Passive Annualized Heat Storage/Annualized Geo Solar-Don't Waste That Heat!

[jeff5may]

I found some interesting data about CaCl PCM in this patent:

Patent EP0478637A1 - Calcium chloride hexahydrate formulations for low temperature heat storage ... - Google Patents

It describes a formulation that is stable enough for the inventor's purposes: mainly greenhouse thermal storage. Highlights include adding the following ingredients to industrial CaCl + 6H2O:

(i) the added strontium chloride comprises about 0.3 per cent by weight of the calcium chloride hexahydrate (nucleating agent - minimum of 0.1%);
(ii) the added fumed silica comprises about 0.1 per cent by weight of the calcium chloride hexahydrate (thickening/gelling agent - minimum of 0.02%);
(iii) the excess water over the stoichiometric quantity included in the calcium chloride hexahydrate comprises about 1.5 per cent by weight of the calcium chloride hexahydrate; and
(iv) sodium chloride is also added, the added sodium chloride being about 0.4 per cent by weight of the calcium chloride hexahydrate.

Such a formulation has a solid/liquid transition temperature of 29.6 +- 0.2 C. This transition temperature can be reduced down to about 22 C by the addition of up to 10 wt per cent each of ammonium chloride and potassium chloride.

This range is a little low for the heat store being considered here, but for a bulk store that provided (ultra) low-grade heat, this stuff would stay above 85 degF for a long time.

With the sodium acetate pcm, the relevant papers I blew through that referenced affordable, inorganic mixtures pointed to the same basic formulation, which melts at 58 C (137 degF):

sodium acetate trihydrate - 100 units by weight;
tetrasodium pyrophosphate decahydrate (Na4P207.10H20) - 2 units by weight;
fumed silica - 2 units by weight;
excess water - 0.27 mole per mole of sodium acetate trihydrate.

Additionally, urea can be added to depress the phase change temperature down to 30 degC. One paper noted that the fumed silica not only acted as an emulsifier, but also aided in reducing the supercooling effect.

Another paper omitted the pyrophosphate and silica, using cellulose gum as an emulsifying agent. The resulting (edible) mixture could be heated, and would supercool freely to frigid temperatures without changing phase, storing the latent heat (with claims of zero latent heat loss) for later discharge at or near the phase change temperature. THIS SOUNDS EXACTLY LIKE THE STUFF USED IN MY HOT-HANDS BAGS.

Nearly all of the papers I found from the last 10-15 years took these two recipes and incorporated other materials into them to try to improve something. Some were incorporating graphite or metal powders to increase thermal conductivity of the gel or paste. Others omitted the silica and made some sort of polymer out of the recipe at about 80% recipe, 20% polymer. Still others took the recipe and encapsulated it somehow, to make foam or pellets or beads or microbeads or nanobeads. Very few had any luck trying to modify the above recipes.