• Zerush@lemmy.ml
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    3 years ago

    A water bottle (~1 liter Water) need a lot of energy to heat the Water and stores the heat for1 Houre, not more. This patches don’t need any energy to heat, no more than the humidity of the environment, to start an exothermic process that lasts between 8-12 Hours, depending on the size of the patch. It requires no more energy than taking it out of the package so that it starts to heat up on its own. It does not require any external energy input, such as heating water electrically, with gas or other fuel, to heat it first, as is the case with the hot water bottle.

    • rcbrk@lemmy.ml
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      3 years ago

      You’re not considering the energy required to smelt the iron.

      Iron filings (in a collected quantity high enough to make manufacturing these heat packs worthwhile) are not a waste product, they are recycled – saving the smelting of that much new iron.

      Sawdust+iron heat packs are a very useful and non-hazardous product, for sure, but aside from situations where a hot water bottle is impractical, hot water bottle still wins.