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«Tantasavasdi, C., Srebric, J., and Chen, Q. 2001. Natural ventilation design for houses in Thailand, Energy and Buildings, 33(8), 815-824. Natural ...»

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Tantasavasdi, C., Srebric, J., and Chen, Q. 2001. "Natural ventilation design for houses in

Thailand," Energy and Buildings, 33(8), 815-824.

Natural Ventilation Design for Houses in Thailand

Chalermwat Tantasavasdia, Jelena Srebricb, Qingyan Chenc*

a

Architecture Program, Thammasat University, Room 713 Faculty of Engineering

Building, Rangsit Campus, Pathumthani 12121, Thailand

b

Department of Architectural Engineering, The Pennsylvania State University, 222

Engineering Unit A, University Park, PA 16802, USA c Building Technology Program, Department of Architecture, Massachusetts Institute of Technology, Room 5-418, 77 Massachusetts Avenue, Cambridge, MA 02139, USA * Phone: (617) 253-7714, Fax: (617) 253-6152, Email: qchen@mit.edu Abstract This paper explores the potential of using natural ventilation as a passive cooling system for new house designs in Thailand. The characteristics of past and present Thai houses are analyzed in terms of climate, culture, and technology. Based on the thermal comfort requirements for the Thai people and the climate conditions in Bangkok, the study found that it is possible to use natural ventilation to create a thermally comfortable indoor environment in houses in a Bangkok suburb during 20% of the year. This study also develops comprehensive design guidelines for natural ventilation at both the site planning and individual house levels by using computational fluid dynamics.

Keywords: Natural ventilation; Computational Fluid Dynamics; Air velocity; Thermal comfort; House; Thailand

1. Introduction Natural ventilation has served as an effective passive cooling design strategy to reduce energy used by air-conditioning systems. For tropical regions, where the air temperature and relative humidity are generally high, the effectiveness of natural ventilation is always questionable. The vernacular architecture of Thailand, such as the traditional Thai house in Fig. 1a, was designed to take advantage of the prevailing winds.

The traditional Thai house is normally built with three notable characteristics: an elevated floor, a steeply pitched roof with long overhangs, and a large open terrace. The elevated floor prevents sudden flooding, protects occupants from dangerous animals, and allows more wind to flow through the living space. The long overhangs provide shade protect the house from rain. There is a thermal stratification in the house; the air temperature in the lower part of the house is lower than that in the upper part. The occupant usually stays in the lower, more comfortable part of the space. The large open terrace, which occupies approximately 40% of the total floor area, serves as a space for outdoor activities, and was an important space for the typical extended families of the Thai people of the past.

Lives in the past two decades have changed dramatically. People tend to be more accustomed to air-conditioned environments. Nearly all cars, urban offices, schools, and houses are air-conditioned if the owner can afford one. About 90% of urban indoor environments are air-conditioned. However, people are concerned with the rising costs of electricity and fuel, especially when they have to pay utility bills for their own residences.

In suburban houses, air-conditioning is used during the hot hours of the days. During the 1 cooler hours, most people are still willing to open their windows and let fresh air in.

Therefore, almost all windows in suburban houses can be opened.

Passive cooling design elements are mostly ignored in modern house designs (Fig.

1b). Dense areas in large cities such as Bangkok create a hot microclimate and discourage the use of natural ventilation because the buildings block each other from the wind.

Current designs used many ideas and building materials from Europe and America without careful consideration for the Thai climate. For example, most new houses have no overhangs, resulting in easy penetration of the sun into the living area. The building envelopes are made of materials with a high thermal mass, such as concrete and brick.

These materials store a large amount of heat during the day, and transfer it into the living space at night. As a result, houses are too hot without the assistance of an air-conditioner.

The problems with current designs have prompted designers to rethink their designs, especially because of an increasing awareness of sustainability. To conserve energy and reduce CO2 emissions, it is important to design energy-efficient buildings.

This paper explores the potential to reduce energy consumed by air-conditioners through the use of natural ventilation in building design.

