Farming fuel is pretty easy in Trigon: Space Story if you know what to do. There are ways to get 3 fuel for every 1 fuel you expend, but remember that this only works if you have good weapons. You must also be okay with having a high bounty.
How to get a lot of fuel in Trigon: Space Story
First, you need to fly around and mess with pirates whenever you can. You can attack them openly, stop them from raiding stations, or look for the red skulls on the map and kill them. Eventually, they'll start attacking you, giving you more chances to raise your bounty.
As soon as your bounty reaches 4.0, you'll get a pirate every time you leave the system. Every pirate ship has at least 2 to 3 fuel to give you after you win a battle. Keep your bounty between 4.0 and 7.0 and the pirates won't be hard to beat, so just keep flying near a station and killing pirates.
For more on Trigon: Space Story, why not check out What's the difference between Trigon: Space Story and FTL? or Is there multiplayer or co-op in Trigon: Space Story? Here on Pro Game Guides.