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

Li X., Li K., Sun Y., Wilson R., Peng J., Shanks K., Mallick T., Wu Y.

Integrating PV solar cells with concentrators into window systems can not only generate electricity for a building, but also has the potential to enhance the thermal resistance of the building's windows without a significant sacrifice of light transmittance for passive daylight. A novel photovoltaic window system, crossed compound parabolic concentrator photovoltaic window, has been recently studied for its electrical properties. However, its thermal and optical performance, including factors such as the overall heat transfer coefficient (U-value) and total optical transmittance when integrated into a building, have not been studied yet. These window factors are crucial for predicting its impact on a building's energy efficiency and indoor comfort. Therefore, this paper aims to investigate the U-value of this window system under various temperature scenarios and the optical transmittance of the window under different incident angles. The thermal conductance was assessed through numerical simulations using a computational fluid dynamics model, which was validated by experimental measurements conducted in a large climate chamber. The optical transmittance was investigated using a validated 3D ray-tracing model, and the total optical transmittance and electricity generation were calculated for typical sunny days in winter and summer under London's climate conditions. Additionally, new configurations were designed to explore the impact of the pitch between adjacent optics on the thermal conductance and optical transmittance of the window. The results showed that the window with a structure of Dx = Dy = 5 mm (where Dx and Dy represent the horizontal and vertical pitches between two adjacent solar optics) has the lowest U-value (2.566 W/m2·K). This U-value is slightly lower than that of the original window (2.575 W/m2·K). The original window with a structure of Dx = Dy = 1.77 mm produces the highest power output. Specifically, it generates 499.25 Wh/m2 on a typical sunny day in winter and 162.73 Wh/m2 on a typical sunny day in summer. However, it exhibits the lowest transmittance (14.6 % on a typical sunny day in winter and 25.2 % on a typical sunny day in summer, respectively), indicating that it is more suitable for buildings with a higher window-to-wall ratio to ensure an adequate amount of natural light. For buildings with a lower window-to-wall ratio, the CCPC-PV window should be designed with a larger horizontal pitch, such as 15 mm and 30 mm, to meet indoor illuminance requirements while also providing enhanced thermal performance and additional power output.

Liu H., He W., Liu X., Zhu J., Yu H., Hu Z.

Building integrated concentrating photovoltaic window (BICPVW) has been proven to be used in sustainable building transparent envelopes. Unfortunately, the balance between optical and electrical properties is difficult. This paper proposes a building integrated concentrating photovoltaic window coupling luminescent solar concentrator and thermotropic material (LT-BICPVW). The luminescent solar concentrator and thermotropic layer were fabricated by bulk polymerization and glass sandwich structure, respectively. Optical models are built based on the optical properties of optical waveguide and thermotropic layer, and optical simulations are performed with Monte Carlo ray tracing technology. Window prototypes are characterized under controlled indoor experimental conditions. The experiment found that as the temperature of the thermotropic film increased from 30 °C to 42 °C, the solar transmittance of LT-BICPVW decreased by 43.3%, and the maximum output power increased by 25%. Compared with, building integrated concentrating photovoltaic window coupled with thermotropic material (T-BICPVW), the power conversion efficiency was improved by 59% at the termination temperature. The LT-BICPVW system can effectively reduce solar heat gain and avoid glare hazards, while using renewable energy to significantly increase power output. A series of predictions are made using validated optical model and tracking technique to better guide the design of PV windows in actual buildings.

Hong X., Zheng X., Lin J.

Wu S., Sun H., Duan M., Mao H., Wu Y., Zhao H., Lin B.

Thermochromic and electrochromic smart windows are increasingly receiving attention for their specific ability to regulate the dynamics of light and heat. The goal of improving smart-window materials is chiefly to achieve building applications. This paper reviews the progress of existing material technologies and summarizes their experiments and simulations related to buildings. To facilitate the leap from material to building, the concept of performance regulation of ideal smart windows is proposed, and the applicability distribution of existing technologies is mapped. Based on the completely different core logic of thermochromic and electrochromic smart windows, the discrepancies and consistencies between the two are discussed. Performance requirements of smart windows in materials and buildings are proposed. Bridging the gap between materials and buildings for interdisciplinary promotion is the key to further exploration of the potential application of smart windows and accelerating their applications in the future.

Hong X., Yang Y., Chen H., Tao Q.

Transparent heat mirror which allows the transmission of visible sunlight while reflects the infrared thermal energy is an effective building energy efficiency technology for hot climatic regions. In this work, a five-layer dielectric/metal/dielectric/metal/dielectric (DMDMD) coating of Si3N4/Ag/Si3N4/Ag/Si3N4 structure is proposed. The radiative properties of the five-layer coatings are theoretically investigated by transfer matrix method. The thicknesses of the layers are optimized by using particle swarm optimization method. The sample of the designed Si3N4/Ag/Si3N4/Ag/Si3N4 coating is prepared and the building energy performance when applying the DMDMD coating in a simple office room is also investigated, taking the hot weather condition of Guangzhou, China as an example. The simulated results show that the Tave + Rave value of the five-layer coating is 8% higher than that of the three-layer coating, and the long-wave emittance of the five-layer coating is 24.8% lower than that of the three-layer coating. And the application of the five-layer coatings on the glazed window could provide the highest energy saving rate of 8.9% when compared with the traditional low-e coatings.

