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Ball State Architecture | ENVIRONMENTAL SYSTEMS 1 | Grondzik 1

SOLAR GEOMETRY(AND RADIATION)


Solar Radiation Components

SHGF (above) is “solar heat gain factor” — a measure of the quantity ofsolar energy that will pass through a sheet of single-pane clear glassat a given time of the year for a given orientation

glass will reflect someincoming radiation;

absorb some; and transmit some

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Solar Radiation:A Resource or a Problem?

mitigating element:

building envelope

result

force

radiation is an environmental force;it can be an asset or a liabilitydepending upon site/project/intent/context


Solar Context—Distance

solar “constant”(Btuh/sq ft)

the “solar constant” is a mean value of the solar radiationintensity found at the edge of the Earth’s atmosphere

Earth

Sun

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Solar Context—Tilt

the tilt of the Earth’s axis, relative to the sun’sposition accounts for the seasons (as solar angle of incidence changes)


Site Context—Latitude

Muncie’s latitude is approximately 40 deg N(a “convenient”latitude often used in design data tables)

latitude locates a site in the north-south context,relative to the equator

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Site Context—Longitude

Muncie = apx. 85 deg W

longitude locates a site in the east-west context,relative to the prime meridian (in Greenwich, England)


Greenwich, England

prime meridian

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Solar-Site Context

the sun “follows” defined and predictable paths on a daily and annual basis

NORTHERN HEMISPHERE

summer

spring/fall

winter


Solar Angles—Altitude

ALTITUDE ANGLE

the angle between a horizontal ground planeand the line describing the position of the sun in the sky vault

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Altitude Angle Patterns

altitude angle; describes height of the sun in the sky – varies with month and time of day

noon

noon


Altitude Angle Patterns

for any given month and time, altitude angle varies with latitude

<< site latitude

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Solar Angles—Azimuth

AZIMUTHANGLE

the angle between south and the position of the sun in the sky vault asprojected onto the horizontal ground plane


Azimuth Angle Patterns

azimuth angle varies with month and time of day; and is most “extensive” in summer

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Solar Position Relative to Site

conceptually simple, but three-dimensional


Solar Position Conventions

• Azimuth angle is usually measured from South (in architecture) … but sometimes from the North in other disciplines (or in the Southern hemisphere)

• South is always “solar” South; not “magnetic”South or “plan” South … see next slide

• Altitude angle is measured from the horizontal

• Times are always expressed as “solar” time … see following slide

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South, Souther, Southest

• Plan South is usually an arbitrary designation used for convenience

• Solar South is—by definition—the position of the sun at solar noon

• Solar South marks the center of symmetry of the daily solar path

• Solar South differs from Magnetic South (as read on a compass) by the local magnetic deviation value


Magnetic Deviation

solar reference

compassreference

magnetic deviation varies with site location

magnetic deviationmagnetic deviation

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Magnetic Deviationzero deviation line

designing for east-central Indiana is not too deviant


Solar versus Clock Time

CT (clock time) = local solar time + equation of time value+ (4)(local standard time meridian – local longitude)

+/- daylight saving time adjustment

bottom line: solar and clock time are rarely identical, and can differ by +/- an hour;this matters when solar loads are combined with clock-based loads

CT

example, Muncie, June 21: latitude adjustment = (4)(75-85) = -40 minutes; DST adjustment = 60 minutes; ET adjustment = -3 minutes

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Clock Time Influences

US time zones (political devices) and time zone reference meridians; the sun is not influenced by politics

equation of time

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Sunpath Information

The sun’s location at any time can be accurately predicted. Such information is readily available in several formats:

Graphic sunpath diagrams:• Horizontal projection diagrams

(including the SBSE Sun Angle Calculator)

• Vertical projection diagrams• Sunpeg diagrams

Tabular data (as in the ASHRAE Handbook)

Online “calculators” (interactive databases)

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Sunpath Diagrams

horizontal projection sunpath diagram

sun paths for selected days can be projected downward onto a horizontal plane(the plane is a surrogate for the Earth’s surface)

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Sunpath Diagrams

Pilkington Sun Angle Calculator

sun paths for multiple latitudes are provided, with usability enhancements

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available from SBSE

Via CERES

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Sunpath Diagrams

vertical projection sunpath diagram

sun paths for selected days can be plotted on a vertical surface(acting as a surrogate for the site horizon)


Sunpath Diagrams

primarily used to properly orient a scale model to the sun (real or simulated);the shadow cast by the peg (gnomon) shows date and time

sunpeg diagram

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Tabular Solar Position Data

Mechanical and Electrical Equipment for Buildings (10th ed)


Online Solar Position Data

susdesign.com/sunangle/

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for the upcoming QUIZ

Focus on the material above for the topical quiz associated with these notes

— while not disregarding the very important information below.


