1、2009 ASHRAE 31This paper is based on findings resulting from ASHRAE Research Project RP-1311.ABSTRACTActivities and findings arising from ASHRAE Research Project 1311-RP are summarized. This project included three main goals, (a) development of models for pleated drapes, venetian blinds, roller blin
2、ds and insect screens, (b) imple-mentation of these models in the ASHRAE Loads Toolkit, and (c) compilation of results suitable for direct application in building cooling load methods such as Radiant Time Series (RTS). The solar and heat transfer interactions present in multilayer systems are compli
3、cated and the corresponding models entail significant complexity. This work produced the ASHRAE Window Attachment (ASHWAT) model that uses a simplified approach to the way in which radiation interacts with each glazing or shading layer. Each layer is assigned spatially-averaged “effective” optical p
4、roperties so that glaz-ing and shading layers can be arranged in any combination. ASHWAT offers wide scope in the design process, the possi-bility of active control (e.g., slat angle adjustment), fast compu-tation, and facilitates the implementation of additional shading layer types. Very few input
5、data are needed to model any layer. Measurement-based validation was undertaken at both the subcomponent level and at the complete system level with documentation in the technical literature. The ASHWAT model has been added to the ASHRAE Loads Toolkit and coupled to the heat-balance room model, supp
6、orting accurate calculation of cooling load impact of fenestration shading. Simplified correlation models were developed to allow shaded fenestration performance estimates via spreadsheet-tractable formulas. The model was also used to generate greatly expanded simplified data for inclusion in Fundam
7、entals and suitable for direct use in widely-used engineering procedures.INTRODUCTIONIt is well understood that buildings account for a large portion of the greenhouse gas production and energy consumption in the developed world. Approximately 25% of this consumption can be attributed to windows. Th
8、e potential for improvement in this sector is enormous. This becomes especially clear when it is recognized that buildings can be more than just energy efficient - they can be designed as net-zero or even net energy producers. Conservation is the key step in a shift to sustainability. Conserved ener
9、gy is the greenest renewable resource.The increased levels of insulation associated with green building design decrease heating loads but augment cooling loads. Well-insulated buildings can easily overheat. Solar gain is especially troublesome because it is often the largest and most variable heat g
10、ain. Fortunately, a properly designed and controlled shading device can be used to admit solar energy when and where heating is required, and reject it otherwise. This paper summarizes ASHRAE research project 1311-RP, “Improving Load Calculations for Fenestration with Shading Devices.” The purposes
11、of this work were to a. develop models for pleated drapes, venetian blinds, roller blinds and insect screens - the ASHRAE Window ATtachment (ASHWAT) models,b. implement the ASHWAT models in the ASHRAE Loads Toolkit and Improving Load Calculations for Fenestration with Shading Devices Charles S. Barn
12、aby John L. Wright, PhD, PEng Michael R. Collins, PhD, PEngMember ASHRAE Member ASHRAE Associate Member ASHRAECharles Barnaby is the vice-president of research at Wrightsoft Corporation, Lexington, MA. John Wright is a professor and Michael Collins is an associate professor in the Department of Mech
13、anical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada. LO-09-003 (RP-1311) 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Addition
14、al reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.32 ASHRAE Transactionsc. compile results suitable for simplified building analysis (e.g., the Radiant Time Series (RTS) method) and for rat-ing the performance of
15、various shading devices. These goals have been achieved. The most visible evidence is in the Indoor Attenuation Coefficient (IAC) tables prepared for the 2009 ASHRAE Handbook - Fundamentals. However, the underlying research has generated benefits well beyond the original intent of the project - acti
16、ng as a catalyst leading to new and more general ways of structuring the prob-lem, new ways of making measurements, new data, new corre-lations, new ways to characterize components, new ways to assess the performance indices of multi-layer systems and new insight regarding the way in which the analy
17、sis of shaded windows can be efficiently coupled with heat-balance cooling load calculations and building energy simulation. This new information is well documented in the technical literature, including more than a dozen ASHRAE Transactions papers. A more thorough summary of the 1311-RP work can be
