Thermopiles are a Blutricity generator included with the RedPower mod. These blocks generate Energy from the thermal differences of adjacent blocks (like lava vs. water). Though it provides even less energy than a solar panel, you are not restricted to having daylight available, so it's very useful to power low-footprint machines like a Sorter or Retriever.
The most efficient stable design is lava on one side and water on four others, providing roughly 0.5 Amps of Blutricity. It is possible to use designs with greater power generation, but they will be unstable, melting adjacent snow or ice blocks and/or solidifying lava into obsidian.