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Comment by grecy | Hacker News Reader

Something I've always thought about.

Northern Canada we run our wood stoves for ~8 months a year. Temp on the surface of the stove is 200C to 300C. Outside is -20C to -40C.

Let's say an average of 200C temp delta across the TEC.

Throw a radiator on the outside of the cabin, use coolant with a tiny pump to circulate. How much power am I going to generate with a square foot (or two) of TECs strapped to the wood stove, using the coolant loop to cool the other side?

It feels like we have such a huge temperature delta we should be able to do something with.

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To steelman your proposal, I should mention that Low-Tech Magazine thinks you're on to something, although they suggest putting the panels on the stove inside the cabin: https://solar.lowtechmagazine.com/2020/05/thermoelectric-sto...

I disagree with that article for the simple reason that I do not consider burning wood to be sustainable, or healthy for anything with lungs. Wood should be used for long-lasting construction or left alone / carefully nurtured to aid our forests in rebuilding biodiversity.

https://www.nytimes.com/interactive/2022/09/07/world/europe/...

https://www.theguardian.com/commentisfree/2022/dec/27/wood-b...

But if you're going to burn wood anyhow, perhaps thermoelectric stoves aren't a terrible way to generate electricity for yourself.

> I disagree with that article for the simple reason that I do not consider burning wood to be sustainable, or healthy for anything with lungs. Wood should be used for long-lasting construction or left alone / carefully nurtured to aid our forests in rebuilding biodiversity.

In Northern Canada there is almost no other way. Once you are ~10minutes out of town there is no natural gas. Also note wood stoves now are double (or triple) burning with catalytic combustors that extract waaaay more heat from the wood and emit waaaay less particles in the air (because they burn it)

Most burned wood is wood unsuitable for construction. No one goes into the woods and cuts down a 200-year oak to burn in their stove.

Short lengths, widths, knots, imperfections, wood waste pressed into pellets, etc.

Although you do have a point about it being bad for air quality. Especially old stove installations.

Here in Europe there is so much demand for wood that forests are being grown just to be cut down and turned into pellets - it's not just waste that goes into them.

Maybe it's not as bad as fossil fuels, but it still has the same issues as commercial monoculture farming, which is why there are issues with bark beetles now. And most of these forests start by tearing down old growth forests.

In my vacation house, I’ve got a Hearthstone Heritage wood stove, neither old nor new, and I believe it has been refurbished a bit. When I ran it over Thanksgiving I put an air purifier next to it and ran it on auto. I’d been concerned about air quality with the wood stove. Pleasingly it ran on low and barely blipped when I’d open the door to add logs. Weirdly, it -would- climb up to medium if I opened the door to the garage — I guess it doesn’t like all the varnish and whatnot from there.

So anecdotally, a stove that’s not a rusty old potbelly and has a good draft seems like it could be okay for air quality.

You can sell the trunk of a 200 year oak (though often they’re hollow if they’re that old) but you will have PLENTY of good firewood left behind from harvesting it, with all the branches. The best use of a 200 year old oak is of course to let it be, the backbone of an entire ecosystem. An 80 year old walnut is another story…

It's not the wood stove owner that is exposed to the wood smoke, it's his neighbors.

"Northern Canada" -- he doesn't have any neighbours. Wood stoves are fine in such a place. They aren't fine at scale.

I think a lot of these could be used for building construction, or for smaller wood objects, if we were sufficiently motivated.

For example, here is an MIT team experimenting with using forked tree branches in construction: https://energy.mit.edu/news/using-natures-structures-in-wood...

I thought pellet stoves got hot enough to be pretty clean. Co2, yes, but not much else?

Residential pellet stoves sound great but in practice they are not reliable (moving mechanical parts that operate at high temps, electronics poorly shielded from said temps—the manufacturers are pretty shady in my experience), and the cost of fuel varies quite a bit and can be very high. Compare to a wood stove that needs no electricity and can burn just about any combustible solid. Modern wood stoves are pretty efficient, too.

> but in practice they are not reliable (moving mechanical parts that operate at high temps, electronics poorly shielded from said temps

25 years ago my grandmother bought a pellet stove that lasted for 15 years with semi-regular maintenance.

Her next stove, same company, broke all the time and lasted less than 10 years, with much higher ongoing costs.

Enshitification strikes again!

If you run it properly. A good wood stove will run cleanly as well. It’s all about the secondary combustion. An open fireplace will not get hot enough for secondary and be dirty.

you're not wrong per se but no one cuts down a 200 year old oak for construction either. softwood like spruce, fir, and pine is the primary wood for construction (at least in north america).

> I do not consider burning wood to be sustainable

You can't really make such a broad statement. Sure, burning wood isn't a sustainable solution for everyone everywhere, but there's plenty of situations where it's the least-worse option for heating. My parents heat their house with wood they gather from their property which mostly falls naturally. It's impractical to do anything else with it.

Perhaps they could let it decompose - releasing approximately the same amount of carbon it releases in burning - but while that might benefit the local ecosystem marginally, it would also mean they were heating their house with fossil fuels instead.

Doing that pumps energy outside your house without heating your house. A better option is to use the temperature differential between the stove and your house. You get less electrical power, however waste energy from both the stove and the device still heats your house effectively making a system over 100% efficient.

Ie convert 1% of the stoves energy into electricity which you then use to power a fan. The fan‘s waste heat also heats the room thus you get to use both the utility of the fan (1%) + waste heat from stove (99%) + waste heat from fan (1%) = 101%.

