EVALUATION OF EVAPORATION ABILITY OF THE SYSTEM FORMITIGATING URBAN HEAT ISLAND

Ikusei Misaka*, Ken-ichi Narita**, Hitoshi Yokoyama****Takenaka Corporation, Chiba, Japan;**Nippon Institute of Technology, Saitama, Japan

***Tokyo Metropolitan Research Institute for Environmental Protection, Tokyo, Japan

Abstract

A lot of techniques to mitigate urban heat island using evaporative cooling have been developed. In order toevaluate the cooling effects by these techniques, on site estimation method of evaluating efficiency wasdeveloped, as the system for mitigating heat island, rooftop greening, wall greening, and water retentivepavement. For quantifying surface heat balance, convective mass and heat transfer coefficient was measure byseveral methods. Using evaporation efficiency for the index of water holding ability, it is able to estimate thecontinuity of evaporation effects with the systems.

Key words: evaporation efficiency, Heat balance, measurement

1. INTRODUCTION

Recently, the urban heat island has comes to bring more and more serious urban problems. It is seems thatone of the main factors to progress urban heat island is little penetration of grand surface according tourbanization. In order to mitigate urban heat island, a lot of techniques using evaporative cooling already havebeen executed, such as greening building surface or surrounding area of buildings, watering building or road,water retentive pavement, and so on.

Evaluation cooling effects by those techniques, it seems to be necessary to clarify the characteristics ofevaporation and heat balance. However, it is not enough to examine because of difficulties to estimate thesecharacteristics.

Then, in this paper, on site estimation method of evaluating efficiency was developed, as the system formitigating heat island, rooftop greening, wall greening, and water retentive pavement.

2. EVALUATION APPROACH

In this study, evaluation approach of the effectscorresponding to an individual technology wasexamined, and the experiments have been executedusing examination systems such as building greeningsystem and the water retentive pavement, and theevaluation of the heat balance and the evaporationcharacteristic was tried.

The fundamental equation used for the evaluationand the analysis is as shown in Fig. 1.

For quantifying surface heat balance, convectivemass and heat transfer coefficient was measure byseveral methods. Moreover, using evaporationefficiency for the index of water holding ability, it isable to estimate the continuity of evaporation effectswith the systems.

3. HEAT BALANCE OF BUILDING GREENINGSYSTEMS

3.1. Heat balance of rooftop greening

In order to evaluate the effects of mitigating urbanheat island by light and thin type rooftop greeningsystem which could be applied in existent buildings,basic experiment using examination rooftop greeningsystem at the building in Tokyo were carried out.

The greening examination system producedpalette type planting base (500mm×500mm×80mmH)made of the vinyl chloride, filled an artificial light soilby 80mm in that, set up the plant for which each

③Sensible heat flux:　　a, ε:albedo,emissivity　　S↓,L↓：Solar radiation 1ong-wave radiation from atmosphere(W/m2)　　δ：Stefan-Bolzmann constant　　Ts：Surface Temperature(K)

TaTsH

①Heat balance equation:　　Rn:Net Radiation (W/m2)　　H,LE,G:Sensible, latent, conductive heat flux(W/m2)

LEGHRn

②Radiation balance equation:　　a, ε:albedo,emissivity　　S↓,L↓:Solar radiation 1ong-wave radiation from atmosphere(W/m2)　　σ:Stefan-Bolzmann constant　　Ts:Surface Temperature(K)

4TLS)a1(Rnｓ

④Analogy between heat and mass transfer:　　C:Specific heat of moist air (J/kg.k)　　k:mass transfer coefficient (kg/m2.g.(kg/kg’)

k83.0C/

⑤Mass transfer coefficient:　　E:Evaporation rate (kg/m2･ｓ)　　Xs,Xa:Absolute humidity of surface and air temperature (kg/kg’)⑥Latent heat flux:　　L:Latent heat of vaporization (J/kg)　　B:Evaporation Efficiency

XaXs/EK

XaXskLLE

③Sensible heat flux:　　a, ε:albedo,emissivity　　S↓,L↓：Solar radiation 1ong-wave radiation from atmosphere(W/m2)　　δ：Stefan-Bolzmann constant　　Ts：Surface Temperature(K)

