The integration of variable renewable energies (VREs) such as wind and solar (PV) in biofuels production is a key component of the green transition, particularly in island settings. Many non-interconnected islands rely heavily on fossil fuels, making renewable solutions both economically and environmentally viable. Recent studies on Greek islands like Anafi and Ai Stratis highlight the potential of hybrid systems combining PV and wind energy for sustainable electricity generation. To further enhance biofuel production, hydrothermal treatment methods are being explored. Hydrothermal carbonization (HTC) and hydrothermal liquefaction (HTL) are emerging technologies that convert agricultural waste into valuable biochar and biofuels. A novel approach, PV-assisted hydrothermal pyrolysis (HYDROPYR), merges HTC with pyrolysis to optimize biocarbon production while utilizing renewable energy sources. This process significantly reduces the energy footprint of hydrothermal conversion by integrating solar power. Additionally, hydrothermal carbonization is being investigated as a promising method for reducing persistent pollutants like per- and polyfluoroalkyl substances (PFAS) in wastewater sludge. By coupling VREs with advanced biofuel processing technologies, the green transition of biofuels production becomes more efficient and sustainable.