Could you please elaborate on the source of fuel for the APU? I'm curious to understand the mechanism behind its operation and how it derives the energy required for its functions. Is the APU fueled by a traditional fuel source such as gasoline or diesel, or does it utilize a more advanced and environmentally-friendly alternative? Additionally, what factors determine the efficiency and longevity of the APU's fuel source, and how does this impact its overall performance and cost-effectiveness?