Designing an Energy Efficient Fan by Dimensional and Physical Design Parameters

By Dr. Abdul Malik Abdul Rahman
January 2006

The Author is the Deputy Director of the Center for Education, Training and Research in Renewable Energy and Energy Efficiency (CETREE), as well as Practicing Architect and Lecturer at the School of Housing, Building & Planning of Universiti Sains Malaysia (USM) in Penang. His special interests is in indoor thermal comfort for Malaysian homes without the use of mechanical aids. Identifying which mode is most effective in a climate where fifty percent of the wind condition is calm. → See also:

Abstract
To experience a thermally comfortable natural environment in a hot-humid climate one is to be under shade and experience some ample air movement concurrently. The Malaysian climate lacks the much needed air velocity for comfort and therefore have to rely on mechanical aids that uses paid energy. The chlorofloroucarbon (cfcs) emitted from air-conditioning is detrimental to the already worrying state of the ozone layer. The power comes from the use of electricity which comes from depleting the fossil fuels. The hypothesis is to create a lot of air movement relying on the electric fan without increasing the energy consumption to cool the building interior and indirectly the building fabric. Between the electric fan and air-conditioning the electric fan is more environmental friendly. This paper presents research initiatives to design a fan that can produce a wider cone of air movement by its physical and dimensional design and also verified that existing fan designs in Malaysian markets are unsuitable for its climate because they need more electrical energy to generate air movement for a bigger catchment area.

Keywords: Tropical thermal comfort, Energy efficient fan, Air movement.

Introduction

Pungkah Fan
Figure 1: The Colonial ‘Pungkah’ Fan

Earlier efforts for indoor thermal comfort were initiatives to reduce heat absorption on the building fabric by passive building design elements and dependent on natural wind to provide air movement indoors. Then it was realized that some forced effort was needed to generate air movement and the ‘pungkah’ fan was introduced by the British, literally using the manpower of the Indian coolie to swing the vertical ceiling fan by hand (Figure 1). With the advent of electricity, electric fans became popular and also convenient. Fan designs gradually improved in many different shapes, sizes and materials, whether they are ceiling, standing, table or wall fans.

After observing existing designs of ceiling fan in the market it was found by personal observation and measurement of air velocity using a pocket anemometer, that the designs are inefficient in creating the cone of air movement. One has to turn to a higher wind speed dial for a wider cone of air movement. The higher the air movement generated by switching to a higher speed the more energy it consumes.

The characteristics of Malaysian wind are that they are erratic in its wind velocity, multi-directional and lacking in the required wind speed. The required velocity should be about 1.2 meter per second and above. Hence sole dependence on natural wind for thermal comfort indoors is mere impossible. One then resorts to using electric fan at a faster velocity than required and/or using air-conditioning. Air-conditioning is a commonplace item which not only is an energy guzzler but also releases chloroflorocarbons (CFCs) that is detrimental to the environment. As population increases the usage of air-conditioning also increases thus further aggravate the ozone layer condition. This is not a low-cost behavior and neither is it an energy efficient one too. The trend should be reversed without compromising human thermal comfort. So an energy efficient electrical fan is deemed paramount to generate air movement indoors. Malaysians need a lot of air movement to rid off the radiant heat during the daytime and also plenty of air movement at night to rid off the high relative humidity. The ideal comfort condition for Malaysians without resorting to mechanical aids is to be under shade and experiencing air movement concurrently. To emulate this situation indoors may not be possible because when walls are erected wind is prevented from entering the interior. To have openings in walls is a temporary solution as mentioned earlier the characteristics of Malaysian wind is not contributive.

Free Wind Fan
Figure 2: The Free Wind Fan

What has been done?

An attempt to use wind energy to turn the wind vane which then turns the fan blades has been experimented upon but was found to be ineffective in areas where the required wind speed is lacking especially in urban areas. The concept of the ‘free wind fan’ (Figure 2) was tested on top of a flat roof of a sixteen meter building where there are no obstruction from any wind direction and speed.

The result was dismal due to the lack of the required the wind speed and it is this criteria that makes designing of buildings orientated to wind influence very difficult. Designing of new buildings orientated towards perpetual winds are possible only at such places as by the sea and the valleys slopes because of the natural low and high pressure differences occurring naturally.

The proposed design of an energy-efficient fan

It has been observed that with the present design of a typical fan where the fan blades are horizontal, the cone of air movement (depending on the length of the fan blades) is only to a certain radius. Anyone sitting beyond this cone would not be able to feel the air movement generated by the fan blades. To increase the fan speed by turning the speed dial to a higher rotation per minute (rpm) the cone of air movement gets bigger and covers a bigger catchment area. Increase of speed consumes more electricity and thus not energy efficient.

