A 3D CAD-Based Simulation Tool for Prediction and Evaluation

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Solar Energy 83 (2009) 1064–1075 www.elsevier.com/locate/solener

A 3D CAD-based simulation tool for prediction and evaluation of the thermal improvement effect of passive cooling walls in the developed urban locations Jiang He *, Akira Hoyano Interdisciplinary Graduate School, Tokyo Institute of Technology, 4259-G5-2 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan

Received 2 July 2008; received in revised form 1 December 2008; accepted 14 January 2009 Available online 24 February 2009 Communicated Communicated by Associate Associate editor: Matheos Santamouris

Abstract

As a passive cooling strategy aimed at controlling increased surface temperatures and creating cooler urban environments, the authors have developed a passive cooling wall (PCW) constructed of moist void bricks that are capable of absorbing water and which allow wind penetration, thus reducing their surface temperatures by means of water evaporation. Passive cooling effects, such as solar shading, radiation cooling and ventilation cooling can be enhanced by incorporating PCWs into the design of outdoor or semi-enclosed spaces in parks, pedestrian areas and residential courtyards. The purpose of the present paper is to detail the development of a 3D CAD-based simulation tool that can be used to predict and evaluate the thermal improvement effect in urban locations where PCW installation is under consideration. Measurement results for the surface reduction effect of a PCW are introduced in the first part of the paper. In the second part, thermal modeling of a PCW is proposed based on analysis results of experimental data. Following that, a comparison study that integrates the proposed thermal modeling was conducted to validate the simulation method. In order to demonstrate the applicability of the developed simulation tool, a case study was then performed to predict and evaluate the thermal improvement effect at an actual urban location where PCWs were installed. Simulations were performed by modeling the construction location in two scenarios; one where the PCWs were composed of dry bricks, and another where the bricks were wet. The results show that, in terms of surface temperature and mean radiant temperature (MRT), this simulation tool can provide quantitative predictions and evaluations of thermal improvements resulting from the installation of PCWs. Ó

2009 Elsevier Ltd. All rights reserved.

Keywords: Passive cooling; Void brick; Evaporation; Thermal environment; Surface temperature; Simulation

1. Introduction

Summer thermal environments in urban areas have been dete deterio riora rati ting ng as urba urbani niza zati tion on prog progre resse sses. s. This This can can be understood by taking into consideration the environmental problems resulting from the urban heat island effect, which in recent years has had a major impact on only large cities, but also on mid-sized cities and small towns as well. Additionally, the number of very hot days and the number of people people sufferin sufferingg from from heat heat stress stress have have increa increased sed signifi signifi-*

Corresponding author. Tel.: +81 459245510; fax: +81 459245553. E-mail address: [email protected] (J. He).

0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.01.006

cant cantly ly in rece recent nt year years. s. As poin pointe ted d out out in a numb number er of reports, the latter has something to do with the former. It is well known that the worsening urban thermal environment ment is prima primari rily ly due due to the the ma massi ssive ve am amou ount nt of arti artifificiallycially-gen genera erated ted heat heat from from human human activit activities ies as well as from the changes in land coverage that occur when natural cover (such as trees and plants) are replaced by buildings and pavement. From From theviewpoint theviewpoint of land-c land-cove overag ragee change changes, s, theheating theheating and heat storage effect of pavement and building surfaces resulting from sunlight have been recognized as a major cause of urban heat island formation. This is because most types types of paveme pavement nt and buildin buildingg exterio exteriors rs are fabrica fabricated ted from from

J. He, A. Hoyano/ Solar Energy 83 (2009) 1064–1075

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Nomenclature

a c p cm D L RS RL t T x X a X s

solar absorptance specific heat (J/(kg K)) humid air specific heat (J/(kg K)) thickness of a brick (m) latent heat (J/kg) solar insolation on the brick surface (W/m 2) long-wave radiation from the surroundings (W/ m2) time (h) temperature (K) distance in normal to an external surface (m) absolute humidity mixing ratio at temperature of T a (kg/kg(DA)) absolute humidity mixing ratio at temperature of T s (kg/kg(DA))