Shading DevicesThe term “shading device” describes an element of the building design intended to reduce the amount of solar radiation falling on, or making its way into, a building; there are several broad categories of such “devices”

• External devices– Are the most thermally-effective approach; these block radiation before it can engage glazing or opaque wall/roof surfaces; many types of external device are available

• Integral devices– Next most effective approach; these engage radiation within thebuilding envelope; various glazings or window inserts are available

• Internal devices– The least thermally-effective approach; these engage radiation only after it is within the building; many popular examples (such as drapes or blinds) are available

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External Shading Devices

external – overhang; can be very effective with direct radiation

blocked

generally admitted

overhang



external – tree; can be very effective (and, if deciduous, can act as a switch)

vegetation

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external – overhang (in section) and fin (in plan)

a fin is an “azimuth-responsive” device

an overhang is an “altitude-responsive” device

overhangvs. fin


Integral Shading Devices

integral – radiation-absorbing glass, increases glass surface temperature

reduced solarradiation transmission

increased heatconvection

tinted glass

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Internal Shading Devices

internal – draperies; radiation is only blocked once it is within the envelope;solar energy (heat) is allowed to enter the building

complex thermalinteractions

draperies


Shading Devices

California State Office Buildings:San Jose

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Shading Devices

shading by inherent building form

Dallas City Hall, TX

Georgia Power offices, Atlanta


Shading Devices

blinds placed between two sheets of glazing (integral)

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Shading Devices

reverse shading (oops) layered shading

1

2

3

1 = vegetation; 2 = overhang; 3 = blinds


Shading Devices?

Florida State Capitol, Tallahassee, FL

New Capitol

Old Capitol

North >>>

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Shading Device Design

• Establish design intent– For example … block sun in summer (the “overheated” period)

• Establish design criteria– For example … full shade, all day from May through September– For example … no shade from November through January– For example … no blockage of views

• Decide upon design method– Design can evolve rationally through the use of various tools– Or, design can be based upon trial and error using precedents

• Verify success of design in meeting criteria– Numerous tools are available: physical models and heliodons, hand calculations and sketches, computer simulations


Shading Device Design Tools

shading masks, these provide a pattern language for shading devices:from Solar Control and Shading Devices (Olgyay and Olgyay)

shading pattern provided by device type; projected onto sunpath diagram

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shading mask for a south-facing overhang plotted ona vertical projection sunpath diagram

lighter area indicates timesof the year when device

does NOT shade window



a shading mask for a specific overhang, showing period of sun penetration

shaded

shaded

no shade

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fisheye photo of sky vault from a site…and similar image with sunpath superimposed:Design with Climate, Olgyay

Solar Pathfindertool

shaded

not shaded



overhang design using trigonometry (or, even easier, using a scale drawing)

L

H

overhang L set to provide full shade (from direct sun) during times of

the year when the solar altitude angle is β (or greater)

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shading device design analysis using a scale model and sunpeg diagram;what you see is what you’ll get

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shading device design analysis using a scale model and sunpeg diagram

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devices and inquisitive designers



three types of heliodons (solar position simulators)

fixed-table, moving sun fixed table, selectable suns fixed sun, moving table

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a heliodon provides information on shading performance and direct sun patterns;it provides no information on daylight factors

photos from the Pacific Energy Center


Expressing Shading Performance

An historic (and still useful) performancemetric for shading systems is:

Shading Coefficient (SC)

SC = solar heat gained through glazing “x”solar heat gained through reference glazing

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Shading Coefficient (SC)

• The “reference” glazing is single-pane, clear glass • As a ratio, SC is dimensionless (it is stated as a

percent or more commonly as a decimal value)• SC values may be found in manufacturers’

literature, in handbooks (such as from ASHRAE), or may be derived from calculations

• SC, as generally used, applies only to glazing (not to the opaque frame or mullions)

• An overall SC may be assembled from “parts”SC overall = (SC1)(SC2)(SC3) …where, SC1 may be for an overhang, SC2 for glazing, …


Expressing Shading Performance

A newer (and need-to-know for code compliance) shading performance metric is:

Solar Heat Gain Coefficient (SHGC)

SHGC = the fraction of incident solar radiation that is admitted through a window or skylight (including the frame)—both that

directly transmitted, and that which is absorbed and subsequently released inward

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Solar Heat Gain Coefficient (SHGC)

• SHGC is a dimensionless value (it is stated as a percent or most commonly as a decimal value)

• SHGC applies to glazing and frame/mullions• SHGC is a laboratory-measured value (the National Fenestration Rating Council—NFRC—tests and certifies products)

• Determining (figuring) an overall SHGC (considering site additions to shading) is a problematic issue

• SHGC ≠ SC• SHGC ≈ (SC)(0.87)

SHGC is self-referential; while SC references a base product for comparison


NFRC Window Label

label is removed from window after code inspection

if product is “residential”

if product is“non-residential”

full solar spectrum

visible spectrum

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Window Performance Indicators

a selective transmission product

Flo

rida

Sol

ar E

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SHGC entire unit

SC glazingonly


movable shading device (sail); Alpine House, Kew Gardens, UK

be creative

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