Solar Optical Properties of Roller Shades: Modeling Approaches, Measured Results and Impact on Daylighting Performance and Visual Comfort

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1 Solar Optical Properties of Roller Shades: Modeling Approaches, Measured Results and Impact on Daylighting Performance and Visual Comfort Ying Chieh Chan Athanasios Tzempelikos, PhD Brent Protzmman, PhD 1

Introduction High performance commercial

2 Introduction High performance commercial facades rely on fenestration systems to control glare and solar heat gains Studies have shown that roller shades can reduce energy consumption and glare problems associated with fenestration systems through proper design and control 2

3 Introduction Roller shades come in a wide variety of colors and patterns with varying degrees of shading and weave construction Fabric Color Openness Factor Ts Rs As Tv Charcoal /Charcoal 1% White/Linen 1% Steel Grey 4% Oyster 5% Charcoal % Linen/Linen % The term openness factor used by manufacturers refers to the open or see through percentage of the shade (fabric) 3

4 Introduction When solar radiation strikes the shade, it is split into: Beam beam portion (unobstructed, directly transmitted) Beam diffuse portion (interrupted, forward scattering) Reflected portion (interrupted, backward scattering) Absorbed portion Glass Roller Shades Transmittance is a function of the solar incidence angle Manufacturers only provide a single total transmittance value (normal incidence) Need a systematic approach to estimate the off normal and diffuse properties from limited information, for annual thermal and daylighting modeling and product characterization 4

5 Objectives Review and evaluate assumptions and methods for modeling shade solar optical properties Present detailed property measurements for different fabrics and evaluate differences between existing approaches; compare their performance with fullscale experiments Use the best performing models with annual simulation to evaluate the impact of roller shades on daylight performance and glare 5

6 Existing Models Model Reference Required Input Data Simplified nonangular properties model Radiance trans function (Reinhart and Andersen 2006) Beam total transmittance normal incidence Diffuse diffuse transmittance Beam/diffuse ratio (specular transmitted) Semi Empirical Model Ray Tracing Model Geometrical Radiosity Model EnergyPlus (2007) Kotey et al. (2009) Radiance genbsdf (McNeil et al. 2013) Window Software (Carli Inc. 2006) Beam total transmittance normal incidence Beam total transmittance normal incidence Beam beam transmittance normal incidence Detailed geometry of the fabric Geometry of the fabric (spacing and thread thickness) 6

7 Kotey et al. (2009) Semi Empirical Model Beam beam Transmittance Normalized beam beam transmittance τ bb ( ) : norm bb bb( ) b cos ( ) 0 2 bb cutoff cutoff 0.3 Exponent b 0.6cos ( bb 0 ) 2 Cut off angle: 65 (95 65 )(1 cos( 0 cutoff bb )) 2 τ bb ( =0) =Openness factor Beam total Transmittance / Diffuse diffuse Transmittance /2 2 ( )cos( )sin( ) dd bt d 0 7

8 BSDF Ray Tracing Model (McNeil et al. 2013) Angular projection of incident and transmitted hemispheres with direction patches Perforated shades/screens

9 LBNL Geometrical Radiosity Model (Carli Inc. 2006) Direct hemispherical transmittance differences in models (Jonsson et al. 200) 9

10 Detailed Property Measurements Fabric # White Steel Grey Detailed Fabric Colorintegrated Charcoal sphere measurements to measure Oyster shade Charcoal spectral normal Linen /Linen /Silver τ bt (Manufacturer) and off normal solar 1% optical % properties 5% (based on % Collins et al. 11% 20) 23% τ bt (Measured) 6 different 1.3% fabrics.5% with different 5% openness 24.2% factors.9% and colors 22.2% Error 30.0% 5.0% 0.0% 72.9% 0.9% 3.5% Openness Factor 1% 1% 4% 5% % % τ bb (Measured) 1.3% 1.6% 4.2% 6.9% 9.7% 11.2% Error 30.0% 60.0% 5.0% 3.0% 3.0%.0% Total Normal Reflectance Total Normal Transmittance Specular Transmittance %R,T,A Wavelength (nm)

11 Comparison of Semi empirical Model with Measurements of Angular Solar Optical Properties Fabric 3 dark color + silver color, low direct diffuse transmission Measured Solar Optical Properties Semi-empirical Model Measured Solar Optical Properties Semi-empirical Model Transmittance (%) Transmittance % Beam total Transmittance Beam beam Transmittance Incident Angle (deg) Incident Angle (deg) Fabric 4 light color, high direct diffuse transmission Measured Solar Optical Properties Semi-Empirical Model Measured Solar Optical Properties Semi-Empirical Model Transmittance (%) Beam total Transmittance Incident Angle (deg) Transmittance (%) Beam beam Transmittance Incident Angle (deg) 11