materials with low albedo and high thermal retention capacities, such as asphalt and concrete. As a result, they absorb a great deal of solar heat during the day, and their surfaces remain warmer than the surrounding air at night due to the retained solar energy. This indicates that reducing the heat absorbed by these surfaces, or otherwise lowering their surface temperatures, would be an effective method of improving outdoor urban thermal environments and mitigating the urban heat island effect. As a passive cooling strategy for controlling the increase of urban surface temperatures and creating a comfortable thermal environment, the authors have developed a passive cooling wall (PCW) constructed of moist void bricks capable of absorbing water (Shirai, Hoyano et al., 1995, Hoyano and Shirai, 1995, 1997). A PCW has features that allow wind to pass through it, and thus reduce its temperature by facilitating the evaporation of water stored in the bricks. As shown in Fig. 1, the PCW provides a shaded area free from direct solar radiation while the PCW surface itself can be cooled by evaporation. This results in cooler surfaces in outdoor locations on summer days. Furthermore, the air passing through the PCW can be further cooled when a breeze is blowing. As a result, the following passive cooling effects can be created: (1) solar shading, (2)

v

air velocity (m/s)

Greek symbols convection coefficient (W/(m 2 K)) ac b evaporation efficiency (surface wet ratio) e emissivity k thermal conductivity (W/(m K)) q density (kg/m3) Stefan-Boltzmann constant (W/(m 2K4)) r Subscripts air a i windward side leeward side o surface s

radiation cooling and (3) ventilation cooling. It is expected that PCWs will be increasingly installed in outdoor locations or semi-enclosed locations such as parks, pedestrian areas, patios and residential courtyards. As described in literature (e.g. Givoni, 1994), porous materials such as unglazed pottery have long been used to improve thermal comfort in residential spaces by assisting passive evaporative cooling. However, previous studies on passive cooling walls that allow wind penetration are scarce. In order to investigate the cooling effects of the developed PCW, experiments have been conducted in the laboratory and at outdoor locations using mock-ups and prototype walls. The experimental results were documented in our previous papers (Shirai et al., 1997, 2000, 2002). This paper focuses on the development of a numerical simulation tool that can be used to predict and evaluate the thermal improvement effect in developed urban locations where PCW installation is under consideration. The measured results of surface temperature reductions resulting from a PCW will be introduced in the first part of the paper. In the second part, we will describe thermal modeling for predicting external surface temperatures of a PCW. A comparison study, in which the proposed thermal modeling was integrated, will then be conducted to validate the simulation method. In addition, in order to demonstrate the applicability of the developed simulation tool, a case study will be carried out to predict and evaluate temperature distributions of all external surfaces in an actual outdoor space where PCWs have been installed. 2. Description of the PCW

2.1. Void bricks

Fig. 1. Schematic description of a passive cooling wall constructed of moist void bricks.

As shown in Fig. 2, we developed two types of waterpermeable void bricks: (1) slit-type brick (Type 1) and (2) open-type brick (Type 2). A slit-type brick has 11 slit-

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Fig. 2. Photos and diagrams of void bricks developed by the authors.

shaped ventilation channels while an open-type brick has three square-shaped channels. The temperature reduction effect of air passing through a PCW constructed of slit-type bricks is greater than that of the open-type bricks. On the other hand, the volume of air passing through a slit-type PCW is smaller than that for an open-type PCW under the same inflow condition. To examine the difference of cooling effect between slit-type and open-type bricks, experiments were conducted in the laboratory using an experimental setup illustrated in Fig. 3. Fig. 4 shows experimental results under the experimental conditions that air temperature and relative humidity of inflow were kept to be 32 °C and 40%, respectively. As seen in Fig. 4, the air temperature were reduced by 3 and 1 °C after passing through the slit-type and open-type bricks, respectively. From this result, it can be understood that the temperature reduction effect for a slit-type PCW is greater than that for an open-type PCW. Specifications for the developed brick’s material are listed in Table 1. Fig. 5 shows the variations of water penetration height for a brick material sample over a period of several hours. After soaking the bottom of the brick in water, it took about 3 h for water penetration to reach a level equal to the height of a standard brick (84 mm).

Fig. 4. Experimental results of cooling effect of the developed bricks.

