Volume 3 solar thermal systems components and applications 3 10 – glazings and coatings

Volume 3 solar thermal systems components and applications 3 10 – glazings and coatings Volume 3 solar thermal systems components and applications 3 10 – glazings and coatings Volume 3 solar thermal systems components and applications 3 10 – glazings and coatings Volume 3 solar thermal systems components and applications 3 10 – glazings and coatings Volume 3 solar thermal systems components and applications 3 10 – glazings and coatings Volume 3 solar thermal systems components and applications 3 10 – glazings and coatings

G Leftheriotis and P Yianoulis, University of Patras, Patras, Greece

© 2012 Elsevier Ltd All rights reserved

3.10.2.1.2 Optical properties of a glazing

3.10.2.1.3 Definitions of useful terms

3.10.2.1.4 Basic laws for solar and thermal radiation

3.10.2.2 Optical Analysis of Glazing and Coatings

3.10.2.2.1 Basic laws for the refraction and transmission of radiation

3.10.2.2.2 Combined absorption and reflection for total transmittance

3.10.3.4.1 Doped metal oxides

3.10.3.4.2 Coatings with metal layers

3.10.3.4.3 Use of interface layers

3.10.3.4.4 Application of a chemically and mechanically resistant top layer

3.10.3.4.5 Development of asymmetrical coatings

3.10.3.4.6 Development of Ag-based coatings resistant to high temperatures

3.10.3.4.7 Development of coatings with double Ag layers

3.10.4.2.1 Manufacture and properties

3.10.4.3 Use of Glass in Solar Collectors

3.10.4.3.1 Light admittance

3.10.4.3.2 Weather protection and heat loss suppression

3.10.4.4 Windows in the Built Environment

3.10.4.5.1 Clear single glazing

3.10.4.5.2 Tinted single glazing

3.10.4.5.3 Reflective single glazing

3.10.4.5.4 Low-emittance single glazing

3.10.4.5.5 Self-cleaning single glazing

3.10.4.7.1 Aluminum

3.10.4.7.2 Wood and wood composites

3.10.4.7.3 Plastics (vinyl, fiberglass, thermoplastics)

3.10.4.7.5 Effect of frames on the window thermal properties

3.10.4.8 Spacers and Sealants

3.10.4.10 Conclusions – Epilogue

3.10.5.1 Operating Principles

3.10.5.2 Technology and Related Problems

3.10.5.3 The State of the Art

3.10.5.4 Comparison with Conventional Glazing

3.10.5.5 Electrochromic Evacuated Glazing

3.10.6 Transparent Insulation

3.10.6.1 Historical Background

3.10.6.2 Optical and Thermal Properties

3.10.6.3 Types of Available Materials

3.10.6.3.1 Granular aerogels

3.10.6.3.2 Monolithic silica aerogel

3.10.6.3.3 Glass capillary structures

3.10.7 Chromogenic Materials and Devices

3.10.7.3 Electrochromic Devices: Principles of Operation and Coloration Mechanisms

3.10.7.4 Materials for Electrochromic Devices

3.10.7.4.1 Transparent electrical conductors

3.10.7.4.2 Active electrochromic film

3.10.7.4.3 Ion storage and protective layers

3.10.7.4.4 Protective layers – magnesium fluoride

3.10.7.5 Performance of a Typical EC Device

3.10.7.9 Metal Hydride Switchable Mirrors

3.10.7.10 Other Switching Devices

3.10.7.10.1 Suspended particle devices

3.10.7.10.2 Polymer-dispersed liquid crystal devices

3.10.1 Introduction

3.10.1.1 Summary

Windows are key elements of a building as they play an important role in many of its functions: They allow the continuity of indoor/outdoor space by visible light admittance (which is very important esthetically and psychologically) They play a significant

They contribute to the daylighting of rooms and present a shield from weather elements (rain, wind, dust, noise) By proper design, the windows can perform all of the above functions Glazing also plays a significant role in solar thermal collectors by admitting solar radiation and by reducing thermal losses to the environment

The most important breakthrough in the flat glass industry is undoubtedly the development of the float process It has revolutionized glass manufacturing and led to the production of high-quality windows Nowadays, multiple glazing with high visible transmittance and increased thermal insulation is the state of the art in the fenestration market The incorporation of various thin film coatings (such as low emissivity (low-e), reflective, self-cleaning) has added value to the glazing products Emerging technologies such as evacuated glazing (EG), aerogels, and chromogenics promise that in the years to come, new improved products with even better properties will appear

Nowadays, the technology of windows has advanced to such an extent that optimum performance windows are produced commercially, each type tailored for a specific need Different optimization criteria apply for windows depending on climate, use of the building (residential/commercial), dimensions, and other characteristics Thus, there is a multitude of solutions available in the market, ranging from low-performance, inexpensive single glazing to highly insulated triple glazing; and furthermore, to

demand, and so on Emerging technologies such as EG, chromogenics, and aerogels promise that in the years to come, new improved products with even better properties will appear

3.10.1.2 Historical Development of Glass Manufacture

It is of interest to know that primitive windows were just holes in the walls In the next step of the development, they were covered with cloth, wood, or paper and then came the possibility to be closed or opened by the use of appropriate shutters Later, windows were built so that they could accomplish a double task: to transmit light and protect the inhabitants from the extreme environ­mental conditions Glass was used for this function The Romans used glass as a material for windows in Alexandria in the second century AD They used cast glass windows (with poor optical properties.) for this purpose [1]

The word window appears for the first time in the early thirteenth century, and it was referring to an unglazed hole in a roof In English the word fenester was used in parallel until the eighteenth century Today, to describe the array of windows within a facade,

we use the word fenestration

Until the seventeenth century, window glass was cut from large disks of crown glass Larger sheets of glass were made by blowing large cylinders that were cut open and flattened, and then cut into panes Most window glass in the early nineteenth century was

of glass could be cut, and resulting in windows divided by transoms into rectangular panels The first advances in automated glass manufacturing were patented in 1848 by Henry Bessemer, an English engineer His system produced a continuous ribbon of flat glass by forming the ribbon between rollers This was an expensive process, as the surfaces of the glass needed polishing If the glass could be set on a perfectly smooth body this would cut costs considerably Attempts were made to form flat glass on a molten tin bath, notably in the United States Patents were awarded in 1902 and 1905 to H Hill and H Hitchcock [2], but this process was unworkable Before the development of float glass, larger sheets of plate glass were made by casting a large puddle of glass on an

through a lengthy series of in-line grinders and polishers, reducing glass losses and cost Glass of lower quality, sheet glass, was made

by drawing upward a thin sheet from a pool of molten glass, held at the edges by rollers As it cooled the rising sheet stiffened and could then be cut The two surfaces were not as smooth or uniform, and of lower quality than those of float glass This process was in use for many years after the development of float glass [3] Between 1953 and 1957, Sir Alastair Pilkington and Kenneth Bickerstaff

a molten tin bath on which the molten glass flows unhindered under the influence of gravity [4] The success of this process lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight [4] In January 1959, Pilkington made public its new technology, which led to rapid growth in the production of high-quality glass Full-scale profitable sales of float glass were first achieved by Pilkington in 1960 In the Soviet Union, a two-stage molding method was developed in 1969 (USSR

manufacturing commercial products In 1974, PPG Industries (the United States) patented its own method for float glass production (US patent no 3843346) [2]

