Eames, Philip

Fang, Yueping

English

Ulster University's Research Portal

Heat SET 2005Heat Transfer in Components and SystemsFor Sustainable Energy Technologies5-7 April 2005, Grenoble, FranceA COMPARISON OF THE PERFORMANCE OF VACUUM GLAZINGAND ELECTROCHROMIC VACUUM GLAZINGYueping Fang, y.fang@ulster.ac.uk (​mailto:y.fang@ulster.ac.uk​)Philip C. Eames, pc.eames@ulster.ac.uk (​mailto:pc.eames@ulster.ac.uk​)Centre for Sustainable Technologies, School of the Built Environment,University of Ulster, Newtownabbey, BT37 0QB, N. Ireland, UKABSTRACT       The thermal performance of an electrochromic vacuum glazing and a vacuum glazing with a range of low-emittance coatings were simulated for insolation levels between 0 to 1000 Wm-2 using a three-dimensional finite volume model. The vacuum glazing simulated comprised two 0.4m by 0.4m glass panes which were separated by a 0.12mm wide evacuated space sealed contiguously by a 6mm wide metal edge seal. The vacuum space formed between the glass sheets was supported by a 0.32mm diameter square pillar grid spaced at 25mm and had either one or two low-emittance coatings. For the electrochromic vacuum glazing a third glass pane on which the electrochromic layer was deposited was sealed to the evacuated glass unit. An “electrochromic vacuum glazing” combines an electrochromic smart window with a vacuum glazing providing both very low heat transmittance (less than 1 Wm-2K-1) and the potential to control visible light transmittance and solar heat gain and thus thermal comfort for building occupants. It may reduce peak energy demands both for cooling during summer and heating during winter. Simulations show that for both vacuum glazing and electrochromic vacuum glazings, when the coating emittance value is very low (close to 0.02), the use of two low-emittance coatings gave limited improvement in glazing performance. The use of only one currently-expensive low-emittance coating in both glazings provided acceptable performance. The significant performance improvement that resulted from the use of suitable rebate depths in solid wood frames for both glazing systems were investigated quantitatively.1. INTRODUCTION       The first working vacuum glazing (Robinson and Collins, 1989) used a low melt solder glass to form a contiguous edge seal at temperatures above 450oC. Many types of soft low-emittance (low-e) coatings and tempered glass will degrade at such a high temperature, they cannot therefore be employed to fabricate vacuum glazing using this method. To remove these restrictions, the Centre for Sustainable Technologies at the University of Ulster developed a low temperature method (i.e. less than 200oC) for producing an edge seal for vacuum glazing (Griffiths et al., 1998). A detailed description of the fabrication of vacuum glazing at low temperature and the performance properties were reported by Hyde et al., 2000 and Fang et al., 2000 respectively. Electrochromic glazing technology research is well documented (Lampert, 1998), visible light transmittance by electrochromic films can be controlled to be down to 8% at the coloured and up to 80% in the bleached state respectively by applying a 1-2V DC switching voltage. The switching time between the coloured and bleached states can vary from approximately 30 seconds to 10 minutes (Fischer et al., 2003). Electrochromic films have many potential applications for example, in the automotive industry to prevent overheating due to sunroofs, as windshields and rear view mirrors to provide glare protection (Fischer et al., 2003). Recent work for building applications has considered the effect of control strategies on energy savings (Kailsson et al, 2000). An electrochromic vacuum glazing combines an electrochromic smart window with a vacuum glazing as shown in Fig. 1.               This novel glazing system combines the low-heat-loss properties of vacuum glazing with a U-value of less than 1 Wm-2K-1 with the variable transmittance of an electrochromic glazing, allowing control of solar heat gain. The combined properties may enable optimal thermal comfort to be provided with limited use of auxiliary space heating and artificial light due to very low heat loss, daylightling and glare attenuation. It may reduce both the peak energy demands for cooling during summer and heating during winter. To date no detailed and systemic studies have been reported on the effect of low-e coatings, insolation and frame rebate depth on the performance of both vacuum glazing and electrochromic vacuum glazing systems. A finite volume model (Eames and Norton, 1993) was modified to analyse heat transfer through a vacuum glazing and an electrochromic vacuum glazing using ASTM standard winter conditions (ASTM, 1995) for the boundary conditions. 