How to use TEGs to create power with your exhaust heat
 

PART 1

http://ecomodder.com/forum/attachmen...1&d=1500555942


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. :thumbup:


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

This is what wikipedia has to say about them:

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 200°F 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.

http://ecomodder.com/forum/attachmen...1&d=1500555965



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.

http://ecomodder.com/forum/attachmen...1&d=1500643277



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:

http://ecomodder.com/forum/attachmen...1&d=1500744275



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:

http://ecomodder.com/forum/attachmen...1&d=1500744283



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.

http://ecomodder.com/forum/attachmen...1&d=1500744298



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. :)

http://ecomodder.com/forum/attachmen...1&d=1500900097



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.

http://ecomodder.com/forum/attachmen...1&d=1500900097



PART 5

http://ecomodder.com/forum/attachmen...1&d=1500985579
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. :D 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. :thumbup:



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

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

How to use TEGs to create power with your coolant heat

I like this thread idea a lot. I swear I saw on digikey.com a tile that generated 14.5 volts on its own and more amps. And there seemed to be lots of them. Have you looked at that site?

Anyway, I think an ideal spot for placing such a system would be near the rear wheel well along the exhaust pipe but in front of the rear wheel well. It'll be a little hotter there than it would be down by the muffler but mufflers often rot out on this you get a really good stainless steel one and I think all of that compromising of the structure of the muffler and the leaking of exhaust can't be good for the tegs.

One or 2 tiles just to trickle charge a battery is a small system and just shouldn't be all that hard to produce.

Thanks. It is pretty fun going through the design requirements for me. Of course I'm a mechanical designer and quite enjoy it too, so that make sense. :)

TEGs are really cool little devices.

Tec is a thermoelectric cooler and a teg is a thermoelectric generator.

Tecs are relatively cheap compared to tegs. Their major difference is their heat tolerance. I think creating a setup with tec's and coolant is a more controllable.

I bought quite a bit of materials for this a long time ago. I really should finish the project at some point.

hi current needed
 

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.

http://ecomodder.com/forum/attachmen...1&d=1500643277



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.

I would suggest placing these TEGs in the immediate vicinity of your upstream O2 sensor, and just behind and downstream of your downstream O2 sensor. If the O2 sensors can take the heat, the TEGs should also be fairly happy there.

Edit: And are you sure you want to rely on ambient air to generate your temperature differential? Seems like you're going to want to put some cooling fins on to take away this heat, and use some sort of forced airflow, at a minimum. Maybe a coolant loop would be better.

Thanks for continuing the thread. That's a lot of TEGs at $20-$25 each!

I think this description so far suggests significant complexity in using TEGs to replace an alternator entirely. A much simpler set up would contribute to reducing alternator need. It could be smaller and cheaper.

To tvago's good points, I assume we'd use cooling fins if we put a TEG unit into the under car airstream. Daox is talking about a lot of TEGs, and I think they would not easily fit around the manifold. Better to be lined-up along the straight middle sections of exhaust piping. Some sort of forced airflow seems possible under the car, along the piping, quite easily. Personally, I would sacrifice temp differential for system simplicity. Coolant circulation makes for complexity.

Any system would need some kind of controller, yes? And there would be a need for a way to prevent the TEGs from drawing power when the car is cool, maybe? There might be situations in which ambient air is warmer than exhaust piping, no?

No problemo. I have it mostly written out, its just a matter of rolling out all the parts of it. There are 5 parts in total.

This is possible, but is there much point in saving 5A from an alternator? The gains will be fairly small.

As Oil Pan mentioned in one of the other threads, doing a temporary alt delete will give you a good idea of the gains you'll get from doing a mod such as this.

The engineers' version of Cuba Gooding, Jr's famous Jerry McGuire movie line: "...show me the DATA..."

My old truck with its carbureted engine probably has some of the lowest running electrical loads of any vehicle on ecomodder, short of an old VW diesel.

To improve the TEG's contact with the exhaust tubing it would be beneficial to change over to square SS tubing and strap them on with stainless steel hose clamps, and of course they would need big ol' heat sinks on them, or else mill aluminum stock with a flat face on one side, and a half-circle on the other, to attach to the existing tubing.

