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Thread: Theremo-electric energy recovery

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    Default Theremo-electric energy recovery

    Its been a while but you guys have always been a wealth of info so I thought I'd give it a try.

    Ok so I'm working on a thermoelectric system that recovers waste heat off "stuff" and directs it through some thermo-electrics and then a solar panel charge controller to recharge batteries.

    Whats my best bet with the chips ? I found 12V 1.5A generator chips but man are they expensive, any ideas ?

    I found the heat sink guide so I should be good there
    Going to run them parallel ( why I want 12V chips ) so I don't run into resistance problems

    Anyone done anything like this around here and how did it work out ???

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    Default Re: Theremo-electric energy recovery

    Well, we do that in reverse with TE coolers - put energy in and cool equipment. There are commercial companies doing this on a larger scale with waste heat to electricity.
    "The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails."
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    Default Re: Theremo-electric energy recovery

    Yup, I'm working with generator chips rather than the cooling chips. It's important to keep the temp below a certain level so I'm going to need a thermal switch and a fan in the system but other than that it looks pretty easy.

    I was just hoping to see if anyone here has built this system before and if so, what insights they might have for me

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    Default Re: Theremo-electric energy recovery

    It sounds like you are looking at a thermoelectric generator, not a thermopile. The Powerpot appears to have a thermoelectric generator. With any luck, the keywords will help in a Google search. Other than that, this isn't a lot of help. The thermoelectric generator as described in the link above looks complicated enough to be expensive. If no one is using really large numbers of them, they will not be cheap. By the way, Wikipedia appears to be pretty screwed up on their thermopile page.

    EDIT:
    Sigh. Nobody ever reads my links. You can find the specs and some prices for commercially available thermoelectric generators. You would also see that a thermopile is a 'pile' of thermocouples, and not a semiconductor device.
    Last edited by MN Dave; 10-20-2017 at 03:29 PM.
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    Default Re: Theremo-electric energy recovery

    Yeah, I wasn't looking for info on thermopiles, not sure I used the term. I'm curious if anyone has built a thermo-electric generation system to convert waste heat and charged their batteries with it.

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    Default Re: Theremo-electric energy recovery

    If you're looking for design info for an electronic system, some info a bit more specific than you've provided would help. I'm assuming you are talking about peltier effect devices, as the generating component?
    I didnt think there was any difference between generating and cooling with those things, but in any case, a supplier and part number would be helpful.
    Using an off the shelf solar regulator is a smart idea, so in essence, your thermo electrics need to look (electrically) like a solar panel. That means you need enough of them in series to achieve a similar no-load voltage as a solar panel (18 - 21V ish), at whatever your "normal" hot end-cold end temperature differential works out at. Then you need enough of those series strings paralleled up, to get the current that you need to meet your power budget.
    Attention to detail with the insulation between the hot and cold sides of the peltier device is critical to getting the best performance out of them. Assuming that is what we're actually talking about.

    Eta the data sheets for the devices you are using will have all the info needed to work this out, except for the hot/cold differential in your system, how many hours per day you can achieve this for, and what your power budget is. These are all questions that you need to answer first, to assess if the system will do what you need it to do, and at a price you can live with.

    Actually, it is probably the full load voltage of a solar panel, that you need to match. Google is your friend.

    Pete
    Last edited by epoxyboy; 10-20-2017 at 04:45 AM.
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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by Boston View Post
    Yup, I'm working with generator chips rather than the cooling chips. It's important to keep the temp below a certain level so I'm going to need a thermal switch and a fan in the system but other than that it looks pretty easy.

