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Windows are one of the significant indicators of the energy efficiency of a building and have undergone extensive research since the last decades. This paper reviews the performance of various window technologies covering the physical and optical properties of traditional windows and advanced window technologies. In window technologies, one of the most critical parameters is its thermal transmittance value or also known as U-value. In this paper, we discuss the relationship between the physical and optical parameters of the different types of windows and its U-value. Additionally, this paper will also provide interested readers with a wide range of information, including the research gaps in window technologies. Among the main conclusions, we found that, although several advancements have been achieved in this field in the last decade, further research is needed to develop window technologies that not only have high insulating properties but also can generate power.
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Jorge Luis Aguilar-Santana,
International Journal of Low-Carbon Technologies 2020, 15, 112–120
© The Author(s) 2019. Published by Oxford University Press.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which
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doi:10.1093/ijlct/ctz032 Advance Access publication 5 December 2019 112
Review on window-glazing technologies and
future prospects
Jorge Luis Aguilar-Santana*,†, Hasila Jarimi, Mariana Velasco-Carrasco and Saa Riat
Department of Architecture and Built Environment, University of Nottingham, University
Park, Nottingham NG7 2RD, UK
Windows are one of the signicant indicators of the energy eciency of a building and have undergone
extensive research since the last decades. This paper reviews the performance of various window technologies
covering the physical and optical properties of traditional windows and advanced window technologies. In
window technologies, one of the most critical parameters is its thermal transmittance value or also known
as U-value. In this paper, we discuss the relationship between the physical and optical parameters of the
dierent types of windows and its U-value. Additionally, this paper will also provide interested readers
with a wide range of information, including the research gaps in window technologies. Among the main
further research is needed to develop window technologies that not only have high insulating properties but
also can generate power.
Keywords: glazing technologies; vacuum glazing; U-value; optical properties
Corresponding author:
jorge.aguilarsantana Received 4 January 2019; revised 3 May 2019; editorial decision 9 May 2019; accepted 9 May 2019
Energy consumption of buildings accounts for around 20–40%
of both commercial and residential sectors, impacting at least
on 30% of the global CO2accounted to developed countries
[1]. This is the result of the growth of energy use in emerging
economies related directly to the annual population expansion
estimated at 1.18% [2]. Achieving thermal comfort has proven to
be a complicated endeavour for locations with extreme climate
conditions, specially the electric load consumed in appliances to
maintain thermal comfort (Heating, Ventilation and Air Condi-
tioning (HVAC) systems).
This paper reviews the recent advancements in window tech-
nologies that can be categorised into static and dynamic windows.
Static windows (also considered as traditional) include passive
technologies that can improve heat and optical window perfor-
mance such as tinted glazing [3], low-E glazing [4], self-cleaning
glazing [5], anti-reective glass [6] and insulated glass [7]. Mean-
internal interaction of materials to adapt or change to external
conditions; these include electrochromic glazing [8], photovoltaic
glazing [9][10], thermochromic glazing [11], gasochromic glaz-
ing [12] and liquid crystal glazing [13].
The important roles of windows in buildings are that, they provide
ventilation, daylighting, solar heat gain and aesthetics. In general,
the thermal and optical properties of a window can be presented
in terms of the following key parameters: U-value, Solar Heat
Gain Coecient (SHGC) and visible transmittance. The evalu-
ation the aforementioned parameters are governed by the three
well-known heat transfer mechanisms: conduction, convection
and radiation. In this section, we will discuss the key parameters
in detail.
2.1. U-value
Also known as the total heat transfer coecient or total thermal
transmittance, the U-value of a window is used to measure its
eectiveness as an insulator. Measured in W/m2K, the U-value
is evaluated based on the air to air heat transfer through the
window components (i.e. glazing, frame, the gap between the glass
panes (if any) and spacers) from or into the building based on
the ambient temperature dierence. As can be seen in Ta b le 1,
the incorporation of highly ecient windows to reduce the total
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A comprehensive state of the art analysis
Tab le 1. Average U-values in building components. Adapted from [14].
Element U-value (W/m2K)
Floor 0.25
Roof 0.16
External wall 0.30
Windows 2. 00
Tab le 2. Typical U-values on dierent glazing types [16](Lolli&Ander-
sen, 2016).
Glass conguration U-value (W/m2K)
Uncoated single glass 6 mm 5.70
Uncoated double glass 12 mm cavity 2.80
Uncoated double glass 15 mm air cavity 1.40
Uncoated double glass 15 mm argon cavity 1.20
Uncoated triple glass 16 mm with argon 0.79
Uncoated double glass 22 mm monolithic aerogel 0.65
Uncoated double glass 33 mm granular aerogel 0.44
Figure 1. Heat transfer through double-glazing windows.
U-value of a building’s fabric will certainly improve the building’s
energy performance and occupants’ comfort [15].
conduction through the glass panes. Therefore, as summarised in
Table 2 , dierent conguration of glass panes (i.e. single-, double-,
or triple-glazed window), with and without low-emissivity coat-
ing, and with or without additional insulating materials between
the glass panes, results in dierent U-values. Other than the
glass panes, additional heat can also be conducted through the
window’s spacers and the frames. On the other hand, convection
and radiation occur specically in three places in multi-pane
windows: the interior and exterior surfaces of windows as well as
the internal cavities between glazing layers, as shown in Figure 1.
2.2. G-value
The SHGC, also described as G-value is the transmittance of
energy as a result of solar radiation; it is determined by the
solar transmittance/energy absorbed by window materials and
reemitted inwards [17]. This factor is used to quantify the solar
shading capabilities of these transparent components to the short-
wave radiation [18], and it is important for reducing the use of
Tab le 3. Overall relation of glazing thickness and heat transfer for Pilking-
ton OptioatTM clear.
