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.

[Mobile Master Tech]

Thanks, Jeff!

What I have found so far:

Some work by Heinz & Streicher showed that over a 15C temp swing, SAT with just enough additives/extra water to stabilize and prevent supercooling (5% or less additives, and 2-3% extra water) held 7-8x more heat than water alone. Adding graphite, etc to increase conductivity reduced this to about 5x of water. Others said more than 5% extra water started to significantly limit latent heat storage.

I'm not worried about improving conductivity. My PCM containers will have 3.5" cross sections and the fastest I could use up that much heat is over an 8hour period. I'd rather have more storage capacity.

Everybody seems to recommend 2-5% of a thickening agent to prevent phase separation-bentonite, fumed silica, superabsorbent polymer like in diapers (SAP), carboxymethylcellulose like the food additive (CMC) are most common, but everything that could be put in a kitchen sink has been tried. They also seem to recommend up to 2% of a nucleating agent or combination of nucleating agents to prevent supercooling, and again nearly everything has been tried, including silver nanoparticles. 3% extra water helps prevent crystallization.

Pure SAT with no nucleators or thickeners can store heat indefinitely, and yes, that's probably the formulation in the heating pads. Great seasonal storage idea, but I think I like dirt better for that, as it would be expensive to only cycle a few times per year.

CMC seems to work best as a thickener, and strontium sulfate seems to work best as a nucleator, demonstrating 0-2C supercooling. Boy, lots of patents and choices. What works best, though?

My goals: additives that are cheap and common which make SAP work indefinitely or at least 1000 cycles with little degradation, backed up by practical experience.

Hope some others chime in as well!

[jeff5may]

"My goals: additives that are cheap and common which make SAP work indefinitely or at least 1000 cycles with little degradation, backed up by practical experience..."

You realize that these goals are at odds with each other, don't you? A more predictable and/or stable formulation can cost as much as you would care to spend...

From what I have seen so far, it looks like the nucleating agent will be much more expensive than the acetate(if it's exotic), and maybe more than the emulsifier(depending on ultimate stability vs price). For your purpose of imposing a lower temperature boundary on the phase change, it seems this ingredient will be the most important to consider. The more rigid your spec, the more important this ingredient becomes.

I like the pyrophosphate, since it is a food additive. In case some pcm containers burst or leakage occurred, there would be no cause for alarm of contamination. Same thing with the cellulose gum. In fact, knowing that the TETRA form worked, I might try some experiments of my own with baking powder. Who knows, the stuff might actually work.

OTOH, if a nucleating agent exists that vastly improves phase change consistency that is not so benign, I might choose a more impervious sequestering agent. Much modern work has been done in this realm. When mixed hot, it has been proven that one can make jelly, rubbery or plastic solid polymers that remain stable pretty darn easily out of the acetate and nucleating agent. The stuff is being made into building materials today that can be cut, drilled, or nailed through with no measurable leakage or performance degradation.

Another thing I have found in general is that the organic(waxes and fatty acids) pcm's serve a better purpose as a blanket or jacket of some sort. In their solid form, the thermal conductivity is low enough to consider them as a layer of insulation. So as they cool to below their meta-critical temperature, they help hold the heat inside the container. The salt formulations don't insulate so well, so they are better purposed as you propose, inside the container.

[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.

[Mobile Master Tech]

HERE is an interesting paper from Osaka University in 1985 describing SAT behavior. A few things jump out at me.

One, phase separation isn't quite as big a deal as I thought. While guaranteed grade SAT quickly drops to 160J/g latent heat then stabilizes, technical grade SAT with 1% Tetrasodium Pyrophosphate Decahydrate (used in toothpaste and cat food), quickly drops also but stabilizes at 200J/g. The trend line shows that it is unlikely to degrade below that. Adding polyvinyl alcohol, acetone and liquid paraffin as thickeners kept SAT stable at 230J/g practically forever.

Two, many nucleating agents work, but none will until first forced into crystallizing the SAT, apparently by lowering the temp. All can be caused to fail by overheating (don't sustain any temp over 69C, and don't take above 79C at all), or by poisoning due to contamination with certain compounds. AC, I wonder if this might be why your Glauber's salt didn't work as described?

[Mobile Master Tech]

Couple of nuggets from "Thermal Storage of Solar Energy", a conference compilation by Ouden:

1. Adding aluminum strips (1.6% by volume) cut heat addition/extraction time in paraffin PCM's fourfold.

2. SAT changes volume less than 5% over 20-80C, well within what hot-fill beverage packaging can handle.

[Mobile Master Tech]

I tested my 64oz Ocean Spray bottles. They actually hold 67oz filled to the brim. I can squeeze out 7 oz of water, or 11% volume change, and have them satisfactorily hold their shape. They can withstand the expansion/contraction of any PCM. Interestingly, the bottles stayed collapsed where I squeezed them easier than they would collapse at the "panels" that were designed to flex. Even with that, they were nice and rigid at the perimeter.

At 2 or so per week personal use, it will take me a while to gather 400+ of them for the entire tank. I might find some neighbors that use products in similar bottles and ask them for theirs.

[Mobile Master Tech]

THIS LINK and also THIS ONE shed some amazing light on real-world insulation.

Whether for a storage tank or a building envelope, the diminishing returns of extra insulation and the dramatic degradation in its effective R or U value with even tiny amounts of uninsulated/less insulated area are surprising. So much so, that improving more poorly insulated areas (by using low-e windows, fixing insulation gaps around obstacles, designing without thermal bridges, etc) is WAY more important than adding extra insulation past a certain point.

An uninsulated pulldown attic staircase (or gaps around insulation) of only 1% of the total area increases heat flow for an attic with R30 by around 25%, while adding insulation to R50 reduces heat flow by only about 5%!

I will be making sure there are no thermal bridges between the bottom of my tank and my basement slab....

[jeff5may]

If you are in need of beverage containers, now is the time to hunt. Forecasted highs are in the 80's and 90's all month long where I'm at (KY). Just stop by any non-conditioned venue and fill up. The 1.5 liter gatorade/powerade bottles are much like the ocean spray containers you described earlier. Sports parks and manufacturing plants top my list. Many of these places have recycle bins especially labeled for plastic bottles, so the need to sort through garbage is highly reduced. Those hard working and hard playing citizens go through a lot of bottles staying hydrated.

As for me, I'm working evening shift, building F150 and Expedition/Navigator frames this year. They figured out I can weld up valleys, schisms and gorges without scrapping any work, so guess what I'm doing this summer? 7 days, 60+ hours a week, end of the mainline repair in full gear. I'm going through at least 3 liters worth of bottles a day, as are the 3 others (1 on each corner) in my spot. The guy across from me drinks bottled water, so he brings a gallon container in every day. It goes in the bin at the end of the shift whether it's empty or not...

[Ron342]

Thanks for your post MMT on how much heat is lost in the simple things - its easy too get excited about complex heat/cool experiments but not the obvious ones - AC brought this out somewhere at the beginning of his long thread - before doing the complicated stuff, close the damn door!