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This thesis is based on workconducted within the interdisciplinarygraduate school Energy Systems. Thenational Energy Systems Programmeaims at creating competence insolving complex energy problems bycombining technical and social sciences. The research programme analysesprocesses for the conversion, transmission and utilization of energy, combinedtogether in order to fulfill specific needs.

The research groups that participate in the Energy Systems Programme are

the Department of Engineering Sciences at Uppsala University, the Division

of Energy Systems at Linköping Institute of Technology, the Department

of Technology and Social Change at Linköping University, the Division

of Heat and Power Technology at Chalmers University of Technology in

Gothenburg as well as the Division of Energy Processes at the Royal Institute

of Technology in Stockholm.

http://www.liu.se/energi

List of Papers

This thesis is based on the following papers, which are referred to in the text

by their Roman numerals.

I Simulations of the energy performance of smart windows based on userpresence using a simplified balance temperature approachJonsson A. and Roos A.

submitted to Energy & Buildings, 2009

II Evaluation of control strategies for different smart window combinationsusing computer simulationsJonsson A. and Roos A.

Solar Energy, 2009, in press

III Visual and energy performance of switchable windows with antireflectivecoatingsJonsson A. and Roos A.

accepted for Solar Energy, 2009

IV The effect on transparency and light scattering of dip coated antireflectioncoatings on windowsJonsson A., Roos A. and Jonson E.K.

submitted to Solar Energy Materials & Solar Cells, 2009

V Optical characterization of anisotropically scattering samples using anintegrating sphere in combination with a diffuse filmJonsson A., Jonsson J.C., Nilsson A.M. and Roos A.

in manuscript, 2009

VI Optical characterization of fritted glass for architectural applicationsJonsson J.C., Rubin M.D., Nilsson A.M., Jonsson A. and Roos A.

Optical Materials, 2009. 31(6): p. 949-958.

VII Investigation of side shift and side losses of surface scattering samplesJonsson A. and Roos A.

submitted to Applied Optics, 2009

Reprints were made with permission from the publishers.

My contributions to the appended papers

I Simulations of the energy performance of smart windows based on userpresence using a simplified balance temperature approachMethod development and most of the writing.

II Evaluation of control strategies for different smart window combinationsusing computer simulationsAll simulations and most of the writing.

III Visual and energy performance of switchable windows with antireflectivecoatingsAll experimental work, all simulations and most of the writing.

IV The effect on transparency and light scattering of dip coated antireflectioncoatings on windowsSome of the sample preparations, all measurements and most of the writ-

ing.

V Optical characterization of anisotropically scattering samples using anintegrating sphere in combination with a diffuse filmMost of the writing and of the measurements.

VI Optical characterization of fritted glass for architectural applicationsSome of the measurements.

VII Investigation of side shift and side losses of surface scattering samplesAll of the measurements and most of the writing.

Other Publications

i Simulations of energy influence using different control mechanisms forelectrochromic windowsJonsson A. and Roos A.

In proceedings of World Sustainable Buildings, 2008, Melbourne, Aus-tralia.

ii Evaluation of the energy efficiency of smart windows with electrochromicglazing and external shading devices using different control strategiesJonsson A. and Roos A.

In proceedings of ISES - Solar World Congress, 2009, Johannesburg,South Africa.

iii Active versus passive solar heating in residential buildingsJonsson A. and Roos A.

In proceedings of Northsun, 2004, Vilnius, Lithuaniaiv Belagd folie reglerar inflöde av solenergi

Jonsson A.

Husbyggaren, nr 3, 2007v Framtidens smarta fönster

Jonsson A.

Presented at Energitinget, 2007, Stockholm, Swedenvi Hushåll med solvärme - ett svenskt pilotprojekt i Anneberg

Jonsson A., Lundh M. and Löfström E.

Program Energisystem förlag, Linköping, 2005vii Method for direct measurement of sample absorptance using the

reflectance port of an integrating sphere.Jonsson A. and Roos A.

In manuscriptviii Homogenisation of scattered light in integrating spheres - a way of reduc-

ing errors caused by port edge lossesJonsson A. and Roos A.

In proceedings of Colloquium Optische Spektrometrie, 2004, Berlin, Ger-many

ix Visual transmittance and energy performance of smart windows with anti-reflective coatingsJonsson A. and Roos A.

In proceedings of Eurosun, 2006, Glasgow, Scotlandx Antireflective coatings on different window surfaces

Jonsson A. and Roos A.

In proceedings of the International Conference on Coatings on Glass andPlastics, 2006, Dresden, Germany

xi Applications of coated glass in high performance energy efficient windowsRoos A., Jonsson A. and Nilsson A.M.

In Proceedings of Glass Performance Days, 2009, Helsinki, Finland

Contents

Det är ingen ordning på allting,

man hittar inte vartenda dugg.

Pippi Långstrump

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.1 Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.2 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1 History of glass and windows . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 Window types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3 Window physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.3.1 Solar spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.3.2 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3.3 Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.3.4 Low-e windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.3.5 Solar control windows . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.3.6 Two-way mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.4 Smart windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.4.1 Control strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.4.2 Smart window technologies . . . . . . . . . . . . . . . . . . . . . . . 25

2.4.3 Electrochromic foil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3 Antireflective treatment using dip-coating . . . . . . . . . . . . . . . . . . . . . 27

3.1 Physics behind antireflection coatings . . . . . . . . . . . . . . . . . . . . 27

3.1.1 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.2 Single layer interference coating . . . . . . . . . . . . . . . . . . . . 28

3.1.3 Multi layer interference coating . . . . . . . . . . . . . . . . . . . . . 30

3.1.4 Moth-eye structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2 Dip-coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2.1 Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.2.2 Plasma treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.2.3 Heat treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.3 Scratch resistance and adhesive testing . . . . . . . . . . . . . . . . . . . 32

3.4 Antireflection coatings on windows . . . . . . . . . . . . . . . . . . . . . . 33

3.4.1 Antiscattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.4.2 Antireflection treatment of smart windows . . . . . . . . . . . . 34

4 Energy simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.1.1 Verification and validation . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.1.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.1.3 Choosing a suitable model . . . . . . . . . . . . . . . . . . . . . . . . 38

4.2 WinSel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.3 Case study - Anneberg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.4 Control strategies for smart windows . . . . . . . . . . . . . . . . . . . . . 41

4.5 Comparison of smart window combinations . . . . . . . . . . . . . . . . 45

4.5.1 Cooling energy balance . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.5.2 Heating energy balance . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.5.3 Total energy balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.5.4 Antireflection coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5 Optical characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.1 Material optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.2 Diffuse and specular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.3 Measuring optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.4 Instruments for optical measurements . . . . . . . . . . . . . . . . . . . . 55

5.4.1 Optical components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.4.2 Goniophotometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.4.3 Bidirectional scattering distribution function . . . . . . . . . . . 57

5.5 Integrating spheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.5.1 Double beam instruments . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.5.2 Single beam instruments . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.5.3 Error sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

7 Summary in Swedish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.1 Introduktion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.2 Antireflexbehandling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.3 Energisimuleringar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

7.4 Optisk karakterisering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

8 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Glossary

absorptance The fraction of incident light that is taken up by the matter, nei-

ther transmitted nor reflected.

antireflection Antireflection coatings are a type of optical coating, commonly

used on eye-glasses and LCD screens, reducing the reflection from the

surface and thereby also increasing the transmittance.

diffuse Diffuse solar radiation reaches earth after first being scattered by

clouds or through the atmosphere. Also a surface can be diffuse, for

example a white paper that reflect incident light at various angles, in

opposite to a mirror.

dip-coating A wet chemical process which can be used to deposit thin coat-

ings on surfaces. The substrate is immersed and slowly withdrawn from

a solution of the coating material, which then is let to dry, and also often

annealed.

electrochromic Materials, which reversibly change transmittance due to an

applied voltage.

float process The glass manufacturing used today. Molten glass floats on on

molten tin and produces glass with a mirror-like reflection without any

distortion.

g-value A measure of the total radiation from the sun entering through the

window, directly transmitted radiation plus the fraction of absorbed ra-

diation entering the room. Also referred to as the solar factor, solar gain

or solar heat gain coefficient, SHGC.

integrating sphere A hollow cavity used for measuring the total light inten-

sity independent of angular distribution.

11

interference Light interference is when two or more light rays interact with

each other. This interaction can be either constructive or destructive,

resulting in a wave having higher or smaller amplitude, respectively.

low-e window Low-emitting window – A window, suitable in cold climates,

with a coating on one or more of its glass surfaces preventing heat from

being transferred.

monochromator An optical component for selecting a narrow band of wave-

lengths chosen from a wider range of wavelengths, i.e. for example se-

lecting green light from the light of a light bulb.

radiation Heat transfer through scattering of particles and/or electromagnetic

radiation, usually infrared radiation.

reflectance The fraction of incident light that is reflected by an object.

refractive index A measure of how much the speed of light is reduced in a

material. This affects the trajectory of the light.

scattering Light scattering occurs when light deviates from a straight trajec-

tory – this usually happens at the rough surface of an object.

smart window For smart windows, suitable in warm and/or varying climates,

the transmittance of daylight can be regulated.

sol-gel Short form of solution-gelation, which is a chemical solution of solid

particles dispersed in a solvent.

solar control window A window, suitable in warm climates, with a coating

on one or more of its glass surfaces that reduces the invisible parts of

the solar radiation to pass through the window.

spectrophotometer Measurement equipment for conducting optical

measurements at several wavelengths of light.

transmittance The fraction of incident light that passes through an object.

U value Measure of the conductance of a material or object. Indicates how

much heat, that is transferred through a wall or window, measured in

W/m2K.

12

1. Introduction

It’s not that I’m so smart;

it’s just that I stay with problems longer.

Albert Einstein

The emergence of the global warming has been addressed by several organiza-

tions [1–3]. The International Energy Agency predicts that oil will remain the

leading energy source for years to come, but the era of cheap oil is over [4].This addresses the importance of energy savings and in Europe the building

sector offers the largest single potential for energy efficiency according to a

United Nations report [1].

The importance of buildings as a significant part of the energy system has also

been addressed in several other studies [5, 6]. The operational phase of the

building is the most important. As much as 85% of the building’s energy use

occurs during this phase and only 15% during the construction phase [7, 8] and

it has also been recognized that windows are the weakest parts in the building’s

energy systems [1]. Therefore an energy perspective arises.

Windows can be studied from several other points of view. Windows are in-

stalled in buildings mainly to give outside view and daylight. Therefore an

optical perspective is of interest. Optical measurements are important for quan-

tifying product properties for comparisons and evaluations. It is important that

new measuring routines are simple and applicable to standard commercial in-

struments and it is important to avoid different systematic error sources for

optical measurements.

Coating windows in different ways to reduce heat radiation and/or the invisible

parts of the solar radiation is common. Low-emitting windows, suitable in cold

climates have a coating on one or more of its glass surfaces preventing heat

from being transferred through the window. Solar control windows, suitable in

warm climates have one or more coatings blocking both heat radiation and the

invisible parts of the solar radiation.