The float method is the standard method for glass production nowadays: Over 90% of flat glass produced worldwide is float glass As of 2009, the world float glass market, not including China and Russia, is dominated by four companies: Asahi Glass, NSG/ Pilkington, Saint-Gobain, and Guardian Industries Other companies include PPG, Central Glass, Hankuk, Visteon, and Cardinal Glass Industries The flat glass market is expected to reach 39 million tons by 2010 [3]

3.10.1.3 Modern Windows

Modern windows became possible with the perfection of the industrial process for glassmaking and the deposition of appropriate thin films on transparent surfaces leading to the use of low-e coatings Low-e coatings are spectrally selective thin films that add value to plain glass enabling it to perform multiple functions as part of fenestration systems: daylighting of buildings and at the same time suppression of radiative heat losses There are two broad categories of coatings: doped metal oxides and metal-based stacks The former are less expensive, they can be deposited on glass by spray pyrolysis immediately after it leaves the float line, and they are better suited for thermal insulation, in cold climates The latter comprises three to five thin film stacks, which require advanced equipment for their production (such as sputtering in high vacuum) and accurate thickness control They are more expensive, but more versatile: they can be tailored on demand either for solar control or for thermal insulation Recent advances in the glazing industry (especially in the metal-based coatings field) have led to widespread production of low-e coatings for fenestration, automotive, and architectural applications Furthermore, these coatings exhibit electronic conductivity and are being used as transparent conductors (TCs) in a multitude of devices, such as light-emitting diodes, displays, dye-sensitized and organic solar cells, smart switchable windows, and gas sensors This wide range of applications brings these films in the forefront of high technology

3.10.1.4 Emerging Technologies

In recent years, materials science and technology have gained a great impetus New materials and devices with amazing properties and functions are being developed Research teams worldwide continuously come forward with new concepts These advances

example, a building with the capacity to adapt itself to the prevailing weather conditions to save energy and to improve the

their optical properties on demand To that end, a multitude of materials are being developed, under the collective name of

‘chromogenic materials’ Coming from the Greek ‘χρώμα’ (chroma) and ‘γεννώ’ (genno), their name implies that they ‘create color’ Indeed, these materials, switch from a transparent state to a colored-absorptive one, or to a reflective-mirror-like one, under the influence of electrical potential (electrochromics), heat (thermochromics), gases (gasochromics), or light (photochromics and phototelectrochromics) Furthermore, there is a large variety of chromogenic material at different degrees of maturity Some others have found their way to the markets, while yet others are unlikely to ever leave the laboratory bench Chromogenic devices are believed to become the smart windows of tomorrow and to eventually dominate the fenestration market, much as float low-e glass

is a standard today Their widespread use in buildings could improve living conditions of inhabitants, reduce the building energy

3.10.2 Thermal and Optical Properties of Glazing and Coatings

3.10.2.1 General Considerations

We give in this section the basic equations and definitions related to the thermal and optical properties of the solar radiation and the general environment Windows are used to permit the entrance of natural light into the buildings (daylighting) and at the same time

to allow visual contact with the outside environment For these reasons, large windows create a pleasant feeling to the inhabitants

On the other hand, we can have huge thermal losses through them in cold climates (winter case) and undesired heat gains in hot climates, especially if they receive direct solar radiation (summer case) However, the solar heat gains are very welcome during winter and for this reason the appropriate arrangement of windows is a basic element for the bioclimatic design of buildings In principle, walls can be insulated thermally very well, but the same is very difficult for windows as they must be transparent Simple, single-pane windows may exhibit, in some cases, about 10 times larger heat loses compared to a standard wall of the same surface area Advanced double-pane windows have only about 3 times the corresponding losses of a wall or even less Special products have been developed for this purpose as we describe them in detail in this chapter We note here that usually transparent materials are used, but for special uses both translucent and transparent materials can be used

3.10.2.1.1 Solar irradiation

We start with some important considerations about the solar irradiation data, as they are needed for the study of optical and thermal properties of glazing and coatings For this purpose, we rely mainly on field-measured meteorological data or predictions from well-known models that are capable for providing such data for the regions that have been modeled Meteorological hourly data exist for several locations of most countries These files usually include solar irradiation, ambient temperature, relative humidity, wind direction, and speed, and they are very useful for long-term energetic predictions

The solar irradiation consists of two components: (1) direct and (2) diffuse irradiation The direct irradiation component (symbol Ib, from beam radiation) is the solar radiation coming directly from the sun to the point of observation without scattering

or absorption from the molecules and particles of the atmosphere The diffuse irradiation is the irradiation received after it has been scattered by these molecules and particles For example, during a cloudy day the light consists mainly of diffuse irradiation The instruments used for the measurement of direct irradiation are called actinometers or pyrheliometers They consist of a tube directed





toward the sun with collimators inside, which do not permit the diffuse rays into the instrument, and a black absorbing surface at the bottom of the tube The solar direct irradiation is absorbed at the base of the tube heating the instrument Appropriate thermocouples placed there give a signal in millivolts that is calibrated to give the accurate reading of the direct irradiation

there is a filter wheel so that spectral measurements can also be taken in various spectral regions

The total irradiation (I or Itot) is measured using an instrument called a pyranometer It usually consists of black (absorbing) and white regions and multiple thermoelectric elements connected to them, all under a double glass dome, which is transparent to the solar radiation The output is calibrated to give the total solar irradiation With the same instrument, we measure the diffuse irradiation (Id) by placing a small disk (or a band) to shadow it at a distance before the pyranometer glass dome However, if the direct component Ib (the beam irradiation) is known from the measurement that we have described before using the pyrheliometer,

we can find the diffuse component Id indirectly from the following formula:

It is obvious that these three quantities are related and it is usually preferable to measure I and Ib directly and get Id from eqn [1] If

Equation [2] may be used to find the total radiation received by a surface (as a windowpane) when we have measured the radiation components on the horizontal plane Assume that the diffuse radiation has an isotropic distribution as an approximation Corrections have been provided for an even better approximation (e.g., see References 5 and 6]) They are based on the fact that the diffuse solar radiation in an area around the direction of the sun is more intense (circumsolar) In general, we should take three components for the diffuse radiation from the sky: isotropic, circumsolar, and horizon (coming from a belt near the horizon)

3.10.2.1.2 Optical properties of a glazing

The optical properties of a glazing depend on material properties and on the incidence angle of the irradiation on them The incidence angle of the direct irradiation is measured experimentally, but it can also be calculated for a certain place and time by finding the position of the sun and, consequently, the direction of beam radiation in relation to the surface normal [5] For the diffuse radiation, we can proceed by an approximation as we have described before [5] Modern technology allows the deposition

of thin film coatings on large areas of glass Low-e coatings can be applied reducing heat loss problems as well as problems with overheating because they also reflect in the far infrared (IR) radiation [7, 8] For this reason large-area windows can be used now

in buildings

The optical properties and the energy performance of a glazing are interrelated Double-glazed units (DGUs) are common, while triple-glazed units (TGUs) are rather uncommon, their use being restricted to very harsh environments The surfaces and panes are numbered starting with 1 for the outside surface of the outer pane (the surface facing the environment outside that belongs to the first pane) Then 2 is the inner surface of the outer pane and so on Also, it is common now to seal the glazed unit using spacers and

to create what is called an insulated glass unit (IGU) The handling of the window is more efficient in this way In some cases the air gap is filled with inert, low thermal conductivity, gases such as Ar or Kr Advanced glazing, creating and maintaining vacuum in the gap, has been proposed and prototypes have been studied, for extremely low heat transfer [9] The unit is completed with a frame

