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Team Members:

Ho Sze Yin (4A)

Lam Leong Kwan (4A)

Tsang Tsz Chun Victor (2B)

Heung Yat Hei (2C)

2010-11 Hong Kong Budding Scientists Award(Secondary School)

Honourable Mention

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Content

1. Introduction

2. Causes of the high energy consumption of buses

3. Proposed solutions

4. Conclusion

5. Reference list

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A. Introduction

A report (Appendix 1) shows that the temperature in the compartment could reach as

high as 48 degree Celsius when bus stops at a bus stop in an open area under strong

sunlight with the engine turned off. Much energy is needed to cool down the

compartment to provide a comfortable environment for passengers. Moreover,

air-conditioning facilities which enable passengers to stay in a comfortable

environment cause high energy consumption. So, we would like to suggest some

solutions for energy saving to bus companies.

We first analyzed the causes of high energy consumption of buses through

information searching on the internet, books, and discussions among teammates and

teachers. Then, we made some suggestions according to the causes identified. We also

conducted experiments to investigate the effectiveness of the suggested solutions.

Finally, we came up with practical solutions.

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B. Causes of the high energy consumption of buses

After analyzing the situation, we found certain causes for high energy consumption of

buses.

1. Improper use of materials

The choice of materials in the production of buses leads to its high energy

consumption.

a. Material making up the windows of the bus

The windows of the bus are single-glazed tinted glass; it may not be the best

heat insulation glass, which makes the compartment heat up faster.

More energy is required for air-conditioners to maintain a comfortable

temperature for passengers.

b. Colour of the bus top

Sun radiation reaches a bus compartment and heats it up mainly through prolonged exposure of the bus top to sunlight. Colour of bus top highly affects the rate of radiation reaching and heating up the bus compartment. Most of the buses’ tops nowadays are painted green or white. They

are not reflective enough to reflect most of the sun radiation. More energy is required for air-

conditioners to cool down the bus

compartment.

c. Mass of the bus

The mass of the materials making up the bus may affect the engine efficiency.

The heavier the bus is, the more the energy is required to move the bus.

Figure 1 Buses with green and white painted tops

(Photos from http://www.shekouonline.com/html/getting_there.html

and http://www.skyscrapercity.com/showthread.php?p=42326862)

White bus top

Green bus top

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2. Energy wasting of engine

Nowadays, buses are equipped with the diesel engines. Although diesel engines

are relatively efficient, it needs to run continuously to keep the air inside the

chamber hot. When bus stops at a bus stop, its engine is still running. During the

idling time, the energy may be wasted.

Figure 2 Diesel engine

(Photo copied from http://www.benzinsider.com/2009/12/daimler-trucks-north-america-

to-offer-detroit-diesel-engines-in-us/)

3. Energy wasting of air-condition

There is report (Appendix 2) saying that temperature in the bus compartment can be as low as 18 degrees Celsius when the air-conditioner is on, which is too cold. Much energy is used to cool down the compartment to a very cold environment,

which is not necessary, and causes a waste of energy.

All the above reasons lead to high energy consumption of buses.

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C. Proposed Solutions

After analyzing the causes, we proposed some solutions to solve the problems.

1. Change the materials used

a. Material making up the windows of the bus

We propose to replace the materials for making the bus windows with higher

heat insulation. We did a series of experiments to investigate which material is

the most suitable to make the windows of the bus.

There are different types

of possible glasses:

single-glazed normal

glass, double-glazed

normal glass, tinted

glass, reflective glass,

and double-glazed glass

filled with water.

Figure 3 Experimental set-up for testing which material is the

most suitable for making the windows of buses

Procedure:

As shown in figure 3, a temperature sensor was put into a sealed box. A piece

of glass was put on the side of the sealed box for testing. Further away from

the box a temperature sensor was placed as reference, to see whether there is

heat gain from the surroundings. A 250 W light bulb was put between two

temperature sensors. It was switched on for 10 minutes and the temperature

change was recorded. The temperature change with that detected by the

temperature sensors inside the sealed box with different glasses tested was

compared.

