[grecy]

I've always wanted to build a little hobby project where I put TECs on top of my wood stove and have a radiator outside with coolant to get a nice big heat difference (maybe 100C to 200C) across them and make power in winter when solar isn't so great in the Yukon.

I know it won't be a massive amount of power, but given it will be 24/7 for about 6 months of winter when the wood stove runs, I think it will be a useful amount.

Does anyone know where I can buy TECs that will handle extremely high temperatures like this?

All the ones I see say they're rated at about a max temp delta of ~67C-72C

[arghwhat]

You'd be actively cooling your wood stove by much more than the power you'd extract, and you won't get more than enough to charge a phone or laptop. TECs are just terrible.

You'd get much better result by making a small steam plant with the same setup: boil water on stove, drive plant (turbine or piston), condense outside, repeat.

However, if you really want to do this with TECs, stack them to lower the per-unit temperature differential, or distribute the heat energy over a larger area and run them in parallel.

[debrice]

I’d suggest doing that operation on the outdoor side exhaust where the heat loss would be irrelevant.

[arghwhat]

I doubt there's enough heat to extract there. Low-pressure exhaust gas is useless, and I don't think a metal chimney exiting out through a wall would conduct enough heat on the outside on its own.

[learc83]

A small stirling engine would be easier and safer than a steam setup.

[8bitsrule]

The good ol' Stirling engine was tried out -- at a few locations in the US Southwest 10 years back -- to generate megawatts of solar-powered electricity (25kW per engine) at the focus of parabolic mirrors.

It got beat out by subsidized Chinese-produced PV panels (SES was forced into bankruptcy).

https://en.wikipedia.org/wiki/Applications_of_the_Stirling_e...

https://en.wikipedia.org/wiki/Mount_Signal_Solar

[abdullahkhalids]

My senior year project ten years ago was exactly this - solar thermal + a Stirling engine. When we started the project, we estimated it 20% or so cheaper than solar PV. In that one year solar prices halved or so, leaving us at a dead end.

I ended up consulting with leather plant for their hot water needs via solar thermal. At least that is still viable.

[8bitsrule]

Interesting. I just learned about that energy combo, and Stirling also makes a lot of sense for someone with a non-solar heat source!

Amazing how China came along at the time it did. The hardball politics aimed against US solar startups since Carter has been a fascinating tale with little mainstream coverage.

[arghwhat]

I'd argue that a steam setup is considerably easier and more proven, but a Stirling engine is certainly safer and more efficient.

(A homemade setup would not be very efficient regardless of technology chosen, so I'm ignoring complexities of elaborate high-performance boilers.)

[icegreentea2]

There are some single layer units that can get up to ~90 of delta_T, but I don't think they're worth dealing with. As a sibling comment points out, you can stack TECs - indeed, you can buy integrated units with very high delta_T that are just pre-stacked TECs.

I think it'd certainly be fun to play with. Some bits of warning - most TECs are physically quite fragile. Not in the sense that you need to be super careful when handling them with your hands, but that heat stresses can totally wreck them. Do NOT hard mount both sides (hot and cold) to different materials - the differential thermal expansion on the two sides has a potentially of ripping your TEC apart. You may be able to get away with a hard mount (such as solder) on one surface, and some sort of thermal transfer material on the other.

Also warning, your hot side is hot enough to start doing weird things with normal solders, which might be the bigger limiting factor.

[esotericn]

Won't this reduce the thermal output of your stove quite significantly? I suppose that might not be a problem in practice (most wood stoves I've used end up overheating the room if you're not careful), but it's not like it's free energy :P

[grecy]

> Won't this reduce the thermal output of your stove quite significantly

I'm going to chime in and say that I think it will be insignificant. I don't know for sure, but I really doubt you could do much to cool the stove. When it's cold in the Yukon, I'm literally stuffing in ~8-10 big logs every ~12 hours.

It's not at all uncommon for the base of the chimney to be glowing red. Obviously not ideal, but it happens.

When it's past -40, I set my alarm for 1am to get up and put in more wood. The stove would just last the night if I didn't, but it's a pain to light it again from scratch every morning, so it's easier to just keep it going.

All of that is to say the wood stove is absolutely pumping out heat 24x7 from ~September to ~April.

If I already have batteries and charge controllers and inverters, why not wire in a handful of TECs.. even if I only get a combined total of 100W, that's worth having over those months the sun is not up for long, and not strong.

[bradknowles]

Glowing red?

As an Amateur Blacksmith (and lifetime member of the Blacksmiths Guild of the Potomac), I was taught that color meant you were hitting 500-600 degrees Fahrenheit, which is usually about the lowest temperature you normally want to have when working on a piece of metal.

If you’re hitting those kinds of temperatures, I’d think you would need some substantial work done on the thermo electric components to keep them from melting, much less being able to operate.

