Works when placed between Lava and Water (or Ice). Place lava under it and water on all four sides, and have a Blue Alloy Wire coming out of the top.
Be careful when placing thermopiles under wood or other flammable materials, as the heat from the lava is not blocked by the thermopile. Placing flammable materials directly over a thermopile may cause them to catch fire.
Using a Voltmeter, it is possible to read the power output of a Thermopile. Connecting one Thermopile (with a lava block under it and four surrounding water blocks) to a Battery Box through a Blue Alloy Wire will produce an output between 0.46A and 0.50A. Although this is not a lot of power, it provides a consistent charge to a Battery Box. One Thermopile takes a very long time to charge a Battery Box and cannot sustain charge being drained from a Blulectric Furnace. At least 10 Thermopiles are needed to for constant furnace use. Other machines using Blutricity, such as the Sorting Machine do not not require as much power and can be sustained when placed next to a Thermopile, without the use of a Battery Box.
Bear in mind that each Blue Alloy Wire between the Thermopile and the Battery Box will reduce the output by ~0.01A.