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Comment by Animats | Hacker News Reader

2.5 kg soft pine wood per hour ... average power output of 4.2 watts

That's 1.68wh/kg of wood.

Pine has a stored energy of about 15 million BTU/Cord (2380 lb)[1], or 4,396Kwh / 1080 kg, or 4000 wh/kg of wood. 0.04% efficiency.

That's terrible by the standards of 19th century steam engines, let alone anything newer. It might be a way to get enough power to charge a cell phone from a stove used for other purposes. But burning wood just to make electric power this way is going to need a lot of forest for very little energy.

Note that the article has drawings, but not pictures, of a vacuum and scooter being powered with a thermoelectric unit. They probably can't do that.

Thermoelectric systems are often used to power heating controls, where little power is required. Some bigger ones can power heating fans as a backup. The U.S. Army has a portable heater with a fan powered this way.

[1] https://www.engineeringtoolbox.com/wood-combustion-heat-d_37...

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Plus, the conversion of sunlight to wood is maybe 0.5-1% efficient if you’re lucky, compared to ~20% for solar.

So about 0.001-0.002% as efficient as solar, up to 5 orders of magnitude worse. With the advantage of less intermittency, I suppose, but both are solid state energy converters; I wouldn’t claim thermoelectric is “lower tech” except that it’s way less efficient and with more environmental footprint due to that inefficiency. A battery to solve the intermittency would be still a much better trade IMHO.

However, solar panels don't make more solar panels at 20% efficiency

They do, if you power a solar panel making plant with solar panels.

Neither do thermoelectric converters grow on trees.

I’m not saying that biology isn’t great. Self-replicating machinery is pretty awesome. But if you’re using advanced machines anyway, you should skip the biological middleman if you can and go right for the source of energy.

Huge losses at every stage, I guess. A good chunk of the heat is going to go right up the chimney, then most of the heat that doesn't escape is going to directly heat the room the stove is in without going through the thermoelectric elements, and then only a tiny fraction of the tiny fraction that passes through them is going to get converted to electricity.

What might be better is something more like a steam engine, except without the complicated maintenance and operation requirements and the annoying tendency to explode if not carefully maintained. Sadly no-one seems to have been able to design such a device.

> Sadly no-one seems to have been able to design such a device.

Why not a Stirling engine?

https://www.microgen-engine.com/ seems to be a commercial source, and https://www.kickstarter.com/projects/672465444/low-cost-ster... was a (failed?) attempt at developing a cheap 1kW generator. The more recent remains are in this blog: http://volodesigns-sterlingproject.blogspot.com/

It looks like the Kickstarter turned into https://seftonmotors.com/ , which are about 2000$ for a 1.2kW (full temp, pressurized) engine.

Yeah, that's the problem - all the Sterling engine solutions end up as expensive, exotic, low production natural gas powered devices. Which means that you need a supply of natural gas, and at that point you have other generation options. There was a slightly lower-power, but much more compact and finished Australian product along the same lines a decade or so ago called the WhisperGen which was intended to replace a hot water and heating boiler. They ended up going out of business unfortunately.

Not sure where you're getting the nat-gas-fired idea from. The more-commercial link I provided seems available as a micro-CHP setup, with a wood-pellet furnace. The Melvin Package from Selfton Motors has a combined propane/wood fire box, but the Melvin engine itself is a separate part, allowing use of a custom heat source.

I'm not sure though why you want to burn wood on that scale. It's really expensive to properly filter the exhaust from a wood stove, and economy of scale doesn't hit in the kW range. I mean, yes, if you're in the middle of nowhere, sure, but I'd claim that an install cost of 3~4$/W(electric) for the Melvin Package, including shipping (even to a potentially-remote location, only 100kg for the largest piece allows transport with e.g. a paramotor or two humans, with additional trips for the cooling jacket/firebox/vents/piping) is acceptable for a reliable wood-fueled electricity source.

> A good chunk of the heat is going to go right up the chimney

It's probably not much.. but I imagine you could boost the efficiency by lining the chimney with copper tube and running water through it, using the waste heat there to heat this water. Then use your TEG as a heat-exchanger between that loop and another loop connected to your "domestic water heater" or other cold water supply.

I honestly thought this article might be a joke – burn wood to generate a trickle of electricity.

Maybe it's merely teasing the Ig Noble Prize committee though. :-)

That's because you're living in a rich country where energy is cheap and abundant. That 40% of the global population that needs to walk for miles to charge their phone for a hefty price, or light their shelter with torches, are very happy with that trickle of electricity. Maybe we -- cause I am also in a rich country -- could learn from that.

Looks like these piezo chips are used for cordless stove fans. Just enough to run low power fan.

I have one - they're more a cool decoration than a fan. If you shape the fan blades to actually move air, the motor slows down to nearly stopped under the load. That's why when you buy them in the shop the fan blades are usually perfectly flat so they don't blow any air at all, but look cool on top of your stove.

Kinda thought they look too good to be true. Market is filled with them and I saw one referred in a sailing channel.

We’ve got mains powered one in our current place and it makes a big difference.

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