Experimental results:

Temp. (°C) of the temp. sensor

inside the sealed box

Temp. (°C) of the temp. sensor

of the reference

initial final Temp. diff. initial final Temp. diff.

one-glazed normal glass 19.6 30.4 10.8 19.6 25.0 5.4

double-glazed normal glass 19.8 29.8 10.0 19.4 24.6 5.2

tinted glass 20.5 23.8 3.3 21.0 28.0 7.0

reflective glass 21.4 24.3 2.9 21.2 27.4 6.2

double-glazed glass filled

with water

22.4 25.8 3.4 22.6 24.8 2.2

Table 1 Results of our experiment for testing which material is the most suitable for

making the windows of buses

Reference

Sealed box

Temperature

sensor

Data logger

Connected to

the computer

250W light

bulb

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From the experiment, we found that single-glazed and double-glazed normal

glasses have the lowest heat insulation. It is not suggested to use normal glass

on buses as windows. However, tinted glass, reflective glass, and water in

double-glazed glass have better heat insulation. Since most of the light and

heat are reflected by tinted glass and reflective glass, and the high specific heat

capacity of water allows water in double-glazed glass to insulate heat.

However, nowadays buses are already using tinted-glass. Replacing

tinted-glass with reflective glass may not have a significant effect on reducing

heat gain by the bus. Using double-glazed glass filled with water may increase

mass of the bus, thus increasing the energy consumption. Tinted-glass should

be used continuously.

b. Colour of the bus top

Colour of bus top may affect the rate of sun radiation reaching the bus,

which heats up the bus compartment. Therefore, we have to consider

which colour absorbs the least radiation, so as to minimize the heat gained

by the bus. We proposed that using more reflective colour, such as silver

and gold, can lower the rate of sun radiation reaching the bus

compartment, thus, less energy is required to cool down the compartment.

We have done experiments to investigate which colour-top has smaller

increase in temperature while exposed under sunlight.

Figure 4 Experimental set-up for testing which colourof bus tops allows the

least incoming heat energy by solar radiation

4 model

buses

Data logger

Computer

Reference

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Procedure:

We made four identical model buses with the scale of 1:100 and put

temperature sensors into them separately. Also, a temperature sensor was put

outside as reference. Then, different colour tops was installed on the model

buses. We compared their temperature rises in each case to study the insulating

effect.

Experimental Results:

Colour of the

bus tops

Temperature (°C) of the temperature sensors Temperature

difference (°C) Initial Final

Reference 19.3 27.2 7.9

Silver 19.4 27.6 8.2

Gold 19.4 27.6 8.2

Green 19.2 27.9 8.7

White 19.3 27.7 8.3

Table 2 Results of our experiment for testing which colourof bus tops allows the least

incoming heat energy by solar radiation

Figure 5a Results for temperature sensors put into Silver, Gold topped bus and as control put

under the sun

Figure 5b Results for temperature sensors put into Green and White topped bus put under the

sun

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Data logger

250W light

bulb

Same distance

We also repeated the same

experiment using a 250W

light bulb instead of the sun.

Figure 6 Experimental set-up for testing which colourof bus tops

allows the least incoming heat energy by 250W light bulb

Experimental results:

Colour of the bus

tops

Temp. (°C) of the temp. sensors Temp. diff.

Initial Final

Reference 20.5 23.8 3.3

Silver 20.5 22.4 1.9

Gold 20.5 21.7 1.2

Green 20.8 24.0 3.2

White 20.5 22.5 2.0

Table 3 Results of our experiment for testing which colourof bus tops allows the least

incoming heat energy by 250W light bulb

From the experiments, it was found that silver, gold and white colour-topped

bus have similar rise in temperature. They are all suitable for heat insulation,

better than green-topped bus. However, gold and silver colours are not easily

painted on buses. White colour top may be the most suitable one.

The possible error of this experiment is that the model buses are made of paper

instead of fiberglass or aluminium, using a different material may result in a

difference in the rate of heat gain. However, we are just comparing the heat

gain, in other words, the temperature difference in the compartment of

different colour-toped bus. The materials use will not affect much.

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c. Mass of the bus

As proposed, the heavier the bus is, the more the energy is required to move

the bus. In order to find out whether it is true or not, we did an experiment to

simulate this.

Figure 7 Experimental set-up for testing the relationship between weight and

energy required to move the vehicle

Procedure:

First, we prepared two identical used-up rechargeable batteries. Then, two

batteries were charged for 10 minutes to ensure they provide the same amount

of energy. Then, a track for model train was set up. We measure how many

rounds the train can run of 500g and 1000g respectively by a photogate and a

data logger.

Results:

Stopwatch

Rechargeable

batteries

Track photogate

Model train

Weight

Figure 8 Results of our experiment for testing the relationship between weight and

energy required to move the vehicle

A stick is

attached to

interrupt the

photogate

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From the above experiment, we found that the train can run more rounds with a

lighter mass. As a result, we can deduce that the energy consumption of the bus

will be lowered if the mass of the bus is decreased.