Mousavi S., Rismanchi B., Brey S., Aye L.

AbstractThe recent significant rise in space cooling energy demand due to the massive use of air-conditioning systems has adversely changed buildings’ energy use patterns globally. The updated energy technology perspectives highlight the need for innovative cooling systems to address this growing cooling demand. Phase change material embedded radiant chilled ceiling (PCM-RCC) has lately acquired popularity as they offer more efficient space cooling together with further demand-side flexibility. Recent advancements in PCM-RCC applications have increased the necessity for reliable simulation models to assist professionals in identifying improved designs and operating settings. In this study, a transient simulation model of PCM-RCC has been developed and validated using measured data in a full-scale test cabin equipped with newly developed PCM ceiling panels. This model, developed in the TRNSYS simulation studio, includes Type 399 that uses the Crank-Nicolson algorithm coupled with the enthalpy function to solve transient heat transfer in PCM ceiling panels. The developed model is validated in both free-running and active operation modes, and its quality is then evaluated using several validation metrics. The results obtained in multiple operating scenarios confirm that the model simulates the transient behaviour of the PCM-RCC system with an accuracy within ±10%. Aided by this validated model, which offers the user detailed flexibilities in the system design and its associated operating schemas, PCM-RCC’s potentials regarding peak load shifting, energy savings, and enhanced thermal comfort can be investigated more reliably.

Xue P., Shen Y., Ye S., Peng J., Zhang Y., Zhang Q., Sun Y.

The secondary solar heat gain, defined as the heat flows from glazing to indoor environment through longwave radiation and convection, grows with the increasing of glazing absorption. With the rapid development and application of spectrally selective glazing, the secondary solar heat gain becomes the main way of glazing heat transfer and biggest proportion, and indicates it should not be simplified calculated conventionally. Therefore, a dynamic secondary solar heat gain model is developed with electrochromic glazing system in this study, taking into account with three key aspects, namely, optical model, heat transfer model, and outdoor radiation spectrum. Compared with the traditional K-Sc model, this new model is verified by on-site experimental measurements with dynamic outdoor spectrum and temperature. The verification results show that the root mean square errors of the interior and exterior glass surface temperature are 3.25 °C and 3.33 °C, respectively, and the relative error is less than 10.37%. The root mean square error of the secondary heat gain is 13.15 W/m2, and the dynamic maximum relative error is only 13.2%. The simulated and measured results have a good agreement. In addition, the new model is further extended to reveal the variation characteristics of secondary solar heat gain under different application conditions (including orientations, locations, EC film thicknesses and weather conditions). In summary, based on the outdoor spectrum and window spectral characteristics, the new model can accurately calculate the increasing secondary solar heat gain in real time, caused by spectrally selective windows, and will provide a computational basis for the evaluation and development of spectrally selective glazing materials.

Haratoka C., Yalcin R.A., Erturk H.

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 O 2 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. • Building energy efficiency with thermochromic glazings compared to regular glazings. • Optimal size, concentration of V O 2 pigments in thermochromic glazings determined. • Optical properties are estimated using four-flux method in conjunction with Mie theory. • Energy performance of the building considers heating, cooling and lighting costs. • Visual comfort is considered together with energy efficiency for different climates

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.

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.

Gan L., Ren H., Cai W., Wu K., Liu Y., Liu Y.

In accordance with China's commitments to reduce greenhouse gas emissions under the Paris Agreement, it is important to establish a fair and effective carbon emission allocation mechanism; this would help to achieve China's 2030 “carbon peak” target. Here, the fixed cost allocation model (FCAM) was used to allocate carbon emission quotas to public buildings in China in 2030, based on the principles of equity and efficiency. By comparing these provincial allocation results with those obtained using zero-sum data envelope analysis (ZSG-DEA), optimal carbon emission quotas were proposed for the public building sectors of thirty Chinese provinces. The results revealed large differences, with the public buildings of Guangdong having the highest quota (11.39%) and those of Xinjiang having the lowest (0.08%). Additionally, Heilongjiang, Inner Mongolia, and Liaoning were revealed to be under high pressure with regard to the reduction of emissions from their public buildings; Heilongjiang was predicted to require the highest reduction, exhibiting a need to reduce 34 million tons of emissions. These findings will help provide new ideas for relevant policy makers to formulate fair and effective carbon allowance allocation plans. • We propose a carbon emission allocation plan for public buildings in China in 2030. • Proposal of optimal carbon emission allowances using fixed cost allocation model. • Provinces with higher input-output efficiency, such as Guangdong, Jiangsu have higher allocations. • Proposes a low-carbon development path for China in 2030 by region.

Bai X., Zhang M., Jin Z., You Y., Liang C.

