1、4763 Integrated Thermal and Daylighting Analysis for Design of Office Buildings Athanassios Tzempelikos Student Member ASHRAE ABSTRACT This work is part of a research project on optimization of faades of commercial buildings in the early design stage. Shading devices are utilized to control solar ga
2、ins and simul- taneouslyprovide adequate daylight to the interior: A method for quantzhing the efects of shading patterns on interior conditions and overall building energy performance is presented. Integratedperformance indices are obtained by the continuous interaction between transient thermal an
3、d lighting simulation. Fenestration and shading systems have a major impact on visual and thermal comfort in perimeter spaces but also on energy consumption, peak loads, and possibly HVAC system sizing. Moreovel; automated operation of shading devices in conjunction with dimmable electric lighting s
4、ystems and HVAC system components could lead to minimization of energy consumption for lighting, heating, and cooling. An inte- grated approach should be followed when designing and controlling fenestration andshading systems in order to obtain optimal solutions. INTRODUCTION Building design is a co
5、mplex process in which critical decisions concerning the different systems related to the build- ing are made at the early stage. The impact of unsteady and continuously variable exterior climatic conditions on indoor environment is determined by the building envelope. The daylighting and thermal pe
6、rformance of perimeter spaces depends on fenestration design. Fenestration area in commer- cial buildings is continuously increasing, driven by the higher demand for buildings with much daylight. Utilization of daylight in buildings may result in significant savings in elec- tricity consumption for
7、lighting while creating a higher quality Andreas K. Athienitis, PhD, PE Member ASHRAE indoor environment (Lee et al. 1998), provided that electric lighting has automatic photocell controls that have been commissioned. The benefits in terms of higher productivity and reduced absenteeism of office wor
8、kers probably exceed the energy savings (Heschong 2002). Nevertheless, many designers do not realize the need to balance the energy consumption between lighting, heating, and cooling. Large fenestration areas often result in excessive solar gains and highly varying thermal loads throughout the year,
9、 especially when inadequate amounts of thermal mass are present. In addition, intense daylight leads to glare problems for south faades of office buildings under clear sky. Innova- tive daylighting/shading systems and dynamic building enve- lope elements such as prismatic panes and light-redirecting
10、 systems (Lorenz 2001; Beck et al. 1999), sun ducts, anidolic zenithal openings, holographic optical elements, etc., were invented and employed during the last decade in order to control solar gains and create a high-quality indoor environ- ment. Advanced glazing products such as electrochromic, the
11、rmochromic, gasochromic, and thermotropic have been also studied for the same reasons. A major factor in the eval- uation of the performance of advanced fenestratiodshading systems is the determination of their optical and thermal prop- erties. These are usually not provided by manufacturers, and th
12、ere is no standard procedure for measuring the transmittance of such devices. These properties can be estimated using vari- ous experimental techniques (Aleo et al. 1994; Collins et al. 2001; Rosenfeld et al. 2001) or using complicated theoretical models (Rheault and Bilgen 1989; Pfrommer et al. 199
13、6; Molina et al. 2000) and with the aid of advanced software (Reinhari and Walkenhorst 2001). Athanassios Tzempelikos is a PhD candidate and Andreas K. Athienitis is a professor in the Department of Building, Civil and Environ- mental Engineering, Concordia University, Montreal, Quebec, Canada. 0200
14、5 ASHRAE. 227 Shading provision should be considered as an integrai part of fenestration system design, particularly for south- facing faades of buildings. Shading devices are multi- purpose: they can block direct sun and solar gains during the cooling season, allow the maximum amount of daylight (a
15、nd solar heat) during the heating season, control the sunlight by diffusing it into the space without causing glare on clear days, while, at the same time, transmitting all the available daylight on overcast days (Tzempelikos and Athienitis 2002). Dynamic control of motorized shading devices, fenest
16、ration systems, electric lighting, and HVAC system components may lead to minimization of energy consumption for lighting, heating, and cooling while maintaining good thermal and visual comfort under continuously changing outside conditions (Lee et al. 1998; Athienitis and Tzempelikos 2003a, 2003b).