    I was just hoping to see if anyone here has built this system before and if so, what insights they might have for me
    Why don't you tell me your understanding of the difference between the two. Both are arrays of thermocouple junctions - they may be designed for different temperature ranges, but both work via the Seebeck Effect.
    "The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails."
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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by Canoez View Post
    Why don't you tell me your understanding of the difference between the two. Both are arrays of thermocouple junctions - they may be designed for different temperature ranges, but both work via the Seebeck Effect.
    Thermocouples and peltier devices are not even close. A thermocouple is a low voltage, low current device, but capable of running at extremely high temperature - K type (alumel/chromel iirc) was good for up to around 1200C, but mV/degree and sod all current. You'd need hundreds of them to get any meaningfull amount of electrical power.
    A peltier junction is a semiconductor device, put current in (amps, at low voltage), and you get around 30C temperature differential across one. Quite common in those 12V electric chillers.
    But the neat thing is, they work almost as well in reverse, provided you can keep one side hotter than the other - hot in this context is quite low, around 100C from memory. But you get a decent bit of power out, and they are a viable way of charging a battery - at a cost. There are usually better ways, like wind, solar, engine driven alternator etc.

    Pete
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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by epoxyboy View Post
    Thermocouples and peltier devices are not even close. A thermocouple is a low voltage, low current device, but capable of running at extremely high temperature - K type (alumel/chromel iirc) was good for up to around 1200C, but mV/degree and sod all current. You'd need hundreds of them to get any meaningfull amount of electrical power.
    A peltier junction is a semiconductor device, put current in (amps, at low voltage), and you get around 30C temperature differential across one. Quite common in those 12V electric chillers.
    But the neat thing is, they work almost as well in reverse, provided you can keep one side hotter than the other - hot in this context is quite low, around 100C from memory. But you get a decent bit of power out, and they are a viable way of charging a battery - at a cost. There are usually better ways, like wind, solar, engine driven alternator etc.

    Pete
    That is true - our devices used for cooling are TE - or Peltier (Also known as Peltier-Seebeck...)

    For power generation, because relatively high temperatures are often used in exhaust stacks, etc to generate the electricity, arrays of thermocouple junctions are used wired in series and parallel to get the power output that is desired. A physics professor had an early version that used a candle to generate the heat to make enough power to run a radio - was used during WWII power outages. I've seen some larger ones used in industrial applications to scavenge energy from waste heat going up stacks and the like that. (No, not Peltier.) They've likely got millions of junctions.

    While the Peltier junction is reversible to get heat, I'm not sure it is as efficient. Some TE Coolers we buy from TECA have resistive heaters to help improve the heating. (https://www.thermoelectric.com/) No personal interest in the business.
    "The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails."
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    Default Re: Theremo-electric energy recovery

    I knew you guys would come through

    OK Yes the generator and cooler type chips are nearly identical. Generator chips are a bit more expensive because they're designed to resist more heat.

    max high temp is ~200C on a high quality chip and ~150C on the cheaper ones

    You don't want to run to many chips in series or the resistance starts to add up. Thus the idea of using a solar panel charge controller, from a small solar panel system

    A heat sink base is often used to moderate the hot side temp and a cold sink with a fan is also common.

    The output of the chips varies as temp varies but at optimum operating temp yes, I want the output to resemble the same solar panel system the charge controller is designed to deal with.

    I've built a prototype just to check the where the correct temp regime exists in the system so yeah. Working that out now. The correct temp range wasn't where I thought it would be by the way.

    I bought a thermo-electric fan and fiddled with it a little so, learned a lot there. Also I'm in a machine shop so I can adjust the cold sink side quite a bit to meet optimal temp when the system is running at normal operating temps.

    Long story short I don't know much about this area of expertise but it looks pretty simple. The chips, get the temp right. Get a fan or two on the system. Hot sink, cold sink. Arrange the output accordingly to fit the charge controller.
    Last edited by Boston; 10-20-2017 at 11:06 PM.

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    Default Re: Theremo-electric energy recovery

    There are different temperature range materials for thermoelectric generators according to the link on my earlier post. You may need to match the chips to the heat source. I don't know much about the details, but there seems to be some good information on that site.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Theremo-electric energy recovery

    Yes, the trick is to make sure the heat side doesn't exceed the limits of the chips used. Thats kinda the million dollar question

    I have several areas of the device which would be capable of supporting thermo-electric exchange, one has an average temp of between 200C and 100C and another about 25% less. I'm going to try the lower temp regime area first and go from there.
    Last edited by Boston; 10-21-2017 at 07:25 PM.

