Example of thermal radiation spectra for black and gray
surfaces at room temperature
The CMB has a thermal black body spectrum at a temperature of 2.72548±0.00057 K.[4]
(2.72 oK = 4.90 o R)
lmax = 5216 microns - oR
4.90 oR
lmax =1064.49 microns
l
max
4. The
= 5216 microns
T(o R)
Temperature of the Cosmic Microwave
Background, Fixsen, D. J. ,The Astrophysical
Journal, Volume 707, Issue 2, pp. 916-920
(2009)
CMB*
1,000,000 microns
10,000 microns
10 microns
micron
0.01 micron
*
0.0001 micron
0.000001 micron
Penzias story by Arthur Schawlow
The invention of the laser, which stands for light amplification by stimulated
emission of radiation, can be dated to 1958 with the publication of the scientific
paper, Infrared and Optical Masers, by Arthur L. Schawlow, then a Bell Labs
researcher, and Charles H. Townes, a consultant to Bell Labs.
3/23/2016
2
Near-infrared
NIR, IR-A DIN
0.75–1.4 µm
0.9–1.7 eV
Defined by the water absorption, and commonly used in fiber optic telecommunication
because of low attenuation losses in the SiO2 glass (silica) medium. Image intensifiers are
sensitive to this area of the spectrum. Examples include night vision devices such as night
vision goggles.
Short-wavelength
infrared
SWIR, IR-B DIN
1.4-3 µm
0.4–0.9 eV
Water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the
dominant spectral region for long-distance telecommunications.
Mid-wavelength
infrared
MWIR, IR-C DIN;
MidIR.[7] Also called
3–8 µm
intermediate
infrared (IIR)
150–400 meV
In guided missile technology the 3–5 µm portion of this band is the atmospheric window in
which the homing heads of passive IR 'heat seeking' missiles are designed to work, homing
on to the Infrared signature of the target aircraft, typically the jet engine exhaust plume.
This region is known as thermal infrared, but it detects only temperatures somewhat
above body temperature.
Long-wavelength
infrared
LWIR, IR-C DIN
8–15 µm
80–150 meV
The "thermal imaging" region, in which sensors can obtain a completely passive image of
objects only slightly higher in temperature than room temperature, (for example, the
human body), based on thermal emissions only and requiring no illumination such as the
sun, moon, or infrared illuminator. Forward-looking infrared (FLIR) systems use this area of
the spectrum. This region is also called the "thermal infrared."
Far infrared
FIR
15–1,000 µm
1.2–80 meV
(see also far-infrared laser and far infrared).
Microwave
1 mm – 1 meter
300 GHz – 300 MHz
1.24 meV – 1.24 µeV
Extraterrestrial daily horizontal irradiation as function of
day of year, for north latitudes
It is measured by satellite to be roughly 1366 watts per square meter,[2]
though it fluctuates by about 6.9% during a year - from 1412 W/m2 in early
January to 1321 W/m2 in early July, due to the earth's varying distance
from the sun.
Watts/m2 reach earth is considerably less and nearer to 1000 watts/m2
*m = number of air masses light must traverse
Clear day irradiance as function of zenith angle, under midlatitude conditions
for visibilities of 5 km and 23 km at January 21 and July 21 for northern hemisphere
Normal
I dir
Diffuse
I diff
ASHRAE Clear Sky Model
G t = G ND + G d + G R = ID + Id + IR
ID =G ND =
A *CN
where G ND = normal direct radiation
exp (B/ sin B )
A = apparent solar irradiation at
zero air mass
B = atmospheric solar extinction
coefficient
Beta = solar altitude
C N = clearness number
ID= G D = G ND * cos (q ) where theta is angle between sun’s rays
and the normal to the surface
CLEAR SKY RADIATION ( H.C. Hottel)
I dir = Io[a o = a1* exp( -k/cos q s)], Io = extraterrestial irradiance
Liu and Jordan Diffuse Radiation on a Horizontal Surface
I diff = (0.271*Io – 0.239 I dir)* ( cos qs )
Correlation between diffuse irradiance and global irradiance
(hourly averages, on horizontal) and hourly clearness index
Hourly Clearness Index :
kT=
Iglo
IoCos(q) zenith
KT = Daily Equivalent of k T
I diff = 1.00 – 0.09kT ,
I glo
0<k T<0.22
I diff = 0.09511-0.0164kT+4.388kT2 –16.638kT3+12.336kT4
I glo
I diff = 0.165 ,
I glo
0.80 <k T
0.22<KT <0.80
Monthly average daily total irradiation on vertical surfaces in
January, April and July, as function of monthly average clearness
index and latitude
Importance of thermal diffusivity of curtain wall.
