FlectoLine Facade, Freiburg, 2024

Photographs by ITKE/ ITFT, University of Stuttgart

Photographs of the Fabrication

Diagrams of ITKE/ITFT, University of Stuttgart

Amid the growing challenges of climate change, the built environment demands a transformative approach that prioritizes energy efficiency in architecture and construction. Façades must evolve into dynamic systems that harmonize with their environment, employing strategies to minimize building energy consumption while maximizing comfort and functionality. Reimagining façades as active responsive components, presents opportunities to enhance building performance and foster innovative architecture that reduces energy consumption and carbon footprints.

The international research project Flectuation focused on the development of advanced responsive façade elements for real-world application. The resulting façade shading system, FlectoLine, located at the Botanical Garden of the University of Freiburg, employs compliant mechanisms for hingeless, elastic deformation, effectively eliminating the mechanical complexity and geometric constraints typical of conventional rigid-body systems. This innovative design is complemented by an advanced control system that incorporates user- and environment-specific response models. The system records these responses to inform a machine learning algorithm capable of predicting ideal actuation levels for various scenarios.

Covering an area of 83.5 m², this façade system explores a future oriented alternative approach to our build environment by retrofitting the current building stock through their response to changing environmental conditions and fluctuating indoor usage patterns. Incorporating Building-Integrated Photovoltaic (BIPV) into these responsive shading elements offers a holistic solution that combines energy generation with a reduction of building energy consumption during operation. The system can optimize interior comfort while simultaneously adjusting the orientation of the photovoltaic cells to maximize solar energy generation.

Building on over a decade of research by the University of Stuttgart's ITKE and ITFT, FlectoLine represents the latest step of development in responsive building envelope design, utilized by compliant mechanisms. Previous demonstrators, such as Flectofold and FlectoSol, showcased the potential of fiber reinforced compliant mechanisms but were limited to indoor testing. Thus, FlectoLine is the first fully functional exterior responsive façade shading, utilized by compliant mechanisms, combining sustainability and enhanced architectural performance.

Responsive Façades

Responsive façades represent a cutting-edge approach in architectural design, enabling building envelopes to actively respond to environmental and user stimuli. By integrating programmed material build-ups, advanced actuators, and sensor networks, these dynamic systems adapt in real time to optimize energy performance, enhance occupant comfort, and foster new interactions between users and their built environments.

Their ability to regulate solar gain, ventilation, and thermal performance positions them as a critical component in reducing energy demands and supporting sustainable urban development.. As the emphasis on sustainable design grows, these façades are poised to play a pivotal role in shaping future buildings, exemplifying the seamless fusion of technological innovation and architectural functionality.

Biomimetic Research

The FlectoLine demonstrator is informed by biomimetic research that has been carried out in collaboration at the universities of Stuttgart, Freiburg and Tübingen.. The biological role model of the water trap (Aldrovanda vesiculosa) features turgor pressure-driven motor zones aligned parallel to the linear midrib (backbone) of the trap. This concept is adapted into the design of the FlectoLine modules, where the linear actuator zone replicates the functionality of the motor zones in the biological system. On the other hand, the veins in bug wings (Graphosoma italicum) were identified as second biological role model with focus on the transfer of the material setup. The veins, initiating the folding motion, is surrounded by stiff and flexible material (sclerotin/resilin). Analysis revealed that, by implementing flexible areas in the actuation zone, stresses in the material structure caused by increase of internal pressure are reduced. Furthermore, it has been shown that the direction of actuation is set by different accumulation of more or less stiff material (sclerotin) around the vein: the bending deformation is higher on the less stiff side so that it is folded in this direction. The motion direction is thereby implemented directly in the material structure itself.

Integrated Actuation

FlectoLine consists of fiber reinforced composite plates with built-in hinge zones that have been specially developed for operation with integrated pneumatic actuators. The pneumatic actuator is integrated directly into the composite in the form of a cushion. The composite's material structure can be divided into a stiffer part below the actuator and a more flexible part above the actuator. When pressurized, the cushion deforms stronger in direction of the more flexible plate, causing the entire composite plate to bend in that direction. By clamping on one side next to the hinge zone, a bending motion of the free, non-clamped end can be achieved. Since the actuation system is integrated directly into the composite plate, no mechanical connections between the folding elements and the actuation mechanism are required. The flexible hinge zones require only a low pressure (0.3 to 1.5 bar) to achieve angular positions from 0° to 90°. During the folding process, elastic energy is stored in the flexible hinge zones, which allows the module to rotate back to its original position as soon as the pressure is released.

Material System

Two different material systems were developed for the technical implementation on the façade. Firstly, the biologically inspired material system, abstracted into elastic and stiffer material layers, was transferred into a hybrid composite with elastomer components and a thermoset-based fiber reinforced composite. Similar to the wing vein, the actuator chamber is enclosed by elastomer layers. This ensures interlaminar adhesion in the actuation plane. The direction of motion during actuation is determined by the asymmetrical distribution of the stiffening glass fiber reinforced material above and below the actuation plane.

