# State of the art of advanced solar control devices for buildings

Tilmann E Kuhn

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## To cite this version:

Tilmann E Kuhn. State of the art of advanced solar control devices for buildings. Solar Energy, 2017, 154, pp.112 - 133. 10.1016/j.solener.2016.12.044. hal-05083480

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# State of the art of advanced solar control devices for buildings

Tilmann E. Kuhn

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany

## ARTICLE INFO

**Article history:**

Received 20 October 2016

Received in revised form 16 December 2016

Accepted 18 December 2016

Available online xxxx

**Keywords:**

Solar-control system

Solar control device

Solar heat gain coefficient

g-value

Total solar energy transmittance

TSET

Solar factor

SHGC

g

q₁

Passive solar gains

Building-integrated PV

BIPV

BIST

Complex glazing

Complex fenestration system

## ABSTRACT

This paper deals with the state of the art of advanced solar control devices for buildings, with the comparative evaluation of solar-control systems and with guidelines for the development of new solar control systems. It includes multifunctional systems with building integrated photovoltaic (BIPV) and/or building integrated solar thermal (BIST) energy conversion. In order to facilitate and to structure the understanding of solar control systems two multidimensional spaces are introduced: the design space and the evaluation space. The design space contains the design parameters which are to be selected by the designer when solar control devices are to be chosen for specific buildings or when new systems are to be developed, such as the color of the slat of a venetian blind or the fraction of holes (direct transmittance) of a fabric for a roller blind. The evaluation space contains the performance parameters or evaluation criteria, which indicate the design's ability to satisfy the functional and aesthetic requirements, such as passive solar gain control or visual comfort. All the design parameters and evaluation criteria are explained in detail in the paper. A chapter with examples of advanced solar control systems completes the overview of the state of the art of solar control systems.

© 2017 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

Solar-control systems can help to reduce the cooling energy consumption of buildings, to reduce the energy consumption of the artificial lighting system, to provide visual comfort, to ensure healthy natural lighting and to generate solar electricity and solar heat at the same time. All of these multifunctional aspects have to be assessed realistically and reliably when systems are chosen for specific buildings and when new systems are being developed. Well-designed solar-control systems are therefore important elements that help to achieve the CO₂ reduction goals worldwide and especially in the European Union as defined in the Energy Performance of Buildings Directive (Directive 2002/91/EC and the recast (2010/31/EC), EPBD). It is important to note, that solar-control systems should not be regarded purely as an unavoidable energy-saving measure. Well designed solar-control systems can also contribute positively to the architecture of the buildings and

to the well-being of the users. This paper aims to contribute to achieving these goals with a description of the state of the art of advanced solar control devices for buildings and with guidelines for the comparative evaluation and the development of new solar control systems.

## 2. Design space and evaluation space - the key to the understanding of solar control systems

The author of this paper has developed several solar control systems (Kuhn, 2016; Clauss and Kuhn, 2001; Baumann et al., 2002; Bläsi et al., 2002; Kuhn, 2006; Hermann and Kuhn, 2006; Kuhn and Hermann, 2007; Kuhn et al., 2007; Maurer et al., 2011; Nestle et al., 2013), theoretical evaluation methods (Kuhn et al., 2000; Kuhn, 2006; Kuhn, 2006; Kuhn, 2007; Kuhn and Herkel, 2011) and measurement procedures (Kuhn, 2014; Kuhn et al., 2016). This was the background for contributions to several standards (EN 14500, 2008; EN 14501, 2005; ISO/CD 19467, 2015; ISO TC160 and TC163, 2016). The author is convinced that the key to the understanding of solar control systems is to introduce two multidimensional spaces: the design space and the evaluation

<footnote>E-mail address: tilmann.kuhn@ise.fraunhofer.de</footnote>

<footnote>URL: http://www.ise.fraunhofer.de</footnote>

<footnote>URLs: http://www.ise.fraunhofer.de</footnote>
space. Fig. 1 provides an overview of the design space. The design space contains the *design parameters* which are to be selected/specified by the designer when solar control devices are to be chosen for specific buildings or when new systems are to be developed, such us the color of the slat of a venetian blind or the fraction of holes (direct transmittance) of a fabric for a roller blind. The choice of a set of design parameters is similar to the choice of a co-ordinate system with orthogonal (= linearly independent) co-ordinate directions in a vector space. It is important to find meaningful and independent design parameters, such as *W* (relevant for the necessary installation space) together with the dimensionless ratio *W/D* (relevant for the optical and visual properties) when a venetian blind with slat width *W* and slat distance *D* is to be specified. The parameter set [*W*, *D*, *W/D*] would not be independent since changes of the parameter *W* would always change implicitly also the parameter *W/D*. The set of design parameters should not only be independent but also complete. The evaluation space contains the *performance parameters or evaluation criteria*, which indicate the design's ability to satisfy the functional and aesthetic requirements, such us passive solar gain control or visual comfort. Fig. 2 shows the aspects of the evaluation space which are related to energy and user comfort. The relative weight of the different evaluation criteria cannot be defined independent of the actual building context, such as small/large window or open plan/single office etc. A generally valid metric (like metrics in vector spaces) therefore does not exist in the evaluation space and the individual dimensions have to be evaluated separately. The best design (or the relative rating of different designs) therefore depends on the specific building and the preferences of the architect and the building owner. The author is nevertheless confident that the systematic overview and the structured procedure greatly helps to select solar control devices in building projects more objectively and to optimize newly developed systems. It is essential to clearly distinguish between evaluation criteria and design parameters in order to keep the overview.

Chapter 3 discusses the list of requirements for solar control systems within the context of the evaluation space. Chapter 4 discusses design options for advanced solar control systems within the context of the design parameter space.

### 3. Requirements for solar-control systems - the evaluation space

Transparent components are essential to the design and performance of a building. They influence its indoor comfort and energy budget in many diverse ways: daylight illuminates indoor rooms throughout the year, solar energy can be used to heat buildings passively, excessive solar gains can cause glare and overheating, transparent areas allow visual contact with the exterior and the transparent areas are also important for the architectural appearance of a building. The following sub-sections are providing an overview on the different criteria which are relevant for the evaluation of solar-control systems with respect to energetic aspects and user comfort including health issues. The criteria are summarized in Fig. 2.

#### 3.1. Criteria for passive solar gain control

Passive solar gains, transmitted through the facade, are determined primarily by:

* the size of the glazed areas,

* the orientation of the glazed areas with respect to the sun,

* external obstructions by surrounding buildings and trees,

* the insulation properties of the facade system (U-value),

* the solar heat gain coefficient *g* of the complex fenestration system/facade system,

* the operation of the facade system.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Design parameters for solar-control systems.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Evaluation criteria for solar-control systems (Kuhn, 2016).</figcaption></figure>

The first three items are building-related/architectural criteria which are not discussed further in this chapter. The amount of heat which enters modern buildings by conductive heat transfer through the envelope is usually small in the summer/cooling season due to the small temperature differences in summer and the level of thermal insulation and air tightness which is already common in many countries. Conductive heat gains between the indoor and outdoor environment may still be relevant in warmer climates with less need for thermal insulation. The solar heat gain coefficient g is defined and explained in chapter 3.1.1. Chapter 3.1.2 provides the criterion $g_{eff}$ with which the operation mode or control strategy can be taken into account when the performance of passive solar gain control is evaluated.

### 3.1.1. Solar heat gain coefficient g - the fundamental evaluation criterion

The solar heat gain coefficient g is the fraction of the incident solar radiation that enters a room after passing through the building skin, it quantifies the passive solar thermal gains. In other words: A solar heat gain coefficient of g = 0.3 means that 30% of the incident radiation enters the room in the form of transmitted radiation or as secondary heat gains due to the inward flowing fraction of the radiation that has been absorbed in different layers of the building skin. g therefore consists of two parts, the solar transmittance $\tau_e$ and the secondary internal heat transfer factor $q_i$. Fig. 3 illustrates this. These solar gains passively heat the room, which is desired during the heating season and unwanted in the case of summer conditions. g is therefore a key parameter for minimization of the building's energy consumption and for correct dimensioning of the heating, ventilation and air-conditioning system (HVAC). g can be used to characterize transparent, translucent or opaque building components with or without integrated active solar energy conversion technologies such as building-integrated PV systems (BIPV) or building-integrated solar thermal collectors (BIST). Several names are used internationally for g: it is called

the “solar heat gain coefficient (SHGC)”, “g-value”, “total solar energy transmittance (TSET)” or “solar factor”. All of these terms are used synonymously in this paper. From autumn 2016, it will be mandatory to include a statement on the solar-control performance in the CE labeling of all retractable solar-control systems as specified in EN13561 (EN 13561, 2015) and EN13659 (EN 13659, 2015). The performance class to be stated in the CE conformity declaration is defined in EN14501 (EN 14501, 2005). The determination of the performance class is based on the effect of the solar-control system in combination with a certain reference glazing as specified in EN14500 (EN 14500, 2008).

### 3.1.2. Effective solar heat gain coefficient $g_{eff}$

g can be used to describe the properties of the facade/roof as a whole, of parts of it (e.g. windows) or of sub-components (e.g. insulating glazing units (IGU)). For the case of components or combinations of components (e.g. combinations of glazing and venetian blind) it is important to know the “center of glazing” value which excludes the influence of frames, lateral losses and spacers at the glazing edges. The “center of glazing” value is especially meaningful since it is independent of the glazing area and therefore generally valid (Kuhn, 2014). In many cases the influence of the frame etc. can be taken into account in a second step by calculation methods such as ISO 15099 (2003), EN 13363 (2007) or AFNOR (2014).

It is important to understand, that g is not a material constant. g is influenced by several boundary conditions such as the external and internal heat transfer (governed mainly by the wind conditions), the spectrum of the incident radiation and the internal and external temperature. The goal of measurements or calculations is to determine g for a certain set of reference boundary conditions. These reference boundary conditions can be national or international reference conditions for product comparisons or product ratings. However, the reference conditions can also be special conditions valid for a specific building and location under consid-
<figure><img src="image_6.png" /><figcaption>Fig. 3. The meaning of the solar heat gain coefficient g. g is the sum of the solar transmittance τe and the secondary internal heat transfer factor qI (Kuhn et al., 2000).</figcaption></figure>

eration. An example of special conditions are the very windy conditions in higher stories of some high-rise buildings. In general it is important to understand the difference between reference conditions, experimental conditions for measurements and the actual boundary conditions at a certain building with a specific location and facade orientation (Kuhn, 2014). Functions or models, such as EN 410, ISO 9050, EN 13363 (2007) and ISO 15099 (2003), the black-box model (Kuhn and Herkel, 2011 or Kuhn, 2006) for the angle-dependence, are needed to determine the value of g for the actual boundary conditions.

