Performance rating of glass windows and glass windows with films in aspect of thermal comfort and heat transmission

This article is about a study on glass window and glass window with film of different types in aspect of thermal comfort and heat transmission. Different types of glass window, clear glass, tinted glass, reflective glass, double pane glass, and low-e glass were investigated. Films with different spectral optical properties were then adhered to the glass windows of different types and studied. The analysis was done based on the outside design weather condition which selected from 12 years of Bangkok meteorological data. Predicted percentage of dissatisfied (PPD) was selected as the thermal comfort index. The relative heat gain (RHG) based on local weather condition was selected as the heat transmission index. The PPD can be subdivided into the PPD due to surface temperature effect and the PPD due to solar radiation effect. The analysis indicated that, for most of the glass windows considered except the reflective glasses, the values of PPD due to solar radiation effect were much larger than the values of PPD due to surface temperature effect. And the most discomfort condition occurred when using a clear glass as window. Adhered films to the glass windows caused the PPD due to surface temperature effect increase and cause the PPD due to solar radiation effect decrease. It was also found that the PPD values due to solar radiation effect for glass windows and glass windows with films were varied linearly with the total transmittance of glass windows and glass windows with films. The PPD values due to surface temperature effect were varied with the total absorptance of glass windows and glass windows with films in an almost linear fashion. The heat transmission index, RHG, based on chosen design weather condition can be subdivided into the RHG due to conduction effect and RHG due to solar radiation effect. The analysis indicated that the values of RHG due to solar radiation effect were larger than the values of RHG due to conduction effect for all glass windows and glass windows with films considered in this study. Adhered film to the glass windows resulted in lowering the relative heat gain due to solar radiation in the amount corresponding to the film properties. But the film had very few effect on the relative heat gain due to conduction. The relative heat gain values were varied linearly with the total transmittances of the glass windows and glass windows with films. The relative heat gain values were also varied inversely with the absorptances of glass windows and glass windows with films in a linear fashion.     

Heat transfer modelling on windows and glazing under the exposure of solar radiation

Due to the effect of solar radiation on windows and glazing system the evaluation of heat flow is of primary importance in modeling the thermal performance within building interiors to account thermal comfort and overall energy consumption of a building. In this context the optical properties of window glazing are measured to determine the percentage absorption of incident solar radiation. An experimental study was performed in a room to measure the glazing surface temperature due to the global radiation on it. The corresponding window plane global radiation and horizontal global radiation were measured outside for simulation. Mathematical models have been developed to simulate the window plane solar radiation and corresponding glazing surface temperature aiming at validating the measured values. The thermal model is concerned with laminar heat transfer for natural and forced convection process according to the ambient conditions. The estimated errors between experimental and simulated values of window plane radiation and glazing temperature are shown to be within ±5%. Using the developed thermal model the heat flow inside the room through windows is determined. Thus overall heat transfer coefficient of glazing (U-factor) and the Solar Heat Gain (SHG) of building interior have been predicted from the simulation.     

炎热地区夏季窗户的热过程研究

以重庆地区为例，探讨了炎热地区夏季南向和西向窗户的太阳辐射得热问题。认为辐射得热是导致室内热环境恶化的首要原因，西向铝合金双玻窗的辐射得热高达520W／m^2，而基于室内外温差的热流最大也不超过50W／m^2；对于南向窗，太阳直射对室内热环境影响相对较小，天空散射与环境反射是窗户得热的主要来源。对炎热地区的窗户节能而言，有效控制辐射得热、采用遮阳装置是问题的关键。     

A simple model for assessing the energy performance of windows

A simple model for the annual energy balance of the window taking solar radiation and heat losses into consideration has been further developed and analysed. Hourly meteorological data for the solar irradiation and the outside temperature are used together with the optical and thermal performance of the window to evaluate the net energy heat flow through a window. The model renders a very simple way to compare different advanced windows in different geographical locations, orientations and buildings using basically only the balance temperature as building input. The energy balance and the cost efficiency for several glazing combinations are evaluated for buildings with different balance temperatures in a typical mid-Swedish climate. This model has a potential to be used for energy rating of windows.     

