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TABLE OF CONTENTS 1. EXECUTIVE SUMMARY ...........................................................................................9
2. PROPOSED CHANGES TO CODE LANGUAGE....................................................11
2.1 Proposed Changes to Chapter 2. Scope ...........................................................11
2.2 Proposed Changes to Chapter 3. Definitions, Abbreviations, and Acronyms .12
2.3 Proposed Changes to Chapter 9 Lighting ........................................................15
2.4 Proposed Changes to Chapter 5 Envelope for Minimum Skylight Area Requirement.....................................................................................................18
3. REASON AND SUBSTANTIATION FOR PROPOSED CHANGES TO DEFINITIONS SECTION (CHAPTER 3)..................................................................20
3.1 Move “Daylighted Area Under Skylights” to Definitions Section .................20
3.2 Address obstructions in “Daylighted Area under Skylights” ..........................21
3.3 Definition of “daylight area under rooftop monitors”....................................24
3.4 Definition of “primary sidelighted area”........................................................30 3.4.1 Sidelighting Photocontrols Study ............................................................ 31 3.4.2 Daylighting Autonomy Studies ................................................................ 32 3.4.3 Literature Review Conclusions................................................................ 33
3.5 Move skylight effective aperture to definitions section...................................34
3.6 Definition of sidelit effective aperture.............................................................34
4. REASON AND SUBSTANTIATION FOR CHANGES TO LIGHTING SECTION (CHAPTER 9)..............................................................................................................35
4.1 Moving Skylight Area Definition to Definitions Section................................35
4.2 Adding Photocontrol Requirement for Large Primary Sidelit Daylight Areas35
4.3 Reduced Daylight Area under Skylights Triggering Photocontrols Requirement.....................................................................................................38
4.4 Minimum Daylit Area under Rooftop Monitors Triggering Photocontrols Requirements ...................................................................................................43
4.5 Exemption for Photocontrols Requirements Based on Effective Aperture (EA)43
5. REASON AND SUBSTANTIATION FOR CHANGES ENVELOPE SECTION (CHAPTER 5)..............................................................................................................47
5.1 Minimum Skylight Area Requirement Analysis..............................................47
9. APPENDIX 1 – PLOTS OF ILLUMINANCE UNDER SKYLIGHTS WITH DIFFERING SHELVING HEIGHTS..........................................................................76
10. APPENDIX 2 – PLOTS OF ILLUMINANCE UNDER SAWTOOTH MONITOR WITH DIFFERING SHELVING HEIGHTS ..............................................................81
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LIST OF TABLES Table 1: NRCC Analysis Variables....................................................................................32
Table 2: Costs and savings from sidelit photocontrols (derated to 65%) in Chicago office
building ........................................................................................................................37
Table 3: Minimum threshold area required for cost-effective sidelit controls in Chicago
when savings are not derated. .....................................................................................37
Table 4: Lighting controls B/C ratios by climate zone and effective aperture for a 4,000 sf
daylit space in a warehouse.........................................................................................42
Table 5: Lighting controls B/C ratios by climate zone and effective aperture for a 4,000 sf
daylit space in an office ...............................................................................................42
Figure 23: Explanation of Effective Aperture Exemptions Graphs...................................44
Figure 24: Annual energy savings for a warehouse with low white over clear acrylic
skylights relative to same warehouse but without photocontrols ................................44
Figure 25: Annual energy savings for a warehouse with prismatic over prismatic clear
acrylic skylights relative to same warehouse but without photocontrols ....................45
Figure 26: Annual energy savings for an office with low white over clear acrylic skylights
relative to same warehouse but without photocontrols ...............................................45
Figure 27: Annual energy savings for an office with prismatic over prismatic clear
acrylic skylights relative to same warehouse but without photocontrols ....................46
Figure 29: ASHRAE Skylight Size, Frame, Edge and Center of Glass Dimensions .........59
Figure 30: Regression Plot of Cost of Triple Glazed Plastic Dome Skylight vs. Size.......64
Figure 31: Regression Plot of Cost of Double Glazed Glass Skylight vs. Size .................65
Figure 32: Control Panel Diagram for a 3-level switching control..................................70
Figure 33: Trend line of RS Means estimated costs for RTU’s – 6 to 12.5 tons ...............74
Figure 34: Trend line of RS Means Estimated Costs for Unit Heaters - 20 to 320 MBH .74
Figure 35: Skylight Case 0 ft Stacks ..................................................................................76
Figure 36: Skylight Case 5 ft Stacks ..................................................................................77
Figure 37: Skylight Case 7 ft stacks ..................................................................................78
Figure 38: Skylight Case 10 ft stacks ................................................................................79
Figure 39: Skylight Case 15 ft Stacks ................................................................................80
Figure 40: 3 ft Sawtooth Case 0 ft Stacks..........................................................................81
Figure 41: 3 ft Sawtooth Case 5 ft Stacks..........................................................................82
Figure 42: 3 ft Sawtooth Case 7 ft Stacks..........................................................................83
Figure 43: 3 ft Sawtooth Case 10 ft Stacks........................................................................84
Figure 44: 3 ft Sawtooth Case 12 ft Stacks........................................................................85
Figure 45: 3 ft Sawtooth Case 15 ft Stacks........................................................................86
Figure 46: 6 ft Sawtooth Case 0 ft Stacks..........................................................................87
Figure 47: 6 ft Sawtooth Case 5 ft Stacks..........................................................................88
Figure 48: 6 ft Sawtooth Case 7 ft Stacks..........................................................................89
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Figure 49: 6 ft Sawtooth Case 10 ft Stacks........................................................................90
Figure 50: 6 ft Sawtooth Case 12 ft Stacks........................................................................91
Figure 51: 6 ft Sawtooth Case 15 ft Stacks........................................................................92
Acknowledgements The project is funded by the US Department of Energy and administered by the Pacific Northwest National Laboratory. Eric Richman is the project manager for Pacific Northwest National Laboratory. This report was developed by Mudit Saxena, Derrick Leung and Luke Suhr of the Heschong Mahone Group and Jon McHugh of the McHugh Energy Group. DOE-2 simulations were conducted by Paul Reeves of Partnership for Resource Conservation. This report is updated based on comments from the Standard ASHRAE 90.1 Lighting and Envelope Committees. This is truly a collaborative with assistance from a number of entities.
This report builds upon earlier analytic work funded by Wal-Mart that was the basis of Addendum d to ASHRAE 90.1-2007. Ralph Williams was the project manager for Wal-Mart.
This analysis also borrows heavily on the Pacific Gas and Electric sponsored daylighting code change proposals to the 2005 and 2008 versions of the California Title 24 energy code.
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1. EXECUTIVE SUMMARY
Heschong Mahone Group, Inc. (HMG) sponsored by funding provided by the Department of Energy and administered by the Pacific Northwest National Laboratory, present this code change proposal to upgrade the ASHRAE 90.1 Nonresidential Building Energy Efficiency Standard in terms of its treatment of daylighting. This report is the first of two reports.
The purpose of this report is to provide the technical and economic analysis in support of expanding the requirements for toplighting in the ASHRAE 90.1-2010 building energy code. Toplighting is the admittance of daylight into the building interior through penetrations in the roof (skylights, roof monitors etc) and the reduction of electric lighting energy consumption through the use of daylight responsive controls (photocontrols). The proposal here is focused on the control requirements. Further life cycle cost analysis is being conducted to confirm the breadth of requirements related to requiring that certain spaces be illuminated with skylights (or have other energy features which result in equivalent or less energy consumption).
This report also contains a preliminary analysis of the advisability of requiring photocontrols for areas near perimeters windows (sidelit spaces). This analysis was based on the results of detailed simulations for the wide varieties of climates in California from overcast and mild near the Oregon border to hot and sunny near the border with Mexico.
The second report following this report will be to expand the requirements for sidelighting. Sidelighting is the admittance of daylight into the building interior through penetrations in the perimeter walls (windows) and the reduction of electric lighting energy consumption through the use of daylight responsive controls (photocontrols).
The proposal for skylighting has five (5) principal elements:
1. Prescriptive requirements for skylights in large (greater than 10,000 sf) tall (ceiling height taller than 15 feet) spaces directly under a roof in most climate zones. We are conducting
2. Change the scope of the standard so that the minimum skylight area requirement applies to unconditioned and conditioned spaces. Prior to this proposal there was no reason to regulate the envelope properties in unconditioned spaces. However, with the addition of the skylighting measure, which primarily saves lighting energy, there is now a reason to regulate the daylight admittance of unconditioned spaces.
3. Reduce the daylit area threshold above which photocontrols are required from 5,000 sf1 down to 4,000 sf. This threshold area for the cost-effective use of photocontrols is reduced due to higher forecasts for energy costs and the “scalar” (present worth factor) changed from 8.0 to 8.8.
1 Requirements for photocontrols were introduced in ASHRAE 90.1-2007 Addendum d.
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4. Revisit the definition of the extents of the daylit area under skylights when permanent racks or stacks are used. This new definition is based on detailed Radiosity simulations1 of interior spaces with stacks of different heights.
5. Provide a new definition of the extents of the daylit area under rooftop monitors. The distribution of light from this toplighting source is asymmetric and at high vertical angles from the nadir.
This report also contains three sidelighting proposals:
1. The primary sidelighted area is described in a manner which is relatively easy to understand and maintains the geometric relationship between window s and the area where lights can be turned off. The sidelighted area extends from the window horizontally inwards by one window head height (distance from the floor to the top of the window).
2. Require that general lighting in the primary sidelighted area be controlled by multi-level photocontrols when the total primary sidelighted area in a space exceeds 1,000 sf.
3. A definition of the Sidelight Effective Aperture. This definition relates the fraction of electric lighting based on the fenestration area and the glazing transmittance. In this proposal, when the Sidelight Effective Aperture is very low, one is exempted from the photocontrol requirements.
1 Simulations used the Lumen Designer (Lighting Technologies Inc.) lighting software as well as photometric files of
skylights developed by the PIER (Public Interest Energy Research Program)
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2. PROPOSED CHANGES TO CODE LANGUAGE
We propose the following changes to the 90.1-2007 prescriptive requirements for skylights and lighting controls. Note that this proposal builds on both ASHRAE 90.1-2007 and Addendum d to 90.1-2007. Addendum d contained a number of requirements that both made it easier to effectively save energy with skylighting and required photocontrols when skylights were installed in buildings. We will be differentiating the date of modifications as follows:
ASHRAE 90.1-2007 (black font),
ASHRAE 90.1-2007 Addendum d (blue font underlined).
Proposed new language for ASHRAE 90.1-2010 is shown in green font, and double underlined.
In this proposal, stricken language will be shown as follows:
stricken ASHRAE 90.1-2007 (black font with strikethrough),
ASHRAE 90.1-2007 Addendum d (blue font underlined and with strikethrough).
Comments about the proposal are enclosed in <triangular brackets>
2.1 Proposed Changes to Chapter 2. Scope <Note: this proposed change has not yet been reviewed by the 90.1 committee. This is an initial proposal to expand the scope of the standard so that skylighting (skylights and automatic daylight controls) can be required in unconditioned buildings.>
2. SCOPE
2.2 The provisions of this standard apply to:
a. the envelope of buildings, provided that the enclosed spaces are
1. heated by a heating system whose output capacity is greater than or equal to 3.4 Btu/h·ft2 or
2. cooled by a cooling system whose sensible output capacity is greater than or equal to 5 Btu/h·ft2, and
b. the following systems and equipment used in conjunction with buildings:
1. heating, ventilating, and air conditioning,
2. service water heating,
3. electric power distribution and metering provisions,
4. electric motors and belt drives, and
5. lighting.
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c. The requirements for minimum skylight area as described in 5.5.4.2.3 apply to all buildings (including unconditioned buildings) notwithstanding the criteria in Section 2.2(a).
2.2 Proposed Changes to Chapter 3. Definitions, Abbreviations, and Acronyms < These changes were approved by the full 90.1 committee on Monday January 21, 2008 and will be posted for public review >
Section 3.2 Definitions Daylight area:
a) Under skylights: the daylight area under skylights is the combined daylight area under each skylight without double counting overlapping areas. The daylight area under each skylight is the opening beneath the skylight, plus horizontally in each direction, the smallest of (See Figure 1 & 2):
i.70% of the ceiling height [0.7 x CH], or
ii.the distance to any primary sidelighted area, or the daylight area under rooftop monitors, or
iii.however, the distance to the front face of any vertical obstruction where any part of the obstruction is farther away than 70% of the distance between the top of the obstruction and the ceiling [0.7 x (CH-OH)].