External
Or
Internal Surface ??
Absorbed
solar
radiation
What if I lived in a tin shack in “the mission?”
3/23/2016
10
U roof layer = 0.110 BTU/hr-ft 2 -F
N
N = 16 floors above 1st floor
13’
U curtain wall = 0.180 BTU/hr-ft 2 F
25
U window installation = 0.60 BTU/hr-ft 2 F
’
6’
8
’
4
’
U door = 0.200 BTU/hr-ft 2 F
70
50
‘
Radiant Heat and Convective Transfer
A Simplification of the Complex, External Surface Boundary Condition
Radiant Energy must first be absorbed by surfaces that enclose the space and the
objects in the space. When these objects and surfaces become warmer than the
surrounding air, some of their heat transfers to the air by convective heat transfer.
Itotal(t) = Idir(t)+Idiff(t)+Iref(t)
Sol-Air Temperature Concept for Opaque Surfaces
q/A = hconductance* (T sol air
surface
– T es)
= hconductance * (T oa – T es ) + aI sin(q) – e*cos(q)*D(IR)long
T sol air surface = Toa + a(I/hcond ) – (e/hcond)*cos(q)*D(IR)long …..Sol-Air Temp. to simplify wall heat transfer B.C.
Tsi (t)
T sol air temperature = Toa + aI sin(q) - e*cos(q)*DIR;
ho
T air (t)
Normally, D(IR)lw = 21 BTU/hr-ft2
ho
Resulting in the temperature effects for various e and ho
DT = 7 o F for e = 0.9 and ho = 3 BTU/hr ft2F , i.e., a/ho = 0.30 for dark colored surfaces)
If a/ho = 0.15 ( light colored surfaces) , then DT = 3 – 4
o
F
What is relationship between Io(t) and cooling load (t), T air(t)?
3/23/2016
12
40 o N Latitude ; July 21; T o = (T o, max + T o, min)/2 = 85.0 o F
T o, max = 95 o F, h o = 3. 0 BTU/hr-ft 2 – F, h inside = 1.46 BTU/h- ft 2 –F
T inside = 78 o F
t(hr)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
BTU/hr-ft2
SHGF
(glazing)
Io (wall)
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
2
2.3
137
157.5
204
234.5
216
248.3
193
221.8
146
167.8
81
93.1
41
47.1
37
42.5
35
40.2
31
35.6
26
29.9
20
23.0
11
12.6
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
Toa(oC)*10
Toa(oC)
Toa(oF)
T sol air (oF)
156
150
172
172
161
172
194
217
233
244
256
261
267
267
261
256
244
244
239
233
217
211
194
187
15.6
15
17.2
17.2
16.1
17.2
19.4
21.7
23.3
24.4
25.6
26.1
26.7
26.7
26.1
25.6
24.4
24.4
23.9
23.3
21.7
21.1
19.4
18.7
60.08
59
62.96
62.96
60.98
62.96
66.92
71.06
73.94
75.92
78.08
78.98
80.06
80.06
78.98
78.08
75.92
75.92
75.02
73.94
71.06
69.98
66.92
65.66
60.1
59.0
63.0
63.0
61.7
110.2
137.3
145.5
140.5
126.3
106.0
93.1
92.8
92.1
89.7
87.0
82.8
79.7
75.0
73.9
71.1
70.0
66.9
65.7
Sol-air temperatures for horizontal and vertical surfaces as function of
time of day for summer design conditions, 21 July at 40° latitude
Solar Related Loads : Orientation Specific ; Loads Realized Out of Phase
3/23/2016
15
aIsol
rIsol
hrad
aD(IR)lw
T Sol Air
tIsol
ho
hi
eD(IR)lw
aD(IR)lw
3/23/2016
16
Mechanism of Radiation Heat Transfer via Fenestration
= components of
Solar Heat Gain Factor
qo
ho
a T glass
hi
qi
U glaz = 1/hi + 1/(DX glass/k glass) + 1/ho
T glazing
3/23/2016
17
Radiant Heat Gain Associated with Phase Lags
Radiant Energy must first be absorbed by surfaces that enclose the space and the
objects in the space. When these objects and surfaces become warmer than the
surrounding air, some of their heat transfers to the air by convective heat transfer.
