evaluation of the effects of greening and highly...

ASI 2 ‐ Prof. Akashi Mochida ‐ Lecture 1

1

Evaluation of the effects of greening and highly reflective materials from three perspectives - mitigation of global warming, mitigation of UHIs, and adaptation to urban warming -

• Akashi Mochida

• Professor

• Department of Architecture & Building Science, Tohoku University, Japan

1. Background

2. Outline of assessment system

3. Examples of total assessment

1) Effects of greening and highly reflectivematerials applied to vertical walls

2) Effects of roadside trees

3) summary of assessments

4. Evaluation of the effects of windows with heat ray retro‐reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection

2


2

Annual mean temperature change in East Asian cities

(Source: Japan Meteorological Agency & China Meteorological Administration)

Ann

ual m

ean

tem

pera

ture

[ºC

]

3

1911 1931 1951 1971 1991 2011

1.3ºC/25years

0.8ºC/100years

2.8ºC/100years

The temperature increases in East Asian cities are much more rapid than the pace of global warming.

This graph was made using the data from website of Tokyo district meteorological observatory(http://www.jma‐net.go.jp/tokyo/sub_index/tokyo/kikou/t_ts/t_ts.html)

4

[Year]

Number of extreme hot days in Tokyo on which daily maximum temperature exceeds 35oC

Furthermore, the number of extremely hot days has been increasing and this caused the increase in health hazard risk in Japan.


3

Number of patients of heatstroke (hyperthermia:熱中症）taken to hospital by ambulance service

5

This graph was made using the data from National Institute for Environmental StudiesBulletin Report on Heatstroke Patients (http://www.nies.go.jp/health/HeatStroke/spot/maps.html)

0

1000

2000

3000

4000

5000Tokyo

Yokohama

Nagoya

Osaka

The number of heatstroke patients has increased sharply.

• To improve such situation, various countermeasure techniques against urban warming have been adopted.

6

Tree planting in urban area

Increasing the solar reflectance of urban surface (High‐albedo surfacing)

Introducing the wind from sea, river, planted park into the inside of city


4

7

Author Focused countermeasure Energy savings Effluent sensible heat

Thermal environment in pedestrians spaces

Akbari et al.(2001) Cool surface 〇

Akbari, H. (2002) Trees 〇

Ali‐Toudert & Mayer (2006) Control of building shapes 〇

Ichinoseet al. (2006) Roof greening 〇〇

Sasaki et al.2006) Greening& White painting 〇

Hataya et al. (2007) Roadside trees 〇

Takebayashi& Moriyama (2007) Greening& White painting 〇

Kondo et al. (2008a,b) Highly reflective painting 〇

Hwang et al. (2011) Control of building shapes 〇

Shushua‐Bar et al.(2011) Trees& grass 〇

Xuan et al. (2012) Control of pitch of buildings 〇

Allegrini et al. (2012) Control of building shapes 〇

Saneinejad et al. (2012) Evaporativecooling 〇

• In recent years, many studies have been done to evaluate the performance of these countermeasure techniques.

• But, in most of studies, performance of the techniques was assessed from different single viewpoint, i.e.

1) Energy savings, 2) The suppression of effluent sensible heat3) The improvement of the thermal environment in pedestrian space, etc.

• However, these aims often conflict with one another. • For example, enhancing the solar reflectivity of vertical building

walls has a positive impact on energy savings, but it has a negative impact on the outdoor thermal comfort of pedestrians, because reflected solar radiation from a building surface tends to be incident to pedestrians.

• Therefore, great difficulties still remain when policymakers and urban planners attempt to select proper countermeasure techniques, despite the enormous accumulation of knowledge

8

Heat ray is reflected to pedestrian

Heat stress of pedestrian

Cooling load


5

• To overcome the difficulties associated with the selection of proper countermeasures against urban warming, the purposes of such countermeasures should be considered well and defined clearly.

• First, there are two aspects to these countermeasures: mitigation and adaptation.

9

Mitigation refers to the removal of the causes of the

phenomenon

Adaptationrefers to the

reduction in the effects of the phenomenon

even though the magnitude of the phenomenon does

not change.

• Second, ongoing urban warming is being caused by both global warming and urban heat islands (UHIs).

• Global warming is caused by the rising concentration of greenhouse gases.

• A UHI is caused by a) the modification in the land-use from a natural

environment into a built environment and b) the intensive energy consumption in urban area

resulting in anthropogenic heat release

⇒Completely different countermeasures are needed to mitigate these two phenomena, global warming and UHIs,and to simultaneously adapt to urban warming.

10


6

• Thus, we must recognize that there are three different perspectives of countermeasures for urban warming:

11

Mitigation of urban heat island

adaption to urban warming

(global warming + UHI)[ex. Create the shade]

Mitigation ofglobal warming[ex. Reduce the CO2

emission]

However,

their distinction remains unclear among researchers.

[ex. Increase the solar reflectance on thebuilding surface]

This study aimed to propose a total assessment method to assess the effects of the countermeasures to 1) mitigate global warming,

2) mitigate UHIs, and 3) adapt to urban warming

simultaneously.

12


7

1. Background







13

• The confusion between the countermeasures to mitigate global warming, to mitigate UHIs, and to adapt to urban warming arises because… the focus regions for discussing urban warming differ among researchers.