Table 1 Specifications of the developed brick’s material. Baking temperature Density in dry condition Density in water-saturated condition Maximum absorbed water content in weight

) 250 m m ( t 200 h g i e h 150 n o i t a r 100 t e n e p r 50 e t a W 0

1000 °C 1750.55 kg/m3 2012.60 kg/m3 14.9%

Brick Water penetration height

water

0

1

2

3

4

5

6

7

8

10(hour) 24

Fig. 5. Variation of water penetration height after soaking in water.

2.2. Water supply method

A U-shaped steel liner (tray) is sandwiched between upper and lower bricks as shown in Fig. 6. The ends of

Fig. 6. Schematic description of water supply method for a PCW.

Fig. 3. Section of experimental setup and locations of measurement points.

these liners are connected to a vertical rectangular duct at each corner of a PCW. Water is supplied into one edge, flows along the liner tray where some of it is absorbed into the brick bottoms, then out from the other edge. A sponge


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is sandwiched between the bricks and the tray. This water supply method ensures that each brick can be independently water-supplied, and that the surface of the supply water will remain stable and at a certain level.

2.3. Characteristics of surface temperature distribution

Prototype PCWs were constructed on the roof of a twostory building. A thermograph for the PCWs is shown at the right of Fig. 7. The thermograph was taken at a time when the PCWs were completely wet. It can be seen that the surface temperature for the south-facing PCW was 29–30 °C, which is lower than the dry-bulb temperature (DBT) by 3–4 °C and higher than the wet-bulb temperature (WBT) by 2–3 °C. The surface temperature for the east-facing PCW, which was not exposed to direct solar radiation, was approximately 25 °C, nearly equal to the wet-bulb temperature. Fig. 8 shows the diurnal variations of the surface temperature on the windward and leeward sides of the south-facing PCW on a sunny day. The temperatures indicated in Fig. 8 were the results measured by thermocouples that provide an accuracy of 0.1 °C. It is obvious that the surface temperature on the leeward side, which was not exposed to direct solar radiation, was equal to the WBT throughout the day. The interior surface temperature of the brick was also equal to the WBT.

Fig. 8. Diurnal variations of brick surface temperature, ambient dry-bulb temperature (DBT), wet-bulb temperature (WBT) on a sunny day (Sep. 7).

of water consumption between slit-type and open-type PCWs was not found. 3. Numerical simulation

2.4. Water consumption of a PCW 3.1. Methodology

Two slit-type PCWs and one open-type PCW were used to measure water consumption. These test PCWs were facing south during measurements. Measurement results of water consumption for a period of four summer sunny days were presented in Fig. 9. From the figure, it can be seen that the maximum of water consumption was approximately 35 g/(m 2 min). The maximum of diurnal water consumption was 15 kg/(m 2 day) during the measurement period. A significant difference

As can be understood from the measurement results described above, PCW surface temperatures can be reduced below ambient air temperature by water evaporation. In order to predict and evaluate the expected thermal improvement of a PCW in a developed urban environment during design stages, numerical simulations are a necessary and practical alternative to physical experiments. As a design tool for supporting the prediction and evaluation

Fig. 7. The left and right are a photo of PCWs and thermograph taken at noon on a sunny summer day (Aug. 11), respectively.

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J. He, A. Hoyano/ Solar Energy 83 (2009) 1064–1075 (a) Weather conditions

35

1200 Dry-bulb temp.

1000

Wet-bulb temp.

30

800

25

600

20

400

15

200

10

0

5 Total horizontal solar radiation Wind speed

Wind direction N

3.0 2.5

E

2.0 S

1.5 1.0

W

0.5 N

0 (b) Water consumption 45 Slit-type

Slit-type

Open-type

35 25 15 5 -5 0:00

12:00

0:00

12:00

Aug. 9

0:00 Aug. 10

12:00 Aug. 11

0:00

12:00

0:00

Aug. 12

Fig. 9. Measurement results of water consumption of test PCWs on summer sunny days.

of thermal improvements resulting from the installation of PCWs, we adapted a numerical simulation method using the 3D CAD-based simulation tool that was previously developed by our research group. A PCW was modeled and a calculation algorithm for its surface temperature was integrated into the simulation tool. The simulation methodology is described below. 3.2. Description of the developed simulation tool

The simulation process is outlined in Fig. 10. The simulation is performed using 3D CAD models for buildings, trees and other structures in the area being analyzed. Three-dimensional spatial forms of the buildings, trees, etc., and two-dimensional ground surfaces are divided into mesh grids, and thermophysical data of construction materials, such as albedo and conductivity and solar transmittance, are assigned to the grids. An automatic mesh-dividing process has been integrated and only uniform mesh can be used in the present version of the tool. A uniform mesh size of 0.2 m was used in the simulation.