Obstruction Height (OH)
(CH – OH)
(iii) > 0.7 x (CH-OH)
DAYLIGHT AREA UNDER SKYLIGHTS
Ceiling Height
(CH)
(i) CH x 0.7
Figure 1: Section View of Daylight Area Under Skylights
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0.7
x C
H
(i) 0.7 x CH
(i) 0.7 x CH
Edg
e of
day
light
are
a un
der
skyl
ight
s
Edge of daylight area under skylights
(iii) > 0.7 x (CH-OH)
(ii) Primary sidelighted area
Obstruction
= skylight
Figure 2: Plan View Daylight Area Under Skylights
b) Under rooftop monitors: the daylight area under rooftop monitors is the combined daylight area under each rooftop monitor without double counting overlapping areas. The daylight area under each rooftop monitor is the width of the vertical glazing above the ceiling level plus in the horizontal direction of the inward facing normal of the glazing, the smallest of (See Figure 3):
i.the monitor sill height, MSH, (the vertical distance from the floor to the bottom edge of the monitor glazing), or
ii.to the edge of any primary sidelighted area or
iii.the distance to the front face of any vertical obstruction where any part of the obstruction is farther away than the difference between the height of the obstruction and the monitor sill height (MSH-OH).
Obstruction Height (OH)
Monitor Sill Height (MSH) (i)
MSH
DAYLIGHT AREA UNDER ROOFTOP MONITOR
MSH - OH
(iii) > (MSH -OH)
DAYLIGHT AREA
Figure 3: Daylight area under monitors
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c) Primary sidelighted area: the total primary sidelighted area is the combined primary sidelighted area without double counting overlapping areas. The floor area for each primary sidelighting area is directly adjacent to vertical glazing below the ceiling with an area equal to the product of the sidelighted area width and the sidelighted area depth (see Figure 4).
The sidelighted area width is the width of the window plus, on each side, the smallest of:
i.two feet, or
ii.the distance to any 5 feet or higher vertical obstruction.
The primary sidelighted area depth is the horizontal distance perpendicular to the glazing which is the smaller of:
i. one window head height, HH (head height is the distance from the floor to the top of the glazing), or
ii. the distance to any 5 feet or higher vertical obstruction.
Primary Sidelighted
Area
Primary Sidelighted Area
Head Height (HH)
1 x (HH)
Primary Sidelighted
Area
2 ft
2 ft
a) Section View b) Plan View
Head Height (HH)
1 x (HH)
Vertical Obstruction > 60 inches Primary
Sidelighted Area
c) Section View d) Plan View with obstructions with obstructions
Vertical Obstruction > 60 inches
2 ft
Figure 4: Section and Plan View of Primary Sidelighted Area
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Skylight Effective Aperture: the overall amount of visible light transmittance of the roof via skylights. Skylight effective aperture is calculated according to the following formula.
Formula for Skylight Effective Aperture
skylightsunder areaDaylight x WF VLTSkylight x AreaSkylight x 0.85Aperature EffectiveSkylight =
Where,
Skylight area = total fenestration area of skylights
Skylight VLT = Area weighted average visible light transmittance of skylights as determined in accordance with Section 5.8.2.6
WF = Area weighted average well factor, where well factor is 0.9 if light well depth is less than 2.0 ft, or 0.7 if light well depth is 2.0 ft, or greater
Light well depth is measured vertically from the underside of the lowest point on the skylight glazing to the ceiling plane under the skylight.
Effective Aperture: relationship of daylight transmitted through windows to the primary and secondary sidelighted areas.
Formula for Primary Sidelighting Effective Aperture
AreaDaylightSidelitimaryofAreaVLTWindowAreaWindow
ApertureEffectivengSidelightiimaryPr
Pr ∑ ×=
Where window VLT, is the visible light transmittance of windows as determined in accordance with Section 5.8.2.6
Rooftop monitors: vertical fenestration integral to the roof
Toplighting: lighting building interiors with daylight admitted through fenestration located on the roof such skylights and rooftop monitors.
2.3 Proposed Changes to Chapter 9 Lighting Reduce criteria from 5,000 sf to 4,000 sf when photocontrols are required. Add photocontrol requirements for daylight areas below rooftop monitors and in the primary sidelit daylight area by windows. Note: daylight area definitions and definitions of effective aperture have been moved to definitions section.
< These changes were approved by the full 90.1 Committee on Monday January 21, 2008 and will be posted for public review >
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9.4.1.3 Daylit area under skylights. The daylit area under each skylight shall be:
(a) The outline of the opening beneath the skylight, plus 70% of the ceiling height in
each horizontal direction when there are no permanently installed partitions and racks with heights greater than 1.5 m within a horizontal distance of 70% of the ceiling height to the edge of the skylight (See Figures 9-1 and 9-2) - or
(b) The outline of the opening beneath the skylight, plus 40% of the ceiling height in the horizontal directions perpendicular to the racks or partitions, plus 70% of the ceiling height (See Figure 9-1)in directions parallel to the racks or partitions.
(c) Identified on the building plans.
Daylit area under a skylight
H
0.4 x H
> 1.5 m ( > 5 ft)
35°
0.7 x H < 1.5 m (<5 ft)
< 0.7 x H
Figure 9-1: Elevation of Daylit Area under Skylights.
0.7 X 6 m = 4.2 m (0.7 x 20 ft = 14 ft)
0.4 X 6 m = 2.4 m (0.4 x 20 ft = 8 ft)
Edge of daylit area
Edg
e of
day
lit a
rea
Ceiling Height, H = 6 m (20 ft)
Partition or rack > 1.5 m ( 5 ft) tall
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Figure 9-2: Plan View of Daylit Area Under Skylights – 6 m High Ceiling and Partitions Greater Than 1.5 m Tall.
9.4.1.3 Automatic Daylighting Controls for Primary Sidelighted Areas. When
the combined primary sidelighted area in an enclosed space exceeds 1,000 ft2, the lamps for general lighting in the primary sidelighted area shall be separately controlled by at least one multi-level photocontrol (including continuous dimming devices) having the following characteristics:
(a) the light sensor for the photocontrol shall be remote from where calibration adjustments are made;
(b) the calibration adjustments shall be readily accessible; and
(c) the multi-level photocontrol shall reduce electric lighting in response to available daylight with at least one control step that is between 50% and 70% of design lighting power and another control step that is no greater than 35% (including off) of design power.
Exceptions to 9.4.1.4 (a) Primary sidelighted areas where the top of the existing adjacent structures
are twice as high above the windows as their distance away from the windows.
(b) Daylight areas where the sidelighting effective aperture is less than 0.1 (10%).
(c) Retail spaces
9.4.1.4 Automatic Daylighting Controls for Toplighting Skylighting. When
the combined total daylit daylight area under skylights for all skylights plus the total daylight area under roof top monitors in an enclosed space exceeds 5,000 4,000 ft2, the lamps for general lighting in the daylight area daylit area under skylights shall be separately controlled by at least one multi-level photocontrol (including continuous dimming devices) having the following characteristics:
(a) the light sensor for the photocontrol shall be remote from where calibration adjustments are made;
(b) the calibration adjustments shall be readily accessible; and
(c) the multi-level photocontrol shall reduce electric lighting in response to available daylight with at least one control step that is between 50% and 70% of design lighting power and another control step that is no greater than 35% of design power.
Exceptions to 9.4.1.4 (a) Daylit areas under skylights where it is documented that existing adjacent
structures or natural objects block direct beam sunlight for more than 1,500 daytime hours per year between 8 am and 4 pm.
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(b) Daylit areas where the skylight effective aperture of glazing is less than 0.006 (0.6%).
Formula for Effective Aperture (EA)
skylightsunder areaDaylight x WF VLTSkylight x AreaSkylight x 0.85Aperature Effective =
Where,
Skylight area = total fenestration area of skylights
Skylight VLT = Area weighted average visible light transmittance of skylights as determined in accordance with 5.8.2.
WF = Area weighted average well factor, where well factor is 0.9 if light well depth is less than 2.0 ft, or 0.7 if light well depth is 2.0 ft, or greater
Light well depth is measured vertically from the underside of the lowest point on the skylight glazing to the ceiling plane under the skylight.
(c) Buildings in climate zone 8 with daylight areas totaling less than 10,000 8,000 square feet in an enclosed space.
(d) If all general lighting in the enclosed space is controlled by occupant sensor(s) that reduces lighting power at least by 50% and the SHGC of the skylights complies with Section 5.5.4.4.2 without the use of exceptions to Section 5.5.4.4.2
2.4 Proposed Changes to Chapter 5 Envelope for Minimum Skylight Area Requirement <Note this proposal is preliminary pending additional review of simulation studies. This proposal has not yet been reviewed by the 90.1 Committee. Values highlighted in yellow are being reviewed.>
5.5.4.2.2 Maximum Skylight Fenestration Area. The total skylight area shall be less than 5% of the gross roof area.
5.5.4.2.3 Minimum Skylight Fenestration Area. In enclosed spaces that are:
i. greater than 10,000ft2, and
ii. directly under a roof with ceiling heights greater than 15 ft, and
iii. one of the following space types: office, lobby, atrium, concourse, corridor, storage, gymnasium/exercise center, convention center, automotive service, manufacturing, non-refrigerated warehouse, retail, distribution/sorting area, transportation, or workshop.
The total skylight area shall be either;
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a. a minimum of 3% of the roof area of that enclosed space with a skylight VLT at least 0.40, or
b. such that the daylight area under skylights will be a minimum of half the floor area and provide a minimum skylight effective aperture of at least 1%.
These skylights shall have a glazing material or diffuser with a measured haze value greater than 90% when tested according to ASTM D1003. General lighting in the daylight area shall be controlled as described in Section 9.4.1.4.
EXCEPTIONS to 5.5.4.2.3 1. Enclosed spaces in climate zones 8,
2. Enclosed spaces with designed general lighting power densities less than 0.5 W/ft2
3. Areas where it is documented that existing adjacent structures or natural objects block direct beam sunlight on at least half of the roof over the enclosed area for more than 1,500 daytime hours per year between 8 am and 4 pm.
4. Enclosed spaces where the daylight area under rooftop monitors is greater than 50% of the enclosed space floor area.
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3. REASON AND SUBSTANTIATION FOR PROPOSED CHANGES TO DEFINITIONS SECTION (CHAPTER 3)
3.1 Move “Daylighted Area Under Skylights” to Definitions Section The skylight area definition was moved from the lighting section 9.4.1.3 to the definitions section because this definition is used in both the lighting section and the envelope section and it was desirable to reduce redundancy and group all of the daylit area definitions together.
The reason for defining the daylit area under skylights is that you want electric lighting to be controlled where there is sufficient daylight and you don’t want the photocontrol system to turn off lights where there is not enough daylight.
The method used to evaluate the appropriate spacing for skylights was the use of the calculated spacing criterion (SC). These spacing criterions were calculated from measurements of the luminous intensity distributions from white skylights under clear skies1. The method used to calculate the spacing criterion is described in the IESNA Handbook and compares the illuminance between two luminaires (skylights) to that directly below a single luminaire (skylight). This method provides a simple method of evaluating the appropriate spacing of skylights for uniform illumination. This method provides a conservative (overly large) metric for spacing skylights because, “When other criteria are considered, such as overlap
1 Available from the New Buildings Institute, http://www.newbuildings.org/pier/ Heschong Mahone Group,
“Construction and Calibration of Skylight Photometric Test Facility”: Final Report for Task 5.3.5a Skylight Photometry Lab and Calibration. PIER Integrated Design of Commercial Building Ceiling Systems project report.
White skylights 1 ft light well no diffuser
05
1015
2025
30
1 1.2 1.4 1.6 1.8 More
Spacing Criterion (Across)
Freq
uenc
y
0%
20%40%
60%80%
100%120%
Cum
ulat
ive
freq
uenc
y
White skylights 1 ft light well no diffuser
0
5
10
15
20
25
30
1 1.2 1.4 1.6 1.8 MoreSpacing Criterion (Along)
Freq
uenc
y
0%
20%
40%
60%
80%
100%
120%
Cum
ulat
ive
freq
uenc
y
Figure 5: Frequency distribution of spacing criterions
for white skylights
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between luminaires, vertical illuminance, shadowing and illuminance distribution above the workplane, it generally is found that luminaires be installed at some spacing-to-mounting-height ratio less than the value of the luminaire spacing criterion.”1
Photometric measurements of diffusing skylights or skylights with a diffusing lens on the bottom of the light well found that for most samples and for most sun angles, the spacing criterion was calculated to be between 1.2 and 1.4 (McHugh et al. 2002). The distribution of spacing criterions for typical white diffusing skylights is shown in Figure 5. The spacing criterion is a basic indication that for uniform light distribution the luminaires (in this case skylights) should be spaced no further apart than around 1.4 times the mounting height.
As a result, the daylit area is defined as the skylight “footprint” plus additional distance in each longitudinal and lateral dimension – and that additional distance is one half of the spacing criterion or 70% of the floor to ceiling height. The “spread angle” that describes how the skylit area increases with ceiling height is the arctangent of 0.7 or 35 degrees. This is very similar to an earlier definition of the daylit area in ASHRAE 90.1-1989 except the older definition expanded the daylit area out 1 ceiling height away from the footprint of the skylight. This smaller definition of the daylit area under skylights gives the designer the correct signal for maintaining sufficient illuminance uniformity.
