Development of Window Shading Models in ESP-r

Bartosz LomanowskiSolar Thermal Research LabDepartment of Mechanical EngineeringUniversity of Waterlooblomanow@engmail.uwaterloo.ca

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