Submitted by ranman12953 t3_10dg022 in askscience
I just installed a woodstove and I bought a heat activated fan to go on top. This fan will literally spin super fast in just a minute of getting hot. How does this work?
Comments
willdood t1_j4l9q8r wrote
They usually have a thermoelectric module embedded in them, which generates electricity using a temperature difference over a semiconductor. The heat from the stove warms one side while the air flow from the fan cools the other. This is why they usually have some form of fin array on the top to make heat transfer to the air more efficient. The module drives an electric motor attached to the fan.
The same principle could also be applied with a Stirling engine - the temperature difference between parts of the engine can be used to produce mechanical work and drive the fan directly, without electricity.
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Blrja6040 t1_j4lepeq wrote
What you are describing (and what OP is asking about) is actually a Peltier element. Aka thermoelectric effect.
Piezo electric is different and uses a quartz crystal to generate a brief electric current from a physical impact. This is the mechanism of many igniters used in gas BBQ grills and heaters.
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bgraham111 t1_j4li6y3 wrote
Boom. You won't get a better, simpler answer than this.
Even tossed in the Stirling engine answer at no cost. (your fan is thermoelectric, but technically a Stirling engine would work.)
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nakrimu t1_j4ljh8v wrote
I built Eco-Fans for years. They use a thermoelectric module that creates a hot and cold side when in use. When your fan heats up with this module it causes what’s known as the Seebeck Effect which causes electrons to flow and power the fan.
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gh0stwriter88 t1_j4ll5do wrote
Actually there are probably a lot more of these with sterling engines than TECs... since that's the Amish solution. It acutally makes way more sense than running electric motors to move the heat too since you arent' wasting electricity to do something that the motion of heat can do for you.... and all the heat is going into the room anyway.
bgraham111 t1_j4lmegs wrote
Wait? I wasn't aware that the Amish used sterling engines. I'd love to hear more. Do you have any info on these?
Did you mean steam engine? Sterling and steam are different...
Of course, steam engines are a solution that the Amish use... I've just never heard of the Amish using high tolerance machining to build a stirring engine.
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noclue72 t1_j4lmkck wrote
I watched a video recently and you can change the direction the fan spins by applying heat to different parts too. Not sure if it was a stirling one or peltier.
bkinstle t1_j4lns1a wrote
Stirling engines have lots of moving parts but peltier fans just have a small white square about 4mm thick between two metal structures without why moving parts safe for the fan itself
bostwickenator t1_j4lo68x wrote
The Amish aren't a huge market verses the whole planet and useful Stirling engines are difficult to build down to a price. The peltier ones are so cheap they are almost certainly more plentiful.
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pm_me_good_usernames t1_j4lt0n0 wrote
Most Amish can use low voltage electricity--you'll often see them using electric calculators at markets, at least in southeast Pennsylvania.
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dr_reverend t1_j4lu1jb wrote
Yes but not quite. Peltier effect is using electricity to drive a temperature difference. It’s the Seebeck effect that is using a temperature differential to drive a voltage potential.
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rayfound t1_j4lzp5a wrote
Yeah the most common thing as I understand it is that they are to be "un-yoked" from modern world. Thus off grid electricity is okay, grid tie isn't. ... Varying degrees here but many also use cell phones.
FluxD1 t1_j4m1fvw wrote
I collect and restore antique steam engines, the Amish really do love these things. Some of the best, and worst, running engines I've seen have come from Amish hands. They either meticulously take care of them, or they run them into the dirt.
Can't say that I've ever seen them use a Stirling engine, however large Stirlings are pretty rare. Wouldn't be surprised to see one in Amish country though
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not-on-your-nelly t1_j4m5vrx wrote
This ingenious stovetop fan for wood-burning stoves is completely silent and costs nothing to operate. Based on a phenomenon known as the Seebeck effect, the fan uses thermocouple technology to generate an electrical current by exploiting the temperature difference between the stovetop and the ambient air. The fan speed varies with the temperature difference (greater difference, greater speed), quietly moving warm air from the stovetop. Made largely from anodized aluminum, it is an efficient way to disperse heat.