• In this study, three domains for evaluating the effects of countermeasures for mitigating global warming, mitigating UHIs, and adapting to urban warming are set in an assessment domain. Assessment indices corresponding to each domain are calculated.

14

Domain B

Urban atmosphere

Entireassessment domain

Domain C

Outdoor pedestrian space

Domain A

Building interior


8

Domain A for assessing the mitigation of global warming

• The cause of global warming is greenhouse gases, especially CO2.CO2 emission increases with the energy consumption.

• In order to assess the mitigation of global warming,domain A, which is the interior of buildings, is defined, and the energy consumption of its Heating, Ventilation, and Air Conditioning (HVAC) system is calculated.

15

• In this study, the total energy consumption over the running time per day of HVAC system (QHVAC_sum) is used as an index for assessing the mitigation of global warming.

• The energy consumption per unit time of building j is calculated from the cooling load of building j(Qin,j)

16

_ ,

Qin, j : Amount of influent heat into building j( cooling load of building j)[W]COPj : Coefficient of performance of HVAC system of building j [‐]

jmax :Total number of buildings in assessment areaQHVAC, j :Energy consumption of HVAC of building j [W]QHVAC_sum:The amount of QHVAC for 24 h [MJ]


9

Index for domain A : energy consumption of HVAC(=Qin /COP)

17

Qin = amount of influent heat into building (cooling load)

= amount of influent heat from outside

(by convection, transmission through windows and ventilation)

+ amount of heat generated in building

• The cooling load of building j is composed of influent heats by convection, transmitting radiation, and ventilation and heat generated inner building.

• Each component is expressed as follows:

18

, , , + , , + , + ,

, , , , , , ,

, , , , , ,

, , , ,

imax :Total number of surface elements of buildings in assessment areaQin :Amount of influent heat into building [W]Qconv,inside :Influent heat by convection from building interior wall

surface to indoor air [W]Qr,trans :Influent heat by radiation transmitting window [W]Qvent :Influent heat by ventilation [W]Qinner :Amount of heat generated inner building [W]


10

Domain B for assessing the mitigation of a UHI• The cause of UHI is the increase in sensible heat in urban space due to

the modification in land-use and anthropogenic heat release.

• To assess the mitigation of a UHI, domain B, which is the urban atmosphere, is defined, and the amount of effluent heat from the urban surface to the outdoor air is evaluated.

19

Domain B for assessing the mitigation of a UHI

• The total effluent heat over 24 h (Qout_sum) is used as an index for assessing the mitigation of a UHI.

20

_

Qout :Net effluent heat from urban surface to outdoor air [W]Qout_sum :The amount of Qout for 24 h [MJ]


11

Index for domain B: net effluent heat from urban surface to outdoor air (Qout)

21

Qout= net effluent heat from urban surface to outdoor air

= effluent heat from urban surface by convection

+ wasted heat from HVAC system

‐ influent heat from outside to indoor

= Qemi ‐ Qabs

Index for domain B: net effluent heat from urban surface to outdoor air (Qout)

• The net effluent heat from an urban surface per unit time is estimated from the balance between the effluent heat (Qemi) and the influent heat (Qabs) from the urban surface.

22

_

Qout :Net effluent heat from urban surface to outdoor air [W]Qout_sum :The amount of Qout for 24 h [MJ]Qemi :Effluent heat from urban surface to outdoor air [W]Qabs :Heat absorbed by urban surface [W]


12

• Each component of the effluent heat (Qemi) and the influent heat (Qabs) is expressed as follows:

23

,

, , , , ,

, ,

, , ,

jmax :Total number of buildings in assessment areaiMAX :Total number of surface elements of building and ground in assessment areaQHVAC :Energy consumption of HVAC [W]Qin :Amount of influent heat into building [W]Qr,trans :Influent heat by radiation transmitting window [W]Qvent :Influent heat by ventilation [W]Qemi :Effluent heat from urban surface to outdoor air [W]Qabs :Heat absorbed by urban surface [W]Qconv,outside:Effluent heat by convection from urban surface to outdoor air [W]

Domain C for assessing the adaptation to urban warming

• In this study, the effectiveness of countermeasures to adapt to the urban warming was assessed by thermal comfort of pedestrians.

• The pedestrian space within domain B is defined as another domain, domain C, to assess the adaptation to urban warming.

24


13

Index for domain C: acceptable volume ratio on the basis of SET*

25

• In this study, Standard Effective Temperature (SET*) was selected as thermal comfort index.

• The space within a height of 2m above ground is defined as pedestrian space.

• Spatial distributions of SET* within pedestrian space were simulated by 1) unsteady heat balance simulation at urban surface coupled with radiation and conduction computations, and 2) CFD simulation.

• The acceptable volume ratio is used as an index for assessing the adaptation to urban warming.

• The acceptable volume is defined as the volume in which the value of SET* is less than its acceptable maximum limit.

• The target time is the hour when the maximum air temperature is reached.

26

∗ ∗

Index for domain C: acceptable volume ratio on the basis of SET*


14

Assessment Procedure• Indices corresponding to each domain are derived from the

results of unsteady heat balance simulation at urban surface coupled with radiation and conduction computations and non-isothermal CFD simulation.

