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Vacuum glazing—A new component for insulating windows
Abstract
The substantial progress in the science and technology of vacuum glazing that has occurred over the past few years is reviewed. Vacuum glazing up to 1 m × 1 m in size has been produced with an air-to-air, mid-device thermal conductance as low as 0.90 W m−2 K−1, compared with 1.3 W m−2 K−1 for high performance double glazing. The mechanical tensile stresses in vacuum glazing due to pressure and temperature differentials are well understood, and appear to be tolerably small. The internal vacuum is high, and extremely stable over long times at moderate temperatures. The likely cost of volume production of vacuum glazing should be only slightly greater than for high performance double glazing.
... By creating a vacuum within the gap between glass panes, VIG significantly reduces heat transfer, effectively minimizing convection and conduction losses. The concept of VIG was initially proposed by Zoller in 1913 and later patented in 1924 [26]. Despite its innovative thermal insulation performance, VIG's commercial deployment faced prolonged delays. ...
... Significant advancements in VIG technology were realized in the 1980s, notably in 1989, when a research team led by Collins at the University of Sydney successfully created a vacuum seal by employing low-melting-point glass at the perimeter of the glass panes, which melts at 450 °C. This breakthrough paved the way for NSG in Japan to develop and introduce SPACIA, marking the launch of the world's first commercially available VIG product [26,27]. SPACIA VIG comprises two glass panes, each ranging from 3-5 mm in thickness, with spacers that are 0.5 mm in width and 0.2 mm in height, placed at intervals of several tens of millimeters. ...
... For instance, Collins used lead(Pb) glass with a melting temperature of around 450 °C to join two glass panes effectively [26]. However, the industrial use of lead is severely restricted due to environmental regulations [38,39]. ...
Enhancing energy efficiency in buildings is a pivotal strategy for reducing energy consumption and mitigating greenhouse gas emissions. As part of global efforts to achieve carbon neutrality by 2050, there is a heightened focus on improving window insulation because windows are a significant source of thermal loss, representing nearly 40% of a building's heat dissipation. This study explores the development and application of vacuum insulation glazing (VIG), a cutting-edge insulation technology, to substantially reduce heat transfer through windows, thereby contributing to building energy savings. With its superior insulation performance, achieving thermal transmittance levels around 0.5W/m ² ·K, VIG technology presents a promising advancement over traditional double-glazed or gas-filled insulating glass units (IGUs). However, the adoption of VIG is challenged by economic factors, with costs significantly higher than standard IGUs and triple-glazed windows meeting passive house standards. The production of VIG, characterized by lengthy evacuation times and high processing temperatures, contributes to its elevated price. This research identifies the potential for cost reduction through optimizing manufacturing processes, including using low-melting-point solders for hermetic sealing and localized heating techniques to shorten production times. Despite the high initial cost, the potential for integrating VIG with other smart technologies suggests a promising future for achieving carbon neutrality in buildings. The study calls for further research and standardization in VIG production to overcome current technical and economic barriers, paving the way for its wider adoption and realizing next-generation energy-efficient building materials.
... As seen in Fig. 1, the VG consists of two glass sheets, the vacuum gap between them that is thermally insulating and stable, and a series of mechanical support pillars that keep the glass sheets apart under the influence of atmospheric pressure [11]. Although Zoller introduced the idea of VG in 1913 [12,13], a research team from the University of Sydney was the first to report on its successful production in 1989 [9,11,12]. ...
... As seen in Fig. 1, the VG consists of two glass sheets, the vacuum gap between them that is thermally insulating and stable, and a series of mechanical support pillars that keep the glass sheets apart under the influence of atmospheric pressure [11]. Although Zoller introduced the idea of VG in 1913 [12,13], a research team from the University of Sydney was the first to report on its successful production in 1989 [9,11,12]. A solder glass edge sealing material with a 500°C melting point was used to create that glazing with a 1 m by 1 m glazing size, the center of glazing U-value was reported to be 0.80 W/m 2 .K [12]. Further, a low temperature (<200°C) edge sealant called indium alloy is also used by the University of Ulster to successfully produce VG with a soft Low-E coating on tampered glass [13,14]. ...
... In a VG, glass sheets are supported by an array of support pillars and those support pillar forms a thermal bridge between two glass sheets [11,67,68]. To maintain a high vacuum pressure between two glass sheets, the pillars require enough physical strength to withstand the huge extrusion force of two glass sheets and proper thermal properties to minimize the heat flow. ...
As a building envelope, glazing materials play a vital role in designing an energy-efficient building. Recently, PV combined vacuum glazing has become a popular research topic and attracted researchers to resolve the drawbacks of commonly used glazing products. In the last decade, researchers have conducted several studies combining PV glazing with vacuum glazing to develop an energy-efficient glazing product. In this review, we discussed the different constructions of PV combined vacuum glazing, recent advancements of this product, the influence of a few key design factors on thermal performance, as well as its prospects in designing an energy-efficient building.
... But the first successful VG was developed by a research group at the University of Sydney in 1989 [23]. VG is similar to double glazing in which the gas filled space is evacuated to a low pressure (<0.1 Pa) and place an array of support pillar between two glass sheets to maintain the separation under the influence of the atmospheric pressure [24,25]. A schematic diagram of a VG is presented in Fig. 1. ...
... Moreover, to maintain a thermally insulated and stable vacuum in the gap, a hermetic edge seal is essential around the periphery of two glass sheets. At the beginning, the research group at the University of Sydney used solder glass to seal the glass edge, which required a high temperature process [23,25,29]. But the high temperature (>450 C) degrades the low-e coating and tempered glass [30,31]. ...
... A schematic diagram of a vacuum insulated glazing[25]. ...
Building integrated photovoltaic (BIPV) glazing is currently regarded as a promising building material with a wide range of benefits. Photovoltaic combined vacuum glazing is a relatively new innovative concept in BIPV glazing. On the other hand, photovoltaic combined hybrid vacuum glazing (PVCHVG) is a rarely studied topic in which an air gap exists between vacuum glazing and photovoltaic glazing to form an insulated glazing unit. This paper investigates the overall energy-saving performance of a CdTe-based semi-transparent PVCHVG. A dynamic simulation model was developed and validated with an outdoor experiment to explore the energy-saving performance of the PVCHVG under five different climate conditions in China, and the results were compared with commonly used window systems. The results indicated good insulation properties against both heat loss and heat gain due to the combined action of vacuum glazing and semi-transparent photovoltaic glazing. Compared to clear single-glazing and double-glazing window systems, PVCHVG can save overall energy consumption up to 59.39% and 39.97% in heating-dominated region, and 76.33% and 73.766% in cooling-dominated region, respectively. Furthermore, the PVCHVG window system generated electricity with a good performance ratio and total system efficiency ranging from 85.7% to 85.78% and 7.45%–7.55%, respectively, considering five climate conditions.
... Therefore, the study of vacuum insulation technology that is applied to buildings has been performed since the 1990s. A representative study of the vacuum insulation technology is the study of vacuum glazing by R. E. Collins [6][7][8][9][10][11][12]. In the corresponding research, pillars are used between two pieces of glasses to form a vacuum layer, and a low emissivity (low-e) film is employed to reduce the thermal emissivity, which is approximately 0.9 for glass [6]. ...
... A representative study of the vacuum insulation technology is the study of vacuum glazing by R. E. Collins [6][7][8][9][10][11][12]. In the corresponding research, pillars are used between two pieces of glasses to form a vacuum layer, and a low emissivity (low-e) film is employed to reduce the thermal emissivity, which is approximately 0.9 for glass [6]. Then, the durability of the pillars is investigated and their influence is discussed [7]. ...
... The calculation conditions and specific size of the VIP for the standard model are also depicted in Figure 12. In this calculation, the thermal conductivities of the plastic cylinder (pillar) spacer, acrylic plate, and vacuum layer are 0.3 W/(m · K), 0.2 W/(m · K), and 0.106 mW/(m · K) (the thermal conductivity can be calculated by Equation (12)) [6,22], respectively, and the thermal emissivities of the acrylic plate and low-e film are 0.9 and 0.3, respectively. The temperature difference for the calculation is 25 °C. ...
The authors develop slim and light-weight vacuum insulation panels (VIPs) by producing vacuum layers with spacers and plastic plates. The developed VIPs have the advantages of a low cost and easy installation, thus facilitating retrofitting insulation of existing buildings. In addition, one of the developed VIPs is slim and translucent so it can be easily used for windows in an internal installation. In this paper, the authors first propose a vacuum layer type slim translucent VIP and focus on a reasonable design method. Next, the authors introduce the design process in which the structural design is obtained with element mechanical analysis and a three-dimensional analysis is conducted for the VIP element. In the study, a heat transfer model is used to predict the insulation performance through finite element analysis (FEA). Subsequently, the authors perform an experiment to measure the thermal conductivity in a guarded hot plate apparatus to validate the performance prediction. Finally, case studies are performed to confirm how the different design conditions affect the insulation performance. The optimum design of the vacuum layer type slim and translucent VIP will have a sufficient structural strength to hold and maintain the vacuum layer. The thermal conductivity is approximately 0.007 W/m·K, which can effectively improve the insulation performance in applications.
... An array of small support pillars ensures that the glass sheets do not come into contact under the large atmospheric pressure. Internal transparent low-emittance (low-e) coating reduces radiative heat transfer to a sufficiently low level [22,23]. The concept of vacuum glazing was first described in a 1913 patent [24]. ...
... The first reported successful production of vacuum glazing was in 1989 [19]. Since that time, many samples of vacuum glazing have been produced under the ''SPACIA" brand using high temperature (450°C) solder glass edge sealing [19][20][21][22][23]. This high temperature process degrades both soft low-e coatings and tempered glass [23]. ...
... Since that time, many samples of vacuum glazing have been produced under the ''SPACIA" brand using high temperature (450°C) solder glass edge sealing [19][20][21][22][23]. This high temperature process degrades both soft low-e coatings and tempered glass [23]. An indium alloy edge seal technique [25,26] at temperatures less than 200°C enables soft low-e coating and tempered glass to be used. ...
Outdoor characterization of thermal and daylight performance of an evacuated (vacuum) glazing has been conducted using an outdoor test cell for clear sunny day, intermittent day and overcast days. An average overall heat transfer coefficient of 1.4 W/m2 K was found for vacuum glazing. Solar heat gain coefficient of vacuum glazing varied between 0.58 and 0.19. Vacuum glazing has potential to reduce 53% heat loss compared to a same area of double-glazing while offering nearly equal amount of heat gain. Internal daylight illuminance was found to be similar to double glazing indicating that the presence of small pillars inside vacuum glazing had no significant visual impact.
... Vacuum insulating glass (VIG), shown in Fig. 1, is a thermally insulating glazing consisting of two sheets of glass that are hermetically sealed together around the edges, with a thin highly evacuated internal space (Collins and Robinson 1991;Collins and Simko 1998;Collins et al. 1995). The separation of the glass sheets under the forces due to atmospheric pressure is maintained by an array of small support pillars. ...
... Since 1989, the University of Sydney has undertaken a comprehensive program of research and development on VIG science and technology (Collins and Robinson 1991;Collins and Simko 1998;Collins et al. 1995;Ashmore et al. 2016). An essential part of this program is the ability to characterise the heat flow through VIG specimens. ...
