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I am currently trying to do body temperature thermal harvesting using thermoelectric module. The voltage obtained from the module is around 50-60mV. There are not many chip out there is able to boost such low voltage to 5V. My requirement is actually harvest body heat to charge an electronic device, say a smartwatch/ smartphone(need at least 5v,1amp). I am not sure it's possible until i found the ltc3108 from linear technology. The chip is powerful as it can boost mV up to 2.3, 3.3, 4.1 or 5v. However, the current output from ltc3108 is super small, where i think i can get only about a few milliamp. Anyone has idea to improve this ? I saw some said using a supercapacitor could work (from youtube), i don't really understand how a supercapacitor could work, as it is charge up to its rated voltage via current input right ? meaning if using a few milliamp could take a very long time to charge. And when it is discharging, could it harm the load device ? because the current from supercapacitor is very big right ? I saw a quite similar project posted in youtube. The schematic is shown below. Anyone mind to explain how it's circuit work ? I think he is not using ltc3108 here. I would really appreciate your help. Thanks in advance

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  • \$\begingroup\$ If you will get a "few milliamp" at 5V from this proposal, I will call it super-efficient. \$\endgroup\$ – Eugene Sh. Nov 21 '16 at 16:11
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    \$\begingroup\$ Use a bigger body. \$\endgroup\$ – brhans Nov 21 '16 at 16:27
  • \$\begingroup\$ You get 50-60mV and how much current? If you want 5V and 1A then that's 5W of power. At 50mV that will require a current of 100A from your thermometric module. Since you haven't given any details of the part i can't say for certain but I strongly suspect you can't get that much current out of it. I also suspect that your power budget calculations (assuming you did any) were off by a few orders of magnitude. \$\endgroup\$ – Andrew Nov 21 '16 at 16:56
  • \$\begingroup\$ If you cover an entire adult body with Peltiers you might be able to get 400mW (less than 10% of what you would like), and it would be highly impractical and uncomfortably cold. \$\endgroup\$ – Spehro Pefhany Nov 21 '16 at 18:52
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The problem is not with the chip (well, not the main problem anyway - but we'll get to that), the problem is with the rubbish efficiency of peliters at small temperature differences, first, the problems.

Peltiers are normally used to create temperature differences when a current is applied to them, peltiers are not known for their stellar efficiency, and when used as generators they're even worse, I've seen figures like 5% labeled as "cutting edge efficiency", so that's our first problem. So you might need to push 100W of heat through a peltier to get 5W of electrical power.

Our second problem is that peltiers have low thermal resistances, see thermal resistance is the temperature difference created by a flow of heat energy (measured in degrees per watt 'C/W). A low thermal resistance means you need a powerful heat flow (not necessarily a high temperature, but a powerful source, lots of watts) to generate any meaningful temperature difference across the device, and we need a big temperature difference across the peliter if we want to generate any meaningful amount of power. This is part of the reason why their efficiency is so low, you need a big temperature difference to get lots of power, but they conduct heat too well - they maintain a high temperature difference about as well a screen door on a submarine maintains a high pressure difference.

This leads to the third and least intuitive problem: Getting a decent temperature drop across the peltier in the first place. See, there's not a great deal of thermal energy available for harvesting from a person in the first place, maybe 200W max across their whole body (which I think is something like 2m^2) so first of all, that puts an upper limit on power generation (the most common peltier is 40x40mm or 1/2500th of a person, so that's already ~0.1W per peltier MAX). And once you take into account the pitiful generating efficiency of peltiers, suddenly you're looking at only a few milliwatts per 40x40mm device (stacking them helps a bit, but while 10mW is twice as good as 5mW, in the grand scheme of things...) This is made worse by the fact that the ability of the peltier to dump the waste heat (all 90-95% of it) into the environment, which is very strongly dependent on the temperature difference, hot things lose heat faster and more easily than mildly warm things, so you need big heatsinks to compensate.

Now you know why chips like the LTC3108 are only designed to spit out a few mA, the kind of applications that thermal harvesting often gets used in uually have so little potential generating capacity that's it'd be pointless using anything bigger. (and mainly the term "energy harvesting" is only applied to really * really* low power stuff)

Now I could be out by an order of magnitude in my calculations, but even then, 5W is 1000 times greater than what I'd expect to be able to generate from a single peltier strapped to a person. If you stuck a big ol' CPU cooler on one side and put a blowtorch on the other, then I'd expect to get 5-10W out of each peltier.

(as for a supercapacitor helping things along, it's not that having a big cap allows you to generate more power, it's just that it allows you to store the energy you've harvested and then release it all in one big burst - like sending a radio packet - of course, now that the cap's been drained, you have to wait a while for it to be recharged)

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  • \$\begingroup\$ But based on what I found in youtube, the schematic diagram shown , it claimed it is doable, which I why I asked. I don't really understand how the flow of the schematic works. Anyone mind to explain ? What is the super capacitor there for ? And the linear regulator 7805 ? Is it charges the super capacitor to certain potential enough to trigger the 7805? And 7805 it self is able to produce 5v 1amp ? \$\endgroup\$ – Dion Tan Nov 22 '16 at 4:55
  • \$\begingroup\$ @DionTan It's not that it won't work at all, it's just that it won't work continuously, (unless you have a really strong heat source like a fire) there just isn't the kind of heat flow from a person. A lot of 78xx regulators don't have undervoltage lockout so they're always on, it's just when you get below their drop-out voltage, their output voltage drops off. As I said in my answer, "it's not that having a big [supercapacitor] allows you to generate more power, it's just that it allows you to store the energy you've harvested and then release it all in one big burst" \$\endgroup\$ – Sam Nov 22 '16 at 21:42
  • \$\begingroup\$ Hi Sam. I appreciate your time and really Thx you for helping me to understand. Okay, for the supercapacitor part, as I know my output from TEG is small. Which is why I used a voltage booster. Let say that ltc3108 is able to allow me regulate 5v, 0.3mA, obviously it is not enough to power up any electronic devices (say a microC) , right ? Which is why we put a supercapacitor there to allow it to fully charge (rated voltage 5.5V), and when it is fully charge, it will allow my microcontroller to operate for a small period of time ? Is that correct ? \$\endgroup\$ – Dion Tan Nov 23 '16 at 10:13
  • \$\begingroup\$ @DionTan Yep, that's basically it, though there's plenty of micros that will work off 0.3mA (you'll probably get better efficiency at 3.3V) The Atmel SMART SAM L22 requires 0.039mA per MHz, so if you ran it at 5V you could hit 5MHz+ before running out of power, and at 3.3V (most modern micros are all 3.3V or less these days) you'd be able to get to ~10MHz. There are plenty of other options (I just Googled "ultra low power micro"), the super cap is most useful when you want to run a big load, like sending a radio burst, for that you might need 50+ mA, but only for a fraction of a second \$\endgroup\$ – Sam Nov 23 '16 at 22:01
  • \$\begingroup\$ Hi sam, so if i going to change my application to detecting heart rate of a rehab patient and send the data to a system base. Will it work ? how long will my microcontroller run on the supercapacitor? However, i read that supercapacitor discharging will produce a lot of current, wont it spoil the microcontroller? I thought of maybe sending heartrate of patient every 2 to 3 minutes ? but im afraid my supercapacitor wont allow my microcontroller to run long enough to measure the heartbeat and send the data. \$\endgroup\$ – Dion Tan Nov 24 '16 at 6:03

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