[peppercorn (OP)]

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?

[RD]

Like a chemical heating pad ? ...

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 264–289 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

[peppercorn (OP)]

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?

[Bored chemist]

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?
"264–289 kJ/kg."

[peppercorn (OP)]

Is that a trick question?
"264–289 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.