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    Default Re: Theremo-electric energy recovery

    OK I can't find the modules I originally saw but these are cheaper and do the same job.
    Sp1848
    27145
    SA

    100 temp difference 4.8V 0.67A so I'm thinking of using 20 for the initial testing. I'll wire 4 in series and 5 sets of 4 in parallel

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    Default Re: Theremo-electric energy recovery

    On a theoretical level: seawater in Maine, or Great Lakes water is cold, even on a hot and sunny day, so a physicist would naturally think that there is energy to be sucked up from the difference if one can get the collector and the heat-sink worked out. Please keep us posted. On another track, I have heard that solar cells are more efficient if they don't get too hot, so I wonder if anyone has put a simple raw-water cooler under a normal solar cell to boost the efficiency? In thermodynamic terms, bringing cold seawater up to some gizmo and sending warmer water back down is going to take some energy, but it might not take much water, like a very small wind turbine water pump?

    Ken

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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by kbowen View Post
    On a theoretical level: seawater in Maine, or Great Lakes water is cold, even on a hot and sunny day, so a physicist would naturally think that there is energy to be sucked up from the difference if one can get the collector and the heat-sink worked out. Please keep us posted. On another track, I have heard that solar cells are more efficient if they don't get too hot, so I wonder if anyone has put a simple raw-water cooler under a normal solar cell to boost the efficiency? In thermodynamic terms, bringing cold seawater up to some gizmo and sending warmer water back down is going to take some energy, but it might not take much water, like a very small wind turbine water pump?

    Ken
    You are describing an OTEC power generation system https://www.makai.com/ocean-thermal-energy-conversion/
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Theremo-electric energy recovery

    I haven't had much time to do to much on it but a cursory look, but given the room I have to install chips I can squeeze 28 chips in. I'd like to use a powered cold sink so I'll loose 500mA at 12V twice which eats chips but it means I don't have fins hanging out the windows. We'll just have to see how the prototype works and go from there. The design is pretty simple.

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    Default Re: Theremo-electric energy recovery

    MN Dave

    Not sure what happened to your latest post but in answer to your suggestion ( and a good one ) of using a glycol bath I was planing on using 1" thick surfaced copper 3" x 12" with two rows of 40mm chips for a total of 28 chips over 3/4 surfaced hot rolled steel with another chunk of copper on the cold side between the chips and the fins, also, and "maybe" and its a big maybe, but ;-) it would look cool a series of powered CPU coolers. I'd lose efficiency but the wow factor would be awesome

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    Default Re: Theremo-electric energy recovery

    Boston,

    My expertise in this area is at the level where the elbow and posterior are difficult to differentiate. I tried to conform that my suggestion was feasible when I realized that I had posted it already.

    I mean, I didn't even know that you could get an appreciable voltage by cooling one side before you started to heat the other. (you tube) And I am giving anyone advice? Sheesh.
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    Default Re: Theremo-electric energy recovery

    Yeah, there need only be a temp difference, it doesn't matter what side its on. It works either way.

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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by kbowen View Post
    On a theoretical level: seawater in Maine, or Great Lakes water is cold, even on a hot and sunny day, so a physicist would naturally think that there is energy to be sucked up from the difference if one can get the collector and the heat-sink worked out. Please keep us posted. On another track, I have heard that solar cells are more efficient if they don't get too hot, so I wonder if anyone has put a simple raw-water cooler under a normal solar cell to boost the efficiency? In thermodynamic terms, bringing cold seawater up to some gizmo and sending warmer water back down is going to take some energy, but it might not take much water, like a very small wind turbine water pump?

    Ken
    I saw an article years ago (like, 20+ years ago), possibly Popular Mechanics, where they put a single solar cell on a water cooled heatsink, under a cheapo plastic fresnel lens of about a square foot. This thing was supposedly cranking out several amps, but had a life expectancy of seconds if the cooling system fell over. I'm tempted to give it a try.