13

Smart windows having switchable electrochromic glazing are suitable in warm

and/or varying climates since the transmittance of daylight can be regulated to

a comfortable level. When nobody is present a smart window can be regulated

to a state which is optimized from an energy perspective. How to control such

windows is an important issue for these products to be accepted by users and

also to reduce the energy consumption of buildings.

The number of coatings on modern windows together with the number of panes

being used, can reduce the transmittance of visible light through the window.

Another type of glass coating, antireflection coatings, can instead increase the

transmittance of visible light. There are several ways to deposit an antireflec-

tion coating. In this thesis a dip-coating technique was used to put antireflec-

tion coatings on glass and plastics.

It is easy to forget the most important factor in all the technical details, namely

the user. The perception of a smart window, for example, is not determined

by whether the transmittance is one percent higher or lower, or if it reduces

energy consumption by 10 or 11 percent, but above all the control system.

Does the user experience that the window is in dark and in bright state when it

is desired? Is it possible to change the state manually and is it sufficiently easy

to change? How is the color from the window experienced? It is not always

certain that the measured data and the way the user experiences the window

are consistent.

14

1.1 Aims

The aims of this PhD project have been threefold:

• Improve the optical performance of windows through depositing antireflec-

tion coatings on glass and plastic surfaces using dip-coating. This method

can be used as a cost effective way of improving the visual transmittance of

windows and the energy performance of other solar energy components.

• Establish the potential of smart windows, whose transmittance can be reg-

ulated. Develop the tools necessary to evaluate how smart windows should

be controlled to both be accepted by users, let in daylight, avoid glare andsave energy?

• Improve the methods used for characterization of surfaces for solar energy

applications, i.e. optical measurements. In particular measurements of light

scattering samples using integrating spheres.

1.2 Outline

This thesis is divided into first a general background regarding windows. Then

there are three main chapters covering antireflection coatings, energy simula-

tions and optical measurements. A brief summary of the conclusions from this

thesis follows together with some suggestions for future work.

15

2. Windows

When the wind changes direction,

there are those who build walls and those who build windmills.

Chinese proverb

Awindow is an opening in a wall, roof or door to allow passage of visible light.

The origin of the word window is from Norwegian “vindauga”, from ‘vind -

wind’ and ‘auga - eye’. Many Germanic languages refer to modern windows

using derived versions of the latin word for windows, “fenestra”. A modern

window has several functions; let in light, give an outside view, act as heat and

sound insulation and might also function as part of the ventilation system of

the building.

2.1 History of glass and windows

The first “windows” were just holes in the wall. Then the holes were covered

with cloth, wood or animal hide. The story of glass started long before being

used in windows when stone-age man is believed to have used cutting tools

made of natural glass around 5000BC. The earliest man-made glass objects

are thought to date back to around 3500BC.

A major breakthrough in glass making was the discovery of glassblowing

around the first century. The long thin metal tube used in the blowing pro-

cess has changed very little since. Soon after the Romans began to use glass

for architectural purposes, with the discovery of clear glass around 100AD.

The production of sheets of glass evoluted during the 11th century. By blowing

a hollow glass sphere and swinging it vertically, gravity would pull the glass

16

into a pod form. The ends of the pod were cut off and the resulting cylinder cut

lengthways and laid flat.

In 1688, a new process was developed for the production of glass sheets. The

molten glass was poured onto a table and rolled out flat. After cooling, the

glass sheet was then ground using increasingly fine abrasive sands and then

polished, resulting in flat glass with good optical properties.

During the Industrial Revolution the technologies for mass production were

developed. In the end of the 19th century an automatic bottle blowing machine

were invented. The effects of different chemical elements in the glass on the

optical properties were scientifically studied. In 1905 a method to vertically

draw flat sheets of glass out of a melt were invented. This manufacturing tech-

nique made it possible to manufacture large areas of glass sheets. [9]

In the 1950s the float process, also known as the Pilkington process were

invented by Sir Alastair Pilkington. In this process molten glass “floats” on

molten tin and produces glass with a perfectly mirror-like reflection, without

any distortion. The low cost and the good optical properties of this production

method have made the window market go from single pane windows to double

pane windows. Today also three pane windows are common in certain parts of

the world.

Since the 1950s the process for glass manufacturing has not changed dramat-

ically but different coating techniques, making it possible to achieve different

window types with various thermal and optical properties, were developed,

for example low-e windows and solar control windows. One of today’s most

promising technological breakthroughs regarding windows is the switchable

glazing used in smart windows.

2.2 Window types

This thesis mainly considers the glazing part of the window and does not deal

with different construction of windows; such as fixed windows, openable win-

dows, roof windows, jalousie windows, etc. This leads to the following divi-

sion: Low-e windows, solar control windows and smart windows.

Low-e windows, suitable in cold climates, have a coating on one or more of

the glass surfaces preventing heat transfer through the window. Solar control

17

windows, suitable in warm climates, have a coating that prevents the invisible

parts of the solar radiation to pass through the window. For smart windows,

suitable in warm and/or varying climates, it is possible to change the daylight

transmittance between a light and a dark state.

2.3 Window physics

For windows there are both optical and thermal aspects that must be specified

in order to know how well they will function regarding energy and daylight.

Windows are primarily used as daylight sources and to create visual contact

with the surroundings. There are also further aspects to take into consideration,

for example durability and heat and sound insulation.

This thesis is mainly considering the energy aspects and touching upon day-

light issues of windows. To be able to compare different types of glazing from

these aspects, it is necessary to introduce a few fundamental parameters such

as thermal and solar radiation properties.

2.3.1 Solar spectrum

Solar radiation is the total spectrum of electromagnetic radiation given off by

the sun and then filtered through the atmosphere. This radiation is usually di-

vided into three major parts, UV radiation, visible radiation, commonly re-

ferred to as daylight, and infrared radiation. The solar radiation reaching us

can be divided into two further subgroups; diffuse and direct solar radiation.

Diffuse radiation reaches the earth by first being scattered in clouds or through

the atmosphere.

The process when the sun gives off energy through radiation is referred to as

emittance. Not only the sun emits electromagnetic radiation. All objects radiate

infrared radiation, referred to as blackbody radiation. The infrared region is

therefore often divided further in different ways.

One way is to denote infrared radiation from the sun as “near infrared”, NIR,

and to keep the infrared notation, IR, for infrared radiation emitted by objects

common on earth, i.e. 0 - 100 ◦C, sometimes referred to as thermal infrared.

18

The solar spectrum at sea level and the radiation emitted by a 20◦C warm ob-

ject is depicted in figure 2.1. The definition of the different wavelength regions

can also be seen in the figure.

0

25

50

75

100

Bla

ckbo

dy r

adia

tion

(W/m

2 μm)

0.4 0.7 30

500

1000

1500

2000

Wavelength (μm)

Sol

ar r

adia

tion

(W/m

2 μm)

UV Vis. NIR IR

Solar radiation, sea levelBlackbody radiation, T = 20°C

Figure 2.1: Solar spectrum at sea level together with blackbody radiation of a

20 ◦C warm object.

Solar radiation entering through the window can either contribute to the heat-

ing or generate extra cooling needs, depending on if there is a heating need or

cooling need in the building. Other aspects on the solar transmittance might

come into consideration. For example the response from a floor heating sys-

tem has been investigated in [10] showing too slow response from the heating

system.

2.3.2 Optical properties

The optical properties of a window are deduced from how the glazing interacts

with the electromagnetic radiation. Howmuch sunlight is reflected, transmitted

and absorbed in the different panes. Then there is also the possibility of re-

emittance of absorbed radiation from the window.

Windows are the most common source of daylight or visible light inside a

building. Modern windows often have two or three panes together with differ-

ent selective coatings. Each pane and the selective coatings reduce the amount

of transmitted light. This leads to reduced use of daylight and might increase

the use of artificial light and also to a darker appearance of the window. An-

other type of coating, an antireflection coating, can instead increase the trans-

mittance of daylight leading to a brighter window. The AR coating reduces

19

the reflectance from the outside. This is particularly important on the outside

since glare should be avoided and also the color rendering is important from

an aesthetic point of view. The effect on the daylight factor by using antireflec-

tion coatings in windows has been studied by for example Rosencrantz [11].

Other daylight sources can be light shelves or light pipes, which are effective at

increasing the light level at a further distance from the wall and windows. [12]

The optical properties of a window are commonly summarized in transmit-

tance of daylight, Tvis, transmittance of all solar radiation, Tsol and the g-value.

The g-value or solar heat gain coefficient, SHGC, is a measure of the total ra-

diation from the sun entering through the window, i.e. directly transmitted plus

the fraction of absorbed radiation entering the room.

2.3.3 Thermal properties

The thermal properties of the window are independent of the optical proper-

ties, and are therefore sometimes referred to as “dark values”. Heat can be

transferred in three different ways, through radiation, through convection and

through conduction.

2.3.3.1 Radiation

Radiation is heat transfer through emittance and scattering of electromagnetic

radiation, usually infrared radiation. Further reading on solar radiation and ap-

plications can be studied in [13]. For a window the radiation part of the heat

transfer can be blocked by different glass coatings in an effective way as de-

scribed in sections 2.3.4 and 2.3.5.

2.3.3.2 Convection

Convection is heat transfer caused by the collective movement of molecules or

particles in fluids. The reason that radiators traditionally are placed under the

windows is that air near the windows is heated up and rises to the ceiling. The

reason that warm air rises is that the particles in warm air moves around more

and therefore takes up more space and is less dense and therefore “lighter”.

This causes a circulation of air and cold air flows from the floor up to the

radiators and warms up. In this way the sense of draught from the windows

can be avoided. Whenever windows are not the least insulated part of the wall

20

and there is no intake of air in connection with the window there is no need to

place the radiators underneath the windows to avoid draught.

2.3.3.3 Conduction

Heat conduction is similar to electric conduction. Heat is transferred through

vibrations and energy transport by free electrons in solids and by collisions and

diffusion in the material in gases or liquids. Heat is thereby transferred with-

out the transport of any bulk material. For a window this heat transfer mainly

occurs in the frame of the window and how to calculate this heat transfer can

be found elsewhere [14].

2.3.3.4 U value

The thermal properties of a window can be summarized in one single param-

eter, the U value. The U value is a measure of the heat conductance in a ma-

terial or object and can be calculated according to the international standard,

EN673 [15]. This quantity is measured inW/m2K and a smaller number cor-

respond to a better insulating window or wall element.

21

2.3.4 Low-e windows

��

Figure 2.2: Position of the coating for alow-e window.

Low-e windows have a

low-emissivity coating on the

outer surface of the inner pane,

as illustrated in figure 2.3. Energy

radiation is absorbed in the glass

pane, but the pane does not re-emit

the radiation outwards due to

the coating. Instead most of the

radiation is re-emitted inwards.

This makes this kind of window

appropriate for cold climates with

a dominating heating season. An

ideal low-e window has high solar

transmittance to let in as much

energy as possible from the sun, according to figure 2.3. The coatings used

for this are normally based on silver, Ag, or tin oxide, SnO2.

0.4 0.7 30

25

50

75

100UV Vis. NIR IR

Tra

nsm

ittan

ce /

Ref

lect

ance

(%

)

Wavelength (μm)

TransmittanceReflectance

Figure 2.3: Ideal low-e window.

