Switchable glazing with a large dynamic range in total solar energy transmittance (TSET)

Modern, energy-saving buildings incorporate large areas of highly insulating glazing. The resulting solar gains lead to major savings in heating energy during winter, but protection against overheating in summer is also needed. Usually this problem is solved by using mechanical shading devices, with the disadvantages of high cost and low durability.The work on switchable glazing at Fraunhofer Institute for Solar Energy Systems, in cooperation with industrial partners, aims to present new and viable alternatives. Two types of switching layers, which are quite different in their structure and function, but are similar in having a large dynamic range in TSET, are being investigated—gasochromic and thermotropic.Gasochromic windows are actively switched between a clear and a coloured (but image-preserving) state by alternately introducing strongly diluted O₂ and H₂ gases. In contrast to classic electrochromic configurations, only one tungsten oxide film with a very thin catalyst coating is needed. At present, prototype windows with an area of 1.1×0.6 m² are being produced by sputtering. Careful adjustment of the layer structure, the gas concentration and its flow velocity is needed to obtain the desired switching rate. Homogeneous colouring of the whole area within seconds has been achieved. In addition to information on the colouring kinetics, the paper also discusses system aspects of these windows.Thermotropic layers switch reversibly and automatically, from a clear state with high transmittance to a milky white state with high diffuse reflectance, when their temperature rises. Depending on the composition of the material, the switching temperature can be chosen in the range needed. The measured optical properties of glass laminates with a thermotropic layer are presented. These are combined with the measured values for further panes to calculate the characteristic data for thermotropic insulated glazing units. The results are compared with those measured on a 1.1×1.65 m² prototype window. Stability results are also included.The effect of the two different types of switchable glazing on building energy savings is explored for a residential building model, using the TRNSYS building energy simulation program.     

Copper based thin films to improve glazing for energy-savings in buildings

This paper presents research results for better window materials for energy saving purposes. Three alternatives of glazing treatments were developed in this project: Cu₂O, CuS + Cu₂O, and Cu + Cu₂O. The second and third alternatives present an adequate control in the infrared irradiance, achieving a significant reduction of the heat transfer through them. These windows allow the transmission of about half of the heat under hot weather condition, thus reducing the thermal load inside the building. The same windows also reduce the outwards flow of heat at half of the rate of windows without coating, improving comfort and reducing heat loss under cold weather conditions. The thin layers were produced using a sputtering technique with planar magnetrons. A high purity copper target (99.99%) was evaporated through plasma of argon–oxygen (70–30% respectively) in order to obtain Cu₂O. Soda-lime glass substrates of 600 × 400 mm were used. About 50% of the infrared is blocked by the Cu₂O window, when it is applied for thermal comfort. Thus, it was necessary to add another layer (CuS), which allowed a reduction of up to 20% in the NIR. The spectral behavior of the Cu + Cu₂O window showed a transmittance of about 50% in VIS range, while in the NIR, the transmittance decreased from 40% to 20%. The composite windows, which contained CuS, Cu, and Cu₂O are suitable solutions for solar control. Simulations of annual energy savings were carried out with the Energy-10 software for the Cu₂O + CuS window for two cities, Mexico City and Mexicali, the latter representing extreme weather conditions (over 35 °C in summer and below 6 °C in winter). The total energy saving was approximately 20% of the total energy demand for Mexicali. Thermal transmission measurements were also carried out at the center of these windows, giving values of about 4 W/m² K.     

Sky type classification in Central England during winter

In the present work, we studied the exterior daylight conditions in Sheffield in Central England (53.38° N, 1.50° W) during winter. The study makes use of the 15 standard sky types defined by Kittler et al. These sky types are presented as diagrams of the ratio of the zenith luminance (L_z) to the diffuse horizontal illuminance (D_v) against solar altitude (γ). The illuminance parameters used are time series of half-hourly values of D_v, L_z, global (G_e) and diffuse (D_e) horizontal irradiance and γ for the period from November 1, 1993, to February 10, 1994.This study shows that the most frequent winter sky types in Central England are: (i) I.2 (overcast, with steep gradation and slight brightening towards the sun), with a frequency of occurrence of 19.5%, (ii) I.1 (overcast, with steep gradation and with azimuthal uniformity) (10.5%) and (iii) II.1 (overcast, moderately graded, with azimuthal uniformity) (9.7%). The most predominant cloudless sky is V.5 (cloudless, polluted, with a broad solar corona), with a frequency of occurrence of 7.6%.     

