Double Glazing Unit Research Articles

Active thermal insulators: Finite elements modeling and parametric study of thermoelectric modules integrated into a double pane glazing system

Active thermal insulators (ATI) represent a new thermal control technology that uses solar energy to compensate for the passive heat losses or gains in building envelopes. This effect is accomplished by integrating photovoltaic (PV) and thermoelectric (TE) systems into a wall assembly. A parametric study is presented considering the implementation of TE-modules into the air cavity of a double pane glazing system. The main objectives of this study are to explore design configurations that maximize the coefficient of performance (COP) of the TE-heat pump units, and maximize the ability of the system to perform as a heating and cooling system for use in buildings. A finite elements model (FEM) was developed and experimentally validated to calculate the steady-state heat transfer for the ATI-system. A parametric study was undertaken to determine: (i) suitable TE-modules for this application, (ii) the optimal spreading of the TE-heat pumps, and (iii) the composition of the double glazing unit. The results of our study indicate that the system can be properly designed for heating purposes, however, a more optimal design will need to be realized in order to make the approach effective for cooling applications.

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.

Effect of switchable glazing on discomfort glare from windows

In this paper we report the results of experimental tests carried out to study the effect of electrochromic (EC)-glazing on discomfort glare from windows. The research is performed by a small scale test-cell equipped with a small area double glazing unit (DGU) where one pane consists of an EC device with a visible transmittance τ v ranging from about 7 to 70% and the other of an ordinary clear float glass ( τ v ∼ 90%), the overall DGU transmittance dynamic range resulting about 4–58%. The performance of the device is investigated under real sky conditions as a function of time, solar radiation intensity, test-cell orientation and switching strategies (static and dynamic). The evaluation methods adopted for assessing the discomfort glare degree are the “New Daylight Glare Index” (DGI N) and the “Glare Subjective Rating” (SR). Results deriving from application of these two methods are compatible and show that for south facing windows and under the involved climatic conditions EC-glazing can be very effective in reducing discomfort glare caused by high window brightness originating from diffuse skylight. This glare reduction can be realized without compromising much of the available daylight so without necessarily increasing the need for artificial lighting and, at the same time, preserving an unobstructed outside view. Occurrence of glare effects from west (or east) facing windows is more difficult to be controlled due to the lower sun angles which entail the use of EC devices switchable to very low transmittance states ( τ v < 7%) and/or integration of traditional sun protection devices. This would probably compromise indoor daylight availability (and potential energy savings associated to reduced use of artificial lighting) as well as exterior view although the positive effect of the EC-glazing could be a substantial reduction of the frequency of use of these additional shading systems.

On-site performance of electrochromic glazings coupled to an anidolic daylighting system

This work takes benefit from two advanced building technologies through the integration of an anidolic daylighting system and an electro-chromic glazing in a building facade for achieving an optimal control of the daylight flux entering in an office room. Computer simulations of different systems configurations, based on ray-tracing techniques, were used to identify the optimal anidolic system design. Once installed in an experimental building the visual and thermal performance of the anidolic system coupled to an electrochromic glazing was assessed by way of on-site monitoring. These performances were compared to those offered by a conventional double glazing unit installed in an identical neighbouring room. An evaluation of users comfort was achieved by way of a survey involving 30 different subjects. Their visual performance during specific tasks performed on a VDT screen and/or on a sheet of paper was used to compare the lighting conditions. The main results of this detailed experimental study, showing the capability of such a combined daylighting system to control the daylight flux entering a working space, are presented in this paper.

