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1 Emergence of Smart Urban Structures over the Course of Time

Smart/intelligent cities have been fantastic, futuristic concepts for quite some time. But they can also be understood as emergent phenomena, i.e. the consequential outcome of the application of the ongoing technological progress in digitalization, sensor and actuator technology and other fields, to the urban planning and development processes. In this section, we will take a brief excursion to the history of smart city concepts and address how their manifestation as a realistic urban development option has come about in recent years.

The formation of urban structures is a fascinating sophisticated process. Apart from the natural process of emergent growth, driven by the fundamental needs of the inhabitants and bound by the constraints and boundary conditions of the geographical and ecological surroundings, like rivers, ports, market squares and trade routes, urban structures like towns and cities have also been object to selective and purposeful planning and design. These processes of controlled planning and urban development commonly follow certain paradigms and specified goals.

Over the course of time, the paradigms of urban development have undergone several shifts, ranging from royal prestige structures with wide avenues and large squares to the worker neighbourhoods of the industrial production age. It shifted later to the mobility-centred dogma of the traffic-friendly city to spanning new urbanism and more recently developing towards a citizen-friendly synergetic approach, influenced by mostly citizen-driven grassroots movements for reclaiming the common space like the Transition Network (Transition Network 2020), the Critical Mass movement (CriticalMass 2013) or the Occupy movement initiated by Adbusters (Adbusters 2020) and urban gardening aspects (Raskin 2017). In order to achieve certain paradigm-specific goals, certain structures like representative-wide avenues and large squares for prestigious representation, suburban single household settlements, industrial districts, neighbourhoods with affordable housing for workers, malls and parks were actualized in each epoch within the overall urban structure.

While the processes of urban planning and development are mostly locally constrained and operate on the scale of neighbourhoods and single districts, the challenge to design cities as a whole in an holistic approach has also been addressed by several great utopians in the past. The Experimental Prototype Community of Tomorrow (EPCOT), designed by Walt Disney around 1959 (E.P.C.O.T. 2012), resembles one example for the efforts to design attractive cities with high quality of life for its inhabitants while simultaneously utilizing then most recent technological developments, e.g. a monorail transportation system and agile personal movers within an holistic approach. The geometric circular design of the city complex seemed to be inspired and influenced by the “Radial Garden City” designs of Ebenezer Howard (Howard 1898) and has also found resemblance in predecessors like the Octagon city project (Gambone 1972). Another more recent example for the holistic approach to the development of circular urban structures is the Venus Project, supported by Jacques Fresco and Roxanne Meadows in about 1970 (Fresco 1995). These concepts covered the relevant aspects of functioning city structures like the arrangement of districts for working, recreation and housing and even agriculture as well as providing transportation systems for the inhabitants and for the logistic of goods. The provision of utilities and all relevant maintenance processes were supposed to be coordinated by a central hub, supported by a vast range of sensor data. The Venus Project even aimed for a novel model of a futuristic society expanding on considerations regarding the economic system of the inhabiting society, proposing a resource-based economic model.

Apart from the circular structures, there have also been other futuristic concepts in the past like domed, floating or even walking cities, as presented in 1964 by Ronald James Herron of the seminal English experimental architecture collective Archigram (University of Westminster 2020). These concepts have been mostly addressed within science fiction literature and movies, apart from rather solitary attempts to apply them to real scenarios from bold futurists like Buckminster Fuller with the dome of Manhattan in 1960 (Carlson 2012) and the tetrahedral floating Triton City for Tokyo Bay in 1967 (Hays and Miller 2008) or the domed Eco-City 2020 (evolo 2010), proposed in 2010 by the Russian innovative architectural studio AB Elis Ltd., projected for 100,000-people in the crater of the Mir diamond mine in the Yakutia Republic in Siberia.

Due to technological and organizational restraints, none of these projects were able to be realized on the scale of a functioning entity. The approach to generate a complete city structure from scratch, as appealing it might seem from a conceptual point of view, poses an obstacle since it is not applicable to the transformation of existing metropolitan city structures. Still these efforts can be seen as precursors to the smart city concept arising from an approach of understanding a city as a synergetic life form with manifold data streams to support its metabolism. The increased prevalence of miniaturized interconnectable sensor systems and the general availability of computing capacity and network bandwidth led to a revitalization of the idea of the “intelligent” city in recent years. This reflects, for instance, in the development of the number of publications during the recent years. A brief survey, for the combined key words “smart city”, on the number of publications from the reputable publisher Elsevier at sciencedirect.com illustrates this development. The search returns about 9364 overall publications between the years 1992 and 2020. The number of publications shows a noteworthy increase since about 2010 and a rather steady annual growth by a factor of about 1.4 in the number of publications between 2015 and 2020. On another note, the term circular city, used by Fresco in connection to the spatial configuration of the city and only secondary referring to the underlying resource base economic system (The Venus Project 2020), has recently seen a renaissance in press articles (Wenzel Elsa and GreenBiz 2019), research programmes (BOKU, The University of Natural Resources and Life Sciences 2018) and scientific literature (Prendeville, Cherim and Bocken 2018) mostly associated not only with smart city concepts but mainly with circular economy aspects. Finally, compilations of recommended procedures for the implementation of smart city concepts have been defined within smart city charts for Germany in 2017 (Günthner, Schweitzer and Jakubowski 2017) and the European Union in 2019 (Wouters, et al. 2019).

