Quote:
Originally Posted by dbhosttexas
So... kind of the same idea as what they use for those fans powered by a wood stove? Sort of a Peltier device kind of thing, but instead of the current creating the difference in temperatures, the difference in temps creates the current? That could be very interesting...
Theoretically a couple of these if they were small enough, could be attached to a larger size muffler to generate power for some low power tank heaters, so waste energy from the heater could be redirected to keep the tanks above freezing...
Have you seen this done?
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From the Wikipedia link above
Thermoelectric systems
Using thermoelectric modules, a thermoelectric system generates power by taking in heat from a source such as a hot exhaust flue. To operate, the system needs a large temperature gradient, which is not easy in real-world applications. The cold side must be cooled by air or water. Heat exchangers are used on both sides of the modules to supply this heating and cooling.
The main challenge is maintaining a 66 degC temperature gradient across the 3 or so mm PTC device.
If you go to the web and look at the fumblings of various Youtube experts you will see that they struggle to get much energy out of such an approach because they generally use a heat sink with an electric fan to get the heat out of the cold side.
Heat flows by one or more of the following three methods:
- Radiation
- Convection
- Conduction
The heat-side is usually always conduction (e.g. putting the PTC hot side against a stove or exhaust pipe)
Cold-side Convection (using heat sink fins and fan) is very common but not that great for this application. I saw one low-volume commercial application for an ice house heater and it was limited to 45 Watts. Enough to charge a phone and an LED light but not enough for 100 watts of thermoelectric power generation.
I'm going to try conduction on the cold-side as well. The frame of the van is a large piece of metal that has a whole lot of cooling surface area.
So the challenge is to get the heat conducted into the cold side to disperse through the frame quickly so that the cold side remains reasonably below 100 degF within a few inches of the PTC.
I did a few calculations to see if it is feasible.
Assume a 300 deg F muffler (149 DegC)
By the time you couple this to the Hot-side you might lose 25 degC putting the hot-side at 124 degC.
Then the max temperature differential for the PTC is 66 degC putting the cold side at 58 degC (136 degF);
If you are sitting in cold weather under threat of a freeze warning the frame will certainly be in the range of 40 degF. Assuming the heat can be dispersed well enough you should be able to keep the frame in close proximity to the cold side PTC well below 136 degF and be able to maintain the full 66 degC temperature gradient across the PTC.
I have a couple of these somewhere along with some 3/16" alum plate. They are rated at 6Amp @ 12V. As mentioned previously I would use a voltage-controlled relay to switch between the Haus battery +12v and the +12V PTC to power the immersion heaters in the tanks. If the heater goes off and the PTC cools down the VCR will switch back to house power when the PTC voltage drops.
https://www.ebay.com/itm/234301912631
You can get an idea of how much energy you will need to raise the temperature of the water in the holding tanks.
https://www.omnicalculator.com/physics/water-heating
For my 18.5 gal and 9-gallon tanks, I picked 48V 600W and 300W respectively. At 13.2V the current draw is 3.5A and 1.8A (total 5.3A) and 45 W and 23W (total 68W ) respectively.
At 1/2 full it should take 5 hours to raise the temp 10 degF. When full it will take double that.
https://www.aliexpress.us/item/32568...yAdapt=glo2usa
If you compare these direct immersion heater elements to the foam type that you strap to the outside of a tank, I think the former is going to be much more effective.
This FACON is speced at 65W (13.5 VDC and 4.8A); it is supposed to effectively warm up to a 50-gallon tank. The biggest issue that got me off these things is the very loose tolerance there is on the thermal switch control. The heater will come on temperatures drop to 45 degF, but it will probably never go off because, with the hysteresis, the heater will not shut off till 68 degF!! Unless you shut these off manually, the amp-hour load to heat the holding tanks from 40 degF to 68 degF (+28 degrees) is probably twice what it takes to just hold above freezing from 45 degrees F.
Secondly, the conduction of the foam pad through the plastic=/ABS tank wall is not going to be nearly as efficient as the immersion heating element which would account for yet another 50% reduction in Amp-hr heating efficiency.
https://www.ebay.com/itm/18598847107...Bk9SR9Cks8C7Yw
The picture below shows a frame mounting location for the muffler on the passenger side rear wheel well. The exhaust will come from a petrol heater right in front of the wheel well and exit behind that rear tire where the engine exhaust pipe is. I'm using a 1" piece of galvanized fence pipe because it will fare better than the thin stainless stuff if rocks and mud come flying up from the tire.
In any event, this is still an experimental system that I plan to prototype in the next month. Suffice it to say there are several details to be worked out and various design choices that should make this an efficient way to reduce the requirements for excess battery capacity to heat holding tanks.
As mentioned before if this fails, I would use the recirculation method where heat comes from the on-demand water heater.
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CSC TT250 + homebuilt trailer
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