Currently, fuel cells have no purpose because the cost to create them (~105 energy plus the cost of searching for the materials and teleporting to their location and back home) is higher than the cost to simply take apart the machine and put it back together again (105 energy straight up). Either make fuel cells cheaper to create, or make it more expensive to disassemble and reassemble a machine (or a third option, don't give free fuel when a machine is fully reassembled or give less fuel; alternatively, fuel cells could add more fuel than they currently do).