Selecting the Right Glass for Solar Shading Keeping cool in summer, - - PDF document

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Selecting the Right Glass for Solar Shading

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

Back to Basics: Specifying the Right Windows for Your Job ASHRAE Seminar Sunday, June 27 10:15 a.m. to 12:15 p.m.

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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

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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.

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Breaking sunlight into its various colors

Red 700 nm Orange Yellow Green Blue

400 nm

Glass prism

Sir Isaac Newton 1723

Invisible infrared Invisible ultraviolet

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Electromagnetic Spectrum

Wave - length 1pm 1nm 1m 1mm 1m 1km Cosmic rays Gamma rays X rays UV Visible spectrum Microwaves Radio IR 400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm 750 nm Solar spectrum 320 nm 3500 nm UV IR 6

Parts of the solar spectrum

1.6 0.0

Wavelength in nm

0.2 0.4 0.6 0.8 1.0 1.2 1.4 2500 2000 1500 1000 Solar spectrum Human eye sensitivity (Visible portion of the spectrum)

Near Infrared (NIR)

500 UV VIS NIR

Ultraviolet (UV) Far Infrared (FIR)

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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

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Warm Objects Emit Radiation

Wavelength in micrometers 0.02 1 10 107 106 105 104 103 102 101 100 10-1 10-2 50 108 0.1

Room temperature Solar Spectral range 0.3 3.5

VIS

FIR

Blackbody radiation spectra from 80 to 35,000 deg Fahrenheit

NIR

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Blackbody Radiation

Previous slide was on a log scale. This is on a linear one.

Wavelength 3.5 m 30 m

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• F curve

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Why black body radiation is important

Warm Cold

Warm panes radiate toward cold

• nes

The wave- lengths are in the far IR spectral range We can take advantage of this in designing the glass panes

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Spectral Selectivity for Cold Climates

Wavelength

Cold climate glass transmittance Room temperature surface emission spectrum Solar spectrum Human eye response

VIS NIR FIR

Visible light Invisible solar IR Invisible IR emitted by room temperature surfaces

UV

Ultra Violet 200 nm 380 nm 760 nm 3.5 m 30 m

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Spectral Selectivity for Hot Climates

Wavelength

Hot climate transmittance Cold climate transmittance Room temperature surface emission spectrum Solar spectrum Human eye response

VIS NIR FIR

Visible light Invisible solar IR Invisible IR emitted by room temperature surfaces

UV

Ultra Violet 200 nm 380 nm 760 nm 3.5 m 30 m

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Quantifying Heat Flows

Ts = Qdirect

Glazing-absorbed solar radiant heat Reflected solar radiation Outward flowing fraction of glazing absorbed radiation

Ni As = Qinward Qg = Ug × Area × t Eo Eo Eo

Glazing conduction heat transfer Visible Transmittance

VT (%) As = Qabsorbed RsEo Heat flux, Q in W/m2

Incident solar irradiance Transmitted solar radiation

Total glazing solar heat gain

Inward fraction

Eo

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Glazing Performance Indices

Ts

Glazing-absorbed solar radiant heat Reflected solar radiation Outward flowing fraction of glazing absorbed radiation

Ni As 1

Visible Transmittance

VT As Ts + Ni As = SHGC

U-factor

U (R-value = 1/U) U VT

Primary Indices

Rs

Solar Heat Gain Coefficient

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Quantifying Spectral Selectivity

Spectral selectivity: Optical properties vary with wavelength Not needed in northern Alaska 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

Wavelength

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Spectral Selectivity of Real Glazings

Clear plate Bluegreen #1 Bronze coated Bluegreen #2 Spectrally sel.-1 Spectrally sel.-2 2,500 2,000 500 1,000

1,500

Wavelength in nanometers 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances of Various Window Glazings

Little Little Mild Strong

Similar IR spectra Lower VT, higher LSG

VIS

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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

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Color Limits

2,500 2,000 500 1,000

1,500

Wavelength 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances 500 1,000 Wavelength 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances 300 700 Blue Red Green 500 1,000 Wavelength 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances 300 700 2,500 2,000 500 1,000

1,500

Wavelength 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances Very Green

Very High LSG Very Low LSG Higher LSG

Low LSG Plate glass LSG

• 1.2

LSG

• 1.6

SHGC high VT quite low

VIS

VT and SHGC relationships for spectrally selective glazings Visible transmittance