The fan can produce up to 125 cfm of air movement, stands 9" tall and has an 8" diameter brass blade; it operates on surface temperatures from 212° to 650°F (100° to 345°C).
Borrowed from Lee Valley Tools
raptorphile t1_j4m9h5r wrote
Can you recommend a good brand? I’ve been on the fence for years because the review are all over the place
bgraham111 t1_j4m9v5b wrote
I grew up in Amish country (even been to an Amish wedding reception), and know they use steam engines... never a Stirling engine. But those steam engines are beautiful.
I interviewed at a company that was planning on building Stirling engines back in 2008. They told me (no idea how true) that other than little toys and novelty sterling engines, there were less than 300 large sterling engines in the world (and they wanted to produce 3000 a month). They went out of business.
Blazin_Rathalos t1_j4mavdk wrote
...What was even their business plan for making that many then?
northwoodsman t1_j4mayt3 wrote
I’ve owned eco fans for years. After 5-7 years I had replace the electric motor behind the blades , but that was pretty cheap and very simple. They are great fans, just pay attention to their intended use. I bought one that was meant for lower temperature and burned it out quickly on a wood stove.
bgraham111 t1_j4mbg0j wrote
Solar power. Use parabolic mirrors, track the sun, heat up the Stirling engine. The prototypes worked, and worked well. Better than photovoltaic cells.
The trick is manufacturing the Stirling engines, which.... is not easy.
55_peters t1_j4mbhm9 wrote
I bought one out of idle curiosity a few years ago. I don't think they make any difference - they don't pull enough air through.
OogoniuM t1_j4mbma3 wrote
When I worked at GameStop a decade ago, the Amish were the ONLY people buying PSP games/movies. It was and still is fascinating to me
nakrimu t1_j4mbo7x wrote
Caframo Eco-Fan 800, the first ever patented wood stove fan or the Eco-Fan 802 meant for gas stoves. They have high quality standards for building them and stand by their products.
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Blazin_Rathalos t1_j4mcms9 wrote
Very interesting, thanks for explaining!
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jaxdraw t1_j4mdmu3 wrote
Any tips on reducing fan noise? I've had one for about 5 months and it produces an audible sound now when it spins. Was debating using w40 or some other lubricant to grease the propeller shaft.
nakrimu t1_j4mdro9 wrote
Ahh, the 802 meant for gas stove would cause the module to crack if used on a wood stove. 800 is meant for wood stove. I agree they do a great job, I had one that lasted me about 8 yrs before I had to swap out the module and motor and it never stopped running.
nakrimu t1_j4medfk wrote
That honestly sounds more like a motor issue. I would contact Caframo since you haven’t had it long.
Inutilisable t1_j4mfa65 wrote
What is tricky about the manufacturing exactly?
severe_neuropathy t1_j4mg64h wrote
Some use electric power tools as well, as you said they mainly care about being off grid so they run generators when they need electricity.
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jaxdraw t1_j4mi9cl wrote
Most wood stove fans rely on something called the Seebeck effect. The Seebeck effect is the inverse to the Peltier effect, and the two are often discussed in tandem.
In lay terms you can draw a small amount of voltage from the movement of heat. On a woodstove this is accomplished by having a large surface area in contact with the metal of the wood stove. The top of the stove fan is usually thinner, with fins or a grid to assist in drawing up and radiating the heat away from the surface of the stove. In the middle of this temperature gradient is a wafer of material with wires inside of it, known as a thermocouple. As the heat is conducted from the stove and up the stove fan the heat gradient produces a small amount of electricity that is captured in the thermocouple and transferred to a small motor that powers the fan blades.
The process is wildly inefficient for most other practical applications, however for something like a woodstove the intent is to direct some of the radiant heat forward via convection. Any loss in efficiency is in the form of radiant heat, so it's not a loss in the sense that the stove is still heating/warming a given area.
