acting as electrochromic layer or as counter electrode layer depending on the polarity of the applied voltage. Devices of around 500 mm² were analysed in this work. They can switch from 0.5 to 2 V, obtaining a spectral change of around 21% mainly in the red wavelength interval. Measured electrochemical impedance was fitted to an equivalent circuit, made by a Randles circuit, with Warburg impedance simulating ionic diffusion at low frequencies. Results show problems in the contact layers, not seen in normal operation. This will serve as a method to improve the devices construction.     

Angular selective thin film glazing

Angular selective control of daylight, solar heat gain and visual performance is the aim of a variety of emerging technologies for windows, roof and wall glazing. Certain oblique thin metal and metal/insulator films on glass have transmittance as a unique function of direction of incidence and they are also spectrally selective. A variety of angular selective control options result according to choice of film materials covering one or a combination of (i) light and glare from high angles (ii) solar heat gain (iii) visual amenity (iv) glare from lower angles (iv) emittance control. They are thus able to be adapted by materials choice to the needs of different latitudes and window orientations. A new and simple way of categorizing performance will be presented.     

Durability of polymeric glazing materials for solar applications

The economic viability of solar collector systems for domestic hot water (DHW) generation is strongly linked to the cost of such systems. Installation and hardware costs must be reduced by 50% to allow significant market penetration. An attractive approach to cost reduction is to replace glass and metal parts with less expensive, lighter weight polymeric components. Weight reduction decreases the cost of shipping, handling, and installation. The use of polymeric materials also allows the benefits and cost savings associated with well established manufacturing processes, along with savings associated with improved fastening, reduced part count, and overall assembly refinements. A key challenge is to maintain adequate system performance and assure requisite durability for extended lifetimes. Results of preliminary and ongoing screening tests for a large number of candidate polymeric glazing materials are presented. Based on these results, two specific glazings with moderate and poor weathering stability are selected to demonstrate how a service lifetime methodology can be applied to accurately predict the optical performance of these materials during in-service use. A summary is given for data obtained by outdoor exposure and indoor testing of polyvinyl chloride (PVC) and high temperature modified polycarbonate copolymer (coPC) materials, and an initial risk analysis is given for the two materials. Screening tests and analyses for service lifetime prediction are discussed. A methodology that provides a way to derive correlations between degradation experienced by materials exposed to controlled accelerated laboratory exposure conditions and materials exposed to in-service conditions is given, and a validation is presented for the methodology based upon durability test results for PVC and coPC.     

Progress in chromogenics: New results for electrochromic and thermochromic materials and devices

Chromogenic device technology can be used to vary the throughput of visible light and solar energy for windows in buildings as well as for other see-through applications. The technologies can make use of a range of “chromic” materials - such as electrochromic, thermochromic, photochromic, etc - either by themselves or in combinations. The first part of this paper points at the great energy savings that can be achieved by use of chromogenic technologies applied in the built environment, and that these savings can be accomplished jointly with improved indoor comfort for the users of the building. Some recent data are presented on a foil-type electrochromic device incorporating tungsten oxide and nickel oxide. In particular, we consider the possibilities of controlling the near-infrared transmittance and optimize this property for specific climates. To that end we discuss Au-based transparent conductors for electrochromics as well as high-transmittance thermochromic multilayer films incorporating VO₂ and TiO₂.