Quote from: K-Dog on Apr 14, 2024, 09:43 PMA large quantity of scrap iron stores heat almost as well as water. 

If you had a volume of 1 cm³ of each material:

Water will have a mass of 1 gram and can store about 4.18 J of heat per degree Celsius.
Iron will have a mass of 7.87 grams and can store about 3.54 J of heat per degree Celsius.
Sand will have a mass of 2.2 grams (about) and can store about 0.703 J of heat per degree Celsius.

Due to its very high heat capacity, for the purpose of home heating water works great.  You can store a lot of heat in a small volume, which is fully filled with the liquid.  With sand or iron balls, besides the lower heat capacity/gram, you have the issue that there is a lot of air space between the balls or grains of sand, depending on how fine the grains of sand or small the balls.  So it takes a lot bigger volume to store the same amount of total heat.

The problem with water for purposes other than home heating (like producing electricity or running a pump) is the phase change to steam (vapor) at 100C/212F.  Once it changes to a gas, the heat capacity drops off the map.  It essentially gives up all the stored heat in the phase change from liquid to gas.  So if you want to store heat at the temps that are efficient for industrial processes or generating electricity, you need either a solid or liquid that doesn't change phase until it reaches 500F or more.  Sand is good because it comes cheap, though the crushed rock/gravel probably works better with a smaller total volume.  Mercury might be a choice, but it would be expensive and mercury is poisonous as all get out so you wouldn't want so much of that shit around.

In any event, according to the article on sand, well insulated it could hold heat between seasons, but you would need a huge thermal mass to hold a winter's worth of heat.  Like a mountain. lol.  I think this type of storage is limited to the day/night cycle.

RE