    Pete
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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by Boston View Post
    MN Dave

    Not sure what happened to your latest post but in answer to your suggestion ( and a good one ) of using a glycol bath I was planing on using 1" thick surfaced copper 3" x 12" with two rows of 40mm chips for a total of 28 chips over 3/4 surfaced hot rolled steel with another chunk of copper on the cold side between the chips and the fins, also, and "maybe" and its a big maybe, but ;-) it would look cool a series of powered CPU coolers. I'd lose efficiency but the wow factor would be awesome
    The heatsinks on both sides would normally be aluminium plate or extrusion. Way less spendy than copper, and thermally almost as conductive. You would only use copper if it was already in your shed, or you were engineering a very highly optimised solution - no offence, but your proposal sounds a bit more empirical than that.
    Steel is about the last material you want to use, unless it is part of an engine exhaust hotbox. It probably doesn't seem that way when you burn you fingers on it, but as a thermal conductor, it is rubbish. FWIW, stainless is even worse, so bad in fact, that it is used as a thermal break in some of the stuff we do, where a strong mechanical connection is still required but thermal transfer needs to be minimised.
    You'll also want thermal grease between the devices and heatsinks - that white goopy stuff that you find under the cooler on your PC CPU. Any electronics hobbyist or supplier should have it. Less is more with this stuff.

    Pete
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    Default Re: Theremo-electric energy recovery

    I'm not be an expert on the advisability of connecting Peltier devices in series. I am more comfortable with heat transfer. Pete is absolutely right about aluminum vs copper as the heat sink material and using thermal grease to couple the devices to the heat sinks. The largest loss in heat transfer can be between the device and the support. Thermal grease has very poor thermal conductivity, but it is much better than air or plain grease. The surfaces need to be smooth and very flat so the grease can be as thin as possible.

    Low carbon steel has about one third of the thermal conductivity of most aluminum alloys, which are about one third that of pure copper. The kicker is the thermal resistance at the interfaces, which may well dominate the equation to the point where steel and aluminum are only marginally different. I have looked at the numbers for titanium 6Al-4V and aluminum heat exchangers, and they are not all that much different once you factor in the interfacial resistance. (Ti 6/4 has a thermal conductivity half that of stainless, which is one third of low carbon steel).

    In this case,the heat may need to be spread laterally through the material, so that does make the thermal conductivity of the metal more important. Just saying that while I agree that copper is way over the top on cost vs. performance, and aluminum is perfectly adequate, steel isn't all that bad for the prototype.

    W/m K; Copper 386, Al 204, Al-6061 167, Steel 54, Stainless ~10, Ti6/4 ~5
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    Default Re: Theremo-electric energy recovery

    Its a gravity fed pellet stove thats made of welded steel. So the area between the pellet entry ports and the burn box has a significant heat difference. It's steel. So I can machine a nice flat surface on it, thermal grease a plate on the top of that to even out the heat ( that particular piece of steel is 3/8 x 10 x 12 ) and go. Oh and yes I do have a couple pieces of copper laying around ( machine shop ) so I could use them but if aluminum works just nearly as well then yeah, its a world cheaper and I've got tons of the stuff.

    Heat exchangers could be CPU coolers ( powered, use about 12V 500mA ) or I could just put some godawful huge passive heat sinks on it. I think it'll need a fan no matter what so rather than toast some chips right off the bat I thought I'd start with some fans ;-) As it is I can jamb about 28 chips onto the area and I should come up with at least 100W out of it once it hits operating temp.

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    Default Re: Theremo-electric energy recovery

    Oh and thanks Dave.

    Yeah, the body is all welded steel and the area of the exchanger is a big flange of steel sticking out from the stove. I can mate a piece of aluminum by machining the surface and using thermal grease. The temp is concentrated in an area of about 8" by maybe 4" and the flange is 12 x 10. So I was hoping to use something that would even out the temp and keep the chips all at roughly the same operating temperature.

