1. P1b(ii) Keeping Homes warm
P1b(ii) Keeping Homes warm
You will learn about:
How energy is lost from uninsulated homes
About payback time
About energy efficiency
About Sankey diagrams

2. Saving Energy www.PhysicsGCSE.co.uk
The roof is insulated with fibreglass. Fibreglass contains air pockets which prevent heat escaping via conduction.
As less energy escapes and more energy stays in the house it means it is energy efficient.
This log cabin has a snowy roof. It tells us that it must have really good loft insulation. If it didn’t then the snow would have melted.
The fireplace in this house is NOT efficient. Most of the heat conducts through the walls out of the room. Only the heat facing forward can get into the room.
The fireplace in this house IS efficient. The heat radiates into the room in all directions. Less heat is wasted so is more efficient.
For every 100J of energy released as heat only 25J are transferred to the room.
For every 100J of energy released as heat 90J are transferred to the room.
efficiency = 𝑢𝑠𝑒𝑓𝑢𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑢𝑡𝑝𝑢𝑡 𝑡𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛𝑝𝑢𝑡 𝑥 100
efficiency = 25 𝐽 100𝐽 𝑥 100=25% 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
efficiency = 90 𝐽 100𝐽 𝑥 100=90% 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡

3. Sankey Diagrams
The light energy arrow is 9 times smaller than the heat energy arrow. This is because 10J is 9 times smaller than 90J. This means the Sankey diagram is to scale.
Sankey diagrams are used to show energy transformations.
An arrow to the right shows the USEFUL energy output.
Remember: there may be more than one arrow so you would need to add up all of the values
Inside the arrow shows the TOTAL INPUT energy
An arrow pointing downwards shows the WASTE energy output.
Remember: there may be more than one arrow so you would need to add up all of the values
The values outside = the values inside.
100J = J J

4. Payback Time www.PhysicsGCSE.co.uk
There are many ways a home loses heat: Roof Walls Doors Floors Windows
The house is the source of the energy
The atmosphere is the sink. It can absorb lots of energy without changing its temperature by very much.
The Payback Time equation will tell you which insulation method is the most cost-effective.
𝑃𝑎𝑦𝑏𝑎𝑐𝑘 𝑇𝑖𝑚𝑒 𝑦𝑒𝑎𝑟𝑠 = 𝑐𝑜𝑠𝑡 𝑜𝑓 𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑎𝑛𝑛𝑢𝑎𝑙 𝑠𝑎𝑣𝑖𝑛𝑔
Insulation
Typical cost in £
Typical annual fuel saving in £
Cavity wall
250
125
Double glazing
5000
100
Loft
150
Double glazing
𝑃𝑎𝑦𝑏𝑎𝑐𝑘 𝑇𝑖𝑚𝑒 𝑦𝑒𝑎𝑟𝑠 = £5000 £100 =50 𝑦𝑒𝑎𝑟𝑠
This means that it will take 50 years to earn back the initial start up cost of £5000. After 50 years all savings are profit.
Cavity wall insulation only takes 2 years to pay back the initial cost

5. Quick Questions
A coal fire transfers 5000 J of energy stored in the coal into 3000 J of useful heat. Calculate the efficiency of the fire.
Using the table on slide 4, calculate the payback time for loft insulation.
Draw a Sankey diagram for a lightbulb that has 100 J of electrical energy that transfers into 75 J of useful light energy.
A fire is 25% efficient. It burns 20kg of coal each day. The energy output into the room is 46.3 MJ. Calculate the energy in 1 kg of coal

6.
Quick Questions
A coal fire transfers 5000 J of energy stored in the coal into 3000 J of useful heat. Calculate the efficiency of the fire.
Using the table on slide 4, calculate the payback time for loft insulation.
Draw a Sankey diagram for a lightbulb that has 100 J of electrical energy that transfers into 75 J of useful light energy.
A fire is 25% efficient. It burns 20kg of coal each day. The energy output into the room is 46.3 MJ. Calculate the energy in 1 kg of coal
20kg coal releases 46.3 MJ
so 1kg coal releases 46.3/20 = MJ
Turn 25% to 100% need to multiply by 4
So MJ x 4 = 9.26 MJ
efficiency = 𝑢𝑠𝑒𝑓𝑢𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑢𝑡𝑝𝑢𝑡 𝑡𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛𝑝𝑢𝑡 𝑥 100
efficiency = 𝐽 5000 𝐽 𝑥 100=60%
𝑃𝑎𝑦𝑏𝑎𝑐𝑘 𝑇𝑖𝑚𝑒 𝑦𝑒𝑎𝑟𝑠 = 𝑐𝑜𝑠𝑡 𝑜𝑓 𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑎𝑛𝑛𝑢𝑎𝑙 𝑠𝑎𝑣𝑖𝑛𝑔
𝑃𝑎𝑦𝑏𝑎𝑐𝑘 𝑇𝑖𝑚𝑒 𝑦𝑒𝑎𝑟𝑠 = =1.67 𝑦𝑒𝑎𝑟𝑠