2. THERMAL PERFORMANCE OF A VACUUM GLAZING AND AN ELECTROCHROMIC VACUUM GLAZING       The thermal performance of a vacuum glazing and an electrochromic vacuum glazing were simulated. In the simulations, the indoor air set-point temperature and the outdoor ambient air temperatures were set to be constant at 21.1ºC and -17.8ºC respectively, the convective heat transfer coefficients on the indoor and the outside surfaces were set to be 8.3Wm-2K-1 and 30Wm-2K-1 respectively corresponding to those in ASTM measurement standards for winter conditions (ASTM, 1995). The simulated vacuum glazing and the electrochromic vacuum glazing were both 0.4m by 0.4m in size and comprised two and three 4mm thick glass panes for the vacuum glazing and electrochromic vacuum glazing respectively. The simulated vacuum glazing component had one or two low-e film coated glass panes, separated by 0.12mm, supported by a 0.32mm diameter pillar array spaced at 25mm in a regular square pattern. The edge seal was a 6mm wide band of indium. The thickness of the electrochromic layer between the vacuum glazing and the third sheet of glass was 0.1mm. The simulated electrochromic layer in the bleached state absorbed approximately 10% of the incident solar energy; in the opaque state, the energy absorption of the electrochromic layer was 80%. In the simulations the thermal conductivity of the electrochromic layer was set to be 1 Wm-1K-1. The thermal performance of the electrochromic vacuum glazing was investigated for the electrochromic layer facing towards the outdoor environment. Fang et al., (2004) illustrated that if the electrochromic layer faced towards the building interior, if the insolation perpendicular to the glazing surface is 600 Wm-2, the simulated indoor glass temperature will be over 129ºC. Such temperatures are too high for the occupants and will damage the electrochromic vacuum glazing systems.  2.1 Thermal performance of a vacuum glazing       A schematic diagram of a vacuum glazing rebated into a solid wood frame is shown in Fig. 2. The predicted temperatures for a vacuum glazing subject to insolation of between 0 and 1000 Wm-2 incident normal to the glass surface were calculated using the finite volume model and are presented in Fig. 3. The emittance of the coating on two glass panes were both 0.18 and the rebate depth in a solid wood frame was 15.4mm. Figure 3 shows that the solar energy absorbed by the vacuum glazing leads to an increase in the mean temperature of the outdoor and indoor glass pane surfaces from –15.6ºC to –8.5ºC and from 14.0ºC to 29.8ºC when the insolation increased from zero to 1000 Wm-2K-1. The temperature difference between the indoor and outdoor glass panes increases from 29.6ºC to 38.3ºC. The rate of increase in the outdoor glass pane surface temperature was larger than that for the indoor glass pane surface, this is because the heat absorbed by the outdoor glass pane cannot be effectively transferred to the indoor glass pane due to the high resistance to heat transfer provided by the evacuated space. When the insolation increases to 448 Wm-2, the indoor glass temperature increases to more than the indoor set-temperature of 21.1ºC, consequently the indoor glass pane begins to transfer heat to the indoor environment.       The predicted overall heat transfer coefficient of the total window and the centre glass area (with boundaries 63.5mm from each sightline (Anon, 1992) in which edge effects have little influence on heat transfer as a function of the emittance of vacuum glazing with one and two low-e coatings were calculated and are presented in Fig. 4. It can be seen that with increasing emittance of low-e coating, the U-value of both the total window and the centre glass area increase. The U-value of vacuum glazing with two low-e coatings is lower than that with one low-e coating and one uncoated float glass (emittance of 0.87), however when emittance is close to 0.02, the use of two low-e coatings gives marginal improvement in the U-values of vacuum glazing.        