Thermo-physical behaviour and energy performance assessment of PCM glazing system configurations: A numerical analysis

The adoption of Phase Change Materials (PCMs) in glazing systems was proposed to increase the heat capacity of the fenestration, being some PCMs partially transparent to visible radiation.The aim of the PCM glazing concept was to let (part) of the visible spectrum of the solar radiation enter the indoor environment, providing daylighting, while absorbing (the largest part of) the infrared radiation.In this paper, the influence of the PCM glazing configuration is investigated by means of numerical simulations carried out with a validated numerical model. Various triple glazing configurations, where one of the two cavities is filled with a PCM, are simulated, and PCM melting temperatures are investigated. The investigation is carried out in a humid subtropical climate (Cfa according to Köppen climate classifi-cation), and “typical days” for each season are used.The results show that the position of the PCM layer (inside the outer or the inner cavity) has a relevant influence on the thermo-physical behaviour of the PCM glazing system. PCM glazing systems (especially those with the PCM layer inside the outermost cavity) can be beneficial in terms of thermal comfort. The assessment of the energy performance and efficiency is instead more complex and sometimes controversial. All the configurations are able to reduce the solar gain during the daytime, but sometimes the behaviour of the PCM glazing is less efficient than the reference one.     

Conjugate heat transfer analysis of double pane windows

This study numerically analyzes conjugate heat transfer through a double pane window using a finite difference technique. The aim of the study is to determine the thermal optimum air layer thickness between the two panes for different climates. Four different cities of Turkey, representing different climate conditions are considered: Ankara, Antalya, Kars and Trabzon. Here, much more realistic boundary conditions considered for panes than those considered in Ayd?n (Energy and Buildings 2000; 32:303–8). Ayd?n (2000) assumed panes as isothermal surfaces, while, here, a conjugate heat transfer analysis is applied. Two different boundary conditions are applied for the outer surfaces facing inside and outside: constant temperature and convection. The height of the window, H is chosen 80 cm and the thickness of each pane is set 4 mm. The effect of air layer thickness varying between 3 and 40 mm on the average Nusselt number and the heat flux through the inner pane. It was shown that energy losses through the double pane windows can be considerably reduced by optimizing thickness of the air layer. It is also shown that filling the space between the glass panes with a gas having a lower thermal conductivity instead of air reduces the insulating value of the window.     

Dielectric/Ag/dielectric coated energy-efficient glass windows for warm climates

Energy-efficient glass windows for warm climates were designed and fabricated using a three-layer system of dielectric/metal/dielectric (D/M/D) on glass. Silver was used as a metal layer. The design parameters for optimum performance of D/M/D on glass-systems for dielectrics, having refractive indices in the range 1.6–2.4, were obtained by numerical calculations. Based on these parameters, D/M/D films on glass substrates were deposited using dielectrics such as TiO₂, WO₃, and ZnS. Upon testing these coated glass windows, it was concluded that the window with any of the three dielectrics performed well and the efficiencies of the windows with different dielectrics were nearly the same.     

Monitoring results of an interior sun protection system with integrated latent heat storage

An interior sun protection system consisting of vertical slats filled with phase change material (PCM) was monitored from winter 2008 until summer 2010. While conventional interior sun protection systems often heat up to temperatures of 40 °C or more, the monitoring results show that the surface temperature on the interior side of the PCM-filled slats hardly ever exceeded the PCM melting temperature of 28 °C even in case of long-term intense solar radiation. As long as the PCM is not fully melted, the latent heat storage effect reduces the solar heat gain coefficient (g-value) of the sun protection system to 0.25 for a totally closed blind, and 0.30 for slats set at 45° (the g-values of the same system without PCM are 0.35 and 0.41, respectively). This reduced the maximum air temperature in the offices by up to 2 K in contrast to a reference room with a comparable conventional blind. The sun protection system with PCM therefore considerably improves thermal comfort. In order to discharge the PCM, the stored heat must be dissipated during the night. In climates with sufficiently low outside air temperatures, this is best achieved using a ventilation system in combination with tilted windows.     

Thermal analysis of super insulated windows (numerical and experimental investigations)

Windows are crucial for people's experience of the indoor climate, especially in the Nordic countries with cold climate and short days during the winter. This paper reports the first results from an ongoing research project focused on an improved integration of windows with the indoor air climate and people's perception of the windows. The thermal performance of a well-insulated window has been investigated both numerically and experimentally in a full scale test room. The window under consideration is a low-emissive triple-glazing window with two closed spaces filled with the inert gas krypton. An oxidised metal with low emissivity factor coats one pane in each space. Experimental and numerical investigations on the thermal performance of the window have been conducted for different winter cases. Temperature data obtained by direct temperature measurement using thermocouples and through numerical analysis are presented. The heat transfer through a window construction depends on three mechanisms i.e., conduction, convection and radiation. In this paper the convection-conducting mechanisms have been closely investigated. The numerical predictions agree well with the results from the measurements.     

