Which bottle is best? selection per unit of function

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.