Numerical Simulation of Passive Cooling Strategies for ... · PDF filemultiple passive cooling techniques that prevent external heat on the outer building envelope and night ventilation,

D.H.C. Toe is senior lecturer in the Faculty of Built Environment, Universiti Teknologi Malaysia, Skudai, Malaysia. S. Sugiyama and S.

Yasufuku are graduate students in the Graduate School for International Development and Cooperation, Hiroshima University,

Hiroshima, Japan. T. Kubota is associate professor in the Graduate School for International Development and Cooperation, Hiroshima University, Hiroshima, Japan.

Numerical Simulation of Passive Cooling

Strategies for Urban Terraced Houses in

Hot-Humid Climate of Malaysia

Doris Hooi Chyee Toe, Dr.Eng. Susumu Sugiyama Satoshi Yasufuku

Universiti Teknologi Malaysia Hiroshima University Hiroshima University

Tetsu Kubota, Dr.Eng.

Hiroshima University

ABSTRACT

The objective of this study was to determine energy-saving modifications through passive cooling to

urban terraced houses in Malaysia. Effects of two strategies, i.e. complete natural ventilation (NV)

strategy and partial air conditioning (AC) strategy, were simulated using TRNSYS and COMIS. The

complete NV strategy relied fully on naturally ventilated condition in the whole house for achieving

thermal comfort in the master bedroom while the partial AC strategy was aimed at reducing the cooling

load in the air-conditioned master bedroom by applying passive cooling techniques to the whole house.

The results revealed that indoor thermal comfort was achieved in complete NV strategy by applying

multiple passive cooling techniques that prevent external heat on the outer building envelope and night

ventilation, even under heated urban climatic conditions. In partial AC strategy reductions of about 39%

to 56% in the sensible cooling load compared to the current scenario were obtained by using several

techniques including night ventilating other spaces and insulating inner surfaces of the master bedroom.

INTRODUCTION

Energy savings are important in the global building sector due to concerns about energy security

and effects of global warming. In hot developing regions such as Southeast Asia, cooling demand in

residential buildings is a major concern since it is predicted to rise sharply in the coming decades in line

with rapid urbanization and population and economic growth (Liu et al., 2010; Sivak, 2009). It can be

seen widely in the region that brick-walled buildings are becoming a common construction for urban

houses in recent years. In Malaysia, a nationwide census in 2010 showed that 85% of the existing urban

houses used brick and another 5% used brick and plank for their outer walls (Department of Statistics

Malaysia, 2012). Unlike traditional lightweight constructions, the high thermal mass building envelope

of brick houses might be difficult to be cooled in the hot-humid climate. It has been reported in 2009 that

space cooling in brick houses accounted for 29% of the annual household energy consumption on

average in the city of Johor Bahru, Malaysia (Kubota et al., 2011). It is thus crucial to apply passive

cooling strategies wherever possible to these urban houses for energy-saving.

Passive cooling encompasses techniques for solar and heat control, heat modulation and heat

30th INTERNATIONAL PLEA CONFERENCE16-18 December 2014, CEPT University, Ahmedabad

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dissipation using naturally driven phenomena such as natural ventilation, radiative cooling, evaporative

cooling and ground cooling (Santamouris and Kolokotsa, 2013). Passive cooling techniques have been

studied in various climatic regions. However, few comprehensive studies were made outside moderate

and hot-dry climates, including field monitoring and numerical modeling exercises with regard to

existing Malaysian houses (Kubota et al., 2009; Mohd Isa et al., 2010; Sadafi et al., 2011). Some of the

main climatic factors negatively affecting the efficiency of the different cooling approaches are high

night ambient temperature, cloud cover, high humidity and insufficient wind speeds (Dimoudi, 1996).

These conditions are usually prevalent in hot-humid climate. Due to dependency on climatic conditions,

further local studies are required to predict effects of a passive cooling system before implementation.

