Optimal transition temperature maps of thermotropic glazing in a reference office room of China considering the building performance

You Y., Yang X., Deng X., Shi F., Wang C., Hong X.

Hong X., Yang X., You Y., Shi F., Yang F., Cui C., Gong Y., Qian D.

Wang C., Yang H., Ji J.

Wu S., Sun H., Song J., Liu S., Shi S., Tso C., Lin B.

Detsi M., Atsonios I., Mandilaras I., Founti M.

Ghamari M., Sundaram S.

Building-integrated photovoltaic (BIPV) glazing systems with intelligent window technologies enhance building energy efficiency by generating electricity and managing daylighting. This study explores advanced BIPV glazing, focusing on building-integrated concentrating photovoltaic (BICPV) systems. BICPV integrates concentrating optics, such as holographic films, luminescent solar concentrators (LSC), Fresnel lenses, and compound parabolic concentrators (CPCs), with photovoltaic cells. Notable results include achieving 17.9% electrical efficiency using cylindrical holographic optical elements and crystalline silicon cells at a 3.5× concentration ratio. Dielectric CPCs showed 97.7% angular acceptance efficiency in simulations and 94.4% experimentally, increasing short-circuit current and maximum power by 87.0% and 96.6%, respectively, across 0° to 85° incidence angles. Thermochromic hydrogels and thermotropic smart glazing systems demonstrated significant HVAC energy savings. Large-area 1 m2 PNIPAm-based thermotropic window outperformed conventional double glazing in Singapore. The thermotropic parallel slat transparent insulation material (TT PS-TIM) improved energy efficiency by up to 21.5% compared to double glazing in climates like London and Rome. Emerging dynamic glazing technologies combine BIPV with smart functions, balancing transparency and efficiency. Photothermally controlled methylammonium lead iodide PV windows achieved 68% visible light transmission, 11.3% power conversion efficiency, and quick switching in under 3 min. Polymer-dispersed liquid crystal smart windows provided 41–68% visible transmission with self-powered operation.

Ming Y., Sun Y., Liu X., Liu X., Wu Y.

Windows significantly influence a building's indoor environment and energy consumption due to their high optical transmittance and low thermal resistance comparing with walls, affecting both indoor daylight comfort and heat gain or loss. Thermotropic (TT) materials can offer dynamic regulation of solar energy, improving building energy efficiency. Parallel slats transparent insulation materials (PS-TIM) integrated into the air cavity of double-glazing windows can effectively increase the thermal resistance of a window. In this study, these two advanced technologies were combined to develop a new type of adaptive fenestration system, named Thermotropic Parallel Slat Transparent Insulation Material (TT PS-TIM) system. The selected TT hydrogel, 5 wt.% poly(N-isopropyl acrylamide) (PNIPAm), was sandwiched within polymethyl methacrylate (PMMA) slices. The slats were subsequently proposed between two glass panes to form a parallel slat transparent insulation materials system. To investigate the thermal and optical properties of the TT slats as well as the thermal performance (i.e., solar heat gain coefficient, and dynamic thermal performance in summer and winter) of the TT PS-TIM system with different slats intervals, a novel model combining Ray-tracing technique and Computational Fluid Dynamics (CFD) has been proposed. The findings of this study reveal that a TT PS-TIM system incorporating 5 wt.% PNIPAm slats can offer a substantial reduction (up to 0.5) in solar transmittance as it transitions from a clear to a translucent state. Accompanied with the phase transition of the TT slats, there is a significant reduction in the Solar Heat Gain Coefficient (SHGC) of the window system. This effectively decreases the unwanted solar heat gain, thus improving the overall buildings thermal performance. According to the advanced transient simulation results based on the weather conditions of London, the TT PS-TIM system exhibits superior performance evidenced by over a 30% reduction in heat gain during summer and approximately a 20% reduction in heat loss during winter in contrast to a conventional Double Glazing (DG) system. This is evident in the form of reduced heat gain during summer and minimized heat loss during winter, resulting in a more thermally balanced environment throughout the year. The comprehensive thermal investigation of TT PS-TIM can effectively help to increase the accuracy of the further energy and daylight analysis.

