Thermotropic Glazing Research Articles

Comparative analysis of hydrogel-based thermotropic glazing and fixed horizontal overhangs in building performance

Thermotropic (TT) glazing is able to achieve energy-saving capabilities similar to external shading devices by dynamically regulating solar heat gain transmitted through windows in response to ambient climatic conditions. However, the building performance of TT glazing compared to traditional shading devices remains underexplored. This study aims to bridge this knowledge gap by comparing the building performance of hydrogel-based TT glazing and fixed horizontal external shading devices in terms of building energy consumption, daylighting performance, and thermal comfort. Experimental tests were conducted over a continuous ten-day period, focusing on the indoor temperature and illumination distribution within experimental rooms equipped with either TT glazing or fixed horizontal external shading device. Building performance models were developed using EnergyPlus and validated with experimental data. Subsequently, a comparative analysis was then conducted to evaluate the impact of TT glazings and fixed horizontal external shading devices on building energy consumption, daylighting performance, and thermal comfort across five typical climate conditions in China. The findings indicated that: (1) In five climate zones, TT provided the greatest energy savings in West-facing orientations, followed by East-facing orientations. (2) TT can replace fixed horizontal external shading device with a certain length to achieve similar building energy efficiency and indoor thermal comfort performance. (3) In regard to daylighting performance, TT achieved the desired values of sUDI<100lx,50 % and sUDI300–3000lx,50 % across all four orientations in the five climates. (4) In terms of thermal comfort, TT improved thermal comfort by reducing the annual daytime average PPDoverheating across all orientations in the five climate zones.

Annual evaluation of the visual-thermal comfort and energy performance of thermotropic glazing in a reference office room of China

Hydrogel-based thermotropic glazing, which achieves the dynamic characteristics by maneuvering the light-scattering behaviors, has a great potential in improving building energy performance, visual and thermal comfort. The visual-thermal comfort and energy performance of the thermotropic glazing with different transition temperature was evaluated by using an experimental validated building performance simulation model. The performance of the thermotropic glazing was compared with conventional double-clear glazing and low-emissivity double glazing under five cities within the five major climate zones across China. The objective is to comprehensively understand the applicability of the thermotropic glazing in different climatic conditions of China as well as to determine the optimal transition temperature. The results showed that: (1) The thermotropic glazing with a transition temperature of 30 °C could achieve better energy performance, visual and thermal comfort in Xiamen, Nanjing, and Kunming. (2) Low transition temperature is not an essential requirement for the application of the thermotropic glazing, which could result in increasing energy use intensity and this effect is more significant in colder regions. (3) The thermotropic glazing could contribute to building energy savings (up to 16.3% and 2.3% i.e., in Xiamen) compared with conventional double-clear glazing and low-emissivity double glazing with low SHGC. (4) The thermotropic glazing applied in Xiamen, Nanjing and Kunming could obtain better visual and thermal comfort (i.e., increase of the desired range of illuminance sUDI500–2000lux, is up to 85.19% and a promotion of 29.18% in time proportion of desirable thermal comfort in Kunming) when compared with low-emissivity double glazing and conventional double-clear glazing.

Thermotropic overheating protection for façade-integrated solar thermal collectors

The capability of thermotropic glazing to reduce stagnation temperatures in solar thermal collectors is evaluated in in-situ testing (south-façade of an office building). Therefore, a thermotropic formulation which is curable by sunlight and which exhibits both, excellent light-shielding characteristics (transmittance reduction by ~20%) and long-term stability is specifically designed. Application relevant overheating protection efficiency is given: absorber temperatures are reduced by up to 29K in collector stagnation. Performance characteristics are not changed during the six month period of in-situ testing. The application demonstration emphasizes the high innovative capacity of thermotropic materials.

Stepwise Activation of Switchable Glazing by Compositional Gradient of Copolymers.

Thermotropic glazing is one of the most promising developments for adaptive solar control; however, a monotonic transparent-opaque transition limits its practical application. In this work, to render stepwise activation of the switchable glazing, we prepared multicomposition copolymers having a compositional gradient. By slow addition of the monomers in the reaction mixture during free-radical polymerization, the blend of copolymers with each polymer having different compositions of the monomers could be prepared. We found that the developed copolymers exhibit different thermal behaviors according to the monomer composition, yielding the nearly linear transmittance change over a wide temperature range due to the gradient hydrophilic-hydrophobic balances. By combining prepared copolymers with photothermal graphene oxide as a heat transducer, we demonstrated gradual solar control of the smart window in response to sunlight intensity in outdoor testing.

Comprehensive study on the light shielding potential of thermotropic layers for the development of new materials.

In recent years thermotropic overheating protection glazings have been the focus for both solar thermal collector technology and architecture. A thermotropic glazing changes its light transmittance from highly transparent to light diffusing upon reaching a certain threshold temperature autonomously and reversibly. In thermotropic systems with fixed domains (TSFD) the scattering domains are embedded in a polymer matrix, which exhibits a sudden change of the refractive index upon reaching a threshold temperature. The aim of the present study was to comprehensively investigate the light shielding characteristics and potential of TSFD materials by applying simulation of light scattering in particle-filled layers. In random walk simulations a variety of parameters were varied systematically, and the effect on the light transmission behavior of TSFD was studied. The calculation steps of the simulation process are shown in detail. The simulations demonstrate that there is great potential for the production of functional materials with high overheating protection efficiency.