2. Thermal Comfort Analysis In addition to the difference between traditional Thai and modern western houses, the comfort expectations of westerners and the Thai may be different. It is important to understand the Thai’s thermal comfort expectations in order to design a proper ventilation system. The ASHRAE Standard 55a-1995 on “Thermal Environmental Conditions for Human Occupancy” gives the upper limit of comfortable temperature as 26°C, and the wet bulb temperature upper limit as 20°C [1], as shown in Fig. 2. However, many studies have concluded that the comfort temperature is higher in tropical regions, since humans have the ability to acclimatize and/or have physiological variation [2]. Field study results show the discrepancies between thermal comfort requirements of the Thai people and westerners. Busch’s study [3] shows that, based on 80% of Thai workers being satisfied, the upper limit of the comfortable temperature can be as high as 28°C for people in airconditioned buildings, and 31°C in naturally ventilated buildings.





The upper limit of comfortable temperature, which was developed for the 40°N latitude, can be 1.0 K higher for every 12 degrees change in latitude to the south [4].

Since Bangkok is located at 14°N latitude, the corresponding comfort temperature upper limit is 28.2°C. This temperature is close to that found in Busch’s study.

Another experiment by Jitkhajornwanich et al. [5] shows that the upper limit for thermal comfort is as high as 31.5°C. This result corresponds to Busch’s results for the naturally ventilated case.

Kwok [6] conducted a survey of 3,544 students and teachers in 29 naturally ventilated and air-conditioned classrooms in Hawaii. He found that naturally ventilated classroom occupants accept a wide operative temperature range (22.0 – 29.5oC).

Based upon the fact that most people now live in air-conditioned spaces, the thermal comfort limit used for the Thai should be that found for the air-conditioned group in Busch’s study, 28°C.

Besides air temperature, strong air movement can increase the rate of convective and evaporative heat loss from the human skin to the environment. Thus, one feels cooler with a higher air velocity. Lechner [7] gives the equivalent temperature reduction (ETR) 2 according to air speed. ETR represents the temperature increase necessary to maintain the same thermal sensation with a given air velocity. For example, the ETR is 1.1, 1.9, and

3.3 K at air velocities of 0.2, 0.4, and 1.0 m/s, respectively. The upper limit of air movement for an indoor space is 1 m/s, at which the wind starts to pick up light objects, such as loose paper. The comfort zone can be shifted along the relative humidity line [8].

Therefore, the upper limit at wind speeds of 0.2, 0.4 and 1 m/s, is 29.1, 29.9 and 31.3°C, or 23, 23.5 and 24.5°C wet-bulb temperature, respectively. These new comfort zones are shown in Fig. 2 for V = 0.2, 0.4, and 1.0 m/s. A recent study in Thailand [9] has found similar trends.

Because the Thai have a tolerance for higher air temperatures, there is some potential for using natural ventilation in modern Thai houses through careful design. The following section analyzes if and when natural ventilation can be used in Thailand.

3. Climate Analysis House designs using natural ventilation require appropriate climates. It is well known that natural ventilation can work in a moderate climate, such as those in western and northern Europe, California, and others. However, Thailand is located between the 6 and 20°N latitudes, and has a typical tropical climate. In general, Thailand is hot and humid, with small seasonal changes throughout the year. There are three recognizable, although not completely distinct, seasons. The hot summer months from March to June are characterized by a high sun angle, high temperature, and moderate south wind. The rainy season from July to October has a lower temperature but a higher humidity than the summer months. The remaining months, November to February, are the winter months, where the sun angle is the lowest and the temperature is moderate. The air temperature for the year ranges from 21 to 35°C, while the relative humidity varies from 45 to 95%.

This investigation used metropolitan Bangkok conditions as a reference case.