Windows with low-emissivity coatings do not transport as much heat. In a

cold climate this means that the heat from inside the building does not reach

the outer pane to such an extent that outer pane i warmed up. The outer surface

of the window might therefore become colder than the ambient air leading to

water condensation especially in mornings after clear nights, giving a diffuse

view through the window. This is by design and nothing that affects the energy

performance of the window. The visible performance can be regarded deteri-

orated, but this deterioration can be avoided with an additional coating on the

external surface [16, 17].

22

Another issue with thermally very efficient windows is the shortening of the

heating season and that the use of solar gains thus becomes smaller. Overheat-

ing problems might exist in energy efficient houses and highly glazed office

buildings. The problem of overheating shows a need for low-e windows with

also lower g-values [18].

2.3.5 Solar control windows

��

Figure 2.4: Position of the coating for asolar control window.

Solar control windows are most suit-

able in warm countries with a dom-

inating cooling season. In a simi-

lar way as low-e windows, the so-

lar control window should block all

blackbody radiation, in this case not

to let in heat. This is achieved with a

low-e coating on the inner surface of

the outer pane, see figure 2.4. Energy

absorbed in the outer pane is then

re-emitted outwards. The solar trans-

mittance should be as low as possi-

ble, as can be seen in figure 2.5, to

block as much of the invisible solar

radiation as possible. The coatings are similar to the low-e case but are usually

somewhat thicker.

0.4 0.7 30

25

50

75

100UV Vis. NIR IR

Tra

nsm

ittan

ce /

Ref

lect

ance

(%

)

Wavelength (μm)

TransmittanceReflectance

Figure 2.5: Ideal solar control window.

23

2.3.6 Two-way mirrors

��

��

��

��

��

��

Figure 2.6: The principle ofa two way mirror.

Two way mirrors, used in for example police in-

terrogation rooms, commonly seen on TV and

in crime movies consist of glazing coated with

a thin aluminum or silver layer, similar to low-e

windows. In two way mirrors a slightly thicker

metal coating is used to get a partially reflec-

tive, partially transparent glass. It is used with

a “dark” room on one side and a “light” room

on the other side. The people in the dark room

can see the person in the light room, but not vice

versa.

This technique can also be used to hide surveil-

lance cameras. The effect can be stronger with

an anti-reflective coating on the dark side. As

can be seen in figure 2.6, the reflectance can be

different depending on from which side you are

looking. With much stronger illumination in the

light room than in the dark room, the glass sur-

face appears as a mirror for a viewer in the light

room. For a viewer in the dark room the glass

pane appears to be a window. The mirror effect

can also be noted in regular windows, if the light

conditions are very different on the two sides.

2.4 Smart windows

Smart windows [19–22], also referred to as switchable windows, refers to win-

dows which can be changed between light and dark states. Smart windows can

provide dynamic illumination control of daylight [23]

A number of field tests on smart windows have been made [24, 25] and shows

that “Occupants found the electrochromic window system significantly more

desirable than the reference window, where preferences were strongly related

to perceived reductions in glare, reflections on the computer monitor, and win-

dow luminance.” Also surveys of window manufacturers have been made and

24

the researchers behind the study believes that the interest in switchable glazing

technologies among end-users will grow significantly [26].

2.4.1 Control strategies

A smart window is not smart without a clever control system and to be able

to both reduce heating needs and have comfortable levels of daylight, a well

functioning control system [27] allowing windows to be controlled individu-

ally [28] is necessary. The windows could of course be controlled completely

manually, but probably with the side effect that the energy aspect would be

missed out.

Physical presence can function as a dominating control strategy [29]. When

entering the room the windows can be regulated to let in daylight to a comfort-

able light level and create visual contacts with the surroundings. If the user for

some reason does not want the windows light, the windows can be switched

to any comfortable level. When there is nobody present the windows can be in

a state which is best from an energy perspective. If it is necessary to heat the

building the windows can be bright to let in solar radiation. If there is a cooling

need the windows can be dark to block the solar radiation from entering the

building.

The importance of the control strategy has been investigated in for exam-

ple [22], where it is stated that the control system can “balance energy effi-

ciency and visual comfort, demonstrating the importance of intelligent design

and control strategies to provide the best performance.” The control system

has been studied by many research groups from several different aspects, for

example in [30–32] and in paper II.

2.4.2 Smart window technologies

There are several different techniques available for manufacturing smart win-

dows. Electrochromic [33] devices change light transmittance in response to

a small voltage. The materials in the electrochromic device then change their

opacity. Electrochromic devices provide visibility even in their dark state. In

the dark state the windows can either absorb or reflect light.

Electrochromism is the dominating technology for switchable windows today

and a couple of companies have initiated introduction of their respective prod-

25

ucts on the market [20, 34]. Less than one year of operation of an electrchromic

window is needed to compensate for the production energy of the plain elec-

trochromic device [35].

Suspended particle devices consist of rod like particles in a fluid. When no

voltage is applied the rods have a random distribution in the liquid and can

make the light diffuse or it might absorb the light depending on the optical

properties of the rods.

Polymer dispersed liquid crystal devices, PDLCs, consist of liquid crystals in

a polymer. With no voltage applied, the liquid crystals are randomly arranged,

resulting in scattering of light. By applying a voltage the liquid crystals are

aligned and forming droplets allowing light to pass with very low levels of

light scattering.

In a wider sense there are even further techniques that can function as smart

windows. LCD used today in TV, computer and mobile phone screens can be

used and also photochromism and thermochromism. Thermochromic Tungsten

doped Vanadium dioxide, VO2, reflects infrared light when the temperature

rises over a certain transition temperature, which through doping can be made

lower than 30 ◦C [36, 37].

2.4.3 Electrochromic foil

One promising technique for achieving smart or switchable windows is to use

electrochromic materials deposited on plastics, i.e. an electrochromic foil [38].

This can be advantageous during manufacturing as it can be made in a roll-to-

roll process and for certain applications it is desireable that the foil can be bent.

The smart foil is also suitable for the retrofit market of windows, since it can

be laminated onto existing glass. This means that windows can be upgraded

with a smart foil without the need to replace the whole window.

26

3. Antireflective treatment using dip-coating

In the right light, at the right time,

everything is extraordinary.

Aaron Rose

Today’s modern windows usually consist of two or more panes, leading to four

or more glass surfaces. All surfaces introduce a reflection of light and thereby

reduces the light transmittance. An antireflection coating is a type of optical

coating, which can be applied to any surface to reduce the reflectance of the

material and thereby increase the transmittance.

This technique is commonly used on eye-glasses, LCD screens and optical

lenses. Normally these coatings are made through rather expensive methods

such as sputtering [39]. Silicon solar cells have a high refractive index which

leads to a solar reflectance of 36%. This reflection loss can be significantly

reduced with an AR coating [40]. To make such coatings commercially avail-

able for larger low-cost applications, such as windows, cheaper methods have

to be developed. For a double glazed window an antireflection coating could

increase the transmittance of visible light by as much as 15%.

3.1 Physics behind antireflection coatings

Reflectance from a material occurs when there is a sudden change of the re-

fractive index. This happens, for example, at the boundary between air and a

material. There are two ways to create materials with no reflectance at certain

wavelengths.

27

One method to get zero reflectance from a material is to have a surface with a

graded index, i.e. with no sudden change in refractive index. Another method

is to use the concept of destructive interference.

3.1.1 Interference

Interference is when two or more waves interact with each other. This inter-

action results in a superposition of waves, resulting in a new wave. If the two

waves are in phase with one another, this interaction is constructive, resulting

in a wave having higher amplitude. If the waves are out of phase the resulting

wave has smaller amplitude and the interaction is called destructive. These two

kinds of interaction are depicted in figure 3.1.

+

+

Figure 3.1: Constructive and destructive interference of two waves.

3.1.2 Single layer interference coating

The phenomenon of interference can occur when there are several simultane-

ous sources of waves. For a thin coating on a material each boundary acts as

light source of reflected light, light is reflected both in the boundary between

air and the coating and also in the boundary between the material and the coat-

ing. To get destructive interference, in the case of a single layer coating, the

optical thickness, nd, should be a quarter of a wavelength, as can be seen infigure 3.2.

The thin coating can only be a quarter of a wavelength for one single wave-length, which is called the design wavelength. Around this wavelength the

reflectance is low, but not zero. The irradiation from the sun peaks at around

550 nm, or visible green light. This wavelength almost coincides with the peak

28

n1

ns

n0

λ

λ/4I

R2

R1

T

Figure 3.2: Destructive interference in a quarter-wave single layer interferencecoating.

of the sensitivity of the human eye, as can be seen in figure 3.3. For visible ap-

plications, such as windows, 550 nm is therefore usually selected as the design

wavelength.

Hum

an e

ye s

ensi

tivity

(ar

b.)

0.4 0.55 0.70

500

1000

1500

2000

Wavelength (μm)

Sol

ar r

adia

tion

(W/m

2 μm)

UV Vis. NIR

Solar radiation, sea levelHuman eye sensitivity

Figure 3.3: Sensitivity curve of the human eye together with the solar spectrumat sea level.

The two reflections must be of equal amplitude to fully cancel each other out.

This is achieved if the refractive index of the coating is n1 =√n0ns, where n0

is the refractive index of the surrounding media and ns is the refractive indexof the substrate, according to figure 3.2. For float glass, with a refractive index

of around 1.52 surrounded by air, the optimal refractive index of the coating is

n1 =√1 ·1.52≈ 1.23.

An optical thickness of a quarter of the design wavelength gives perfect antire-

flection properties only at the design wavelength with normal incidence angle

of the light. Around this angle, the reflectance is low but not zero. This is due

29

to the fact that the optical path length is smallest for light coming in at normal

incidence and is then increased as the angle of incidence increases. To com-

pensate for the longer path lengths at higher incidence angles and get better

overall antireflection properties, the coating can be made somewhat thinner.

No solid material, which can be deposited on glass with such a low refractive

index as 1.23, can be found in nature. Magnesium fluoride, MgF2, has a refrac-

tive index of 1.38 in the visible range and is commonly used for AR coatings.

Teflon R© has a refractive index of 1.31 but is very difficult to deposit as a thin

non-absorbing film. To achieve a lower refractive index it is necessary to have a

porous structure [41] where the material is mixed with air on a subwavelength

scale [42]. Effective medium theory [43–45] can be used to describe the optical

properties of such materials. This theory is based on mathematical models that

describe macroscopic properties of materials based on properties and relative

volume fractions of the components.

3.1.3 Multi layer interference coating

A single layer interference coating can give perfect antireflection properties

for the design wavelength, but around it the antireflection coating is not as

effective. For a broader and near perfect antireflection treatment it is necessary

to put several coatings on the surface – a multi-layer stack. A multi-layer stack

has a drawback since it increases the reflectance at a further distance from

the design wavelength, while a single layer coating gives lower reflectance for

the whole spectrum. Multilayer coatings can be designed to cover the visible

range, but not the solar spectral range.

