Case study: an electric kettle

Figure 7.4 shows a typical 2 kW electric kettle. The kettle is manufactured in Southeast Asia and transported to Europe by air freight, a distance of 12,000 km. Table 7.5 lists the materials. The kettle boils 1 liter of water in 3 minutes. It

Table 7.5 Electric kettle bill of materials, life: three years

Component

Material

Process

Mass, kg

Material energy, Hm MJ/kg*

Process energy, Hp MJ/kg*

Kettle body

Polypropylene

Polymer molding

0.86

94

8.6

Heating element

Ni-Cr alloy

Def. processing

0.026

130

2.6

Casing, heating

Stainless steel

Def. processing

0.09

81

3.4

element

Thermostat

Nickel alloys

Def. processing

0.02

72

2.1

Internal insulation

Alumina

Power forming

0.03

52

27

Cable sheath, 1 meter

Natural rubber

Polymer molding

0.06

66

7.6

Cable core, 1 meter

Copper

Def. processing

0.015

71

2.0

Plug body

Phenolic

Polymer molding

0.037

90

13

Plug pins

Brass

Def. processing

0.03

72

2.3

Packaging, padding

Polymer foam

Polymer molding

0.015

110

11

Packaging, box

Cardboard

Construction

0.13

28

0.5

Other small

Proxy material:

Proxy process:

0.04

110

11

components

polycarbonate

polymer molding

Total mass

1.3

*From the data sheets of Chapter 12.

is used to do this, on average, twice per day, 300 days per year, over a life of three years. At end of life, the kettle is sent to landfill. How is energy con­sumption distributed across the phases of the kettle’s life?

Figure 7.5 shows the energy breakdown. The first two bars—materials (120 MJ) and manufacture (10 MJ)—are calculated from the data in the table by multiplying the embodied energy by the mass of each component and then summing. Air freight consumes 8.3 MJ/tonne. km (Table 6.7), giving 129 MJ/ket – tle for the 12,000 km transport. The duty cycle (6 minutes per day, 300 days a year for three years) at full power consumes 180 kWhr of electrical power. The corresponding consumption of fossil fuel and emission of CO2 depends on the energy mix and conversion efficiency of the host country (Table 6.6). If this were Australia, for example, the factors from Table 6.6 give, for the use phase, 1800 MJ oil equivalent and 128 leg CO2, as shown in the figure. At the end of life the kettle is dumped, at an energy cost of 0.2 MJ.

FIGURE 7.5

The use phase of life consumes far more energy than all the others put together. Despite using it for only 6 minutes per day, the electric power (or, rather, the oil equivalent of the electric power) accounts for 88% of the total. Improving ecoperformance here has to focus on this use energy— even a large change, 50% reduction, say, in any of the other uses makes insignificant difference. Heat is lost through the kettle wall. Selecting a polymer with lower thermal conductivity or using an insulated double wall could help here; it would increase the embodied energy of the mate­rial bar, but even doubling this leaves it small. A full vacuum insulation would be the ultimate answer; the water not used when the kettle is boiled would then remain hot for long enough to be useful the next time it is needed. The seeming extravagance of air-freight shipping accounts for only 6% of the total energy. Using sea freight instead increases the distance to 17,000 km but reduces the transport energy per kettle to a mere 0.2% of the total.

This dominance of the use phase of energy (and of CO2 emission) is characteristic of small electrically powered appliances. Further examples can be found in the next case study and the exercises at the end of this chapter.