Anybody Using Thermo Electric in their PV System?

paulstamser · November 2007

Hi,

I run a small (180 watt) 12 volt stand-alone PV system at my remote homestead with a backup gasoline engine generator.

This time of year the days are short with seemingly endless cloudy periods when there is almost no solar PV input to my battery bank so I have to fire up the noisy, offensive, and fossil fuel burning gasoline engine.

However, at this very same time of year I also have a woodstove operating to heat my home. This woodstove will be running almost constantly for the next 5 months. I probably am not the only one running this type of combined PV/wood heat stand alone system.

My question is whether there is a practicable and affordable thermo-electric element for a woodstove that produces 12 volt electricity from HEAT. When I Google the question it seems that the technology does exist, but is it available and affordable?

For a small PV system like mine a thermo-electric unit putting out just a couple of amps at 12 volts would be very useful. For example, today is cloudy AGAIN and my laptop computer at 2 amps is more than my PV array is putting out so my batteries are discharging and this has been going on for days now, but my woodstove has been blazing all along with LOTS of energy!

Is is practical to generate electricity and charge a battery bank from a (non) fossil fuel burning woodstove?

Has anyone here seriously investigated or tried doing it in conjunction with a PV & battery bank system?

If it is even remotely practicable and affordable I would love to try it!

westbranch · November 2007

Re: Anybody Using Thermo Electric in their PV System?

here is a link with some cautionary notes near the bottom for your idea.
http://www.kk.org/cooltools/archives/001525.php

we use one of these small stove fans at our cabin and it is great but I dont think it would put our much juice for charging as it is quite small. you might want to contact them re output.

http://www.caframo.com/ecofans.htm

good luck
Eric

mike95490 · November 2007

Re: Anybody Using Thermo Electric in their PV System?

It's real hard to get USEABLE volts from heat. There are some small fans that run from woodstove heat, but they put out milli-amps, not amps. There is some concern about their longevity too
http://www.fluesystems.com/sundries/info/ecofan.htm

niel · November 2007

Re: Anybody Using Thermo Electric in their PV System?

melcor has some useful info on them and i agree that you won't get much bang for the buck, but in the dead of winter with little being produced you may think even that little bit to be worthwhile.
http://www.melcor.com/index_melcor.html

SolarJohn · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Since you have 'heat' available, have you considered a sterling engine? It seems that if you're burning wood for five months at a time, it may be worth the trouble/expense to have one built.

John

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Great minds think alike. I like you have a small stand alone system, with wood heat. I do use a thermo-electrtic fan and it works great. Several years ago, I had the idea of, "wouldn't it be neat to power a muffin fan on the ceiling from such system. I built a proto-type, with perhaps a dozen modules, wired together in series to get 12vdc. ( I don't remember any of the specifics however). I had a giant finned aluminum heat sink on the cold side and a piece of steel on the hot side. Try as I might, I could never get it to do any thing. The only way I could get it to put out enough open circuit voltage to measure was by putting a large block of lake ice on the heat sink, which of course melted so fast as to be useless, sitting on the stove. I tried using water as a transfer medium and finally gave up. I suspect that one of the problems is designing a system with a way to create a big enough Delta T between the hot and the cold side, considering it has to sit on a stove.

While the idea is still intreaging, I will have to find a greater mind than mine to solve it. If you make progress please let us know.

I now run a 12vdc ceiling fan that draws about 5 watts and works quite well off the 12vdc solar system.

Good luck,

Icarus

BB. · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Probably about the most efficient way to generate electric power from your wood lot would be to use wood gas from a wood gas generator burned in a internal combustion engine powered generator.

And if your generator is too noisy--have you looked at the new inverter type generators (Honda, Yamaha, and other knock-offs)... They are real quiet and much more fuel efficient than the old standard 5kW $399 generators. And can save you lots of money in fuel costs (short of a good diesel gen-set).

-Bill

paulstamser · November 2007

Re: Anybody Using Thermo Electric in their PV System?

I should have known that thermo-electric wasn't very practical or it would be a commonly used feature of stand-alone PV systems in northern (forested) climes like this area. It seems like it SHOULD work perfectly well.

