Thermochromic Glazing Research Articles

Assessment of Gel-Based Thermochromic Glazing for Energy Efficiency in Architectural Applications.

With the increasing global focus on energy efficiency and environmental sustainability, intelligent building materials such as thermochromic glazing have emerged as a hot topic of research. The intent of this paper is to explore the utilization of gel-type thermochromic glazing within the realm of architectural energy conservation calculations. It conducts an exhaustive examination of the material's attributes, its capacity for energy savings, and the obstacles encountered in real-world applications. Through simulation studies and case analyses, this paper assesses the energy efficiency of gel-type thermochromic glazing across various climates and suggests strategies for optimization. The study revealed that the incorporation of gel-based thermochromic glazing leads to a marked reduction in energy usage within buildings, an improvement in indoor comfort levels, and significant environmental advantages.

Simulation-Based Evaluation of the Impact of an Electrochromic Glazing on the Energy Use and Indoor Comfort of an Office Room

Electrochromic glazing alters its optical properties in the absence/presence of an electrical charge, varying from clear to dark to control daylighting and solar heat gains. This study aims to evaluate the impact of an electrochromic glazing, with indoor glare or temperature control, on the energy performance and thermal and visual comfort of an office room under three European climates, using a calibrated simulation model. The novelty of the paper lies in its combined performance assessment, using different standards and metrics. The results showed reduced climatization energy requirements with temperature control, but significantly increased artificial lighting energy use. Glare control achieved useful illuminance levels during 74–80% of working hours. Concerning temperature control, working hours within thermal comfort increased (21–43%) under a free-float regime. Moreover, the performance of this glazing was compared to that of a clear glazing with/without a reflective film and a thermochromic glazing for different solar orientations. The electrochromic glazing with glare control showed the highest energy savings (14–36%) for a western orientation, and the lowest negative impact on daylighting for a northern orientation. The best glare reduction was achieved with the reflective film. Considering the free-float regime, the electrochromic glazing, with temperature control, showed the highest increase in working hours within thermal comfort (6–9%) for a western orientation.

Optimization of the thermal-optical performance of a PCM-integrated thermochromic glazing system

Facades that employ adaptive technologies have shown great potential in enhancing building energy efficiency. This study investigates a novel phase-change material integrated thermochromic glazing (PCM-TCG), and applies it on skylights of an exhibition building in Shanghai. Thermal performance and daylighting performance of the building were analysed using Fluent and Grasshopper/Honeybee respectively and they were optimized through Non-dominated Sorting Genetic Algorithm-II. It was found that on summer solstice and winter solstice, the optimized solution provided an excellent trade-off between thermal and daylighting performance, reducing the total energy demand by 88% and increasing the daylighting satisfaction time by 5 hours compared to low-E glazing on summer solstice and winter solstice.

Multi-objective optimization of thermochromic glazing properties to enhance building energy performance

Glazings systems are responsible for significant building gains and losses regarding energy and thermal loads. Thus, current research has converged on finding glazing solutions that minimize heating, cooling, and lighting needs. One example of innovative glazing systems is thermochromic glazings. These glazings change their optical and thermal properties according to their surface temperatures by darkening at higher temperatures and consequently decreasing visible and solar transmittance. These property transitions have an impact not only on the heating and cooling needs of a building but also on the electric lighting needs. This research aims to study the impact that different switching temperature ranges and thermochromic coating transmittance values have on the energy use of an office room in different climates. This is accomplished with the annual energy simulation for heating, cooling, and electric lighting energy use of an office with a thermochromic glazing system in two different climates. A multi-objective optimization process is integrated to minimize the office’s thermal, lighting, and total energy use according to the thermochromic glazing transition temperatures and transmittances. Optimization results show highly conflicting values between the office room’s electric lighting and climatization energy use, showing that electric lighting energy use can increase up to 200% with low transition temperatures. Additionally, optimum solutions show improvements of 15% in total energy use against one off-the-market thermochromic glazing.

Energy modelling and saving potential of polymeric solar-responsive thermochromic window films

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/m2 and 12 kWh/m2 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/m2, a 5 °C hysteresis reduced the annual cooling energy use intensity by up to 0.2 kWh/m2 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.