27

Building form

LocationMeteorological

data

Unsteady radiation and conduction simulation

Non‐isothermal CFD simulation

MRT calculation

SET* calculation

•Three dimensional distribution of velocity, air temperature, and absolute humidity

•Surface temperature atground and buildings


•Amount of exhaust heat from HVAC

Evaluation index of domain B

Amount of net effluent heat from urban surface

Evaluation index of domain C

Acceptable volume(SET*<Acceptable maximum limit)

ratio

•Three dimensional distribution of MRT

•Effluent heat by convection from surfaces

•Exhaust heat from HVAC

Evaluation index of domain A

Amount of energy consumed by HVAC

•Influent heat by conduction•Influent heat by radiation•Influent heat by ventilation

1. Background







28


15

Simulation Target• In the 1st example, the assessment system was applied to

the ideal town block model where the height and width of buildings were set to be identical.

• Building coverage ratio: 25%Building height, road width: 20mBuilding use: Business Office

• Meteorological conditions of Otemachi in Tokyo were used as boundary conditions.

29

Details of the office building

30

Building

Structures

Number of stories [-] 5

Floor height [m] 4

Ceiling height [m] 2.8

Rentable area ratio [-] 0.8

Building volume available for work

purposes [m3] 4390

HVAC

System

Ventilation rate [m3/s] 2.18

COP [-] 3

Air temperature set point [°C] 28

Running time [h] (7-21)

Settings

Related to

Indoor Heat

Balance

Interior heat generation Given by the guidelines for

calculating building energy

consumption in Japan

(NILIM and BRI, 2013)

Heat capacity of materials inside the

building, such as furniture and documents


16

Calculation Cases• Three situations were simulated using different physical properties for

the vertical wall surfaces of the town block model: (1) concrete, (2) highly reflective materials, and (3) green

• For all the cases compared, the physical properties of the roof and road surface were identical.

31

Case name

Material Albedo

[-] Emissivity

[-] Surface

wetness[-]Window ratio [-]

Concrete Concrete 0.2 1.0 0.0 0.0

Highref Highly reflective

material 0.6 1.0 0.0 0.0

Greening Greening 0.2 1.0 0.3 0.0

(1)concrete (2)highref (3)green

Albedo‐>Solar reflectance

Moisture availability [‐]

Calculation conditions for unsteady radiation and conduction Simulation

32

Calculation date(Related to solar altitude)

August 30th

Calculation periodFor 48 h from 0:00 a.m. on August 29th

Meteorological Data data for air‐conditioning design [3] was usedMesh Number

(x×y×z)34×34×9

Domain size(x[m]×y[m]×z[m])

505.6×505.6×226.4

Boundary conditions

GroundVertical gradient of soil temperature was set to zero in 0.5 m of earth

Solid surfaceIn: Total heat transfer coefficient, =9[W/m2K], Out: Convective heat transfer coefficient, c=12[W/m2K] was imposed.

Expanded AMeDAS Weather Data for air‐conditioning design ( for typical summer condition ) was used as meteorological data.


17

Non-isothermal CFD Simulation

33

N

S E

W

Assessment area

• In this simulation, 7 × 7 buildings were modeled, and the result for the central area was used for calculating the assessment index.

• Target time was 2 p.m.,

• Inflow conditions at this time wind direction: southwind speed: vertical profile was given by power law, 2.7m/s at a height of 6.5m

air temperature: 33.9 o Cabsolute humidity: 0.018[kg/kg’]

Calculation conditions for CFD Simulation

34

Calculation time 2 p.m.Mesh Number (x×y×z) 105×96×43

Domain size (x[m]×y[m]×z[m]) 792×356.4×554.9

Turbulence model Durbin type revised k‐model

Discretization scheme of convective teams of transport equations

First‐order upwind scheme


18

Boundary conditions of CFD Simulation

35

In‐flow

wind direction South

Wind velocity a=0.27 [4],zs=6.5[m], Us=2.7[m/s] [3]

Turbulence energy and dissipation rate The vertical distributions are given in accordance with [4]

Air temperature 33.9[°C] [3]Absolute humidity 0.018[kg/kg’] [3]

Outflow and lateral <u>, <v>, <w>, k, ε, q, q: zero gradient

Upper <u>, <v>, k, ε, q, q: zero gradient, <w>=0

Solid surface

Velocity The generalized log law was applied

Temperature

Surface temperatures of each mesh are given by the result of unsteady radiation and conduction simulation.Sensible heat flux from solid surface hw:

ac=12[W/m2K], qp: The air temperature of the first mesh from solid surface

Absolute humidity

Latent heat flux from solid surface Lw:

aw=αc×6.0×10‐6 [W/m2K], b: Surface wetness,fp: The water vapor pressure of the first mesh from solid surface

ss z

zUzu )(

pwcwh

pwww ffL

Simulation Results

36


19

Simulation results (1)Unsteady radiation and conduction simulation-> Unsteady heat balance simulation coupled with

radiation and conduction computations

37

Building form


data



MRT calculation

SET* calculation









ratio







38

• We considered the multi‐reflections of short‐ and long‐wave radiation by calculating Gebhart factors.