The general quantity, g<sub>wholewindow</sub> for the whole window including frames is defined by the following equation:

$$g_{\text{wholewindow}} := \frac{Q_{\text{withsun}} - Q_{\text{withoutsun}}}{A_{\text{ref}} E_{\text{ref}}} \quad (1)$$

where Q<sub>withsun</sub> or Q<sub>withoutsun</sub> is the heat flow rate [W] through the window with or without sun, respectively. A<sub>ref</sub> is the window area including frames. E<sub>ref</sub> is the irradiance on the facade. The boundary conditions “without sun” are identical to the conditions with sun with the only difference that E<sub>ref</sub> = 0 W/m². The center of glazing (cog) value of g is defined by:

$$g := \frac{q_{\text{withsun}} - q_{\text{withoutsun}}}{E_{\text{ref}}} \quad (2)$$

where q<sub>withsun</sub> or q<sub>withoutsun</sub> is the heat flux density [W/m²] through the center of the glazing with or without sun, respectively. This document focuses mainly on the center of glazing value. g can either be measured directly or calculated based on the properties of the individual layers and components of the facade. The fundamental differences between the two methods are described in Table 1. Very often, a combination of calorimetric measurements and model calculations is appropriate for an efficient and reliable characterization of g.

A comparison of the performance of different complex fenestration systems should be realistic and reliable. The standard conditions (e.g. from EN 410; ISO 9050 or ISO 15099, 2003) with normal incidence of the radiation are unrealistic, especially in the case of facades with venetian blinds. In addition to that they do not consider the usage/control strategy in the case of switchable facade systems. It is clear that the real usage can not be foreseen when the facade is controlled manually or when manual override is allowed. Based on experience from many real building projects, it can be stated that it is nevertheless very helpful to assess the performance for different assumed user modes in order to determine the robustness of a facade concept. This is especially true for buildings with large glazed areas of the facade. Especially in open-plan offices or rooms which are located at the corners of a

<table><caption>**Table 1**
Comparison of different methods to determine the solar heat gain coefficient g.</caption><thead><tr><th></th><th>Calorimetric measurement</th><th>Calculation methods</th></tr></thead><tbody><tr><td>Advantages</td><td><strong>Direct measurement</strong> without model assumptions. g is therefore well defined also for very complex fenestration systems<br>Since <strong>real samples</strong> are assessed, deviations from the ideal properties of the sample are included in the result (e.g. unequally tilted slats of venetian blinds)</td><td>When the model has been prepared and validated, the effect of modifications can be assessed quickly and easily. (e.g. different tilt angle or color of the slats)<br>Samples of the whole facade segment are not required</td></tr><tr><td>Challenges</td><td>Many time-consuming measurements are necessary to cover all the required combinations of operation modes (e.g. tilt angle of slats) and directions of the incident radiation (considering different x<sub>s</sub> and γ<sub>f</sub>)<br>A sample of the fenestration system has to be constructed and sent to the laboratory<br>In some cases, additional difficulties are raised by the not perfectly uniform and parallel light in laboratory measurements<br>In some cases, additional difficulties are raised by the thermal capacity of the facade because of transient, constantly changing boundary conditions in the case of outdoor measurements</td><td>No standard models are available for very complex fenestration systems with e.g. thick absorbing and scattering layers with non-negligible thermal resistance<br>Validation with calorimetric measurements is mandatory<br>Models very often do not consider the angular dependence of g correctly</td></tr></tbody></table>
building, careful design of solar-control systems is therefore essential. In some rooms of the fully glazed German high-rise building GALLILEO, in Frankfurt (Main), 70% of the peak cooling load is caused by solar gains (Berchtold). Solar gains can therefore have a huge impact on occupant comfort, building energy demand, CO₂ emissions, investment costs for HVAC systems and additional costs.

A realistic and reliable approach for the evaluation of the solar-control performance is based on the so-called “effective g-value” or “effective Solar Heat Gain Coefficient” g<sub>eff</sub>. It is defined as

$$g_{\text{eff}} := \frac{\text{monthly(or hourly)solar gains}}{\text{monthly(or hourly)total incident irradiation}} \quad (3)$$

A detailed description of a method for the determination of g<sub>eff</sub> is given in Kuhn et al. (2000) and Kuhn (2016). The method can also be used to determine hourly or daily effective g-values.

## 3.2. Criteria for thermal comfort

Thermal comfort in rooms is influenced by the following factors:

* the thermal energy balance on the surface of the person's body:

- the activity of the person and the resulting metabolic rate/heat generated by this task (see also ISO 7730).

- the convective heat exchange between the person and the surroundings, depending on the air movement and clothing conditions (see also ISO 7730).

- the radiative heat exchange between the person and the surroundings, depending on the infrared and solar radiation and on clothing conditions.

* uneven or asymmetric radiative temperature distribution (see also ISO 7730).

* local differences in the air temperature (e.g. cool air temperatures at floor level and hot temperatures at head height), (see also ISO 7730).

* drafts, determined by the wind speed and the turbulence level (see also ISO 7730).

* relative humidity of the air which determines the possibility of evaporative cooling by perspiration (see also ISO 7730).

* cultural and individual aspects such as physical, physiological and psychological factors.

Effective solar-control systems therefore do not only have to reduce the solar gains, they also have to prevent high local temperatures on the indoor surface of the facade in order to avoid high peaks and asymmetric radiative temperature distributions. In addition, solar-control systems should also be able to protect people against direct solar irradiation which causes strongly asymmetric radiative temperature distributions and high heat gains into the body of the person. Detailed calculation formula and criteria are given in ISO 7730 and EN 15251 (2007), but these criteria are only applicable to moderate differences in the local radiative temperature and therefore not valid for situations with direct insolation of persons with high irradiation levels. It seems to be clear nevertheless, that it is a mandatory requirement for facade systems that they should be able to protect people against direct solar irradiation with high irradiation levels, especially in summer conditions. There are several studies which prove this influence, especially with respect to thermal comfort in cars (see e.g. Hodder and Parsons, 2007), but this aspect is generally not taken into account when the thermal comfort in rooms is assessed with building simulation programs. And what about venetian blinds with perforated slats or electrochromic glazing that do not completely block the direct solar radiation? There are always asymmetric irra-

diation conditions when daylight enters a room (also in overcast situations) and these radiative heat gains in the solar spectral range are not taken into account in the calculation methods given in ISO 7730. It is not clear how these radiative gains should be taken into account in thermal comfort assessments with building simulation programs, although there are studies on the biomedical effects available (see e.g. Hodder and Parsons, 2007). In principle they could be simply added to the energy balance on the body of the person as given in e.g. ISO 7730 as contribution to the mean radiant temperature and to the local radiant temperature of the window. However, the solar radiation is not distributed diffusely into the room like the thermal radiation and has to be treated as directional radiation. However, it is not at all obvious that asymmetric directional solar radiative gains are perceived in the same way as asymmetric thermal radiative gains from opaque walls. The reason is that people are accustomed to this type of asymmetric heat gain and that they probably expect and accept some asymmetry due to daylight, especially in winter. Further investigation of this topic by user assessments under daylight conditions is strongly recommended to further analyze this issue.

## 3.3. Criteria for daylighting

Electric lighting is often responsible for approximately one third of the total primary energy demand in energy-efficient buildings such as passive houses. The use of daylight by optimized facade systems (including optimized control strategies for these systems) is therefore an important measure to reduce the energy demand for the electric lighting. The daylight, transmitted through the facade, is determined primarily by:

* the climatic conditions/location,

* the size of the glazed areas,

* the position of the glazed areas (the parapet area is for example generally less efficient for daylighting than the upper parts of the facade),

* the orientation of the glazed areas with respect to the sun,

* external obstructions by surrounding buildings and trees,

* the light transmittance τ<sub>v</sub> of the complex, fenestration system/-facade system including the light distribution function on the indoor surface of the facade

* the operation/control of the facade system.

Therefore, daylighting can not be evaluated independently of the building. In many countries (e.g. Germany), daylighting requirements are based on minimum requirements for the daylight factor D. D is defined by

$$D := \frac{E_{v, \text{global, horizontal, indoor}}}{E_{v, \text{global, horizontal, outdoor}}} \quad 100\% \quad (4)$$

for overcast conditions. The main idea was already introduced early in the 20th century (Waldram, 1909) and is still in use in many standards (see e.g. DIN 5034, 2007). E<sub>v, global, horizontal, indoor</sub> is the indoor global horizontal illuminance on the working plane [lux]. E<sub>v, global, horizontal, outdoor</sub> is the outdoor global horizontal illuminance. Different definitions for the diffuse sky are used in different standards being either uniform or with lower luminance at the horizon (Moon and Spencer, 1942). In all cases the sky is assumed to be purely diffuse without direct sun and without differentiation between different orientations like e.g. south or north. However, orientation factors have been proposed in the British Standard BS 8206-2:2008 “Lighting for buildings -Part 2: Code of practice for daylighting” (British Standard BS 8206-2, 2008). In most cases the “CIE standard general sky” (ISO 15469, 2004) is used. D can not be transformed directly into indoor illumination levels under real
or typical weather conditions for a given location and a specific facade orientation. D can therefore not be used directly to assess how often the required illuminance levels, as specified in e.g. (EN 12464-1, 2011), are reached by daylight alone. It is therefore also not possible to determine the energy demand for artificial lighting based on D alone. A research program for the California Energy Commission CEC in the US (Heshong, 2012) showed that there is almost no correlation between D and user satisfaction with the daylighting situation, which means that D is not a reliable metric. In addition to that, it is impossible to compare the daylighting performance of different moveable shading devices or different control strategies since blinds are usually fully retracted when the sky is overcast.