17、 This paper presents an integrated approach for fenestra- tion and shading design analysis and optimization at the early stage. First, optimum window size-defined as window-to- wall ratio for generalization-is determined based on inte- grated performance indices obtained by the continuous inter- act
18、ion between transient hourly thermal and lighting simulation. Daylight availability ratio and reduction in peak thermal loads and energy consumption for heating and cooling are identified as initial criteria. The impacts of different shad- ing properties on interior visual and thermal comfort and ov
19、er- all energy performance of perimeter spaces of office buildings are shown by means of simulation, and the significance of control strategies is discussed. Key parameters are identified, and a methodology for optimum design is proposed. FENESTRATION AND SHADING SYSTEMS: THE KEY PARAMETERS Fenestra
20、tion systems are the link between daylighting and thermal performance of perimeter spaces. They are the most important building envelope element in office buildings. They can provide interior spaces with daylight, view, and solar heat while at the same time they could be the cause of thermal and vis
21、ual discomfort, excessive heat gains-or losses-and highly varying thermal loads throughout the year. The balance of positive and negative influence of solar radiation on build- ing energy use and human comfort is something difficult to deal with. Solar radiation accompanies the admission of daylight
22、, which contributes to visual comfort and reduction in electric lighting energy consumption (depending on orienta- tion). High solar gains may result in increase of cooling load but also reduction of heating load for near south-facing perim- eter zones. Also, appropriate control of electric lights c
23、an reduce peak cooling load. Although an adequate amount of daylight is ensured, problems associated with glare and visual discomfort are inevitable if direct solar radiation enters the room. In addition, thermal problems arise if no shading is used. Shading devices can be the solution to the above
24、prob- lems. The selection of the type and properties of these devices is critical; it will determine daylighting and shading perfor- mance; energy consumption for heating, cooling, and electric lighting; and peak loads. It also has a significant impact on visual and thermal comfort. Fixed shading de
25、vices are usually employed in the building envelope to exclude solar radiation in the summer and admit it during the winter. However, they block a significant amount of diffuse daylight and they are not effective under cloudy skies. On the other hand, movable shad- ing devices can be adjusted to cha
26、nging outdoor conditions. Moreover, in the case of automated systems, each control strategy has its own impact on all the above-mentioned param- eters, as discussed next. As a result, the design of advanced fenestratiodshading systems is a complicated task. Normally, the optical and ther- mal charac
27、teristics of these devices should first be evaluated in order to estimate their effect on interior conditions (Andersen et al. 2003; Tzempelikos and Athienitis 2001). The situation becomes more complicated for faades with high solar gains. Impacts on work plane illuminance levels, peak heating and c
28、ooling loads, and electricity consumption for lighting should all be taken into account when designing a faade. Since the above parameters are interrelated, an integrated approach must be followed in order to attain an optimal solution. The significance of integration in building design is well expl
29、ained by Clarke et al. (1998). Performances on advanced glazing systems based on integrated simulation with widely used soft- ware are described by Citherlet and Scartezini (2000). An opti- mum cooling and lighting energy balance between the window and the lighting system may be identified and utili
30、zed (Lee and Selkowitz 1995). Integrated daylighting and thermal analysis shall lead to selection of optimum system properties and dynamic control (if any), with the objective to lower energy consumption for lighting, heating, and cooling while at the same time maintain good thermal and visual comfo
31、rt under continuously varying exterior conditions (Johnson et al. 1984; Athienitis and Tzempelikos 2002). This may result in the design of multifunctional faades, which will fulfill all desired requirements for energy efficiency and comfort (Vartiainen et al. 1999; Tzempelikos and Athienitis 2002).
