This is why I love this group, everyone here thinks outside the box
I posted one other post on sand battery in this group, but it was not my creation.
I have been trying various methods of heating the sand, I have used a solo stove, and I am not testing out used nichrome wire in the form of hot water elements, and stove heating elements…
Like you last year I figured out that after my battery is charged I could take the solar energy and dump that power ( direct DC ) from my solar panels into my hot water tank. I am now getting solar powered hot showers with propane instant hot water heater as backup. Using this system I set the propane on the lowest heating as possible because even on cloudy days I get enough heating to make luke warm hot water, which the propane heater does not have to work hard to heat, saving me propane. When I get a few sunny days in a row, I take very long hot showers with no guilt
I also play with solar ovens, sounds like you are a brother from a different mother I have a sun oven, vacuum glass oven, and I have played around with making a Pex hot water heater which did not work out very well for me…
At present I went to the dollar store and bought a $20.00 single stove top burner. I took this and then I used 2 1/2 gal stainless steel pots to hold sand inside a trash can, inside a 55 gal barrel. I found the heat was too intense and melted thru the stainless pots. So, I have moved to a dutch oven design with fiber glass insulation around the dutch oven and a stainless pot below filled with sand.
With this setup, I have dried wood ( before I used insulation ) that I use for starter material for my rocket stove, temps were 200 - 320 degrees F before I added fiberglass insulation…
I have been searching for better types of insulation, I would like to get some rockwool, have been watching craigslist close…
What I have been doing is preparing for the next step, which is to make one far larger than this with additional insulation of perhaps styrocrete which uses cement with chopped up styrofoam for insulation. I have this all ready chopped, but in my research I am considering using chopped up cardboard instead, or dust crete which is cement mixed with sawdust.
I bought thermometers to guide me as to what would give me the best results. I have also been looking at used kilns as I am considering making a solar kiln, or a rocket stove kiln to move up to the next level of working with metals, as I would like to do casting with my 3d printer for moulds.
After watching a documentary of special made bricks they use graphite combined with metal to store heat, I am considering putting a bucket of old bolts I have into the dutch oven to see how it retains heat and at what rate it would give it out.
I also follow a solar project called ISEC where they take 100 watt panels and modify them to cook with.
I have been scaling up their ideas…
For me to be at this point of now having over 200 degrees for over 24 hours is earth shattering for me. I was looking into heat retention cooking but moved into much more.
I want to be able to store heat chemically. So far quicklime looks to be the most reasonable option, but there are other chemical reactions I am looking at. I want a reaction that I can simply store the heat chemically and then pull it out years later.
Love to chat about this stuff as I am seeing real world results from all of my efforts.