18、 found in (Wright et al. 2009). THE ASHWAT MODELSTo 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. Shading layers are represented by an equival
19、ent homog-enous layer that is assigned spatially-averaged “effec-tive“ optical properties. This approach has been used in a number of studies (e.g., Parmelee and Aubele 1952, Farber et al. 1963, Rheault and Bilgen 1989, Pfrommer et al. 1996, Rosenfeld et al. 2000, Yahoda and Wright 2004b, 2005) and
20、has been shown to provide accurate characterization of venetian blinds (e.g., Huang et al. 2006, Wright et al. 2008, Kotey et al. 2008b). Some portion of the incident solar radiation passes undisturbed through openings in a shading layer and the remaining portion is intercepted by the structure - ya
21、rn, slats, or some other material. The portion of the inter-cepted radiation that is not absorbed is scattered and leaves the layer as an apparent reflection or transmission and these components are assumed to be uniformly dif-fuse. In addition, a shading layer will generally transmit longwave radia
22、tion (i.e., it is diathermanous) by virtue of its openness, and effective longwave properties are assigned accordingly.Using effective optical properties and a beam/diffuse split of solar radiation, this framework provides freedom to consider many types of shading layers.The ASHWAT models require ve
23、ry little input data because subcomponent models are used to calculate effective layer properties instead of relying on empirical information about the entire layer. For example, the effective solar optical properties of a venetian blind can be calculated as a function of slat geometry plus the sola
24、r and longwave properties of the slats. Effective properties of a pleated drape are calculated as a function of various fabric properties and a specified value of fullness. Methods to obtain convective heat transfer coefficients for glazing cavities are well established. The convection coeffi-cients
25、 for exposed glazing/shading layer surfaces cannot be predicted with the same accuracy. In the ASHWAT code these coefficients must be specified by the room model of the build-ing simulation program. This approach provides the opportu-nity to differentiate between natural and forced convection and pe
26、rhaps between different types of forced convection caused by different types of diffusers. Established values are available in the limiting cases where the shading layer is spaced well away from the window or where the spacing approaches zero. The method for specifying convection coefficients at an
27、inter-mediate spacing is presented in (Wright et al. 2009) and Kotey et al. (2009b) mention this is a possible area of future research. THE MULTI-LAYER ANALYSISStructureEach glazing/shading layer system is treated as a series of parallel layers separated by gaps (Wright 2008, Wright et al. 2009). Se
28、e Figure 1. First, the flux of absorbed solar radiation at each layer, Si, is determined. Second, an energy balance is applied at each layer in order to obtain the set of layer temper-atures, Ti , and the corresponding heat flux values. The long-wave radiant exchange algorithm is noteworthy because
29、it allows for the possibility of diathermanous layers and because the mean radiant temperature can differ from the air temper-ature on the indoor and/or outdoor side.Figure 1 Glazing/shading multi-layer analysis structure.ASHRAE Transactions 33Solar AnalysisAn algorithm has been devised, extending t
30、he work of Edwards (1977) by which beam and diffuse components of solar radiation can be tracked as they interact with a multi-layer system of glazing and/or shading layers (Wright and Kotey 2006). The method is sufficiently general to include beam and diffuse insolation on the outdoor side as well
31、as diffuse insolation on the indoor side.Heat BalanceAn energy balance is applied at each layer to obtain layer temperatures, radiosities and convective heat transfer rates. The known heat transfer coefficients are then used to construct a resistance network making it possible to calculate U-factor
32、and SHGC for a system that includes one or more diatherma-nous layers. This hybrid calculation provides the opportunity to calculate U-factor, SHGC (Wright 2008, Collins and Wright 2006) and IAC under many different environmental conditions.Glazing cavities are treated as sealed enclosures and the a
33、ssociated convective heat transfer coefficients are calculated using the correlation of (Wright 1996). Airflow between the first two layers, on the indoor side and/or the outdoor side, can be modeled in order to deal with unsealed shading attach-ments. Details regarding the way in which unsealed gap
34、s are included in the heat balance can be found in (Wright 1986). The models for a glazing cavity with an enclosed venetian blind are documented in (Huang et al. 2006, Wright et al. 2008, Yahoda et al. 2004a).Overview of the Layer Models and Input RequirementsInput data needed for each glazing layer