LonelyPerceptron t1_j4mjn40 wrote
Generators that are fueled by the kerosene they make from pine sap on forests that they control, right? They don’t just buy gas/diesel from the corner store who buys it from the refinery that makes it from grid-connected power, right?
jaxdraw t1_j4mkh91 wrote
I did previously because the motor had to be cranked to start. Now the fan shaft seems to oscillate no matter how I tighten it down
joanzen t1_j4mkuor wrote
Thanks! There's some that are built to run higher temps that work fine, but on a wood stove the temps vary too much for the average elements to work reliably.
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pm_me_good_usernames t1_j4mp68u wrote
For at least some Amish I think the main rule is they're not allowed to enter contracts. So they can buy gas and batteries but they can't have mains electricity, they can use a phone as long as it's pay-as-you-go. I'm pretty sure that varies between groups; each Amish community basically has its own rules. I know some of them don't use electricity at all, but they can use pneumatic power tools. And there's still a fair few pay phones in Amish country they can use to call doctors and veterinarians and people like that. There's also other groups of Anabaptists like Mennonites that are similar to Amish in some ways but different in others, even some that drive cars and work in offices but just always wear long sleeves no matter the weather.
Painting_Agency t1_j4mp7o9 wrote
Trying to gotcha the Amish when you have a superficial understanding of their beliefs isn't really a useful activity.
Their society has its own sins but they believe technology shouldn't alienate people from community and their heritage. They adopt new ways cautiously and with reservation, for instance, a telephone used to sell crops or to call for aid in an emergency. Using gasoline as needed to aid farming or the household is something they have considered and in some cases accept.
Wedoitforthenut t1_j4mpbsc wrote
Ironic that the Amish will allow heat to do the work for them, but no electricity. I wonder if thats just due to a lack of understanding, or if there is some principle of electricity that the Amish don't like? I am ignorant of Amish culture tbh.
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nakrimu t1_j4n10yk wrote
I would request a new fan if you are having that many issues, it’s still under warranty!
SacredRose t1_j4n4dfc wrote
You need to build something with as little friction as possible and it needs to run super smooth all the way around. I imagine the precision machining needed to build larger versions is gonna be pretty high and tricky so it won’t be cheap.
5kyl3r t1_j4n62f4 wrote
spoiler alert, they're still electric
they use thermoelectric devices, also known at TEC or Peltier (pell-tee-ayy). they're flat square plates. give it electric current and it moves heat from one side to the other. reverse the electric current and the direction it moves the heat also reverses. but like most things electrical, they work as the opposite type of device. (meaning led makes light but can technically be used to detect light, and speakers make sound but can technically be used as a crude microphone, and motors make motion from electricity, but if you spin them they can generate electricity, etc). if you force heat to move through a peltier, from one side to the other, it will generate electrical current. heat moves from hotter to colder. so they just put a heatsink and fan on the "cold" side, and the "hot" side attaches to the stove. the heat moves through the peltier and into the heatsink. even without the fan, a heatsink will still remove heat. that eventually is enough to move decent amount of heat, to where it starts to generate enough to power the little efficient fan. once the fan starts spinning, it ADDS to the effect, as it helps the heatsink on the cold side remove heat even more quickly, so the faster you move heat through the think, the more juice it generates. that's how they work
before you start thinking of million dollar world saving ideas related to energy, these things are horrendously inefficient. silent dorm room coolers use these. (the ones that make noise are normal compressor and refrigerant based, but the ones that are dead silent use peltiers. well, silent other than maybe the sound of a fan). yes they move heat, if you want to use them as a cooler, but they're like 65% efficient or something, so to move 10 watts worth of heat through the peltier, it will output the 10 watts you move on the hot side, as well as an addition 4.5 watts in efficiency losses, so you have to dissipate 14.5 watts to move 10 watts. that's really bad compared to compressor heat pumps. compressors for example, can do something like move 50 watts of heat while using only 10 watts of electrical power. this is why you don't see peltiers used often for large scale things. they've been used in spacecraft where they are contantly radiating heat out of the crafts, and doing it through a peltier is a free way of generating extra power. they're used in some specific industrial and medical devices too, but usually in specific situations where it needs to be small, simple, silent, etc. they're also limited on how