32、The window itself (without shading) initially determines the daylight levels in the interior. The type of window and its optical properties, size, shape, location, and orientation are the parameters affecting the daylighting performance. Among these, the window size varies significantly from buildin
33、g to building. Before analyzing the effects of shading, a method for selecting optimum window relative size is presented in the next section. SELECTION OF OPTIMUM WINDOW SIZE A parametric sensitivity analysis is performed to investi- gate the effect of window size and orientation on combined dayligh
34、ting and thermal performance of office buildings. A single perimeter office space in Montreal with one exterior wall is used as a base case, to systematically study the effect of window size on daylighting, peak heatinglcooling loads, and overall energy consumption. The effect of window size on 228
35、ASH RAE Transactions: Research electric lighting demand is also presented. If electric lights are dimmable, energy consumption is further reduced and the effect on cooling load is discussed. Results allow the selection of window size for each orientation, based on priority criteria. Window size is e
36、xpressed as window-to-wall ratio for gener- alization of results. Initially, two factors are considered for selection of window-to-wall ratio of the faade: (i) the ability to provide adequate daylight into the space and (2) the reduc- tion in peak heating and cooling load and energy consumption. Alt
37、hough there are many other parameters that should be taken into account when selecting window size, such as glare, ther- mal comfort, or even aesthetics, those will be evaluated in a second step, when shading devices are also considered in the integrated design process. Daylighting Considerations We
38、 refer to daylight as the visible portion of solar radia- tion, to separate the luminous from the thermal effects of solar energy. A base-case scenario is created; a typical 4 m x 4 m x 3 m high perimeter private office space in Montreal is consid- ered, with one exterior wall and one window. All pa
39、rameters in the model can vary: location, orientation, climatic data, room dimensions, window size and properties, target work plane illuminance. For the base case, only the window-to-wall ratio and the orientation will vary in order to isolate the effect of these parameters for a typical office spa
40、ce. A typical double-glazed window (clear glass) is assumed for the base case, according to MNECCB recommendations for Montreal. The window-to-wall ratio is used as a continuous function (0% to 100%) for all orientations. The daylight availability ratio (DAR) is used to quanti the daylighting “effic
41、iency” of the window. DAR is defined as the fraction of time in a year during which sufficient daylight (more than a pre-specified setpoint) is available on the work plane. The calculation of DAR is done as follows. First, the available amount of incident daylight on the faade is computed by hourly
42、simulation (Tzempelikos and Athienitis 2002). This depends on location, orientation, and climate, and it is a function of time. Because the amount of incident daylight depends on the sky conditions, it is appropriate to separate between clear and cloudy days. For this reason, the average number of c
43、lear and cloudy days in a year is calculated based on statistical data and then a separate analysis for clear and overcast days is performed. This could be achieved using correlations of average daily difise fraction of radiation with average monthly clearness index (Klein and Duffie 1978) or using
44、daily average cloud cover data (if available). Then, hourly simulations are performed to find illuminance values incident on a faade, using existing models for clear and over- cast sky conditions (CIE 1973). The transmitted daylight in the room is computed next using effective beam and difise effect
45、ive transmittances of the window. Hourly transmittance values are calculated, taking into account hourly changes in solar incidence angle. Since solar incidence angle can be expressed as a function of day number and solar time for a specific surface, transmittance functions are eventually expressed
46、only as a function of time and day number. Direct and diffuse portions of daylight are treated separately. The total amount of daylight transmitted into the room is equal to the sum of the product of each trans- mittance with the respective amount of incident daylight (Equation 1). G(n,t) = Ed(n,t)
47、rd(n,t) +Eb(n,t) “Cb(n,t) (1) where G = transmitted daylight (lx) Ed = diffuse daylight incident on window (k) Td = window difise transmittance E6 = beam (direct) daylight incident on window (lx) zb = window beam (direct) transmittance n day number in the year, n = O. 365 t = hour index, t = 1 . 24
48、The daylight distribution on the work plane-assumed at 0.8 m height-is calculated next. Under overcast sky, the difise light entering the space is assumed to be distributed uniformly on all room surfaces, and a radiosity-based method is applied to find luminous exitances on all surfaces (Tzem- pelik
49、os and Athienitis 2002). Configuration factors are calcu- lated for a selected grid on the work plane area, and the final work plane illuminance distribution is computed. For clear sky, a division must be made between the direct and the diffuse part of transmitted light. For the difise portion, the same method as for the overcast sky is applied. For direct light, the surface(s) on which it is incident are considered first and their luminous exitances are calculated. Then the method continues as for the overcast sky case. If there are mirror surfaces or specular reflections in the r
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