玻璃窗内加布帘后传热性能的研究

对玻璃窗内加布帘后的传热系数进行了测定。实验的窗系统分别为单层玻璃和双层玻璃内加布帘。实验中考虑了布帘边缘自然松弛和布布边缘密封的影响。通过对１００％玻璃的窗系统进行实验，得出该条件下传热系数的经验关系式，然后对窗框类型和环境风速的影响进行修正。修正后的传热系数经验公式可以方便地用于实际窗系统的瞬态传热计算。     

A Study of the U-Factor of the Window with a High-Reflectivity Venetian Blind

The study focuses on the heat transfer coefficient (U-factor) of a window with a long-wave high-reflectivity venetian blind (LWHRVB). Two side-by-side hot boxes are built in a room for testing the window U-factors. The tested window systems are 100% glass area without frames, which are equivalent to the center-of-glass of common windows. The slat of venetian blinds is made of plastic with aluminum film glued on each side, and the long-wave reflectivity is 0.93. The two types of window systems tested are the single glazing and the double glazing with the LWHRVB, respectively. The effect of slat slope-angles of venetian blinds on U-factors is also studied. The empirical equations of the center-of-glass U-factors are derived from the experimental data. The corrections for effects of window frames and outdoor wind velocity are made thereafter. The corrected empirical equations can be easily used to estimate the U-factor of the practical window with the LWHRVB.     

Novel glazing technologies to mitigate energy consumption in low‐carbon buildings: a comparative experimental investigation

Buildings play a key role in total world energy consumption as a consequence of poor thermal insulation characteristics of facade materials. Among the elements of a typical building envelope, windows are responsible for the greatest energy loss because of their notably high overall heat transfer coefficients. About 60% of heat loss through the building fabric can be attributed to the glazed areas. In this respect, novel cost‐effective glazing technologies are needed to mitigate energy consumption, and thus to achieve the latest targets toward low/zero carbon buildings. Therefore in this study, three unique glazing products called vacuum tube window, heat insulation solar glass and solar pond window which have recently been developed at the University of Nottingham are introduced, and thermal performance analysis of each glazing technology is done through a comparative experimental investigation for the first time in literature. Standardized co‐heating test methodology is performed, and overall heat transfer coefficient (U‐value) is determined for each glazing product following the tests carried out in a calibrated environmental chamber. The research essentially aims at developing cost‐effective solutions to mitigate energy consumption because of windows. The results indicate that each glazing technology provides very promising U‐values which are incomparable with conventional commercial glazing products. Among the samples tested, the lowest U‐value is obtained from the vacuum tube window by 0.40 W/m²K, which corresponds to five times better thermal insulation ability compared to standard air filled double glazed windows. Copyright © 2015 John Wiley & Sons, Ltd.     

Visual and energy performance of switchable windows with antireflection coatings

The aim of this project was to investigate how the visual appearance and energy performance of switchable or smart windows can be improved by using antireflective coatings. For this study clear float glass, low-e glass and electrochromic glass were treated with antireflection (AR) coatings. Such a coating considerably increases the transmittance of solar radiation in general and the visible transmittance in particular. For switchable glazing based on absorptive electrochromic layers in their dark state it is necessary to use a low-emissivity coating on the inner pane of a double glazed window in order to reject the absorbed heat. In principle all surfaces can be coated with AR coatings, and it was shown that a thin AR coating on the low-e surface neither influences the thermal emissivity nor the U-value of the glazing. The study showed that the use of AR coatings in switchable glazing significantly increases the light transmittance in the transparent state. It is believed that this is important for a high level of user acceptance of such windows.     