If sunlight hitting a PV panel puts out current, why not heat on some different kind of panel? But it's not as simple or easy as my wish would have it to skip the mechanical stage like PV does so elegant and well!

I guess that I'm stuck with my generator which should be running right now. Plus getting another PV module to beef up my cloudy weather output somewhat.

Someone mentioned a Stirling engine, which is a good idea. The ironic thing is that my gas generator (a home-built Honda 4hp engine with a Chevy 12 volt alternator) is sitting on the heavy cast iron stand of an antique Erickson hot air engine. A big massive thing with a huge cylinder and piston but apparently it worked.

A little "toy" steam engine turning a very small generator would also seem a possibility. (I can hear water steaming in a kettle on my stove right now).

Years ago I looked into wood gasification and that would be somewhat practical for me, altho the conversion and added labor would be a big job, but I could do it. Yet it's so easy to pour gas into a engine's gas tank. Plus I'm already using wood full-time right now in two different applications.

Recently I've put wood to a 2nd use and that is for a dedicated summer hot water heater. This heater burns lower quality wood and scraps that normally I wouldn't bother with in my home woodstove but of which I have an unlimited supply. Since using this wood-fired water heater in the summer time, I have cut my propane useage in half or more. In winter I also sometimes cook on my woodstove and that helps too. Of course in winter I have all the hot water I want from my home woodstove.

The less fossil fuel I use, the better I like it!

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

The difference between SHOULD and does is a long distance! I've spent my life working on things that SHOULD work, only to find that they don't.

The problem with thermo-electric is that it relies on the DIFFERENCE between the hot side and the cold side. In addition to putting the heat into the hot side, you have to remove the heat from the cold side. With the thermo-electric eco-fans, they solve this problem with a very small draw fan, coupled with a large heat sink on the cold side. The added benefit is that the fan draws cooler air over the heat sink creating the Delta T required to make it work. If you have an eco-fan that is running slow because the fire is low, try putting a ice cube on the heat sink and see how much faster it turns!

I had an interesting experiance with mine one time. We had been away for a couple of weeks. The temp was ~ -20 and had been for weeks. My neighbor came down and lit our fire earlier in the day. When we came home, the
room was about zero F. The stove was cold to touch, but the fan was whirring away. I thought, what the heck, why is the fan going when the stove is cold? It turns out that while the stove felt cold to touch, it was perhaps 50F, while the room was Zero! The power of the device is defined by the DIFFERENCE in temp, not the amount of heat you can put in. In fact too much heat will damage the unit. The fans have a bi-metal spring on thier bases to raise the base off the stove if it gets to hot.

I'm sure there are keener minds than mine that COULD come up with a system that maintains that Delta T, and puts out SOME useable power. My gut feeling is that it is not a very effecient way of generating power. (You could argue that it is truely FREE power in the sense that it doesn't "use up" the heat of the fire and therefore effeciency doesn't really matter, but from a production/sales point of view it would be a loser).

As for steam, my argument there is, how complicated a system do you want to have? My feeling is always KISS. Unless you love tinkering with systems for the joy of it, you would be best to look at the consumption side and make that as effecient as possible, and then add panels and batteries as you can.

While applaud your home made generator, you might gain considerable effeciency by using a modern inverter generator. My honda Eu 1000 runs my xantrax 20 amp charger, all the power tool chargers, the laptop chargers, modem etc at once, all while running on idle power. It uses ~1 litre of fuel in 4 ours with that load.

Keep thinking, perhaps you will come up with the next big thing!

Icarus

niel · November 2007

Re: Anybody Using Thermo Electric in their PV System?

i was going to try this for myself, but as you said that delta t or temperature difference gives you the power. i thought that in the winter that isn't a problem as there's plenty of cold air out there. the trouble with it is that it passes heat from the hot side to the cold side and that means it's wasting heat by heating the outside air. in the winter most of us want to keep our heat because of how expensive it can be or difficulty in producing/maintaining it and i never did it because the benefits didn't outweigh the drawbacks.

paulstamser · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Interesting story about the "cold" woodstove and the fan...

