Development of a novel thermal insulation system for building envelope application

Abstract

The building sector is responsible for 38% of global energy and process-related greenhouse gas emissions. In recent decades, a rapid increase in energy consumption in buildings has been witnessed, making energy reduction a pressing issue. Thermal insulation systems, that can operate dynamically in response to changing transient conditions, by alternating between thermally conductive and insulated states, are gaining attention in their architectural applications, since they constitute an effective mean for reducing energy consumption while simultaneously improving occupants comfort levels. Despite the pioneering work that has been conducted by researchers to develop adaptive insulation technologies in different engineering fields, currently none of the adaptive insulation technologies are embedded in real-world buildings for a number of reasons. The aim of this thesis, therefore is to identify these reasons and on this basis, to explore, the different ways in which an adaptive insulation system can be created. There are no established standards or requirements for the development of adaptive thermal insulation technologies. Consequently, this research initially focuses on the creation of design criteria that address aspects related to thermal performance, design feasibility, and technological complexity of the examined systems. Subsequently, the exploration of different design concepts was initiated by first creating a scheme of aspects, concerning the geometry of the core, the mobility of the outer layer, the number of cavity compartments, the type of actuation and the types of mechanisms that facilitate the transition between thermal insulation states. From the combination of these aspects, six design alternatives emerged, the principle of operation of which is based on the idea of a structure that can inflate and deflate during the transition of the system from the insulated to the conductive state. The core geometry of the concepts was modeled within TRISCO steady-state 3D software tool, where various parameters were tested in order to obtain the final topology. The aim was to achieve a thermal resistance comparable to that of state-of-the-art insulating materials in their insulated state and furthermore, to achieve a large range of shift in order to acquire a significantly low thermal resistance in the conductive state comparable to the case of an uninsulated concrete block. Simultaneously, the thermal transmittance (U-value) for the insulated and conductive states of each concept was estimated through numerical simulations in TRISCO and validated using analytical models for comparison. The results of the analysis indicated that, the thickness of the air cavity and the emissivity of the membrane material greatly affect the examined thermal resistance. Additionally, the range of shift depends significantly on the degree of evacuation of the air cavity. It has been found that technologies whose structure and working principle allow full compression of the core in the conductive state, can lead to a lower thermal resistance which is governed by solid conduction. The results of the simulations showed that one of the examined concepts satisfied the initial goal, achieving thermal transmission which ranged from 0.13 W/𝑚2K to 2.79 W/𝑚2K in the insulated and conductive states respectively. Whereas, the thermal resistance ranged from 7.5 𝑚2/KW to 0.4 𝑚2/KW in the insulated
and conductive states respectively. The obtained values of thermal transmittance from TRISCO, were compared with the thermal transmittance from analytical formulas. In this way, the accuracy of the numerical simulations was validated. The final stage of the design exploration phase led to the selection of the final design through a multi-
criteria analysis that is aligned with the aforementioned initial design criteria. The final design is a 1.5 m by 0.75 m opaque panel of low emissivity honeycomb core made up of 10 mm cells. In the insulated state the panel has a thickness of 0.18 m while after being compressed the thickness is reduced to 1/3. Actuation of the system is achieved using a dual-function pump that supplies air to a system of channels that access the core. An approximation of the sizes of the pump and pipes were given for the design of the air-supply system. In this way, the system can be reversibly switched between an inflated state governed by gaseous conduction and a deflated state, where the solid conduction of adjacent surfaces governs. The architectural application of the adaptive insulation system, considers the element as an infill component in which the outer skin is integrated into the system, implying that the transition between thermal states is visible from the façade. To conclude, this research explores the potentials of using the parameters influencing heat transfer capability for the development of an adaptive insulation system intended to be used in the building envelope. The research through design resulted in a promising design based on the resulting thermal performance indicators. However, to realize the proposed technology, tests on physical model need to be conducted. The thermal performance of the system should be validated experimentally in order to derive statistical data for its safe application and the actual dimensions of the air-supply system need to be determined.