Analysis of ten-year weather data shows that natural ventilation is most suitable for the winter months. Fig. 3 shows a bio-climatic chart of winter temperatures and humidities along with the comfort zones for different indoor air velocities. Each spot on the chart represents the hourly averaged air conditions in Bangkok on a ten-day basis. The averages were taken at the same time over a ten-day period from the ten-year weather database. The air in winter mornings and evenings is already within the comfort zone without the wind effect included For the rest of the time, appropriate air movement is needed to achieve a comfortable condition. The temperature in some afternoon hours in January and February exceeds 31.3°C, which is too high for comfort, even with a 1 m/s indoor air velocity. The

climate can be classified into five groups, as shown in Fig. 3:

1. Hot air; natural ventilation does not work

2. Warm air; high natural ventilation is needed

3. Comfortable air; moderate ventilation is appropriate

4. Humid air; natural ventilation is not appropriate

5. Cool, humid air; minimal ventilation will help The chart suggests that if the indoor air velocity is 0.4 m/s, natural ventilation can provide comfort for 1,825 hours per year (approximately 20% of the time). Although this is not a large number, it is worth to pursuing, because natural ventilation has a very low cost.

3 Note that the indoor air temperature is normally higher than the outdoor air temperature, due to internal heat gains and heat transfer through the building enclosure in such a climate. If all the walls are shaded, and the walls and roof insulated with a 0.075 m thickness of microfiber [10] (the total U-value for the roof and walls is 0.44 W/m2 °C), the design cooling load is 4.5 kW for a typical Thai family home with a floor area of 160 m2. The latent load is about 10%. If the wind inlet apertures in the building are 10% of the floor area, an average wind velocity of 1.0 m/s at the apertures will lead to an indoor air temperature of only 0.2 K higher than the outdoor air temperature. If the wind velocity at the apertures is reduced to 0.4 m/s, the temperature difference only increases to 0.5 K.

However, if the air velocity is further decreased to 0.1 m/s, the indoor air temperature will be 2.0 K higher than the outdoor air temperature, which will eliminate most of benefits of natural ventilation in such a climate.

The above analysis suggests that it is important to know the local wind patterns.

The prevailing wind comes from two predominant directions in the winter months. In November and December, the wind is mostly from the north-northeast (NNE) and the north (N). The wind starts to change direction in January to the south-southwest (SSW) and the south (S). The wind direction changes completely into SSW and S in February.

The wind speed in the winter months varies from 0.4 to 3.2 m/s. The five groups of climate conditions shown in Fig. 3 are plotted on a timetable in Fig. 4. Each climate condition is plotted with the corresponding prevailing wind speed. Since group 2 needs maximum ventilation, it is important to have sufficient wind velocities for this group. Fig.

4 indicates that the lowest wind speed for group 2 occurs in the evenings of January and February. The corresponding average wind speed is 1.4 m/s from the SSW direction. If a house can provide enough wind under this condition, it can be comfortable for all other conditions except group 1.

4. Site Planning Designing houses with good natural ventilation in urban areas is challenging, because the surroundings have a significant impact on the wind pattern and indoor air velocity. In the downtown area, air pollution and disturbing noises discourage the use of natural ventilation in residential buildings. Therefore, our investigation focuses on suburban residential areas. These suburban houses have a larger potential for taking advantage of the prevailing winds, because of the cleaner and less dense neighborhood environments. In fact, houses in the suburbs are perceived as the norm in modern Thailand.

Most of mater plans at today’s typical housing projects do not encourage the use of natural ventilation. Houses with a typical land lot of 240 to 250 m2 are mostly aligned with the street grid. This makes the houses upstream block wind from those downstream.

Natural ventilation is not suitable for housing projects with this kind of alignment.

In order to reduce the wind blockage by the houses, they should be staggered to encourage natural ventilation [11]. Linear alignments create a wind shadow, which is a low-pressure area at the back of each house. The best arrangement is to stagger the houses according to the prevailing wind direction. It reduces the wind shadow area at the back of each house, giving the maximum airflow to the downstream houses.

However, solar heat gain plays an equally important role in deciding the house orientation. In order to minimize the solar gain in Thailand, the houses should be oriented 4 with long sides facing north and south. If the prevailing wind direction is not north-south, this could creat a possible conflict between these design factors.



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