Now that I understand better how thermo-electric works, the hurdles for its practical use become apparent. This does recall to mind stuff I read years ago about it and why I gave it up back then. By now I thought some new break-through had been made. Guess not.

It seems to me, however, that there was a company making T.E. power producing units years ago. Not something you ran off a woodstove, but a boxy type of appliance that perhaps ran off natural gas or propane. My memory of it is hazy and could be off and apparantly if they did build one it didn't catch on.

I'll have to look at the modern inverter generator. Tell the truth, I don't even know what they are. I built this Honda engine/Chevy alternator charger back around 1993 and have been using it ever since with no problems. I'll have to check its fuel consumption against 4 hrs per liter. I run it at around 20-25 amps and it does the job charging my 4 golf car batteries. But I were to spend money on some addition it would probably be for another PV panel.

I agree with the KISS philosophy. The idea of a small steam engine is probably a whimsical one similar to producing electricity of any kind from a woodstove, esp. direct conversion of heat into electricity without the dreaded mechanical step.

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

The only specs I have ever gotten on TE devices are amperage in-temp out specs. I have never seen the reverse.

The new inverter gennys are built by honda, Kawasaki, and a bunch of chinese knockoffs. The bulk of thier added effeciency comes from the fact that they can reduce the speed of the engine as the load drops. They keep maintain the phasing electronicly rather than from the speed of the gen-set. A Honda Eu-1000 can run a good Xantrax 40 amp charger at idle. They can be had for ~$350 in good used condition, and ~$600 new.

I agree, more panels is always a good option. I wish for a couple more for Christmas.

Icarus

BB. · November 2007

Re: Anybody Using Thermo Electric in their PV System?

TEG's (Thermal Electric Generators) are in use around the world... Here is an article and a manufacturer of TEG's...

Just out of curiosity, figuring out fuel costs for a small TEG (21 watt--PDF file), fuel consumption is 0.8 gallons of propane per day...

Watt*Hours/gallon=21w*24 hours per day / 0.8gpd = 630 watt hours per gallon

Now, take a standard Honda eu2000i 1,600 watt inverter/generator at 1,600 watts for four hours at 1.1 gallons of fuel (gasoline) consumption:

W*H/G = 1,600 w*4 h / 1.1 gallons = 5,818 Watt*Hours / gallon (gasoline)

Or, at 1/4 power (400 watts), 15 hours, at 1.1 gallons per fill up:

W*H/G = 400 w*15 h / 1.1 gallons = 5,454 Watt*Hours / gallon (gasoline)

So, it appears that a Honda eu2000i is over 8x as fuel efficient as a TEG...

The TEG would be interesting if you need 21 watts continuously. The units are supposed to be reliable (20+ years), but there have been reports that they fail pretty often (like once every year or two)--and these units (from link) where going to be replaced by a diesel generator and batteries because of the problems (1998 report).

Now, if you really want a high-end TEG--look no farther than Plutonium TEGs... Just 3.8 kG of the stuff will give you decades of 70 watts of power...

The Wikipedia entry for Nuclear TEGs has some interesting information/pictures of Russian ones that use Strontium 90---in years past--to power remote light houses and navigation equipment...

Back to your generator with the car alternator... Assuming 25 amps at 12 vdc... To be equivalent to the Honda eu2000i--:

12 vdc * 25 amps * 1 hour / 5,454 w*h per gallon = 0.055 gph

or about 7 oz (or 1 cup) of gasoline per hour equivalent with your setup (assuming the eu2000i efficiency). Or, about 18 hours per gallon (at 12v*25a=300watts)...

A non-inverter type generator from Onan, 2kW, 4 gallons, 13 hours of runtime:

2,000w*13h/4g=6,500 w*h/gallon

Not bad--I am surprised that it is that efficient... However, if running at lower loads (like 1/2 or 1/4 loads), I would expect the efficiency to go down dramatically (I could not find the 1/4 power fuel ratings).

There are several advantages to the Inverter/Generators... One is that the inverter has very accurate frequency control vs that of a standard generator with mechanical governor. Sometimes nice for something that needs accurate line frequency (motors, clocks, some electronics and transformer equipment).