Thickness mm U-value (W/m2K) G-value (SHGC) Visible transmittance (Tv)
2.0 5.8 0.89 0.91
4.0 5.8 0.87 0.90
6.0 5.7 0.82 0.88
8.0 5.6 0.80 0.87
10.0 5.6 0.77 0.87
12.0 5.5 0.74 0.85
19.0 5.3 0.66 0.81
heating loads in winter. Typical SHGC values range between 0.2
and 0.7 as presented in Figure 2.
2.3. Visible transmittance
This optical property comprises of the visible portion of the light
spectrum that passes through a given glazing material. It typically
ranges from 90% for clear glazing (see Figure 3) to 10% for highly
reective coated glazing. This factor is determined by the type of
the glazing, the number of panes and the presence of coatings
that can aect transparency [15]. A high visible transmittance
reduction in electric lighting and heating loads. Typical double-
glazing windows show a visible transmittance of 78% and could
show further reductions if low-E coatings [19]andtintedlms
[20] are included in its composition.
This section reviews the static or traditional glazing technologies.
In recent years, the research on this type of glazing has been
focusing on enhancing the thermal and optical performances. The
glazing includes soda lime glass, multi-pane windows, laminated
glass, reective coatings, low-E coatings, suspended lms and
vacuum windows.
3.1. Glass thickness
For a single-glazed window, as summarised in Table 3 ,theinu-
ence of the glass thickness to its thermal performance was eval-
uated. A signicant reduction in the U-value of glass panes was
found for glass samples with thicknesses above 12 mm. However,
the reduction in the heat transfer comes with the penalty in the
visible transmittance and the overall weight of the window.
3.2. Multipane glazing
Double-glazed windows have been widely manufactured in the
UK since the last quarter of the 20th century [21]. They can
reduce energy consumption in buildings and improve the material
insulating properties, since the gap between panes is used as a
thermal barrier [22].TheimportanceofAguilarsstudyliesinthe
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J. Luis et al.
Figure 2. Solar heat gain coecient of single glass for dierent tinting types.
Figure 3. Visible transmittance for dierent coatings in double-glazing units.
Figure 4. Glazing components for static windows.
modelling of four dierent congurations of double-glazing panes
on an air gap (using clear, absorbent, low-E and reective glass).
The study established that reective glass could save up to 72.9%
of energy consumption whilst having a median payback period of
8.75 years for hot and dry climates.
An air gap introduced between glass panes would act as the
thermal insulation layer, reducing heat transfer through the win-
dow components [23]. This also represents a 50% reduction in
heat losses when compared to single-glazing [24], with the advan-
tage of maintaining the visible transmittance and G-value at rela-
tively high level [20](seeFigure 4). Moreover, the heat transfer
rate a double-glazed window is 2.5 times lower than a single-
glazed window and hence reducing heat gain by about 50–67%
for warm climates [25].
3.3. Tinted glass
Tinting glazing panes help to retain material transparency from
the internal layer of the window, absorbing a portion of the
solar heat whilst blocking daylight [26]. Glass tinting involves the
addition of metallic components on the glass during the oating
process. This process reduces the window transmittance, visibility
and colour [27], maximising the absorption of solar energy to up
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A comprehensive state of the art analysis
Tab le 4. Tinted glass specications for various colours and thicknesses [27].
Colour Thickness mm U-value
Visi bl e
Bronze 4 5.8 0.68 0.60
6 5.7 0.60 0.49
8 5.6 0.53 0.40
10 5.6 0.48 0.33
Green 4 5.8 0.63 0.79
6 5.7 0.55 0.73
8 5.6 0.50 0.68
10 5.6 0.46 0.63
Grey 4 5.8 0.66 0.55
6 5.7 0.57 0.43
8 5.6 0.50 0.34
10 5.6 0.45 0.26
to 50% for single panes [20] whilst eectively reducing the solar
This absorption property on the glass can be adjusted to spec-
tral selectivity, meaning that the tinted glass pane can absorb the
near-infrared solar spectrum whilst transmitting solar daylight
only. Tabl e 4 summarises the tinted glass available in the market
based on their colour and thickness with the corresponding level
of transparency, insulation and heat gain coecients [27].
3.4. Low-emissivity coatings
Like a heat mirror, low-emissivity surface reects long infrared
wave. When applied in the internal surface of a window, the
glazing will allow visible light and short infrared to pass through
(heat energy), returning it back to the internal space. As a result,
the heat transfer between the inside and outside environments
is reduced and the insulating property of the window is
Low-emissivity (low-E) coated glass is manufactured with a
thin transparent coating made from metal oxide such as tin, silver
or zinc to reduce the emissivity of glass surfaces [28]. The man-
ufacturing of low-E coatings includes the sputtered and pyrolytic
methods [20]; the rst method includes the multi-layered coatings
of either metal (usually silver), oxide or nitrides deposited by
physical vapour sputtering, most commonly known as ‘so-coat
low-E’. Meanwhile, the pyrolytic technology includes tin-oxide
chemical molten deposition or ‘hard-coat low-E’ that results in
thicker coatings than the so-coat low-E.
Hard-coat low-E has reported savings of up to 13–17% com-
pared to so-coat low-E (8–10% respectively) and overall energy
savings for both heating and cooling loads as reported on as study
of Canadian dwellings [29]. Derived from the previous research,
the consideration of a hard-coat low-E solution would result in
improved performance for its utilisation in UK dwellings.
Similar to tinted glass, low-E coatings can be tailored to spe-
Figure 5. Heat transfer, SHGC and visible transmittance adapted from Cardinal®
glass models.
cooling, heating and daylighting performance. A disadvantage
presented by low-E coatings [30] is that they can reduce the
solar heat gain to compensate any existent heating loads [31].
