Experimental validation for thermal transmittances of window shading systems with perimeter gaps

Virtually all residential and commercial windows in the U.S. have some form of window attachment, but few have been designed for energy savings. ISO 15099 presents a simulation framework to determine thermal performance of window attachments, but the model has not been validated for these products. This paper outlines a review and validation of the ISO 15099 centre-of-glass heat transfer correlations for perimeter gaps (top, bottom, and side) in naturally ventilated cavities through measurement and simulation. The thermal transmittance impact due to dimensional variations of these gaps is measured experimentally, simulated using computational fluid dynamics, and simulated utilizing simplified correlations from ISO 15099. Results show that the ISO 15099 correlations produce a mean error between measured and simulated heat flux of 2.5?±?7%. These tolerances are similar to those obtained from sealed cavity comparisons and are deemed acceptable within the ISO 15099 framework.     

Description of ParaSol v3.0 and comparison with measurements

ParaSol is a computer program for calculating the solar and thermal properties of windows with sunshades and the energy demands of a room with a window/shading system. The program has three main features. One of them is a calculation of g, T and U for normal incidence of beam irradiation, which is performed as soon as a window or an internal/interpane sunshade is selected. The other two are based on yearly simulations, using DEROB-LTH, a building energy simulation program on which ParaSol is based. One of the applications gives the monthly average of g and T for the window glazing and the glazing/sunshade system. The other application gives the heating and cooling demands for a room with a window, with and without a sunshade, where input data are given for the internal heat, ventilation settings, shading control and temperature set-points. Version 3.0 of ParaSol, which has some new and improved models, is described in this paper. The g-values obtained with this program version are compared with measurements on windows with internal/interpane screens/venetian blinds. The absolute deviation is less than 0.03 for the venetian blinds. The measured values of dark internal screens with closed air gaps exceed those simulated by Parasol, but are lower than the ones simulated with open air gaps.     

百叶外遮阳太阳散射辐射计算模型及程序实现

窗户外部的遮阳系统可以减少进入室内的太阳辐射得热量。目前,在计算遮阳系统对太阳辐射的遮挡作用时,仅仅从几何角度上考虑遮阳构件对直射辐射的减少作用,没有考虑对散射辐射的影响,但在实际中,遮阳构件的存在,对散射辐射的影响不能忽略,特别是对于百叶类型的遮阳构造,由于其布置较为紧密,对直射辐射遮挡较多,进入室内的太阳辐射主要为散射辐射,因此建立详细的散射辐射计算模型,可以正确反映和评估其遮阳效果,并为计算遮阳构件对建筑能耗的影响提供基础。通过分析散射辐射入射到百叶遮阳板上的光学特性,建立用于计算太阳散射辐射透过和反射的数学模型,并根据该模型得到用于计算机编程的算法。该模型和算法可以应用于不同的百叶遮阳系统,为建筑能耗模拟中考虑百叶遮阳板对空调能耗的影响提供理论基础。     

Correlating thermal transmittance limits of walls and roofs to orientation and solar absorption

The presented research addresses the association of maximum thermal transmittance (U value) of walls and roofs with orientation and solar absorption. The study is performed on walls and a roof typically used in Kuwait when subjected to local hot climate conditions. A computer program employing the total equivalent temperature difference (TETD) method is developed to estimate the U values corresponding to solar absorption coefficients ranging from 0 to 1. At zero solar absorption coefficient, the maximum U values stipulated in the existing national energy code are used to induce the constant total daily heat flux for the developed correlations. Furthermore, the thermal inertia parameters affected by the established U values are also predicted. The results show that the orientation and absorption coefficient have a profound effect on the U value particularly for the roof and west and east wall orientations. Moreover, the decrease in the thermal transmittance significantly enhances the thermal inertia parameters of the aerated autoclaved concrete wall type.The developed correlations can be added to the Kuwait building energy code to extend the application scope of the U value limits based on surface orientation and external solar properties.     