The objective of this study is to determine energy-saving modifications through passive cooling to

Malaysian urban houses. The target houses are terraced houses, which formed majority (42% as of 2010)

of the existing urban housing stock (Department of Statistics Malaysia, 2012). This study analyses the

effects of two passive cooling strategies, i.e. complete natural ventilation (NV) strategy and partial air

conditioning (AC) strategy, on thermal comfort and cooling load, respectively, through numerical

simulation using TRNSYS and COMIS programs.

METHODS

Description and Modeling of the Case Study Terraced House

One of the case study terraced houses from a previous field experiment (Kubota et al., 2009) was

modeled in this simulation study. The selected terraced house represents typical modern terraced houses

in terms of spatial design and building structures (Toe, 2013). The house measures 6.7 m by 13.1 m with

a total floor area of 155 m2, which is an average sized double-storey terraced house (Figure 1). Floor-to-

ceiling height of rooms is 3.05 m. The total nett air volume of the whole house is 538 m3; that of the

master bedroom is 65 m3. The building was oriented towards northwest, which means that the external

façade of the master bedroom faces northwest. It was constructed of brick and concrete and had single

glazing windows. The entire house was not insulated. Description of the constructional layers of the

terraced house and their reference U-values in the computer model is given in Table 1.

The whole house was modeled as TRNSYS Type 56 ‘Multi-zone Building’ in three dimensions

using the TRNSYS 3D plug-in in Google SketchUp interface (Klein et al., 2012). The building model

comprised 17 thermal zones with corresponding air flow zones in COMIS to represent each partitioned

room or functional space including attic spaces. All protruding elements on the building facades and

immediate surrounding objects, i.e. neighbouring houses, that might shade the studied house were also

modeled in three dimensions. A time base of 1 h was set for the transfer function to represent the thermal

Figure 1 (a) Exterior view and (b) floor plans of the case study terraced house.

(a) (b)

Ground floor

First floor

30th INTERNATIONAL PLEA CONFERENCE 16-18 December 2014, CEPT University, Ahmedabad

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mass behavior of the brick walls. The party walls on both sides of the house were modeled as boundary

walls with identical zone temperatures assumed on both sides of the walls. Meanwhile, the boundary

condition for the ground floor was the constant soil temperature assumed to be the average air

temperature at the site over the whole simulation period. Thermal properties of building materials and

parameter/input values for air flows were obtained from Malaysian manufacturers or reference data to

correspond with the local construction (Toe, 2013). Wind pressure coefficients were estimated using a

parametrical model developed by Grosso (1992) known as CPCALC+. Coupling between the TRNSYS

and COMIS models were implemented via Type 157 in TRNSYS so that air flow rates per zone and

zone air temperatures were iterated in each time step until the mass and energy balance per zone reached

convergence.

Model Validation

Empirical validation of the terraced house model was performed using the above-mentioned field

experiment data from June-August 2007 (Kubota et al., 2009). This study focuses on the results in the

master bedroom because existing households used air conditioners mainly in master bedrooms (Kubota

et al., 2009); the study interest is to reduce this cooling energy. Figure 2 shows temporal variations of the

simulation results compared to the measurement data at 1.5 m height above floor in the master bedroom.

Two ventilation conditions, i.e. night ventilation and daytime ventilation, are shown. Overall, the

Table 1. Constructional Layers and Reference U-values of the Terraced House Model.

Building Element Constructional Layers Reference

U-valuea

(W/m2K)

External and internal

walls

20 mm thick cement plaster + 100 mm thick clay brick + 20 mm

thick cement plaster

2.75

Party wall 20 mm thick cement plaster + 200 mm thick clay brick + 20 mm

thick cement plaster

2.07

Ground floor 8 mm thick ceramic tile + 22 mm thick cement screed + 100 mm

thick concrete slab + soil layer

3.75b

First floor (family and

bedroom zones)

15 mm thick timber flooring + 15 mm thick cement screed +

100 mm thick concrete slab + 20 mm thick cement plaster

2.81

First floor (bath

zones)

