1、18 2009 ASHRAEThis paper is based on findings resulting from ASHRAE Research Project RP-1311.ABSTRACTShading devices offer a cost saving strategy in dynami-cally controlling solar gain through windows. As such, there is an ongoing effort to accurately quantify the thermal perfor-mance of shading dev
2、ices. In the present study, solar gain through various shading devices attached to a conventional double glazed window was measured using the National Solar Test Facility (NSTF) solar simulator and solar calorimeter. The shading devices include two venetian blinds, a roller blind, a pleated drape an
3、d an insect screen. More specifically, the solar heat gain coefficient (SHGC) and the solar transmit-tance, sys, of each system were measured; and the interior attenuation coefficient (IAC) was calculated from the SHGC measurements. Furthermore, SHGC, sysand IAC were calcu-lated for the same experim
4、ental conditions using models devel-oped for building energy simulation and performance rating. The calculations agreed very well with the measurements. INTRODUCTIONIn buildings with significant cooling loads solar gain is especially troublesome because it is generally the largest and most variable
5、heat gain the building will experience. As a result, window shading attachments that can be used for solar control are drawing attention and a renewed effort is being made to develop models for devices such as venetian blinds, drapes, roller blinds and insect screens (e.g., van Dijk et al. 2002, Ros
6、enfeld et al. 2000, Pfrommer et al. 1996, ISO 2004, Yahoda and Wright 2004, 2005, Kotey et al. 2009a, b, c, d). Window shading attachments also offer the benefit of being operable and many devices such as venetian blinds and roller blinds can be automated. Thus, shading attachments can be used effic
7、iently to admit solar energy when and where heating, and possibly lighting, is required but reject it otherwise. Computer simulation offers a means to evaluate the energy saving performance of shading attachments, their potential to reduce peak cooling loads and the effectiveness of various control
8、strategies. However, until recently, the detailed simulation of shading attachments was routinely neglected. Research is currently geared toward the modeling of shading attachments for building energy simulation but these models are also useful for design and rating. Such an effort has led to the de
9、velopment of various models for complex fenestrations systems (i.e., systems containing glazing and shading layers) in building energy simulation and performance rating soft-ware like ParaSol v3.0 (Hellstrom et al. 2007), EnergyPlus 2007, WINDOW 6.1/THERM 6.1 (Mitchell et al. 2006) and WIS (van Dijk
10、 et al. 2002). However, some of the models in the aforementioned software are either limited in their capabilities or not general enough to handle certain combinations of glaz-ing/shading layers.To expand the scope of shading attachment modeling to include more common devices, an ASHRAE Research Pro
11、ject 1311-RP (Wright et al. 2009, Barnaby et al. 2009) was under-taken. This research project has led to the development of fenestration shading models designated ASHWAT (ASHRAE Window ATtachment). ASHWAT models are currently imple-mented in an enhanced version of the ASHRAE Loads Toolkit (Barnaby e
12、t al. 2004, Pedersen et al. 2001).The ASHWAT models were developed for four specific types of window attachments: drapes, venetian blinds, roller blinds and insect screens. There are significant differences Solar Gain through Windows with Shading Devices: Simulation Versus MeasurementNathan A. Kotey
13、 John L. Wright, PhD, PEngStudent Member ASHRAE Member ASHRAECharles S. Barnaby Michael R. Collins, PhD, PEngMember ASHRAE Associate Member ASHRAENathan A. Kotey is a PhD student, John L. Wright is a professor, and Michael R. Collins is an associate professor in the Department of Mechanical and Mech
14、atronics Engineering, University of Waterloo, Waterloo, Ontario, Canada. Charles S. Barnaby is the vice-president of research at Wrightsoft Corporation, Lexington, MA.LO-09-002 (RP-1311) 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published
15、 in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 19between these categories and any one of these categories repre
16、sents a very large variety of products. In order to retain generality and practicality while striking a balance between complexity and computational speed a simplified approach was taken regarding the way in which radiation interacts with a shading layer. Two points are worth mentioning.Shading laye
17、rs are characterized by making the assump-tion that each layer, whether homogeneous or not, can be represented by an equivalent homogenous layer that is assigned spatially-averaged “effective“ optical proper-ties. This approach has been used in a number of studies (e.g., Parmelee and Aubele 1952, Fa
