Keywords

Introduction

To the thermally and environmentally comfortable shelter of buildings, sunlight can be a welcome trouble sometimes in that overheating and visual glare are liable to appear in summer due to excessive solar gain. Overheating becomes a major concern for modern office buildings, even in cold climates. Energy consumption is unavoidable for buildings to be a comfortable shelter in order to satisfy space cooling, space heating, lighting, and appliances, which makes buildings one of the largest electricity consumers in a modern city. Energy demands for space heating in cold climates and space cooling in hot climates are among the world’s biggest concerns.

According to studies conducted by EU, buildings account for 40–45% of the total energy demand [1]. Of the total global energy consumption in the building sector in 2010, 20% was consumed by China, 19% by the United States, 15% by the OECD countries, 6% by Russia, and 40% by other countries [2]. The increase of worldwide building energy consumption is estimated to be an average rate of 1.5% per year from 2012 to 2040 [3].

Previous studies proved that the thermal performance of buildings in winter has been greatly improved by applying energy-saving measures and developing new technologies. However, pursuing higher life standards, better affordability of air-conditioning, inevitable urban temperature rising, and global climate change have drastically increased energy needs for cooling [4]. In Asian countries, by 2050, the estimated average cooling energy consumption of residential and commercial buildings will have increased by up to 750% and 275%, respectively [5].

High electricity consumption has brought about a higher greenhouse gas (GHG) emission rate and urban heat island (UHI) phenomenon. What is more, the rise in air temperatures lowers heating demands but enlarges cooling demands [6]. This is one factor of the increasing energy demands nowadays. Another factor increasing the energy demands is the preference for lightweight building components and high transparency in facade systems in modern times. The reason for such preference lies in the fact that large windows and highly glazed facades in new buildings can provide desired natural light, solar gain, and external view by which an international look can be forged and leave with visitors the impression of success and corporate imagery [7]. There is a risk of creating high heating and cooling loads in these buildings.

It is reported that 88% of solar radiation directly incident on a glass surface can be transmitted into the building [8]. Grynning et al. [9] revealed that among all the building envelops, windows are responsible for the most amount of heat gain/loss inside the building, i.e., almost 45%, whereas roof, wall, and floor for 8%, 8%, and 9%, respectively. Twenty-two percent of energy consumption can be contributed by window systems of residential buildings [10].

In recent years, sustainable building design was emphasized by building designers to foster energy conservation, reduce GHG emission, and pursue a healthy living environment. In most buildings, careful design is required to enjoy the benefits of sunlight and daylight and effective control of negative impacts. Poorly designed buildings are more likely to absorb solar radiation, resulting in quick heat gains difficult to emit out, thus resulting in additional needs for natural, renewable, or mechanical systems and increasing the cooling energy demands [5, 11]. By contrast, highly designed buildings, apart from the ability to control the impacts of solar radiation, can slow down such heat transfer, thereby reducing the internal energy demands and realizing energy savings for air-conditioning systems (HVAC) [12].

Passive cooling strategies (PCS) are sustainable design concepts which use zero or low energy input to keep indoor spaces cooled. In a broad sense, PCS for buildings can be classified into three groups: (a) heat prevention, (b) heat modulation, and (c) heat dissipation [13]. Heat prevention strategies are mostly related to solar radiation control, blocking the Sun’s radiation from entering the building to eliminate the possibility of heat gains. Heat modulation strategies take heat storage techniques as a priority in order to control heat flux indoor. Usually, heat is stored during the day and slowly released during the night. Heat dissipation strategies seek to remove indoor heat using on-site natural cooling approaches before releasing it into a natural reservoir (air, water, ground) [14].

Articles regarding different types of cooling strategies between 1990 and 2014 have been analyzed, and it can be seen that passive cooling researches have led scientific interest in different climate contexts [15]. Among the PCS are two interesting bioclimatic approaches for controlling insolation for cooling modern buildings [16]: (1) solar control techniques (SCT) that are designed for solar protection and (2) renewable solar cooling technologies (RSCT) that are designed for solar collection with cooling purposes as the final goal. SCT encompasses a large number of strategies that are highly designed to decrease the impacts of solar radiation on the building envelope while on the interior space, reducing cooling energy demands [12].

Some of these classes of passive strategies, such as orientation, shape, shading, insulation, and their incidences on total energy demand, have been examined by Pacheco et al. [3] whose conclusion is that orientation, shape, and shading strategies are effective at blocking insolation and have a great effect on the ultimate energy demands. The papers between 1990 and 2014 dealing with different passive cooling strategies by means of diverse methods for evaluation have been presented in [12]. It can be observed that solar control strategies (orientation, glazing, and shading) are the most evaluated passive strategies, and the shading analysis has the particularly highest results among them [12]. The importance of shading technology used in buildings can be demonstrated by numerous studies conducted on this topic.

It is evident that window is the most vital part for a building to minimize heat gain/loss. As is known, the most effective method to control solar radiation is applying proper external shading to intercept direct solar radiation before it strikes the window [17]. Shading devices can reduce the solar heat gains and thus provide comfortable indoor temperature, reducing the cooling loads. Drop of almost 6 °C in the room temperature has been observed [18]. Kumar et al. [19] evaluated various passive cooling techniques and found that solar shading alone can decrease the inside temperature by about 2.5–4.5 °C. They can also function as an esthetic element aside from day lighting if properly designed. Solar shading is a highly cost-effective way to control overheating when compared to the alternative of installing air-conditioning system, which makes its utilization be an important aspect of many energy-efficient building design strategies.

Shading devices have been in use since ancient times. The Mughal architecture (India) used the form of inclined and deep shades to cover wider surface area with deep carvings on building exteriors. These carvings can assist with creating mutual shading, and the extended surface helps to increase the convective heat transfer [20].Peebles (1940) firstly explored the relationship between shading and energy use through experiments [21]. Sun shading systems have been studied since the 1960s from which time on criteria has been established for the proper design of the shadow in line with the latitude of a given location [22, 23]. By the end of the twentieth century, Sun shading systems have been properly classified and described in existing literature.

Solar Radiation and Solar Geometry

Solar Radiations

The Sun emits a tremendous amount of energy into space in the form of electromagnetic radiation including X-rays, ultraviolet and infrared radiation, and radio emissions, as well as visible light. The Sun provides free solar radiation and heat for the planet Earth.

The Earth orbits the Sun in an elliptical path and constantly spins around its 23.5° inclined axis. The winter solstice for the Northern Hemisphere occurs on around December 21, when the Sun is directly overhead at noon at the south latitude of 23.5°. The summer solstice occurs about June 21 when the Sun is directly overhead at noon at the north latitude of 23.5° [24]. As the Sun is at one focus of the elliptical path, the distance between the Earth and the Sun is closest when the Earth travels to one end of the major axis in December and farthest when to the other in June. Solar radiation intensity inversely varies with the square of the distance from the source by a range of 3.5% around a mean value of 1353 W/m2 on Earth [24]. Given the amount of solar radiation people received, it is only a small seasonal variation.