The external surface temperature for each mesh can be calculated by solving a non-steady-state one-dimensional heat balance equation in normal to the surface. In the heat balance equation, three-dimensional radiation irradiated on the surface is taken into account. The short-wavelength radiation on the surface is direct solar insolation, sky solar radiation and reflected solar radiation. Reflected solar radiation includes both specular reflection and isotropic diffuse reflection. Only the first reflected solar radiation is considered in the present study. Atmospheric radiation and longwavelength radiation from the surroundings are considered in the long-wavelength radiation irradiated on the surface. Sky solar radiation and atmospheric radiation are calculated from the sky view factor for each mesh. The sky view factor is calculated by the multi-tracing simulation from the mesh toward multiple hemispherical directions. The tracing direction is established so that the tracing density (interval) comes to have the same shape factor. The sky view factor is estimated by counting the number of tracers reaching the boundary surfaces. The shape factor for calculating the reflected solar radiation and long-wavelength radiation from the surroundings is deter-

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The backward-difference method is used for solving the non-steady-state heat balance equation. One simulation is run using 5-day weather data in 5-min time steps in order to obtain a periodic steady-state solution. The simulated results of surface temperatures for the 5th day are output and used for analysis. As outputs of the simulation, temperatures of all external surfaces can be predicted and visualized on the 3D models (see the lower left corner of Fig. 10). From the calculated results of surface temperatures, mean radiant temperature (MRT) at a point can be estimated. The mean radiant temperature at a point is the measure of the combined effects of the temperatures of all surfaces that surround the point. The larger the surface area is and the closer one is to it, the more effect the surface temperature of that surface has on the individual. The MRT at a height of 1 m above the ground was used to evaluate thermal comfort in outdoor human activity 

Àk

@ T   

@ x x¼0 

Àk

@ T 

@ T @ t

Fig. 10. Description of the simulation tool.

mined by the same method used in the estimation of the sky view factor. Convective heat transfer is calculated on the assumption that ambient air temperature and wind velocity are uniformly distributed in the outdoor spaces at the time of analysis. This assumption is valid under weather conditions with low wind velocity. The surface convection coefficient is considered to be a function of air velocity and is given by Jurges’ equation. The non-steady-state one-dimensional heat conduction equation for each mesh is solved using the above-mentioned heat balance data as boundary conditions for external surfaces. Boundary conditions for internal surfaces are the indoor air temperature for the buildings and the underground temperature for the ground. Rooms on the same floor of a building are considered to be a single room, and the indoor air temperature is uniformly distributed at the time of analysis. The tree shape is modeled as a 3D CAD model and the crown is composed of meshes containing solar transmittance data. Solar transmission radiation decreases as it passes through the tree mesh model. This mesh model makes it possible to quantify the influence of the position and distance of sunlight passage within the crown on solar transmission. The surface temperature of a tree’s crown is calculated by empirical formulas derived from the experimental data, and can be expressed as a function of the solar radiation incident on the surface, ambient air temperature, and wind velocity (Shimokawa et al., 1996).



@ x  x¼ D

¼

¼ aR si þ eð R L À rT si4 Þ þ aci ðT ai À T si Þ À Lb

aci ð X si À X ai Þ cm

4 ¼ aR so þ eð R L À rT so Þþ aco ðT ao À T so Þ À Lb

ð1Þ

aco ð X so À X ao Þ ð2Þ cm

k @ 2 T qc p @ x2

ð3Þ

spaces in the present study. A detailed description of the simulation methodology can be found in Asawa et al. (2008). 3.3. Energy balance equation on the PCW surface

Fig. 11 shows the energy flow on a PCW surface. Nonsteady-state one-dimensional heat balance equations for external surfaces of a PCW can be expressed by Eqs. 1 and 2. The heat balance equation inside the brick is written by Eq. 3. In Eq. 1, the left term is the conduction heat transferred into the brick. At the right of Eq. 1, the first term is the solar insolation, the second term is the net long-wave radiation, the third term is the convective heat flux and the fourth term is the latent heat by evaporation. In the latent heat term, evaporation efficiency ( b) was used to express the wetting condition of the brick surface. This is because part of the brick surface may become dry due to active evaporation when exposed to direct solar radiation, causing the surface temperature to rise, as can be seen in Fig. 12. The shaded brick surfaces, such as the internal surfaces in the ventilation channels, can be kept completely

Fig. 11. Energy flowpaths at the PCW surface.

