Imagine a personal heating system that works indoors as well as outdoors, can be taken anywhere, requires little energy, and is independent of any infrastructure. It exists – and is hundreds of years old.
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.
Huh. So maybe heat packs are a reasonable use of scrap iron’s embodied energy after all. Assuming you have a sufficient source of uncontaminated steel filing waste and that it’s economical to collect and process into heat packs.
…But only if you’re heating your water using fossil fuels using an inefficient method! If your water is heated using solar or waste heat capture or a heat pump[4], which would swing the balance way over to hot water bottles again.
I know that iron filings are recycled in foundries, if not used for these patches or for other uses. I am referring to waste that is used by residual products from other manufacturing, which is obviously the case with iron filings and sawdust.
It is true that a well-wrapped hot water bottle can store a little more heat than 1 hour, but not by far the 12 hours of these patches, which are also much more practical in their application, since they can be worn for a day cool comfortably under clothing, or on a sore part of the body, as they are primarily intended, without getting in the way of a job or activity during the day.
For a hot water bottle, you first need to heat the water in the kitchen, which obviously implies an energy expenditure that we have to invest first and that is not necessary in these patches, which in themselves are a source of energy due to a chemical process. .
Besides, they are still recyclable afterward, since the iron oxide, product of this chemical reaction, is still usable in foundries, since in any case iron oxide is the form in which this element appears in nature.
These patches are obviously a 100% ecological and recyclable product, containing neither before nor after toxic or polluting residues, not necessarily the same in a hot water bottle, in the manufacture of which various chemical and plastic products are used, which are difficult to recycle (PVC in the cap, several toxic products used in the rubber…)
Iron oxide is not recyclable in a foundry. It would need to go back to a smelter to be smelt into iron again, which I assume is about the same efficiency as smelting iron ore. (Smelting hematite requires ~20 GJ / tonne [1] = ~ 4460 kJ / 0.223kg).
Spent iron oxide heat packs are probably best thrown in the compost.
I agree that an iron powder heat pack can be far more practical where a highly portable slow emission of heat are both important for the application. The heat output for something 10% the weight of a water bottle is pretty impressive:
1 litre water bottle cooling from 80 C to 40 C:
~4kJ/kg⋅K[2] * 40 K * 1 kg = 160 kJ
100g heat pack containing 50g iron[3]:
50g * 1648kJ / 223g = 369 kJ [4]
Iron powder heat packs need to be stored in suitable packaging (usually plastic) to protect them from moisture and oxygen until their single use.
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.
So… I really don’t know chemistry, and these aren’t the highest quality references, but here goes:
Huh. So maybe heat packs are a reasonable use of scrap iron’s embodied energy after all. Assuming you have a sufficient source of uncontaminated steel filing waste and that it’s economical to collect and process into heat packs.
…But only if you’re heating your water using fossil fuels using an inefficient method! If your water is heated using solar or waste heat capture or a heat pump[4], which would swing the balance way over to hot water bottles again.
I know that iron filings are recycled in foundries, if not used for these patches or for other uses. I am referring to waste that is used by residual products from other manufacturing, which is obviously the case with iron filings and sawdust.
It is true that a well-wrapped hot water bottle can store a little more heat than 1 hour, but not by far the 12 hours of these patches, which are also much more practical in their application, since they can be worn for a day cool comfortably under clothing, or on a sore part of the body, as they are primarily intended, without getting in the way of a job or activity during the day.
For a hot water bottle, you first need to heat the water in the kitchen, which obviously implies an energy expenditure that we have to invest first and that is not necessary in these patches, which in themselves are a source of energy due to a chemical process. . Besides, they are still recyclable afterward, since the iron oxide, product of this chemical reaction, is still usable in foundries, since in any case iron oxide is the form in which this element appears in nature.
These patches are obviously a 100% ecological and recyclable product, containing neither before nor after toxic or polluting residues, not necessarily the same in a hot water bottle, in the manufacture of which various chemical and plastic products are used, which are difficult to recycle (PVC in the cap, several toxic products used in the rubber…)
Iron oxide is not recyclable in a foundry. It would need to go back to a smelter to be smelt into iron again, which I assume is about the same efficiency as smelting iron ore. (Smelting hematite requires ~20 GJ / tonne [1] = ~ 4460 kJ / 0.223kg). Spent iron oxide heat packs are probably best thrown in the compost.
I agree that an iron powder heat pack can be far more practical where a highly portable slow emission of heat are both important for the application. The heat output for something 10% the weight of a water bottle is pretty impressive:
Iron powder heat packs need to be stored in suitable packaging (usually plastic) to protect them from moisture and oxygen until their single use.
A hot water bottle needn’t be rubber or plastic.