Thermoelectric generation

[smelly]

Hello, I'm a newb to the forum but have done a bit of hunting on my woefully slow 3g connection before posting and hope I'm not treading well trodden ground.

I live on a narrowboat in the UK and have a 4kw solid fuel burner that's lit pretty much 24/7 during the darker months when my 130W PV panel's not producing much.

We over at canalworld.net are playing with the idea of using peltier modules to generate electricity from our fires and I'm wondering whether anyone knows a bit more about these modules.

I appreciate that seebeck is viewed as rather inefficient source, however generating any significant current will be useful; a couple of amps over a day saves an hour's engine running while charging the batteries, this would mean a 25% reduction in our electricity generation cost over the winter.

Any significant differences between the modules sold as TEC rather than TEG would be of particular interest, and ideas as to how hot we can get them and the like would also be useful.

I've found an article in make magazine where someone using a 37watt cooler module with a candle managed 400ma, if there's a linear relationship then that would mean roughly 6amps for me which is most of our domestic consumption over 24 hours and would mean a wood powered laptop

Our electricity comes from 12v truck batteries that when new totalled 440Ahr but ours are pretty old now.

To save people doing too much reading, the side of the fire i intend to mount to runs at around 150 degrees C at a minimum; that's quite cool for lot of boats as we've got a 2Kw boiler in there that heats our hot water and 1.75Kw of rads; a lot of boats don't have such boilers.

For a bit of context, the last time someone tried guessing how many residential boats there are in the UK they reckoned at about 15000; the vast majority of which have solid fuel as their primary heat source as diesel's unreliable and expensive while LPG is unwieldy when you need to change bottles every few days.

 

[Blars]

You can't just look at amps, you also need to look at volts to get the amount of power generated. To get a reasonable amount of power, I expect you would need quite an investment in peltier junctions. Have you considered some older tech like a steam tubine or steam engine driving a generator? RV boondockers have similar problems, (but no burning solid fuel) and frequently use solar panels supplimented when needed by gasoline, desil, or propane genoraters.

 

[dknguyen]

People have asked this before (though nowhere near as well defined or with a specific application in mind as you have). I've never heard of anything come of it. It's been done but by aerospace/military types only when there was no alternative, let alone home users. Read this for what it is and why it doesn't work well if you can get it to work at all:
Radioisotope thermoelectric generator - Wikipedia, the free encyclopedia

37watt @400ma sounds really far fetched to me. To put that in perspective, that works out to be 92.5V@400mA which is enough to shock and kill you as well as power an incadescent light bulb. That is FAR better than any solar panel and I would think if that were true you'd hear about them everywhere...yet most people don't even know what a thermoelectric unit is. I have a really hard time believing that unless it was a unit of gigantic cost and size (or a fancy aerospace radioactive one).

I see on the forum that people are saying things like "even 1A at 14V would make a difference" as if 1A or 14V was small. It 1A@14V might be small for a boat, but to us over here (well, me at least) it sounds like someone trying to do the job of a gas generator with some AA batteries. I'm actually really impressed by the link that the one guy on your forum posted about the hand powered LED (if it is genuine). I didn't think they could do even that.

I agree that as long as you're using a wood fire, you might as well go with a steam generator. And of course there are solar panels but you already mentioned those and said it was for dark months. Though I suppose you might get somewhere if you can afford to get a few hundred of them to link together. But make no mistake, a steam generator would kick it's ass.

And just a thought...but what about a solar boiler maybe driving a steam generator? I heard solar boilers are much much more efficient than solar panels and can be quite effective on cloudy days. Don't know how well they would pair up with a steam generator though. Still a rather exotic approach though.

 

[smelly]

wow, a title in a reply, I've never seen that before...

cheers guys!

I should probably clarify that the 37 watt example was that of a 37 watt cooler with the polarity reversed. Working that through as there was no voltage mentioned, at 6% efficiency that's give ~ .22 watts, so not many volts, I may well need more modules... Ah well, the thought process develops

Regarding steam, the logistics are against us; the majority of our burners are either in the middle or in the bow. Any turbine gear would need to be with the engine/batteries (or in the living quarters that are usually 6ft wide) that in the vast majority of examples are 10 to 15 metres away from the fire and that makes for a lot of heavily insulated pipe losing a lot of heat along the way; it's also diverting a lot more heat from the cabin and in the harsh winter last year that would've left the boat uninhabitable, it took serious care of the fire to keep it burning hot enough.

As to solar; certainly we've got 130w polycrystalline that looks after the summer days; but it's the winter nights where this design is best suited, i.e when the fire's working hardest.

With voltage I plan to raise it by running the modules in series; I wonder whether the spare PWM controller I've got hanging around might be needed and also I wonder whether, if open circuit they're running at say 17 volts, fastening them into a battery would naturally suppress the voltage to a useful level. (I may have just demonstrated my naivete in these matters...)

I've already ordered some modules; they're 136 watt coolers and I intend once again to reverse the polarity. Other than the inherent limitations in the system one serious consideration is how hot I can get them. Most suppliers of the modules quote max temps between 200 and 230C, solder junctions seem to have an uppermost melting point of around 230 using high temp solder, that's a concern as when the fire ran away earlier today the fire sides hit 250...

One last comment about the solar steam generator; there is a big debate on the Sietch blog about similar; I draw from that that to make best use of solar heat you need glass as it's best at trapping solar energy; glass and canal boat roofs do not mix well as in urban areas the canals run through it's not unusual to come "under fire" from brick/air rifle wielding kids. I was choosy of which PV panel we bought with this in mind (it's an evergreen btw) on a personal note as well we've no space on the roof; it's our shed with firewood, coal, the wind genny when it's not up etc...