2. Choice of the engine

As mentioned, diesel engine consumes a lot of fuel. A possible solution is to

change the bus into a Hybrid electric vehicle (HEV). The HEV uses its internal

combustion engine to generate electricity by spinning a motor-generator to either

recharge their batteries or to directly power the electric drive motors. Many HEVs

reduce idle emissions by shutting down the Internal Combustion Engine at idle

and restarting it when needed, which is known as a start-stop system.

However, the cost is very high. Also,

the HEV technique may be too

advanced to be used on buses. From the

economic point of view, it’s not

suggested to be used on buses.

Figure 9 A Hybrid Electric Vehicles

(Photo from http://inhabitat.com/first-russian-hybrid-car-coming-in-2012/)

3. Air-conditioner

We propose to place a variable-frequency drive, which is a system that controls

the frequency of energy supplied to the alternate current motor to verify its

rotational speed (Finney, 1988), into the air-conditioners of the buses to lower the

energy consumption.

The variable-frequency drive may enable the air-conditioner work more efficiently,

completely, which enables the room to achieve the setting temperature, and

reduces energy consumption to maintain the temperature in the bus. The

air-conditioner will only work in the lowest revolution area most of the time.

Compared with the air-conditioner with no variable-frequency driver, it reduces

30% to 40% of energy consumption (McQuay International).

When the temperature achieves the assigned setting, the variable-frequency drive

will finely adjust the power used, which maintains the temperature, then a

comfortable environment will be achieved, and less energy will be wasted.

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4. Water circulation system

We suggest using water pipes with water circulation to surround the bus outer

shell, as shown in figure 11. As water has high specific heat capacity, much energy

is required to heat it up. Thus, the bus compartment will increase a small degree of

temperature under sunlight, this adversely help lower the energy required for

air-conditioner to cool down the bus compartment.

Moreover, cooler water can replace the warm water in the pipes when the bus gets

to the terminus, this help reduce the energy required to cool down the water.

Figure 10 Bus circulated with water pipes

Water is heavy, which may increase the burden of the engine; however, we may use a

smaller amount of water and change the water with cooler water more frequently. In

practice, we may find the optimized amount of water used by experiment.

5. Moving the terminus into a covered area

To prevent the buses from heating up under strong sun at the bus stops, we

propose to move the bus-stops into covered areas, so the buses will not be exposed

to sunlight directly and heat up fast. Less energy is required to cool down the bus

compartment.

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D. Conclusion

From the experimental results and our investigations, we came up with

three suggestions to lower the energy consumption of buses.

First, paint the bus tops in white colour, and circulate the bus with water pipes with

water circulation as to lower the rate of heat transferred into the bus compartment,

thus lower the energy required for air-conditioner to cool down the compartment.

Second, add variable-frequency drive into the air-conditioners as to increase the

efficiency of air-conditioner.

Last but not least, move the terminus into covered areas to prevent the buses from

heating up under strong sunlight at the bus stops.

Although all of the above proposed solutions are quite costly, they are useful in the

long run to lower the energy consumption, which helps bus companies to save more

by saving energy and protecting the environment.

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E. Reference List

1. (Appendix 1) Type: Newspaper, accessed online

陳 凱 迎 ( 2 J u n e 2 0 1 0 ) . “車廂 48 度 議員質疑停車熄匙 ” The Hong

Kong Economic Times.

http://www.hket.com/eti/article/4deec7b8-fb78-4255-a3d0-0d475c54fb79-107035

?category=green_news. 13 February 2011.

2. (Appendix 2) Type: Articles, accessed online

李祥 168 (2 November 2006). “澳门金店的冷气” iask.sina.com. http://iask.sin

a.com.cn/b/6909158.html. 13 February 2011.

3. Type: Book

Finney, D. (1988) Variable frequency AC motor drive systems. IET

4. Type: Article, accessed online

McQuay International (2005). “Reduce Your Chiller’s Energy Usage Up to 30%

by Adding a VFD” www.mcquay.com

http://www.mcquay.com/mcquaybiz/literature/lit_service/Brochures/VFDRetrofit_

13F-2315_0805_FINAL.pdf. 15th April 2011.

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Comments

The students could demonstrate good analysis of the causes of high-energy consumption of buses. The proposed solution is highly related to one of the suggested causes – the air conditioning of buses. Then, the student contestants used model buses to conduct some well-designed experiments to investigate on how the material of a bus may affect the temperature increase of a bus. The data presentation was clear. The student contestants also suggested other methods to improve the energy consumption of buses. However, it would be better if the validity and the cost-effectiveness of the methods could also be discussed.

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