18、rber et al. 1963, Rheault and Bilgen 1989, Pfrommer et al. 1996, Rosen-feld et al. 2000, Yahoda and Wright 2004, 2005) and has been shown to provide accurate characterization of venetian blinds (e.g., Huang et al. 2006, Wright et al. 2008, Kotey et al. 2008a).Some portion of the incident solar radia
19、tion passes undisturbed through openings in a shading layer and the remaining portion is intercepted by the structure of the layer. The structure may consist of yarn, slats, or some other material. The portion of the intercepted radiation that is not absorbed will be scattered and will leave the lay
20、er as an apparent reflection or transmission. These scattered components are assumed to be uniformly dif-fuse. In addition, a shading layer will generally transmit longwave radiation (i.e., it is diathermanous), by virtue of its openness, and effective longwave properties are assigned accordingly. U
21、sing effective optical properties and a beam/diffuse split of solar radiation at each layer, the framework used to represent multi-layer systems provides virtually unlimited freedom to consider different types of shading layers. This framework also delivers the computational speed needed in the cont
22、ext of build-ing energy simulation (Wright et al. 2009, Barnaby et al 2009).To evaluate and validate the ASHWAT models, solar gain through various shading devices attached to a conventional double glazed (CDG) window was measured using the National Solar Test Facility (NSTF) solar simulator and sola
23、r calorimeter. Performance parameters including solar heat gain coefficient (SHGC), interior attenuation coefficient (IAC) and solar transmittance, sys, were obtained for a conventional double glazed (CDG) window as well as various CDG/shading layer combinations. The shading devices include dark and
24、 light coloured venetian blinds, a medium coloured roller blind, a medium coloured drape, and a dark coloured fibreglass insect screen. Performance parameters were also obtained for the same conditions using the ASHRAE Toolkit simulations that incorporate ASHWAT models. PERFORMANCE PARAMETERSWhen so
25、lar radiation is incident on a fenestration system a portion will be directly transmitted to the indoor space while other portions are absorbed by the individual layers, some of which is redirected to the indoor space by heat transfer. For a given fenestration system, the solar gain is characterised
26、 by the SHGC which is the ratio of the solar gain to the solar irra-diance. In a multi-layer fenestration system consisting of n layers, the SHGC can be expressed as(1)where sysis the solar transmittance; Aiand Niare respectively the absorbed portion of incident solar radiation and the inward flowin
27、g fraction of the absorbed solar radiation in the ithlayer.A shading attachment will generally reduce solar gain and this effect may be conveniently represented by the IAC. (2)where SHGCcfsand SHGCgare SHGC values for the shaded and unshaded glazing system, respectively. Historically, IAC has been p
28、resented as a constant depending only on glazing and shade properties (e.g., ASHRAE 2005). However, IAC also depends on solar incidence angle, especially for shades having non-uniform geometry (e.g., venetian blinds, pleated drapes). The IAC is an important parameter since it is required to determin
29、e solar gain using cooling load calculation proce-dures such as ASHRAEs Radiant Time Series (RTS) method.MEASUREMENTSFacilityThe experiments were performed using the NSTF solar simulator and solar calorimeter. This measurement facility is capable of measuring the SHGC and the U-factor of a full scal
30、e window with or without shading layers. Figure 1 is a sche-matic of the measurement apparatus. Measurements can be carried out under a variety of imposed weather conditions using a solar simulator arc-lamp source and a solar calorimeter positioned in a large environmentally-controlled chamber. The
31、lamp, in combination with an optical reflector system, provides a uniform irradiance over the test area with a spectral irradiance distribution that approximates the ASTM AM1.5 solar spectrum (ASTM E891-87 1987). The intensity of the incident flux at the test section can be adjusted from 100 to 1100
32、 W/m2(32 to 350 Btu/ft2h). The angle of incidence can be varied from 0 to 30 above the horizontal. The calorimeter consists of an outer and an inner cell with an absorber plate within the inner cell. The outer cell is designed to provide a stable temperature environment for the inner cell while the
33、absorber plate adds or removes heat from the inner cell. The amount of heat entering or leaving the inner cell can be accurately measured using the heat exchanger loop connected to the absorber plate. The environmental chamber can be maintained at temper-atures ranging from -20 to +50C (-4 to 122F).