We focus mainly on the glazed part of windows in this chapter

3.10.2.1.3 Definitions of useful terms

At this point it is useful to introduce some definitions that are commonly used for the optical and thermal properties of glazing and coatings We then give the equations for the dependence of various physical quantities on the angle of incidence

Total solar transmittance (Tsol, expressed as a percent or a number between 0 and 1) is the ratio of the total solar energy in the solar

energy falling on it In other words, solar transmittance is the portion of total solar energy that is transmitted through the glazing Total solar reflectance (Rsol, expressed usually as a percent or a number between 0 and 1) is the ratio of the total solar energy that is reflected outward by the glazing system to the amount of total solar energy falling on it We should note that for windows with

different films on the two sides the reflectance will depend on the side of the window surface exposed to the sun In a similar manner for DGU and TGU, it depends on the sequence of any existing films

Total solar absorption or absorptance (Asol, expressed usually as a percent or a number between 0 and 1) is the ratio of the total

We should point out here that the solar transmittance and solar reflectance can be measured directly It is usually then easier to calculate solar absorption from the following basic equation, which is an expression of the energy conservation:

Solar transmittance is one of the most important physical parameters as it gives the entry of solar energy through the glazing, or any protective envelope in general It affects the total heat transfer, but other factors are needed also to determine the total heat transfer Test methods exist that can give the value of transmittance in situ or ex situ A measurement method of solar transmittance for various materials can be devised by using the sun (or artificial sun) as the energy source, an enclosure, and a pyranometer Sometimes, in addition to transparent, we have to design methods that are appropriate for special cases such as for translucent, patterned, or corrugated materials Some methods can be applied at a small sample area, or others may give an average over a large area, as the need arises Methods also exist that are used to measure transmittance of glazing materials for various angles of incidence up to nearly 80° relative to the normal incidence However, some methods allow measurements of the solar transmittance only at near-normal incidence

can pass through a glazing system to the amount of total visible solar energy falling on the glazing system

Visible light reflectance (Rvis) is the percent of total visible light reflected by a glazing system

previous three quantities is 1

that is allowed to pass through a glazing system to the amount of total UV solar energy falling on the glazing system

There are practical reasons for the interest in the absorption of light in the UV region of the solar spectrum because it contributes

to the deterioration and fading of materials (as, e.g., fabrics and furnishing) Obviously we can define the other two quantities for the UV part of the solar spectrum

Luminous transmittance (Tlum) is defined as

glazing system The luminous transmittance is in effect the visible transmittance weighed by the human eye sensitivity It provides a quantitative representation of the impression of a glazing system to our vision

Similarly, we can also define the luminous reflectance (Rlum) and luminous absorption (Alum) Again, the sum of the previous three quantities is 1

3.10.2.1.4 Basic laws for solar and thermal radiation

The electromagnetic radiation (solar and thermal that is of interest here) is a flow of photons with energy

Using this equation we find that the spectral distribution of thermal radiation emitted by bodies at or around ambient temperatures

radiation: symbol s) The solar spectrum is available from measurements with the attenuation caused by atmospheric absorption at sea level or extraterrestrial from satellites without it The equivalent temperature of the sun (more specifically of the photosphere that emits most of the solar spectrum) is at 5900 K In Figure 1, we show the spectral distribution of solar radiation after passing

Solar spectrum at sea level

Sensitivity of the photopic vision (a.u.)

Blackbody radiation at 70 °C

λ (μm)

show thermal radiation at a typical temperature of a body with ε = 1 (black body) Note the different scales on the two sides The relative efficiency of the human eye is also shown (in arbitrary units, red line) at the region near the maximum of the solar spectral distribution

two sides The relative efficiency of the human eye is also shown (in arbitrary units, red line) at the region near the maximum of the solar spectral distribution

∞

0

The blackbody, for example, the perfect absorber and emitter, is an ideal concept All real materials have a no zero reflectance and

as a result, they emit less radiation than a blackbody To express this fact, eqn [4d] can be modified for the case of a gray body with radiation properties independent of wavelength:

intensity that would be emitted by a black body at the same temperature (T) Emittance can be monochromatic (for a given

over all wavelengths (total) and over all directions of the hemisphere enclosing the emitting surface (hemispherical)

giving the fraction of radiation emitted by surface 1 that is intercepted by surface 2

exchange of thermal radiation is [5]

1

3.10.2.2 Optical Analysis of Glazing and Coatings

3.10.2.2.1 Basic laws for the refraction and transmission of radiation

two terms in the bracket represent the reflectance for the perpendicular and parallel polarization, respectively For example, for glass

We have assumed zero absorption up to now and we must consider it next The system of N parallel plates is very useful in modeling

of the transparency of double and triple glazing

transmittance is decreasing at a fast rate with the angle of incidence [5]

3.10.2.2.2 Combined absorption and reflection for total transmittance

We can follow again the ray-tracing method and get a general equation The exact result is not very useful here Instead, we can use the approximate final result [5]:

For the practical applications, as solar energy, the total transmittance is approximately equal to the product of the two transmit­tances due to reflection and absorption separately

And for the reflectance

Equations [13], [16], and [17] can be used for more than one plate In this case, we should use the total thickness of the system for L

3.10.2.3 Thermal Properties

3.10.2.3.1 Theoretical background

Our purpose is to establish the energy evaluation of windows and show how we can find the energy gains and losses In our analysis,

we are using the well-known thermal resistance concept that simplifies calculations It is based on the solution of the differential equation for heat conduction:

∂T

∂ t where A is the rate of heat production per unit volume in the material, at the point we consider, c the specific heat of the material,

being small for insulating materials and large for good heat conductors It depends also on temperature, but for relatively small variations of temperature and the kind of materials we consider it is constant to a very good approximation The heat flow per unit

on the emissivity of the corresponding surfaces involved for each of them and a number of other factors For hi, which also depends

may vary considerably because it depends on wind velocity and direction as well on the rest environmental conditions We may take

of the gas, density, specific heat, viscosity, and the width of the gas space The inclination to the vertical also affects its value [12] 3.10.2.3.2 Practical considerations

The U-value (or factor) is the overall heat transfer coefficient for a glazing system It is defined as the rate at which heat is transmitted through it, per unit surface area per unit temperature difference between its two sides It is measured in watts per square meter per

heat flow, and the better is its insulating value The symbol U (or Uw) is used for the value referring to the whole window, UC the

In some cases we may encounter the term R-value, which is the inverse of the U-value The R-value is usually cited when discussing wall and ceiling insulation values and rarely for windows and other fenestration products The higher the R-value, the better insulated is the wall or window, and it is more effective in keeping out the heat (and cold)

In practice, to facilitate thermal calculations for a window, we consider three zones: glazed, frame, and edge zone (Figure 2) The edge zone is approximately taken to be about 6 cm wide [11, 13, 14]

3.10.2.3.3 Other useful terms

At this point it is useful to mention some other terms that are related to the energy performance of glazing

