Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: peppercorn on 30/09/2009 16:14:50

Title: If I can store electricity through chemical reactions why not heat?
Post by: peppercorn on 30/09/2009 16:14:50
Heat is a difficult thing to store for any reasonable time - especially on small-scale applications & where weight is an issue: i.e. A large heavily insulated tank.

So, of the many reversible chemical reactions that involve heat (endothermic & exothermic) - is there one (or a class) that would allow light, compact storage from a heat source?
Title: If I can store electricity through chemical reactions why not heat?
Post by: RD on 20/10/2009 20:33:18
Like a chemical heating pad ? ...

Quote
Sodium acetate is also used in consumer heating pads or hand warmers and is also used in hot ice. Sodium acetate trihydrate crystals melt at 58 C, dissolving in their water of crystallization. When they are heated to around 100 C, and subsequently allowed to cool, the aqueous solution becomes supersaturated. This solution is capable of supercooling to room temperature without forming crystals. By clicking on a metal disc in the heating pad, a nucleation center is formed which causes the solution to crystallize into solid sodium acetate trihydrate again. The bond-forming process of crystallization is exothermic, hence heat is emitted. The latent heat of fusion is about 264289 kJ/kg. Unlike some other types of heat packs that depend on irreversible chemical reactions, sodium acetate heat packs can be easily recharged by boiling until all crystals are dissolved. Therefore they can be recycled indefinitely.
http://en.wikipedia.org/wiki/Sodium_acetate
Title: If I can store electricity through chemical reactions why not heat?
Post by: peppercorn on 21/10/2009 14:31:45
Like a chemical heating pad ? ...
Yes, of course! Thanks RD!
Used to have one in fact -stuck it in the microwave to 'reset'.

Anyone want to hazard a guess what energy density they have?
Title: If I can store electricity through chemical reactions why not heat?
Post by: Bored chemist on 21/10/2009 18:46:01
Like a chemical heating pad ? ...
Yes, of course! Thanks RD!
Used to have one in fact -stuck it in the microwave to 'reset'.

Anyone want to hazard a guess what energy density they have?

Is that a trick question?
"264289 kJ/kg."
Title: If I can store electricity through chemical reactions why not heat?
Post by: peppercorn on 22/10/2009 12:34:06
Is that a trick question?
"264289 kJ/kg."

Ah, you've spotted my deliberate mistake! [note to self: must pay attention!]

Wikipedia rates high power NiMH car batteries 250 kJ/kg.  Obviously electricity is a much wider application than heat, but the comparison is interesting nevertheless.