34、 The SHGC sysNii 1=nAi+=IACSHGCcfsSHGCg-=20 ASHRAE Transactionstemperature set point in the chamber can be maintained within 1C (1.8F). A variable speed fan incorporated in the cham-bers air-circulating system provides wind with speeds rang-ing from 0.5 to 4.0 m/s (1.6 to 13 ft/s). The wind directio
35、n is normal to the plane of the test sample. A detailed description of the theory and the operating principles of the NSTF solar simulator and solar calorimeter can be found in several refer-ences (e.g., Harrison and Dubrous 1990, CANMET 1993, van Wonderen 1995).ProcedureMeasurements were taken usin
36、g the window in combina-tion with various shading devices. The test method is similar to the method prescribed by CSA A440.2-98 (1998). First, the window was mounted in the mask wall of the calorimeter test cell. The test cell was then placed inside the environmental chamber with the mask wall facin
37、g the solar simulator. Test conditions including solar irradiance, Ginc, indoor air temperature, Tin, and outdoor air temperature, Toutwere maintained at steady state while the net energy transfer through the window, Qnet, was measured. During each test a still air condition was maintained on the in
38、door side of the window with a small fan mounted near the top of the inner cell to eliminate stratification. Wind, with a steady speed of 2.9 m/s (9.5 ft/s) perpendicular to the window, was mechan-ically maintained at the outdoor side of the window. The experiment was carried out with solar irradian
39、ce at normal incidence. In subsequent experiments, shading devices were attached to the window and the test was repeated. Table 1 shows a summary of glazing/shading system test combina-tions and associated test conditions. Estimating the Surface Convection Heat Transfer Coefficients Previous experim
40、ents under similar convection condi-tions using a Calibration Test Standard (CTS) gave a total (i.e., including both convection and radiation) indoor surface heat transfer coefficient of htot,in= 9.6 1.9 W/m2K (1.7 0.3 Btu/ft2hF) and a total outdoor surface coefficient of htot,out= 16.5 5.3 W/m2K (2
41、.9 0.9 Btu/ft2hF) (van Wonderen 1995). Given the indoor surface temperature of the CTS (glass), Tg,in, and the indoor mean radiant temperature, Tin(assumed equal to the air temperature), the indoor radiative heat transfer coefficient, hr,in, was estimated by treating the window as a small object in
42、a large enclosure. See Equation 3.(3)(a) (b)Figure 1 Schematic of measurement apparatus: (a) SI units and (b) I-P units.hrin,glass Tgin,2Tin2+()Tgin,Tin+()=ASHRAE Transactions 21where is the Stefan-Boltzmann constant and glass= 0.84 is the emissivity of glass. The outdoor radiative heat transfer coe
43、fficient, hr,out, was estimated in a similar manner.(4)In this case Tg,outand Toutare respectively the outdoor surface temperature of the CTS glazing and the outdoor mean radiant temperature (again assumed equal to the air tempera-ture). The temperatures obtained during calibration, i.e., Tin, Tg,in
44、, Tg,outand Tout, are listed in Table 2. Since the surface coefficient, htot, is of sum of the radiative and the convective components, the values of hc,inand hc,outwere estimated using Equations 5 and 6 (5)(6)giving hc,in= 4.6 W/m2K (0.8 Btu/ft2hF) and hc,out= 10.0 W/m2K (1.8 Btu/ft2hF). These conv
45、ective heat trans-fer coefficients were needed as input data for the ASHWAT simulation models.Test SamplesThe window used in this study was a pre-fabricated insu-lated glazing unit (IGU) mounted in a fixed wooden frame. The shading devices that were attached to the window include commercially availa
46、ble insect screen, pleated drape, venetian blinds and roller blind. The distance between glazing/shading layers is given in Table 3. The window and shading devices are described below and detail is also provided in Table 4. Insulated Glazing Unit and Frame. The air-filled IGU consists of two 3 mm (0
47、.12 in.) layers of clear glass separated by a commercially produced edge seal comprising foam spacer and butyl sealant to give an air gap of 12.7 mm (0.5 in.). The IGU was mounted in a wooden frame (unpainted pine). The frame design enabled easy attachment of shading devices. Figure 2 shows a cross-
48、section of the window and the mount-ing details in the mask wall of the solar calorimeter.The projected area of the window was divided into three sub-areas: the centre-glass area, Acg, the edge-glass area, Aeg, and the frame area, Afr. The centre-glass area is defined as that part of the view area m
49、ore than 63.5 mm (2.5 in.) from the sight line (e.g., CSA A440.2-98 1998, ASHRAE 2005) and the edge-glass area consists of the remaining part of the view area. The frame area consists of the portion lying outside the sight line. Figure 2 also shows the sub-areas of the window. The total projected window area, Aw, is the sum of Acg, Aegand Afrwhile the total glass area (view area), Ag, is sum of Acgand Aeg. The dimensions of Awwere 1665 1665 mm (65.6 65.6 in.) and the dimensions of Agwere 1590 1590 mm (62.6 62.6 in.). In
