Kinetic Solar Envelope

Rapid urbanization during the last decades has had several environmental, economic and social consequences. Among these, an issue of great concern has been the development of the so-called Urban Heat Island (UHI) phenomenon, characterized by higher temperatures in the density of built areas than the ones of the rural surroundings [Santamouris, 2007]. In the European context, this phenomenon is especially intense in the Mediterranean basin, with a fast growth of energy consumption in the last years due to the widespread use of air conditioning systems and the increase of cooling demand. This situation gets even more worrisome in the face of global warming and the significant rise of Heat Waves (HW) [Salvati et al., 2017].

Especially in Athens, Greece, UHI has been present already since the 1980s. Many research studies have shown that there has been an increase of the energy building demands, thermal risk and vulnerability of urban population and it has been reported that during the HWs, there is even an intensification of the average UHI magnitude by up to 3.5^oC. The heat that is dissipated from the buildings to the external environment increases the UHI phenomenon, and therefore, has a strong indirect impact. More specifically, in Athens an average increase of the cooling load of about 13% is estimated, with an annual global energy penalty for unit of city surface and degree of UHI intensity of 0.74 kWh m^-2 K^-1 [Santamouris et al., 2001].

The UHI is a complex phenomenon and is directly and indirectly related to serious energy, environmental, health, and economic problems [Golden et. al., 2004] [Phelan et. al., 2015]. When it comes to the influence of buildings, it has been established that there is both a direct and an indirect impact of the building’s energy performance on the increase of the UHI in the cooling-dominated areas, such as Athens [Phelan et. al., 2015]. More specifically, the direct impact relates to the reflected solar radiation originating from the building’s surface towards the urban microclimate, whereas the indirect one concerns the released heat which is generated from the building’s cooling systems and is then accumulated in the urban environment. An improvement of the building envelope and the energy efficiency might, therefore, reduce the ambient temperature and building’s impact and, consequently, decrease the amplitude of the phenomenon.

In this direction, a certain level of climatic responsiveness and adaptiveness to extreme heat changes in an energy efficient way can arise as a promising strategy, in order to reduce the building’s energy consumption. This gives way to the broad development of responsive technologies, such as passive dynamic adaptive facade systems. Thanks to their adaptive mechanisms and the ability to implement smart technologies and autoreactive materials, they are favored due to the real-time responsiveness to the also dynamic and unpredictable environmental changes, acting as the threshold between building and exterior environment. The above-mentioned framework, composed by problem and promising mitigation strategy, is the direction that was followed in the current research study. This was further explored and developed with a focus on the incorporation of smart and shape-changing materials, such as Shape Memory Alloys (SMAs), by means of a case study in Athens, Greece, and design concept to evaluate their potentials for facade application.

In short, Shape Memory Materials (SMMs) belong to the energy-exchanging smart materials and are one of the major elements of intelligent composites because of their unusual properties, such as the Shape Memory Effect (SME), autoreactivity, large recoverable stroke (strain) and adaptive properties, which are due to the reversible phase transitions in the materials. More specifically, thermo-responsive SMMs, and SMAs belonging to the same category, can sense thermal stimulus and exhibit actuation or some predetermined response, making it possible to tune some technical parameters such as shape, position, strain, stiffness and other static and dynamical characteristics [Wei et al., 1998]. An input of thermal energy alters the microstructure through a crystalline phase change, which enables multiple shapes in relationship to the environmental stimuli, making them promising materials for the integration in passive responsive facade applications [Addington, 2012].

SMMs have been broadly used in a wide range of fields, with a large interest in aerospace, automobile industry and biomedicine, with applications that include hinges, trusses and morphing skins. In architecture and engineering, the application of SMMs is still in an initial stage, however, there are already developments and interest in this direction as well. Most common examples are self-healing systems, actuators, sensors and vibration control systems, as well as smart and solar morphing envelopes to enhance the building’s thermal comfort and energy performance and saving, which is the research scope that the current study focused on [Li et al., 2018]. More specifically on the facade applications, most realized projects concern kinetic shading systems, whereas solar morphing shading skins are not broadly explored. Where an actuator is required, this is completely embedded into the device or strategically located to trigger a specific action [Dakheel et al., 2017].

Examples of the above principles applied in facade systems in practice can be found in the actuation systems developed in the Solar Kinetic, Air Flow(er) and Piraeus Tower projects, to name a few. All of them present one or two degrees of freedom in the desired movement, varying between two extreme positions, open and closed modes, with continuous transition between the two. Where movement is three-dimensional, as in the first two examples, a constrained swivel motion produced by bending and buckling of elastic materials characterizes the rotation. An exception is the case of the Air Flow(er) project, in which hinges are used to rotate stiff wings. Most of those performing three-dimensional movements have also been developed either at component or subcomponent scale, due to the discrete sizes and capabilities of the smart materials as actuators. Figure 1 illustrates diagrammatic schemes with the principles of the operational movements, using SMAs as actuators. In the Solar Kinetic and Air Flow(er) cases, the stimulus originates from a heat source provided through electrical current, whereas in the Piraeus Tower project, the system is activated by the changes in solar radiation [Suralkar, 2011] [Payne et al., 2013] [Doumpioti et al., 2010].

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