0.0 0.2 0.4 0.6 0.8 1.0

SHGC

0.0 0.2 0.4 0.6 0.8 1.0 LSG = 0.6 0.8 1.0 1.2 1.4 1.6 1.8

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SHGC versus VT

0.3 0.13 0.33

Single-pane clear glass Target for hot climate glazings

Forbidden zone Forbidden zone

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Coatings and Tints 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

• ptions

One can use

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Cold-climate low-e coated windows

Low-emissive configuration

Cold Warm One way to do the job

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Cold climate glazings Admit and trap solar heat

Insulated gas space (air, argon, krypton) High solar gain low-e coating. Transmits solar, doesn’t emit FIR, so it keeps the heat inside, where it is needed Total solar spectrum

FIR

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Cold climate glazings Admit and trap solar heat

Cold-climate low-e coated windows

High-reflective configuration Low-emissive configuration

Cold Warm Two ways to do the job

1 2

Cold Warm Cold climate low-e coating.

FIR FIR FIR not emitted FIR reflected

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Hot Climate Glazings Admit visible, reject invisible solar

Solar near IR

Warm Cool

Reflective

Hot-climate coated windows

One way to do it

Visible light

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Hot-climate near-IR reflective coating (Also called “hot-climate low-e coating) (or a low-solar-gain low-e coating)

Visible

• nly

By rejecting nearly half the incident solar radiation with reflection, the SHGC is nearly half as large

NIR

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Hot Climate Glazings Admit visible, reject invisible solar

Long-wavelength IR Solar near IR

Cold-climate low-e coating

Solar near IR absorber Warm Cool

Reflective

Hot Cool

Absorptive

Hot-climate coated windows

Two ways to do it

Visible light

1 2

Hot-climate near-IR reflective coating

NIR FIR VIS VIS

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Coatings for Energy Control

Cold-climate low-e coated windows

Absorptive longwave conversion High-reflective configuration

Long-wavelength IR Solar near IR Cold-climate low-e coating Hot-climate solar near IR reflective coating

Low-emissive configuration *Second pane optional

in principle Cold Warm Cold Warm Hot Cool Warm Cool Warm Cool Solar near IR absorber (longwave convertor)

*

Hot Cool

Or Solar direct reflection

b. a. *

Hot-climate coated windows

c. d.

Or Or

Putting it all together

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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

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When exterior shading is neither permitted, nor desired, nor possible

Use High-Performance Glazing Systems

To minimize solar heat gain, use low solar gain low-e coated glazings with high LSG ratio Choose VT to fit the situation

VT high for north-facing, and exposures already shaded fairly well VT low for east- and west-facing exposures inadequately shaded

To reduce peak load, enhance comfort and allow smaller air conditioners, use double pane windows

Impact resistant for coastal zone Insulated frames to reduce condensation and improve comfort further 28

Glass Spectral Choices

Clear plate Bluegreen #1 Bronze coated Bluegreen #2 Spectrally Spectrally

2,500 2,000 500 1,000

1,500

Wavelength in nanometers 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances of Various Window Glazings

VIS

High VT, low SHGC Medium VT, lower SHGC Low VT, lowest SHGC

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Window Recommendations in Summary

All windows: Insist on high-LSG glazings and double-pane, insulated windows throughout the building—for energy savings, comfort, reduced peak load, and smaller A/C capacity (and cost). North-facing: Use a side-wall, or a deep window reveal to block low rising and setting sun on hot summer days South-facing:

• Use a modest overhang if you like winter sun
• Use a wide overhang to avoid sun year round
• High-LSG glazings are especially important if shading’s inadequate

East- and West-facing, a menu of choices: For hot climates:

• Dense tree shading where possible Awning shade

Exterior shade screen Exterior roller shutters Highest-LSG glazing system, VT between 0.2 and 0.4 Interior reflective operable shade For cold climates: Well-insulated multiple pane windows with insulated frames Laminated glass for impact resistance if exterior shade is not enough for this 30

Proper Glazing Choices Promote

Good energy efficiency Protection against future energy price shocks Protection against peak demand charges from utilities Reduced global warming Lower energy costs Visual and acoustic comfort Thermal comfort Higher building values More productive employees