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Thread: Using a peltier to generate electricity

  1. #1
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    Using a peltier to generate electricity

    Today I was thinking about turbo chargers and using the waste heat that engines make, then I got an idea. Why not use the engine or exhaust heat to generate electricity?

    For anyone that does not know, a peltier is a solid state electric device that can generate electricity when heat flows from the hot side to the cold side. It can also cause heat to flow from the cold to the hot side when electricity is applied. The greater the heat difference between the two sides, the more power is generated. Peltiers are used in areas with lots of geothermal energy to power remote homes.

    If you put a peltier on a hot flat part of the engine block that was inside the plenum or another place that had good air flow, and added a heat sink to the cold side, I believe you could generate all the power you could want. Peltiers are very reliable. There would be zero moving parts in the entire electric system. You would need a few peltiers running in parallel to generate the electricity you needed. This would make the system redundant.

    You could also try the exhaust pipe. If the pipe is too hot, insulating it just enough to get the ideal temp would be easy. A small heat sink located in the slip stream designed to not disrupt the air flow would probably work well.

    The entire system would weight a fraction of what a light weight alternator does, would not have any parasitic losses, and should be more reliable.

    What do you think? Do you see any problems? I think I may try to generate some electricity with my car. The electricity would not be used in the car's electrical system at first. If I can reliably generate power then I will try running my car from it.

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    Registered User StRaNgEdAyS's Avatar
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    I like it a lot!
    It certainly bears investigating, I'm surprised it hasn't been thought of before
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    It seems the maximum hot side would be about 300 F / 150 C. The temperature change could not be greater than 160 F / 70 C using a single stage. You can multi stage by stacking peltiers on top of each other. This would work well on the engine block, or head. The units would be 40mm (1.57 inches) by 40mm by 4mm.

    http://www.kryotherm.ru/generat_termo_moduls.htm has 20 watt (under ideal conditions) units, given thier size I could make an array to power everything, I think.

    http://www.powerchips.gi/fwdlook.shtml If this technology works it would change the transportation industry over night.
    Last edited by dustind; July 25th, 2004 at 04:45 AM.

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    Registered User StRaNgEdAyS's Avatar
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    The more I look at it, the more I like it.
    Being an electricity board trained sparky by trade (#1) you'd have thought I'd have thought about this earlier, but there is not a great call for thermodynamic electricity generation over here in Australia.
    There is certainly no shortage of heat avaliable under the cowl, and putting some of that wasted radiant energy to use is certainly a great idea. The only problem I could see is possible heat retention problems, where areas that would normally radiate heat might loose a little of this ability, and the heat then gets reflected back into the engine.
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    I was thinking about that. Would the heat sink on top of the peltier cause more or less heat loss inside the engine? I doubt it will change too much either way. I guess if there was some space between the peltier / heat sink units it would not matter too much.

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    Registered User Bob Kelly's Avatar
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    Howdy !
    Sounds like a great idea to me ...where can i get one ?
    hehehe
    In my experience playing with Fuel-vaporization system on automobiles
    the exhost tempitures can reach and or exceed 800deg.f thats mighty hot ! (cherry red i think ! )
    however, as you mentioned insulating would be no problem..

    The only problem i could see is the lack of heat when the engine is cold
    or in extream cold conditions, as in high altitude !

    Do not discount the engine getting cold while running ! ...
    I remember vividly my car quitting one cold day driveing home with the vaporizer in line... i promptly got out and put my hand on the exhost header ( where i had the vaporior tubes attached) and i could keep my hand there easily ! the exhost pipe was that COLD !
    Granted that only happened once and after insulating the entire manifold it never happened again....

    I believe your best bet would be to cut a short section of square tubeing and attach it just below the headder of your car... mount your
    electricity goodies there and try that... when your done messing with that you could weld the original section back to the exhost pipe and it will be as good as new.
    Or perhaps buy a piece of flexable exhost pipe and use it ?

    Sense the avrage altenator uses somewhere arround 5 hp to turn it at full charge it is very wastfull... useing the latent heat is a briliant idea !
    GO FOR it ! and sell the idea to general motors and get Rich ! HAHAHA!

    Bob.....

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    Registered User wsimpso1's Avatar
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    My thermomachine background is lacking on this one. A little more of a description of how the gadget is constructed would help all of us, and some idea of its thermodynamic efficiency is helpful too.

    Certainly we have waste heat well in excess of the electrical load we carry. You have to place this device where enough waste heat is available and can be detoured through the peltier before being exhausted.

    Keep in mind a few issues:

    All engines tend to be fussy about being able to be adequately cooled during high power low speed operations (take-off and climb). This is because the engine is making its maximum waste heat while the cooling air mass flow is low for cooling the oil and either the cylinder heads or the radiator...

    When the electrical power required is low, the heat that would otherwise be rejected to the electrical system would have to be rejected anyway, so the waste heat path through the peltier would have to be also available to get heat overboard anyway.

    A large percentage of the drag is incurred by cooling current systems. This type of device may add more to the cooling drag - be careful, this could swipe speed from you than other methods...

    Billski

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    Registered User wsimpso1's Avatar
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    Well, I could not resist, and did a web search. Bismuth based semiconductor junctions are the basic place where energy exchanges occur in a Peltier, libeating electricity. High temp versions can go to 200 C on their hot surface, but I could not tell if this was hot side of the junction, or just the outside of some insulative layer to allow the junction to survive with that outside temperature. Efficiency of commercial units are on the order of 10% as a power generators. That means that you need to flow 10x the energy you would capture as electricity. 200 C is not as hot as our cylinder heads get, but is hotter than our crankcases.

    One outfit marketed for low maintenance remote power generation worked out to 11% efficiency at producing 500 W and and was a box 61 x 61 x 40 inches... Now that particular unit is interesting for comparison because 500 W is the neighborhood we need to support a small IFR airplane. If we assumed that by omitting the extraneous parts, this gadget would only take up an eighth of the original volume, our IFR airplane's Peltier would only be 30 by 30 by 20 inches, which is about the size of the engine it would be taking heat from...

    Hmm, while tantilizing, this gadget appears to be too bulky yet for supporting IFR aircraft, so let's check it out for Sport Pilot and that ilk. Well, you still need something in the way of a radio, transponder, and an anti-collision strobe, so lets say 100 W, which means about 18 x 18 x 12 inches, which is still on the order of the volume taken up by the engine that would power such an aircraft...

    Being as they are semiconductors, the maximum working temperature for the hot side is pretty well restricted. If we assume generous hot and cold side temps, we can get to an upper bound on efficiency of approximately 35-40%, so while they may improve, they will likely remain bulky, and that could make their application tough. And I have not done any investigations into their weights...

    Now as to cooling drag, we currently run hot surfaces higher than 200 C and thus delta T's of 160C. In the case of a Peltier, you are trying to transfer heat with a delta T of 80 C, so you would need to double the airflow for these components, actually raising cooling drag.

    Now when the hot side can go to 300C and the cold side to -100 C, our cooling can be as efficient as current systems and cover the engine temperature range. At these temps, the power density will begin to be high enough to allow acceptable packaging in an airplane.

    My perspective? Hmmm, a 500W alternator might actually require close to 1 HP to to turn, costing you something like 0.42 pounds of fuel per hour at 500 W output. This is less than 1% of cruise fuel flow. Where they are now, Peltiers will cost both form drag and cooling drag. Even with the temps spread apart enough to equalize cooling flows, the form drag penalty is still there for running air around the added bulk.

    I will have quite enough to do getting my ship flying and reliable. When the Peltier gets there as an alternative to rotating power units, let me know.

    Billski

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