When I was a kid, I made a potato battery. Basically I stuck a galvanized nail and a copper wire into a potato and hooked it to a light. This was interesting, but the amount of electricity was so small and the slowly rotting potato was so unpleasant that I wrote it off as being fun but totally impractical.
So now being a little older I come across the concept of the Earth Battery. This is pretty much the same as the potato battery except that you just stick some copper and galvanized steel in the ground somewhere and get about 0.5 volts of power with very low amperage. I also now know that you can increase the voltage by putting several in series or increase the amperage by wiring them in parallel... or increase both by doing a combination of the two.
There is also a little gadget called a joule thief http://en.wikipedia.org/wiki/Joule_thief that will boost the voltage at the cost of amperage. You can apparently run over 100 led's or even a florescent tube with a single earth battery cell using one of these.
So I wanted a light for my doorway, and I had a couple of old LED flashlights. The LED I had required three volts to function, so I thought I would just keep hooking more cells up in series until it ran the light. I had some copper and galvanized pipe that I cut into pieces a few feet in length, and I had some plastic 5-gallon buckets.
Basically, all I did was fill the bucket with dirt, add water to make mud, push a copper pipe in one side and a galvanized (zinc) pipe in the other. This generated about 0.2 volts. I added some salt and vinegar (about a cup of each) and the voltage went up to about 0.6 each. I hooked 5 of these up in series and ended up with about 3.2 volts at 30 milliamperes = 0.096 watts.
Now 0.096 watts is not much power, a AA batter is about 1.5 volts at 4 amps or about 6 watts, so it would take about 60 of my buckets of dirt to generate the same current as a AA battery. However, there are a lot of low power appliances that can make use of this, such as small lights, radios, telegraphs and fans to name a few. These are not that important here, but are vital in most third world countries, and may become important in a SHTF scenario here someday.
For comparison, I have a 50 watt solar panel worth about $250. So it would take about 2600 buckets (or cans) of dirt to accomplish the same thing, except that they work 24 hours a day instead of about 6 for solar so really it would only take 650 cans of dirt to accomplish the same thing as a $250 solar panel.
The real question is how long will it last. I have had it running now for about 48 hours and it looks about the same. If it lasts for a week it may have potential, if it lasts a month it will definitely be useful. In any case there are several factors that will influence this that I will have to play with. I believe that if I stuck the pipes directly in the ground with several feet of damp dirt between them (instead of in my buckets) then it should last indefinitely. I also made a smaller lower cost version by cutting the top off of an aluminium soda can, filling with mud and sticking a pencil lead in it. This version actually worked a little better on voltage (about 1 volt per can), but not so good on amperage (about 0.5 milliamperes). I tried substituting an old lump of wood charcoal that I pulled out of a camp fire and it produced about the same voltage but about 6.3 millamperes... much better, I suspect this is due to the increased surface area of the lump of charcoal.
I will post more after I try a few more combinations and see how they work over time. I also found some similar methods of generating electricity from static, radio, and cosmic rays... basically from the air, that I intend to try out. For more information start here: http://en.wikipedia.org/wiki/Earth_battery and then do a youtube search on "earth battery".
part 2 here