The TEG wearable
This post is going to describe my goals and technical information that I have been able to gather. A thermoelectric generator is a small device that can produce energy based on a differential of temperature. Hot on one side, cold on the other. The greater the difference, the more energy that the device will produce.
There are a large number of different TEG modules that are better served for different applications. Such as extreme cold, extreme hots, or minimal delta T’s. Unfortunately, these devices don’t produce that much energy and are therefore not good for energy production (like a wind turbine). What they are good for though, is ultra low power sensor applications. (Such as bio-sensors on a human, remote destinations, industrial equipment) Basically any area that it wouldn’t be suitable to have a device that requires servicing. Such as charging a battery.
For my purposes I need to find a TEG module that is the most effective given a very minor ΔT (change in temperature)
- TEG2-126LDT – Designed specifically for Body & Sensor Power Thermoelectric Harvesting Applications! Seems perfect to me
- Can handle up to 356° F
- Can handle up to 356° F
^the module is typically configured as a 40x40mm square
This TEG can produce voltages around 0.5V at 100mA. That’s 0.05 watts. We can try and boost this by expanding the thermal contact with the body (like a heat sink) and also adding a literal aluminum heat sink on the non-contact side. Until I can do further testing, I will assume I only have 0.05watts to work with. My current plan is to integrate this module in a hat, since the head produces a lot of heat and a hat will be able to encompass a large surface area. Inside the hat, I am planning on lining the inside with copper foil, to act as a large heat sink that is ‘sinked’ through the TEG module.
Based on this tiny power output, it’s worth investigating how to deal with this. For starters, we need to boost and store this energy somewhere. A battery is not ideal as we will lose far too much energy in heat in order to charge one. A capacitor seems perfect given they can deal a relatively large amount of power (compared to the output power) and can survive a stupendous amount of energy cycles. A 5 farad capacitor will do great. To ‘harvest’ or scavenge this energy, we are lucky enough to already have an IC that’s built for pulling this energy and storing it in a capacitor. The LTC3108 chip will be perfect
There’s even a breakout board ready for use
This LTC3108 chip is capable of storing energy into an external capacitor, and then supplying it to an application. The intent is for it to harvest energy, and then handle periodic bursts of energy output.
- Vsrc: -0.3v to 2.0v
- Vout: 2.35 V, 3.3 V, 4.1 V or 5 V. (selectable)
- Iout: ?? maybe 100mA ??
- To charge a 5f cap to 5V, given our power input…
- ~163 minutes to fully charge
- This should be easy to reduce given some tricks to make the charge linear
- To discharge
- ~10.65 minutes to power a very bright LED
- This assumes you aren’t wearing the TEG anymore! (so it should last longer)
I will use an LED as a proof of concept, it can also serve as a fun ‘wearable’, but it isn’t terribly useful. Unless you want a flashlight on the go that will never require a battery replacement.. so maybe.. anyway, Beyond making light, we can use this TEG device to power sensors that transmit data. At this point, we now turn to ultra low power sensors and microcontrollers
An arduino Uno will run for 1 day on a 9 volt battery. If you put the arduino to sleep, it will run for 4 years. We can step this further and use a lower power arduino, and cut some of the middlemen out. We don’t need on-board LED’s, so we can cut those out. We can also use a weaker Arduino, such as the 3.3v @ 8MHz Arduino Tiny Pro. We don’t want to use the voltage regulator either, those lose about 70% of our efficiency. We can power the board off of the 3.3v output from our LTC3108 mentioned earlier. The voltage & clock speed can be reduced. Finally, we can use code/software to disable parts of the ATMEGA328 that we are not using. Such as the ADC, which apparently draws a lot of power.
Sparkfun has a great library that we can use to disable the previously mentioned parts. More importantly, it will allow us to easily put the Arduino to sleep! This is the most crucial part to dealing with the power consumption. When the Arduino is active, it pulls 6.5mA. When it is asleep, it pulls 4.3uA! Which is pretty negligible. Given this library, we can wake the arduino up every couple minutes to read and send data, or do whatever task that it is designed to do.
I think the best example, is a stripped arduino that intermittently wakes up to read a sensor, and send the data out via a radio frequency, then goes back to sleep.
A nRF24L01+ transceiver and receiver would be perfect. Any sort of data could be transmitted in random, intermittent bursts. Some kind of biosensors that monitor things? Perhaps it could send data to the internet if you are in a wifi zone. There’s a hookup guide here
What other kind of applications could a human body based TEG be used for? I have a few fun ideas, but a serious application eludes me.
I’m thinking a battery free LED sunglasses is appropriate..!!!