Non-uniformities in the heat flow through the support pillars in vacuum insulating glass (VIG) can lead to significant errors in the measurement of the thermal insulating properties of these devices. This paper discusses these errors in instruments for which the measurement area is in direct thermal contact with the glass sheets. The spatial non-uniformities of the heat flow in different VIG designs are modelled using the finite element method. For measuring areas with large dimension compared with the separation of the support pillars, the errors are unacceptably large for all practical designs of VIG when using guarded hot plate instruments. These errors are less for heat flow meter instruments due to the construction of the heat flux transducer.
... The building envelope is one of the critical parts of retrofitting plans so 75% of heat loss/ gains depends on the building façade and thermophysical properties of the materials and the rest are due to the orientation of the buildings and the ventilation openings [263]. [264] also showed that on average, in USA and Sweden, respectively, about 7% and 3% of energy consumption is wasted through the windows. Many sustainable design solutions can be applied to design and form of the facade to improve the energy performance of the buildings, such as considering a double layer's air corridor, installation of external and internal insulation layers, improving Uvalues of the windows and solar heat gain coefficient, which all can reduce energy demand by about 30% [265]. ...
Given the rapid trend of technological advancement, the concept of a smart sustainable city has emerged to offer smart solutions to the efforts and activities designated to ensure ESG and sustainability achievement. Although several rating systems have been proposed to assess the performance of different projects at different scales, there is still no comprehensive framework to support and measure the level of smartness in the neighborhood building stock, with specific reference to the social housing context, in which all measures and policies can be effectively realized without any managerial challenges, being in charge by just one unique entity.
To bridge this gap, this research project aims to propose an assessment framework to measure the level of smartness in the neighborhood. This framework is developed using a mixed method approach that encompasses three qualitative and quantitative phases, including, i) creating the comprehensive taxonomy of smartness KPIs; ii) formulating determinant indexes to assign weights to KPIs; iii) using an output-based measurement method to control and measure the level of smartness within a set of checklists of required technologies, at five levels of smartness, for each single KPI. The novelty of this thesis is also reflected in these three phases, in which, a comprehensive list of smart solutions that can be applied in the neighborhood context is collected; a new set of indexes that reflects the main goals of smartening is developed to assign weights to the smartness KPIs; and a new checklist of smart technologies that are required to deliver the expected smart services are solidified. In the end, the applicability of the final framework is tested on a case study, through three scenarios that reflect the current status of the neighborhood, the needs of residents, and the priorities of the community’s managers.
This framework empowers policymakers, engineers, and community managers to clarify the objective level of potential smartness through different elements of the neighborhood. Moreover, this pioneering framework can be leveraged to quantitatively evaluate the smartness of neighborhoods and distinguish the critical difference between the definition of smart and smarter neighborhood building stocks. As this research is committed to the social housing context, the last section evaluates the applicability of smartness KPIs in the context of Italy, by using qualitative methods, such as a focus group approach and SWOT analysis.
... Besides reducing thermal convection between the glass sheets, using appropriate coatings and spacer pillar and seals can reduce radiation and conduction across the vacuum glazing. Collins et al. [113] presented the results of pioneer work on vacuum glazing where the thermal conductance was reduced to 0.90 Wm -2 K -1 , the internal vacuum was stabilized for long periods of time, the mechanical and thermal stresses were dominated, and production costs were reduced. Fang et al. [114] investigated both experimentally and numerically the effects of low emissivity coatings on the thermal performance of vacuum glazing. ...
The building sector is rated as a big consumer of electric energy and emissions, responsible for about 40% of final electric energy consumption. As a result, the Paris Agreement 2015 set a goal for buildings and the construction sector to reach a nearly zero-carbon stage by 2050. This urged most countries to create regulations for the construction sector and invest in energy efficiency programs. The present paper aims to present an updated review of building energy-saving solutions and techniques to contribute to carbon emission mitigation in the building sector. The high energy consumption of a building is mainly due to heating and cooling, which is directly related to the thermal properties of the materials used. Natural ventilation and illumination are other aspects that contribute to the high energy consumption. Considering these issues, the review covers energy-efficient construction materials such as mortars, concrete with PCM, new construction materials with PCM such as 3d printing concrete and geopolymer concrete, and bricks usually used in buildings. Also, the review covers the methods and solutions for energy saving for building heating and cooling. Since transparent windows and façades are essential for structures, their thermal and visual performance is crucial. Established and under-development techniques for windows and façades are presented and discussed. Walls and roofs are usually rated at the top of the weak barriers against a building's heat losses and energy gains. The present paper reviews existing and still under research and development techniques to improve the thermal performance of walls and roofs, such as cool roof and cool walls, walls and roofs with phase change materials (PCM), and ventilated walls and ceilings.Some authors’ comments are presented at the end of each topic. Some possible opportunities for future research and developments are also presented.
... Convection and radiation effects are heavily interpreted by researchers in the literature since the transmission from window frames is mainly dependent on material properties. In order to reduce the radiation effects, a vacuum environment is created by evacuating the double glazing (Collins et al., 1995). A schematic view of the system is given in Figure 2a. ...
In daily life, people spend a lot of time indoors. For this reason, it is important to design closed environments in such a way that they do not adversely affect human health. Especially temperature adjustment is one of these designs. The main goal is to keep indoor environments cool in the summer and warm in the winter. This study investigated window design, which is an important part of the indoor environments. Air, argon and krypton gases used in double glazing applications reduce the window heat transfer coefficient. In the study, a 2D CFD model was created and gas movement was prevented with a transparent barrier placed in a gas environment. In this way, convection effects were reduced and the heat transfer coefficient of the window was interpreted. In the reference state, the window heat transfer coefficient was determined for air, argon and krypton at a gas thickness of 20mm. Then, the gas movement was prevented by the transparent barrier and the new value of the window heat transfer coefficient was compared with the reference situation. It was seen that the window heat transfer coefficient, which was 1.605, 1.122 and 0.875 w/m 2 K for the 3 gases in the reference case, was 1.398, 0.956 and 0.687 W/m 2 K with the transparent barrier, respectively. In addition, when the velocities of the gases between the double glazing were considered, it was seen that the fluid velocity was higher for argon and krypton than for air when there was no obstruction. A similar situation was observed when a blocker was placed.
... Conventional windows used in most buildings are comprised of glass panes that are separated by air or inert gas to limit the heat transfer between them. Vacuum-insulated glazing (VIG) offers the promise of superior performance by taking advantage of vacuum to limit the conduction and convection between the layers of glass [1]. Even though windows occupy between 10 and 15% of the total envelop area of a home which is exposed interacts with outside environment, they account up to 30% of the heat loss [2], i.e., windows are responsible for more than five to six times of energy loss, compared to walls. ...
Although vacuum-insulated glazing (VIG) has been proposed as a promising solution towards developing energy-efficient buildings, VIGs have not become popular in the market due to several technical challenges including the complexity of the fabrication process. In particular, the edge-seal is a key component that significantly affects the thermal insulation and mechanical performance, and the development of edge-seal with adequate thermal insulation, mechanical strength, and reasonable processing cost is essential to overcome such technical issues in VIG. For this purpose, effects of edge-seal design parameters on the VIG performance should be identified. In this research, we analyzed the edge-seal for thermal transport as well as structural stresses to study the effects, and then identified and evaluated the material mixes for the edge-seal requirements. The finite element simulations showed the significance of VIG corner calculation on overall thermal transmittance and the importance of seal conductivity below 1 W/m.K. The experiments with the flexible seals with different ratios of fine glass powder demonstrated that the measured shear strength values for the seal with less than 30% glass powder were more than 10 times larger than the calculated shear stress values. Based on these simulation and experimental results, a flexible sealant was developed using a proprietary mix of ceramic materials that meets the requirements of the designed VIG edge-seal, including structural as well as thermal stress resistance and a low conductivity. Moreover, the sealant is self-curing under atmospheric conditions, and thus it does not require costly inline process of laser curing or oven baking.
... One of the key improvements that can be made towards the energy efficiency of buildings especially during winter is balancing the reduction in heat loss (which includes having more efficient insulation and a considerable reduction in infiltration) with the benefits of solar gains received by a building (7). There are various methods to reduce heat transfer from buildings including glazing optimisation such as vacuum glazing, gas-filled glazing, triple vacuum glazing and multilayer glazing; methods for frame optimisation can also make a difference in thermal retention within a building (8,9). One method which has been investigated in various studies is the use of shading devices which can act as an extra layer of insulation for windows resulting in reduced heat loss whilst maintaining solar gain when needed. ...
Managing thermal loss is a key topic that needs further investigation as it has a direct link to reducing the energy load in buildings. One of these thermal loss management methods can be the use of shading devices. Dynamic thermal models normally used at the early stages of the building design can play an important role in the decision-making process regarding the use of shading devices. This paper presents the results of a real-world study assessing the potential of using a sealed cellular blind as a passive energy conservation method, where the real-world results are compared with the simulated results generated with EDSL Tas. During the real-world study, a positive impact of having blinds was seen whereby the window surface temperature increased and office heating energy consumption was lowered. EDSL Tas was able to predict a similar trend of results for the window surface temperature but not for the energy consumption. This was mainly due to the inability of the software in demonstrating the effect of infiltration of the blind.
... The presence of vacuum, conduction and convection losses are reduced while the presence of low emission coating abates the radiative heat transfer (Memon et al., 2019). Support pillars are most often stainless steel (Wilson et al., 1998) (Collins et al., 1995;Griffiths et al., 1998) made while recently transparent support pillar made from Bismuth boron glass powder (Zhao et al., 2013), translucent aerogel (Büttner et al., 2019) were also investigated. Edge sealing is a critical task for vacuum glazing to maintain the low pressure between glasses for the high durability of the system. ...
Building fenestrations are the key components maintaining the connection between building exterior and interior. However, they are also the weakest allowing heat loss, gain and light. To tackle the enhanced building energy demand, active and passive both ways must be included. Benign energy-generating components and passive energy-saving are both concomitantly possible using photovoltaic (PV) window fenestration. In this work, three different generations PV based fenestration integrated photovoltaics (FIPV) have been reviewed to understand how effective FIPVs are for low energy building. Later advanced technologies suitable for FIPV applications are also discussed.
... This construction consists two glass panes separated by an [26]. A highly insulating vacuum glazing must include excellent features in hermetic edge seal, maintaining stable vacuum space, low-e coating, support pillars and mechanical strength [27]. Researches on vacuum glass also focuses on the above characteristics and requirements. ...
The application of PV double skin ventilated windows (PV-DSV) can reduce intense sunlight from entering room and meantime lower the cell temperature. Vacuum glazing is superior in thermal insulation. To further strengthen the thermal insulation of the PV-DSV, this paper proposes an integrated vacuum glazing with PV double-skin ventilated window (VPV-DSV), which employs the CdTe PV glazing and the vacuum glazing as the front and back glazings. Thermal and electrical performance of the VPV-DSV was investigated and compared with the PV vacuum insulating glazing unit (VPV-IGU) through experiment and simulation. Comparison of PV-DSV with different types of back glazing was made to explore the influence of back glazing types in PV-DSV. Furthermore, influence of the window design factors was explored. Results indicated that the use of vacuum glazing improved the thermal insulation of PV-DSV and resulted in 59.8% reduction of the U-value. VPV-DSV reduced more cooling load, but VPV-IGU outperformed VPV-DSV in thermal insulation. Owing to a small temperature coefficient of the CdTe cell, their power output was similar. In addition, the WWR and PV coverage had more obvious impacts on the performance of VPV-DSV than the emissivity of low-e coating.
