Fluids play a crucial role in energy systems. Understanding the principles of fluid dynamics is therefore essential for analyzing these systems and developing effective solutions to provide sustainable energy. In this unit, two main forms of energy are considered: mechanical and thermal. Fundamental principles are described and case studies are presented for application. These case studies focus on offshore Wind Turbine, pumped-storage energy transfer stations and geothermal energy (low and high energy), three promising solutions for a sustainable future on Reunion Island.