ReviewDesign approaches and typologies of adaptive facades: A review
Graphical abstract
Introduction
Windows as conventional daylighting elements are able to provide daylight into spaces in the perimeter zones, but within a limited distance from windows or skylights [1]. Thus, different daylighting systems are extensively used in researches and experiments such as prismatic panels [2], Laser-cut panels (LCP) [3], light shelves [4], louvers and blinds [5], anidolic systems [6] and so forth. These systems are often integrated either with a ‘shading system’ (e.g. venetian blinds) or ‘light-guiding system’ (e.g. prismatic panels) that are capable of collecting daylight and then redistributing or reflecting it into the interiors. The entire daylighting strategies often take into account the environmental factors (e.g. sky's luminance distribution, sun) and physical components (e.g. buildings, landscape, surrounding obstructions).
In other words, daylighting is conjoined with solar gain, which means depending on the context, allowing solar gain may be an advantage, but in some cases, it must be controlled. Thus, the most ambitious question in choosing a daylighting strategy is to counterbalance daylight harvesting and controlling discomfort risks (e.g. glare and overheating). Until recently, different conventional daylighting systems and technologies have been developed like solar shades [7], venetian and roller blinds [8], that are commonly applied due to their lower costs and ease of applications [9]. Fixed or static shading systems have limited potentials to respond to indoor or outdoor environmental variations throughout a day or season and may result in unacceptable performance if the operating requirements change over time series. As a consequence, non-flexible facades that increase daylight penetration through maximizing façade transparency often lead to occupant modifications due to glare or overheating problems over the building life cycle, which, in turn, decrease the anticipated indoor comfort and energy savings in long term [10,11]. Therefore, a conventional window may be enough to let the daylight enter the space, but providing more natural light into the depth of the space requires more advanced design alternatives.
On the other hand, a building's facade is the most visible element that defines the aesthetical appearance of a building itself. Also, it is responsible for ensuring a physical barrier and an interface between inside and outside, and therefore exposed to uncontrollable meteorological variations throughout a year such as solar radiation, precipitation, wind and extreme temperatures that affect indoor comfort conditions of occupants. To that end, facades are responsive to different design scenarios and functional performances that may contradict each other: shading vs. artificial lighting, views vs. privacy, solar gain vs. overheating, daylight vs. glare. Thus, from an environmental point of view, there are several parameters that should be considered during the design stage of a building façade in general:
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Solar gain control: The solar radiation admitted to the building influences directly its indoor temperature, and consequently the comfort level of the occupants.
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Natural ventilation: The building skin can control the natural air exchange and circulation and thus reduce the mechanical system usage under specific conditions.
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Daylighting vs. artificial lighting: Admitting natural light into space through fenestrations and shading systems are the key components of any envelope, influencing indoor illuminance that affects user well-being. A proper combination of daylight with artificial lighting is the main goal to minimize electrical energy consumption considerably based on three main reasons. First, artificial lighting systems such as LED packages considering breakthroughs in their color efficiency are capable of delivering 160 up to 330 lm/W [12]. On the other hand, daylight itself has a relatively acceptable luminous efficacy (110–130 lm/W) that depending on sky can reach up to 150 lm/W [13]. Second, daylight is originating from a renewable source; sun, while electricity is mainly generated from fossil fuels in most under-developed or developing countries. Third, up to 20% of the overall building energy load refers to artificial lightings [14]. According to literature, lighting energy savings regarding daylight harvesting vary from 20 to 87% [15]. However, daylight penetration solely is not enough to lead energy savings automatically, unless it is linked to shading and lighting control strategies [15]. Several reviews investigated different types of static and dynamic control strategies as the main potential to counterbalance daylight penetration and lighting usage [16,17]. Therefore, the challenge is permitting daylight deeply as possible while reducing the overall energy consumption and keeping individual visual sensation in comfort.
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View out: Opening the walls serves the psychological visual connection of users to outdoors.
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Heat control: Controlling the heat flow between interior and exterior has a great implication on building's thermal performance. This is typically done by employing insulation within the opaque part of the envelope, while the glazing part requires more attention due to the heat transmission through windows which can be improved by shading the windows during summer.
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Moisture control: Generally, there are two types of moisture that a building skin is dealing with; rain and condensation. While rain exposes the façade to humidity from the outside, condensation forms on cold surfaces inside the room, when a large temperature swing occurs between inside and outside of the room due to weak envelope insulation.
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Noise: Acoustic insulation is another fundamental role of a building façade that is subjected to outdoor noise caused by temporal variabilities.
