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3 To my children...

4 This thesis is based on work conducted within the interdisciplinary graduate school Energy Systems. The national Energy Systems Programme aims at creating competence in solving complex energy problems by combining technical and social sciences. The research programme analyses processes for the conversion, transmission and utilization of energy, combined together 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.

5 List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I II III IV V VI VII Simulations of the energy performance of smart windows based on user presence using a simplified balance temperature approach Jonsson A. and Roos A. submitted to Energy & Buildings, 2009 Evaluation of control strategies for different smart window combinations using computer simulations Jonsson A. and Roos A. Solar Energy, 2009, in press Visual and energy performance of switchable windows with antireflective coatings Jonsson A. and Roos A. accepted for Solar Energy, 2009 The effect on transparency and light scattering of dip coated antireflection coatings on windows Jonsson A., Roos A. and Jonson E.K. submitted to Solar Energy Materials & Solar Cells, 2009 Optical characterization of anisotropically scattering samples using an integrating sphere in combination with a diffuse film Jonsson A., Jonsson J.C., Nilsson A.M. and Roos A. in manuscript, 2009 Optical characterization of fritted glass for architectural applications Jonsson J.C., Rubin M.D., Nilsson A.M., Jonsson A. and Roos A. Optical Materials, (6): p Investigation of side shift and side losses of surface scattering samples Jonsson A. and Roos A. submitted to Applied Optics, 2009 Reprints were made with permission from the publishers.

6 My contributions to the appended papers I II III IV V VI VII Simulations of the energy performance of smart windows based on user presence using a simplified balance temperature approach Method development and most of the writing. Evaluation of control strategies for different smart window combinations using computer simulations All simulations and most of the writing. Visual and energy performance of switchable windows with antireflective coatings All experimental work, all simulations and most of the writing. The effect on transparency and light scattering of dip coated antireflection coatings on windows Some of the sample preparations, all measurements and most of the writing. Optical characterization of anisotropically scattering samples using an integrating sphere in combination with a diffuse film Most of the writing and of the measurements. Optical characterization of fritted glass for architectural applications Some of the measurements. Investigation of side shift and side losses of surface scattering samples All of the measurements and most of the writing.

7 Other Publications i ii iii iv v vi vii viii ix x xi Simulations of energy influence using different control mechanisms for electrochromic windows Jonsson A. and Roos A. In proceedings of World Sustainable Buildings, 2008, Melbourne, Australia. Evaluation of the energy efficiency of smart windows with electrochromic glazing and external shading devices using different control strategies Jonsson A. and Roos A. In proceedings of ISES - Solar World Congress, 2009, Johannesburg, South Africa. Active versus passive solar heating in residential buildings Jonsson A. and Roos A. In proceedings of Northsun, 2004, Vilnius, Lithuania Belagd folie reglerar inflöde av solenergi Jonsson A. Husbyggaren, nr 3, 2007 Framtidens smarta fönster Jonsson A. Presented at Energitinget, 2007, Stockholm, Sweden Hushåll med solvärme - ett svenskt pilotprojekt i Anneberg Jonsson A., Lundh M. and Löfström E. Program Energisystem förlag, Linköping, 2005 Method for direct measurement of sample absorptance using the reflectance port of an integrating sphere. Jonsson A. and Roos A. In manuscript Homogenisation of scattered light in integrating spheres - a way of reducing errors caused by port edge losses Jonsson A. and Roos A. In proceedings of Colloquium Optische Spektrometrie, 2004, Berlin, Germany Visual transmittance and energy performance of smart windows with antireflective coatings Jonsson A. and Roos A. In proceedings of Eurosun, 2006, Glasgow, Scotland Antireflective coatings on different window surfaces Jonsson A. and Roos A. In proceedings of the International Conference on Coatings on Glass and Plastics, 2006, Dresden, Germany Applications of coated glass in high performance energy efficient windows Roos A., Jonsson A. and Nilsson A.M. In Proceedings of Glass Performance Days, 2009, Helsinki, Finland

