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Solar & Alternative Energy

Electrochromic vacuum glazing may improve thermal comfort in buildings

The low heat loss of vacuum glazing can be combined with the variable transmittance of electrochromics to control solar gain.
15 November 2006, SPIE Newsroom. DOI: 10.1117/2.1200610.0404

In general, more heat escapes through glass (windows) than through the other materials in a structure. Thus, finding ways to make windows more thermally insulating is important to improving the energy efficiency of a building. We have combined two window construction techniques—vacuum glazing and electrochromic glazing—to provide improved thermal comfort while limiting the use of auxiliary space heating and artificial light. This performance is thanks to the system's very low heat loss and variable light transmission, which can also control glare from daylighting.

A vacuum glazing (VG) comprises two sheets of glass that are separated by a very narrow evacuated space. An array of metal or ceramic pillars holds the sheets apart, and the edges can be sealed with solder glass or indium. The interior faces of one or both glass sheets usually have a transparent, low-emittance coating. Windows made from vacuum glazing are much more thermally insulating than either single pane or conventional double glazed windows.

Electrochromic (EC) glazing causes glass to change its tint in response to an applied voltage change. Research in this area and its many potential applications are well documented.1 Visible light transmittance by EC films can be varied between 8% in their colored state and up to 80% in the bleached state by applying a 1–2V DC switching voltage. An ‘EC VG’ combines EC and VG technologies, as shown in Figure 1. Our novel glazing system combines the low-heat-loss properties of VG—a U-value (heat transmittance) of less than 1Wm-2K-1—with the variable transmittance of EC glazing to control solar gain.

Figure 1. This schematic diagram shows the various components of an electrochromic vacuum glazing.

The first working vacuum glazing2 using a low melt solder glass to form a contiguous edge seal at temperatures above 450°C was reported in 1989. Many types of soft, low-emittance coatings and tempered glass degrade at such high temperatures and so cannot be used in this kind of process. To remove these restrictions, a low temperature method (less than 200°C) was developed using indium to create the edge seal. A detailed description of this process and its experimentally determined performance has been reported previously.3 Our novel glazing system uses this method.

An extensively validated finite volume model4 was used to analyze the heat transfer through an EC VG for American Society for Testing and Materials (ASTM) standard winter boundary conditions. Symmetry considerations enabled a model of one quarter pane of the EC VG to represent the full pane. The array of pillars supporting the glass is incorporated and modelled directly in the finite volume method with a graded mesh. A high density of nodes in and around the pillars are used to represent of the heat transfer. The EC VG simulation assumed that the EC layer faced the outdoor environment. Fang et al.5 showed that when the EC layer faced inward, glazing surface temperatures would be too high for occupant comfort and would result in damage to the EC VG system.

Figure 2 presents isotherms calculated using the FVM within a 0.4m by 0.4m EC VG with 80% absorption, rebated 15.4mm into a solid wood frame under insolation of 300Wm-2. The temperature difference between the two glass panes of the VG resulted from the high thermal resistance of the vacuum gap; the temperature difference between the panes separated by the EC layer resulted from the EC layer absorbing heat.

Figure 2. Under ASTM winter conditions, these isotherms are predicted for a 0.4m by 0.4m EC VG.

Figure 3 shows that when the level of insolation incident perpendicular to the glazing surface increases from 0 to 1000Wm-2, the predicted surface temperature of the indoor glass pane increases from 13.4°C to 56.0°C and that of the outdoor glass pane increases from -16.0°C to 99.2°C. The rate of increase in mean surface temperatures is significantly larger for the outdoor pane than the indoor one because the electrochromic layer on the outdoor pane absorbs solar radiation. At insolations greater than 200Wm-2, the indoor pane absorbs radiation such that it transfers heat to the indoor environment. The temperature of the outdoor pane is greater than that of indoor pane at an insolation of 370 Wm-2.

Figure 3. Under insolation between 0 and 1000 Wm-2 incident on the outdoor glass surface, these are the mean surface temperatures of the indoor and outdoor glass panes of a 0.4m by 0.4m EC VG.

Figure 4 shows the calculated U-values of the 0.4 m by 0.4 m EC VG with one (Window 2) and two (Window 1) low-emittance coatings with different emittance values. When the emittance approached 0.02, the two low-emittance coatings gave limited improvement in the U-values of the total and center glazing areas as compared to glazing with one low-emittance coating.

Figure 4. The U-values predicted for varying emittance of an 0.4m by 0.4m EC VG depend on whether the evacuated gap contains one or two low-emittance coatings.

These simulations and experimental studies of EC VG windows suggest that they are more thermally insulating than a standard double pane window and comparable to a good triple pane window. Windows made from EC VGs should improve the energy efficiency of buildings while keeping occupants thermally comfortable.

Yueping Fang, Trevor J. Hyde
School of the Built Environment, University of Ulster
Jordanstown, Newtownabbey, Northern Ireland, UK

Yueping Fang is a research associate in the School of the Built Environment, University of Ulster, Northern Ireland, UK. His research interests include energy efficiency in buildings, finite element analysis, and nonimaging optics.

Philip C. Eames
School of Engineering, University of Warwick
Coventry, UK

Philip C. Eames, who now works in the School of Engineering at the University of Warwick, Coventry, was previously the director of the Centre for Sustainable Technologies (CST) and the Built Environment Research Institute (BERI) at the University of Ulster.