The definitions of the well efficiency and daylight area under skylights are used to define the effective aperture of the skylighting system. The effective aperture describes what fraction of the exterior illuminance makes it through the roof above the daylit area. When the effective aperture is below a certain threshold then it is not cost-effective to require photocontrols.
3.2 Address obstructions in “Daylighted Area under Skylights” The current language in addendum d to ASHRAE 90.1-2007 sets the daylit area as the rough opening of the skylights and expanded in each direction 70% of the ceiling height. However, when there are the permanent stacks or partitions in the space that are greater than 5 feet tall, the daylit area is limited to 40% of the ceiling height in directions perpendicular to the stacks.
At the time this was originally proposed, the authors had suggested to the lighting committee that this distance should be a function of the gap between the ceiling and the top of the obstruction. We are re-proposing this concept based on the results of simulations conducted using the lighting software Lumen Designer.
The results of these simulations show that there is at least 1/4 of the maximum horizontal daylight illuminance levels in the aisles that fit inside of the daylit areas when the definition is modified to include the lesser of 70% of the ceiling height or any obstruction that is further way than 70% of the distance between the top of the obstruction and the ceiling.
In Lumen Designer we created a 100 ft × 100 ft room with a 25 ft ceiling. A single 4 foot by 8 foot skylight was placed above the center of the room and we assumed overcast conditions to reduce non-uniformity of the distribution.
1 P. 9-50 to 9-51 IESNA 2000. IESNA Lighting Handbook Ninth Edition, New York.
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Next, we used various arrangements of stacks to see how they affected the daylit zone. In all cases, each stack was 6 ft wide and extended to the end of the room. The stacks were also spaced 7 ft apart from each other. While the stacks were restricted on width and spacing, they could all be shifted left or right. We used three arrangements the skylight located over the stack, to the edge of one stack and centered over the aisle between the stacks. Figure 6 gives an overhead view of these stacks, and the black square indicates the position of the skylight.
Figure 6: Skylight (black square) location relative to stacks
For each of the arrangements shown, five different stack heights were used: 0 ft, 5 ft, 7 ft, 10 ft and 15 ft (a 0 ft stack height implies an empty room). When analyzing each room, we used a calculation grid that was 2.5 ft above the floor and had a spacing of 2 ft.
To make the Lumen Designer results more convenient to work with, they were exported to Excel. Then through a process of formatting the spreadsheets, trends in the daylit area could be observed. The initial simulation had no stacks and we selected the horizontal illuminance at the edge of the daylit zone without stacks as the metric for the illuminance levels we would use to define the edge of the daylit zone with stacks. In the graph below, the plotted points represent the length of the daylit area vs. gap-height. The colored points represent different stack arrangements. One can see that all three cases follow the same general trend line. The solid lines, “70% Gap Height” and “40% Room Height”, show how current conventions compare to simulations.
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Daylit Zone as a Function of Gap Height
05
1015202530
0 5 10 15 20 25 30
Gap Height (ft)
Day
lit Z
one
(ft)
Between Stacks Above Stack EdgeAbove Stack Proposed DefinitionExisting Definition
Figure 7: Daylit Zone as a Function of Gap Height
When looking at this graph, one must consider the fact that the daylit zone would often end abruptly at the edge of a stack, and a large change in stack height would be necessary to allow light into the next aisle. The daylit zone under skylights appears to have a liner relationship with the gap height in a proportion of about 0.9. The convention of 70% of gap height is consistent with the measured spacing criterion of skylights and maintains conservativism of the definition. It is desirable that the definition of the daylit area under skylights is conservative as we want to err on the side that occupants are not left in the dark – something that might result in the lighting controls being disabled. The current definition which defines the edge of the daylit area under skylights as 40% of he ceiling height when the stacks are greater than 5 feet tall can result in daylit areas that are either too large (represented at points with gap height of 10 feet) or too small (when gap height is 18 feet).
The next figure is a direct comparison of the simulated daylit zone to the predicted zone using the proposed definition that the daylit area under skylights is the area directly beneath the skylight of the rough opening of the skylight and extended horizontally in each direction the lesser of:
1) 70% of the ceiling height or
2) to the edge of any stack or partition that is within 70% of the distance between the top of the partition and the ceiling (70% of gap).
If the prediction was perfect all points would lie on the diagonal line with a slope of 1 (i.e. a correlation coefficient of one between the simulated edge of daylit zone and predicted edge of daylit zone). The interpretation is as follows: if a point is on the line
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then the 70% convention accurately predicted the daylit zone; if a point is below the line then the 70% convention overestimated the daylit zone, and if a point is above the line then the 70% convention underestimated the daylit zone. As the graph shows, nearly all points lie above the line, so the 70% gap convention is underestimating the extent of the daylit zone.
Daylight Zone Comparison
05
1015202530
0 5 10 15 20
Proposed Daylit Zone (ft)
Sim
ulat
ed D
aylit
Zo
ne (f
t)
Between Stacks Above Stack EdgeAbove Stack 45 Degree Line
Figure 8: Simulated Edge vs. Predicted Edge
3.3 Definition of “daylight area under rooftop monitors” The objective of this analysis was to determine how stack height affected the extent the daylit zone, where the only source of light was a single sawtooth structure.
The building as shown in Figure 9, was modeled in Lumen Designer, a Radiosity based lighting software program. This model did not include any electric lighting luminaires, it contained only one daylight aperture, and a three foot tall vertical glazing element facing north, that was centered on the vertical side of the sawtooth.
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Figure 9: Building Schematics
Sawtooth width, stack height and stack positions were varied to determine their effect on the daylit area. The sawtooth widths were 3, 6 and 9 ft. Stack heights were 0, 5, 7, 10, 12 and 15 ft (0 ft implies an empty room). Finally, we used 5 different stack positions. Using every possible combination of sawtooth width, stack height and stack spacing, there were a total of 90 simulations. These three variables can be seen in Figure 10, which is a cross-section of the building.
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The stacks were all 2 ft wide, extended to the end of the room along the E-W axis, and were spaced 8 ft apart from each other, so there was always room for 10 stacks. Stack width and spacing were kept constant while they were shifted to the sides to get five floor plans as shown in Figure 11. The black bars represent an overhead view of the stacks and the red, dashed line shows the position of the daylight aperture. Each new floor plan was achieved by shifting all 10 stacks to the right in 2 ft increments.
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Floor Plan 2 Floor Plan 3
Floor Plan 4 Floor Plan 5
Figure 11: Floor Plans 1-5 showing different stack positions (black) with respect to daylight aperture (red dotted line)
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As in the previous analysis, the Lumen Designer iso-footcandle results were exported to a spreadsheet. Then by formatting the spreadsheet so that cells containing illuminances that were one half, one third and one fourth of the maximum illuminance were given different colors, the trends in the daylit zone were observed. In this case there was a grid spacing of 1 ft. From this method one could determine the extent of the daylit region from the aperture. The following two graphs show some typical results; the x-axis is the distance from the top of a stack to the ceiling (Gap Height), and the y-axis is shows the horizontal distance from the daylight aperture to where the light level is one-half of the maximum amount. The first graph shows the trend when the sawtooth was 3 ft wide, and the second graph shows the trend when the sawtooth was 6 ft wide. We looked for “backscatter” of light; light reflecting off of the roof and being directed backwards behind the plane of the glazing. This backscatter was relatively small for all sawtooth opening modeled.
3 ft Sawtooth Trendy = 1.1019xR2 = 0.6661
05
1015202530
0 5 10 15 20 25 30
Gap Height (ft)
Dist
ance
to 1
/2 m
ax
FC (f
t)
Floor 1 Floor 2 Floor 3 Floor 4 Floor 5
Figure 12: Extent of Daylight with 3 ft Sawtooth
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6 ft Sawtooth Trend
y = 1.3398xR2 = 0.7454
0
10
20
30
40
0 5 10 15 20 25 30
Gap Height (ft)
Dist
ance
to 1
/2 m
ax
FC (f
t)
Floor 1 Floor 2 Floor 3 Floor 4 Floor 5
Figure 13: Extent of Daylight with 6 ft Sawtooth
By studying these figures, one could clearly see a trend in the daylit zone. It was, however, not linear. In all cases the trend appeared to follow something like a statistical S-curve, but it could be approximated with a simple line.
The next two figures give a direct comparison of the simulated daylit zone to the predicted one. If the prediction was perfect all points would lie on the diagonal line with a slope of 1 (i.e. a correlation coefficient of one between the simulated edge of daylit zone and predicted edge of daylit zone). . The interpretation is as follows: if a point is on the line then the convention accurately predicted the daylit zone; if a point is below the line then the convention overestimated the daylit zone, and if a point is above the line then the convention underestimated the daylit zone. As the graphs show, most points lie along the line so the convention is fairly reliable.
3 ft Sawtooth Comparison
05
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0 5 10 15 20 25 30
Calculated Daylit Zone (ft)
Dis
tanc
e to
1/2
max
FC
(ft)
Floor 1 Floor 2 Floor 3 Floor 4 Floor 5
Figure 14: Simulated vs. Calculated Daylit Zone with a 3 ft Skylight
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6 ft Sawtooth Comparison
0
10
20
30
40
0 5 10 15 20 25 30
Calculated Daylit Zone (ft)
Dist
ance
to 1
/2 m
ax
FC (f
t)
Floor 1 Floor 2 Floor 3 Floor 4 Floor 5
Figure 15: Simulated vs. Calculated Daylit Zone with a 6 ft Skylight
We considered basing the metric of the extent of the daylit zone being based on the head height of the glazing in the sawtooth monitor. This would have been consistent with the definition we use for the sidelit daylit area, but this definition did not match the data as well. Note that the “gap” as defined in this analysis is the distance between the ceiling and the top of the stacks. Note that this is only ½ of foot different that the gap as defined in our proposal; we are proposing that the gap as the difference in vertical heights between the monitor sill height (MSH) and the stack (partition) height. We decided to use the monitor sill height (MSH) as the basis of the calculation of the extents of the daylit area under monitors because these structures typically have sloping ceilings and it may be hard to determine what the “ceiling height.” is
3.4 Definition of “primary sidelighted area” ASHRAE 90.1-1989 and the versions of the California Title 24 building efficiency standards prior to 2008 defined the daylight area by windows with a daylit zone depth of 15 feet regardless of the window head height. The authors of this report reviewed this assumption on behalf of Pacific Gas & Electric for a revision proposal for the California 2008 Title 24 energy code.
Our hypothesis was that the daylit zone depth is not a single number (15’), but rather is determined by a DZ Depth/HH ratio where there is an optimum distance from the window inside which savings are guaranteed. This assumption of geometric similarity for the daylit zone is what makes using scale models possible when conducting daylighting studies. The assumption is that with all other things equal, that the depth of the daylit zone is proportional to other architectural elements, primarily window heights.
We used two separate studies that looked that at the relationship between the DZ Depth/HH ratio and photocontrols performance to guide the DOE2 based analysis we conducted for the Title 24 revision proposal and for this proposal.
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3.4.1 Sidelighting Photocontrols Study This study set out to describe the current status and performance of photocontrols in those daylit buildings utilizing a “sidelighting strategy”, i.e. with daylight entering a space from windows along the walls rather than from above. Since the study was funded by two California utilities and the Northwest Energy Efficiency Alliance, it focused on buildings in California, Oregon and Washington State along the west coast of the USA. (HMG 2005)
In order to gather candidate buildings for this field study, extensive professional networks were tapped to identify 369 buildings that would potentially fit the study criteria, with daylight provided primarily from the side, and photocontrols installed to reduce electric lighting energy use. A phone survey was conducted with the building managers of 162 of these buildings to verify the status of daylighting, to collect preliminary information and to recruit sites for more detailed on-site surveys. Ultimately, 56 of these buildings were visited, and the monitored performance of lighting energy in 123 spaces in 49 of these buildings was included in the analysis.
The monitored lighting energy performance of the spaces was then compared to an idealized savings estimate based on a DOE2 simulation of the space. The performance for each space was then rated through a Realized Savings Ratio (RSR). The RSR is the ratio of the monitored lighting energy performance onsite against the idealized lighting energy performance from DOE2. A RSR ratio of less than 1 indicates that photocontrol system saves less than the DOE2 predictions.
Following are the results of the study comparing the RSR with the DZ Depth/HH ratio and depth of daylit zone:
Figure 16: Sidelighting Photocontrols Study Results
A B C D E F G H I
Min Max Mean Std. Dev. Min Max Mean Std. Dev.All 4.00 58.75 17.48 9.19 0.40 4.65 1.57 0.86 =0 4.00 41.50 16.57 7.93 0.40 4.65 1.66 0.90 >0 6.00 37.50 17.72 7.43 0.50 3.75 1.46 0.72 >0.5 6.00 24.25 14.51 3.62 0.53 2.00 1.28 0.40
Daylit Zone Depth Daylit Zone Depth to Head Height RatioRSR
Daylit Zone Depth
-5
101520253035404550556065
Min Max Mean Std. Dev.