Numerical study of free convection turbulent heat transfer in an enclosure

Abstract Turbulent flow in an enclosed cavity or box is a model for many flows of practical interest: heating of a room; flow in a double glazing unit or ventilation, that are encountered in a number of situations of practical importance in our every day encounters. A three dimensional enclosure in the form of a rectangular enclosure contains a convectional heater built into one wall and having a window in the same wall. The heater is located below the window and the remain walls are insulated. The size of the window is varied while its position is fixed, and the heater position and size remain constant. The localized heating and cooling induces two boundary layers that collide in the region between the window and the heater. The timed averaged equations for continuity, momentum and energy, which are coupled to the turbulence equations, were solved using a finite difference technique. The vorticity vector potential formulation has been employed. A further use of difference false transient factors in different flow regions coupled with non-linear partial differential equations has been employed to hasten convergence of the numerical solution. The results were that the enclosure is stratified into three regions: a cold upper region, a hot region in the area between the heater and the window and a warm lower region.

AN INFLUENCE OF OUTDOOR TEMPERATURE ON THERMAL PERFORMANCE OF WINDOWS WITH A LOW-EMISSIVITY GLAZING

Heat economy is a pressing issue in all countries with severe climate. The most frequent method to solve the problem is to decrease heat losses in buildings by increasing the thermal resistance of enclosures. The most popular measure to improve windows is the use of low-emissivity coatings and inert gases. The U-value of such glazing unit is mostly dependent on the possibility of coating to reflect IR (infrared) radiation. These coatings are very effective when surface temperatures are high. During service of windows with a change of outdoor conditions (mostly temperature and wind), an U-value of it will suffer changes. The aim of this article is to show an influence of outdoor temperature on thermal performance of windows with a low-emissivity glazing and to determine the differences of declared and design U-values of modern effective windows substantial enough to introduce a concept of a normative design U-value for windows in Lithuania. We have found that differences between declared and design values (under the conditions of Lithuania) reach on average up to 17% for double glazed IG units with one low-E coating. Thus in case of very effective windows, it makes sense to introduce a concept of design U-value for windows in our country.

AN INFLUENCE OF OUTDOOR TEMPERATURE ON THERMAL PERFORMANCE OF WINDOWS WITH A LOW-EMISSIVITY GLAZING

Heat economy is a pressing issue in all countries with severe climate. The most frequent method to solve the problem is to decrease heat losses in buildings by increasing the thermal resistance of enclosures. The most popular measure to improve windows is the use of low-emissivity coatings and inert gases. The U-value of such glazing unit is mostly dependent on the possibility of coating to reflect IR (infrared) radiation. These coatings are very effective when surface temperatures are high. During service of windows with a change of outdoor conditions (mostly temperature and wind), an U-value of it will suffer changes. The aim of this article is to show an influence of outdoor temperature on thermal performance of windows with a low-emissivity glazing and to determine the differences of declared and design U-values of modern effective windows substantial enough to introduce a concept of a normative design U-value for windows in Lithuania. We have found that differences between declared and design values (under the conditions of Lithuania) reach on average up to 17% for double glazed IG units with one low-E coating. Thus in case of very effective windows, it makes sense to introduce a concept of design U-value for windows in our country. First Published Online: 26 Jul 2012

Thermal transmittance of multiple glazing: computational fluid dynamics prediction

Computational fluid dynamics (CFD) is applied for predicting the convective heat transfer coefficient, thermal resistance and thermal transmittance for a double glazing unit. The predicted thermal resistance of glazing is compared with reference data and good agreement is achieved. The convective heat transfer coefficient and thermal transmittance vary with the air space width and the temperature difference across glazing. The CFD technique can be used to gain insight into multiple glazing performance and also optimise the design and operation of novel multiple glazing systems such as air flow windows or double skin facades in terms of energy efficiency and thermal comfort.