A dynamic metric is therefore necessary to evaluate the daylighting performance of switchable complex fenestration systems realistically and reliably. This is especially true for systems with venetian blinds or with other slatted or louvered devices with tiltable slats. Such an evaluation is based on time series of illuminance values for certain points on the working plane in a room. In many cases the hourly average illuminance is not representative for the quickly fluctuating daylighting conditions and the time step has to be decreased from one-hour to one-minute mean values by using validated interpolation algorithms to generate meteorological data for the intermediate time steps (Walkenhorst et al., 2002). A discussion of different “dynamic daylight performance metrics” can be found in Reinhart et al. (2006). The “daylight autonomy” metric DA (Reinhart and Walkenhorst, 2001) is defined as

$$DA := \frac{\text{working hours with daylight illuminance} > \text{threshold}}{\text{total working hours}} \quad (5)$$

and is therefore the “percentage of the working hours of the year when the minimum illuminance requirement at the sensor point is met by daylight alone”. In the case of facade systems with venetian blinds it has been shown that the illuminance values on the working plane in a room can be calculated reliably using the RADIANCE daylight simulation program (Ward and Shakespeare, 1998; Reinhart and Walkenhorst, 2001; Reinhart and Andersen, 2006). A discussion of different variants of daylighting simulation programs based on RADIANCE can be found in Reinhart and Andersen (2006). In 2012, the CEC-research program (Heshong, 2012) showed good correlation between user satisfaction with the daylighting situation and the daylight autonomy DA. However, there is an ongoing discussion if visual discomfort from excessive brightness should be taken into account in the evaluation of daylight sufficiency (like e.g. in the useful daylight index UDI (Nabil and Mardalievic, 2006)), but due to the findings in Heshong (2012) this does not increase the prediction of daylight sufficiency. With DA, it is therefore possible to take the control strategy for the complex fenestration system into account and to evaluate the daylighting performance. It is essential to use the same control strategy for the assessment of daylighting, visual comfort and passive solar gain control. In the case of “manual control” or possible “manual override”, the control strategy is not known a priory. It is nevertheless possible to evaluate different control strategies to assess the robustness of the performance of a complex fenestration system against user interventions (see also the discussions in chapter 4.3.3 on page 33). In chapter 3.1.2, the effective solar heat gain coefficient g<sub>eff</sub> has been defined for the evaluation of the solar-control performance. There are also ongoing activities to define an “effective daylight autonomy” DA<sub>eff</sub> (Reinhart and Andersen, 2006) which takes a reference control strategy for a venetian blind system into account. It is possible to calculate the daylight autonomy DA for many points in a room in order to analyze the distribution of the DA. It is an

actual research topic to find metrics to evaluate the DA distribution in the room with respect to overall user satisfaction with the daylighting situation in the room as a whole (not only at certain positions in the room). One of these approaches is the ‘spatial Daylight Autonomy’ sDA (Heshong, 2012), another one is the Annual Sunlight Exposure (ASE) (Illuminating engineering society IES LM, 2013).

### 3.3.1. Conclusion on criteria for daylighting

Dynamic metrics are necessary to evaluate the daylighting performance of switchable complex fenestration systems realistically. Static metrics like the daylight factor D are not appropriate, especially because they are based on overcast sky conditions which are not relevant for the use of solar-control systems. Dynamic metrics exist together with validated modeling tools. The daylight autonomy DA seems to be the most appropriate metric. It has the additional advantage that the results can also be used to determine the energy demand for artificial lighting.

### 3.4. Criteria for visual comfort

Visual comfort is an important parameter that influences the well-being and performance of human beings. It becomes even more important in working environments that include computer monitors or other visual display terminals. The key aspects of visual comfort are:

* visual contact with the exterior,

* glare protection and avoiding reflections on computer screens,

* contrast between visual task and background,

* sufficiently high illuminance values,

* color rendering of objects in the room.

Often conflicting aspects have to be optimized case for case, e.g. visual contact with the exterior and providing glare protection.

### 3.4.1. Criteria for visual contact with the exterior

Visual contact with the exterior is a very important aspect of visual comfort. User assessments with different venetian blinds in Freiburg, Germany (Wienold and Christoffersen, 2006) have shown, that many people even accept some disturbance by glare in order to be able to see the outside. Nevertheless, a validated and generally accepted metric for visual contact does not exist at present. Such a metric would at least have to address the rating of the following aspects:

* Sufficiently large view of the exterior. It is obvious that the size of the view depends not only on the size of the window but also on the distance between observer and facade. User satisfaction also depends on the position of the image relative to the person’s viewing direction (an observer will not be satisfied e.g. with a window behind himself).

* Quality of the image. The quality of the image is influenced at least by the following aspects:

- Color rendering of the outside image. The color rendering index R<sub>A</sub> (EN 410) can be used as metric also for this criterion although it is intended to be used for the color rendering of objects inside the room which are illuminated by daylight which has passed through the facade. The reason is that the color stimulus (tristimulus value) of a colored surface is identical for both situations (Kuhn, 2016).

- Sharpness and image distortion (the quality could be reduced e.g. in the case of some daylight re-directing glass patterns).

- No disturbing reduction of contrast by haze (which can occur e.g. in the case of some light-scattering white fabrics) or
sources with disturbingly high luminance in the viewing area (like the glossy edges of some perforated slats of venetian blinds when the sun is shining on the edges). It is important to note that all of these effects are generally irrelevant without direct irradiation. The effects are most evident when relatively dark images are being observed (e.g. a park in the shadow of large trees) and when the facade is at the same irradiated directly by the sun with high illuminance values. This should be taken into account when such effects are evaluated.

### 3.4.2. Criteria for glare protection

Glare can be categorized in the following way:

* **Glare causing ocular health problems** like temporary blinding or permanent damage of the eye. This can happen for example in the case, when people are observing a solar eclipse phenomenon using glasses with damaged filter or when they look into devices that concentrate solar radiation (like lenses or curved mirrors). A discussion of criteria for the evaluation of high glare levels, which can be produced e.g. by small concentrating heliostat fields in building facades, can be found in Gonzales-Pardo et al. (2015).

* **Disability glare** impairs the visual perception of objects, in most cases by reducing the contrast between colors or black and white, but it is also possible that strong light sources (like the flash of a camera) disturb the visual perception by reducing the size of the pupil and therefore reducing the brightness of the rest of the image. Contrast reduction is mainly caused by stray light within the eye, within the air between the eye and the viewed object or by light reflections on surfaces (veiling glare, see below). The stray light in the eye can be caused by light scattering effects at the iris cornea, the lens or the vitrous humor. The intensity of stray light in the eye varies greatly from person to person and it generally becomes stronger when people get older. Stray light between the eye and the viewed object caused by scattering (e.g. by atmospheric particles) creates additional luminance in the field of view that reduces the contrast of the original image. Criteria for the evaluation of disability glare caused by contrast reduction are therefore directly linked to the evaluation of the minimum contrast requirements in a given scene (e.g. the contrast between black letters and white background on a computer screen or on paper). A discussion of minimum contrast requirements on computer screens can be found in ISO 9241-303 (2011), Moghbel (2012) or Schierz and Schmits (2012). Moghbel (2012) considers both, the contrast on the screen and the background luminance. Schierz and Schmits (2012) takes into account different direct and global illuminance levels on different types of computer screens and evaluates the effect not only for black and white but also for contrast ratios between different colors. The determination of the maximum allowable illuminance on the computer screen is a mandatory part the GS-certification (Basis for the GS-testing of visual display terminals EK1-ITB 2000:2015, 2015) of the device, it therefore has to be determined for every visual display terminal. It can be concluded, that illuminance levels of up to 1500 lux are tolerable for ordinary screens and that illuminance levels above 3000 lux are difficult, even for advanced screens.

* **Discomfort glare** is a subjective rating of a lighting situation. The glare level is in many cases below the level of disability glare. It has indirect consequences like fatigue or headaches. It is normally not directly measurable and is therefore determined by user assessments with questionnaires. It is generally rated into four different classes:

- imperceptible,

- perceptible, but not disturbing,

- disturbing, but tolerable,

- intolerable.

Since discomfort glare is a highly subjective and individual phenomenon, it is clear that it is not possible to predict, whether an unknown person will be disturbed by discomfort glare in a particular situation. All discomfort glare rating schemes therefore try to predict statistically the average glare rating of a large group of people. An overview of current work on discomfort glare for daylight conditions can be found in Wienold and Christoffersen (2006) and Clear (2013). Discomfort glare rating schemes have the following general structure:

$$ 
\begin{aligned}
\text{discomfort glare} = & \text{[large glare source term]} \\
& + \text{[small glare source term]} \\
& = \underbrace{\frac{f(\text{overall brightness of the scene})}{\text{adaptation level}}}_{\text{large glare source term}} \\
& + c \log \left( 1 + \sum_{i} \underbrace{\frac{(\text{brightness}_i)^y \times \text{solid angle}_i}{\text{adaptation level} \times \text{position rating}_i}}_{\text{term for all small glare sources i}} \right)
\end{aligned}
\quad (6) $$

The term for small glare sources contains the constant c and takes into account how bright and how large a glare source is. It weights this result by the adaptation level of the eye and by the relative position in the field of view of the observer. This position rating has to take into account the direction and the angular displacement of the glare source from the observers line of sight since the human eye is more sensitive to light coming from below the horizontal plane that contains the eye, than from above. It should also take into account that the eye is much more sensitive in the central region of the visual field than in the peripheral regions of the visual field. The large glare source term consists of a constant and a function depending on the overall brightness of the scene, weighted by the adaptation level. There are strong indications from the work in Wienold and Christoffersen (2006) that both, the experienced brightness of the room and the adaptation level are closely related to the global vertical illuminance at eye level $E_{\text{vert. glob}}$.

Currently, the most widely accepted and best validated glare metric is the daylight glare probability DGP, developed by Wienold and Christoffersen (2006). The DGP is based on extensive user assessments in Germany and Denmark with 74 subjects, more than 110 h tests and in total more than 349 different situations. These user assessments have been carried out in the European research project Ecco-build. The computational effort can be reduced when the enhanced simplified DGP is used instead of the DGP (Wienold, 2009).

$$ 
\begin{aligned}
\text{DGP}:= & c_1 \times E_{\text{vert. glob}} + c_2 \log \left( 1 + \sum_{i} \frac{L_i^2 \times \omega_i}{(E_{\text{vert. glob}})^{1.87} \times P_i^2} \right) + c_3 \quad (7) \\
& = \underbrace{c_3 + c_1 \times E_{\text{vert. glob}}}_{\text{largeglaresourceterm}} + c_2 \log \left( 1 + \sum_{i} \underbrace{\frac{L_i^2 \times \omega_i}{(E_{\text{vert. glob}})^{1.87} \times P_i^2}}_{\text{smallglaresourcesi}} \right)
\end{aligned}
\quad (8) $$

where $(c_3 + c_1 E_{\text{vert. glob}})$ has been interpreted as a second order approximation to the large glare source term. $P_i$ is the Guth position index, $L_i$ and $\omega_i$ are the luminance and the solid angle of each glare source respectively.
* Reflex glare or veiling glare generally creates discomfort or disability glare. When the contrast of an object (e.g. on a visual display terminal) is reduced by additional light stemming from reflected radiation, this is called "reflex glare". In general reflex glare has a stronger impact on the contrast between different colors than on the contrast between black and white (Schierz and Schmits, 2012).

