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Old 07-20-2017, 09:14 AM   #1 (permalink)
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How to use TEGs to create power with your exhaust heat

PART 1




So, you'd like to use a thermal electric generator (TEGs) to eliminate, or reduce the load on your alternator? We have actually talked about it quite a bit over the years here on EM. Here are just a few examples:

http://ecomodder.com/forum/showthrea...ator-4538.html
http://ecomodder.com/forum/showthrea...les-22368.html
http://ecomodder.com/forum/showthrea...tegs-3124.html

But, this isn't a thread discussing if its possible. This is thread that shows you how to do it! So lets see what it would take to get there.


First off, what the heck is a TEG / Seebeck / Peltier generator anyway?

This is what wikipedia has to say about them:

Quote:
A thermoelectric generator (TEG), also called a Seebeck generator, is a solid state device that converts heat flux (temperature differences) directly into electrical energy through a phenomenon called the Seebeck effect (a form of thermoelectric effect). Thermoelectric generators function like heat engines, but are less bulky and have no moving parts. However, TEGs are typically more expensive and less efficient.
So, we need a temperature differential because that is what makes these TEGs thingies work. You put heat on one side, and cool on the other an they generate a voltage. Pretty neato stuff. The larger the temperature difference, the more power they create. So, what is the largest temperature differential we can find on our vehicles? That would be heat from the exhaust for the hot side, and ambient air temperature for the cool side. This gives us several hundred degrees Fahrenheit of temperature differential. Yes, we could use coolant, but that only has at the very most about a 200F differential. 600F vs 200F is going to give us a lot more power. Sadly, TEGs are not crazy powerful or efficient, so we need all the temperature differential we can get.



Okay, where the heck can I find TEGs?

Now that we know where we are getting our heat from, we can go try to find some TEGs. The biggest challenge now is finding TEGs that hold up to exhaust temperatures. The closest thing I've been able to find are TEGs sold at TecTeg.com and CustomThermoelectric.com. Both of these sites have TEGs capable of handling 320C (608F). This may still be on the low side for heat resistance. Exhausts can definitely get hotter than 600F, so you would have to be sure to position the generator downstream far enough so that it would not see above 600F. This will require testing by you on your vehicle in the middle of summer as that'll give the worst case scenario.



Thats great, which one do I choose?

Now, we have a whole bunch of TEGs to choose from, but which one will work the best for us? Well, first we have to figure out what size we want to look at. I chose to look at the 40mm (1.57 inches) units as I think that would fit best on the exhaust of a smaller car. More on this later though.

To compare them, I made the spreadsheet you see below. It contains all the 40mm TEGs that the two companies have listed. Ideally, we want about a 13.5-14.5V output from the TEGs. This will ensure that the 12V battery gets fully charged. To get these specs, I used the "matched load" numbers, not the open circuit numbers on their spec sheets.





Out of all of these TEGs, I like the second one the best. Here are the full specs on it. We will need 2 of them wired in series to get 13.4V output which should be great for getting a battery charged up. This doesn't mean it won't charge above that. It just means that 13.4V is where its most efficient (putting out the most amps). So, I will be using this one as the example going forward.



PART 2

How many of these TEG thingies do I actually need?

Great, we have a TEG selected that we'll use! Now, how many do we need? As you can see, the TEG listed above puts out 7.5W. Two of them in series will create 15W. This is not an amazing amount of power by any means. A standard alternator puts out 13.5V and lets say 80 amps for a total of 1080W. To match that output, we would need 144 TEGs! That is a bit much, and actually probably not required the vast majority of the time. The alternator is sized to be able to handle every electrical load on the car at the same time. The liklihood of this happening is small and if you're aware of it, you can ensure it never happens. So, the next step really is to figure out how much power you want your TEGs to generate. The best way of doing this is to figure out what kind of electrical load your car has while its in operation. As a rough guide, MetroMPG has posted some information on typical electrical loads. The best way is to measure it with an ammeter though.