... To maintain a vacuum, edges of glass sheets are sealed hermetically [42] by lowtemperature indium alloy edge sealing techniques [43,44]. This low-temperature process enables soft low emission coating and tempered glass to be used compared to high-temperature solder glass edge sealing [45,46] technique which degrades the soft low emission coating and tempered glass. Indoor characterisation of vacuum glazing under a controlled environment using hotbox calorimeter [47], simulation work using finite elements [48], and outdoor thermal [36], optical [49] and daylighting [36] characterisation using outdoor test cell at temperate climate were investigated earlier, which confirmed that insulated vacuum glazing is suitable for low energy building application. ...
In this work, thermal and visual comfort of low heat loss switchable suspended particle device-vacuum
(SPD-vacuum) glazing was investigated for less energy-hungry adaptive building’s glazing or façade
integration at temperate climate. This SPD-vacuum glazing had 38% visible transmittance in the presence
of 110 V applied an alternating voltage and 2% visible transmittance in the absence of electrical power.
Outdoor test cell characterisation was employed to measure the thermal and daylighting parameters of
this glazing. Solar heat gain or solar factor was calculated using non calorimetric methods and varied
between 0.38 (Switch OFF/opaque) to 0.51 (Switch ON/transparent). Test cell indoor and ambient pa-
rameters (incident solar radiation and ambient temperature) were engaged for thermal comfort analysis
by using PMV and PPD methods. Visual comfort was analysed from glare potential, useful daylight index,
and colour properties. The comfortable thermal environment was attainable using both states of this
glazing for a clear sunny day. Acceptable daylight throughout the day was possible for a clear sunny day
for opaque state; however clear state offered allowable/comfortable correlated colour temperature (CCT)
of 5786.18 K and colour rendering index (CRI) of 94.83
... The filling type VIPs are often used in new buildings to improve the insulation performance of the walls. Collins [6][7][8][9][10][11][12] studied vacuum glazing which maintains a vacuum layer by using spacers. Vacuum glazing consists of low-emissivity glass and a clear float, with an evacuated space in between rather than air or noble gas. ...
Insulation performance in older buildings is usually poor, so retrofitting the insulation in these buildings would reduce the energy required for heating, resulting in cost and energy savings. Windows account for a significant amount of the heat loss, therefore, we have developed vacuum layer type vacuum insulation panels (VIPs) with a frame structure that is also slim and lightweight. The developed VIPs are inexpensive and easy to install, as well as being slim and translucent, so retrofitting the window insulation of existing buildings can be easily performed. In this paper, we propose a frame covering with a low emissivity film and a gas barrier envelope coating, with a focus on a reasonable design method. Firstly, a structural model was created to evaluate the safety and specifications of the frame using element mechanical analysis. Next, a finite element model (FEM) was created to predict the insulation performance. Subsequently, experimental validation was completed and the insulation performance was evaluated with the measured thermal conductivity by a guarded hot plate (GHP) apparatus. Finally, case studies were used to evaluate the insulation performance under different conditions. The optimum design included a reasonable frame-structure to hold the vacuum layer with a high insulation thermal conductivity performance of approximately 0.0049 W/(m·K).
... This is equivalent to 2.55 Pa for a 1 mm gap or 0.255 Pa for a 10 mm gap. Below this level of pd the effective conductivity is approximately proportional to pd (Collins, 1995), and the conductive heat loss will be proportional to k pd p dd . ...
The concept of an evacuated flat plate collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. This paper demonstrates the reduction in heat loss coefficient and increase in efficiency resulting from evacuating a flat plate collector: it is hoped that these results will stimulate interest in the concept. Evacuated tubes are now mass-produced in large numbers; evacuated flat plate collectors could in principle replace these tubes if the technical difficulties in creating extended metal-glass seals can be overcome. The experimental experiences described here should indicate targets for future research.
Two different designs of evacuated flat plate solar thermal collector, each with a 0.5 × 0.5 m flooded panel black chrome plated absorber, were tested under a solar simulator. The cover glasses were supported by an array of 6 mm diameter pillars. Inlet and outlet temperatures were monitored via PT100 RTDs and glass temperatures were measured using thermocouples. Inlet temperature was controlled by a fluid circulator connected to a header tank with a Coriolis mass flow meter to measure fluid flow rate. Testing was conducted indoors with and without the use of a fan to cool the top cover glass. The test conditions spanned the range 200 < G < 1000 W/m², 0⩽TM⩽52°C.
Evacuating the enclosure reduced the measured heat loss coefficient by 3.7 W/m² K: this was a close match to predictions and corresponds to an increase in aperture efficiency from 0.3 to 0.6 at TM/G=0.06m2K/W. The poor efficiency under non-evacuated conditions was due to the black chrome absorber coating being less selective than commercial panel coatings.
The solder seals were developed from experience with vacuum glazing but the increased gap led to reliability issues. A vacuum pump maintained the enclosures under a high vacuum (<0.1 Pa) during testing. The enclosure based on a thin rear metal tray proved to be more effectively sealed than the more rigid enclosure with glass on both sides: the latter developed leaks as the front to rear temperature difference increased. The biggest challenge in the manufacture of evacuated flat plate collectors is to ensure a long-term hermetic seal such that no pumping is required.
... Next generation technologies in the glazing industry are vacuum and multichamber glazing. A vacuum insulation glazing (VIG) is a double glazing unit with a high vacuum in between which completely eliminates gas conduction [9]. To prevent the two panes from touching under the load of the atmospheric pressure (10 tons/m 2 !), an array of thin spacers -so-called pillars-maintains the vacuum gap which is typically 0.2-0.7 mm thin. ...
This lecture summary provides a short but comprehensive overview on the “energy and buildings” topic. Buildings account for roughly 40% of the global energy demands. Thus, an increased adoption of existing and upcoming materials and solutions for the building sector represents an enormous potential to reduce building related energy demands and greenhouse gas emissions. The central question is how the building envelope (insulation, fenestration, construction style, solar control) affects building energy demands. Compared to conventional insulation materials, superinsulation materials such as vacuum insulation panels and silica aerogel achieve the same thermal performance with significantly thinner insulation layers. With low-emissivity coatings and appropriate filler gasses, double and triple glazing reduce thermal losses by up to an order of magnitude compared to old single pane windows, while vacuum insulation and aerogel filled glazing could reduce these even further. Electrochromic and other switchable glazing solutions maximize solar gains during wintertime and minimize illumination demands whilst avoiding overheating in summer. Upon integration of renewable energy systems into the building energy supply, buildings can become both producers and consumers of energy. Combined with dynamic user behavior, temporal variations in the production of renewable energy require appropriate storage solutions, both thermal and electrical, and the integration of buildings into smart grids and energy district networks. The combination of these measures allows a reduction of the existing building stock by roughly a factor of three —a promising, but cost intensive way, to prepare our buildings for the energy turnaround.
... Vacuum glazing is a new deep-processing products of glazing [1] . The use of the vacuum principle to achieve the effect of thermal insulation, sound insulation and having excellent anti-aging, visible and infrared light through the reflection properties, can be widely used energy-efficient buildings, greenhouse project aspects, solar collectors, heat insulation and other energy-saving cars transparent material, and has broad prospects for development. ...
Microstructure properties of PbO-TiO2-SiO2 system sealing solder for vacuum glazing was studied. Sealing solder was placed on the sealing glass substrate. Samples were obtained by sintering at different temperatures. The microstructure properties and binding properties with soda lime glass substrate at different temperatures was observed and studied. Micro hardness of the sealing solder after sintering were tested. The results show that in the solder sealing temperature, a transition layer was formed gradually between the sealing solder and the glass substrate as the sintering temperature increases. PbO and SiO2are main ingredients of the transition layer. This layer performed tight connection with the glass substrate after it was completely formed. The interface between sealing solder and soda lime glass substrate showed good performance. PbO-TiO2-SiO2-RxOy system is an ideal sealing material for the production of vacuum glazing.
... In response to manufacturing problems of longevity, increased multi-layer thickness of units, and perhaps even cost, the prototyping of evacuated glazing was pursued (Collins, 1995). ...
Advancements in engineering and space technology are increasingly finding application in our buildings. Building envelopes are utilising components of high-technological solutions resulting in better vision, light transmission, energy generation, storage capacity, shading, ventilation and integration with the external environment. This report summarises several technological advancements and suggests forthcoming directions for building envelope design. Attention is drawn to the danger of over-sophistication, at the expense of architecture, attempting to serve technology. A new method for testing integrated building systems and their control is proposed.
... However, fabrication process of vacuum glazing is considerably complicated compared to other glazing technologies. Especially challenges in developing 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]. ...
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.
Increased urbanization in China has caused tall residential buildings to become an inevitable building form. Consequently, the façade and especially the windows and glazing areas have attained paramount importance for the overall heat loss and heat gain of the building. Recently, sustainable design principles have begun to drive certain macro-trends in construction that are also starting to affect façade design, requiring reduced energy consumption, cradle-to-grave material and component assessment, and measures to improve the well-being and safety of building occupants. The substantial technological advances in windows and glazing systems made during the past 25 years have improved the possibility for designing well-functioning windows that also provide improved sustainability and lower life-cycle costs. The paper reviews the recently developed European energy efficiency rating and labelling system for windows and discusses how structurally bonded window technology allows to meet or to exceed the strict European building code requirements, while providing further sustainability benefits.
Managing thermal loss is a key topic that needs further investigation as it has a direct link to reducing the energy load in buildings. One of these thermal loss management methods can be the use of shading devices. Dynamic thermal models normally used at the early stages of the building design can play an important role in the decision-making process regarding the use of shading devices. This paper presents the results of a real-world study assessing the potential of using a sealed cellular blind as a passive energy conservation method, where the real-world results are compared with the simulated results generated with environmental design solutions limited thermal analysis software (EDSL Tas) and integrated environmental solutions virtual environment (IES VE). During the real-world study, a positive impact of having blinds was seen whereby the window surface temperature increased and office heating energy consumption was lowered. Both software tools were able to predict a similar trend of results for the window surface temperature in with and without blind scenarios whereas for energy consumption although in the presence of a blind a consistent correlation is seen between measured and calculated values but not without a blind. This can be attributed to the inability of the software tools in demonstrating the effect of infiltration in the absence of a blind or shading device i.e., a clear window scenario.
The performance gap analysis regarding thermal loss between dynamic thermal models and real-world settings within buildings can enhance the predictability of the building energy software tools used by designers. Early design inputs within buildings can prevent costly building re-work to improve the building’s energy performance. This can also improve the understanding within the building industry of the importance of reducing thermal loss through the use of shading devices and ensuring the software tools used to model these devices are as close to real-world settings as possible.
This is the second of two papers concerning errors in the measurement of the thermal insulating properties of Vacuum Insulating Glass (VIG) due to non-uniformities in the heat flow due to the support pillars. Part 1 deals with the situation where the measurement area is in direct thermal contact with the glass sheets. This paper discusses how the non-uniformities and associated measurement errors can be reduced using thermally insulating buffer plates on each side of the specimen. A single parameter is developed that characterises the maximum error for measurement areas of all sizes. Values of this parameter are given for all practically relevant designs of the VIG and properties of the buffer plates. Methods are developed for selecting measurement conditions that lead to acceptable tradeoffs between reducing the errors associated with non-uniformities in the heat flow and errors due to heat flow through the edges of the specimen.
