Thermal Battery Systems Inc.
"For the low temp applications we are pursuing water is working out very well as the storage medium. We find the following benefits with using water.
1: It is basically free and available everywhere.
2: It is a good conductor into heat exchangers that are placed in it.
3. High specific heat.
4. A latent heat capacity that is 143x the specific heat and this latent aspect is at the "perfect" temperature because it "protects the bottom" of the acceptable entering water temperature to the heat pump.
The other thing to mention is that these water based "thermal batteries" are buried in uninsulated tanks. (The top is insulated) So they are thermally interacting with the surrounding earth. If the application intended to store high temp in the system this would obviously result in net losses to the ground. In low temp applications the uninsulated aspect results in net gains from the ground. In other words it acts like a geothermal ground loop which absorbs heat from the earth when cold water is circulated through it.
The idea of thermal storage is an interesting one if you think about it from the perspective of a conventional "ground source" heat pump system. In that case the design are focused towards the un-containment of thermal energy. Too small of an area results in a failed system from a depleted or a temperature saturated loop. All the heat is gone / bottled up!
The solar thermal approach is the opposite. Everything is done to contain thermal energy. The idea of a non-insulated tank being buried in the earth is preposterous to many in the solar thermal trade. All the heat will go away!
The application is the aspect that defines the design. If the application concept is presumed to be something other than what it is then the design approach will be off."
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