Energy Efficiency

Calculating Efficiency

Energy efficiency is a measure of how much energy is transferred into useful energy to perform a task or function compared to the waste energy produced. This is given as a percentage figure of the initial energy transferred.

Energy efficiency = (Useful energy transfer to a device / Total energy supplied to the device) x 100%

Example 1: A LED bulb is supplied with 300 J of electrical energy over an hour, from this 285 J of light energy is emitted. What is the efficiency of the LED bulb?

Energy Efficiency, figure 1

Efficiency = (Useful energy / Total energy supplied ) x 100 %

= (285 / 300) x 100 % = 95 %

Example 2: A car supplied 5 x 105 _J _of energy from the petrol consumed. The car is 30% efficient, how much of the petrols chemical energy has been transferred to the kinetic energy of the car?

Efficiency =(Useful energy / Total energy supplied) x 100%

Useful energy = Total energy supplied x Efficiency

=5x105 x 30% =1.5x105 J of Kinetic energy

Example 3: A kettle is 80% efficient and needs to produce 1.68 x 105 J of heat energy to boil ½ litre of water from 20oC to 100oC. How much electrical energy must be supplied to the kettle?

Energy Efficiency, figure 2

Efficiency = Useful energy / Total energy supplied x 100%

Total energy supplied = Useful energy / efficiency

Total energy supplied = 1.68 x 105 / 80% = 2.1 x 105 J of electrical energy.

Increasing Efficiency

To increase the efficiency of a device it needs to transfer more of the supplied energy into the useful energy required for the function of the device.

Energy Efficiency, figure 1

This data from the Department of Energy compare the efficiency of light bulbs over a three year period. The LED have improved from about 60% to nearly 95% efficiency. This is due to changes in technology resulting from research and development. This is one way to improve the efficiency of a device.

For any device with moving parts, heat due to friction is a source of waste energy. Reducing this frictional heat loss by the use of lubrication is a good way to improve the efficiency of the device. Lubrication can be in the form of oils, grease and even plastic coatings.

Did you know that the Ancient Egyptians improved the efficiency of their carts that hauled the stones to build the pyramids by pouring water onto the sand, this acted as a lubricant and meant a team of 10 men could pull the huge blocks of stone.

Adding insulation to a house or other object that is being heated will improve the efficiency too as this allows less energy to be dissipated as heat into the environment.

Energy Sources

Our energy sources can be divided into two major categories:

Non-Renewable: Once used it can not be replaced.

Renewable: The source can be replaced or is ever present.

Energy Efficiency, figure 1

Non-Renewable: Coal, oil and natural gas (Fossil Fuels), Nuclear energy from Nuclear fission (using uranium and plutonium both of which are minerals dug from the ground, like coal).

These are the traditional sources of energy that have been used by people throughout history, especially since the industrial revolution in the middle of the 18th Century. Nuclear fission as a power source for electricity began in Obinsk, Russia in 1954.

Fossil fuels still form the major energy source, but a recognition that they will run out and of their impact on Global Warming due to Carbon Dioxide emissions, have led to many countries moving to renewable sources and Nuclear power.

Sweden, for example, plans to ban the sale of petrol and diesel cars in the next couple of years and the UK Government intends to stop the sale of new petrol and diesel vehicles in 2040.

Renewables: Hydro-electric, solar, wind, tidal, wave, Nuclear fusion and biofuels.

Energy Efficiency, figure 2

One of the advantages of these sources is that they are not going to run out, they are sustainable energy sources for the future, and at the point of use they are mostly carbon neutral (no Carbon dioxide is emitted).

However, as many of them require concrete in their construction and other manufacturing processes they are not Carbon free. Biofuels are burnt, like all fuels and do produce Carbon Dioxide, but the plants that are grown to make the fuels remove carbon dioxide too, so biofuels are considered to be carbon neutral and they are renewable.

They are increasingly common sources of energy, in 2017 the UK had its first-ever fossil fuel free day of electricity production, all of that day’s electrical energy supplied to homes, schools etc came from sustainable sources.

Nuclear fusion is still only a theoretical process, it takes Hydrogen and pushes them together to make Helium, this is the same process that powers our sun. If it can be made to work it will be able to make large amounts of electrical energy from seawater.

There is a trend for the increase in the use of renewables compared to non-renewables, but this trend is very different in different countries, depending on their political and economic situation.

Explain the reasons a country might wish to move away from reliance on gas as an energy source for producing electricity and move to wind and solar power.
Your answer should include: reducing / reduction / carbon dioxide / emissions
Explanation: Reduction in Carbon Dioxide emissions to reduce the impact on Global warming. Solar and wind are renewable and sustainable, gas is non-renewable and will run out.
A coal-fired power station uses 2.5 x108 J of chemical energy per minute to generate 3.75 x 107 J of electrical energy, calculate the efficiency of the power station. (in %)
Your answer should include: 15% / 15
Explanation: Efficiency = useful energy / total energy x 100% Efficiency = 3.75 x10⁷ / 2.5 x10⁸ x 100% = 15%
Explain how a house can be made more energy efficient.
Your answer should include: insulating / cavity walls / foam / window / double glazing / light bulbs / LED / solar power / loft space