Solar gain (or solar heat gain) (SHG) in general refers to the heat increase of a structure (or object) in a space that results from absorbed solar radiation Objects intercepting sunlight absorb the radiation and as a result their temperature is increased Then, of course, part of the heat is reradiated at far-IR wavelengths If a glass pane (or other material) is placed between the solar irradiation and the objects intercepting it, that is, transparent to the shorter wavelengths and not to the longer, then the solar irradiation has as net result an increase in temperature (solar gain)

This is the general principle on which the greenhouse effect is based and has become well known in the context of global warming The amount of solar gain increases with increasing incoming irradiation from the sun and with the ability of the intervening materials to transmit short-wavelength (solar) radiation and in part to absorb small fraction of it It is useful to include

a low-emittance coating in order to reflect the long-wavelength (thermal) radiation back into the space protected by glazing In passive solar building design, for example, the aim is to maximize solar gain from the building in order to reduce space heating demand (winter) and to control it in order to minimize cooling requirements (summer) The composition and coatings on glass for the building glazing can be manipulated to optimize the greenhouse effect, while the pane size, position, and shading can be used to optimize solar gain Solar gain can also be transferred to the building by indirect or isolated solar gain systems Objects having large thermal capacity are used to smooth out the fluctuations during the day, and to some extent between days

Solar heat gain coefficient (SHGC) is the fraction of incident solar irradiation admitted through a window, both directly

SHGC, the less solar heat it transmits in the protected space SHGC is used in the United States

g-Value is the coefficient commonly used in Europe while shading coefficient (SC) is an older term that is still sometimes used in

SC values are calculated using the sum of the primary solar transmittance and the secondary transmittance Primary transmit­tance is the fraction of solar radiation that directly enters a building through a window compared to the total solar insolation, the amount of radiation that the window receives The secondary transmittance is the fraction of heat flowing inside the space from the window, compared to the total solar insolation

Total solar energy rejected (%) is the percent of incident solar energy rejected by a glazing system It is to the sum of the solar reflectance and the part of solar absorption that is reradiated as thermal energy outward

Shading coefficient (SC) is the ratio of the SHG through a given glazing system to the SHG under the same conditions for clear, double-glass window The SC defines the solar control capability of the glazing system and it is useful when discussing the properties of fenestration and shading devices In other words, the SC gives the solar energy transmittance through windows

3.10.3 Low-Emittance Coatings

3.10.3.1 General Considerations

Low-e coatings are thin films that exhibit spectral selectivity: they are highly transparent in the visible (VIS) part of the electro­

law and the Wien displacement law, which state that the heat exchange by radiation between surfaces is characterized by their thermal emittance and that the maximum of emitted radiation from a body occurs at a specific wavelength, related to

materials has a cutout frequency below which all incoming electromagnetic radiation is rejected (e.g., reflected) and that the

IR reflectivity is directly related to the electrical conductivity of the material Thus, it is possible to decouple the visible light spectrum from that of thermal radiation and to have surfaces with properties being entirely different with regard to thermal and visible radiation Furthermore, it becomes clear that for a film to exhibit low-e properties it is necessary to possess electronic conductivity

Low-e coatings were first envisaged for use in transparent heat-insulating glazing Although they were already known from the 1960s, the main thrust in their development took place after the petroleum crisis in 1974, as is the case for most

of the renewable energy materials In the 1980s and 1990s, low-e glass products dominated the markets Nowadays, use of low-e glass in architecture is very common, and in many countries it is mandatory by law to increase the energy efficiency

are Pilkington, PPG, Saint-Gobain, AGC (former Glaverbel), Nippon Sheet Glass Co (NSG), and Guardian Links to their Internet sites appear in the webpage list

Apart from their use in buildings, these coatings have a number of diverse applications, emerging from their electrical conductivity, which enables them to serve as TCs in a multitude of devices such as dye-sensitized and organic solar cells, smart switchable windows, gas sensors, light-emitting diodes, and displays This wide range of applications brings these thin films in the forefront of high technology

3.10.3.2 Solar Control Versus Thermal Insulation

In Figure 3 appear the transmittance and reflectance spectra of plain glass and of a typical low-e coating The intensity of the solar

absorptance (and consequently the emittance) of glass is considerable On the other hand, the transmittance of a typical low-e coating follows closely the solar spectrum in the visible and diminishes in the IR The coating reflectance has the opposite behavior:

low, as it exhibits high reflectance Thus, the low-e coating acts as a spectrally selective filter, which allows passage of the visible (and

λΤ ≈ 2 μm, which transmit the solar IR The former are suitable for warm climates, the latter being suitable for cold climates in which solar gains are desirable

Low emissivity coatings on architectural glass Surface and Coatings Technology 93: 37–45, with permission

3.10.3.3 Deposition Methods

methods are presented

3.10.3.3.1 Thermal evaporation

The raw material of the film is placed in a crucible and heated in vacuum so that a vapor transfers material to an adjacent surface (the substrate) at a sufficient rate [19]

3.10.3.3.2 Electron beam gun evaporation

Instead of thermally heating the raw material, which can be ineffective in cases of compounds with low thermal conductivity (as are dielectrics), an electron beam springing from a metal at high temperature can be deflected into the crucible by a magnetic field [20] Momentum transfer from the incident electrons heats the material This method is versatile as the electron beam can be manipulated (in a way similar to that of a cathode ray tube) to heat uniformly all the material in the crucible The beam intensity can also be altered in order to maintain constant deposition rates Electron guns with multiple crucibles are available for the sequential deposition of different materials within the same vacuum chamber, without interrupting the vacuum To improve the stoichiometry of compounds, reactive evaporation can be used, with the presence of a gas (usually O2) in the vacuum chamber that

is incorporated in the film structure To enhance the film packing density, ion-assisted evaporation can be used: The substrate is bombarded by energetic ions of an inert gas (usually Ar) during deposition and the resulting film becomes more compact The e-gun method is widely used in the optics industry for the production of antireflection coatings, optical filters, etc

3.10.3.3.3 Sputtering

In this method, a plasma of inert and/or reactive gases (such as Ar, O2) is created in a low pressure, and energetic ions in the plasma dislodge chunks of the raw material from a solid plate (known as the target) These chunks are deposited on the substrate [21, 22] Depending on the gas discharge method used to create the plasma, one can distinguish between radio frequency (RF), medium frequency (MF), or direct current (DC) sputtering Furthermore, magnetrons are used to increase the efficiency of the electrons in ionizing the Ar atoms by trapping the electrons near the target, and thus we have Twin-mag, pulsed magnetron sputtering, etc The sputtering method is applied for the deposition of industrial metal-based low-e coatings on large areas

Evaporation and sputtering are also known as physical vapor deposition (PVD) methods