35、 include three solar properties (transmittance, front/back reflectance), eval-uated at normal incidence, plus three longwave properties (transmittance, front/back emissivity). These data are readily available (e.g., IGDB 2008). Off-normal solar properties of glazing layers are estimated according to
36、 the behavior of an uncoated reference glass (Wright et al. 2009).The beam/diffuse characterization of solar radiation necessitates an expanded set of solar optical properties for shading layers (Wright and Kotey 2006). A portion of incident beam radiation will leave the layer without being scattere
37、d. The properties associated with this unscattered portion are called beam-beam properties. Beam-diffuse properties are needed to describe the scattered components of beam insola-tion. Despite this added complexity, the only input data needed to characterize drapery fabric, roller blinds and insect
38、screens, including off-normal properties, are openness, total solar transmittance and total solar reflectance at normal incidence. These three properties are routinely used to specify drapery fabric (e.g., ASHRAE 2005, Keyes 1967) and can be measured with inexpensive instrumentation. ASHWAT models c
39、alculate the corresponding effective solar properties of pleated drapes and venetian blinds using information about geometry and the solar properties of the slats or fabric. The off-normal beam-beam and beam-diffuse properties of drapery fabric are used in the pleated drape model. Venetian blind sla
40、ts are assumed to reflect and transmit in a purely diffuse manner. The ASHWAT code includes integration routines to obtain diffuse-diffuse properties of materials and layers.A method has been devised to estimate longwave proper-ties of drapery fabric, roller blind material and common insect screens
41、knowing only the openness of the material (Kotey et al. 2008a). These material properties are converted to effective longwave properties for pleated drapes (using the same net radiation balance used to obtain diffuse solar properties (Kotey et al. 2009a) and venetian blinds (Yahoda and Wright 2004a,
42、 b). Gaps can be specified as sealed or vented. Any gap thick-ness can be used. Any fill gas can be specified as long as prop-erty data (molecular mass, viscosity, specific heat and thermal conductivity) are available. These data are readily available for gases including air, argon, krypton and xeno
43、n. Properties can be calculated for fill gas mixtures (e.g., Rohsenow and Hartnett 1973).Shading Layer ResearchA new measurement technique was developed for this study. Special sample holders, small tubes with their end open-ings cut at various angles, were designed for use in the inte-grating spher
44、e of a commercially produced spectrophotometer. These sample holders provide the unique capability of measuring beam and diffuse components of solar transmission and reflection with respect to beam radiation at various incidence angles. Semi-empirical models were formu-lated to evaluate the off-norm
45、al properties of drape, roller blind and insect screen materials (Kotey et al. 2009c, d, e). Effective layer properties of venetian blinds (Yahoda and Wright 2005, Kotey et al. 2008b) and the effect of pleating in drapes (Kotey et al. 2009a) are evaluated using a more funda-mental net radiation sche
46、me.The models formulated for the off-normal solar proper-ties of drape, roller blind and insect screen materials are based on measurements using samples with front/back symmetry. Drapery materials and roller blinds are generally symmetric. Insect screens are always symmetric. Exceptions include line
47、d drapes and blackout roller blinds but these have zero openness (i.e., no beam-beam transmission) and little or no diffuse transmission. Under this circumstance the models for solar transmission become trivial and the models for reflection can be applied equally well to the individual sides of the
48、fabric or roller blind material.Energy Performance IndicesThe ASHWAT heat balance includes a provision to calcu-late indices of merit for the multi-layer system. These include U-factor and SHGC. The code is based on the theory developed in (Wright 2008) and provides all of the generality of that the
49、ory except for two restrictions: (1) SHGC cannot be calculated for 34 ASHRAE Transactionssituations with zero solar radiation and (2) even though the system can include any combination of glazing and shading layers, indices of merit cannot be calculated for systems with more than one consecutive diathermanous layer.Two additional indices of merit are calculated by the ASHWAT code, Fr,inand Fr,out. The development of these parameters is also given in (Wright 2008). The values of Fr,inand Fr,outgive a measure of the relative strength of radiative heat transfer, with respect to the total, bet