Day
lit Z
one
Dep
th (f
t)
All =0 >0 >0.5Daylit Zone Depth to Head Height Ratio
-0.501.001.502.002.503.003.504.004.505.00
Min Max Mean Std. Dev.
All =0 >0 >0.5
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The results are presented in aggregate for various bins of RSR values for the monitored spaces (Column A). The ‘All’ bin represents all the spaces monitored, “=0’ bin represents spaces where the photocontrol system is not operational, “>0” represents spaces where the photocontrol system is working, and “>0.5” represents spaces where the photocontrol system is working very well (at least 50% of the predicted savings). Only those spaces where the RSR>0.5 can be called as spaces where the photocontrols are working properly, and ones where the energy savings are fairly guaranteed.
Figure 1 illustrates the values for daylit zone depth. The best functioning systems (RSR>0.5) had daylit zone depths averaging 14.5 feet with a standard deviation of 3.6 feet. Thus, 11’ to 18’ was the norm for well functioning systems.
Figure 1 also illustrates the ratio of the daylit zone depth to head height (DZ Depth/HH ratio). The best functioning systems (RSR>0.5) had ratios averaging 1.3 with a standard deviation of 0.4. Thus, 0.9 to 1.7 was the normal ratio for well functioning systems, with a ratio of 2 as the maximum observed, i.e. a control zone depth that was twice the window head height. This suggests that limiting daylit zone depth for effective photocontrols to 1.7 or 2 times the window head height is a reasonable guideline.
3.4.2 Daylighting Autonomy Studies Studies done at the National Research Council Canada (NRCC) by Christoph Reinhart have also shed more light on the appropriateness of various DZ Depth/HH ratios. In a paper presented at the Building Simulation 2005 conference, Reinhart presents the results of a simulation-based analysis of the daylit zone depth and DZ Depth/HH ratio. He does so by establishing a link between the depth of the daylit zone and the simulated daylight autonomy distribution in a space. (Reinhart 2005)
Daylight autonomy is a percentage number that indicates the percent of occupied day-time hours when a task-specific minimum illuminance is maintained by daylight alone. In Reinhart’s analysis, the depth of the daylit area corresponds to points at which the daylight autonomy falls to half of its maximum value.
Using this basis, daylit zone depths of rectangular sidelit spaces were simulated using Radiance for a variety of climates, facade orientations, façade geometries, and usage patterns as seen in Table 1 below. For all the combination of these variables, a standard window head height was maintained.
Table 1: NRCC Analysis Variables Variable Range # Climates centers Daytona Los Angeles, New York, Vancouver, Winnipeg, 5 Beach, FL CA NY BC MB Facade orientation North South West East 4 VT of windows [%] 35 75 2 Balustrade yes no 2 Sill yes no 2 Occupancy Office classroom 2 Min illuminance [fc] 30 50 2
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Based on a parametric analysis of all these variables (640 design combinations spread over a variety of climate regions), the study presents the distribution of the appropriate daylit zone depth to window head height ratio as seen in Figure 17 below.
Figure 17: NRCC Analysis Results
The results show that in the absence of blinds, predicted daylit zone depths range from as little as 0.5 to over 3.3 times the window-head-height. Over 85% of predicted zone depths fall into the ratio band of 1.0 to about 2.5. In the presence of blinds however, overall daylit zone depths decrease, with 85% of all predictions now lying between 0.8 and 2. Since some type of shading device is necessary in most spaces, an upper boundary of 2 seems to be preferable to the optimistic 2.5 that can be achieved without blinds.
3.4.3 Literature Review Conclusions Based on the results of the two studies, it is clear that energy savings from daylighting controls with windows are largely dependant on how deep the control zone is in relation to the window head height. The DZ Depth/HH ratio expresses this relationship, and results show that a DZ Depth/HH ratio of 2.0 is the maximum allowable ratio for a successful daylighting control system. Results also show that savings are most optimal around a DZ Depth/HH ratio of 1.2.
In considering how the results of this literature review are applied to the Title 24 and proposed ASHRAE standards, it should be recognized that these definitions of the “daylit zone” are different from what is called the “daylight area” in Title 24 and in this proposal. The daylight area defines whether a luminaire should be on a separate circuit or be controlled by a daylighting control. As such one can conceive of the “daylit zone” to extending past the controlled luminaire to halfway to the next row of luminaires. Thus the energy code “daylight area” can be thought of being one half the luminaire spacing less than the “daylit area” in the studies described above. If light fixtures are approximately spaced one head height apart (10 feet apart in a 10 foot ceiling), the
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proposed ASHRAE 90.1 daylight area is approximately one half head height less than the daylit zone considered in these studies.
3.5 Move skylight effective aperture to definitions section The effective aperture definition was previously used only for defining the exceptions to Section 9.4.1.4 for when photocontrols were not required under skylights. We are now also using this definition in a new envelope section 5.5.4.2.3 as an alternative method for defining the minimum skylight area for large open spaces directly beneath a roof. Since this definition is used in both sections it was thought that rather than having redundancy in the stand that this term be defined once.
3.6 Definition of sidelit effective aperture The effective aperture (EA) is a good proxy for the amount of daylight entering the sidelit zone of interest and accounts for the control zone depth, window area, window head height, and window visible transmittance.
The effective aperture is the product of the window area and its visible light transmittance divided by the area of the sidelit zone. This definition of effective aperture matches quite closely to the definition of daylight “lunes” for windows that are infinitely wide. That is the effective aperture gives a good representation of the effect of window size and window location on the fraction of daylight entering a space.
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4. REASON AND SUBSTANTIATION FOR CHANGES TO LIGHTING SECTION (CHAPTER 9)
In most of the cases that we simulated, skylights without photocontrols increase total energy costs. In all of the cases we looked at, skylights matched with a photocontrol system will save total energy costs as compared to the same building with skylights but with no photocontrols. Similar finding were found for sidelighting with and without photocontrols, namely that adding the photocontrol reduces energy costs.
4.1 Moving Skylight Area Definition to Definitions Section The skylight area definition is used in more than one section of the standard and thus to reduce duplication it is defined in the definitions section.
4.2 Adding Photocontrol Requirement for Large Primary Sidelit Daylight Areas As described in the entitled, Definition of “primary sidelighted area” prior research has indicated where photocontrols are most likely to save energy. The highest likelihood of saving energy with photocontrols is under the daylit area under skylights. For sidelighted spaces, savings is most likely if the area controlled is within one window head height of the windows. Energy and energy cost savings were derated to 58% of ideal savings, the same amount of derating that was observed for working controls installed within one window head height of vertical windows.
It should be noted that the areas that are typically controlled in sidelit applications are smaller than the spaces that are toplit. Thus the type the controls that we considered as the basis of this analysis were for smaller stand-alone controls as compared to the larger relay panel type of controls assumed for the toplighting analysis.
The benefit cost analysis of requiring photocontrols in sidelit spaces was to find the balance point where the initial costs are paid for by the product of the annual energy cost savings and the “scalar” the discounted present worth factor for a 15 year period of analysis. As described in the Life Cycle Costing section, the calculated scalar for a 15 year period of analysis is 8.8 present valued years.
There are two types of first costs, fixed first costs that are relatively independent of the area served and variable fixed costs that are a function of the sidelit area. For dimming controls, the variable costs are those associated with the cost of the dimming ballast – the larger the areas served the more ballasts are needed. The installed cost of the dimming controller and sensor are fixed costs as their cost does not increase as a function of area. For multi-level switching systems, the variable costs are those associated with the relays and additional wiring (ASHRAE 90.1 does not yet require bi-level switching and thus this is an added cost).
As a result of discussion with the ASHRAE 90.1 Lighting Subcommittee, the following variable and fixed costs were associated with dimming and switching controls systems. For switching control systems the additional circuiting costs associated with bi-level
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switching is $0.108/sf (for more information see the section entitled “Cost of Multi-Level Switching,” in Section 7.1.5 Life Cycle Costing). The Committee recommended that we use a conservatively high value of $400 for the multi-level switching control, $400 to install the control and $50 to commission the control for a total fixed cost of $850.
For dimming control systems, the Committee recommended that we use a $50 incremental cost per 59 W dimming ballast or for a space with a lighting power density of 1 W/sf, the variable cost would be ($50 per ballast)/ (59 Watts per ballast) x (1 W/sf) = $0.85 per sf. The fixed cost for the dimming controls was conservatively estimated to e approximately $150 for the control, $150 for installation and $50 for commissioning or $350 in fixed costs.
A DOE-2.2 simulation (eQUEST version 3.60) of a 10,000 sf office building located in Chicago was used to estimate savings. This building had 10 foot ceilings, a lighting power density of 1.0 W/sf and a design light level of 30 fc. Continuous strips of windows with sill heights of 4 feet and head heights of 8 feet and with 58% visible light transmittance admitted daylight into the space. Glare was controlled by light colored horizontal blinds which would close whenever the glare index for an observer 12 feet from the windows exceeded 20. Both continuous dimming controls (10% minimum light output 20% minimum power input), and two level plus off switching controls (100%, 50% and 0% steps) were modeled and compared to a “base case” building with no lighting controls.
The dimming controls reduced electricity energy consumption by 1.83 kWh/sf but increased natural gas consumption by 0.024 therm/sf. Similarly the two level plus of switching control saved 2.16 kWh/sf but increased natural gas consumption by 0.027 therm/sf. The increase in gas consumption is expected, in the winter when lights are turned off, the reduced internal loads results in more gas that must be burned to heat the space. Similarly in the winter the reduced internal loads from lighting reduces air conditioning consumption. Approximately 7% of total electricity savings is due to air conditioning reductions.
To better match real savings, and to be more conservative we have derated the theoretical energy savings by 65% to account for some systems not working correctly. This is based on a survey of 123 sidelit spaces and eliminating any systems saving less than 10% of predicted energy. Thus for dimming controls the derated savings are 1.19 kWh/sf and -0.016 therm/sf. For switching controls the derated savings are 1.40 kWh/sf and -0.018 therm/sf. With an electricity cost of $0.0942/kWh, and a natural gas cost of $1.25/therm the annual cost savings are approximately $0.0927/sf·yr for dimming controls and $0.110/sf·yr for dimming controls. The scalar (present worth factor) for a 15 year period of analysis is 8.8. Thus the present valued energy cost savings for the dimming controls is PV$0.815/sf and PV$0.965/sf. See Table 2 for summary results of these calculations.
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Table 2: Costs and savings from sidelit photocontrols (derated to 65%) in Chicago office building
Thus for the daylighting control to be cost-effective it needs to control lighting that is serving at least the amount of area as given in the breakpoint sf equation above.
The breakpoint square footage needed to make the multi-level switching controls is:
Switching breakpoint sf = $950 / ($0.965/sf - $0.108/sf) = 992 sf
For Chicago, which has less daylight than many other cities, the derated savings are less than the variable costs associated with dimming ballasts. However, as shown in Table 3, if one does not derate the energy savings, then the dimming ballast is cost-effective when the controlled area is greater than 860 sf.
Table 3: Minimum threshold area required for cost-effective sidelit controls in Chicago when savings are not derated.
Description Dimming2 level switching
Fixed cost of measure $350 $850Variable cost per sf $0.8475 $0.108DOE-2 elec savings (kWh/sf-yr) 1.83 2.16Elec cost savings $/sf-yr $0.1726 $0.203DOE-2 gas savings (therm/sf-yr) -0.024 -0.027Gas cost savings $/sf-yr -$0.0300 -$0.034Energy cost savings ($/sf) $0.1426 $0.169PV cost savings, scalar = 8.8 (PV$) $1.2550 $1.485Threshold sf for B/C = 1 859 617
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Given that the switching control is minimally compliant control, we have set the threshold to be approximately 1,000 sf. In most other climates the threshold area would be smaller because in more sunny climates the savings per sf would be greater.
4.3 Reduced Daylight Area under Skylights Triggering Photocontrols Requirement A relatively stable way of normalizing the electric lighting impact of a skylighting system with a given climate zone is to consider its effective aperture. The effective aperture approximates the net daylight transmittance of the skylighting system. The effective aperture accounts for the fraction of roof that is not opaque, the transmittance of the skylights and the transmittance of the light well (well efficiency).
The annual energy cost savings resulting from installing photocontrols in skylit building is between $0.10 to $0.35/sf per year in most climates with minimal levels of skylighting (low effective apertures). Under the worst case conditions and a daylit area of 5,000 sf, the energy savings correspond to approximately $500/yr.