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9 Contents Det är ingen ordning på allting, man hittar inte vartenda dugg. Pippi Långstrump 1 Introduction Aims Outline Windows History of glass and windows Window types Window physics Solar spectrum Optical properties Thermal properties Low-e windows Solar control windows Two-way mirrors Smart windows Control strategies Smart window technologies Electrochromic foil Antireflective treatment using dip-coating Physics behind antireflection coatings Interference Single layer interference coating Multi layer interference coating Moth-eye structure Dip-coating Cleaning Plasma treatment Heat treatment Scratch resistance and adhesive testing Antireflection coatings on windows Antiscattering Antireflection treatment of smart windows... 34

10 4 Energy simulations Simulations Verification and validation Limitations Choosing a suitable model WinSel Case study - Anneberg Control strategies for smart windows Comparison of smart window combinations Cooling energy balance Heating energy balance Total energy balance Antireflection coatings Optical characterization Material optics Diffuse and specular Measuring optical properties Instruments for optical measurements Optical components Goniophotometer Bidirectional scattering distribution function Integrating spheres Double beam instruments Single beam instruments Error sources Conclusions and outlook Summary in Swedish Introduktion Antireflexbehandling Energisimuleringar Optisk karakterisering Acknowledgements Index

11 Glossary absorptance The fraction of incident light that is taken up by the matter, neither 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 coatings 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 radiation 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 intensity independent of angular distribution. 11

12 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 wavelengths chosen from a wider range of wavelengths, i.e. for example selecting 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 trajectory 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/m 2 K. 12

13 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 organizations [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 installed in buildings mainly to give outside view and daylight. Therefore an optical perspective is of interest. Optical measurements are important for quantifying product properties for comparisons and evaluations. It is important that new measuring routines are simple and applicable to standard commercial instruments 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

14 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 antireflection coating. In this thesis a dip-coating technique was used to put antireflection 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

15 1.1 Aims The aims of this PhD project have been threefold: Improve the optical performance of windows through depositing antireflection 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 regulated. Develop the tools necessary to evaluate how smart windows should be controlled to both be accepted by users, let in daylight, avoid glare and save 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 simulations and optical measurements. A brief summary of the conclusions from this thesis follows together with some suggestions for future work. 15

16 2. Windows When the wind changes direction, there are those who build walls and those who build windmills. Chinese proverb A window 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 5000 BC. The earliest man-made glass objects are thought to date back to around 3500 BC. A major breakthrough in glass making was the discovery of glassblowing around the first century. The long thin metal tube used in the blowing process has changed very little since. Soon after the Romans began to use glass for architectural purposes, with the discovery of clear glass around 100 AD. The production of sheets of glass evoluted during the 11 th century. By blowing a hollow glass sphere and swinging it vertically, gravity would pull the glass 16

17 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 19 th 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 technique 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 dramatically 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 windows, roof windows, jalousie windows, etc. This leads to the following division: 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

18 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 daylight 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 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 divided into three major parts, UV radiation, visible radiation, commonly referred 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 C, sometimes referred to as thermal infrared. 18

19 The solar spectrum at sea level and the radiation emitted by a 20 C warm object is depicted in figure 2.1. The definition of the different wavelength regions can also be seen in the figure. Solar radiation (W/m 2 μm) UV Vis. NIR IR Wavelength (μm) Solar radiation, sea level Blackbody radiation, T = 20 C Figure 2.1: Solar spectrum at sea level together with blackbody radiation of a 20 C warm object Blackbody radiation (W/m 2 μm) Solar radiation entering through the window can either contribute to the heating 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 system has been investigated in [10] showing too slow response from the heating system Optical properties The optical properties of a window are deduced from how the glazing interacts with the electromagnetic radiation. How much sunlight is reflected, transmitted and absorbed in the different panes. Then there is also the possibility of reemittance 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 different 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. Another type of coating, an antireflection coating, can instead increase the transmittance of daylight leading to a brighter window. The AR coating reduces 19

