1、2009 ASHRAE 803ABSTRACT Shading devices are important design elements of glazed faades to reduce energy consumption of buildings and improve thermal and visual comfort of occupants. Although there has been significant development in the evaluation and modeling of the thermal performance of shading d
2、evices, current methodologies are limited to a few shading products and types. Furthermore, current fenestration thermal models do not account for radiation emission and absorption through-out shading layers, and allurements for energy generation and conversion imbedded in glazing layers. This paper
3、 presents a general methodology to compute the thermal performance of fenestration systems incorporating permeable shading devices and elements for energy generation and conversion. The meth-odology assumes each shading layer as porous with effective radiation and thermal properties. The effective p
4、roperties account for the geometrical and thermal characteristics of the shading layer, and the effect of the convective heat transfer within the layer porous structure. Using the concept of the thermal penetration length, effects of porous shading layers on the convective heat transfer from their b
5、oundary surfaces to the adjacent gas spaces are also accounted for. A validation study is carried out, in which the U-factor of a double-glazed window with between-pane Venetian blinds are compared with the available laboratory measurement. The comparison results show that the model predictions are
6、in good agreement with the measurement.INTRODUCTIONShading devices are important design elements of glazed faades to reduce energy consumption of buildings and improve thermal and visual comfort of occupants. Shadings may be placed between glazing layers, or attached to the inte-rior or exterior faa
7、de surfaces to control natural illumination, solar heat gains, glare, view out, heat loss through facades. In some applications of double skin facades, shading devices are used to manage energy flows to/from buildings. Most popular types of shading devices include slat-type blinds, roller screens an
8、d draperies. Although there have been significant advancement in the performance evaluation of shading devices, predictions of their thermal performance remain a challenge to be addressed due to their complex geometries and effect on the heat transfer mechanisms.In the past decades, there has been s
9、ignificant work devoted to the evaluation of the optical, daylighting and energy performance of shading devices. The ISO standard 15099 (ISO, 2003) presents a validated model to compute the optical and long-wave radiation characteristics of slat-type blinds. The IEA Task 27 (Kohl, 2006) carried out
10、a compre-hensive assessment of the solar optical and thermal perfor-mance of several product types of interior and exterior blinds and roller shades through measurement and computer simu-lation using the ISO-like and simple models. The IEA Task 34/43 (Loutzenhiser et al., 2007) carried out empirical
11、 validations of building-energy simulation tools for daylighting perfor-mance and thermal loads of interior and exterior Venetian blinds and shading screens. Most of the tested simulation tools used simple models for the prediction of the optical and ther-mal performance of the shading devices. Howe
12、ver, models to predict the thermal performance (e.g., U-factor) of fenestration systems incorporating shad-ing devices are at the early stage, particularly those related to convection flows in open gas cavities adjacent to permeable shading layers. The current methodology is based on the Thermal Mod
13、eling of Shading Devices of WindowsA. Laouadi, PhDMember ASHRAEA. Laouadi is a research officer at the Institute for Research in Construction, National Research Council of Canada, Ontario, Canada.LO-09-078 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ash
14、rae.org). Published 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.804 ASHRAE Transactionsthermal-resistance approach developed for
15、simple fenestra-tion systems, made up of essentially thermally opaque glaz-ing (ISO, 2003; Wright, 2008). A one-dimensional conduction heat transfer model is used for glazing layers coupled with radiation and convection models at the layer boundary surfaces. Radiation emission is assumed at the boun
16、dary surfaces of layers. Convection models use the existing correlations for the convective film coefficients in gas cavities or around flat surfaces. Complex shading layers are treated as individual layers with effective radiation prop-erties, but with assumed uniform layer temperature (no ther-mal
17、 resistance of shading layer). These simple models have been implemented in currently available fenestration tools such as WINDOW (LBNL, 2008) and WIS (WinDat, 2008). Recent research showed, however, that these simple models were not accurate for slat-type blinds (Yahoda and Wright, 2004a), and for