• A chiller FDD method combining model-based and data-based methods was proposed. • The deviation was used as a diagnostic parameter, and its contribution to FDD was verified. • The effect of sample size on the accuracy of the feature recognition model was explored. • Compared with the PCA, FDA, and SVM, the proposed method had excellent performance. Reliability of chillers is of great significance to maintaining sustainability buildings and reducing carbon emissions. The cause of chiller performance degradation was found in early fault detection and diagnosis technology, and the measure can be taken to save energy. This paper proposed an automatic diagnosis technique for Chiller based on the feature-recognition model and Spectral Regression Kernel Discriminant Analysis (SRKDA). Feature-recognition model would be used to calculate diagnostic parameters, deviations( D ) between normal and fault data. At the same time, SRKDA would be applied for mapping original nonlinearly separable feature space to the separable features that are linear and improving the computational speed. For one thing, compared with principal component analysis (PCA) and Fisher Discriminant Analysis (FDA), the proposed method has the lowest false alarm rate and the highest detection rate for fault detection. For another, compared with the FDA and Support vector machine (SVM) for fault diagnosis, the proposed method has excellent accuracy and training time performance. In addition, experiments show that model-based data processing improved the separability of original data and further improved FDD accuracy.

Nie Y., He W., Liu X., Hu Z., Yu H., Liu H.

Building integrated concentrating photovoltaic (BICPV) windows have attracted numerous studies in recent years. However, there is a tradeoff between the light transmittance and power generation efficiency in the design of BICPV window. In this paper, a smart luminescent solar concentrator (LSC) is introduced as the BICPV window. The proposed smart LSC system features on the combination of fluorescent dyes with thermochromic materials to enhance photoelectric conversion efficiency as well as form a dynamic response mechanism to ambient solar radiation and environmental temperature. In this study, a BICPV smart window system consists of the waveguide doped with organic dye Lumogen F Red-305 (BASF) and the thermochromic hydrogel membrane has been developed. The research on analytic design parameters is executed through optical simulation by ray tracing technology along with outdoor comparative experiments. From simulations for a smart LSC of 100 mm × 100 mm × 3 mm with a bottom-mounted solar cell of 100 mm × 10 mm, the optical effective concentration is found to be with the range of 1.23 to 1.31 when a highest gain of 1.26 in power over the bare solar cell is obtained from experiments.

Castillo M.S., Liu X., Abd-AlHamid F., Connelly K., Wu Y.

Buildings are responsible for over 40% of total primary energy consumption in the US and EU and therefore improving building energy efficiency has significant potential for obtaining net-zero energy buildings reducing energy consumption. The concurrent demands of environmental comfort and the need to improve energy efficiency for both new and existing buildings have motivated research into finding solutions for the regulation of incoming solar radiation, as well as ensuring occupant thermal and visual comfort whilst generating energy onsite. Windows as building components offer the opportunity of addressing these issues in buildings. Building integration of photovoltaics permits building components such as semi-transparent façade, skylights and shading devices to be replaced with PV. Much progress has been made in photovoltaic material science, where smart window development has evolved in areas such as semi-transparent PV, electrochromic and thermochromic materials, luminescent solar concentrator and the integration of each of the latter technologies to buildings, specifically windows. This paper presents a review on intelligent window technologies that integrate renewable energy technologies with energy-saving strategies contributing potential solutions towards sustainable zero-energy buildings. This review is a comprehensive evaluation of intelligent windows focusing on state-of-the-art development in windows that can generate electricity and their electrical, thermal and optical characteristics. This review provides a summary of current work in intelligent window design for energy generation and gives recommendations for further research opportunities.

Feng Y., Lv M., Yang M., Ma W., Zhang G., Yu Y., Wu Y., Li H., Liu D., Yang Y.

Thermochromic smart windows technology can intelligently regulate indoor solar radiation by changing indoor light transmittance in response to thermal stimulation, thus reducing energy consumption of the building. In recent years, with the development of new energy-saving materials and the combination with practical technology, energy-saving smart windows technology has received more and more attention from scientific research. Based on the summary of thermochromic smart windows by Yi Long research groups, this review described the applications of thermal responsive organic materials in smart windows, including poly(N-isopropylacrylamide) (PNIPAm) hydrogels, hydroxypropyl cellulose (HPC) hydrogels, ionic liquids and liquid crystals. Besides, the mechanism of various organic materials and the properties of functional materials were also introduced. Finally, opportunities and challenges relating to thermochromic smart windows and prospects for future development are discussed.

You Y., Yang X., Xu Z., Cui C., Shi F., Hong X.

Ming Y., Hu M., Jiang F., Yuan Y., Wu Y.

Ming Y., Hu M., Liu X., Yuan Y., Wu Y.

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

Ullah Z., Gruosso G., Aziz M.O., Scacciante A.

Wang C., Liu S., Li X., Shi Q., Wang W., Fu Y., Chen J., Yan Tso C.

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

Wang C., Yang H., Ji J.

Bai X., Tang Q., Luo J., Mao Y., Liang C., Zhang X.

Mesloub A., Hafnaoui R., Hassan Abdelhafez M.H., Seghier T.E., Kolsi L., Ben Ali N., Ghosh A.