Electrically switchable chromogenic materials for external glazing

The development of chromogenic materials is important for their potential applications as electrically adjustable or thermally self-adjusting light and heat filters in the external glazing of buildings. Recently, we have reported our investigations on electrolyte-liquid crystal dispersions, which show independent electro-optical and electrochromic properties characterized by fast bleaching times. Nevertheless, for the external glazing of buildings, it is recommended to have a transparent off state, which turns opaque upon application of the external field. In this paper, we present our efforts to homeotropically align by rough surfaces fluid mixtures of a low molecular mass liquid crystal with a negative dielectric anisotropy, a liquid crystalline monomer, and electrolyte molecules in order to obtain transparent films after a polymerization process. Upon application of an ac electric field, the device becomes opaque and if a dc field is added, the cell changes colour too. We have also investigated the relationship between electro-optical/chromogenic response and physical chemical properties of the components.     

Optical characterization of commercial large area liquid crystal devices

Three chromogenic glazed units made of a polymer dispersed liquid crystal (PDLC) flexible film laminated between two glasses have been optically characterized. These glazed units are based on the same PDLC device inserted in different bulk-colored (clear, grey, bronze) glasses. The applications of this device are intended for large area glazing in building and transportation industry. The general electro-optic characteristics of the device (spectral transmittance at 550 nm as a function of voltage, switching time, self-transparency temperature) are presented. The optical characterization has been performed at normal incidence in the solar range with an integrating sphere spectrophotometer and in the visible range with a large integrating sphere taking care of distinguishing between the regular and the diffuse components of the transmitted and reflected radiation. Integrated values of luminous and energetic parameters are compared and discussed.     

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.     

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.     

High-efficiency multistable switchable glazing using smectic A liquid crystals

Presently, nematic polymer dispersed liquid crystals (PDLCs) that are used in switchable glazing technology require constant power to operate and, moreover, exhibit unwanted haze at wide viewing angles. In this paper, a novel switchable glazing technology, based around a bistable electro-optic effect in the Smectic A liquid crystal phase, is described which does not require constant power to operate or exhibit haze. The application of a low-frequency (100 Hz) voltage induces an optically opaque state due to the motion of ionic material whereas the application of a higher frequency AC (1 kHz) voltage induces a haze-free clear state. Multistable (greyscale) operation is possible through the application of intermediate frequencies or voltages; the threshold voltages of the effect were found to range from 36 to 66 V rms. Any voltage-induced state is preserved indefinitely after removal of the voltage leading to low power consumption.     

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.     

Optimum glazing combination for solar space heating

A study is made for comparing the maximum seasonal energy yield obtainable by solar collectors for space heating application. Different glazing combinations with glass and plastic as glazing materials are considered. The study is made for four different locations. The performance of eight glazing combinations with covers ranging in number from one to three is compared to obtain the optimum combination for each location. The results show that selecting the optimum glazing combination improves the performance significantly. In general, plastic covers give higher yield. The study confirmed that the use of two covers is justified in cold, cloudy climates while a single cover is suitable for temperate climates. In most cases three covers lead to a significant reduction in the yield. Replacing plastic by glass as a top cover for longer life results in a small yield reduction. Some of the other conclusions are that the ratio of average to normal transmittance-absorptance product changes significantly with location and month of the year. However, the seasonal average value of this ratio is almost constant for any number of covers but changes with location.     

Temperature-induced stresses in vacuum glazing: Modelling and experimental validation

A temperature difference across a sample of vacuum glazing causes differential expansion of one glass sheet relative to the other. In vacuum glazing with a fused edge seal, this results in tensile and compressive stresses in the glass sheets, and bending of the structure. The physical origins of these stresses and deflections are discussed, and a finite element model is used to determine their magnitude. The model has been validated by comparison with experimental data for a well-characterised sample of vacuum glazing under accurately defined external conditions. Modelling data are presented for two glazing designs which have properties that are characteristic of the extremes of performance of this type of glazing. It is shown that mechanical edge constraints can profoundly alter the spatial distribution of stresses in the glazing.     

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.     