A study of the U-factor of a window with a cloth curtain

Heating and cooling energy calculations and building energy simulations need the overall heat-transfer coefficient (U-factor) of windows. This study focuses on the U-factor of a window with a cloth curtain indoors. Two side-by-side hot boxes are built in a laboratory for testing the window U-factors. The tested window systems are 100% glass area without frames which are equivalent to the center of glass of practical windows. The two types of tested window systems are the single glazing and the double glazing with a cloth curtain, respectively. The effect of curtain edge sealing conditions on U-factors is also studied. The empirical equations of the center-of-glass U-factors are derived from the experimental data. The corrections for the effects of window frames and outdoor wind velocity are made thereafter. The corrected empirical equations can be easily used to estimate the U-factor of a practical window with a cloth curtain.     

Use of a variable parameter test-cell for the study of latent-heat solar walls

A paraffin Trombe wall with double glazing, is studied experimentally with the help of a test-cell allowing several working conditions to be checked. The efficiency of the wall itself is shown to be very similar whether thermocirculation is on or not. But an important additional contribution comes from the glazing in the first case. The analysis of the results points out that a controlled air circulation would give much better results than the two cases considered. Indeed the overheating due to thermocirculation can be cancelled by some controlled air circulation only, and the important convective front losses observed at the begining of the night have to be decreased by getting the heat back to the room through forced air circulation.     

Energy efficiency of a dynamic glazing system

The reduction of air-conditioning energy consumptions is one of the main indicators to act on when improving the energy efficiency in buildings.In the case of advanced technological buildings, a meaningful contribution to the thermal loads and the energy consumptions reduction could depend on the correct configuration and management of the envelope systems. In recent years, the architectural trend toward highly transparent all-glass buildings presents a unique challenge and opportunity to advance the market for emerging, smart, dynamic window and dimmable daylighting control technologies (Lee et al., 2004).A prototype dynamic glazing system was developed and tested at ITC-CNR; it is aimed at actively responding to the external environmental loads. Both an experimental campaign and analyses by theoretical models were carried out, aimed at evaluating the possible configurations depending on different weather conditions in several possible places. Therefore, the analytical models of the building-plant system were defined by using a dynamic energy simulation software (EnergyPlus).The variables that determine the system performance, also influenced by the boundary conditions, were analysed, such as U- and g-value; they concern both the morphology of the envelope system, such as dimensions, shading and glazing type, gap airflow thickness, in-gap airflow rate, and management, in terms of control algorithm parameters tuning fan and shading systems, as a function of the weather conditions.The configuration able to provide the best performances was finally identified by also assessing such performances, integrating the dynamic system in several building types and under different weather conditions.The dynamic envelope system prototype has become a commercial product with some applications in façade systems, curtain walls and windows.The paper describes the methodological approach to prototype development and the main results obtained, including simulations of possible applications on real buildings.     

Reassessment of fenestration characteristics for residential buildings in hot climates: energy and economic analysis

This paper attempts to resolve the reported contradiction in the literature about the characteristics of high-performance/cost-effective fenestration of residential buildings, particularly in hot climates. The considered issues are the window glazing property (ten commercial glazing types), facade orientation (four main orientations), window-to-wall ratio (WWR) (0.2–0.8), and solar shading overhangs and side-fins (nine shading conditions). The results of the simulated runs reveal that the glazing quality has a superior effect over the other fenestration parameters and controls their effect on the energy consumption of residential buildings. Thus, using low-performance windows on buildings yields larger effects of WWR, facade orientation, and solar shading than high-performance windows. As the WWR increases from 0.2 to 0.8, the building energy consumption using the low-performance window increases 6.46 times than that using the high-performance window. The best facade orientation is changed from north to south according to the glazing properties. In addition, the solar shading need is correlated as a function of a window-glazing property and WWR. The cost analysis shows that the high-performance windows without solar shading are cost-effective as they have the largest net present cost compared to low-performance windows with or without solar shading. Accordingly, replacing low-performance windows with high-performance ones, in an existing residential building, saves about 12.7 MWh of electricity and 11.05 tons of CO₂ annually.     

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.     

Smart switchable glazing for solar energy and daylight control

The exciting field of chromogenic materials for smart windows and other large-area applications is discussed. A selection of switchable glazing devices that change color electrically are detailed. The types of devices covered are the electrochromic which change color electrically, covering electrochromic, dispersed liquid crystal and dispersed particle glazing that switch under an applied electric field. Device structures and switching characteristics are compared. The status of prototype and commercial devices from commercial and university labs through out the world are covered. A discussion of the future of this technology is made including areas of necessary development for the realization of large area glazing in excess of 1m².     