8 mm thick ceramic tile + 22 mm thick cement screed + 100 mm

thick concrete slab + 20 mm thick cement plaster

3.29

Ceiling (master

bedroom)

6 mm thick ceiling board 4.55

Ceiling (other zones) 3.2 mm thick ceiling board 5.54

Pitched roof 20 mm thick concrete roof tile + 25 mm thick timber batten +

aluminium foil

2.67

Flat roof 22 mm thick cement screed + 100 mm thick concrete slab + 20

mm thick cement plaster

3.37

Window 6 mm thick single layer float glass 5.61 a Includes convective and radiative heat transfer coefficients of 7.7 W/m

2K for inside surface and 25

W/m2K for outside surface.

b Excludes soil layer.

Figure 2 Temporal variations of the simulation and measurement data in the master bedroom in (a)

night ventilation and (b) daytime ventilation conditions.

222426283032343638

Air

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mp

. (°C

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24262830323436

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. (°C

)

13/7 14/7 15/7 16/7 17/7 18/7

Date / Time

Measurement

SimulationOutdoor

Simulation

Measurement

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mp

. (°C

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Date / Time

Measurement

SimulationOutdoor

Simulation

Measurement

(a) (b)


3

validation results are satisfactory in terms of air and operative temperatures with root mean square errors

(RMSE) of 0.31-0.55 °C and coefficients of determination (R2) of 0.89-0.96.

Simulation Test Cases and Weather Conditions

Table 2 summarizes the simulation test cases of this study. The techniques were selected by

considering their practicality to be applied to existing terraced houses through relatively simple building

modification and/or behavioural adjustment. In particular, night ventilation is considered a potential

passive cooling technique for brick houses while daytime ventilation emulates the window opening

behavior of the majority of existing households (Kubota et al., 2009). The complete NV strategy relies

fully on naturally ventilated condition in the whole house for achieving thermal comfort in the master

bedroom. Meanwhile, the partial AC strategy attempts to reduce the cooling load in the master bedroom

by applying passive cooling techniques to the whole house. It was assumed that air conditioning was

used only in the master bedroom for nine hours per day (21:00-6:00) with a set temperature of 23 °C.

This study deals with the sensible cooling load only. Internal heat gains from occupants (4 persons;

seated at rest), lighting (5 W/m2) and common household appliances were considered in all simulations.

It is noted that infiltration rates in the master bedroom average 0.1 ACH when no ventilation was applied

for both complete NV and partial AC strategies.

Weather conditions for the simulation were taken from an actual weather data set measured at the

centre of a heat island in Johor Bahru, Malaysia to represent urban climate of typical terraced housing

neighbourhoods (Kubota and Ossen, 2011). The geographical location is 1°29’19” N and 103°45’41” E

at an elevation of 26 m above sea level. A wind velocity profile exponent of 0.25 was used to represent

the urban location (Counihan, 1975). The simulation time step was set to coincide with the weather data

at 10-minute intervals. The simulation was run using the above weather file for two whole months, i.e.

January-February 2010. Subsequently, simulation results for a 10-day period of continuous typical fair

weather days are analysed in this study. As shown in Figure 3, outdoor air temperature ranges from 25-

36 °C while outdoor relative humidity ranges from 50-90% over the period. The analysis period begins

several days after the simulation start time, thus allowing the model to acquire sufficient thermal history.

Output files were generated and post-processed in Excel spreadsheets after the simulation.

Table 2. Simulation Test Cases.

Technique Test Conditions

Natural ventilation (open

window period)

Night ventilation (20:00-8:00); daytime ventilation (8:00-20:00); no

ventilation (0 h); full-day ventilation (24 h)

Forced ventilation 10 ACH or 30 ACH night (20:00-8:00) in master bedroom

Attic ventilation 10 ACH night (20:00-8:00) or 30 ACH full-day (24 h) in attic

Thermal insulation Roof; ceiling; external wall – outside surface; external wall – inside

surface; internal wall; party wall; floor (R-value: 4 m2K/W)

Window shading External shading; internal shading (Shading factor, SF: 0.75)

High reflectivity roof coating Solar reflectance: 0.8, longwave emissivity: 0.9

Window glazing Low-E glass (U-value: 2.54 W/m2K, G-value: 0.44) or heat barrier

film (U-value: 5.73 W/m2K, G-value: 0.48)

Figure 3 Temporal variations of weather data for the simulation analysis period.

