Selecting the Right Glass for Solar Shading Keeping cool in summer, warm in winter, comfortable all the time,... and saving energy too Back to Basics: Specifying the Right Windows for Your Job ASHRAE Seminar Sunday, June 27 10:15 a.m. to 12:15 p.m. Ross McCluney, Ph.D., Prinicipal Research Scientist Florida Solar Energy Center 1 Background � I teach a half-day short course on Energy Smart Windows for residences � Short Course Outline: � Fundamentals of heat transfer � Dealing with the sun – orientation and shading � Solar spectrum fundamentals – � Spectral selectivity for hot and cold climates � Intro to daylighting & glare � Interior, exterior, and glazing shading options � Hourly energy performance � Web sites for energy ratings and hourly performance estimation � Advice on selecting the right windows for your residence � This presentation: � Material I present dealing with glazing systems � Emphasis is on reducing solar heat gain � while admitting adequate daylight illumination 2
Solar Spectrum Fundamentals � Solar radiation covers a range of colors and wavelengths � Important for the design and performance of windows in different climates. � Solar radiation physics � Needed to fully understand the variety of window products now on the market. � We begin with the electromagnetic spectrum. 3 Breaking sunlight into its various colors Sir Isaac Newton 1723 Glass prism Invisible infrared Red 700 nm Orange Invisible Yellow ultraviolet Green Blue 400 nm 4
Electromagnetic Spectrum Wave - 320 nm length Cosmic rays UV Gamma 400 nm 1pm rays 450 nm X rays 1nm 500 nm UV Visible 550 nm 1 � m Solar spectrum IR 600 nm spectrum 1mm 650 nm 700 nm 1m Radio 750 nm Microwaves 1km IR 3500 nm 5 Parts of the solar spectrum 1.6 1.4 Solar spectrum 1.2 1.0 Human eye sensitivity (Visible portion of the 0.8 spectrum) 0.6 0.4 0.2 NIR UV VIS 0.0 0 500 1000 1500 2000 2500 Wavelength in nm Near Infrared (NIR) Ultraviolet (UV) Far Infrared (FIR) 6
Emission of Heat Radiation � Warm objects emit radiation � The hotter they are, the more they emit � As their temperature increases, the spectral distribution shifts as well, as shown on the next slide 7 Warm Objects Emit Radiation Blackbody radiation spectra from 80 to 35,000 deg Fahrenheit 10 8 10 7 10 6 FIR NIR VIS 10 5 10 4 10 3 Room 10 2 temperature 10 1 10 0 Solar Spectral 10 -1 range 10 -2 3.5 0.02 0.1 0.3 1 10 50 Wavelength in micrometers 8
Blackbody Radiation Previous slide was on a log scale. This is on a linear one. 75 � � � � F curve 3.5 � m 30 � m Wavelength 9 Why black body radiation is important Warm panes The wave- radiate lengths are in the toward cold far IR spectral ones range We can take advantage of this in designing the glass panes Cold Warm 10
Spectral Selectivity for Cold Climates Cold climate glass transmittance Room temperature surface emission Solar spectrum spectrum Human eye response Wavelength UV VIS NIR FIR Ultra Visible Invisible Invisible IR emitted by Violet light solar IR room temperature surfaces 3.5 � m 200 nm 380 nm 760 nm 30 � m 11 Spectral Selectivity for Hot Climates Hot climate Cold climate transmittance transmittance Room temperature surface emission spectrum Solar spectrum Human eye response Wavelength UV VIS NIR FIR Ultra Visible Invisible Invisible IR emitted by Violet light solar IR room temperature surfaces 3.5 � m 200 nm 380 nm 760 nm 30 � m 12
Quantifying Heat Flows Incident solar Heat flux, irradiance Q in W/m 2 E o Transmitted solar radiation Total Reflected solar T s = Q direct E o glazing radiation R s E o solar Glazing-absorbed heat E o A s = Q absorbed solar radiant heat gain Inward fraction Outward flowing N i A s = Q inward E o fraction of glazing absorbed radiation Visible Transmittance VT (%) Glazing conduction Q g = U g × Area × � t heat transfer 13 Glazing Performance Indices Primary Indices 1 Solar Heat Reflected solar R s T s Gain radiation Coefficient Glazing-absorbed T s + N i A s = SHGC A s solar radiant heat Outward flowing N i A s fraction of glazing absorbed radiation VT VT Visible Transmittance U U-factor U (R-value = 1/U) 14