20 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 antireflection 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 transmittance of daylight, T vis, transmittance of all solar radiation, T sol and the g-value. The g-value or solar heat gain coefficient, SHGC, is a measure of the total radiation from the sun entering through the window, i.e. directly transmitted plus the fraction of absorbed radiation entering the room Thermal properties The thermal properties of the window are independent of the optical properties, and are therefore sometimes referred to as dark values. Heat can be transferred in three different ways, through radiation, through convection and through conduction Radiation Radiation is heat transfer through emittance and scattering of electromagnetic radiation, usually infrared radiation. Further reading on solar radiation and applications 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 described in sections and 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

21 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 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 without 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] U value The thermal properties of a window can be summarized in one single parameter, the U value. The U value is a measure of the heat conductance in a material or object and can be calculated according to the international standard, EN673 [15]. This quantity is measured in W/m 2 K and a smaller number correspond to a better insulating window or wall element. 21

22 2.3.4 Low-e windows 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 Figure 2.2: Position of the coating for a low-e window. 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, SnO 2. Transmittance / Reflectance (%) UV Vis. NIR IR Wavelength (μm) Transmittance Reflectance 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 deteriorated, but this deterioration can be avoided with an additional coating on the external surface [16, 17]. 22

23 Another issue with thermally very efficient windows is the shortening of the heating season and that the use of solar gains thus becomes smaller. Overheating 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] Solar control windows Solar control windows are most suitable in warm countries with a dominating cooling season. In a similar way as low-e windows, the solar 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 transmittance should be as low as possible, as can be seen in figure 2.5, to block as much of the invisible solar Figure 2.4: Position of the coating for a solar control window. radiation as possible. The coatings are similar to the low-e case but are usually somewhat thicker. Transmittance / Reflectance (%) UV Vis. NIR IR Wavelength (μm) Transmittance Reflectance Figure 2.5: Ideal solar control window. 23

24 2.3.6 Two-way mirrors Two way mirrors, used in for example police interrogation 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 reflective, 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 surveillance 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 surface 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. Figure 2.6: The principle of a two way mirror. 2.4 Smart windows Smart windows [19 22], also referred to as switchable windows, refers to windows 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 window luminance. Also surveys of window manufacturers have been made and 24

25 the researchers behind the study believes that the interest in switchable glazing technologies among end-users will grow significantly [26] 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 individually [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 comfortable 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 example [22], where it is stated that the control system can balance energy efficiency 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 Smart window technologies There are several different techniques available for manufacturing smart windows. 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

26 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 electrochromic 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, VO 2, reflects infrared light when the temperature rises over a certain transition temperature, which through doping can be made lower than 30 C [36, 37] 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-toroll 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

27 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 available 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 refractive 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

28 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 Interference Interference is when two or more waves interact with each other. This interaction 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 Figure 3.1: Constructive and destructive interference of two waves Single layer interference coating The phenomenon of interference can occur when there are several simultaneous 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 coating. 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 in figure 3.2. The thin coating can only be a quarter of a wavelength for one single wavelength, 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

29 I λ/4 λ T R 1 R 2 n 0 n 1 n s Figure 3.2: Destructive interference in a quarter-wave single layer interference coating. of the sensitivity of the human eye, as can be seen in figure 3.3. For visible applications, such as windows, 550 nm is therefore usually selected as the design wavelength. Solar radiation (W/m 2 μm) UV Vis. NIR Wavelength (μm) Solar radiation, sea level Human eye sensitivity Figure 3.3: Sensitivity curve of the human eye together with the solar spectrum at sea level. Human eye sensitivity (arb.) 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 n 1 = n 0 n s, where n 0 is the refractive index of the surrounding media and n s is the refractive index of 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 n 1 = An optical thickness of a quarter of the design wavelength gives perfect antireflection 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

