It’s tricky. Iron, and steel would be similar, has 6 times the density of sand; sand has almost twice the specific heat. Iron and mild steel have 200 times the conductivity. So by weight, sand is a better storage medium, but steel wins by volume. If getting the heat from the heating element is a concern, steel is much better. Still, sand is 10 times more conductive than air, and easy to pour around the heater.
I think (emphasis on think) hot air rises (buoyant convection), but for conduction, heat goes in all directions. I think (again, not sure) the air in sand won’t move enough that heat will go up more than any other direction.
Vacuum is a great insulator, but you need a pretty good vacuum. Google wasn’t very helpful for this, but one source, https://apps.dtic.mil/sti/tr/pdf/AD0298997.pdf, shows about a 10% reduction in conductivity for a pressure of 13 mm of mercury, 1/60 of atmospheric pressure. It works, but it’s a tough task keeping enough air out of something like a sand battery to really make a difference…
There is a bunch of research and attempts to get them efficient from across the globe for thermal storage. the latest is a group from Finland.
it sounds like they are just using a resistance heater to heat it, then they have a heat exchanger inside it to circulate air or fluid around. Essentially a radiator.
it might be best to align the heater vertically in the middle more like a rod, then use heat exchangers tubes spaced around it.
You need a perfect vacuum which is basically impossible, however, any molecules sucked out will help because it is the molecules bouncing off each other that transfers the energy. A vacuum tube is 8×10−8 Torr down to 8×10−11 Torr and it is roughly 1mmHG to 1 torr. So in theory it is possible but hard to do.
You can get the needed vacuum with a diffusion pump or turbo-molecular pump, and an oil-sealed mechanical vacuum pump to back them (get the pressure low enough for them to work). For the volume of this sand battery, the pumps will be fair sized. If you need a source of roughly three kilowatts, it’s easy. Just turn off the vacuum pumps, and use the power they would have used .
edit: And if you think leaks are a problem for gasifiers, imagine trying to pump out the air from a leak that expands maybe 10,000 times as it enters your system .
(Maybe 10 000 000 000 to 10 000 000 000 000 if we go with Sean’s numbers)
It isn’t just that, if you use a continuous piece of material, it will conduct all the way around. It really needs a break between the inside and outside. Which means it needs to be sealed. Whatever it is used to seal it with has to be able to withstand the temperature.
The numbers are what some website said was normal for vacuum tubes. Those were made in like the 1920s so I figured the technology should be relatively feasible without molecular pumps and such.
Overall the concept of the sand battery isn’t different then the whole house thermal batteries which aren’t much more then a pit with a bunch of rocks and other backfill in it. then you circulate air through it. The sand battery is just made for higher temperatures so you don’t need as much volume.
I was watching this video as this guy came close to what I was thinking only he used a light bulb to give radiant heat to the objects inside…
I just shook my head when I noticed he left a cell phone in the chamber with a radiant heat source… WITH A LITHIUM BATTERY ???
But at least he showed me some of my ideas might work
As I thought more about it, light bulbs do what I am thinking as they are in a vacuum, and they are heated. But being connected to wire the heat dissipates…
They cheat
They use not-that-great vacuum pumps, but they add a “getter,” usually a thin film of Barium metal evaporated on the inside of the tube after it’s been sealed. The thin film is highly reactive, and absorbs (“gets”?) any gas remaining in the tube. You start with an okay vacuum, and wind up with a really great one.
So trying to store the heat that is in the sand battery…
Well, my quck lime has arrived…
In my first test I added too much water and did not get much heat out…
Then I added just a few drops of water to 1/2 cup of quick lime, got temps from 90 - 210 degrees…
Now after adding the the water, I have from what I read slate lime. So, now I am testing trying to take this and see if I can get back to quick lime with the addition of heat. My first try at this failed tried at 300 deg… Today trying at 400 deg…
From what I am reading to go from limestone to quicklime it is 900 C or about 1800 F I can try this but I wanted to find the lowest temp first before I went up to that high of a temp…
Since I added 5 lbs of bolts to thermal battery temps have not been as high as before. Min temp is right around 300 deg. amazes me days at a time above 300 degrees 24 /7…
Today I got a reply on another board of someone else in process of building a sand battery. Thought I would share here →
Hi. I am very happy to find this chat
We are building the same thing but for schools so they can cook and not use any biomass. Schools will have zero cost zero pollution cooking
Economics on this are pretty awesome. Schools in central Africa spend $1500usd a year. Varies greatly to feed 300 students buy wood
A sand battery cooker costs $5000 to build and will run for 20 years.