The thermal performance of a vacuum glazing with rebate depths from 0 to 22.4mm in a solid wood frame were calculated and are presented in Fig. 5, the U-value of the total window area decreased from 1.58 Wm-2K-1 to 1.09 Wm-2K-1; that of the centre glazing area decreased from 1.16 Wm-2K-1 to 0.92 Wm-2K-1. The rate of decrease in U-value of the total glazing area was obviously larger than that of the centre glazing area. The frame rebate significantly reduced heat transfer through the edge area of the vacuum glazing, thus the total window area. The decrease of the U-value of the centre glazing area was relatively small.2.2 Thermal performance of Electrochromic vacuum glazing        The thermal performance of an electrochromic vacuum glazing with the electrochromic layer coloured, with an 80% absorption, facing the outdoor environment was simulated. For insolation of between 0 and 1000 Wm-2 perpendicular incident on the glazing surface, the mean surface temperatures of the outdoor and indoor glass pane surfaces were calculated and are shown in Fig. 6. It can be seen that with increasing insolation from 0 to 1000 Wm-2, the predicted surface temperature of the indoor glass pane increased from 11.4ºC to 57.3ºC and that of the outdoor glass pane increased from –15.6ºC to 99.7ºC. Due to the solar energy absorbed by the outdoor glass pane and the high thermal resistance of the evacuated space reducing the heat transfer from the outdoor glass pane to the indoor glass pane, the rate of increase in mean surface temperatures of the outdoor glass pane is significantly larger than that of the indoor glass pane. When insolation is larger than 200 Wm-2, the indoor glass pane will transfer heat to the indoor environment; when the insolation is greater than 380 Wm-2, the temperature of the outdoor glass pane is larger than that of indoor glass pane.       The U-values of the electrochromic vacuum glazing with one and two low-e coatings with various emittance were calculated and are shown in Fig. 7. Comparing Fig.7 to Fig. 4, the emittance of the low-e coating has a very similar influence on the U-value of the electrochromic vacuum glazing. When the emittance approached 0.02, two low-e coatings gave limited improvement in the U-values of the total and centre glazing areas. The difference between Figs. 4 and 7 is that the U-value of the electrochromic vacuum glazing is larger than that of the vacuum glazing, the reason for this is that the electrochromic vacuum glazing has effectively a thicker glass pane than the vacuum glazing, this leads to heat conduction through the edge seal in the electrochromic vacuum glazing being larger than that of vacuum glazing (Fang et al., 2000). From Figs. 3 and 6 when both emittance values were 0.02, the U-values of the total window and the centre glazing areas of the electrochromic vacuum glazing are 1.22 Wm-2K-1 and 0.80 Wm-2K-1 and those of the vacuum glazing are 1.07 Wm-2K-1 and 0.76 Wm-2K-1 respectively.                              The effect of rebate depth in a solid wood frame on the thermal performance of the electrochromic vacuum glazing is shown in Fig. 8. The emittance of the coating on both glass panes of vacuum glazing within the electrochrommic vacuum glazing was 0.18. When the rebate depth increases from 0 to 22.4mm, the U-value of the total window reduces from 1.64 Wm-2K-1 to 1.22 Wm-2K-1 (i.e. by 25.6%) and that of the centre glazing area reduces from 1.17 Wm-2K-1 to 0.96 Wm-2K-1 (i.e. by 17.9%). The frame rebate depth effectively reduces the U-value of the total window system and the centre glazing area. 3. CONCLUSIONS       The thermal performance of vacuum glazing and an electrochromic vacuum glazing and the effects of solar insolation, emittance of low-e coatings and rebate depth into a solid wood frame were simulated using a finite volume model and compared in detail. It was found that when the incident insolation was over 448 Wm-2 for vacuum glazing and 200 Wm-2 for the electrochromic vacuum glazing, the temperature of the indoor glass was larger than that of the indoor environment and heat is thus transferred from the glazing into the indoor environment. When the insolation was over 380 Wm-2, the temperature of the outdoor glass pane became larger than that of the indoor glass pane for the electrochromic vacuum glazing due to the solar energy absorbed by the