Shi F., You Y., Yang X., Hong X.

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.

Li J., Sun R., Cheng J., He X., Zhang Y.

Human thermal responses can vary based on individuals' thermal histories. Although comparative studies on thermal comfort have been conducted in different regions, research is still limited regarding the differences in thermal responses among people living in the same city but having emigrated from diverse climates. Addressing this gap, this study, conducted in Beijing between 2021 and 2022, involved on-site meteorological measurements and questionnaire surveys, collecting a total of 1,370 questionnaires. The objective was to examine the differences in thermal response among individuals from different climatic backgrounds post-migration and assess the implications for urban design improvements. The study findings revealed the following: 1) Southern migrants exhibited a higher neutral Universal Thermal Climate Index (UTCI) compared to northern migrants (20.2 °C vs. 18.7 °C). 2) In terms of the acceptable temperature range (TAR) for 1 h, southern migrants showed a greater tolerance for hot conditions compared to northern migrants (4.4–30.8 °C vs. 0.6–28.6 °C), but less tolerant for cold conditions. 3) The transient acceptable range was considerably wider than the 1-h TAR. 4) Residents feeling thermally neutral at present had lower demands for urban design improvements. Our results highlight the importance of considering diversity of thermal response and implementing targeted management and planning interventions. The study is expected to provide practical recommendations for creating more livable and sustainable urban environments.

Wang J., Li G., Zhao D.

The thermochromic smart window has shown potential for building energy saving by regulating optical characteristics according to real-time temperature. However, most existing studies on smart windows were conducted using an annual building energy-saving analysis. However, the ideal performance of a smart window that strikes a good balance between reducing energy consumption, eliminating the discomfort glare, and providing adequate daylight remains unclear. In this study, a novel AlN/VO2/AlN (AVA) multilayer thermochromic glass with improved transmission by using the thin-film interference principle is proposed, and the energy consumption and daylighting performance of the AVA windows are investigated using a multi-objective optimization method, considering different structural parameters in heating/cooling modes. The effect of different optical characteristics of the AVA glass on building energy consumption and the required annual daylight hours within the useful daylight illuminance (UDI300–2000) is analyzed to find out the Pareto frontier of the AVA windows. The Pareto Optimal Solution (POS) shows the best energy-saving AVA window should possess τlum = 0.58 and ΔτNIR = 0.33, which results in a maximum energy saving of 12.13 % for the office. This solution will strike a good balance between reducing energy consumption, eliminating the discomfort glare, and providing sufficient daylight.

Khaled K., Berardi U., Liao Z.

