## 1 Introduction
[18] proposed a flywheel energy-saving system (FESS) that can store the redundant energy at no-load stages and low-load stages and then release the stored energy at high-load stages. [PERSON] et al. [19] applied a hydraulic accumulator to a 20 MN fast forging hydraulic press to realize energy conversion by absorbing large flow-pressure pulses and hydraulic shock. The results show that the hydraulic accumulator has promising energy-saving effects. [PERSON] and [PERSON] [20] utilized a hydraulic accumulator and a flywheel to realize energy recovery and presented a control strategy for this energy-saving method. [PERSON] [21] developed a novel hydraulic accumulator that can keep the hydraulic system pressure constant by using a piston with an area that varies with stroke. Compared with conventional recovery accumulators, this new accumulator could significantly increase the energy storage density. [PERSON] et al. [22] proposed an  
Figure 1: Energy dissipation of each energy conversion unit during a working cycle. (E-M, electrical–mechanical energy unit; M-H, mechanical–hydraulic energy unit; H-H, hydraulic–hydraulic energy unit; H-M, hydraulic–mechanical energy unit; M-F, mechanical-forming energy unit). integrated drive and energy recuperation system for a hydraulic excavator, and the large gravitational potential energy of the boom was recovered by a three-chamber hydraulic cylinder. [PERSON] et al. [23] combined the advantages of hydraulic accumulators and electric accumulators and presented a compound energy recovery system to improve the energy efficiency of hydraulic equipment. Through a series of experiments, they validated that the compound system could increase the energy efficiency by approximately 39%. [PERSON] et al. [24] studied the energy-saving potential of the boom cylinder with an accumulator in the hybrid excavator system. The results of simulations and experiments showed that the closed hydraulic regeneration system had a high recovery efficiency. Examples of applying energy recovery systems also include generator-super capacitor energy recovery and the gas cylinder energy recovery system [25]. Figure 2 gives the energy density of different energy recovery circuits. However, [PERSON] [26] pointed out that the low utilization efficiency of the recovery energy is also an important problem that cannot be ignored. In general terms, an energy recovery process is composed of two sub-processes: the recovery process and the reutilization process. The number of energy conversions increases with the integration of an energy recovery system, thereby increasing the system's structural complexity and causing low energy efficiency as well. In summary, energy efficiency and mismatch between the demanded power and the installed power can be significantly improved by using the aforementioned methods. However, most of those methods only consider energy efficiency and ignore the impact that they have on the hydraulic press itself. Therefore, most of the energy-saving presses present complicated structures and poor practicality. Furthermore, most of the existing studies in the literature only focus on the hydraulic power units and hydraulic control system; they ignore that the hydraulic press is a forming machine composed of various functional systems, such as the hydraulic actuating system, hydraulic cooling system, and other auxiliary systems. Each of these systems waste a large amount of energy during the forming process and has great potential to save energy. Therefore, in contrast to existing energy-saving methods, the system in this paper focuses on the hydraulic cooling system as a novel starting point and proposes an energy-saving buffer system to improve the energy efficiency of a single press. A reduction of the installed power and improvement in energy efficiency can be achieved by integrating the prefill system into the hydraulic cooling system, whereas noise pollution, processing properties, and structural complexity can be improved by adding a buffer system.