lecture objectives:
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Lecture Objectives:
Finish with Solar Radiation and Wind
Define Boundary Conditions at Internal Surfaces
Solar radiation
• Direct • Diffuse• Reflected (diffuse)
Externalsurface
Sky DiffuseDirect Normal
radiation
Reflected
n
Solar Angles
Vertical surface
Normal to verticalsurface
S
E
NSun beam
W
S
z
- Solar azimuth angle– Angle of incidence
Direct and Diffuse Components of Solar Radiation
Window
External wall
Horizontal shading
Ver
tical
sha
ding
Ver
tical
sha
ding
Ashaded
Aunshaded
Global horizontal radiation IGHR
and Diffuse horizontal radiation measurements
)cos( DNRGHRationzontalRadiDifusseHoi III
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
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
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
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
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
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
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
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 _)]}([/)({
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
Surface Balance
Conduction
All radiation components
Convection
Convection + Conduction + Radiation = 0
For each surface – external or internal :
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
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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