Another interesting feature is the ability to add two Honda eu*000i generators together in parallel for twice the amount of power. Not sure if I would want that (vs just a single larger generator), but it is an option.

Another feature typical of the inverter generators (at least the Honda, Yamaha, and the knockoffs) is that they are very quiet. In a typical street vendor/musician setting in San Francisco, you cannot even hear them running unless you are standing right next to one and listening for it.

Lastly, the one I like, is the "eco throttle" (or economy throttle) mode. Because there is not an inverter, the engine speed is no longer fixed--instead there is an alternator that then powers an inverter, which provides the fixed AC output. Because of this, the engine can now run slower at lower power outputs for lower power outputs. Also, tends to reduce noise quite a bit at lower power too.

Look at the Honda numbers above--the efficiency at 100% power and 25% power is not much different--Not many gasoline generators can match that range of efficiency.

With the small Honda generators, because they have a small internal fuel tank, people (and even companies) have taken to adding a siphon to the fuel cap so that they can attach a 5 gallon jerry can to the generator over its own internal 1.1 gallon fuel tank (for the eu2000i).

But for pure fuel efficiency, a good diesel is hard to beat--although, I have not found any smaller units (in the 2kW rating area) yet. They all seem to be quite a bit larger (and more expensive).

-Bill

Just to highlight from the Wiki link regarding TEGs:

RTGs use thermoelectric couples or "thermocouples", to convert heat from the radioactive material into electricity. Thermocouples, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3-7%. However studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations.
...
Dynamic generators can provide power at more than 4 times the conversion efficiency of RTGs. NASA and DOE have been developing a next generation radioisotope-fueled power source called the Stirling Radioisotope Generator (SRG) that uses Free-Piston Stirling engines coupled to linear alternators to convert heat to electricity. SRG prototypes demonstrated an average efficiency of 23%.

Which seems to confirm the fuel efficiency differences between a Propane powered TEG and a gasoline powered generator...

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Bill once again, I sit in awe,,,

Now if you could figure out a way to use waste heat for the input, as we have been discussing. Other than a woodstove, how 'bout the idea of using the pilot light(s) from a stove or water heater? As I said above, the effeciency doesn't really matter if you are are using waste heat from some other source.

I contend the biggest issue is trying to figure out how to dump the heat FROM the cold side, if the hot side has to live in the heat. I suppose I could see some sort of insulated box that protects the cold side. As I said in my prototype, I could get it to work only with blocks of ice on the cold side. The ice production is not a problem since we have 48" of the stuff all winter. On the other hand, a block of ice every few minutes doesn't make much sense.

I'll keep thinking, and post if I come up with any genius ideas, ha, ha.

Tony

niel · November 2007

Re: Anybody Using Thermo Electric in their PV System?

bill,
well done and researched by you. i'm impressed. i did not know of the rtgs so i learned something today. most of the tegs out there do not handle the extreme temps that the rtgs do and i can only imagine the costs involved even without the radioactive materials.

BB. · November 2007

Re: Anybody Using Thermo Electric in their PV System?

You guys are very welcome--as you can tell, my reading growing up was all over the map (except things by Emily Bronte or other types of high class fiction

)... I even bought several of those "handbooks" for Engineering, Machinists, and such in college and read through those (kind of skipped the details of the math--just made me realize how little I know in life...) just because I found those so interesting (some of the entries where a century or more old).

Regarding TEG's--the closest I have seen are the "thermopiles" that are used in gas appliances to power the pilot valves in home furnaces.

The standard ones seem to be about 0.750 volts (750 millivolts) that run in a gas pilot... Very hard to get much useful energy out of one (enough for a red LED--perhaps to thermolpiles in series?)...

I really don't think you are going to get very much power from this method of energy extraction...

You can read this link about the various common materials used for thermocouples (this article is more about using for temperature measurements).

The best way to generate useful energy conversion is to draw heat from the hottest source and dump it to the coolest sink... So, placing something in the flue is not going to work very well (efficiency wise) as the flue gas (should be) relatively cool. And the problem is that running a "cool" surface in a hot flame is that it condenses carbon onto the surface--so you end up with problems of soot and corrosion forming on your heat collector.
Other than using a Stirling Fan (like mentioned earlier) or "wood gas", I cannot think of anything that would get more useful energy from a wood stove (other than air, hot water, heating)...