Nevertheless, the benecial reduction of glare and solar control
set these coatings as an option for emittance optimisation and
radiation control, as summarised in Figure 5.
3.5. Anti-reective coatings
A modern technique to increase daylight transmittance uses anti-
reective coatings, which can increase the visible transmittance
up to 15% (from 0.74 to 0.89) in double pane windows. Low-E
coating using SnO2, demonstrating no impact in the overall U-
value factor and increasing the G-value by 7%. The use of this
technology helped to decrease the annual heating demand by 4%
[32]; a summary of the technical data presented on manufactured
products by IQ glass is illustrated in Tab l e 5 (IQ [33]).
3.6. Self-cleaning coatings
The eld of self-cleaning coatings uses the action of water as a
labour-saving method for the cleaning for glazing elements, and
it has been a technique used especially for those windows that
include a power generating solution, as photovoltaic panels.
Commercially self-cleaning coatings are constrained into two
main categories: hydrophobic and hydrophilic (as described in
Figure 6). In hydrophobic coatings, water droplets tend to roll
on the glass surface due to a high-static contact angle between
the surface and the water; this is achieved by means of moulded
polymers [34], waxes and chemical vapour deposition (CVD)
[35]. Meanwhile, hydrophilic coatings includes a thin transparent
layer of titanium dioxide (TiO2) to generate a static contact angle
of less than 90between the glass material and the water, which
chemically breaks down glazing dirt when exposed to light; this
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J. Luis et al.
Tab le 5. Anti-reectivewindowscurrentlyinthemarket,comparisonofopticalandthermalproperties.
Name Technology U-value (W/m2K) G-value (SHGC) Visible transmittance (Tv)
IQ anti-reective single glass SG/anti-reective low iron glass 5.80 0.90 0.98
IQ anti-reective double glass DG/anti-reective low iron glass/Ar-lled cavity/low-E coating 1.20 0.64 0.85
IQ anti-reective double glass DG/anti-reective low iron glass/Ar-lled cavity/solar control 1.20 0.48 0.85
Tab le 6. Triple-glazing heat transfer solar coecient and transmittance comparison for noble gas cavity llings.
Manufacturer Product U-value (W/m2K) Visible transm. (Tv)G-value (SHGC)
Modelling (Optitherm-Air) 4/12/4/12/4 air 0.90 0.58 0.34
Nippon Sheet Glass®Co. Pilkington 6/12/4/12/4 Ar 90% 0.70 0.73 0.61
Nippon Sheet Glass®Co. Pilkington 6/12/4/12/4 Kr 90% 0.60 0.73 0.61
Modelling (Optitherm-Xenon) 4/12/4/12/4 Xe 90% 0.53 0.58 0.34
Figure 6. Droplet angle comparison for untreated, hydrophobic, specially cleaned
and hydrophilic glass (Midtdal & Jelle, 2013).
3.7. Substitute gases
ll the gap between glazing panes [24]. Noble (inert) gases are
to transmit heat and availability in nature. Xenon gas is the
most ecient of the solutions, but the scarcity of the material
value of 0.64 W/m2K) has a relatively medium-use due to its
improved performance but a higher price when compared to
argon solutions [14]. For multi-pane gas-insulated windows,
the double- and triple-glazed modules can achieve U-values
from 1.0 to 2.2.W/m2K, and 0.5–0.8 W/m2K, respectively [16],
a comprehensive summary of these technologies can be found
in Table 6 .
Previous experimentation and modelling have proven that
argon-lled solutions could reduce the window conductivity by
67% when compared to air-based mixtures [38]; their odourless
and non-toxic properties have made possible to incorporate
these technologies in residential households in colder regions
[39]. Krypton-based gap llings reduce the overall U-value by
17.18% from argon gaps in theoretical comparisons, although
viable option for mass production. Alternatively, xenon gap ll
U-values from them all (estimated at 0.28 W/m2K), although this
element faces a similar future as krypton, having more expen-
sive manufacturing costs than argon or krypton technologies
combined [15].
Multi-layered windows are recently seen as a technology suit-
able for replacing window glazing in historical buildings, where
the actual need for a thin and lightweight solution is required.
They represent an environmentally acceptable, non-degradable
and narrow solution that rarely presents condensation in the
middle gap during its lifecycle [24].
3.8. Vacuum glazing
The use of vacuum glazing is considered a very promising solution
to reduce the heat transfer via air conduction and convection
within the evacuated gap. This is due to the reduction in the
number of gas particles responsible for the heat transfer within
the evacuated gap.
Nowadays, the manufacturing industry is pursuing the devel-
opment of evacuated glazing [40][41]whichcanreportalower
U-value by reducing heat conduction through the support pillars.
Commercially available vacuum windows can achieve U-values as
low as 0.7 W/m2K (as included in Tab l e 7 ).
Heat transfer in vacuum glazing occurs by contact of the
support pillar array and edge sealing by means of conduction
(Figure 7); therefore, methods to include novel materials in these
components are tested in order to reduce these U-values in
windows [42].
Aerogel is a highly insulating material discovered early in the
year 1930 ([43]); it is a silica-based material composed of 4% silica
preventing heat convection existence whilst allowing a high light
transmittance. The use of this material in the window industry
has proven to reduce the heat transfer via conduction, reaching
U-values as low as 0.05 W/m2K[20]inthematerialproperties
solely [44].
The dynamic window concept comprises module fenestrations
that integrate movable or switchable devices for shading and
energy harvesting (Figure 8). This includes variable optical and
thermal properties that in most cases are climate-dependant.