Energy efficiency of a dynamic glazing system

The reduction of air-conditioning energy consumptions is one of the main indicators to act on when improving the energy efficiency in buildings.In the case of advanced technological buildings, a meaningful contribution to the thermal loads and the energy consumptions reduction could depend on the correct configuration and management of the envelope systems. In recent years, the architectural trend toward highly transparent all-glass buildings presents a unique challenge and opportunity to advance the market for emerging, smart, dynamic window and dimmable daylighting control technologies (Lee et al., 2004).A prototype dynamic glazing system was developed and tested at ITC-CNR; it is aimed at actively responding to the external environmental loads. Both an experimental campaign and analyses by theoretical models were carried out, aimed at evaluating the possible configurations depending on different weather conditions in several possible places. Therefore, the analytical models of the building-plant system were defined by using a dynamic energy simulation software (EnergyPlus).The variables that determine the system performance, also influenced by the boundary conditions, were analysed, such as U- and g-value; they concern both the morphology of the envelope system, such as dimensions, shading and glazing type, gap airflow thickness, in-gap airflow rate, and management, in terms of control algorithm parameters tuning fan and shading systems, as a function of the weather conditions.The configuration able to provide the best performances was finally identified by also assessing such performances, integrating the dynamic system in several building types and under different weather conditions.The dynamic envelope system prototype has become a commercial product with some applications in façade systems, curtain walls and windows.The paper describes the methodological approach to prototype development and the main results obtained, including simulations of possible applications on real buildings.     

A test method to evaluate the thermal performance of window glazings

An apparatus and a methodology to evaluate the thermal performance of window glazing are presented. Single glazings commercially available in Mexico are currently tested. During the test sequence, the glass samples are mounted in a specially designed calorimeter apparatus. The test is conducted in controlled laboratory conditions at the National Centre for Research and Technology Development in Mexico using a solar simulator test lamp. The calorimeter apparatus and the solar simulator test lamp were characterised and the overall heat loss coefficient U_c measured was of 1.7±0.1 W/m²°C. Overall heat transfer and shading coefficients are derived from the experimental results. The test method described allows the testing of practically any kind of glazing array. Glasses under investigation were of the reflective, absorbing coloured and common ones. This test method can be adequate to evaluate film coating glazing or multiple solar control coating glazing. Also, it can be of assistance to researchers, glass manufacturers and building designers in the development of rating and comparing of glazing options.     

Glazing edge-seal using foamglass as spacer and frameless window design

This article presents a new design for the edge-seal of multiple-glazings with spacers made of foamglass and a new concept for frameless windows to reduce the heat loss through windows significantly. Thus the energy demand for heating is reduced or covered by solar energy gains through the window to a higher extent. The thermal performance of window assemblies with foamglass spacers and with and without frames is compared with that of the common window design. For the calculations of the heat flux a finite element analysis computer program has been used to account for the 2D-effects in the glazing, edge-seal and frame heat transfer patterns. The total heat transfer through an example window with a glazing 1 m×1 m is reduced by 45% using the window design presented. The objective of this article is not only to quantify the heat fluxes for different combinations of glazing, edge-seal and frame. The major part of the article focuses on practical aspects that are important for the durability of edge-seals, such as mechanical stress within the materials, water vapour and gas tightness, as well as on new design concepts of window–wall joints. A frameless window construction is an important aspect to enhance the thermal performance of windows. The costs for this kind of frameless windows are estimated to be less than or equal to windows commonly used now.     

Energy rating of different glazings for Indian climates

In this study, energy rating of different window glazings, available in the Indian market, has been carried out. This rating is helpful in selecting the best window for a given building and a given climate. It is shown that savings by a window w.r.t. the base window (single glazed, clear glass, 6 mm thick), depend upon window type, its orientation, climatic conditions of the place, buildings dimensions and thermal transmittance of its walls and roof. The study has been performed for five different climatic zones of India. Ten types of windows have been studied which include clear glass, tinted glass, low-e coated and solar control windows. Three types of buildings are considered with U-value of their walls ranging 0.52–2.07 W/m²K and U-value of their roof ranging 0.54–2.34 W/m²K. Finally, regression analysis is performed to develop energy rating equations for different glazings, buildings and climates.     