The atmosphere acts as thermal energy storage medium and heat exchange agent at the Earth’s surface, which is characterized by two unique properties [25]: transmitting short-wavelength solar radiation (0.23–2.26 μm) and serving as opaque for long-wavelength radiation (>2.26 μm). The atmosphere is able to intercept, absorb, and redirect incoming solar radiation. About 30% of the solar radiation coming from the Sun is reflected back into space by the Earth (approximately 4%) and its atmosphere (26%) [25, 26]. The main conditions that influence how much solar energy a place receives during the day are Sun angle, air mass, day length, cloud coverage, and pollution [26].

The solar radiation in the sky can be direct, diffuse, or reflected radiation, as indicated in Fig. 1. “Direct radiation” is also called “beam radiation” or “direct beam radiation” in that solar radiation travels on a straight line from the Sun down to the Earth’s surface. Shadows can only be produced when direct radiation is blocked. “Diffuse radiation,” on the other hand, describes the sunlight that has been scattered by molecules and particles in the atmosphere but that has reached the Earth’s surface. When the sky is clear and the Sun shines high in the sky, direct radiation accounts for 85% of the total radiation striking the ground, and the diffuse radiation is about 15%. As the Sun travels lower in the sky, the percent of diffuse radiation keeps going up until it reaches 40% when the Sun is 10° above the horizon [26]. Atmospheric conditions such as clouds and pollution also contribute to increased percentage of diffuse radiation, which can reach as much as 100% of the solar radiation on extremely overcast day. Besides, the ratio of diffuse radiation to direct radiation is far greater at higher latitude but cloudier places than in lower latitude but sunnier places, and it tends to be higher in the winter than in summer at these higher latitudes, cloudier places. The sunniest places, by contrast, tend to encounter less seasonal variation in the ratio between diffuse and direct radiation [26]. “Reflected radiation” describes sunlight that has been reflected off of non-atmospheric things such as the ground. Asphalt reflects about 4% of the sunlight that touches it and a lawn reflects about 25% [26]. Very snowy conditions can sometimes raise the percentage of reflected radiation to rather a high level, for instance, 80–90% of the radiation reflected by fresh snow.

Fig. 1
figure 1

Solar radiation from the Sun [25]

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Solar Factor of Glazing

Solar radiation falling over glass is divided into three parts: partly reflected back, partly absorbed depending on the glass thickness, and partly transmitted into building, as shown in Fig. 2. The ratio of each of these three parts to the incident solar radiation defines the reflectance factor, the absorptance factor, and the transmittance factor of the glazing. The solar factor of glazing is the percentage of the total solar radiant heat gain entering the room through the glass. It is the sum of the solar radiant heat energy penetrated through by direct transmittance and the proportion of the energy absorbed and reemitted by the glazing to the interior space [27].

Fig. 2
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Solar radiation and glazing [24, 27]

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Solar radiation without effective shading on building facades during peak hours might induce significant problem of overheating. Ordinary window glass is transparent for the shortwave radiation emitted by the Sun but opaque to the longwave radiation emitted by the objects in the room. Hence, solar radiation, once entered through the glass, gets trapped. The direct solar gain through windows is the greatest heat gain source, which makes the indoor temperature far higher above the outdoor air temperature even in moderate climate. Overheating of indoor spaces represents increasing of energy demands for cooling loads and for air-conditioning.

The main principle of shading design is to control direct sunlight, to maintain thermal comfort, and to introduce enough daylight without causing overheating. In most cold and dry climate, the common practice of shading design is according to direct radiation as it is dominant on clear days and also that diffuse radiation is desirable most of the time as a source of daylighting in the building. However, in hot and humid climates, or when shading mainly used for glare control, the diffuse component should also be considered in the shading design process.

Solar Geometry

Solar geometry is the determining factor of heat gain, shading and the potential of daylight penetration… [24]

Sun angles and window/shadow angles are two most important parameters for shading design.

Sun Angles

To properly design shading devices, it is necessary to be well informed of the position of the Sun in the sky. The most important characteristic of solar position is its seasonal variation. As seen in Fig. 3, in the Northern Hemisphere, the Sun rises slightly northeast and sets slightly northwest in summer. In winter, it rises slightly southeast and sets slightly southwest. It also rises much earlier and sets much later in summer than in winter.

Fig. 3
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Sunrise and sunset throughout the year [24]

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Azimuth and altitude angles shown in Fig. 4 express the Sun’s position relative to a location on the Earth’s surface in the polar coordinates. “Solar azimuth,” also known as the “bearing angle,” is the angle of the Sun’s projection onto the ground plane relative to due south [24]. The solar azimuth angle is zero at solar noon and negative in the morning. “Solar altitude” is the angle of the Sun above the horizon. The solar altitude angle is zero at sunrise and sunset, which reaches its highest value at solar noon. It is a function of latitude and declination of the Earth as well as the hour of a day [24].

Fig. 4
figure 4

Angles between solar and surface azimuths [24]

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Window/Shadow Angles

Building surfaces or windows receive differential amount and quality of light depending on their orientation, time of year and day, as well as the Sun angles. Usually speaking, architects are more concerned about the solar position with relation to a window or solar collector. When attempting to shade a window, the horizontal and vertical shadow angles relative to the window plane are more important than the absolute azimuth and altitude of the Sun. Horizontal shadow angle (HSA) is the horizontal angle between the normal of the window pane, or the wall surface, and the current Sun azimuth, as shown in Fig. 4. Vertical shadow angle (VSA) is more difficult to describe. And up till now, it can be best explained as “the angle a plane containing the bottom two points of the wall/window and the centre of the Sun, makes with the ground when measured in a normal mode to the surface” [28].

Traditional design tools such as Sun path diagrams and shading masks [29, 30] are useful to determine the time period when the window receives solar radiation and the appropriate shading mask to block insolation. Figure 5 demonstrates an example of Sun path diagrams for latitude 30° in the Northern Hemisphere. Information on Sun angles and solar path diagrams can be found in Architectural Graphic Standards [32].

Fig. 5
figure 5

Sun path diagrams for latitude 30° in the Northern Hemisphere [31]

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Types of Shading Technology

Shading of solar radiation can be achieved by a wide range of building components, including facade self-shading, shading devices (overhangs and Venetian blinds, to name just a few), special glazing with high performance, as well as deciduous climbing plants. There is also a considerable amount of research focused on developing and testing new technologies, such as phase change materials (PCM) technology.

Facade Self-Shading

As an esthetic and functional skin of a building, facade self-shading (FSS) is aimed to lower the insolation on building envelop during a certain period [33, 34]. Architects proposed many simple designs effective in blocking insolation during noon and other more complex designs capable to protect buildings from severe insolation between early morning and late afternoon [12]. One typical example of FSS, the inverted pyramidal buildings, was constructed in many cities, for instance, the Bank of Israel (Israel), City Hall (USA), Canada Water Library (UK), and City University of Hong Kong (Hong Kong). They are built with inclined walls instead of the traditional vertical facades. In terms of thermal performance, inclined walls can provide a self-shading effect against the direct solar radiation of high intensity, which will reduce solar heat gain as well as building cooling loads while enhancing daylight utilization [35, 36]. However, the heating demand of inverted pyramidal buildings will be higher in winter.