LEi LEi

Ci Ci Ci

Ci

Ci

Hi Ri Ri Ri

Si

Si

LEi

Si : Solar radiation [W] Ri : Longwave radiation [W] Hi : Sensible heat flux [W] Ci : Heat gain by heat conduction [W] LEi : Latent heat flux [W]

Ci

Ci

Ci

Ci Hi

Hi

Hi

Monte–Carlo simulation

Heat balance components considered in the coupled analysis


20

Surface Temperatures of Exterior Building Surface- Results of Radiation and Conduction Simulation -

• The surface temperatures of the four vertical surfaces were averaged.

• The surface temperatures of the greening and highly reflective materials were lower than that of concrete during daytime.

39

Tem

pera

ture

[°

C]

24

28

32

36

40

44

0:00 4:00 8:00 12:00 16:00 20:00 0:00

concrete

highref

green

airtemp

Simulation results (2)Estimation of Mean Radiant Temperature (MRT) using the results of unsteady radiation and conduction simulation

40

Building form


data



MRT calculation

SET* calculation









ratio








21

MRT is…

41

Mean Radiant Temperature (MRT) is the uniform surface temperature of a black enclosure with which an individual exchanges the same heat by radiation as the actual environment considered.⇒ In outdoor space, MRT indicates the radiant heat (both solar (short‐wave) radiation and long‐wave radiation) coming from sky, building walls, ground and absorbed by human body.

Horizontal Distribution of Mean Radiant Temperature (MRT)(2 p.m. , h=1.25m)

- Results of Radiation and Conduction Simulation-

• The MRT values around the highly reflective material surface were higher in the overall assessment area than those for concrete, while the MRT values around the greening were lower than those for concrete.

42


22

Simulation results (3)CFD simulation

43

Building form


data



MRT calculation

SET* calculation









ratio







Horizontal Distribution of Wind Vector and Air Temperature (2 p.m. , h=1.25m)

- Results of Non-isolated CFD Simulation-

• The air temperatures for the greening case was lower than those of the concrete and highly reflective material cases.

44


23

Comparison of absolute humidity and air temperaturein “greening case” (2 p.m. , h=1.25m)

45

Absolute humidityWind velocity and air temperature

The absolute humidity is high in the same area of low air temperature.

N

EW

S

N

EW

S

Simulation results (4)Estimation of thermal comfort index SET*

46

Building form


data



MRT calculation

SET* calculation









ratio








24

SET*(Standard Effective Temperature)

1) Wind velocity

2) Temperature

3) Radiation (MRT)

4) Humidity

5) Clothing

6) Metabolism

given from the resultsof CFD and heat balance simulation coupled with radiation and conduction computations

assumed

Human thermal comfort index

A.P.Gagge, J.A.J.Stolwijk, Y.Nishi: 1971, An effective temperature scale based on a simple modelof human physiological regulatory response, ASHRAE Transactions, 77, pp.247‐262, 1977

Horizontal Distribution of SET* (2 p.m. , h=1.25m)

• The SET* values in the case of the highly reflective material is the highest because of the worsening radiant environment.

• The SET* values for the greening case increases in the area near the west wall of the building because of the increase in the humidity.

48


25

Assessment Results

49

Indices for each domain

• From now, assessment results using these simulation results are shown.

• Domain A was set to assess the mitigation effect of global warming:⇒Index: the energy consumption of HVAC

Domain B was set to assess the mitigation effect of a UHI:⇒Index: the effluent heat from urban surface

Domain C was set to assess the adaptation effect to urban warming:⇒Index: the acceptable volume ratio on the basis of SET*

50

Domain B

Urban atmosphere


Domain C


Domain A

Building interior


26

Assessment results (1)Index of domain A

51

Building form


data



MRT calculation

SET* calculation









ratio







Assessment of Global Warming Mitigation ( Domain A) - total energy consumption of HVAC system over running time per day-

• In the cases of the greening and highly reflective materials, the energy consumptions were lower than in the concrete case.

52

The total energy

consumption of HVAC over

running time [M

J/day]

0

200

400

600

800

1000

1200

1400

concrete highref green

Qinner/COP Qintial/COPQvent/COP Qconv,inside/COP


27

Assessment results (2)Index of domain B

53

Building form


data



MRT calculation

SET* calculation









ratio







Assessment of UHI Mitigation (Domain B) - the total effluent heat from the urban surface over 24 h -

• In the case of the greening, the amount of net effluent heat decreased significantly because of the reduction in the effluent heat by convection from the wall surface (Qbuild).

54

The total effluen

t heat

over 24 h [MJ/day]

-2000

0

2000

4000

6000

8000

10000

12000

14000

concrete highref green

Qvent Qan

Qroad Qbuild


28

Assessment results (3)Index of domain C

55

Building form


data



MRT calculation

SET* calculation









ratio







Assessment of Adaptation to Urban Warming (Domain C) - the frequency distribution of SET*

within pedestrian space (2 p.m.) -• In the case of the highly reflective material, the volume where SET*

exceeded 35oC increased overall, and the acceptable volume ratio in the highly reflective material case was 70%, while that of the concrete case was 84%. In the greening material case, the acceptable volume ratio was 79%.

56

the maximum acceptable SET* value (SET*max) ⇒ 35°C.

