Vacuum glazing for highly insulating windows: Recent developments and future prospects

Renewable and Sustainable Energy Reviews 54 (2016) 1345–1357

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Vacuum glazing for highly insulating windows: Recent developments and future prospects Erdem Cuce a,b,n, Pinar Mert Cuce a,c a

Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, University Park, NG7 2 RD Nottingham, UK Department of Mechanical Engineering, Faculty of Engineering, University of Bayburt, 69000 Bayburt, Turkey c Republic of Turkey Ministry of National Education, Ankara, Turkey b

art ic l e i nf o

a b s t r a c t

Article history: Received 13 September 2015 Received in revised form 21 October 2015 Accepted 26 October 2015

A comprehensive review of vacuum glazing technology from state-of-the-art developments to future prospects has been presented. The review has been conducted in a thematic way in order to allow an easier comparison, discussion and evaluation of the findings. First, a thorough overview of historical development of vacuum glazing has been given. Then, numerous experimental, theoretical, numerical and simulation works on the scope have been evaluated and the characteristic results from the said works have been analyzed. Commercial vacuum glazing products in market have been assessed in terms of several performance parameters such as overall heat transfer coefficient, visible light transmittance, solar heat gain coefficient and cost. Techno-economic and environmental aspects of vacuum glazing technology have also been discussed. It can be concluded from the results that overall heat transfer coefficient of a vacuum glazing can be reduced up to 0.20 W/m2K through optimized integrations with low-e coatings. The incomparable U-value range of vacuum glazing enables significant mitigation in energy consumption levels and greenhouse gas emissions. Retrofitting 25.6 million homes in the UK with vacuum glazing can provide a carbon abatement of about 40 million tonnes a year, which is very promising. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Vacuum glazing U-value Buildings Energy consumption Carbon abatement Retrofit

Contents 1.

2.

3.

4.

5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Windows and heat loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Global necessity to novel window technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vacuum glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Description of vacuum glazing concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. History of vacuum glazing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Performance investigation of vacuum glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Experimental works. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Theoretical works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Numerical works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Simulation works. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Commercial products of vacuum glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Pilkington SPACIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Pilkington SPACIA-21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Laminated SPACIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparative in-situ testing performance of vacuum glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental impact of vacuum glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further works on vacuum glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

n Corresponding author at: Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, University Park, NG7 2RD Nottingham, UK. Tel.: þ 44 115 951 4882.

http://dx.doi.org/10.1016/j.rser.2015.10.134 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356

1. Introduction 1.1. Windows and heat loss Recent reports released by International Energy Agency (IEA) clearly indicates that buildings are responsible for about 40% of total world energy consumption in 2014 [1], and current predictions demonstrate that the trend will continue if decisive measures are not taken. Besides the remarkable role of buildings in global energy consumption, growing significance of environmental issues related to buildings is also unequivocal [2,3]. Greenhouse gas emissions emitted by buildings in many developed countries account for more than 30% of total emission as reported by Baetens et al. [4]. In this respect, numerous attempts are made worldwide to mitigate energy consumed in buildings, and thus to halt greenhouse gas emissions in the atmosphere. The most prominent measure of the world in recent years can be considered as putting extra attention to renewables [5,6]. However, renewable energy resources currently supply only about 14% of total world energy demand as previously revealed by Hasan and Sumathy [7] and Panwar et al. [8]. Therefore, additional measures such as developing high-efficient, cost-effective and environmentally friendly building elements are notably required. It is well-documented in literature that the greatest percentage of the energy consumed in buildings belongs to the energy losses through building envelope as a consequence of poor thermal insulation characteristics of existing building elements notably windows [9]. Windows are indispensable components of building envelope which provide air ventilation, vision, day-lighting, passive solar gain and the opportunity to leave the building in extreme situations. However, they are responsible for a significant amount of energy used in buildings due to their remarkably higher U-values compared to other components of building envelope. As reported by Cuce [9] for a typical building, the U-values of roof, floor, external walls and windows are around 0.16, 0.25, 0.30 and 2.00 W/m2K, respectively. Windows play a significant role in heating and cooling demand of buildings, particularly when their overall area is large. According to the results of comprehensive review on fenestration systems by Jelle et al. [10], about 60% of total energy consumption in buildings is attributed to the windows. Due to the significance of windows in reducing heating and cooling demand of buildings, considerable attention at global scale is given to improving their performance. 1.2. Global necessity to novel window technologies As global energy prices have a remarkably increasing trend, there is a growing awareness of the energy efficiency requirement in all areas, and as emphasized previously buildings are of significant relevance. In this respect, new building standards are invoked by many developed countries to be able to improve the fabric efficiency of new buildings and in some cases to also enhance the thermal performance of existing buildings through a process of energy-efficient retrofitting [1]. The UK is one such country, where the energy efficiency is considered as being crucial for both new and existing buildings [11]. A recent program of tightening building standards is agreed for the majority of the UK regions, where all new buildings will have to achieve an energy rating label scale in terms of energy consumption and emissions [12]. In light of this

