Figure 7.1 shows the procedure for the eco-audit of a product. The inputs are of two types. The first are drawn from a user-entered bill of materials, process choice, transport requirements, duty cycle (the details of the energy and intensity of use), and disposal route, shown at the top left. Second, data for embodied energies, process energies, recycle energies, and carbon inten­sities are drawn from a database of material properties; those for the energy and carbon intensity of transport and the use energy are drawn from lookup tables like those in Chapter 6 (top right of the figure). The outputs are the energy or carbon footprint of each phase of life, presented as bar charts and in tabular form. The procedure is best illustrated by a case study of extreme simplicity—that of a PET drink bottle—since this allows the inputs and outputs to be shown in detail. The later case studies are for more complex products, presented in less detail.

The inputs. One brand of bottled water—we will call it Alpure—is sold in 1-liter PET bottles with polypropylene caps (see Figure 7.2). One bottle weighs 40 grams; its cap weighs 1 gram. The bottles and caps are molded, filled with water at a source of sparkling purity located in the French Alps, and transported 550 km to London, England, by 14-tonne truck. Once there, the bottles are refrigerated for two days, on average, before appearing on the tables of the restaurant where they are consumed, adding signifi­cantly to the diners ‘ bills. The restaurant has an environmental policy: all


plastic and glass bottles are sent for recycling. We use these data for the case study, taking 100 bottles as the unit of study, requiring 1m3 of refrigerated space.

The eco-audit procedure has five steps, described here for energy. An audit for CO2 follows the same steps. [26]

Table 7.1

The bill of materials and processes for a PET bottle (100 bottles)

Bill of materials








Mass m kg

Energy Hm MJ/kg*

CO2 kg/kg*

Energy Hp MJ/kg*

CO2 kg/kg*

Bottle, 100 units









100 units








Dead weight (100 liters of water)






*From the data sheets of Chapter 12.

retrieved from the database—here, the data sheets of Chapter 12, using the means of the ranges listed there (right side of the table). Multiplying the mass of each component by its embodied energy and summing gives the total material energy—the first bar of the bar chart.

2. Manufacture. The audit focuses on primary shaping processes since they are generally the most energy-intensive steps of manufacture. These are listed against each material, as in Table 7.1. The process energies and CO2 per unit mass are retrieved from the database, as on the right side of the table. Multiplying the mass of each component by its primary shaping energy and summing gives an estimate of the total processing energy, the second bar of the bar chart.

On a first appraisal of the product, it is frequently sufficient to enter data for the components with the greatest mass, accounting for perhaps 95% of the total. The residue is included by adding an entry for "residual components," giving it the mass required to bring the total to 100% and selecting a proxy material and process. "Polycarbonate" and "molding" are good choices because their energies and CO2 lie in the midrange of those for commodity materials.

3. Transport. This step estimates the energy for transportation of the product from manufacturing site to point of sale. For the water bottle, this is dominated by the transport of the filled bottles from the French Alps to London, a distance of 550 km. The energy demands of transport modes were described in Chapter 6 (Table 6.7); that for a 14-tonne truck is 0.9 MJ/tonne. km. Multiplying this figure by the mass of the product and the distance travelled provides the estimate. It is not just the bottles that travel 550 km, it is also the water they contain. This is included in the bill of materials of Table 7.1 to ensure that its mass is included in auditing the transport phase.

4. The use phase. The use phase requires a little explanation. There are two different classes of contribution:

■ Some products are (normally) static but require energy to perform their function; electrically powered products such as hairdryers, electric kettles, refrigerators, power tools, and space heaters

are examples. Even apparently unpowered products such as household furnishings or unheated buildings still consume some energy in cleaning, lighting, and maintenance. The first class of contribution, then, relates to the power consumed by, or on behalf of, the product itself.

■ The second class is associated with transport. Products that form part of a transport system or are carried around in one add to its mass and thereby augment its energy consumption and CO2 burden. The transportation table, Table 6.7, lists the energy and CO2 penalty per unit weight and distance. Multiplying this figure by the product weight and the distance over which it is carried gives an estimate of the associated use-phase energy and CO2.