Or am I missing something obvious here?

[arghwhat]

> Even if I only get a combined total of 100W

Quoting Wikipedia:

> The typical efficiency of TEGs is around 5–8%

To get 100W out at 5% efficiency, you'd need to put 2kW in, 1.9kW of which has to be moved away to keep the cold side cold with perfect cooling.

Realistically speaking, the cooling will not be that efficient, and will probably cost you a bit more energy. I'd ballpark that to 2.5kW lost total for 100W of charging.

That's pretty significant.

[starpilot]

Thermoelectrics have terrible efficiency, that's why they're hardly used despite the fact that waste heat is literally everywhere. Your computer, your phone, your car engine, everywhere there is energy to be recovered. In theory, any heat-based powerplant could use them, but instead they all boil water and drive steam through turbines, which is much more efficient.

[scottlocklin]

Thermoelectrics you can order from a catalog have real world efficiency ~6%, which is on the order of what a well engineered practical Sterling engine will do for converting heat to electricity without any moving parts to wear out, exotic working fluids/seals, or ... complete lack of supply chain for any of the above. If this new material pans out it could be on the order of 24% efficient, which would be preposterously better than any heat engine you can fit in an Alaska cabin.

I hate the computer programmer weeb meme of suggesting a Sterling engine for every damn thing. None of you jokers suggesting this has ever built a real world heat engine, let alone attempted to engineer an efficient one from scratch, which is what you'd be doing in the case of log cabin Sterling engines. There are excellent reasons they're not commonly used, and less thermodynamically efficient external combustion engines (like steam) are used. The problems with making them work efficiently are immense.

[trhway]

>would be preposterously better than any heat engine you can fit in an Alaska cabin.

Or a cabin on Mars. Solar and nuclear would be used on Mars. For nuclear - with 20% efficiency i think it would allow to use simple RTG (Matt Damon style) instead of full blown nuclear reactor with working fluids, pumps, turbines, etc.

[grecy]

> Thermoelectrics have terrible efficiency

I agree with what you're saying, but the point is that the woodstove is running 24x7 in the winter, and the solar is very pool. Whatever energy I can get from the TECs is an order of magnitude more than I'll be making without them - i.e. zero.

[lgats]

I think you could build a panel of TECs and just set it up near your wood stove. The radient hear from the stove would be enough to keep the TECs at high temp on one side and room temp (maybe with some heatsink) on the other side.

You could adjust the distance from the stove to keep things within operating range.

[phkahler]

>> Won't this reduce the thermal output of your stove quite significantly?

Yes. You want to have the "cold" side indoors where the heat is going anyway. You might think it's all going outside in the end, but we dont want to create a new path for it to get there.

BTW a sterling engine running a generator seems like a good idea in this case.

[dghughes]

Hello fellow Canadian :)

Wouldn't a home made Sterling Engine be easier and cheaper? Then use the motion to power a small generator.

But like others commented you may just end up cooling your stove. The Yukon can get pretty cool.

Maybe an old washer motor, a prop to generate power from wind? Or solar in the summer when light is ample.

[asdfman123]

Yeah, I've seen people who use small wind turbines to generate electricity far north. Seems like the best way to do it.

[grecy]

They're very noisy, and you have to get them above the trees, which is quite high. Also where I live is not very windy.

[asdfman123]

That's interesting. I'm curious about your living situation, though. Do you live out there most of the year (are you there right now?), or is it kind of a vacation home?

Sounds cool... I'm always interested in off-the-grid living arrangements.

[grecy]

I don't have enough money for a vacation home! I'm full time in the Yukon when I'm not on the road.

Many people up North have solar setups and are off grid, usually with older lead-acid battery setups which work just fine.

Solar is great in the summer when the sun is up for 20+ hours a day, but slim in the winter when we only get ~4 hours of sunlight. Most people supplement with a generator for a couple of hours a day which is what I really hope to avoid with ideas like this TEC on the woodstove.

The stove is already pumping out so much heat, and it's so cold outside I feel like the temperature difference is just begging me to do something with it!

My other dream is to by land on moving water (river, stream) and make a little water wheel that turns an old pickup truck alternator. It will be killer in summer, but it's a problem in the winter because the water will be solid for ~5 months.

[asdfman123]

Are you software/do tech work? Do you work remotely? And are you completely off-grid, or live in a small community? I just find it very interesting.

By the way, I wonder if it's possible to use waste heat from a generator to warm your house, so you can turn down the stove.

(Also, I was curious about your project idea too and it turns out they do already build wood stove thermoelectric generators if you Google that term...)

[ragerino]

That's a cool idea.

I came across this link because I want to build cheap small sensor stations using LoRa to transmit data (gps location?, temp, humidity, air quality) to a central server.