Io (t)
aI
a+t+r=1
Tsi
Ti
3/23/2016
What is equation of thermal conduction and heat storage for block?
What is relationship between T block (t)and Ti (t)?
What is relationship between Io(t) and cooling load (t)?
18
Shading Coefficient, SHGF, SHGC and Radiative Heat Load
q/A) fenestration = q/A) conduction gain + q/A) solar gain
q/A = Uglaz(T oa design – T ia design) + tI + hi DT glaz inside env
aI = hi DT inside env+ ho DT outside env
…..(1)… Single pane glass
……….(3)….Energy balance of glass pane at steady state
Combining equations (1) and (3) , with assumption DT inside env ~ DT outside env = DTenv
q/A = Uglaz (T oa design – T ia design) + tI + aI hi /(hi + ho) = q/A) cond + q/A) solar
q/A) solar = tI + (aI * hi )/(hi + ho) = I*[ t + a hi /(hi + ho )]
F = t + a hi /(hi + ho ) single pane, double strength, 3mm thickness = 0.87………………ASHRAE reference glazing (measured)
Fdp = t + (ao /ho)*fdp + (aifdp)*(1/ho + 1/hs) ……..double pane where 1/fdp = (1/ho + 1/hs + 1/hi)
F DSA = 0.87 (measured)
SHGF = F DSA * I incident = solar heat gain factor
Shading Coefficient = SC = F glazing in use / FDSA = F glazing in use / 0.87 = 1.15*F glazing in use
(q/A) radiative solar heat gain = SC*SHGF = SC* F DSA * I incident = SHGC * I incident
SHGC
Center of Glass and Edge-of-Glass U values for a Variety of Space Materials
“Ideal” = space material with same U value as the glazing
Shading Coefficient Analysis
Transmissivity, reflectivity, absorptivity as a function of angle of incidence
A = DSA (double strength sheet glass) a = 0.055 ,t = 0.86 , r = 0.085
B = 6 mm clear glass
C =6mm gray, bronze or green tinted heat absorbing glass
The Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from
sunlight. The SHGC is the fraction of the heat from the sun that enters through a window. SHGC is
expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it
transmits.
The fraction of external solar radiation that is admitted through a window or skylight, both directly
transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient
(SHGC) has replaced the shading coefficient as the standard indicator of a window's shading ability.
It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the
less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of
the glass alone or can refer to the entire window assembly.
Shading Coefficient: A measure of the ability of a window or skylight to transmit solar heat,
relative to that ability for 3 mm (1/8-inch) clear, double-strength, single glass. Shading coefficient
is being phased out in favor of the solar heat gain coefficient (SHGC), and is approximately equal
to the Shading Coefficient multiplied by 1.15.
The shading coefficient is expressed as a number without units between 0 and 1. The lower a
window's solar heat gain coefficient or shading coefficient, the less solar heat it transmits, and
the greater is its shading ability.
