GoSeigen wrote:Bubblesofearth wrote:
Yes, you can slow heat loss by insulating. However, given the big difference in thermal conductivity sand will be a much better insulator itself than water and therefore require less external insulation. Ultimately the choice of material will come down to a number of different practical considerations.
You don't want sand to be insulating. What is the use of that? You want it to be able to absorb heat and release it fast. And you want it to store a lot of heat. Compared to water sand is inferior in both departments.
As mentioned earlier it seems to have other advantages: it doesn't leak so readily and you can practically raise its temperature higher than water.
GS
Let’s first clear up temperature & heat energy.
Temperature measures the hotness or coldness of something & at an atomic level it is the average energy (kinetic, rotational, vibrational) of the atoms or molecules in the body being measured. The more the average atomic/molecular energy the higher the temperature & the more it will cause mercury or alcohol in a traditional thermometer to expand.
Heat by contrast is a measure of energy, the capacity to do work.
As an example the temperature of a teaspoon of boiling water is a lot higher than a bucket of water at cold tap water temperature. If two teaspoons of boiling water are tipped into the bucket of water, the bucket will then contain twice as much heat energy, but that energy will be spread over the large volume of the bucket & the temperature will not change within the limits of most thermometers ability to measure.
For a thermal battery the more self insulating the better. In operation you supply heat via e.g. electrical heating elements & you want as much as possible of this heat energy to stay inside the thermal battery. You do not want it to release the energy quickly, the exact opposite, you want it to release the energy very slowly. In operation you raise the sand to several hundred degrees & then pass a central heating pipe through the sand which has water at the temperature needed to heat the house. So long as the sand is hotter than the water in the central heating pipe, there is a transfer from the sand to the water & this water is then pumped through the radiators to warm the house.
Water is not a useful reservoir for this type of application, with numerous drawbacks, especially that to go beyond the boiling point you have to pressurize it, whereas sand will happily support much higher temperatures without any similar complications.
We are talking about technology very familiar to our Victorian ancestors who would keep food warm for hours by surrounding a pot of e.g. stew with towels & hay beyond that to make a well insulated meal that will keep for hours. Cool boxes are a more modern variant on the idea.
Regards,