Figure 19 through Figure 22 show the minimum daylit area for which the ratio of the benefit to cost of providing photocontrols is 1.0 for the various climate zones and effective apertures. For every climate zone and each effective aperture, a daylit area was calculated that gave a benefit to cost ratio of 1.0 using the formula below.
In this case we have recalculated the scalar to be 8.8. This based on a period of analysis of 15 years, a discount rate of 7%, a tax rate of 37%.…. For these calculations the cost of photocontrols is conservatively estimated at $3,000 for a two zone switching system. The cost of adding additional wiring for bi-level switching has been estimated to be $0.084/sf for warehouse, $0.116/sf for retail and $0.124/sf for office (for more details see the cost of multi-level switching in the life cycle costing section). Thus for a 4,000 sf the maximum cost of installed controls and wiring is approximately $3,500
We have evaluated the minimum daylit area for a high transmittance (Prismatic over prismatic - 72% VLT) and a low transmittance skylight (Low white over clear - 29% VLT) to assure we are not overlooking any effects by normalizing by effective aperture. The results are consistent by effective aperture for a given building model and climate zone. Different buildings have different target illuminances and different climates have different availabilities of daylight. But within a given climate and building type, effective aperture normalizes the results well over different skylight types. A vertical dashed line through the graph represents the typical effective aperture for the given building type. Figure 18 explains this further with a schematic of the graph.
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MINIMUM DAYLIT AREA TRIGGERING PHOTOCTRLS REQ.
Typi
cal
Effective Aperture
Are
a (s
f), B
/C ra
tio =
1
PHOTOCTRLS NOT COST EFFECTIVE
PHOTOCTRLS COST
EFFECTIVE
COST EFFECTIVE
NOT COST
EFFECTIVE
Figure 18: Schematic of Min Daylit Area Requirement Graphs
Warehouse (Heating & Cooling)Low White over Clear Skylight (29% VLT)
Figure 20: Daylit area for which ratio of benefit to cost of providing photocontrols is 1
for various effective apertures, for a warehouse with prismatic over prismatic clear acrylic skylights
In Figure 19 and Figure 20, the warehouse model shows cost effectiveness for areas as low as 2,000 sf for most of the climate zones. Criteria requiring photocontrols in spaces above 4,000 sf floor area are easily cost-effective for all climate zones including climate zone 8 (Alaska).
OfficeLow White over Clear Skylight (29% VLT)
Scalar = 8.8
Typi
cal0
5,000
10,000
15,000
20,000
0.0% 0.4% 0.8% 1.2%
Effective Aperture
Are
a (s
f), B
/C r
atio
=1 1A
2B3C4A5A6A78
Figure 21: Daylit area for which ratio of benefit to cost of providing photocontrols is 1 for various effective apertures, for an office with low white over clear acrylic skylights
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OfficePrismatic over Prismatic Skylight (72% VLT)
Scalar = 8.8
Typi
cal
0
5,000
10,000
15,000
20,000
0.0% 0.4% 0.8% 1.2% 1.6% 2.0% 2.4% 2.8%
Effective Aperture
Are
a (s
f), B
/C r
atio
=1 1A
2B3C4A5A6A78
Figure 22: Daylit area for which ratio of benefit to cost of providing photocontrols is 1 for various effective apertures, for an office with prismatic over prismatic clear acrylic
skylights
The office model shows cost effectiveness at areas lower than the required minimum of 4,000 sf in almost all climate zones except climate zone 8 (Alaska) for its typical effective aperture of 1.2%.
From the above figure one can see that the proposed minimum daylit area requirement of 4,000 sf is providing cost effectiveness for the photocontrols in all climate zones and all building types at their respective typical effective apertures.
The following tables provide an analysis of the benefit to cost ratios for adding photocontrols, to a 4,000 sf daylit area, at various effective apertures for a high transmittance (Prismatic over prismatic - 72% VLT) and low transmittance skylight (Low white over clear - 29% VLT)
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Table 4: Lighting controls B/C ratios by climate zone and effective aperture for a 4,000 sf daylit space in a warehouse
From looking at Table 4, it is clear that controls in a 4,000 sf daylit zone in a warehouse are cost effective in any climate zone. Typically we have found warehouses are at 1% EA, which is indicated on the table with the dashed box.
Table 5: Lighting controls B/C ratios by climate zone and effective aperture for a 4,000 sf daylit space in an office
In office spaces, when one is above 1% effective aperture, the controls are cost effective in all climate zones except CZ 8 (Alaska). For a 4’ x 4’ skylight with a 50% transmittance in a light well with a 75% well efficiency, above a 12 foot ceiling in an office, the effective aperture is 1.2%, which is indicated on the table as typical. The effective aperture is only higher if the skylights are spaced so that their daylit areas overlap (spaced closer together than 20 ft).
The energy savings associated with photocontrols of $0.10 to $0.35/sf·yr ($500 to $1,700 per year) when compared to reasonable skylight effective apertures would pay for the cost of the control system in 2 to 7 years. This payback is less than the ASHRAE scalar of 8.8, used to determine the cost-effectiveness of measures.
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Concerns that photocontrols may not reliably save energy have proven to be unfounded in toplit spaces. Recent monitored power consumption of lighting circuits in 33 toplit spaces by McHugh et al (2004) found that the average savings from photocontrol systems was 98% of that predicted by the SkyCalc simulation software. SkyCalc is based upon the DOE-2 energy simulation software used in this analysis of photocontrol savings.
It should be noted that a similar field survey of sidelit spaces, found substantially lower fractions of saved lighting energy as compared to that predicted by the DOE-2 energy simulation software. We believe there are two reasons for this: 1) it is harder to adequately control electric lighting in sidelit spaces due to the greater daylight gradients in sidelit spaces and 2) the DOE-2 daylighting simulation overestimates the amount of reflected light available deep into a space. A skylighting system with evenly spaced skylights provides more even levels of daylight and is more accurately modeled by DOE-2.2.
4.4 Minimum Daylit Area under Rooftop Monitors Triggering Photocontrols Requirements Requirements are added for rooftop monitors to be consistent with the requirements for skylights.
4.5 Exemption for Photocontrols Requirements Based on Effective Aperture (EA) Figure 24 through Figure 27 detail the average annual energy cost savings from skylighting systems with photocontrols versus those without photocontrols for various effective apertures. The horizontal dashed line through the graph represents the threshold point above which we found the benefit to cost ratio was greater than 1.0 for a 10,000 space. $0.045/sf⋅yr savings corresponds to saving at least $4,000 (the approximate cost of wiring and photocontrol systems) discounted over 15 years using the ASHRAE economic variables or having at least a B/C ratio of 1.0 when using a scalar of 8.8. Though we have shown that 5,000 sf is the appropriate daylit area to start requiring photocontrols, this evaluation of an effective aperture below which skylights are not required for any daylit area regardless of size.
The vertical dashed line through the graph corresponds to the threshold effective aperture point below which the savings are not substantial enough to warrant a requirement of photocontrols. Figure 23 explains this further with a schematic of the graph.
The effective aperture as defined in this proposed change is the net transmittance of the roof which includes the fraction of skylight area, and the transmittance of the skylights and light wells.
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EFFECTIVE APERTURE EXEMPTION FOR PHOTOCTRLS REQ.
Effective Aperture
Ener
gy C
ost S
avin
gs
Due
To
Con
trol
s ($
/sf.y
r)
PHOTOCONTROLS REQUIRED AND COST
EFFECTIVE
PHOTOCONTROLS NOT REQUIRED BUT COST
EFFECTIVE
PHOTOCONTROLS NOT REQUIRED AND NOT
COST EFFECTIVE
PHOTOCONTROLS REQUIRED BUT NOT
COST EFFECTIVE
PHOTOCTRLS REQUIRED
PHOTOCTRLS NOT REQUIRED
PHOTOCTRLS NOT COST EFFECTIVE
PHOTOCTRLS COST
EFFECTIVE
Figure 23: Explanation of Effective Aperture Exemptions Graphs.
Warehouse (Heating & Cooling)Low White over Clear Skylight (29% VLT)
Scalar = 8.8
$0.045
$0.00
$0.05
$0.10
$0.15
$0.20
$0.25
$0.30
$0.35
$0.40
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014
Effective Aperture
Ener
gy C
ost S
avin
gs D
ue T
o C
ontro
ls ($
/sf.y
r)
1A2B3B4A5A6A78
Figure 24: Annual energy savings for a warehouse with low white over clear acrylic
skylights relative to same warehouse but without photocontrols
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Warehouse (Heating & Cooling)Prismatic over Prismatic Skylight (72% VLT)
Scalar = 8.8
$0.045
$0.00
$0.05
$0.10
$0.15
$0.20
$0.25
$0.30
$0.35
$0.40
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035
Effective Aperture
Ener
gy C
ost S
avin
gs D
ue T
o C
ontro
ls ($
/sf.y
r)
1A2B3B4A5A6A78
Figure 25: Annual energy savings for a warehouse with prismatic over prismatic clear
acrylic skylights relative to same warehouse but without photocontrols
OfficeLow White over Clear Skylight (29% VLT)
Scalar = 8.8
$0.045
$0.00
$0.05
$0.10
$0.15
$0.20
$0.25
$0.30
$0.35
$0.40
0 0.002 0.004 0.006 0.008 0.01 0.012
Effective Aperture
Ene
rgy
Cost
Sav
ings
Due
To
Con
trol
s ($
/sf.y
r)
1A2B3B4A5A6A78
Figure 26: Annual energy savings for an office with low white over clear acrylic skylights
relative to same warehouse but without photocontrols
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OfficePrismatic over Prismatic Skylight (72% VLT)
Scalar = 8.8
$0.045
$0.00
$0.05
$0.10
$0.15
$0.20
$0.25
$0.30
$0.35
$0.40
0 0.005 0.01 0.015 0.02 0.025 0.03
Effective Aperture
Ene
rgy
Cost
Sav
ings
Due
To
Con
trol
s ($
/sf.y
r)1A2B3B4A5A6A78
Figure 27: Annual energy savings for an office with prismatic over prismatic clear
acrylic skylights relative to same warehouse but without photocontrols
From the above figure one can see that skylighting systems with effective apertures greater than 0.006 (0.6%) are saving more than $0.066/sf⋅yr for most climate zones. We have evaluated the effective aperture values for a high transmittance (Prismatic over prismatic - 72% VLT) and low transmittance skylight (Low white over clear - 29% VLT) to assure we are not overlooking any effects by normalizing in this manner.
For warehouse areas with relatively low light level requirements, 0.6% EA (effective aperture) results in savings that exceed this threshold for all climate zones. In offices with higher light level requirements and thus higher set points for switching controls, this 0.6% EA threshold still allows cost-effective savings in most of the climate zones, with the exception of climate zone 8 (sub-arctic). In retail, with the highest lighting power density and greatest setpoint, the switching control system is not able to provide cost effectiveness at 0.6% EA for the low transmittance skylight. However with a high transmittance skylight, the switching system begins to respond to the available daylight at lower EAs enabling greater savings. Although not cost effective at 0.6%EA, it can be seen that for higher EAs savings are clearly over the threshold of 0.066$/sf-yr.
The next section also develops plots that show that at high daylit areas, that photocontrols are cost-effective for effective apertures greater than 0.6% for all occupancies except retail with high general lighting power densities. This proposal defines a single effective aperture at which photocontrol requirements are exempted. This is a policy decision for ease of enforcement. If the lighting committee so determined, a more complex rules set could b e created that considered occupancy or the lighting power density of general lighting.
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5. REASON AND SUBSTANTIATION FOR CHANGES ENVELOPE SECTION (CHAPTER 5)
Data developed for this proposal indicate that high transmittance skylights applied in conjunction with photocontrols reduce the life cycle cost of buildings. We believe that prescriptively requiring a minimum skylight area and photocontrols for large open buildings such as warehouses and big box retail can assist ASHRAE in its goal of cost-effectively reducing energy consumption in building by 30%. This prescriptive requirement for skylights in big box buildings is similar to the minimum skylighting requirements in the 2005 version of the California Title 24 energy code.1 [NOTE: This proposal is preliminary pending additional review of simulation studies. This proposal has not yet been reviewed by the 90.1 Committee.]
5.1 Minimum Skylight Area Requirement Analysis For the analysis of requiring skylights, we looked at results from the DOE2 simulations and analyzed the savings from adding skylights to each of the three building types. The runs with skylights and photocontrols were compared to the runs without skylights and no photocontrols. The total costs of adding skylights were added up, which included the following components:
1. Cost of the skylights. This includes the cost of skylight wells for Office only, which had a dropped ceiling.
2. Cost of photocontrols
3. Cost of adding bi-level wiring
4. Cost of increased (or decreased) equipment capacity for heating and cooling due to increased (or decreased) heating/cooling loads.
We also calculated the savings from skylights obtained from the DOE2 runs, and used a scalar of 8.8 for a 15 yrs analysis period. A benefit to cost ratio was calculated for each climate zone.