30 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 compensate 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, MgF 2, has a refractive 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 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 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 material. 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

31 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 design [49]. 3.2 Dip-coating Dip-coating is a wet chemical process which can be used to deposit thin coatings. 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 constant 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 properties [51] but this process can also degrade the optical properties to some degree [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

32 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 process 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 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 surface, which helps in the film forming process. [55] Heat treatment AR coatings on glass can be made more adhesive through heating the glass after 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 problem with plastic materials is that most have a melting point at around 100 C and 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

33 Weathering tests are another way of testing antireflection coatings for solar energy applications [58, 59]. 3.4 Antireflection coatings on windows 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 materials 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 decreases from 0.3 % to 0.2 % at 550 nm with 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. Diffuse transmittance With AR coating, measured on SnO side 2 With AR coating, measured on opposite side Without AR coating, measured on SnO 2 side Without AR coating, measured on opposite side Wavelength (nm) Figure 3.5: The diffuse transmittance for an antireflection coated low-e glass, measured on both SnO 2 side and on the opposite side. 33

34 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 Transmittance With AR coating Without AR coating Wavelength (nm) Figure 3.6: Transmittance of a switchable foil with AR coating. This increase in the visible transmittance of the electrochromic foil can increase the total daylight transmittance for the whole window to such high levels that the windows become more acceptable even in climates with very dark periods, similar to the Swedish. 34

35 Transmittance dark clear EC window with 4 AR coatings EC window with 2 AR coatings EC window without AR coatings Solar control window Wavelength (nm) Figure 3.7: Transmittance of a double glazed smart window with different numbers of antireflection coatings. 35

36 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 simulation 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 simplified 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

37 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 system 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 Verification and validation Verification is the process of determining that a computer model and simulation software accurately represent the developer s description and specification. To be certain that obtained results correspond to reality, it is essential to validate [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 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 transparency 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

38 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 account. This means that other values might be lost, such as behavior and experience. 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 results 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 Choosing a suitable model There are several different techniques available for making energy simulations [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

39 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 simple tool for selecting windows. Using the window properties solar gain and U value, different windows can be compared for a building located in a specific 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 simulations presented 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

40 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 Energy (kwh/m 2 a) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month 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 106kW h/m 2 a 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 solar collectors would be around 200kW h/m 2 a when considering losses in the storage system. The results show that the heating output from the solar collector 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

41 50 40 Energy (kwh/m 2 a) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Figure 4.2: The influence on the energy balance of a south facing roof window during 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 components 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 controlling 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

42 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 electrochromic component in the window to a maximum of 200 W/m 2. 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 optimization 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 optimization 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 optimization 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 optimization 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 optimization 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

43 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 X X X O O X X X X O X X O X O X X X X O X X O X X X X X X O X X O X X X X X O O X O O X X X X O O O X X O X X X X X O O X O O X X X X O O O X X O X X X X X O O X O O X X X X O O O X X O X X X X X O O X O O O O X X O O O X X O X O X X X O O X X X O O X X O O O X X X X O X X X O O X X X O O X X O O O X X X X O X X X O O X X X X X X X X X O X X X X X X X X X O X O - Energy optimization mode - 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 T sol R sol A sol Abs outer Abs inner T vis R vis U-value (W/m 2 K) g- value Light ,63 Dark 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 and the 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/m 2 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

44 Energy (kwh/m 2 a) Heating Cooling Total -300 EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO N E S W Window configuration for each orientation 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. Energy (kwh/m 2 a) Heating Cooling Total -400 EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO EO O2 O1 DO N E S W Window configuration for each orientation Figure 4.4: Energy balance of a double glazed smart window in an office building with a balance temperature of 8 C located in Denver for different orientations and for different control strategies. through the window is less than 300 W/m 2 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 cooling needs, in a similar way as the solar radiation [67]. The additional cooling need and heating contribution should be compared to the corresponding values 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

