Hydrogen fuel cell systems can be a drop-in solution to battery electric vehicles (there is a business in converting battery forklifts and battery drones to hydrogen). Other than those, with the possible exception of diesel engines, hydrogen is generally not a drop-in replacement for fossil fuels in vehicles. Hydrogen can be a drop in solution for many industrial uses of fossil fuels, although some retrofit (e.g., boiler tips and piping) may be required.