TaTsH

①Heat balance equation:　　Rn:Net Radiation (W/m2)　　H,LE,G:Sensible, latent, conductive heat flux(W/m2)

LEGHRn

②Radiation balance equation:　　a, ε:albedo,emissivity　　S↓,L↓:Solar radiation 1ong-wave radiation from atmosphere(W/m2)　　σ:Stefan-Bolzmann constant　　Ts:Surface Temperature(K)

4TLS)a1(Rnｓ

④Analogy between heat and mass transfer:　　C:Specific heat of moist air (J/kg.k)　　k:mass transfer coefficient (kg/m2.g.(kg/kg’)

k83.0C/

⑤Mass transfer coefficient:　　E:Evaporation rate (kg/m2･ｓ)　　Xs,Xa:Absolute humidity of surface and air temperature (kg/kg’)⑥Latent heat flux:　　L:Latent heat of vaporization (J/kg)　　B:Evaporation Efficiency

XaXs/EK

XaXskLLE

Fig. 1 Fundamental equation of heat balance

Fig. 2 Mesurement apparatus of rooftop greening

Solar radiationLong-wave radiation

from atmosphere

Wind directionWind velocity

Thermo-couple Heat flow meter

Weather conditionmeasurement

without greening systemRooftop with greening system

70mm

Radiation and thermalenvironment

Evapotranspirationmesurement

TemperatureHumidity

10mm

Electronic balance

Short and long waveradiation meter

Heat flow meterThermo-couple

Radiation and thermalenviroment

Thermo-camera

Thermo-camera

The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan

system was specified, and planting was set up. Themeasurement apparatus and cross section ofmeasurement point are shown in Fig. 2. The greeningexamination system and the measuring instrumentswere set up in the end of July 2003, and themeasurement began on August 10.

The radiation balance and the heat balance wereanalyzed to quantify the cooling effects by rooftopgreening. The net radiation, in heat balance equation② , calculated by measurements of solar radiation,atmospheric radiation, and the surface temperaturewith albedo and emissivity. Moreover, the conductiveheat flux to the ground was measured by heat fluxmeter. The latent heat flux was calculated from theevapotranspiration rate obtained according to theweight change value. And sensible heat flux wascalculated as residual of heat balance equation①. Inaddition, evaporation efficiency β was tried tocalculate from the value of sensible heat and latentheat flux with equation③〜⑥.

The results of heat balance analysis are shown inFig.3. The results of heat balance analysis show thelatent heat consumption of evapotranspiration atgreening system to prevent increase sensible heat.

Fig.4 shows daily change of evaporation efficiencyin each examination plants and accumulated solarradiation from August 19 to September 20. It isconfirmed that evaporation efficiency is different fromplants varieties and times of watering. Mitigatingeffects by rooftop greening system were varied in kindof plants, water condition and so on.

Using evaporation efficiency as an index ofevaporation ability, it is able to evaluate the differenceof the effects by plants and water condition.

3.2. Heat balance of wall greening

In order to evaluate the mitigating effects of urbanheat island by wall greening, an experiment tocharacterize the evaporation effect of a greeningsystem and to quantify its heat balance was carriedout.

At heat balance analysis, sensible heat flux iscalculated using heat transfer coefficient estimated bymeasurements with SAT (Sol-air Temperature) meter.The experiments were carried out with theexamination greening panel shown in Fig.5 in the wallof the first floor rooftop department look south in thebuilding in Tsukuba city. The plants were assumed tobe two kinds, Hedera herix and Euonymus fortunei.Leaf area index LAI was 1.45m2/m2 at Hedera herix,and 1.18m2/m2 at Euonymus fortunei. The installationof the greening panel and the measuring instrumentswas done in the end of July 2005, and measured fromAugust 1 to the 31.

The SAT meter fixed a mock plant made of plasticto the heat insulator to become similar shape to theplants and almost the same leaf area as the greeningpanel by the same size as the unit, and colored thewhole with a matt black. The SAT meter built inpartially of the greening panel, and the measurementapparatus is shown in Fig.6. It is assumed that thereare neither evaporation nor conduction in the SATmeter, so sensible heat flux is equal to net radiation.