The Experiment

The hypothesis:
To increase the cone of air movement with the same fan speed of the same type of ceiling fan.
The equipment:
Two types of experimental fan blade as illustrated in Figure 3. The dimensions are as follows; blade length 8 inches and blade width 3 inches.
The method:
  1. Using a hand-held anemometer the velocity is taken at every increment of 300mm interval outwards from the distance beginning along the tip of the fan blade (Figure 4a).
  2. Also readings are taken from ground level and increment of 300mm vertically from every horizontal point respectively (Figure 4b).
The reading:
  1. the horizontal blade
  2. the 15 degree inclined blade
  3. the improved 15 degree inclined blade.
Horizontal Blades
Figure 3: (a) the horizontal blade
15° Inclined Blades
Figure 3: (b) the 15 degrees inclined blade

Before readings were taken we identified the dependents and independents for air velocity comparisons. The independents are (see Diagram 1):

Vertical Reading
Figure 4: (b) vertical reading
  1. identify zones A, B and C for measuring air speeds.
  2. all measurements are taken at 300mm intervals vertically and horizontally.
  3. the rotation per minute was set to 1140.
  4. current set to 1.4 ampere
  5. vertical distance between floor level to fan is fixed at 2250mm
  6. vertical and horizontal readings are taken at 300mm distance intervals.
Horizontal Reading
Figure 4: (a) horizontal reading
Parameters
Diagram 1: The Parameters
Air Velocity at Horizontal Fan
Diagram 2: The Air Velocity at Horizontal Fan Design
Air Velocity at Fan Blades with 15 Degree Pitch
Diagram 3: The Air Velocities at Fan Blades with 15 Degree Pitch
Comparison between Fan Blades
Diagram 4: Comparison between the Fan Blades with 0 degree and 15° Pitch

The Analysis

(a) The Horizontal Blade (refer to Diagram 2)

To generate an air movement of 1.5m/s or thereabouts is sufficient enough for thermal comfort. Air movements of 1.2m/s and 1.37m/s beginning from 900mm high are already acceptable at position B and 1.3m/s, 1.74m/s, 2.13m/s and 2.5m/s are also acceptable figures at position A. It is observed that as we get further from the center of the fan the air movement gets less above approximately 700mm height. A difference of 1.13m/s (i.e. 2.5m/s – 1.37m/s) is recorded at 1800mm high. Position C determines the extent of the cone of air movement which begins from position B. At 600mm and 1200mm high an air velocity of 0.36m/s and 0.34m/s respectively are experienced. This is about the height where a fan design should be aiming at because it is within the zone for human comfort. It seems that with a horizontal fan blade one experiences lower air velocities less than 0.5m/s at all vertical levels. A distance of 600mm away from position B only 0.22m/s of air velocity is experienced and none at all above 300mm from floor level. It is then assumed that in order to feel the air movement beyond the cone the speed of the fan is to be increased. Increasing the fan speed entails more electrical energy and thus is not energy efficient.

(b) The 15° Inclined Blade (refer to Diagram 3)

Air movements of 1.13m/s and increasing to 1.37m/s beginning from 900mm to 1800mm height are acceptable at position B and 1.34m/s, 1.63m/s, 1.96m/s and 2.02m/s are also acceptable figures. No air movement is recorded above the height of 900mm from position C. But an increase of air velocity is reversed compared to position A and B where the increase happen with increase in height up to 900mm level i.e. from 0.43m/s at level 300mm to 0.78m/s at level 900mm. Though the recorded air speeds do not actually comes within the requirement for thermal comfort but some air movement verifies the hypothesis that by inclining the fan blade the distribution of air movement would span a greater area as was shown at level 600mm of horizontal distance, air velocities can be recorded at level 900mm, unlike the horizontal fan where only at level 300mm is there air movement. Beyond which there is none.

(c) Comparison between the two Fans (refer to Diagram 4)

Conclusions for the horizontal fan blade design (0° fan design):

Conclusions for the inclined fan blade design (15° fan design):

Conclusion

From the experiment we have verified that economy can be achieved without affecting the standards of comfort indoors by installing at strategic locations for the fan to cover a bigger cone of air movement. Number of fans can be reduced and still achieve the air movement needed for thermal comfort. For example instead of having four fans installed in a four-bedded for a hospital room only two would be sufficient to give the same amount of air movement. It is hoped that future fan designs would take this into consideration as there is no extra cost needed to have a fifteen degree pitch for all types of existing fans in the Malaysian market today. Please note that the experiment was done with three 200mm fan blades. The ceiling fans in the market are normally averaged at least 600mm to 700mm in length. With these measurements and bent at a 15° pitch, it would very highly likely that air movements of more than 1m/s can be experienced at the 0.78m/s reference point of the 200mm experimental fan blade. At that position and with air speed of more than 1m/s the experiment has verified the hypothesis.

References

  1. Abdul Rahman A.M. (2004), Low Energy Cooling Technology for Malaysian Homes, Penerbit Universiti Sains Malaysia, ISBN 983-861-274-X.
  2. Abdul Rahman A.M. Natural Cooling of Malaysian Urban Homes by Free Wind, Green Pages, Editorial.
  3. Abdul Rahman A.M. & Noordin N. (1995), Cooling Malaysian Homes by Free Wind, Seminar on Building Design and Construction Innovative Technologies and New Techniques, 27 – 29 July. Conference Room, School of Housing , Building & Planning, Universiti Sains Malaysia.
  4. Abdul Rahman A.M. (2001), Energy-efficient buildings of the future, News Straits Times, Saturday 26 May.
  5. Abdul Rahman A.M. (2002), Aim for energy-efficient designs for our buildings, News Straits Times, Thursday September 5.

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