Microclimate Design : Urban Heat Island

MicroClimate Design Study Of University Town, NUS Department of Building School of Design and Environment

Introduction  What is UHI and its effects?  Measures to mitigate UHI effects

Objectives

• To study the impact of UHI in the U-town region due to existing built-environment • To Identify the critical areas with hotspots and also to propose its mitigation measures to reduce the key effects of UHI • To select relevant mitigation measures that include addition of green/cool roof, vertical greenery & adding vegetation to existing topography to critical areas of the site

Methodology •

Understand the climate of the chosen site through the following parameters – Outdoor temperature mapping, wind profile & wind speed, thermal comfort index, total heat load, solar radiation using below software tools.

TSV with Tmrt TSV without Tmrt RayMan – PET

STEVE tool – Mapping out site temperature & total heat load ANSYS Fluent- Site wind flow map EnviMET –Mean radiant temperature

•

Identifying hot spot through studying the Daytime and Nighttime UHI effect in STEVE tool

•

Mitigating measures applies on a building in the hotspot region including building martials and vegetation IES – Peak cooling load comparison between different building materials & vegetation EnviMET – Mean radiant temperature between different building materials

•

Optimal strategy proposed

Site Information

• University Town (UTown) • Site Area Of 140825 Sqm. • Integrated with the Kent Ridge Campus • On South, it is linked with Ayer Rajah Expressway • On West, it is linked with Sub-Arterial Clementi Road

UTown Information Residential Colleges Yale - Green NUS Colleges Education Town Create Stephen Tower Raidy Resource Centre U Town Residences Centre It Undergraduate Students  include ItFor CREATE contributes tower to1,700 is the anpulse international of activities research in Housing up to residents  campus House more than 600 students University andTown innovation hub  two SAGA College multipurpose sports halls  On 67,000 sqCollege m plot  arock Cendena climbing wallit house some 1,200 researchers  gym, ELM College recreational pool  an auditorium  dance studios and practice rooms.

Residential Yale NUS U Town Colleges Residences Education Resource Centre

Stephen Town Create Green Tower Raidy Centre

Site Components

West Skyline

East Skyline

Overview of Software Implementation

Steve tool

PET

Rayman

ENVI-met

TSV

Manual

Fluent

Energy

IES-VE

Microclimate Analysis

STEVE Tool Analysis

Scope • • • •

Ambient air temperature Total heat load CO2 storage& sequestration Define hot spot

Input Input using default setting wind 0.69m/s, albedo concrete 0.16

Total Heat load 　

Thermal load unit (Wh/m4)

Total building footprint area(m2)

Total buildings envelope area(m2)

SCL ECG SG FAIG

4.08816 0.242066 0.3925 0.40085

100190.6 100190.6 100190.6 100190.6

389646.2 389646.2 389646.2 389646.2

Annual total (MWh) 159597.2 9450.0 15322.8 15648.7

Tree Species Diospyros buxifolia

Fagrea fragrans

Brownea grandiceps

Maniltoa browneoldes Hopea odorata

Archontophoneix alexandrae

CO2 Storage and Sequestration

Total Carbon Storage =7221 kg C Total carbon sequestration = 526 kg C/year

Define Hot spot- Daytime

Yale-NUS college Town Green

Temp(max)

Temp (avg-day)

Define Hot spot- Daytime 33.2

Yale-NUS college

33.4

Town Green

Define Hot spot- Daytime Causes •

These areas exposed to sun directly (High SVF) , results to receive large solar radiation during the daytime.

•

More trees planted in four residential colleges than Yale-NUS college, which provides shading effects during the daytime.

•

Four residential colleges relatively dense and building can provide selfshading effects due to heights variation.

Define Hot spot- Nighttime

Residential college 4

Yale-NUS college Create buildings

Temp(max)

Temp (avg-day)

Define Hot spot- Nighttime

Causes

25.9

•

Less plants (GnPR), heat emission from walls of building, heat is easy to be trapped

•

High GnPR in open area Town Green

EnviMet Analysis Scope Mean Radiant Temperature

Mean Radiant Temperature (deg C) in Utown on March 20, 2016 (12pm- 05pm)

12:00 PM

01:00 PM

02:00 PM

03:00 PM

04:00 PM

05:00 PM

Mean Radiant Temperature (deg C) in Utown on March 20, 2016 (06pm- 11pm)

06:00 PM

07:00 PM

08:00 PM

09:00 PM

10:00 PM

11:00 PM

Mean Radiant Temperature (deg C) in Utown on March 21, 2016 (12am- 05am)

