1.3.5 Reducing Energy Loss · 1. Electricity, Energy and Waves | Wjec GCSE Science (Double Award): Physics

Section 1 of 9Why reduce energy loss?

Heating a home is expensive and, when fuel is burned, produces carbon dioxide. A well-insulated house needs less energy to stay warm, saving money and reducing emissions. In devices, reducing wasted energy makes them more efficient. To reduce losses we must understand where the energy escapes, and block the relevant method of transfer.

Section 2 of 9Insulating buildings

Houses lose thermal energy through the roof, walls, windows, doors and floor. Common measures and the physics behind them are:

Insulating buildings

Loft insulation

(mineral wool or foam): traps air, which is a poor conductor, and stops convection in the loft space. Because hot air rises, the roof is often the biggest source of loss, so loft insulation is the most cost-effective measure.

Cavity wall insulation

Cavity wall insulation foam or fibre placed in the gap between brick layers stops convection currents in the air inside the cavity and reduces conduction across it.

Double glazing

Double glazing two panes with a trapped air or inert gas gap, or a vacuum, reduce conduction. The gap is narrow so convection is limited.

Draught excluders and carpets

Draught excluders and carpets stop warm air escaping around doors and through the floor.

Reflective foil behind radiators

Reflective foil behind radiators reflects infrared radiation back into the room rather than letting it be absorbed by the wall.

Section 3 of 9Thickness and conductivity

The rate of energy transfer by conduction through a layer depends on three things.

Thickness and conductivity

It is greater when the material has a higher thermal conductivity, when the temperature difference across it is larger, and when it is thinner.

So to reduce loss use a material with low thermal conductivity and make it thicker.

Doubling the thickness of an insulating layer roughly halves the rate of conduction through it, but you get diminishing returns in cost after a certain thickness.

Payback time compares the cost of a measure with the annual saving: payback time = cost of installation ÷ annual saving. A cheap measure with a large saving (loft insulation) has a short payback time, whereas expensive double glazing pays back over many years.

Section 4 of 9Reducing waste in devices

In machines, friction and resistance cause waste.

1

Lubricants separate moving surfaces and reduce friction.

2

Thicker, low-resistance cables lower electrical heating losses.

3

Fans and heat sinks are not about reducing waste but about removing unavoidable waste before it damages components.

4

Streamlined shapes reduce air resistance in vehicles.

5

Regenerative braking in electric and hybrid cars stores kinetic energy as electrical energy instead of wasting it as thermal energy in brake discs.

Section 5 of 9Evaluating measures

Evaluating insulation measures involves cost, payback time, disruption, lifetime and environmental impact.

Evaluating measures

For example, a measure that saves 400 kWh a year and costs a lot may still be worth it if it lasts for 40 years.

In an exam you must use the data given and link the saving to the physical mechanism, for example "trapped air stops conduction and convection".

Section 6 of 9Link to energy resources and everyday numbers

Reducing energy loss is part of the bigger picture of saving energy.

1

If less energy is wasted by buildings and machines, less must be generated, so fewer fuels are burned.

2

Energy is billed in kilowatt-hours: an appliance of power 2 kW used for 3 hours uses 6 kWh.

3

If a measure cuts the heating energy needed by 20% on a bill of 12 000 kWh a year, it saves 2400 kWh.

4

At an example price of 25 pence per kWh that is £600 a year, which you can compare with the installation cost to find the payback time.

Also remember that insulation cannot stop transfer completely; it only slows it.

Link to energy resources and everyday numbers

A hot-water tank with a thick jacket still cools overnight, just more slowly.

And a house needs some ventilation to remove moisture, so draught-proofing must be balanced against air quality.

Combining measures (loft, walls, windows, draught strips) gives the largest savings, but each has a different payback time, so a householder will normally do the cheapest, highest-saving measures first.

Section 7 of 9Worked examples

Cavity wall insulation costs £900 and saves £150 per year. Calculate the payback time.
Loft insulation 10 cm thick lets energy escape at 120 W under certain conditions. Estimate the rate if the thickness is doubled to 20 cm of the same material and temperature difference.
Explain why the air gap in a cavity wall is filled with foam rather than left empty.

Section 8 of 9Key terms

Term	Meaning
Insulation	A material that reduces the rate of thermal energy transfer.
Thermal conductivity	A measure of how well a material conducts thermal energy.
Payback time	Cost of a measure divided by the annual saving it produces.
Double glazing	Two panes of glass separated by a gap that reduces conduction.
Draught	Unwanted flow of air that carries thermal energy through gaps.
Lubricant	Substance that reduces friction between moving surfaces.

Section 9 of 9Exam technique

1

Saying that insulation "keeps cold out"; it slows the transfer of thermal energy out (or in).

2

Forgetting convectiontrapped air is only a good insulator if it cannot circulate.

3

Dividing the annual saving by the cost when finding payback time.

4

Thinking that thicker insulation always has a short payback; returns diminish.

5

Mixing up the roles of foil (reflects radiation) and foam (stops conduction and convection).