0

400

800

1200

528 552 576 600 624 648 672 696 720 744 768

24

26

28

30

32

34

36

Air

Te

mp

. (°

C)

So

lar

Ra

d.

(W/m

²)

Rain period

Date / Time (Hour of the Year)

23/1 24/1 25/1 26/1 27/1 28/1 29/1 30/1 31/1 1/2

50

60

70

80

90

100

RH

(%

)


4

RESULTS AND DISCUSSION

Complete Natural Ventilation Strategy

Figure 4 presents the simulated indoor air temperatures in the master bedroom for the four natural

ventilation conditions that represent night ventilation, daytime ventilation, no ventilation and full-day

ventilation. It is noted that temporal variations of indoor temperatures in each simulation have similar

patterns over the 10-day analysis period. Thus, results are shown in statistical summaries for the whole

period. As expected, night ventilation provides the lowest indoor air temperatures among the tested open

window conditions (Figure 4). This is due to the nocturnal ventilative cooling through open windows

and thermal mass effect of the cooled building structures that lowers the night-time and peak indoor

temperatures of the following day. Daily maximum (95th percentile) and minimum (5

th percentile) indoor

air temperatures in night ventilated condition are 1.7 °C and 1.3 °C lower than those of daytime

ventilation, respectively. Nevertheless, the daily minimum air temperature in the night ventiled room is

still 2.7 °C higher than the outdoors.

Further passive cooling techniques are applied consecutively as shown in Figure 5 in addition to

night ventilation and daytime ventilation, respectively. The most effective technique in reducing the

daily maximum air temperature in night ventilated condition is roof insulation; the said temperature is

decreased by 0.9 °C compared to applying night ventilation only (Figure 5a). Most of the solar heat gain

in the master bedroom, which is on the first floor, probably comes through the roof due to its relatively

large surface area and high noon solar altitude at the location. With less heat gain during the day and a

cooler adjacent attic space for the whole day, the building structures maintain cooler and serve to reduce

the minimum air temperature as well. Techniques that reduce solar radiation through roof into the

building would be important. In fact, high reflectivity roof coating reduces the mean indoor air

temperature most among all of the techniques in Figure 5a, i.e. by 0.6 °C. The high reflectivity coating

probably improves nocturnal cooling additionally by virtue of the less heated roof surface on exposure to

the sun and absence of thermal insulation at night. Nevertheless, all of the solar control techniques are

Figure 4 Statistical summary (5

th and 95

th percentiles, mean and ± one standard deviation) of simulated

indoor air temperatures in different natural ventilation conditions for complete NV strategy.

Figure 5 Statistical summary (5

th and 95

th percentiles, mean and ± one standard deviation) of simulated

indoor air temperatures in (a) night ventilated and (b) daytime ventilated conditions with respective passive cooling techniques for complete NV strategy.

25 26 27 28 29 30 31 32 33 34 35

Outdoor

Night ventilation

Daytime ventilation

No ventilation

Full-day ventilation

Air Temperature ( C)

27 28 29 30 31 32

Night ventilation

Roof insulation

High reflectivity roof coating

Ceiling insulation

External shading

Low-E glass

Internal shading

External wall-outside insulation

External wall-inside insulation

Forced ventilation (10 ACH night)

Attic ventilation (10 ACH night)


27 28 29 30 31 32 33 34

Daytime ventilation

Roof insulation


Ceiling insulation

External shading

Low-E glass

Internal shading



Forced ventilation (10 ACH night)

Attic ventilation (10 ACH night)


(a) (b)


5

less effective in daytime ventilated condition compared to night ventilated condition (Figure 5b). The

inflow of hot outdoor air through open windows during daytime increases the indoor air temperature and

diminishes their cooling effects. On the other hand, forced ventilation with an air change rate of 10 ACH

in the room at night lowers the daily minimum air temperatures most, i.e. by 0.6 °C and 1.4 °C in night

ventilated and daytime ventilated conditions, respectively.

