Development of Window Shading Models in ESP-r

1. Development of Window Shading Models in ESP-r Bartosz Lomanowski Solar Thermal Research Lab Department of Mechanical Engineering University of Waterloo blomanow@engmail.uwaterloo.ca

2. Background

3. Outdoor Side (i=n) ¼ ¼ Indoor Side (i=1) n-1 i+1 i i-1 2 Glazing Layer • Center-glass models are well established for multiple, parallel, planar layers

4. Solar-optical calculation [1] • Determines transmitted and absorbed amounts of solar radiation ¼ ¼ i

5. i=2 i=3 • Heat transfer analysis [1] • Energy balance performed at each layer • Net heat transfer from a layer must equal amount of absorbed solar radiation • Solution of energy balance equations yields: • Temperature at each layer • Values of heat flux at each location

6. Venetian Blind Models

7. Model expansion to include venetian blind layer • Venetian blinds influence both solar and longwave radiation • Blind layer is assigned spatially averaged or “effective” properties • Treated as homogenous planar layer

8. Calculation of effective longwave properties of blind layer [2] • Models for longwave emittance, reflactance and transmittance were developed • Calculation of effective solar-optical properties of blind layer [3] • Models for beam-to-beam, beam-to-diffuse reflectance and transmittance were developed

9. Convection model of a between-the-glass venetian blind • Correlation development [4,5,6] • Models show good agreement with experiments. • Models developed based on GHP (Guarded Heater Plate) measurements and CFD simulations

10. Multi-layer Solar Optical Model • Algorithm devised to track beam and diffuse components of solar radiation [7] i notation

11. Allows presence of both specular and diffuse components • Shading layer scatters portion of incident beam solar radiation Shading layer: beam-to-diffuse conversion

12. Expanded set of optical properties assigned to each layer • Beam-beam • Beam-diffuse • Diffuse-diffuse • Effective solar optical properties describe shading layer

13. Sample analysis results [7]

14. Heat Transfer Analysis • Capabilities: • Solve for complex resistance network • Any combination of: • radiative and convective resistors • indoor/outdoor air and mean radiant temperature • diathermanous layers • Theory and code for solver and indices of merit (e.g., U-value,SHGC) developed and running

15. Ongoing research – venetian blinds • Indoor venetian blind convection models • Currently, indoor/outdoor shade convection models are crude • Anticipated that uncertainty of these convection models has little bearing on the inward flowing fraction, which itself is often a small component of solar gain • Anticipated that uncertainty of these convection models has little bearing on the convective/radiative split • None the less, better models are under development (CFD, interferometry) [e.g. 8,9]

16. ESP-r Development -Venetian Blinds

17. ESP-r: Transparent Multilayer Constructions (TMCs) • Current treatment of glazing systems: • Based on optical properties database • Blind descriptions not supported • Describes entire system transmittance and reflectance • TMC control based on replacing sets of optical properties • Convective/radiative heat transfer through air gaps of TMC assigned constant R value • Angle dependency • Glazings are rotationally symmetric – i.e. optical properties are function of incidence angle • Optical database – properties listed at 5 incidence angles

18. Proposed ESP-r Development – Venetian Blinds • Advanced Glazing Systems facility • User specified glazing-shading system • Indoor, between-the-glass, outdoor venetian blind • Eventually add definitions for common configurations • User specified venetian blind geometry (slat width, angle, spacing, solar reflectivity, emissivity) • “system” optical properties • Calculated at simulation time-step within ESP-r • More accurate treatment of air-gaps (fill gases, convective/radiative heat transfer) • Introduce profile angle dependency for venetian blinds (slat type blinds are rotationally asymmetric)

19. Proposed ESP-r Development – Venetian Blinds • Benefits • Flexibility of varying blind geometry within ESP-r • More accurate treatment of heat transfer problem • Foundation for more sophisticated blind control • Lay groundwork for more accurate, dynamic treatment of shading systems • Implementation Scope • Implementation of solar and thermal models for venetian blind systems only • Blind operation and control not part of the current scope

20. Points of discussion

21. (1) Input file for glazing/shading systems • TMCs - described by optical properties sets and replacement sets for blind control • Held in zone tmc file • Proposed “Advanced Glazing System” - described by individual glazing optical properties and blind properties • System optical properties and layer absorptance calculated at simulation time-step • Require new input file to store glazing/shading system properties

22. (2)Treatment of indoor/outdoor venetian blinds • Longwave exchanges • Is blind lumped within the transparent construction or is it an explicit surface within the zone? • Could lump the blind and flag the blind surface as “longwave transparent” • Then calculate longwave exchanges (hrad) for both blind and innermost glazing surface

23. (3) Is this a replacement of externally generated optical property sets? • Currently, can use WINDOW 4 to import optical property data sets • Optical database does not support profile angle dependency • Required input for multi-layer solar optical model • Reflectance and transmittance – for EACH glazing layer • Blind slat optical properties • Could make use ofoptical database for single glazing entries • Use as input to assemble glazing/shading system

24. (4) Integration with other developments • SHOCC, DDS, Daylight123 • How do these facilities interact with TMCs? • Is there possible overlap with proposed venetian blind models?

25. References [1] Wright, J.L.; 1998, “Calculating Center-Glass Performance Indices of Windows”, ASHRAE Transactions, Vol. 104, Part 1. [2] Yahoda, D.S., and Wright, J.L., 2004, “Methods for Calculating the Effective Longwave Radiative Properties of a Venetian Blind Layer”, ASHRAE Transactions, 110, Part 1. [3] Yahoda, D.S., and Wright, J.L., 2005, “Methods for Calculating the Effective Solar-Optical Properties of a Venetian Blind Layer”, ASHRAE Transactions, vol. 111, Part 1. [4] Tasnim, S.H., Collins, M.R., Wright, J.L., “Determination of Convective Heat Transfer for Glazing Systems with Between-the-Glass Louvered Shades”, in review, International Journal of Heat and Mass Transfer [5] Tasnim, S.H., Collins, M.R., Wright, J.L., “Numerical Analysis of Convective Heat Transfer in Fenestration with Between-the-Glass Louvered Shades”, in review, International Journal of Heat and Mass Transfer [6] Wright, J.L., Huang, N.Y.T., Collins, M.R., “Thermal Resistance of a Window with an Enclosed Venetian Blind: A Simplified Model”, in review [7] Wright, J.L., and Kotey, N.A., 2006, “Solar Absorption by Each Element in a Glazing/Shading Layer Array”, ASHRAE Transactions, vol. 112, Part 2. [8] Roeleveld, D., Naylor, D., Oosthuizen, P.H., “Empirical Correlation for Free Convection in an Isothermal Asymmetrically Heated Vertical Channel”, in review [9] Collins, M.R.., 2004 "Convective Heat Transfer Coefficients from an Internal Window Surface and Adjacent Sunlit Venetian Blind", Energy and Buildings, Vol 36 (3), pp. 309-318