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Tab le 7. Sample comparison of vacuum-glazing windows commercially available (Pilkington-NSG, 2014) (NSG-Group, 2017).
Name Technolog y Thickness mm U-value (W/m2K) G-value (SHGC) Visible transmittance (Tv)
Pilkington SpaciaTM
(double glazing)
VDG with 0.2 mm pillars 6.2 1.4 0.66 0.76
Pilkington SpaciaTM
Shizuka (double g.)
VDG with laminated glass 9.2 1.4 0.61 0.73
Pilkington SpaciaTM
Shizuka Cool (double g.)
VDG with Ar interlayer
single low-E coating
6.2 1.0 0.49 0.7
Pilkington SpaciaTM 21
thermal control (triple g.)
low-E coating
18.2 0.9 0.58 0.64
Pilkington SpaciaTM 21
VTG Ar interlayer double
low-E coating
18.2 0.7 0.46 0.59
Figure 7. Typical vacuum-glazing components.
Figure 8. Thermochromic colour switching phases [45].
Tab le 8. The summary of electrochromic, photochromic, thermochromic and gasochromic windows.
Type Structure Behaviour U-value (W/m2K) Visible transmittance
G-value (SHGC) Comment
Electrochromic Liquid crystal droplets
in a conductive glass.
Requires constant
energy to maintain clear
0.86 [48]ReducesVtinresponse
to electric changes
Uses dynamic control Requires constant
energy to maintain a
clear state
Photochromic Polymeric matrixes Blocks sunlight in
response to the natural
light incidence
[Wu, et al., 2017].
Reduces Vt in the
response of Light
Darkens when exposed
to SHGC higher than
Blocks sunlight in
response to the natural
light incidence
Thermochromic Uses vanadium dioxide
to change the opacity
Polymer lm [45] - Reduces Vt When
exposed to higher
Gasochromic Uses Tungsten oxide
WO3in gas
encapsulated molecules
Regulates transparency
in variations to
- Reduces 5–6% in visible
transmittance [49].
- Regulates transparency
in variations to
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Figure 9. Diagrams for triple glazing (b), double and triple glazing with PCM (a and c). Adapted from [Li, et al., 2018].
Figure 10. Diagram of c-Si Photovoltaic (PV) window model [Park, et al., 2010].
Some examples of these technologies are electrochromic and pho-
tochromic [Wu, et al., 2017], thermochromic as shown in Figure 8
[45][46], gasochromic, phase change materials and liquid crystal
devices [20][47]. Table 8 summarises the properties and charac-
teristics of some of the dynamic glazing windows.
4.1. PCM windows
Phase change materials [Li, et al., 2018] are substances that can
absorb store and release energy from surrounding materials; their
use in reducing heat transfer in windows plays an important role
as described by its structure included in Figure 9.Recentstudies
show that double-glazing window with PCM llings can help to
reduce solar heat gains in buildings whilst increasing up to 3.0C
the temperature on the window interior surface [Li, et al., 2018].
4.2. Building Integrated Photovoltaics (BIPV)
Windows with integrated solar photovoltaic modules face the
challenge of maintaining a high electricity yield, without com-
promising the visible transmittance of glass [50]. Therefore, most
of the novel developments in PV glazing are limited to the semi-
transparent PV cells (as presented in Figure 10). Dye-sensitised
solar cells (DSSC) are conductive glazing materials that include a
TiO2lm that is capable to reach an average energy eciency of
16–20% [51]. Recent technologies on windows are aimed to har-
vest solar energy for electricity conversion, becoming an attractive
option for its lower environmental impact, cost reduction and
eective land use.
Manufacturing energy ecient windows according to local
energy codes require the improvement of U-value factors on all
external glazing units, as well as cost, weight and dimensions.
Finally, a market-based summary on fenestration technologies is
presented in Figure 11,focusingontheU-values based on the
data presented by major international window companies. The
important reduction achieved by substitute gases, electrochromic
and vacuum glazing windows is remarkable and mainly driven
from their eort to reduce the heat transfer by conduction and
radiation in the glazing samples.
This review comprises of a detailed study of commercially avail-
able window products that were assessed based on the window’s
heat transfer, visible transmittance and SHGC. From the literature
review, the following conclusions can be drawn:
Energy loss through the building envelope is directly linked
to the interaction of exterior surfaces to the environment.
Traditional windows are manufactured with materials of high
heat transfer rate, and their U-values are particularly high in
comparison to other elements (walls, roofs and oors). Present
and future researches are focused on reducing heat transfer
on fenestration elements by the use of static and dynamic
on reducing the U-valueofwindowsasitmakesthesamples
extremely heavy and bulky. Tinted windows have a similar
inuence on heat transfer, aecting only the visible transmit-
tance and solar heat transfer coecient when used as a stand-
alone solution.
Window coatings, on the other hand, are eective solutions to
reduce the impact of SHGC in buildings. An example of this
is the use of low-E, anti-reective and self-cleaning coatings,
which can reduce the SHGC factor to 0.48 (for low-E) without
having a signicant repercussion in the glazing transparency.
Gas-lled cavities can reduce signicantly the U-value of win-
dows (by 22.2% with argon, 33.3% using krypton and
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Figure 11. Average U-value comparison according to commercially published reports (IQ [33]) [39].
41.1 for xenon, respectively), in comparison to air-lled
cavity values (0.62 W/m2K).
Amongst the compared window technologies, multi-pane
glazing can signicantly reduce the heat transfer on windows
(double, triple and vacuum windows). Nevertheless, vacuum
glazing with 0.7 W/m2K can reduce the U-value by almost 42%
when compared against standard double glazing with argon
lling (1.2 W/m2K). This eectively improves the insulating
characteristics of the nowadays widely used double-glazing
Although dynamic windows can provide control on visible
transmittance and heat transfer of windows; manufacturing
processes, material availability and climatic conditions are to
be considered when selecting the most appropriate solution for
each scenario.