cold the cold side can get. they have a rating of the temperature difference they can create between the hot and cold side. if you stack two of them in series, they call that a cascade. but due to the inefficiency i mentioned, the first stage is usually tiny, and the second stage much larger. if a peltier can pump 10w of heat, but outputs 14.5 watts due to only being 65% efficient, the second peltier has to pump 14.5 watts, and it will also have 45% in losses added to that, so it needs to be larger than the first peltier. if one peltier can make a 40 degree temp difference between the hot and cold side, by cooling the hot side of the first stage with a second peltier, you can get a lower temperature. it a room is 70F and your peltier can get the cold side 30F colder than the hot side, your cold side will hit 40F. but that's not quite freezing, so if you want it to get below freezing, you'd size an even larger peltier to cool the hot side, so it can move the heat from the first stage, as well as the extra heat generated from efficiency related losses, but if it can also create a difference of 30F, then it cools the hot side of the main peltier to 40F, so now it can get another 30F below that, so now it can get to 10F. you can get even colder by stacking layers of these, but 65% efficiency is pretty bad, so this turns into an upside down pyramid of wasted energy pretty quickly and scales out of reason, especially when you consider the cost of aluminum and copper for heatsinks. ok enough rambling, i figured some people who have never heard of these devices might immediately get ideas for things you could do with them, so i thought going into a deep dive ramble might be interesting for some people. if you're into electronics hobby, you can get peltiers on amazon for around $5 per. but don't expect some world changing device. they're neat, but are hindered by their inefficiency. they still have their uses, but they're limited
dogswontsniff t1_j4n7f20 wrote
Unfortunately, the heat rising off the stove produces way more vertical force than these produce horizontal force.
It's better than nothing if you got one for free, but a simple box fan can move wood stove air at a much lower cost effective price.
Looks like r/woodstoving is leaking. We get questions about these things weekly.
Merely a neat looking gimmick
Inutilisable t1_j4n7mm4 wrote
I designed lab equipments with precision pistons made of graphite in glass tubes. It’s really good but it is expensive, especially in low quantities, something like >40$ for 1/2” diameter piston, a few inches long. There was no other way to get low friction. I imagine that other constraints gets involved when you want to get any useful energy from it, at large scale.
Chagrinnish t1_j4n9kr9 wrote
If it's not under warranty you can get a replacement if you google "CD motor" or just tear apart an old CD player.
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ranman12953 OP t1_j4nbu7z wrote
Now thats a really informative answer. This person knows some things.
Painting_Agency t1_j4ncuve wrote
> Mennonites
There are at least a few Mennonites attending the veterinary school where I work. Women, otherwise honestly I might not have recognized them as such. They show up wearing their home sewn dresses and bonnets, and go to classes in a teaching hospital where they learn about every high-tech treatment that veterinarians now have access to.
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edjumication t1_j4nildw wrote
Actually in this case the wiring and electric motor are 100% efficient. Any energy lost will eventually end up as heat which you are trying to release into the room anyway.
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ZeroTrousers3D t1_j4nq1vg wrote
The few Mennonites I've known have all had phones, electric lights, fridges, stoves, etc. The basic, functional stuff. One guy even kept an old laptop for doing his books.
The way it was explained to me is that modern stuff that's used to a 'good' purpose like phones for urgent communication and business, or electric refrigeration to keep food from spoiling are okay; but things meant to entertain or replace "the work of human hands" is not.
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MyMomSaysIAmCool t1_j4nswy9 wrote
TL/DR: They don't reject electricity or any other technology outright. They pick and choose the technology that will benefit them, and reject anything they feel is detrimental.
Long version: There's many flavors of Amish, and all follow different rules. Technology is allowed or forbidden depending on its impact on the community. Telephones? Some communities love them, it brings distant neighbors together. Cellphones? Yes, for the same reason. A smartphone that lets you spend all day scrolling Reddit, that's probably not going to fly because it'll separate you from your community rather than bringing you closer.
And there's also rules for what's allowed at work. A friend of mine bought a trailer from a PA company, and she was surprised to see Amish people driving forklifts, running power tools, arc welding, etc, as part of the manufacturing process. The Amish aren't held to the same standards when on the job, because doing so would make them unemployable.