Fig. 3 Heat balance of rooftop greening

Fig. 6 Mesurement apparatus of wall greening

With Greening(Lawn)

-200

0

200

400

600

800

1000

9/140

9/146

9/1412

9/1418

9/150

9/156

9/1512

9/1518

9/160

Flu

x(W

/m2)

Rn H lE G

Witout Greening

-200

0

200

400

600

800

1000

9/140

9/146

9/1412

9/1418

9/150

9/156

9/1512

9/1518

9/160

Flu

x(W

/m2)

1)Watering 5L/m2 for a day

0.00.10.20.30.40.50.60.70.80.91.0

19 20 21 22 23 24 25 26 27 28 29 30 31Aug.

Ev

apo

ratio

nE

ffic

ien

cy

0481216202428323640

Da

ilyS

ola

rR

aid

atio

n(M

J)

Daily Solar Radiation Lawn Phylia nodifova Sedum Soil

2)Watering 5L/m2 for 3 days

0.00.10.20.30.40.50.60.70.80.91.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18Sep.

Ev

ap

ora

tio

nE

ffic

ien

cy

0481216202428323640

Da

ilyS

ola

rR

aid

atio

n(M

J)

Fig. 4 Evaporatopm efficiency of each plants

300×6=1,800mm

1,000mm

Wall

Front elevation Cross section

Plan

SAT meter

Wall

300mm

300mm

300×6=1,800mm 300×6=1,800mm

425mmSAT meter

Wall greening panel (Hedera Herix)

Rooftop of the first floor

Wall greening panel (Euonymus fortunei)

Fig. 5 The examination wall greening system

500mm

Greening panel(×2)

2,000mm

Front elevation Cross section

4,500mm1,500mm

Solar radiation meter

Thermo-couple

Heat flow meter

Thermo-couple

Short and long waveradiation meter

Temperature Humidty Wind direction and velocity

Short and long waveradiation meter

Solar radiation meter

Temperature Humidty Wind direction andvelocity

Rooftop of the first floor

Wall

The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan

And, assuming the albedo and emissivity to be 0.0 and 1.0 because the surface is a black, the net radiation canbe calculated from measurement value of solar radiation, atmospheric radiation and the long wave radiation fromthe surface. Convective heat transfer coefficient α and mass transmission rate k were calculated from therelation between sensible heat flux and the temperature difference of surface and air, and the analogy betweenheat and mass transfer(Refer to the equation③〜⑤). The relation between wind speed and convective heat ormass transfer coefficient by measurement of SAT meter, was applied to calculate these coefficients of wallgreening panels.

The net radiation and the conductive heat flux were measured by long and short wave radiation meter andheat flow meter. Sensible heat flux is calculated by multiplying convective heat transfer coefficient by thetemperatures difference of surface and the air. And latent heat flux was calculated as residual of heat balanceequation①. In addition, evaporation efficiency was calculated with equation⑥.

Fig. 7 shows daily change of evaporation efficiency in each examination plants and accumulated solarradiation from August 1 to 31. The evaporation efficiency of wall greening panel changes in about almost 0.15 to0.25, regardless of daily accumulated solar radiation. The daily average of the evaporation efficiency in Augustbecomes 0.15 at Hedera herix, and 0.21 at Euonymus fortunei , and Euonymus fortunei is slightly high throughthe experiment period. Moreover, it is possible to confirm the tendency that evaporation efficiency will increasethe next day of rainfall and watering.

It could be confirmed that the evaporation efficiency of wall greening system was different according to plantvarieties and the water conditions.

4.HEAT BALANCE OF WATER RETENTIVE PAVEMENT

To evaluate the effects of mitigating urban heat island by thermo-sensitive pavement system, which is,developed from the point of view of using rainwater effectively, basic experiment about evaporation efficiencyusing as an index of thermal and evaporation characteristics was carried out.

Fig. 8 Composition and expected function ofthe pavement system

Fig.9 Mesurement apparatus for waterretentive pavement

Fig. 10 air and surface temperature, andEvaporation Efficiendy

Fig.11 Relative of evaporation efficiency andsurrface temperature

Fig. 7 Relation between evaporation efficiency of wall greening sysytem and daily solar radiation

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31Aug.