2 The Building Envelope: From Skin to Sculpture to Sensor

Since the emphasis of this essay lies on the prospects of the building envelope within the context of the smart city approach, we will not further expand on the various intriguing aspects of smart city concepts but will rather elaborate on the specifics of the building envelope. We will sketch the progression of the building envelope from its static pragmatic nature towards aesthetic and dynamic responsive element.

The building envelope as an indispensable component of the physical building classically constitutes two main functionalities. On the one hand, it covers the rather obvious role of providing shelter and protection from the outer environmental impacts and the control of energy transport including solar radiation and the flow of air between inside and outside. And on the other hand, it resembles the aesthetic vision of its creators and owners. The measures to fulfil the role of physical shelter and protection mostly depend on the available technological and materialistic means, whereas the artistic design is heavily impacted by the predominant spirit of its era. Still these roles are not strictly separable and intertwine to some degree, since available technological measures enable certain aesthetic creations, the necessity to meet certain energetic and comfort goals impact the choice of aesthetic means and the preferences in creative means push certain technological developments. In most functional everyday buildings, these aspects are merged in an amalgamation to a functional equilibrium. A departure from the functional interpretation of the building skin towards the artistic aspects is present in the works of several iconic creators like the masterworks of Jean Prouvé, the wide span lightweight structures of Frei Otto, the sculptural designs by Shigeru Ban or the elaborated structures of Carlo Scarpa accompanied by his interpretation of the building envelope as the third skin of its inhabitants, to just name a few. The general manifestation of the building envelope was mostly that of a static element. This rigid approach was transcended in the early twentieth century by several creators by applying dynamic aspect to the mostly static building and its envelope. In the beginning, the ideas were merely explored in concepts and drawing, for example, by the Russian constructivist architect and graphic designer Chernikhov in 1933 (J. G. Chernichov 1933). The concepts were later picked up and implemented by innovators such as Buckminster Fuller around 1940 (Marks and Fuller 1960), who probably also processed them into the Tensegrity concept (Lalvani 1996). The dynamic approach found application in experimental works, for example, the US Pavilion at Expo 67 in Montreal in 1967 from Buckminster Fuller (Carlson 2012). It took until 1970 for the dynamic concept to gain foothold and be coined by the term “kinetic architecture” by William Zuk (Zuk and Clark 1970). The application of cybernetic ideas and computational power to the dynamic approach by Negroponte gave rise to the branch of “responsive architecture” (Negroponte 1970) and finally found its youngest manifestation in the concept of “climate adaptive building shells” by Loonen (Loonen, Trčka and Hensen 2011). The integration of modern sensor and actuator technology and control algorithms enables creators to develop buildings towards a vision of a dynamic entity, interacting with its inhabitants and surrounding. Since the establishment and refinement of the ideas of dynamic architecture, a great number of individualistic examples of responsive and climate adaptive buildings have already been built (Loonen 2010).

3 Future Prospects of the Building Envelope: Examples from Research and Commerce

The future of the building envelope within the context of smart/intelligent cities is linked to an increase in its functionality by utilizing state-of-the-art sensor and network technology and several other aspects to adapt its capabilities to the needs of the inhabitants and the impacts of the environment. Some aspects of this extension in functionality can be derived from the concepts of dynamic architecture.

One prevalent goal of climate adaptive building shells is to adjust the impact of radiative heat transfer into the building by dynamic shading. The applied shading mechanisms can be realized by architectural means via kinetic elements or can be integrated into the transparent envelope elements like windows and glass facades. A comprehensive review of various technological shading solutions like electro-, thermo- and photochromic elements in modern window applications is given in the work of Casini (Casini 2016). Novel concepts of combining microfluidic channels and magnetic particle suspension with transparent building elements to achieve switchable shading have also been realized (Heiz, et al. 2017).

Apart from shading, there are also other beneficial aspects that can be addressed by the building envelope. The generation of energy by energy harvesting methods via photovoltaic/photothermic elements has reached technological maturity and can be expected to become more wide spread in the coming years. The term “Building Integrated Photovoltaic/Thermal” (BIPV/T) is already firmly established in the research community. A survey of the term “BIPV” at Elsevier alone returns an overall of 2773 publications for the timespan between 1997 and 2020, with a steady number of 200 to 300 publications per year for the timespan between 2015 and 2020. Commercial PV manufacturers, e.g. Heliatek (Heliatek 2020), have specialized their product portfolio to this application. The application has reached the maturity to fully equip whole building facades with PV elements, e.g. the office building Z3 of the Züblin AG in Stuttgart (Erban, Oman and Popp 2016) and the institute building of the Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW) in Stuttgart (Geuder 2018). The combination of photovoltaic/photothermal energy harvesting and shading by integrating PV/T elements in shading slats and curtains is subject of research projects like Arkol (Fraunhofer ISE 2020) as well as part of the business concept of startups like SunCurtain (SunCurtain 2020).