program, The UK aims at reducing its greenhouse gas emissions by 80% by 2050, where the residential sector accounts for 27% of total emissions [13]. The program also adopts a fabric energy efficiency standard, where the new buildings have to achieve an annual energy demand below a certain maximum level. For instance, a detached dwelling in the UK would have to achieve annual energy requirement for heating below 46 kWh/m2year resulting to the Uvalues of windows in the region of or lower than 1.2–1.4 W/m2K to be able to meet the fabric energy efficiency standard as recently reported by Cuce et al. [14]. Similarly in Germany, the Passivhaus Standard set out by the Passivhaus Institute of Darmstadt indicates that the buildings are expected to require an energy amount less than 15 kWh/m2year for space heating/cooling per year corresponding to a U-value range for windows below 0.85 W/m2K [15]. Unfortunately, conventional window technologies are not capable of meeting such high thermal standards and this situation results in high energy consumption and consequently increases the CO2 emissions to the atmosphere. Current fenestration market is dominated by air or Argon filled double glazed windows due to their remarkably better thermal insulation performance compared to conventional single glazing, and well-documented fabrication process. However, their U-values are still very high as illustrated in Table 1, and insufficient to fulfill the requirements of low-carbon building concepts adopted by many developed countries [16,17]. Therefore, there is a consensus among scientists on the global necessity of low-cost, efficient and environmentally friendly window technologies. The objective of this research is to provide the state-of-the-art developments on vacuum glazing, which is one of the most promising glazing technology developed for low/zero carbon building concept.

2. Vacuum glazing 2.1. Description of vacuum glazing concept Vacuum glazing is a unique and high performance fenestration technology which enables minimum heat loss and high visible transmittance in a slim window product [18]. The idea was first introduced by Zoller in 1913 [19,20] but was not successfully fabricated until 1989 [21]. The first successful manufacturing of vacuum glazing was achieved by Robinson and Collins [22] at the University of Sydney. The glazing configuration utilized a contiguous solder glass edge seal which can be produced only at a process temperature above 450 °C. At a further stage, they elucidated this prerequisite by producing an edge seal vacuum glazing at below 200 °C [23–25]. Vacuum glazing technology does not have complex fabrication details. A conventional vacuum glazing consists of two sheets of glass separated by a vacuum medium with an array of support Table 1 U-values of commercial glazing products. U-value (W/m2K)

Pilkington [16]

Ref. [17]

Air filled double glazed window Air filled double glazed window with low-e Argon filled double glazed window with low-e

2.70 2.00 1.80

2.53 2.10 1.90

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pillars keeping the two sheets of glass apart as shown in Fig. 1 [9]. The support pillars are mostly imperceptible from a distance of about 2–3 m, hence their influence on vision is negligible [26–28]. The key role of the vacuum gap between the glass sheets is to eliminate the conduction and convection which play a significant role in the U-value of fenestration products. 2.2. History of vacuum glazing The concept of vacuum glazing, which is essentially based on minimizing conductive and convective heat transfer in a glazing via a vacuum gap, is not new and its origination goes back to the 1910s. The idea was first proposed by Zoller in 1913 and granted with a patent in 1914 [19]. However, the first successful vacuum glazing products could only be developed by the end of 20th century due to the considerable difficulties in fabricating a working vacuum glazing sample. The working principle of vacuum glazing is similar to that of double glazing in which the gas-filled space is evacuated to a particular pressure to be able to eliminate convection and gaseous conduction [29]. However, fabrication process of vacuum glazing is considerably complicated compared to other glazing technologies. Especially challenges in developing