* Outdoor glare with possible impact on workplaces in other rooms, street traffic or aviation. There are no generally accepted criteria for the evaluation of outdoor glare. In Germany there is only a guideline for plants (like e.g. photovoltaic power plants) (Bund-Laender-Arbeitsgemeinschaft fuer Immissionsschutz (LAI)) but it does not consider realistic weather conditions and it assumes ordinary flat glass surfaces into account. It therefore cannot take anti-reflective coatings, patterned glass or anti-gloss structures on the outer surface. Since some of the patterned glass surfaces create stronger glare effects than flat glass because of low-angle scattering, the guideline underestimates the glare situation for these cases. Since it neglects all existing anti-glare surface treatments, it overestimates glare effect drastically in many cases. It can be concluded that there is no valid criterion at the moment, there have only some initial attempts to improve the situation (see e.g. Wienold, 2013 or Brotas and Wienold, 2014).

It is important to understand that a specific glare phenomenon generally belongs to more than one of the categories mentioned above. Evaluation of glare protection can be divided into glare protection in the room and outdoor glare. Glare evaluation in the room can be limited to the evaluation of disability glare and discomfort glare in almost all practically relevant cases, since veiling glare creates discomfort or disability glare and since ocular health problems created by glare normally do not occur in offices or similar situations.

**Conclusion:** In situations like offices with computer screens, glare evaluation can be reduced in most cases to two tasks:

* **Disability glare:** calculate the illuminance on the computer screen (e.g. with RADIANCE) using typical meteorological data for the whole year, to determine whether the illuminance is within the specifications for the screen. Assume 1500 Lux as maximum, if the type of the screen is not known. A carpet plot (temporal raster plot) provides a good overview. If the data is analyzed statistically, it often makes sense to allow a certain number of hours with higher illuminance than required by the specification of the screen, e.g. 1–5% of the working hours.

* **Discomfort glare:** calculate (e.g. with RADIANCE) with typical meteorological data for the whole year, whether the DGP (or the enhanced simplified DGP) is within the limits given in Wienold and Christoffersen (2006), Wienold (2009). This means that DGP values below 0.35 are desirable and correspond in many cases to imperceptible glare. 0.35 ≤ DGP < 0.40 corresponds in many cases to perceptible, but not disturbing glare. 0.40 ≤ DGP < 0.45 corresponds in many cases to disturbing, but tolerable glare. Measures should be taken to prevent DGP values above 0.45 from occurring, since they correspond to intolerable glare in most cases. Again, a carpet plot provides a good overview and it often makes sense to allow a certain number of exceptions, e.g. 1–5% of the working hours.

### 3.4.3. Contrast between visual task and background

This section deals with the contrast between the visual task (e.g. computer screen or printed paper) and the ambient background behind the visual display terminal or the paper. The contrast between e.g. the black letters and the white paper on which the letters are printed is not covered here, as it has been discussed in

chapter 3.4.2. In general, it is desirable for the background to have a lower luminance than the visual task itself in order to minimize reflex glare/veiling glare. The difference between the ambient luminance of the background behind the visual task and the luminance of the visual task should be limited in order to avoid tiring darkness-brightness re-adaptation of the eye. However, this does not mean that a monotonic, completely homogeneous and tedious luminance distribution is desirable. A good compromise has to be found and large differences in the adaptation level over the viewing field should be avoided.

### 3.4.4. Sufficiently high illuminance levels

Sufficiently high illuminance values are necessary in order to perform visual tasks accurately and effectively. The necessary illuminance value depends on the type of activity and the duration of the visual task. Necessary illuminance levels for indoor work places are specified in the EN12464-1 (EN 12464-1, 2011) standard. They range from 100 lux for corridors etc. up to 5000 lux for e.g. special medical tasks. The necessary illuminance values can be provided by daylight and/or artificial lighting. Artificial lighting has to be designed to ensure that the illuminance requirements can always be met, not only when the lighting system is new (maintenance factor). However, this does not mean that the artificial lighting has to be switched on automatically when the illuminance value falls below the required level in daylight situations. The requirement is only that the user must have the possibility to switch on additional artificial lighting in order to reach the required level. Experience shows that a large amount of energy can be saved when users manually switch on the additional artificial lighting instead of an automatic switch, without the user satisfaction with the lighting situation decreasing. This is especially relevant in the case of energy-efficient office buildings, where artificial lighting often accounts for approximately 1/3 of the total primary energy consumption of the building.

### 3.4.5. Colour rendering of objects in the room, viewed by an observer who is also in the room

The color rendering of objects, viewed by an observer who is in the room, is in general different from the color rendering of objects in the room viewed by an observer who is outdoors, looking through a spectrally selective glazing unit into the room (see Section 3.5). The color rendering of objects within the room for an internal observer is evaluated by the general color rendering index $R_a$ as defined in the EN410 standard (EN 410, 2011) and CIE13.3 (CIE 13.3, 1995):

$$R_a := \frac{1}{8} \sum_{i=1}^{8} R_i \qquad (9)$$

where $R_i$ is the specific color rendering index for each of eight different test colors. $R_i$ is calculated for the CIE1931 colorimetric 2° standard observer within the CIE 1964 (U*, V*, W*) color vector space. It is related to the color difference $\Delta E_{i,U,V,W}$ in this color vector space between the test color i illuminated directly by the reference illuminant D65 and by the D65 illuminant transmitted through glazing. The eight test colors are relatively weakly saturated colors distributed over the whole color range (chromaticity/hue range). The test colors 9–14 and the corresponding specific indices $R_9$ to $R_{14}$ have been excluded from the general color rendering index $R_a$ in EN410 (EN 410, 2011). Colors 9–12 are saturated red, yellow, green and blue. Color 13 is close to the color of Caucasian human skin and color 14 is a saturated green that is similar to the leaves of trees. It is therefore clear that $R_a$ undervalues the rendering of saturated colors and light colored human skin. If these colors are important, the color rendering index could be extended or the additional $R_i$ can be evaluated separately. From the evaluation of
the color rendering of LED lamps, it is known that the rendering of weakly saturated colors can be very different from the rendering of saturated colors, as conventional LEDs typically reach $R_a > 80$, but often $R_9 \ll 50$. In general, it can be stated that $R_i$ or $R_a > 90$ corresponds to very good color rendering EN410 (EN 410, 2011). $R_i$ or $R_a > 80$ corresponds to good color rendering (EN 410, 2011).

### 3.5. Color rendering of objects in a daylit room viewed from outdoors

The color rendering index $Ra$, as specified in EN410 (EN 410, 2011) cannot be used to characterize the color rendering quality when objects in a room are viewed by an observer standing outside the room when the source of illumination is also outside the room. Therefore a new index $Ra_{out-in}$ has been defined. It would be advantageous if the color of human skin were to be rendered with high quality when people are looking indoors from outdoors, so that the users in a room do not look pale or ill. Color rendering of objects in a room viewed by an observer who is outdoors is also relevant for the case of internal “white venetian blinds” which are white for observers in a room and which also should appear white when the observer is outside. Instead, “white venetian blinds” often look greenish from the outside when they are mounted behind spectrally selective solar-control glazing.

A new criterion $Ra_{out-in}$ for color rendering of objects in a room, illuminated by daylight and viewed by an observer outside the building therefore has been proposed in Kuhn et al. (2016).

$$Ra_{out-in} := \frac{1}{8} \sum_{i=1}^{8} Ri_{out-in} \qquad (10)$$

where $Ri_{out-in}$ is the specific color rendering index for each of eight different test colors when they are viewed from outdoors. Eq. (10) can also be applied to the supplementary test colors with the numbers 9–14 as discussed in Chapter 3.4.5 and Kuhn et al. (2016). Colors 9–12 are saturated red, yellow, green and blue. Color 13 is close to the color of Caucasian human skin and color 14 is a saturated green that is similar to the leaves of trees. As people tend to react sensitively to the appearance of other people, it may be appropriate to give high priority to color 13, so that outdoor observers do not misinterpret the room occupants to be pale or unwell. In analogy to Chapter 3.4.5 and EN410 (EN 410, 2011), it can be noted that or $Ra_{out-in} > 90$ corresponds to very good color rendering. $Ra_{out-in} > 80$ corresponds to good color rendering. Similar thresholds apply for the $Ri_{out-in}$ for individual colors (Kuhn et al., 2016). Application of the new criterion for several solar control systems in Kuhn et al. (2016) showed that the quality of color rendering is lower in almost all cases, when the observer is looking from outdoors into the room. The difference is significant in many cases and can be much larger than 25%, with the effect that the quality of the color rendering changes from “very good” to “good” or it loses the attribute “good”. The new criterion can therefore help to optimize the facade with respect to both, solar-control performance and user acceptance.

### 3.6. Criteria for outdoor color specifications and color uniformity

In construction projects it is very important that no unexpected differences in the perceived color of the building skin are noticed by observers. First of all the term ‘perceived color’ of the facade has to be defined. The color impression can be expressed as a point in one of the three-dimensional color vector spaces (e.g. RGB, XYZ, L*a*b*, ...). Since perceived color differences are to be evaluated, it is important that equal Euclidean distances in the color vector space correlate with equal visually perceived color differences, which means that the color vector space should be ‘perceptually

uniform'. None of the color vector spaces is perfect with respect to this issue (Wyszecki and Stiles, 2000) and color perception has individual variations but the L*a*b*-system (CIELAB, either for 2° small-field foveal view or 10° large-field view) was specially designed with the goal of being 'perceptually uniform'. The dimensions in the L*a*b* vector-space are the lightness of the color (the L*-value), the contribution in the red-green dimension (a*-value) and contribution from the yellow-blue dimension (b*-value). Within the color vector space it is therefore possible to evaluate also the chromaticity (or color saturation, or colorfulness) and hue in addition to the color distance. The hue is related to the relative portion from the red/green dimension and the relative portion from the blue/yellow dimension. This means that hue is determined by the fraction a*/b*. Normally, there will be no dispute about construction work, if the subjective color impression is acceptable. The problem of individual human color difference evaluation is that impressions of color differences are highly subjective since

* The perception of color can differ strongly from person to person.

