Post on 12-Apr-2017
transcript
Rhea ThompsonPhD Candidate
Marine Estuarine & Environmental Science ProgramUniversity of Maryland, College Park
Retention and Thermal Performance of a Thin Sloped Green Roof on a Sustainable Home
Location: Rockville, MD Manufacturer: Live RoofArea: 312 ft2
Depth: 2.5”Slope: 13o
Roof Membrane: R-Value 50Soil Components : • 84.4 % Shale• 4.4 % Sand• 11.1% Compost
Stormwater
Management
Air Quality
Biodiversity
Energy Savings
Aeshetics
Urban Heat
Island
Evapotranspiration links water and energy
Learning Objectives1. Research the effect of storm
characteristics (size, frequency, intensity) and soil moisture on retention
2. Determine the trade-offs of soil moisture on thermal performance and green roof cooling
3. Research the effect of soil moisture on evapotranspiration (ET) rates
Image Credit: Scott Tjaden
Calculating Retention & ET
±ΔS =P-RO-ET
Retention increases with event size
0 10 20 30 40 50 60 70 80 90 10005
101520
f(x) = 0.203940750803771 x − 0.0374074408015517R² = 0.524725985012699
f(x) = 0.13037413874999 x + 0.934963415301213R² = 0.614330734942987
July-Nov
Precipitation (mm)
Rete
ntio
n (m
m)
P<0.0001
0 5 10 15 20 25 30 35 40 45 50020406080
f(x) = 2.53381212643459 ln(x) + 10.5350877319616R² = 0.00482214401161447
f(x) = NaN ln(x) NaNR² = NaN
Precipitation (mm)
Rete
ntio
n (%
)Larger storm events produced less retention as a percent of precipitation in July-November
P<0.0001
Low antecedent water content improved retention in July-November
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.1605
101520
f(x) = − 29.6301845392849 x + 5.89798756445569R² = 0.104686168954636
f(x) = − 22.2020693910455 x + 6.05682953745975R² = 0.255163150048322
July-Nov Linear (July-Nov)
Pre event VWC (m3/m3)
Rete
ntio
n (m
m)
P<0.05
Summary: Stormwater Retention•Green Roof retention increased with event size and low soil moisture•Green roof was less efficient at retaining water as storm size increased• Storm intensity and frequency had no effect
𝑅−𝑉𝑎𝑙𝑢𝑒𝐺𝑅=∆𝑇 /𝑄
𝟏𝟎𝟎𝐎𝐅
𝟕𝟓𝐎𝐅
¿¿
Ambient Temperature
Soil Heat Flux
Under Tray Temperature
Measuring ThermalPerformance
Image Credit: Scott Tjaden
Image Credit: Scott Tjaden
Reflected Radiation
Incident Solar Radiation
Evapotranspiration
Measuring ThermalPerformance
Mean Green Roof R-Value (2.40 Btu/hr · ft2 · F)
Jun Aug Oct Nov Jan Mar0.010.020.030.040.050.060.0
Green Roof WaterShedMaryland Standard
R Va
lue
(Btu
/hr ·
ft2
· F)
The potential for energy savings is greatest for poorly insulated buildings
Existing Roof Membrane
R-Value (hr · ft2 · F/Btu)
Roof Membrane + Green Roof
R-Value (hr · ft2 · F/Btu)
Percent Increase
(%)
50 52.4 4.8130 32.4 8.0125 27.4 9.6120 22.4 12.0215 17.4 16.02
0 0.02 0.04 0.06 0.08 0.1 0.12 0.1401234567
f(x) = − 24.118305956841 x + 3.95471855648063R² = 0.610449732617138
July-September
Average Daily Soil Moisture (m3/m3)
R Va
lue
(hr ·
ft2
· F/
Btu)
P<0.0001
Thermal Performance decreases with water content
0 0.02 0.04 0.06 0.08 0.1 0.12 0.1402468
10
f(x) = − 38.8752122769247 x + 5.22253190485296R² = 0.476150584854126
July-Septe...
Average Daily VWC (m3/m3)
Cool
ing
(oC)
Green Roof provides cooling benefit up to 9.3o C
P<0.0001
Summary: Thermal Performance• The green roof (R-Value 2.40 hr · ft2 · F/Btu) on WaterShed
provides little thermal relief in comparison to the home’s insulated membrane (R-Value 50 hr · ft2 · F F/Btu) • The potential for energy savings is greatest for poorly
insulated buildings • Thermal performance decreased with water content• Cooling benefit up to 9.3o C with low water content
• Assigning a green roof an R-Value is an imperfect method of assessing its thermal performance, as evapotranspiration is key to cooling
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14012345
f(x) = 22.4900534510692 x − 0.00490467998602218R² = 0.721985550161787
July-Sept Linear (July-Sept)
Average Daily VWC (m3/m3)
ET (m
m/d
ay)
P<0.0001
ET increases with water content, and may be key to improving retention and thermal performance
Conclusion: To Irrigate or Not?• The strong links between plant evapotranspiration rates, stormwater retention, thermal performance and soil moisture indicate:• Plants may be the key to improved retention and thermal performance
Conclusion: To Irrigate or Not?• The strong links between plant evapotranspiration rates, stormwater retention, thermal performance and soil moisture indicate:• Plants may be the key to improved retention and thermal performance• Irrigating may be important to improving plant growth, which in turn may improve retention and thermal performance due to increased evapotranspiration
Conclusion: To Irrigate or Not? ET
ET• The strong links between plant evapotranspiration rates, stormwater retention, thermal performance and soil moisture indicate:• Plants may be the key to improved retention and thermal performance• Irrigating may be important to improving plant growth, which in turn may improve retention and thermal performance due to increased evapotranspiration
Conclusion: To Irrigate or Not? ET
ET• However, there are trade-offs to be considered for irrigating green roofs• Too much water will impair
retention and thermal performance• A delicate balance between
the above factors must determined
• Findings important as greater rainfall and more frequent and intense storms are expected under climate change, which could further impair retention and thermal performance