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vacuum-tight and thermally insulating materials to be utilized in edge seal notably delayed the commercialization of vacuum glazing [30]. However, theoretical works and continuing stream of patents in the field of vacuum glazing over the last decades indicated the high level of interest in this unique fenestration technology as a consequence of the global awareness of the role of windows in total energy consumption [31]. The first samples of vacuum glazing were fabricated in the early 1980s, however the thermal performance achieved was poor as the level of vacuum, which needs to be below 0.1 Pa to eliminate gaseous conduction, was not sufficient [32]. Following the development of a new sealing technique, the first successful vacuum glazing sample was fabricated at the University of Sydney in 1989 [33]. The first commercialization attempts of vacuum glazing started in 2000 s by Nippon Sheet Glass (NSG) Group. Three different vacuum glazing products called SPACIA, SPACIA-21 and Laminated SPACIA were fabricated in Japan and intensive efforts were made in the following years to enhance the thermal insulation characteristics and cost-effectiveness of the aforesaid vacuum glazing products. The SPACIA was constructed from two 3, 4 or 5 mm sheets of glass, with pillars 0.2 mm high and 0.5 mm diameter. A single low-e coating was utilized as shown in Fig. 2a yielding to a U-value of

Fig. 1. The 3D schematic of a conventional vacuum glazing.

Fig. 2. Structural details of (a) SPACIA and (b) SPACIA-21.

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1.5 W/m2K for the entire glazing. In the SPACIA-21, an Argon gap was used in collaboration with single or double silver low-e coating as illustrated in Fig. 2b resulting in a U-value around 0.7 W/m2K. For the applications where the strength is requisite, the vacuum glazing was laminated with an additional glass sheet, thus the Laminated SPACIA was produced having a U-value similar to the standard SPACIA [34].

3. Performance investigation of vacuum glazing The first samples of vacuum glazing successfully fabricated at the University of Sydney remarkably stimulated the research on this novel technology, and as a consequence this, a group at the University of Ulster investigated the potential of developing a lower temperature sealing method to notably overcome the problems of the high-temperature method such as coating degradation, loss of temper and high embodied energy. The technique developed enabled vacuum glazing to be fabricated at temperatures lower than 160 °C. The glazing sample developed through this method at the University of Ulster utilized an indium or an indium alloy seal with a secondary seal used to prevent moisture ingress from occurring. The best U-value achieved was reported to be 0.86 W/m2K. From the 2000s to present, research was mostly focused on improving the U-value of the existing vacuum glazing products. In this respect, numerous experimental, theoretical, numerical and simulations works were conducted especially in recent years. In this section, a comprehensive assessment of the aforementioned works is presented. 3.1. Experimental works Fang et al. [35] conducted an experimental research to evaluate the thermal performance of a novel hybrid vacuum glazing integrated with an air gap and low-e coating. They investigated the impact of vacuum gap configuration on the overall thermal insulation performance as shown in Fig. 3. The lowest U-value was reported to be 0.24 W/m2K for the sample integrated with three low-e coatings. A similar work was conducted by Manz et al. [36] in Switzerland on a triple vacuum glazing sample. The impacts of emittances of glass sheet surfaces inside the cavity, support pillar radius, support pillar separation and thermal conductivity of support pillar material on the average U-value of vacuum glazing were investigated experimentally as illustrated in Fig. 4. The U-value was found to be 0.20 W/m2K by using stainless steel support pillars and