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wetted. As understood from the above-mentioned measurement results, the surface temperature for the shaded brick surface can be considered to be equal to the wet-bulb temperature. Because the simulation methodology is valid under low wind velocity weather conditions (wind speed less than 3 m/s), the wind velocity on the windward and leeward sides of a PCW is assumed to be the same as the input weather data. The PCW surface temperature is calculated by solving Eq. 3, using Eqs. 1 and 2 as boundary conditions. The brick openings are composed of quadrangle surfaces as shown in Fig. 13. Each square is 20 Â 20 cm and equal to the size of the minimum mesh used in the calculation. Temperatures of the quadrangle surfaces are considered to be equal to the wet-bulb temperature. The number of the squares is determined by the ratio of openings to the sectional area of a brick. The ratio of openings is 34% and 48% for a slip-type and open-type brick, respectively. 3.4. Validation of the simulation method

In order to validate the proposed simulation method, we carried out a comparison study of the simulated and mea-

The PCW (Type2,Ratio of openings=48%)

Visualization of surface temperature 20cm m c 0 2

Brick

Fig. 13. Description of surface temperature calculation method.

sured surface temperatures. Measurements were conducted using the experimental PCW shown in Fig. 14. A comparison study was carried out to determine the evaporation efficiency. Surface temperatures of the experimental PCW were calculated using the proposed simulation method under calculation conditions in which the evaporation efficiency was assumed to be a value between 0.0 and 1.0. The results of the comparison study showed that strong agreement was found between the simulated and measured surface temperatures when the value of evaporation efficiency was between 0.4 and 0.6. As an example, Fig. 15 shows diurnal variations of simulated and measured surface temperatures on a sunny day. The evaporation efficiency was assumed to be 0.5 in the simulation of surface temperature. Weather conditions for the day are indicated in Fig. 16. Wind speeds were measured below 2 m/s throughout the day. As shown in Fig. 15, the measured surface temperature was slightly different at different measurement points and the maximum temperature difference was approximately 5 °C. The average values of the measured surface temperature for three measurement points ((S1–S3) provided in Fig. 14) were compared with the simulated results. The correlation between the simulated surface temperature and the average measured surface temperature is presented in Fig. 17. From an examination of this figure, it can be determined that the simulated surface temperature agreed within

Water supply

Measurement points S1 S2 S3

12 8 10 6 8 4 6 4 2 2 0 0 -2 -2 -4 -4 Difference between surface temperature and ambient air temperature

Fig. 12. The top photo shows the test PCWs. The middle and bottom are thermographs of the PCWs that were exposed to direct daytime solar radiation.

openings

M2 M3 M4 M5

8 0 0 1

M1

8 8 5

Water tank 660

100

100

Fig. 14. An experimental PCW and locations of measurement points.

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J. He, A. Hoyano/ Solar Energy 83 (2009) 1064–1075 30

Simulated sureface temp.

S3 (measured)

)

S2 (measured)

S1 (measured)

25

( e r u 20 t a r e p m e T 15

building with a meeting room is located at the North end of the location. A tree with a height of 7 m was growing at the center of the developed urban location. A roof covered rest station with walls composed of PCWs was constructed at the South end of the location. Illustrations showing the

Ambientair Average of (S1+S2+S3)

Table 2 Thermal properties of the void brick. Solar reflectance

Thermal conductivity (W/m K)

Specific heat (kJ/(kg K))

0.3

0.8

840

10 0

2

4

6

Fig. 15. Comparison temperature.