Vacuum-insulated glazing (VIG) with a low-emittance coating has a great market potential as an effective transparent insulator. The thermal insulating performance of VIG is determined by its design, including material selection and configuration of different components. Thermal conductance of the vacuum gap as a transport bottleneck is one of the primary factors controlling the thermal transport across VIG. In particular, because support pillars provide the main thermal transport channels across the vacuum gap, increasing the pillar thermal resistance is a key strategy for creating effective thermal insulation while maintaining the vacuum space. In this research, the effects of various pillar design parameters, such as thermal conductivity, geometry, and arrangement, on the VIG thermal performance were comprehensively investigated via the finite element method. In addition, analytical models for thermal transport were examined and thermal conductance across the VIG unit was experimentally measured for validation. The pillar design parameters, especially the height, shape, spacing, and arrangement of the pillars, showed significant effects on the thermal performance of VIG. This research also shows that the smaller contact area for horizontal pillars can effectively decrease the heat loss by more than 30%. Because current VIG analytical equations of thermal performance are only applicable to cylindrical pillars, an analytical equation that can better describe the thermal performance of rectangular parallelepiped pillars is presented, along with a discussion about the mechanism of thermal transfer for different pillar shapes. Thermo-mechanical analyses based on 3D FEM simulations can provide valuable insights into the effect of various design parameters on the overall performance the VIG, allowing for the development of an optimal VIG design.
Increased urbanization in China has caused tall residential buildings to become an inevitable building form. Consequently, the façade and especially the windows and glazing areas have attained paramount importance for the overall heat loss and heat gain of the building. Recently, sustainable design principles have begun to drive certain macro-trends in construction that are also starting to affect façade design, requiring reduced energy consumption, cradle-to-grave material and component assessment, and measures to improve the well-being and safety of building occupants. The substantial technological advances in windows and glazing systems made during the past 25 years have improved the possibility for designing well-functioning windows that also provide improved sustainability and lower life-cycle costs. The paper reviews the recently developed European energy efficiency rating and labelling system for windows and discusses how structurally bonded window technology allows to meet or to exceed the strict European building code requirements, while providing further sustainability benefits.
The paper discusses the increasing urbanization leading to the prevalence of tall buildings and highlights the importance of facades, particularly glazing, in managing heat loss and gain. Over the past 25 years, significant advances in insulating glass and glazing technologies, such as low-e coatings, gas fill options, and warm-edge spacers, have led to the efficient design of building envelopes constructed primarily of glass. The rise of sustainable design principles is now influencing facade designs to prioritize reduced energy consumption, life cycle assessment, and occupant comfort.
Urbanization, particularly in countries such as China, is associated with increased economic activity in urban areas, where more than half of national income is generated, creating challenges in infrastructure and resource management as populations migrate to cities. Urban structures have a significant environmental impact, contributing to greenhouse gas emissions and high resource consumption, necessitating a shift toward sustainable building practices.
The concept of sustainability is framed within a broader definition that emphasizes achieving a balance between resource extraction and renewal while minimizing the overall environmental impact. The building sector is integrating green strategies that focus on efficient resource use, life-cycle thinking, and human-centered design to improve the sustainability of buildings. In particular, life-cycle costing (LCC) emphasizes the long-term financial benefits of energy-efficient design and operation.
The paper also examines the complexities of defining "green" versus "sustainable" buildings and points to the emergence of standards such as the LEED rating system to guide sustainable building efforts. It explores the benefits of integrating new glazing technologies that not only improve aesthetics, but also address energy management, natural ventilation, and indoor comfort.
Emerging sustainability-focused glazing technologies include improved thermal glazing, adaptive and light-scattering glazing, and the integration of photovoltaics into facades, allowing buildings to function as energy suppliers as well as energy consumers. Key innovations add value through integrated design approaches that improve both energy efficiency and occupant comfort.
In addition, silicone materials play a critical role in improving the energy efficiency and longevity of glazing systems, particularly through the introduction of warm-edge spacers and structural glazing techniques. This results in improved thermal performance, sound reduction, and overall building envelope durability.
Looking to the future, the author anticipates continued technological advances that will further enhance glazing performance, facilitating the evolution of facades into energy-efficient and environmentally sustainable building elements while promoting occupant health and comfort. These changes underscore the paradigm shift toward a more integrated and sustainable approach to the design and construction of urban environments.
In order to solve the problem of building energy loss and actively respond to the implementation of the national energy conservation and emission reduction policy, this system, according to the international standard of vacuum glass JC/T1079-2008, combined with the development status and production demand of the measuring instrument, developed a set of instruments that can meet the design requirements to quickly and accurately measure the heat transfer coefficient of vacuum glass. At the same time, the constant pressure measurement conditions are put forward and implemented on the basis of the system. The circuit, software and mechanical structure of the system are studied and designed. Through the above design, the design and test of the vacuum glass heat transfer coefficient measuring instrument are completed. The experimental data show that the measurement error is 0.34% under the condition of adopting effective control strategy, which is about 16 times lower than the measurement error of low-end control method, which is 5.6%. The conclusion shows that the instrument can be used to measure the target more accurately, which is of economic and practical significance.
Vacuum flat plate (VFP) solar thermal collectors exhibit excellent optical and thermal characteristics due to a combination of wide surface area and high vacuum thermal insulation offering a high performance and architecturally versatile collector with a variety of applications for industrial process heat and building integration. A vacuum flat plate solar collector consists of a solar absorber in a flat vacuum enclosure comprising glass or glass and metal covers sealed around the periphery with an array of support pillars to maintain the separation of the enclosure under atmospheric pressure. The edge seal must be both mechanically strong and hermetic to ensure the durability of the internal vacuum over collector lifetime. This presents several challenges for the fabrication of flat vacuum enclosures. In this study a novel sealing technique is presented using a tin-based alloy, Cerasolzer 217, to create the vacuum seal between two glass panes and an edge separating spacer. The sealing process is undertaken at temperatures ≤250 °C allowing the use of thermally tempered glass panes. The mechanical strength of the edge seal was investigated using a tensometer. It was demonstrated that the bond between glass and edge spacer was sufficiently strong to withstand induced stresses in the edge seal region. The edge seal was leak tested using a conventional Helium mass spectrometer leak detector and was shown to possess leak rates low enough to maintain an adequate vacuum pressure to supress conductive and convective heat transfer in the collector. A finite element method (FEM) is developed and validated against the experimental results and employed to predict the stresses in different regions of the enclosure. It was found that the mechanical strength limits of the seal and glass are higher than the stresses in the edge seal region and on the glass surface, respectively.
Amorphous silicon-based semi-transparent photovoltaic windows can produce renewable electricity and offer a certain amount of natural daylight for occupants. However, it has a deficiency as the absorbed solar energy would be partially transferred into additional cooling demand in summer. In this respect, a novel semi-transparent photovoltaic vacuum glazing is proposed to improve energy performance. The selection of appropriate glazing of an energy-efficient building should take into consideration the specific climate conditions. The daylighting behaviour of the glazing will also affect the daylighting performance as well as the lighting consumption. In this paper, the thermal performance, daylighting performance and overall energy performance of the proposed vacuum PV glazing in different climate regions have been investigated. A daylighting model was conducted by DAYSIM to evaluate the annual daylighting performance. It was found that the vacuum PV glazing can balance daylighting availability and visual comfort by providing sufficient daylight in the anterior half of the room and reducing daylight glare to the minimum level. The energy simulation by EnergyPlus demonstrated that the vacuum PV glazing has the energy-saving potential up to 43.4%, 66.0%, 48.8%, and 35.0% in Harbin, Beijing, Wuhan and Hong Kong, respectively. However, the applications of the vacuum glazing lead to additional cooling consumption in the moderate climate zone, such as Kunming. The results advanced the understanding on the applicability and limitation of the vacuum PV glazing in different climate backgrounds. Furthermore, the reversed and the reversible vacuum PV glazing were proposed to enhance the adaptability. The results suggest that the reversible vacuum PV glazing can act energy response in a more efficient way and fully utilize the energy-saving potential of the integration of the PV glazing and the vacuum glazing.
This paper aims to investigate a novel design of vacuum insulated semi-transparent thin-film photovoltaic (PV) glazing called ‘PV VG-2L’, focusing on its thermal performance. The PV VG-2L consists of two parallel glass panes; a semi-transparent thin-film PV glass and a 4 mm hard low-E coated glass, separated by a 0.3 mm vacuum gap. Unlike the conventional vacuum glazing that uses stainless steel, ceramic or alumina as the support pillars, strong and low thermally conductive aerogel material was employed to prevent the glass panes from collapsing under atmospheric pressure. To investigate its performance, a mathematical model for PV VG-2L was developed and numerically solved in MATLAB. The developed model was then validated against indoors and outdoors lab-scale experiments. To conduct the experiments, two PV VG-2L prototypes were manufactured using an innovative low-temperature high-level vacuum method. Using the validated mathematical model, at its optimum design, the performance of the PV VG-2L was numerically simulated, yielding an average U-value as low as 0.60 W/m²K. The promising U-value implies that the PV VG-2L investigated in this research has a wide range of potential applications depending on the energy needs and application.
The heat transfer process of vacuum glass is very complicated. In particular, the heat transfer process of functional vacuum glass, which includes the coupling of heat conduction, convection, and radiation, does not have an exact mathematical solution. The most important parameter representing the thermal properties of vacuum glass, the heat transfer coefficient, is difficult to measure online because it increases over time, thereby decreasing the thermal-insulation performance. Thus, measuring it quickly and accurately for a vacuum glass in use is difficult. This study was conducted to develop an efficient method to simulate heat transfer through vacuum glass. To this end, based on advanced numerical-simulation technology, a computational fluid dynamics software was used to analyse the heat transfer process, and the simulation results applied to guide and analyse the non-steady-state test method. It was found that when a circular heating plate is used to heat the side of the vacuum glass, the ratio of the radius of the heating plate to the thickness of the vacuum glass should exceed three. This approach guarantees that the centre of the heating plate undergoes one-dimensional heat transfer, and the temperature measurement at the centre of the non-heated surface is of practical significance.
Vacuum glazing (VG) is a super-insulator which can effectively reduce energy consumption in buildings. It consists of two glass panes and the gap in between is evacuated. To withstand the atmospheric pressure, spacers are arrayed in the gap and the edge is sealed. This study conducts heat transfer and stress analysis in the central part of VG. Previous studies used simple one-dimensional thermal resistance model to analyze the heat transfer. This study applies multi-dimensional heat transfer analysis and the effects of various geometrical parameters on the thermal resistance are concisely expressed in a fitted formula from the computational results. In addition, the stress concentration caused by the glass-spacer contact is numerically found and a few methods to avoid it are suggested. Finally, the heat transfer is analyzed again for a modified spacer to confirm the applicability of the previously fitted formula.
Non-concentrating solar thermal collectors are generally available in two forms, flat plate or evacuated tube. Recently a third configuration, the evacuated flat plate, has attracted interest due to enhanced performance and aesthetic characteristics. By isolating a solar absorber in a vacuum space (<1 Pa) heat loss from the absorber can be minimized resulting in improved efficiency. In addition the improved thermal insulation performance of evacuated panels over conventional glazing systems makes them attractive solutions for integration into energy efficient building facades.
This two part paper describes the design, construction techniques and thermal performance of two vacuum enclosures, fabricated at Ulster University, as prototype components for evacuated flat solar collectors. The first enclosure consists of two glass panes sealed to an edge spacer and separated by an array of support pillars on a regular square grid to form a narrow evacuated space. The second enclosure incorporates an uncooled copper sheet to represent a solar thermal absorber. The enclosures were tested at three conditions i.e. with an internal pressure of high vacuum (0.0021 Pa), low vacuum (8.4 Pa) and no vacuum (atmospheric pressure).