four low-e coatings. Cuce and Riffat [37] developed a unique vacuum glazing concept called vacuum tube window at the University of Nottingham, and conducted several experiments on different samples fabricated for an accurate thermal insulation performance assessment. Vacuum tube window concept was basically the combination of a particular amount of evacuated glass tubes at optimized dimensions, and integration of them into a double glazed frame as illustrated in Fig. 5. The evacuated tubes at a particular vacuum pressure were fixed between two glass window panes and for the external connection between vacuum tubes, an insulating adhesive was utilized. The results indicated that the U-value of vacuum tube window is highly dependent on the tube diameter (D) as shown in Fig. 6. The U-value was found to be around 0.30 and 2.00 W/m2K for D¼80 and 20 mm, respectively. There was not a remarkable difference between the U-values for D¼60, 70 and 80 mm. Thus, 60 mm was considered as the optimum tube diameter with a U-value of 0.40 W/m2K. 3.2. Theoretical works It is well-documented in literature that the radiative heat transfer between the two interior surfaces of the glass needs to be kept as low as possible to be able to obtain good thermal performance from a vacuum glazing. In this respect, Eames [21] provided an indication of the radiative heat transfer rate that will occur for various combinations of different low-e coatings for a temperature difference between the inner and outer glass surfaces of 20 °C (20 °C inside and 0 °C outside). The values of emittance presented in decreasing magnitude in Table 2 are for untreated glass, a hard low-e coating, a soft silver coating and a double silver coating. It was achieved from the theoretical work that a notable reduction occurs in the radiative heat transfer rate due to the inclusion of a single-hard low-e coating. Subsequent improvements achieved by using better performing low-e coatings were found to be less significant [21]. Cuce and Riffat [29] theoretically investigated the thermal insulation performance of the world's first commercially available vacuum glazing product. Their results were compared with the manufacturer’s thermal performance report, and an excellent agreement was achieved as given in Table 3. Within the scope of their research, translucent aerogel support pillars were recommended for commercial vacuum glazing, and impact of this replacement on the thermal performance of the glazing was numerically analyzed. The U-value of vacuum glazing was determined as a function of thermophysical properties of the window components as shown in Fig. 7. The results revealed that the

Fig. 3. Different configurations of hybrid vacuum glazing developed by Fang et al. [35].

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Fig. 4. Impact of pillar separation (s), pillar radius (r), thermal conductivity of pillar material (λ) and emittance (ε) on the U-value of vacuum triple glazing [36].

Fig. 5. Schematic of the vacuum tube window (on the left) and the sample developed (on the right) by Cuce and Riffat [37].

U-value of vacuum glazing exponentially increases with the thermal conductivity of glass material. In addition, it was also found that the U-value of vacuum glazing remarkably changes with the thermal conductivity of support pillar for any type of glass material. Significant enhancements were achieved at the thermal

performance of vacuum glazing after aerogel retrofit as illustrated in Table 4. Memon et al. [38] developed a novel low-temperature hermetic composite edge seal for the fabrication of triple vacuum glazing depicted in Fig. 8. The fabrication process was found to be successful in achieving a vacuum pressure of 0.048 Pa in the two

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Table 4 Enhancement at the thermal performance of vacuum glazing after aerogel retrofit [29]. U-value (W/m2K)

Conventional support pillars Aerogel support pillars Enhancement (%)

Fig. 6. Numerical and experimental U-values of vacuum tube window for the tube diameters of 50 and 80 mm [37]. Table 2 The effect of low-emittance coatings on radiative heat transfer rate in a vacuum glazing [21]. The emittance of 1st glass surface

The emittance of 2nd glass surface

Heat transfer rate (W/m2K)

0.9 0.9 0.16 0.9 0.05 0.9 0.02

0.9 0.16 0.16 0.05 0.05 0.02 0.02

4.21 0.81 0.45 0.26 0.13 0.10 0.05

Table 3 U-values of vacuum glazing through manufacturer data report and CFD analysis.

Pilkington Cuce and Riffat [29]

kglass (W/mK)

kpillar (W/mK)

Uwindow (W/m2K)

0.10 0.10

0.04 0.04

1.20 1.24

Fig. 7. U-value of vacuum glazing as a function of thermophysical properties of the window components [29].

gaps between the three glass sheets. D They utilized a theoretical approach based on a three-dimensional finite element model in order to determine thermal insulation performance of the triple vacuum glazing with dimensions of 300 mm  300 mm and with vacuum pressure of 0.048 Pa. Central and total thermal transmittance of the triple vacuum glazing were predicted to be 0.33 and 1.05 W/m2K, respectively. They also concluded that potential enhancements in thermal insulation performance can be achieved

kglass (W/mK) 0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.87

1.24

1.47

1.63

1.75

1.84

1.91

1.97

0.54

0.67

0.74

0.79

0.83

0.86

0.89

0.91

37.7

46.4

49.9

51.6

52.6

53.2

53.5

53.7

by reducing the total width of the edge seal from 14 to 8 mm and by utilizing better low-e coatings on the glass surfaces.