* Human vision is not objective and sometimes recognizes color differences between completely identical colors if there is a non-uniform borderzone between them. For example, if there is a large gray area, a black stripe, a white stripe and another large gray area with exactly the same gray as the first gray zone, then the second gray area appears to be brighter than the first one.

* Color differences can be recognized much more easily, if the colored areas are adjacent to each other without any border line between them.

* Color differences can be recognized much more easily, if the colored areas are on a background with a uniform color.

Since the subjective impression is not a reliable criterion, objective criteria have to be used in order to evaluate color differences. First of all, the term ‘perceived color’ of the facade has to be defined. In order to decide if the differences are within the tolerances or if they are bigger than permitted. In general, color differences can be considered to be insignificant when $\Delta a^* \ll 1$, $\Delta b^* \ll 1$ and $\Delta E = \sqrt{(\Delta L^*)^2 + (\Delta a^*)^2 + (\Delta b^*)^2} \ll 1$ when surfaces are illuminated by the unpolarized reference illuminant D65 and evaluated with the sensitivity of the CIE1931 colorimetric 2° or 10° standard observer within the CIE1964 L*a*b* color vector space. But for other boundary conditions, color differences can appear:

* Metamerism: Colors with the same reflectance spectrum have the same apparent color under all illumination conditions. By contrast, metameric colors with different reflectance spectra can have the same apparent color when they are illuminated with one specific illuminant and a different apparent color when they are illuminated with another illuminant.

* Polarization: Sometimes the polarization of the illuminant can have a strong impact on the perceived color. A strongly polarized illuminant, for example, is the blue skylight which is polarized by Rayleigh scattering at air-particles. This can have a strong effect on the polarization dependent color of some types of toughened (tempered) glass panes with solar-control coatings consisting of thin films. Because of practical relevance, it should be noted here that obviously people should not wear polarizing sunglasses when evaluating color differences under daylight conditions.

* Influence of the viewing angle and the angle of incidence: The reflectance spectrum (and therefore the color) of many surfaces strongly depends on the angle of incidence and the angle of the
reflected light that is reaching the eye of the observer. Color can be a bi-directional property. Color differences can therefore appear when identical surfaces are viewed under different viewing angles and/or with different angles of incidence of the illuminant. Non-identical colored surfaces can e.g. have the same color when they are viewed under normal incidence, but they can have different colors when they are viewed under oblique angles of incidence.

It can be concluded, that it is definitely not easy to define gen-
erally valid criteria for outside color specifications and color uni-
formity in construction projects. One important aspect is, that
manufacturers want to have well defined tolerances for lab-
measurements, which can be used for the quality management in
the production. But building owners and architects expect color
uniformity under real illumination conditions, which also includes
for example polarized blue skylight or red-shifted sunlight in the
evening. It can be concluded that it is neither appropriate nor
advisable to use a small number of human observers as the basis
for color difference evaluations because of the subjective color per-
ception of the human eye. Instead of this, all involved parties
should agree on measurable criteria in an early stage of the plan-
ning process and the criteria should take into account all the above
mentioned aspects. It also seems to be clear, that the frequently
used criterion $\Delta E \le 1$, with diffuse illumination with a spectral dis-
tribution corresponding to D65, is too simple in many cases.

3.6.1. *Optional room darkening*

In some situations it is necessary to darken the room to a cer-
tain extent. This ranges from darkening of bedrooms during day-
time, up to high-level darkening of scientific laboratories or
photographic studios. The darkening performance can be differen-
tiated between the performance of the moveable part itself and the
fixing or guiding system. Sometimes even the windowsill is rele-
vant for the darkening performance, especially in the case of win-
dow sills made of e.g. marble, which is partly translucent and
therefore allows some light to be transmitted through the stone
itself. It is also important to note that the adaptation level of the
eye in scotopic vision determines the level of darkening necessary.
In bedrooms, if LCD displays of clocks, radios or TV are present,
there is always a background illuminance of several $10^{-3}$ lux so
that the eye does not reach the maximum sensitivity. The require-
ments are therefore much lower under such conditions than for
laboratory-grade darkening and this background illuminance level
of several $10^{-3}$ lux should be considered when darkening products
for home applications are tested with human observers. For the
moveable part of the curtain, possibly existing "pin-holes" are
the most relevant aspect for laboratory-grade darkening. Both,
the moveable part of the curtain and the complete system includ-
ing the fixing system can be characterized with the criteria and
methods specified in EN14500 (EN 14500, 2008) and EN14501
(EN 14501, 2005). New versions of these standards with enhanced
darkening performance evaluation are currently under preparation
with the involvement of the author of this paper. The new versions
will be published at the end of 2015 or in 2016.

3.7. *Influence on circadian and neuroendocrine regulation through the*
"circadian photoreceptor"

All mammals have an internal timing mechanism that co-
ordinates biochemical, physiological and behavioral processes to
maintain synchrony with the ambient cycles of light, temperature
and nutrients. This also applies to human beings. Several studies
have shown that light is the most potent cue to synchronize daily
activities (Ketema et al., 2009). In mammals, light perception

occurs only in the retina (Ketema et al., 2009). Three different types of photoreceptors are present within this part of the eye: cones, rods and the newly discovered circadian photoreceptor. The circadian photoreceptors were first discovered in 1991 in mouse eyes (Foster et al., 1991). The circadian photoreceptor consists of intrinsically photosensitive retinal ganglion cells (ipRGCs) which are a kind of nerve cell (Berson et al., 2002). Their existence and important role in the human eye have been verified and documented in 2007 by Zaidi et al. in the outstanding paper (Zaidi et al., 2007). The sensitivity of the human ipRGCs peaks in the blue area of the solar spectrum (at ≈481 nm) (Zaidi et al., 2007). Researchers believe that the classical photoreceptors (the rods and the cones) are responsible for image-forming vision, whereas the ipRGCs play a key role in the non-image forming vision (Ketema et al., 2009). This non-image-forming photoreceptive system communicates not only with the master circadian pacemaker located in the suprachiasmatic nuclei of the hypothalamus, but also with many other brain areas that are known to be involved in the regulation of several functions (Ketema et al., 2009). In studies with people who conventionally were considered to be completely blind (they did not have active retinal rods and cones, but active ipRGCs) it has been shown (Zaidi et al., 2007) that this non-image forming ocular sense can affect several aspects of conscious awareness, subconscious awareness and health independently from the circadian system. It can be concluded that it is essential that

* Complex fenestration systems should not block the blue area of the solar spectrum in order to allow the people in the room to correctly synchronize their internal clock with the environmental cycles of light and probably also because of other health aspects. Venetian blinds with colored slats (especially with red slats) or colored roller blinds (especially with red fabrics) are examples of systems that should be avoided or at least used carefully.

* Artificial lighting designers should be careful with blue light, especially with wavelengths around 480 nm, in order to minimize unwanted impact on the synchronization of the internal clock of the people in the room. It is clear that the now almost obsolete incandescent lights had very low energy efficiency and a small portion of blue light. It can be very useful to adapt the spectrum of the artificial lights depending on the daytime.

3.8. *Influence of building envelope materials on the increased*
* temperature of urban agglomerations relative to the surrounding rural*
* areas (urban heat island effect)*

It is well-known that the operative temperature in urban
agglomerations can differ significantly from the temperature of
the nearby rural areas. The difference has a typical diurnal pattern
with increased temperatures, especially in the late afternoon and
the night, known as the urban heat island (UHI) effect and with
sometimes negative temperature differences in the morning which
is called the urban cool island (UCI) effect (Oke, 1987; Bueno et al.,
2012). The UHI effect typically leads to 4 K higher temperatures
compared to the surrounding rural areas which aggravates prob-
lems during heat waves, especially in warm climates, by increasing
sickness and mortality rates. The UHI effect also leads to higher
energy consumption for cooling and to overheating of buildings.
An analysis of the interaction between "energy and climate in
the built environment" was published by Santamouris et al.
(2001). In general the following reasons are known for the UHI

* lower albedo due to darker surfaces (e.g. asphalt streets) and increased multiple reflections in urban canyons,
* higher density of local (waste) heat generation,

* lower heat loss due to convection because of reduced wind speed caused by greater roughness in the urban environment,

* increased surface area because of the morphology/roughness of the urban environment,

* lower evaporation due to the reduction of vegetated areas,

* increased radiative temperature because of buildings with warm surfaces in the surrounding environment.

The building skin (the facades and especially the roof) can help to minimize this effect by lowering the absorption of solar radiation (increased solar reflectance) and by increasing the infrared (IR) heat exchange with the clear sky, which is especially relevant in the case of warm climates with clear sky conditions in summer. The effects of this approach have been the subject of thorough research since the 1990s. An overview of the topic is provided by the Cool Pavements Compendium distributed by the U.S. Environmental Protection Agency and the references there-in US Environmental Protection Agency (2005).

The solar reflectance index (SRI) was introduced by the ASTM task group to promote the development and exploitation of cool construction materials by combining both effects, the solar reflectance properties and the emissivity into a single number (ASTM E1980-11). The SRI is therefore a measure of the effectiveness of a horizontal or low-sloped opaque surface in mitigating UHI overheating effects (Wilson et al., 2015). It is currently defined by the ASTM 1980-11 standard as the relative temperature of a surface ($T_s$) with respect to the temperature of a standard white surface ($T_w$) and the temperature of a standard black surface ($T_b$) under reference solar and ambient conditions. The definitions of the standard white surface (SRI = 100, solar reflectance = 0.80, thermal emissivity = 0.90) and the standard black surface (SRI = 0, solar reflectance = 0.05, thermal emissivity = 0.90) are such that SRI values greater than 100 and less than 0 are possible (Wilson et al., 2015). ASTM 1980-11 also specifies standard solar irradiation conditions and an ambient air temperature of 310 K, a sky temperature of 300 K and convective coefficients corresponding to low, medium and high wind speeds. Using these parameters, a heat-balance equation is solved iteratively to determine the steady-state temperature of the surface in question. Alternatively, the SRI for surfaces with a solar reflectance between 0.2 and 0.9 can be determined using an approximate equation with an average error of 0.9 and a maxi-mum error of 2, which is also cited in ASTM 1980-11. The SRI is therefore a measure for the daily peak temperature but it does not evaluate the effectiveness of radiative cooling during the night. It is clear, that the SRI is not meaningful, if the actual conditions are not similar to the reference conditions. In addition to that, the SRI is a simplified static measure for the daily peak temperature, as already mentioned above. If the impact of specific materials for building envelopes on the diurnal pattern of the UHI effect is to be evaluated, more sophisticated dynamic models (like e.g. Bueno et al., 2012) have to be used. With respect to the choice and design of solar control devices this means, that light colors with a high solar reflectance are preferred with respect of UHI.

