Choosing a fuel for a new machine is an important part of the design process and must take into consideration any environmental impact or waste produced by using the fuel. Fossil fuels, resources including , , and , produce large amounts harmful when burned. Sustainable fuels, resources including and , are considered more environmentally friendly because they do not produce much of any waste. These fuels, however, can still have a large impact of the environment aesthetically by requiring large tracts of land for power collectors. So the pros and cons of each each fuel must be weighed before choosing a fuel for the project.