Shading Devices

Given its practicality and low maintenance features, different types of shading devices (SDs) are increasingly used to weaken insolation impacts, like overhangs [37], louvers [38], roller shades [39], awnings [40], and Venetian blinds [41]. Proper designs of shading devices could block direct solar radiation during summer and allow maximum daylight in during winter [42, 43], aiming to keep indoor spaces conditioned, lower energy demands, and reduce glare levels near windows [12].

Classification of Shading Devices

As summarized by Tsangrassoulis [44], the conventional shading devices can be classified in different ways, for example, by the angular selectivity, the placement location in relation to the glazing, or controllability.

  1. 1.

    By Angular Selectivity

    Shading devices such as overhangs, louvers, fins, egg-crates, and blinds can block direct radiation from the Sun of certain incidence angle while diffuse radiation can still be used for space illumination [44]. Other types of shading devices including screens, curtains, and roller shades reduce both direct and diffuse radiation, whose performance is almost free from the Sun’s incidence angle [44].

  2. 2.

    By Placement Position

    Shading devices can be classified into external type, mid-pane type (in between double glazing), or internal type, according to their placement location in relation to the glazing. Typical commercially available external shading devices manifest themselves as overhangs, vertical fins, egg-crate, louvers, blinds, awnings, roller shades, and perforated metal sheets [44, 45]. Internal shading devices have fabric roller shades, curtains, and blinds as their kinds. The mid-pane systems often refer to louvers, Venetian blinds, and shades mounted in between panes.

    External shading devices of different types are summarized by and illustrated in Fig. 6 [12, 24, 42, 46]. External shading devices are efficient in blocking direct solar radiation before it transmits through the glazing, and orientation is crucial for their performance. Horizontal shading is preferable for south-orientated windows, and vertical shading is more suitable for east/west orientation. They are particularly appropriate for heavily glazed buildings and extensively implemented in regions where heat gain from solar radiation plays a prominent role in the cooling load of a building.

    Fig. 6
    figure 6

    Schematic diagrams of different types of external shading devices: (a) overhang; (b) brise-soleil; (c) filled egg-crate with panels; (d) horizontal panel and louvers; (e) light shelf [12]

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    As for the internal shading devices, direct solar radiation has already entered the room, and far-IR radiation reemitted from the interior surfaces cannot pass through the glass without being absorbed [44]. The solar heat is trapped inside the room, which increases cooling energy consumption during the overheating period [47]. The main purpose of internal shading is to reduce glare and privacy revelation. In most cases, internal systems cost less than the external ones. Most designers have enjoyed the convenience of internal shadings regardless of the advantages for external shadings. However, since they are operated by users, their inefficient use is unavoidably common.

    The mid-pane shading system has larger solar heat gain coefficient compared to the external one. However, they can combine easy handling with simultaneous weather protection and use highly reflective material without having their reflectance properties disturbed by the weather. Solar radiation can be redirected to create a better daylight distribution in the building.

  3. 3.

    By Controllability

    Shading systems can also be categorized as movable/dynamic or fixed/static counting on their controllability [44, 48]. Users may have more control over the former type, but its maintenance costs are higher instead. The most common movable elements are motorized louvers or blinds [44].

    Fixed shading devices are available to be mounted on both the outside and the inside of the windows in buildings, i.e., external and internal. External ones are effective for decreasing solar radiation heat gain. However, they block the daylight from entering the room, which means artificial light is needed, and healthy winter solar radiation would be also blocked [49]. The use of fixed shading devices should take the balance between the decrease of cooling loads and the unfavorable increase of heating loads into consideration [23, 39]. In Mediterranean climate, fixed shading devices are favored as long as there are openings on the south facades, as they can manipulate intense summer daylight, reduce cooling loads, decrease glare, and promote visual comfort [50,51,52].

    Movable/dynamic shading devices often refer to conventional components such as louvers, roller shades, vertical blinds, and Venetian blinds, which can be mounted in such flexible ways as internally, externally, or in between panes [53]. On account of their controllability, they offer more advantages both to allow the favorable sunlight in during winter and to prevent or control the undesirable direct Sun during summer. Therefore, many high-rise office buildings with glass curtain wall systems favor movable/dynamic shading devices to regulate solar radiation and daylight [54, 55].

Typical Shading Devices

After reviewing 109 papers on the use of shading devices in buildings between the years 1996 and 2015, Kirimtat et al. [55] concluded that Venetian blinds, external fixed louvers, roller shades, and overhangs are the top four most commonly studied shading devices.

  1. 1.

    Overhangs

    An overhang is a fixed horizontal surface mounted upon window exteriors, which projects over a window to provide shade. Its type, shape, and size depend on the Sun conditions. Simple overhangs can be highly effective at blocking summer sunlight of high angles, particularly for south-oriented glazing. They do not hinder window opening/closing and never obstruct a full view out. What is more, they can protect exterior windows, doors, and walls from rainwater while keeping the foundation dry in the meantime [56]. In temperate climates, it is effective both in shading a window for reducing glare and solar heat gain during summer and in allowing sunlight in for warming a building during winter, considering the fact that the summer sunlight is of higher angle than the winter sunlight [57].

  2. 2.

    Louvers

    Louvers can be designed in forms of horizontal and vertical slats mounted on the exteriors of a glazed opening or window. External horizontal louvers can be used as a south shading device. Louvers extend just far enough to keep out the highly shining summer sunlight and to permit low warm winter sunlight to transmit heat into the room. The optimum projection (width) of louvers depends on the distance between slats, the latitude, and the local climate type [55].

    The vertical louvers provide effective shading performance mainly on the eastern and western facades. They also act as a wind break to increase the glass insulation in cold seasons. Vertical fins can be arranged to be adjustable and to change their angle relative to the wall in accordance with the Sun’s position, which can either block or allow daylight and solar radiation in for any angle [55, 57]. An example of the vertical louvers in a library building can be demonstrated in Fig. 7.

  3. 3.

    Egg-Crates

    The egg-crate shading devices combine both vertical and horizontal shading elements together and block sunlight and solar radiation from all directions, which make them own a high shading efficiency and could provide optimal shading in regions of hot climate. The horizontal elements control the ground glare. They are extremely massive structures and block plenty of natural light, view out, and winter solar gains [55, 57].

  4. 4.

    Light Shelves

    Light shelves are light-reflecting overhangs. They cannot only shade the space from direct sunlight to avoid overheating but also reflect such sunlight onto the ceiling deeper into the building, which provide more uniform distribution and increase daylight illuminance levels at a distance of even 5–10 m from the window. Light shelves are useful primarily to tall conventional windows and clerestory windows that have large amount of glazing area at a height greater than 2.2 m. They are most effective when installed on the south façade in regions of mild climate although not recommended for tropical or desert climate because of usually intense solar radiation [42].