12:00 AM

01:00 AM

02:00 AM

03:00 AM

04:00 AM

05:00 AM

Mean Radiant Temperature (deg C) in Utown on March 21, 2016 (06pm- 11pm)

06:00 AM

07:00 AM

08:00 AM

09:00 AM

10:00 AM

11:00 AM

Result_Analysis Mean Radiant Temperature (deg C) Max at Daytime – 79.05

Max at Nighttime – 25.95

Urban Ventilation Analysis

1

2 3

Note: Solution method: First order upwind for Momentum and Standard for Pressure North wind: 2m/s at reference height of 15m; Wind profile: Log law

Wind Profile-North

Source: Prevailing wind direction & speed obtained from NEA over a period of 18 years

1 Hot spot -Residential college 4

Area 1

Area Weighted Wind Velocity at 2m above ground= 0.477 m/s

2 Hot spot – Yale NUS colleges

a e r A

2

Area Weighted Wind Velocity at 2m above ground= 0.985 m/s Vortices continuously circulated in the courtyard

3 Hot spot – Town Green

Area 3

Area Weighted Wind Velocity at 2m above ground= 1.02 m/s Strong wind flow in the open area

Solar Exposure- Analysis

Sun Path Diagram of Singapore

Solar Exposure- Analysis 40%

1.62% 30%

40% 70% 90% 100% 30% 60% 70% 30% 60%

90%

Results The solar exposure percentage • Roof level ranges from 70% to 100% • North sides ranges from 1.62% to 40% • South sides ranges from 40% to 70% • East sides ranges from 30% to 60% • West sides ranges from 30% to 60%

Need for cool roof and green roof

Outdoor Thermal Comfort

Methodology-TSV Equations TSV Equation TSV without Tmrt

TSV with Tmrt

Perception Scale TSV range -3 ～ -2 -2 ～ -1 -1 ～ 0 0 ～ 1 1 ～ 2 2 ～ 3

Note: TSV without Tmrt is adopted by Steven Tool

Perception cold to cool cool to slightly cool slightly cool to neutral neutral to slightly warm slightly warm to warm warm to hot

Methodology-PET

Results-TSV Equations

Percentage of Dissatisfaction (PD) Outdoor thermal comfort-Daytime

TSV with Tmrt Location Ta[C] Area 2 33.2 Daytime Area 3 33.4

V [m/s] Tmrt 0.985 58.82 1.02 74

100% 90%

TSV 4.4 5.0

P 9% 5%

PD 91% 95%

Area 1

25.9

0.477

23.68

0.3

91%

9%

Area 2

25.9

0.985

23.68

0.1

92%

8%

Nighttime

Sensation Warm to hot Warm to hot Neutral to slightly warm Neutral to slightly warm

80%

91%

95%

70% 60% 50% 40% 30% 20%

17%

10% 0%

PD wi th Tmrt

PD wi thout Tmrt Area 2

TSV without Tmrt

Daytime Nighttime

Area 3

Outdoor thermal comfort- Nighttime

Location

Ta[C]

V [m/s]

TSV

P

PD

Area 2

33.2

0.985

0.9

83%

17%

Area 3

33.4

1.02

0.9

83%

17%

Area 1 Area 2

25.9 25.9

0.477 0.985

-1.0 -1.4

98% 99%

2% 1%

Sensation Neutral to slightly warm Neutral to slightly warm Cool to slightly cool Cool to slightly cool

10% 9% 8% 7%

9% 8%

6% 5% 4% 3% 2%

2%

1%

Note: Area 1, 2 3 corresponds to hot spots 1, 2, 3

17%

0%

PD wi th Tmrt

1%

PD wi thout Tmrt Area 1

Area 2

Results-PET

Daytime Night time

Area

PET

Perception (SG)

Perception (Europe)

2

45.8

Very Hot

Very Hot

3

54.1

Very Hot

Very Hot

1

23.9

Slightly Cool

Slightly Warm

2

26.3

Neutral

Slightly Warm

Comparison-Two TSV equations VS PET

Daytime

Nighttime

Location

TSV with Tmrt

Area 2

Warm to hot

Area 3

Warm to hot

Area 1 Area 2

Neutral to slightly warm Neutral to slightly warm

TSV without Tmrt Neutral to slightly warm Neutral to slightly warm

PET Very hot Very hot

Cool to slightly cool

Slight cool

Cool to slightly cool

Neutral

Mitigation Strategy-Estate Level Cool roof with higher albedo

Material properties of Roof in ENVI-met

Original

R=0.3

Improved

R=0.48

Decrease of Air temperature (Ta) Min: 26. 61 Max: 31.89

Min: 19. 89 Max: 31.13

Decrease of Mean Radiant Temperature (Mrt) Min: 53. 76 Max: 79.05

Min: 50.39 Max: 75.63

Causes • More shortwave radiation directly reflected by roof from surface to space