Vacuum glazing provides a high-performance solution to
reduce the heat transfer on windows by eectively reducing the
thermal conduction and convection within the window gap.
Favourable conditions for this technology to be developed in
coming years are predicted from this review.
Building integrated photovoltaic modules are a promising
and attractive solution to energy harvesting, providing lower
From the review, we may conclude that vacuum-glazing tech-
nologies with aerogel applications could appear in the market
in the coming years, as stated by Cuce [15] when the challenges
on the materials strength and stability are overcome. Similarly,
the use of semi-transparent crystalline PV modules integrated
into window prototypes may be considered as a method to
achieve net zero carbon buildings for future fenestrations.
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... To achieve this with glazing is especially challenging because of the typical stringent requirements on visible-range transparency and haze 5,6 . While current approaches to this challenge utilize the insulating glass units (IGUs) with air or fill gas [5][6][7][8] , high thermal-barrier performance of such IGUs requires large gap thickness between glass panes, which in turn is limited by gas convection, number of panes and structural constraints. The use of much thinner vacuum-insulated glass units, on the other hand, is limited by the seal integrity and high costs 9,10 . ...
... The use of much thinner vacuum-insulated glass units, on the other hand, is limited by the seal integrity and high costs 9,10 . Low-emissivity silver and other coatings allow for limiting the energy loss due to black-body-like electromagnetic emissivity originating from the room-temperature building's interior [5][6][7][8][9][10] , though they can capture only a fraction of escaping energy at a cost of deteriorating visible-range transparency. Aerogels, highly thermally insulating materials used in applications ranging from pipe insulation to a Mars rover [11][12][13] , have been highly sought after for applications inside IGUs as a solid material replacement for gas fillers [14][15][16][17][18][19] because they stand out as a class of materials capable of outperforming still air and other gas fillers as efficient thermal barriers [20][21][22][23][24] . ...
... Development of transparent aerogels, including cellulose-based ones 25,[28][29][30][31] , remained limited to small scales while also featuring haze and transparency characteristics still inadequate for uses in most types of glazing. While the technological solutions for controlling thermal-range emissivity are highly adequate and widely used [5][6][7][8][9] , and the recent advent of the electrochromic approaches promise to address the needs of solar gain and privacy control [32][33][34] , the lack of good ...
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To maintain comfortable indoor conditions, buildings consume ~40% of the energy generated globally. In terms of passively isolating building interiors from cold or hot outdoors, windows and skylights are the least-efficient parts of the building envelope because achieving simultaneously high transparency and thermal insulation of glazing remains a challenge. Here we describe highly transparent aerogels fabricated from cellulose, an Earth-abundant biopolymer, by utilizing approaches such as colloidal self assembly and procedures compatible with roll-to-roll processing. The aerogels have visible-range light transmission of 97–99% (better than glass), haze of ~1% and thermal conductivity lower than that of still air. These lightweight materials can be used as panes inside multi-pane insulating glass units and to retrofit existing windows. We demonstrate how aerogels boost energy efficiency and may enable advanced technical solutions for insulating glass units, skylights, daylighting and facade glazing, potentially increasing the role of glazing in building envelopes.
... Overall, based on the Global Status Report, buildings and constructions are assumed to be responsible for more than 55% of the total global energy consumption [6]. It was reported that the main share of energy consumption in buildings goes to heating and cooling spaces to compensate for the weak performance of building envelopes [7][8][9]. Therefore, the need to improve the thermal performance of building numerical solutions have been identified, and those solutions should be complement and/or validated by reliable experimental methods [27]. ...
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One of the most important parameters that indicate the energy performance of a window system is the thermal transmittance (U-value). Many research studies that deal with numerical methods of determining a window’s U-value have been carried out. However, the possible assumptions and simplifications associated with numerical methods and simulation tools could increase the risk of under- or over-estimation of the U-value. For this reason, several experimental methods for investigating the U-value of windows have been developed to be used either alone or as a supplementary method for validation purposes. This review aims to analyze the main experimental methods for assessing the U-value of windows that have been published by national and international standards or as scientific papers. The analysis criteria include the type of the test in terms of boundary conditions (laboratory or in situ), the part of the window that was tested (only the center of glazing or the entire window), and the data analysis method (steady-state or dynamic). The experimental methods include the heat flow meter (HFM) method, guarded hot plate (GHP) method, hot box (HB) method, infrared thermography (IRT) method, and the so-called rapid U-value metering method. This review has been set out to give insights into the procedure, the necessary equipment units, the required length of time, the accuracy, the advantages and disadvantages, new possibilities, and the gaps associated with each method. In the end, it describes a set of challenges that are designed to provide more comprehensive, realistic, and reliable tests.
... [18,19], which both reduced the transmission of heat even more. However, VG is considered a very good solution to reduce heat transfer through air convection and conduction within the evacuated gap [20]. Though, heat transfer occurred in vacuum glazing mainly by conduction through the support pillar array and edge sealing [19,21,22]. ...
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.
... A study of the literature on windows heat transfer indicates many different approaches to numerical simulation. This is due to the variety of possible geometric configurations, types, and orientations, as well as to the diversity of the heat transfer modes (natural or forced convection, with or without consideration of thermal radiation and incident solar radiation) [57][58][59]. Table 1 shows overview of the topically relevant numerical investigations of heat transfer in windows. It provides information on the window type, the kind of dimensional simulation (one-, two-, or three-dimensional), the fluid dynamics condition (steady or transient), the classification of fluid flow (laminar or turbulent), the aim of the study, the mathematical modeling approach with the parameters considered, and whether the thermal radiation and incident solar radiation were or were not taken into account. ...