45 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. Energy (kwh/m 2 a) Heating Cooling Electricity -80 O2 O1 R2 R1 O2 O1 R2 R1 O2 O1 R2 R1 O2 O1 R2 R1 N E S W Window configuration for each orientation 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 calculated using a combination of the Fresnel formalism and experimental data. The international standards ISO 9050 [68] and EN 673 [15] were used to calculate the solar factor (g-value) and the thermal conductance (U value), respectively. For the electrochromic layers, refractive indices were taken from the Windows and Daylighting Group at Lawrence Berkeley National Laboratory [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 surface 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

46 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/m 2 K) g-value (%) Double pane reference DG No Double pane EC combination DG + EC Light Dark Low-e reference 1 LE1 No Low-e EC combination 1 LE + EC1 Light Dark Low-e reference 2 LE2 No Low-e EC combination 2 LE + EC2 Light Dark Solar control reference 1 SC No Solar control EC combination 1 SC + EC1 Light Dark Solar control EC combination 2 SC + EC2 Light Dark Solar control EC combination 3 SC + EC3 Light Dark 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 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 somewhere in between Stockholm and Miami the annual cooling need can be decreased 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

47 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 investigated. 0 DG + EC Energy (kwh/m 2 a) LE + EC1 LE + EC2 SC + EC1 SC + EC2 SC + EC3 Energy opt. Office 2 Office 1 Daylight opt. No EC -400 N E S W Optimization mode for each orientation Figure 4.7: Cooling energy balance of the selected window combinations in Denver for different orientations. Reference cases in figure 4.6 are represented by the No EC case 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

48 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 balance. 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. 300 DG + EC Energy (kwh/m 2 a) LE + EC1 LE + EC2 SC + EC1 SC + EC2 SC + EC3 Energy opt. Office 2 Office 1 Daylight opt No EC -200 N E S W Optimization mode for each orientation Figure 4.8: Heating energy balance of the selected window combinations in Denver for different orientations 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 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 considering 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

49 DG + EC LE + EC1 LE + EC2 Energy (kwh/m 2 a) SC + EC1 SC + EC2 SC + EC3 Energy opt. Office 2 Office 1 Daylight opt No EC -400 N E S W Optimization mode for each orientation Figure 4.9: Total energy balance of the selected windows in Denver for different orientations. 200 DG + EC Energy (kwh/m 2 a) LE + EC1 LE + EC2 SC + EC1 SC + EC2 SC + EC3 Energy opt. Office 2 Office 1 Daylight opt. No EC -200 N E S W Optimization mode for each orientation Figure 4.10: Total energy balance of the windows with the best energy performance 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 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 artificial lighting would not be necessary as often. The AR coating does not affect 49

50 0 SC + EC3 Energy (kwh/m 2 a) Energy opt. Office 2 Office 1 Daylight opt. No EC N E S W Optimization mode for each orientation Figure 4.11: Cooling energy balance of the windows with the lowest cooling need in Miami for different orientations. Energy balance (kwh/m 2 a) Heating Cooling Total -300 EC AR LE SC DG EC AR LE SC DG EC AR LE SC DG EC AR LE SC DG N E S W Window configuration for each orientation Figure 4.12: Energy balance for double glazed windows in an office building in Denver, 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 differences, as can be seen in figure 4.12, which is a comparison of the windows in table

51 Table 4.4: Optical and energy related parameters for the different windows. Window Short name EC coating Tsol (%) Rsol (%) Asol (%) Tvis (%) Rvis (%) U(W/m 2 K) g-value Low-e EC combination EC Light Dark Low-e EC + 2 AR coatings AR Light Dark Low-e reference LE No Solar control reference SC No Double pane reference DG No

