Heat transfer and efficiency: Sankey diagrams
Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.
This Sankey diagram for an electric lamp shows that most of the electrical energy is transferred as heat rather than light.
Energy can be transferred usefully, stored or dissipated. It cannot be created or destroyed. Notice that 100 J of electrical energy is supplied to the lamp. Of this, 10 J is transferred to the surroundings as light energy. The remainder, 90 J (100 J – 10 J) is transferred to the surroundings as heat energy.
The energy transfer to light energy is the useful transfer. The rest is ‘wasted’: it is eventually transferred to the surroundings, making them warmer. This ‘wasted’ energy eventually becomes so spread out that it becomes less useful.
Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat energy instead of light energy. This is the Sankey diagram for a typical filament lamp.
Modern energy-saving lamps and LEDs (light-emitting diodes) work in a different way: they transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp.
From the diagram, you can see that much less electrical energy is transferred, or 'wasted', as heat energy from the energy-saving lamp. It's more efficient than the filament lamp.
The efficiency of a device such as a lamp can be calculated:
efficiency = useful energy out ÷ total energy in (for a decimal efficiency)
efficiency = (useful energy out ÷ total energy in) × 100 (for a percentage efficiency)
The efficiency of the filament lamp is 10 ÷ 100 = 0.10 (or 10 percent).This means that 10 percent of the electrical energy supplied is transferred as light energy (90 percent is transferred as heat energy).The efficiency of the energy-saving lamp is 75 ÷ 100 = 0.75 (or 75 percent). This means that 75 percent of the electrical energy supplied is transferred as light energy (25 percent is transferred as heat energy).