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Windows are important structural components that determine the energy efficiency of buildings. A significant parameter in windows technology is the overall heat transfer coefficient, U. This paper analyzes the methods of numerical determination of the U-value, including for windows that use passive technologies to improve thermal performance. The analysis was intended to evaluate the heat flux and temperature distribution across glazed surfaces and the accuracy of traditional approaches to the determination of heat loss through window structures. The results were obtained using the heat flux measurement method described in the international standard ISO 9869-1:2014. The paper shows that the non-uniformity of the heat flux density on a window surface can be as high as 60%, which in turn generates an error in the calculations based on stationary heat transfer conditions.
... Xenon has the strongest heat resistance, but it is the most expensive to produce, followed by krypton and argon, making argon the best option among all inert gases. Previous study has shown that argon-filled glass can lower window conductivity by 67% when compared to air-filled glazing [8]. Aerogel, on the other hand, is known as one of the most promising thermal superinsulation materials due to its porous structure with nanometre pore size, which permits the material to have thermal conductivity lower than that of still air [9]. ...
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The building industry accounts for almost 40% of the world's energy consumption. To reduce the global heat transfer coefficient, sustainable buildings should use highly insulated enclosures. As the building envelope serves as a barrier between the exterior and interior of the building, integration of passive solar design principles in its construction, such as smart windows with low thermal conductivity materials are essential. Smart windows may assist to reduce energy consumption by minimizing heat gain by the building, which able reduce the cooling loads while maintaining the thermal comfort for the building users. This study features smart double-glazed windows filled with low thermal conductive materials which are argon and aerogel to improve window insulation in pursuit of energy efficiency improvement. A numerical model is developed in ANSYS Workbench to evaluate thermal insulation performance of argon-filled and aerogel-filled windows by measuring the indoor surface temperature of the building at three critical times of the day. Newton's Law of Cooling is used to compute the empirical value of the heat transfer across the window to compare and validate the numerical data. This study shows that argon-filled and aerogel-filled window able to reduce the heat transfer across the building up 21% and 59% respectively. Aerogel is proven to resist more heat transfer as compared to argon
... The vacuum can also be filled with inert gases like argon. A recent innovation is use of light absorbing suspended particles in the cavity between the glass panes [95,102]. These suspended particles align in a defined orientation to prevent heat and light exchange. ...
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Passive buildings are proving to be a solution to menaces of energy crisis and greenhouse gas emissions across the world. Such buildings tend to exhibit low energy demand owing to their cleverly designed envelopes, which comprise of walls, roofs, doors, windows and other openings. This requires use of new materials and technology, leading to an increased initial construction cost. However, with reduced energy consumption, the lifecycle cost of a passive building may be lower than that of a conventional building. These passive buildings also need to cater to occupants’ comfort which is subject to local climatic conditions and climate change. This article discusses economic feasibility and climatic adaptability of a passive building, in addition to advances in passive building strategies. Owing to lack of general awareness and standards related to passive building construction, these buildings have not achieved enough popularity. While many countries are striving hard to bring passive buildings to common masses, a large number of countries are yet to initiate the move. This article outlines several active organizations, standards and rating systems for passive buildings. This article also presents some of the recent research trends and a comprehensive bibliography for the benefit of researchers and practitioners.
... Thin silver films are used in a wide range of applications including antibacterial coatings [1] and different optical devices [2]. Silver thin films are deposited on a large scale as low emissivity (low-E) multilayers to improve the insulating properties of glazings [3,4]. Another application field of this type of multilayers is as transparent anode materials with low sheet resistance and high transparency which could replace the more expensive indium-based materials [5,6]. ...
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Silver films with a thickness below 50 nanometer were deposited on glass using DC magnetron sputtering. The chemical stability of the films was investigated by exposure of the film to a droplet of a HCl solution in a humid atmosphere. The affected area was monitored with a digital microscope. The affected area increases approximately linearly with time which points to a diffusive mechanism. The slope of the area versus time plot, or the diffusivity, was measured as a function of the acid concentration, the presence of an aluminum seed layer, and film thickness. The diffusivity scales linearly with the acid concentration. It is shown that the diffusivity for Al-seeded Ag films is much lower. The behavior as function of the film thickness is more complex as it shows a maximum.
... Insulation can be improved by, inter alia, the use of advanced glazed units containing E-low or electrochromic coatings [5][6][7], vacuum glazing [8][9][10], as well as noble gases in the spaces between the glazed units. These methods may considerably reduce the heat transfer coefficient of glazing, Ug (with argon by −22.2%, with krypton by −33.3%, and with xenon −41.1%, respectively), compared with the Ug of glazed units filled only with air [11]. In their experiments on revolving glass panes, Saleh et al. [12] found that the azimuthal rotation of glazing, while maintaining the same original direction of the wall, is an effective means of ameliorating the solar heat gain to the space (alleviating or augmenting the heat gain for cooling or heating purposes, respectively). ...
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The energy crisis, the risk of interruptions or irregular supplies of conventional energy carriers, and the need to protect the environment stimulate the search for new solutions to improve the heat balance of buildings with the use of solar energy. In this paper, direct and indirect solar gain systems integrated with the building envelope are discussed. In the context of the identified operational problems, the evolution of the classic Trombe wall was shown in the period 1967–2022. Modifications to the windows and Trombe wall proposed in the reviewed works can significantly reduce the risk of an insufficient supply of heat in the winter season. This review also indicates the impact of climate conditions on the decision-making process involved in the selection of the Trombe wall design with respect to energy–effects optimization. The insufficient thermal insulating capacity of Trombe walls has been diagnosed as the reason why they do not enjoy much popularity in cold and moderate climates. As the main directions of development of solar gains systems, the search for solutions that maximize solar gains while ensuring high standards of thermal insulation and the implementation of intelligent technologies were indicated.