You could always take the Stirling Fan, pop off the fan and replace it with a, probably, home made alternator (hamster powered example here) to generate some power for LED's or something else...

-Bill

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

I suppose you could, in theory, build a large system into the wall of a building. The hot side being "near" the heat source, the cold side outside the building. If you were in our climate where -40 is not uncommon and -20c is the norm you MIGHT have a big enough delta t. I suspect however that any energy gained by the teg would be more than lost by the heat loss through the wall. If you could somehow figure out how to make the hot side, and the cold side seperatly, and pull them apart. I guess then it wouldn't be a teg device.

Bill, I like you have eclectic interests, and in many of them I know almost enought to be dangerous. In few of them do I have much confidence.

Tony

Icarus

paulstamser · November 2007

Re: Anybody Using Thermo Electric in their PV System?

icarus wrote: »

I suppose you could, in theory, build a large system into the wall of a building. The hot side being "near" the heat source, the cold side outside the building. If you were in our climate where -40 is not uncommon and -20c is the norm you MIGHT have a big enough delta t. I suspect however that any energy gained by the teg would be more than lost by the heat loss through the wall. If you could somehow figure out how to make the hot side, and the cold side seperatly, and pull them apart. I guess then it wouldn't be a teg device.

Icarus

I was thinking (and BB seems to alude to it below) that one could get the hot/cold thermo-electric junction by placing the device in the outdoor portion of ones chimney.

Woodstove exhaust gases are pretty hot and northern climates are dang cold in winter. My stack is a thin stainless steel insulated unit and the hot/cold areas are only about an inch apart.

I wonder if anyone has attempted to build a thermo-electric chimney?

crewzer · November 2007

Re: Anybody Using Thermo Electric in their PV System?

I wonder if anyone has attempted to build a thermo-electric chimney?

A Google search on the terms "thermo electric chimney" (without the quotation marks) yields some interesting hits. Check the Italian power plant! And, here's an interesting gadget: http://www.pgiint.com/pdf/IOM-TEC-8C.pdf

HTH,
Jim / crewzer

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

The one thing I know from experiance, is that if you put something cold in the flue you will have MASSIVE creosote buildup. I suspect that the cold side will make for a cold spot in the flue. I had water coils in a wood stove, and (as Bill suggests) the gas condensed big time.

Interesting idea however.

Icarus

BB. · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Regarding Creosote problem... The new stoves have a catalytic converter just before the chimney to burn the unburned gases left behind from the original combustion...

But reading the EPA's Do's and Don'ts indicate that there is still a fair amount of maintenance and some buildups even when using a catalysts equipped stove. It appears it would reduce the problem--but you still will need to have yearly (+/-) cleaning of water coils and such...

While looking around, found a wood burning forum here... Sounds like a wood burning boiler and 1,000 gallon +/- storage tank is a popular solution. Some people have a gasifier pre-stage for their (outdoor) wood burning boilers (OWB's)... This is a hard core forum (lots and lots of posts and home made solutions). Looks easy to get lost in...

I ran across this site before when looking at various fuel comparisons (wood, gas, coal, etc.)... First time I looked at their forums.

Looked at the TEC Jim posted... From the specs:

Fuel Consumption at ~7 watts:
72 ft3/day Natural Gas
0.66 gal./day Liquid Propane

7w*24 hours/d 100cf / 73cf/d = 230 watt*hours per 100 cuft

For me, I am billed in Therms (~97 cuft/therm) @ $1.21 to $1.42 per therm...

$1.21 per therm / (230wh per 100 cf / (97/100cfpertherm)) = 0.0051 $/WH or $5.10 per kWhr--if I got that right....

Honda eu2000i Generator running at 1/4 load or 400 watts:

($3.35/gallon of gas) / (5.454 kWhr/gallon) = $0.614 per kWhr for gasoline generator...

For the propane version of the TEC:

7w*24 hours/d / 0.66g/d propane = 255 watt*hours per gallon of propane (less than 1/2 the efficiency of earlier posted TEG)...