  5. 5.

    Venetian Blinds

    As a type of most popular shading device [55, 58], Venetian blinds are made up of separate horizontal louvers with equal space, which are widely used for adjusting the daylight penetrated through the glass into the room. Standard Venetian blinds have louvers of 2.5 cm width and 2.2 cm vertical space and are installed 2.5 cm away from the glass surface in commercial buildings. They are effective in ensuring privacy, as well as visual and thermal comfort . The optical and thermal properties of Venetian blinds integrated window are complex and vary with louver characteristics of color, shape, size and configuration, rotation angle, as well as incidence angle of solar radiation [59,60,61]. Figure 8 demonstrates Venetian blinds mounted in a residential building.

  6. 6.

    Roller Shades

    Roller shades are extensively used to manage daylight and control glare in both residential and commercial buildings and have significant impact on energy use of space lighting, heating, and cooling [55]. Either manual or automated top-down roller shades can be deployed continuously from top to bottom. Bottom-up roller shades is a less common configuration. Top-down roller shades can be arranged either with left open for high solar altitude to block the top part of the window while still providing views or closed completely for low solar altitude while potentially eliminating useful daylight and views depending on shade transmittance. The automated versions show appreciable performance in energy savings, according to recent studies [55, 62]. Figure 9 demonstrates an applied sample in a library.

  7. 7.

    Vertical Blinds

    As popular internal shading devices, vertical blinds are used on large windows or patio doors, mainly in commercial buildings and sometimes in residential buildings. The blind slats are either manually or automatically controlled to shade out the direct sunlight entering through window glazing. They are made up of various types of fabric, plastic, wood, or metal materials, with wide range options of texture, color, and design. Another advantage is its quick and easy installation. Vertical blinds can simultaneously satisfy occupant needs for solar radiation control and energy savings daylight and window view [54].

Fig. 7
figure 7

Vertical louvers mounted in Dusa library building of Nanjing University

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Fig. 8
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Venetian blinds mounted in a residential building

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Fig. 9
figure 9

Roller shades mounted in Dusa library building of Nanjing University

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Special Glazing as Shading Devices

Nowadays, high-performance glazing is available to control solar heat gain by manipulating the solar optical properties of glazed surfaces, glazing of high reflectivity, thermochromic glasses, electro-chromic glasses, and holographic glasses, to name just a few. These new glass products may relieve people of the need for external shading devices.

  1. 1.

    Low-E Glazing

    Low-e glass has a special metal coating on one side – either pyrolytic (hard coating) or sputtered (soft coating). The microscopically thin and almost invisible coating acts as a filter that blocks longwave radiation (e.g., infrared heat) and lets in shortwave radiation (e.g., UV and visible light) [63]. During the summer months, the low-e glass reduces the penetration of outside infrared thermal radiation and the overall solar heat gain of a building, while in winter, it retains inside heat (longwave radiation) within the room and keeps the room warm. Eskin and Turkmen (2008) reported cooling demand savings as high as 16% after using low-e double glazing compared to clear double glazing in a simulated office building located in several cities in Turkey [64].

  2. 2.

    Electro-chromic Glazing

    Electro-chromic (EC) glazing is a kind of smart glass whose visible transmittance can be controlled with a small electric charge [65, 66]. Its optical functionality derives from the EC films that change their optical absorption via the insertion or extraction of ions from a centrally positioned electrolyte. The minimum voltage required for their operation can easily be provided by integrated photovoltaics [67]. Use of EC windows may lead to a cooling demand reduction by almost 50%, as manifested by previous studies [68,69,70]. Besides, EC glazing contributes to indoor visual and thermal comfort of occupants [71, 72]. However, in extreme glare situations when the Sun sits low in the sky, providing visual comfort cannot go without the assistance of shading devices [73, 74].

New Shading Technologies

  1. 1.

    PV-Integrated Shading Device

    Photovoltaic (PV) cells utilize sunlight to generate electricity. PV applying in buildings was brought in the late 1970s [75], first featured as a building integrated component in the late 1990s [76], and first proposed to be integrated with shading devices probably in 1998 by Yoo and Lee [77].

    Photovoltaic panels, traditional types or more recent transparent types, can be combined with external solar shading devices in both new and existing buildings [78]. Such panels offer advantages by generating electricity directly from the incident sunlight, by reducing indoor overheating, by providing visual comfort, and by saving energy [79,80,81]. Their technical advantages are also proved over other PV integrations like roof stand-alone PV systems [82], for example, ease of inspection and maintenance, the roof space freedom for other uses, and higher possibilities to integrate dynamic Sun-tracking technologies for optimizing solar gain throughout the year.

    However, it is quite challenging to apply PV-integrated shading devices due to the complexity and adaptability of these systems in varying conditions [83]. Their configurations and dimensions would influence the performance of buildings with PV system [49]. For appropriate application of this technology into a building, the main influential parameters have been highlighted, which are optimal incline angle of the devices with the correct size and right distance from the glazing [75, 78].

  2. 2.

    Phase Change Materials (PCM)

    Phase change materials (PCM) are capable of storing and releasing large amount of energy, and the introduction of PCM in building components is an efficient method to uplift the thermal energy storage capacity in latent form and the thermal inertia [84, 85]. PCM technologies were developed for the translucent and transparent building envelope such as windows [85], shutters [86, 87], and blinds [88, 89], which have been increasingly applied in facade over the last 20–25 years. A literature review on PCM by Silva et al. [90] reveals that research studies on prototype solutions show PCM’s potential to enhance the thermal performance of buildings through the glazing and shading devices and to improve thermal comfort and therefore occupants’ satisfaction.

Deciduous Climbing Plants as Window Bioshader

Deciduous climbing plants are good at shading the facades in summer. They are sensitive to season changes of climate and function as dynamic solar window bioshader, which demonstrate considerable advantages over engineered shading structures, say awnings, louvers, and screens [91]. As their peak growth approaches in summer, they absorb most of the solar radiation via photosynthesis and evapotranspiration on leaves [92] and reflect or transmit only a small amount of solar radiation into the room. Thus, overheating and undesirable glare in the building is decreased. In winter, their work flow is the other way around. The bioshader sheds off its leaves and the solar radiation can penetrate through the glass into the building, which reduces heating demand [55] (Fig. 10).

Fig. 10
figure 10

Sectional view of a window bioshader [93]

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The shading performance of a bioshader depends heavily on the density of the foliage and the coverage ratio. In an experimental study on the shading coefficient of deciduous climbing plants conducted in the UK temperate climate, the value of solar transmittance through one leaf layer covering southwest window initially reached around 45% and then dropped to 12% when it passed over five leaf layers [93]. As a matter of fact, the morphological plasticity, species diversity, seasonality, and dynamic response of these deciduous climbing plants to diurnal changes in light conditions and ambient temperatures make quantifying and predicting their thermal performance much more complex [91]. Although climbing plants used for moderating the microclimate of buildings originates from rather a long time ago, very few scientific researches on window bioshader have been done for evaluating such influences [55, 93].