Table 6 through Table 8 show the Benefit to Cost ratios calculations for all climate zones for two building types: Warehouse and Retail. We added an additional analysis for a warehouse with lower ceiling height: Warehouse (32ft Ceiling), Warehouse (24ft Ceiling).
By lowering the ceiling height, additional skylights are required to maintain daylight uniformity, which adds to the cost part of the equation. We calculated that for a 24ft ceiling height a total of 64 skylights will be required, as compared to 36 skylights require for a warehouse with a 32 ft ceiling height. These skylights would be of a smaller size to maintain the same skylight to floor area ratio (SFR). A detailed explanation of the other three models is provided in Section 7.1.3.
1 Section 143c Title 24, Part 6
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To calculate the savings, we compared a DOE2 run with no skylights and no photocontrols to a run with skylights and added photocontrols. We used a thermally broken, double glazed medium white skylight for Climate Zones 1 through 5 and thermally broken, triple glazed medium white skylight for Climate Zones 6 through 8. For Climate Zones 6 through 8, a triple glazed skylight was chosen to keep with the minimum prescriptive skylight u-factor requirements in ASHRAE 90.1.
Skylight to Floor area Ratio (SFR) of 3% was chosen for Retail and Warehouse, which results in an Effective Aperture (EA) of approximately 0.010 for all three building types.
The photocontrols used were switching controls with On/50%/Off stepped switching pattern for warehouse and On/66%/33% stepped switching for Retail.
The period of analysis was 15 yrs (Scalar of 8.8)
Cost of energy used was $0.0942/kWh and $1.25/therm.
In the tables the following nomenclature has been followed:
“Lighting Savings” Savings in kWh from reduction in lighting energy use due to skylights
“Cooling Savings” Savings in kWh from cooling energy reduction due to skylights (negative means cooling energy use increased with skylights)
“Total kWh Savings” The total of all electric energy savings. This includes lighting, cooling, and ventilation energy use due to skylights
“Heating Savings” Savings in therms from heating energy reduction due to skylights (negative means heating energy use increased with skylights)
“Lifecycle Energy Cost Savings (Scalar 8.8)”
Shows the Energy Savings in $, calculated from the DOE2 simulations multiplied by the scalar of 8.8 for a 15 yrs analysis period.
“Cost of Controls”
Gives the cost of photocontrols. When the period of analysis equals or exceeds 15 yrs, an additional cost of replacement for the photocontrols is calculated using a single present value factor of 0.362. Calculated using the following formula:
( ) ofAnalysisYrsRate Discount11Factor Value Present Single .+
=
“Cost of Skylights” Gives the costs for skylights in $
“Cost of Bi-level Wiring” Gives the incremental cost of adding additional wiring for a bi-level switching control
“Cost of Extra Heating / Cooling Capacity”
Gives the incremental cost of higher (or lower) capacity heating or cooling equipment based on the increased (or decreased) cooling and heating requirement. If the loads
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decreased due to the addition of skylights, this was treated as a negative cost.
“Total Costs” Gives the sum of all the above cost
“Benefit to Cost Ratio”
(BC Ratio) Gives the ratio of the energy cost savings and the total costs. BC ratios above 1 are indicated with yellow, lower than 1, but greater than 0 are indicated with grey, and lower than 0 are indicated with pink.
All the costs mentioned here are explained in detail in Section 7.1.5
Finally we also calculated a “Breakpoint Area” based on BC ratio of 1.0. This is the minimum building area for which the BC ratio will be at least 1.0, calculated using the following formula:
( ) Area BuildingCosts Other All - ingsEnergy Sav
Controls of CostArea Breakpoint ×=
Here:
“Energy Savings” is the lifecycle energy cost savings (Scalar 8.8)
“All Other Costs” is the sum of all other costs except “Cost of Controls”, namely Cost of Bi-level Wiring, Cost of Skylights, and Cost of Extra Cooling/Heating Capacity. These costs are dependent on the area of the building, while cost of controls is independent of the building area.
We also calculate the “Percent of Total Cost Reduction” using the following formula:
Skylights withoutCostEnergy Skylightsto due SavingsCostEnergy Reduction Cost Total of Percent =
Here:
“Energy Cost Savings due to Skylights” is the difference in total energy cost with skylights and without skylights.
“Energy Cost without Skylights” is the total energy cost for the no skylight run.
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From the above tables, it can be seen that the BC ratios are safely above 1.0 for all Climate Zones from 1 through 5 for the Warehouse (36 ft Ceiling), Warehouse (24ft Ceiling), and Retail models. For colder climate zones, Climate Zones 6 through 8, with more stringent U-factor requirements that call for a triple glazed skylight, the cost of skylights increase, and energy savings benefits of skylights decrease compared to the hotter climate zones. As a result, the BC ratios are lower than those in the hotter climate zones, but are still above 1 for Warehouse (36ft ceiling) and Warehouse (24ft Ceiling). In the big box Retail model BC ratio goes below 1 for Climate Zone 8.
The breakpoint area calculated for each climate zone shows that for Climate Zones 1 through 5, the areas are less than 10,000 sf. For the colder Climate Zones 6 through 8, the breakpoint area gets above the 10,000 sf in the Retail model. The breakpoint area represents the minimum building area required to maintain a BC ratio of 1 or more. Hence buildings that have an area greater than the breakpoint area for each climate zone will have a BC ratio greater than 1. From this analysis it can be seen that for all three building types, in Climate Zones 1 through 5, 10,000 sf can be the minimum area requirement for requiring skylights.
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6. BACKGROUND
The ASHRAE 90.1-1989 Standard provided for large areas of skylights that to be exempt from the U-factor requirement for roofs, provided that they were used in conjunction with photocontrols. However, based on the erroneous belief that photocontrols rarely save energy, the requirement for photocontrols was removed in the 1999 version of the Standard and the material properties for skylights were made more stringent.
In 2003, at the behest of the Southern California Edison Company, the Heschong Mahone Group (HMG) monitored the energy savings from 33 photocontrol systems in skylight buildings. This study found that 32 out of 33 systems were working and were saving 98% of the energy as predicted by the SkyCalc6 software. Soon thereafter, the Pacific Gas & Electric Company proposed changes to the California Title 24 energy standards that would require the use of photocontrols in large skylight spaces and the use of skylights in large open low rise spaces. The proposed changes were adopted and took effect in California in 2005.
Wal-Mart has a very effective skylighting design that has been refined over the years that saves substantial amounts of energy costs, as compared with no skylights. This design is based on inexpensive, plastic skylights having high visible light and solar transmittance, and a photocontrolled lighting system that turns down lights in response to daylight. In 2006, a proposal was submitted on behalf of Wal-Mart to allow an exception to the ASHRAE/IESNA 90.1 Standard’s prescriptive SHGC requirements when skylights are diffusing and general lighting is controlled by photocontrols. This proposal identified that the exception to the SHGC requirements would increase energy savings as SHGC and visible light transmittance of plastic skylights are highly correlated. This proposed modification was unanimously approved by the full 90.1 committee and adopted as addendum d to the ASHRAE 90.1-2007 standard.
Thus this proposal is based upon the 2007 standard including all approved addenda.
This proposal seeks to reduce the energy consumption of buildings by turning of lights when daylight is available. The requirement for automatic daylighting controls (photocontrols) is only proposed for those applications where the controls are clearly cost-effective over their projected life.
For large open spaces directly under roof, we have also identified those applications where it is clearly cost-effective (i.e. reduces the life cycle cost of the building) to add skylights and automatic daylighting controls to reduce energy costs.
Whether it is a requirement for photocontrols or skylights and photocontrols, there is a minimum applicable area before these measures are cost-effective. Detailed economic and energy calculations were required to identify this threshold area for each climate zone.
6 SkyCalc is a special-purpose software tool developed by the utilities for use in skylighting sizing and energy
optimization. It is available at www.energydesignresources.com/resource/129/
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7. ANALYSIS METHODOLOGY
The code proposal outlined above was supported by an extensive energy savings and cost-effectiveness analysis. This analysis supported key decisions made in the following areas:
For each climate zone, which skylight cases, in conjunction with photocontrols, yield energy cost savings relative no skylights at all. This is evaluated at the skylight-to-floor area ratio (SFR) of maximum energy savings and also at 5% SFR. The answer to this question would help define which skylight cases should be allowed when used in conjunction with photocontrols.
Under what conditions (climate zone, building type, LPD etc) do skylights and photocontrols have a lower life cycle cost than an opaque roof? The answer to this question may indicate that skylighting could be required for certain occupancies in some climate zones
Under what conditions (space size, effective aperture of skylights etc) do the energy savings from photocontrols pay for their additional cost? The answer to this question would form the basis of a proposal to require photocontrols under cost-effective conditions.
The analysis was based on energy simulations and supported by Life Cycle Costing of the measures promoted through this proposal. The purpose of the simulations was to calculate energy and life cycle cost savings from various skylight parameters, building types and lighting controls.
7.1 Simulation Parameters A total of approximately 40,000 energy simulation runs were conducted by Paul Reeves of The Partnership for Resource Conservation using the DOE2.2 energy simulation engine. The DOE2.2 simulation engine is same engine as used in the popular eQUEST energy simulation tool.
A summary of the simulation parameters is included in Table 9 below. Detailed descriptions of each of the parameters are included in the following sub-sections.
Measure # Parameters List of Parameters
Climate Zone 15 Reference: Table 13
Building Type 3 Warehouse, Retail, Open-Plan Office
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7.1.1 Baseline Assumptions For each of the three buildings, the baseline building was modeled with no skylights and therefore no daylighting controls. Energy cost and life cycle cost savings for each climate zone, building type and skylight configuration was then based on the corresponding baseline building with no skylights and no daylighting controls.
7.1.2 Skylight Specifications Skylight properties were varied to account for high, medium, low and very low transmissivity. This will impact SHGC and visible light transmittance (VLT).
Single, double and triple glazing layers will be modeled. Various skylight frames will also be modeled as a function of the number of glazing layers. Combinations are: Single glazed skylights with metal frames. Double glazed skylights with metal and thermally broken frames. Triple glazed skylights with both thermally broken frames and vinyl frames. The number of glazing layers affects SHGC, VLT and U-factor. The frame configuration affects U-factor only.
Most of the glazing configurations are from a database developed by Joe Deringer for an analysis of ASHRAE 90.1 fenestration requirements. The few modifications come from manufacturer supplied skylight property data.
Skylight areas are incremented as follows: 1%, 2%, 2.5%, 3%, 3.5%, 4%, 5%, and 6% of roof area
U-factor is changed by altering the frame type, the number of glazing layers and for glass skylights by low-e films. Single glazed skylights have metal frames. Double glazed skylights can be either metal or thermally broken metal frames. Triple glazed skylights can be either thermal broken metal frames or vinyl frames.
SHGC and VLT are changed together by investigating common diffusing glazings. For plastic skylights the choices are high white, medium white and low white. Clear layers are added when additional U-factor is desired.
Since we are only interested in diffusing skylights for providing relative uniform daylight into the space glass skylights have either a sheet of laminated glass with a white interlayer or a sheet of prismatic plastic installed in the skylight frame or at the bottom of the light well. If the prismatic plastic diffuser is installed a the bottom of the light well, it is assumed that all of the solar heat gain enters the space directly or by passing through the sides of the light well into the plenum and is not partially filtered out by the diffuser as is the case when the diffuser is installed in the skylight.
A skylight is considered diffusing if the glazing layers have a haze rating greater than 90%. All the skylights modeled have haze ratings greater than 90%.