3.1.4 Moth-eye structure

A moth-eye surface can be considered a layer in which the refractive index

varies gradually from that of the surrounding material to that of the bulk mate-

rial. The name moth-eye comes from the fact that this type of antireflection was

first discovered in nature on the cornea of night-flying moths [46] by Bernhard

in 1967 [47].

The refractive index at any depth follows the effective medium theory in a

similar way as for the porous structures used in porous coatings deposited with

dip-coating. Contrary to the dip-coating layers a moth-eye structure is achieved

30

by removing material from the surface and the technique is therefore called a

subtractive method. The total reflectance of the material with the coating is the

interference of an infinite series of reflections at an infinite number of refractive

indices. For a transition over distance of λ/2 these reflections mostly interfere

destructively and reduce the reflectance. [46, 48] In theory the antireflection

properties achieved with this technology are superior to the quarter-wave de-

sign [49].

3.2 Dip-coating

Dip-coating is a wet chemical process which can be used to deposit thin coat-

ings. This process can be divided in five steps [50] as depicted in figure 3.4.

First the substrate is immersed in a solution of the coating material at con-

stant speed. The substrate is left to settle and is then withdrawn at a constant

speed while the deposition occurs. The coating is left to drain and finally dry.

Afterwards the substrate can also be baked to improve the mechanical prop-

erties [51] but this process can also degrade the optical properties to some de-

gree [52]. This wet-chemical sol-gel process creates an integrated network of

discrete particles or network polymers. The composition of the sol-gel can

vary, and some examples can be found in [53, 54].

1. Immersion 2. Start up 3. Deposition 4. Drainage 5. Drying

Figure 3.4: The different steps of the dip-coating process.

If the withdrawal speed and liquid viscosity are not high the thickness of the

coating is determined by the viscosity of the sol, the withdrawal speed, the

density of the sol and the surface tension of the sol. [40]

31

3.2.1 Cleaning

To avoid the forming of droplets on the material and thereby having uneven

coatings it is necessary to have absolutely clean surfaces. Dishwashing and

mechanical cleaning is in many cases not sufficient. Ozone cleaning is a pro-

cess where a substrate is immersed in a highly oxidative atmosphere. Organic

residues react with the ozone and are thereby removed and this enhances the

film forming process.

3.2.2 Plasma treatment

One way of improving the adhesion is surface activation and cleaning with

plasma before the coating process. This also improves the wetting of the sur-

face, which helps in the film forming process. [55]

3.2.3 Heat treatment

AR coatings on glass can be made more adhesive through heating the glass af-

ter the coating process. This heating process is limited by the material having

the lowest melting point of the materials being used. Glass, for instance, can

be heated to well above 500 ◦C, which is the softening point of glass. A prob-

lem with plastic materials is that most have a melting point at around 100 ◦Cand some only slightly higher. To avoid deformation of plastic materials very

low temperatures are necessary [56]. This temperature is not sufficient for the

coating material to create strong bondings. It is, however, not always necessary

to have good mechanical properties for an antireflection coating, for example

on the inner surfaces of an insulated glass unit.

3.3 Scratch resistance and adhesive testing

A common way of testing the adhesion of an antireflection coating is to use

a simple tape test. One method for testing the scratch resistance is to try

to scratch the surface using a pencil. Numerous methods and standards are

available for standardized scratch resistance and adhesive testing routines. [57]

32

Weathering tests are another way of testing antireflection coatings for solar en-

ergy applications [58, 59].

3.4 Antireflection coatings on windows

3.4.1 Antiscattering

Amra, et al [60] have shown that a single antireflection coating, can also be

perfectly antiscattering, i.e. no light is scattered at the surface boundary for

certain wavelengths. For the case of the glazed parts of a building a scattering

surface might be of interest to let in daylight. But for the case of windows, the

scattering of light should always be kept at a minimum to give a clear view

to the outside. Single layer interference coatings made from porous materi-

als can give also this positive side effect: Haze is a well known problem with

hard tin oxide based coatings. This is caused by the dentrific growth during the

pyrolytic process. The diffuse transmittance from a hard coated low-e glass de-

creases from 0.3% to 0.2% at 550 nmwith a single-layer antireflection coating

as can be seen in figure 3.5. The antiscattering properties could be optimized

further by having smaller silica spheres.

400 450 500 550 600 650 700 750 8000

0.005

0.01

0.015

0.02

Wavelength (nm)

Diff

use

tran

smitt

ance

With AR coating, measured on SnO2 side

With AR coating, measured on opposite sideWithout AR coating, measured on SnO

2 side

Without AR coating, measured on opposite side

Figure 3.5: The diffuse transmittance for an antireflection coated low-e glass,measured on both SnO2 side and on the opposite side.

33

3.4.2 Antireflection treatment of smart windows

Switchable glazing generally has lower transmittance of visible light than other

glazing components. Switchable window foil were coated with antireflection

coatings to study the effect on daylight transparency in the clear state, showing

an increase from 77% to 81% at 550 nm, as can be seen in figure 3.6.

400 450 500 550 600 650 700 750 8000

0.2

0.4

0.6

0.8

1

Wavelength (nm)

Tra

nsm

ittan

ce

With AR coatingWithout AR coating

Figure 3.6: Transmittance of a switchable foil with AR coating.

This increase in the visible transmittance of the electrochromic foil can in-

crease the total daylight transmittance for the whole window to such high lev-

els that the windows become more acceptable even in climates with very dark

periods, similar to the Swedish.

34

500 1000 1500 2000 25000

0.2

0.4

0.6

0.8

1

Wavelength (nm)

Tra

nsm

ittan

ce

dark

clear

EC window with 4 AR coatingsEC window with 2 AR coatingsEC window without AR coatingsSolar control window

Figure 3.7: Transmittance of a double glazed smart window with different num-

bers of antireflection coatings.

35

4. Energy simulations

Life is a flame as long as the oil lasts.

Carl Linnaeus

In energy simulations, a computer model of a building is made to investigate

how different components would function in the building before it is even built.

Whole-building simulation tools can be practical to deduce the total energy

use of a building and thereby help in selecting the most appropriate heating

and/or cooling system. To decide which windows are the most appropriate in

a building it is not necessary to perform whole-building simulations; A sim-

ulation software tool which simplifies the building, but has a more advanced

window model, can be used as a window selection tool. Such software can

also function as an energy rating tool for windows and give indications on

how well future products perform. Smart windows, see section 2.4, are not yet

well established on the market, and to be able to compare such windows with

traditional windows for different climates and weather conditions and also to

evaluate different control strategies, energy simulations are necessary.

4.1 Simulations

Examining the effects of different factors and components using real-world

studies are in many cases not practical and too costly. To avoid these obstacles,

it is possible to instead construct a computer model of the system. This sim-

plified description of a real system can provide a clearer overall picture and

provide a better understanding of a system and its properties.

By carrying out simulations, it is possible to process a large amount of data in

a relatively short time and easily change or modify the input data. Moreover,

36

it is often easy to modify the model to simulate similar systems. It also creates

the opportunity to change the physical environment of the technical system,

such as climate, geographical location and orientation. Simulation results can

also easily be used for comparisons with other simulation results and actual

measurements. The complexity of a system can be reduced by simulating sys-

tem components interconnected. A major problem can be reduced to a smaller

problem or sub-problems.

The simulation is a process of designing a model of a real system and carry out

experiments with the model. The aim can be to understand the behavior of the

system or evaluate new strategies. Computer simulations have become useful

parts of modeling many technical applications and natural systems in physics,

chemistry and biology, and also anthropogenic systems, such as in economics

and in social sciences.

4.1.1 Verification and validation

Verification is the process of determining that a computer model and simula-

tion software accurately represent the developer’s description and specifica-

tion.

To be certain that obtained results correspond to reality, it is essential to vali-

date [61] the model. This is done by comparing the results with a real system.

If a real system is not available some reasonability check should be made. It is

also possible to make a sensitivity analysis to examine how much results are

affected when parameters are varied.

4.1.2 Limitations

Even if the model is verified and validated it is not certain that the simulations

give reasonable results. There might be errors that are unknown and which do

not show in the validation cases. Also user errors and misinterpretations might

lead to incorrect information from a simulation.

A restriction on the use of simulation models can also be the lack of trans-

parency of the tool. As users of a simulation tool have limited insight in the

model it is more difficult to achieve an understanding of how the results should

be interpreted. Many simulation tools might work as closed black boxes. You

put in some data and you get some results from the software. What happens

37

in between is often not very clear. It is always preferable to have open source

simulation software or at least well documented and well tested software so

that it is possible to figure out why the calculations give the results they do.

Using a simulation model requires a relatively large basic knowledge on the

technical system and how the model works to reduce the risk of errors both in

the input of data but also in the analysis of results. A good basic knowledge of

the technical system makes it possible to estimate the correctness of the input

data and results, which increases simulation reliability and validity.

In a numerical simulation only quantifiable parameters can be taken into ac-

count. This means that other values might be lost, such as behavior and ex-

perience. There always have to be some system boundaries to the model and

only a limited number of parameters and couplings can be taken into account.

The programmer has decided on which parameters, were to be included in the

model and which were not. The programmer has also decided how many and

which variables should be possible for the user to modify. The assessment of

what is important is always subjective. Because of this, combined with the fact

that the calculations are based on simplifications, it should be noted that results

from a simulation do not give a complete picture of the real system. The re-

sults from a simulation give rather an idea of how a real system works within a

given framework and should therefore always be set in a wider context where

other aspects are taken into account.

4.1.3 Choosing a suitable model

There are several different techniques available for making energy simula-

tions [62] and which to use should of course depend on the requirements of

the user [63]. If you are interested in the air quality in a crowded building you

might want to use computational fluid dynamics [64] to be able to simulate the

air flows within the building. Some programs can do energy simulations of a

whole building while other focuses on just particular components. The choice

should be made depending on what results you would like to get.

38

4.2 WinSel

WinSel is a simulation tool to calculate the energy for heating and cooling

caused by the windows as a building component. The purpose is to be a sim-

ple tool for selecting windows. Using the window properties solar gain and

U value, different windows can be compared for a building located in a spe-

cific climate using just balance temperature and a climate data file as input.

The model that WinSel is based upon is presented in Karlsson, et al [65]. Due

to the simplicity of the program, it is suitable as a tool for selecting the right

type of window for a certain building. Meteorological input can be taken from

various sources, in this thesis data has been obtained from Meteonorm [66].

The results achieved from the program is the energy balance for the heating

season and the cooling season. The energy balance is calculated per square

meter glazing area from the equation:

Energy balance= Solar heat gain−Thermal losses.

Note that it is the energy balance per square meter glazing area that are

achieved and not the energy use per square meter floor area. In the simulationspresented in this thesis, negative values indicate that energy must be supplied

in order to heat or cool the building. Positive values imply that more energy

is gained through the windows than what is lost. The values are presented as

kWh per square meter window area.

4.3 Case study - Anneberg

During 2000 and 2001 a new residential area was built in Danderyd outside

Stockholm, Sweden, having a heating system consisting of solar heat stored in

the rock for use during winter time as space heating. Solar heating was then

complemented with electrical heating. Through this solution almost 70% of

the heating and hot water was estimated to be covered by solar energy. To

increase the delivered energy from the solar collectors a rather large south

facing solar collector area has been placed on the roofs.

