Laundry day today. I rise, drain the three soapy twenty-five litre pails, then head to the laundry room and distribute the three pails amongst two machines and start a thirty-minute rinse cycle. I extract the “smalls” and “elastics” and pop the rest in tumble dryers.
Downstairs I spread the socks, briefs and other small items on a clothes-horse to dry out and to humidify my apartment. As if it needed it.
Charles Goodall’s book “How to Live a Low-Carbon Life” reminded me of the Latent Heat of Evaporation.
Latent Heat of Evaporation is the amount of heat needed to raise a liquid across the boundary to a vaporous state. In the case of water we are talking 2,265 units. Units of what? For us it doesn’t matter. What does matter is that it takes 2,265 units to convert a gram of water in liquid form at 100ºc to (the same gram of) vapour at 100ºc.
Note that the temperature doesn’t change; the 2,265 units of energy are what it takes to break the molecules of water away from each other and free them as free-standing molecules of water in vaporous form, or “water-vapour”.
It took ONE unit to raise the liquid water from 98ºc to liquid water at 99ºc, and it took ONE unit of energy to raise the liquid water from 99ºc to 100ºc, and it will take ONE unit of energy to lift the water in vapour form from 100ºc to 101ºc, and so on.
But it takes a whopping TWO THOUSAND, TWO HUNDRED AND SIXTY FIVE units of energy to change the water from liquid from to vapour while the whole lot remains at the same temperature of 100ºc.
Back to my clothes on the clothes horse.
Over the next twenty-four hours, the air circulating into, around, and then out of my apartment will be chilled by the amount of energy required to boil off the water from my damp spun-dry clothes.
That means that although today’s forecast says 12ºc and right now the air in my apartment is a heady 23ºc, drying my clothes on a clothes horse will reduce the temperature in my apartment dramatically.
Now you know why I crank up the heat in my apartment at the time I pop the laundry in the washing machines.
Meanwhile, back at the ranch, those two tumble dryers are consuming vast amounts of energy to boil out the water from my spun-dry bed sheets, towels, and dress shirts.
Energy is energy, and it doesn’t go away. I can reduce use of electricity by drying smalls on a clothes-horse. But I compensate for that by needing electricity to re-heat my apartment.
There is, you should know, a latent heat figure for the boundary between liquid and solid.
In the case of water, think 334 units. Not as bad as the 2,265 for evaporation, but bad enough.
At the time your liquid water at 0º freezes to form ice at 0º, it requires 334 units of energy to cross that boundary.
Making ice cubes is expensive in terms of energy.