Development of Window Shading Models in ESP-r
Bartosz LomanowskiSolar Thermal Research LabDepartment of Mechanical EngineeringUniversity of [email protected]
Background
Center-glass models are well established for multiple, parallel, planar layers
Glazing Layern-1 i+1 i i-1 2
OutdoorSide(i=n)
IndoorSide(i=1)
f i, ( ) b i, ( )
i ( )
Solar-optical calculation [1] Determines transmitted and absorbed amounts of solar radiation
i
In
1 Ii Ii
1 I1
In
1 Ii Ii
1 I1
if , ib,
i
Heat transfer analysis [1] Energy balance performed at each layer Net heat transfer from a layer must equal amount of absorbed
solar radiation
i=2
S1
i=3
S2S3
R1R2R3
Q3 Q2 Q1
T1
T2T3
T4
Solution of energy balance
equations yields: Temperature at each layer Values of heat flux at each
location
Venetian Blind Models
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
P l a n a r , S p e c u l a r G l a z i n g L a y e r
P l a n a r , N o n - S p e c u l a r S h a d i n g L a y e r
2i - 1ii + 1n - 1
I n d o o rS i d e(i = 1 )
O u t d o o rS i d e(i = n )
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
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
Multi-layer Solar Optical Model Algorithm devised to track beam and diffuse components
of solar radiation [7]
1nB
1nD
iD
iD
1iD
i
1iD
iB
iB
1iB
1iB
notation
Allows presence of both specular and diffuse components Shading layer scatters portion
of incident beam solar radiation
Shading layer: beam-to-diffuse conversion
Expanded set of optical properties assigned to each layer Beam-beam Beam-diffuse Diffuse-diffuse
Effective solar optical properties describe shading layer
f,dd
b,dd
dd
dd
b,bd
b,bb
f,bb
f,bd
Incident beamradiation (front)
f,bb
f,bd
b,bb
b,bd
Incident beamradiation (back)
Incident diffuseradiation (front)
Incident diffuseradiation (back)
SUN
Sample analysis results [7]
Beam Fluxes (W/m2)
Diffuse Fluxes (W/m2)
Absorbed Solar Radiation (W/m2)
IndoorSpace
Absorbed Solar Radiation as apercentage of incoming flux
Diffuse Source Terms due toBeam Radiation (W/m2)
SUN600.0 501.8 417.2 7.9
0.48.847.587.4
75 85.7 100.8 128.5
7.7254.4205.3166.5
127.9127.079.986.3
13.0% 12.0% 19.0% 19.0%
0.00.00.00.0 0.0286.1208.3
Gla
ss
Gla
ss
Transmitted Solar Radiation to indoor space as a percentage of incoming flux = 20.0%
Reflected Solar Radiation from the system as a percentage of incoming flux = 38.0%
Ven
etia
n bl
ind
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
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]
ESP-r Development -Venetian Blinds
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
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)
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
Points of discussion
(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
(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
(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 of optical database for single glazing entries Use as input to assemble glazing/shading system
(4) Integration with other developments
SHOCC, DDS, Daylight123 How do these facilities interact with TMCs? Is there possible overlap with proposed venetian blind models?
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
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