I would have to have some really good reasons to use a TEG/GEC for generating electricity... I can understand why TEGs (in at least one project) was being replaced in Antarctica with Diesel Generators and battery banks (hybrid power system--I posted this link earlier)--even then, roughly, more than 1/2 the fuel was being used just to keep the motors and batteries warm when the gensets were not running (assuming the wind/solar pv panels are operating to plan).

-Bill

PS: I should add for those outside of the US, we are billed from $0.07 per kWhr in "cheap" states" to $0.12-$0.35 per kWhr in the more expensive states...

PPS: Another wood burner's link: http://woodheat.org/

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Bill,

In fact most new EPA rated stoves DON'T have catalytic sytems on them anymore. What most of them do to obtain a clean burn is use an auxiliary ari injection so that the gasses are re-burned before the flue. The reason that most stove makers got away from the catalyst is that it was expensive an more inportantly it could be easily damaged and destroyed by burning unclean fuel, ie painted wood, lead ink paper etc.

A well regulated EPA burning stove will emit virtually NO visable emissions. When my Pacific Energy stoves are well warmed up the only thing you can see out of the flue is a heat wave. You are correct in that there is a dramatic reduction of creoste as a result of this process, but it is not zero. Part of the reason that this is achievable is by keeping the flue tempature higher than the old smouldering fire stoves. (I have this constant battle with my sister in law about her stove. She burns it choked down, full of smokey emissions because she claims the "wood burns too fast" if she burns it hot. I tell her to burn it hot for shorter periods, or get a smaller stove!) In a modern stove if you cool down the flue with a cold element you will still introduce creosote.

Icarus

PS. Outdoor wood hot water furnaces are a popular option in many parts of the US and Canada. The problem that people have run into is that they tend have very slow smouldering burn rates, using large pieces of wood that are not always spit and properly dried. Many jurisditions (sp?) are looking into limiting or banning outdoor furnaces for the said same reasons.

IF you burn wood, (as I do) you have to be environmentaly responsible both to your near neighbors as well as the rest of us. Dry wood, properly fired is indeed a reasonable alternative, especially if you can burn wood waste that might otherwise go to the land fill. I burn logging slash and trees that are left after logging opperations. In another house on the west coast we burned mill ends, 6 cords of kiln dried mill ends heats two buildings for 3 seasons, not bad.

BB. · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Sounds interesting... Air injection in the exhaust port was one of the improvements in early car emissions systems (1970 or even a bit earlier)...

I don't use wood heat here--and have not lit off a fire place in probably 25+ years. My parents used a fireplace insert for 20 years and it worked OK--never has been serviced so I can only imagine what it must be like in there now... Getting close here to banning regular fireplaces in new construction (California, or at least SF Bay Area) because of the pollution during the winter. Just a matter of time, I am sure...

Catalyst fireplaces always sounded like a problem waiting to happen with all of the different materials people tend to burn (besides just seasoned wood).

In one of the sites I posted, scroll down here to see a collector that was substituted into flue stack (double wall stainless steel pipe, inside is flue with a 1" water jacket)... Looks like a neat unit--but, has to be the right stove, hot fire, and a good straight flue...

-Bill

GreenerPower · November 2007

Re: Anybody Using Thermo Electric in their PV System?

TEC are basically designed to optimally used as heat pump. They are much less efficient to be used as electric generator. Popular TECs (as sold on eBay) are operating at <200C and delta T ~ 67C whereas TEGs (Themo-Electric Generators) are designed to operate normally > 300C to have efficiency in the teens. TEG are still fairly expensive.

However, TECs can be used for "low-grade" heat when they are "free" with efficiency about 4% or less. So, they normally generate about 2% of the rated voltage and current. For example, a TEC operate at 15V, 10A (i.e. 150W) would generate at ~0.3V @ 2A (6W). You can string enough of them in series and parallel the strings to give sufficient voltage and current for an inverter.

Be caution to make sure the operating temperature is < 200C (not to melt the solder of the TEC elements) and delta T ~67C get the TEC to work at optimal efficiency point as generator.