Quantifying Spectral Selectivity � Spectral selectivity: Optical properties vary with wavelength � Not needed in northern Alaska Wavelength � Can be very helpful in hot and warm climates � Useful in cold climates when buildings are internal load dominated and have trouble losing heat � In these cases we need low solar heat gain � So Just lower the solar transmittance � But this also lowers visible transmittance � Spectral selectivity allows dropping solar gain without dropping visible transmittance as much 15 Spectral Selectivity of Real Glazings 1.0 Spectral Transmittances of Various Window Glazings Clear plate Bluegreen #2 Spectrally sel.-1 Bluegreen #1 Spectrally sel.-2 Bronze coated 0.8 Little 0.6 Little 0.4 Similar IR Mild spectra 0.2 Strong VIS 0.0 0 500 1,000 2,000 2,500 1,500 Lower VT, Wavelength in nanometers higher LSG 16
Light to Solar Gain ratio - A measure of spectral selectivity VT Visible transmittance: Fraction of incident light transmitted SHGC Solar heat gain coefficient: Fraction of incident solar radiation admitted as heat gain LSG Light-to-Solar Gain ratio: Ratio of visible transmittance to solar heat gain coefficient LSG = VT SHGC 17 Color Limits Higher Spectral Transmittances Spectral Transmittances LSG 1.0 1.0 Low LSG Blue Red 0.8 0.8 Plate glass 0.6 0.6 LSG � � 1.2 � � 0.4 0.4 Green 0.2 0.2 VIS LSG � � 1.6 � � 0.0 0.0 0 500 1,000 2,000 2,500 0 300 500 700 1,000 1,500 Wavelength Wavelength Very Very Spectral Transmittances High Spectral Transmittances Low 1.0 1.0 LSG LSG 0.8 0.8 Very Green 0.6 0.6 SHGC high VT quite low 0.4 0.4 0.2 0.2 0.0 0.0 0 300 500 700 1,000 0 500 1,000 1,500 2,000 2,500 Wavelength Wavelength 18
VT and SHGC relationships for spectrally selective glazings SHGC versus VT LSG = 0.6 0.8 1.0 1.0 Forbidden Single-pane clear glass 1.2 zone 0.8 1.4 1.6 0.6 SHGC 1.8 0.4 Target for 0.2 hot climate glazings Forbidden 0.33 zone 0.0 0.3 0.13 0.0 0.2 0.4 0.6 0.8 1.0 Visible transmittance 19 Coatings and Tints One can use � High solar gain low-e coatings for cold climates � Low solar gain low-e coatings for hot climates � IR-absorbing glass for hot climates � A variety of ways to coat and tint glass � Here’s a detailed rundown on the options 20
Cold climate glazings Admit and trap solar heat Cold-climate low-e Low-emissive configuration coated windows FIR High solar gain One way to low-e coating. do the job Transmits solar, doesn’t emit FIR, 1 so it keeps the Total solar heat inside, where spectrum it is needed Insulated gas space (air, argon, krypton) Cold Warm 21 Cold climate glazings Admit and trap solar heat Low-emissive High-reflective Cold-climate low-e configuration configuration coated windows Two ways to do the job FIR FIR 2 1 Cold Warm Cold Warm Cold climate FIR reflected FIR not emitted low-e coating. 22
Hot Climate Glazings Admit visible, reject invisible solar Hot-climate coated windows Reflective One way to do it Visible NIR only By rejecting nearly half the incident solar radiation with 1 reflection, the SHGC is nearly half as large Warm Cool Solar near IR Hot-climate near-IR reflective coating Visible light (Also called “hot-climate low-e coating) (or a low-solar-gain low-e coating) 23 Hot Climate Glazings Admit visible, reject invisible solar Hot-climate coated windows Reflective Absorptive Two ways to do it VIS FIR NIR 1 2 VIS Warm Hot Cool Cool Long-wavelength IR Solar near IR Hot-climate near-IR reflective coating Visible light Cold-climate low-e coating Solar near IR absorber 24
Low-emissive Cold-climate low-e High-reflective Coatings for Energy Control configuration configuration coated windows Putting it all Or together b. a. Cold Warm Cold Warm Hot-climate coated windows Absorptive longwave conversion Solar direct reflection Or Or * * c. d. Hot Hot Cool Cool Warm Warm Cool Cool Long-wavelength IR Cold-climate low-e coating * Second pane optional Solar near IR Hot-climate solar near IR in principle reflective coating Solar near IR absorber (longwave convertor) 25 Exterior Shading Though we’re talking about glazing systems, I can’t fail to mention the value of exterior shading. It is generally better to block the sun before it strikes the glass � But we cannot always do this, due to � Subdivision restrictions � Aesthetic considerations � Multi-story building � Desire not to block an important scene 26
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