The impact of bead milling on the thermodynamics and kinetics of the structural phase transition of VO2 particulate materials and their potential for use in thermochromic glazing

The thermodynamics and kinetics of the structural phase transition from monoclinic VO 2 (M) to rutile VO 2 (R) and vice versa were studied for particulate materials obtained by bead milling of VO 2 (M) powder. Using wet bead milling, we decreased the particle size of VO 2 (M) powder from ∼1 μm to 129 nm. With progressive milling, the switching enthalpy decreased from 47 J g −1 to 29 J g −1 due to a loss of crystallinity. The switching kinetics were studied using Friedman's differential isoconversional method. The activation energy | E α | decreases with increasing difference between the actual temperature of the material and its switching temperature ( T 0 ). Furthermore, | E α | decreases with progressive milling, and kinetic asymmetry is induced. For milled particulate materials, | E α | is lower for the switch from VO 2 (R) to VO 2 (M) than for the opposite switch. For hydrothermally synthesized nanoparticles, | E α | is in the same order of magnitude, albeit with inverse switching asymmetry. Latter may result from different defects that are introduced during both preparation techniques. Applying layers of milled particulate material to glass sheets yielded thermochromic coatings with luminous transmission of 40.7% and solar modulation of 8.3%. This demonstrates that milled VO 2 particles have potential for use in energy efficient thermochromic windows. • Switching enthalpy of VO 2 particulate material decreases with progressive milling. • Activation energy of the structural phase transition decreases with particle size. • Progressive milling induces asymmetry in switching kinetics of particulate material. • Hydrothermally synthesized particles display inverse asymmetry in kinetics. • Milled VO 2 particulate materials are suited for use in thermochromic windows.

Design of thermo-chromic glazing windows considering energy consumption and visual comfort for cellular offices

Optical behavior of thermo-chromic glazings can be modulated with changing temperature based on phase transmission of embedded pigments. Such pigmented glazings can be used in residential or office windows to alter visible and solar transmittance of the window, based on environmental conditions. This study focuses on numerical design of thermo-chromic glazings with V O2 pigments used in office buildings so that energy consumption can be minimized, while considering visual comfort. Optical properties of pigmented glazing depends on pigment radius, volume fraction and glazing thickness. In this study, two main design variables that are pigment radius and volume fraction are examined by applying parametric analysis for a constant glazing thickness that is identical to a standard ordinary glass. The radiative transfer through glazings are solved using four flux method to predict the optical properties of the glazings. The optimum thermo-chromic glazings are identified and energy efficiency along with resulting daylight illuminance are compared with that of an ordinary double glazing for several geographical locations. It is observed that thermo-chromic windows for office buildings have significant energy saving potential which can lead decreases in energy consumption from 17% for cold climates to 42% for hot climates.

Assessment of the visual, thermal and energy performance of static vs thermochromic double-glazing under different European climates

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%).

CFD analysis of environmental impacts on a thermochromic smart window

Thermochromic (TC) glazing is an advanced smart window technology that can help to achieve nearly zero-energy buildings. The glazing temperature that affects TC thermo-optical properties directly impacts its energy-saving performance. However, the impacts of solar radiation and natural convection occurring on this temperature-sensitive material are not yet fully understood. Compared to previous focuses on material properties and energy evaluation, this study aimed to provide a different view from thermo-fluid dynamics on the heat transfer characteristics of a TC window, with impacts of environmental factors addressed. A validated Computational Fluid Dynamics (CFD) model identifies the environmental impacts on the TC glazing temperatures under the coupled radiative and convective heat transfer. Furthermore, a theoretical model was developed with an empirical factor obtained from CFD results. It enables a simple calculation to obtain the TC glazing’s temperature by using solar factors, outdoor temperatures and optical properties. The model suggests that the temperature difference between the TC glazing and the ambient is a function of absorbed solar irradiation (∝q̇total0.75). The predictive model is then tested using typical climate conditions in three cities in China with different solar angles, which demonstrates good agreement with an average absolute error of 5% compared to simulated glazing temperature. This estimation method can further assist in material design, product selection and Building Energy Simulations to achieve a TC window design that is ‘smart’ on seasonal change requirements for different regions.

