Friday, July 26, 2013

Screen Printing and Strip Casting a Thermoelectric Module or Thermocoupler

I have recently seen a few things that sparked my interest in this subject, again. One was a the Canadian girl who made a flashlight to work from body heat.



This could have been done using a thermocouple and a voltage booster as shown here:


This got me thinking about how to build and construct the actual thermocouple.

The basic idea of a thermocouple is that two different types of conductive metals that are put in contact with each other will create electricity when heated. Since the metals aren't creating the electricity, the heat is, then somehow electrons flow through the metal which can be read and potentially harnessed as in the case of the led lights shown above. It's not much electricity, but this method is commonly used to create thermometers and even some waste energy retrieval systems. This Seebeck effect does not require electricity it is almost mechanical from my standpoint.

I have had a couple of problems relating to my frames in that they seem to generate heat on the top surface when exposed to the sun. Also I have been using plaster inside to stiffen these pieces which has been working as a cooling surface, which is almost in direct contact with the exterior, except for a few veneers. The flashlight and voltage booster shown above seem to have a thermocouple embedded in a tile that transfers the heat from the top surface through and to the thermocouple. From there the electricity is fed to the led light with some regulation and boosting to get the voltage to where the led light can operate. I don't think I can make anything as efficient as the ceramic tile and I am not sure if the thermocouple inside the tile is complex or simple, but I have seen that some of the thermocouples shown online are basic metal wire wrapped around each other and heated, which also means this should work as a flat contact. There are some homemade thermocouples that are linked in series and others that generate electricity to run a fan when they are placed on a wood stove. I haven't seen much detail about thermocouples that work from the heat generated from the sun, but if hand heat can create a flow then the heat caused by reflecting sunlight should generate that much heat as well. Ideally if the heat is generated over a larger flat area then it may be enough to actually read on a meter and then theoretically do something.

I purchased a thermocouple from Grainger and could not get it to generate a current when I put it in a toaster oven. I have also started coating some surfaces with metals to see if I can get any sort of current between them. This also reminds me of electrolysis which is something you don't want on boats in the water. Electrical currents can be created between vessels of different materials like an aluminum hull parked near a steel hull and if there is any corrosion the salt water allows a current to flow between the vessels like a large battery. The end result is that the electrolysis wears away the metal causes a leak or worse. I imagine the same thing can happen in buildings when they are constructed with different metals being attached as the contacts could create electricity and that could also cause corrosion over time.

My problem has been to simply put together the pieces so that I can feel confident that I am going to get a current out of the different metals with the low amount of heat that I am going to be able to apply. I am not sure if the cooling surface below the exposed metals is also relevant or not, as it seems to be with the tiles around the thermocouples mentioned above. Heat flows to cool, which is the absorption of the energy from the heat and that converted energy can be electricity in the case of the differing metals connected to each other. As electricity will flow towards the path of least resistance then the contacts and cool surfaces are providing a direction for the heat energy to go. Heat is not directly electricity, so electrons must be getting released between the two metals which seems to go through the metal creating the current. Assuming I have this kinda right, then if the contact spaces are larger then the reaction can create a cumulative effect as long as the energy has a place to go.

This is where I am confused since all of the pieces I see are concentrating the effect on small rod light devices or wire that comes to a point. These types of thermocouples are devised to make an easy transition from the couple to the wire and therefore the path of least resistance is clear and the wires are treated like a positive and a negative. I am not sure how you decide what is positive and negative, but the underlying theory or Seebeck effect seems very simple with the metal wire and direct heat. The air around the wires may be the cooling element with the exposed wires and therefore the amount of current being created is less than with the ceramic tile effect, but for me any current would be neat. The cooling area or contact area may be a surface that simply pulls the heat into the surface and then the wires and metals are able to conduct the heat and transform it into loose electrons. That's the best I've got so far and the only metals I have are aluminum and steel. I am trying to figure out what pattern to use for the contacts and if the contacts need to be limited to a small area or not.

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