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Glazing Description SHGC Tvis Baseline Size
Total Horizontal U-factor
U-factor Center Glass
U-factor Edge Glass
Metal, Single glass, Med. wht. interlayer 0.666 0.608 4 x 4 1.285 1.149 1.148 Metal, Single glass, 8% SS 0.219 0.081 2 x 4 1.980 1.190 1.190 Metal, Single glass, Bronze 0.593 0.417 2 x 4 1.980 1.190 1.190 Metal, Single glass, Clear + prism. diff. 0.709 0.785 4 x 4 0.803 0.539 0.632 Metal, Single glass, Evergreen + prism. diff. 0.410 0.591 4 x 4 0.803 0.539 0.632
Metal, Dbl glass, 20% TI over clear + prism. diff 0.237 0.189 2 x 4 1.300 0.570 0.650
Metal, Dbl glass, 8% SS over clear + med. wht. diff. 0.138 0.054 2 x 4 1.300 0.570 0.650
Th.Brk., Dbl Low-e glass, Evergreen over med. wht. 0.265 0.369 4 x 4 0.519 0.412 0.475 Th.Brk., Dbl Low-e glass, Clear over med. wht. 0.353 0.474 4 x 4 0.519 0.413 0.476 Th.Brk., Dbl Low-e glass, Argon, Evergreen over med. wht. 0.255 0.369 4 x 4 0.471 0.339 0.415 Th.Brk., Dbl Low-e glass, Argon, Clear over med. wht. 0.351 0.474 4 x 4 0.471 0.340 0.416 Th.Brk., Dbl Low-e glass, Clear over clear + prism. diff. 0.349 0.613 4 x 4 0.430 0.284 0.374
Th.Brk., Dbl Low-e glass, Argon, Clear over clear + prism. diff. 0.347 0.613 4 x 4 0.402 0.247 0.349
Th.Brk., Dbl glass, 20% TI over clear + prism. diff. 0.237 0.189 2 x 4 1.100 0.570 0.650
Th.Brk., Dbl glass, 8% SS over clear + med. wht. diff. 0.138 0.054 2 x 4 1.100 0.570 0.650
Table 10: Glass Skylight Properties
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Glazing Description SHGC Tvis BaselineSize
Total Horizontal U-factor
U-factor Center Glass
U-factor Edge Glass
Metal, Single plastic, Prism. 0.802 0.826 4 x 4 1.330 1.056 1.055 Metal, Single plastic, Med. wht. 0.589 0.615 4 x 4 1.330 1.113 1.112 Metal, Single plastic, Low wht. 0.387 0.320 2 x 4 1.920 1.110 1.110 Metal, Single plastic, Ultra low wht. 0.181 0.150 2 x 4 1.920 1.110 1.110 Metal, Dbl plastic, Clear over med. wht. 0.542 0.490 2 x 4 1.290 0.570 0.650 Metal, Dbl plastic, Clear over high wht. 0.619 0.750 2 x 4 1.290 0.570 0.650 Metal, Dbl plastic, Low wht. over clear 0.344 0.290 2 x 4 1.290 0.570 0.650 Metal, Dbl plastic, Ultra low wht. over clear 0.161 0.136 2 x 4 1.290 0.570 0.650 Th.Brk., Dbl plastic, Clear over med. wht. 0.542 0.490 2 x 4 1.120 0.570 0.650 Th.Brk., Dbl plastic, Clear over high wht. 0.619 0.750 2 x 4 1.120 0.570 0.650 Th.Brk., Dbl plastic, Low wht. over clear 0.344 0.290 2 x 4 1.120 0.570 0.650
Vinyl, Triple plastic, Clear over clear over prism 0.765 0.890 2 x 4 0.650 0.360 0.510
Th.Brk., Dbl plastic, Ultra low wht. over clear 0.161 0.136 2 x 4 1.120 0.570 0.650
Vinyl, Dbl plastic, Clear over med. wht. 0.542 0.490 2 x 4 0.840 0.570 0.650
Th.Brk., Dbl plastic, Prism. over prism. 0.690 0.719 4 x 4 0.710 0.516 0.573
Th.Brk., Triple plastic, Prism. over prism. over prism. 0.614 0.631 4 x 4 0.666 0.333 0.411
Th.Brk., Triple plastic, Clear over clear over med. wht. (U 0.91) 0.499 0.450 2 x 4 0.910 0.360 0.510
Th.Brk., Triple plastic, Clear over clear over med. wht. (U 0.66) 0.560 0.414 4 x 4 0.666 0.339 0.415
Th.Brk., Triple plastic, Clear over clear over high wht. 0.585 0.690 2 x 4 0.910 0.360 0.510
Table 11: Plastic Skylight Properties
Actual U-factors versus rated or default U-factors
The tables in the ASHRAE Handbook of Fundamentals and the default skylight properties and the U-factor requirements for skylights are all based on the old NFRC residential rating criteria for skylights. This old criterion assumed skylights were over a 2’ by 4’ opening. The recent NFRC criteria require that skylights be rated for a 1.200 x 1,200 mm size (48” x 48”). More heat loss occurs through skylight frame and edge of glass per unit area than through the center of glass.
As skylights get larger there is a greater ratio of center of glass area to edge of glass and frame. As a result, the tables in the Fenestration Chapter of the ASHRAE Handbook of Fundamentals, the default tables and the required skylight U-factors are all higher than current NFRC ratings for the same product configurations. This analysis adjusts U-factors with respect to the actual size of skylights modeled. As the skylight increases in size, the U-factor drops relative to the ratios of: frame, edge of glass and center of glass areas. We assume that when the skylight size is greater than 32 sf,
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that additional skylights would be used rather than making a larger skylight. Thus skylight U-factors do not drop any lower than what would be the case for a 32 sf skylight.
The edge of glass area is assumed to be 0.7 inches wide and the frame area is assumed to be 2.5 inches wide. See Figure 28 for an illustration of the frame, edge of glass and center of glass areas.
Figure 28: ASHRAE Skylight Size, Frame, Edge and Center of Glass Dimensions
7.1.3 Building Types and Specifications
Warehouse Model
The warehouse modeled is 82,944 sf with 32 ft ceiling height. The stacks in this space are 15 feet tall and have an effective reflectance of 40%. The walls, floor and ceiling of the space have reflectances of 50%, 20% and 60% respectively.
The warehouse is modeled with sidelit windows with a 3% Window/Wall ratio.
We are interested in looking at the impact of skylights on a heated only warehouse with gas unit heaters.
General lighting systems have a maximum lighting power density of 0.9 W/sf. based on Table 9.6.1 of the ASHRAE 90.1-2004 Standards.
Lighting systems and controls:
High or medium bay HID lighting
a. Single level on/off control
24”
48”
48” – 2 x 0.7” =
46.6” – 2 x 2.5” = 41.6”
Center of glass
Edge of glass Frame
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b. Two level on/off control (On/50%/OFF)
Retail Model
The big box retail store is modeled is 46,656 sf with 24 ft ceiling height. The stacks in this space are 15 feet tall and have an effective reflectance of 40%. The walls, floor and ceiling of the space have reflectances of 50%, 20% and 60% respectively.
The building we are considering is heated and cooled with roof top air cooled roof top units. Outside air rates are determined as according to ASHRAE 62.
The retail space is modeled with sidelit windows with a 1.5% Window/Wall ratio.
Lighting power density of general lighting set to 1.7 W/sf based on Table 9.6.1 of the ASHRAE 90.1-2004 Standards.
Lighting systems and controls: There are two likely types of lighting systems with their associated controls:
High or medium bay HID lighting
c. ON/67%/33%
T-8 fluorescent industrial strips
d. Fluorescent dimming 20% power at 10% light level
Office Model
The office is modeled as the core zone of an open plan office with 11,664 sf area and 12 ft high ceiling. The office prototype has a dropped ceiling, and the skylights are modeled with a 4’ light well. The partitions in this space are 5 feet tall and have an effective reflectance of 40%. The walls, floor and ceiling of the space have reflectances of 50%, 20% and 80% respectively.
The building we are considering is heated and cooled with roof top air cooled roof top units. Outside air rates are determined as according to ASHRAE 62.
The office model is modeled with no sidelighting, since the space is assumed to the core zone of a larger open-office space.
Lighting power density of general lighting set to 1.1 W/sf based on Table 9.6.1 of the ASHRAE 90.1-2004 Standards.
Lighting systems and controls:
T-8 fluorescent recessed troffers
e. Two level + Off (ON/50%/OFF)
f. Fluorescent dimming 20% power at 10% light level
7.1.4 Weather Locations We will use the representative US locations that were used to define the weather zones for ASHRAE. Below is an extract from the “Climate Classification for Building Energy Codes and Standard” document generated by PNNL.
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B. Thermal Zone Definitions Zone No.
Climate Zone Name and Type
Thermal Criteria(1,8) Representative U.S. City*
Köppen Class.
Köppen Classification Description
1A Very Hot – Humid 5000 < CDD10ºC Miami, FL Aw Tropical Wet-and-Dry 1B(7) Very Hot – Dry 5000 < CDD10ºC --- BWh Tropical Desert 2A Hot – Humid 3500 < CDD10ºC ? 5000 Houston, TX Caf Humid Subtropical (Warm Summer) 2B Hot – Dry 3500 < CDD10ºC ? 5000 Phoenix, AZ BWh Arid Subtropical 3A Warm – Humid 2500 < CDD10ºC ? 3500 Memphis, TN Caf Humid Subtropical (Warm Summer) 3B Warm – Dry 2500 < CDD10ºC ? 3500 El Paso, TX BSk/BWh/H Semiarid Middle Latitude/Arid
Subtropical/Highlands 3C Warm – Marine HDD18ºC ? 2000 San Francisco, CA Cs Dry Summer Subtropical (Mediterranean) 4A Mixed – Humid 2500 ? CDD10ºC AND
Table 12: Thermal Zone Definitions for ASHRAE 90.1 Standards
The ASHRAE requirements are specific to 8 climate zones where 1 is the warmest and mildest climate and climate zone 8 is the coldest. In addition, these climate zones are further subdivided by A, B and C based on humidity. We are only modeling those zones which have a representative city in the US. Thus climates 1B and 5C are not modeled.
In reviewing simulation results we found that models using the Chicago weather files did not match the general trends of energy consumption with respect to climate. Perhaps the Chicago weather file has anomalies. Thus we decided to use another city in climate zone 5A. The city we selected is a TMY2 “primary” weather station, Indianapolis, IN. TMY2 primary weather stations make use of measured solar radiation data, most of the other files make use of modeled solar radiation.
Zone# City, State
1A Miami, FL 2A Houston, TX 2B Phoenix, AZ 3A Memphis, TN 3B El Paso, TX 3C San Francisco, CA 4A Baltimore, MD 4B Albuquerque, NM 4C Salem, OR
5A Chicago, IL Indianapolis, IN
5B Boise, ID 6A Burlington, VT 6B Helena, MT 7 Duluth, MN 8 Fairbanks, AK
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Table 13: Climate zones summary
7.1.5 Life Cycle Costing For our purposes the life cycle cost LCC is:
LCC = FCS + FCPC + FCAC + (Scalar x Annual Energy Cost)
Where,
FCS = first cost of skylights, $
FCPC = first cost of photocontrol system, $
FCAC = first cost of air conditioning (associated with the skylight loads), $
Scalar = present worth multiplier for energy costs, 8.8 present valued years for the 15 year analysis.
Annual Energy Cost = annual cost of operating heating (natural gas), cooling and lighting systems (electricity) in a prototype building in a typical weather year, $/yr
It is assumed that the presence of skylights and photocontrol systems will have negligible effects on maintenance costs for the first 15 years.
Cost of Energy
Cost of electricity - blended electricity rate of $0.0942/kWh
Cost of natural gas - $1.25/therm
Scalar (Present Worth Factor)
The source of the fuel prices and the basis of the scalar ratio is a memo developed by Merle McBride entitled, “SSPC 90.1-2010 Fuel Prices and Scalar Ratio,” dated January 19, 2007. These variables and their outcomes are contained the table below generated by the spreadsheet “901Scalar Ratio Calculations (12-18-06).xls” provided by Eric Richman at PNNL.
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Table 14: Economic variables as the basis of scalar calculations
Scalars Heating Cooling11 First Cost Scalar - S2 0.835 0.83512 Htg & Clg Scalars - Sh & Sc 7.39 7.3913 Scalar Ratio - SR 8.8 8.8
Light yellow are user input variablesLight blue are calculated values
Input Variables for Scalar Ratio Calculations
Period of Analysis and Life of Equipment
The following estimates of the life of equipment have helped define the parameters used in the life cycle cost analysis.
Life of Skylights – 20 years In talking with Wal-mart’s construction manager, they expect the skylights to last 20 years but could be longer. Wal-Mart has approximately 300,000 skylights – oldest are about 13 years old. Deterioration is small and they expect the skylights to be intact for at least 20 years. A small percentage or damaged by hail. On new buildings in those areas of the country with higher likelihoods of hail they are specifying skylights tougher plastics.
This analysis is conservative in that our period of analysis was only 15 years.
Life of photocontrols – 15 years In talking with the lighting subcommittee the consensus is that photocontrols should be reasonably expected to last 15 years. When the Heschong Mahone Group conducted their sidelighting survey, one the controls with the greatest energy savings was installed in 1989 or about 15 years prior to the survey. Bi-level wiring would not have to be installed. We expect that the incremental costs will be lower in 15 years but to maintain the conservative approach to estimating life cycle cost, we assume the photocontrol system is replaced after 15 years but that there is no costs associated with bi-level wiring or skylight maintenance.
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First Cost of Skylights
Skylight costs were derived from and manufacturers’ cost for installed skylights as well as the costs developed by the envelope committee7 for ASHRAE standard 90.1. The ASHRAE figures all were in terms of incremental costs above a single glazed skylight with a metal frame. We then fit a regression line through the data to average the costs. In general the ASHRAE costs were quite close to the average of the manufacturers we interviewed. An example of this is shown in Figure 29 and Figure 30, where the squares represent the ASHRAE cost values and the diamonds represent the manufacturer estimates of installed costs.