Optical and thermal performance of glazing with integral venetian blinds

Shading devices have long been used to control solar gain and daylighting through windows. The optimal design and use of such devices requires a detailed knowledge of their optical and thermal properties, including their variation with the angle of incidence of the solar radiation. We describe the experimental methods used to characterise two types of double-glazing units (DGU) incorporating Venetian blinds. The first consists of a DGU with an adjustable blind located in the space between the panes. The second incorporates a fixed blind element bonded between two glass sheets. The spectral bi-directional transmittance functions (BRTF) were measured for a range of incidence angles, using the Cardiff goniospectrometer. The thermal properties were measured with an illuminated hot box. These measurements are laborious, and it is desirable to develop simple models, that could be used for product development to estimate the effect of changing the properties of the blind material and its design, without the need to make detailed measurements on each variation. A model is described, that calculates the properties of the DGU using as input the optical properties at normal incidence of the component elements (glass and the blind’s slats). The model is applied to the variable blind DGU, varying the angle of rotation of the blind. Despite the simple nature of the model, it is shown to satisfactorily predict the properties of the DGU. The model can easily be incorporated in building energy simulation programs.

Variable angle photometric characterization of a laminated glass embedding a lamellae system

Glazing units containing sunshading devices (vertical blades, roller shutters, Venetian blinds, etc.) are available on the market nowadays. These products are roughly divided into two families: double-glazed units with automatically controlled devices in the interspace and monolithic glasses embedding fixed devices. The luminous and solar behaviour of the incorporated sunshading devices is intentionally strongly dependent on the position of the sun in the sky, and therefore the optical characterization of this kind of advanced glazing requires transmittance and reflectance measurements as functions of at least one angular coordinate. In this paper a lamellae system embedded in a plastic matrix and sandwiched between two glass sheets is considered. Transmittance and reflectance of the sample computed from the geometrical parameters of the lamellae system are compared to the corresponding values measured with global and bidirectional variable angle measurement systems. As an example of application of the obtained data in real conditions, the laminated glass containing the lamellae system closes the window of a room and daylighting inside the room is evaluated.

On the study of energy performance and life cycle cost of smart window

With worldwide energy cost rising significantly, there has been a pressing need to reduce the burning of fossil fuels and subsequently energy consumption. This, coupled with the prospect of global warming threatening human habitation, has made countries including Singapore more conscious and aware of the energy problem at hand. This paper deals with smart window, a double glazing unit where one pane consists of a high-performance heat reflective glass and the other coated with low-emissivity (low-e) coating. This combination of glazing provides optimum energy efficiency and a high level of daylight transmission with minimal reflectance. A study is made on the benefits derived from smart window done on a hypothetical 20-storey building. This encompasses a description of its quantitative impact on cooling load, energy consumption and energy savings achieved as compared with other forms of glazing. Following this, a detailed life cycle costing is done to determine the economic benefits attained from this type of glazing. The reduction of atmospheric pollutants as a result of using smart window is also analysed, and the future application of the glazing in hot and humid climates is discussed. In conclusion, it is observed that the smart window meets the technical and economic targets set, thus making it a viable long-term investment for high-rise commercial buildings.

Monolithic silica aerogel in superinsulating glazings

Silica aerogel is an open-pored porous transparent material with optical and thermal properties that makes the material very interesting as an insulation material in windows. A number of different aerogels have been investigated for their optical and thermal performance. High thermal resistance of aerogel was found for all the investigated samples and the samples showed very high solar as well as light transmittance. However, all the investigated aerogel samples showed a tendency to scatter the transmitted light, resulting in a reduced optical quality when the aerogels are integrated in glazings. This phenomenon is considered as being the main obstacle to incorporating the material in clear glazings, but a significant improvement of the optical quality of aerogel has been observed during the last five years. A number of prototypical evacuated 500×500×28 mm 3 aerogel double glazed units employing a new edge seal technique were manufactured and characterized for their optical and thermal properties. As expected, the same scattering of light was found in the aerogel glazings as in the aerogel samples, but excellent thermal performance was found, indicating a glazing type that, from a thermal point of view, is without competition in heating dominated climates.