A single-thin-film model for the angle dependent optical properties of coated glazings

Technology for the manufacture of coated glazings with spectrally selective optical properties, such as low-e and solar-control glazings, has been developed in the last few decades. This is leading researchers to develop new optical and thermal models in order to ascertain glazing performance. These new models must accurately reproduce the optical properties for any incident solar angle by using the available experimental data, which often means only information for normal incident radiation. In this paper, a new model is presented that characterizes the angular dependence of coated glazings. To provide a simple, intuitive understanding, this model uses only one thin film to characterize optical performance. In addition an optimization algorithm has been developed to obtain the spectral optical properties of that equivalent film using spectral experimental data under normal incidence. Finally, the model is validated with experimental data and the results are compared with other known models.     

Angle-selective glazing for radiant heat control in buildings: theory

The theory and predicted performance of a new type of glazing exhibiting strong angle-selective reflectance and high viewing transparency are presented. The glazing comprises a fixed venetian blind style arrangement of laser-cut panels. The theory shows that the transition from high transmittance at near normal incidence to high reflectance at larger incident angles can be varied in magnitude and over a wide angular range by altering the laser-cut spacing and panel spacing. Calculated radiant transmission and daylighting performance for the glazing on southern and east/west facades in buildings at temperate and sub-tropical northern latitudes predicts excellent performance for a single practical configuration of the system.     

Experimental evaluation of ventilated glazing performance in Hong Kong

Window glazing affects much the indoor environment and the energy use in buildings. While double glazing has better thermal performance than single glazing, the airflow window options carry additional advantage of directly removing the absorbed solar energy in glass panes. This paper reports an experimental study in Hong Kong in evaluating the thermal/energy performance of the above‐mentioned glazing systems. A new approach of using numerical simulation technique to improve the quality of experimental analysis was introduced. Our findings show that the natural‐ventilated glazing system has a better thermal performance than the double‐glazing system since the latter received 13.6% more convective heat gain. The main advantage of the exhaust‐ventilated glazing system lies in the decrease of convective heat transfer to 34.3% of the double glazing, and 19.4% of the single‐glazing types. The results showed that the ventilated glazing schemes in association with daylight utilization could lead to substantial electricity savings in the office environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Modern insulation requirements change the rules of architectural design in low-energy homes

In the design of very well-insulated homes, there is a need for a more nuanced design that takes into account winter and summer conditions. In this paper, we compare a traditional design for a typical Danish single-family house with large glazing areas oriented towards the south and smaller glazing areas towards the north, and a design with an even window distribution where the glazing-to-floor ratio is the same for each room. We found that the use of solar gains through south-oriented windows is not as important as is traditionally believed because, in well-insulated homes, space heating demand is not reduced much by having larger south-facing windows. Furthermore, we found that there is a g-value above which the additional solar gains through south-oriented windows do not help reduce space heating demand, and it becomes important to use solar shading or glazing with solar-control coating as a cheaper alternative to reduce overheating. Maximum window sizes from an overheating perspective were identified that are larger than the optimal window sizes for space heating demand. However, we show that the difference in space heating demand with optimal window size and with larger window sizes is small, so it is up to the building owner to decide whether or not he wants larger glazing areas to allow for more daylight. And windows can be positioned in the façade with considerable architectural freedom. However, we do recommend an even distribution of the glazing-to-floor ratio, because this generally provides an improved thermal indoor environment in south-oriented rooms and will ensure a better daylight level especially in north-oriented rooms. We also show that the optimal window size is influenced by thermal zone configuration and that there is a need for models in which a difference is made between zones with direct and with non-direct solar gains.     

The use of transparent insulation materials and optical switching layers in window systems

The new class of transparent insulation materials is classified and characterisation methods are summarised. An overview on available materials and a comparison of these new materials with conventional glazing systems is given. The high potential of these materials in collector systems is briefly discussed. In the final section an overview on new optical switching materials is given, which may help to achieve economically interesting system costs.     

On the effect of glazing and code compliance

The trend for modern buildings to be designed with large glazing areas in the exterior envelope is becoming more popular than ever before. At the same time, the power utilities in most of Kuwait demand code compliance through targeting of peak electric requirements for heating, ventilation and air-conditioning (HVAC) systems. The lack of detailed and accurate information on the thermal performance of glazing systems coupled with limits set by the code of practice can result in bad building design with respect to indoor comfort conditions. This paper presents a methodology for selecting glazing areas, while keeping the peak-load demand fixed. Provisions for the selection of large glazing areas are based on the type of glazing used. Finally, using a multi-storey building in Kuwait as an example, analysis has been conducted for seven different types of glazing. The choice of glazing offers the designer significant flexibility in opting for a glazing area of between 20 and 100%, and yet achieving full compliance with the code.     