Optical and thermal performance of glazing with integral venetian blinds

Shading devices have long been used to control solar gain and daylighting through windows. The optimal design and use of such devices requires a detailed knowledge of their optical and thermal properties, including their variation with the angle of incidence of the solar radiation. We describe the experimental methods used to characterise two types of double-glazing units (DGU) incorporating Venetian blinds. The first consists of a DGU with an adjustable blind located in the space between the panes. The second incorporates a fixed blind element bonded between two glass sheets. The spectral bi-directional transmittance functions (BRTF) were measured for a range of incidence angles, using the Cardiff goniospectrometer. The thermal properties were measured with an illuminated hot box. These measurements are laborious, and it is desirable to develop simple models, that could be used for product development to estimate the effect of changing the properties of the blind material and its design, without the need to make detailed measurements on each variation. A model is described, that calculates the properties of the DGU using as input the optical properties at normal incidence of the component elements (glass and the blind’s slats). The model is applied to the variable blind DGU, varying the angle of rotation of the blind. Despite the simple nature of the model, it is shown to satisfactorily predict the properties of the DGU. The model can easily be incorporated in building energy simulation programs.     

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.     

Improvement of thermal environment and reduction of energy consumption for cooling and heating by retrofitting windows

Various techniques for creating a comfortable thermal environment and saving energy have been proposed and employed in residential buildings in many countries, including Japan. For these techniques to be introduced, existing houses should be renovated. Among the techniques available, installation of additional inner windows is effective in creating a comfortable and energy-efficient living environment. In the present research, the effect of additional inner windows on the thermal environment and energy saving was investigated by measuring indoor windows on the thermal environment and energy saving was investigated by measuring indoor concrete building. Air temperatures, the humidity, the global solar radiation on horizontal and vertical surfaces, radiant temperatures, and the electricity consumption of air-conditioners were measured. A comparison of these values before and after the installation of inner windows showed that the thermal environment and energy saving had improved. Results obtained from a thermal model agreed well with measured results by changing the value of solar transmittance and heat transmission coefficient of the glazing following renovation. Furthermore, in a questionnaire survey conducted in summer, more than half of the occupants answered ‘‘comfortable’’ to a question on the overall thermal comfort.     

Field study on indoor thermal environment in an atrium in tropical climates

Solar penetration through the transparent envelope can severely deteriorate indoor thermal environment inside an atrium building particularly in tropical climates. This paper reports the application of two low-cost measures, namely high level internal solar blinds and water spray, to minimise overheating problems on the three levels inside the atrium of a guesthouse in Southern China, where summer is hot and humid. The blinds reduce direct solar penetration at the top of the atrium whilst the evaporative spray system cools down the glazed surfaces of the atrium envelope. A site test was undertaken over 10 consecutive days covering both overcast days and clear days in July 2004. Measurement of indoor thermal environmental parameters was conducted on three levels in the atrium and the recorded data represent the internal conditions: with and without internal blinds protection from solar, and with and without water spray. This study has shown that on hot and clear summer days, with water spray and without blinds the average air temperature difference from 1200 to 1800 h between both first floor and second floor, and second floor and external were 5.7 and 1.7 K, respectively; whilst with blinds and without water spray the average air temperature differences were 8.7 and 4.8 K, respectively.     

Subject responses to electrochromic windows

Forty-three subjects worked in a private office with switchable electrochromic windows, manually operated Venetian blinds, and dimmable fluorescent lights. The electrochromic window had a visible transmittance range of approximately 3–60%. Analysis of subject responses and physical data collected during the work sessions showed that the electrochromic windows reduced the incidence of glare compared to working under a fixed transmittance (60%) condition. Subjects used the Venetian blinds less often and preferred the variable transmittance condition, but used slightly more electric lighting with it than they did when window transmittance was fixed.     

Analysis and modelling of window and glazing systems energy performance for a well insulated residential building

The energy performance of a window depends on its thermal transmittance, the glazing solar transmittance, and the air leakage due to the frame and installation airtightness.In new installations air leakage represents a quite small term which is almost independent from the window and in particular from the glazing system selection.The contributions of the two other terms to the building thermal balance are not independent to each other: the most effective thermal insulating glazing, as triple glazings, are generally characterized by low solar transmittance reducing solar gains. The thermal energy balance of the building is then affected not only in summer but also in winter, potentially increasing heating energy need.This work evaluates the impact of different kinds of glazing systems (two double and two triple glazings), window size (from 16% to 41% of window to floor area ratio), orientation of the main windowed façade and internal gains on winter and summer energy need and peak loads of a well insulated residential building. The climatic data of four localities of central and southern Europe have been considered: Paris, Milan, Nice and Rome. A statistical analysis has been performed on the results in order to identify the most influencing parameters.     

