Demonstrating the Seebeck Effect Using Friction and Tap Water

Have you ever wanted to witness the boundaries set by the laws of thermodynamics firsthand, and on a budget? If so, this project is perfect for you! For a project at Delft University of Technology, my group and I were tasked with designing a physics demonstration for high school students. We decided to showcase the conversion of heat generated from friction into electrical energy using Peltier components. Typically, Peltier components utilize the Peltier effect, which involves creating a temperature gradient at the electrified junction of two different conductors. While these components are generally used for cooling, we opted to reverse their function: by applying a temperature gradient, we can generate a small voltage! This reverse effect is called the Seebeck effect. By cooling one side with water and by heating another using friction, we could measure a voltage of around 100 mV.

A list of supplies you probably need to recreate our design:

• Plates of wood, we used 6 mm thick MDF.
• Peltier elements, we used three TEC1-12706 elements. These elements can be ordered for super cheap off aliexpress. The more you can fit in your design, the higher the maximum voltage you can generate!
• A metal axle.
• Bearings. We used two in our design, with an inner diameter of 10 mm and an outer diameter of 22 mm. You can use whichever you like, or whichever size fits your axle.
• A sturdy piece of fabric to which you hold no emotional attachment. It's probably going to wear down over time. We used some thick RPET fabric.
• Two thin plates of aluminium or copper. These act as heatsinks for the hot and cold side of the components and make sure the heat transfer from both thermal reservoirs is optimal (somewhat).
• Some nuts and bolts to attach a handlebar to the crank.
• A piece of plastic which could be used as a handlebar, or just an old handlebar. Any sturdy cilindrical shape you can fit your hand around should do the job.
• A plastic or metal container which can hold ice water without leaking all over the place.
• Water and ice.
• Wood glue.
• Thermal paste or thermal glue.
• 2 part epoxy, to glue the metal heatsinks to our wooden construction.
• A breadboard to construct the simple circuit on. It can be done without a breadboard, but this is not recommended for beginners.

DISCLAIMER

The first two steps involve constructing wooden parts, and it's important to include a disclaimer: we had access to a laser cutter at our university, which allowed us to easily create precise and complex wooden shapes without any manual labor. If you're building this design without a laser cutter, consider simplifying some of the components. The device's functionality is straightforward, so making modifications shouldn't be too difficult if you have some experience with a wood saw. For those inexperienced with a saw: please be careful. Woodworking tools don't just look sharp. Search up a guide for a specific procedure if you're having any doubts!

Constructing the stand

For our stand, we need two pieces of wood which can stand up straight. We cut out two rectangular parts which are about 20 cm tall, and cut 22 mm holes in the middle of them to accomodate one bearing each. In one of the rectangles we also cut a square window to attach a small square of aluminium. The size of this aluminium square and the size of your window depends on how many Peltier components you use and how you decide to place them. We used three components which are 4 x 4 cm and stacked them on top of each other.

This piece of aluminium will make contact with the spinning friction disk and transfer heat to the Peltier stack. Copper also works.

The two wooden rectangles could either be glued to a baseplate, or you could construct some tiny feet so they can stand on their own. We did both.

The most important part is making sure the bearings are aligned so our spinning axle can rotate freely. The design of the stands themselves isn't as important, just make sure the aluminium plate covers the entire surface area of your Peltier stack and there's enough room to mount said stack. Also keep enough room between the two stands to fit your cooling reservoir.

Now that we have our stand ready and, well, standing, we can construct the spinning wheel. This spinning wheel will constantly rub a piece of fabric against our aluminium heatsink.

Constructing the wheel

For the wheel, we cut out 7 disks, each with a 10 mm hole (size of the axle!) in the middle:

• 8 cm radius, 1 piece, our big spinning disk
• 6 cm radius, 2 pieces
• 4 cm radius, 2 pieces
• 2.5 cm radius, 2 pieces

You could use other sizes, just make sure they're in descending order. The idea is to stack the 6 smaller disks on top of our largest disk for added sturdyness and stability. Using only one 6 mm thick disk might be to unstable and makes it harder to get a good grip on the axle.

Stack them on top of each other one by one in descending order and glue them together using wood glue. Tip: sliding the axle through the disks makes sure they're neatly aligned during the glueing process.

Cut out your piece of fabric and glue it to the largest wooden wheel.

Constructing the crank

For the crank, we cut out 3 rounded rectangles (just for aesthetic purposes, you could also make them rectangular) and three disks with the same diameter as the round, circle part of the rounded rectangle.

Once again, we stack all the pieces, for the same reasons as before. The disks provide a better grip on the axle.

As you may have noticed in the photos, we also used some screws in both the wheel and the crank. This makes the connections between the wooden parts even stronger. It also allowed us to better connect the wooden parts to our axle.

Now to attach the Peltier elements. We stacked three elements on top of each other. Make sure you pay attention to the orientation: the hot side of the Peltier elements is usually the side on which the serial number of the component is written. Make sure the cool side of the components is facing away from the hot side of your device.

Always make sure to check this beforehand, otherwise you might damage the component!

Should you choose to stack the components, make sure the cold sides with the text on them are all pointing in the same direction.

Apply thermal paste to the metal plate you've built in the stand. There are plenty of tutorials out there for proper thermal paste application, just make sure you don't use a really uneven pattern. The hot side of the top component should be firmly pressed against the metal heatsink and locked in place.

If mounted correctly, the cool side of your peltier stack should now be exposed inside the window. This is where we'll mount our cooling element.

The cooling part is more up to you. However you wish to cool your Peltier component is fine. Just remember: the greater the temperature difference between the hot and cold side, the higher the generated voltage will be! Always be careful when using liquids for cooling, as we're using electronics which we preferably do not want to short (and destroy). Make sure there's no dripping or leakage.

We cut up a plastic cup and attached a layer of aluminium foil to it. Press your cooling element firmly against the cold side of the stack, add thermal paste if you want.

It works best if the part of the container in contact with the Peltier stack is a thermally conductive material, such as aluminium, which we used.

We filled the container with regular tap water. You could add ice cubes to increase the temperature gradient in the system. However, as the cold reservoir gets colder, the effect of friction heating will become less noticeable.Try to strike a nice balance.

The way we designed this circuit is by wiring all the Peltier elements in series, to make sure the voltages the individual components generate add up. Because of the small current, there's usually no need to add an extra resistor to the circuit.

We did do this however, because our multimeter was made in the cretaceous period and has a large internal resistance.

Once you connect everything, you should be able to measure a voltage immediately, and see it rising as you spin the wheel!