Figure 6a shows the simulated indoor operative temperatures for combinations of the most effective

technique for each of the building elements. The techniques are applied accumulatively and step-by-step

from more effective ones to less effective ones in night ventilated condition. The results are evaluated

for thermal comfort using an adaptive comfort equation (ACE) for naturally ventilated buildings in hot-

humid climates (Toe and Kubota, 2013). The 80% comfortable upper limits predicted using daily mean

outdoor air temperatures of the analysis period average 29.6 °C. Figure 6a indicates that the daily

maximum indoor operative temperature is reduced by 2.2 °C and meets the 80% comfortable upper limit

when roof and external wall-outside surface insulation (R-value 4 m2K/W), and external window

shading (shading factor 0.75) are applied in addition to night ventilation under the heated urban climatic

conditions. Alternatively, the comfort limit is also met by substituting the roof insulation with high

reflectivity roof coating, though daily maximum temperature is higher in the latter. It is implied that

introducing these four techniques to existing urban terraced houses may satisfy indoor thermal comfort

in naturally ventilated condition on fair weather days (Figure 6b).

Partial Air Conditioning Strategy

Figure 7 shows the simulated sensible cooling loads in the air-conditioned master bedroom by

considering different natural ventilation conditions for the master bedroom and other zones. The cooling

load is 50.2 MJ/day when daytime ventilation is applied to the whole house (Case 1), which represents

the current behaviour of most households. By applying night ventilation to the whole house except the

master bedroom, the cooling load is reduced by about 5% even when the master bedroom is daytime

ventilated (Case 5). Building structures that are cooled at night keep adjacent indoor temperature low

and reduce the cooling load indirectly. The highest reduction in cooling load, i.e. 8%, is seen when the

master bedroom receives no natural ventilation and other zones are night ventilated (Case 6).

Further passive cooling techniques are applied consecutively as shown in Figure 8 in addition to the

ventilation conditions of Cases 1 and 6, respectively. For Case 1 the most effective technique in lowering

Figure 6 (a) Thermal comfort evaluation and (b) conceptual illustration of combined passive cooling techniques for complete NV strategy.

Figure 7 Simulated sensible cooling loads in different natural ventilation conditions for partial AC

strategy.

27 28 29 30 31 32

Night ventilation

+ Roof insulation

+ External shading

+ External wall-outside insulation

+ Low-E glass

+ High reflectivity roof coating

+ Forced ventilation (10 ACH)

+ Attic ventilation (10 ACH)

+ Internal shading

Operative Temperature ( C)

ACE 80% comfortable upper limit

0 10 20 30 40 50 60

1

2

3

4

5

6

Cooling Load (MJ/day)

Master bedroom Other zones

Daytime ventilation Daytime ventilation

Daytime ventilation No ventilation

No ventilation No ventilation

No ventilation Daytime ventilation

Daytime ventilation Night ventilation

No ventilation Night ventilation

Case

External shading

Roof insulation

External wall-outside

insulation

Master bedroom

Night ventilation

Night ventilation

(a) (b)


6

the cooling load is floor insulation; the reduction is about 8% compared to the current condition (Figure

8a). Applying high reflectivity roof coating and roof or ceiling insulation give reductions of 6% and 5%

each, respectively. For Case 6 ceiling insulation decreases the cooling load most by about 7%, followed

by roof insulation and high reflectivity roof coating (Figure 8b). Besides, wall insulation is more

effective on internal wall, followed by external wall-inside surface. Overall, all of the passive cooling

techniques except floor insulation, party wall insulation and attic ventilation give greater reductions in

the cooling load in Case 6 compared to Case 1, likely due to exclusion of hot outdoor air in closed

window conditions during daytime.