3.10.3.3.4 Chemical methods

In order to avoid vacuum that entails expensive (and sensitive) equipment, chemical deposition methods have been developed A

withdrawal at a controlled rate, and subsequent heat treatment [23] Alternatively, the chemical solution can be applied by spray or

by spin coating In the latter, the substrate is rotating and as a result, the chemical solution is spread evenly on its surface Chemical vapor deposition (CVD) uses heat to decompose a vapor of a precursor chemical to make a thin film of a desired composition [24, 25] A variation of the CVD technique is called spray pyrolysis; a fluid containing the precursor is then sprayed onto a hot

substrate This method is used on a large scale for deposition of tin oxide-based films on hot glass as it comes out from the float glass production and is transferred to the cooling stage Electrochemical techniques include cathodic electrodeposition from a chemical

methods is the inevitable presence of traces in the resulting films of the precursors used, due to limitations in the heat treatment phase These traces may impede the film performance or reduce their service life by unwanted side reactions Furthermore, environmental and health hazards from the compounds involved can be a concern

3.10.3.4 Types of Coatings

Two groups of materials are of particular interest for use as low-e coatings: (1) doped metal oxides and (2) film combinations that

adherent to glass, chemically inert, their luminous and near-IR absorbance can be low, and their thickness does not affect electrical

substrates, and chemically reactive In this group of films, the electrical resistance is strongly thickness-dependent

3.10.3.4.1 Doped metal oxides

Low-e coatings based on doped metal oxides comprise a host lattice (usually In2O3, SnO2, or ZnO) that is doped by metal or halide atoms The most common representatives of this group of films are tin-doped indium oxide (In2O3:Sn, usually called ITO), fluorine-doped tin oxide (SnO2:F, usually called TFO), and gallium-doped zinc oxide (ZnO:Ga, usually called GZO) Doping is accomplished either by adding a higher-valence metal, by replacing some oxygen with fluorine, or by oxygen vacancies The compounds mentioned above have wide enough band gaps to allow considerable transmission in the visible and doping is feasible to a level high enough to render the materials IR reflecting and electrically conducting In doped metal

used mostly for thermal insulation The main advantage of the doped metal oxides, compared to metal-based films, is the chemical and mechanical stability, which allows their use on glass surfaces exposed to ambient conditions This is why these

The development of these materials has reached maturity, and numerous commercial products are available in the market for windows and other architectural applications Most of these films are prepared by spray pyrolysis and their typical thickness is on

Although doped metal oxide coatings are well established, intense research activity continues in the field The research effort is directed toward the development of alternative host materials and dopants, as well as multiphase mixtures of known materials, in order to improve various properties of the coatings, such as electrical conductivity, optical transmission, hardness, and adherence [15] Some of the combinations reported in the literature are the following [15]: ITO:ZnO, ITO:Ti, In2O3–ZnO (IZO), In2O3:Ti,

3.10.3.4.2 Coatings with metal layers

In this type of coatings, the highly reflective metal film (that would otherwise be opaque in the visible) is sandwiched between two dielectric layers that have an antireflective effect: with an appropriate index of refraction and thickness, the light beams reflected on the front and back surfaces of each of the dielectric layers are of opposite phase and of nearly equal amplitude Thus, they interfere destructively and as a result, the film reflectivity is diminished One is then led to dielectric/metal/dielectric (D/M/D hereafter) multilayers Dielectrics with high refractive indices, usually metal oxides, such as TiO2, ZnO, ZnS, SnO2, Bi2O3, and In2O3, are

all these metals, Ag is the most suitable, due to its low absorption in the visible Coatings based on Au or Cu have inferior optical properties and a characteristic golden brown color

To achieve high transmittance, the metal layer needs to be as thin as possible The growth mechanism of metal layers on glass [26–28] imposes the limit: in the initial stages of their development on glass and other dielectric materials, metal films form tiny nuclei With material continuously added on the substrate, these nuclei grow via surface diffusion and direct impingement, into islands that are discontinuous and possess a fractal nature Further thickening of the metal film leads to large-scale coalescence and

to continuous films The coalescence thickness is about 15 nm for thermally evaporated Ag films [26, 27] and can be reduced to about 9 nm using other methods such as sputtering and ion-assisted deposition [27]

In these stacks, the metal layer thickness governs the coating properties With the metal thickness increasing, the coating electronic conductivity increases, and its thermal emittance and luminous transmittance decrease In Figure 4 appears the measured emittance of evaporated ZnS/Ag/ZnS coatings versus Ag layer thickness [8] An abrupt decrease of emittance with increasing Ag

indicates that for this type of films, an optimum can be found at around 22 nm of Ag

λ (nm)

and stability of low emittance multiple coatings for glazing applications Solar Energy Materials and Solar Cells 58: 185–197, with permission

The D/M/D coatings are more versatile than doped metal oxides It is possible to optimize them either for thermal or for solar control through proper selection of the thickness of each individual layer with use of standard thin film optics software and the

control (Tlum ≈ 85%, Tsol ≈ 50%, ε ≈ 0.05) and 30/10/30 nm for thermal insulation (Tlum ≈ 87%, Tsol ≈ 72%, ε ≈ 0.15) Deposition of such extremely thin stacks requires exact growth control of each individual layer Furthermore, optical interference between different layers causes the whole coating to fail, should only one layer deviate from the desirable thickness The major disadvantage of D/M/D coatings is their lack of durability They are sensitive to environmental exposure and degrade with time as atmospheric gases and Ag

research work is being carried out worldwide [15] Indeed, work has been reported recently on ZnS/Ag/ZnS, ZnO/Ag/ZnO, ZnO/Cu/ ZnO, IZO/Al/GZO, ZnO:Al/Ag/ZnO:Al, TiO2/Ag/TiO2, and many others [15]

Typical optical transmission properties of Ag-based low-e coatings are shown in Figure 5 Therein appear ZnS/Ag/ZnS stacks optimized for maximum transmittance at 550 nm, ZnS/Cu/Ag/ZnS and ZnS/Al/Ag/ZnS coatings with ultrathin (less than 5 nm) Cu

or Al films added on Ag to improve emittance and provide thermal stability Finally, five-layer stacks of the form ZnS/Ag/ZnS/Ag/ ZnS have been developed to provide nearly zero emittance (less than 0.02) These coatings and their properties are indicative of the recent technological developments in the field, which have already been adopted by the glazing industry Indeed, Ag-based thin films for energy-efficient fenestration are now highly optimized, and a very large number of products with specified thermal, solar, and luminous properties are available on the market Commercial metal-based coatings are prepared by sputtering, as this technique permits accurate thickness control and in-line coating of large areas It is remarkable that with the sputtering method, thickness control approaching atomic precision is feasible in a high-performance production environment and handling of glass

several breakthroughs presented in the following paragraphs [8, 30]

Deposition and optical properties of optimized ZnS/Ag/ZnS thin films for energy saving applications Thin Solid Films 306: 92, with permission

3.10.3.4.3 Use of interface layers

Interfaces are ultrathin, optically neutral layers, used to improve the coating properties: A 5 nm-thick ZnO interface (also called the

‘seed layer’) intervenes between the dielectric film and the Ag layer It is used as substrate for the Ag layer to enhance the Ag film uniformity, pushing the coalescence threshold down to 10 nm Thus, coatings with the lowest thermal emissivity and the highest

prevents oxidation of the Ag layer during the reactive deposition of the covering oxide layer It also protects the Ag film from oxygen permeation that decreases the age resistance of the coating This development was implemented industrially in the mid-1990s 3.10.3.4.4 Application of a chemically and mechanically resistant top layer

An oxynitride (such as SiNxOy) improves the mechanical and age resistance of Ag-based low-e coatings This development was industrially applied after the implementation of the dual magnetrons as a sputtering tool since the end of the 1990s