A modified model for estimation of daylight factor for skylight integrated with dome roof structure of mud-house in New Delhi (India)

The daylight factor model given by Charted Institute of Building Services Engineers (CIBSE) was modified in this paper to incorporate time variations with respect to zenith angle (θ_z) and vertical height (h) of working surface above ground surface which was normalized with central height (H) of skylight dome. The modified model contains constant exponents which are determined using linear regression analysis based on hourly experimental data of inside and outside illuminance for each month of the year 2007–2008. The prediction of modified model is found in good agreement with experimental observed inside illuminance data on the basis of values of root mean square percentage error (e) and correlation coefficient (r). The annual average daylight factor values for big and small dome skylight rooms are determined as 2.3% and 4.4% respectively. The energy saving potential of skylight rooms for selected climatic locations in India is also presented in this paper.     

Large-area smart glass and integrated photovoltaics

Several companies throughout the world are developing dynamic glazing and large-area flat panel displays. University and National Laboratory groups are researching new materials and processes to improve these products. The concept of a switchable glazing for building and vehicle application is very attractive. Conventional glazing only offers fixed transmittance and control of energy passing through it. Given the wide range of illumination conditions and glare, a dynamic glazing with adjustable transmittance offers the best solution. Photovoltaics can be integrated as power sources for smart windows. In this way a switchable window could be a completely stand alone smart system. A new range of large-area flat panel display including light-weight and flexible displays are being developed. These displays can be used for banner advertising, dynamic pricing in stores, electronic paper, and electronic books, to name only a few applications. This study covers selected switching technologies including electrochromism, suspended particles, and encapsulated liquid crystals.     

Energy conversion efficiency of a welded Cu/Bi–Te/Cu composite under periodically alternating temperature gradients

It was reported recently by us that the energy conversion efficiency η_ATG of thermoelectric (TE) generators working under the alternating temperature gradient (ATG) produced by switching at an optimum period of τ = 120 or 240 s is several times higher than η_STG of them operating under the steady temperature gradient (STG), where the generators are sandwiched between two Peltier modules. However, the reason for η_ATG > η_STG has not yet been explained explicitly. In order to clarify the relation between the optimum τ and η_ATG or η_ATG/η_STG in the wide range of τ, long p- and n-type Cu/Bi–Te/Cu composites with t_Bi–Te = 4.0 mm and t_Cu = 5.0 mm were fabricated so that they have an optimum τ longer than the previous periods. The maxima η_ATG of the p- and n-type composites were 0.073% at τ = 960 s and 0.057% at τ = 480 s, respectively, and their η_ATG/η_STG were approximately 1.4. Owing to this experimental result, η_ATG/η_STG is related closely to the thermal diffusion time τ_D of a composite or a generator, so that η_ATG and η_ATG/η_STG were found to be in inverse proportion to τ_D. The reason for this low η_ATG results from the high external resistance and the long τ caused by a thick t_Bi–Te, in addition to too fat Bi–Te sample giving the high thermal conductance. The maximum η_ATG was also found to be expressed successfully by a theoretical expression.     

Electrochromic windows

Electrochromic windows can be described as devices based on electrochromic materials that work as reversible switching optical shutters, different device concepts and their electrochemical principles will be described.The relevance of electrochromic windows to low energy architecture will be discussed. A detailed analysis about the general requirements and the specific performances of electrochromic glazing for architectural applications will be introduced. The challenging issues of the present electrochromic technology approaching architectural glazing will be identified from the technological point of view and from the applications perspective.Emerging electrochromic glazing designs will be critically examined. A final discussion will be introduced about the different technical and marketing problems that slow down the time to market of electrochromic windows.     

Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance

Thermochromic films of VO₂, as well as multilayer films of VO₂ and TiO₂, were made by reactive DC magnetron sputtering. Spectrophotometrically measured transmittance and reflectance were used to determine optical constants pertinent to temperatures below and above a temperature-induced structural change at τ_c≈60 °C. We then used computations to optimize multilayer films for specific applications and, specifically, demonstrated that TiO₂/VO₂/TiO₂ films could display a luminous transmittance significantly higher than that of bare VO₂ films, and that TiO₂/VO₂/TiO₂/VO₂/TiO₂ films could yield a large change of solar transmittance for temperatures above and below τ_c. Our data can serve as starting points for developing novel coatings for windows with superior energy efficiency.     