R=0.3

Effects • Lower Air temperature (Ta) and Mean radiant temperature (MRT) at the outdoor • Reduction of Ta is more significant than MRT

R=0.48

• Improve outdoor thermal comfort • Reduction of UHI effects

Cool ROOF

Mitigation Strategy-Estate Level Void Deck

Yale-NUS

U Town Residence

Yale College-Increase of Wind velocity

Original

Improved

• Not very effective in terms of velocity magnitude • But the ground wind movements are improved, 0.985removing m/s the ground level pollutants • Improve uniformity of wind distribution

0.773 m/s

Town green-Increase of Wind velocity

Original

Improved

• Increase of velocity magnitude • Enhance wind movements 1.020 m/s • Improve uniformity of wind distribution

1.112 m/s

Yale NUS-Decrease of Air temperature Tmax

Tmax

• Decrease 32.8 33.2 of Ta at both daytime and nighttime

Daytime

Original (Daytime)

Improved (Daytime)

• A good way to lease the trapped heat between buildings • Effects on reductionTmin of Ta Tmin maybe more significant when 25.9 25.6 combines the wind simulation

Nighttime Original (Nighttime)

Improved (Nighttime)

Mitigation Strategy-Estate Level Planting Greenery

Town Green – Daytime hot spots Tmax Medium greenery density

Lower greenery density

33.4

0.2

33.2

Higher greenery density

0.6

32

Yale-NUS

Daytime hot spots Lower greenery density

Tmax

Medium greenery density

33

33.1

33.2

Higher greenery density

Nighttime hot spots Lower greenery density

Tmin

25.9

Medium greenery density

25.7

Higher greenery density

25.6

Location

Summary of Mitigation (Estate Level)

Table: TSV and PET for each mitigation strategy

Time

Mitigation

Yale-NUS Daytime Cool roof Town Green Yale-NUS Void Daytime Town deck Green Medium greenery Town density Daytime Green Higher greenery density Medium greenery density Daytime Higher greenery density Yale-NUS Medium greenery density Nighttime Higher greenery density

Ta[C]

V [m/s]

Tmrt

TSV with Tmrt

33.2

0.985

55.44

4.3

TSV without Tmrt 0.9

33.4

1.02

71.5

4.9

0.9

52.7

Very Hot

32.8

0.773

58.82

4.3

0.9

45.7

Very Hot

33.4

1.112

74

5.0

0.8

53.8

Very Hot

33.2

0.985

58.82

4.4

0.9

45.8

Very Hot

32.6

0.985

58.82

4.1

0.7

45.3

Very Hot

33.1

0.985

74

4.9

0.8

53.9

Very Hot

33

1.02

74

4.9

0.8

53.7

Very Hot

25.7

1.02

23.68

0.0

-1.5

22.7

Slightly Cool

25.6

1.02

23.68

0.0

-1.6

22.7

Slightly Cool

PET

PET sensation

43.8

Very Hot

Mitigation Strategy-Building Level Green Roof and Vertical Greenery

Existing Building Study-ERC

Roof Type

Addition Layer Turfing: Turf Roof Soil Substrate (40% Moisture Content): Shrubs Shrub roof Soil Substrate (40% Moisture Content): Trees Tree roof Soil Substrate (40% Moisture Content): Education Resource Centre - NUS

Property R-value thickness R-value R-value thickness R-value R-value thickness R-value

Value 0.36 100 0.063 1.61 300 0.19 0.57 700 0.443

Total R-value 2.372

3.749

2.962

Ref: N.H. Wong, etc., the effects of roof top garden on energy consumption of a commercial building in Singapore

Existing Building Study-ERC Peak Cooling Load (kW)

Reduction Percentage

223.3 220.3 214.1 216.4

1.34% 4.12% 3.09%

Flat Roof Turf Shrubs Trees

Space Conditioning Peak Sensible Load (kW) 580

Peak cooling Load (kW)

570 560 550 540 530 520 510 Flat Roof

100% Turf

100% Shrubs

Roof Type Floor 1

Floor 2

Floor 3

Floor 4

100% Trees

Mitigation for Yale-NUS: Green Roof and Vertical Greenery

Cluster of Yale-NUS Buildings High-rise Building: small roof area large wall surface