Both the solar collector technique and heat storage in the bedrock are well

established, although the combination has never been used previously. The

aims for the Anneberg study were to investigate the energy solutions from a

39

broad perspective by examining how well the system functions and how the

large solar collector area on the south facing roofs affect the energy balance of

the buildings.

Large south-facing roofs covered with solar collectors reduce the available area

for south-facing windows. One aim of the Anneberg study was to investigate

how the lack of passive solar gain through south-facing windows affects the

energy balance of the buildings. The south-facing windows have a positive

total energy balance of 69 kWh per square meter window area annually. The

monthly values are shown in figure 4.1.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec−20

−10

0

10

20

30

40

50

Month

Ene

rgy

(kW

h/m

2 a)

Figure 4.1: The influence on the energy balance of a south facing window during

a year.

Installing roof windows instead of solar collectors would reduce the energy

need, according to figure 4.2. Over a whole year this would reduce the energy

need of the building by 106kWh/m2a using the windows installed in Anneberg.Replacing the windows installed by better performing windows would further

increase the energy savings with south facing windows.

The solar collector system was not fully functioning during the evaluation of

the system, but simulations show that the heating contribution from the so-

lar collectors would be around 200kWh/m2a when considering losses in thestorage system. The results show that the heating output from the solar col-

lector system is larger than what could have been achieved by installing roof

windows.

The solar collectors can provide both heat and hot water and the windows

can provide both heat and daylight to the buildings. The two components are

looked upon differently by the users. The solar collectors are seen as energy

40

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec−20

−10

0

10

20

30

40

50

Month

Ene

rgy

(kW

h/m

2 a)

Figure 4.2: The influence on the energy balance of a south facing roof windowduring the heating season.

collectors, while the windows are seen as building elements for creating a nice

living environment. This makes the contributions from the different compo-

nents hard to compare. The results from the window simulations show that

there is a potential for better energy efficiency and better indoor environment

by having larger glazed south facing window areas and also by installing larger

and better performing windows.

4.4 Control strategies for smart windows

The simulation tool, WinSel for simulating and comparing windows, have been

further developed so that the software also can simulate smart windows with

the ability to vary the g-value or the solar heat gain coefficient. The g-value

can be controlled using different control strategies, which can be based on

time control, user control and different types of daylight control. Six different

control strategies were developed to exemplify different approaches for con-

trolling smart windows. This new functionality of the software makes it easy

to compare smart windows between themselves and also to make comparisons

with static windows.

41

The following six control strategies were implemented:

EO “Energy optimization” means that the windows are always kept in the

state which is best from an energy perspective. In the simulations the

windows are kept in a dark state whenever there is a cooling need and

in a light state whenever there is a heating need.

DO “Daylight optimization” means that the windows are in a state which is

optimized from a daylight perspective. The perpendicular component

of the transmitted direct solar radiation was thus regulated by the elec-

trochromic component in the window to a maximum of 200W/m2. This

mode of the control mechanism reduces annoying glare when the sun is

low in the sky and when the solar irradiation is close to perpendicular to

the window. Solar radiation at glancing incidence angles does not turn

the window into a dark state.

O1 “Office 1” mode corresponds to having the window in “daylight optimiza-

tion” mode between 7:00 a.m. and 6:00 p.m. and otherwise in “energy

optimization” mode.

O2 “Office 2” mode corresponds to having the window in “daylight optimiza-

tion” mode during half of the time between 7:00 a.m. and 6:00 p.m. and

otherwise in “energy optimization” mode. This is a simplified way of

simulating that the office is occupied only during half of the time.

R1 “Residential 1” mode corresponds to having the window in “daylight op-

timization” mode between 6:00 a.m. and 8:00 a.m. and also between

4:00 p.m. and 10:00 p.m.and otherwise in “energy optimization” mode.

R2 “Residential 2” mode corresponds to having the window in “daylight opti-

mization” mode during half of the time between 6:00 a.m. and 8:00 a.m.

and also between 4:00 p.m. and 10:00 p.m.and otherwise in “energy op-

timization” mode. This is a simplified way of simulating that rooms in

the building are only occupied during half of the time.

The different control strategies, which can be seen in more detail in table 4.1,

can easily be modified. Over a year the time resolution of an hour is assumed

to be averaged and the simplifications of the strategies is a way to make the

results more comprehendable. Switchable windows can then be evaluated and

compared to static windows at different locations and in different buildings.

The results in figure 4.3 are for the smart window presented in table 4.2 and

for a residential building located in Denver. Since the heating season is quite

long, the energy balance for heating is strongly positive for the south facing

window. We can also see that the choice of control strategy has a consider-

42

Table 4.1: Detailed list of how the control strategies were implemented.

Weekdays Weekends

Time EO O2 O1 R2 R1 O2 O1 R2 R1 DO

06 - 07 X X X O O X X X X O

07 - 08 X X O X O X X X X O

08 - 09 X X O X X X X X X O

09 - 10 X X O X X X X X O O

10 - 11 X O O X X X X O O O

11 - 12 X X O X X X X X O O

12 - 13 X O O X X X X O O O

13 - 14 X X O X X X X X O O

14 - 15 X O O X X X X O O O

15 - 16 X X O X X X X X O O

16 - 17 X O O O O X X O O O

17 - 18 X X O X O X X X O O

18 - 19 X X X O O X X O O O

19 - 20 X X X X O X X X O O

20 - 21 X X X O O X X O O O

21 - 22 X X X X O X X X O O

22 - 23 X X X X X X X X X O

23 - 06 X X X X X X X X X O

X - Energy optimization mode

O - Daylight optimization mode

able impact on the cooling balance for east, south and west facing windows.

This is as expected, but the simulation can give quantitative estimations for

the differences. It should be remembered that the two extreme cases are not

so realistic and that the most interesting results are to be found for the mixed

control strategies. An interesting and perhaps less expected result is that the

choice of control strategy has a significant impact also on the heating energy

balance for the south facing window.

Table 4.2: Optical and thermal parameters for the window simulated.

EC state Tsol Rsol Asol Abs

outer

Abs

inner

Tvis Rvis U-value

(W/m2K)

g-

value

Light 0.51 0.11 0.38 0.26 0.12 0.68 0.13 1.6 0,63

Dark 0.09 0.08 0.83 0.80 0.03 0.11 0.08 1.6 0.17

In figure 4.4 the corresponding results are shown for the office building with

a balance temperature of 8◦C. In this case the heating season is shorter andthe cooling season is longer. The choice of control strategy is then even more

important. The difference between daylight and energy optimization strategies

is as high as 200 kWh/m2 of window area per year.

Artificial lighting can also be included in the different control strategies. In

figure 4.5, twenty watts of artificial lighting per square meter window area

was assumed. The artificial light is switched on when the total solar irradiation

43

-300

-200

-100

0

100

200

300

400

Ene

rgy

(kW

h/m

2 a)

Window configuration for each orientation

EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO

N E S W

Heating

Cooling

Total

Figure 4.3: Energy balance of a double glazed smart window in a residential

building located in Denver for different orientations and for different control

strategies as defined in table 4.1.

-400

-300

-200

-100

0

100

200

300

Ene

rgy

(kW

h/m

2 a)


EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO

N E S W

Heating

Cooling

Total

Figure 4.4: Energy balance of a double glazed smart window in an office buildingwith a balance temperature of 8 ◦C located in Denver for different orientations

and for different control strategies.

through the window is less than 300W/m2 and someone is assumed present.

Presence was following the same pattern as in table 4.1.

Depending on the time of year and on whether there is a heating or cooling

need, the artificial lighting can contribute to the heating or generate extra cool-

ing needs, in a similar way as the solar radiation [67]. The additional cooling

need and heating contribution should be compared to the corresponding val-

ues caused by solar radiation as shown in figures 4.3 and 4.4. It can be clearly

seen, in figure 4.5, that the solar contribution is around an order of magni-

44

tude larger. Note the different ordinate scale in figure 4.5 compared to figures

4.3 and 4.4. This indicates that artificial lighting is less important for the total

energy balance than solar irradiation.

-80

-60

-40

-20

0

20

40

Ene

rgy

(kW

h/m

2 a)


O2 O1 R2 R1 O2 O1 R2 R1 O2 O1 R2 R1 O2 O1 R2 R1

N E S W

Heating

Cooling

Electricity

Figure 4.5: Electricity for artificial lighting and how it affects the annual heating

and cooling of studied office and residential buildings in Denver.

4.5 Comparison of smart window combinations

The optical properties of different combinations of smart windows were cal-

culated using a combination of the Fresnel formalism and experimental data.

The international standards ISO 9050 [68] and EN673 [15] were used to cal-

culate the solar factor (g-value) and the thermal conductance (U value), re-

spectively. For the electrochromic layers, refractive indices were taken from

the Windows and Daylighting Group at Lawrence Berkeley National Labora-

tory [69]. The refractive indices were used together with Fresnel formalism to

determine the transmittance and reflectance of the complete windows that were

“constructed”.

The window surfaces are labeled 1 to 4 from the outer surface to the inner sur-

face according to common practice. Four double pane reference windows were

identified: A window without any coatings, two windows with low-e coatings

on surface 3, one with a tin oxide coating and one with a silver based coating,

and finally a window with a silver based solar control coating on surface 2.

45

Table 4.3: Optical and energy parameters for the simulated windows.Window Short name EC coating Tsol (%) Rsol (%) Asol (%) A1 (%) A2 (%) Tvis (%) Rvis (%) U (W/m2K) g-value (%)

Double pane reference DG No 69 13 18 10 7.4 80 15 2.8 75Double pane EC combination DG+EC Light 59 10 31 25 6.0 74 11 2.8 65

Dark 11 8 81 79 1.3 12 7.7 2.8 18

Low-e reference 1 LE1 No 59 16 25 11 15 74 18 1.6 71Low-e EC combination 1 LE+EC1 Light 51 11 38 26 12 68 13 1.6 63

Dark 9.3 8.2 82 80 2.8 11 7.8 1.6 17

Low-e reference 2 LE2 No 52 23 25 12 13 77 13 1.3 64Low-e EC combination 2 LE+EC2 Light 46 13 41 27 13 71 10 1.3 59

Dark 8.1 8.5 83 81 2.9 11 8 1.3 15

Solar control reference 1 SC No 34 41 25 22 2.4 63 25 1.3 38Solar control EC combination 1 SC+EC1 Light 31 40 29 22 7.0 58 23 1.3 38

Dark 5.1 41 54 22 32 9.2 24 1.3 33

Solar control EC combination 2 SC+EC2 Light 31 29 40 33 7.8 58 18 1.3 39Dark 5.1 9.5 85 84 1.4 9.2 5.3 1.3 11

Solar control EC combination 3 SC+EC3 Light 29 26 45 43 1.9 55 23 1.3 33Dark 4.7 9.8 85 85 0.3 8.5 10 1.3 10

These reference windows were then combined with electrochromic coatings

forming another set of six different windows: The uncoated double pane

window and the two low-e windows were combined with an electrochromic

coating on surface 2. The solar control window was combined with

an electrochromic coating on surface 3 and also switched so that the

electrochromic coating was on surface 2 and the solar control coating on

surface 3. In addition a solar control window with both the electrochromic

layer and the solar control layer on the outer pane was designed. The

different window combinations are summarized in table 4.3 and presented

schematically in figure 4.6. All the investigated electrochromic windows can

be manufactured with today’s known technologies.