The use of fluoropolymer materials like polytetrafluoroethylene (PTFE, colloquially known from the DuPont brand name Teflon)-coated textiles and ethylene tetrafluoroethylene (ETFE) copolymer foil in membrane structures has also seen successful application in the combination with photovoltaic elements (Cremers and Lausch 2008), artificial lighting elements (ETFE 2014), dynamic shading by pressure controlled switching of complementary printing patterns (Gabler, et al. 2010) and low emission coating (Cremers, Palla, et al. 2016). The advancement of these materials for the application in architecture and buildings has been subject to several previous research projects at HFT Stuttgart like MESG (Manara, Beck and Cremers 2012), FMESG (Manara, Kehl, et al. 2018) and FOLLOW-E2 (Hildebrandt, et al. 2019). The combination of fluidic systems and membrane structures has been subject to numerical investigations within research works, for example, at Transsolar Energietechnik GmbH (Ganji Kheybari and Lam 2017).

A vast number of other potential beneficial uses of the building envelop can be unlocked in conjunction with agricultural use by facade greening of curtain walls or compact greening elements integrated into the building skin. The beneficial effects of façade greening range from wastewater treatment (Eisenberg, Ludwig and Well 2020) to regulation of ambient temperature and humidity, air purification with special moss wall elements (Artificial Ecosystems 2020), compensation of carbon dioxide by cultivation of algae (Kerner 2017) and the straightforward provision of edible fruits and leafy greens (Fraunhofer UMSICHT 2011). The biomass produced from algae is versatile in its use and can be processed for food, cosmetics and even fuel. Some of these concepts are closely related to the concepts of vertical farming (Despommier 2010). The incorporation of the roof in conjunction with the synergetic combination of agriculture and fish farming, known as aquaponic, constitutes a highly efficient method and would yield further benefits (Goddek, et al. 2019). Additional benefits arise from a more holistic approach to integrate other streams from the building like air and heat into the system (Windcloud 2020). Another approach is the direct production of hydrogen via solar photoelectrochemical water splitting (May, et al. 2015).

The aspect of air purification by means of special photocatalytic TiO2 coatings has been addressed by researchers (RWTH Aachen 2018) and even been realized within singular building projects (TDMA 2018). The support of the ventilation system by utilizing natural ventilation phenomena like the chimney effect in conjunction with the well-understood concept of the trombe wall structure is another probable benefit of the additional redirection of the airflow through vortex structures would even enable a separation of airborne particles to support the air purification system.

On a final note, it should also be addressed that the concept of thermal activation of building skin also bears some noteworthy potential for the building envelope beyond its static functionality. Thermal activated building envelope elements have the capability to act as dynamic heat exchanger and in combination with proper heat pump application enable a reduction of heating and cooling load on the building.

Many of the above concepts and technologies are not exactly novelties. In fact, an overview of many of the mentioned concepts was compiled and presented by Cremers in the past (J. Cremers 2017). Still virtually all of the above-mentioned concepts either will benefit or will only now be efficiently realized through the increased availability of sensor/actuator systems and data-based control algorithms of the smart city concept in its interpretation as a massive IOT entity.

4 Upcoming Investigation Within iCity Phase 2

Within the upcoming phase 2 of the iCity research project, the possibilities and capabilities of different variations of novel building envelope elements and their beneficial impacts for the inhabitants and the environment will be assessed. In phase 1 of the iCity research project, some aspects like optimization of natural ventilation via windows and noise reduction measures in the construction material have already been successfully addressed. The insights from these preceding works will be valuable inputs due to the overlap in the topics since windows are an integral part of the building envelope.

The combination of various solutions in addition to optimized algorithmic control is expected to release additional beneficial synergetic effects. The investigations will be supported by numerical dynamic building simulation in order to quantify the impact on energy balance and environmental parameters like temperature and daylight irradiance. The research will be substantially intensified to identify other main actors in research and commerce on the field and harness interdisciplinary knowledge exchange.

5 Conclusion and Outlook on the Necessity for Further Research

The main takeaway from the presented, somewhat abbreviated, considerations, concerning the role of the building envelope within the context of the smart city concept, is that a shift in the functionality seems to be immanent. The potential for an extension of its functionality and beneficial impact is rather high and diversified. A decent amount of research needs to be done to cover the multitude of aspects and quantify the possible advantages of their application.