52 5. Optical characterization There is no success without hardship. Sophocles Optical measurements can be used to quantify optical characteristics of materials, such as transparency, and thermal properties of a window or the color and surface roughness of any material. These quantified numbers make it possible 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 evaluations to be fare it is important that all measurements are accurate and performed 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 interaction of light with matter. This can be described by the famous Snell s relation: N 1 sinθ 1 = N 2 sinθ 2 (5.1) The subscripts 1 and 2 denote the first and second medium respectively. N 1 is thus the refractive index of the incoming medium. N 2 is the refractive index of the second medium. θ 1 and θ 2 correspond to the angle of incidence and angle of refraction, respectively. The principle of light refraction is depicted in figure

53 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 different 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 = E R /E A (5.3) t = E T /E A (5.4) 53

54 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: r p = N 2 cosθ 1 N 1 cosθ 2 N 1 cosθ 2 N 2 cosθ 1 t p = r s = N 1 cosθ 1 N 2 cosθ 2 N 1 cosθ 1 +N 2 cosθ 2 t p = 2N 1 cosθ 1 N 1 cosθ 2 +N 2 cosθ 1 2N 1 cosθ 1 N 1 cosθ 1 +N 2 cosθ 2 (5.5) The angle between the ray and the surface normal is denoted θ. The intensity reflectance, R and transmittance, T, are given by the squared complex ratios as R p = r p r p and T p = N 2 N 1 t p tp. For normal incidence and unpolarized light these relations can be simplified to: ( ) N1 N 2 2 R = (5.6) N 1 + N 2 54 T = N ( ) 2 2 2N1 (5.7) N 1 N 1 + N 2

55 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 = I T /I I (5.8) R = I R /I R 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 absorptance 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 T using equation Instruments for optical measurements 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 properties separately at different wavelengths of the light. This corresponds to different 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

56 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 difficulties that arise. The light source has to be controlled and stable. To avoid other light sources interfering with the measurement an opaque cabinet is often 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 Goniophotometer A goniophotometer, or just goniometer, is an instrument to measure light intensity 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

57 Figure 5.4: Schematic drawing of a goniometer equipment Bidirectional scattering distribution function The parameter to use for describing the scattering properties is the bidirectional 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 transmittance distribution function, BTDF, and the bidirectional reflectance distribution function, BRDF, respectively. 57

58 5.5 Integrating spheres Integrating spheres, also referred to as Ulbricht spheres [70], are optical components with hollow cavities and interiors coated with a highly diffuse and highly reflective material, usually barium sulfate, BaSO 4 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 information. 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] 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 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

59 for conducting transmittance measurements, one entrance port for incoming light and one port for the detector. On the other hand, in a single beam instrument, 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 changing the sphere throughput. Reference measurement to the left and sample measurement to the right 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 reflections can come up both in the sample and between optical components in the measurement equipment. 59

60 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 sample measurement. To compensate for this the sample signal should be multiplied 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] 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 functions 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 integrating 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. BTDF (sr 1 ) Sample Sample + Diffuser Diffuser Scattering angle ( ) Figure 5.8: Goniometer measurement showing the BTDF of a low-angle scattering 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

61 Figure 5.9: Image of the scattering of laser light from the sample. and a measurement using the diffuser method and correcting for multiple reflections Transmittance 0.9 Glass Diff Corr Wavelength (nm) Figure 5.10: Measurement of a clear glass sample. Glass is an ordinary measurement. Diff corresponds to a diffuser measurement, Corr to a diffuser measurement 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

62 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 Transmittance 0.9 Diff Corr Corr+Comp T sphere L Wavelength (nm) Figure 5.11: Transmittance spectra for the low-angle scattering glass sample showing very good agreement when applying the k-factor according to paper V 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 underestimates 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 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

63 Figure 5.12: Schematic illustration of how the sample was moved into the integrating 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 sphere were 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