Once you know what your typical electrical load is, you can figure out how many TEGs you need. For my example, I am going to say that I need 15A (200W) of charging. We already know that we need 2 TEGs in series, and that series of 2 TEGs gives us 15W. So, the math is simple at this point. We take 200W of generation divided by 15 which gives us 13.3 TEGs in parallel. I'm going to round that up to 14 to keep the numbers even. 14 in parallel times 2 in series gives us 28 TEGs. That sounds a lot more doable.





Nope, wait! You need more.

BUT, this is their rated output at maximum temperature differential. We surely won't be operating at maximum temperature all of the time. So, we will need to derate them. By how much is a total guess unless you've done testing to see what kind of exhaust temperatures you'll see consistently. Then you can reference the data sheet for the TEG and guess what kind of power output you'll get. For this example I am going to take a total guess and say we will derate them by a third. Now we need 36 TEGs to get our 200W.



PART 3

How do I put this jumble of TEGs together to make some powah?

Yay, we're getting 36 TEGs to generate around 210W and significantly reduce the load on the alternator. But, how do we slap the TEGs on the car and make it work? Lets get to that now.

Note, this is just a quick way I came up with to slap the TEGs on the exhaust. There are many other ways you could possibly do it.

You'll notice I've kept the number of TEGS divisible by four. This is intentional as the mounting of them will be on a square tube. Four sides to the tube, four TEGs keeps things all neat and tidy. This is also why I chose 40mm TEGs as they fit the square tube that kind of matches the pipe size the tube will be replacing. The square tube will replace a section of the exhaust. It would look something like this:





Then, you would need to add heat sinks to the cold side of all the TEGs to help cool them off and maintain the highest temperature differential possible. It would look something like this:





And beyond that, you have to find a way to bolt those heat sinks down to the tubing. The TEG modules are supposed to be under a fairly specific amount of pressure to ensure long life and good performance. Here is some of the guide from TecTegs.





PART 4

OMGz the wires!

Next up we need to take care of the electrical wiring. In this example, you would wire 2 TEGs in series, and 18 of them in parallel. It'll look something like this. Yep, its a bit messy, but really its nothing compared to an EV with BMS wires haha.





Controlling the heat!

Beyond this, if we've done things right, we'll also need some way to control the generator. We can't just hook it up to the battery and let'r rip. Its possible that the generator could overcharge the battery. We also need to stop the TEGs from discharging the battery when they aren't producing power. If you put power through a TEG, it will create heat on one side, and cool on the other. While this is neat, its not really what we're looking to do.
Therefore, we need a charge controller of some sort. This could be as simple as a switch if you want to keep things extremely simple. You'll also want a voltmeter to monitor things if you go that way. However, a much better idea is a purpose made charge controller. Something that is made for a solar panel or wind turbine would be what we're looking for. It will protect the battery from overcharging and from discharging as well. The charge controller will have to handle the max amps of this setup of 15A. Here is a cheap example of what I'm talking about. I just searched amazon real quickly and found one.





PART 5


image by BA1969

Holy cow, it cost how much?!?!

Now, assuming you've done all these things, which is totally possible. I've specifically left out one very important detail to this all, the price. If we need 36 TEGs, those TEGs are going to cost us $31 a piece (that is with their volume discount). That brings us to a total of $1116 just for the TEGs. That doesn't include the pipe, welding work that will need to be done, the mounting system, heat sinks for the cold side, or charge controller and wiring. So, this is where things sadly really fall apart. I've bought entire cars for less than what this system would cost. Granted, you could do this, and transfer the TEG modules from vehicle to vehicle assuming they last that long. In a automotive application I have my doubts though. In these high temperature situations, they estimate the TEGs to last about 4 years.

So, while this is all completely doable, its insanely expensive. But, that is how you do it folks! Hope you enjoyed the engineering exercise.



===============================

Here is my analysis of using TEGs powered by your coolant:

How to use TEGs to create power with your coolant heat

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