The Impact of Bead Milling on the Thermodynamics and Kinetics of the Structural Phase Transition of Vo2 Particulate Materials and Their Potential for Use in Thermochromic Glazing

The Impact of Bead Milling on the Thermodynamics and Kinetics of the Structural Phase Transition of Vo2 Particulate Materials and Their Potential for Use in Thermochromic Glazing

Impacts of thermo-optical properties on the seasonal operation of thermochromic smart window

A type of smart window using thermochromic glazing (TCG) is a promising technology for green buildings owing to the self-regulating feature and low-maintenance need. Its most important feature, thermo-optical properties that regulate the blockage of solar heat, is directly linked to the variation of surface temperatures. However, challenges from the inhomogeneity of thermo-optical properties, the coupled solar radiation and natural convection, and varying outdoor conditions all seriously hinder the understanding of its mechanism. In this paper, a validated Computational Fluid Dynamics (CFD) model achieves the simulation of inhomogeneous tinting of TCG by defining the thermo-optical properties of each finite volume according to the surface temperature. Solar radiation and natural convection at outdoor, indoor and the cavity are solved to reflect glazing temperature more accurately. The case studies compared six different switching temperatures in the range of 20 ∼ 42.5 °C with a transition gradient of 10 °C. Averaged meteorological data for both summer and winter, sunny days and cloudy days are selected to present realistic climate impacts. The result reveals the overall saving in transmitted solar radiation in summer and heating penalties in winter. It suggests the best switching temperatures for each climate condition. With the seasonal operation, the highest saving in solar heat gain is 20.9% when adopting a switching temperature of 25–35 °C, while the lowest saving can be negative, meaning TCG is not suitable for those climate zones. The proposed evaluation criteria help to quantify the applicability of TCG with the input of the summer/winter day ratio and sunny/cloudy ratio. The best region to apply TCG is where summer days are longer and winter solar radiation is significantly lower. The in-depth understanding of this temperature-sensitive process benefits the optimization of TCG in buildings, especially for its seasonal operation needs.

Numerical analysis on the thermal performance of PCM-integrated thermochromic glazing systems

Thermochromic glazing and phase-change materials are capable of adapting their optical and thermal properties in response to temperature variation, without consuming additional energy. They are therefore ideal construction materials for passive building envelopes. In this study, the energy-saving potential of combining the two materials into one adaptive glazing system is investigated numerically. Computational fluid dynamic models were constructed to simulate the thermal performance of different adaptive glazing systems consisting of the two materials. The models were tested against two existing experimental studies, and good agreements were found. Based on the models, the applicability of the novel glazing systems for a south office building façade was investigated for summer and winter in Shanghai, China. It was found that PCM-integrated thermochromic triple glazing units outperform normal double-glazing units by reducing total heat gain by up to 32% and 40% for a sunny and cloudy day in summer, respectively. In winter, however, they tend to block more desirable solar radiation and hence are not as effective as normal double-glazing units.

Innovative approaches to thermochromic materials for adaptive building envelopes

Thermochromic (TC) materials are characterized by a change of their optical response at a specific temperature. They can work based on both, the alteration of solar reflection by temperature, or the change of photoluminescence intensity. In building applications, this type of smart materials enhances the rejection of solar heat for high temperatures to favour cooling of the envelopes and reduces this rejection for low temperatures to improve surface heating. This adaptive optical response improves energy efficiency and reduces environmental impact of urban areas. Most of the current advances in this area are related to TC glazing based on Vanadium oxide, while opaque TC materials have been developed as based on Leuco dyes. The main drawback of these last materials is their significant aging in outdoor applications due to a photo-degradation process. The present work shows the recent results of a multidisciplinary and multinational consortium for research on innovative approaches to thermochromic materials for adaptive building envelopes. Next steps will be focused on building simulation to evaluate material choices across different performance aspects, while physical prototypes will be used for inter-laboratory evaluation of such performance and material durability.

Multi-objective optimization of thermochromic glazing based on daylight and energy performance evaluation

Many efforts have been detected to investigate thermochromic (TC) glazing for improving building energy saving, while only a few approaches for daylight performance analysis. In this study, the performance of TC glazing is investigated based on multi-objective optimization for minimizing energy demand while maximizing daylight availability. The effects of five parameters including transition temperature, solar transmittance in clear state, solar transmittance modulation ability, luminous transmittance in clear state, and luminous modulation ability on the building energy consumption and useful daylighting illuminance (UDI300–3000) are examined. Linear Programming Technique for Multi-dimensional Analysis of Preference (LINMAP) is used for the decision-making of Pareto frontier. This research aims to explore the ideal thermochromic glazing by considering the daylight and energy performance of a typical office room, taking the weather condition of Xiamen, China as an example. Although it is impossible to achieve both optimal values of energy consumption and UDI300–3000 simultaneously, the proposed multi-objective optimization method could still provide low energy consumption with sufficient daylight. The non-dominated sorting of Pareto optimal solution (POS) demonstrated that the optimum building energy consumption and UDI300–3000 for single glazed windows are 46.64 kWh/m2 and 70.92%, respectively, while the values for double glazed windows are 44.40 kWh/m2 and 71.88%, respectively. The selected hypothetical TC windows can improve the building energy and daylighting performance simultaneously when compared with traditional clear single and double glazed windows. The presented framework provides a multi-objective optimization method to determine the most suitable TC glazing technologies for designers and architects during the design and retrofit procedure.

