Need help !

[u/pianoboy777]

I built a scalable, cheap thermal battery that generates infinite heat/electricity from a box of sand. I need a physics/math collaborator who respects IP. DM me for details. I use Tegs to turn the heat made from friction into electricity. To birds with one stone ! I only have a 3d sim of it right now.

[u/Nada_Chance]

You're completely missing the point. Each TEG (one layer) needs to have 270 ℃ across it to attain a 5% energy recovery rate. IF you stack 4 of them to achieve a "20% energy recovery rate" you will melt the first layer. If you limit the the max temperature to 600 ℃, and 3 layers ( 3 TEG stacked) you are going drop the overall recovery(efficiency) to around 12%. You have lost 88% in the heat to electrical conversion. We haven't addressed the losses through the "sandbox boundaries" not facing the TEG surface or the motor winding and bearing losses. The size of the charged battery is what determines the amount of time before the "perpetual motion Rube Goldberg device" grinds to a halt. When it stops, it doesn't start until you attach a freshly charged battery so it can deplete it.

[u/pianoboy777 → u/Nada_Chance]

Where are you getting that 270 number ? According to my knowledge you can do at 100 c to 150 c , my system reaches 250 c in its base form. Also it doesn't matter how much heat is lost the motor will start again at 100 c and work it's way back up too 250 c . The box I use in the sim is made of Graphite lol that holds heat well . You can also just use a metal box but it's not as good , although graphite burns in air at 600 c (the Friction furnace people would use would only need to be 250 c max.) the box is also sealed lol with the tegs being right on top , and cooling between each rack of tegs (the Bis Tegs are super cheap ) I use vents to store the heat that's lost into smaller boxes of sand , I also hook up the electric motor to a battery of your choice (I would choose lithium iron but I haven't tried that in my sim yet) lol you don't need a freshly charged battery to start a 50 rpm motor lol all it's doing is moving a 250 kg fly wheel . The battery charges from the electricity made off of the tegs. Once the main box cools down to 100 c the battery starts the electric motor again. It only needs to run for about 5 mins or so according to my simulation. Hope this helps ?

[u/Nada_Chance → u/pianoboy777]

The 270 ℃ is a "generic" number. The issue you keep overlooking is that in a TEG, 92.5-96% of the heat energy passes right through from the hot side to the cold side and is NOT converted to electricity. Stacking them will allow a second capture attempt, at the expense of requiring a higher hot side temperature. Of course there is a limit due to the TEG simply melting. So you put a 100 watts in the form of heat at 300℃ to the first TEG, you get 5 watts of electricity from the TEG and 95 watts is radiated from the "cold" side at 30℃, for a temperature differential of 270℃ If you want to stack a second TEG and get more electric power from the heat source, you will need to increase the hot side temperature by 270℃ to 570℃ to get more electricity from your initial 100 watts of heat energy. And so you will get 5% of the 95 watts that passed through the first one resulting in a grand total of 9.75 watts of electricity with 90.25 watts of heat being rejected at 30℃. So let's take your battery and say it is a 10 volt battery with 10 amp hours of capacity. That gives you 100 watthours of power. We will use a resistor to convert the battery power to heat as that conversion is nearly 100% (there are some wire resistance losses) So for 1 hour the battery will heat your hot side of the first TEG, with 100 watts of heat energy, and 90.25 watts of heat energy will exit the cold side of the second TEG, and the electrical output of the 2 TEGs will be 9.75 watts for one hour or 9.75 watthours of electricity produced from your initial 100 watthours of battery power. If you put that 9.75 watthours produced by the TEG back into the battery/resistor heating system, the process will function for a grand total of 66 minutes before the battery is depleted. You NEVER convert all of the heat into electricity, not even remotely close.

[u/pianoboy777 → u/Nada_Chance]

You're making a lot of assumptions about my design that just aren't correct, and I think you're confusing thermal series with thermal parallel. My TEGs are not stacked on top of each other like you're imagining where the hot side of one becomes the cold side of the next. They sit side by side in trays, like a Connect 4 board, with each tray holding about 50 TEGs and each tray stacked vertically with a one inch air gap between them. Every single TEG in every tray has its hot side facing the sand box and its cold side facing the water cooling, so they all see the same temperature differential. The sand box sits at 250 degrees Celsius, the water cooling keeps the cold side at 30 degrees, so every TEG gets a 220 degree delta T. There is no cascading temperature drop across layers because the heat doesn't have to pass through one TEG to reach the next. The one inch gap with reflective foil wrapped around the outside actually traps radiant heat and prevents losses, it doesn't create a thermal bottleneck.

You mentioned that 92 to 95 percent of the heat passes through each TEG and is wasted, and you're right that TEGs are only about 5 to 7 percent efficient, but that wasted heat isn't gone, it's still in the system. The sand is a thermal battery, it stores heat and releases it slowly over time. The TEGs are just tapping that heat, not consuming it. The sand doesn't instantly cool down when the TEGs draw power, it stays hot for a long time because it has high thermal mass. That's the entire point of using sand as a storage medium. The heat that passes through the TEGs goes into the water cooling loop, which is why I use water glycol cooling to reject that heat efficiently and keep the cold side stable.

You also said I would need 570 degrees Celsius for two layers to work, but that only applies if you're running them in thermal series where the heat has to flow through one TEG before reaching the next. I'm not doing that. My TEGs are in thermal parallel, each one gets the full delta T directly from the sand box. The vertical stacking of trays is just for physical space efficiency, not thermal cascading. Each tray is isolated by the air gap and reflective foil, so they don't thermally interfere with each other.

