[Mobile Master Tech]

Some additives are expensive, some aren't. Hence why I am trying to find an effective, cost efficient method. Strontium sulfate is way cheaper than silver, etc..

I'm sure there is a satisfactory answer, PCM companies are doing it successfully. It wouldn't be the first time someone found a way to do it better (or easier for DIY) when others said it couldn't be done. I don't think my goal is unrealistic.

If any ingredient is benign enough to not panic over if it spilled on the floor, it is good enough. There are far worse things we keep in our houses, including the flammable wood they are made of.

I am not trying to lower the melt temperature or suppress solidification. 136F melt temp is perfect. Not too hot for a GSHP pulling from a 70F or warmer source, hot enough that I will always be able to get 125F or warmer water out of the heat exchanger if the tank stays above 130F , hot enough to prevent Legionella. If my tank temp drops lower than 130, there will be times I get less than 125F output. SAP doesn't complete it's solidification until about 122F due to supercooling, but almost all of it's solidification happens between 130-136F. 6F I can handle.

With a 3.5" cross section and design features allowing for volume change, I think the hot-fill food product PET bottles are a perfect, durable (and free!) PCM container. Even if full solidification took 6 hours due to the PCM self insulating, it will be fine. I won't be using 200+ kbtu that fast when the tank is supplemented by a 2 ton GSHP.

I read of tests of paraffin in 3" dia plastic tubes that was able to completely melt/charge in 2 hours and completely solidify/discharge in 5 hours. The bulk of the heat exchange happened far quicker.

For hydrates, having a PCM and it's container with a specific gravity greater than 1 means the bottles can be stacked in the tank with no need for mechanical restraint, making it easy to rejuvenate someday if needed.

If using non-corrosive pcms, the heavygage foil down the center of the bottle will speed heat transfer up, since the necks of the bottles are 1-1.5" diameter.

Thanks so much for your input, Jeff. I look forward to hearing of any actual usage experience if anyone has it.