P1.2.1 Energy Transfers and Efficiency

Question 1

How are methods of insulation effective and cost-effective?

Answer 1

1. The most effective methods of insulation are ones that give you the biggest annual savings.
2. The most cost-effective methods tend to be the cheapest. They are cost-effective because they have a short payback time - this means the money you save covers the amount you paid really quickly.

Question 2

What is payback time and how do you calculate it?

Answer 2

Eventually the money that you have saved on heating bills will equal the cost of putting in the insulation. The time it takes is called the payback time.
(payback time = initial cost/annual saving)

Question 3

What is the conservation of energy principle?

Answer 3

Energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed.

Question 4

Explain useful and wasted energy.

Answer 4

• Energy is only useful if it can be converted from one form to another. Useful energy is concentrated energy.
• When energy is transferred only part of it is transferred, the rest is ‘wasted’.
• Wasted energy is eventually transferred to the surroundings, which become warmer. The wasted energy becomes increasingly spread out and so becomes less useful.
• The entire energy output by a machine, both useful and wasteful, eventually ends up as heat.
• As the heat is transferred to cooler surroundings, the energy becomes less concentrated - it dissipates.
• The total amount of energy stays the same, but as it becomes increasingly spread out it can’t be used easily or collected back in again.

Question 5

The _ _ _ _ energy that is _ _ _ _ _ _, the _ _ _ _ efficient the device is.

Answer 5

The less energy that is wasted, the more efficient the device is.

Question 6

What are the two equations that allow you to calculate the efficiency of an appliance?

Answer 6

efficiency = useful energy out / total energy in
efficiency = useful power out / total power in
(efficiency in decimal or %)

Question 7

Are all devices 100% efficient? What happens to the wasted energy?

Answer 7

Except for electric heaters, no device is 100% efficient and the wasted energy is usually spread out as heat. Electric heaters are usually 100% efficient because all the electricity is converted to ‘useful’ heat. Ultimately, all energy ends up as heat energy.

Question 8

Evaluate low energy light bulbs and LED lighting.

Answer 8

1. A low energy light bulb is about 4 times as efficient as an ordinary light bulb.
2. Energy-efficient are more expensive to buy but they last much longer.
3. Energy-saving light bulbs are normally more cost-effective than ordinary light bulbs (payback time).
4. LED light bulbs are even more efficient than low energy bulbs, and can last even longer.
5. But they are more expensive to buy and don’t give out much light as the other two types of bulb.

Question 9

Evaluate replacing old appliances with newer energy-efficient ones.

Answer 9

1. New, efficient appliances are cheaper to run than older, less efficient appliances.
2. But new appliances can be cheaper to buy.
3. You have to work out it’s cost effective to buy the new appliance (payback time).

Question 10

Explain how ‘waste’ energy be useful in heat exchangers?

Answer 10

• Heat exchangers reduce the amount of heat energy that is ‘lost’.
• They do this by pumping a cold fluid through the escaping heat.
• The temperature of the fluid rises as it gains heat energy.
• The heat energy in the fluid can then be converted into a form of energy that’s useful again - either in the original device or for other useful functions.