I recently saw an article that claims that heating water to 40° for washing laundry consumes around 5–10kWh per load.
However, it turns out that heating up water doesn't consume energy. You need energy to do it, to be sure — but that energy is still in the water after you heat it up. The Carnot limit prevents you from recycling most of that heat into exergy to, say, drive the washing machine motor, but nothing is preventing you from transferring that heat from dirty water into clean water, or into a heat reservoir that holds it until your next wash. First, is such a reservoir possible? Most definitely. You can buy fractionated paraffin wax that melts at more or less whatever temperature you want, to a few degrees of precision, with a heat of fusion near that of water ice. A big Thermos inside the washing machine, full of paraffin or another phase-change material, could hold a laundry load's full of heat for several days, if not a week. But how to get the heat into it? The key is a clever little machine called a "countercurrent heat exchanger": in its simplest form, two parallel long pipes that have been welded together, with water, or some other fluid, flowing in opposite directions through them. As the hot water flows in one direction, it loses heat to the cold water flowing in the other direction — and the cold water, you might say, loses its cold to the hot water. When the formerly hot water exits, it's just a little warmer than the cold water going in, and similarly, the formerly cold water exits just a little cooler than the hot water was originally. The countercurrent heat exchanger is part of many animals (a nose is a variant that needs only one tube, which acts as a heat reservoir, and the reason you don't get hypothermia just from breathing) and its use in human engineering goes back decades, if not centuries. And indeed devices like cement kilns and Passivhausen use CCHE to reduce heat loss to manageable levels. So why doesn't your washing machine use one? Maybe because 5-10kWh per load is maybe a dollar at residential electricity prices, and saving US$30 a year isn't worth the extra machinery, fragile glass, and extra space. Essentially any process that heats something up to a high temperature, then cools it back down, can have its efficiency improved by the same principle. Firing pottery, making glass, casting metal, powder-coating metal, baking bread, sterilizing water, making cement (as mentioned above), and so on. Many of these processes are not inherently energy-consuming, or inherently consume only a tiny fraction of the energy that we currently spend on them. Kragen -- To unsubscribe: http://lists.canonical.org/mailman/listinfo/kragen-tol
