Coal

Although energy crystals are by far the most common source of energy in the falling city, some older machines use coal to do their job instead. Being a way more reliable energy source for long continuous work that doesn't need to be replaced and recharged, some machines that need to function all day and night burn coal and get their energy from that. The biggest user of coal is the hospitals throughout the city, relying on a constant source of power to keep patients alive. The use of coal is getting more and more restrictive, however, as it is very much a limited resource. The aim is to get everything on a more renewable energy source like the energy crystals being powered mainly by the wind. So far no solutions have been found that don't involve someone having to monitor the power output of the crystals and swapping them while the machine is still running. Something that poses a risk for the worker swapping the crystal and the risk of having to swap multiple crystals at the same time leaving the machine without energy until the new ones are out in. Some machines running on coal also use a type of wood that grows really quickly but is too weak to build out of as an effect. While great as a backup source, the wood doesn't burn as hot as coal and thus makes the machines less efficient. A mix of coal and wood lets a machine run efficiently and still not use as much coal as it otherwise would have. Coal powered machines are required to sit near the top of the building it supplies. This is to prevent the smoke and ash to dirty the streets and worsen the air quality for the people living nearby. Some have built chimneys to lead the smoke higher but the wear and tear they face from the constant wind make it a more temporary solution.