Using sand to store energy: two links
UAE desert sand can store solar energy up to 1000°c
Energy storage system based on silicon from sand
I'm in the process of building heated seedling tables. This is the third iteration.
The first cut just had the heating element from a baseboard heater under the table. Most of the heat escaped around the edges of the seedling trays.
In the second version, I covered the mesh of the greenhouse table with sheets of aluminum, to catch the heat and spread it out. It worked a lot better, but there was a hot spot where the element was, so only about a third of the seedlings got the proper heat.
Now, I've put 3" tall edges on the table and put poly film inside it to make a "bathtub," and filled that with wet sand, the idea being that the sand will serve as a thermal mass and spread the heat more evenly.
We'll see how that works… I'll try to get some photos up…
I strongly doubt the idea with wet sand to spread the heat.The water in the sand will be a good heat conductor, but heat tends to dry out sand quickly.
Sand itself is a really good heat isolator. In fact soo good that you often use sand for molds when you cast metals.
If you keep adding water every day to the sand and only add a tiny amount of heatit could still work ok.
The best heat spreader is metals.
As we don’t need any fast heat transfer any junk metal that you happen to have should do great.
Some ceramics also work quite good for slow heat transfer and of course all liquids works fine.
In this case air could work as well to spread the heat. That is if you have enclosed air and some space from the heater so the air can spread around.