Due to their large application quantities with extremely low efficiency, pollutant emissions, high fuel consumption, and oil price, researches on the environment protection and the energy saving of construction machinery, especially hydraulic excavators, become very necessary and urgent. In this chapter, the authors proposed a complete study for the excavators' hydraulic energy recovery systems. This study is divided into two parts. In the first one, an overview for the energy saving principles is discussed and classed based on the type of the energy recovered. In the second part and once the energy recovery system is selected, the authors proposed a new approach to design the energy recovery system under a typical working cycle. This approach, the global optimization method for parameter identification (GOMPI), uses an optimization technique coupled with the simulated model on simulation software. Finally, results concluded that applying GOMPI model was an efficient solution as it proves its accuracy and efficiency to design any energy recovery patent applied to hydraulic systems.