This could be a good solution for otherwise "waste heat". I wouldn't use NG or LP just to generate heat for TECs to generate power - much inefficient compared to a mechanical heat-engine coupled with an AC or DC generator where you could get ~30-40% conversion efficiency.

Some links of interests:
http://www.tetech.com/modules/high-performance.shtml
http://www.zts.com/node/5034
http://www.research.philips.com/newscenter/archive/2006/060227-woodstove.html
http://www.triz-journal.com/archives/1997/01/a/index.htm
http://www.tetech.com/publications/pubs/1984RJB2.pdf

GP

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Bill,

We're getting a bit off topic here but so what,,,,

Your intuition says that the best route would be to get as many of the btu's out of the stove and flue before it exits the building, ergo creating a more effecient use of those btu's rather than throwing them up the flue.

In reality, most modern stoves, burning well seasoned dry wood, will extract ~60-75% of the available btu's. The "wasted btus are needed for proper draft, as well as to keep the flue temp hot enough to reduce creoste. Out door furnaces have a potential advantage as thier flues can be short (they don't have to navigate a building for example) and they don't have to worry about the consiquenes of a chimney fire. They suffer however due to thier short flues with draft problems and chronic underfiring.

The idea of pulling hot water off the stove is a nice one, but if you extract the btu's to do so, increasing the net effeciency of the system, you introduce a whole host of other problems. I'm not averse to doing some of these things, and do indeed do some, but there aint nothing free.

Most people would be farther ahead, using a conditioned space preheated tank, and a demand water heater. If the preheat tank can get some solar gain as well you would be that much further ahead. If you have a pre-heat tank just picking up room air, you could potentialy raise the incoming temp of the water 5-10f or more. Those are all btu's that you don't have to put in to bathe etc. (I am also a fan of recapturing that heat before you send it on to the sewer or septic. The idea of a grey water heat exchanger is a good one in some applications. One idea I heard of that makes great sense is using the public sewer system as a medium for a water based heat pump system. Sounds funky, but there is a lot of waste heat in a public sewer. It seems a shame to use it just once).

There are lots of creative ways to keep our carbon foot print small. Hail to those that keep thinking and tinkering about them.

Icarus

BB. · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Icarus,

As you well know, adding anything to extract heat for domestic hot water to a wood burning stove/kitchen stove/etc. is going to cause some sort if issues (less clean burning, reducing proper draft, if done wrong--increased risk of energetic disassembly, etc.) and require increased work to keep it running well.

I have yet to see anyone recommend adding devices to extract "extra" heat from well designed wood burning devices to provide for wider distribution of space heating or domestic hot water use. The downsides of these additions frequently increase the risk of fire/explosion/back drafts.

At this point, the Stirling Fan motor with a "hamster alternator" seems to be the recommended limit for after market conversions for electrical energy from wood burning stoves (for those that did not follow the link to the stack water tank heater--it was a web page which included pictures of several exploded hot water heater attachments--improperly installed and used--some of the same warnings that can apply to solar thermal domestic hot water systems too).

-Bill

GreenerPower · November 2007

Re: Anybody Using Thermo Electric in their PV System?

To the OP's questions, certainly using TEC to generate electric power from wood stove waste heat is doable and affordable (about 1$/watt with TEC prices on eBay if you build one yourself). It's commercially available but expensive since they use TEG (Thermo Electric Generator) as in my earlier comment.

There is a technology called CHP (Combined Heat and Power), it's getting popular in Europe and starts to get interests in US. Google for it and you would fine plenty of information (or look at my website). The idea is to use NG to run a generator and the waste heat is used for heating via a heat exchanger. Units small about the size of a diswaher that generates a few KWs is available but still expensive. The combined efficiency is in the 90s.

GP

paulstamser · November 2007

Re: Anybody Using Thermo Electric in their PV System?

GreenerPower wrote: »

To the OP's questions, certainly using TEC to generate electric power from wood stove waste heat is doable and affordable (about 1$/watt with TEC prices on eBay if you build one yourself). It's commercially available but expensive since they use TEG (Thermo Electric Generator) as in my earlier comment.

