Case study: a portable space heater

The space heater shown in Figure 7.8 is carried as part of the equipment of a light goods vehicle used for railway repair work. It burns 0.66 kg of liquid propane gas (LPG) per hour, delivering an output of 9.3 kW (32,000 BTU). The air flow is driven by a 38 W electric fan. The heater weighs 7 kg. The (approximate) bill of materials is listed in Table 7.7. The prod­uct is manufactured in India and shipped to the United States by sea freight (15,000 km), then carried by 32-tonne truck for a further 600 km

FIGURE 7.8

Table 7.7 LPG space heater bill of principal materials, life: three years

Component

Material

Process

Mass (kg)

Mat. CO2 kg/kg*

Proc. CO2 kg/kg*

Heater casing

Low C steel

Def. processing

5.4

2.5

0.19

Fan

Low C steel

Def. processing

0.25

2.5

0.19

Heat shield

Stainless steel

Def. processing

0.4

5.1

0.27

Motor, rotor and stator

Iron

Def. processing

0.13

1.0

0.21

Motor, conductors

Copper

Def. processing

0.08

5.7

0.17

Motor, insulation

Polyethylene

Polymer

0.08

2.1

0.51

extrusion

Connecting hose,

Natural rubber

Polymer

0.35

1.5

0.61

2 meters

molding

Hose connector

Brass

Def. processing

0.09

6.3

0.18

Other components

Proxy material:

Proxy process:

0.22

5.6

0.86

polycarbonate

polymer molding

Total mass

7.0

*From the data sheets of Chapter 12.

to the point of sale. It is anticipated that the vehicle carrying the heater will travel, on average, 420 km per week, over a three-year life, and that the heater itself will be used for 2 hours per day for 10 days per year. At end of product life, the carbon steel components are recycled.

This is a product that uses energy during its life in two distinct ways. First there is the electricity and LPG required to make it function. Second there is the energy penalty that arises because it increases the weight of the vehicle that carries it by 7 kg. How much CO2 does the product release over its life? And which phase of life releases the most?

Figure 7.9 shows the CO2 emission profile. The first two bars—mate­rials (19 kg) and manufacture (1.7 kg)—are calculated from the data in the table. Transport releases only 3.5 kg per unit. The power consumed by burn­ing LPG for heat (9.3 kW) far outweighs that used to drive the small electric fan motor (38 W), so it is the CO2 released by burning LPG that we evaluate here. It is less obvious how the static use for generating heat, drawn for only 20 hours per year, compares with the extra fuel consumed by the vehicle because of the product weight—remembering that, as part of the equipment, it is lugged over 22,000 km per year. The figure shows that the CO2 of use outweighs all other contributions, here accounting for 90% of the total, as it does with most energy-using products. Of this, 34% derives from burn­ing gas and 76% from the additional fuel consumed by carrying the heater to the sites where it is used. The CO2 burden to recycle steel, from the data sheet in Chapter 12, is about 0.7 kg/kg, saving the difference between this and that for primary production (2.5 kg/kg)—a net saving of 10 kg.