Triple glazed plastic skylight: clear clear over white
145
492614
737
983
590
355319
852912
1,180
606644552 571
y = 21.3x + 160.0R2 = 0.5
0
200
400
600
800
1,000
1,200
0 5 10 15 20 25 30 35
Skylight area (sf)
Inst
alle
d co
st p
er s
kylig
ht (
$)
Figure 29: Regression Plot of Cost of Triple Glazed Plastic Dome Skylight vs. Size
7 We thank Joe Deringer for his assistance in obtaining this data.
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Double glazed glass skylight: clear low-e over clear
211
842
1,052993
1,811
1,0301,217
1,517
1,217
1,440
1,888
1,591y = 49.229x + 179.69
R2 = 0.9439
0200400600800
1,0001,2001,4001,6001,8002,000
0 5 10 15 20 25 30 35
Skylight area (sf)
Inst
alle
d co
st p
er s
kylig
ht ($
)
Figure 30: Regression Plot of Cost of Double Glazed Glass Skylight vs. Size
Based upon these regression equations, we then were able to estimate the costs of the skylighting system for each prototype building for each of the skylight to floor ratios from 1% to 6%. These results are given in the tables below.
Building Area>> 82,944 sq.ftWarehouse
Skylight Fraction Index >> 1% 2% 2.5% 3% 3.5% 4% 5% 6%HMG_ID Area per skylight >> 23.04 46.08 57.6 69.12 80.64 92.16 115.2 138.24
Table 18: Installed Plastic Skylight Costs for Retail Big-Box Prototype
Note that typical construction practices limit skylight sizes to 32 sf per skylight. So that even though the estimates are based upon 36 skylights that increase in size, the cost data here is smoothed and provides a cost break on larger skylight areas. This reflects that the costs of a skylight include both fixed costs and variable costs.
Building Area>> 11,664 sq.ftOffice with Dropped Ceiling and Light Wells
Skylight Fraction Index >> 1% 2% 2.5% 3% 3.5% 4% 5% 6%HMG_ID Area per skylight >> 3.24 6.48 8.1 9.72 11.34 12.96 16.2 19.44
Table 20: Installed Plastic Skylight Costs for Office with Dropped Ceiling Prototype
For the office prototype the costs also include the costs of the light well. This is because the office prototype is modeled with a dropped ceiling.
First Cost of Lighting Controls
The installed costs of adding a photocontrol system to each of our prototype buildings is given in Table 21. The costing of controls for the 82,944 sf warehouse, and the 46,656 sf big box retail is based upon 4 controls zones. That is there are 4 different desired illuminance levels in different areas of these buildings. This is perhaps conservative in that there may only be really two zones in a warehouse: one zone that is for shipping and receiving that has higher light levels but also usually without stacks to absorb light, and a second zone for lighting in the stacks. The prototype for the office with a dropped ceiling is only 11,664 sf and thus we consider the costs for this prototype to have only two control zones.
Table 21: Cost of Photocontrol Systems for each Prototype Building
On/off control is a single control step; two levels (100%, 50%) plus off is a two step control as is 2/3 or 1/3 switching; (no off) is also a two step control; three levels (100%, 67%, 33%) plus off is a 3 level control. Potentially in a 4 zone building with three level plus off control their need to be as much as 3 x 4 = 12 separately controlled outputs.
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Dimming controls for fluorescent systems have a dramatically different light and power relationship than those for HID systems. A fluorescent dimming system consumes approximately 20% of rated power at minimum (10%) light output. A metal halide system typically consumes in excess of 50% of power at minimum (25%) light output. Thus a fluorescent dimming system is better suited for savings energy than HID systems, and we have based our analysis on fluorescent dimming.
Cost of Multi-Level Switching
The purpose of this section is to describe the calculation for costing the addition of multi-level daylighting controls to an ASHRAE 90.1-2004 minimally compliant building.
This minimally compliant (base case) building has: Automatic shut-off controls (timeclock) controlling a lighting contactor (relay)
that switches fixtures on and off.
Conduit serving a row of fixtures carries one neutral and one hot conductor
All of the lights in a given section of the building can be turned on and off together. Bi-level switching or control is not required.
The proposed case is the same building and lighting system except: A 3 level plus off photocontrol system is installed. As daylight levels increase,
three circuits are sequentially turned off. As daylight levels decrease the circuits are sequentially turned on.
The lighting system has more lighting contactors but with correspondingly fewer poles per contactor to support the greater levels of control.
Lighting is circuited so that conduit serving a row of fixtures carries one neutral and three hot conductors so that the three levels of control are available in each row of lighting. Fixtures closest to the skylights are turned off first as daylight levels increase and those furthest away from the skylights are turned off last.
Wiring Description:
We assume a 4-wire (3 hot + 1 neutral) home run from the main control box for both the base case and proposed case design.
For the base case, each branch from the home run is 2-wire (1 hot + 1 neutral) For the proposed case, each branch from the home run is 4-wire (3 hot + 1 neutral)
Three, 3-pole lighting contactors are used at the control panel (see diagram below) to provide a three phase, multi-level switching system. When possible, good electrical design tries to balance phases across each stage of lighting control. This is not always possible for small daylighting systems.
The calculation method is described below using the warehouse model as an example (the first line in the spreadsheets shown below).
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Figure 31: Control Panel Diagram for a 3-level switching control.
Fixture and Circuit Layout
Fixture type assumed is 200W PS Metal Halide - Sylvania M200/PS METALARC M200 series ballast - 232 W input, and 0.9 Amps input current. By picking relatively low wattage fixtures, we are being conservative as the fixture spacing is closer together and thus there is more linear feet of wire per square foot of area served.
For a warehouse that is 83,000 sf at 0.9 W/sf,
232W / 0.9W/sf = 258 sf/fixture
If one divides 83,000 sf by 258 sf per fixture, this yields 322 fixtures.
Assuming a rectangular spacing grid, the square root of 258sf/fixture is 16.1 or approximately a 16ft by 16ft spacing of fixtures.
Thus the design has a regular spacing of 16 feet between conduit runs. If one divides the total building area of 83,000 sf by the 16 feet between conduit runs, this results in a total conduit length of 5,187 linear feet of conduit length for the rows of conduit serving lighting. On average there is 1 linear ft of branch conduit length for each 16 sf of area.
To estimate the design amperes on each circuit we use the following calculation:
Design Amps = Rated Amps x Continuous Duty Derating x Circuit Fill Factor
For 12 gauge wire with a nominal rating of 20 amps, the design amps for lighting circuits with a continuous duty derating factor of 80% and a circuit fill factor of 80%, the design amps are:
Design Amps = 20 x 0.8 x 0.8 = 12.8 Amps.
For the 200W PS Metal Halide with a rating of 0.9 Amps/fixture, the total number of fixtures per circuit is 12.8 Amps / (0.9 Amps/Fixture) = 14.2 Fixtures per circuit rounded down to 14 fixtures per circuit. With 322 fixtures in the building, approximately 322 fixtures / (14 fixtures/circuit) = 23 circuits are needed for the warehouse.
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Warehouse 322 277 0.90 232-10 poles + 1-3pole $2,288.86 9-3 poles $3,555.00 $0.015 $0.084 $258
Retail 343 277 0.90 252-10 poles + 1-6pole $2,526.93 9-3 poles $3,555.00 $0.022 $0.116 $201
Office 113 120 0.95 9 1-10pole $946.93 3-3 poles $1,185.00 $0.020 $0.128 $167*12 ga wire, 20 amp rating, 80% derating, 80% fill =net 12.8 Amps per circuit
Table 24: RS Means CostWorks 2005 CD - Cost for 3-Pole Lighting Contactor
Qty CSI Number Description CrewDaily Output
Labor Hrs Unit Bare Mat.
Bare Labor
Bare Equip. Bare Total
Total Incl. O&P Type Year
1 164202000200Lighting contactors, 3 pole, electrically held, 600 volt, 30 amp, AC enclosed NEMA 1 1 Elec 3.6 2.222 Ea. $235.00 $90.50 $0.00 $325.50 $395.00 Union 2005
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Cost Estimation
To calculate the cost difference between base case with single level switching and proposed case with multi-level switching, we are adding 2 wires to the branch conduits and changing from a 10-pole lighting contactor to three, 3-pole lighting contactors at the control panel. See accompanying spreadsheet for cost data. The cost data is from the 2005 version of the RS Means CostWorks construction cost estimating guide.
The RS Means total installed cost of THHN wire is $50 per 100 linear feet including overhead and profit.. We doubled this amount even though labor costs would be less than double for two wires.
We used the total installed cost of lighting contactors including overhead and profit. However, RS means only has pricing for 3 pole lighting contactors. To calculate the pricing for 10 pole and 6 pole contactors, we used the Square D published list prices for 3, 6 and 10 pole contactors from their 2003 catalog to scale up the RS Means costs to these other contactor sizes.
Table 25: Estimation of Lighting Contactor Costs
Lighting Contactor
2003 Sq D Cat.
Cost Factor
2005 Means
2005 Estimate
3-pole $370 100% $395 $395
6-pole $593 160% $633
10-pole $887 240% $947
The installed costs of adding a photocontrol system to each of our prototype buildings is given in Table 26. The costing of controls for the 82,944 sf warehouse, and the 46,656 sf big box retail is based upon 4 controls zones. That is there are 4 different desired illuminance levels in different areas of these buildings. This is perhaps conservative in that there may only be really two zones in a warehouse: one zone that is for shipping and receiving that has higher light levels but also usually without stacks to absorb light, and a second zone for lighting in the stacks. The prototype for the office with a dropped ceiling is only 11,664 sf and thus we consider the costs for this prototype to have only two control zones.
Table 26: Cost of Photocontrols and Total Control System
Building model
Control description
(# levels + off, # of zones)
Cost of Photocontrols
Cost of Photocontrols
($/sf.)
Total Cost (wiring, contactors
& photocontrols) ($/sf.)
Total Cost (wiring, contactors
& photocontrols) ($/cntrl pt.)
Warehouse 2 level, 4 zones $4,522 $0.05 $0.14 $425Retail 2 level, 4 zones $4,522 $0.10 $0.21 $368Office 2 level, 2 zones $2,849 $0.24 $0.37 $483
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First Cost of Air Conditioning/ Heating Equipment
Cost of air conditioning is based off of RS Means catalogue data for a rooftop unit with gas furnace sometimes called a “gas pack.”
The incremental cost of increasing the size of a rooftop unit in the 6 to 12 ton range is approximately $740/ton including materials, labor, overhead and profit. This estimate is derived from a regression analysis of RS Means data, the slope of the line is the incremental cost of increasing the size of the air conditioning unit. This incremental cost does not include the cost of duct work as many of the large spaces that would be using skylights as envisioned here would have relatively short duct runs.
Cost of Rooftop Units Between 6 - 12.5 Tons
y = 741.33x + 2653.1R2 = 0.9858
0
4,000
8,000
12,000
5 6 7 8 9 10 11 12 13
Tons
Tota
l Inc
l. O
&P
($)
Figure 32: Trend line of RS Means estimated costs for RTU’s – 6 to 12.5 tons
Figure 33: Trend line of RS Means Estimated Costs for Unit Heaters - 20 to 320 MBH
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8. REFERENCES
IESNA 2000 IESNA Handbook 9th ed. New York, Illuminating Engineering Society of North America
McHugh, J, Lewin, I, Domigan, J, “Skylights as luminaires: PIER skylight photometric test results.” Proceedings of the IESNA Conference 2002, 427-442. IESNA, New York
McHugh, J., Pande, A., Ander, G.& Melnyk, J. “Effectiveness of Photocontrols with Skylighting,” Proceedings of the 2004 IESNA Annual Conference, Tampa, FL.
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9. APPENDIX 1 – PLOTS OF ILLUMINANCE UNDER SKYLIGHTS WITH DIFFERING SHELVING HEIGHTS
The following figures show the analysis results after being exported to Excel. Numbers on the top row and left column are grid numbers. Yellow represents the daylit zone, blue represents the position of each stack (if any), pink represents the original daylit zone (no stacks), and red represents the position of the skylight. In all of these figures, the skylight is between two stacks. In some cases a stack’s edge will lie between two grid squares, and at other times the edge will lie on top of a square; for this reason, some stacks will appear to have different sizes, even though they are all 6 ft wide.
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10. APPENDIX 2 – PLOTS OF ILLUMINANCE UNDER SAWTOOTH MONITOR WITH DIFFERING SHELVING HEIGHTS
The following figures show the analysis results after being exported to Excel. Numbers on the top row and left column are grid numbers. Looking at all the cells there will be some maximum value; pink represents the cells that are over 1/2 of that maximum value, orange represents the cells that are over 1/3 of that maximum value, and yellow represents the cells that are over 1/4 of that maximum value. Blue represents the position of each stack (if any), the green bar represents the position and width of the sawtooth, the red dashed line represents the position of the aperture, and the gray bar represents the extent of the calculated daylit zone.