90

8

10 12 14 Time (hour)

between

30

simulated

16 1 8

and

measured

Total horizontal solar radiation

) ) % ( y 80 ( 25 t i Relative humidity e d r i u t m Air temperature a u h 70 r e20 e p v i m t Wind speed e a t l e 60 i r 15 R A

50

2

4 6

24

surface

4

800 )

2

10 0

20 2 2

m 600 / W ( n o i 400 t a i d a r r a 200 l o S

3 ) s / m ( d 2 e e p s d n 1 i W

0

0

8 10 12 14 16 18 20 22 2 4 Time(hour)

Fig. 16. Weather data used in the comparison study.

2 °C of the actual measured data. The thermal properties of the brick used in the simulation are provided in Table 2. 4. Case study

4.1. Description of the analysis object

An actual urban location where PCWs could be installed was selected for analysis. As shown in Fig. 18, a one-story 30 ) C º ( e r 25 a r e p m e t e 20 c a f r u s d e t a 15 l u m i S

10 Measured surface temperature (ºC)

Fig. 17. Correlation temperature.

between

simulated

and

measured

surface

Fig. 18. Photos of the actual developed location where PCWs were installed. The top photo is a view from Northwest. The middle photo is a birds-eye view from Southwest. The bottom shows an internal view of the rest station from the Southeast.

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Fig. 19. Plan (left) and section (right) of the developed location with incorporated PCWs.

Fig. 20. Birds-eye views of 3D CAD models for the developed location with incorporated PCWs.

40

100 Absolute humidity

e r 35 u t a r Relative humidity e p 30 m e Air temperature T 25

y t y i t i d i d ) 80 i m ) m u A u h D h e ( t g 60 e u k v l / i t o g a s l b e 40 R A

Global solar radiation incident upon a horizontal surface

/1000 W 800 n o 600 i t a i d 400 a r 200 r a 0 l o 0 S

2.5 Wind speed

2.0 1.5 1.0 0.5

2

4

6

0.0 8 10 12 14 16 18 20 22 24

) s / m d e e p s d n i W

Time hour) Fig. 21. Weather conditions for a sunny summer day (Aug. 5) in Tokyo.

design and layout of the rest station are provided in Fig. 19. 3D CAD models for the location are shown in Fig. 20. 4.2. Simulation results

The developed location was modeled in two scenarios: (1) Case 1 stipulates that the PCW was composed of dry bricks (without the evaporative cooling effect) and (2) Case 2 stipulates that the PCW was composed of wet bricks. Simulations were performed using hourly weather data for a typical sunny summer day (August 5) in Tokyo.

The input weather data is for a reference weather year, and was prepared based on data provided by the Transactions of the Society of Heating, Air-Conditioning and Sanitary Engineers of Japan. The reasons for choosing this day are as follows: (1) high air temperature, (2) low wind speed throughout the day, (3) high solar radiation intensity (clear sky) during the daytime, and (4) cloudy sky at night (cooling by nocturnal radiation is reduced). Urban heat islands form easily during the day. Diurnal variations of air temperature, relative humidity, wind speed and solar radiation are shown in Fig. 21. Fig. 22 shows the simulated surface temperature distribution for both cases at three different times (09:00, 12:00 and 15:00). At 09:00, the surface temperature of the dry brick wall that was not exposed to direct solar radiation was nearly equal to the ambient air temperature, whereas the surface temperature of the wet brick wall was several degrees lower than the ambient air temperature. At 12:00 and 15:00, the surface temperatures of the dry brick wall exposed to direct solar radiation rose above 40 °C and were higher than the ambient air temperature by 5–10 °C. The surface temperature of the wet brick wall, on the other hand, remained below the ambient air temperature. As can be seen at the lower right corner of Fig. 22, the ground surface in the rest station with a roof was also kept at a lower temperature by the surrounding PCWs. Fig. 23 shows diurnal variations of the brick surface temperature for points P1 and P2 indicated in Fig. 19. P1 and P2 were set at a height of 1 m above the ground. As seen in Fig. 23, in Case 2 (with wet brick walls) the surface


Fig. 22. Simulation results of surface temperature distribution for the dry (left) and wet (right) brick walls.

