Natural Cooling of Malaysian Urban Homes by Free Wind

Introduction

Cooling Malaysian urban homes by natural means has always been a great challenge to many architects and mechanical engineers. Ceiling and/or portable fans and air conditions are some of the ways to displace hot internal air with fresh external air thereby cooling off the internal building fabric and also acting as a catalyst in evaporating moisture from the human skin. Both ways entail running costs. Between the two, the fans have always been cheaper than any other means. It is highly unlikely that we can be thermally comfortable indoors without the mechanical aids.

The tropical climate is not only hot throughout the year with little temperature difference, but also very high in humidity (>80%). The average maximum temperatures occurs for about six to seven hours during daytime correspondingly with low relative humidity (approximately +/-60%). Cooling is needed during daytime to counter the radiant heat from the searing heat of the sun. Conversely, the average maximum relative humidity occurs during the midnight hours. Cooling is needed during these hours to counter the high relative humidity in the air which is not conducive for comfortable sleep.

Current Conditions

Most homes in Malaysia are constructed with construction materials which absorbs heat readily from the sun during the daytime. At night time, the air outside cooled rapidly, but the building fabric behaves differently where heat is released from the building mass to the surrounding air indoor and outdoor. This thermal behaviour is typical of houses predominantly built of highly densed materials such as clay and cement bricks, concrete and also lightweight concrete aggregate. Figure 1 shows the thermal performance of the different types of domestic dwelling viz., the traditional Malay attap roof house, the traditional Malay zinc roof house, the low-cost cluster house, the low-cost walk-up flats and the low-cost terrace. The attap and zinc showed extremes of temperature both daytime and night time with temperatures soaring up to above 33^oC internally during daytime and below 26^oC internally during night time. This extremes was due to the nature of the construction materials which are of thin veneer type of timber planks for walls and attap and zinc for roofs. So the absorption and the release of heat was rapid to equal to outside temperatures. Unlike the three other houses of the heavily masonry type of construction where absorption and release of heat was of a graduaal meovement. Figure 2 shows the behaviour of thermal performance of those houses under investigation on the hourly basis. The night time behaviour of the modern houses have failed in releasing heat rapidly from the building fabric thereby causing discomfort having higher internal temperatures than the external condition.

Figure 1: CET mean daily maximum & minimum for the five houses based on the data from the monitoring experiment¹

Figure 2: Period average inside to outside temperature differences for the measured cases²

Possible Solutions

Apart from tapping the energy from the sun to produce electricity via solar cell panels which at the moment only the wealthy can afford, there are other passive and cheaper means of reducing the heat load of the building interiors. One of them is the use of the turbine ventilator (Figure 3) which is gaining popularity. It uses the principle of the stack effect from inside the building and together with the aid of light wind speed from outside to siphon out hot air. One thing it does not do is to generate ample air movement for evaporative cooling.

Figure 3: A typical turbine ventilator

Another possible alternative is to use the uninterrupted wind speed high up in the altitude. It is an acceptable fact that wind speed increases the higher the altitude because of the absence of land mass and built-up areas nearer the ground (Figure 4). The Idea is to use the principle of the wind vane (externally located) by rotating the fan (internally located) simultaneously as the wind vane rotates. This idea is presently being researched into for further development to make it more efficient. Depending on the location in Malaysia wind is relatively calm for almost 30% to 50% of the year. Calm condition is considered to be less than 0.3m/s by definition. And when there is wind, its speed varies and can come from different directions, all within a day (Table A & B). In theory the principle works, as shown in Figure 5. There are variables that needed to be specified before it could reach marketability.

Figure 4: Altitude and wind speed³

Figure 5:
Diagrammatic theory of the free wind fan

These variables would likely to be:

1. At what height of the wind vane should be to capture the constant flow of wind?
2. The weight of the whole contraption, it should be lighter and durable.
3. Should it be employing the stack effect or generate air movement?

Consideration 1, would vary depending on the location of the proposed building. If building were to be by the coastal areas, its height need not be high for obvious reasons. For buildings in the urban context, height is critical.

Several advantages could be gained from this contraption:

1. The speed of wind at the high altitude can be transferred to the lower altitudes to generate the same speed. This required speed is scarce at ground level.
2. The speed can be constanr or vary but never calm. Air movement provide fresh air in an cool the interior structures thereby reducing the internal heat load.
3. Could still generate a good amount of air movement even when windows are closed during rainy days.
4. Reduce electricity bills.
5. Maintenance free.

References

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