A geothermal generator is a structure that can only be built on a geyser and converts the thermal energy into electrical power. It provides a contant 3600W but it is more expensive to build than the other power generators and is limited by the location and number of geothermal geysers. The geothermal generator is available once it is researched. It is the only power generator that requires research.
The placement requirement of the geothermal generator often means it's located far from the safety of the core base, or necessitates a base relocation to a zone harder to defend against raiders. It can be protected against raiders by sealing it off with two layers of stone walls.
In cold biomes it can be useful to build your initial base surrounding a geothermal vent as a source of heat, and eventually power generation.
While you can use vents to pipe the generator's heat into another room, a cheaper alternative is an open door. The open door will release some of the heat, and colonists in adjacent rooms will not notice the generator through the door, even if it's in line of sight. Geothermal generators have a very low beauty score, so if placed indoors, should have their own room.