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You are here: Allan's TIME > D.W.A. Biography > Solar Home > Eutectic Salt

Eutectic Salt for Solar Home Heat Storage

Whatisit? Some salt work very well as a thermal mass to store the sun's heat.  The idea is to select a salt, such as Glauber's Salt, whose phase change from solid to liquid is at around 70 - 90 degrees F. 

On Jan. 18, 2008, David Allan wrote:

Dr. Maria Telkas, U of Delaware, proved that Glauber's Salt technology was sound and economical some 30 years ago. She had a couple of patents in that regard. She built a home based on same at the U. of Delaware, and it was a nice practical validation. This technology could have a major impact on how we deal with energy, but no one has picked it up. Some have tried, but they didn't follow  her patent guidelines, and they failed. They make it in Europe successfully.

PCM is about the only retailer I have found in the states, but I have not researched it recently. I bought some from them for our rental addition that is described on our web site, and the shipping costs from Sweden were like $400!

In Europe it is cost effective, but not here yet because we have been able to get energy so cheap. That will change.

Thermal storage is a fundamental problem in solar designs and phase change materials make tremendous logical sense when you go through the numbers. For example, stone, brick and the like have about 0.2 cal./gram/deg C heat capacity. The different mixtures of Glauber's salts, for example, have about 50 to 80 cal./gram during the phase change. Water is 1 cal./gram/deg C. So if you cycle brick, for example, through about 5 degree Celsius change, you only have 1 cal./gram. In this case you have an advantage of 50 to 80 times the thermal storage with Glauber's salts as you do with brick.

As described on our web site, I can heat the north side of our home (passively) with these salts. Because of Dr. Telka's patented techniques, the salts cycle indefinitely. I have had mine for about 20 years.

My salt tubes are about 2 1/2 inches in diameter and 36 inches long and are black-heavy PVC -- sealed with caps on the ends. They work well. As you can see on our web site, I have them on the South side so that the winter sun gives them good exposure -- melting them during the day time, so that they will give off 90 degree F heat at night. If you had them in a circulating water bath, that would probably work as well.


On March 11, 2008, David Allan wrote:

The salts that I have melt at 90 degrees F and have a heat capacity of about 80 calories per gram.

I don't know their current cost, but here is the link to buy them: 

The last I knew PCM actually gets their salts form Europe, and if I recall correctly, it may be Sweden. You may call them and get their source.

As shown by Dr. Telkas, you cannot use strait Glauber's salt. She came up with a patented mixture to make them work properly. I believe PCM uses her technique. There are lots of sources for Glauber's salts on the internet, but you would have to search out her patents and make the mixture yourself for it to work properly, and that is not trivial. We have done it, and I simply decided to buy them.

When you put them in your storage chamber, make sure you have a lot of surface area.


On October 05, 2010, David Allan wrote:

Unfortunately, Glauber's salts properly mixed for eutectic heat-of-fusion work are not available that I know of in the states. Dr. Maria Telkas had a couple of patents that we followed in our initial investigations in Boulder, CO, when I decided to use them in our home. She built a home at the U of Delaware using her mixture and it worked well. I talked to her a couple of times back in the 1980s -- marvelous lady. She was then living in Gathersburg, Maryland. You may be able to find her patents; they have undoubtedly expired. I had copies and lent them out, and they didn't get returned. She used borax to act as a seed to keep the salts from super cooling, and she added clay (I believe bentenite) to stabilize the mixture. The deca-hydrate water bond to the sodium sulfate is a week bond, and if it breaks then you loose the heat-of-fusion properties of about 80 calories per gram. The melting point is about 90 degrees F. The salts I have in our home came from England. Some companies in the states tried her idea, but bypassed the patents and they all failed.

We built our home in 1992 with no furnace and it often gets to -20 degrees F here. It did last winter.

We did an experiment last winter. We usually always build a fire in our living room fireplace for ambience and warmth when we have company three or four times over the course of a winter. Last winter, we never did, and we did okay. We wear sweaters and layer a bit.


On June 25, 2012, Kannazuki Shimura wrote: 

"If you are in the U.S., they are sold by PCM Products, if you are in Europe, just ask around or you can buy a heat storage/exchange system that already uses them.

"Coincidentally, we use eutectic salts in our thermal energy storage and ground-air/solar heat exchanger too (it is pretty mainstream here in Europe. Our neighbors use it too. Not everyone can afford a commercial one, though)."

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Page posted January 18, 2008 by Sterling D. Allan
Last updated June 25, 2012
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