0

0.1

0.2

0.3

0.4

0.5

Volume ratio [‐]

concrete

highref

green

SET*[°C]

Acceptable Unacceptable


29

Summary of assessment results ( Example 1 )In the conditions assumed in this study,

greening and highly reflective material have good impact on mitigating global warming and UHIs.

However, in terms of adapting to urban warming, greening was not effective and highly reflective material had obviously negative impact.

57

Case

Energy consumption

of HVAC

[MJ/day]

[Domain A]

Net effluent sensible heat

from urban surfaces

[MJ/day]

[Domain B]

Acceptable volume ratio

evaluated on the basis of

SET* (<35oC)

[Domain C]

Concrete 950 12400 84[%]

Highref 930 11400 75[%]

Greening 860 5100 79[%]

1. Background







58


30

Simulation target

59

Shinbashi Sta.新橋駅

• In the 2nd example, the effects of roadside trees in an actual urban area were evaluated.

• The thermal environment around wide intersections is often considerably worse.• In this study, an area around a wide intersection in Shinbashi, typical business

district in Tokyo, was selected as the simulation target

This image was extracted from Google earth

Computational domain

60

Two situations with and without roadside trees were simulated.

Tree crown height [m]

Treeheight[m]

Tree crown width [m]

Pitch of tree planting [m]

Leaf area density [m2/m3]

SurfaceWetness [‐]

3 13.5 5 10 0.56 0.44


31

61

(1) Aerodynamic effects of the planted tree(2) Thermal effects of the planted tree:a) Shading effects on solar radiation and

long‐wave radiation,b) Generation of water vapor from tree canopy.

Various effects of trees considered by tree canopy model(Yoshida, Ooka, Mochida, Murakami, Tominaga (2000))

solar radiation

(d)

longwaveradiation

(c)

(a)

(b)

(turbulence)

(penetration)

(a) aerodynamic effects of tree canopy

(b) latent heat from tree canopy

(c) shading effect on long‐wave radiation

(d) shading effect on short‐wave radiation

62

In order to reproduce the aerodynamic effects of stationary small scale obstacles that are smaller than the grid size, such as trees and small buildings, various models have been developed based on the methodology of canopy flow modeling

Canopy model for reproducing aerodynamic effectsof Tress


32

63

In Canopy model, we consider the situations where small obstacles (solid) are included in the computational grid, and fluid and solid are coexisting.

Tree Canopy

Outline of canopy model (1)

64

Instead of reproducing the configurations of small obstacles by computational grids, the model equations used in CFD are modified to include the extra terms expressing their effects.

Outline of canopy model (2)

Tree Canopy


33

65

k – model with tree canopy model decreases in velocity increases in turbulence increases in dissipation

0

i

i

x

u

ii

j

j

it

jij

jii Fx

u

x

u

xk

p

xx

uu

t

u

3

2

kkj

t

jj

jFP

x

k

xx

ku

t

k

FCPCkxxx

u

tk

j

t

jj

j

21

j

i

i

j

j

itk x

u

x

u

x

uP

[Continuity equation]

[k transport equation]

[ transport equation]

[Momentum equation]

: fraction of the area covered with trees

Cf: drag coefficient for canopya : leaf surface area density

Cp1: model coefficient for F

－Fi: extra term added to the momentum equation

+ Fk: extra term added to the transport equation of k

+ F: extra term added to the transport equation of

Fi

Fk ii Fu

F kp FCk

×

2

jif uuaC

aa

66

Fi

Fk

Fε

2

jif uuaC

ii Fu

parameters to be determined according to the real conditions of trees

, a , Cf：

Expressions of extra terms Fi, Fk, F in tree canopy model

ii FuCk

: fraction of the area covered with trees

a : leaf surface area density

Cf: drag coefficient for canopy


34

67

Leaf surface are density a

a = 2

1(leaf surface area)

Volume of tree crown

area of one side leaf surface

Tree Crown（樹冠）

68

C：

Fi

Fk

Fε

2

jif uuaC

ii Fu

a model coefficient in turbulence modeling, which should be optimized, for prescribing the time scale of the process of energy dissipation in canopy layer

parameters to be determined according to the real conditions of trees

, a , Cf：

Expressions of extra terms Fi, Fk, F

ii FuCk


35

69

Comparison of vertical velocity profiles behind tree

(x/H=5)(x1/H=4)(x1/H=3)(x1/H=2)(x1/H=1)

(x/H=5)(x/H=4)(x/H=3)(x/H=2)(x/H=1)

(x1/H=5)(x1/H=4)(x1/H=3)(x1/H=2)(x1/H=1)

(x/H=5)(x/H=4)(x/H=3)(x/H=2)(x/H=1)

(x/H=5)(x1/H=4)(x1/H=3)(x1/H=2)(x1/H=1)(x1/H=5)(x1/H=4)(x1/H=3)(x1/H=2)(x1/H=1)

: measurement : CFD with type B model

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

(1) C=1.5

(2) C=1.6

(3) C=1.7

(4) C=1.8

(5) C=1.9

(6) C=2.0

a=1.17[m2/m3]Cf =0.8[-]

70

Simulation (C=1.8)measurement

Comparison of vertical velocity profiles behind tree(Cpe1=1.8)

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]

0

6

12

0 0.7 1.4U/UH

Height[m]


36

71

(1) Aerodynamic effects of the planted tree (2) Thermal effects of the planted tree:a) Shading effects on solar radiation and

long‐wave radiation,b) Generation of water vapor from tree canopy.