electrochromic layer. For both simulated glazing systems, when the emittance approached 0.02, two low-e coatings gave limited improvement in thermal performance of glazing systems compared to one low-e coating. Using one currently-expensive low-e coating will provide acceptable thermal performance. Increasing frame rebate depth improves the thermal performance of both vacuum glazing and electrochromic vacuum glazing. ACKNOWLEDGEMENTS The research was partly supported by an Overseas Research Student Award of the Committee of Vice-Chancellors and Principals, UK and a Vice-Chancellor’s scholarship of the University of Ulster through a postgraduate studentship for Dr.Yueping Fang. The authors acknowledge the support provided  by the European Commission through Framework Programme V: EESD – ENERGY for the “Electrochromic Evacuated Advanced Glazing" project (Contract number: ENK6-CT-2001-00547).REFERENCES       Anon, 1992. A PC program WINDOW 4.0 for analysing window thermal performance. Lawrence Berkeley Laboratory Berkeley, CA94720 USA p.11.       ASTM, 1995. Standard test method for thermal performance of building assemblies by means of a calibrated hot box (C976), 1995 Annual Book of ASTM Standards, Vol. 04.06, pp. 463-481.        Eames P.C. and Norton B., 1993. A validated unified model for optics and heat transfer in line-axis concentrating solar energy collectors. Solar Energy 50, pp. 339-355.       Fang Y. Eames P.C. and Norton B., 2000. Influence of insolation level and glass thickness on the thermal performance of evacuated glazing, In Proc. World Renewable Energy Congress VI (WREC 2000), Brighton, U.K. pp. 1878-1881.       Fang Y., Eames P.C., Hyde T.J., Norton B., 2004. Thermal analysis of electrochromic vacuum glazing, accepted by the “World Renewable Energy Congress VIII (WREC2004) Denver USA (available on CD ROM).       Fischer U., Hausler T., Rogab H., Rottmann M., Kraft A. and Heckner K.N., 2003. Heat transport and thermal expansion of electrochromic glazing systems with voltage controlled transmission due to solar irradiation. Proc. of Fifteenth Sym. on Thermophysical Properties, June 22-27, 2003, Boulder, Colorado, U.S.A.       Griffiths P.W., Leo Di M., Cartwright P., Eames P.C., Yianoulis P., Leftheriotis G. and Norton B., 1998. Fabrication of evacuated glazing at low temperature. Solar Energy 63, pp. 243-249.        Hyde T.J., Griffiths P.W., Eames P.C. and Norton B., 2000. Development of a novel low temperature edge seal for evacuated glazing. In Proc. World Renewable Energy Congress VI (WREC2000), Brighton, U.K. pp. 271-274.       Kailsson J., Kailsson B. and Ross A., 2000. Control strategies and energy savings. Proc. EuroSun 2000, Copenhagen, Denmark, June.       Lampert C. M., 1998. Smart switchable glazing for solar energy and daylight control, Solar Energy Materials and Solar Cells, 52, pp. 207-221.       Robinson S. J. and Collins R.E., 1989. Evacuated windows-theory and practice, ISES Solar World Congress, International Solar Energy Society, Kobe, Japan.Fig. 1 Schematic diagram of an electrochromic vacuum glazingFigure 1 Schematic diagram of an electrochromic vacuum glazing        Fig. 2. Schematic diagram of a vacuum glazing rebated into a solid wood frame with rebate depth of W.Fig. 4 Variation of glazing surface temperatures and U-values with various frame rebate depth. Fig. 3 Predicted glazing mean surface temperatures with insolation intensity between 0 and 1000Wm-2 incident perpendicular to the outdoor glass pane surface.    HhhFig. 4 U-values of vacuum glazing with various emittance values of one and two low-emittance coatings. Fig. 5 Variation of glazing surface temperatures and U-values with various frame rebate depth. Fig. 6 The mean surface temperatures of the indoor and outdoor glass panes under insolation incident on the outdoor glass surface between 0 and 1000Wm-2.Fig. 7 U-values of electrochromic vacuum glazing with emittance of one and two low emittance coatings.Fig. 8 The mean surface temperatures of the glass panes and U-values of window under various rebate depth.PAGE  

A comparison of the performance of vacuum glazing and electrochromic vacuum glazing

https://pure.ulster.ac.uk/ws/files/11819372/heatset-Fang1.doc

A comparison of the performance of vacuum glazing and electrochromic vacuum glazing

Abstract

Similar works

Full text

Available Versions

Ulster University's Research Portal