You're not going to achieve an over-unity system. Assuming no other losses, you'll be using 100% of heat energy from the stove. The equation is missing a -1% term somewhere.

You’re allowed to make use of the same energy at different points in time. The 1% used by the fan is the same 1% that’s heating your room.

A computer converts 99.9% of the electricity to heat a room just like a space heater. However, you can play video games on the PC while it’s heating your room.

That's why all electric heating should be devices that do work or compute. Do not waste the Gibbs free energy of the universe.

I want a cottage industry of aluminum smelters in cold climates.

Pick up a pile of aluminum oxide from a depot, electrolyze it into aluminum at home, and drop off the finished aluminum at the depot for a cheque that covers your electricity.

"The theoretical minimum energy requirement for this process is 6.23 kWh/(kg of Al), but the process commonly requires 15.37 kWh."

https://en.wikipedia.org/wiki/Hall%E2%80%93H%C3%A9roult_proc...

That's quite a few kWh of heat per kg of finished product. You're going from Al2O3 to Al, so your returns will weigh less than your pickups.

Comparable to coal or oil heat:

"With a complete combustion or fission , approx. 8 kWh of heat can be generated from 1 kg of coal, approx. 12 kWh from 1 kg of mineral oil"

https://www.euronuclear.org/glossary/fuel-comparison

Neat idea. Aluminum smelting isn’t really hands off enough to make this viable at home.

It might make sense at industrial scale while pumping waste heat to nearby office buildings or something. But even slightly higher electricity costs would be a huge issue.

The problem is that heat pumps exist, so resistive heating is never the best option.

An aluminium smelting at home heating solution would have a maximum CoP of 2 (as you're reducing the energy usage of the smelter by the same amount). A heat pump could easily have a CoP of 3 or 4.

You have that math backwards. A smelter turns ~1/2 the energy from electricity into chemical energy and releases the remaining 1/2 as heat thus the COP would be 0.5 not 2. However smelting is profitable because aluminum oxide is less valuable than aluminum which more than compensates for the waste heat having zero value at the smelter.

Thus various schemes for combined heat and power etc which take the valueless waste heat and use it for some other purposes like warming homes or greenhouses etc. So while the COP would be 0.5 the electricity would be subsidized by the smelling process. https://en.wikipedia.org/wiki/Cogeneration

Where such schemes fail is generally economies of scale.

That’s a lot more efficient than an industrial aluminum smelter.

The use of electricity as an input is inescapable for this process, so may as well use the waste heat as directly as possible.

I should mine BTC in my heatless basement! Love it.

this was the only sane argument for blockchain mining but it was never competitive with heat pumps

Depends how cold it is outside, but yes, heat pumps usually win.

They didn't miss anything. They're just double counting utility. Same thing would happen to get warm next to a computer. You pay for electricity once, and use the computer while getting cozy.

You misunderstood. For comparison, a typical cheap AC has an efficiency (coefficient of performance) of 300%. For every 1W consumed, the AC moves 3W.

That's an AC. This is a heat engine run normally. You're not getting >= 100%.

The "over 100% efficiency" is technically incorrect, but the OP is right.

Carnot limits the efficiency to make work from a thermal process (much less then 100%, in fact).

Carnot does not prohibit you from enjoying the inefficiencies as waste heat.

Yes there is a massive potential for power generation. Stirling engine would give you the maximum efficiency but maybe not the max power output.

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

https://www.youtube.com/watch?v=Ge3cerSVEI0

When it's -40 often, and you live off grid, no moving parts trumps complexity every time

Wouldn't that effectively just suck the heat from your home?

I feel like that's not a great use of your stove?

  Heating with a wood stove doesn't really have a 'medium' setting.  You get 100% while it's roaring away, and that slowly diminishes as the fuel burns down.  (Yes you can adjust dampers, and air-flow valves to "slow it down"; but that doesn't really work/happen.)

  I've lived in a home heated by a wood stove for 10 years.  You would build-up the stove so it blasts away, goto bed, and by morning there would be enough coals left to start it up again but quite chilly in the house.

  There is nothing like it.  I love it.  I don't have the same access my father did to family farms, wood lots, and trees.  Today I would look into rocket mass heaters.  Spread that heat out and allow it to more slowly heat a house.

Yes I'm aware. I grew up with a wood stove as a primary source of heat, and hated it.

But that doesn't really address my issue. For an ambient wood stove, the heat is pushed through the house from one central spot (the stove).

If you use a system to use that heat for power or something else, aren't you just going to cool your home?

Or you do what slavs did for century and build a massive mud oven to trap the heat in.

Better yet, bring back Feolite for ridiculous heat capacity in Calories per gram.

This sounds all very "homesteading". If you automate various components including wood feed you could get a better experience.

The solution to this is and has been for centuries is to add mass to the stove.

For the PD to deliver work it would have to transport heat outside. Heat you're burning wood to have inside.

Better to store the heat somehow, and have it be slowly released.

Heat we have a massive, massive, massive excess of.

It's electricity I want, in the winter, when the sun is only on the solar panels for ~3 hours a day.

ya, but not enough to overcome PD's limitations.

You're better off buying a little steam turbine and generating electricity directly off of the stove. On top of the electricity, you get to keep the heat.

Or, better yet, buy a propane tank and a suitably large generac. Then pipe the cooling water into radiators in the house.

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