Implementation of energy-efficient windows in Swedish single-family houses

A questionnaire survey of 1010 homeowners in Jämtland and Västernorrland, which are two counties in central Sweden, was conducted to understand the factors influencing their decision to install energy-efficient windows. We complemented this survey with an interview of 12 window sellers/installers in the county Jämtland. The annual energy cost reduction, age, and condition of the windows were the most important reasons for the window replacement decision. Approximately 80% of the respondents replaced their windows with energy-efficient windows with U-value of 1.2 W/m² K. Condensation problems, perceived higher prices, and lack of awareness about windows with lower U-values were important reasons for non-adoption of more energy-efficient windows. Window sellers/installers have a strong influence on homeowners’ window selection that was indicated by the 97% of homeowners who bought the windows that were recommended to them. Sellers/installers revealed that they did not recommend windows with U-value of less than 1.2 W/m² K because they thought that investing in such windows was not economical and because windows with U-value less than 1.2 W/m² K could cause water condensation on the external surface of window pane.     

Daylighting performance of an electrochromic window in a small scale test-cell

In this paper, the results of an experimental investigation aimed at assessing the performance of electrochromic (EC) windows with respect to daylighting control in buildings are presented. The research is performed under real weather conditions by a small scale test-cell equipped with a small area double glazing unit (DGU) where one pane consists of an EC device with visible transmittance τ_v ranging from 6.2 to 68.1% and the other of an ordinary clear float glass (τ_v ≈90%). Experimental tests are carried out as a function of time, weather conditions, test-cell orientation and switching strategies. These data are integrated with spectrophotometric measurements. Results show that the angle selectivity of the glazing combined with its active switching effect allows a wide range of selectable transmission states to suit the latitude and orientation of a building in relation to the local climatic conditions. For south facing windows and under the involved climatic conditions EC glazing driven by a dynamic control strategy can be very effective in reducing discomfort glare caused by high window brightness. Glare reduction can be realized contemporarily maintaining the work plane illuminance to adequate level for computer based office tasks so without compromising much of the available daylight. Furthermore, since EC glazing is never switched to heavily darkened states (τ_v >20%), colour rendering of inside objects should be always acceptable, although internal illuminance level could be slightly lower than to what users prefer in relation to the correlated colour temperature of the incoming light. These results change when considering west orientation for which high-luminance direct sunlight patches are registered on the work-plane even for EC glazing switched to its lowest transmitting state letting suppose that EC windows cannot provide full control of uncomfortable direct sunlight effects without integration of additional shading devices.     

Complex multimaterial insulating frames for windows with evacuated glazing

The thermal performance of a complex multimaterial frame consisting of an exoskeleton framework and cavities filled with insulant materials enclosing an evacuated glazing was simulated using a two-dimensional finite element model and the results were validated experimentally using a guarded hot box calorimeter. The analysed 0.5 m by 0.5 m evacuated glazing consisted of two low-emittance film coated glass panes supported by an array of 0.32 mm diameter pillars spaced 25 mm apart, contiguously sealed by a 10 mm wide metal edge seal. Thermal performance of windows employing evacuated glazing set in various complex multimaterial frames were analysed in detail. Very good agreement was found between simulations and experimental measurements of surface temperatures of the evacuated glazing window system. The heat loss from a window with an evacuated glazing and a complex multimaterial frame is about 80% of that for a window comprised of an evacuated glazing set in a single material solid frame.     

含相变材料双层玻璃窗光热分析模型

建立了含相变材料双层玻璃窗的光热传输模型,考虑相变材料和玻璃半透明特性,采用有限差分求解方程。在通过实验数据验证模型准确的基础上,分析了相变材料的融化温度对含相变材料双层玻璃窗光热性能的影响。结果显示:建立的模型可模拟相变材料双层玻璃窗的光热传输过程;随融化温度升高,含相变材料双层玻璃窗温度衰减因子则逐渐增大,相变材料融化时间延后,相变材料呈液态的时间变短,但温度滞后值、热流密度和太阳透射能则呈不规则变化。     