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:

http://ecomodder.com/forum/attachmen...1&d=1500744275



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:

http://ecomodder.com/forum/attachmen...1&d=1500744283



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.

http://ecomodder.com/forum/attachmen...1&d=1500744298

If you KNOW the engine RPM speed that you're most likely to be operating at, you can also "gain" some heat (and thus more output) by locating the TIG array around the square pipe at the acoustic 1/2-wave length down the pipe from the engine, ie: the spot where, at a constant engine speed, the exhaust-gas pulses create a "standing-wave."

This is basically an 'acoustic' version of adjusting antenna feed-tuning as done by CB'ers for their antennas. Where the "standing-wave" occurs, the pulses will be strongest & hottest...the "sweet-spot" for those TIGs.

Of course, this is only valid for one RPM (ie: highway cruising at a set speed), but something possibly worth considering?

How about the inner part as you describe, but instead of the heatsinkds it is encased in another square peofile tube which is a thermal syphon filled with water under a vacuum or lpg.

Or alternatively, what about a flat aluminum plate like part of a belly pan?
Or two L-shaped plates with TEGS to the sides, or for that matter, a plate with a square channel folded in to bond with TEGS on 3 sides, so the exhaust runs under it (I'm not fond of enclosing any part of the exhaust system between a belly pan and the rest of the car)?

About clamping force - no chance to use heat paste? That would provide full thermal conductivity even if the clamping force is low?

And last but not least - any estimates on how much weight it all will add?
This seems fairly lightweight so far - but it needs to be light to be worthwhile.

(subscribed!)

That is not a lot of torque per screw to clamp them down: 0.072 kg per m only equals about 6 inch pounds.

On my car, the TEGs close to the undercarriage would probably not fit. If they did fit, I would think the fins would not cool the TEG as well because they might be wedged between the heat source and the underside of the car. If they won't cool as well they won't produce the hoped-for power, either, no?

Very cool idea. :thumbup:

I think if you're going to water cool them, a tiny pump low power would do vastly better than thermal syphon. I've evaluated thermal syphon systems in solar hot water setups, and active systems are able to produce way more power than the pump actually uses. My guess would be the same for this type of system.

Those are all quote possible. Experimentation would show you what works best of course.

I also wonder how feasible it is to make the inner tube out of aluminum for greater heat transfer. I have no idea how it would hold up to the corrosive exhaust environment, but it would perform quite a bit better than steel or stainless. I think if I were actually building this, I'd go that route.


They claim that they do not need thermal paste. There is a layer of graphite on these TEGs. The ones with ceramic require thermal paste (they look white), but the graphite layer (they look grey metalic) negates the need for it per their instructions.


The TEGs themselves are not that heavy at all. I have a 40mm one, and I just weighted it at .8 oz (23g).

In the installation manual, they say that the TEGs grow and shrink a decent amount during the heating and cooling. The specified torque rating is to ensure their longest life.


Correct, you want to keep them in the air stream as much as possible to keep the cold side of the TEG as low as possible.

320C is not enough, I would not want to take chances with roasting them. Even the muffler on my car easily melts some mineral wool insulation and fiberglass.

What you want is this one:
https://www.alphabetenergy.com/product/powercard/

I also would use a boost converter to extract maximum power from the units.

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. :)

http://ecomodder.com/forum/attachmen...1&d=1500900097



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.

http://ecomodder.com/forum/attachmen...1&d=1500900097

http://i7.tinypic.com/4th0v2f.jpg
http://www.layitlow.com/forums/16-post-your-rides/65803-show-pipes-4.html

Bellfower pipes (named for Bellflower, CA), with spikes or a high-bypass shroud would look [Valerian/Mad Max/Fifth Element/pick one]. I saw a 59 Chebby two-door with Bellflower pipes on I-5 this weekend.

Why isn't this on the front page yet? It's hard-core Ecomodder. :thumbup:

PART 5

http://ecomodder.com/forum/attachmen...1&d=1500985579
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. :D 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. :thumbup:

That may very well be true. We wouldn't know without testing. Tecteg does have what they call a 'hybrid' TEG that is good for 360C / 680F. It is actually a bit cheaper than the TEG I selected at $3.18/watt vs the one I selected which is $4.67/watt. I didn't go with it because it is 56mm vs the 40mm I wanted, and it also required 3 in series vs 2 or 4 which is a bit more ideal for the pipe setup I wanted to go with. Certainly doable though, and probably for a bit less cashola.