4.5.1 Cooling energy balance

The simulations were made for three different locations: Denver, Miami and

Stockholm. Only some of the results are presented here and the rest can be

found in paper II. For Denver having both heating and cooling needs some-

where in between Stockholm and Miami the annual cooling need can be de-

creased by as much as 300 kWh per square meter window area by using a solar

control or low-e coating in combination with an electrochromic layer instead

of an uncoated double glazed window. It can be seen that the cooling need is

almost eliminated, for the window combinations with the lowest energy use,

with the optimum choice of control strategy. This is illustrated in figure 4.7.

46

Double pane reference 1 Double pane EC combination��

��

��

��

Low-e reference 1 Low-e EC combination 1

Low-e reference 2 Low-e EC combination 2

Solar control reference 1 Solar control EC combination 1 Solar control EC combination 2 Solar control EC combination 3

Figure 4.6: Figures of the different window configurations that were investi-

gated.

En

erg

y (

kW

h/m

2a

)

Optimization mode for each orientation

DG + EC

LE + EC1

LE + EC2

SC + EC1

SC + EC2

SC + EC3

Energy opt.

Office 2

Office 1

Daylight opt.

No EC

-400

-300

-200

-100

0

N E S W

Figure 4.7:Cooling energy balance of the selected window combinations in Den-ver for different orientations. Reference cases in figure 4.6 are represented by the

“No EC” case.

4.5.2 Heating energy balance

For the heating balance, as can be seen in figure 4.8, the static low-e windows

outperform the other window combinations. This is obviously due to the higher

g-value of these windows and the fact that the Denver climate is characterized

by cold winters but still a fair amount of solar radiation throughout the win-

47

ter. The g-value is thus more important in Denver than in Stockholm, and all

windows except the north facing one contributes considerably to the heating

balance. We can see that for the south facing window with the highest solar

irradiation, the control strategy actually also affects the heating energy bal-

ance. This is because there are periods also during the winter when the control

system puts the window in a state which is not optimized for highest energy

gain.

En

erg

y (

kW

h/m

2a

)


DG + EC

LE + EC1

LE + EC2

SC + EC1

SC + EC2

SC + EC3

Energy opt.

Office 2

Office 1

Daylight opt.

No EC

-200

-100

0

100

200

300

N E S W

Figure 4.8:Heating energy balance of the selected window combinations in Den-

ver for different orientations.

4.5.3 Total energy balance

For the total energy balance the cooling season becomes important and the

‘best’ windows depend on the control strategy. In “energy optimization” mode

the electrochromic low-e combinations clearly outperform the others as shown

in figure 4.9.

The windows having the lowest energy use for each of the control strategies are

shown in figure 4.10. If we shift the control strategy from “office 1” to “office

2”, the window with the lowest energy use facing north, east and west changes

from SC+EC3 to LE+EC2. This clearly indicates the importance of consid-

ering different windows and different control strategies for the electrochromic

windows depending on the building type and activity in the building.

The same simulations have also been made for other locations. For the case of

Miami, the most energy efficient way to combine a smart window is always

to have the solar control coating and the electrochromic layers on the outer

48

En

erg

y (

kW

h/m

2a

)


DG + EC

LE + EC1

LE + EC2

SC + EC1

SC + EC2

SC + EC3

Energy opt.

Office 2

Office 1

Daylight opt.

No EC

-400

-300

-200

-100

0

100

200

N E S W

Figure 4.9: Total energy balance of the selected windows in Denver for differentorientations.

En

erg

y (

kW

h/m

2a

)


DG + EC

LE + EC1

LE + EC2

SC + EC1

SC + EC2

SC + EC3

Energy opt.

Office 2

Office 1

Daylight opt.

No EC

-200

-100

0

100

200

N E S W

Figure 4.10: Total energy balance of the windows with the best energy perfor-mance in Denver for different orientations.

pane. The potential for smart windows is very large. The cooling energy can

be decreased by 200 kWh per square meter window area annually compared to

the static window resulting in the lowest cooling need, as can be seen in figure

4.11

4.5.4 Antireflection coatings

An antireflection coating, see section 3, can give higher daylight utilization in

energy efficient windows [11]. This can lead to lower energy usage since arti-

ficial lighting would not be necessary as often. The AR coating does not affect

49

Ene

rgy

(kW

h/m

2 a)


SC + EC3

Energy opt.Office 2Office 1

Daylight opt.

No EC

-500

-400

-300

-200

-100

0

N E S W

Figure 4.11: Cooling energy balance of the windows with the lowest coolingneed in Miami for different orientations.

-300

-200

-100

0

100

200

300

400

Ene

rgy

bala

nce

(kW

h/m

2 a)


EC AR LE SC DG EC AR LE SC DG EC AR LE SC DG EC AR LE SC DG

N E S W

Heating

Cooling

Total

Figure 4.12: Energy balance for double glazed windows in an office building inDenver, USA.

the emissivity properties of the window, thus the heating and cooling balance

of the window is rather unaffected by an antireflective coating. Heating and

cooling energy balance calculations have been made to analyze the energy

properties of windows with AR coatings. The results show very small differ-

ences, as can be seen in figure 4.12, which is a comparison of the windows in

table 4.4.

50

Table 4.4: Optical and energy related parameters for the different windows.Window Short name EC coating Tsol (%) Rsol (%) Asol (%) Tvis (%) Rvis (%) U (W/m2K) g-value

Low-e EC combination EC Light 49 15 36 63 19 1.6 0.59Dark 9 11 80 10 11 1.6 0.16

Low-e EC+ 2 AR coatings AR Light 54 12 34 71 12 1.6 0.63Dark 10 11 79 11 11 1.6 0.17

Low-e reference LE No 60 15 25 75 17 1.6 0.72Solar control reference SC No 34 41 25 63 25 1.3 0.38Double pane reference DG No 69 13 18 80 15 2.8 0.75

51

5. Optical characterization

There is no success without hardship.

Sophocles

Optical measurements can be used to quantify optical characteristics of ma-

terials, such as transparency, and thermal properties of a window or the color

and surface roughness of any material. These quantified numbers make it pos-

sible to compare different products and can also be used in the evaluation of

new products or as input in simulation software. For the comparisons and eval-

uations to be fare it is important that all measurements are accurate and per-

formed under similar conditions. It is beneficial if new measuring routines are

simple and applicable to standard commercial instruments.

5.1 Material optics

Material optics describes the behavior and properties of light and the interac-

tion of light with matter. This can be described by the famous Snell’s relation:

N1 sinθ1 = N2 sinθ2 (5.1)

The subscripts 1 and 2 denote the first and second medium respectively. N1 is

thus the refractive index of the incoming medium. N2 is the refractive index of

the second medium. θ1 and θ2 correspond to the angle of incidence and angleof refraction, respectively. The principle of light refraction is depicted in figure

5.1.

52

��

��

��

��

��

Figure 5.1: Refraction according to Snell’s relation.

Optically a material is characterized by its complex refractive index which

states its ability to refract and absorb electromagnetic radiation. The refraction

of electromagnetic radiation is described by the refractive index n, and the

absorptance of the radiation by the extinction coefficient k.

N = n+ ik (5.2)

5.2 Diffuse and specular

Light reflected from or transmitted through a medium can be scattered at the

surface or in the bulk of the material. This happens if the surface is not flat or

if the bulk is inhomogeneous. Macroscopic surface scattering is due to the dif-

ferent incidence angles that the different rays of light have against the medium.

Each ray of light is transmitted according to Snell’s relation.

At a flat interface between two uniform media of different refractive indices

the incident, reflected and transmitted rays are all in one plane: the plane of

incidence. The ratios of the amplitudes are defined as the amplitude reflectance

and transmittance, according to

r = ER/EA (5.3)

t = ET/EA (5.4)

53

where E is the amplitude of the electric field of the wave.

Figure 5.2: The principle of s- (left) and p-rays (right) of light.

Light interaction with matter are different for s-rays, where the oscillations are

perpendicular and p-rays, where the electromagnetic oscillations are parallel

to the plane of incidence, according to figure 5.2. The ‘s’ in s-rays are short for

the German word for perpendicular, ‘senkrecht’. Fresnel’s equations state the

ratios for the amplitudes of the reflected and transmitted rays and the incident

ray:

rp = N2 cosθ1−N1 cosθ2N1 cosθ2−N2 cosθ1

tp = 2N1 cosθ1N1 cosθ2+N2 cosθ1

rs = N1 cosθ1−N2 cosθ2N1 cosθ1+N2 cosθ2

tp = 2N1 cosθ1N1 cosθ1+N2 cosθ2

(5.5)

The angle between the ray and the surface normal is denoted θ . The intensityreflectance, R and transmittance, T, are given by the squared complex ratios as

Rp = rpr∗p and Tp = N2N1tpt∗p. For normal incidence and unpolarized light these

relations can be simplified to:

R=(N1−N2N1+N2

)2

(5.6)

T =N2N1

(2N1

N1+N2

)2

(5.7)

54

5.3 Measuring optical properties

Deducing the transmittance of a sample is made through two measurements.

One without the sample and one with the sample in place, the ratio then gives

the transmittance of the sample and the principle is the same for reflectance

according to

T = IT/IIR= IR/IR

(5.8)

The law of conservation of energy states that the sum of reflected, transmitted

and absorbed light must equal the amount of the incoming light,

R+T +A= 1 (5.9)

Reflectance and transmittance can easily be measured in this manner, while ab-

sorptance cannot be measured directly. Therefore the absorptance of a material

generally is acquired from the the measurements of R and T as A = 1−R−Tusing equation 5.9.

5.4 Instruments for optical measurements

5.4.1 Optical components

To measure the optical properties of materials a number of components that

are needed. There needs to be some kind of controlled light source, which can

be for example an incandescent lamp, a gas discharge lamp or a laser. Some

kind of detector is also needed in order to put a value on the optical properties.

This can be a photovoltaic or solar cell, a photoresistor or a photomultiplier

tube. In the most primitive case the sun can act as the light source and the eye

as the detector.

A common instrument for optical measurements is the spectrophotometer,

which is sketched in figure 5.3. A spectrophotometer measures optical prop-

erties separately at different wavelengths of the light. This corresponds to dif-

ferent colors in the visible region. To obtain a spectrum it is necessary to step

over many monochromatic (single-colored in the visible region) wavelengths.

55

This can be achieved with a grating, which reflects or transmits different colors

of the light at different angles. To select only a limited angular sector, a slit is

used. A slit is simply an opaque object with a small opening. The width of

the slit thus determines the wavelength resolution. These two components are

referred to as a monochromator. A filter is then used to reduce the second

order reflection from the grating. The filter is simply a glass or plastic material

with coatings or colored in such a way that it only transmits or reflects light of

certain wavelengths.