3.3. Numerical works Han et al. [39] investigated the thermal performance of vacuum glazing by using three-dimensional finite element method. Heat conduction through the support pillars and edge seal and the radiation between two glass sheets were evaluated. The heat conduction of residual gas in vacuum gap was ignored for a low pressure of less than 0.1 Pa. Two samples of vacuum glazing with sizes of 300 mm  300 mm and 1000 mm  1000 m were numerically modeled. The heat transfer coefficients of this unit obtained from simulation and numerical prediction were found to be 2.19 and 2.26 W/m2K, respectively, with a deviation of 2.79%. Fang et al. [40] numerically investigated the thermal performance of a triple vacuum glazing consists of three 4 mm thick glass panes with two vacuum gaps, with each internal glass surface coated with a low-emittance coating with an emittance of 0.03. The constructional details of the triple vacuum glazing and the heat flow through the novel design are shown in Fig. 9. They modeled the temperature distribution inside the vacuum glazing for the boundary conditions and model parameters given in Tables 5 and 6, respectively. The three-dimensional isotherms of the triple vacuum glazing illustrated in Fig. 10 clearly shows the temperature gradient across the three glass panes due to the high thermal resistance of the two vacuum gaps. The isotherms of the three glass surfaces A–C described in Fig. 9 are shown in Fig. 11. It is observed from the results that the increased heat conduction through the edge seal results in greater temperature gradients at the edge areas of the three glass surfaces. The thermal transmission of the entire triple vacuum glazing system was found to be 0.65 W/m2K whereas it was 0.26 W/m2K at the center-of-glazing area. Due to the significant impact of heat conduction through the edge seal, the thermal transmission of the entire glazing is approximately two times larger than that at the central glazing area. Fang et al. [41] numerically and experimentally investigated the potential enhancement in thermal performance of triple vacuum glazing with low-emittance coatings. The triple vacuum glazing considered consisted of three, 4 mm thick glass panes with two vacuum gaps, sealed with indium metal and separated by an array of stainless steel pillars, 0.2 mm high, 0.3 mm diameter and spaced at 25 mm. Their results indicated that decreasing the emittance of the four low-e coatings from 0.18 to 0.03 reduces the U-value at the center-of-glazing area from 0.41 to 0.22 W/m2K for a 400 mm m by 400 mm triple vacuum glazing rebated by 10 mm within a solid wood frame. Comparison of the U-values for different number of coatings and emittance values is shown in Fig. 12.

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Fig. 8. Schematic diagram and three-stage composite-edge-sealing design process for the fabrication of triple vacuum glazing [38]. Table 5 Boundary conditions of the triple vacuum glazing [40]. Ambient temperature (°C) Indoor Outdoor

20 0 Glazing surface heat transfer coefficient (W/m2K)

External surface Internal surface

25 7.7

Table 6 Model parameters of the triple vacuum glazing [40]. Vacuum glazing dimensions

Fig. 9. Constructional details of triple vacuum glazing (a) developed by Fang et al. and heat conduction through the glazing elements [40].

Thickness Width Length Glass pane thickness Emittance Edge seal width Pillar diameter Pillar height Pillar separation Frame rebate depth

12.24 mm 500 mm 500 mm 4 mm 0.03 6 mm 0.3 mm 0.12 mm 25 mm 10 mm Thermal conductivity (W/mK)

Indium Glass and solder glass Pillar Wood frame

83.7 1 20 0.17

3.4. Simulation works Vacuum tube window, which is a novel design of tubular vacuum glazing, was comprehensively investigated by Cuce and Riffat [37]. Several CFD simulations were conducted to determine the impact of design parameters such as pane thickness, tube

thickness, tube diameter and Argon gap on the overall thermal insulation performance of the vacuum glazing. The 2D schematic of the vacuum tube window utilized in CFD research is given in Fig. 13. Parametric study was carried out for several values of tube diameter as shown in Fig. 14. For Argon thickness of 1 and 2 mm,

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Fig. 10. Isotherms of triple vacuum glazing with boundary conditions and model parameters shown in Tables 5 and 6 [40].