The green building movement has emphasized the importance of sustainable building systems design practices to improve the performance of new and existing building envelope components. It is known that fenestration accounts for the majority of heat transfer across the building envelope. This highlights the relevance of fenestration technologies to improve the thermal performance of existing glazing units. A state-of-the-art literature review was conducted to identify the current research directions of fenestration-related technologies with the aim to identify research opportunities related to their implementation in cold climate zones. A keyword-guided literature search was executed to collect current research works regarding various types of fenestration technologies including automated shading systems, glazing films and coating, and smart glazing systems. The review provides a technology overview of these fenestration technologies, the methodologies used to evaluate their energy performance, and the building energy savings obtained through their implementation. Fenestration technologies were assessed based on an evaluation of their season-specific energy performance and an assessment of limitations for cold climate zone implementation. Finally, future research opportunities were identified to improve the energy performance verification procedure of fenestration technologies. Research gaps related to the cold climate applicability of fenestration technologies were also highlighted. Findings from this work provide insight into future research endeavors related to the energy savings quantification procedures for fenestration technologies and the technology's development for cold climate zone applications.
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The application of tempered glass has made it possible to significantly reduce the support pillar number within evacuated glazing (EG) since tempered glass (T-glass) is four to ten times mechanically stronger than annealed glass (A-glass). The thermal transmittance (U-value) of 0.4 m by 0.4 m double evacuated glazing (DEG) with 4 mm thick T-glass and A-glass panes with emittance of 0.03 were determined to be 0.3 Wm−2K−1 and 0.57 Wm−2K−1, respectively (47.4% improvement) using previously experimentally validated finite volume model. The thermal transmittance (U-value) of 0.4 m by 0.4 m triple evacuated glazing (TEG) with 4 mm thick T-glass and A-glass panes with emittance of 0.03 were determined to be 0.11 Wm−2K−1 and 0.28 Wm−2K−1, respectively (60.7% improvement). The improvement in the U-value of EG with T-glass is due to a reduction in support pillar number, leading to reduction in heat conduction through pillar array. The impact of tempered glass on the thermal transmittance for TEG is greater than that of DEG since radiative heat transfer in TEG is much lower than that in DEG, thus the reduction in heat conduction resulted from the reduction of support pillar number in TEG is much larger than that in DEG.
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In subtropical Hong Kong, solar heat gain through glass windows contributes to a significant proportion of building cooling load. Therefore, in cooling dominated areas, the principal of window design includes both utilizing natural daylight and eliminating solar heat gain. In this study, a new heat insulated coating combined antimony-doped tin oxide (ATO) and cesium tungsten bronze (Cs0.33WO3) was developed. Based on measured properties of different glass samples, thermal and daylighting performance of different glass windows, including a clear glass window, a double-layer glass window, a low-e glass window and a coated glass window, were evaluated based on numerical simulation. Results indicate that the developed heat insulated coating can effectively reduce the solar heat gain while maintain the indoor daylighting illuminance at a high level. Up to 9.5% of building energy use can be saved if the coating is applied to south facing clear glass windows.
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With the rapid development of photovoltaic technologies, building-integrated photovoltaic (BIPV) windows could be used to replace traditional glazing, especially semi-transparent amorphous silicon (a-Si) photovoltaic (STPV) windows which can generate electricity in situ and admit daylight into the indoor environment. The utilization of semi-transparent PV modules provides the benefit of low solar heat gain coefficient (SHGC) as a key characteristic of window products. Meanwhile, it also produces a drawback as the remaining solar energy could be converted into heat gain which increases cooling load. Due to the excellent thermal insulation performance of vacuum glazing, the integration of STPV and vacuum glazing provides the potential to achieve the best energy-efficient performance by the low solar heat gain of the PV modules and low heat losses of the vacuum glazing. However, the determination of a suitable glazing of a building in different locations must consider the climate background. In this paper, the thermal performance of the proposed vacuum photovoltaic insulated glass unit (VPV IGU) in different climate zones has been investigated. The simulation work has shown that the vacuum PV glazing can provide a significant energy saving potential in Harbin, Beijing, Wuhan and Hong Kong, which represent the severe cold, cold, hot summer and cold winter, and hot summer and warm winter regions, respectively. However, it is not suitable for the moderate climatic region like Kunming. The results have indicated the advantages of utilizing the vacuum PV glazing in different climates as well as its limitations.
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The application of tempered glass has made it possible to significantly reduce the support pillar number within evacuated glazing (EG) since tempered glass (T-glass) is four to ten times mechanically stronger than annealed glass (A-glass). The thermal transmittance (U-value) of 0.4 m by 0.4 m double evacuated glazing (DEG) with 4 mm thick T-glass and A-glass panes with emittance of 0.03 were determined to be 0.3 Wm-2 K-1 and 0.57 Wm-2 K-1 , respectively (47.4% improvement) using previously experimentally validated finite volume model. The thermal transmittance (U-value) of 0.4 m by 0.4 m triple evacuated glazing (TEG) with 4 mm thick T-glass and A-glass panes with emittance of 16 0.03 were determined to be 0.11 Wm-2 K-1 and 0.28 Wm-2 K-1 , respectively (60.7% improvement). The improvement in the U-value of EG with T-glass is due to a reduction in support pillar number, leading to reduction in heat conduction through pillar array. The impact of tempered glass on the thermal transmittance for TEG is greater than that of DEG since radiative heat transfer in TEG is much lower than that in DEG, thus the reduction in heat conduction resulted from the reduction of support pillar number in TEG is much larger than that in DEG.