Computation of glass-cover temperatures and top heat loss coefficient of flat-plate solar collectors with double glazing

A set of correlations for computing the glass-cover temperatures of flat-plate solar collectors with double glazing is developed. A semi-analytical correlation for the factor f₂—the ratio of outer to inner thermal resistance of a double-glazed collector—as a function of collector parameters and atmospheric variables is obtained by regression analysis. This relation readily provides the temperature of the second (outer) glass cover (T₂). For estimating the temperature of first (inner) glass cover (T₁), another relation for the factor f₁—the ratio of thermal resistance between the two glass covers to the thermal resistance between the absorber plate and first glass cover—is developed. A wide range of variables is covered in the present analysis. The results are compared with those obtained by numerical solutions of heat-balance equations. Using the proposed relations of glass-cover temperatures, the values of top heat loss coefficient (U_t) can be computed and are found to be very close to those obtained by numerical solutions of heat-balance equations. The maximum absolute error in the calculation of U_t by the proposed method is only 1.0%, so numerical solutions of heat-balance equations for the computation of U_t are not required.     

Numerical evaluation of the mixed convective heat transfer in a double-pane window integrated with see-through a-Si PV cells with low-e coatings

This study presents a two-dimensional numerical analysis for thermal control strategies on potential energy savings in a double-pane window integrated with see-through a-Si photovoltaic (PV) cells with low-emittance (low-e) coatings. Both heat transmission through the air gap by combined convection and radiation, and air flow patterns within the cavity of the window were considered. The convection-conducting mechanisms in the cavity of the double-pane window have been closely investigated in this paper. Based on numerical predictions, the effect of Rayleigh number on airflow patterns was investigated for low Rayleigh numbers in the range of 10³ ? Ra ? 10⁵. The effect of the low-e coatings on the glazing U-value was also explored in this paper. It was found that a large quantity of heat transfer by radiation could be reduced. This novel glazing system could help engineers’ design in more advanced window systems with building-integrated photovoltaic (BIPV) applications in modern buildings.     

Spectral effects on the transmittance, solar heat gain, and performance rating of glazing systems

This study investigates the potential changes in Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT) ratings of vertical or tilted glazing systems that would result from a deliberate change in the reference spectrum used as Spectral Weighting Function (SWF). This SWF is necessary to evaluate broadband-average optical properties from their spectral values, and obtain the desired rating of such bulk properties. The SWFs currently specified by rating institutions in Europe and North America for SHGC and VT are now outdated, and their inadequacies are discussed. Six potential replacements, which have been recently adopted by ASTM are described, including three direct irradiance spectra and three global irradiance spectrum incident on tilted surfaces of various tilts (20°, 37° and 90°). Some of these spectra have been tailored for use in building energy applications, including Building-Integrated Photovoltaics (BIPV). The effect of tilt on the U-factor and hence SHGC of glazing systems used for skylights on roofs is discussed, using a representative dataset of 37 glazing system specimens. The spectral effects on SHGC induced by a change in the current North American SWF are also obtained for this dataset, and show small to moderate deviations from current ratings (−2% to +7% for windows, and −3% to +11% for skylights). The variations in VT are within ±2% for most glazing systems. To remove the current inconsistency in the SWFs used for SHGC and VT, it is recommended that a single SWF be used for both properties. For improved accuracy and reliance on active standards, it is also recommended that the SWF for SHGC and VT be either one of the two recent ASTM G197-08 global irradiance spectra, depending on application (incidence on a vertical surface for window applications, and incidence on a 20°-tilted surface for skylight applications). No change in colorimetric calculations (based on the D65 illuminant) is recommended, however.     

