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 sailor@pdx.edu 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
Regulations/Laws: Toronto Green Roof Bylaw (energy & stormwater) Image from: toronto.ca ESRI Canada Ltd., 12 Concorde Place, Toronto 4
Incentives: Portland, OR ($5/sq. ft) Image from: greenroofs.com 5 OHSU Bldg., Portland OR
Codes & Standards: LEED (sustainable sites water, UHI; energy) Image from: atlurbanist.tumblr.com Clough Building at Georgia Tech., Atlanta 6
Green bragging rights and market value Image from: gardenvisit.com Vancouver Convention Ctr. (Canada s largest at 6 acres) 7
A typical extensive green roof 8
The literature asserts a wide range of energyrelated performance claims 9
~11 o C ( 20 o F) warmer at night Average rooftop (membrane) surface temperatures for a standard and a green roof from the Penn State University field experiment. Denardo et al., ASAE, 2003. 10
Temperature Heat Flux Maximum Summer Roof Temperatures: * green roof avg. of 22 o C (39 o F ) cooler than a conventional roof. * green roof warmer at night Student Union, Univ. Central Florida. J. Sonne, ASHRAE J., 2006 11
Results from simplified models energy savings (up) to 48%... majority of existing and new buildings Modeled green roof as a simple reduction in U-Value (increase in R-value of about 2.5 ft 2 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. Nichaou et al, Energy and Bldgs., 2001 12
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. Desjarles et al, RCI Proceedings, 2010 13
The Roof is only one Contributor to HVAC Loads Roof Windows Infiltration Internal loads & ventilation Walls 14
Heat Transfer on a Green Roof Latent heat (evapotranspiration) Sensible heat (convection) Shading (LW & SW) Evapotranspiration Insulation Thermal storage Moisture balance Shortwave radiation Longwave radiation Conduction Protection & Drainage Layers and Roof Construction 15
Thermal Conductivity and Soil Moisture Meles and Sailor, Energy and Buildings 2011 16
Conventional Roof Day Conventional Roof -- Night T surf = T membrane ~ 120-150 o F Heats up rapidly during summer day but cools off rapidly at night. 17
Cool White Roof -- Day Cool White Roof -- Night T surf = T membrane ~ 90-110 o F Doesn t heat up as much during summer day and cools off significantly at night. 18
Green Roof-- Day Green Roof- Night T surf ~ 90-110 o F T membrane ~ 80 100 o F Doesn t heat up much during summer day but remains warm at night due to stored heat. 19
Whole-building analysis approach
A Physically-Based Approach: Green Roofs in EnergyPlus 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. 21
FOLIAGE 4 σ f ε gε fσ 4 4 [ I S (1 α f ) + ε f Iir ε fσt f ] + ( Tg Tf ) + H f L f F f = σ f + ε + ε ε ε f g f g GROUND SURFACE F g = (1 σ ) f σ f ε gε fσ 4 4 4 [ I (1 α ) + ε I ε T ] ( T T ) s g g ir g g ε + ε ε ε f g f g g f + H g + L g Tg + K * z H f = ( 1.1* LAIρ af C p, ac fwaf ) * ( Taf T f ) H g = ρ ag C C W ( T T p, a hg af af g ) L = l * LAI ρ C W r ( q q ) f f af f af af f, sat L g = C l W ρ e, g g af ag ( q q ) af g L LW SW H Storage Sailor, D.J., Energy and Buildings, 2008. G 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 m 2 of conditioned area with a roof area of 744 m 2 Office - three floors, 4982 m 2 of conditioned space, and 1660 m 2 of roof area Both building types have electric direct expansion (DX) cooling and natural gas heating Internal loads and schedules are different 23
Modeling Study Design Cities 24
Modeling Study Design Roofs 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 5 cm 15 cm 30 cm LAI= A Leaf / A (0.5, 2.0, 5.0) 25
PHOENIX BUILDINGS Energy Use in Baseline Buildings (conventional roof) PORTLAND BUILDINGS Sailor et al., JBP, 2011 26
Energy and Cost Savings for Baseline Green Roofs Compared to Conventional Roofs 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 28
Energy and Cost Savings for Baseline Green Roofs Compared to White Roofs 80,000 a) Energy Savings $0.40 b) Energy Cost Savings kj/m 2 60,000 40,000 20,000 0 $0.20 $0.00-20,000 -$0.20-40,000-60,000 -$0.40 Elec Savings Gas 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 29
Annual Energy Savings Green Roof Soil Depth 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 32
Annual Energy Savings Green Roof Soil Depth 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 33
Annual Energy Savings Green Roof LAI 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 34
Annual Energy Savings Green Roof LAI 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 35
Roof Design affects Sensible and Latent Flux to the Urban Environment 36
The Urban Heat Island Sensible heat (S) Solar radiation Waste heat (Q f ) Long-wave radiation (LW) Evaporative cooling Thermal storage (G) 37
Summer Mean Peak (a) and Total (b) Sensible Flux to the Urban Environment TOTAL PEAK Scherba et al., Bldg. & Environ., 2011 38
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 sailor@pdx.edu 39