### 3.9. Special aspects of transparent building-integrated PV (BIPV) envelope components

Multifunctional transparent building-integrated PV (BIPV) envelope components include the additional function to convert solar energy into solar electricity. The passive solar gains and the impact on thermal comfort are not independent from the solar electricity yield, because the passive solar gains and the temperature of the inner surface of the facade are affected by the portion of the absorbed solar energy that is converted into electricity and

which is therefore no longer available to heat the facade components. All other (visual and optical) criteria are not affected by the additional PV function, since the PV-cells have the same visual and optical effect as e.g. screen-printed patterns with the same reflectance and transmittance. A methodology to simulate the electricity yield with special focus on the simulation of complex BIPV systems and the determination of the cell temperature has been developed under the guidance of the author of this document (Sprenger, 2013; Sprenger et al., 2016).

### 3.10. Special aspects of building-integrated transparent solar thermal (BIST) collectors

Glazing-integrated solar thermal collectors present a particularly effective way to extract absorbed energy from the facade. The inlet temperature of the absorber and the flow rate have a very large influence on the passive solar gains and the temperature of the inner surface of the building envelope. Maximum solar gains and temperatures occur under stagnation conditions with a negligible flow rate. It is therefore impossible to decouple the active and passive solar gain evaluation for such systems and the g-value is not a constant any more, it becomes a function of the inlet temperature $T_{in}$ and the fluid flow $\dot{q}$: g-value = $f(T_{in}, \dot{q})$. All other (visual and solar) parameters are not affected by the additional solar thermal function. A methodology for the evaluation of transparent solar thermal collectors and the integration of the model into the building simulation program TRNSYS has been developed under the guidance of the author of the present paper (Maurer, 2012; Maurer and Kuhn, 2012).

## 4. Design options for solar-control systems - the design space

This section describes the design parameters that have to be specified when solar-control systems are to be chosen for a certain construction project or when new systems are to be developed. The structure of this chapter follows the structure of the design space as presented in chapter 2 and Fig. 1. SubSection 4.1 discusses technology options. Chapter 4.2 discusses positioning and control aspects. Chapter 4.3 presents some examples of advanced solar control systems.

### 4.1. Available technologies

Different technologies are available for solar control systems:

* **Venetian blinds** are defined by the slat design, the slat distance and the optional subdivision of the curtain into two or more separately controllable sections (see also figure 1). An optional vertical subdivision of the curtain into a separately controlled upper section and a lower section would be very good for the performance, but it is omitted in most cases for financial reasons. The upper section is very important for daylighting and the lower-section could be controlled separately according to the visual comfort requirements. Many different slat shapes are available ranging from simple c-shaped slats to sawtooth-shaped retroreflecting slats (e.g. from Retrosolar). The surface properties of the top-side of the slats determine not only the reflectance of the blind, they also have a strong influence on the light transmittance when the slats are not closed. Glossy or mirror-finished surface properties should be used very carefully because of possible glare due to light reflections and possibly non-robust performance (see Kuhn, 2006). However, completely matt surfaces are also not advisable because of possible aesthetic problems with fingerprints. The surface properties of the bottom side of the slats are very important for the
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Venetian blind with spectrally selective slats with high light reflectance and high absorbance in the solar NIR spectral range. A large portion of the solar NIR radiation is transmitted through the cover lacquer and absorbed in the black paint below. Most of the incident visible light is reflected by the slats and can therefore - depending on the tilt angle of the slats - contribute to the daylight supply of the room. picture ©Warema.</figcaption></figure>

brightness (luminance) on the indoor-facing surface of the blind. This brightness in turn, is not only significant with respect to glare protection but also affects visual contact to the outside, since visual contact can be disturbed considerably if the individual slats are too bright. It is possible to use spectrally selective surface properties for the slats of venetian blinds (see Fig. 4). The selective slats with high light reflectance and low NIR-reflectance within the solar spectral range transmit light much better than the radiation with longer wavelengths than within the visual range (≥ 780 nm). This means that external venetian blinds with these slats provide the same daylighting and better overheating protection than conventional blinds with slats with similar reflectance values for both solar NIR radiation and light.

* The performance of roller blinds is determined by both the properties of the curtain and the light-tightness of the mounting system (in some cases with vertical guide rails at the sides of the curtain). The reflectance of the outer surface of the curtain determines the solar-control performance. Another important aspect of the curtain is its direct-direct transmittance, which determines both the ability of the system to block direct vision of the very bright solar disc (with luminances of ≈ 10⁹ cd) and the possibility for visual contact to the exterior. It is clear that both criteria cannot be fulfilled perfectly at the same time and that a compromise has to be found. An extreme case is presented by completely non-diffusing and very transparent coated thin polymer films which can be used instead of fabrics. These foils provide very good visual contact but only limited glare protection and often poor color rendering. The direct-diffuse transmittance determines the brightness of the inner surface of the curtain and therefore has a major influence on glare protection and visual contact to the exterior, since visual contact is not possible in the case of very bright curtains. It is clear that a visual inspection of the properties of a curtain should therefore include situations with direct illumination of the system. The light-exclusion properties of the mounting system are relevant in bedrooms, for glare protection in offices and for the darkening performance in the case of laboratories etc. The requirements are very different for the different types of use (chapter 3). Roller blinds can be mounted internally or between the panes of an insulating glazing unit, a closed cavity

facade or a double-facade. "External roller blinds" are often called "window facade awnings".

* Vertical louvre blinds consist of vertical strips of fabrics that can be rotated around vertical axes. The requirements on the curtain material are similar to the requirements for roller blinds with the difference that it is not important to provide visual contact through the curtain since visual contact is possible between the slats. The slats can be retracted horizontally.

* Awnings can be mounted internally or externally. Internal awnings are mounted horizontally below roof windows, often in big entrance halls etc. External awnings are used to shade conservatories/winter gardens or patios. The solar-control performance of patio awnings is determined by the solar transmittance and the solar absorbance of the curtain, since the device should reduce the direct insolation but should not become too hot in order to avoid uncomfortably high radiative temperatures.

* Pleated blinds can consist of a pleated fabric. They are folded when they are retracted. The design parameters for fabrics of pleated blinds are similar to those of roller blinds. However, there are no light-tight mounting systems for pleated blinds.

* External or internal **curtain panels** consist of a fabric and upper and lower fixings. They can be moved horizontally, but they cannot be retracted. The design parameters for the fabric are similar to the parameters for roller blinds. Since the blinds cannot be retracted, they are always visible and consume space. External curtain panels are often metal fabrics with horizontal sliding fixings at the top and at the bottom. Sometimes perforated metal plates are used instead of fabrics.

* Roller shutters are external devices which in most cases, are designed to provide shelter during the night and during absence of the occupants (burglary protection). However, in many cases, especially in private homes, they are also used for solar-control in summer. They consist of horizontal laths that are hinged together and can be retracted by a roll at the top. The laths can be made of polymer material or metal. The laths can be filled with insulation material. A new daylighting and solar-control roller shutter with translucent rods has been developed by the author (see chapter 4.3.4).

* Externally mounted fixed or rotatable individual slats are typically much larger than the slats of a venetian blind. They can be made of metal, glass or polymer material. There are a
lot of similarities between glass slats (with or without screen printed patterns) and fabrics of roller blinds, therefore a similar reasoning applies for the discussion of the design parameters. In many cases glass slats with a high direct transmittance are used, which means that the glare protection performance of such systems is limited. The individual slats can be mounted horizontally or vertically. There are also newly developed shape morphing shading systems with “slat” movements based on hingeless flapping mechanisms inspired by nature (Lienhard et al., 2011).

* **Overhangs** are static cantilever extensions of a building, formed by the roof overhang, integrated into the facade or especially designed to provide shelter for an entrance or a patio. They effectively provide solar protection in summer, especially in the case of southern orientation because of the high profile angle of the sun in summer. They are much less effective in western or eastern orientation.

* **Sun sails** consist of a piece of fabric and a fixing system, forming something like a freestanding roof. They can be rigid and not retractable or retractable. Rigid sun sails also provide rain protection for outdoor areas. The discussion about the design parameters of fabrics of sun sails is very similar to the discussion for awnings. Rigid sun sails have to be windproof.

* **solar-control glass panes** can have switchable or constant properties. They can be used in glazing units, glass slats or as ventilated glass panes in double-skin facades.

**Classic solar-control glass panes with constant properties** have spectrally selective transmittance. The spectral selectivity of single, double or triple glazing can be characterized by the ratio S between the light transmittance and the g-value $S = \tau_v/g$. Typical values for double glazing are S ≈2, e.g. S = 66/34 or S = 50/25. Higher values of S are difficult to achieve under the boundary condition of high values of the general color rendering index $Ra$. The spectrally selective glass panes are normally nano-coated with interference coatings. The nano-coating consists in most cases of two or three silver layers which provide low-emissivity at the same time. Triple-silver coatings normally provide better color rendering properties than double-silver coatings. Most high-performance coatings are not stable under normal atmospheric conditions and therefore have to be integrated into sealed glazing units or laminated safety glass.

**Switchable layers on glass panes** can change their visual and solar-control properties because of intrinsic or extrinsic triggers. The switching mechanism can be based on the following principles:

- The transmittance of **electrochromic** layers can be switched externally by applying a certain voltage to the glazing. The optical properties are influenced by changing the oxidation state of tungsten oxide electrically. Different darkening levels are possible depending on the voltage. Color rendering properties are challenging in many cases because of the deep blue color of the tungsten oxide. Different measures are taken to compensate this effect. Electrochromic glazing provide good visual contact with the exterior but limited glare protection because of the non-scattering properties and because it does not block visual contact with the solar disc. Electrochromic layers on glass are available on the market and used in the construction industry and the automotive industry.