��

��

��

��

��

��

��

��

Figure 5.3: Optical measurement system

When measuring the optical properties of materials there are a number of dif-

ficulties that arise. The light source has to be controlled and stable. To avoid

other light sources interfering with the measurement an opaque cabinet is of-

ten used. An additional way to avoid other light sources is to use a chopper

together with a phase sensitive detector, a lock-in-amplifier. The chopper gives

a pulsed light at a specific frequency. The lock-in-amplifier can filter out all

light which is not pulsed at the specific frequency.

5.4.2 Goniophotometer

A goniophotometer, or just goniometer, is an instrument to measure light in-

tensity at various angles of the outgoing light with the detector at different

positions. A goniometer can thus be used to acquire the scattering distribution.

A schematic drawing of a goniophotometer measurement equipment can be

seen in figure 5.4

56

��

��

Figure 5.4: Schematic drawing of a goniometer equipment.

5.4.3 Bidirectional scattering distribution function

The parameter to use for describing the scattering properties is the bidirec-

tional scattering distribution function, BSDF, see figure 5.5. This parameter

describes the relation between the incident irradiance and outgoing radiance at

a specific angle. A BSDF value is generally described as dependent on incident

and outgoing angle, but can also depend on wavelength and polarization.

��

��

��

Figure 5.5: The bidirectional scattering distribution functions.

There are several subsets of the BSDF functions defined. The two describing

the distribution of transmitted and reflected light are the bidirectional trans-

mittance distribution function, BTDF, and the bidirectional reflectance distri-

bution function, BRDF, respectively.

57

5.5 Integrating spheres

Integrating spheres, also referred to as Ulbricht spheres [70], are optical com-

ponents with hollow cavities and interiors coated with a highly diffuse and

highly reflective material, usually barium sulfate, BaSO4 or Spectralon R© .

This optical component can be used to measure optical power or intensity

by collecting scattered light. Contrary to a goniometer, which measures the

scattering distribution, an integrating sphere does not provide any spatial in-

formation.

Integrating spheres are used for measuring total transmittance or reflectance

of surfaces, integrating over all angles of illumination and observation. The

inside of the sphere should ideally scatter light evenly in all directions, i.e. the

surface should be Lambertian. The build-up of an instrument using integrating

spheres can be found in [71].

5.5.1 Double beam instruments

In a double beam instrument the instrument shifts between a reference light

beam and a sample light beam to compensate for changes in sphere response,

light intensity and other deviations in the optical components. The principle of

a double beam configuration is shown in figure 5.6.

��

��

� ��

Figure 5.6: The principle of a double beam instrument. The reference and sample

beam are constantly alternating.

5.5.2 Single beam instruments

All ports that are introduced result in deviations from a homogeneous and ideal

integrating sphere. In a single beam instrument there only needs to be two ports

58

for conducting transmittance measurements, one entrance port for incoming

light and one port for the detector. On the other hand, in a single beam instru-

ment, there is no control mechanism, for example if the light intensity changes

or if the sphere throughput is changed. When placing a sample in front of any

port the sphere throughput is changed, which means there is a change in sphere

response between the reference and the sample measurement since the sample

is removed for the reference measurement.

An alternative way of doing the reference measurement is to put the sample in

front of the port but so that the light spot totally misses the sample. To get the

same sphere response for the sample measurement the same part of the port

should be covered but with the light spot fully hitting the sample, according to

figure 5.7. This procedure requires a light spot which is at least less than half

of the port in size and even smaller if scattering samples are to be measured,

since the light spot then must be far away from the sample edge and port edge.

Figure 5.7:Measuring transmittance in a single beam instrument without chang-

ing the sphere throughput. Reference measurement to the left and sample mea-

surement to the right.

5.5.3 Error sources

There are several issues when dealing with optical measurements and Clarke

and Compton have published a thorough description of many possible error

sources regarding integrating sphere measurements [72], and has also included

some suggestions on sphere design. Some other issues are: Stray-light that

passes through the chopper but still does not follow the expected beam path

can contribute to the signal [73]. The detector has to be linear. Multiple reflec-

tions can come up both in the sample and between optical components in the

measurement equipment.

59

For homogeneously or near homogeneously scattering samples the light is

scattered once already at the sample, which is equivalent to the scattering in

the sphere wall for the reference signal. This means that light is scattered by

the sphere wall one more time for the reference measurement than for the sam-

ple measurement. To compensate for this the sample signal should be multi-

plied by the reflectance of the sphere wall. [74] Many scattering samples have

a large fraction of low-angle scattered light. Regarding this compensation only

the part of the light that is high-angle scattered should be multiplied by the

reference [75, 76].

5.5.3.1 Inhomogeneously scattering samples

For inhomogeneously scattering samples the sphere response is not the same in

the sample case as in the reference case since the scattering distribution func-

tions are not the same and therefore the sphere response is not the same. One

way to avoid this can be to introduce a diffuser between the sample and the in-

tegrating sphere. This would give more or less the same scattering distribution

functions in both reference and sample case, as can be seen in figure 5.8.

−20 −15 −10 −5 0 5 10 15 200

10

20

30

40

Scattering angle (°)

BT

DF

(sr

−1 )

SampleSample + DiffuserDiffuser

Figure 5.8: Goniometer measurement showing the BTDF of a low-angle scatter-ing sample, of a diffuser and of the sample together with the diffuser.

The scattering from the sample is also depicted in figure 5.9. The main goal

of the diffuser method is to avoid losses around the sphere port edges. Instead

another issue arises that has to be taken care of for the measurement, namely

multiple reflections between the sample and the diffuser.

To show that the principle works, a clear glass sample was measured both with

and without the diffuser. The results from these measurements are presented

in figure 5.10 showing very small deviations between a regular measurement

60

Figure 5.9: Image of the scattering of laser light from the sample.

and a measurement using the diffuser method and correcting for multiple re-

flections.

350 400 450 500 550 600 650 700 750 800 8500.85

0.9

0.95

Wavelength (nm)

Tra

nsm

ittan

ce

GlassDiffCorr

Figure 5.10: Measurement of a clear glass sample. ‘Glass’ is an ordinary mea-surement. ‘Diff’ corresponds to a diffuser measurement, ‘Corr’ to a diffuser mea-

surement with correction made for multiple reflections.

A low-angle scattering sample was also measured and a regular measurement

now shows a deviating result as can be seen in figure 5.11, bottom curve. The

transmittance obtained using the diffuser method is at a similar level as the

T-sphere measurement. The T-sphere is used as “reference”, as such a sphere

can be assumed to depend less on the scattering distribution of the incoming

light since the sphere has no reflectance port and hence no port losses at such

a port.

61

Reflectance values are higher for higher incidence angles than for near normal

incident light. A compensation factor, k, has been calculated in paper V to

compensate for insufficient correction for multiple reflections between sample

and diffuser. Applying this compensation factor for the measurement shows

excellent agreement with a measurement with a T-sphere dedicated integrating

sphere.

350 400 450 500 550 600 650 700 750 800 8500.85

0.9

0.95

Wavelength (nm)

Tra

nsm

ittan

ce

DiffCorrCorr+CompT−sphereL900

Figure 5.11: Transmittance spectra for the low-angle scattering glass sampleshowing very good agreement when applying the k-factor according to paper

V.

5.5.3.2 Side shift and edge losses

Some of the scattered light from scattering samples might hit the edge of the

sample and exit in such a fashion, that it is not collected by the integrating

sphere. The detected signal from the light entering the sphere then underesti-

mates the real transmittance or reflectance of the sample.

To investigate the magnitude of this possible error a sample was gradually

moved into an integrating sphere according to figure 5.12, so that the measured

signal also included the “edge-loss”. The sample was machined into a circular

shape of a size slightly smaller than the sphere entry port and the edge was

polished. The cross section of the beam was varied during the experiments,

but was always smaller than the sample size. The sample was mounted on a

thin metal arm allowing the sample to be moved into the sphere as illustrated

in figure 5.12. The sample edge was also painted black to get a reference value

where the “edge-loss” was absorbed by the paint, as illustrated in figure 5.13

and prevented from entering the sphere.

62

Figure 5.12: Schematic illustration of how the sample was moved into the inte-

grating sphere.

Monitoring the intensity of transmitted light, while moving the sample into

an integrating sphere, shows the effect of edge-losses and provides a way of

actually measuring the intensity escaping through the edge. When the sample

gets just inside the sphere the signal intensity increases as the edge-loss gets

included in the signal. The same experiment was also performed with the edge

painted black to suppress the edge losses. The detected signal versus position

is shown in figure 5.14, for the flat surface facing the light source.

Figure 5.13: Integrating sphere measurements moving the sample into the spherewere made for the sample without modification (left) and also with sample edge

painted black (right).

The differences depending on the light spot size is small as can be seen. This

is probably due to the fact that the light spot in all cases was much smaller

than both the sample and the transmittance port. With the sample edge painted

black the signal is not going up as much when the sample is moved into the

63

sphere, as can be seen in figure 5.14. The sample thickness was 3mm, which

means the sample is fully inside the sphere at the position 3mm in the graphs.

It should be noted that the change of signal is not only an effect of edge losses

but is also caused by changes in the sphere throughput as the sample moves

deeper into the sphere. It is also an effect of some reflected light from the

sample not escaping back through the entry port, but is caught by the sphere

wall around the entry port when the sample is moved further into the sphere.

It can be seen that in the case with the clear edge the signal goes up by about

2%, when the edge is just inside the sphere port, compared to the signal for

the blackened sample.

0 3 6

0.8

0.9

1

Position (mm)

Inte

nsity Ø

beam = 7 mm

Øbeam

= 11mm

Øbeam

= 18mm

Øbeam

= 18mm B

Figure 5.14: Integrating sphere measurements moving the sample into the

sphere for different light spot diameters. Also doing this when sample edge has

been painted black – denoted ‘B’ in the legend. Light impinging on the clear

surface.

64

6. Conclusions and outlook

Trying is the first step towards failure.

Homer Simpson

Success is dependent on effort.

Sophocles

This thesis looks at one important component of a building’s energy system,

the window. This component has been looked at from four different perspec-

tives: Antireflection and switchable coatings, optical characterization and en-

ergy performance.

The fact that less daylight is transmitted through modern windows make it in-

teresting to find ways of increasing the transmittance. Antireflection coatings

have been shown to give higher light transmittance through windows without

affecting the thermal performance. For large area applications, such as win-

dows, it is necessary to use techniques which can easily be industrialized in

large scale at low cost. Such a technique is dip-coating in a sol-gel of porous

silica. Single layer antireflection coatings have been deposited on glass and

plastic materials to study visual and energy performance. It has been shown

that antireflection coatings can be a key component for achieving higher trans-

mittance in windows. This is one way of getting brighter windows that can give

higher exchange of daylight, while still not causing higher heating or cooling

needs.

Further investigations on how to increase the durability of such coatings, es-

pecially on plastic materials are needed. Regarding daylight there are many

aspects to consider. How daylight could be used more, without increasing

the heating and/or cooling need of the building, how annoying glare can be

avoided. The issue of evenly distributed light is another difficult task before

daylight can be further used to light up our homes and offices. The impact of

an increased daylighting level and how it affects the electricity usage, without

65

negatively, and preferably positively affecting the visual comfort is another

interesting field of research.