Environmental conditions in window frames with double-glazing units

The use of double-glazing units has increased steadily over the years and now sealed units are installed in most new buildings and used in replacement of windows. However, failure of double-glazing units due to condensation between the panes has occurred on many construction projects and this has demonstrated the need to use good quality units that are glazed into appropriate frames using the correct methods. A long-term exposure experiment began in 1994 to monitor the conditions around double-glazing units in various types of window frames. This experiment uses instrumentation to measure the relative humidity, temperature and the presence of liquid moisture in the cavities around the edge seal of double-glazing units. Results are reported for a 2-year monitoring period (from October 1994 to September 1996), these demonstrate differences between winter and summer periods and the effect of the external and internal environment on conditions in the glazing cavity. The implications for the durability of the double-glazing units are discussed, although, the results presented here are interim.

A note on the net radiation method applied to a system composed of a semitransparent film between two glazings

The net radiation method is applied to obtain an expression for the thermal radiation heat transfer between a double glazed window unit which contains a semitransparent grey film in the cavity. The results are interpreted in terms of network modelling equations. The total heat transfer equals the result as obtained by A. Pfluger in 1984. Although the results as presented are not new, it is the first time the net radiation technique is used for this special purpose.

Thermal performance assessment of an advanced glazing system

The four different techniques which were used to test an advanced, four-pane glazing system and standard double-glazed unit are described. The results from each test are compared. Where agreement is not good, explanations are suggested. The advanced glazing system was found to have a U-value of 0.9 W/m 2 K and a shading coefficient of 0.48. The glazing simulation models WINDOW (Lawrence Berkeley Laboratories, Berkeley, CA, U.S.) and MULTB (Pilkington Glass, U.K.) were used to predict glazing performance. Simulation of the two glazing systems which were experimentally assessed allows comparison between models, and between predicted and measured performance. Agreement was within the error bands associated with each assessment.

Bendable silver-based low emissivity coating on glass

A detailed investigation of a multilayer design for a temperature resistant low emissivity and highly electrical conductive coating is performed. The optical transmittance, reflectance and the heat emissivity are discussed. A good agreement with computer simulated spectra and experimental data was achieved. The layer system was prepared by high rate magnetron sputtering on large float glass sheets. By gravity bending in normal atmosphere these coated substrates were formed and then further processed to heated windshields and curved double glazed units. The performance characteristics of these final products are discussed.

Flat glass products

In developing glass products, principal goals are esthetic appeal, reduced energy consumption, and reduced capital and operating costs. For architectural markets these are primarily obtained with well-designed, multiple-glazed heat-absorbing and heat-reflecting units. Recently developed double-glazed units with Sungate 100® coated glass for both commercial and residential markets are 30% more energy-efficient compared to triple-glazed units. Herculite II® glass, the first ion exchange glass made by the float process (PPG), produces unabraded strengths up to 75 000 psi and abraded strengths of 45 000 psi mostly for aircraft applications. Although plastics have made some inroads in the past due to lighter weight and in some cases better impact resistance, there will be no great rush to replace flat glass due to its superior abrasion resistance and substantially lower price.

Improving the reliability of cemented double-glazing units

Improving the reliability of cemented double-glazing units

Silica low-reflection coatings for collector covers, by a dip-coating process

A procedure for the deposition of antireflection coatings onto collector cover plates, by means of a dip-coating process using an ethanol-based colloidal silical sol, is described. By use of this process the solar transmittance of such cover plates can be increased by an increment 0.05 thus eliminating over 60 per cent of the reflectance losses. This increased transmittance increases the daily heat output for a single-glazed flat plate collector by a factor of approximately 1.1, and for double-glazed units by a factor of approximately 1.2. These antireflection films can be deposited on glass and on some types of plastic substrate. The abrasion resistance of the deposits on glass can be improved greatly by baking to 500°C. Outdoor weathering tests have shown the films probably have adequate stability for use in solar collector applications.

Monitoring the hermetic sealing of double-glazing units

Monitoring the hermetic sealing of double-glazing units