the U-value was found to be around 0.40 W/m2K. On the other hand, the U-value exceeded 1.00 W/m2K while β increased 2–3 mm. The U-value was found to be around 0.30 and 2.00 W/ m2K for D ¼80 and 20 mm, respectively. There was not a remarkable difference between the U-values for D ¼60, 70 and 80 mm. From this point of view, it was concluded that 60 mm is the optimum value for tube diameter which results in a U-value around 0.40 W/m2K. In another work, Cuce and Riffat [29] simulated the thermal insulation performance of a commercial vacuum glazing product after replacing existing support pillars with translucent aerogel. Firstly, they provided a CFD model to determine the U-value of vacuum glazing as a function of thermal conductivity of the glass pane as illustrated in Fig. 15. Then, the number of aerogel support pillars was optimized to be able to achieve the lowest U-value from the vacuum glazing. It is concluded from their results that if three aerogel support pillars are utilized at per section of short edge (5 cm gap between each pillar), the enhancement in the U-value of vacuum glazing is found to be 31%, as clearly shown in Fig. 16. Fang et al. [34] emphasized that the edge seal in vacuum glazing is a thermal bridge transferring relatively large amounts of heat compared to the high insulation vacuum gap. In this respect, they developed highly efficient complex multimaterial frames to reduce the total heat flow through vacuum glazing by 20%. The minimum U-value achieved from the vacuum glazing samples fabricated through the said method was reported to be 0.20 W/m2K [36,42,43]. Berardi [44] indicated the practicality of a vacuum glazing integrated with 13.5 mm thick aerogel panel. The overall heat transfer coefficient and the transmissivity of the vacuum glazing were reported to be 0.66 W/m2K and 0.85, respectively. Fig. 11. Isotherms of three glass surfaces A (a), B (b) and C (c) of the triple vacuum glazing [40].

4. Commercial products of vacuum glazing In this section, commercially available vacuum glazing products are introduced. Constructional details of each commercial product are provided as well as thermal insulation and optical performance parameters. Practicality and reliability of vacuum glazing technology

is evaluated in terms of commercial aspects. It needs to be noted that commercialization of vacuum glazing is still challenging, however there are many attempts in progress such as TopTherm 90 as a consequence of several research projects. In the near future, it is expected to have a wide range of products in market. Currently, it is

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Fig. 12. Comparison of U-values of the triple vacuum glazing (TVG) with one, two, three and four emittance coatings for the emittance value of (a) 0.18 and (b) 0.03 [41].

even longer life expectancy for this high performance vacuum glazing technology. 4.2. Pilkington SPACIA-21

Fig. 13. Cross-sectional view of the vacuum tube window for CFD simulation [37].

unequivocal that the vacuum glazing market is dominated by three products, which are Pilkington SPACIA, Pilkington SPACIA-21 and Laminated SPACIA as reported by several researchers [9,10,52]. 4.1. Pilkington SPACIA Pilkington SPACIA vacuum glazing consists of an outer pane of low-emissivity glass and an inner pane of clear float glass, separated by a microspacer grid of small pillars each measuring 0.5 mm diameter, set 20 mm apart, which are robotically positioned, with intelligent camera checking. This grid ensures that the two glass panes are kept a fixed distance apart. The edges are welded to achieve a hermetic seal. Air is extracted to create a vacuum via the extraction point, rather than being air or gas filled. High insulation performance is achieved through a slim entire glazing thickness, which is only slightly thicker than single glass. Pilkington SPACIA provides similar energy efficiency performance with a standard double glazed unit containing low-emissivity glass, but in a much thinner profile. It is therefore perfectly suited for use in original, refurbishment or new thin profile frames, allowing the property to maintain its characteristic appearance [45]. Actual thickness of 6 mm Pilkington SPACIA is reported to be 6.5 70.1 mm. Center pane U-value of the product is 1.10 W/m2K whereas light transmittance and solar heat gain coefficient are 0.78 and 0.67, respectively according to the latest manufacturer data report. Pilkington SPACIA provides five times better thermal insulation than single glazing. In this respect, it has a strong potential to mitigate energy consumed in buildings, thus to halt carbon emissions. The company ensures 10 year warranty with an

