Materials and the Environment: Eco-Informed Material Choice

Energy and CO2 footprints of energy, transport, and use

Energy is used to make materials and to shape, join, and finish them to make products. Energy is used to transport the products from where they are made to where they are used, and the products themselves use energy during their lifetimes; some use a great deal. This energy is provided pre­dominantly by fossil fuels and by electric power, much if it generated from fossil fuels. These sources differ in their energy intensity and carbon release.

Energy intensities. Energy intensities of fossil fuels and their carbon foot­prints are listed in Table 6.5. Reading across, there is the fuel type, the oil equivalent (OE, the kg of crude oil with the same energy content as 1 kg of the fuel), the energy content per unit volume and per unit weight, and the CO2 release per unit volume, weight, and MJ. The units (kg, KJ, etc.) used here are those standard to the SI system. Conversion factors to other sys­tems (lbs, Btu, etc.) can be found at the end of this book.

The oil equivalence of electric power. Electricity is the most convenient form of energy. Today most electricity is still generated by burning fossil fuels, but the pressure on these fuels and the problems caused by the emis­sions they release are urging governments to switch to nuclear and renewable sources, and most of these generate electric power. The energy mix in a country's electricity supply is the proportional contribution of each source

to the total. The first four columns of Table 6.6 give examples of this mix for countries that span the extremes. Australia relies on fossil fuels for almost all its electricity; in France it is predominantly nuclear, and in Norway it is almost wholly hydroelectric. The United States and Japan are

the world's two largest economies, but the two most populous and fastest growing are China and India.

The relevant numbers from an environmental point of view are those in the last three columns: the efficiency of electricity generation from fossil fuels and the oil equivalence and CO2 release per unit of delivered electrical power. These figures differ greatly from country to country, mainly because of the differing energy mixes, to a lesser extent because of the differing efficiencies. We'll use these numbers in later chapters. To keep things simple we will use values for a "typical" developed country with an energy mix of 75% fossil fuel and a conversion efficiency of 38%, giving an oil equivalence of 7 MJ (or 0.16 kg) and a carbon footprint of 0.5 kg CO2 per kW. hr.

Transport. Manufacturing is now globalized. Products are made where it is cheapest to do so and then transported, frequently over large distances, to the point of sale. Transport is an energy-conversion process: primary energy (oil, gas, coal) is converted into mechanical power and thus motion, some­times with an intermediate conversion to electrical power. As in any energy conversion process, there are losses. We express the energy of transport as an energy per tonne. km, carrying with it an associated CO2 footprint per tonne. km.

Table 6.7 lists data for these quantities, drawn from reports of the U. S. Department of Transport, the U. K. Network Rail, and Transport Watch U. K. Transport energy and carbon footprint are calculated by multiplying the weight of the product by the distance traveled and the fuel-vehicle coef­ficients listed in the table.

Use energy. Many products consume energy, or energy is consumed on their behalf, during the use phase of life. As we shall see in Chapter 7, this use-phase energy is often larger than that of any other. Most of it derives from fossil fuels, and some is consumed in that state, as primary fossil fuel. Much is first converted to other forms of energy before it is used, of which the most obvious is electricity.

When fossil fuels are used directly (as in the use of gasoline to power cars), the primary energy and CO2 can be read directly from Table 6.5 . When instead it is used as electricity, relevant fossil fuel energy and CO2 depend on the energy mix and generation efficiency, and these differ from country to country. It is then necessary to convert the electrical energy, usually given in kW. hr, to MJ and CO2 oil equivalent by multiplying by the conversion factors like those of Table 6.6.