Figure 9a presents the simulated sensible cooling loads for combinations of the most effective

techniques in the ventilation condition of Case 6. As before, the techniques are applied accumulatively in

step-by-step basis. Compared to the current condition (Case 1), the cooling load of the master bedroom

is reduced by about 39% to 30.9 MJ/day when the ceiling, internal wall, external wall-inside surface and

floor are insulated (R-value 4 m2K/W) for Case 6 (Figure 9). The cooling load is lowered by 56% to 21.9

MJ/day when all of the techniques are used simultaneously, although the further reductions by high

reflectivity roof coating, party wall insulation and internal shading are only about 3% or less each.

It is implied from the above simulation results that changing from daytime ventilation to night

ventilation is fundamental to gain better effectiveness of other passive cooling techniques for both

complete NV and partial AC strategies. Due to the intense solar heat gain through the roof, roof

insulation for complete NV strategy and ceiling insulation for partial AC strategy provide the greatest

cooling effects. In particular, for complete NV strategy techniques that prevent external heat on the outer

building envelope are relatively effective to keep the indoors cool (Figure 6b). On the other hand, for

partial AC strategy insulating the inner surfaces is relatively effective to reduce the cooling load (Figure

9b). Since the master bedroom is air-conditioned in this strategy, these techniques aid to prevent the

mechanically cooled indoor air from being transferred outward.

Figure 8 Simulated sensible cooling loads in ventilation conditions of (a) Case 1 and (b) Case 6 with

respective passive cooling techniques for partial AC strategy.

Figure 9 (a) Simulated sensible cooling loads and (b) conceptual illustration of combined passive

cooling techniques for partial AC strategy.

0 10 20 30 40 50 60

Daytime ventilation (all zones)


Roof insulation

Ceiling insulation

Attic ventilation (30 ACH full-day)

Floor insulation

Internal wall insulation


Party wall insulation


External shading

Heat barrier film

Internal shading


0 10 20 30 40 50 60

+ Night ventilation (other zones)

Ceiling insulation

Roof insulation


Attic ventilation (30 ACH full-day)




Floor insulation

Party wall insulation

External shading

Low-E glass

Internal shading


No ventilation (master bedroom)(a) (b)

0 10 20 30 40 50 60

Daytime ventilation (all zones)

+ Ceiling insulation

+ Internal wall insulation

+ External wall-inside insulation

+ Floor insulation

+ Low-E glass

+ External shading

+ High reflectivity roof coating

+ Party wall insulation

+ Internal shading


No ventilation (master bedroom) + Night ventilation (other zones)

Case 1

Case 6


Ceiling insulation


Floor insulation

Night ventilation

AC

Master bedroom

(a) (b) Case 6 No ventilation (master bedroom)

+ Night ventilation (other zones)


7

CONCLUSIONS

The simulation results of a typical Malaysian terraced house reveal that indoor thermal comfort

may be achieved in naturally ventilated condition by applying multiple passive cooling techniques that

prevent external heat on the outer building envelope and night ventilation, even under heated urban

climatic conditions. When air conditioning is used in the master bedroom, reductions of about 39% to

56% in the sensible cooling load compared to the current scenario can be reached by using several

techniques including night ventilating other spaces and insulating inner surfaces of the master bedroom.

Further consideration of different building orientations, annual performance, cost-and-benefit

effectiveness, and effects on indoor humidity as well as latent cooling load would be useful to realize

their practical implementation in the urban terraced houses. Such modifications are expected to

contribute largely to energy savings and carbon emission mitigation.

ACKNOWLEDGEMENTS

We gratefully acknowledge financial support from the Nichias Corporation, Ministry of Education

Malaysia and Universiti Teknologi Malaysia for Research University Grant Program 2014 (Vot

Q.J130000.2721.01K22), and The Hitachi Scholarship Foundation.

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Numerical Simulation of Passive Cooling Strategies for ... · PDF filemultiple passive cooling techniques that prevent external heat on the outer building envelope and night ventilation,