Pilkington SPACIA-21 is a triple glazed vacuum glazing consisting of two low-e coatings in the unit along with Argon filling. Its characteristic design provides a highly energy efficient unit with a similar thickness to a conventional double insulating glass unit. Basically Pilkington SPACIA-21 is a hybrid vacuum glazing composed of Pilkington SPACIA and low-e glass. The cavity is injected with Argon gas that is lower in thermal conductivity by about 30% compared to air, thus achieving remarkable thermal insulation performance. The product is available with a solar control low-e coating for enhanced solar control. For improved thermal insulation performance, utilization of different types of inert gases like Krypton in the airspace is also available. Pilkington SPACIA-21 is fabricated in standard clear or green glass with an entire thickness varying from 18.2 mm to 21.2 mm. For 18.2 mm thick Pilkington SPACIA-21, center pane U-value of the product is reported to be 0.90 W/m2K while light transmittance and solar heat gain coefficient are 0.64 and 0.58, respectively. On the other hand, center pane U-value is 0.70 W/m2K, light transmittance is 0.58 and solar heat gain coefficient is 0.34 for the 21.2 mm thick Pilkington SPACIA-21 [45]. Enhanced thermal insulation characteristics of Pilkington SPACIA-21 can be explained through the sketch shown in Fig. 17 4.3. Laminated SPACIA Laminated SPACIA is a newly developed commercial product for sound reduction and anti-burglar. There are two specific examples of Laminated SPACIA called Shizuka and Mamoru. Shizuka includes a laminated structure with a single pane enabling multi-benefits like sound reduction, thermal insulation and safety, regardless of its thin structure. In Mamoru, the laminated structure is provided by a poly-carbonate sheet. Both Shizuka and Mamoru are ideal vacuum glazing products to provide comfortable dwelling and office spaces even in the noisy areas. They can be fabricated in standard clear or light green glass with a total glass thickness of 9.7 and 10.7 mm, for Shizuka and Mamoru, respectively. U-value, visible light transmittance and solar heat gain coefficient of Shizuka are noted to be 1.50 W/m2K, 0.72 and 0.62, respectively whereas they are 1.20 W/m2K, 0.63 and 0.53 for Mamoru according to the data report of NSG Group [46].

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Fig. 14. Contours of static temperature for the tube diameter (D) of 80, 70, 60 and 50 mm [37].

Fig. 15. U-value of vacuum glazing after replacing existing support pillars with aerogel [29].

Fig. 16. Optimization of number of aerogel support pillars for the U-value of vacuum glazing [29].

5. Comparative in-situ testing performance of vacuum glazing

UK, energy saving performance and payback period of vacuum tube window were compared with two different high performance glazing technology called heat insulation solar glass [50,51] and solar pond window [52]. As a consequence of its highly insulating feature [53], vacuum tube window technology provided the shortest payback period with maximum energy saving as illustrated in Fig. 18 and Table 7. Current fabrication cost of vacuum tube window is €130/m2, which is somewhat competitive with that of conventional double glazed windows in the UK as comprehensively reported by Cuce [52]. Compared to the special and energy-efficient design of vacuum tube window developed by Cuce and Riffat [37], Pilkington SPACIA and other commercial vacuum glazing products seem much more expensive. Current cost of the said technologies range from €272/m2 to €476/m2 depending upon volume, sizes and quantities as stated by Pilkington [54].

Vacuum glazing technologies are techno-economically evaluated by several researchers within the scope of different retrofitting projects like HERB (Holistic energy-efficient retrofitting of residential buildings), which is an EU funded research project coordinated by the University of Nottingham [47]. The novel vacuum tube window technology developed as one of the target outputs of the HERB project was integrated into a test house, which is one of the creative energy homes in the University Park Campus at the University of Nottingham, and the thermal insulation, energy saving and thermal comfort performance of the vacuum glazing was analyzed through another research project funded by E.ON [48]. In a recent research, Cuce et al. [49] extended the aforesaid previous research to a comparative techno-economic analysis of vacuum glazing technology. In this respect for a typical three bedroom semi-detached house in the

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Fig. 17. Schematic to explain the enhanced thermal insulation feature of Pilkington SPACIA-21 [45].

Table 7 Payback periods for novel glazing technologies for a typical UK house [49]. Glazing type

Capital cost [€] Annual savings [€]

Vacuum tube window 2600 Heat insulation solar glass 3000 Solar pond window 2400

Fig. 18. The U-values of novel glazing technologies for the same entire glazing thickness [49].