Performance of Shading Technology

Methodology

As summarized by Kirimtat et al. [55], who have looked through all the relevant studies on static and dynamic shading devices from 1996 to 2015, 54% of reviewed studies have been conducted in a theoretical way, and only 20% were carried out experimentally on site or in test cell facilities, and the 26% left combined both analytical and experimental methods.

Experimental Studies

Based on the research objectives addressed, experimental studies are carried out either in test bed facilities with full-scale mock ups or in real buildings, under preset experimental conditions. Post-occupancy evaluation (POE) studies are performed under preestablished patterns of usage and aim to examine the buildings’ actual performance as well as their occupants’ behavior intentions and comfort level [58].

Test bed facilities have been widely used to examine the performances of static/fixed and movable/dynamic shading devices. The Lawrence Berkeley National Laboratory (LBNL) in California is famous for test cell experiments and boasts of a long history starting with the Los Alamos experiments on configurations of shading systems related to passive solar buildings in large-scale test bed facilities [94] during 1975–1985. Then the MoWiTT (Mobile Window Thermal Test) program [95] carried out a series of experiments on window systems with shading devices such as internal and external Venetian blinds. The Windows and Daylighting Group at LBNL investigated the performances of internal and external shading/daylighting systems for operation improvement and control algorithm development. Visual comfort of occupants is another focus in many studies using technique of high dynamic range (HDR) images. Lee et al. [83] adopted LBNL’s test bed facility and investigated the performances of energy savings and visual comfort using different dynamic shading systems such as internal and external Venetian blinds and roller shades.

In filed study conducted in Oakland Federal building by LBNL, Lee et al. [53] measured two identical rooms’ cooling loads and lighting electricity use and examined the impact of automated blinds on the building’s energy performance, work plane illuminance, and view out. Another study by LBNL [96] has used a 450 m2 full-scale and fully furnished test bed facility at the southwest corner of the New York Times building and tested a facade system that consists of external shading, fritted glass, and shades for sunlight and glare control. Its performance in terms of visual comfort and views to the outside was examined and whether the best control strategy works was verified using automated roller shade system [96].

The “Energy and Building Design (EBD)” department of the University of Lund has a test facility consisting of four climatically controlled south-facing rooms, which has been used by several field studies on the energy performance as well as thermal and visual comfort level of occupants. The BELOK collaboration team at the Swedish Energy Agency and local owners of commercial properties employed the EBD facility to examine the performance of newly developed shading/daylighting systems of motorized conventional blinds in terms of electric lighting consumption and shading performance [97].

Roche [98] adopted test facilities of office building in the British Building Research Establishment (BRE) to examine custom-made motorized roller shades coupled with electric lighting control system, in terms of performance of energy consumption, glare, and illuminance. At the Centre for Window and Cladding Technology building in the University of Bath, Skelly and Willkinson [99] used the test room of a single occupant cellular office with a window wall facing 40° east of south and verified the performance of shading system of motorized Venetian blinds and a controllable lighting system.

Simulation Tools

As summarized by Dubois [23], since the early 1980s, computer programs have been developed to accurately determine the optimal geometry of external shading devices such as overhangs and awnings under conditions of clear sky while not including algorithms to evaluate their energy performance. Since the middle 1980s, dynamic computer programs have been developed to calculate the radiative heat transfer through solar protective glass and shading devices [23]. As one of the most important contributions, Pfrommer et al. (1996) created a dynamic model to calculate radiation flows through internal and external Venetian blinds in which both the diffuse and direct solar radiation can be fully considered [100]. Promising advances on dynamic energy simulation programs were algorithms proposed to determine optical properties of window-shade systems and replace the parameter of shading coefficient, which depend on the Sun angle and accurately represent the radiative energy flows through complex fenestration systems coupled with shading devices [23].

To accurately design shading device for the energy efficiency of a building, simulation modeling is recognized to the easiest and fastest way. Theoretical models of shading system performance including analytical [100] and numerical methods [41] constitute the backbone structure for simulation tools [58]. A wide variety of the energy and lighting simulation tools developed since the 1990s are being widely used by researchers and designers and progressively improved day by day. Crawley et al. [101] compared the features of 20 simulation tools of building energy performance including ESP-r, EnergyPlus, and TRNSYS, based on a series of criteria including daylighting, solar performance, and energy savings. Most of the reviewed simulation tools have allowed for the simulation of controllable window blinds as well as user-specified daylighting control [58]. Radiance and Daysim are mostly used tools to simulate daylighting and shading systems. Kirimtat et al. [55] have made a careful analysis of the simulation tools for shading devices in buildings used between 1996 and 2015 and came to the conclusion that EnergyPlus, Radiance, and DOE-2 are the most popular simulation tools while the least popular tools are Ecotect and DIVA for Rhino. They also summarized and tabularized functions and other detailed information about simulation tools used most widely in scientific researches, including EnergyPlus, DesignBuilder, DoE-2, Ecotect, DIVA for Rhino, IES-VE, Radiance, ESP-r, TAS, TRNSYS, and others.

Simulation is the preferred alternative in order to assess the performance of different shading strategies or systems as well as complex fenestration systems (CFS), which enables the possibility of easily testing different alternatives within a cheap and controlled experimental setup compared with built prototypes and field studies.

Performance Parameters

Previous studies, despite having differentiated research focuses, use a series of performance parameters such as the optical and thermal properties, energy demand, and visual comfort to evaluate the performance of shading systems.

Physics Properties

Thermal and optical elements for glazing include thermal transmittance (U-value), the solar heat gain coefficient (SHGC, g-value), and the visible/solar transmittance and reflectance values. g-value is the coefficient commonly used in Europe to measure the solar energy transmittance of glass. SHGC is used in the United States and most commonly refers to the solar energy transmittance of a window as a whole (Fig. 11). Tsangrassoulis [44] defined SHGC as “the fraction of incident solar radiation that actually enters a building through the entire based on the estimation of a characteristic window assembly as heat gain.” SHGC is analogous to g-value when referred to the solar energy transmittance of the glass alone. Shading coefficient (SC) was succeeded by SHGC in the United States, which is calculated as the ratio of examined glazing g-value to the g-value of a 3 mm clear float glass (0.87). SHGC ranges from 0 to 1, a lower value representing less solar gain. Standards of EN 410-2011 and EN 673:2011 can be applied to measure the values of these characteristics.