Part 1 of this paper describes the fabrication process for the vacuum enclosures and the measurement of their thermal insulation properties using a hot box calorimeter. The theory of heat transfer through an enclosure with support pillars is discussed; experimental results are compared with mathematical models predictions. A fabrication methodology has been successfully established and a U-value of 1.35 W/m² K for an enclosure with an internal pressure of 0.0021 Pa has been demonstrated. The experimental results are in good agreement with the predictions.
Part 2 of this paper describes solar simulator testing of the enclosure containing a copper plate. The highest stagnation temperature (121.8 °C) was reached under steady-state conditions in the high vacuum test and was in good agreement with predictions. The transient plate and glass surface temperatures were measured and found to be consistent with the predicted curves.
Vacuum insulating glazing (VIG) is a relatively new glazing technology that was first developed primarily for its thermal insulation properties. This research aims to continue a preceding study about the acoustic performance of VIG and to analyse the effects of dissimilar thickness and additional damping to the sound insulation performance of VIG. In this research, the airborne sound insulation performance of VIG is examined with theoretical and experimental approaches. Sound insulation performance was measured using the field sound intensity method for 5 configurations of VIG. Measurements were well-predicted by theory at low frequencies, where it is mostly governed by the mass-law with additional corrections. Sound insulation performance was heavily affected by the coincidence dip, and using dissimilar thickness in the VIG does not alleviate the problem. Additional damping affects the coincidence behaviour of the VIG, shifting the critical frequency and increases the sound insulation performance at and above the critical frequency with improvements up to 9 dB.
The development of vacuum glazed windows in recent decades has provided a foreseeable energy saving opportunity in the design of low-energy consumption buildings and the application of building integrated photovoltaic (BIPV) has experienced rapid development for application in buildings. This paper reports our investigations on the combinations of the vacuum glazing and BIPV integration. Semi-transparent photovoltaic windows can convert solar energy into electricity, but most of absorbed solar heat is transferred into indoor environment which becomes additional cooling load. The proposed vacuum photovoltaic insulated glass unit (VPV IGU) in this paper combines vacuum glazing and solar photovoltaic technologies, which can utilize solar energy and reduce cooling load of buildings at the same time. Various experiments were conducted to evaluate the thermal performance and determine the key characteristics of the VPV IGU in this study. It was found that the VPV IGU can achieve very low total heat gain coefficient (U-value) of around 1.5 W/(m² K) and block most of undesired solar radiation from penetrating through the window. Compared with a common double-pane glass sheet, the vacuum PV glazing can maintain the indoor environment at a relatively low temperature due to its excellent thermal insulation performance in summer. A detailed simulation study has been conducted by EnergyPlus and Berkeley Lab WINDOW. The simulation work has indicated that the cooling load can be reduced by 14.2% by a south-oriented VPV IGU compared with common glazing products while power generation is not compromised compared with normal BIPV systems. The results show that the application of the VPV IGU has a huge energy saving potential and can minimize the drawback of common PV insulating glass units.
Building energy reduction requires highly advanced low heat loss, heat gain and comfortable daylight allowing glazing. Presently available glazing systems are classified mainly in two categories, controlling solar heat gain and controlling low heat loss. Low heat loss through glazing systems can be achieved by (i) suppression of convection in the air between the outer panes by use of multiple glass panes or aerogels, (ii) having an inert gas or vacuum between the panes to reduce or eliminate respectively convective heat transfer. In all these systems, low emissivity coatings are also required to reduce the radiative heat transfer. Low heat glazing allows large areas of a building façade to be glazed without large attendant heat losses. However, they require the addition of an ability to switch from transparent to opaque to avoid excessive solar heat gain and to control glare. Electrically actuated electrochromic, liquid crystal and suspended particle device glazing systems and non-electrically-actuated thermochromic, thermotropic, and gasochromic glazing systems offer control of solar heat gain control and daylight. Recent relevant developments are reviewed with the contemporary status of each technology provided.
We have sequentially deposited layers of silver nanowires (AgNWs), silicon dioxide (SiO2) nanoparticles, and polystyrene (PS) nanoparticles on uncoated glass by a rapid low-cost supersonic spraying method to create antifrosting, anticondensation, and self-cleaning glass. The conductive silver nanowire network embedded in the coating allows electrical heating of the glass surface. Supersonic spraying is a single-step coating technique that does not require vacuum. The fabricated multifunctional glass was characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), ultraviolet–visible spectroscopy, and transmission electron microscopy (TEM). The thermal insulation and antifrosting performance were demonstrated using infrared thermal imaging. The reliability of the electrical heating function was tested through extensive cycling. This transparent multifunctional coating holds great promise for use in various smart window designs.
Glazing transmittance variation with clearness index has higher influence than incident angle for soar energy application. This work presents variation of vacuum glazing transmittance with clearness index. Clearness index and transmittance was calculated from measured one year (2014) solar radiation and glazing transmittance data in Dublin, Ireland. Clearness index below 0.5 offer single value of transmittance whereas above 0.5 clearness index glazing transmittance varies with clearness index. For different azimuthal orientation, clearness index associated with vertical plane glazing transmittance has been proposed. In Dublin for south facing vertical plane, vacuum glazing has 35% transmittance below 0.5 clearness index. Yearly usable single transmitted solar energy and solar heat gain coefficient for vertical plane south facing vacuum glazing are 87 W/m² and 0.22 respectively.
The lateral sealing method of vacuum sealing used to fabricate Low-E vacuum glazing. The optical performance and surface morphology of the film at each sealing temperature were tested. The heat transfer rate to the central vacuum glazing was also tested. High-temperature sealing treatment may increase the density of the Low-E film surface. The visible spectral transmittance of the film and the far infrared spectral reflectance were significantly increased, Whereas the near-infrared spectral absorption ratio decreased slightly. The lowest rate of central heat transfer of vacuum glazing was 0.9 W/(m2·K) when the sealing temperature was 500°C. However, above 500°C, after the Ag film breakage and reunion to form a polymer film, the film performance degrades.
A switchable suspended particle device (SPD) evacuated (vacuum) glazing has low inherent heat loss and can control solar heat gain by changing its transparency. The thermal performance of combined SPD–vacuum glazing has been investigated using a test cell. In this work two different combination of SPD–vacuum glazing was evaluated. In the first combination, SPD glazing was facing the outside ambient environment and vacuum glazing was facing the indoor test cell environment (SPD–vacuum). In the second combination, SPD glazing was facing the indoor test cell environment and vacuum glazing was facing the outdoor ambient environment (Vacuum–SPD). Variation of the SPD glazing position in a combined SPD–vacuum glazing had little impact on either the internal test cell temperature or the glazing surface temperature. This combined glazing system achieved a dynamic transmission range from 2% (opaque state) to 38% (transparent state). Low overall heat transfer coefficients between 1.00 W/m2 K to 1.16 W/m2 K were found for this combined glazing. Dynamic solar heat gain coefficient was possible using this glazing, which varied from 0.045 to 0.27. This type of combined glazing system is suitable for summer and winter both conditions.
Advanced lightweight insulation and window technologies can contribute significantly to achieving industry and government goals of substantially improving fuel economy without loss of vehicle performance or passenger comfort. Two conventional passenger automobiles, a 2001 sport-utility vehicle (SUV) and a 1999 mid-size sedan, were retrofitted with lightweight insulation; the sedan was also fitted with specially designed windows. The body insulation and windows reduce heating and cooling loads, which allows downsizing of heating, ventilation, and air conditioning (HVAC) equipment. Benefits derived from the use of advanced insulation and window technologies include: Demonstrated reductions in cooling loads; Fuel savings for conventional and hybrid vehicles; Extended range for electric vehicles; Greatly improved passenger comfort; Reduced degradation of interior surfaces; and Improved safety. The research vehicles were retrofitted with Lawrence Berkeley National Laboratory's (LBNL) patented gas-filled panel (GFP) insulation. GFPs are multi-layer baffle constructions made from lightweight films and filled with a low-conductivity gas. These lightweight panels can be up to three times as effective as conventional foam insulations depending on the type of gas used. The GFPs used in the automobile retrofits had a krypton or xenon gas fill, which provides an effective thermal resistance of R-12 per inch and R-20 per inch, respectively. By comparison, expanded polystyrene has a thermal resistance of R-5 per inch. The solar-control glazings used in the sedan retrofit consist of special coatings, which create a narrow-band-pass filter that rejects ultra-violet and infrared (IR) wavelengths. The result is much less heat gain into the interior of the car and less degradation of interior surfaces. The retrofitted cars were tested under driving conditions in a wind tunnel and in an outdoor setting. Test results show that vehicle warm up under soak conditions (vehicle parked in the sun) was delayed compared to a factory vehicle.
The development trend of the vacuum glazing technology and its potential applications were tentatively reviewed in a thought provoking way. The discussions centered on: i) the operating principle of the vacuum glazing; ii) the technical limitations and possible solutions of the conventional vacuum glazing technology; and iii) its potential applications in the progress of the low-carbon economy, especially in auto and building industries. The original work involved the technical innovation to significantly improve the vacuum glazing technology. In the newly-developed technique, the uniformly distributed, protruding, self-supporting spacer dot-arrays were successfully prefabricated on one or both sides of the glass substrates;and consequently, vacuum glazing can be easily realized with another piece of ordinary flat glass.
The analytical model of vacuum glazing (VG) is studied compared with the 3D numerical model. The support pillar with different thermal conductivity coefficient, radius, height, separation, and different glass emittance coefficient are used. The residual gas pressure inside VG space is considered. When thermal conductivity coefficient of pillar was lower than 10 Wm-1K-1, the simple pillar heat transfer model is not validated. The U value has the big difference between the analytical model and numerical simulation at 0.1 Wm-1K-1. The U value was little affected as the pillar thermal conductivity higher than 20 Wm-1K-1. But the U value was affected by pillar separation, radius, and glass emittance, except the pillar height. The residual gas pressure has nothing effect on U value as the pressure lower than 0.1 Pa.
The heat transfer mechanism of vacuum glazing has been deeply analyzed firstly, and then a mathematical model for conduction and radiation of heat transfer in vacuum glazing has been established according to the heat conduction and radial heat transfer coupled model theory. Utilizing the process of absorption-emission media transfer, a nonlinear heat transfer governing equation of differential and integral of the glass layer is achieved, which provides a new theoretical approach for the study of the insulation performance of vacuum glazing.
The simulation and experimental measurement method are carried out to study the insulation performance of vacuum glazing. The heat transfer mechanism of vacuum glazing has been simply reviewed firstly. Grid meshing method of heat transfer model for vacuum glazing geometry is achieved by results of different meshing methods in ANSYS WORKBENCH 13.0, which is used in heat transfer simulation of vacuum glazing. To ensure the accuracy of calculation, too dense meshing in pillar region is unnecessary, but grid refinement in the projection area of the upper and lower glass must be uniform and dense enough. Heat transfer coefficient of vacuum glass under different pressures has been gotten on the experiment device. A thermal conductivity curve that changes with pressure is 'S' shape. The pressure value at curve inflection point where the gas state shifts could be estimated by Knudsen coefficient. The gas is at the state of free molecule when pressure of the evacuated space between two parallel glass sheets lower 0.1Pa. Then the reduction of pressure lower 0.1Pa has little effect on heat transfer coefficient which also demonstrates in experiment.
