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

4 Image from: toronto.ca ESRI Canada Ltd., 12 Concorde Place, Toronto

Regulations/Laws: Toronto Green Roof Bylaw (energy & stormwater)

5 Image from: greenroofs.com OHSU Bldg., Portland OR

Incentives: Portland, OR ($5/sq. ft)

6 Image from: atlurbanist.tumblr.com Clough Building at Georgia Tech., Atlanta

Codes & Standards: LEED (sustainable sites—water, UHI; energy)

7

Image from: gardenvisit.com Vancouver Convention Ctr.

(Canada’s largest at 6 acres) Green bragging rights and market value

A typical “extensive” green roof

8

The literature asserts a wide range of energy-related performance claims…

9

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

10

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

11

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

14

Protection & Drainage Layers and Roof Construction

Longwave radiation

Latent heat (evapotranspiration)

Conduction

Shortwave radiation

Heat Transfer on a Green Roof

Sensible heat (convection)

15

• Shading (LW & SW) • Evapotranspiration • Insulation • Thermal storage • Moisture balance

Thermal Conductivity and Soil Moisture

16 Meles and Sailor, Energy and Buildings 2011

Conventional Roof – Day Tsurf = Tmembrane ~ 120-150 o F

17

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

18

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

19

Doesn’t heat up much during summer day…

Green Roof- Night

…but remains warm at night due to stored heat.

Whole-building analysis approach…

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

21

[ ] ( ) 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

23

Modeling Study Design—Cities

24

Modeling Study Design—Roofs

25

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)

26

PHOENIX BUILDINGS

PORTLAND BUILDINGS

Sailor et al., JBP, 2011

Energy and Cost Savings for Baseline Green Roofs Compared to Conventional Roofs

28

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

29

-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

33

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

34

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

35

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

Roof Design affects Sensible and Latent Flux to the Urban Environment

36

The Urban Heat Island

Solar radiation

Evaporative cooling

Long-wave radiation (LW) Waste heat (Qf)

Sensible heat (S)

Thermal storage (G)

37

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

39

sailor@pdx.edu