The geothermal icon is represented with a building, but one part of the building will have higher top where smoke will come out of. The other part of the icon is there will be arrows coming from the bottom of the building or the ground and into the building.
This page shows the different variations of the Geothermal from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.
Idea of Geothermal Icon by Eric Brissette
We've received the idea on January 30, 2013 and created the icons on April 5, 2013
Icons for green/renewable energy sources. The two I'm looking for are a Wind Turbine and Solar Panels. Other possible ideas are Hydroelectric, Geothermal, and Biomass, but those icons may be more difficult to portray with a simple icon.