Among the above mentioned environmental factors, this paper focuses on visual comfort, view and daylighting objectives, as the aspects that can be most improved by an adaptive façade. To address these concepts two alternative approaches have been widely tested; (a) converting conventional daylighting systems into conventional adaptive façade systems (e.g. motorized venetian blinds [18], or light shelf [19]), or (b) using Complex Fenestration Systems (CFS) which are characterized by their ability to change the light angular directions passing through them such as prismatic films or LCP [20]. However, CFS systems often involve multiple diffusive layers in which each layer has its own unique optical properties depending on the incident light angle [21]. Conventional adaptive systems were investigated comprehensively from different perspectives to improve the energy efficiency in office buildings [22,23], while they often address a set of conditions as an average for all users. However, they do not have the ability to change their settings based on environmental conditions or indoor needs of occupants. A more recent third approach are designs focusing on the adaptability of the system which can react independently to short term changes in boundary conditions or user preferences in separate or shared zones [24,25] to maintain occupant's well-being and improve overall building performance.. This is the main feasible advantage of non-conventional adaptive systems that requires innovative design and technological solutions. This paper focuses on this latest approach to ‘adaptive facades’ (AFs).
Designing integrated adaptive façade systems involves a dynamic process, and more specifically they are evaluated based on how they fulfill the defined purposes. Unlike prescriptive functions for conventional shading systems like roller shades (open/close) or venetian blinds (height or slat angle), performance-based functions of AFs allow to potentially employ innovations (e.g. different configurations) and changes based on user demands as they are not prescribed subjects [26]. Eventually, over a hundred adaptive building skins and concepts are reviewed and classified in different schemes [24,27,28].
One of the first publications that used the term ‘adaptive façade’ (AF) for classifications was [29] in its first edition (2007). Then in the literature, several definitions were introduced to define the capabilities and responsibilities of the AF. It was defined as a morphogenetic evolutionary design with real-time adaptation with boundary conditions [30], or a façade that can change its properties passively or actively over time to reduce the building energy consumption [31,32]. In some studies, occupant's requirement was addressed in their definitions and even modified the terminology. The study of Loonen et al. [24] defined AF as climate adaptive building shell (CABS) that can change its functions or behavior repeatedly and reversibly over time in response to outdoor and indoor control variables, or similarly Wang et al. [33] introduced acclimated kinetic envelope (AKE) that can adapt itself through mobile ways reversibly. From industrial perspective, a recent study by Attia [34] in form of interviews with 27 façade experts resulted in similar AF definition that should mainly respond to both indoor and outdoor environment to enhance the building energy efficiency without impairing user comfort.
Therefore, AF permits energy savings by adjusting its elements to prevailing boundary conditions either by passive reactions (intrinsic) or active reactions (extrinsic) to enhance comfort levels by immediate responses to occupant's desires and preferences [24,35]. Thus, in this research as stated by authors, ‘adaptability can be interpreted as the potential of the system to deliver multi-objective comfort criteria under uncertain environmental conditions by changing its physical characteristics within a short timing scale’. These given challenges and their importance in built environment resulted in commissioning the Adaptive Façade Network as a project entitled Cost Action TU-1403 in 2014 to harmonize and share the existing knowledge on AFs at a European level [36] and ended in 2018 [37].
Basically, three phases are recommended by [38] to form a unified and systematic performance classification of AFs (Fig. 1):
Phase one deals with collecting the ambient information. Outdoor and indoor environmental condition and occupancy patterns affect both façade performance and operation from design till construction stage. In particular, each variable incorporates with human comfort desires and building performance that AF can fulfill one or both.
Phase two processes the collected dataset through computational tools and controlling strategies. Successful operation of AFs by effective controlling is the key element that affects the overall performance. Practically, there are two groups of controls [24,35]:
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Extrinsic control
AF can be distinguished through its ability of accepting feedbacks to adjust itself actively. This is called an extrinsic control and typically consists of three main components; sensors, processors (controllers) and actuators, and by integrating logical control to close the loop makes adaptive behavior at two levels:
- a.
Centralized; a supervisory control system linked to groups of actuators, which can be controlled in three ways depending on complexity: direct as the lowest complex system; where actuators are manually controlled without any external inputs, and has the highest capability of user interaction, reactive system; when sensors are determining façade reactions, system-based as the most complex method; when more complexity is added through enabling multi-deterministic processors to solve unpredictable problems with least user interaction such as fuzzy logics or artificial neural network (ANN) that require optimizing multi-objective real-time environmental data [35].
- b.