A numerical study of daytime passive radiative coolers for space cooling in buildings

A passive daytime radiative cooler is made of a sky facing surface which can preserve the indoor air temperature below ambient without energy consumption by simultaneously reflecting solar radiation and emitting thermal radiation to the universe through the atmospheric window located between 8–13 μm of the electromagnetic spectrum. After the first demonstration of radiative cooling under direct sunlight, a solar mirror coated with a mid-infrared (MIR) emissive thin film has become the standard device architecture. This study firstly reviews recent developments in daytime passive radiative cooling, followed by describing the development of an energy balance mathematical model to study the potential application of passive radiative coolers in HVAC systems of buildings. Some micro-channels are fabricated on the back side of the passive radiative cooler, allowing fluid to flow in an isolated loop such that the coolant can be chilled and transported to the demand side for spacing cooling. This leads to the partial replacement of conventional vapor compression refrigeration by the radiative cooling panel. Considering the steady state energy balance within the radiative cooling panel integrated HVAC systems, the cooling performance and indoor air temperature are evaluated by numerical analysis. A 100 m2 passive radiative cooling panel could chill water for the cooling of air, reducing indoor air temperature by 10 °C, equivalent to a net cooling power of 1600 W. This study suggests that the proposed passive radiative cooling system should be used to pre-cool the ambient hot air such that the overall energy consumption of a traditional air-conditioning system can be reduced. The findings promise the application of passive daytime radiative cooling in building HVAC systems.     

Thermal analysis of a building brick containing phase change material

This paper presents the thermal analysis of a building brick containing phase change material (PCM) to be used in hot climates. The objective of using the PCM is to utilize its high latent heat of fusion to reduce the heat gain by absorbing the heat in the bricks through the melting process before it reaches the indoor space. The considered model consists of bricks with cylindrical holes filled with PCM. The problem is solved in a two-dimensional space using the finite element method. The thermal effectiveness of the proposed brick-PCM system is evaluated by comparing the heat flux at the indoor surface to a wall without the PCM during typical working hours. A paramedic study is conducted to assess the effect of different design parameters, such as the PCM's quantity, type, and location in the brick. The results indicate that the heat gain is significantly reduced when the PCM is incorporated into the brick, and increasing the quantity of the PCM has a positive effect. PCM cylinders located at the centerline of the bricks shows the best performance.     

Thermal response of brick wall filled with phase change materials (PCM) under fluctuating outdoor temperatures

Based on the enthalpy-porosity technique, a model of thermal conduction accompanied with solidification and melting processes is developed and numerically analyzed to investigate the thermal response of the brick wall filled with phase change materials (PCM). The thermal response, which is represented by indoor wall surface temperature response, of brick wall filled with PCM is evaluated and compared with that of solid brick wall. The effects of PCM filling and its filling amount on thermal response of brick wall are investigated and discussed. It is indicated that, compared to the common solid brick wall, the thermal storage of brick wall filled with PCM is elevated by the alternate process of melting and solidification under fluctuating outdoor temperatures. The use of PCM in the brick walls is beneficial for the thermal insulation, temperature hysteresis and thermal comfort for occupancy. In addition, with the increasing filling amount of PCM, the fluctuation of indoor wall surface temperature is significantly smoothed. Correspondingly, the hysteresis in response to the outdoor temperature fluctuation is enhanced. Moreover, the present model is verified by experimental data available in the literature.     

Energy performance of a dual airflow window under different climates

Ventilated windows have shown great potential in conserving energy in buildings and provide fresh air to improve indoor air quality. This paper reports our effort to use EnergyPlus to simulate the energy performance of a dual airflow window under different climates. Our investigation first developed a network model to account for the two-dimensional heat transfer in the window system and implemented it in EnergyPlus. The two-dimensional assumption and the modified EnergyPlus program were validated by the measured temperatures of the window and the energy demand of a test cell with the window under actual weather conditions. Then EnergyPlus was applied to analyze energy performance of a small apartment installed with the dual airflow windows in five different climate zones in China. The energy used by the apartment with blinds windows and low-e windows was also calculated for comparison. The dual airflow window can reduce heating energy of the apartment, especially in cold climate. The cooling energy reduction by the window was less important than that by shading solar radiation. The dual airflow window is recommended for colder climate. If improving air quality is a major consideration for a building, the window can be used in any climate.     

建筑节能与玻璃

公共建筑与居住建筑是建筑整体构成的主要部分。建筑物的节能,以外围护结构节能为主;外围护结构节能由屋面、外墙、外窗节能构成。由于现代建筑设计中,外窗占外墙的比例越来越大,导致通过窗户造成的能源损耗约占整体建筑的1/3~1/2。因此,窗户的节能性十分重要。窗户由窗框、玻璃组成,玻璃约占整体窗户的80%~90%,窗玻璃的隔热性能至关重要。阐述了太阳光辐射热能的性质及各种玻璃的节能性,着重介绍了镀膜玻璃及相关节能效果,对建筑节能设计有一定的参考价值。     