Various effects of trees considered by tree canopy model(Yoshida, Ooka, Mochida, Murakami, Tominaga (2000))

solar radiation

(d)

longwaveradiation

(c)

(a)

(b)

(turbulence)

(penetration)

(a) aerodynamic effects of tree canopy

(b) latent heat from tree canopy

(c) shading effect on long‐wave radiation

(d) shading effect on short‐wave radiation

Shading effects of solar and long-wave radiations

The present model is based on the following assumptions:

1. Only the effect of tree crown is modelled. The effects of stem and branches are assumed to be negligibly small.

2. The ratio of absorbed radiations to the total incident radiation on the tree crown is given by the function

321 x,x,xakexp1

Tree crown ･Leaf area density a [m2/m3] ･Absorption coefficient k’ [-]

l [m] ℓ

(1) Distance through the tree crown ℓ [m]

(2) Leaf area density a [m2/m3]

(3) Absorption coefficient k’ [-] (here, k’=0.6)Tree crown=樹冠


37

Generation (transpiration) of water vapor and heat balance at leaf surface

・The heat balance equation at leaves that compose the tree crown

(1)

SP : Absorbed solar radiation [W]

RDP : Absorbed long-wave radiation [W]

HP : Sensible heat [W]

LEP : Latent heat [W]

SP

HP

LEP

RDP

・Using Eqs. (1), (2) and (3), leaf surface temperature TP is obtained. HP, LEP and TP are used as boundary conditions for CFD computation.

PaPcPP TTAH

sPaPPWPP ffLALE

0LEHRS PPDPP

(2)

(3)

Simulation Results

74


38

Horizontal Distribution of wind velocity (2 p.m. , h=1.5m)

75

50 [m/s]

・There is an area of high wind speed around front corner of the tallbuilding.

(1)Without trees

W

Wind direction

H=75m

H=27m

H=27m

H=27m

H=27m

H=75mN

EW

S

Comparison of wind velocity distributions of the cases with and without trees

76

50 [m/s]

(1)Without trees (2)With trees

W

H=27m

H=75m

H=27m

H=75mN

EW

S

・ The areas of high wind speed around the tall building are seen inthe both cases. In the east part of the domain, wind speed is relatively low.


39

Horizontal Distribution of Mean Radiant Temperature (MRT)(2 p.m. , h=1.5m)

77

6530 [ ]

Effects of tree shade is clearly seen on the north side of the road.

(1) Without trees (2) With trees

W

H=27m

H=75m

H=27m

H=75mN

EW

S

In the north west part of the domain, SET* is low in both cases due to high wind speed.In the east part, SET* in the case with trees is lower than that in the case without trees. This difference was mainly caused by the difference in MRT values.

Horizontal Distribution of SET* (2 p.m. , h=1.5m)

78

3525 [ ]

(1)Without trees

W

40

(2)With trees

H=27m

H=75m

H=27m

H=75mN

EW

S


40

Assessment Results

79

Indices for each domain

• From now, assessment results using these simulation results are shown.

• Domain A was set to assess the mitigation effect of global warming:⇒Index: the energy consumption of HVAC

Domain B was set to assess the mitigation effect of a UHI:⇒Index: the effluent heat from urban surface

Domain C was set to assess the adaptation effect to urban warming:⇒Index: the acceptable volume ratio on the basis of SET*

80

Domain B

Urban atmosphere


Domain C


Domain A

Building interior


41

Assessment results (1)Index of domain A

81

Building form


data



MRT calculation

SET* calculation









ratio







Assessment of Global Warming Mitigation ( Domain A) - total energy consumption of HVAC system over running time per day-

• The difference between the cases is very small.

• The influent heat by transmission through windows (Qr, trans) is slightly smaller in the case with roadside trees.

82

Qr,trans/COP

Qinner/COP

Qinitial/COP

Qconv,inside/COP

Qvent/COP

0

20

40

60

80

Without roadside tree

With roadside tree

The total energy

consumption of HVAC over

running time [GJ/day]


42

Assessment results (2)Index of domain B

83

Building form


data



MRT calculation

SET* calculation









ratio







• In the case with roadside trees, the amount of net effluent sensible heat decreases by 5 %.

• This is caused by the reduction in the effluent sensible heat convected from the road surface in the tree shade.

84

Qconv(from road)

Qan

Qr,trans

Qconv(from building)

Qconv(from tree)

Qvent

‐100

0

100

200

300

400

500

600

700

Without roadside tree

With roadside tree

The total effluen

t heat

over 24 h [GJ/day]

Assessment of UHI Mitigation (Domain B) - the total effluent heat from the urban surface over 24 h -


43

Assessment results (3)Index of domain C

85

Building form


data



MRT calculation

SET* calculation









ratio







• In the case with trees, the area where SET* is less than the acceptable limit (35oC) increases.

• The acceptable volume ratios considering the entire pedestrian space in both cases are very high

86

the maximum acceptable SET* value (SET*max) ⇒ 35°C.

