Thermoelectric self starting system?

Projects
#1

So at argos 2023, a young man brought up the concept of using thermoelectric pads to generate power with the heat from the gasifiers. the general consensus was agreed that the pads (at this time in technology) dont generate a significant amount of power by themselves to be something to rely on…but its been scraping at the inside of my head since then.

i started thinking to myself, yea a pad or 2 wont charge any batteries in any significant measure…but what if we change the goal? reduce the power?

After some very basic research, it looks like you can get units that can generate up to 125w, but depending on size you can look at individual pads that can generate between 1-125w, and can THEORETICALLY be setup in a modular connection to get up to 5kw (this amount doesnt seem cost/effort effective from what it looks like is required, but i wouldnt be doing my due diligence if i didnt at least mention it) and they dont seem to be very expensive ranging around 10-30 dollars USD for the average sized ones. but for most setups ive seen online your looking at 125w and under realistically. though im sure someone smarter than me will have already made a much more powerful proof of concept, lol

next concern is temp range. they seem to have a “hot” and “cold” side and they dotn seem to really care about weather its hot or cold, it just cares that there is a temp difference. So the “hot” sides seem to be able to handle around 600 degrees Celsius (1112 F), and a 30mmx30mm (1.2x1.2 inch roughly) unit could put out on average 5-6.5w. adding some sort of heat sink would be ideal i imagine, but heat sinks are ALREADY part of an efficient gasifier system so it just happens to be in the right direction.

the temp can EASILLY be worked around, by adding a separated shield, or just measuring the temp range on the unit and figuring our where the best placement is, 1,112F is VERY forgiving.

now a car ac blower is definitely not something that would be effective as they generally use 750ish watt per HP. BUT there are a TON of power efficient blower style and squirrel cage style fans that use as little as 12w (granted, these ARE relatively small and are most commonly plastic, but there re metal ones too), BUT this opens up an interesting thought experiment of using the initial warming up heat to power a small fan to pull air through the system, thus cutting down on startup time and removes the need to plug in your fan to an external power source such as a car battery while you wait for the system to warm up enough to get the motor/engine up and running, then let it do the work.

granted this would probably take a little longer to get started up without an external power supply, and the thermal pads themselves wont generate enough power BY THEMSELVES to do anything major, but powering a decent sized power efficient fan is VERY doable. The concept may lead to a self starting system that would benefit initial starts. conceptually the idea has been beating me in the back of the skull for half a year, so i figured id toss it out there.

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#2

Do you happen to have a link for the 1000 degrees F thermoelectric modules?

edit: Maybe they’re thermocouples. They would take the temperature, but their efficiency is a lot lower than thermoelectric (Peltier) modules.

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#3

Here’s a company that makes some. I have a folder on my home PC that I can get more links from when I was looking them up, little limited with my phone ATM, lol

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#4

on page 3 of this paper, they have a chart of the relative efficiencies of various teg module technologies, and some pretty graphs of effectiveness.

If you look around you can find the modules for a lot less on like alibaba or aliexpress then you need to add heatsinks, thermal paste, etc for thermal transfer. I wouldn’t pay 950 dollars for one.

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#5

that brings up an interesting thought. using the cheap amazon ones, you can only get away with a couple hundred degrees, but that should still be doable if a shield is installed. granted it would take much more effort, but could still be done

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#6

From the webpage:
 Test Arrays have received bench testing consisting of resistance and
mechanical checks.
 Test modules have received bench testing consisting of multiple temperature
cycles to a temperature difference (DT) of 350C
 Base materials have received bench evaluations to 440C
 Delivered modules have been tested once up to 300C to ensure internal
electrical interconnect forms
 High temperature performance based upon previous test experience
 High temperature electrical connections (up to 700C) are implemented on the
hot side providing better performance stability over multiple heat cycles
and sustained high temperatures

and later in the document:

 Some slow degradation may occur at 600C.
 Tested to hot/cold cycles to 300C with < 15% degradation

I think all this means “you’re on your own above 300C” but it’s hard to tell for sure. 300C is still pretty high compared with conventional materials, but it would be nice to see the whole module tested at rated temperature for a good long time.

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#7

From the middle of the paper:

“Compatibility is most important for segmented generators because the
thermoelectric material properties may change dramatically from one segment to another. Other
factors (not considered here) may also affect the selection such as: thermal and chemical
stability, heat losses, coefficient of thermal expansion, processing requirements, availability and
cost. For this analysis, we consider only the thermoelectric properties in the 1-dimensional heat
flow problem.”

I believe this means that they’ve done thermal modelling, but a real module might melt or come apart due to thermal expansion. So it’s interesting, but you can’t buy one, yet, or maybe ever :slightly_smiling_face:

#8

Since all I’ve done so far is be picky, here’s a flakey suggestion of my own. Build a generator of conventional (available) thermoelectric modules, mount them with a block of aluminum on the bottom and a heat sink on top; maybe a fan on the heat sink. Set this on top of your heating woodstove. Put it on movable legs, so a bi-metal strip or automotive thermostat core can lift it off the stovetop when the temperature gets too high.

Free from the Someday Mental File, and worth every penny.

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#9

even then, with the cheap ones on amazon would still be doable. but with the added requirement of adding some sort of shielding, or maybe having the “thermo generator” on some sort of magnetized plate setup so you can place it where the heat is, and move it accordingly to make sure that it doesnt stay in a place thats too hot

but with that in mind, it reminds me of a shield that my dad had placed around our old stove pipe. he had originally made a hood like shield around the top in order to retain, and disperse heat through the room a little better, but now im wondering how hot it actually got. because even with the more limited thermal range of the cheaper amazon units, it still feels VERY achievable to get some power from heat. even if its running a bunch of these smaller units together in a aluminum heat sink that i can just set onto what oven, or even a retort.

ill have to think on it some more, but there’s definitely some potential to squeeze more power out of our heat here.

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#10

You can recover the energy using teg modules. It has more to do with cost effectiveness, and finding a way to use the power more then anything else. If the module cost 10 dollars, and its output is 10w. If I did the math right, the breakeven is 277 days of continuous operation provided you have a use for the 10w.

If, for example, you plaster them on the wood stove, you have to be careful you aren’t blocking the heat from radiating from the stove which is the primary purpose of the stove. The teg module will most likely act as an insulation layer, and it results in lower overall efficiency of the heat output.

You can squeeze more efficiency out of the system but sometimes on a small scale it is just harder to do and doesn’t end up to be cost effective. A wood fired CHP plant is around 90% efficient, but they are able to use the low grade waste heat for things like district heating or hot water, but they get some efficiencies from the scale.

Where the Teg modules make the most sense are where you want power, but it isn’t readily available and you have a heat source. Like one of those solo type stoves with the teg module for backpacking/camping so you can charge your cell phone while you are boiling your drinking water. They work even better if you have both the hot and the cold source like snowballs and a fire.

The main point is higher efficiency, doesn’t always result in a cost savings. No one is against you trying, and I think everyone would be happy for your success, but it isn’t easy.

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