Illuminance-based slat angle selection model for automated control of split blinds

Venetian blinds play an important role in controlling daylight in buildings. Automated blinds overcome some limitations of manual blinds; however, the existing automated systems mainly control the direct solar radiation and glare and cannot be used for controlling innovative blind systems such as split blinds. This research developed an Illuminance-based Slat Angle Selection (ISAS) model that predicts the optimum slat angles of split blinds to achieve the designed indoor illuminance. The model was constructed based on a series of multi-layer feed-forward artificial neural networks (ANNs). The illuminance values at the sensor points used to develop the ANNs were obtained by the software EnergyPlus™. The weather determinants (such as horizontal illuminance and sun angles) were used as the input variables for the ANNs. The illuminance level at a sensor point was the output variable for the ANNs. The ISAS model was validated by evaluating the errors in the calculation of the: 1) illuminance and 2) optimum slat angles. The validation results showed that the power of the ISAS model to predict illuminance was 94.7% while its power to calculate the optimum slat angles was 98.5%. For about 90% of time in the year, the illuminance percentage errors were less than 10%, and the percentage errors in calculating the optimum slat angles were less than 5%. This research offers a new approach for the automated control of split blinds and a guide for future research to utilize the adaptive nature of ANNs to develop a more practical and applicable blind control system.     

国内外中空玻璃节能特性计算标准对比与分析

本文对比分析了国内外中空玻璃节能特性的计算标准,其中包括遮阳系数Sc的计算标准GB/T2680-94、ISO9050-2003和传热系数U值计算标准ISO 15099-2003、NFRC100-2010、JGJ/T151、ISO10292-1994、EN673-1998,GB/T22476-2008,总结了这些标准的主要区别和联系,并通过计算同一中空玻璃结构在各标准条件的U值,说明了在产品节能性能比较时的注意事项。     

轻型竹木复合墙体的热工性能试验研究

为研究轻型竹墙的热工性能,促进推广此类新型轻质墙材在节能建筑中的应用,设计并制作了2片轻型竹墙、1片轻型木墙以及1片轻型竹木复合墙,并进行了墙体的防护热箱试验,实测获取了墙体的热阻和传热系数。结合已有文献中轻钢龙骨墙体的热工性能的试验研究结果,对轻型竹墙、轻型木墙以及竹木复合墙体等的热工性能进行了比较研究。根据ISO 6946:2007的平均传热系数计算方法对轻型竹墙、轻型木墙以及轻型竹木复合墙体的稳态传热性能进行了计算和有限元分析,验证了有限元分析的可行性。基于有限元模型分别以材料的导热系数、龙骨间距、龙骨厚度及墙内钉间距为参数对3种轻型竹木墙体的热工性能进行了模拟分析。结果表明：轻钢龙骨墙体的传热系数最大,轻型竹墙与木墙的传热系数相近,说明以竹代木的设计理念是可行的;龙骨材料的导热系数垂直积层方向变化对墙体的热工性能影响较大;增加龙骨间距可适当减小墙体的传热系数,而增大龙骨厚度则可大幅减小墙体的传热系数;墙体热工性能受墙内钉间距影响较小,钉间距超过150mm时可忽略其影响。     

Convective heat transfer in domed skylight cavities

Domed skylights are important architectural design elements that deliver daylight and solar heat into buildings, and connect the building's occupants to the outdoor environment. Despite the widespread use of domed skylights, there is limited information on the convective heat transfer within cavities of multi-glazed domes. This information is required to evaluate the thermal performance of domed skylights for product rating purposes, or to evaluate the heat loss or gain of installed skylights in buildings. This article presents a numerical study on the laminar natural convection in horizontal concentric domed cavities heated from the inside surface. A commercial CFD package is used to solve for the flow and temperature fields. The results show that for large cavity gap spacing-to-radius ratios, the cavity flow is mono-cellular and steady state. For small gap spacing ratios, however, the cavity flow may be multi-cellular and transient periodic. Practical correlations for the heat transfer coefficient as a function of the cavity shape and gap spacing ratio are developed for both flow regimes. The critical gap spacing ratio that yields the maximum Nusselt number is quantified for each cavity shape.     