05

1015202530354045

Volume ratio [%]

Without roadside trees

With roadside trees

SET*[°C]

Acceptable Unacceptable

Assessment of Adaptation to Urban Warming (Domain C) - the frequency distribution of SET* (2 p.m.) -


44

• The acceptable volume ratios considering the entire pedestrian space in both cases were very high.

-> This is because the high-wind-speed in the area around a front corner of tall building.

• The modification of wind flow by a tall building, and the resulting high wind speed strongly affected the reduction of SET* under the conditions assumed in this calculation.

Horizontal distributions of wind velocity (at 1.5 m height)

• In the east part of the computational domain, the wind speed is relatively low.

• In this part, SET* in the case with tress is lower than that without tress.

• The acceptable volume ratios:87%(without tress)97% (with tress)

• This result indicates that planting roadside tree is effective for adaptation to urban warming especially in areas of low wind speed.

Horizontal distributions of SET* (at 1.5 m height)


45

Summary of assessment results ( example 2 )

Case

Energy consumption of

HVAC[GJ/day]

[Domain A]

Net effluent sensible heat from urban surfaces[GJ/day]

[Domain B]

Acceptable volume ratio evaluated on the basis of SET* (<35oC)

[Domain C]

Entire pedestrian

spaceEast side

Without trees

77.1 625 97[%] 89[%]

With trees 76.1 592 99[%] 97[%]

Under the conditions assumed in this study, the presence of roadside trees had positive effects on the mitigation of UHIs and for adaptation to urban warming, but had little effect on mitigation on global warming.

1. Background







90


46

1) Most previous studies that compared effects of countermeasure techniques were aimed at a single perspective.

2) However, these aims often conflict one another.

3) Therefore, great difficulties still remain when policymakers and urban planners attempt to select proper countermeasure techniques, despite the enormous accumulation of knowledge.

4) To overcome the difficulties, a new total assessment method was proposed to assess the countermeasures for urban warming from the three different viewpoints, i.e.

a) mitigating global warming,

b) mitigating UHIs, and

c) adapting to urban warming,

and two examples of the assessment using the proposed method were shown.

91

2) Green and highly reflective surfaces had a positive impact on the mitigation of global warming and UHIs. However, in terms of adapting to urban warming, greening was not very effective and the highly reflective material had a clearly negative impact under the conditions assumed in this study.

3) Roadside trees had positive impacts on the mitigation of UHIs and the adaptation to urban warming, but had little effect on mitigation of global warming under the conditions assumed in this study.

92


47

Next target of this study-evaluation of the effects of countermeasuretechniques for window-

Single float glass(no special techniques)

Single float glasswith heat shading film（mirror reflection）

Single float glasswith heat ray retro‐

reflective film（再帰性反射）

1. Background







94


48

• Recent years, in order to reduce the cooling load of building, heat ray reflective glass or heat ray reflective film for window have been widely adopted.(for example, heat shading films(遮熱film))

• But they usually reflect solar radiation to pedestrian space. So, using these modifications have a negative impact on thermal comfort of pedestrian.

95

Heat ray is reflected to pedestrian

outdoor thermal environment

Cooling load

• To avoid the negative impact on pedestrian,“heat ray retro-reflective film (熱線再帰性反射film)” has been developed.

96

Indoor spaceOutdoor space

Heat ray retro‐reflective film

Single float glass

Incident direction

retro reflection(再帰性反射)


49

• “Heat ray retro-reflective film(熱線再帰性反射film)” has been developed.

• It is expected that adopting this film will have positive impacts on both reducing indoor cooling load and mitigating thermal environment in outdoor space.

Heat ray is reflected to incoming direction

Cooling load

outdoor thermal environment

• In this study, to evaluate the effect of heat reflective film on outdoor thermal environment,

a) a new method of radiation simulation which can considerdirectional reflection was developed ,

and

b) radiant analysis of outdoor thermal environment in Shinbashi (新橋), Japanese typical business district, was performed.

98


50

Outline of revised method for radiant computation

99

Problem of previous radiation computation method

• Many researchers have developed and used radiant simulation to evaluate the effects of radiation on outdoor thermal environment.

• However, in most of these methods, each surface in the computational domain is assumed to be a perfectly diffusively reflecting surface.

100

Incident direction

ex) concrete wall


51

Problem of previous radiation computation system

• Therefore, most of the existing methods can not evaluate the radiant field that is strongly affected by the directional reflection, such as the radiant field around a window with a heat ray retro-reflective film.

101ex) retro‐reflective filmex) concrete wall

• In this study, to consider the directional reflection, radiant heat exchanges between urban surfaces were calculated by a method proposed by Yoshida (University of Fukui) et al. (2014).

• This method revised the progressive radiosity method extended to the directional radiant computation by Ichinose (Tokyo Metropolitan University) et al. (2005) for outdoor space.