Fig. 11
figure 11

Schematic diagram of solar heat gains through glazing and shading [44]

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For a shading device, the method to describe thermal and optical characteristics has no significant difference from those used for glazing. Since the 1960s, a number of researchers have tried to evaluate windows’ thermal transmittance change due to shading devices, and many studies have been done to quantify the heat flow reduction through windows equipped with different types of shading devices in the 1970s and 1980s. As Dubois (1997) summarized, for windows with single pane and clear glass, internal shading devices such as Venetian blinds, roller shades, and draperies could reduce heat losses by 25–40% and metallic coated shades by 45–58% [23]; for windows with double pane and clear glass, roller shades, roman shades, and films reduce heat losses by up to 50% [102]. The thermal resistance of the window shade system can be greatly uplifted if the space between shading device and window glass is airtight [23].

Since the end of 1950s, many studies have attempted to define optical properties of shading devices. The optical properties of shading devices or solar protective glass can be also described in terms of solar transmittance and reflectance values, solar heat gain coefficient (SHGC), or shading coefficient (SC), indicating their capacity to block solar radiation, “how well a shade shades” [23]. These studies do not usually include buildings’ annual energy consumption or indicate the optimal shading strategies for any particular climate [23].

According to shading coefficient, Olgyay (1963) rated different shading devices from the least to the most effective one in blocking solar radiation [22]. Dubois’ (1997) text about the ranking of these devices was quoted as follows: “1) venetian blinds, 2) roller shades, 3) insulating curtains, 4) outside shading screen, 5) outside metallic blind, 6) coating on glazing surface, 7) trees, 8) outside awning, 9) outside fixed shading device, 10) outside movable shading device” [23]. This author also concluded that external shading devices are more effective by 30–35% in preventing solar radiation entering the room than internal ones [22, 23]. Heat-absorbing mid- pane shading devices are about 15% more effective than are internal shading devices [23, 103].

Steemers [104] investigated the shading performance in terms of reducing solar radiation using different external fixed shading devices mounted on facades of different orientation. On south, east, and west facades, overhangs are more effective than vertical fins and egg-crate; on east and west facades, the difference between overhangs and fins is too small to be calculated; on the north façade, vertical fins are better than overhangs and egg-crate [23, 104]. Hoyano (1985) found that vegetal vine sunscreens have a weak solar transmittance of 25% [105].

The influence of shading devices on transmitted solar heat gains through window (Fig. 11) can be evaluated by use of the total solar heat gain coefficient (gtot), which refers to the shading system combined with glazing. European standard EN13363-1: 2003 + A1:2007 and EN 13363-2:2005 describe the calculation method of the g-value and Tvis for the combinations of shading and glazing. As the Sun moves across the sky, the incidence angle of direct solar on the shading device constantly changes, which then leads to hourly varied solar heat gain coefficients.

The simplified calculation using the glazing’s U- and g-value alongside solar transmittance and reflectance of the shading system to estimate gtot for an external shading system is as follows [44]:

$$ {g}_{\mathrm{tot}}={t}_{\mathrm{solar}-\mathrm{shading}}\ast g+\left(1-{t}_{\mathrm{solar}-\mathrm{shading}}-{\rho}_{\mathrm{solar}-\mathrm{shading}}\right)\ast G/10+{t}_{\mathrm{solar}-\mathrm{shading}}\ast \left(1-g\right)\ast G/5 $$
(1)

where \( G=\sqrt{1/U+0.3} \), g is the solar gain coefficient of the glazing, U is the U-value of the center of glazing, tsolar − shading is the solar transmittance, and ρsolar − shading is the solar reflectance of the shading system (facing incident radiation).

The shading factor (Fc) is another method to characterize a shading system, defined as the ratio of the g-value of the combined glazing-shading system (gtot) to that of the glazing alone (g) [44]. In general, internal shading systems have higher values (0.3–0.6) of shading factors than external ones (0.09–0.2), and typical values for mid-pane system range from 0.2 to 0.3 [44].

Typical values for glazing properties in terms of U-value, g-value, and Tvis are listed in Table 1, and gtot and Fc values of different shading systems are listed in Table 2 [44].

Table 1 Typical values of U-value, g-value, and Tvis for three types of glazing (according to EN 13363-1:3003+A1) [44]
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Table 2 Typical values of gtot and Fc [44]
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Energy Performance

Attention was first paid on shading’s energy performance after Peebles’s experimental work on the relationship between cooling loads and solar protection [21]. Then, the influence of shading systems on heating loads and annual energy consumption was evaluated. Since the middle 1980s, energy use of lighting was considered with the heating and cooling loads because dimming systems would allow daylighting to replace artificial lighting and shading’s impact on daylighting levels [23].

With the development of computer programs on energy performance, a wide variety of parametric studies of solar shading devices and annual energy use have been conducted before the twenty-first century, which demonstrated that shading devices decrease the cooling demands in buildings while increasing heating demands [23]. A general conclusion can be drawn that with decreasing shading coefficient (better shade), cooling loads reduced and heating loads increased accordingly. Higher shading coefficient (poor shade) leads to higher cooling loads and lower heating loads [23].

Dubois (1997) summarized studies showing that shading devices reduce the energy consumption for cooling [23]. External precast concrete overhangs and fins were found to have the ability of reducing the cooling load by at least 50% in Brisbane, Australia, by Harkness (1988) through computer simulation [106]. Sunscreens were found to reduce cooling loads by 23% in San Diego by Brambley et al. (1981) through field studies [107]. External shading devices mounted on windows of double pane and clear glass demonstrated reduction of the cooling loads by 75% by Halmos (1974) through theoretical calculation [108].

Along with the development of software for multivariable analysis and parametric design, many studies regarding solar shading systems have centered on the optimization of the design of the components to better its performance. The reported performance of Sun shading systems greatly varies on cooling demand reduction values. Hwang and Shu (2011) studied the impact of overhangs of different lengths on the cooling demands of a south-facing office in a lightweight office building located in Taiwan and found a reduction of cooling loads between 7% and 11% [109]. Pino et al. (2012) reported cooling savings of up to 54% following the application of louvers at north, east, and west orientations of a fully glazed office building located in Santiago, Chile [110]. Ferrari and Zanotto (2012) discovered cooling reduction values ranging from 10% to 27% by employing external Venetian blinds in different south-facing offices located in several Italian cities [111]. Manzan evaluated the impact of the application of flat screens parallel to the window in south-facing offices in Italy, obtaining cooling savings ranging from 49% to 56% in the case of Trieste and from 30% to 46% in the case of Rome [112].

Many studies have also shown that most shading devices contribute to increases in energy consumption for heating while reducing the cooling loads [23], one example of which was done by Treado et al. (1984) using building energy simulation of a typical office building in seven different climatic locations of United States through the program DOE-2 [113]. In another study of Bilgen’s (1994) experiments, the heating load was found to increase by 4–6% and the cooling load reduced by 69–89% for mid-pane automated Venetian blinds used in Montreal [114]. Thus the net annual energy savings for a building only occur when the reduction in cooling load exceeds the increase in heating load [113].

