Building Energy Effects of Green Roof Design Decisions
David J. Sailor, Ph.D., T.B. Elley, and M. Gibson Mechanical and Materials Engineering Department
Portland State University [email protected]
Presented at BEST3 – Atlanta
April, 4 2012
Outline
• Overview – Why green roofs? – Performance claims
• Background on surface energy balance – Building – Environment
• Building energy simulation methodology – Energy model – Design variations
• Results and Discussion
2
Why green roofs?
• Aesthetics and recreation • Biodiversity and habitat • Roof life • Air quality • Storm water quality • Storm water runoff reduction • Urban Heat Island (UHI) • Building Energy Consumption
3
4 Image from: toronto.ca ESRI Canada Ltd., 12 Concorde Place, Toronto
Regulations/Laws: Toronto Green Roof Bylaw (energy & stormwater)
6 Image from: atlurbanist.tumblr.com Clough Building at Georgia Tech., Atlanta
Codes & Standards: LEED (sustainable sites—water, UHI; energy)
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Image from: gardenvisit.com Vancouver Convention Ctr.
(Canada’s largest at 6 acres) Green bragging rights and market value
Average rooftop (membrane) surface temperatures for a standard and a green roof from the Penn State University field experiment.
Denardo et al., ASAE, 2003.
~11 o C ( 20 o F)
warmer at night
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Student Union, Univ. Central Florida.
J. Sonne, ASHRAE J., 2006
Maximum Summer Roof Temperatures: * green roof avg. of 22 o C (39 o F ) cooler than a conventional roof. * green roof warmer at night
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Temperature
Heat Flux
Results from simplified models “…energy savings (up) to 48%...”
• Modeled green roof as a simple reduction in U-Value (increase in R-value of about 2.5 ft2 o F /Btu). • Neglected thermal storage, moisture-dependent performance, and differing behavior with respect to environmental conditions. • Savings reported are percent of HVAC energy (not total building energy)
• “Moderate insulation” was R-7 to R-8; “Well insulated” was R-14 to R-22.
… majority of existing and new buildings
12 Nichaou et al, Energy and Bldgs., 2001
Measurements at ORNL
“Results showed vegetative roofs reduced heat gain (reduced cooling loads) compared to the white control system … by approximately 61%... to 67%. “ 4ft by 4ft test sections on ORNL’s RTRA Only insulation was fiberboard with R-3.8.
13 Desjarles et al, RCI Proceedings, 2010
The Roof is only one Contributor to HVAC Loads
Internal loads &
ventilation
Roof
Walls
Windows
Infiltration
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Protection & Drainage Layers and Roof Construction
Longwave radiation
Latent heat (evapotranspiration)
Conduction
Shortwave radiation
Heat Transfer on a Green Roof
Sensible heat (convection)
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• Shading (LW & SW) • Evapotranspiration • Insulation • Thermal storage • Moisture balance
Conventional Roof – Day Tsurf = Tmembrane ~ 120-150 o F
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Heats up rapidly during summer day…
Conventional Roof -- Night
…but cools off rapidly at night.
“Cool” White Roof -- Day
Tsurf = Tmembrane ~ 90-110 o F
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Doesn’t heat up as much during summer day…
“Cool” White Roof -- Night
…and cools off significantly at night.
Green Roof-- Day
Tsurf ~ 90-110 o F Tmembrane ~ 80 – 100 o F
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Doesn’t heat up much during summer day…
Green Roof- Night
…but remains warm at night due to stored heat.
Building Energy Model: U.S. DoE’s EnergyPlus †
Plant Canopy Model: FASST ‡
† www.eere.energy.gov/buildings/energyplus ‡ S. Frankenstein, G. Koenig, FASST Vegetation Models, Technical Report TR-04-25, 2004. Sailor, D.J., “A Green Roof Model for Building Energy Simulation Programs”, Energy and Buildings 2008.