If 360C still isn't good enough they do have higher temp TEGs similar to the ones you linked. They have TEGs that will withstand temperatures of 850C / 1562F. The price is an order of magnitude more outrageous than the two listed above though. We're talking several hundred to over $1k for ONE TEG that produces only a few watts.

One could install them a bit "away" from the exhaust pipe or muffler in order to reduce how much of the exhaust heat can get communicated to the TEG. Obviously any such idea reduces output and efficiency, but if it extended the life of the system significantly and reduced the need for more costly and complex coolant systems ... still... all this points to perhaps why it is not a common mod: kinda complex and fairly expensive without the certainty of being able to fully replace the alternator reliably. My alternator is nearly 20 years old and still kicking, on demand, cold engine or hot engine. If a $1000 TEG system burned-out after 4 years, I would be bummed-out!

Ill dig up what i had found some time ago. I remember it as 1$ / watt

The Alphabet Energy TEGs are supposed to be cheaper because they're silicon based, but because the hot side temperature can be higher they are still effective. The only tidbit of info on cost I found was this:
https://www.alphabetenergy.com/alpha...gy-efficiency/

"Pricing for the PowerModule Developer Kit™ starts at $1,995."

That includes the TEGs, a stainless steel heat exchanger, coolant radiator, pump, and will generate >900W at higher temperature. That means the TEGs themselves are near the 1 dollar/watt range I believe, because that heat exchanger has got to be really expensive with all the little fins.

I think a good strategy would be to use only a few of them to generate say 90W so that you have just enough electrical power for most of the critical things like ignition/fuel pump, and cooling the generators with the heater circuit. That way the alternator isn't struggling at low engine speed, and you can underdrive the alternator for minor efficiency gains. The amount of extra heat going into the coolant is modest and the weight/complexity/cost would be very reasonable.

Engine exhaust at WOT is actually hotter than 700C, so temperature could still be a problem but given they advertise its use for truck exhaust, I am guessing it is probably capable of handling those temperatures. Then again they say 350C maximum continuous operation... I dunno.

I think this will be truly viable the day when someone comes out with a cheap 700C hot side capable TEG.

This sounds like something that an F1 team could develop into a viable system for street usage. They spend a lot of money and the fruits of this are right where they would benefit.

I agree Capri. I think a lot of these TEG places could substantially lower prices if they got some real volume to sell. An OEM looking to do this may be able to build a TEG exhaust for 2-3x the cost of an alternator. This seems like a lot, but to gain 5-10% in fuel economy is a huge feat. On small cars, we know this can be done.

Apparently, they are developing them and have been for a few years (though not TEG, it seems). Check this out:
2014 Formula One exhaust energy recovery system explained | Electric Vehicle News

That's relevant to another thread.

I remember a picture that Google can't find of a Baja Bug with a 'teepee' header (4-into-1 straight up in the air) with a rusty turbo bolted on top as a muffler.

No that's different, the current F1 engine are what are called turbocompounding, where the turbo's excess turbine power is used to generate electricity. The turbine power comes from the excess pressure in the cylinders that is otherwise wasted.

TEGs are what would be called combined cycle, where the waste heat is used to generate power. A more efficient way to do this is BMW's Turbosteamer, which uses a heat exchanger and a separate steam turbine loop. The TEG's beauty is that it is solid state and tiny.

It would be neat if F1 cars started tacking those onto the exhaust, although they probably wouldn't be worth their weight/mass. Maybe it would work if they could be somehow set into the heads so the coolant system doesn't need additional components.

Thermal paste them to the inside of a pice of Al pipe square, hex for 3 searies. then run your hot pipe through the middle. Add some cement for filler .Add a layer of 1/8" foam between the filler and the teg. like the flexable sheet stuff they wrap new dvd player's in . This will vaporize out during the brakein period leaving a small air gap to alow for thermal expansion of the chip sets. Also the cement would act like a capacitor, slowing and evening out the temperature spikes.
Also changing from direct transfer to IR only
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Side effect that hasn't been bought up yet .
It will keep charging when you turn off the car ,the exhaust is hot for a long time after shut down, this may make up for some of the shprt commings. .Active cooling may be necessary for cool down.
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Would just after the catalytic converter be the best place as it is a constant temp when active.? With good temperatures at low engine load.?

