# Energy Regeneration Hydraulic System via a Relief Valve with Energy Regeneration Unit
## 2 Configuration of the PRV with HERU  
### Configuration of HERU of Energy Loss in Relief Valve  
To reduce the energy loss of the relief valve, a hydraulic energy regeneration unit (HERU) is connected to the outlet of the relief valve, which can increase the outlet pressure of the relief valve and thus reduce the pressure drop between the inlet and outlet [18], as shown in Figure 1. As there are many types of relief valves, the PRV is chosen as an example in this paper. The advantages are as follows:(1) The overflow energy loss, which backs into the tank directly in the traditional PRV, is converted into hydraulic energy to be stored in the HA and released when needed. During this process, the outlet pressure of the PRV is far higher than that of the tank, which means that the pressure drop between the inlet and outlet is decreased. Therefore, the pressure drop through the PRV can be reduced. The pressure drop of the PRV is defined as  
\[\Delta p=p_{1}-p_{2} \tag{1}\]  
where \(p_{1}\) is the inlet pressure of the PRV; \(p_{2}\) is the outlet pressure of the PRV; and \(\Delta p\) is the pressure drop between the inlet and outlet.  
When the traditional PRV is connected to the tank, the pressure \(p_{2}\) is approximately equal to zero. Therefore, the pressure drop can be simplified as  
\[\Delta p=p_{1} \tag{2}\]  
When the HERU is connected to the outlet of the PRV, the oil will flow into the HERU and cause an increase in the outlet pressure \(p_{2}\). Therefore, the pressure \(p_{2}\) is far larger than zero, while the pressure drop through the PPRV is smaller than that without HERS.  
The overflow energy loss in PRV is featured by the product of the flow rate and the pressure drop  
\[P=\Delta p\cdot Q \tag{3}\]  
where \(P\) is the power lost through the PRV and \(Q\) is the flow rate through the PRV.  
The flow rate \(Q\) through the PRV remains mostly constant. Hence, the energy loss through the PRV with HERU is less than the traditional PRV. Moreover, the energy stored in the HERU can be reutilized and the efficiency of the hydraulic system can be improved. In general, a higher outlet pressure \(p_{2}\) results in a lower overflow energy loss of the PRV, which is characterized by the pressure drop \(\Delta p\). To ensure the normal operation of the PRV, the outlet pressure \(p_{2}\), which is caused by the HERU, must be smaller than the target relief pressure of the PRV.  
(2) The proposed PRV is different to the traditional PRV in that the outlet of the pilot valve is connected to the tank alone. In comparison, the outlet of the pilot is connected to the outlet of the main valve in the traditional PRV. This difference can ensure that the pilot valve of the proposed PRV is not affected by the pressure fluctuations in the outlet of the PRV.  
(3) As seen in Figure 1, the PRV has a main valve and a pilot valve. The inlet pressure \(p_{1}\) acts on the main valve spool and the pilot valve spool at the same time. Meanwhile, the pressure \(p_{3}\) of the spring chamber is also connected to the pilot valve spool. The outlet port of the PRV is connected to the HERU through a directional valve, which is used to determine whether to recover the overflow energy or not. The pilot oil circuit is connected to the tank alone to avoid an increase in the relief pressure as the PRV and the HERU are in series.