- **Gaschromic** glazing can be switched by inserting a chemically oxidizing or reducing gas into the glazing cavity adjacent to the switchable coating. In most cases, the optical properties are influenced by changing the oxidation state of tungsten oxide with the oxidizing or reductive gas. The optical properties are similar to the properties of elec-

trochromic layers. No energy is required to maintain the switching state. Large areas can be switched more evenly than with electrochromic glazing.

- The transmittance of **photoelectrochromic** layers (Bechinger et al., 1996; Gregg, 1997) can be switched similarly to electrochromic glazing with the difference that the coating itself generates the voltage which is needed for the switching of the optical properties. Opening or closing the electric circuit externally darkens or bleaches the device respectively. Stand alone devices can only be switched into the dark state when they are irradiated by the sun. When the electrodes of photoelectrochromic devices are coupled to an additional external power supply, each state (bleached or colored) can be manually stored or changed by applying an external voltage between the two electrodes.

- No external trigger is possible in case of **photochromic** layers which always switch into the dark mode when they are irradiated by the sun. The main challenge of most photochromic glass panes is the strong temperature dependence of the switching mechanism. Photochromic layers are mainly used in sunglasses and not in facade systems.

- **Thermochromic** layers change their color depending on the temperature of the layer. No application in construction products is known, since most of the layers have little influence on the light transmittance.

- **Thermotropic** layers (Wilson, 2000) change their scattering properties depending on the temperature of the thermotropic layer. The basic physical mechanism is good mixing of two different phases at low temperatures and segregation of the two phases at higher temperatures. The scattering above the phase change temperature is often caused by multiple Mie-scattering by domains with dimensions similar to the wavelength of the incident radiation (Nitz, 1999). Thermotropic layers are e.g. polymer blends, hydrogels or lyotropic liquid-crystal polymers LCP, which have to be encapsulated between two glass panes. Polymer blends consist of two or more polymers which are blended together, switching from a homogeneous miscible phase at lower temperatures to a heterogeneous immiscible phase at higher temperatures. Hydrogels are networks of polymer chains dispersed in water, being evenly distributed or forming discrete domains depending on the temperature. Lyotropic liquid-crystal polymers LCP consist of dissolved polymers in a solvent, forming different liquid crystal phases depending on the temperature. In all cases the differences in the refractive index of the segregated phases at higher temperature create scattering similar to the fat droplets in milk. There have been several research projects in the construction sector dealing e.g. with the application of thermotropic layers in facade elements or as overheating protection for solar thermal collectors in stagnation conditions. Thermotropic layers are not completely clear and non-scattering in the “clear state” in most cases. It is therefore difficult to use these materials in the vision area, where visual contact with the exterior has to be provided. The most difficult situation is when people are viewing a dark scene in the shadow, while the glazing is directly irradiated by the sun (haze).

- **Polymer-dispersed liquid-crystal layers (PDLC)** have properties similar to thermotropic LCP-layers with the difference that the switching is activated by an electric field. They therefore can be switched externally. They also have some residual haze in the “clear-state”, especially under oblique angles of incidence. Glass panes with PDLC-layers are available on the market. Because of the small influence on the g-value and the possibility of haze they are normally used only
for internal applications such as privacy screens, e.g. in some
of the German ICE-trains behind the train driver's cabin.
More details on switchable layers can be found e.g. in BINE
themeninfo 1/2002 (2002).

* Transparent building-integrated PV-elements (BIPV) can be used as a substitute for transparent fabrics, screen-printed glazing or static blinds. The efficiency is higher than the efficiency of conventional solar-control systems, since parts of the absorbed radiation is converted into useful electricity. Several systems are available on the market. They can be integrated into sealed glazing units (warm facade) or mounted externally as ventilated glass panels.

* Transparent building-integrated solar thermal (BIST) collector elementscan be used instead of static blind systems or fabrics to reduce the transmitted solar energy. They can be integrated into glazing units (warm facades). At least one system with glazing-integrated collector was available on the market (Robin Sun, www.robinsun.com). Alternatively, they can be installed additionally in front of the facade like e.g. a balcony parapet. An example of this is given by vacuum tube collectors which provide similar aesthetics to conventional protection systems, that prevent people from falling off balconies, floor-to-ceiling windows or fully glazed facades (see Fig. 16). In the case of additionally mounted systems, care has to be taken to prevent glass pieces from falling down if one of the tubes break. The collectors therefore should be protected with an additional laminated glass pane or other measures. When collectors are mounted on tilted cantilever extensions of facades, the additional glass pane (with its additional reflectance losses) can be omitted by using other protective systems.

## 4.2. Positioning and control aspects

### 4.2.1. Control

The control of sun-shading systems is very important for their real performance, since solar-control systems are only effective when they are in operation and not retracted. External devices are motorized in most cases with different priority levels in the control algorithm. The highest priority level is normally assigned to firefighter and wind-protection triggers. The performance can be enhanced greatly by using presence detection and by closing the blinds completely when nobody is in the room during the cooling season. The same is true for the heating season when the blinds are retracted completely when nobody is in the room.

It is very important to recognize, that the average setting of the solar control systems in a building depends on the initial status in the morning and that the user behavior can be influenced easily without limiting the user interactions and the possibility of manual control. The reason is that most users do not test all possible blind positions and then choose the best setting. Most of the users just change the setting of the blind until it meets their actual requirements. This means that it is very efficient to pre-set the blinds at night in order to ensure that the blinds are completely closed in the early morning during the cooling season and that they are completely retracted during the heating season. The users then might choose their desired setting during the daytime. This procedure has been implemented since 2001 in the main building of the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg (Gerber and Herkel, 2008). The results are shown in Fig. 5. It can be seen that most of the users do not completely retract the blinds in summer and that most of them do not close the blinds in the heating season when the blinds are closed or opened in the morning respectively. The procedure also ensures that the blinds are in an energetically advantageous status when people are absent due to holidays, business travel or week-ends.

More advanced control modes are sometimes used (e.g. cut-off control, where the slats are tilted dynamically in order to maximize visual contact and at the same time blocking the direct irradiation). Silent motors enhance greatly the acceptance of all control algorithms that change the blind position when someone is in the room.

### 4.2.2. Positioning

External solar control systems, such as external venetian blinds, are generally more effective than internal blinds when they are mounted outdoors in front of a well insulated glazing unit. But they have to withstand the environmental wind conditions and sometimes also sand- and hailstorms. External systems are therefore generally more expensive than internal systems. Sometimes it is impossible to install external systems when the external wind conditions are too difficult, like in high-rise buildings. The positive impact on the cooling load is partly compensated with additional heating loads when highly effective external solar control systems are used as glare control systems in the heating period. A combination of external and internal systems is therefore recommended in most cases.

Solar control systems integrated in the facade between the glass panes, such as integrated roller or venetian blinds, can be very effective when insulating glass panes on the indoor side are providing a high thermal resistance between the indoor space and the solar control device and when the thermal resistance between the device and outdoors is much smaller than the thermal resistance on the indoor side (Kuhn, 2007). Such systems are completely wind stable when the distance between the solar control device and the nearest glass pane is large enough to ensure that a collision between the pane and the device can be avoided under all circumstances, including the combined effects of external wind suction/pressure and the pumping of sealed glass panes due to the expansion/compression of the fill gas due to temperature changes. Pumping can be avoided when the pressure difference between the facade and the exterior is compensated by e.g. air exchange between indoors and outdoors. Care should be taken in order to avoid condensation, when air exchange is possible, e.g. with concepts such as closed cavity facades which are always maintained to be at slightly higher pressure than the outdoor environment. The positive impact on the cooling load of highly effective integrated solar control devices is partly compensated with additional heating loads when highly effective solar control systems are used as glare control systems in the heating period. A combination of external and internal systems is therefore recommended in most cases.

Internal solar control devices, such as internal roller or venetian blinds, can be very effective for the control of visual comfort in the room. They can also have a significant effect on the total solar energy transmitted (g-value) when they are able to reflect a large portion of the incident radiation back to the outdoor environment. There are highly effective mirror finished (see chapters 4.3.1) and white internal venetian blinds (see chapter 4.3.3). In general it can be said that venetian blinds with mirror finished systems are more effective when they are in optimal position (see Kuhn, 2006) and that the solar control performance of white systems is more independent from the actual tilt angle (more robust with respect to user interactions in the case of manual control). In case of high requirements on passive solar gain control, internal venetian blinds should be combined with solar control glazing.

## 4.3. Examples of advanced solar control devices

Examples of advanced solar control devices are presented in the next subsections.
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Diurnal profile of the mean opening status of the blinds at the main building of Fraunhofer ISE in Freiburg, Germany (Gerber and Herkel, 2008). 12 rooms with approx. 30 users have been assessed. In the heating period (January and March), the blinds are retracted during night time in order to propose open blinds to the users in the morning and in order to ensure that the blinds are retracted when nobody is in the room e.g. at week ends. In summer (July and September), when overheating has to be avoided, the blinds are closed at 2:00 am. The circle around the blind with the black marks shows one of the monitored blinds. The blind position has been recorded by pictures of a CCD camera. Pictures ©Fraunhofer ISE.</figcaption></figure>

### 4.3.1. Systems for retroreflection of the direct solar radiation

Internal venetian blinds with slats with mirror finished top surface are generally more effective when they are retroreflecting. The reason is, that they reflect almost all light to the outside when they have the optimum slat tilt angle. Even in the case of non-optimum slat tilt angles they also reflect more light to the outside in general, than slats with flat mirror finished top surface. The retroreflection properties can be realized by a retroreflecting structure on the top surface, such as the Retroflex slats (see Köster, 2002 and Fig. 6) or by a retroreflecting shape of the slat (see 7). The bottom surface of retroreflecting slats is matt white or light gray in most cases in order to improve glare protection.

### 4.3.2. Stainless steel roller blind

The shape of the rods of the stainless steel blind s_enn® (see Fig. 8), was developed by the author of this paper, who is a co-inventor of this system (Clauss and Kuhn, 2001). The system selectively shields certain regions of the sky. This external system leads

to very good solar protection (see Kuhn, 2006) and a transparent appearance, while direct insolation of the room and the associated glare is prevented in most cases. s_enn® was awarded prices, e.g. an innovation prize at the 2003 R + T Trade fair in Stuttgart, Germany which is the most important international trade fair for sun-shading systems.