3.10.3.4.5 Development of asymmetrical coatings

Asymmetrical coatings incorporate dielectric layers with different refractive indices Compared to symmetrical layer structures, such layer systems result in higher transmittance (due to better antireflection of Ag) and possess neutral color, granting a higher market acceptance They also exhibit lower color sensitivity for individual layer thickness variations, resulting in a coating process with less waste In this field, fundamentally theoretical and practical developments were performed by Grosse et al [31] Since the end of the 1990s, the changeover from symmetrical to asymmetrical Ag layer systems has been accomplished by degrees by all low-e manufacturers

3.10.3.4.6 Development of Ag-based coatings resistant to high temperatures

Since the 1980s, Ag layer systems resistant to high temperatures have been developed for the production of bent coated car

ultrathin Ti interface layers that protected the Ag layer against oxidation during annealing Later, this knowledge was transferred to the production of heat-strengthened glass Since the end of the 1990s, heat-resistant Ag-based layer systems have been marketed for heat-strengthened architectural glass A specific characteristic of these coatings is that they are opaque and absorbing in the as prepared state (due to the presence of Ti and suboxide films) They become optically transparent and heat reflective after annealing

at high temperatures, as the opaque films are oxidized and become clear (Ti and TiOx absorb oxygen and are transformed into TiO2)

3.10.3.4.7 Development of coatings with double Ag layers

Coatings in the form of D/Ag/D/Ag/D were proposed by Berning [26], in the early 1990s It is essentially two D/Ag/D stacks put together, with the middle dielectric layer being twice as thick as the other two The layer structure also contains interface (seed and sacrificial) layers above and below the Ag films Insulating glass units with such a coating exhibit reduced heat transmittance

Figure 5) It must be stated that double Ag layer coatings are costly to produce At the moment, solar control glasses with such coatings are a trendsetter for car and architectural glazing all over the world

3.10.3.5 Conclusions – Resumé

Low-e coatings are spectrally selective thin films that add value to plain glass enabling it to perform multiple functions as part of fenestration systems: daylighting of buildings and at the same time suppression of radiative heat losses There are two broad categories of coatings: doped metal oxides and metal-based stacks The former are less expensive, they can be deposited on glass by spray pyrolysis immediately after it leaves the float line, and they are better suited for thermal insulation, in cold climates The latter comprises three to five thin film stacks that require advanced equipment for their production (such as sputtering in high vacuum) and accurate thickness control They are more expensive, but more versatile: they can be tailored on demand either for solar control

or for thermal insulation Recent advances in the glazing industry (especially in the metal-based coatings field) have led to widespread production of low-e coatings for fenestration, automotive, and architectural applications Furthermore, these coatings exhibit electronic conductivity and are being used as TCs in a multitude of devices, such as light-emitting diodes, displays, dye-sensitized and organic solar cells, smart switchable windows, and gas sensors This wide range of applications brings these films in the forefront of high technology

3.10.4 Glass and Windows

3.10.4.1 Float Glass

involves the flotation of molten glass on a bath of liquid tin, producing a perfectly flat surface on both sides The glass has no wave

or distortion and is nowadays the standard method for glass production: over 90% of flat glass produced worldwide is float glass

 

3.10.4.1.1 Manufacture

The raw materials used for the production of float glass typically consist of 72.6% sand (silicon dioxide), 13.0% soda (sodium carbonate), 8.4% limestone (calcium carbonate), 4.0% dolomite, and 1.0% alumina Another 1.0% of various additives is also present These are compounds for the adjustment of the physical and chemical properties of the glass, such as colorants and refining agents The raw materials are mixed in a batch mixing process, then fed together with suitable cullet (waste glass), in a controlled ratio, into a furnace operating at approximately 1500 °C Common flat glass furnaces are 9 m wide, 45 m long, and contain more than 1200 tons of glass Once molten, the temperature of the glass is stabilized to approximately 1200 °C to ensure a homogeneous

a delivery canal [32] The amount of glass allowed to pour onto the molten tin is controlled by a gate Once poured onto the tin bath, the glass spreads out in the same way that oil spreads out if poured onto a bath of water In this situation, gravity and surface tension result in the top and bottom surfaces of the glass becoming approximately flat and parallel The molten glass does not spread out indefinitely over the surface of the molten tin Despite the influence of gravity, it is restrained by surface tension effects between the glass and the tin The resulting equilibrium between the gravity and the surface tension defines the equilibrium thickness of the molten glass (T), given by the relation [33]:

respectively For standard soda-lime-silica glass under a protective atmosphere and on clean tin, the equilibrium thickness is approximately 7 mm

Tin is suitable for the float glass process because it has a high specific gravity, is cohesive, and is immiscible into the molten glass

pressure-protective atmosphere consisting of a mixture of nitrogen and hydrogen The glass flows onto the tin surface forming a floating ribbon with perfectly smooth surfaces on both sides and an even thickness As the glass flows along the tin bath, the temperature is gradually reduced from 1100 °C until the sheet can be lifted from the tin onto rollers at approximately 600 °C The glass ribbon is pulled off the bath by rollers at a controlled speed Variation in the flow speed and roller speed enables glass sheets of varying thickness to be formed Top rollers positioned above the molten tin may be used to control both the thickness and the width

of the glass ribbon Once off the bath, the glass sheet passes through a lehr kiln for approximately 100 m, where it is further cooled

the glass is cut by machines A block diagram of the float process is shown in Figure 6

3.10.4.2 Toughened Glass

Toughened glass is physically and thermally stronger than regular glass The principle of toughened glass relies on the fact that the faster contraction of the glass surface layer during cooling induces compressive stress in the surface of the glass balanced by tensile stress in the body of the glass as shown in Figure 7 The greater the surface stress, the smaller the glass particles that will result in case the glass is broken Surface compressive stresses increase the glass strength This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects, which could cause crack initiation For glass to be considered toughened, the surface compressive stress should be a minimum of 69 MPa For it to be considered safety glass, the surface compressive stress should exceed 100 MPa There are two main types of commercial heat-treated glass: heat strengthened and fully tempered Heat-strengthened glass is twice as strong as common glass while fully tempered glass is

Raw material silos

Compressive stressTensile stress

3.10.4.2.1 Manufacture and properties

Toughened glass is made from annealed glass via a thermal tempering process The glass is placed onto a roller table, taking it through a furnace that heats it above its annealing point of about 720 °C The glass is then rapidly cooled by forced convection with use of air drafts As a result, the external layers of the glass plate solidify earlier than the internal part Stresses are generated during this process as shown in Figure 7 An alternative chemical process involves forcing a surface layer of glass at least 0.1 mm thick into compression by ion exchange of the sodium ions in the glass surface with the 30% larger potassium ions, by immersion of the glass into a bath of molten potassium nitrate Chemical toughening results in increased toughness compared with thermal toughening, and can be applied to glass objects of complex shape [34]

Toughened glass has several disadvantages, all due to its pronounced stress pattern:

• It must be cut to size or pressed to shape before toughening and cannot be reworked once toughened The same applies to polishing the edges or drilling holes

• It is most susceptible to breakage due to edge damage as the tensile stress is a maximum there Shattering can also occur

in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (e.g., striking the glass with a point)

