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Example of thermal radiation spectra for black and gray surfaces at room temperature

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 o K = 4.90 o R). max = 5216 microns - o R 4.90 o R. max =1064.49 microns.

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Example of thermal radiation spectra for black and gray surfaces at room temperature

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  1. 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) • max = 5216 microns - oR 4.90 oR • max =1064.49 microns lmax = 5216 microns T(o R)

  2. 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. Extraterrestrial daily horizontal irradiation as function of day of year, for north latitudes

  4. 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

  5. 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

  6. ASHRAE Clear Sky Model G t = G ND + G d + G R = ID + Id + IR ID =G ND = A * C N 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 qs)], Io = extraterrestial irradiance Liu and Jordan Diffuse Radiation on a Horizontal Surface I diff = (0.271*Io – 0.239 I dir)* ( cos ) qs

  7. Correlation between diffuse irradiance and global irradiance (hourly averages, on horizontal) and hourly clearness index Hourly Clearness Index : k T = Iglo IoCos(q) zenith KT = Daily Equivalent of k T Idiff = 1.00 – 0.09kT , 0<k T<0.22 I glo 0.22<KT <0.80 Idiff = 0.09511-0.0164kT+4.388kT2 –16.638kT3+12.336kT4 I glo Idiff = 0.165 , 0.80 <k T I glo

  8. Monthly average daily total irradiation on vertical surfaces in January, April and July, as function of monthly average clearness index and latitude

  9. 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?”

  10. U roof layer = 0.110 BTU/hr-ft 2 -F N N = 16 floors above 1st floor U curtain wall = 0.180 BTU/hr-ft 2 F 25’ U window installation = 0.60 BTU/hr-ft 2 F 13’ 6’ 4’ 8’ 50 ‘ U door = 0.200 BTU/hr-ft 2 F 70 ‘

  11. 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* (Toa– Tes) +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. T sol air temperature = Toa + aIsin(q)- e*cos(q)*DIR; Normally, D(IR)lw= 21 BTU/hr-ft2 ho ho Tsi (t) 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) T air (t) 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)?

  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

  13. Sol-air temperatures for horizontal and vertical surfaces as function of time of day for summer design conditions, 21 July at 40° latitude

  14. Solar Related Loads : Orientation Specific ; Loads Realized Out of Phase

  15. hrad aIsol rIsol aD(IR)lw T Sol Air tIsol ho hi eD(IR)lw aD(IR)lw

  16. Mechanism of Radiation Heat Transfer via Fenestration = components of Solar Heat Gain Factor ho hi qo T glass qi a U glaz = 1/hi + 1/(DX glass/k glass) + 1/ho T glazing

  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 What is equation of thermal conduction and heat storage for block? What is relationship between Tblock(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 + hiDT glaz inside env …..(1)… Single pane glass aI = hiDT inside env+ hoDT outside env Combining equations (1) and (3) , with assumption DT inside env ~ DT outside env = DTenv ……….(3)….Energy balance of glass pane at steady state 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

  19. 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

  20. 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

  21. 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.

  22. 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)

  23. 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

  24. q/A) fenestration = q/A) conduction gain + q/A) solar gain q/A = Uglaz(TOAdesign – TIA design) + tI + hiDT env …..(1)… Single pane glass q/A = Uglaz(TOA design – TIA design)

  25. q/A) fenestration = q/A) conduction gain + q/A) solar gain q/A = Uglaz(TOAdesign – TIA design) + tI + hiDT env …..(1)… Single pane glass q/A) solar = tI + (aI * hi )/(hi + ho) = I*[ t + a hi /(hi + ho )]

  26. Assumptions in Cooling Load Determinations • Weather conditions from a long term database (energy codes often specify what data can be used to minimize degree of oversizing • Solar loads selected are those that occur on a clear day in the month selected for the calculations • Full occupancy assumed • All equipment operating at reasonably representative capacity • Lights operating as expected for a typical design occupancy • Latent as well as sensible loads are considered. • 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.

  27. Internal Heat Gain Expressions 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

  28. Conceptual Diagram of Meeting Heating or Cooling Load of a Space Wind Working Fluid In Thi Infiltration d(q)/dt = 500*GPM*(Thi – Tlo), BTU/hr Ventilation T outside WH20 Infiltration Space of Environmental Control Tset, %RH set W H20 Space of Environmental Control Tset, %RH set W H20 Exfiltration Working Fluid Out Tlo

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