### 4.3.3. White venetian blind with special slat shape

The special shape of the 'Genius slats' (see Fig. 10 and 9) of a new venetian blind ensures good sun-shading properties which are relatively independent of the actual setting of the slat angle over broad ranges, which ensures robust performance despite so-called 'faulty operation' (see chapter Kuhn, 2006). The top side is white with high solar and light reflectance for good solar-control and light re-directing properties. The bottom side is light gray in order to reduce the luminance on the inner side of the blind for improved glare protection. Both sides are matt, with little gloss, in order to minimize irritating and glaring light reflections. The

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** A slat of the Retroflex venetian blind. The direct sunlight is retroreflected by the structure on the top surface of the slat. This improves the solar control function especially in the case of internal venetian blinds. The sun is retroreflected with only one reflection when the slats are in optimum position. Pictures ©Dr. Helmut Köster, Köster Lichtplanung.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Retrolux blinds have special shapes. The zigzag-shape at the outside facing part of the slats retroreflects direct sun. The inner part of the slats is intended to increase the daylighting in the room by redirecting diffuse light or direct light with lower profile angles into the room. Pictures ©Köster-Lichtplanung.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** New stainless steel blind s_enn<sup>®</sup>. The person on the photo at the right-hand side is standing outdoors. The shape of the rods of the new stainless steel blind s_enn<sup>®</sup> was developed by the author of this paper. The blind can be retracted by a roller at the top of the blind, similar to a conventional roller blind. The wind resistance of this blind is much higher than for conventional venetian blinds or roller blinds due to the shape of the rods. Pictures ©clauss-markisen (Clauss Markisen GmbH).</figcaption></figure>

special shape prevents people from looking onto the directly irra-
diated bright area on the top side of the slat, since this area is
shielded in many cases by the "ridge" in the middle of the slat.
Venetian blinds with these slats have been installed e.g. in the
Roche-Tower, Basel, Switzerland, or in the Main-Tower in Frank-
furt/Main, Germany (see Fig. 9).

4.3.4. Roller shutter with daylighting functionality

A daylighting roller shutter (see Figs. 11 and 12) was developed
by the author. It consists of translucent horizontal laths that are
connected by hinges and can be retracted by a roll at the top.
The laths are made of extruded polycarbonate. The inner and outer
surfaces are transparent while the internal bars are translucent
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Internal venetian blind with 'Genius slats' in the 56-storey building Main-Tower in Frankfurt/Main, Germany. The original internal venetian blinds with non-retroreflecting mirror finished surface have been replaced since 2008 step by step by venetian blinds with genius slats in order to avoid overheating (Warema Renkhoff GmbH, 2008).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Internal venetian blind with 'Genius slats'. The 'Genius slats' were invented by Hans-Peter Baumann, Rolf Brunkhorst and the author Baumann et al. (2002).</figcaption></figure>

white. The translucent bars block the direct irradiation. The new roller shutter is intended to be used in all offices or rooms, except bedrooms, instead of conventional roller shutters. The idea behind the development is that the darkening function of conventional roller shutters is not needed in these cases and that the daylighting function of conventional roller shutters is very poor.

### 4.3.5. Angle-selective transparent BIPV-system

The angle-selective transparent BIPV-system PVShade® is shown in Figs. 13 and 14. The system was invented by the author of this paper in 2006 (Kuhn, 2006). It was optimized and further developed by F. Frontini in his Ph.D. thesis under the supervision of the author of the present paper (Frontini, 2009). This multifunctional facade system generates photovoltaic electricity and simultaneously provides solar control and glare protection. It is especially designed to be used in the sill area of completely glazed facades as shown in Fig. 14. This area has very little effect on the daylighting conditions in the room. It therefore can be equipped with a static solar-control system in order to reduce the cooling

load of the building. The angle-selective transmittance originates from the arrangement of the two layers of PV-strips. It selectively shields higher sun positions and allows visual contact in horizontal and downward directions. A methodology for the optical and electricity yield simulation of this complex fenestration system (CFS) with BIPV-functionality has been developed under the guidance of the author of the present paper by W. Sprenger in his Ph.D. thesis (see chapter 2.4.4 and chapter 4.4.2 in Sprenger (2013)).

### 4.3.6. Angle-selective transparent solar thermal facade collectors

Fig. 15 shows prototypes of a new type of building-integrated transparent solar thermal collector (TSTC). These angle-selective solar thermal collectors were invented by Michael Hermann and the author, in 2006 (Kuhn and Hermann, 2007; Hermann and Kuhn, 2006). The basic idea is to integrate openings with angle-selective transmittance into an absorber and to integrate the absorber within a sealed glazing unit or a closed cavity facade. The facade cavity with the absorber is continuously flushed with a very low flux of dry air in the case of closed cavity facades. The multifunctional facade elements allow visual contact to the exterior, provide solar and glare control and generate useful solar thermal heat gains. In summer, during the cooling season, the collector is intended to be used as heat source for thermally driven cooling systems or desiccant de-humidification systems. Desiccant de-humidification systems are particularly interesting since market available systems can already be operated at relatively low temperatures of e.g. 55 °C. The importance of de-humidification will increase, since energy efficient cooling systems like thermally activated building systems (TABS) usually lower the temperature of the ceiling and since condensation has to be avoided under all circumstances. During the heating season the collector is intended to be used as a renewable heat source for low-temperature heating systems, like floor-heating systems. The solar radiation coming from directions with high solar profile angles is selectively shielded by the external surface of the absorber, while visibility through the collector is retained in the horizontal or downward directions for people inside the building. The integration of the new transparent solar thermal facade collectors into a 40-storey high-rise building with desiccant de-humidification and thermally driven cooling systems has been assessed theoretically in Maurer et al. (2011) with co-authorship of the author of this paper. The new multifunctional TSTC systems were installed in a pilot building as part of the European research project Cost-Effective (D3.1.2 Prototype, 2011), together with a thermally driven cooling system for the offices in the uppermost story of this building (see
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Daylighting roller shutter, viewed from outdoors. A cross-section of the translucent lath can be seen in the upper illustration. The new laths were developed by the author. Photos ©Prokuwa Kunststoff GmbH, Dortmund.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Daylighting roller shutter, seen from indoors. On the top, a cross-section of the translucent lath can be seen. Photos ©Prokuwa Kunststoff GmbH, Dortmund.</figcaption></figure>

Fig. 16). The semi-transparent collectors were installed together with air-heating vacuum facade collectors. The pilot building is the building of the Slovenian National Building and Civil Engineering Institute ZAG, in Ljubljana, Slovenia. The energy consumption

and the thermal comfort of the offices in this building has been monitored. The results and a comparison with simulations are described in Jordan et al. (2015) and Jordan et al. (in press) with co-authorship of the author of this document. A detailed physical
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** The angle-selective transparent BIPV-system PVShade® contains two layers of thin-film PV-strips in the outermost glass panes of a triple glazing. The arrangement of the PV-stripes is the origin for the angle-selective transmittance.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** The angle-selective BIPV system installed in the sill area of a seminar room of Fraunhofer ISE in Freiburg, Germany. The solar-control system in the upper facade segment is the stainless steel blind, as described in chapter 4.3.2.</figcaption></figure>

model for this multifunctional facade component was developed by C. Maurer within his Ph.D. thesis (Maurer, 2012; Maurer and Kuhn, 2012) under the guidance of the author of this.

**5. Conclusion and outlook**

**5.1. Summary of the results**

This paper provides a structured overview of the design options and evaluation criteria for advanced solar control sys-

tems with respect to user comfort and building energy con- sumption. The meaning of all the design parameters and evaluation criteria is explained and the technological state of the art with respect to the design possibilities and evaluation methods is presented. The paper also includes multifunctional systems with building integrated photovoltaic (BIPV) and/or building integrated solar thermal (BIST) energy conversion. A chapter with examples of advanced solar control systems com- pletes the overview of the state of the art of solar control systems.
<figure><img src="image_18.png" /><figcaption>**Fig. 15.** New transparent solar thermal collector. The left-hand photo demonstrates the possibility of visual contact from indoors to outdoors. The photo on the right-hand side shows the new facade element mounted in the Testlab Solar Facades at Fraunhofer ISE in Freiburg, Germany. Pictures ©ZAG, Ljubljana, Slovenia and Fraunhofer ISE, Freiburg, Germany.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Fig. 16.** Installation of the angular selective solar thermal facade collectors in a pilot building of the European research project Cost-Effective www_cost-effective-renewables.eu which has been co-ordinated by the author. The angular selective solar thermal collectors have been invented by Michael Hermann and the author, in 2006 (Kuhn and Hermann, 2007; Hermann and Kuhn, 2006). The semi-transparent collectors are installed together with air-heating vacuum collectors. The pilot building is the building of the Slovenian National Building and Civil Engineering Institute ZAG in Ljubljana, Slovenia. photos ©ZAG.</figcaption></figure>

## 5.2. Recommendations for future research

Strongly improved calculation capacities of desktop computers allow much higher complexity in the modeling of complex fenestration systems. One approach for further development is based on goniometric measurements of the bi-directional scattering distribution function BSDF. The exploitation of such complex characteristics together with a strongly improved management of the information flow within the semantic web (internet of things) seems to be very promising in order to enhance accuracy and increase the reliability on the construction site. There are several activities concerning building information modeling (BIM) in the construction industry, like e.g. the 5D-initiative of ENCORD, but much more research is needed.

Another important future research area is the development of new high-performance multifunctional components (with or without active solar energy conversion) for the building skin which can

be produced effectively with industrialized processes that also include the planning, installation and maintenance in order to reduce the life-cycle-costs.

## Acknowledgements

The author thanks the European Commission for funding the Project Cost-Effective (NMP2-LA-2008-212206) in the 7th Framework Programme. See also www_cost-effective-renewables.eu. The author also gratefully acknowledges the financial support by the German Federal Ministry of Education and Research within the framework of the Leading-Edge Cluster Competition and the research cluster Solarvalley Mitteldeutschland under contracts Nos. 03SF0389A and 03SF0406B.

The author is grateful to the European Commission for the funding of the ALTSET research project under the ID SMT4-CT96-2099.

The author also thanks the German federal ministry of education and research for the funding of the REGES project under the ID 0329798A.

The author also thanks the Velux Stiftung, Switzerland which has co-funded the project under the contract number 248.

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