• Using toughened glass can pose a security risk in some situations because of the tendency of the glass to shatter completely

tempered glass is questionable Although it will resist a brick or rock, it is susceptible to sharp instruments such as ice picks or screwdrivers When attacked in this manner, tempered glass tends to crumple easily and quietly, leaving no sharp

• The surface of toughened glass is not as hard as that of plain glass and is more susceptible to scratching To prevent this, toughened glass manufacturers apply various coatings or laminates to the surface of the glass

• Tempered glass has wavy surfaces, caused by contact with the rollers This waviness is a significant problem in the manufacturing

of thin film solar cells [36]

3.10.4.3 Use of Glass in Solar Collectors

Glass plays an important role in thermal solar collectors, serving several purposes: It enables light admittance onto the collector absorber, protects the absorber from weather elements (rain, dust, etc.), and provides some insulation against heat loss from the collector front surface An optimum solar collector glazing must perform well all three functions

3.10.4.3.1 Light admittance

As can be seen in Figure 3, plain float glass exhibits high transmission in the visible and near IR, apart from a reduction in the wavelength range from 700 to 1500 nm This dip in transmittance is associated with iron compounds present in the glass, especially iron trioxide, Fe2O3 The larger the iron content of the glass, the less transparent it becomes Glass with high iron content (0.5% and above) has a greenish appearance and poor transmittance As can be seen in Figure 3, the plain glass reflectance in that range is low, thus the loss of transmittance is due to absorption This is very inconvenient from the solar collector point of view, as at this range, there is a significant amount of solar radiation that cannot reach the absorber In order to rectify this situation, glass with low iron content (about 0.02% Fe2O3) has been developed This improves the glass Tvis value from 88% (a value typical of a 6 mm-thick clear float glass, see Table 1) to 91%

Further improvement in the glass transmittance can be effected by reduction of reflectance (which is about 8% in the visible, as can be seen in Figure 3 and Table 1) There are two types of reflectance: diffuse and specular (mirror-like) The former is caused by

reflectance is caused by refractive index mismatch between glass and air (1.5 and 1.0 respectively) The use of thin films with refractive indices in between 1.5 and 1.0 and of appropriate thickness can cause the reflected light beams originating from the air/ film and film/glass surfaces to have equal magnitude and opposite phase The two beams then cancel out suppressing reflectance

reflection coatings on both sides can achieve Rvis values as low as 1% [5] However, as the production cost of such coatings is rather high, they are not very common in solar collector covers

Table 1 Mid-pane values of optical and thermal performance indicators for various types of coatings

Thermal U-value

Triple glazing (all panes 6 mm thick)

Quadruple glazing (all panes 6 mm thick)

3.10.4.3.2 Weather protection and heat loss suppression

The use of tempered (toughened) glass is common in solar collectors to withstand blizzard attacks Furthermore, appropriate edge sealing is required to prevent moisture ingress and air infiltration

As regards heat losses, glass effectively suppresses convective losses to the environment (compared to an unglazed collector) but

of glass in this respect, one could use the low-e coatings presented in the previous section However, these coatings would also reduce light admittance, as can be seen in Figure 3 A more appropriate solution is to stop the heat from being emitted from the

beyond the scope of this chapter

3.10.4.4 Windows in the Built Environment

pane on the building walls and to act as a peripheral seal

In Table 1 appear the mid-pane values of optical and thermal performance indicators for various types of glazing They have

• EN 410 for optical properties in the visible and solar part of the spectrum (Tvis, Rvis, Tsol, Rsol, Asol) and for g-value

• EN 673 for U-value

The results derived by this piece of software have been cross-checked and validated with equivalent results from the literature

The values of Table 1 are typical for state-of-the-art commercial products that abound in the market They are used to compare different glazing types that are presented in the following sections

3.10.4.5 Single-Glazed Windows

3.10.4.5.1 Clear single glazing

The simplest type of window consists of a single clear uncoated glass It provides the highest visible transmittance but exhibits large thermal losses Furthermore, such a glazing does not provide sufficient sound insulation and suffers from mist condensation Nowadays, the use of such glazing is limited to low-cost solutions or to retrofitting of windows in historical buildings that do not possess thick enough frames to accommodate double glazing

3.10.4.5.2 Tinted single glazing

Tinted glass is a normal float glass containing colorants Colored glass is an important architectural element for the exterior appearance of facades It is also used in interior decoration (doors, partitions, staircase panels, mirrors) Its production is the same as that of clear float glass apart from the addition of appropriate colorants to the standard raw materials Colorants are mostly metals, each producing a different color, depending on its nature and its concentration in the glass Some of the most common colorants and the colors they produce are the following: iron/green, brown, blue; manganese/purple; chromium/green, yellow, pink; vanadium/green, blue, gray; copper/blue, green, red; cobalt/blue, green, pink; nickel/yellow, purple; cadmium sulfide/yellow; titanium/purple, brown; cerium/yellow; carbon and sulfur/amber, brown; selenium/pink, red; gold/red

visible transmittance reduces the quantity of daylight admitted indoors Its primary use in windows is therefore to reduce glare and excessive solar transmission As the reduction in light transmission is effected through absorption, such glazing exhibit high SHGCs: the absorbed radiant energy is initially transformed into heat within the glass, thus raising the glass temperature A significant amount of it is then reemitted indoors Tinted glazing allows a greater reduction in visible transmittance (Tvis) than in SHGC due to reemission, as can be seen in Table 1 In a practical situation, transmittance in the visible and SHGC are required to increase (winter, cold climates) or to decrease (summer, hot climates) simultaneously by a similar degree Thus, single-tinted glazings are far from achieving optimum performance To rectify this situation, other, more appropriate solutions have been developed such as spectrally selective coatings with light blue/green tint having higher visible performance and lower SHG (glazing no 13 in Table 1 is one such example)

3.10.4.5.3 Reflective single glazing

A reflective coating can be added to glass to increase the reflectivity of its surface, in order to achieve a considerable reduction in solar gains The reflective coating usually consists of thin metallic or metal oxide layers, and comes in various metallic colors such as bronze, silver, or gold The SHGC varies with the thickness and reflectivity of the coating, and its location in the glazing system While some reflective coatings must be protected by sealing in cavity (e.g., those based on noble metals), others are durable and can

be added on exposed surfaces It can be seen in Table 1 that a reflective coating changes very little the U-value of single glazing that is dominated by convection (between glass and the surrounding air) and conduction (through the glass) Similar to that of tinted glass, the visible transmittance of reflective glass declines more than its SHGC Architects are generally fond of reflective glazing because of its glare control and appealing outside appearance However, the usage is limited by its sun mirror effect that may cause disturbances on traffic roads and nearby buildings Also in well-illuminated rooms, the loss of visual privacy and outside views at night can be a concern to the occupants

3.10.4.5.4 Low-emittance single glazing

Low-e coatings can be added to float glass to achieve either solar control or thermal insulation The materials and properties of low-e coatings have already been covered extensively The performance of single low-e-coated glazing depends on the position of the coating (indoors or outdoors) The correct placement of the coating is indoors in order to suppress long-wave radiative heat losses to the environment In that configuration, the heat emitted from indoors is reflected back into the room Otherwise (e.g., with the low-e coating facing outdoors), the heat would have been absorbed by the glass, raising its temperature, which would have caused

an increase of convective heat losses The glass thickness also plays a role in the performance of such a glazing, especially in SHGC

conjunction with double glazing The market share is over 30% of the fenestration products installed in the United States [13] 3.10.4.5.5 Self-cleaning single glazing

