Playing with Peltier Elements

Something I’ve been continuously dabbling in is producing electricity. I figure that it’s not enough just to use it and I should look at ways in which I can produce...

Something I’ve been continuously dabbling in is producing electricity. I figure that it’s not enough just to use it and I should look at ways in which I can produce and scavenge it to understand it better.

This is a simple exercise in which we look at Peltier elements and the Seebeck effect. By running electricity through a Peltier element you can make a device which can either heat or cool something. Not only that but by heating or cooling one side of the Peltier element you can generate electricity from something either hot or cold – note that this isn’t free energy and this is only useful for recovering waste energy.

And if you doubt any of this, then give it a go for yourself.

For this project you’ll need the following items (shown above):

  • Heat sinks – I ripped them out of an old computer, from the CPU and the graphics cards – basically you’re looking for anything aluminium or copper based, sheet or section metal will also work just as well.
  • Batteries – I’m using 3 AAA batteries to generate about 100ma / 3.5 V
  • Breadboard – not essential but useful for a quick circuit
  • Low power LED – I’ve added a couple of short leads to mine
  • Electrical tape (just in case) – balanced the heat sink on above the candle and also taped the multimeter probes to the peltier
  • Candles + lighter/matches
  • Thermometer – this is a medical one, use anything you have to hand (get a proper one, mine was rubbish)
  • Multimeter
  • Toothpaste – Yes! This is correct, it’s not a mistake.
  • A Peltier element

You should be able to find everything you need around the house for free, you don’t need anything fancy except of course the Peltier. To get one of these, they are used in some computers to help cool the CPU but probably, like me, you’ll just have to buy one. I got mine from Farnell Electronics – they have a good range of them on the site between £11 and £125, they also ship worldwide which is handy. The one I chose is a bit pricey at about £20, but it has a good temperature differential, low internal resistance and fairly low voltage/current at maximum temperature differential – meaning I won’t need to use a huge power supply to see it working. This is the Peltier element I used and here’s more Peltier elements listed.

Ideally after doing this experiment, I want to get a few more and you may also want to do that as well once you realise what you can do with them. Anyway, I guess I should actually explain what a Peltier Element is…

What is a Peltier Element

In 1821, Seebeck found by using two different metals that are connected by two separate junctions, they will develop very small voltage if the two junctions at maintained at different temperatures.

In 1834, Peltier discovered the opposite of this, he found that if you apply a voltage to the same setup that it caused a different temperature at each junction, allowing you to generate both heat and cold from the voltage. Although what’s actually happening is heat transfer, the heat is transferred from one side to the other, making this a solid state heat pump.

You may also find they are referred to as TEC’s – ThermoElectric Coolers or in some cases TEG’s – ThermoElectric Generators. Essentially the Peltier Element is a combination of lots of very small thermocouples, junctions between 2 different metals or semi conductors and these are sandwiched between 2 ceramic plates and then encased in silicon.

They are in no way as efficient as regular refrigeration and are used for the benefit that there is no maintenance, no moving parts and they can occupy a much smaller space. They are used when the rapid heating or cooling or something is needed – typically lab work.

So if they’re so inefficient why do we care?…

Energy Scavenging with Peltiers

OK, so you can’t get a lot out of these, but the point is by combining them in systems that produce a lot of wasted heat, we could minimise the waste and reclaim this. Granted, this is not going to amount to much, but scale it up and you can see why car manufacturers such as BMW are beginning to combine them around the exhaust – some of that wasted heat from the engine can be converted to electricity. So if you’re going to waste heat, why not get the most out of it?

Imagine an oven lined with these, or a device that could cook your food and chill something at the same time. Of course, it’s much harder than that, unfortunately Peltiers aren’t able to transfer much heat and because they work by creating a temperature differential, you need a way extract the heat and keep the other side cool at the same time. So just sticking them out in the sun or on the side of your oven isn’t going to generate electricity, it works on a car exhaust because of the air flow when the car moves cools one side.

OK, enough talk, on with the demonstration…

Generating temperatures with Peltier Elements

This circuit is really simple, we’re just going to connect the Peltier to the battery cells and measure the voltage. You can see from the picture above, I’m using the breadboard to connect the two, but this is just me being lazy. Be very careful when you connect this to the battery – one side is going to get very hot. For safety I’m resting this on one of the heatsinks. These temperatures are generated from a 3.5v source and to show room temperature I’ve added another thermometer.

Here’s the hot side giving out 42.6 degrees C

And the cold side was too cold for my thermometer to read – need to get a better one but it felt much like something fetched from the fridge

What I did find is that when I had the heatsink on the hot side, I got a much better result, also there was no notable heat from the hot side, however much more heat off the batteries! It seems these are much better at cooling that heating.

Generating electricity from heat

Still a simple circuit, but this time no batteries! the LED is going to be powered by the Peltier (hopefully), it’ll be really dim and if you can’t see anything, use the multimeter to measure the voltage and current – I did warn you not to expect much! The trick of course is to remember basic physics, heat rises, so ideally you want your peltier to the side of the heat source so that only one side is heated, otherwise you’re not creating the optimum temperature differential – this is what the metal is for – conducting heat to the peltier. First time and I get 0.5volts and 260 mA, not enough to light the LED.

Oh and the toothpaste? So, the surface of the heat sinks and the peltier are going to have lots of imperfections and because of this, they won’t transfer as much heat in between the element and the heatsinks. You could use thermal paste but I don’t have any to hand – I found using toothpaste (seriously!) works just as well for a short time, however, under heat it soon dries out. It’s also much much cheaper to use as you experiment. Basically I think any kind of paste will do, whatever you have to hand. I found that with this, I got additional voltage and current generated (0.67V and 350mA), as below – also just to prove I really did use toothpaste, I added in the picture of the nice striped toothpaste being applied.

I also increased the heat source to 2 candles which proved to substantially improved the readings to 1.05 volts and just over 520 mA!!. Still not enough to fully power my LED and I have a feeling that prolonged temperatures like this even with my toothpaste additive is shortening the life of this Peltier.

If I could generate airflow over the heatsink on the non-heated side, I suspect I could further improve the temperature differential and create more electricity and also further distance the heat source, it was still much too close but Idid the best with what I had.

In conclusion

So there you have it, from 2 candles I very inefficiently generated over a volt of electricity, I could really refine this and improve it but unless I’m using candles anyway, then there’s no point other than for demonstration. I’d be interested in adding in multiple elements to generate more, which I may do in the future, but I’d have to run this from something where there is wasted heat – maybe my motorcycle engine block.

Anyway if you want to get some energy back from the heat you’re wasting, want to heat something, want to cool something then have a look at these. They’re probably not that efficient used to cool electronics, such as computers but there are plenty of niche uses that can be found for them and they’re definitely worth playing with if you get the chance.

I was also surprised that the elastic band holding this all together didn’t snap off! :)

One top tip – to remove the toothpaste, the best thing to use is… a toothbrush! And unlike thermal paste, it leaves your heat sink smelling minty fresh.

I now have visions of a candle powered Arduino!!

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