Revision of the Trombe wall calculation method proposed by UNE-EN ISO 13790

The European and International Standard UNE-EN ISO 13790 presents a set of calculation methods for the evaluation and design of energy and thermal performance of buildings. These methods have diverse range of details for calculating the energy use of heating and cooling in different building zones, as well as for calculating the heat transfer and solar heat gains of special elements, such as ventilated solar walls (Trombe walls). In this article, the authors have revised the aforementioned document in order to check the proposed mathematical models and their implementation within Mediterranean climates. This assessment pinpoints the existence of some errors in the equations provided in EN ISO 13790 under steady state conditions. Concurrently, the corrected equations are shown and new correlations are proposed for the ratios δ and ω which are more suitable for Mediterranean climates.     

Occupant control of passive systems: the use of Venetian blinds

The way occupants interact with passive control features is poorly understood. Venetian blinds are a key element in the passive control of a building’s environment. They help to control glare, daylighting, and overheating, all of which affect both the comfort of occupants and a building’s energy consumption. However a building’s environmental design is very dependent on assumptions regarding occupants use of blinds. This paper examines the current assumptions in occupants use of blinds and the impact these assumptions have on a building’s environment. It then compares the assumed use with the actual use from monitored data of blind use in both summer and winter conditions in three buildings. In particular, the impact of façade orientation, sunshine, and electric lighting on blind use is investigated. Preliminary results confirm that on average, 40% of a building’s façade is obscured by blinds resulting in a reduction in daylight. This reduction in daylight is equivalent to an unobstructed glazed area of 68% of the building’s façade assuming that 20% of light penetrates a blind when it is fully drawn. Occupants use of blinds is predominantly not affected by solar availability as often modelled, although there is a weak relationship between orientation and window occlusion.     

Enhancement of natural ventilation in buildings using a thermal chimney

A new module was developed for and implemented in the EnergyPlus program for the simulation and determination of the energy impact of thermal chimneys. This paper describes the basic concepts, assumptions, and algorithms implemented into the EnergyPlus program to predict the performance of a thermal chimney. Using the new module, the effects of the chimney height, solar absorptance of the absorber wall, solar transmittance of the glass cover and the air gap width are investigated under various conditions. Chimney height, solar absorptance and solar transmittance turned out to have more influence on the ventilation enhancement than the air gap width. The potential energy impacts of a thermal chimney under three different climate conditions are also investigated. It turned out that significant building cooling energy saving can be achieved by properly employing thermal chimneys and that they have more potential for cooling than for heating. In addition, the performance of a thermal chimney was heavily dependent on the climate of the location.     

Measurement and simulation of the thermal environment in the built space under a membrane structure

This paper presents a study to clarify characteristics of the summer thermal environment in an actual membrane structure with a semi-closed space underneath by using field measurements and simulations. The first part of this paper describes the following findings from the field measurements conducted during a summer period: (1) the solar transmission through the membrane had a greatest impact on the thermal environment in the living space under the membrane structure during the daytime; (2) the mean radiant temperature (MRT) at the central part of the living space went higher than the air temperature due to heat storage in the ground and walls of surrounding buildings where radiation cooling to the sky was obstructed by the membrane. In the second part of the paper, a 3D CAD-based simulation tool (called the thermal environment simulator) developed by the authors' group was used to simulate the thermal environment in the test membrane structure. A comparison between the measured and simulated surface temperature of the membrane was carried out. As a result, it was found that the simulated results agreed well with the measurement data. In addition, the following subjects were discussed using the simulation tool: how the thermal environment under the membrane is influenced by changing the solar transmittance and absorptance of the membrane or changing the ground surface materials. Simulation results show that the simulation tool is able to provide a quantitative prediction and evaluation of the thermal environment in the living space under the membrane structure in terms of the surface temperature and mean radiant temperature distribution.     

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.