A comparative study on the switching kinetics of W/VO2 powders and VO2 coatings and their implications for thermochromic glazing

Monoclinic VO2 (M) displays thermochromic properties based on its reversible metal-insulator transition. We studied the kinetics of the underlying structural phase transition (SPT) from monoclinic VO2 (M) to rutile VO2 (R) and vice versa both in powders and coatings, using isoconversional kinetic analysis based on datasets obtained through differential scanning calorimetry and UV–vis–NIR spectrophotometry. For VO2 powders, prepared via solution-phase reduction of V2O5 with oxalic acid and subsequent thermal anneal, we show that the activation energy |Ea| of the SPT is temperature dependent, and decreases with increasing difference between the material's temperature and the critical switching temperature T0. |Ea| for both VO2 (M) to VO2 (R) and VO2 (R) to VO2 (M) is similar, and ranges between 138 and 563 kJ mol−1, depending on the temperature of the material. This indicates that similar defects play a key role in both SPTs. Upon doping with tungsten, T0 was lowered from 66.93 °C (0 at. % W) to −28.38 °C (3.5 at. % W). |Ea|, however, remained in the same range. Nonetheless, at W concentrations above 2 at. % a significant asymmetry was observed with higher |Ea| for the switch from VO2 (R) to VO2 (M). The SPT of VO2 (M) in coatings proceeded 4 times slower. This may result from the immobilization of the VO2 domains on the substrate surface and within the coating network, making the SPT more difficult to progress. The findings of this study have important implications for the application of VO2 (M) in energy efficient thermochromic glazing.

The effect of thermochromic windows on visual performance and sustained attention

Thermochromic windows have been widely studied as a technology that can potentially offer increases in energy conservation and provide a desirable luminous environment inside buildings. However, there has been little attention placed on how the tinted states of thermochromic glazing influence occupant behaviour and visual perception. An experiment under controlled conditions was designed to test the influence of different thermochromic tint states on human response. By using a controllable artificial window, five typical luminous conditions were set up, including clear (no tint) and two different levels of blue and bronze tint states, which produced different room colour temperatures. Thirty-one subjects were recruited who completed three visual tasks, including the visual acuity and colour naming tasks using the coloured Landolt ring chart and a sustained attention test using the d2 test. Subjective assessments were also collected using questionnaires. Statistical analyses showed that across the thermochromic window conditions, no significant differences in performance were found for the visual acuity and d2 tests. However, there was a significant effect for the colour naming task from the Landolt ring test. Under blue tint conditions, subjects reported higher alertness and produced fewer errors. More natural and acceptable lighting conditions were found under the bronze-tinted conditions. Therefore, when developing innovative thermochromic windows applied in buildings, it is important to cater for the visual requirements of the occupants in the space, not only energy efficiency goals.

Optimization of the thermochromic glazing design for curtain wall buildings based on experimental measurements and dynamic simulation

Thermochromic (TC) glazing could provide a significant reduction of energy consumption in curtain wall buildings. However, each application requires a design tailored to building’s specifications. This paper proposes a complete approach for designing TC glaze based on building energy simulation starting from the production of thin thermochromic layers and the measurements of their optical properties by means of a customized spectrophotometer. The main focus of this work is to identify the optimal TC optical response that minimises the building energy consumption. Energy simulations have been performed for a virtual mock-up set at two locations with different climates, Italy and Poland. A set of profiles, each one determining thermochromic properties in terms of switching temperature, range of solar transmittance and transition speed, have been created with a fine step of temperature (2 °C) and used to simulate different scenarios. The outcome of the optimization provided the optimal properties to achieve the right balance between cooling energy reduction and heating energy increase due to the application of the thermochromic layer, in comparison to a standard clear glass. The fine step in switching temperature allowed to accurately estimate the subtle differences for the two different climates (25 °C Italy, 24–26 °C Poland). The highest impact has been found for the Italian location with a maximum reduction of total energy consumption of 22.8%. This was achieved with a thermochromic switching at 25 °C, with fast transition and range of transmittance between 0.1 (switched state) and 0.5 (normal state), which is a not extreme behaviour.