Decentralized; embedded local processors that are controlled with or without an external system such as an information hub, and can be categorized into direct and inner systems. Direct system uses artificial external elements like local micro-actuators to move each element of façade individually, while inner system implies the material capability (e.g. hygroscopic) of self-adjustment which leads to an intrinsic control.
- a.
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Intrinsic control
This type of activation is often accomplished by using series of special materials known as ‘smart materials’ that does not allow feedbacks [39]. AFs with intrinsic control is capable of self-adjusting and automatically triggered by environmental inputs. It is also called ‘inner material-based control’ as the system responses are taking place in local level regardless of external decision making controls.
Comparing with extrinsic control systems, intrinsic activation occurs immediately without consuming energy for states transitions which results in low operation cost, but it limits the degree of adaptation of façade within certain pre-defined conditions and unpredicted disturbances may occur. Also, the absence of manual user intervention within the controlling system is regarded as another drawback of intrinsic control.
Phase three includes responsive physical and non-physical actions based on received energies from controllers. The AFs performance relies on their control scenario (phase two) embedded in system to provide the highest influence on façade's reactions as follows:
- 1.
The interactions between building envelopes and outdoor environment occur within four main ‘physical domains’ that enables different responsive functions such as preventing, rejecting, modulating, collecting, or admitting based on initial purposes of AF's in phase one as shown in Fig. 2. In an overview studied by Loonen [27], thermo-optical domain (zone E) exists in almost all systems due to the facts that thermal environment is changing and in most cases a daylighting system is a part of AFs.
- 2.
Directly associated with human comfort is the ‘responsive time scale’ of AF to react. Buildings are subject to different climatic forces that often occur at provisional timing frames that are noticeable during building's life cycle in the range of seconds to seasons however the adaptation speed depends on the implemented technology or designer's decision [24]:
- a.
Seconds
The shortest fluctuations that are randomly determined in nature such as wind direction and speed, or interactive facade that vicinity of occupants changes its configuration in short-time steps.
- b.
Minutes
Daylight availability and cloud cover are characterized in order of minutes, in which most of the AFs that deals with thermo-optical domain alter their transparency at rate of minutes.
- c.
Hours
Sun angular position in sky dome, indoor/outdoor air temperature or solar radiation are constantly changing, as many of adaptive facades that track the sun path are classified in this category.
- d.
Diurnal
Presence of occupants normally follows daily schedules that AFs can particularly taking the advantage of this fixed 24 h' pattern.
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Seasons
Variations across seasons impose different ambient conditions depending on geographical location and latitude.
- a.
- 3.
The ‘adaptation scale’ can be changed either at macro scale, or micro scale, or a combination of both that depend on their spatial resolution that AF reacts as discussed below in more detail [24]:
- a.
Macro scale
‘kinetic’, ‘active’, ‘movable’, ‘responsive’, and ‘biomimetic’ facades are the types that often deliver the adaptation in macro level which the motion is observable via moving parts within a wide range of possibilities such as folding, expanding, creasing, sliding, rolling, rotating, stretching and inflating.
- b.
Micro scale
The other type of adaptation is directly based on material molecular structure and its deformations. In particular, adaptability is either exposed through changes in optical, or thermos-physical properties, or energy conversions that are utilized typically in ‘switchable’, ‘media’ and ‘smart’ facades.
- a.
- 4.
The size of the façade system that is changing in ‘spatial scale’ corresponds to their performance significantly. There are five different scales of adaptation [38]: (i) building material, (ii) externally-attached elements to façade, (iii) façade subsystems, (iv) the entire façade, or (v) the entire building. Attia identified four object-based levels related to façade evaluation; material, component, system, building [28] and concluded the most common findings were in material level, and rarely in building level either in scientific publications or current standards (EN-13830).
- 5.
The ‘visibility scale’ of adaptation affects the appearance of building. The more the façade dynamics become visible, the more it alters the appearance and occupant's view quality and quantity.
Moving from design stage to market adaption adds extra layers of complexity within key performance indicators of AFs to assess them at building level [28]. From in-depth interview analysis of 27 façade experts, four main performance objectives of adaptive facades were derived according to [40]: (1) indoor environmental quality, (2) interactive control of adaptive facades, (3) occupant's acceptance and interactions with adaptive facades, and (4) standardization. These findings outlined the role of occupants' comfort and well-being in the market penetration of future applications of AFs. Although, very limited projects involved occupants to close the feedback loop, and there are very few straightforward approaches to evaluate the impact of AF on occupant's comfort and energy performance [34,41]. On the other hand, future trends emphasized the occupant interactions with AFs through personalization and smart automations as the most essential technological trends [40].