So yeah, it's not as simple as it seems, and every community is a little different.
zebediah49 t1_j4ntrsm wrote
I happened to run into one of them a couple years back, and also happened to have a thermal camera on me at the time, and thought it was neat.
Temperature across a heat-powered fan on a wood stove. Note the nearly constant bottom section temperature, and a sharp 40F delta-T in the center where the thermoelectric pad is.
E: Sorry for the potato quality, but we're talking minimum budget FLIR Lepton here. And my MSX alignment is a bit off.
Redditributor t1_j4nwshp wrote
Are Amish a subset of Mennonite?
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pm_me_good_usernames t1_j4o4etp wrote
The Amish broke off from the Mennonites around 1700. They're related, but I don't think either is a subset of the other.
the_non-binary_child t1_j4o52d8 wrote
Thermoelectric energy. Heat travels up from the base of the fan to heat a Peltier device installed underneath the fan's blades. When the device is heated, it causes thermoelectric energy which works to turn the blades and circulate the warm air around the room.
dinominant t1_j4o6pb8 wrote
Thanks for sharing this. I'm surprised the hot side is only 82°C. Not even hot enough to boil water. However the fan is actively cooling the fireplace, so that does explain why it isn't over 100°C.
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cara27hhh t1_j4oex7f wrote
fans are weird, the propeller design and material makes a lot of difference and so you can get some that claim to be the same wattage, measure at the same wattage, but that move different amounts of air
I imagine it only gets more complicated when you're trying to combine that with an unknown amount of power generated from an odd method
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I_love_hate_reddit t1_j4oirj0 wrote
That's also how motorhome refrigerators are kept cool with nothing but a gas flame from the propane tank
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bigflamingtaco t1_j4oltp7 wrote
How is free fan operation less cost effective than paying for electricity?
I don't think many are expecting tec fans to blast air across the room. Anyone that knows anything about peltier knows you don't get a tremendous amount of power out of them for the same reason they consume a lot of power to cool anything.
Mounting one to my Big Buddy heater is a heck of a lot cheaper than supplying the internal fan with D-Cells, which can't even make it through a single weekend of camping.
tribrnl t1_j4oo27j wrote
The Amish people who built the home next door to my in-laws in Iowa were able to use power tools as long as they were pneumatic. Didn't write make sense to me, but whatever. They also had a non Amish guy drive them to and from the work site in a van.
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BiAsALongHorse t1_j4oud24 wrote
Is the difference just in the ceramic around the semiconductors or are the semiconductors different too?
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polygeekYYC t1_j4ovtv1 wrote
But.....don't they have to charge them?
Turksarama t1_j4oxg1d wrote
I remember when people thought that solar Stirling generation might become a thing. Then the price of PVs never stopped plummeting.
Laslopaniflex t1_j4oztql wrote
Coincidentally, I googled Caframo this will because I own one of their boot dryers. They are a rare example of a very small town Canadian manufacturer. On paper it looks like they treat their employees quite well.
Firegardener t1_j4p07j7 wrote
Aren't those gas refrigerators using the gas to evaporate the cooling medium and so on?
mtandy t1_j4p3qbm wrote
Amish affiliations and their tech use, hand washing clothes is not popular apparently.
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DivideEtImpala t1_j4p8j0i wrote
Bicycle transformer? PSPs probably didn't draw more than 10W or so, so even with mechanical and electrical efficiency losses it should be more than enough.
joalheagney t1_j4pb3u8 wrote
The other big issue is the driving gas. You want something with a really low molecular mass for maximum thermal expansion. Hydrogen gas would be ideal if it didn't have a distressing tendency to diffuse into and through metals. And. You know. Explode in contact with heat and Oxygen.
Helium is half as good (twice the MM) ... but incredibly expensive and almost as hard to contain. Doesn't explode though which is good.
And then you're into N2, O2 and you may as well use air for obvious reasons. At about 14 times the MW of hydrogen gas. :/
joalheagney t1_j4pbw10 wrote
Because you can just allow more heat to go up a tall, well designed/balanced chimney, at a lower fuel to heat efficiency, than taking that heat, converting it to electricity, then to kinetic energy.
