Soft-Robotic-Driven Adaptive Photovoltaic Building Envelopes

Current building envelopes are predominantly static, manually operated, and do not have integrated energy harvesting devices. As opposed to this, kinetic building envelopes with integrated photovoltaics, have the potential to improve building energy performance and occupant comfort by regulating solar heat gains and natural lighting while simultaneously generating electricity on site. If equipped with reflecting elements, the adaptive envelopes can be also used to redistribute solar radiation among neighbouring buildings for generation of thermal energy or mitigation of urban heat-island effects. Even though various approaches to kinetic and climate adaptive envelopes exist, they are limited to theoretical studies, small-scale prototypes, and single, exhibition-type prototypes. The reason for this is in the actuation mechanisms used, which are typically very complex and are comprised of many mechanical components, such as electromotors, rails, pulleys, and cables. This increases the need for maintenance and reduces the lifespan of the envelope. To realize the full potential of adaptive building envelopes, a robust and cost-effective way to reliably move their elements is required. Pneumatically-driven elastomeric actuators, as commonly found in soft robotics, have been identified in this work as a simple, low cost, and robust actuation solution. The toughness, relative chemical inertness, and low glass-transition temperature of silicone elastomers make them ideal for tolerating external environmental conditions, such as rain, cold, heat, and wind. Inherently compliant, they are robust to external disturbances. Pneumatically-powered, they exhibit a high force-to-weight ratio, large strokes, and low mechanical impedance. The challenge is in their design, fabrication, and control. In particular, one of the core challenges of soft actuators is to be able to actively vary their stiffness. The variable stiffness, in this case, would enable actuators to stabilise the envelope panels in wind. In this work, a novel hybrid, soft-hard-material, pneumatically-driven, two-axis actuator with variable stiffness is introduced. It comprises a single body three-chambered elastomeric actuator and a metal two-axis joint from classical mechanics. The metal joint provides structural stability for the soft actuator, thus enabling its more precise and controlled movements, higher forces, and variable stiffness, while all the advantages of pneumatically-driven elastomeric actuators are retained. The corrugated design of chambers’ walls allows fast (0.1 s) actuation and reduction of strain in the material, thus increasing the lifespan of the actuator. The actuator achieved 30,000 cycles in laboratory conditions without breaking; assuming four cycles a day, this is equivalent to a 20-year lifespan. The inherent compliance, pneumatic actuation, and compact design allowed for the reduction of actuator’s size, weight, and complexity compared to an actuator of the same performance that would be constructed in classical mechanics, for example using two electromotors, gearboxes, and spring. As such, this actuator enabled construction of modular, lightweight adaptive building envelopes with integrated energy harvesting devices, such as thin film photovoltaic panels. The soft actuators were produced using a novel method for fast and precise manufacturing of elastomeric actuators, called Quick-cast, introduced also in this work. A novel distributed, modular pneumatic control system is also introduced in this work. It allows pressure to be locally controlled at each envelope module by receiving control signals from a central controlling unit, while the measurements from the orientation sensor, an inertial measurement unit, are sent to the central unit. Such a pneumatic orientation control system enables expansion of the envelope to a wide range of shape and sizes, with only a few steps required to connect a new envelope module with the soft actuator. Several envelope prototypes have been constructed and tested in real weather conditions in solar tracking experiments. Electricity gains of 30-50% have been measured for a South oriented envelope with 16 widely distributed panels; these gains correspond well with theoretical estimates. The gain in solar tracking depends largely on envelope orientation, climatic region, and the distribution of the elements within the envelope. However, the total building energy savings also include the effects of active shading. Estimated energy savings for an office in Zurich with such an adaptive photovoltaic envelope are 20-80% more than an equivalent static system, depending on the building type. We estimated the self-energy consumption, that is, the energy used for pneumatic control vs. the total energy produced, to be about 3%, based on measurements