Developing non-conventional adaptive systems requires performance to be quantified during early stages of design and since its concept is becoming the main interest of many recent researchers, it is vital to introduce an efficient way to predict the benefits and performance. Essentially, all the adaptive shading systems are developed to answer four main human needs [42]; (1) thermal comfort, (2) daylight level, (3) glare, and (4) view out, along reducing lighting and energy loads, in which any of these can contradict to the other one. Therefore, assessing their applicability during design process is extremely significant.
To that end, different approaches have been used to evaluate both visual and thermal comfort in the past that can be categorized into three methods: (1) field measurements, that are done either by full scale experiments/mock-ups, or surveys for subjective assessments [43,44], (2) numerical calculations or estimations [23], and (3) building performance simulation (BPS) [42,45]. However, each method has its own strengths and weaknesses:
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Experiments are the most accurate but the cost is relatively high and time-consuming, and the design parameters are mostly inflexible due to the equipment availability. Similarly, surveys require long period of measurement and data collection by people that need further calculation to achieve a conclusion.
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Numerical calculations (empirical formulas) is a fast and cost effective method, although they are not applicable to all cases that use simplified estimation criteria (e.g. using standards).
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Software simulations are fast and economically effective, especially during early stages of design when design parameters are flexible and there is no building and occupants to conduct field measurements, but they are prone to low accuracy if many assumptions and simplifications are made. Therefore, this method requires a validation process.
The methods for designing efficient AFs, and ensuring effective performative strategies that are integrated within the design process are the most critical aspects. AFs are not only barriers between interior and exterior environments, but also building components that can create comfortable spaces by dynamically responding to the building's external environment, and significantly reduce buildings' energy consumption. However, achieving these design aims requires a high level of integration at the early stages of the design process. In the literature, there are few papers that reviewed a number of definitions of AF's typologies [30,40,46], case studies [47], their current trends of applications [48], structural requirements and performance [49], or their interactions with occupants [50]; however, the terminology around differences, limitations of applications and basics of design approaches towards AFs are still ambiguous. Therefore, this paper aims to conduct a review based on a systematic search and answer two main questions:
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What are the differences between existing AF's typologies?
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What are the available design workflows and technologies towards non-conventional AFs?
Section snippets
Methodology
With respect to a review with a systematic search definition [51], the research methodology includes a critical analysis of existing studies that focused on AFs in office buildings, while the selected papers must meet the following criteria (Fig. 3):
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Findings should focus on proposing a new adaptive façade system excluding conventional ones such as venetian blinds, roller shade, or light shelf that are extensively reviewed in previous studies [17,42,52].
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There are many studies which focused on
Existing typologies
Emerging different terms of adaptive definitions of facade shows that scientific research on dynamic skins is developing towards innovative design solutions by utilizing the possibility of integrating mechanical and automated actuators, material sciences, and IT systems. Thus, technological solutions are moving from static elements to adaptive elements capable of rapidly changing function and configuration in relation to physical requirements.
Despite the technological aspects of AFs,
Design workflows and technologies towards non-conventional AFs
As defined earlier, AFs are a set of responsive elements that aim to enhance energy saving and user comfort, either manually or automatically. In particular, the authors categorize AFs based on their motion typologies into two main groups: (1) simple motion type, and (2) complex motion type. In terms of energy involvement, both groups employ passive and active strategies. The passive approach requires no initial power to create adaptation, such as using manual control or responsive surfaces,
Discussion
Current definitions of AF and existing typologies involve innovative technologies that along their given potentials, need to be supported during design stage. Among reviewed studies, the most common applied physical domain is the integration of thermal and optical feature, in which responses at an hourly basis are often implemented for successful AFs to guarantee outdoor view while maintaining visual comfort and energy efficient building.
Moreover, Fig. 27 decomposed conventional and
Conclusion
The study illustrates the potentials of employing an AF to improve both indoor environment and energy consumption, in which two questions are answered through a review with systematic searching as follow:
- (1)
What are the differences between the existing AF's typologies?
This paper reviewed the current interpretations of AF from the perspective of their adaptation mechanisms and possible user interactions with the façade. Ten different typological approaches are derived from the literature. The
Declaration of Competing Interest
None.
Acknowledgment
The study is part of a PhD research and is not subject to any specific funding. The aim of this research is to contribute to the body of knowledge of the International Energy Agency (IEA) Energy in Building and Communities (EBC) Annex 79; Occupant-Centric Building Design and Operation.
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