    Temp at max is 700F to about 1000F if I use a blower just under the hot side flange with there being about a 30% drop in temp at the flange. So I'm mating a steel surface thats max 500F at its hottest point and more like 350F average to a piece of aluminum, or thats the plan to even out the heat over the entire piece ( two actually 4 x 12 and say 3/4 thick or whatever I have out in the yard I can find. We have tons of aluminum. I'd mate that to the chips ( machine surface to say +-1/1000 ) and then again with a cold sink. I was thinking of using CPU coolers.a

    At the moment I'm trying to work out the best burn chamber design. I thought a venturi shape of a minimum of 1.5 inches diameter would work great, made out of 1/2 FAS steel and suspended within the pellets. Didn't work, or at least didn't work well as a passive air system. Maybe if I'd have powered some combustion air through it but as a passive system it just smoked like crazy

    Anyway now I'm onto a more conventional basket, although still suspended in the burn chamber so air can circulate all around it. I've tried about a half dozen burn chamber configurations so far and the best ones ( on a passive system ) are about 75% air and 25% fuel by volume with as much surface area of the fuel exposed to combustion air as possible

    Staring at a new screen now all marked up and ready to be shaped ;-)

    Anyway thanks for all the input. Those bits about heat conductivity and interface interference are most helpful
    Last edited by Boston; 10-30-2017 at 04:18 PM.

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    Default Re: Theremo-electric energy recovery

    OK so I have the next rendition of this thing just about ready to weld up. The thermo electric chips should work well but I'm not exactly sure what temps to expect at the plate where they are located. I'll just measure it once I get this next rendition fired up but I'm thinking I should be somewhere between 150 and 200 F or less max.

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    Default Re: Theremo-electric energy recovery

    I must have fallen into the wrong forum! It's great the stuff that comes up here sometimes. Cars, guitars, motorbikes, tractors, amplifiers, biochar, greenhouses, bathrooms, and now something as close to perpetual motion as I can imagine. Carry on

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    Default Re: Theremo-electric energy recovery

    LOL no thermo electric chips have been around for a long long time.

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    Default Re: Theremo-electric energy recovery

    Thermo electric generator chips appear to be an intriguing means of generating electricity. I'm curious what motivated you to use thermo electric generators as opposed to solar panels?
    Will

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    Default Re: Theremo-electric energy recovery

    It's a gravity fed pellet stove, but I still needed a blower, so I used heat from the body of the stove to generate some juice to run the blower. that way I didn't have to plug it in.

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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by Boston View Post
    It's a gravity fed pellet stove, but I still needed a blower, so I used heat from the body of the stove to generate some juice to run the blower. that way I didn't have to plug it in.
    Question - does the manual for this stove say that it is safe to operate without the blower?

    If not, you're starting the stove from "cold" and it may take a bit to generate enough heat to run the blower - unless you've got a back-up battery to get you started.
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    Default Re: Theremo-electric energy recovery

    Um, its an R&D project, there is no manual.

    All stoves cold start. This one uses chips which go to a solar panel charge controller which go to a batter which power a blower which can be actuated prior to the chips reaching generating temps. So yeah. Got that covered. Although the blower settings are still manual, eventually I'll have a thermal switch slaved to a thermo which regulates BTU output.

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    Default Re: Theremo-electric energy recovery

    I would be in favor of a Sterling engine driving a generator. I know it is not steam but ...

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    Default Re: Theremo-electric energy recovery

    Way more complex than a simple chip.

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    Default Re: Theremo-electric energy recovery

    Quote Originally Posted by Boston View Post
    Um, its an R&D project, there is no manual.

    All stoves cold start. This one uses chips which go to a solar panel charge controller which go to a batter which power a blower which can be actuated prior to the chips reaching generating temps. So yeah. Got that covered. Although the blower settings are still manual, eventually I'll have a thermal switch slaved to a thermo which regulates BTU output.
    I meant a manual for the stove. Buddy has a pellet stove with gravity feed where they don't want it "started cold" without the blower started - that's the only reason I ask. I didn't see why it couldn't be started without the blower, but that's what the instructions said. As a woodstove guy, it made little sense to me.

    I get that the rest of it is an R&D project.
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    Default Re: Theremo-electric energy recovery

    The stove powered blower sounds like an interesting project. It sounds like the blower will come on and ramp up to speed as the stove heats up so there's never a real issue of overheating.
    I looked at some of these generating chips on Ebay to try out in a small project that would circulate solar heated water on one side and cold seawater on the other to generate electricity. I should see how much I can generate that way. It looks like a possible way to supplement solar panel generated power.
    Will

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