Window Options Keeping cool in summer, warm in winter, comfortable all the time,... and saving energy too Ross McCluney, Ph.D., Prinicipal Research Scientist Florida Solar Energy Center ! Many factors affect the design and choice of windows for the Florida home. ! After some background information, we’ll take a tour through the options .
Are windows just “holes in the insulation?” Some are,but . . . “it ain’t necessarily so!” ! Good windows can out-perform opaque insulated walls, energy-wise. ! Windows provide much more than energy savings! ! A building is there to provide comfort and protection from the elements, not just to save energy. ! If energy can be saved too, that’s even better. ! We’ll start with some basics ! Then we’ll cover energy and economics ! And finish with a summary of window option recommendations
Finding the Right Window P It is more than just choosing a pretty window. P We must also deal with the heat and the cold, as well as the glare and overheating of direct sunlight < The heat and cold: insulation and shading < The glare and overheating of direct sunlight: orientation and shading P Other issues < Choice of window frame and glazing < To insulate or not? < Acoustic isolation? < Impact resistance? < Utility concerns
Dealing with the Sun P The Good : Big windows provide a bright and open room with great views and good daylight illumination P The Bad : Overheating, fading of furnishings, blocked views P The Ugly : Killer glare from the sun, big energy bills, thermal discomfort P Three strategies for dealing with the sun < Know where the sun is < Shape and orient the building properly relative to the sun < Shade the windows and walls properly
Heat Transfer The three modes of heat transfer T hot T cold Radiation Conduction Convection
Heat Flows Through Windows Absorbed solar radiation conducted through the frame Directly transmitted solar Reflected solar radiation through the glazings radiation (includes both light & heat) Glazing-absorbed solar radiant heat Outward flowing Inward flowing fraction of fraction of glazing glazing absorbed radiation absorbed radiation
Heat Flows Through Windows Absorbed solar radiation conducted through the frame Directly transmitted solar Reflected solar radiation through the glazings radiation (includes both light & heat) Glazing-absorbed solar radiant heat Outward flowing Inward flowing fraction of fraction of glazing glazing absorbed radiation absorbed radiation Heat conducted through the glass Heat conducted through the frame
Insulated windows reduce conduction, convection, and radiation Heat conducted through the glazing system Coatings reduce radiation transfer Insulating gas reduces conduction Proper spacing minimizes convection Winter Cold Warm Summer Cool Hot Insulation reduces heat conduction through the frame
Knowing Where the Sun is P Radiation from the sun is generally much stronger than that from the sky, except on hazy and partially overcast days P The sun moves through the sky in a known way each day P Radiation coming directly from the sun’s “disk” is called “direct beam radiation.” P Orienting the building and its windows is important to maximize the benefits and minimize the problems produced by direct beam solar radiation. P First we look at a generic drawing of the sun’s path through the sky on the summer and winter solstices P Then we consider how to orient a house properly relative to the sun’s positions in the sky
SUMMER WINTER Sun rises south of due east, Sun rises north of due east, sets south of due west, sets north of due west, and is low in the sky at and is high in the sky at noon noon Shade: southwest to west to Shade: protect west window on overhang for noon warm winter days east to northeast morning west to northwest afternoon
Orientation and shading N Minimize east and west exposure Shade the facade Wide overhangs Fence Closet Garage Buffer East and West Exposures Utility room
Solar Spectrum Fundamentals P The sun’s radiation covers a range of colors, and beyond. P This electromagnetic radiation has important features for the design and performance of windows in different climates. P We need to know a little more about the physics of solar radiation to fully understand the variety of window products now on the market. P We begin with the electromagnetic spectrum.
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
Electromagnetic Spectrum Wave - 320 nm length Cosmic rays UV Gamma Gamma 400 nm 1pm rays rays 450 nm X rays 1nm 500 nm UV Visible 550 nm 1 : m Solar spectrum 600 nm IR spectrum 1mm 650 nm 700 nm 1m Radio 750 nm Microwaves 1km IR 3500 nm
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)
Emission of Heat Radiation P Warm objects emit radiation P The hotter they are, the more they emit P As their temperature increases, the spectral distribution shifts as well, as shown on the next slide
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 0.02 0.1 1 3.5 10 50 0.3 Wavelength in micrometers
Why black body radiation is important Warm panes The wavelengths radiate are in the far IR toward cold spectral range ones We can take advantage of this in designing the glass panes Cold Warm
Spectral Selectivity for Cold Climates Cold climate glass 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
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
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 R s E o radiation solar heat Glazing-absorbed 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 Frame conduction Q f = U f × Area × ) t heat transfer
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 U-factor (R-value = 1/U)
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
Spectral Selectivity of Real Glazings 1.0 Spectral Transmittances of Various Window Glazings Clear plate Bluegreen #2 Spectrally Bluegreen #1 Spectrally Bronze 0.8 Spectral selectivity: 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
Coatings and Tints One can use P High solar gain low-e coatings for cold climates P Low solar gain low-e coatings for hot climates P IR-absorbing glass for hot climates P A variety of ways to coat and tint glass
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
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.
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)
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