This paper shows a special test system, and it is applied to thermal performance study of vacuum glazing. Firstly, the heat box device are defined and realized. Secondly, the control and text strategy of temperature, voltage, current, power, electrical energy are described in detail. Thirdly, the soft system sketch and principle are shown. Experimental results demonstrate that the proposed scheme satisfies the requirement for insulation thermal performance of vacuum glazing testing with high accuracy parameter value gotten. The proposed scheme has been used in evaluating the insulation performance of vacuum glazing.
Vacuum glazing is an innovative transparent thermal insulator that has applications in high performance windows. Two sheets of glass are separated by an evacuated space maintained by a regular array of support pillars, and the structure is sealed around the edges with solder glass or a metal bond. The evacuated internal gap reduces the heat transfer across the glazing by eliminating air conduction and convection between the glass sheets. This review paper traces the development of vacuum glazing from the initial proposal 100 years ago, to the first reported manufacture in 1989 by the University of Sydney, the subsequent commercialisation of that design in 1996 and recent advances. The various design challenges are discussed along with the innovative ways they were successfully addressed by two research groups. Several key non-technical factors that have been equally important to the commercialisation and acceptance of this new technology are also presented. The future prospects for vacuum glazing are discussed in the context of the growing interest in energy efficient buildings and higher standards for the thermal performance of windows.
Double-silver low-e vacuum plate glass was made by high temperature sealing process at sealing temperatures of 460, 480, 500 and 520℃. The microstructure of surface and cross section of the low-e films was examined by field emission scanning electron microscopy, and the mechanical properties of the samples were tested by nano-indention method. Meanwhile, the critical load of the samples was tested by a nano scratch tester. The results show that the density of the films increases at a appropriate sealing temperature. The elastic modulus and nano-hardness of the films, and the binding strength between the films and glass can be improved at a proper sealing temperature, and the good combination of properties can be obtained for the films at sealing temperature of 480℃. The creep stress exponent of the films increases with increasing sealing temperature, and when the sealing temperature is higher than 500℃, the higher creep stress exponent causes deterioration of fatigue properties of the films.
Presently, the technique of edges sealing of vacuum glass panels is being utilized in a variety of industrial applications, such as in displays and home appliances. The sealing conditions of a vacuum glass panel strongly affect its key performance parameters, such as its insulation and strength. In the present study, edges of two glass panels were melted and sealed using a hydrogen mixed gas torch. The edges were sealed after appropriately setting process parameters that affected the shape of the sealed edges. Regression models were established for estimating the edge thickness, deflection, and maximum radius of the sealed part, which were considered as shape parameters. The effects of the process parameters on the shape parameters, as well as the interactions among the process parameters, were analyzed, and a polynomial regression model that considered these interactions was established. The feasibility of all the regression models was verified through analysis of variance.
In most areas of the United States, windows are by far the poorest insulating material used in buildings. As a result, approximately 3 percent of the nation's energy use is used to offset heat lost through windows. Under cold conditions, conventional double glazings create uncomfortable spaces and collect condensation. However, with the recent introduction of low-emissivity (low-E) coatings and low/conductivity gas filling to respectively reduce radiative and conductive/convective heat transfer between glazing layers, some manufacturers are beginning to offer windows with R-values (resistance to heat transfer) of 4 hr-ft²-F/Btu (0.70m²-C/W). This papers presents designs for and analysis and test results of an insulated glass unit with a center-of-glass R-value of 8-10; approximately twice as good as gas-filled low-E units, and four times that of conventional double glazing. This high-R design starts with a conventional insulated-glass unit and adds two low-emissivity coatings, a thin glass middle glazing layer, and a Krypton or Krypton/Argon gas fill. The unit's overall width is 1 in. (25 mm) or less, consistent with most manufacturers' frame and sash design requirements. Using state-of-the-art low-emissivity coatings does not significantly degrade the solar heat gain potential or visible transmittance of the window. Work to date has substantiated this concept of a high-R window although specific components require further research and engineering development. Demonstration projects, in conjunction with utilities and several major window manufacturers, are planned. This high-R window design is the subject of a DOE patent application.
A study has been made of the rate of heat transport through a short pillar connecting two separated plates. Three separate approaches to the problem are discussed-analytic, numerical and experimental. The results of the different methods are in good agreement, and confirm the classical result for heat flow through a short, circular contact between semi-infinite bodies. These results are of particular relevance to the design of flat evacuated glazings incorporating short support pillars.
A recent innovation in evacuated tubular solar collector technology is described. A desorbable gas, which specifically adsorbs on the graphitic solar selective surface, can reversibly degrade the vacuum by providing a heat conduction path at elevated temperatures. The stagnation temperature of the system is thus limited in a controllable manner without significantly degrading the low temperature performance. A simple theory incorporating the Langmuir adsorption isotherm and the Knudsen free molecule transport regime is used to describe the phenomenon.
When the heat transfer coefficient of multiple glazings is to be determined, it is important to know the exact thermal conductivity value of the glasses used for building purposes. The values given for flat glass lie between 0.8 and 1.15 W/(m K). The thermal conductivity of 11 float glass samples of 4 different colours, manufactured in 5 European countries, has been determined in the temperature range between 10 and 90°C with an uncertainty of less than 1.5%. At 10°C, the mean value of all glasses investigated amounts of 1.022 W/(m K): the individual values vary between 1.033 and 1.017 W/(m K). On the basis of the composition, the basicity and the density, the measurement results could be well interpreted by known theoretical considerations. The slight variation of the thermal conductivity values obtained by measurement is in agreement with the values calculated from the composition and the density and can be explained by the fact that all float glasses are of nearly the same composition. By interpolation of the measurement data, coefficients could be determined to calculate the thermal conductivity from the oxide composition. The investigation has shown that - at 10°C - a value of 1.02 W(m K) can be applied for the thermal conductivity of flat glasses used for building purposes, independent of their colour or manufacturer.
Current research and development offers important opportunities for improved energy efficiency of architectural windows. This chapter covers the design criteria for different climates and reviews means to fulfill these criteria by proper materials selection. We discuss glass properties, the importance of multiple glazing, the many uses of surface coated glass, and possibilities connected with materials interposed between glass panes. Among the coatings, we treat those based on noble metals and on doped oxide semiconductors, both of which have static spectrally selective properties, as well as electrochromics-based and thermochromic coatings which enable a dynamic control of the throughput of radiant energy. The fascinating possibilities with electrochromic “smart windows” are pointed out. Angular-selective coatings are discussed briefly. Novel antireflection coatings make it possible to boost the transmittance both of coated and uncoated glass. Regarding materials interposed between the panes, the discussion includes gases and gas mixtures, coated plastic foils, silica aerogels, photochromic plastics, thermochromic cloud gels, and electrically switched liquid-crystal-based materials.
This chapter presents the results of the recent analytical and experimental investigations of heat transfer in rarefied gases. It concentrates on the heat transfer through the electrically neutral nonreacting gases. The effects of ionization, dissociation, etc., that may arise in high-speed high temperature gas flows are not considered. The term “rarefied” means that the molecular mean free path is not small compared to a characteristic dimension. The parameter that describes the degree of rarefaction is the Knudsen number. In defining the Knudsen number, it is important to select the appropriate characteristic mean free path and length. When the Knudsen number is very small, then, in the vicinity of the body, the number of collisions between the molecules is large compared to the number of collisions between the molecules and the body. In this case, the usual continuum concepts are applicable and the Navier-Stokes equations and the Fourier heat conduction law are valid. When the Knudsen number becomes sufficiently large, then the continuum concepts must be modified for calculating the heat transfer. At very high Knudsen numbers where the number of collisions between the molecules and the wall is much larger than the number of collisions between the molecules, the flow is termed “free molecule.”
The design and fabrication of a highly insulating window glazing is being studied at SERI. Computer aided design analyses indicate that an all glass, edge sealed vacuum window with spherical glass interpane spacers and a low emittance, coating,on one internal surface could exhibit a thermal conductance of less than 0.6 W/m2K (thermal resistance, R > 10°F ft h/Btu). Cost effective means of mass-producing such a glazing have been explored. A CO2 laser has been used to produce a continuous, leak tight welded glass perimeter at 580°C, and this process appears to be a promising approach. However, at this temperature in vacuum, few low-emissivity coatings retain their desirable properties. Systemmatic measurements were made on tin oxide (fluorine doped) and indium-tin oxide low-e coatings. The indium-tin oxide was shown to be improved by vacuum heating. The ratio of solar weighted transmittance to emittance (313 K black-body weighted), a measure of performance in this application, is shown to have a sharply defined maximum at a coating sheet resistance of approximately 5 ohms per square in both of these oxide semiconductor coatings.
Evacuated glazing consists of two plane sheets of glass which are hermetically sealed around the edges and which contain a narrow evacuated space. An array of support pillars maintains the separation of the glass sheets under the influence of atmospheric pressure. The vacuum effectively eliminates gaseous heat transport through the structure, and radiative heat transport can be reduced to a low level by the incorporation of transparent, low emittance coatings on one or both of the inner glass surfaces. The application of evacuated glazing is in insulating windows. Evacuated glazing contains steady or long term stresses as a result of the combined effects of atmospheric pressure and temperature differentials. A method is developed for determining the magnitude of such stresses due to these separate influences. The stresses are calculated analytically, and by finite element modelling. The results of the calculations are validated by experimental measurements. These stresses are used with a model of delayed fracture in glass to estimate the probability of mechanical failure of evacuated glazing over long periods of time. It is shown that evacuated glazing which is well designed, manufactured and installed should be no more susceptible to mechanical failure than conventional double glazing, for similar operating conditions.
Flat evacuated glazing with very good insulating properties requires: (i) a leak-free edge seal around the periphery of two glass sheets; (ii) a low, and stable pressure within the space between the sheets; (iii) a low thermal conductance array of support pillars to maintain the separation of the glass sheets under the influence of atmospheric pressure forces; and (iv) transparent, low emittance coatings on the inner surface of one, or both glass sheets. Experimental samples of evacuated glazing up to 200 mm × 250 mm have been produced which incorporate the first three features. It has been shown that an array of pillar supports having thermal conductance less than 0.3 W m−2 K−1 can be produced which is quite unobtrusive and which results in tolerably low levels of mechanical stress within the glass sheets. The optical properties of windows incorporating such supports are very similar to conventional windows. Levels of vacuum have been achieved without getters in several devices for which contributions to thermal conductance due to heat flow through the gas are less than the limit of resolution of the measuring device (0.2 W m−2 K−1). Durable, vacuum compatible low emittance coatings exist which should result in a thermal conductance due to radiative heat flow of less than 0.3 W m−2 K−1. The overall thermal conductance of a window incorporating such coatings should be less than 0.6 W m−2 K−1. The ability of these windows to withstand mechanical load and temperature differentials appears to be adequate for most architectural applications.