6. Environmental impact of vacuum glazing The role of windows in global energy consumption, thus in greenhouse gas emissions is unequivocal. Among the elements of a typical building envelope, windows are responsible for the greatest energy loss due to their notably high overall heat transfer coefficients. About 60% of heat loss through the fabric of residential buildings can be attributed to the glazed areas [1]. Therefore,

181.33 89.02 107.60

Payback [year]

14.34 33.70 22.30

energy-efficient retrofitting of conventional windows with novel glazing technologies is of vital importance for carbon abatement. Vacuum glazing technology is very promising in this respect for decisive mitigation of energy consumed in building sector, thus for substantial reduction of greenhouse gas emissions in the atmosphere (notably CO2). The U-values of windows in the UK buildings would have to be in the region of or lower than 1.20–1.40 W/m2K in order to meet the latest fabric energy efficiency standard [55]. According to the report of Energy Saving Trust [56], if all of the 25.6 million homes in the UK are retrofitted with energy-efficient windows, a notable carbon abatement of 12.8 million tonnes a year can be achieved. In other words, 12.8 million tonnes of CO2 being saved is equivalent to 840,000 cars being taken off the road. As the vacuum glazing technologies can provide at least 3 times better insulating feature, the annual carbon abatement can reach

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almost 40 million tonnes if the retrofit case is considered for a glazing technology like vacuum tube window.

7. Further works on vacuum glazing The electrochromic vacuum glazing is a promising concept, which is of prime interest especially in recent years [57,58]. Linking an electrochromic glazing with a vacuum glazing in a single window can provide a notably low heat loss glazing which can be switched from transparent to opaque thus allowing daylighting and solar gain to be adjusted. The preliminary research on triple vacuum glazing clearly shows that the thermal insulation performances of the samples are very high especially when the glass panes are integrated with low-e coatings. In further works, composite aerogel and vacuum glazing configurations can be studied for better thermal and sound insulation characteristics. Several numerical and simulation works are already in progress regarding this novel idea [29,52]. The material of support pillars plays a significant role in overall thermal insulation performance of vacuum glazing due to the thermal bridging effects. In this respect, aerogel-assisted support pillars can be utilized in existing commercial products to enhance the insulation feature of the vacuum glazing as reported by Cuce and Riffat [29].

8. Conclusions In this research, a comprehensive review of vacuum glazing technology has been presented covering state-of-the-art developments and future prospects. Following the evaluation of thorough historical process of vacuum glazing, experimental, theoretical, numerical and simulation attempts on the scope have been summarized and the key results from the said works have been discussed. Current commercial vacuum glazing products have been investigated in terms of various performance parameters such as overall heat transfer coefficient, visible light transmittance, solar heat gain coefficient and cost. Techno-economic and environmental impacts of vacuum glazing technology have also been addressed within the scope of this research. Some bullet results which can be concluded from the review are as follows: 1. Vacuum glazing is a unique and high performance fenestration technology which enables minimum heat loss and high visible transmittance in a slim window product. Additionally, it does not have complex fabrication details. 2. Overall thermal insulation performance of vacuum glazing is a strong function of constructional parameters such as number and material of support pillar, thermal conductivity of glass panes, edge seal technology utilized and existence of low-e coatings on glass panes. For instance in a research, the U-value of vacuum glazing is reported to be 1.50 W/m2K with a single low-e coating, whereas it is enhanced to 0.70 W/m2K with double low-e coating supported by an Argon gap. 3. The lowest experimental U-value of vacuum glazing is noted to be 0.20 W/m2K by using stainless steel support pillars and four low-e coatings. 4. More than 30% enhancement in the U-value of vacuum glazing can be achieved if highly insulating aerogel support pillars are utilized instead of conventional support pillars. 5. Novel designs of vacuum glazing such as vacuum tube window provides an energy-efficient, low-cost and environmentally friendly solution for both existing buildings and new-build applications. The experimental U-value of vacuum tube window is reported to be 0.54 W/m2K, whereas its fabrication cost is about €130/m2.

6. The U-value of vacuum tube window is highly dependent on the tube diameter (D) as it is found to be around 0.30 and 2.00 W/ m2K for D¼80 and 20 mm, respectively. 7. Vacuum pressure needs to be lower than 0.1 Pa in any vacuum glazing to be able to eliminate conductive and convective heat transfer. 8. Retrofitting 25.6 million homes in the UK with vacuum glazing can provide a carbon abatement of about 40 million tonnes a year, which is very remarkable.

Acknowledgments Corresponding author gratefully acknowledges the financial support of TÜBITAK (Scientific and Technological Research Council of Turkey) through Grant BIDEB 2219 2015/1.

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