The cup or mug equation

I’ve been wondering about paper cups for a while now. Is it more environmentally friendly to drink from a disposable paper cup, or to drink from a mug and then use a dishwasher to clean it?

At first I figured it must all come down to energy: does it cost more energy to produce the cup, or to clean the mug? But of course there’s more to it than that. My simple equation becomes a bit more complicated when you factor in the embodied energy* in the paper cup (that is, the total energy that was required to manufacture, transport and dispose of the cup from start to finish of its life), and try to weigh that against the potential for reuse of the mug. Then you would have to balance that against the energy cost of each mug reuse, and factor in frequency of use of each, along with the volume and efficiency of production and running… and all of a sudden my notebook is covered with scribbles and I still have no idea what to drink my coffee out of.

I couldn’t find anything conclusive online, and decided to look further. First stop, Lazyweb. A few months ago I posted my question to Yahoo Answers, but didn’t really get anything more than guesses in response.

Then I asked my friend Dom, who is actually a bit of an authority when it comes to paper cups**. She has been collecting them for years now, I think planning to one day make some sort of action-art based work out of her collection. Any time I meet up with her, she’s usually got a couple of cups that she picked up stowed away somewhere. I emailed her about cups and mugs. She said:

I have to check on the latest, but it was, as long as the dishwasher is packed to its limit each time, and it is a well designed dishwasher i.e. energy/water efficient, the mug is best.

but, as part of some new cup work in the future, I will have to re-investigate the facts.

Pretty good, but still not conclusive. I sort of forgot about it for a while.

Then today, via a mailing list I’m on, the link I was looking for for ages. Here’s the science bit:

The energy of manufacture of reusable cups is vastly larger than the energy of manufacture of disposable cups (Table 1). In order for a reusable cup to be an improvement over a disposable one on an energy basis, you have to use it multiple times, in order to “cash in” on the energy investment you made in the cup. If a cup lasts only ten uses, then each use gets “charged’ for one-tenth of the manufacturing energy. If it lasts for a hundred uses, then each use gets charged for only one-hundredth of the manufacturing energy.

But in order to reuse a cup, it has to be washed. The efficiency of the dishwasher, and the efficiency of the energy system that powers it, determine how much energy is required for each wash. Hocking assumed a new, commercial dishwasher running on Canadian electricity, requiring about 0.18 MJ/cup-wash. The total amount of energy per use is this wash energy plus the appropriate fraction of manufacturing energy, depending on the cup’s lifetime. Figure 1 shows how the energies per use of the three reusable cups decline, the more you use them.

The lifetime needed for the energy per use of a reusable cup to become less than for a disposable cup, is called the ‘break-even point.” In Table 2, the break-even matrix shows how many uses are required for each reusable cup to do better than either disposable cup.

The results are extremely sensitive to the amount of energy the dishwasher requires for cleaning each cup. Hocking’s choice for the dishwasher, requiring 0.18 MJ/cup-wash, is barely less than the manufacturing energy of the foam cup, 0.19 MJ/cup. If Hocking had chosen even a slightly less energy-efficient dishwasher as his standard, then the reusable cups would never have broken even with the foam cup.

The lesson of this life-cycle energy analysis is that the choice between reusable and disposable cups doesn’t matter much in its overall environmental impact. One should use one’s best judgement.

Which is to say, today I brought a mug to work. They’re nicer to drink out of anyway.

  • A note on embodied energy: here’s something I posted on another blog in response to someone who claimed that buying a hybrid is worse for the environment than keeping your inefficient car because it costs more energy to manufacture the hybrid than it will save over the course of its lifetime:

It’s true that the manufacturing of ‘environmentally friendly’ products involves an energy spend in itself. For example, a solar panel would have to be used for 20 years to compensate for the energy footprint it cost to produce the panel.

However, this cost is the case with all products; just because an item does not recoup the full cost of it’s own production, doesn’t mean it has a greater negative impact than standard products. It just has less of a negative impact, which is better than nothing.

This highlights some of the complications of the ‘carbon neutrality’ idea. At the moment, the best one can realistically look to do is offset their effect of their consumption to a certain degree (although this is almost certainly better achieved through more direct and ongoing means than planting trees every time you catch a plane). Even this still leaves us individually and collectively far from compensating for our environmental effects, or living off the means of one planet (in Europe right now, we’re living off three planet Earth – that is, our rate of environmental consumption would require three planets to sustain).

It’s just kind of a bugbear of mine.

** As an interesting aside, Dom once met with the head of purchasing for a very large fast food franchise in the UK to try to get them to move from using plastic-coated paper cups to the squishy white foam type, which are apparently much greener. He said that they knew that the change would make sense environmentally and economically, but that the public had the opposite perception and the marketing campaign required to educate their customers and offset the damage to the company’s perceived greenness would cost too much.

— 06 Apr 2007