Eva

pra

tio

nef

fici

en

cy

0

3

6

9

12

15

Dil

yso

lar

rad

iati

on

(MJ)

Daily sorlar radiation Precipitation or watering Euonymus Fortunei Hedera herix

Waterproof sheet

Urethane mat withthermo-sensitivehydoro gel

Water retentive block

Receiving counter

300mm

300mm

25mm50mm

Waterproof sheetUrethane mat

block

Precipitation Evaporation

InfiltrationWater rise by

capillarity

Water absorption to the gel(below sensitive temperature)

Water desorption abovethe sensitive temperature

Waterproof sheetUrethane mat

block

Rooftop

Heat flow meter

Short and Long-wave radiationWind direction and Speed

TemperatureHumidityThermo-couple

Heat flow meterThermo-couple

-0.5

0.0

0.5

1.0

1.5

0 3 6 9 12 15 18 21 0E

vap

ora

tio

nE

ffic

ien

cy

20

30

40

50

60

Tem

per

atu

re(℃

)

Evaporation Efficiency Air TemperatureSurface Temperature

Sensible Temperature(=29.2℃）

-0.5

0.0

0.5

1.0

1.5

20 25 30 35 40 45 50

Evaporation Efficiency

Ev

ap

ora

tio

nE

ffic

ien

cy

Evaporation Efficiency

The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan

The section composition of the pavement system and the conceptual diagram in the function and the effectare shown in Fig.8. The urethane mat of 50mm in thickness that has the function of water storage was set upunder the inter-rocking block. To attempt effective use for water, thermo-sensitive hydro gel is filled to theurethane mat. Thermo-sensitive hydro gel is a gel in which water can be absorbed or desorbed depending on itstemperature, that is, water absorption in case its temperature below a certain sense temperature(set temperature),and water desorption above the sense temperature. The experiments were carried out with the examinationpavement system at the building roof in Tokyo. The pavement system was constructed on August 9, 2005, andthe size of system district was 3m×3m. Fig. 9 shows the layout of the measuring instruments.

The experiment with paper filter is executed to estimate heat and mass transfer coefficient of the pavementsurface. From the results of experiments with paper filter, it can be confirmed that the relation between heat ormass transfer coefficient and wind speed is good. So, each heat flux and evaporation efficiency was calculated byusing the heat and mass transfer coefficient using equation③〜⑥.

Results of heat balance analysis on the pavement and the rooftop at August 19, is shown in Fig.10. This figureshows the latent heat consumption of evaporation at pavement system to prevent increase sensible andconvective heat flux. Because sensible heat flux at the pavement surface is about 20% at the rooftop surface indaytime, it seems to be effective for improvement urban thermal environment. From the relation between theevaporation efficiency and the surface temperature(Fig.11), the tendency that the evaporation efficiency increasewhen the surface temperature had been above 25℃, that is the sense temperature, was confirmed.

Fig. 12 shows daily change of accumulated solar radiation and evaporation rate for a day at the pavementsystem from August 14 to September 22. The evaporation effects continue, though watering is not executed tothe pavement since it waters about 30L/m2 on August 10.

The effect mitigating urban heat island by this pavement system, was confirmed , and this effect is found tobe kept up by thermo-sensitivity.

5. CONCLUSION

In order to evaluate heat balance and the evaporation characteristic, the experiments have been executedusing examination systems such as building greening system and the water retentive pavement. Then on siteestimation method of evaluating efficiency was developed, as the system for mitigating heat island, rooftopgreening, wall greening, and water retentive pavement.

For quantifying surface heat balance, convective mass and heat transfer coefficient was measure by severalmethods. Using evaporation efficiency for the index of water holding ability, it is able to estimate the continuity ofevaporation effects with the systems to mitigate urban heat island.

It is important that the data to evaluate the effects for improvement urban thermal environment, for exampleevaporation efficiency and heat balance characteristic and so on, will be enhanced to enough to apply to varioustechnique for mitigating urban heat island.

Acknowledgement

We expressed our profound gratitude to Dr. Hirotaka Suzuki for his kind guidance on experiments.

Fig.11 Relation between evaporation efficiency of water retentive pavement system and daily solar radiation

Precipitation(mm)

0

2

4

6

8

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Ev

apo

rati

on

rate

(L/m

2/d

ay)

0

10

20

30

40

Dai

lyso

lar

rad

iati

on

(MJ

)

Dialy solar radiation

Evaporation Rate

8.53.5 5.0 87.520.517.0 19.0 11.0

4.0

22.5

Aug. Sep.

The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan