Advances In Glazing Materials Essay, Research Paper

Table of Contents

Section Page

List of Figures 2

Introduction 3

Definitions 4

Current Options In Widow s Energy Efficiency 5

Understanding How Energy Moves Through Windows 5

Different ways Energy Travels 5

How is Energy in Windows Measured 6

R-values / U-values 6

Types Of Glazings In Windows 6

Low-e Glazing 7

Spectrally Selective Coatings 7

Heat-Absorbing Glazings 8

Reflective Coatings 8

Tomorrow s Options for More Efficient Windows 8

Superwindows 8

Summary/Conclusions 10

Recommendations 11

References Cited 12

List of Figures

Figure Page

1. How energy flows through windows: Radiation 6

2. How energy flows through windows: Convection … .6

3. How energy flows through windows: Conduction .7

4. Three Routes to Switchable Windows. 10

Introduction

Until recently, clear glass was the primary glazing material used in windows. Although

glass is durable and allows a high percentage of sunlight to enter buildings, it has very little

resistance to heat flow. During the past two decades, though, glazing technology has

changed greatly.

Research and development into types of glazing have created a new generation of materials

that offer improved window efficiency and performance for consumers. While this new

generation of glazing materials quickly gains acceptance in the marketplace, the research

and development of even more efficient technologies continues.

Definitions

Gas Fill A heavier-than-air gas such as argon or krypton is used to fill the space between panes to slow heat transfer.

Glazing The glass and/or plastic in a window unit that provides visibility yet blocks air leakage and some of the heat flow.

Infrared Radiation Invisible radiation that humans perceived as heat.

Low-e Coating Low-emissivity (low-e) coatings on glass surfaces reflect heat energy, but transmit visible light.

Pyrolytic (Hard Coat) Low-e Durable metal oxides that are fused into the surface of window glass.

Sputtered (Soft Coat) Low-e A coating on the inside pane of window glass in a sealed unit. It is made of reflective metal deposited in a vacuum.

R-Value A measure of a window s resistance to heat flow.

Total Solar Transmittance The total amount of all light spectra that is admitted by the window glazing.

U-Value The amount of heat transmitted by the window.

Center Of Glass R-Value/U-Value Performance of a window measured through the center of the glass only; not the entire unit.

Unit R-Value/U-Value A measure of thermal resistance/ heat transmittance for an entire window, including the frame.

Visible Light Transmittance A measure of the amount of that portion of the total solar radiation visible to the human eye.

Understanding How Is Energy Moves Through Windows

Different Ways Energy Travels

Understanding how windows work and how energy travels through them is the first step necessary to comprehend this concept of windows and energy. Windows start with the glass, or glazing. According to Krahn glazings do three basic things: They light in, they let you look out, and they isolate indoor environment from outdoors. (Wilson, p.94). Glass is transparent to sunlight, in other words short wavelength radiation. Glass absorbs radiant heat, or long wavelength infrared radiation, warming up in the process. (Wilson, p.94). The glass radiates this heat both inside and outside.

Figure 1. How Energy flows through windows: Radiation.

Figure 2. How Energy flows through windows: Radiation.

Figure 3. How Energy flows through windows: Conduction.

How is Energy in Windows Measured

R-Value / U-Value

Manufacturers usually represent the energy efficiency of windows in terms of their U-

values (conductance of heat) or their R-values (resistance to heat flow). If a window’s R-

value is high, it will lose less heat than one with a lower R-value. Conversely, if a window’s

U-value is low, it will lose less heat than one with a higher U-value. In other words, U-

values are the reciprocals of R-values (U-value = l/R-value).

Usually, window R-values range from 0.9 to 3.0 (and U-values range from 1.1 to 0.3), but

some highly energy-efficient exceptions also exist. When comparing different windows,

you should ensure that all U- or R-values listed by manufacturers: (1) are based on current

standards set by the American Society of Heating, Refrigerating, and Air-Conditioning

Engineers (ASHRAE), (2) are calculated for the entire window, including the frame, and

not just for the center of the glass, and (3) represent the same size and style of window.

Types of Glazings In Windows

Today, several types of advanced glazing systems are available to help control heat loss or

gain. The advanced glazings include double-and triple-pane windows with such coatings as

low-emissivity (low-e), spectrally selective, heat-absorbing(tinted), or reflective; gas-filled

windows; and windows incorporating combinations of these options.

Low-e Glazings

Low-e glazings have special coatings that reduce heat transfer through

windows. The coatings are thin, almost invisible metal oxide or semiconductor films that

are placed directly on one or more surfaces of glass or on plastic films between two or more

panes. The coatings typically face air spaces within windows and reduce heat flow-between

the panes of glass.

When applied inside a double-pane window, the low-e coating is placed on the outer surface

of the inner pane of glass to reflect heat back into the living space during the heating season.