Impact of Different Modeling Approaches on Indoor Illuminance Levels and Daylight Glare Evaluation Model + Experiments Weather Data Perez Sky Model Incident Direct and Diffuse Illuminance on Window

12 Impact of Different Modeling Approaches on Indoor Illuminance Levels and Daylight Glare Evaluation Model + Experiments Weather Data Perez Sky Model Incident Direct and Diffuse Illuminance on Window Shading Controller Building Description Occupancy Information Complex Fenestration Model Hybrid Ray Tracing & Radiosity Model Multiple Interreflection of Daylight in Interiors Initial Luminous Exitance from Interior Surfaces Interior Illuminance & Luminance Distribution Glare Model Glare Index Daylight Glare Probability

13 Comparison of Different Modeling Approaches with Full Scale Experiments Work Plane Illuminance Fabric 4 was tested in full scale office spaces Wrokplane Illuminance (lux) Experiments Constant (Fixed Beam/Diffuse Ratio) Cosntant (No Specularity) Measured Solar Optical Properties Semi-Empirical Model /27/13 0:00 /27/13 :00 /2/13 0:00 /2/13 :00 /29/13 0:00 Time Constant/fixed ratio properties models fail to predict illuminance levels accurately The semi empirical model performs well compared to results modeled with measured properties and with full scale experiments 13

14 Comparison between Modeling Methods Using Annual Daylighting Simulation Hybrid ray tracing and radiosity method (Chan, 2013) Fabric 3 work plane illuminance results for winter days Constant (No Specularity) Semi-Empirical Model Constant (Fixed Beam/Diffuse Ratio) Measurement Work Plane Illuminance (lux) /7 1/7 1/ 1/ 1/9 1/9 1/ 1/ 1/11 Time The semi empirical model performs well compared to results modeled with measured properties

15 Comparison between Modeling Methods Using Annual Daylighting Simulation Calculation of Daylight Glare Probability (DGP) throughout the year Annual DGP distribution with Fabric 3 Hour 1 Measurement Days 1 Semi-Empirical Model Days Annual DGP distribution with Fabric 4 Hour 1 Measurement Days 1 Semi-Empirical Model Days 1 1 Constant (Fixed Direct/Diffuse Ratio) Days Constant (Fixed Direct/Diffuse Ratio) Days Constant (No Specularity) Days Constant (No Specularity) Days 15

16 Summary metrics comparison between different modeling methods for Fabrics 3 and 4 Detailed Measurements Semi Empirical Model Constant (Fixed Beam/Diffuse Ratio) Constant (No Specularity) Continuous Daylight Autonomy Fabric 3 Fabric 4 Illuminance RMSE (lux) Annual Time DGP > 0.35 Continuous Daylight Autonomy Illuminance RMSE (lux) Annual Time DGP > #N/A

17 Conclusion Overview and comparison of current approaches for modeling solar optical properties of roller shades Manufacturers data are not always accurate more detailed information is needed The impact of shading properties models on daylighting and glare are significant simplified models are inaccurate Kotey s semi empirical solar optical properties model is quite reliable for incidence angles <

18 Future Work Categorization and classification of shading products according to overall performance and revisions in ASHRAE Handbook/design guides Shading properties and control selection and optimization: o Parameters: control thresholds, openness, transmittance o Objectives: maximize daylight autonomy and minimize glare probability cda S65 1S65 27S DGP95 1

19 Thank you! ACKNOWLEDGMENT Thanks to Lutron Electronics Co Inc for supporting this work, and to Kawneer/Alcoa Inc, Viracon Inc, and PPG Industries for providing the infrastructure for the full scale experiments. Thanks also to Prof. Michael Collins from the University of Waterloo for conducting the integrated sphere measurements. 19

20 Impact of modeling approaches on the effectiveness of advanced shading controls Advanced shading controls can be used to reduce excessive illuminance and protect from direct sunlight and glare (Shen and Tzempelikos 2013) The impact of modeling approaches on the performance of spaces with controlled shades was evaluated Constant Annual DGP differences semi empirical model vs simplified models Hour 1 Semi-Empirical Model (Fixed Direct/Diffuse Ratio) Days Days Annual unobstructed view differences semi empirical model vs simplified models Differences of Shade Closed Percentage Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec20 Time

Window shades: selecting optical properties for visual comfort

Window shades: selecting optical properties for visual comfort Ying-Chieh Chan School of Civil Engineering, Purdue University ychan@purdue.edu Athanasios Tzempelikos School of Civil Engineering, Purdue