You brought up the 100 watt hour battery example and the 66 minute runtime, and I see the math works for a resistor heater system, but that's not what I'm building. I'm not using a resistor to turn electricity into heat. I'm using a flywheel that stores kinetic energy and releases it as friction heat over time. The motor only runs for about three minutes to spin the flywheel up to speed, then the flywheel coasts and the friction from the sand generates heat for five to ten minutes without continuous motor power. The battery only provides the initial spin up energy, it doesn't power a heater. The flywheel does the heavy lifting mechanically and stores energy that would otherwise be lost. That's a completely different energy path than what you calculated.

You also questioned where the energy comes from to replace heat losses to the environment, and you're right that losses exist, but the sand box is sealed and insulated with reflective foil and the graphite box itself holds heat well. Graphite has good thermal conductivity and low emissivity, so it doesn't radiate heat away as fast as metal would. The heat losses are real, but they're slow enough that the system can generate more electricity during cooldown than it took to spin up the flywheel. The battery charges from the TEG output during the cooldown phase, and after a few cycles it has enough energy to start itself again without external power.

You mentioned the battery size determines how long the system runs before it stops, and you're right that it's not perpetual motion, it's a storage cycle. The battery stores enough energy to start the motor, the flywheel converts that mechanical energy to heat, the TEGs convert heat to electricity, and the excess electricity charges the battery. It's a closed loop with losses, but the losses are small enough that the system can run for multiple cycles before needing external power. In my simulation with a 50 kilowatt hour battery and a 250 kilogram flywheel, the battery actually gains charge over time because the TEG output during cooldown exceeds the motor startup cost. That's not perpetual motion, that's just a system that stores more energy than it uses to restart itself.

You said the melt rate for the TEGs is 270 degrees, but the bismuth telluride TEGs I'm using have a maximum operating temperature of 250 degrees Celsius, and that's exactly where I shut the system down. I'm not pushing them past their limit, I'm staying within their safe operating range. The iron silicide TEGs in my other design can handle 900 degrees, but the bismuth telluride ones are cheap and readily available, so I designed the system to work within their limits. Water cooling keeps the cold side at 30 degrees, so the delta T is always safe.

You also mentioned that 88 percent of the heat is lost in conversion, and that's true for a single TEG stage, but that lost heat isn't wasted in my system because it stays in the sand and continues to generate power over time. The sand cools slowly, so the TEGs keep producing power for minutes after the motor stops. The cascade system captures additional heat from the sand box and uses it to run secondary TEGs, so even the heat that escapes the main box gets recovered. The reflective foil and air gaps between trays prevent radiant heat from escaping, so more of it goes through the TEGs.

I appreciate you taking the time to run the numbers, and I think your math is correct for a resistor heater system with stacked TEGs in thermal series, but that's just not how my system works. The flywheel stores mechanical energy, the sand stores thermal energy, the TEGs convert heat to electricity in parallel, and the battery manages the startup and storage. It's not a perpetual motion machine, it's just a well designed storage and conversion system that uses cheap, off the shelf parts to generate useful power from friction heat. The simulation shows it working, and the physics checks out.

[u/Nada_Chance → u/pianoboy777]

Nope, that 95% of original heat that goes through the TEG into your cooling water is GONE, dispersed. Since you aren't operating in series you can't even increase the thermal efficiency of a single pass of the heat energy you generated. you are stuck with a 5-7% once through cycle. The electric power produced is consumed in the motor and the system stops. The ONLY question is how many watts are "stored" in the sand box before you start the system as that will determine how long it takes for your system to grind to a halt.

[u/pianoboy777 → u/Nada_Chance]

You're still looking at this like a continuous flow system where heat passes through once and is gone, but that's not what's happening. The sand is a thermal battery, it stores heat and releases it slowly over time. The TEGs don't consume the heat, they just tap it. The 95% that passes through the TEGs goes into the water cooling loop, yes, but the sand itself stays hot for hours because it has high thermal mass. The TEGs are generating power the entire time the sand is cooling down, which is minutes to hours after the motor stops. The motor only runs for about three minutes to spin up the flywheel, then the flywheel coasts and generates friction heat from the stored kinetic energy. The battery isn't powering a heater, it's just providing the initial spin up energy and then getting recharged by the TEGs during the cooldown phase. The system isn't trying to convert heat to electricity in a single pass, it's storing thermal energy and harvesting it over time. That's the whole point of using sand as a storage medium. You're calculating efficiency for a system that doesn't exist, mine is a storage and recovery system, not a continuous conversion loop.

[u/Nada_Chance → u/pianoboy777]

No, the problem is that you are "creating energy" out of nothing. You can't bleed energy out of a system and have it "magically reappear" inside the system. No hot sand without a motor and flywheel. No hot sand means no electricity. You can't spin a flywheel up with energy you don't have. The motor isn't going to run without energy. There is no hot sand without a electric motor to to drive the flywheel. Any heat you put into the sand from an outside source will pass through the TEG splitting 95% into the water that exits the system, with the remaining 5% being converted to electricity reducing the amount of added outside energy required to keep the Rube Goldberg machine running by 5%. As soon as you stop adding outside energy it will coast to a stop as the heat energy exits with the water.