The fracture of a brittle solid under a spherical indenter is the best studied case of fracture in a strongly inhomogeneous, well defined, stress field. Two principal topics are discussed. the path of a crack in a field of non-uniformly directed stress, and the stability of cracks of various length when the prior stress on the crack path is non-uniform. For the first, it is shown that the crack growth should, to a first approximation, be orthogonal to the most tensile principal stress, and thus correspond, in a torsion-free stress field, to a surface delineated by the trajectories of the other two principal stresses: while, to a second approximation, the crack should deviate from this path by having a larger radius of curvature at every bend. thus exhibiting a pseudo-inertia even in slow growth. This is in accordance with the known experimental facts about the Hertzian crack, particularly the fact that the crack at the surface forms systematically outside the edge of the circle of contact, at which the maximum tensile stress occurs. On the second question, it is found that there are four crack lengths, c_0, c_1, c_2, c_3, corresponding to stationary values of energy. c_0 and c_2 represent unstable equilibria, and diminish with increasing load; c_1 and c_3 represent stable equilibria and increase with increasing load. With small indenters, c_0 soon becomes less than the size of pre-present surface flaws, and an unobserved shallow ring crack of depth c_1 is produced: the critical condition for observed fracture is then the merging of c_1 with c_2, allowing unstable growth to the cone crack of depth c_3. This explains Auerbach's law, that the critical load for production of a cone crack is proportional to the radius. r, of the indenter sphere. With larger indenters, of several centimetres radius for a typical case, c_1 and c_2 merge and disappear before c_0 exceeds the size of pre-present flaws. The critical load for cone fracture then becomes nearly proportional to r^2, as observed. The previous calculations of Roesler (1956a, b) relate to the second stable crack dimension, c_3, though his energy scaling principle is also applicable to the critical condition at which c_1 and c_2 merge. The Hertzian fracture test, within the validity range of Auerbach's law, affords a means of measuring surface energy at the fracture surface independent of knowledge about the pre-present flaws.
This paper discusses the performance limitations of evacuated glazing. Evacuated glazing up to at least 1 m2 in area can be rapidly evacuated. The mechanical tensile stresses in evacuated glazing are strongly dependent on the level of insulation achieved, particularly with respect to the pillar array during manufacture. Highly insulating evacuated glazing contains stresses, in regions which can lead to fracture, that are comparable with conventional double glazing. It appears likely that such glazing can withstand temperature differentials of at least 40 degree(s)C. Evacuated glazing consisting of two 1 m2 sheets of 4 mm thick glass, with internal pyrolytically deposited low emittance coatings, has been manufactured with an air-to- air mid-device thermal conductance of approximately 0.85 Wm-2K-1, of which approximately 2/3 is due to radiation and 1/3 to pillar conductance. These samples have negligible gas conductance and an extremely stable internal vacuum at room temperature. Substantially lower values of radiative heat flow would require sputtered transparent low emittance coatings that can withstand the high temperature currently used in the solder glass edge sealing process. Lower pillar conductance can only be achieved using thicker glass sheets, or tempered glass.
Evacuated glazing is a form of double glazing, utilizing the internal vacuum between the two glass sheets to eliminate heat transport by gas conduction. To achieve the target thermal conductance, the internal gas pressure must be 10 Pa) by baking at temperatures close to those used in the evacuation procedure during the construction of the glazing. Such samples show an improvement in the vacuum with time if stored at temperatures significantly below the bakeout temperature. The experimental data are consistent with a qualitative model of the outgassing of evacuated glazing which involves diffusion of molecules within the volume of the glazing, and adsorption/desorption of gas molecules on the surfaces.
Flat evacuated glazing consists of two plane sheets of glass separated by a narrow evacuated space. These structures must incorporate an array of support pillars in order to maintain the separation of the glass plates under the influence of atmospheric pressure forces. A design procedure is outlined for determining the dimensions of this pillar array. Two important constraints in the design process are the mechanical tensile stress on the outside of the glass plates near the pillar, and the thermal conductance through the array of support pillars. A third constraint arises because of stress concentration near the pillars on the inside of the window. Evacuated windows having usefully low values of thermal conductance through the pillar array and tolerably small levels of exterior tensile stress can only be produced if large stresses exist on the inside of the structure in the region of the glass plates near the support pillars. The implications of these stresses are discussed. It is concluded that it is possible to design a pillar array for which the localised tensile stresses and overall thermal conductance have usefully small values.
Transparent evacuated insulation utilizes the same operating principles as the Dewar flask - gas conduction and convection are essentially eliminated by the evacuated space, and radiative heat transport is small because of internal low emittance coatings. These insulating structures consist of two flat sheets of glass with a hermetic edge seal. An array of support pillars is necessary to maintain the separation of the glass sheets under the influence of atmospheric pressure. The extensive literature on transparent evacuated insulation is reviewed. The design of these devices involves trade-offs between the heat flow through the pillars, and the mechanical stresses. A design methodology for determining the dimensions of the pillar array is developed. An analytic method is described for calculating the stresses and bending produced by a temperature difference across the structure. The results are in reasonable agreement with experimental measurements. The stresses within the structure are shown to be less than conventionally accepted levels over a wider range of operating conditions. Many samples of transparent evacuated insulation have been built and tested in which the heat transport through the evacuated space is due entirely to radiation, to the limit of resolution of the measuring device (0.2 W mâ»Â²Kâ»Â¹). No increase in heat transport has been observed over a period of 18 months. Much higher accuracy measurements have commenced. It appears likely that transparent evacuated insulation will achieve mid-plane insulating values of 0.6 W mâ»Â²Kâ»Â¹, and possibly somewhat lower.
There are significant differences in glass strength information presentedby major United States glass manufacturers. These differences have causedconsiderable confusion among glass designers. The differences are the resultof different simplifying assumptions in formulation of the respective glass designprocedures. Presented in this paper is a glass failure prediction model whichrelates the probability of failure of glass to surface flaw characteristics and inducedstresses. This is accomplished by combining a statistical failure theoryfor brittle materials with results of a geometrically nonlinear plate analysis. Themodel is formulated to include all factors which are known to significantly affectthe strength of glass, such as load duration, surface area, etc. As such themodel provides a basis for resolving current conflicts in the glass design process.
A unifying treatment of environment-sensitive crack velocity functions for intrinsically brittle solids is presented. The formalism is soundly based on the concept of thermal activation barriers, but is phenomenological in that it does not attempt to identify the explicit underlying physical and chemical processes responsible for these barriers. Equations prescribing the v-G (crack velocity versus mechanical energy release rate) characteristics at specified chemical concentrations (partial pressures) and temperatures are thereby presented. These equations incorporate the familiar three velocity regions into a composite function: region I, chemically assisted fluctuations over stress-enhanced energy barriers; region III, similar but in the absence of environmental species; region II, a connecting flow-limited transport branch. In addition, the equations include provision for healing and repropagation branches in unloading-reloading cycles. Central to the argument is the assertion that zero-velocity thresholds are quiescent configurations, defined by appropriate Dupré work of adhesion terms, W. These W terms serve as reference baselines for the entire v-G function, such that changes in chemical concentration (relative humidity) or interface type (virgin versus healed) may be considered in terms of simple curve shifts along the G axis. Data for selected brittle solids, principally mica but also glass and sapphire, in moist environments are used to illustrate the formalism.
A comprehensive set of optical constants and a partial set of bulk optical properties are listed for clear, absorbing, and low-iron glasses used for windows. The measurements extend from the near ultraviolet to the far infrared, covering the range of interest for calculating solar and thermal radiative transfer through windows. This information is also needed to calculate the properties of thin-film coatings on glass substrates. Large variations in solar absorption are observed among these glasses, whereas the far-infrared properties are almost constant for all glass types. An important quantity, the hemispherical total emissivity, is 0.837 at 20°C, as determined from reflectance data measured with an IBM Fourier-transform spectrometer. Solar properties are determined from conventional transmission and reflection measurements.
The technical feasibility of a patented, laser-welded, evacuated insulating window was studied. The window has two edge-sealed sheets of glass separated by 0.5-mm glass spheres spaced 30 mm apart in a regular array. A highly insulating frame is required and several designs were analyzed. The vacuum window's combination of high solar transmittance and low thermal conductance makes it superior to many other windows in cold climates. In the US Pacific Northwest, the vacuum window could save about 6 MJ of heating energy annually per square meter of window in comparison to conventional, double-glazed windows. A large, vacuum laser-welding facility was designed and installed to conduct glass welding experiments and to fabricate full-sized vacuum windows. Experiments confirmed the feasibility of laser-sealing glass in vacuum but identified two difficulties. Under some circumstances, bubbles of dissolved gases form during welding and weaken the seal. Glass also vaporizes and contaminates the laser beam steering mirror. A novel moving metal foil mirror was developed to circumvent the contamination problem, but it has not yet been used to complete welding experiments and fabricate full-sized vacuum windows. 63 refs., 53 figs., 19 tabs.
A general formalism is developed by means of which the radiative heat transfer between macroscopic bodies of arbitrary dispersive and absorptive dielectric properties can be evaluated. The general formalism is applied to the heat transfer across a vacuum gap between two identical semi-infinite bodies at different temperatures. The peculiarities arising when the gap width is of the order of, or smaller than, the dominant thermal radiation wavelengths are studied and quantitatively evaluated for the case of two metal bodies. The predicted strong increase with diminishing gap width is in qualitative agreement with experimental results.
A recent innovation in evacuated tubular solar collector technology is described. A desorbable gas, which specifically adsorbs on the graphitic solar selective surface, can reversibly degrade the vacuum by providing a heat conduction path at elevated temperatures. The stagnation temperature of the system is thus limited in a controllable manner without significantly degrading the low temperature performance. A simple theory incorporating the Langmuir adsorption isotherm and the Knudsen free molecule transport regime is used to describe the phenomenon.
The design and fabrication of a highly insulating, evacuated window
glazing were investigated. A thermal network model was used to
parametrically predict the thermal performance of such a window.
Achievable design options are predicted to provide a glazing with a
thermal conductance less than 0.6 W/sq m K which is compact, lightweight
and durable. A CO2 laser was used to produce a continuous, leak tight,
welded glass perimeter seal around 25 x 25 sq cm test specimens. Various
diameters of regularly spaced spherical support spacers were
incorporated in the specimens as well as an integral SnO2:F transparent,
low emissivity coating for suppression of radiative heat transfer. Laser
sealing rates of .06 cm/s were achieved at a 580 C glass working
temperature with 400 W of continouous wave (CW) laser power.
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.
An analysis is presented which permits the accurate prediction of component lifetimes after proof testing. The analysis applies to crack propagation controlled fracture but can be used as a conservative prediction when crack initiation is predominant. The analytical predictions are confirmed in a series of time-to-failure measurements.Man berichtet von einer Analyse die die genaue Voraussagung der Lebensdauer eines Keramikbestandteiles, nach den Abnahmeversuchen, ermglicht. Die Analyse wird auf den durch Riausdehnung kontrolierten Bruch angewandt, kann aber auch als eine konservative Voraussagung bentzt werden im Fall wo die Rieinleitung ausschlaggebend ist.Die analytische Voraussagung wurde durch eine Reihe von Zeit bis zum Bruch Messungen besttigt.On prsente une analyse permettant de prdire avec prcision la dure de vie d'un composant cramique aprs preuve de rception. L'analyse a trait aux ruptures dont la propagation des fissures est contrle, mais est utilisable en tant que prdiction conservative, dans le cas o l'amorage de la fissure est la phase prdominante.Les prdictions analytiques se voient confirmes par des sries de mesures de dure de vie rupture.
The indentation strength of brittle solids is traditionally characterized by Auerbach's law, which predicts a linear relationship between the load required to initiate a Hertzian cone crack and the radius of a spherical indentor. This paper reviews both the energy balance and flaw statistical explanations of Auerbach's law. It is shown that Auerbach's law in the strictest sense only applies to well-abraded specimens. A novel application of Weibull statistics is presented which allows the distribution of fracture loads to be predicted for any specimen surface condition for a given indentor size. The indentation strength of a brittle solid, for both spherical and cylindrical indentors, is shown to be influenced by both its surface flaw statistics and the degree of interfacial friction. It is observed that the indentation strength of soda-lime glass is increased by a factor of about three times that expected for frictionless contact, and that for a fully bonded indentor, conical fractures cannot occur.