Switchable glazing with a large dynamic range in total solar energy transmittance (TSET)

Modern, energy-saving buildings incorporate large areas of highly insulating glazing. The resulting solar gains lead to major savings in heating energy during winter, but protection against overheating in summer is also needed. Usually this problem is solved by using mechanical shading devices, with the disadvantages of high cost and low durability.The work on switchable glazing at Fraunhofer Institute for Solar Energy Systems, in cooperation with industrial partners, aims to present new and viable alternatives. Two types of switching layers, which are quite different in their structure and function, but are similar in having a large dynamic range in TSET, are being investigated—gasochromic and thermotropic.Gasochromic windows are actively switched between a clear and a coloured (but image-preserving) state by alternately introducing strongly diluted O₂ and H₂ gases. In contrast to classic electrochromic configurations, only one tungsten oxide film with a very thin catalyst coating is needed. At present, prototype windows with an area of 1.1×0.6 m² are being produced by sputtering. Careful adjustment of the layer structure, the gas concentration and its flow velocity is needed to obtain the desired switching rate. Homogeneous colouring of the whole area within seconds has been achieved. In addition to information on the colouring kinetics, the paper also discusses system aspects of these windows.Thermotropic layers switch reversibly and automatically, from a clear state with high transmittance to a milky white state with high diffuse reflectance, when their temperature rises. Depending on the composition of the material, the switching temperature can be chosen in the range needed. The measured optical properties of glass laminates with a thermotropic layer are presented. These are combined with the measured values for further panes to calculate the characteristic data for thermotropic insulated glazing units. The results are compared with those measured on a 1.1×1.65 m² prototype window. Stability results are also included.The effect of the two different types of switchable glazing on building energy savings is explored for a residential building model, using the TRNSYS building energy simulation program.     

Approximate method for computation of glass cover temperature and top heat-loss coefficient of solar collectors with single glazing

An improved equation form for computing the glass cover temperature of flat-plate solar collectors with single glazing is developed. A semi-analytical correlation for the factor f—the ratio of inner to outer heat-transfer coefficients—as a function of collector parameters and atmospheric variables is obtained by regression analysis. This relation readily provides the glass cover temperature (T_g). The results are compared with those obtained by numerical solution of heat balance equations. Computational errors in T_g and hence in the top heat loss coefficient (U_t) are reduced by a factor of five or more. With such low errors in computation of T_g and U_t, a numerical solution of heat balance equations is not required. The method is applicable over an extensive range of variables: the error in the computation of U_t is within 2% with the range of air gap spacing 8 mm to 90 mm and the range of ambient temperature 0°C to 45°C. In this extended range of variables the errors due to simplified method based on empirical relations for U_t are substantially higher.     

Top heat-loss factor of double-glazed box-type solar cooker from indoor experiments

The top heat-loss factor (U_t) of a box-type solar cooker varies with plate temperature, wind heat-transfer coefficient and ambient temperature. A method for correlating U_t with these variables is presented for a cooker with double glazing. A set of equations is developed for correlating data obtained in indoor experiments at different plate temperatures and wind speeds.     

Reassessment of fenestration characteristics for residential buildings in hot climates: energy and economic analysis

This paper attempts to resolve the reported contradiction in the literature about the characteristics of high-performance/cost-effective fenestration of residential buildings, particularly in hot climates. The considered issues are the window glazing property (ten commercial glazing types), facade orientation (four main orientations), window-to-wall ratio (WWR) (0.2–0.8), and solar shading overhangs and side-fins (nine shading conditions). The results of the simulated runs reveal that the glazing quality has a superior effect over the other fenestration parameters and controls their effect on the energy consumption of residential buildings. Thus, using low-performance windows on buildings yields larger effects of WWR, facade orientation, and solar shading than high-performance windows. As the WWR increases from 0.2 to 0.8, the building energy consumption using the low-performance window increases 6.46 times than that using the high-performance window. The best facade orientation is changed from north to south according to the glazing properties. In addition, the solar shading need is correlated as a function of a window-glazing property and WWR. The cost analysis shows that the high-performance windows without solar shading are cost-effective as they have the largest net present cost compared to low-performance windows with or without solar shading. Accordingly, replacing low-performance windows with high-performance ones, in an existing residential building, saves about 12.7 MWh of electricity and 11.05 tons of CO₂ annually.