Lecture Objectives: Finish with Solar Radiation and Wind Define Boundary Conditions at Internal...
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Transcript of Lecture Objectives: Finish with Solar Radiation and Wind Define Boundary Conditions at Internal...
![Page 1: Lecture Objectives: Finish with Solar Radiation and Wind Define Boundary Conditions at Internal Surfaces.](https://reader030.fdocuments.net/reader030/viewer/2022032704/56649d705503460f94a526aa/html5/thumbnails/1.jpg)
Lecture Objectives:
Finish with Solar Radiation and Wind
Define Boundary Conditions at Internal Surfaces
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Solar radiation
• Direct • Diffuse• Reflected (diffuse)
Externalsurface
Sky DiffuseDirect Normal
radiation
Reflected
n
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Solar Angles
Vertical surface
Normal to verticalsurface
S
E
NSun beam
W
S
z
- Solar azimuth angle– Angle of incidence
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Direct and Diffuse Components of Solar Radiation
Window
External wall
Horizontal shading
Ver
tical
sha
ding
Ver
tical
sha
ding
Ashaded
Aunshaded
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Measurement of Direct Solar Radiation
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Global horizontal radiation IGHR
and Diffuse horizontal radiation measurements
)cos( DNRGHRationzontalRadiDifusseHoi III
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HW1 Problem
8 m 8 m
2.5 m
Internal surfaces
You will need Austin weather data:http://www.caee.utexas.edu/prof/Novoselac/classes/ARE383/handouts.html
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Solar components
cosDNRDIR II
2/)cos1()cos(_ DNRGHRskydif III2/)cos1(_ groundGHRreflecteddif II
reflecteddifskydifdif III __
• Global horizontal radiation IGHR
• Direct normal radiation IDNRDirect component of solar radiation on considered surface:
Diffuse components of solar radiation on considered surface:
Total diffuse solar radiation on considered surface:
z
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m/s 2for U 0.25
m/s 2for U 0.5
U
u
05.03.0 Uu
uh 6.55.3
Velocity at surfaces that are windward:
Velocity at surfaces that are leeward :U -wind velocity
u u
Convection coefficient :
windward leeward)( surfaceair TThAQ
External convective heat fluxPresented model is based on experimental data, Ito (1972)
Primarily forced convection (wind):
surface
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Boundary Conditions at External Surfaces
1. External convective heat flux
Required parameters:- wind velocity- wind direction - surface orientation
U
windward
leeward
Energy Simulation (ES) program treats every surface with different orientation as separate object.
Consequence:
N
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Wind Direction
Wind direction is defined in TMY database:
“Value: 0 – 360o Wind direction in degrees at the hou
indicated. ( N = 0 or 360, E = 90, S = 180,W = 270 ). For calm winds, wind direction equals zero.”
U
windward
leeward
Wind direction: ~225o
N
http://rredc.nrel.gov/solar/pubs/tmy2/http://rredc.nrel.gov/solar/pubs/tmy2/tab3-2.html
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Internal Boundaries
Room
F
C
L R
1
1
11
2
2
22
3
3
33
A air node
internal surface node
external surface node
element-inner node
Co
nve
ctio
n
Rad iati on
Window
TransmittedSolar radiation
Internal sources
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Surface to surface radiation
ψi,j - Radiative heat exchange factor
Exact equations for closed envelope
44,, jiijiiji TTAQ
n
kkikjkjijji FF
1,,,, 1
nji ,...,2,1,
nji ,...,2,1, Closed system of equations
Ti TjFi,j - View factors
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Internal Heat sourcesOccupants, Lighting, Equipment
• Typically - Defined by heat flux – Convective
• Directly affect the air temperature
– Radiative• Radiative heat flux “distributed” to surrounding surfaces
according to the surface area and emissivity
radiationsourceiiiiiisource QAreaSUMAreaQ _)]}([/)({
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Internal Heat sources
• Lighting systems– Source of convective and radiative heat flux – Different complexity for modeling
above structure
lamp surf ace A , T surf
Plamp
qshort_wave
qlong_wave qconvection
P la mp
qsh or t_w a ve
ql on g_ w av eq co n ve ctio n
qsh o rt_w ave
ql on g_ wav e
qco n ve ctio n
Pla m pP la m p
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Surface Balance
Conduction
All radiation components
Convection
Convection + Conduction + Radiation = 0
For each surface – external or internal :
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Air balance - Convection on internal surfaces + Ventilation + Infiltration
h1
Q1
h2
Q2
Affect the air temperature- h, and Q as many as surfaces- maircp.air Tair= Qconvective+ Qventilation
miTs1
Tair
Uniform temperature Assumption
Qconvective= ΣAihi(TSi-Tair)
Qventilation= Σmicp,i(Tsupply-Tair)
Tsupply
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Distribution of transmitted solar radiationDIRECT solar radiation
diffuse reflectionfi rst refle
ct ion
third reflect ion
s econd refle ct ion
di rect s un r adiatio
n
Floor absorpt ion
absorpt ion
abso
rptio
n
diffuse reflection
diff
use
refle
ctio
n
totally absorbed
iiiii ARAAASF 321floorfloorA 1
)()1(2 ,_ iiiFfloorfloorisurfaces FA
.....3 A
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Distribution of transmitted solar radiationdiffuse solar radiation
diffuse sunradiat ion
sec on d re fle ction
absorpt ion
abso
rptio
n
lighting
window
diff
use
emis
sio
n
diffuse reflection
diff
use
refle
ctio
n