18、diathermanous - infrared transparent- layers (Collins and Wright, 2006). Furthermore, these models do not account for elements imbedded in glazing layer for energy generation and conversion, which are getting more popular in todays high performance building designs. Due to the limitations of existin
19、g prediction models of shadings, ASHRAE has sponsored a research project (RP 1311) to develop validated optical and thermal prediction algorithms for several types of shading devices, including slat-type blinds, drapes, roller blinds and insect screens (Kotey et al., 2009a,b). For the purpose of val
20、idation studies, Garnet (1999) and Huang (2005) conducted laboratory measurement using the guarded heater plate apparatus of the U-factor of clear and low-e double-glazed windows with between pane metallic Venetian blinds. They found that the slat-tip-to-glaz-ing spacing and slat angle positions had
21、 significant effect on the U-factor of the window and blind system. The thermal bridging effect of the metallic blinds reached its maximum when the slat angles were horizontal, resulting in a higher U-factor than that of closed slats. Recently, Wright et al. (2008) developed a simplified model to co
22、mpute the film coefficient of a double glazed window cavity with between-pane metallic blinds. The blinds divide the window cavity into two sub-cavi-ties. Wright et al. (2008) used the existing cavity correlations to compute the film coefficients of the sub-cavities based on a modified sub cavity wi
23、dth. The latter, which is larger than the true sub-cavity width (equal to slat tip-to-glazing spacing), was found proportional to the slat width and its cosine angle. The proportionality constant was determined by comparing the model predictions of U-factor with the measurement results of Huang (200
24、5). The proposed model, termed Reduced Slat Length, yielded exceptional results for the window cavity spacings of 17.78 mm and 25.4 mm. However, the model failed to accurately-predict the U-value of the window with the larger cavity spacing of 40 mm. Despite this drawback, the model of Wright et al.
25、 (2008) indicates an important conclusion that existing correlations for cavity film coefficients may be safely used to predict the thermal perfor-mance of window and shade systems without recurring to the time-consuming and computationally intensive CFD simula-tions. Furthermore, the model of Wrigh
26、t et al. (2008) allows more flexibility to address other combinations of shading types and tilted window configurations. CFD computer simulations have also been used for vary-ing purposes: (1) to investigate the flow and temperature patterns in gas cavities between glazing and shading layers, (2) to
27、 validate the simulation results with the measurement, and (3) to develop useful correlations for the convective film coef-ficient in cavities between glazing and shading layers. Lami-nar, two-dimensional flows were generally considered in the investigated work. Convective flows with and without dir
28、ect radiation coupling in windows with internal and between-pane Venetian blinds were addressed by several researchers, includ-ing Ye et al. (1999), Phillips et al. (2001), Collins et al. (2002a,b), Collins (2004), Shahid (2003), Shahid and Naylor (2005), Naylor and Collins (2005), Naylor et al. (20
29、06), Avedissian (2006), and Avedissian and Naylor (2007). The maximum effect of blinds on the heat transfer through the window and blind system was found when the blinds were closed. The obtained simulation results for the temperature and flow patterns compared favorably with the available measureme
30、nt of similar window and blind configurations. Correlations for the average Nusselt number of the window-blind cavity were also developed for various slat angles, and used in thermal-resistance models to compute the U-factor of window and shading systems. The predictions of the U-factor of window us
31、ing such improved models compared generally well with the full CFD simulation.It should be noted that the current advanced empirical and CFD-based thermal models are limited to single or double glazed windows with specific metallic blinds. They do not account for other thermal properties of blinds s
32、uch as thermal conductivity and slat spacing. Furthermore, they cannot be applied to other types of shading devices such as drapes, or to other window tilt configurations.The aim of this paper is to develop a general methodology to compute the thermal performance of fenestration systems incorporatin
33、g shading devices for implementation in fenestra-tion computer programs.OBJECTIVESThe specific objectives are:To revisit the current models for heat transfer mecha-nisms through fenestration systems, and include the peculiarities of shading devices, such as permeability to mass and thermal radiation