Low-rise Buildings: Large roof area

Focusing on 1. Green roof 2. Green Faรงade

External wall Glazing Roof

Property

Value

U-value (W/m2K)

1.46

U-value (W/m2K)

1.47

Shading Coefficient

0.7

U-value (W/m2K)

0.475

Green Roof 1130 1120 1110 1100 1090 1080 1070 1060 1050

1105.09 3130.59 1080.61 3096.93

3160 3150 3140 3130 1088.96 3112.24 3120 3110 3100 3090 3080 3070

Building Load (mWh)

Peak Cooling Load (kW)

Comparison of different types of roofs for the building

Roof Type Peak Cooling Load

Building Load

Conclusion 1. Green roof has positive impact on lowering down Peak Cooling Load and annual Total Building Load 2. Roof with 100% cover of shrubs has most significant positive impact

Green Roof 205

680

200

670

195

191.22 653.63

660

190

650

185

181.87 177.14

180

638.57

630

175

626.05

170

620 610

165 160 Flat roof

640

Roof with 100% Turf

Roof with 100% Shrubs

Roof Type Peak Cooling Load

600 Roof with 100% Trees

Building Load (mWh)

Peak Cooling Load (kW)

Comparison of different types of roofs for the Top Floor

Total Building

Peak Cooling Load (kW)

Reduction Amount

Flat roof Roof with 100% Turf Roof with 100% Shrubs Roof with 100% Trees

1122.77 1105.09 1080.61 1088.96

17.68 42.16 33.81

Top Floor Flat Roof Roof with 100% Turf Roof with 100% Shrubs Roof with 100% Trees

Peak Cooling Load (kW) 201.55 191.22 177.14 181.87

Reduction Amount 0.0% 10.33 24.41 19.68

Building Reduction Load(MWh) Amount 3147.56 3130.59 3096.93 3112.24

16.97 50.63 35.32

Building Reduction Load(MWh) Amount 667.22 0.0% 653.63 13.59 626.05 41.17 638.57 28.65

Building Load

Conclusion: 1. Green roof has most significant impact on top story in terms of both peak cooling load and total building load 2. It has limited impact on lower stories in term of building load 3. Further investigation needed on the effectiveness of greenroof

Vertical Greenery System Comparison of vertical greenery impact on peak cooling load and building load 1140

1120

3200 3100 3009.95

Layers

R-value

Turfing Layer

0.36

Substrate Layer (0.1m)

1.923

Air gap (0.1m)

0.16

1100

2900

2793.61

1080

1060

2700

1056.15 1048.86

1040

1020 Baseline

2800

2595.45 2600 1042.26

Building Load (MkW)

Peak Cooling Load (kW)

3000

2500

Green Faรงade

Green Faรงade with SC of 0.5

Vertical greenery systems Peak Cooling Load

Building Load

2400 Green Faรงade with SC of 0.3

R-value increases from 0.516 to 0.926

WWR= 0.25 Baseline Shading Coefficient = 0.7 SC=0.5, SC=0.3 are also simulated

Summary of Green Roof and Vertical Greenery Performance Comparison of peak cooling load and building load reduction 20.0% 18.0% 16.0% 14.0% 12.0% 10.0% 8.0% 6.0% 4.0% 2.0% 0.0%

19.0% 16.7%

17.5%

11.2% 5.9% 4.4%

6.6%

7.2% 1.6% 0.5%

3.8% 1.6%

3.0% 1.1%

Green roof

Cool roof Peak cooling load reduction

Building load reduction

Pros

Cons High Biodiversity maintenance fee Dampness problem, need Noise reduction good water proofing Increase Reduce storm building water runoff structure load Increase

aesthetic Low structure Probably facing load glare issue

Research limitation

Accuracy of simulation

• Integration between wind distribution and air temperature were not assessed by this study • Building energy consumption were assessed based on weather file, was not integrated with microclimate computed from this study

• Results of wind simulation can be improved by implementing the second order upwind discretization and the enclosure size can be extended to minimize the influence from boundary on the wind flow, mesh independency test should be conducted prior to the final simulation

• Difference of air temperature results between Steven tool an ENVI-met was not covered in this study

• Terrain was not considered in this project, which will influence the results of air temperature and wind flow simulation

• Different software were required to use in the project, results in less integration

• Adding construction layers will enlarge thermal mass

• TSV calculation only conducted for the worst scenario

• Shading effect from surrounding building • Further study on green roof impact on buildings with different building sizes

Thank You..