Energy simulations were used to compare different windows and the potential

for switchable or smart windows were investigated. A simulation tool, WinSel,

were extended to be able to use different control strategies for smart windows.

WinSel is a tool that can be used to evaluate and compare windows without

the need for complete building simulations using only balance temperature

and climate data as input. The results from this thesis show the potential of

the emerging technology of smart windows, but it was also shown that the

control system really is the key factor for energy efficient smart windows. It

was also shown how to optimally combine switchable glazing with static panes

in different climates and buildings.

Characterizing different glazing and plastic materials are important from both

an energy and a daylight perspective and this thesis include several efforts and

possibilities to characterize materials optically – in particular scattering sam-

ples using spectrophotometers with integrating spheres. The effect of different

scattering profiles have been investigated and several obstacles and possibili-

ties for optical measurements have been discussed. An advanced technique to

measure side shift and edge losses has been presented. An easy-to-use method

to characterize anisotropically scattering samples is also presented.

Reflectance measurements using the same principle, as described in section

5.5.3, could also be investigated. Scattering samples can give even higher er-

rors when measuring reflectance since, for instance, the reflection can have a

large component that is scattered directly out through the entrance port.

66

7. Summary in Swedish

Genom att försöka med det “omöjliga”,

når man högsta graden av det möjliga.

August Strindberg

7.1 Introduktion

Bakgrunden och syftet med detta avhandlingsarbete har varit att titta på en

viktig komponent i vårt energisystem, nämligen fönster. Fönster kan stud-

eras utifrån ett flertal vetenskapliga synvinklar. Fönster har vi främst för ut-

sikt och dagsljuinsläpp. Ett optiskt perspektiv blir därmed intressant. Fönster

är också den svagaste länken i en byggnads energisystem, varför ett energi-

perspektiv också uppenbarar sig. Att belägga fönster på olika sätt för att min-

ska värmestrålningen genom fönstret är en teknik som används idag. Fönster-

beläggningar kan även göra att genomsläppligheten eller transmittansen av

synligt ljus kan öka. Detta kan åstadkommas med en antireflexbeläggning, som

kan appliceras på glasytan på olika sätt.

7.2 Antireflexbehandling

Alla solfångare som används idag har någon form av konvektionsskydd,

som även fungerar som ett korrosionsskydd för själva absorbatorytan eller

solcellen. I regel består den av vanligt glas eller någon polymer.

67

Antireflexbehandlade glas kan även finna tillämpningar inom fönsterområdet.

I dagens fönster används ofta två eller tre rutor. I de bägge ytorna på varje

glasruta uppstår en reflex, vilket minskar genomsläppet av dagsljus. Detta kan

göra att transmittansen av dagsljus blir låg, och fönstret upplevs som mörkt.

I bägge dessa fall kan man antireflexbehandla ytorna för att minimera

reflektionsförlusterna och därigenom öka transmittansen. För ett täckglas kan

transmittansen ökas med 5 - 6 procentenheter och för ett tvåglasfönster med

upp till 15%.

Det finns många olika sätt att belägga glas med ett antireflexskikt.

Traditionellt används en metod som kallas sputtring för att belägga glasögon,

mobiltelefondisplayer, TV- och datorskärmar. Sputtring är en komplicerad

och förhållandevis dyr metod när det handlar om att belägga stora ytor.

I detta arbete har antireflexbehandling av glas och plast genom doppning

studerats. Metoden går ut på att man doppar ned materialet som ska beläggas

i en lösning av beläggningsmaterialet. Materialet dras sedan upp med

kontrollerad hastighet ur lösningen. Tjockleken på skiktet bestäms sedan

av en mängd parametrar som viskositeten på lösningen, koncentrationen

och uppdragshastigheten. På det sättet kan bägge ytorna på en glasskiva

eller plastfilm beläggas samtidigt. Studier har även visat på att denna

behandling, som förbättrar de visuella egenskaperna, endast påverkar

fönstrets energiprestanda marginellt. Egenskaperna hos ljusspridning har även

studerats och visar att oönskad ljusspridning från energieffektiva fönster i

vissa fall kan minskas genom antireflexbehandling.

7.3 Energisimuleringar

Energisimuleringar kan användas för att modellera en byggnad eller

byggnadsdel för att avgöra hur den fungerar. Det är möjligt att göra redan

innan byggnaden finns annat än på ritbordet. Simuleringar av hela byggnader

är praktiska för att avgöra totala energibehovet hos en byggnad och kan

därigenom hjälpa vid valet av värme- och/eller kylsystem. För att avgöra

vilka fönster som är mest lämpliga i en byggnad är det inte nödvändigt

att simulera hela byggnaden. Istället kan byggnaden förenklas till ett fåtal

beskrivningsparametrar. Ett simuleringsverktyg som förenklar byggnaden,

men istället har en mer avancerad fönstermodell är WinSel.

68

I denna avhandling har WinSel vidareutvecklats så att programmet även kan

hantera fönster med varierbar genomsläpplighet av ljus, så kallade smarta fön-

ster. Dessa fönster är fortfarande mycket ovanliga på marknaden och enda sät-

tet att göra rättvisa jämförelser med andra fönster i olika klimat är energi-

simuleringar. Simuleringarna i denna studie visar på potentialen till energi-

besparingar med smarta fönster, samt vikten av ett väl fungerande kontroll-

system, som även kan fungera som länken mellan goda energiegenskaper och

visuella egenskaper.

7.4 Optisk karakterisering

Optiska mätningar kan användas för att kvantifiera optiska egenskaper, till

exempel transmittans och termiska egenskaper hos fönster, eller färg och

ljusspridningsegenskaper hos material i allmänhet. Dessa mätningar kan

sedan användas för att jämföra olika produkter och kan också användas för

att utvärdera nya produkter eller för att kunna utvärdera hur en komponent

fungerar i ett system. För att få rättvisa jämförelser och utvärderingar är

det väsentligt att mätningarna är korrekta och genomförda under likvärdiga

villkor.

Inom solenergiforskningen behövs ständigt nya metoder för utvärdering av nya

material. Detta gäller inte minst inom den optiska mättekniken när nya mate-

rial och materialkombinationer introduceras. Det är också av stor vikt att mät-

rutiner är enkla och kan genomföras på standardiserade kommersiella instru-

ment. Detta ställer stora krav på den utrustning som används och ofta måste de

använda instrumenten modifieras för att ett visst prov skall kunna mätas.

Inom byggnadsindustrin finns ett stort intresse för att utnyttja glas i

byggnader, dels till fönster men även för andra komponenter för ljusinsläpp.

I denna avhandling har mätningar av framförallt ljusspridande prover

analyserats. Spridande glas- och plastmaterial kan vara intressanta både som

täckglas för solceller, men även för insläpp av dagsljus i byggnader. Att mäta

spridande prover i kommersiella mätinstrument är något som ofta leder till

mätavvikelser.

Mätfel som beror på ljusspridning i material har studerats och ett förslag till

lösning för mer korrekta mätningar i kommersiella instrument har presen-

terats. Förslaget går ut på att applicera en diffusor framför detektorn både

69

under mätning av prov och under mätning av referenssignal. Detta innebär

att ljusspridningsbilden blir likadan i både referens- och provfall och att mä-

tavvikelserna på detta sätt undviks.

Det är lätt att glömma bort det viktigaste i alla tekniska detaljer, nämligen

brukaren. Upplevelsen av hur ett smart fönster fungerar, till exempel, bestäms

inte av om transmittansen är en procent högre eller lägre eller om det sänker

energiförbrukningen med 10 eller 11 procent utan framför allt av hur kontroll-

systemet fungerar. Upplever brukaren att fönstret är i mörkt och ljust tillstånd

då det önskas? Finns det möjlighet att ändra och är det tillräckligt enkelt att

ändra?

Hur upplevs färgskiftningen hos fönster? Det är inte alltid säkert att mätdata

och upplevelser av fönstret är överensstämmande. Det finns stor potential att

spara energi genom valet av fönster. Valet är upp till brukare, installatörer,

fastighetsägare med flera. För dem betyder inte simuleringsresultat eller mät-

data allt utan valet görs utifrån ett mer subtilt perspektiv.

70

8. Acknowledgements

Heja pappa! Pappa bäst! ...Hugo bäst! ...Mamma bäst! ...Alice bäst! ...Många bäst!

Alice, 2 years

Bla, bla, bla, bla, bla, bla, bla,...

Hugo, 8 months

I have had a lot of support during my PhD studies and I would like to thank you

all. Without you, this would not have been possible. Especially I would like to

thank my supervisor Arne Roos for being such a relaxed, but still competentand helpful professor. Thank you for the work we did together and for all your

guidance. Many thanks to the head of the department, Clas-Göran Granqvist.It has been a pleasure to be part of your highly recognized research group.

I would also like to send my gratitudes to Per Nostell for his engagement andguiding in the field of antireflection coatings. A special thanks to Jacob Jons-son for your advice on optical measurements and for all your help.Mari-LouisePersson, Anna Werner and Tobias Boström are greatly acknowledged for being

such great role models within the field of energy systems and for all trips and

fun we had together.

My “twin” PhD student during these years, Magdalena Lundh, is greatly ac-knowledged for the work we did together, but most of all for the joy we had

during the study and conference trips we spent together. Annica Nilsson isacknowledged for the work we did together both as a student and now as a

researcher. I am glad that you decided to come back and I would like to wish

you all the best for the rest of your PhD studies.

Thanks to Ewa Wäckelgård for being enthusiastic and for convincing me tobegin my PhD studies, and thanks to everyone else at the Solid State Physicsdepartment and at the Energy Systems Programme, especially Joakim Widén,Magnus Åberg and Erica Löfström.

Finally I would like to send many thanks to all my family and friends. Mumand dad for being enthusiastic about my work. My children, Alice & Hugo,for cheering for me, supporting me and for all the joy you bring. Kristin forbringing so much sunshine and delight in my life.

71

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78

Index

absorptance, 55

adhesive testing, 32

antireflection coatings, 27

antiscattering, 33

antireflection treatment

cleaning, 32

heat treatment, 32

moth eye structure, 30

ozone cleaning, 32

plasma treatment, 32

plastics, 34

computer simulations, 37

conduction, 21

convection, 20

detector, 55

diffuse, 53

dip-coating, 31

effective medium theory, 30

electrochromic devices, 25

electrochromic foil, 26

energy simulations, 36

float process, 17

g-value, 20

goniometer, 56

integrating spheres, 58

interference, 28

constructive, 28

destructive, 28

light scattering, 53, 56, 57

light sources, 55

low-e windows, 17, 22

monochromator, 56

moth-eye structure, 30

optical measurements, 52

porous structure, 30

radiation, 20

refractive index, 27, 30

SHGC – solar heat gain coefficient, 20

smart windows, 17, 24

control strategies, 25, 41

energy simulations, 42

sol-gel process, 31

solar control windows, 17

solar radiation, 18

solar spectrum, 18

spectrophotometer, 55

specular, 53

suspended particle devices, 26

two-way mirrors, 24

U value, 21

windows, 16

history, 16

low-e, 22

manufacturing, 16

physics, 18

WinSel, 39

79

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