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temperature (P1) of the Southeast-facing brick remained lower than the ambient air temperature throughout the day. Furthermore, the former was 5 °C lower than the latter during the daytime. The surface temperature (P2) of the southwest-facing wet brick wall was about 1 °C lower than the ambient air temperature, except during the period of 13:00–17:00. During the period of 13:00–17:00, the surface temperature (P2) of the wet brick went higher than the ambient air temperature because the brick surface was exposed to direct solar radiation. In order to thoroughly comprehend the thermal improvement of the PCWs on thermal comfort in the developed location, the mean radiant temperature (MRT) at a height of 1 m above the ground was used as an evaluation index. The simulated results of the MRT distribution for Case 1 and Case 2 at 12:00 are shown in Fig. 24. The MRT for Case 2, (wet brick walls), was 2–3 °C lower than that for Case 1 (dry brick walls). Furthermore, from the lower thermal image seen in Fig. 24, it can also be stated that the MRT near the wet brick walls was the lowest and measured 2–4 °C less than the ambient air temperature. 5. Conclusions

In order to predict and evaluate the thermal improvement effect in a developed urban location where application of a passive cooling wall (PCW) is under design stage consideration, a numerical simulation method was developed and presented in this paper. A PCW was constructed of moist void bricks and was found to be capable of providing the following passive cooling effects: solar shading, evaporative cooling and ventilation cooling. Our measured results show that the daytime surface temperature of the PCW that was exposed to direct solar radiation was lower or slightly higher than the dry-bulb temperature, and that the surface temperature for the PCW that was not exposed to direct solar radiation was nearly equal to the wet-bulb temperature when the PCW was completely wet. Based on the above-mentioned measurement results, a thermal model for calculating the surface temperature of a PCW was proposed. The calculation algorithm based on the proposed thermal model was integrated into a 3D CAD-based simulation tool previously developed by the authors’ group. An examination of a comparison between the simulated and measured results for a test PCW determined that the simulated surface temperature of the PCW agreed with the actual measured data within a range of 2 °C. Furthermore, a case study was conducted to predict and evaluate the thermal improvement of PCWs on the environment of an actual developed location. Simulations were performed by modeling the developed location in both wet and dry scenarios. The surface temperature reductions caused by the PCWs and the places where a cooler radiant environment was formed in the examined installation loca-

tion can be visually understood by comparing the simulation results of the two cases. In conclusion, in terms of surface temperature and mean radiant temperature (MRT) distribution, the authors found that the developed simulation tool can be used during design stages to evaluate the thermal improvements that might result from installation of PCWs in developed urban environments. 6. Final remarks

The main goal of the study presented in this paper is to describe a simulation tool for quantifying the improvement effect of the PCW on the thermal radiation environment from surface temperature reductions. Except the radiation cooling effect, the PCW can also provide air temperature reduction effect and ventilation cooling effect as well. In 50 P1 (dry brick surface)

P2 (dry brick surface)

45 ) C º ( 40 e r u t a r 35 e p m30 e T

P2 (wet brick surface) Ambientair

25 P1 (wet brick surface)

20 0

2

4

6

8

10

12

14

16

18

20

22

24

Time (hour)

Fig. 23. Diurnal variations of dry and wet brick surface temperature.

Fig. 24. Comparison of MRT at 1 m above ground for the cases with dry (top) and wet (bottom) brick walls.


order to evaluate these effects, a coupled simulation method is being developed (Kakuya et al., 2007). This coupled simulation method makes it possible to predict solar and thermal radiation, airflow and humidity distribution. However, the calculation load is too great (the computing time is too long) to carry out the coupled simulation on a PC at the present time. It still requires a lot of research and development efforts to integrate the coupled simulation method into a design tool that can be used by a general designer. In such situation, the simulation tool proposed in this paper is very useful for the designers who require a quick analysis of the improvement effect by the difference of surrounding materials and spatial forms on the thermal radiation environment during the design phase. Acknowledgement

This work was in part supported by the New Energy and Industrial Technology Development Organization of Japan (NEDO) under Contract No. 0827001. References Shirai, K., Hoyano, A., Horiguchi, T. 1995. Development of waterpermeable ventilable bricks as the basis for a new evaporative cooling technique, the International Solar Energy Society 1995 Solar World Congress, Harare, Zimbabwe. Hoyano, A., Shirai, K., 1995. Design and development of an evaporativecooling prototype wall made of water-permeable ventilable bricks, the International Solar Energy Society 1995 Solar World Congress, Harare, Zimbabwe.

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