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The energy performances of c-Si PV windows and a-Si PV windows have been investigated comprehensively worldwide. However, CdTe thin film PV windows which are supposed to achieve better performance due to its higher efficiency are rarely studied. This study evaluated the energy performance of semi-transparent CdTe thin film PV window applying on commercial buildings. The results show that CdTe PV windows have large energy saving potential in Hong Kong, and the annual energy generation of per unit CdTe PV windows is 52.3kWh/m². Compared to the common window and a-Si PV window, the saving in net energy consumption is 19.6% and 15.3%, respectively.
Conventional window technologies tend to have poor thermal transmittance coefficients (U-values)which cause significant heat loss during the winter season and undesired heat gain in the summer. This study reports a new procedure to calculate center glass U-values of triple and quadruple windows which include low-emissive radiant energy veils – shades and blinds with spectrally selective coatings – between the outermost glass panes. A numerical zonal model is developed to simulate a net radiation system coupled with finite difference and ray tracing methods using validated derived experimental equations. A parametric investigation is carried out wherein the influence of different parameters such as optical properties, inter-pane distances and slat angles of the blinds on the thermal performance of the glazing system are analyzed. Three different gas fill types are examined under realistic boundary conditions. It is shown that this window system has a compelling U-value compared to ordinary multilayer glazing products.
This paper introduces a novel c-Si based building integrated photovoltaic (BIPV) laminate. It was produced by cutting standard crystalline silicon solar cells into narrow strips and then automatically welding and connecting the strips into continuous strings for laminating between two layers of glass. The overall energy performance of the BIPV insulated glass unit (IGU) including power, thermal and daylighting performance was investigated experimentally. The daily energy conversion efficiency of the active solar cell area was about 15% on sunny days, but it was less than 12% on cloudy days and overcast days. Due to the combined effect of both the semi-transparent PV laminate and the insulated glass unit, the solar heat gain coefficient (SHGC) of the BIPV IGU was as low as 0.25, which was much lower than commonly used glazing windows. Daylight analysis by means of high dynamic range (HDR) cameras and daylight glare probability (DGP) indicated that the BIPV IGU could reduce discomfort glare to some extent compared to clear glass windows. The net energy production of the BIPV IGU was estimated without considering the differences in HVAC energy use in this study. The artificial lighting energy consumption was about 431 W h/day while the average BIPV electricity production for the same period was 1940 W h/day. The net power generation was therefore 1509 W h/day for this BIPV IGU in Berkeley, California. Shading tests for the BIPV IGU were also conducted and the results revealed that the vertical configuration of solar cell wiring in the BIPV laminates was much more sensitive to horizontal shading patterns than vertical shading models. Thus, if shading was unavoidable in some cases, a reasonable arrangement of PV strings should be considered to bring down the energy loss as much as possible. Also, the impacts of environmental factors on the energy conversion efficiency of BIPV IGU were analyzed. Specifically, the power output declined by 0.42% of the peak power for each Celsius degree temperature rise. Thus, if more attention was paid to the heat dissipation issue of BIPV IGU, the overall energy conversion efficiency would be improved.
We focused on developing penetration-type semitransparent thin-film solar cells (STSCs) using hydrogenated amorphous Si (a-Si:H) for a building-integrated photovoltaic (BIPV) window system. Instead of conventional p-type a-Si:H, p-type hydrogenated microcrystalline Si oxide (p-μc-SiO x :H) was introduced for a wide-bandgap and conductive window layer. For these purposes, we tuned the CO 2 /SiH 4 flow ratio (R) during p-μc-SiO x :H deposition. The film crystallinity decreased from 50% to 13% as R increased from 0.2 to 1.2. At the optimized R of 0.6, the quantum efficiency was improved under short wavelengths by the suppression of p-type layer parasitic absorption. The series resistance was well controlled to avoid fill factor loss at R = 0.6. Furthermore, we introduced dual buffers comprising p-a-SiO x :H/i-a-Si:H at the p/i interface to alleviate interfacial energy-band mismatch. The a-Si:H STSCs with the suggested window and dual buffers showed improvements in transmittance and efficiency from 22.9% to 29.3% and from 4.62% to 6.41%, respectively, compared to the STSC using a pristine p-a-Si:H window.
A short pitch cholesteric liquid crystal (CLC) acts as one-dimensional photonic crystals. It was widely studied for electro-optical switching devices. Although it was proposed for a switchable smart window, its spectral characteristics have critical limitations for window applications. In this report, we present dichroic-dye-doped short pitch CLC films, suitable for switchable window applications. The spectral limitations of a conventional CLC film in the voltage-off screening states were resolved by combining two different approaches of doping dichroic dye and increasing film thickness. Although each adjustment showed insufficient improvements, the implementation of a combined approach resulted in a drastically enhanced shielding performance in the voltage-off state, desirable for privacy windows. The increased film thickness promoted both the scattering focal conic and reflective multi-domain planar configurations for two different voltage-off states. The modified planar configuration caused a specific colored reflection with some turbidities. Furthermore, the dichroic dyes in both states effectively absorbed transmitting and scattered lights, resulting in the dark gray focal conic state and opaque colored reflection in the multi-domain planar state. Such combined efforts brought a great synergetic effect for enhancing a shielding efficiency. As a result, spectral properties of the voltage-off screening states were greatly different from the conventional CLC films, which show a milky white and tinted color, respectively, in a bright environment. The spectral performances of three switchable states were dramatically improved for the application of electrically switchable smart windows. The dichroic-dye-doped CLC films may be usefully adopted for switchable ultraviolet and heat windows if the cholesteric pitch and dichroic dyes are optimized for the corresponding wavelengths of electromagnetic radiation.