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.     

A test method to evaluate the thermal performance of window glazings

An apparatus and a methodology to evaluate the thermal performance of window glazing are presented. Single glazings commercially available in Mexico are currently tested. During the test sequence, the glass samples are mounted in a specially designed calorimeter apparatus. The test is conducted in controlled laboratory conditions at the National Centre for Research and Technology Development in Mexico using a solar simulator test lamp. The calorimeter apparatus and the solar simulator test lamp were characterised and the overall heat loss coefficient U_c measured was of 1.7±0.1 W/m²°C. Overall heat transfer and shading coefficients are derived from the experimental results. The test method described allows the testing of practically any kind of glazing array. Glasses under investigation were of the reflective, absorbing coloured and common ones. This test method can be adequate to evaluate film coating glazing or multiple solar control coating glazing. Also, it can be of assistance to researchers, glass manufacturers and building designers in the development of rating and comparing of glazing options.     

Optimization of insulation thickness for different glazing areas in buildings for various climatic regions in Turkey

Insulation is one of the most effective methods intended for reducing energy consumption in both heating and cooling of buildings. Selecting the right materials and determining the optimum insulation thickness in building insulation application is an important issue. In 2000, the “Thermal Insulation Requirements for Buildings” was enacted in Turkey, energy saving by limiting the energy amount used for heating in buildings being the target. In this study, the effect of the alteration of windows and exterior wall areas on the heating energy requirement of the building and on the optimum insulation thickness has been examined by using P₁–P₂ method. The study has been carried out for four degree-day regions of Turkey for various insulation materials, glazing areas, and fuel types; the results have been presented in charts. In the rest of this study, effects of different insulation thicknesses and fuel on fuel consumption and thereby on emissions of pollutants such as CO₂ and SO₂ are evaluated. For example, in the building where XPS (extruded polystyrene foam) insulation material and natural gas are used and where the ratio of glazing area to exterior wall area is 0.2 (glazing area percentage), energy saving for the four regions has been found to be 13.996, 31.680, 46.613, and 63.071 $/m², respectively, and the payback period of investment has been found to be 2.023, 1.836, 1.498, and 1.346 years, respectively. The emissions of CO₂ are decreased by 50.91% for the cases in which optimum insulation material (XPS) and natural gas are used. The emissions of CO₂ and SO₂ are decreased by 54.67% for the cases in which optimum insulation material (XPS) and fuel oil are used.     

Chemical oxidation and spectral selectivity of austenitic stainless steel AISI 321 (for use in solar-energy applications)

Polished samples of austenitic stainless steel AISI 321 were chemically oxidized in 0.1–0.6 M aqueous solutions of chromic and sulphuric acids at 90 °C for an oxidation time of 25 min, as well as for oxidation times ranging from 10 to 32 min in 0.6 M solution. Chemical oxidation of the stainless steel produced interference films ranging in colour from light-blue to deep-blue, purple-red and peacock-green. Measurements of solar absorptance (α_s) and total hemispherical thermal emittance (ϵ_h) showed that all the oxidized surfaces were spectrally selective. The most favourable spectrally selective surface was deep-blue in colour and it had values of α_s = 0.85 ± 0.01 and ϵ_h = 0.18 ± 0.01. This surface was prepared after an oxidation time of 12 min at a concentration of 0.6 M of chromic and sulphuric acids. For the unoxidized polished stainless steel, the α_s and ϵ_h were 0.38 ± 0.01 and 0.15 ± 0.01 respectively. No significant changes in the optical properties of the films were observed after 6 months exposure to highly humid tropical weather conditions.     

Visual and energy performance of switchable windows with antireflection coatings

The aim of this project was to investigate how the visual appearance and energy performance of switchable or smart windows can be improved by using antireflective coatings. For this study clear float glass, low-e glass and electrochromic glass were treated with antireflection (AR) coatings. Such a coating considerably increases the transmittance of solar radiation in general and the visible transmittance in particular. For switchable glazing based on absorptive electrochromic layers in their dark state it is necessary to use a low-emissivity coating on the inner pane of a double glazed window in order to reject the absorbed heat. In principle all surfaces can be coated with AR coatings, and it was shown that a thin AR coating on the low-e surface neither influences the thermal emissivity nor the U-value of the glazing. The study showed that the use of AR coatings in switchable glazing significantly increases the light transmittance in the transparent state. It is believed that this is important for a high level of user acceptance of such windows.