Prediction of solar heat gain of double skin facade windows

A linear model involving the solar heat gain coefficient and U value is often used to calculate the solar heat gain of traditional windows; however, it is not known if such linear model is applicable to double skin facades, which is typically featured by ventilated cavity and often with blinds inside. This paper reports on an experimental investigation into the relation between the two coefficients and the energy gain by a double skin facade without blinds. A small-scale solar calorimeter was constructed to measure the energy gain of a double skin facade window about 1.1 m high. Four tests with solar radiation intensity ranging from 205 to 793 W/m² showed that the energy gain can be represented by such linear model. The solar heat gain coefficient can be determined from the data fitting process with more accuracy than the U value, which, being a minor determinant of the heat flow in the presence of the solar radiation, may require more data for reasonable accuracy. The advantage of this linear model lies in its simplicity, which makes it easy to incorporate into present building energy simulation tools.     

Energy and visual comfort performance of electrochromic windows with overhangs

DOE-2 building energy simulations were conducted to determine if there were practical architectural and control strategy solutions that would enable electrochromic (EC) windows to significantly improve visual comfort without eroding energy-efficiency benefits. EC windows were combined with overhangs since opaque overhangs provide protection from direct sun which EC windows are unable to do alone. The window wall was divided into an upper and lower aperture so that various combinations of overhang position and control strategies could be considered. The overhang was positioned either at the top of the upper window aperture or between the upper and lower apertures. Overhang depth was varied. EC control strategies were fully bleached at all times, modulated based on incident vertical solar radiation limits, or modulated to meet the design work plane illuminance with daylight. The EC performance was compared to a state-of-the-art spectrally selective low-e window with the same divided window wall, window size, and overhang as the EC configuration. The reference window was also combined with an interior shade which was manually deployed to control glare and direct sun. Both systems had the same daylighting control system to dim the electric lighting. Results were given for south-facing private offices in a typical commercial building.     

建筑双层外窗复合传热过程交叉效应的研究

应用不可逆热力学分析并实验测试了双层窗的复合传热过程 ,得出 :在不考虑日射得热的条件下 ,自然对流换热及其对辐射换热的交叉效应是传热过程的主要因素 ,说明降低窗间气体的自然对流将是提高外窗的绝热性能的主要方向之一。     

Analysis of different models to estimate energy savings related to windows in residential buildings

A Window Energy Rating System (WERS) provides a simple, approximate method to compare the energy performance of the various windows and to determine the different potential savings for the various weather conditions. The main aim of this paper is to obtain a WERS for two climatic zones in Spain.For this purpose, the heating loads and energy savings of a residential building with different types of windows were obtained by three ways. Firstly, the energy through the window was evaluated considering only the climatic conditions. Secondly, the study was performed taking into account the energy useful for the heating system considering the climate and the type of building. Finally, the different cases were simulated using TRNSYS16 and WINDOW5. This study was performed for different European climates.The WERS proposed here is based on the second method. It takes into account the U factor of the window, U factor of the frame, absortivity of the frame, solar heat gain of the glazing and infiltration.     

建筑负荷预测中太阳辐射透过窗户引起的冷负荷

通过对建筑物玻璃窗所受太阳辐射得热的性能分析,建立了太阳辐射得热模型;根据《民用建筑热工设计规范》中划分的5个建筑热工分区,建立了典型玻璃窗的太阳辐射得热系数（SHGC）数据库,由此构建了透过玻璃窗的瞬时太阳辐射得热模型。参考DeST中的家具平板模型,建立了玻璃窗的室内蓄热平板模型,通过模型将太阳辐射得热量转化为瞬时冷负荷,并与相同条件下冷负荷系数法中的透过玻璃窗的日射得热形成冷的负荷进行对比,其相对误差不超过20％,满足城市规划阶段负荷预测的要求。从而为解决太阳辐射透过玻璃窗形成的冷负荷提供了一个新的思路,为修正用于区域能源规划中建筑负荷预测用的负荷因子模型提供了一个实用方法。     

Modeling windows in DOE-2.1E

The most recent version of the DOE-2 building energy simulation program, DOE-2.1E, provides for more detailed modeling of the thermal and optical properties of windows. The window calculations account for the temperature effects on U-value, and update the incident angle correlations for the solar heat gain properties and visible transmittance. Initial studies show up to a 35% difference in calculating peak solar heat gain between the detailed approach and a constant shading-coefficient approach. The modeling approach is adapted from Lawrence Berkeley Laboratory's WINDOW 4 computer program, which is used in the National Fenestration Rating Council (NFRC) U-value rating procedure 100-91. This gives DOE-2.1E the capability to assess the annual and peak energy performance of windows consistent with the NFRC procedure. The program has an extensive window library and algorithms for simulating switchable glazings. The program also accounts for the influence of framing elements on the heat transfer and solar heat gain through the window.