There was some discussion on the kiara server about a possible geothermal generator mod. this is an outline of the ideas to compare them and get feedback.
The basic concept is that a special block would be placed at bedrock. then, it would be fed mining pipes and energy (and a mining drill?) to increase its "depth". as the depth increases, higher tier mining pipes would be required, made out of more advanced materials. once a redstone signal is applied (or removed?) it will start to generate energy based on its depth. the greater its depth, the more heat is generated. there also may be a modifier based on the biome. the heat will be output the top in either GT or IC2 heat units. this can then be used to make steam, which can be piped to the surface to generate power.
however, as with most renewable energy sources, some form of limitation must be imposed. there are currently two ideas on how this can be done:
1. Some bedrock blocks in the world would be replaced with "geothermal tiles" the frequency of this would be based on biome, with volcano biomes having the most, mountains and boreal forests some, and the rest of the world will have nearly none. these would be distinguishable by sight, and the generator must be placed directly on top of this tile.
the pros of this approach are:
it is a lot simpler to search for a tile directly underneath rather than having to look at the whole surrounding area.
it makes these areas "special". while you could find a ton in a volcano biome, you might get lucky and find one somewhere else, which would provide a nice power source, and more variety.
it is much easier for the player to understand
The cons of this approach are:
it requires worldgen.
it would need flat bedrock to be in effect for it to work optimally
2. instead of having special blocks, there would be a limit of having only one generator per X tiles. This could be based on biome, alternatively a larger biome based modifier to the heat production could be applied, and have the distance requirement a flat number
The pros of this approach are:
It would not require worldgen
it allows geothermal energy to be used anywhere, though certain biomes would be more effective.
The cons of this approach are:
It could be very complicated for a player to get working, since they could cross biome layers, and there is no easy indication of biome borders at bedrock level
It requires checking the surrounding area every so often, which could cause lag
it allows it to be spammed indefinitely, though requiring a very large area to do so.
potentially too similar to bloodasp's proposed oil system
some ideas for future expansion are:
a scanner similar to the OV scanner to locate geothermal tiles from the surface
having disastrous collapses if they are built too close together
retro-gen to place geothermal tiles after the world is generated
having the output slowly diminish if too many are in close proximity
using a second pump to create a coolant circuit
having surface features such as hot springs to show areas where geothermal spots might be.
EDIT: some proposed numbers for the first approach:
a basic mining well generator at its maximum depth will generate 12 HU/t
invar(or aluminium?): 25
a well will take 10-20 pipes to reach the next tier of depth
the frequency of geothermal spots would be about the following:
volcanoes: 1 per chunk
mountains, boreal forest (any others?): 1 per 5 chunks
everywhere else: 1 per 25 chunks