I vote for:
1) Solar collectors on Baren planets and not turbines... unless they're "solar wind" turbines.
2) Wind turbines on desert planets.
3) Deuterium collection on Gas Giants. Not turbines! In fact, no use for gas giant planets except deuterium... Their moons, if they have moons in this game, would be good for dome projects. But that's it.
4) "Minor" races shold be able to spy on other races... and sell that info to whom the like.
However, in the spirit of inspiring better arguements....
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On Gas Giants.
1) Not failed stars. To say taht a gas giant is like a star except for less mass is like saying a puddle is like an ocean except less volume. Technically, both statements are somewhat accurate. However, the behavior of the larger is nothing like that of the smaller. They are not similar. And to say a gas giant is a failed star is, again, like saying a puddle is a failed ocean. It's a mental trap you should not allow yourselves to enter.
2) Cores. We don't know. It's all guesswork. The probability is we are not currently capable of predicting what the core of a gas giant will be like because it's so far away from our ability to do experiments in large quantities that whatever is there is likely to be far more complex than we can yet predict. We do know, however, that the core is under pressures impossible to imagine.
3) Pressure. The pressure in a giant's atmosphere goes from "unimaginable" at the core to the "zero" of the space that surrounds it... And *everywhere* in between. That human made objects can survive within the atmosphere of a gas giant is not questionable: it has been done. December 7, 1995, Galileo's Jupiter Probe entered the atmosphere of Jupiter and lasted close to an hour, plunging to a depth of some 130km and 22atm! As such, the only differce 22nd century technology will achieve is the depth and time a structure will be able to stand. Certainly that translates to a permanent floating structure in the upper levels of the atmosphere. If not, how are you going to extract deuterium from a giant?
4) Energy. Frankly I don't see any reason why we should spend time on devising new methods of getting energy from a gas giant: it's providing deuterium and the energy you'll get from the fusion of that is likely to be more than you'll get from anywhere else on the planet. Still...
a) Pressure Bottle. Lower from your platform a pressure bottle to 75% its burst strength, let gas in, seal the bottle at that depth, hoist it back up and let the pressurised gas out through a turbine.
b) Sea Anchor. Different layers are likely to have different wind speeds. A floating platform that dangles a sea anchor -- like a parachute made of carbon filiments -- into a lower layer would travel at a different speed than the layer it was floating in. Such a difference in speed would allow wind turbines to work.
c) Electropotential. Lightning is continuous on gas giants. That means electrical potentials between layers. An electric tether lowered to a diffent level should allow for these potentials to flow from one layer to the next, allowing you to collect some of the energy... Of course, you're inviting electrical storms to come your way. But, with large enough relays -- good old 20th century relays -- you should be able to avoid damage to sensitive duotronics from the giga-voltage.
d) Thermal Difference Engines. Lower a large tube to a layer of a different temperature. Pump gas from that layer up and use the heat difference to make energy.
e) Orbital Tethers. An electrically conductive wire orbiting a gas giant with a magnetic field -- and they're all likely to have an magnetic field -- can create an electrical curent.
f) The list goes on.
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Despite all this xyu, I *still* vote gas giants remain useful for one thing: the major source of deuterium... Even though I find the idea scientifically unsound... Blast it all! DEUTERIUM IS FOUND EVERYWHERE!!!!! Oh, well. I lost this fight long ago.