All energies are related back to primary energy, meaning oil, via oil – equivalent factors for energy conversion discussed earlier (Tables 6.5 and 6.6). Retrieving these and multiplying by the power and the duty cycle—the usage over the product life—gives an estimate of the oil- equivalent energy of use.

The PET bottle is a static product. Energy is consumed on its behalf via refrigeration for two days. The energy requirements for refrigeration, based on A-rated appliances, are 10.5 MJ/m3 per day for refrigeration at 4°C and 13.5 MJ/m3 per day for freezing at -5°C,

using electrical power in both cases. The use energy is chosen to give the value for refrigeration.

5. Disposal. As explained in Chapter 4 there are five options for disposal at the end of life: landfill, combustion for energy recovery, recycling, re-engineer and reuse (Figure 4.2). A product at end of first life has the ability to return part or all of its embodied energy. This at first sounds wrong – much of the "embodied" energy was not embodied at all but was lost in the inefficiencies of the processing plant, and even when it is still there, it is, for metals and ceramics, inaccessible: the only easy way to recover energy directly is by combustion, not an option for steel, concrete or brick. But think of it another way. If the materials of the product are recycled or the product itself is re-engineered or reused, a need is filled without drawing on virgin material, giving an energy credit. Carbon release works in the same way, with one little twist: one end-of-life option, combustion, recovers some energy but in doing so it releases CO2.

Table 7.2 lists the path for each option and first-order estimates for the energies involved to allow their approximate evaluation in the case studies and exercises that follow. The energy cost of transport to landfill is negligi­ble compared with the other energies associated with life. Recycling recov­ers the difference between the original embodied energy and the energy of recycling. Re-engineering and reuse recover (by filling a need without using new material) almost all the original embodied energy. The data sheets of Chapter 12 provide estimates for recycle energy Hrc as well as for the origi­nal embodied energy Hm. Where no data appear for recycling it is because the material cannot be recycled. The carbon credit is treated in a similar

Table 7.2 Disposal route and energy balance

Disposal route

First-order estimate for energy

1. Landfill. Collect and transport to landfill site.


2. Combust for heat recovery. Collect, sort combustibles, combust.

Recover calorific value.

3. Recycle. Collect, sort by material family and class, recycle.

Recover difference between embodied energy Hm and recycle energy Hrc.

4. Re-engineer. Collect, dismantle, replace or upgrade components, re-assemble.

Recover most of embodied energy Hm. Use 0.9 Hm as estimate.

5. Reuse. Market as "pre-owned" product via trading outlets, websites etc.

Recover embodied energy Hm.

Table 7.3

Recycle energy and CO2 for PET


Material Mass m kg

Recycle energy Hrc MJ/kg*

Recycle CO2 kg/kg*

m. Htot MJ

m.(CO2)tot kg

Bottle, 100 units






*From the data sheets of Chapter 12.

The energy and the carbon footprint bar charts for bottled water per 100 units.

way. Recovered energy and carbon credit appears as negative on the eco­audit bar-charts.

The outputs. Figure 7.3 and Table 7.4 show the outputs. What do we learn? The largest contribution to energy consumption and CO2 generation

derives from the production of the polymers used to make the bottle. The second largest is that of the short, two-day, refrigeration. The seem­ingly extravagant part of the life cycle—that of transporting water, 1 kg per bottle, 550 km from the French Alps to the diners ‘ tables in London—con­tributes 10% of the total energy and 17% of the total carbon. If genuine concern is felt about the eco-impact of drinking Alpure water, then (short of giving it up) it is the bottle that is the primary target. Could it be made thinner, using less PET? (Such bottles are 30% lighter today than they were 15 years ago.) Is there a polymer that is less energy intensive than PET? Could the bottles be made reusable and of suffi­ciently attractive design that people would want to reuse them? Could recycling of the bottles be made easier? These are design questions, the focus of the lower part of Figure 3.11. Methods for approaching them are detailed in Chapters 8 and 9.

An overall reassessment of the eco-impact of the bot­tles should, of course, explore ways of reducing energy and carbon in all phases of life, not just one, seeking the most efficient molding methods, the least energy-intensive trans­portation mode (32-tonne truck, barge) and—an obvious step—minimizing the refrigeration time.