 

[dknguyen]

What about a wood fire steamed generator?

 

[smelly]

What about a wood fire steamed generator?

Yo no comprendez, what's the difference between that and a heat exchanger/turbine arrangement?

 

[dknguyen]

Oh nothing, nvm. Difference in terminology and it didn't register.

 

[tcmtech]

The fatal flaw behind steam turbine generators for private use is that they still need a pressurized steam source to make them work and that requires loads of safety related components needing to be built in to the system along with the bureaucracy of certification and other legal issues related to having a pressurized boiler system in general.

The peltier coolers running in reverse as thermoelectric power cells works but its not all that efficient. Its cheap and easy to build though.
The return on power is far less than what they take when powered to work as heat pumps. You may only see 5 -15% returns at best on electrical power with them working as a power cell in relation to what they take in heat pump mode. That is a 100 watt peltier unit will probibly only put out 5 -15 watts of usable electric power working as a power cell with a wide heat differential from one side to the other.

I made one for a elementary school science room demonstration device that used an 12 volt 20 watt cooler from a old CPU heat pump. It used hot water on one heat sink and snow on the other and pit out around 1.5 volts at about 1 amp. Basically it was the same as a D battery but the kids thought it was pretty amazing that snow and hot water could make electricity!

Also they have a upper limit to how hot the can get due to the type of solder they are assembled with. Much over 300 - 350 degrees F and it melts and the cell is destroyed.

Fortunately for what they cost they are cheap and simple things to buy and play with so I say pick up a few higher powered ones and see that you can do with them!

 

[smelly]

The upper temperature limit is going to be the biggest, or most expensive challenge; we've found a 300C intermittent cell for a sensible price now, I'm just waiting for the supplier to contact me.

If they're still the price advertised we'll be in business.

 

[cobra1]

care to share

 

[Oznog]

It IS a neat field. However, as noted, there's lots of limitations.

There's very little power generated at low temp differentials. High temp differentials are readily available from combustion sources, but the lifespan is much more limited at higher temps where it actually produces useful amounts of power. Also, it may be difficult to regulate the temp to accurately prevent even higher temps. Such as a candle flame source- suppose 300C is ideal, 250C is just too low for performance, and 325C exceeds the temp limit. Can you really guarantee 300C without a feedback control mechanism?

It's not just a matter of the heat itself, either. The module and the thermocouples undergo thermal expansion, and the number of cycles it can go from 25C to 300C may be mechanically limited by the structure of the thermocouple. It's not the whole module which expands, either- the "cold side", when well-cooled to get proper performance, won't expand, but the "hot side" will. That's an uncomfortable geometric change for it to undergo.

 

[smelly]

care to share

Termo-gen.se are advertising 200 deg TEGS for 8 euros, they've a higher temp range (230 cont 320 intermittent I doubt the fire will make 300C) that that they don't advertise but considering US suppliers are asking >$50 for 200C modules I reckon Sweden's the place to go... I await their price list.

 

[smelly]

It IS a neat field. However, as noted, there's lots of limitations.

There's very little power generated at low temp differentials. High temp differentials are readily available from combustion sources, but the lifespan is much more limited at higher temps where it actually produces useful amounts of power. Also, it may be difficult to regulate the temp to accurately prevent even higher temps. Such as a candle flame source- suppose 300C is ideal, 250C is just too low for performance, and 325C exceeds the temp limit. Can you really guarantee 300C without a feedback control mechanism?

It's not just a matter of the heat itself, either. The module and the thermocouples undergo thermal expansion, and the number of cycles it can go from 25C to 300C may be mechanically limited by the structure of the thermocouple. It's not the whole module which expands, either- the "cold side", when well-cooled to get proper performance, won't expand, but the "hot side" will. That's an uncomfortable geometric change for it to undergo.

The geometrics may well break it assuming we can get high enough temp tolerance, but maybe stacking would limit the stresses. I plan, when the first TECS arrive, to compare the differential and insulating properties with a couple of thermometers; a flat plate magnetic stove therm' and a probe therm'... Stacking might be the compromise but tbh I'm yet to see; the 12709 modules should land this week and the top plate of the fire shouldn't kill them so the data will start flowing. The higher temp ones will have to wait until next payday and even that may be abated by new batteries as the current ones are really struggling

Folk on canalworld will start to swap stats around then

 

[cr0sh]

Have you thought about using a Stirling engine generator? I know there was one company out there that made such a generator in a fairly small package meant to be mounted at the focus of a parabolic reflector. It used a "captured piston" design (in which the piston, which was in essence a permanent magnet, shuttled back and forth through a coil of wire) - not a reciprocating engine coupled to a generator.

 

[Pommie]

I found these pdfs a while ago. They should give you some ideas.

Mike.

 

[Oznog]

Hi-Z technologies is a small tech company which has been around for many years. They put a bunch of proper, honest design docs online. I'm not really gonna sort through it right now to find the right ones, or even if they still have the good ones online, but this is them:
**broken link removed**

If this is a boat, you're gonna want to use water-cooling. The system efficiency is poor and/or degrades the modules with high "cold side" temps and you will probably have many watts of heat rejecting out the cold side. It'd take a really big heatsink and a lot of fans to air-cool, and it'll probably not be able to approach the cold side temps possible with water cooling.

The fans themselves- or the water pump- can easily consume more power than the TEGs generate and that can invalidate the whole system's purpose. A heasink which is just a straight path of aluminum/copper/heatpipe into the water would do it without expending power for a fan/pump, but it would require some genius to do this in a practical manner.