We will begin installing April 2024
The system is a cubic yard of sand mixed with shredded aluminum cans 24” of mineral wool around that. Stucco then shreded plastic trash 24” thick and more stucco
10kw of panels. The cooks use 25 gallons of water a day to boil beans corn rice etc.
Our system is charged to 400c in 5 days
We have 100 gallons of water I. The insulation too but in a region so it does not boil
Cook comes in uncovers the hot cooking pots adds food and adds hot water from the tanks. The pots are surrounded by the 400c sand and aluminum. The food cooks for as long as needed. 1/2 the 25 gallons goes up as steam
The system consumes / losses about 50 kWh per day. The power from pv ranges from 16 to 80. The sand and water storage buffers us through the cloudy days
30% of the world uses wood to cook. Expensive polluting and causes deforestation. We are working to switch the schools and institutions over to solar to save them money. Stop the air pollution and deforestation
Any suggestions please let us know! Thanks. Dan wastefree23.org
I think that means 60% of the energy goes to steam production. It would save a lot if the heat flow to the pots could be limited to what’s needed to simmer. Maybe a steam jacket system, or adjustable heat pipes? Maybe the pots could be raised or lowered to regulate heating.
something doesn’t seem right. it takes roughly 1/3 of a kw to bring water to a boil from 20C. which is like 8 kw for 25 gallons.
They would probably be better served to get rid of the plaster for their insulation and use a mix of clay, straw, ash (ash contains aluminum oxide which is a refractory material) and charcoal like you would use for refractory firebricks. Aluminum cans have a layers of plastic in the walls, which acts as an insulator which probably isn’t good for inside the sand since it melts at like 250C. Since they have 300 kids to find cans, they might be better served to use solid aluminum since the specific heat capacity is about the same but it conducts heat really well. It is just larger (and more expensive) because it is light so the density isn’t as high as sand.
For the insulator material, they are better off finding styrofoam or the expandable polyurethane foam or both because they are expanded foam and has air trapped in it already. And for insulation it is about trapping air to limit movement and limiting conduction between the solid particles, and two different substances heat doesn’t conduct as well between the layer interfaces.
Charcoal is also a good insulator because it has air spaces inside, but it is all connected so it would need to be crushed.
don’t know for sure, but that makes more sense to me.
This sounds like it is in the ballpark of being a sand battery, as the numbers somewhat jive… I have requested pictures of his setup.
He is using the right insulation. What he could do far better is to use heat retention cooking, get the pots up to boiling then pull them off and put in an insulated cooker thus saving all of that heat stored in the thermal mass, and let that heat cook his food…
As for the KW, if you have a cubic yard of sand mixed with aluminum and you are only using that energy when you are cooking, then the heat in the sand would build up… I have not done the math, but I am very interesting to see his configuration, and what does $5000 buy in Kenya for solar panels. and insulation.
I think it was in Jamaica, in the 80s. They did a concentrated solar system that was parabolic and boiled salt water to generate steam to power a generator, and they probably got some fresh/distilled water out of it as well. It was only like a kw or two for output.
I built a very early version that used 12" square mirror tiles from Home Depot. This sort of thing would best be used where lots of food could be cooked or baked every day. Now that I have so many used solar panels all over the yard, we either cook with an induction portable stovetop, or light up the high powered TLUD to make charcoal in the process of cooking. Last week, we had visitors, so I cooked outdoor over the TLUD three times. Once for a big skillet of Pork Chops, another time for a large pan of potatoes, and a third time for Asian stir-fried long beans.
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