• Climate-responsive coatings adjust their thermo-optical properties in response to boundary conditions. • A transient heat transfer model was developed in MATLAB/Simulink. • Annual heating and cooling energy performance of commercial thermochromic coatings are assessed. • Spectral-selectivity, gradual transmittance change, hysteresis , and switching time are considered. • Results are more promising during cooling seansons, but are evident yearound in continental climates. Windows are often considered the weakest components in the thermal envelope driven by their low thermal resistances and static transmittances to solar gains. While low-E coatings improve the former, climate-responsive coatings that adjust their thermo-optical properties in response to changing boundary conditions are promising to address the latter. In particular, thermochromic films are passive technologies that rely on intrinsic material properties to adapt to varying ambient conditions and are more accessible with simpler structures than their active counterparts; hence, they prevail for solar control applications. However, the nature of current building energy simulation tools imposes limitations on evaluating their performance. In this article, a 1-D transient heat transfer model was developed in MATLAB/Simulink to evaluate the annual heating and cooling energy performance of thermochromic glazing while overcoming several limitations of current building energy simulation tools by accounting for spectral-selectivity, gradual transmittance change, hysteresis behaviour, and delayed switching time. The model was benchmarked against EnergyPlus, and the annual energy performance of a representative room was then evaluated for several double-glazing configurations, window-wall ratios, and exposures in the cold and hot climates of Toronto, ON and Abu Dhabi, UAE, respectively, while quantifying the effects of varying the hysteresis width and switching time. The results showed that commercial thermochromic glazing outperformed clear and low-E windows for both climate conditions. In particular, for a window-wall ratio of 80 % and compared to clear reference glazing, annual energy use intensity reductions up to 6.3 kWh/m 2 and 12 kWh/m 2 were realized in Toronto and Abu Dhabi, respectively, utilizing a glazing configuration that combined exterior thermochromic and interior low-E coated panes, where the latter helped in increasing the former’s temperature causing it to switch at lower ambient temperatures.. While the typical 30-minute switching time of thermochromic coatings was found to increase the annual energy use intensity by up to 0.5 kWh/m 2 , a 5 °C hysteresis reduced the annual cooling energy use intensity by up to 0.2 kWh/m 2 in Abu Dhabi. Less significant effects and savings were found for lower window-wall ratios, particularly in Toronto, where the coating rarely reached temperatures higher than 45 °C.

Lei Q., Wang L., Xie H., Yu W.

Windows are the least energy-saving part of the building envelope. To realize building energy-saving, smart windows have been developed. However, traditional fully passive thermochromic smart windows cannot adjust their transparency according to the complex outdoor climate “intelligently”, and only adjusts the solar radiation, ignoring the indoor temperature increase caused by heat entering the room in the form of convection. Here, we embed poly ( N -isopropylacrylamide) (PNIPAm) microgel into a highly transparent polyacrylamide (PAM) matrix, the PNIPAm-PAM hydrogel exhibits an ultrahigh luminous transmittance of 90.6% and solar modulation of 65.5%. By introducing nanoparticles into thermochromic hydrogels and combining the advantages of fluid glass in heat convection control, we develop a new type of active and passive dual-control smart window (APDC smart window) for the first time. In the indoor demonstration, it is proved that the smart window injected with 1-cm PNIPAm liquid has the best energy-saving ability, and the greenhouse installed with a 1-cm PNIPAm liquid smart window reduces the indoor air temperature by 15 °C compared with normal glass. In the outdoor demonstration, the indoor air temperatures of the APDC smart windows are reduced by ∼14 °C, ∼16 °C, ∼8.5 °C, and ∼9.5 °C respectively in different orientations of east, west, south, and north than normal glass window, and it is reduced by ∼4–8 °C compare with the 1-cm PNIPAm liquid smart window. This opens a new avenue for energy-efficient buildings and greenhouses. • PNIPAm-PAM hydrogel is presented as a hard gel with attractive thermochromic properties. • Doping of nanoparticles provides strong photothermal conversion properties for hydrogels. • Combination of thermochromic smart windows and fluidic glass provides unprecedented energy-saving performance.

Liu X., Wu Y.