34、, and elements for energy generation and conversion.To develop models to compute the effective thermal properties of permeable shading layers.To develop models to compute the convection film coeffi-cients of gas spaces adjacent to permeable shading layers.To validate the methodology by comparing its
35、 predic-tions with the available measurement.ASHRAE Transactions 805THE PROPOSED METHODOLOGYShading devices make a simple glass fenestration system a far more complex system with complex heat transfer mech-anisms to handle. Shading layers may be permeable to gas and thermal radiation, and the result
36、ing cavity spaces may be open so that gas can move from one space to another. Shading layers may also emit or absorb thermal radiation throughout their media, so that local temperature gradients may be reduced compared to thermally opaque layers. Heat transfer may be further complicated if glazing l
37、ayers include elements for heat generation (such as electric thin films to control mois-ture condensation) or energy conversion (such as photovoltaic conversion), which are getting more popular in todays high performance building designs. Current fenestration thermal models (ISO, 2003), which are im
38、plemented in existing fenes-tration design tools such as Window (LBNL, 2008) and WIS (WinDat, 2008), do not account for such effects of complex fenestration systems. This section sheds some light on the heat transfer mechanisms in fenestration systems with shading devices, and develops appropriate m
39、odels to compute their thermal performance.AssumptionsThe following assumptions are considered.Each fenestration layer is assumed solid and porous with calculated effective radiation and thermal proper-ties to account for any convection and radiation effect in a layer medium.Heat transfer through a
40、layer medium is by one-dimen-sional conduction. Layer Heat TransferConsider a multi-layer fenestration system consisting of (N) layers as shown in Figure 1. Layer 1 faces the exterior envi-ronment and layer (N) faces the interior environment. Each layer (j) is surrounded by gaseous spaces at its bou
41、ndary surfaces. The layer exchanges heat with the adjacent environ-ments by convection to the gaseous spaces and radiation to the adjacent layers. The exchanged heat at the layer surface is then transported through the layer medium by conduction, and radiation. By virtue of the foregoing assumptions
42、, the transient energy balance of an elemental control volume at node (i) within a layer (j) is expressed by the following relation (Siegel and Howell, 2002):(1)wherecj,i= effective specific heat at node i of layer j, J/kgK (Btu/lbF)kj,i= effective thermal conductivity at node i of layer j, W/mK (Bt
43、u/hftF)Tj,i= temperature at node i of layer j, K (F)qr,j,i= net radiation flux per unit surface area at node i of layer j, W/m2(Btu/hft2)qsol,j,i= absorbed solar radiation per unit volume at node i of layer j, W/m3 (Btu/hft3)q0,j,i= heat generation per unit volume at node i of layer j W/m3(Btu/hft3)
44、j,i= effective density at node i of layer j, kg/m3(lb/ft3)If the front (facing the exterior environment) and back (facing the interior environment) surfaces of layer j are both subject to beam and diffuse incident solar radiation, the absorbed solar heat per unit volume at node i is expressed as fol
45、lows:(2)with(3)(4)whereqf,beam= beam solar radiation flux density incident on the front surface of fenestration system, W/m2(Btu/hft2)qb,beam= beam solar radiation flux density incident on the back surface of fenestration system, W/m2(Btu/hft2)qf,dif= diffuse solar radiation flux density incident on
46、 the front surface of fenestration system, W/m2(Btu/hft2)qb,dif= diffuse solar radiation flux density incident on the back surface of fenestration system, W/m2(Btu/hft2)SRPV= ratio of photovoltaic surface area to layer surface area, TRf,1:j1= front solar transmittance of layer stack 1 to j 1, TRb,N:
47、j+1= back solar transmittance of layer stack N to j + 1, f,j,i= absorption coefficient per unit length of layer j at node i for the front incident beam solar radiation, m1(ft1)b,j,i= absorption coefficient per unit length of layer j at node i for the back incident beam solar radiation, m1 (ft1)f,d,j
48、,i= absorption coefficient per unit length of layer j at node i for the front incident diffuse solar radiation, m1(ft1)b,d,j,i= absorption coefficient per unit length of layer j at node i for the back incident diffuse solar radiation, m1(ft1)ji,cji,Tji,t-x- kji,Tjx- qrji,qsolji,q0 ji,+=qsolji,qfsolj
49、i,qbsolji,+=qfsolji,fji,qfbeam,fdji,qfdiff, SRPVPV f j i,+=TRf 1:j 1, qfbeam,qfdif,+()qbsolji,bji,qbbeam,bdji,qbdif, SRPVPV b j i,+=TRbN:j 1+, qbbeam,qbdif,+()806 ASHRAE TransactionsPV,f,j,i= photovoltaic cell efficiency of layer j at node i to convert the front incident solar energy into electric energy, PV,b,j,i= photovoltaic cell efficiency of layer j at node i to convert the back incident solar energy into electric energy, The intermediate stack transmittance val