Self-cleaning glazing [43, 44] have an additional coating on their external surface that is exposed to weather elements and gets dirty

Under UV light irradiation, electrons and holes are produced in the conduction and valence band of the film, respectively, in

the film surface as follows: The holes oxidize water molecules to hydroxyl radicals and the electrons react with oxygen to form peroxide radicals, which in turn can react with electrons and protons to produce hydrogen peroxide These highly reactive radicals

Distance (mm)

was first reported by Fujishima and Honda [45] Furthermore, TiO2 thin films possess another interesting property: they are water repellent Rainwater that lands on the glass surface does not adhere on it Instead, droplets are formed that drip down the window washing away the decomposed dirt particles Hence, the self-cleaning action works in two stages: breakdown of dirt under UV irradiation and removal of dirt by the rainwater Self-cleaning glass is particularly suited for highly glazed, tall buildings in which glass cleaning is time consuming and costly The reduction of glass cleaning costs brought about by the use of self-cleaning glass can counterbalance (or even overcome) the increased cost of glass Nowadays, there are several glass manufacturers that produce self-cleaning products However, research is still active in this field, aiming to improve the photocatalytic efficiency by use of visible light instead of UV, by increase of the TiO2 porosity and by use of alternative materials (such as ZnO, SnO2, and CeO2) and their

transmittance, higher reflectance, and slightly lower g-value No significant effect can be observed in U-values

3.10.4.6 Multiple-Glazed Windows

Multiple panes with air-sealed cavities can be used to improve the glazing thermal insulation properties without undue reduction in transmittance and in heat gains The fabrication of multiple glazing (double, triple, quadruple) poses new challenges to the

Furthermore, the spacers must be able to accommodate the thermal stress and the differential expansion of the two (or more) glass sheets They are also required to be thermally insulating, otherwise edge losses may exceed the extra insulation multiple glazing offer To minimize heat losses through multiple glazing, one needs to reduce not only the peripheral heat losses (e.g., conduction through the spacers) but also the heat transferred through the air gap The latter can be caused either by thermal conductance of the air (or gas) through an unduly short gap or by convection in case the gap is too large An optimum of the gap width can be found at

loss is minimized Inert gases have a similar behavior: Ar has an optimum similar to air (14 mm) The optimum for Kr and Xe is 10 and 6 mm, respectively [46] The optimum distance depends on the thermophysical properties of the various gases (e.g., thermal conductivity, thermal diffusivity, and viscosity) Different types of multiple glazing are presented next

3.10.4.6.1 Double glazing

As can be seen in Table 1, the U-value of a double glazing with two clear panes is 49% lower than that of a similar single pane window The improvement of the window thermal properties comes at a price of a 12% reduction in SHGC, which is more than acceptable In situations where further reduction of solar gains is desirable, tinted and reflective glass can offer a decrease in g-value between 30% and

convective heat transport in a glazing cavity as a function of distance between panes for air and inert gases Adapted from Manz H (2008) On minimizing heat transport in architectural glazing Renewable Energy 33: 119–128, with permission

60% As with single glazing, the U-value of double-glazed windows with tinted or reflective glass are not affected considerably Low-e coatings, on the other hand, further improve thermal insulation, halving the clear double glazing U-value Depending on the type of low-e coating, different properties can be obtained (e.g., solar control, with suppressed solar gains, or thermal insulation with high solar gains), each appropriate for a different climate type and application The placement of low-e coating on the window assembly plays a marginal role on the overall window properties Usually these coatings are placed within the air gap for protection reasons The outdoors pane is preferred by glass manufacturers to be the coated one, with the indoors pane being clear glass, as this configuration is more efficient in blocking thermal radiation from the inside (in a similar way as in the case of single glazing but less pronounced) Using two coatings on both panes is not favored by glass manufacturers Indeed, as follows by comparison of windows 10 and 15 of Table 1, the use of two low-e coatings instead of one causes a 23% reduction in SHGC and an 11% reduction in U-value, which are small compared to the increase of the window cost, which could be 1.5 times higher or more Of the double glazing appearing in Table 1, the most effective ones in terms of thermal insulation seem to be those with the solar control coatings

Further reductions in U-values are possible with use of a less conductive and more viscous inert gas Manufacturers use Ar, Kr, Xe, and mixtures of these as filling gases They are nontoxic, nonreactive, clear, and odorless As can be seen in Table 1, the effect of

Thus, highly insulating windows benefit more from the use of inert gases, as the relative change in U-value is larger The biggest problem with inert gases is that their retention in the glazing is questionable As with all gases, they tend to diffuse through the seals and to escape through microcracks in the sealing materials Keeping the gas within the window unit depends largely on the quality

of the design and construction, materials in use, and assembly, particularly the sealing techniques As a result of all these

3.10.4.6.2 Triple and quadruple glazing for ultrahigh thermal insulation

In heating-dominated climates with extremely low temperatures, the U-values of double glazing are not low enough to ensure acceptable thermal losses of buildings In these environments, triple and quadruple glazing are used, having U-values down to

high-tech triple and quadruple glazing, Kr or Xe are used as the filling gases to reduce the overall width of the window These gases allow placement of the glass panes at shorter distances (see the inset of Figure 8)

3.10.4.7 Window Frames

Frames are very important in glazing systems Not only do they hold windows in place but they also act as a peripheral seal The thermal properties of the frame play a significant role in the overall U-value of the window Nowadays, a variety of window frames is available, ranging from wood (the material used traditionally) to aluminum, plastics, and various other composite materials Next,

3.10.4.7.1 Aluminum

Aluminum is a light, strong, and durable material that can be easily extruded into complex shapes Aluminum frames are available

in anodized and factory-baked enamel finishes that are extremely durable and require virtually no maintenance However, aluminum as a window frame material is not very efficient in terms of thermal insulation As a metal, it exhibits high thermal conductance, greatly raising the overall U-value of a window unit For this reason (and also for economizing on materials), the aluminum frame profiles are hollow with complex shapes, in an effort to create air enclaves

In hot climates, where solar gain suppression is often more important than heat losses, improving the insulating value of the frame can be much less important than using a higher-performance glazing system In cold climates, on the other hand, a simple aluminum frame can easily become cold enough to condense moisture or frost on the surfaces of window frames Even more than the problem of heat loss, the condensation problem has led to the development of better insulating aluminum frames The most

components into interior and exterior pieces and use a less conductive material to join them Current technology with standard

3.10.4.7.2 Wood and wood composites

The traditional window frame material is wood, because of its availability and ease of milling into the complex shapes required to make windows Wood is favored in many residential applications because of its appearance and traditional place in house design

disadvantages of wooden frames are that they are not as durable as other materials and require maintenance (with paint or lacquer)

to last longer

A variation of the wood-framed window is to clad the exterior face of the frame with either vinyl or aluminum, creating a permanent weather-resistant surface Clad frames thus have lower maintenance requirements, while retaining the attractive wood finish on the interior

Alternatively, composite wood products, such as particle board and laminated strand lumber, in which wood particles and resins are compressed to form a strong composite material have also been used to produce window frames that have similar thermal