Thermotropic glazings for overheating protection. I. Material preselection, formulation, and light‐shielding efficiency

ABSTRACTThis article presents a systematic strategy for formulation and optimization of thermotropic layers for overheating protection purposes. Specifically, thermotropic systems with fixed domains (TSFD) which consist of a thermotropic additive finely dispersed in a matrix material are considered. Based on systematic material (component) preselection regarding thermoanalytical characteristics and refractive indices, numerous thermotropic layers were formulated. TSFD with thermoplastic matrix were produced by compounding and compression molding. TSFD with resin matrix were produced by UV curing. The thermotropic layers were analyzed as to solar optical properties, threshold temperature, switching process and residual transmittance in the opaque state applying UV/Vis/NIR spectrometry equipped with a heating stage. Best performing materials exhibited solar hemispheric transmittance in the range of 72.2–84.5% and between 59.6 and 83.7% in the clear and opaque state, respectively. Threshold temperatures between 45 and 75°C were realized. Refractive index difference between matrix and additive and solar hemispheric transmittance displayed a close correlation. Hence, refractometry was shown to be an appropriate tool for material preselection. Furthermore, investigations revealed a close correlation of thermal transitions of thermotropic additives recorded by differential scanning calorimetry and threshold temperatures of thermotropic layers formulated therewith. However, thermotropic layers formulated so far have to be optimized with respect to light‐shielding performance for efficient overheating protection. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39950.

Thermotropic glazings for overheating protection. II. morphology and structure–property relationships

ABSTRACTThis article completes a systematic strategy for formulation and optimization of thermotropic systems with fixed domains (TSFDs) for overheating protection purposes. Focus was on characterization of morphology and on revealing optimization potential. A comprehensive characterization of scattering domain size and shape was done applying optical microscopy and scanning electron microscopy. In general, scattering domains exhibited inappropriate size and/or shape for optimum overheating protection performance. Moreover, several TSFD displayed defects (vacuoles, voids) resulting from thermomechanical or physicochemical interaction of matrix material and thermotropic additive during manufacturing. Morphological features along with solar optical and thermorefractive properties allowed for establishment of structure–property relationships. Light‐shielding efficiency of TSFD correlated well with scattering domain size and shape. The majority of TSFD showing defects exhibited an increase of solar hemispheric transmittance upon heating. Several strategies to overcome defect formation and to improve scattering morphology were suggested and proof of concept was shown partially, thus indicating a significant optimization potential of the established TSFD. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39910.

Effect of temperature-cycling on the morphology of polymeric thermotropic glazings for overheating protection applications

In this paper, the morphology of thermotropic systems with fixed domains (TSFD) was evaluated applying high resolution Atomic Force Microscopy (AFM) also upon heating and cooling. Furthermore nano-mechanical characteristics of the samples were determined by means of Force/Distance spectrometry. TSFD formulated with additive types exhibiting a short chain length displayed roughly spherical scattering particles with dimensions between 0.6 μm and 4 μm. By means of Force Distance spectroscopy stiffness values of 0.6 N/m and 7.8 N/m were determined for the scattering domains and the matrix, respectively. Upon heating, melting and deliquescence of the additive along with migration was ascertained. After cooling to ambient temperature the formation and growth of terrace-like additive domains on the surface was recorded. Additive types with long-chain molecules developed anisotropic scattering domains resembling distorted disks without predominating orientation. Diameters up to 50 μm and a thickness between 200 nm and 600 nm were ascertained. Determination of stiffness yielded values of 0.9 N/m and 13.1 N/m of the scattering domains and the matrix, respectively. Upon heating, swelling and deliquescence of the additive were detected. After cooling to ambient temperature a partial recovery of swelling was observed. Force-Distance spectroscopy yielded a 5 to 10 nm thick additive layer which coated wide areas of the surface after the heating cycle for all samples investigated.

Facade systems with variable solar control using thermotropic polymer blends

Thermotropic layers integrated into glazing or transparent insulation systems reversibly reduce the total solar energy transmittance by becoming opaque when heated. Characteristics of both systems have been measured as a function of temperature and incidence angle, including the total solar energy transmittance for a thermotropic heat-mirror insulating glazing unit (IGU). Properties of complex thermotropic glazing systems have been simulated, starting from data characterising the thermotropic layer. The results show good agreement with measured data. A prototype system sample of a thermotropic heat-mirror IGU was integrated into an outdoor test facility and a thermotropic TEIF (transparent exterior insulation and finish) prototype system was installed into the facade of a demonstration house. Both systems have been monitored and showed the expected switching properties, resulting in the desired overheating protection. Computer simulations of the building integration of the thermotropic TEIF system have been performed, showing a strong dependence of the optimum switching temperature on the total solar and internal gains of the building.

Intelligent window using a hydrogel layer for energy efficiency

The intelligent window is a thermotropic glazing based on the sealed laminate of a new type of intelligent material, a highly viscous polymer aqueous solution or hydrogel, which consists mainly of water, a water-soluble polymer with hydrophobic groups, an amphipathic molecule and sodium chloride. The hydrogel was developed after conducting fundamental studies on hydrophobic bonding. The amphipathic molecule enables autonomously reversible light adjustment while retaining its homogeneity. In addition, a cellulose derivative is used selectively in the water-soluble polymer with hydrophobic groups, by which problems such as lack of durability, toxicity and high cost, which had previously prevented the commercialization of the intelligent window have been resolved. Peripheral technologies were also established for sealing and lamination, so that the panel type, autonomously responsive, light adjustment window, the Affinity Intelligent Window (AIW) can be produced with a large area of 1m2 with the properties required for practical use.