64 sphere, as can be seen in figure The sample thickness was 3 mm, which means the sample is fully inside the sphere at the position 3 mm 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. Intensity Ø beam = 7 mm Ø beam = 11mm Ø beam = 18mm Ø beam = 18mm B Position (mm) 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

65 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 perspectives: Antireflection and switchable coatings, optical characterization and energy performance. The fact that less daylight is transmitted through modern windows make it interesting 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 windows, 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 transmittance 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, especially 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

66 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 samples using spectrophotometers with integrating spheres. The effect of different scattering profiles have been investigated and several obstacles and possibilities 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 errors when measuring reflectance since, for instance, the reflection can have a large component that is scattered directly out through the entrance port. 66

67 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 studeras utifrån ett flertal vetenskapliga synvinklar. Fönster har vi främst för utsikt och dagsljuinsläpp. Ett optiskt perspektiv blir därmed intressant. Fönster är också den svagaste länken i en byggnads energisystem, varför ett energiperspektiv också uppenbarar sig. Att belägga fönster på olika sätt för att minska värmestrålningen genom fönstret är en teknik som används idag. Fönsterbeläggningar 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

68 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

69 I denna avhandling har WinSel vidareutvecklats så att programmet även kan hantera fönster med varierbar genomsläpplighet av ljus, så kallade smarta fönster. Dessa fönster är fortfarande mycket ovanliga på marknaden och enda sättet att göra rättvisa jämförelser med andra fönster i olika klimat är energisimuleringar. Simuleringarna i denna studie visar på potentialen till energibesparingar med smarta fönster, samt vikten av ett väl fungerande kontrollsystem, 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 material och materialkombinationer introduceras. Det är också av stor vikt att mätrutiner är enkla och kan genomföras på standardiserade kommersiella instrument. 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 presenterats. Förslaget går ut på att applicera en diffusor framför detektorn både 69

70 under mätning av prov och under mätning av referenssignal. Detta innebär att ljusspridningsbilden blir likadan i både referens- och provfall och att mätavvikelserna 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 kontrollsystemet 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 mätdata allt utan valet görs utifrån ett mer subtilt perspektiv. 70

71 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 competent and 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 and guiding in the field of antireflection coatings. A special thanks to Jacob Jonsson for your advice on optical measurements and for all your help. Mari-Louise Persson, 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 acknowledged 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 is acknowledged 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 to begin my PhD studies, and thanks to everyone else at the Solid State Physics department 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. Mum and 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 for bringing so much sunshine and delight in my life. 71

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78 tance, total solar energy transmittance, ultraviolet transmittance and related glazing factors, [69] Windows Daylighting Group - Lawrence Berkeley National Laboratory. Chromogenic materials - optical constants of electrochromic materials and transparent conductors, [70] R. Ulbricht. Die bestimmung der mittleren räumlichen lichtintensität durch nur eine messung. Elektrotech. Z., 21: , [71] P. Nostell, A. Roos, and D. Rönnow. Single-beam integrating sphere spectrophotometer for reflectance and transmittance measurements versus angle of incidence in the solar wavelength range on diffuse and specular samples. Review of Scientific Instruments, 70(5): , [72] F. J. J. Clarke and J. A. Compton. Correction methods for integratingsphere measurement of hemispherical reflectance. Color Research and Application, 11(4): , [73] D. Rönnow and A. Roos. Stray-light corrections in integrating-sphere measurements on low-scattering samples. Applied Optics, 33(25): , [74] K. Grandin and A. Roos. Evaluation of correction factors for transmittance measurements in single-beam integrating spheres. Applied Optics, 33(25): , [75] A. Roos and C. G. Ribbing. Interpretation of integrating sphere signal output for non-lambertian samples. Applied Optics, 27(18):3833 7, [76] A. Roos. Interpretation of integrating sphere signal output for nonideal transmitting samples. Applied Optics, 30(4):468 74,

79 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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