Same overall effect with extra (unnecessary) steps. Chemical PE -> heat (-> electricity) -> kinetic energy -> gravitational PE.
A better overall strategy is mass heater fires, like mass heater rocket stoves, or Scandinavian masonry stoves. Burn a small fire, hot, fast and about 70% efficient. Let the heat soak into a massive thermal mass and allow the heat to slowly soak out into the dwelling.
An even better solution are Chinese fire-beds, where you sleep directly on a very flat, very short stove. Heat the body, not the house.
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Raul_Coronado t1_j4pfxuk wrote
Theres countless variations of amish and mennonites that all have various standards, often based in interpersonal conflicts more than anything
warriorscot t1_j4phwcw wrote
It really depends on the particular definition of their group. Amish isn't a monolithic group, even most of the monolithic religions and groups aren't particularly well connected.
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luckeratron t1_j4phxjz wrote
Don't bother outside of some very niche uses they don't really add much.
Kraz_I t1_j4pindb wrote
Are you sure they weren’t Mennonites?
OogoniuM t1_j4piuwu wrote
They rode their horse and buggy to the stores. We have a nice sized Amish community in indiana
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nakrimu t1_j4pjxqr wrote
I believe it’s just the ceramic that’s different but don’t know for certain as I just built them. The modules were pre-assembled.
nakrimu t1_j4pl0wv wrote
Yes it was a great place to work nestled on the shores of Lake Huron. You can sit at a work station and look out over the water, when you are not busy of course! It’s very small town and everyone looks after each other!
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ggobrien t1_j4q0mxu wrote
At the risk of being completely pedantic, if it's 65% efficient, it would be need to dissipate 15.38 watts to move 10 watts (10/.65).
Or, I could be completely misreading what you said, which is highly likely.
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gh0stwriter88 t1_j4q2soo wrote
Its because grid electricity makes you beholden to the electric company....generators are usually ok for work use or pnumatic.
gh0stwriter88 t1_j4q34wp wrote
Yes woodstove mounted sterling engines... It's small just enough to drive a fan decently. Sterling engines don't require tight tolerances... For basic ones, you only see that in solar parabolic generators etc too eek out more power.
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Mikel_S t1_j4q3rti wrote
Now that this is answered so concisely, can anybody link to one of these fans? I'd like to see if we could retrofit one onto our flue.
ozspook t1_j4q8opp wrote
Stirling cryocoolers were quite popular for superconducting high-q microwave and mobile phone base station filters, for a while.
Pretty good 2nd hand source for making Stirling engines.
ozspook t1_j4q9iv9 wrote
I use mine to cool the base of the flue a bit, where it exits the heater, hopefully extend the life of the hottest part a few more years.
tribrnl t1_j4qhhcb wrote
Thanks, I always thought that it was about modernity or technology or something!
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bigflamingtaco t1_j4zxrx2 wrote
I don't think we're on the same page here. I'm talking about using a tec fan with a portable propane heater instead of using batteries to run its internal fan, not using tec fans as a solution for all heat distribution requirements. You use tec fans with Mr Heater style propane burners and micro stoves as often used for winter camping. If you're running your buck stove in the living room and want to distribute the heat to the other end of the house, tec fans aren't going to do it.
5kyl3r t1_j51h9qb wrote
yeah as soon as I saw "pedantic", I immediately knew that I probably botched my on-the-fly math haha. thanks for the correction good sir or madam.
ggobrien t1_j51jk6h wrote
Take my advice as much as you paid for it, it's worth about the same :)
On the fly math is always scary, especially in online forums where people are quick to be pedantic (I would never do that).
joalheagney t1_j51th4w wrote
Ah. The OP was talking about installing a stove so I was thinking like a wood stove.
bigflamingtaco t1_j5319nn wrote
Sorry, I probably should have been clearer about my transition to small heaters.
Clutterking t1_j57zpfh wrote
This effect is used on space craft, using the delta temperature between radiation from nuclear material and the vacuum of space -- it's how the voyager space craft had the power to communicate while so far from the sun.
Russia also used it with nuclear material to power light houses, 'cause Russia.
https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_generator
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