Development of catalysts for anaerobic processing of liquid organic waste

In the work, the development of a catalyst carrier of biomaterial for anaerobic processing of liquid organic waste was carried out. A highly porous cellular material (НPСM) made of high-alumina ceramics obtained by thermal destruction of a polymer matrix impregnated with a high-alumina ceramic slip was used as the carrier base. The catalyst coating was applied by blowing a conductive powder through a carrier pre-coated with an adhesive composition or by impregnating the carrier with a suspension of conductive glaze followed by firing. The conductive powder was the product of milling grinding ferromagnetic bodies in a vortex layer apparatus. An experimental setup has been developed and manufactured, including a vortex layer apparatus, a blower, and a chamber with a catalyst carrier. The plant operation was optimized using the response surface methodology. It has been established that it is rational to vary the mass of working bodies to obtain the required concentration while reducing the power consumption of the plant by minimizing the duration of its operation. The next steps are to study the efficiency of an anaerobic bioreactor with an integrated HPCM with a catalyst coating formed by sputtering and glazing to produce biohydrogen and biomethane during anaerobic bioconversion of organic matter of various wastes.     

Glazing edge-seal using foamglass as spacer and frameless window design

This article presents a new design for the edge-seal of multiple-glazings with spacers made of foamglass and a new concept for frameless windows to reduce the heat loss through windows significantly. Thus the energy demand for heating is reduced or covered by solar energy gains through the window to a higher extent. The thermal performance of window assemblies with foamglass spacers and with and without frames is compared with that of the common window design. For the calculations of the heat flux a finite element analysis computer program has been used to account for the 2D-effects in the glazing, edge-seal and frame heat transfer patterns. The total heat transfer through an example window with a glazing 1 m×1 m is reduced by 45% using the window design presented. The objective of this article is not only to quantify the heat fluxes for different combinations of glazing, edge-seal and frame. The major part of the article focuses on practical aspects that are important for the durability of edge-seals, such as mechanical stress within the materials, water vapour and gas tightness, as well as on new design concepts of window–wall joints. A frameless window construction is an important aspect to enhance the thermal performance of windows. The costs for this kind of frameless windows are estimated to be less than or equal to windows commonly used now.     

Solar collectors with colored absorbers

The integration of solar collectors in buildings should be compatible with the architectural design, and solar collectors with colored absorbers would be aesthetically preferable. In our laboratory we constructed and tested flat plate solar collectors with colored absorbers for water heating applications. The study includes collectors in their typical form with the protective glazing, and also collectors without glazing. Unglazed solar collectors are not widely used, although they are cost effective solar devices, suitable for low temperature thermal applications. We tested outdoors the constructed models, glazed and unglazed, with black, blue and red brown absorbers. In order to overcome the high thermal losses of the unglazed collectors and the low optical efficiency of the colored absorbers, we used flat booster reflectors. The additional solar radiation input from the reflectors increases the thermal energy output of the collectors, improving their performance. Theoretical steady state efficiency curves are also given for collectors with or without glazing. The presented experimental and theoretical results determine the range of the effective operation of the proposed solar collector types, which can be used in a variety of applications, instead of glazed or unglazed solar collectors with a black absorber.     

Analysis of a ventilated solar evaporator for concentrating dilute hazardous waste

A mathematical model of a ventilated solar evaporator for concentration of dilute waste is presented. The design is similar to conventional solar still design, but allows for air flow in order to enhance the evaporation rate. The model consists of energy and mass balances on the basin, liquid concentrate, air and glazing. The model shows that the thermal efficiency of the evaporator is enhanced by convective air flow. Single cover evaporators are more efficient than two cover evaporators. Energy storage in the basin reduces efficiency.     

Thermal conductance measurement on vacuum glazing

A method is described for measuring the thermal conductance of vacuum glazing that is well-suited for integration into the manufacturing process of such devices. The sample of vacuum glazing to be measured, initially at elevated temperature, is placed in contact with a second sample of vacuum glazing with a known thermal conductance. The external surfaces of the glazings are then cooled by forced flow of air at room temperature, and a measurement is made of the rate of decrease of the temperature of the contacting glass sheets of the two samples. The method is simple to implement, and can be automated. The results obtained with the method are quite reproducible. The measurement can be made as the production samples of vacuum glazing cool at the completion of the manufacturing process, resulting in significant savings in time and labour compared with other methods.