Shading strategies are strongly climate dependent. Treado et al. (1984) found the respective shares of cooling and heating loads in total energy consumption depend on the location’s climate; so do the shading strategies [113]. According to Emery et al. (1981), fixed overhangs and fins result in a modest reduction in total energy consumption, and the best shading strategies in three cities of the United States (New York, Seattle, and Phoenix) are reflective glazing, heat absorbing glazing, and glazing combined with external aluminum louvers [23, 115].

In heating-dominated climate, Treado et al. (1983) found that window films contribute to larger increases in heating loads than to reductions of cooling loads, thus not resulting in annual energy savings [116]. Hunn et al. [117, 118] simulated several internal and external shading devices in the climate of Minneapolis by computer program with the discovery that a better performance can be obtained with internal shading devices (as opposed to external fixed) when energy use and cost as well as peak energy demand reduction are analyzed. High-performance glazing (heat absorbing, reflective) and external devices (overhang, fin, annual solar screen) almost always yield increased annual energy use [23, 117, 118], which confirm results obtained in residence buildings of Austin, Texas, by Pletzer et al. [119]. Dubois’ [103] study in Stockholm (Sweden) showed that shading devices should not be used when heating is needed because larger energy savings might be achieved with seasonal awning (around 12 kWh/m2/year) than with fixed awning (11 kWh/m2/year) mounted on the south window [120]. But for the climate of Florida, Mc Cluney and Chandra (in Germer, 1984) found the opposite: external devices (overhang, awning, window screen) perform best on energy saving [23, 121].

It is demonstrated that external shading devices are much more effective than internal ones for the climate of South Korea, providing better performance of shading and view [47, 122]. The proposed experimental external shading device provided a 20% reduction of the cooling loads and a 12% reduction of heating loads compared to the basic cases using internal horizontal blinds [122]. In a deeper-level study, Kim et al. [47] found that the above experimental external shading device (fixed tilted overhang) can lower insolation without compromising the visibility and thermal performance by controlling the overhang’s depth and slat angle, and its maximum value of cooling load is lower than the lowest value of cooling load with the conventional blind system [47]. Moreover, its heating loads are the lowest compared to other three shading systems.

Atzeri et al. [123, 124] simulated the performances of internal and external shading devices in an open-plan office located in Rome using EnergyPlus. As the simulation results reveal, for external shading devices, the cooling demands reduced and the heating demands increased slightly, while for internal shading devices, the influential rise in cooling demands cannot be compensated by a corresponding reduction of heating demands [123, 124].

However, external shading device is not popular because of maintenance, cost, and esthetic reasons. External shades have occupied considerable market share in Europe, but they remain rarer in North America on account of reasons such as high wind loads for high-rise buildings and plenty of exposure to snow/ice in winter [125, 126]. They are less accessible for cleaning, repair, and maintenance, which results in higher life-cycle costs of a building.

All these studies on the influence of solar shading on cooling and heating energy uses confirmed that shading strategies are heavily climate dependent. Most of the results show that shading does reduce cooling loads and tends to increase heating loads, while few of them have reached any general conclusion on the amount of energy saved and what is the best shading strategy overall [23].

Visual Comfort and Occupant Comfort

The above two subsections concentrate on the physical and energy aspects of solar shading. Glare is also an important factor for many building occupants, and glare control is a legal requirement where workers regularly use display screen equipment. Glare can appear where there is bright sky or reflections from buildings outside but most commonly from direct sunlight. CIBSE TM37 mentioned that if direct sunlight causes glare, opaque shading is best because transparent shading such as tinted glazing usually cannot assist with much; translucent shading such as a thin light colored fabric blind provides certain solar protection but under sunlight itself may become uncomfortably bright [127]. Various BRE publications [128, 129] offer guidance regarding glare control. It is suggested that the best option is to control overheating using one technique, e.g., external shade or solar control glazing, and provide a separate system to reduce glare, e.g., internal blinds [18].

Fixed/static shading devices (e.g., overhang, fixed louver, light shelve, and side fin) are effective to prevent direct sunlight from entering a room and allow in diffuse radiation largely, often deeper into the space [130, 131]. How much solar radiation transmitted through shaded windows varies with seasons or times in a day. Torcellini et al. demonstrated that designed louvers have strong potential to allow daylighting in 100% of the office spaces during most of the year by reflecting light deep onto the ceiling with white paint of the highest possible reflectance (exceeds 80%) [131, 132]. Yet fixed/static shading devices lack control and privacy and normally cannot give optimal performance all the time, particularly in overcast conditions or for extreme solar altitudes and azimuths [131, 133]. Moreover, all types of fixed/static shading devices performs best for south-facing glazing but not well for non-south-facing glazing where their depth needs to be excessively large to block direct sunlight of low solar altitudes [131, 134].

As reviewed by O’Brien et al. (2013), movable/dynamic shading devices are an appropriate choice to significantly reduce solar radiation and glare for low solar altitude and high solar penetration depth (e.g., late afternoon for west facades) [131]. They are also helpful to reduce glare during winter for south facades, in early summer after sunrise and before sunset for north facades, and for activities requiring highly controlled illuminance levels [131]. Venetian blinds as the commonest movable/dynamic shading devices do offer glare control and can contribute to visual acuity and visual comfort in the work place. Christoffersen et al. (1997) tested both static and manually operated shading devices including Venetian blinds and light shelves and demonstrated that horizontal blinds achieve the best results of daylight utilization [135]. Reinhart et al. (2006) experimentally proved that daylight contribution in an office varies with the distances of Venetian blinds from the facade by 10–60% [136].

However, in line of their own preferences, occupants often adjust the positions and rotation angles of manually operated shading systems such as blinds, which makes them frequently operated in “worst-case scenario” position and remain in that status for a long time [58]. This results in the overprediction of designers about the benefit of shades on energy savings due to optimistic or simplistic assumptions [58, 131, 137, 138]. Automated shading/daylighting systems are believed to resolve this issue because continuous adjustment is impractical or impossible to be realized by occupants in any cases. Its performance of lighting levels and distribution uniformity heavily depends on the type of the control strategy adopted [58]. Chan and Tzempelikos (2013) proved that Venetian blinds using new glare protection control algorithms are efficient in providing high values of daylight autonomy without sparking the risk of glare [139].

Post-occupant evaluation studies on thermal comfort have demonstrated that operable shading devices provide more control freedom [140] allowing occupants to better adapt to indoor environment than fixed ones, and higher levels of comfort tend to be reported if occupants have a perception of significant control over their thermal environment [141]. The interaction of occupants with shading systems should be treated as relevant input for the assessment of both performance of the shading system and occupant comfort [138]. Yet further research is still needed on occupant behavioral patterns and post-occupancy surveys for better shade controls.

Design of Shading Devices

Shading devices can have a significant impact on building appearances, either optimistic or pessimistic. The earlier that shading devices are considered in the design process, the more likely that they are to be attractive and well integrated in the overall architecture of a project.

