Drink containers coexist that are made from many different materials: glass, polyethylene, PET, aluminum, steel—Figure 9.1 shows them. Surely one must be a better environmental choice than the others? The audit of a PET bottle in Chapter 7 delivered a clear message: the phase of life that dominates energy consumption and CO2 emission is that embodied in the material of which a product is made. Embodied energies for the five materials are plotted in the upper part of Figure 9.2 (a plot of CO2 shows the same distribution). Glass has values of both that are by far the lowest. It would seem that glass is the best choice.
But hold on. These are energies per kg of material. The containers differ greatly in weight and volume. What we need are values per unit of function. So let’s start again and do the job properly, listing the design requirements. The material must not corrode in mildly acidic (fruit juice) or alkali (milk) fluids. It must be easy to shape, and—given the short life of a container—it must be recyclable. Table 9.1 lists the requirements, including the objective of minimizing embodied energy per unit volume of fluid contained.
The masses of five competing container types, the material of which they are made, and the embodied energy of each are listed in Table 9.2. All five materials can be recycled. For all five, cost-effective processes exist for making containers. All but one—steel—resist corrosion in the mildly acidic or alkaline conditions characteristic of bottled drinks. Steel is easily protected with lacquers.
FIGURE 9.2
Table 9.1 |
Design requirements for drink containers |
Function |
Drink container |
Constraints |
Must be immune to corrosion in the drink Must be easy and fast to shape Must be recyclable |
Objective |
Minimize embodied energy per unit capacity |
Free variables |
Choice of material |
Table 9.2 Data for the containers with embodied energies for virgin material |
||||
Container type |
Material |
Mass, Grams |
Embodied energy MJ/kg |
Energy/Liter MJ/Liter |
PET 400 ml bottle |
PET |
25 |
84 |
5.3 |
PE 1 liter milk bottle |
High-density PE |
38 |
81 |
3.8 |
Glass 750 ml bottle |
Soda glass |
325 |
15.5 |
6.7 |
Al 440 ml can |
5000 series Al alloy |
20 |
208 |
9.5 |
Steel 440 ml can |
Plain carbon steel |
45 |
32 |
3.3 |
That leaves us with the objective. The last column of the table lists embodied energies per liter of fluid contained, calculated from the numbers in the other columns of the table. The results are plotted in the lower part of Figure 9.2. The ranking is now very different: steel emerges as the best choice, polythene the next best. Glass (because so much is used to make one bottle) and aluminum (because of its high embodied energy) are the least good.
Postscript: In all discussion of this sort, there are issues of primary and of secondary importance. There is cost; we have ignored this because ecodesign was the prime objective. There is ease of recycling; the value of recycled materials depends on differing degrees on impurity pickup. There is the fact that real cans and bottles are made with some recycled content, reducing embodied energy of all five to varying degrees but not enough to change the ranking. There is the extent to which current legislation subsidizes or penalizes one material or another. And there is appearance: transparency is attractive for some products but irrelevant for others. However, we should not let these cloud the primary finding: that the containers differ in their life energy, dominated by material, and that steel is by far the least energy intensive.