Thermal behaviour of switchable liquid crystal glazing under real environmental conditions

Abstract Switchable liquid crystal glazing presents several advantages over conventional glass facades by seamlessly incorporating shading functions directly into the glass structure. This technology eliminates the need for mechanical components, enhancing reliability and reducing environmental impact. It also features rapid response times, within fractions of a second, and allows for variable shading levels, enabling precise adaptation to individual needs, thereby improving thermal and visual comfort. However, the design of these systems presents challenges, particularly with regard to the heating of the glass panes due to the absorption of solar radiation, especially when shading. Since the liquid crystals are embedded directly within the pane arrangement of the glazing unit, heat is transferred to the glass, causing uneven heat distribution, specifically in the clamped, shaded edge areas, which increases the risk of thermal stress fractures. Two types of glazing, double and triple, were installed and tested under real weather conditions in a facade test building to measure and assess the temperature behavior within the glass pane structure over a period of three years using various shading scenarios. The study revealed temperature extremes ranging from -9.9 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C to 79.9 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C for triple glazing units (TGU) and -7.3 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C to 77.2 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C for double glazing units (DGU). Critical temperature differences between center and edge areas reached up to 47.9 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C (TGU) and 46.5 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C (DGU), exceeding the 40 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C resistance limit specified for float glass in DIN EN 572-1. Multiple linear regression analysis demonstrated that 78.7 % of temperature variance could be explained by environmental variables, with direct solar radiation and outdoor temperature being the dominant influencing factors. Despite theoretical risks for thermal fracturing based on temperature difference thresholds, no structural damage was observed during the entire monitoring period, suggesting higher thermal resistance of the LC-cells than anticipated. The results provide valuable data for developing predictive temperature control models and optimizing the reliability and durability of switchable glazing systems under real-world conditions.