A good shading system should have functions to control direct solar radiation, manage diffusive solar radiation, and introduce enough daylight aiming to maintain thermal and visual comfort without triggering overheating and glare. In addition, good shading systems should allow the visual contact with the external environment, provide privacy, but not limit natural ventilation [44]. Therefore, it is evident that the selection and design of a proper shading system can be a complicated and challenging task. In most cases, it is difficult for a single shading system to satisfy all the requirements. External shading, glazing, and internal shading can work collaboratively to achieve the design objectives for energy consumption and occupant comfort [44].

As stated above, the most crucial feature of solar position is its seasonal variation. Shading systems must be adapted to seasonal solar variations that may occur at different locations, usually with complete exclusion in summer and maximum exposure in winter. A brief guidance of the solar angles for sizing overhangs on south-facing facades in the Northern Hemisphere according to the Earth’s latitude is presented in Table 3 [12, 43] and illustrated in Fig. 12 [24, 43].

Table 3 Angles for sizing overhangs on south-facing facades in the Northern Hemisphere according to the Earth’s latitude [12, 43]
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Fig. 12
figure 12

Basic shading strategy for south-facing glazing [24, 43]

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where L° represents the Earth’s latitude on the Northern Hemisphere, χ° the summer angle measured from the window sill to the Sun’s altitude, and γ° the winter angle measured from the window head to the Sun’s altitude [12, 43] which are shown in Fig. 12.

To design effective shading devices, the Sun’s position with respect to the surface normal of a vertical plane needs to be known, which has already been introduced in section “Solar Factor of Glazing.” Two angles, HSA and VSA as indicated in Fig. 13b, can be used to determine the geometry of the horizontal or vertical shading device required for a window. If AZM, ALT, and OSN, respectively, represent solar azimuth, solar altitude, and the facing orientation of a given surface (window), the equations to calculate the HSA and VSA can be given by Eqs. 2 and 3 [28]:

$$ \mathrm{HSA}=\mathrm{AZM}-\mathrm{OSN} $$
(2)
$$ \mathrm{VSA}=\arctan \left(\tan\;\mathrm{ALT}/\cos \mathrm{HSA}\right) $$
(3)
Fig. 13
figure 13

(a) Definition of depth and width of a horizontal shading device; (b) solar altitude and shadow angles for sizing shading devices [12, 28]

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For a horizontal shading device, its size as indicated in Fig. 13a can be calculated using Eqs. 4 and 5 [28]:

$$ \mathrm{DHD}=\mathrm{HW}/\tan \mathrm{VSA} $$
(4)
$$ \mathrm{WHD}=\mathrm{DHD}\times \tan\;\mathrm{HSA} $$
(5)

where the height value HW refers to the vertical distance between the shading device and the window sill, DHD is the depth of the horizontal shading device, and WHD is the width of additional projection from the side of the window as indicated in Fig. 13a, exactly which side depends on the time of day and which side of the window the Sun is on.

To design a horizontal shading device, it is suggested to simply follow the following steps [28]:

  • Determine cutoff date: This is the first or last day of the year on which complete shading is required. Full shading begins at the first shaded day, continues through summer, and ends on a day symmetrical about the summer solstice.

  • Determine cutoff times:They represent the start and end times of day between which complete shading is required. The closer to sunrise and sunset these times are, the exponentially larger the required shade is.

  • Lookup Sun position:Use solar tables or a Sun path diagram to obtain the Sun’s azimuth and altitude at each time on the cutoff date.

  • Calculate HSA and VSA:Using the equations given above, calculate the HSA and VSA at each time.

  • Calculate required depth and width:Once again, using the equations above, calculate the depth and width of the required shading device on each side of the window [44].

The optimum depth and width of an overhang depend on latitude and climate. For the Northern Hemisphere, the basic guidelines of cutoff day for three climate zones are recommended by the EERE (Office of Energy Efficiency and Renewable Energy) [56]:

  1. 1.

    Cold climates: Use Sun angle on June 21, and pinpoint the overhang shadow line at mid-window.

  2. 2.

    Temperate climates: Adopt Sun angle on June 21, and pinpoint the overhang shadow line at the window sill.

  3. 3.

    Hot climates: Employ Sun angle on March 21, and pinpoint the overhang shadow line at the window sill.

The design of shading devices can be extremely complex. The above method just provides a simple method to shape a fixed horizontal shading device that will provide full shading over the selected time period, which cannot examine how much solar gain obtained in winter when some sunlight penetration is usually desirable. In order to understand the full performance of a shading device, the Sun path diagram and shading mask is needed. Over the recent years, computational software has incorporated traditional methods and Sun path diagrams to create precise tools for sizing and shaping shading devices for rather specific purposes.

Considering the wide variety of buildings’ configurations and climates, it is difficult to make sweeping generalizations about the selection and design of shading devices. However, the following design recommendations generally hold true [43]:

  • Shading devices should be selected according to the window orientation. Fixed horizontal shading device is most appropriate to be adopted in south-oriented windows and vertical ones for east-/west-oriented windows. Shading of north-facing glass can not be taken into consideration in those latitudes where very little direct solar gain is received. However, as in the tropics, the north facade also receives much direct solar gains, and this rule-of-thumb must be disregarded.

  • Direct solar radiation on south-facing glazing can be effectively controlled by fixed overhangs, while diffuse radiation should be controlled by other measures, such as low-e glazing.

  • The area of east-/west-facing glass should be limited to the greatest extent possible, because it is more difficult to shade than south-facing glass. Landscaping is recommended for these orientations.

  • Although internal shading devices such as Venetian blinds, roller shades, and curtains can control glare and improve visual acuity and visual comfort, they cannot be exclusively used because they are not efficient in reducing cooling loads. Window shading system should be designed as a whole, combination of internal and external shading devices.

  • Remember that shading would affect daylighting and the view to external environment; consider them simultaneously and balance between such requirements as daylighting, solar gains, and maintaining the view to improve occupants’ comfort. Light shelf is an example, which reflects sunlight deep into a room through high part of window while shades lower part.

  • Carefully evaluate all aspects of operations and maintenance (O&M), safety implications, and durability into any life-cycle cost analysis of shading devices, particularly movable ones.

Summary

After a general introduction of solar radiation and solar geometry, a literature review on different shading technologies used in buildings as well as their performance was made; it has been found that different types of shading technologies were proposed and applied in different altitudes and climates. Many experimental and theoretical studies have been conducted to verify, evaluate, and improve the performance of shading devices. Studies verified that shading devices reduce the cooling energy use, and external shading devices are more effective than internal ones in reducing direct solar radiation. Studies have also shown that most shading devices contribute to increases of heating energy use. The performance of shading strategy is heavily climate dependent. Simple calculations to size a horizontal shading device were provided. Design recommendations were summarized for proper design of shading system to prevent overheating during summer while allowing maximum daylight to enter during winter, thus to maximize occupants’ thermal/visual comfort and minimize energy demands.

Reviewed studies on shading strategies have mostly relied on software simulation methods and monitoring to a lesser extent. While monitoring provides invaluable information about the actual performance of running systems, there is still need for more information about their operation, especially taking users’ perception and their reaction into account.