GP

That sounds promising. Can you give us direct links or more specific info for a home-brew project that generates doable and affordable thermo-electric power from woodstove heat?

Or are you talking about Stirling engine technology?

I can envision the latter with a "hamster" generator as better than the NOTHING I'm generating off my woodstove now which is basically running 24/7 for the next few months....

I'll admit being at the beginning of a renewed learning curve here but do feel than woodstove electricity is a subject worthy of KISS study and experimentation.

paulstamser · November 2007

Re: Woodstove Water Heating

Re: Woodstove Water Heating

I have long-term experience with this related and important subject.

First, many years ago, I built a stand-alone wood fired water heater out of a 30 gallon LP water tank to fill a bathtub. I found it awkward and cumbersome to use and I abandoned it.

Then I built a "coil" (water tubes) that ran thru the interior of my old woodstove and connected to 90 gal. watertank upstairs. This tank I filled with a gasoline engine pump. While it worked okay, the water didn't easily get hot enough for shower use and was not practical in the summertime, so when I got a new woodstove I did not install the coil in it. (But I still use the watertank for gravity-fed sink use).

Now I use a dual KISS system for heating water. In winter when my woodstove runs all the time I just have a stockpot with lid on top of the woodstove which gives me 2 gallons of scalding water that is adequate when tempered in a KISS gravity fed shower (and for clothes washing). This shower is so simple and effective that it is amazing!

In summer I have an old box woodstove (with lids) outdoors with a simple tin-pipe chimney. Here too I use the stockpot with lid method to heat water to scalding temps using waste or low quality biomass fuel of which I have an UNLIMITED amount free for the gathering.

Of course I have a simiple rather rustic lifestyle and am not trying to duplicate grid or urban mansion conditions.

What I don't have yet is a solar water heater, but that is another scheme that I'm working on and long overdue.

GreenerPower · November 2007

Re: Anybody Using Thermo Electric in their PV System?

paulstamser wrote: »

That sounds promising. Can you give us direct links or more specific info for a home-brew project that generates doable and affordable thermo-electric power from woodstove heat?

Or are you talking about Stirling engine technology?

No, I'm not talking about any kind of mechanical engine. This is just pure TECs i.e. heat to electricity.

First, the link I mentioned in earlier post in this thread. It allows you to pick TECs and operate them at their optimum efficiency as generators.

Second, TECs like these from eBay, you can get 10 for about $US 40 and generate about ~40-50 W i.e. ~ 1$/watt

Take one unit and temporarily attach heatsinks to both sides, use a hot gun to blow on one side at a time and a volt meter to determine which side when getting hot would give you + volt on the red wire. Mark that side, that would be the side to get heat from the stove.

Attach them (the marked side) on a "baking pan" with thermal silicon paste (space them apart enough so that you have room for the heat sinks) . Wire 5 of them in series and 2 series in parallel. Attach (and secure) heat sinks on the other side (you can get cheap aluminum CPU heat sinks).

The hard part that needs your creativity is to "efficiently couple" heat from the stove to the "baking pan" side and don't get it too hot (i.e. >200C and to keep the wire isulation from melting, you can use heat-shield sleeves for the TEC wires) and dissipate heat on the other side.

If you have about 70C temp difference between the 2 sides of the TECs (optimum point), 10 TECs (in 2 strings of 5) would give you about 4A (short circuit) and 30V (open circuit). The optimal point (to get most power) is about 1/2 of the open circuit voltage. So, if you have a 12V appliance or an inverter/DC-DC converter that can take 30VDC open circuit (15VDC operating), you would have about ~40-50W of electricity.

I did this about a year ago using heat from the gas fireplace and a 75W auto cigarette-plug type inverter.

GP

icarus · November 2007

Re: Anybody Using Thermo Electric in their PV System?

Sounds just like the system I built. (Decribed on a previous thread) The problem was taking the heat away from the cold side. I think that my problem was that the hot side was too hot. If you can put the hot side on something other than the top of the stove it would help, something like the top of a hot water bucket, or a stack of bricks or stones. That would temper the hot side a bit, and make getting the heat off the cold side a bit easier.

Icarus

Anybody Using Thermo Electric in their PV System?

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