Thermoelectric devices (waste heat electricity generation)

[mskobier]

I just hate it when I get to sit around during the holiday season with nothing but time on my hands. During this time, my brain goes into overdrive and I start thinking of very strange things.

One of those things is the Atwood propane furnace in my CT. It's rated at 12k btu input with only about 9k btu of actual heat generation. The missing 3k btu goes out the exhaust as waste heat. I have been thinking of a way to utilize that waste heat. Currently, I am considering making a removable bracket that would direct the furnaces exhaust to a thermoelectric device. Normally called a peltier device. This is the device that the thermo electric coolers use to make hot or cold. The interesting thing about these devices, is if you heat one side and cool the other, they will generate DC current. I am sure not as much as they would consume making hot/cold, but possibly enough to add a few watts of power back to the battery. The configuration of the device is actually quite simple. You apply heat to a heat sink mounted on the hot side, and have a cooling fan blowing air across a heat sink mounted on the cold side. This transfer of heat is what causes the device to generate electric current. So, the question is, has anyone tried this idea, and if so, how well did it work? Most of my CT adventures has me away from shore power. If I can harness some of the waste heat to help add a little power back to the battery, that would be a very good thing. Running the furnace is the largest load on the battery. assuming a run time of approx 30 minutes each hour, and the furnace consuming 1.8 amps for each hour of run time. After a day of operation, the furnace will have used about 22 amps from the battery. Thats almost 50% of usefull power from the battery. I do not discharge my battery to less than 50% of its rated capacity, so it limits the amount of power I can use to approx 50 amps. So anything I can recover would be helpfull! I have been looking a a 400watt rated device. Thats about 30 amps of generating potential. I doubt that I can generate that much power, but if I could generate just a couple of amps, then running the furnace would have a net positive effect on the battery. Currenty I can get these devices fairly inexpensively on Ebay.

Anyway, just some holiday season pondering. Possibly something to exercise the tired old brain cells with. Any input would be appreciated.

Merry Christmas and Happy New Year!
Mitch

[Greg M]

Interesting idea. I know of a couple of remote radio repeater sites that use thermocouples to power them over the winter months (not enough sun for solar) so I know you can get usable power out of a flame. Sounds like it's time for some research.

-Greg

[Moho]

I've actually researched Thermoelectric Generators before for a project. The biggest problem to get them to perform well is balancing the compression on the plates. A fan isn't a very efficient way for the cold transfer side of the plate. To get them to perform well you want consistent pressure of a hot and cold surface across the entire surface of the dielectric plates. This pressure must be sustained constantly and allow for thermal expansion.

Granted a fan blowing cold air and heat passing across the other side will generate electricity, it just won't do it efficiently. You're working with a device that when working efficiently will transfer about 5-10% of the heat loss into electricity, so less than efficient really affects the performance.

The best way to make them work efficiently is to have a "hot plate" on one side and a "cold plate" on the other. The tension is held on the plates by springs. Around 200 psi compression from what I recall is what most of the plates I researched called for. Quite often the cold plate is a heat sink block recirculating cold water. The hot plate is normally a plate of copper attached to the heat source for good thermal transfer.

I think it is a good idea to apply it to the application, but getting it to operate efficiently will be the biggest problem in a teardrop.

[afreegreek]

where does the power for the cold side fan come from? seems to me it would suck harder on the battery that it would blow and leave you with a net loss..

[droid_ca]

I was researching into the peltier device and thought about uysing what is called a Joule thief to get every last bit of energy that was produced...This will be fun to follow...excited to see more of what you learn... would also like to hear more about your 400 watt device that you mentioned

[Treeview]

This idea has the sound of a perpetual motion pursuit!

[Big Dan]

Have you guys seen thesehttp://www.biolitestove.com/BioLite.html I think its the same thing your talking about.

