# Energy Regeneration Hydraulic System via a Relief Valve with Energy Regeneration Unit
## 6 Conclusions  
The proposed configuration of the PRV with the HERU connected to its outlet was discussed through the simulation and experimental analysis. These useful conclusions are obtained:  
1. To reduce the energy loss of the relief valve, the HERU is connected to the outlet of the PRV, which can increase the outlet pressure and thus reduce the pressure drop between the inlet and outlet. The overflow energy loss that backs into the tank directly in the traditional working conditions is converted into hydraulic energy, which is stored in the hydraulic accumulator and can be released when needed.
2. The PRV with HERU connected to the outlet can still have a better function in regulating pressure. The flow rate of the PRV is almost constant independent of whether the HERU is connected or disconnected to the PRV after a short adjusting time.
3. A higher pre-charge pressure can achieve higher regeneration efficiency, while the lower pre-charge pressure can regenerate more energy. The chosen pressure of the HA should consider the regeneration efficiency, the actual working style, and the installation space.
4. Further research will concentrate on the multiple HA used in the proposed HERU and the control of them to regenerate as much energy as possible with a higher regeneration efficiency.  
**Acknowledgments:** The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Grant No. 51505160), Promotion Program for Young and Middle-aged Teachers in Science and Technology Research of Huaqiao University (ZQN-YX201), Industry Cooperation of Major Science and Technology Project of Fujian Province (2017H6101), Open Foundation of the State Key Laboratory of Fluid Power and Mechatronic Systems (GZKF-201517 & GZKF-201611), and Natural Science Foundation of Fujian Province (2017J01087).  
**Author Contributions:** [PERSON] proposed the idea of using the energy regeneration unit to recovery the energy loss through the relief valve. [PERSON] and [PERSON] designed the experiments. [PERSON], [PERSON] and [PERSON] performed the experiments and analyzed the data. [PERSON] and [PERSON] contributed to the control of the test rig. [PERSON] and [PERSON] wrote the paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclatures and Symbols**  
\(p_{1}\) is the inlet pressure of the PRV; \(p_{2}\) is the outlet pressure of the PRV; \(\Delta p\) is the pressure drop between the inlet and outlet; \(p_{3}\) is the pressure of the spring chamber; \(p_{a0}\) is the pre-charge pressure of HA; \(p_{3}\) is the outlet pressure of the pump; \(p_{a}\) is the pressure of the HA; \(F_{0}\) is the pre-tightening force of the reset spring; \(A_{1}\) is the effective cross-sectional area of the main spoil; \(V_{a0}\) is the rated volume of HA; \(V_{a1}\) and \(p_{a1}\) are the gas volume and pressure when the HA is at the minimum working pressure; \(V_{a2}\) and \(p_{a2}\) are the gas volume and pressure when the HA is at the maximum working pressure; \(\Delta V_{a1}\) is the maximum oil volume that is stored in HA when the pressure in HA increases from \(p_{a1}\) to \(p_{a2}\); \(n\) is the polytrophic exponent; C is a constant; \(E_{\rm p}\) is the energy density of the HA; \(E_{\rm r}\) is the easy loss through PRV without HERU; \(E_{\rm a}\) is the regenerated energy in HA; \(Q\) is the flow rate of the PRV; \(\eta_{\rm r}\) is the regeneration efficiency of the HERU.  
\begin{table}
\begin{tabular}{c c c c} \hline Pre-charge pressure/MPa & 6.7 & 8.2 & 12.5 \\ Energy loss through PRV without HERU/kJ & 206.3 & 166.0 & 94.8 \\ Regenerated energy in HA/kJ & 126.3 & 114.8 & 79.2 \\ Regeneration efficiency/\% & 61.2 & 69.2 & 83.6 \\ \hline \end{tabular}
\end{table}
Table 1: Energy regenerated with different pre-charge pressures.