Topic 2 What is meant by the efficient use of energy?

The Law of Conservation of Energy

This law is an important rule of Physics:

Energy can neither be created nor destroyed. It is turned from one form to another.

What this means is that you cannot get something for nothing.  Nor can you lose it.  Every joule of energy has to be accounted for.  This is what this topic is about.

Useful Energy and Waste Heat

Think of a car.  For every 100 kJ (100 000 J) energy contained in the petrol:

• 60 kJ is lost as heat up the exhaust and radiator;

• 40 kJ is turned into movement.

Of that 40 kJ:

• 35 kJ goes to driving the wheels

• 0.5 kJ to power the electrics;

• 1 kJ to power the steering;

• 3.5 kJ to power the air conditioning.

If you add up all the numbers, you will find that they add up to 100 kJ.

Of the 35 kJ to drive the wheels, some will be lost in friction.  Less than 35 % of the energy we put into the car actually ends up in moving the car along the road.  However none of the energy has been destroyed.  It has simply been turned into other forms of energy.

Question 1 How much energy is wasted?  What fraction is this? What happens to the energy?  ANSWER

Whenever we have any machine to do a job of work, some energy is wasted.  This motor needs to be cooled by a fan, otherwise it gets hot.

Any energy that is not useful is wasted.  It is possible to harvest some of the waste energy to make it useful, for example:

• the heater in a car keeps us warm on cold days;

• the turbo-charger uses hot exhaust gases to pump extra air into the cylinders.

You can see the huge amount of waste heat coming from this plane as it takes off.

This waste heat is simply heating up the air.

So what happens to the energy that a car uses to go along the road?  It is used to:

• overcome friction from the road surface;

• overcome drag from the air;

• to provide kinetic (movement) energy for the car.

So most the energy to make the car move is being turned into heat from friction.  And when the brakes are applied to slow down, the kinetic (movement) energy is turned into heat.  You can see the brakes of this racing car glowing red hot.

All the 100 kJ of energy from the petrol is eventually turned into heat.  On the way we have diverted a little of that energy to do a useful job for us, namely to go somewhere!

Reducing the Waste

One way of reducing wasted energy from brakes is to use regenerative braking. 

This idea has been used with electric trains for many years.  When this locomotive goes down a hill, its motors act as generators and puts current into the wires.  This provides extra current for locomotives going up the hill in the opposite direction.

You can't do that with a petrol or diesel engine, but there are now hybrid vehicles available.  The picture below shows the idea:

Question 2  How do you think this system works?  Answer

In this system the vehicle uses a generator to charge up a battery.  When the vehicle accelerates, the battery gives power to a motor.

For more information about hybrid vehicles, see these websites:

www.visionengineer.com/ env/hv.shtml

www.fueleconomy.gov

Low Grade Heat

There are ways of recovering waste heat.  Many devices have heat exchangers which use waste heat going out to warm up air or water going in.  In the end all energy ends up as low grade heat.  The further along the energy process, the lower the grade of heat.  This low grade heat is hard to extract energy from. 

Why should we worry about waste heat?

There are currently several pressing issues:

• Global warming with all its related issues;

• Rising price of oil (Petrol is £1 (€1.5) a litre and may well rise further);

• Dwindling energy resources;

• Pollution and environmental damage.

We need to think through ways in which we use energy, especially in transport:

• Cars (especially 4 x 4 vehicles) use a lot of energy.  May be there should be more effort to move to hybrid vehicles.  Super fast top speeds and accelerations are possibly luxuries we cannot afford any longer.

• More travel by public transport, although many people dislike buses and there are many places in this country that are no longer accessible by rail.

• How much longer will cheap flights be available or acceptable?

Electric vehicles are quiet and non-polluting.  But we have to generate the electricity somewhere, and that does cause pollution.  We need to think about ways of generating electricity:

• Coal-fired power-stations cause pollution and acid rain;

• Gas-fired stations are running out of gas.  They also emit greenhouse gases;

• Wind farms cause no pollution, but don't generate on a calm day;

• Nuclear powers stations do not give out greenhouse gases, but nuclear waste is dangerous.

We have to think about the efficiency of the energy process.

Efficiency

No energy converting process ever gives out as much energy as is put in.  You always have to put in more energy than you get out.   We can measure the energy efficiency of a device using this simple equation:

The energy efficiency is always a fraction less than 1, which is multiplied by 100 to give a percentage.  (If you get 2/3 correct in you exam, you get 67 %.)

Question 3  Clever Trevor invents a bicycle that has a generator in the front wheel that can give out energy at a rate of 100 J every second.  On the back wheel there is a motor that uses energy at a rate of 100 J every second.  Trevor says that as soon as you have pedalled the bike up to speed, you don't have to pedal any more.  The bike will keep on going.  Do you think he's right?  ANSWER

Examples of efficiency:

You never get devices that are 100 % efficient.  If your answer gives 100 % efficiency or more, you have done it wrong!  Watch out for this bear trap.

Worked Example

Question 4  An electric motor is 55 % efficient.  It uses 1000 J of electrical energy every second.  How much useful work can it do?  How much energy is lost?  What happens to this energy?  ANSWER

Now answer Question 5.

Now try the Topic Quiz.

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