102

For indoor space

For outdoor space


52

103

The equations of the radiant computation considering directional reflection

• The equations of the extended radiosity method are as follows,

Element i

Element kElement j

Element k

Ri(j) : the radiosity per unit solid angle of surface element i intercepted by a surface element j [W/sr]

Ei(j) : the radiation per unit solid angle emitted from surface i to surface j [W/sr]

ρki(j) : the fraction of the radiosity reaching surface j from surface k via surface i per unit solid angle [1/sr]

κki : the correction coefficient of the distribution of the reflected radiosity from surface k to surface i

hemi(k,i) : the reflectivity measurement value from surface k via surface i to the surroundings

∑ κ ･ρ ･･ (1)

κ , ∑ ･･ρ⁄ (2)

104

The equations of the radiant computation considering directional reflection

∑ κ ･ρ ･･ (1)

κ , ∑ ･･ρ⁄ (2)


53

Radiant analysis of outdoor thermal environment in Shinbashi (新橋) district

105

Simulation target

106

Shinbashi Sta.新橋駅

• In this study, an area around a wide intersection in Shinbashi, typical business district in Tokyo, was selected as the simulation target

This image was extracted from Google earth


54

Computational domain

107

In terms of reduction of calculation cost, several buildings located in the same town block were modeled as a lumped building.

新橋駅

160m

166m

40m

40m40m

H=24m

H=32m

H=40m

H=100m

N

EW

S

40m

Wide intersection

Calculation cases

108

新橋駅

160m

166m

40m

40m

40m

H=24m

H=32m

H=40m

H=100m

N

EW

S

40m

Two cases of simulations with two different films put on the western surface of a building are compered here.


55

Calculation cases

109

The other is the case where the conventional heat shading film(遮熱film) is put on the same surface.

H=24m

H=32m

H=40m

H=100m

40m

N

EW

S

40m

One is the case where the retro‐reflective film(熱線再帰性film) is put on the western surface of a building.

Meteorological conditions

110

Thermal environment on a particularly hot summer day was simulated.

Meteorological data Japan Meteorological Agency in Tokyo

The target date 13:00,14:00,15:00,16:00July 23, 2010


56

Meteorological condition on July 23, 2010

0

100

200

300

400

500

600

700

800

900

Global solar radiation[W

/m2]

20

22

24

26

28

30

32

34

36

Air tem

perature[℃

]

July 23 was sunny day.Maximum temperature was 35 .

112

The amount of solar radiation absorbed by ground surface(July 23, 2010)

N

EW

S13:00

14:00

0

900

[W/㎡]

Conventional heat shading filmRetro‐reflective film


57

113

The amount of solar radiation absorbed by ground surface

(July 23, 2010,13:00)N

EW

S13:00

0

900

[W/㎡]


In the case with Heat shading film, the high peak value of solar radiation is observed around the western surface of the building

114

N

EW

S

14:00

0

900

[W/㎡]


The peak is not observed in the case with Retro‐reflective film at 14:00

The amount of solar radiation absorbed by ground surface

(July 23, 2010,14:00)


58

115

0

900

[W/㎡]

N

EW

S15:00

16:00

Retro‐reflective film Conventional heat shading film

The amount of solar radiation absorbed by ground surface(July 23, 2010)

116

13:00 14:00

‐60

‐110

‐70

‐110

‐60

‐90

16:00

‐95

‐50

15:00

‐80 ‐50

The difference of the amount of solar radiation absorbed by ground surface (Retro‐reflective film – Conventional heat shading film)

The amount of solar radiation absorbed by ground surface in the case with the heat ray retro‐reflective film is lower by up to 110 W/m2

than that with the conventional heat shading film.

‐110 0[W/㎡]


59

Conclusions

• The effect of a heat ray-retro reflective film on the thermal environment in outdoor space is evaluated using a numerical simulation on the basis of the radiant analysis method considering directional reflection.

• In this study, the radiant environments around a building in Shinbashi district, with heat ray retro-reflective film(熱線再帰性反射film) and heat shading film (遮熱film), were simulated.

• The result indicated that the amount of solar radiation absorbed by ground surface in the case with the heat ray retro-reflective film was lower by up to 110 W/m2

than that with heat shading film.

117

References

1) Yumino S., Uchida T., Sasaki K., Kobayashi H., Mochida A. “Total assessment for various environmentally conscious techniques from three perspectives: mitigation of global warming, mitigation of UHIs, and adaptation to urban warming”, Sustainable cities and society, 19, (2015), 236‐249

2) Yumino S., Uchida T., Mochida A., Kobayashi H., Sasaki K. “Evaluation of greening and highly reflective materials from three perspectives”, Proceedings of I 9th International Conference on Urban Climate (ICUC9) (2015)

3) Shinji Yoshida, Saori Yumino, Taiki Uchida, Akashi Mochida “Effects of windows with heat ray retro‐reflective filim on outdoor thermal environment and building cooling load”, Journal of Heat Island Institute International, 9(2), (2014), 67‐72

4) Shinji YOSHIDA, Saori YUMINO, Akashi MOCHIDA, Taiki UCHIDA, “An evaluation of the effects of heat ray‐reflective film on the outdoor thermal environment using a radiant analysis method”, Proceedings of the 9th International Conference on Urban Climate (ICUC9) (2015)


60

End, Thank you

Akashi Mochida

Professor, Dept. of Architecture & Building ScienceGraduate School of Engineering, Tohoku UniversityAoba 06, SENDAI 980-8579, Japan

E-mail: [email protected] http://www.archi.tohoku.ac.jp/labs-pages/kankyo/old/index/index_e.html

evaluation of the effects of greening and highly...

Documents