use the natural, stable temperature of the ground to power garden lights and sensors via a thermoelectric device
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I'm a big fan of alternative energy. While solar and wind are the big players in modern alternative energy, I think geothermal energy should be looked into more.
I plan is to use geothermal energy in an unconventional way. Usually geothermal is used to cool and heat buildings or used to make electricity via steam. I am hoping to produce electricity directly via a thermoelectric device.
I have been doing voltage checks throughout the day. I have gotten just about half a volt during the hottest part of the day. I also insulated the exposed part of the ground radiator with earth.
So I tried the setup during day time with thermal paste. And I am making power! It's not much power though. It's defiantly in the energy harvesting range of power output.
This is a picture with my multimeter disconnected.
Here is a side view of the setup.
When the sun started to go down my voltage went down to 0.034 volts. I think this is due to a drop in ambient temp. I also checked the current and I was making about 1.5 miliamps short circuit. This means I'm making about 50 micro amps. Another note, I flipped the TEG over to see if it preferred one side to be the hot side but it didn't matter.
I tried the setup with the ground radiator almost all the way into the ground. Then I placed the TEG on top of the ground radiator and then a PC CPU heat sink on top of the TEG. The test was done in the evening and I used a multimeter to measure voltage of the device. I did not produce any voltage. I will try the test again in the day time so that the ambient air will be hotter.
Also of note is that I did not use any thermal paste on the device. I think thermal paste will be highly important to get decent performance from my setup.
I made a radiator out of scrap steel. This radiator will conduct heat into the ground to create a temperature differential for the thermoelectric device. The radiator is about 14 inches long and is make from two lengths of "angle iron". A cross section of the radiator is 2 inches long. At the top of the radiator is a flat square piece that the TEG will sit on.
So when I first saw this year's Hackaday prize video, I was hyped! Then, I saw the energy harvesting category and thought "I can do this!". But no idea came to me unit months later when I was reading a blog post on here about energy harvesting. I tried to think of all the renewable forms of energy I knew of while I was walking though my house. Then, I looked out the window and into my back yard. I was thinking about geothermal energy and how behind it was compared to wind and solar. That's when I got the idea. I was looking at the ground under a tree and thought how could I put a garden light under it.
A metal anchor in needed because metal is a great conductor of heat and metal can easy be driven into the ground.
A thermoelectric device or TEG is a solid state device that converts heat differentials to electrical energy. They can be bought cheaply from eBay or various other online stores.
Thermal paste is commonly used to bond CPUs to cooling devices in computers. I will try making some home made paste with three parts tooth paste and one part petroleum jelly.
The obvious way of accessing temperature down in the ground would be using a heat pipe. You should be able to build one yourself. May be use a regular steel pipe that is sturdy enough to be hammered into the ground. Close up one side. Put a bit of water into the pipe and boil it so the steam drives out all air. Then close the other side of the pipe, too. If you want to transfer heat downwards you need to add a wick that moves the liquid water back up to the top. Without the wick you have a thermal diode that moves heat only in one direction.
I think a possible problem would be that the ground is only at a constant temperature at a larger depth, the top layers will be closer to ambient. If your spike is in contact with the top soil as well as the bottom it would even out before reaching the generator.
A possible solution would be to use an insulating sleeve for the top half of the spike so only the bottom part (that is deeper underground) would be exposed.
Using aluminium or copper would yield a lot more heat transfer than iron, steel or stainless. I'm curious if you get this working, would be fun to try in my yard too, I have a lot of crappy solar lights as well.
Thanks for the comment! I added some soil to cover and insulate the top part. And I used steel because I had it on hand and I could weld it. But aluminum or copper would be better.
Trey Core
Gos
Jan Waclawek
Mike Rigsby
Brian Sutherland
Don't forget that at different times of the day and year, the temperature difference may be inverted (hotter on top, or colder on top) -- and the scavenging circuit needs to handle that. Also, it would be best to charge all the time and store to a capacitor or battery for nighttime lighting.