Adaptive control of solar heat gain and visible light transmission through windows is perceived to be a potential measure for enhancing energy conservation and visual comfort in buildings. In this study, a novel versatile window, named Building Integrated Photovoltaic (BIPV) smart window, was proposed to offer simultaneous improvement of daylighting control, on-site electricity generation and building energy efficiency, compared to traditional BIPV windows with static optical properties. The key components of the proposed system include an optically switchable thermotropic layer made of Hydroxypropyl Cellulose (HPC) hydrogel, crystalline-silicon photovoltaic cells, clear glass and low-emissivity (low-e) glass covers. The thermotropic layer can respond to heat by autonomously changing its visible and near-infrared optical properties, with which the amount of solar radiation into building spaces can be manipulated and thus the risks of excessive solar heating and illumination can be prevented. Apart from excellent solar modulation, the BIPV smart window can collect a proportion of the light scattered from the thermotropic layer and concentrate it onto the integrated PV cells for extra electricity generation. An innovative methodology has been proposed to predict the optical, thermal and electrical properties of the BIPV smart window under varying ambient conditions. Numerical simulations have been carried out in EnergyPlus to predict the window's performance when it is applied to an office-type environment in the climate of Nottingham, the UK. The influence of different window design scenarios, in terms of Window-to-Wall Ratio (WWR), orientation and transition temperature, has been investigated. It was found that using the BIPV smart window can achieve an annual energy saving of 36.6% but also a more comfortable indoor luminous environment, compared to the counterpart BIPV window (with no thermotropic layer integrated), when installed in the south-oriented office with a WWR of 25%. • A novel BIPV window system with variable visible light and solar transmittance is proposed. • Numerical methods for modelling the proposed system are provided. • The designed system offers better energy and daylighting performance than conventional BIPV glazing and low-e double glazing.

Ke Y., Tan Y., Feng C., Chen C., Lu Q., Xu Q., Wang T., Liu H., Liu X., Peng J., Long Y.

• The first tetra-fish-mimetic design to meet both the energy-saving and artistic demands. • Performance is highly competitive with the best-reported results of thermochromic windows. • The mechanisms of rich color and energy-efficiency improvement are investigated. • Vivid colors are generated by photonic nanostructures instead of pigments. • Energy savings are further proved for buildings in the Asia Pacific as examples. The development of architectural windows with adaptive solar modulation is promising to reduce the energy consumption of heating, ventilation, and air conditioning (HVAC). In the work, we report a Tetra-fish-inspired aesthetic thermochromic window based on phase-changed materials to meet both energy-saving and aesthetic demands. We demonstrate the glasses coated with photonic co-doped vanadium dioxides, which exhibit the angle-dependent vivid colors mimicking the skin of tetra fishes with high transmittance, a practical transition temperature, and an acceptable solar modulation property. The glasses give superior energy-saving performances in representative cities in the Asia Pacific, resulting in annual energy savings of up to ∼ 35.9 kWh/m 2 for a typical office building. The work may inspire the future development of novel materials in building envelopes.

Teixeira H., Glória Gomes M., Moret Rodrigues A., Aelenei D.

Highly glazed buildings are typically responsible for significant solar heat gains/losses and, consequently, considerable cooling and heating energy needs throughout the year. Thermochromic glazing is an innovative passive technology, which autonomously and reversibly modifies its thermal and optical properties when direct sunlight heats it, potentially improving both energy efficiency and comfort. However, there is scarce evidence about the global performance of this glazing when installed in commercial buildings. Therefore, this study aims at assessing the annual visual, thermal and energy performance of a thermochromic glazing (12 + 12 + 6 mm) against a conventional clear glazing (6 + 12 + 4 mm) with and without a reflective solar control film installed in an existing office room, considering different European climates. To this aim, a building simulation model calibrated with experimental data obtained on a previous study was used. The results showed that in respect of the percentage of working hours with useful illuminance levels the thermochromic glazing (80%–88%) is better than the conventional clear glazing (64%–74%). However, the glazing with the reflective solar control film is more effective in reducing potential glare. Regarding the thermal performance assessment, under free-float conditions (no mechanical heating/cooling), the thermochromic glazing shows a better performance (20%–48% working hours within comfort conditions) when compared to conventional glazing (1%–42%). The results also show significant total energy savings (climate control and artificial lighting) in the case of the thermochromic glazing, particularly in the hot climate of Lisbon (50%). • Comfort and energy performance of thermochromic versus conventional static glazing. • Building simulation model of an office room calibrated with experimental data. • Up to 88% of working hours with useful illuminance with thermochromic glass. • Up to 48% of working hours within thermal comfort with thermochromic glass. • Up to 50% of energy savings with thermochromic glass when compared with clear glass.