Case study: a coffee maker

The 640 Watt coffee maker shown in Figure 7.6 makes four cups of cof­fee in 5 minutes (requiring full power) and then keeps the coffee hot for a subsequent 30 minutes, consuming one sixth of full power. The housing

is injection-molded polypropylene, the jug is glass, there are a number of small steel and aluminum parts, a heating element, and, of course, a cable and plug. The control system has some simple electronics and an LED indicator. Each brew requires a filter paper that is subsequently discarded. We take the life of the coffee maker to be five years, over which time it is used once per day. Table 7.6 summarizes the bill of materials. At the end of life the product is returned to the maker, where it is disas­sembled and the polypropylene parts are separated and recycled.

Figure 7.7 shows the breakdown, calculated as in the previous case study. Here electronics (Table 6.4) are included, assigning them an energy of 3000 MJ/kg. The first three bars—materials (170 MJ), manufacture (14 MJ), and transportation (5 MJ)—are all small compared to the energy of use. One use cycle uses the equivalent of 10 minutes of full 640 W power. Over five years this becomes 194 kWhr of electrical power, which, if generated in the United States (Table 6.6), corresponds to an equivalent oil consumption of 1380 MJ and CO2 emission of 105 kg. Each use also consumes one filter paper—1,825 of them over life—each weighing 2 grams. The embodied energy of paper, from the data sheet in Chapter 12, is 28 MJ/kg, so this represents an additional 100 MJ. At the end of life the polypropylene is recycled, requiring 40 MJ/kg to do so, thereby saving the difference between this and the embodied energy of 94 MJ/kg. (54 MJ/kg)—a return of 49 MJ per unit.

There is nothing that can be done to recover the electrical power once it is used, but it is possible to reduce it by replacing the glass jug with a stain­less steel vacuum jug, thereby eliminating the need for a heater to keep the coffee warm. The embodied energy of stainless steel is three times greater than that of glass, so it is necessary to check that this redesign really does save energy over the product’s life—a task left to the exercises at the end of this chapter.