A Physically-Based Approach: Green Roofs in EnergyPlus
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[ ] ( ) fffggfgf
fgfffirffSff LHTTTIIF ++−
−++−+−= ↓↓ 444)1(
εεεεσεεσ
σεεασ
)(*)*1.1( , fafaffapaff TTWCCLAIH −= ρ
FOLIAGE
GROUND SURFACE
,* ( )f f af f af af f satL l LAI C W r q qρ ′′= −
[ ] ( )z
TKLHTTTIIF g
ggfggfgf
fgfggirggsfg ∂
∂+++−
−+−−+−−= ↓↓ *)1()1( 444
εεεεσεεσ
εεασ
)(, gafafhgapagg TTWCCH −= ρ
( )gafagafggeg qqWlCL −= ρ,
Storage
SW H
LW L
G
Sailor, D.J., Energy and Buildings, 2008. 22
Modeling Study Design - Buildings
• Buildings based on US DoE Commercial Building Benchmarks (ASHRAE 90.1-2004; Torcellini et al. 2008)
• Lodging (residential) - four floors, 31 apartments, and an office space - totaling 2824 m2 of conditioned area with a roof area of 744 m2
• Office - three floors, 4982 m2 of conditioned space, and 1660 m2 of roof area
• Both building types have electric direct expansion (DX) cooling and natural gas heating
• Internal loads and schedules are different
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Modeling Study Design—Roofs
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5 cm
15 cm
30 cm LAI= ALeaf
/ A (0.5, 2.0, 5.0)
Membrane Roof Baselines: metal decking, rigid insulation (0.125m thick) and a roofing membrane with solar reflectance of 0.30 (conventional) and 0.65 (white) Green roof cases: 9 cases varying growing media depth and LAI. All non-irrigated. The baseline green roof = 15cm, LAI=2
Energy Use in Baseline Buildings (conventional roof)
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PHOENIX BUILDINGS
PORTLAND BUILDINGS
Sailor et al., JBP, 2011
Energy and Cost Savings for Baseline Green Roofs Compared to Conventional Roofs
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Electricity and gas savings of baseline green roof compared to conventional roof per square meter of roof area. Savings are nominally ½ to 2% of total building energy.
Sailor et al., JBP, 2011
Energy and Cost Savings for Baseline Green Roofs Compared to White Roofs
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-60,000
-40,000
-20,000
0
20,000
40,000
60,000
80,000
kJ/m
2
a) Energy Savings
Elec Savings Gas Savings
-$0.40
-$0.20
$0.00
$0.20
$0.40b) Energy Cost Savings
Elec Cost Gas Cost
Annual energy and energy cost savings per unit roof area of the baseline green roof compared to a highly reflective white roof.
Sailor et al., JBP, 2011
Annual Energy Savings—Green Roof Soil Depth
32
Annual energy and energy cost savings per square meter of roof for green roofs on lodging (L) and office (O) buildings with three different soil depths (5, 15, and 30 cm)compared to a conventional roof.
Sailor et al., JBP, 2011
Annual Energy Savings—Green Roof Soil Depth
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Annual energy and energy cost savings per square meter of roof for green roofs on lodging (L) and office (O) buildings with three different soil depths (5, 15, and 30 cm)compared to a conventional roof.
Sailor et al., JBP, 2011
Annual Energy Savings—Green Roof LAI
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Annual energy and energy cost savings square meter of roof for green roofs on lodging (L) and office (O) buildings with three different levels of LAI (0.5, 2.0, and 5.0)compared to a conventional roof.
Sailor et al., JBP, 2011
Annual Energy Savings—Green Roof LAI
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Annual energy and energy cost savings square meter of roof for green roofs on lodging (L) and office (O) buildings with three different levels of LAI (0.5, 2.0, and 5.0)compared to a conventional roof.
Sailor et al., JBP, 2011
The Urban Heat Island
Solar radiation
Evaporative cooling
Long-wave radiation (LW) Waste heat (Qf)
Sensible heat (S)
Thermal storage (G)
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Summer Mean Peak (a) and Total (b) Sensible Flux to the Urban Environment
38 Scherba et al., Bldg. & Environ., 2011
PEAK
TO
TAL
Conclusions • Green roofs can have lower energy cost than conventional roofs,
and sometimes compete well against white roofing
• Effects are more pronounced for lodging buildings
• Energy benefits more substantial in less insulated buildings
• Deeper soils reduce energy consumption in lodging buildings
• Increased LAI reduces cooling load but increases heating load
• Roof choice can have significant impact on the external energy budget and urban climate system
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