[Moho]

Yep, same technology

[mskobier]

All,
Thanks for your replies. A number of questions were voiced and I can answer some of them. I understand the constraints of thermal expansion and contraction. I had already decided heavy springs were the way to go. 200lbs of pressure would seem to be a bit much, but what do I know. The heat transfer would be accomplished using thermal compound to ensure good thermal contact, and the springs would make sure it stayed together.

I had already decided on a copper plate for the hot side and a large finned aluminum heatsink for the cold side. The only time this device would be installed would be during cold winter weather when the furnace would be in use. As such, I am looking at around freezing temps for ambient and all the DC powered fan would do is move the cold air across the heatsink. I plan on using a DC powered computer type fan to move the air across the cold side. The fan would be powered by the TEG itself. Once the hot side was hot enough, it would start making dc power which would run the fan. There would be a blocking diode or some sort of charge controller in the circuit to prevent the battery from discharging into the system during the periods of time the furnace was not running.

I am not after absolute efficency. I am just trying to increase the times between battery charging.

The TEG's I was looking at on Ebay are items number 170687871156, 170753414420, 310158610445, 310363610856, and 280785393340 to list just a few. The last number I listed is rated at 545 watts. Personally, I think that is wishfull thinking, but who knows.

I have approx 3000 btu's of waste heat I am trying to get some use out of. It may not be worth the effort, but at least I'll give it a try and possibly learn something too.

Yes I have seen those little thermoelectric generators on the internet before. They look really cool. It is an excellent idea to utilize a common heat source to make at least a little useable power. There is also a commercial thermoelectric generator on Ebay. Its number is 220376907961, but please do not die from sticker shock when you see how much they want for it. And its only rated at 100 watts @24v!

Well, I broke down and purchased one of the 62mm devices. It was less than $40.00, so I am not out much if it does not work like I want it to. I'll let the group know when it comes in and I can try a few tests.

Happy New Year to all,
Mitch

[droid_ca]

did you check out the Joule Thief there are kits on Ebay that raise 1.5 volts up to 5 volts they are neat little circuits

[Breytie]

I love the idea of changing low grade (wasted) energy into high grade electricity! Sort of co-generation in reverse! Electricity as by-product from the generation of heat!

Get it going, tweak for double digit improvements first, single digit fine tweaks tend to cost a lot more in time and money!

Keep us up to date with progress!

[Tumbleweed_Tex]

Totally kewl…

My heart envisions an entire bank of TEGs along the side of the trailer, squeezing every last calorie of heat out of the furnace exhaust gas.

My head, however, insists on the boring realization that running the hot exhaust gas back through some sort of small radiator on the inside of the trailer, and blowing that same little computer fan across it, will put a large portion of that 3k BTUs of wasted heat back into the interior space. The furnace will then operate less minutes per hour, equating directly to longer battery life.

But how fun is THAT?

Good luck and keep us posted...

[jstrubberg]

Hmmm....


I remember reading about a cabin (a very LARGE, expensive cabin) a fella had built in Colorado that had the flue for the fireplace and furnace run underneath the floor. They used a small recirculating fan on the far end to keep the cooling exhaust moving in the right direction. At exhaust, the air was basically ambient temperature, making the whole thing highly efficient.

Why couldn't you do something similar with a trailer? Use a small exhaust fan on the far end of the flue to draw, and split the flue in tow and route it under the floor of the trailer. Let all that plywood and bedding become a heat sink and soak up all that heat energy.

[Moho]

I'll be interested to see the return on this.

As far as the 200 PSI that was for the specific models I was looking at, if I remember correctly the recommended pressure varied slightly from product to product and specs should be on the datasheet for recommended pressure.

[eamarquardt]

Me thinks you might be better off with a hand (or foot) powered generator. You burn calories, make electricity, make heat, lose weight, humidify the air, and create less CO2 and particulate emissions.

For what yer gonna get, I think the $40 would have been better spent on a solar panel but await your test results and I will be happy to be proven wrong.

Cheers,

Gus