Smart windows – Transmittance tuned thermochromic coatings for dynamic control of building performance

Thermochromic window technologies promise energy-saving capabilities through intelligent regulation of solar irradiation indoor penetration. Although improvements have been made, nanoparticle thermochromic coated glazing technology has not yet achieved optimal balance between luminous transmittance and solar modulating capability. While luminous transmittance can be increased, this generally comes at the cost of reduced solar modulation, and this trade-off requires exploration. Thermochromic glazing with varying luminous transmittance and corresponding solar modulation has hence been developed through coating VO2 nanoparticles directly onto glazing by adjusting thicknesses of the coatings. The advantages of this method are simplicity of process, high purity of starting materials, compatibility with a wide range of substrates, and reproducibility in switching properties. The effects of the thermochromic glazings on building performance were then explored using building simulation for desert, Mediterranean, and temperate climates. It was found that 7.1–46.4% annual energy-saving can be achieved in comparison to using an uncoated clear double-glazed window. Trade-off between energy-saving capability and UDI500-2000 lx within the space was also explored and results showed that the designed thermochromic glazing reduced visual discomfort 1.2–3.2 m into the space. Overall, responding to external conditions, thermochromic technological advancements can help make buildings more resilient to future weather changes.

Daylight performance analysis of TiO2@W-VO2 thermochromic smart glazing in office buildings

Thermochromic glazing (TC) is able to modulate the solar radiation transmittance through windows in response to temperature, passively. Apart from the significant role of TC windows in thermal energy conservation in buildings, they affect the indoor daylight performance due to their lower visible transmittance rate and tinted appearance. In this paper, we carried out a comprehensive study on the impact of TiO2@W-VO2 thermochromic glazing, fabricated by the authors, on indoor daylighting performance of a typical office room. The spectrophotometry test on the fabricated TC glass shows a significant modulation ability in near-infrared wavelengths. Based on the measured optical transmittance, daylight behaviors of the glazing were elaborated in terms of visual comfort, color quality of the transmitted light, non-visual daylight availability, and artificial lighting load, using computer simulation methods and numerical calculations. Finally, the paper offers an interactive approach between material development and fabrication methods on one hand, and holistic thermal and daylight analysis of the product on the other hand. This approach optimizes the physical properties of a TC glazing in tradeoff between solar modulation ability, visible transmittance, and color appearance.

A multistage recursive approach in time- and frequency-domain for thermal analysis of thermochromic glazing and thermostatic control systems in buildings

The frequency-domain analysis is an effective approach to analyze the steady-periodic heat transfer in buildings. Auxiliary load in buildings can be calculated using the indoor temperature in a frequency-domain analysis. However, both auxiliary load and indoor temperature are usually unknown in building analysis, which makes computing the indoor temperature and auxiliary load with the presence of heating and cooling setpoints are challenging in a frequency-domain analysis. A similar problem applies to the calculation of the auxiliary loads of buildings with dynamic materials, such as thermochromic materials. This paper proposes and implements a recursive approach in time- and frequency-domain to calculate the indoor temperature in an office space with dual setpoint and thermochromic glazing systems with different critical transition temperatures and tinted-to-clear transmissivity ratios. This method allows for capturing the free-floating fluctuations of the indoor temperature within the deadband as well as determining the points-in-time when the indoor temperature equals heating or cooling setpoint temperatures. A multistage procedure is defined to account for the varying window transmittance. The proposed method accounts for the time-dependent response of a thermostat and thermochromic glazing within a finite number of adjustments without requiring to carry out time-domain temperature or load calculations. Results revealed reductions in the annual cooling energy use and increases in the annual heating energy consumption using thermochromic glazing systems with the total annual energy savings of up to 1.22 kWh/m2-yr for the τ-ratio of 0.65, 3.50 kWh/m2-yr for the τ-ratio of 0.30, and 5.22 kWh/m2-yr for the hypothetical case with zero τ-ratio.