Solar is not really an option because it needs to be cleaned once in a while.

[hooande]

how big of a temperature difference do thermoelectric materials usually require? is it like dozens or hundreds of degrees over a given period, or something that's likely to be encountered in a normal environment?

[icegreentea2]

Typical applications span the order of tens of degrees Celsius.

[jcims]

Tear one of these apart? https://www.amazon.com/dp/B007BLN652/ref=cm_sw_r_cp_api_i_5j...

You could also use the coolant to pull heat off the stove (obvs with appropriate safety built in) and run the hot side of the TEC within spec.

I’ve seen wood stoves get red hot after an incorrect damper setting runs for 5-10 mins, direct heating might fry your circuits.

[chrido]

I've been experimenting with this idea too. The idea is to drive a fan, which blows on metal heatsink attached on the cool side of the TEC. The air stream further accelerates the fire, so this becomes a reinforcing circle. Directly wiring it is not enough, you need a battery to start, buffer, then you need a controllers, never had time for this

There is a company making beautiful camping stoves with TECs and fans: www.bioliteenergy.com

[gwbas1c]

How much power are you trying to make? You can already buy "off the shelf" USB power supplies for camp fires

[grecy]

As much as possible. I would probably cover the entire top of the stove in TECs (so maybe something like 0.5m^2). If I wanted to I could play with adding some to the sides, onto the chimney, etc.

Essentially, it's just to supplement the solar which as I said isn't so crash hot in the Yukon in winter. It's a hobby, and I'd like to see what I can get out of it.

Again, because it's 24x7 for ~6 months I think it might be a fun side project to play with and watch what I can get out of it.

[kragen]

> How much power are you trying to make?

> As much as possible.

That is not a useful response. Would it be worth doing if "possible" turned out to be 0.1 mW? 1 mW? 10 mW? 100 mW? 1 W? 10 W? 100 W? 1 kW? 10 kW? Presumably somewhere in that sequence your answer goes from "no" to yes" and that point determines what tradeoffs you're going to be willing to accept in order to increase your power capacity.

People have suggested steam engines. Those would definitely produce more power than TECs, at least twice as much and potentially six times as much. But they are far more likely to kill you. Is that tradeoff worth it to you?

TECs are pretty expensive per watt. Are you really willing to spend tens of thousands of dollars if it will increase your energy output a little? How about hundreds of thousands?

There are tradeoffs in any engineering design. It's obvious that more power is better, but without some idea of the shape of your utility curve, it's impossible to evaluate those tradeoffs in a useful way.

[logfromblammo]

Co-generate with a Stirling engine or steam turbine. Your application is the wrong scope for TECs. It will be a hundred times more expensive, and a tenth of the efficiency.

[hinkley]

Last time I was looking at efficient camp stoves I ran into this guy:

https://www.bioliteenergy.com/products/campstove-2

The idea of putting my phone, or any lithium battery really, that close to fire... this plan is not for me.

[logfromblammo]

Couldn't you get around that with a really long USB-A-to-whatever charger cable?

Slapping a Stirling engine on a rocket stove just seems like a really good idea to me. My major concern would be the in-backpack weight and volume.

[hinkley]

DC doesn’t like long runs, but I believe there’s a battery in that yellow block along with the charge circuitry. If I’m wrong it may warrant another look.

[galangalalgol]

Could you chain 3 of them using a tank of coolant at room temp as the between value. Or make it water and put the TEC on the cold side of the tank

[grecy]

The plan is to have a tank of glycol coolant sitting outside (at around -20C to -40C ambient) and mount a radiator outside. I'll use a fish tank pump or similar to circulate the coolant. Put water cooling blocks onto the TECs so the wood stove heats one side to around 100C to 200C, and the cold coolant coming in brings it back down, to hopefully get a temperature difference in the range of 100C (ish).

It's all just an idea right now, first I have to find TECs that are up to the heat directly touching the stove.

[UncleEntity]

You could use the waste heat that goes up the chimney to warm the fluid with something like an automotive intercooler and set the system up backwards, maybe?

[grecy]

Ohhh, I hadn't thought of that idea. Interesting. I will be sure to experiment with that!

[nullc]

If you cool the exhaust gases too much they will condense in the flu, and the condensation is incredibly corrosive. Stoves/furnaces etc. (except for condensing ones) intentionally waste a lot of heat intentionally keeping the exhaust gases hot just to prevent this.

The hot exhaust also helps create airflow that moves the combustion products out of your living space.

It would be neat to recover otherwise totally wasted heat, just be careful.

[ip26]

Solar & lithium batteries are so cheap from scale, it's worth comparing the cost, even though production is very low in winter.

[grecy]

Don't get me wrong, I'll still have a good deal of solar. I just want to supplement it in the winter, especially the ~20 hours a day when the sun is below the horizon!