Thermo Electric Generator - Variable resistance USB Circuit

[dusseldorf101]

So I was hoping to create a usb phone charger powered by a TEG (Thermo Electric Generator). I haven't designed any circuits since Uni so I am really struggling putting this together and also I think it needs a variable resistor microcontroller to manage the output power/voltage.

So into the details I was looking at using a TEG like this: ThermoEelctricGenerator

The output values at specific temperatures are:

Parametric model: Temperature(℃)/Open-circuit voltage(V)/Current(MA): 20c/0.97V/225mA
40c/1.8V/368mA
60c/2.4V/469mA
80c/3.6V/559mA
100c/4.8V/669mA

The TEG would be operating between 60c and 100c so V(min)=2.4v and V(max)=4.8V and I(min)=469mA and I(max)=669mA.

This is where I need my maths checking so:

First half of problem: Proof of Concept
An iphone chargers output is:
iPhone Charger 5 volts 1.0 amps 5 Watts

So P(max)= 3.21W P(min)= 1.13W

So first of all I think I would actually need two TEG's in series so that the minimum Voltage output meets the required minimum for an iPhone Charger so for two TEG's in series:

V(min)=4.8V
I(min)=469mA (please can you confirm that current is the same in series? ~it's been a while :S)
P(min)= 2.25W

V(max)=9.6V
I(max)=669mA
P(max)=6.42W

Firstly, in principle will this work? I know that the current is alot less than the wall charger but am I also right in saying that this would just charge the phone slower?

Do I need to keep looking for a better power source? Do you know of any better performing TEG's?

Second half: Circuit Design
Because the Voltage and Current are so variable based on the input temperature I need something that is going to smooth out the voltage, what is the best way to do this? Would I be best to use a Voltage regulator or a micro controller to manage the outputs?

Any and all help would be MASSIVELY appreciated, I'm a Product Designer by trade and only have a working knowledge of Electronics and 0% knowledge of circuit design of this type.

Thanks Everyone!!

 

[hevans1944]

You could connect three of them in series, followed by a switching power supply to regulate the output voltage to five volts for the iPhone.

How are you going to heat these puppies up to 60 C? You also need to provide a pretty good sized heat sink to maintain the cold side near ambient temperature (25 C?) if the hot side is between 60 C and 100 C (boiling temperature of water at sea level). A TEG might be useful in an isolated location with no available mains power, but then what kind of cell phone reception would you get there?

Early in the last century the Russians constructed a thermopile to be placed on the chimney of an oil-fueled lamp to provide power for a broadcast band radio. IIRC, it had fins on the outside to maintain the temperature difference between the hot junctions inside and the cold junctions outside the chimney. These could be constructed from iron-copper thermocouples, but it would take a boatload of junctions to create enough voltage to run even a transistor radio. And iron-copper TC junctions are subject to corrosion more so than, say, more expensive chromel/alumel junctions.

Good luck with your project. Also, I seem to recall reading somewhere that TNGs like the ones you are using can be electrically paralleled for higher current capability. If you already have two of them on hand, you might try that using a dummy load resistor and measuring the current supplied from each TNG to verify they can be paralleled and the current load shared between them. No problemo with series connections of course, and you are right about the current capability of series-connected TNGs: same as a single TNG of the same size and temperature difference.

 

[garublador]

Your math looks good to me. I won't get into too many specifics, but here are my initial thoughts.

The way I understand that these phones work with the chargers is there is some sort of resistor divider on the USB data lines that tell the phone what type of current your charger can supply. USB 2.0 ports on computers are only guaranteed to be able to supply 500mA while dedicated charger can supply more. So they put a resistor divider on the USB data lines in higher current chargers to tell the phone it can draw more current. That means you'll have to choose if you want your device to be able to guarantee either 500mA or 1A. I don't have any experience with TEG's so I'm not sure what would happen if the phone tries to draw more current than what the TEG can supply at the given temperature.

You're right that you'll need some sort of voltage regulator. USB spec is from 4.75V to 5.25V and I doubt you'll be able to stay in that range. I'd use some sort of switching power supply. You may need a buck-boost (can convert voltages from either higher or lower than the desired output voltage) depending on the configuration you choose. Remember to take the efficiency of the converter into account when determining how much current your TEG's can supply.

With all that in mind, I wouldn't trust the two in series to charge a phone at 60C. You don't meet the 500mA requirement. How the circuit at that temperature would behave with a 500mA load on it would depend on the voltage regulator you choose and what the phone does with an under voltage condition. I would trust it to charge at 500mA at 100C, but not at 1A.

 

[dusseldorf101]

I should probably explain the back story, I am working on an idea/project that is centred around reusing wasted energy in the home. I saw some of these TEG's that were used to power a Fan on top of a stove to increase the circulation of heat around a room. There is a TEG built into the base of the fan and you simply place it on top of the stove and through latent heat exchange it powers the fan.

So the concept is basically a small magnetic device that you can affix to any hot metal surface to generate power. The idea being that if you were sat at the airport waiting and there weren't any plug sockets nearby but there was a radiator in the corner, or if you are at home and you simply wanted to make use of the heat coming from your radiator then you could snap this device onto the radiator and charge your phone.

60 degrees came from being a rough estimate of the peak output temperature of an average radiator (This was found using some very quick googling), and 100 degrees as the peak temperature because this device could be fixed to a metal kettle to use the heat that is given off from a house hold kettle to generate power. I would probably need to make some consideration to if the product is attached to anything hotter such a stove but for now I just want to know if it is possible for a typical household application.

I wonder if it would be a more effective device if instead of directly charging a phone via USB that it had an internal battery that it charged and then when you needed power it could then be connected to a phone. That way you could also charge it via a USB port and it would work similarly to any external backup phone battery pack?

 

[GPG]

You may need a buck-boost (can convert voltages from either higher or lower than the desired output voltage)

A good idea. Google SEPIC

 

[GPG]

an internal battery that it charged

Still need a converter but perhaps you could switch one from charge (the battery) to charge the phone.