Hmmm, that's a good point. It's not constant temperature as almost all the fuel is burned by the time it enters the exhaust system, but the catalyst will definitely reheat it a little at lower load when the combustion efficiency is lower.

I think residual engine coolant should be enough to keep the TEGs from overheating once the car is off. A metal pipe has very little thermal capacity.

I don't think it is necessary to cool the TEGS after key-off.
Once the engine stops the heat sinks will cool them down quickly even if the power drain is cut off.
The engine generally won't be at full blast shortly before cutoff. I don't park at WOT ;)

WOT to park no no agreed. The cat starts working minutes after start up and is working for some time after shut down till it gets under 6-800f . Theas have started many fires from people driving at slower than ecomodder speeds , was a major problem when they first came out. Finding the happy spot or controlling the hot side temp is the problem the high temp TEG are too spendy and 12-1600f is way too hot .
If your cooling pump or fan used say 10% of power production then why not let it top up the battery recapturing more of the energy already invested? Cost /w is high at this stage of development, so by the eg in the beginning we are only able to use a fraction of whats needed. By charging 120% of running time the % just got bigger.
---
a way to extend cat heat/power production @900f add 1 drop fule. the cat will catalyze the pure hydrocodone yealding heat allowing (just a guess) 10-15mun extension of charge time per drop.alowing for 2-500% charge timeV drive time.

Sense the high temperatures are a product of the rate of energy flow being soo tightly confined to that small volume/area (heating and cooling)

How about spreading that same power (energy rate) over a larger area .. it would dilute that same exhaust heat energy over a larger area .. thus if the cooling were the same , it would yielding a lower temperature hot side for harvesting .. of course leaving it small for the cat which needs the higher temps .. but the rest of the plumbing doesn't need the higher temps .. The larger surface area , might also make mounting easier , as well as more of them on a given exhaust line .. and starting with a larger hot side , also means a larger cold side , thus also increasing the cold side energy flow rate in the same outside environmental conditions , and with that increase it would also further reduce the hot side temperatures.

The other side benefit is decreasing ICE back pressure from the larger plumbing.

Once the engine is killed the flow stops and the reaction in the cats stops too. If it didn't the cats would normally do a bad job, as the exhaust gas has to be treated in the fraction of a second it takes to pass through them.
The (relatively heavy and bulky) cats will remain hot for a while; park in high dry grass on a hot day and you'll start a fire.
The pipe behind it will cool rapidly however, especially with a herd of heat sink TEGs scavenging heat.
The residual exhaust gas has hardly any thermal mass, unlike the TEGs. The only remaining heat source it conductive though the exhaust pipe from the cats. I can't see how the temperature could rise after key-off?
If it is safe to run them, nothing can harm them afterwards.

Waste Heat Recovery
 

Back when I was designing a car to compete in the Auto X-Prize Competition, I was working on a exhaust heat recovery generator. I planned to create steam which would drive a small engine, either piston or turbine, which would turn a generator. My first test boiler was a length of stainless brake line coiled inside of a housing, that the exhaust from a small engine emptied into the middle of the boiler at the bottom. It went up the inside of the coil, flowing over the top of the coil and and exited at the bottom outside of the coil. On the few tests I was able to perform, steam was produced in less than a minute, so the ability to remove heat from the exhaust was easily accomplished. Work was halted when I was forced to move, lost my workspace, and most of my computer files including drawings and calculations. Such is life. The "engine" I was going to test first, was a turbine out of a pneumatic wrench, and the second was a multi cylinder 2 stroke engine for RC aircraft. There would have to be other systems constructed, pressure pump, condenser, and the generator head itself. The F-1 guys have simplified this greatly by have a direct drive generator driven from the turbo shaft, Nice! I wonder what kind of generator works at those RPM levels? Oh well, my two cents worth.

http://ecomodder.com/forum/attachmen...2&d=1502136797

http://ecomodder.com/forum/attachmen...3&d=1502137019