This first material system had already been tested and proven effective in previous demonstrators, exemplifying its reliability and adaptability for large-scale applications. However, to optimize the fabrication process in respect to time and cost, a second alternative material system was developed. To explore a more streamlined alternative, a thermoplastic-based material structure was constructed that works in a similar way: here, two layers of thermoplastic glass fiber reinforced plastic based on polyamide-6 with different stiffnesses were bonded together using an elastic .

Both systems are provided with a protective outer layer that offers excellent weather resistance, thus qualifying them for use in exterior façade elements. In this context, each system was subjected to a weathering and fire test to ensure that the mechanical properties and appearance remain stable for at least 15 years and that the components at least meet the requirements of fire class B2. In view of the different weather conditions on the façade, the components were also subjected to a wind test in which the maximum expected wind load from different directions was applied. To ensure the longevity of the façade elements, each material system was tested cyclically under pneumatic actuation for bending up to 90° with at least 20,000 test cycles.

Control System. User and Environmental Responsiveness

To effectively control the performance of the responsive façade, a digital twin has been developed, enabling real-time simulation of thermal and lighting behaviors as well as energy production from integrated photovoltaic (PV) panels. The digital twin gathers real-time data through embedded sensors, including indoor lighting levels from light sensors, outdoor lighting levels from solar exposure sensors, indoor temperature from distributed temperature sensors, and wind conditions from anemometers on the façade. Forecast data, such as detailed weather predictions (solar radiation, cloud cover, temperature, wind speeds, and precipitation) from meteorological Application Programming Interfaces (APIs) and energy demand forecasts based on previous usage, are also integrated into the system. Using this data, a decision-tree-based control algorithm optimizes three aspects of indoor comfort—lighting for adequate brightness, glare minimization, and thermal regulation—while maximizing PV energy production. The system calculates optimal panel angles by continuously analyzing real-time and forecasted inputs, ensuring efficient operation throughout the day and balancing occupant comfort, energy efficiency, and renewable energy generation.

Large Scale Responsive Façade

The FlectoLine façade serves as a proof-of-concept, showcasing the potential for creating large-scale responsive façades using fiber reinforced plastic laminates with compliant hinge zones and integrated pneumatic actuators. Covering an area of 83.5 square meters, the façade consists of 101 components, with dimensions ranging from 0.81 × 0.86 m to 1.50 × 1.31 m in the x and y axes, respectively. The modules require a pressure of 0.4 bar to fully actuate to a 90° angle, demonstrating their efficiency in motion. Thin-film organic photovoltaic (PV) cells are incorporated into the design to harvest solar energy, ensuring the responsive façade sustains its energy needs independently. Additionally, the demonstrator explores possibilities for fostering direct interaction between the built environment and its occupants through active control systems, made feasible by the seamless integration of computational design, advanced simulation, and fabrication processes.

PROJECT TEAM

ITKE Institute of Building Structures and Structural Design, University of Stuttgart
Edith A. Gonzalez San Martin, Dr.-Ing. Axel Körner, Prof. Dr.-Ing. Jan Knippers,

ITFT Institute for Textile and Fiber Technologies, University of Stuttgart
Matthias Ridder, Dr.-Ing. Larissa Born, Prof. Dr.-Ing. Götz T. Gresser

HELLA Sonnen- und Wetterschutztechnik GmbH
Stephan Moser, Robert Weitlaner, Gerald Lukasser

Jehle Technik GmbH
Sai Konduri, Uwe Boerboom, Alexander Jehle

Formfinder Software GmbH
Robert Roithmayr

SCIENTIFIC COLLABORATORS

PBG Plant Biomechanics Group, Botanical Garden, University of Freiburg
Prof. Dr. Thomas Speck

WITH SUPPORT OF

Kalaivanan Amudhan, Avelia Christina Emilton, Yara Karazi, Deng Ming,
Marcel Rosenfelder, Sarvenaz Sardari, Sai Pranneth Singu, Ivanna Trifunovic,
Aysima Yavuz

IN COOPERATION WITH

Cluster of Excellence IntCDC – Integrative Computational Design and Construction for Architecture, University of Stuttgart

Cluster of Excellence livMatS – Living, Adaptive and Energy-autonomous Materials Systems, University of Freiburg

ASCA GmbH & Co. KG, Kraiburg Gummiwerke GmbH, KREMPEL GmbH, Roy Hohlfeld

PROJECT INFORMATION

FUNDING

Central Innovation Programme for small and medium-sized enterprises (ZIM) |
Federal Ministry for Economic Affairs and Climate Action (BMWK)

Austrian Research Promotion Agency (FFG)
Federal Ministry of Labour and Economy (BMAW) |
Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology (BMK)