Effects of water and a variety of organic liquids on crack-growth rates in soda-lime-silica glass was investigated. When water is present in organic liquids, it is usually the principal agent that promotes subcritical crack growth in glass. In region I, subcritical crack growth is controlled primarily by the chemical potential of the water in the liquid; whereas in region II, crack growth is controlled by the concentration of water and the viscosity of the solution formed by the water and the organic liquid. In region III, where water does not affect crack growth, the slope of the crack-growth curves can be correlated with the dielectric constant of the liquid. It is suggested that these latter results can be explained by electrostatic interactions between the environment and charges that form during the rupture of Si-O bonds.
The mechanics of crack initiation and propagation beneath an axisymmetric flat punch are investigated. The stress tensor given by Sneddon in 1946 is described. Numerical integration along stress trajectories gives the strain energy release rate as a function of both the crack length and its position relative to the indenter. Comparison with Hertzian fracture is made. The initiation of crack outside the circle of contact is shown to be due to the steepest gradient of stresses along the flaws near the circle of contact. The meaning of Auerbach's law is discussed. The Auerbach range is shown to correspond to the relatively flat maximum of the envelope of theG againstc/a curves for various starting radii. The influence of subcritical crack growth is also discussed. The model proposed in 1978 by Maugis and Barquins for kinetics of crack propagation between punches and viscoelastic solids is used. It is assumed that the static fatigue limit corresponds to the true Griffith criterion with intrinsic surface energy , and that the critical strain energy release rateG
c corresponds to a criterion for crack speed instability and velocity jump, so that no stress corrosion is needed to explain subcritical crack growth for 2GG
c. The 1971 experimental results of Mikosza and Lawn are easily interpreted by this model. Finally, experiments performed on a borosilicate glass give results that agree satisfactorily with the theory. Due to kinetic effects, an apparent surface energy of about 4.5 J m–2 is obtained, larger than the intrinsic surface energy and slightly lower than the fracture energy derived from high-speed experiments.
The question of the physical interpretation of Auerbach's law, that the critical load for production of a Hertzian cone fracture is proportional to the radius of the indenting sphere, has aroused interest in recent years (i) because of its implications concerning the validity of certain brittle fracture criteria and (ii) because of its potential use as a means for measuring fracture surface energies. Two distinct schools of thought, one based on a flaw statistical model and the other on an energy balance concept, have emerged as a result of various attempts to account for this law. This paper describes a theoretical and experimental study aimed at testing the validity of each of these two approaches. Whereas quantitative Hertzian fracture tests have hitherto been extensively made only on surfaces of carefully handled specimens, such as commercial plate glass in the as‐received state, in the present case they are made on plate glass surfaces treated by an abrasion process. With as‐received surfaces the obscure nature of the distribution of surface flaws, from which the cone cracks initiate, largely precludes a conclusive comparison between the two theoretical approaches, while for abraded surfaces the experimentally justifiable assumption that the flaw distribution is ``uniform'' leads to simple but widely conflicting predictions from the two approaches, thus providing the basis for a definitive experiment. According to a flaw statistical argument, in conjunction with an empirical critical stress criterion for fracture, Auerbach's law is predicted to break down for tests on uniformly damaged surfaces, with the critical load for cone fracture becoming a sensitive function of the depth of the damage layer. On the other hand, a stepwise application of Griffith's energy balance criterion for fracture to the growth of a cone crack through the inhomogeneous Hertzian stress field predicts strict adherence to Auerbach's law within limits of indenter siz-
e, with the Auerbach constant of proportionality in this law being independent of flaw statistics. Static and impact tests, made with steel balls ranging from 0.08 to 1.9 cm in radius on plate glass surfaces abraded with slurries of Nos. 240, 320, 400, and 600 SiC abrasive powder, confirm the essential constancy of Auerbach's law, thereby providing strong evidence for the energy balance explanation. Moreover, it is found that the abrasion treatment leads to a drastic reduction in the scatter in results; as a result variations of less than 10% in the fracture surface energy, which is proportional to the Auerbach constant, should be detectable in experiments performed on a given material under different test conditions.
We review work on In2O3:Sn films prepared by reactive e-beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is ≳150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions ≳50 nm and a C-type rare-earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50-μm range, by X-band microwave reflectance, and by dc electrical measurements. Hall-effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon-induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corresponding mobility is ∼30 cm2/V s. The complex dielectric function ε is reported.
These data were obtained from carefully selected combinations of spectrophotometric transmittance and reflectance data. It is found that ε can be reconciled with the Drude theory only by assuming a strongly frequency-dependent relaxation energy between the plasma energy and the band gap. We review a recently formulated quantitative theoretical model for the optical properties which explicitly includes the additive contributions to ε from valence electrons, free electrons, and phonons. The theory embodies an effective-mass model for n-doped semiconductors well above the Mott critical density. Because of the high doping, the Sn impurities are singly ionized and the associated electrons occupy the bottom of the conduction band in the form of an electron gas. The Sn ions behave approximately as point scatterers, which is consistent with pseudopotential arguments. Screening of the ions is described by the random phase approximation. This latter theory works well as a consequence of the small effective electron radii. Exchange and correlation in the electron gas are represented by the Hubbard and Singwi–Sjölander schemes. Phonon effects are included by three empirically determined damped Lorentz oscillators. Free-electron properties are found to govern the optical performance in the main spectral range. An analysis of the complex dynamic resistivity (directly related to ε) shows unambiguously that Sn ions are the most important scatterers, although grain-boundary scattering can play some role in the midvisible range.
As a result of this analysis one concludes that the optical properties of the best films approach the theoretical limit. Band-gap shifts can be understood as the net result of two competing mechanisms: a widening due to the Burstein–Moss effect, and a narrowing due to electron-electron and electron-ion scattering. The transition width—including an Urbach tail—seems to be consistent with these notions. Window applications are treated theoretically from detailed computations of integrated luminous, solar, and thermal properties. It is found that In2O3:Sn films on glass can yield∼78% normal solar transmittance and ∼20% hemispherical thermal emittance. Substrate emission is found to be insignificant. Antireflection with evaporated MgF2 or high-rate sputtered aluminum oxyfluoride can give ∼95% normal luminous transmittance, ∼5% normal luminous reflectance, little perceived color and little increase in emittance. A color purity <1% in normal transmission and <10% in normal reflection is achievable for a daylight illuminant within extended ranges of film thickness.
Evacuated glazing consists of two sheets of glass separated by a small gap, hermetically sealed around the edges, and containing a thermally insulating vacuum. This article describes techniques to determine the time necessary to evacuate such a device through a small pump out tube. The time constant for pressure reduction is determined by numerical modeling. An approximate analytic method is also developed which gives results in good agreement with the numerical data. The numerical results are validated by experimental measurements. It is shown that samples of area about 1 m2 and internal gap ∼0.2 mm can be evacuated in a few minutes through a small pump out tube. It is therefore likely that the evacuation time in the production process of evacuated glazing will be dominated by the heating and cooling cycles associated with outgassing of the internal surfaces, rather than by the pumping process itself.
The vacuum glazing project at the University of Sydney has progressed to the point where the main features of the vacuum glazing design are determined well. Over 500 glazings with areas up to one square meter have been formed. The stresses to which these glazings are or may be exposed have been studied extensively. The durability of the glazing structure and the internal vacuum has been demonstrated. Vacuum glazing of the type designed and formed at the University of Sydney has a center-of-glazing thermal conductance as low as 0.85 and 1.2 Wm−2K−1, for glazings with two and one internal low emittance coatings, respectively. A method for the manufacture of the vacuum glazing is outlined from which the cost to manufacture the glazing can be estimated. A cost at the factory of about 32 ± 6 m−2 for glazing using one sheet of low-e glass is obtained, when production volume is approx. 105 m2yr−1 and is partially automated. This is about 25% higher than the estimated manufacturing cost of the high thermal resistance, argon filled, double glazing utilizing low-e glass, which are currently in production and being sold in the United States, Europe and Japan. These glazings typically have center-of-glazing thermal conductances of about 1.1 Wm−2K−1 or more.
Architects and glass designers often refer to published standards when selecting glass thicknesses and areas for glazings in buildings. Glass design standards generally are based upon the results of experiments involving the breakage of standard sized glass sheets under carefully controlled conditions. More recently, some design standards have incorporated mathematical failure models which contain empirical parameters whose values have been estimated from these same experimental studies. In this paper, the technical basis for the recommendations within current architectural glass design standards is discussed. Existing glass failure prediction models are reviewed and it is shown that significant discrepancies exist between their recommendations and those of the design standards. A modified crack growth model is proposed which predicts failure probabilities for both short and long term stresses which are consistent with established design practice.
A guarded hot-plate apparatus has been developed for measuring the local thermal conductance of flat evacuated glazing. Parasitic heat flows in the apparatus have been reduced to below an equivalent thermal conductance of 0.01 W m2 K1. Techniques are described for determining the separate contributions to heat flow through the sample from pillar conduction, conduction through residual gas, and radiation. The accuracy of the measurement system is estimated to be better than ±2% and the reproducibility for sequential measurements is better than ±0.004 W m2 K1 for a measurement area of approximately 1.7 cm2.
State-of-the-art of evacuated glazing. International Energy Agency Solar Heating and Cooling Program, Working paper T18
- G M Tang
- C J Turner
- D A Dey
- E B Clugston
- T Hansen
- B Simko
- P C Norton
- S N G Eames
- V Lo
- M Wittwer
- Brunotte
Tang, G. M. Turner, C. J. Dey, D. A. Clugston, E. B. Hansen. T. Simko. B. Norton. P. C. Eames, S. N. G. Lo, V. Wittwer and M. Brunotte. State-of-the-art of evacuated glazing. International Energy Agency Solar Heating and Cooling Program, Working paper T18/BS/WD1/93 (1993).
Edge conduction in vacuum glazing, unpublished
- T R E Simko
- F Collins
- D Beck
- Arasteh
T. Simko. R. E. Collins, F. Beck and D. Arasteh, Edge conduction in vacuum glazing, unpublished. Standard AS 128881989 (1989).
Radiation heat transfer between parallel surfaces, unpublished. R. J. Corrucini, Gaseous heat conduction at low pressures and temperatures
- R E Zhang
- T Collins
- G M Simko
- C J Turner
- M Dey
- Brunotte
Zhang, R. E. Collins, T. Simko, G. M. Turner, C. J. Dey and M. Brunotte, Radiation heat transfer between parallel surfaces, unpublished. R. J. Corrucini, Gaseous heat conduction at low pressures and temperatures. Vacuum 7-8, 19929 (1957).
A failure prediction model for window glass, NTIS Accession No. PB81&148421
- W L Beason
W. L. Beason, A failure prediction model for window glass, NTIS Accession No. PB81&148421, Institute for Disaster Research, Texas Tech University, Lubbock, TX (1980).
Solder glass with Shottglas
- Schott
Schott Glaswerke, Solder glass with Shottglas. Product Information Sheet, Number 4823/2e (1989).
Measurement of thermal conductance of evacuated glazing
- R E Collins
- C A Davis
- C J Dey
- A C Fischer-Cripps
- S J Robinson
- T Simko
- J.-Z Tang
- G M Turner
- X Zhou
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Measured angular distribution of the emissivity and calculated radiation heat transfer of architectural coated flat glass, Part 2, Results and discussion
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Evacuated windows — theory and practice
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Production and properties of evacuated glazing. ISES Solar World Congress
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Evacuated glazing. SPIE International Symposium on Optical Materials Technologyfor Energy Efficienq and Solar Energy Conversion XI
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