Low-e films are applied in either soft or hard coats. Soft-coat low-e films degrade when

exposed to air and moisture, are easily damaged, and have a limited shelf life, so they are

carefully applied by manufacturers in insulated multiple-pane windows. Hard low-e

coatings, on the other hand, are more durable and can be used in add-on (retrofit)

applications. But the energy performance of hard-coat low-e films is slightly poorer than

that of soft-coat films. Windows manufactured with low-e films typically cost about 10% to

15% more than regular windows, but they reduce energy loss by as much as 30% to 50%.(Energy Design, 1990)

Although low-e films are usually applied during manufacturing, retrofit low-e window films are also widely available for do-it-yourselfers. These films are inexpensive compared to

total window replacements, last 10 to 15 years without peeling, save energy, reduce fabric

fading, and increase comfort.(Wilson, 1993)

Spectrally Selective Coatings

Spectrally selective (optical) coatings are considered to be the next generation of low-e technologies. These coatings filter out from 40% to 70% of the heat normally transmit through clear glass, while allowing the full amount of light to be

transmitted. Spectrally selective coatings can be applied on various types of tinted glass to

produce “customized” glazing systems capable of either increasing or decreasing solar gains

according to the aesthetic and climatic effects desired.

Computer simulations have shown that advanced glazings with spectrally selective coatings

can reduce the electric space cooling requirements of new homes in hot climates by more

than 40%. Because of the energy-saving potential of spectrally selective glass, some utilities

now offer rebates to encourage its use.(Energy Design 1990)

Heat-Absorbing Glazings

Another technology uses heat-absorbing glazings with tinted coatings to absorb solar heat gain. Some heat, however, continues to pass through tinted windows by conduction and reradiation. But inner layers of clear glass or spectrally selective coatings can be applied with tinted glass to further reduce this heat transfer. Heat- absorbing glass reflects only a small percentage of light and therefore does not have the mirror-like appearance of reflective glass.

Gray- and bronze-tinted windows reduce the penetration of both light and heat into

buildings in equal amounts (i.e., not spectrally selective) and are the most common tint

colors used. On the other hand, blue- and green-tinted windows offer greater penetration of

visible light and slightly reduced heat transfer compared with other colors of tinted glass.

When windows transmit less than 70% of visible light, plants inside could die or grow more

slowly. In hot climates black-tinted glass should be avoided because it absorbs more light

than heat.(Wilson 1993)

Reflective Coatings

Like black-tinted coatings, reflective coatings greatly reduce the transmission of daylight through clear glass. Although they typically block more light than heat, reflective coatings, when applied to tinted or clear glass, can also slow the transmission of heat. Reflective glazings are commonly applied in hot climates in which solar control is critical; however, the reduced cooling energy demands they achieve can be offset by the resulting need for additional electric lighting.(Wilson, 1993)

Tomorrow s Options for More Efficient Windows

Superwindows

“Superwindows” now coming on the market can attain high thermal resistance by combining multiple low-e coatings; low-conductance gas fills; barriers between panes, which reduce convective circulation of the gas fill; and insulating frames and edge spacers.

Also, optical properties such as solar transmittance can be customized for specific climate

zones. The heat from even a small amount of diffuse winter sunlight will convert these

super-windows into net suppliers of energy. This first generation of superwindows now

available have a center-of-glass R-value of 8 or 9, but have an overall window R-value of

only about 4 or 5 because of edge and frame losses.(Skerrett, 1993)

Also under development are chromogenic (optical switching) glazings that will adapt to the

frequent changes in the lighting and heating or cooling requirements of buildings. These

“smart windows” will be separated into either passive or active glazing categories.

Passive glazings will be capable of varying their light transmission characteristics according

to changes in sunlight (photochromic) and their heat transmittance characteristics according

to ambient temperature swings (thermochromic). Active (electrochromic) windows will use

a small electric current to alter their transmission properties.

Figure 4. Three Routes to Switchable Windows

Lawrence Berkley Laboratory s Selkowitz thinks that electrochromics should be far easier to sell than low-e, or emissivity, windows, which already appear in more that 25 percent of the new commercial and residential windows. He calls low-e glazing an invisible technology that you just have to trust is working.

Summary/Conclusion

No one type of glazing is suitable for every application. Many materials are available that

serve different purposes. Many factors can determine whether one should be looking to buy the best or cheaper range of insulated windows. The most significant factors should be:

1. Your microclimate: One might live in a part of the country that is generally warm, but your house might be in a cold spot-for example, on the noth side of a hill where is it always windy.

2. How much does energy cost: in Califiornia for example where the rate for electricity per kilowatt-hour will be the most expensive in the nation, one can justify spending a lot more on energy efficient windows than if you live in Colorado where is cheap.

3. How long do you plan to live in your home?: Good efficient windows are a big investment. And if you plan to live in your house for a long time then it will save you a lot of money in the long run, but if you only plan to live in a house for a couple of years only, then the price is hard to justify.