Center-of-Glass U values for double and triple-pane glasses at ASHRAE winter design conditions
What is practical
limiting Dx relative to
Effectiveness?
Dx < 0.3 in
1/U glass = 1/ hi + 1/ h s + 1/ ho where h s = h rad + h con(d, v)
Window Technologies: Low-E Coatings
Low-emittance (low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a
window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal
mechanism of heat transfer in multilayer glazing is thermal radiation from a warm pane of glass to a cooler pane. Coating a
glass surface with a low-emittance material and facing that coating into the gap between the glass layers blocks a significant
amount of this radiant heat transfer, thus lowering the total heat flow through the window. Low-E coatings are transparent to
visible light. Different types of low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low
solar gain.
Spectrally selective glazing. Glazing that is transparent to some wavelengths of the solar spectrum and reflective to
others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared as
well as UV radiation. Spectrally selectivity can be achieved with low-E coatings and/or high-performance tints.
High-solar-gain low-E glass is often made with pyrolytic low-E coatings, although sputtered high-solar-gain low-E is also
available.
http://www.efficientwindows.org/lowe.cfm
q/A) fenestration = q/A) conduction gain + q/A) solar gain
q/A = Uglaz(TOAdesign – TIA design) + tI + hi DT env
…..(1)… Single pane glass
q/A = Uglaz(TOA design – TIA design)
q/A) fenestration = q/A) conduction gain + q/A) solar gain
q/A = Uglaz(TOAdesign – TIA design) + tI + hi DT env …..(1)… Single pane glass
q/A) solar = tI + (aI * hi )/(hi + ho) = I*[ t + a hi /(hi + ho )]
Assumptions in Cooling Load Determinations
a. Weather conditions from a long term database
(energy codes often specify what data can be used to minimize degree of oversizing
a. Solar loads selected are those that occur on a clear day in the month
selected for the calculations
a. Full occupancy assumed
b. All equipment operating at reasonably representative capacity
c. Lights operating as expected for a typical design occupancy
d. Latent as well as sensible loads are considered.
e. Heat flow is analyzed assuming dynamic conditions =
heat storage in building envelope and interior materials considered
Buildings are classified as envelope-load-dominated and interior load dominated.
Internal Heat Gain Expressions
Heat Gain
Building Element
Curtain Wall
Roof
Fenestration
Occupants (sens)
Occupants (lat)
Equipment
Lighting
Infiltration
Ventilation
Motors
Solar
Internal Sources
Conduction Solar Component Enhancement Transmission
UCW*(Toa - T ia)
NA
UCW *I*(a/ho) - 0
URoof*(Toa - T ia) Uroof*[I*(a/ho) – e*cos(q)*DIR/ho)]
NA
Uglaz*(Toa - T ia) hi*DT sol = tI + aI hi /(hi + ho)
SC* SHGF
NA
NA
NA
N occ * Table Value
NA
NA
NA
N occ * Table Value
NA
NA
NA
See ASHRAE Tables
NA
NA
NA
3.41WFulFsa
NA
NA
NA
1.08*CFMinf *DT
NA
NA
NA
1.08*CFMven*DT
NA
NA
NA
Miscellaneous
Compare solar
Fractions of heat gain
through wall vs. glazing
U = 0.042 BTU/hr-ft2-F
a = 0.9, I incident = 120 BTU/hr-ft2
4840*CFMven*DW
2545(P/EM)FumFLM
Conceptual Diagram of Meeting Heating or Cooling Load of a Space
Wind
Working Fluid In
Thi
Ventilation
Infiltration
d(q)/dt = 500*GPM*(Thi – Tlo), BTU/hr
T outside
WH20
Infiltration
Space of
Environmental
Control
Tset, %RH set
W H20
Space of
Environmental
Control
Tset, %RH set
W H20
Working Fluid Out
Tlo
Exfiltration