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An energy-saving method to solve the mismatch between installed and demanded power in hydraulic press Lei Li a, Haihong Huanga,*, Zhifeng Liua, Xinyu Lia, Matthew J. Triebeb,c, Fu Zhaob,c aSchool of Mechanical Engineering, Hefei University of Technology, Hefei, China bSchool of Mechanical Engineering, Purdue University, West Lafayette, IN 47907-2088, USA cDivision of Environmental and Ecological Engineering, Purdue University, West Lafayette, IN 47907-2088, USA a r t i c l ei n f o Article history: Received 15 June 2015 Received in revised form 6 August 2016 Accepted 14 August 2016 Available online 17 August 2016 Keywords: Energy-saving Installed power Demanded power Match Hydraulic press group a b s t r a c t Improving the energy effi ciency of hydraulic presses has become an important fi eld of research in low- carbon manufacturing systems. The mismatch between installed and demanded power is the main cause of low energy effi ciency among hydraulic presses. This study presents an energy-saving method to solve the problem, where a single drive system composed of several motor-pumps, is partitioned into several drive zones corresponding to load profi les. The system is used to supply power to several hydraulic presses with approximately same installed power. Each drive zone is shared by grouped hydraulic presses in the same operation. Furthermore, a method for scheduling drive zones is presented to share drive zone with no confl ict and shorten their idle time. The composition of each drive zone is optimized to match the power demand of each operation to achieve the scheduling schemes. The proposed energy- saving method is applied to a hydraulic press group in the case study. Results show that the energy effi ciency of a single hydraulic press in the group is increased by approximately 20% and the average energy consumption can be reduced by 43% compared with the traditional setup. 2016 Elsevier Ltd. All rights reserved. 1. Introduction Hydraulic presses are widely used in the metal forming process because of their high power-to-mass ratio, high stiffness, and high load capability. Unfortunately, they are also known for their high energy consumption and low energy effi ciency. In 2013, the num- ber of metal forming presses was approximately two million in China. Given that the average power rating of the presses is 40 kW, more than 280 billion kWh electrical energy is consumed per year, which is comparable to the total energy consumed by Spain in 2014 (BP Statistical Review of World Energy, 2014). Considering the vigorous promotion of low carbon and energy saving economies in manufacturing process in recent years, reducing the energy con- sumption of hydraulic systems is crucial. Thus, increasing atten- tions have been focused on energy-saving methods for hydraulic presses. The installed power of the drive system in hydraulic press is designed to meet the maximum power demand of pressing oper- ations. However, as the same drive also serves other operations which have lower power demands, mismatches between installed power and demanded power occur. Valve-controlled hydraulic systems have been widely applied in conventional hydraulic sys- tems to transform installed power into demanded power because of their low cost and simple structure. However, valve-controlled hydraulic systems have many drawbacks, such as considerable energy and pressure loss (Grabbel and Ivantysynova, 2005). An energy-saving, pressure-compensated hydraulic systemwith an electrical approach was proposed to reduce the usage of con- trolling valves, while achieving pressure compensation function and regeneration (Wang and Wang, 2014). A common approach used to circumvent mismatch is to control the fl ow based on load sensing technique (Finzel et al., 2009). And a widely used system is the volume control electrohydraulic system driven directly by various kinds of variable-speed motors, such as variable-frequency motors (Camoirano and Dellepiane, 2005; Su et al., 2014) and servo motors (Zheng et al., 2009). The control of pressure, fl ow, and * Corresponding author. Hefei University of Technology, 193 Tunxi Road, Hefei City, Anhui, China. E-mail address: huanghaihong (H. Huang). Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: /10.1016/j.jclepro.2016.08.063 0959-6526/ 2016 Elsevier Ltd. All rights reserved. Journal of Cleaner Production 139 (2016) 636e645 direction of working fl uid achieved by controlling rotation speed (Zheng et al., 2010). However, conventional control approaches are based on a linear model. This may not guarantee satisfactory con- trol performance for the servo motor direct drive volume control system; therefore, considerable research has focused on adaptive control approaches (Chen et al., 2008; Ferreira et al., 2006; Lin et al., 2013; Wang et al., 2012). However, these methods increase the complexity of control and do not reduce installed power of hy- draulic systems. Another promising approach is the use of digital hydraulics, which had been proposed several decades ago but only achieved signifi cant development recently (Locateli et al., 2014). Digital pump concepts have been analyzed, in which individual cylinders in piston pumps can be switched on or off with valves, allowing fl ow distribution to occur in intervals among several outlets (Heitzig et al., 2012), thereby matching the demanded power of an operation. Digital hydraulics have considerable advantages over analoguetechnologywithregardto effi ciency,redundancy, robustness, and component standardization. Studies have shown that digital hydraulics can signifi cantly reduce energy loss (partic- ularly during partial load) when compared with traditional systems (Huova and Laamanen, 2009; Linjama et al., 2009; Scheidl et al., 2012). Furthermore, digital pumps can be shared by two or more actuators to reduce the amount of partial load, allowing a reduction of installed power (Heitzig et al., 2012). However, as the hydraulic press itself is not a multi-actuator system, digital pump itself will not improve energy effi ciency signifi cantly and is not commonly used in hydraulic presses. In summary, although the hydraulic press is one of the most commonly used manufacturing systems, the work on mismatching between demanded power and installed power has been limited. This study, by adopting the concept of shared digital pumps, de- velops an energy-saving method for the operation of drive system in grouped hydraulic presses. Matching between installed and demanded power is achieved by coordinating operations and optimizing the confi guration of the drive systems. Meanwhile, the waiting time of drive system is reduced by drive zone sharing, which signifi cantly improves energy effi ciency of the hydraulic system. 2. Methods 2.1. Energy characteristics of the hydraulic press In traditional hydraulic press systems, the drive consists of AC (alternating current) asynchronous motors and variable displace- ment pumps. Not all the electrical energy is transformed into hy- draulic energy because of energy loss in a forming process. The energy dissipation and load profi les of a simple drive system are shown in Fig. 1 (Zhao et al., 2015). Generally, hydraulic press operations include fast falling (FF), pressingwith slow falling(PF),pressuremaintaining(PM), unloading (UD), fast returning (FR), and slow returning (SR), and often part or all of these operations are included in a forming process. The function of each operation is shown in Table 1. Among them, FF, PF, FR, and SR are necessary operations, and the others are selected according to the requirements of the forming process. The installed power of the drive of a hydraulic system is designed to meet the maximum power requirement of PF, but the pressing time is much shorter than that of the forming process and the demanded power of other operations is much less than that of PF operation, usually leading to a mismatch between installed power and demanded power, as shown in Fig. 2. Furthermore, between two successive forming processes, there is a waiting (WT) time for loading and unloading the work piece, which is almost equal to the time of the forming process. When the drive system is waiting, the demanded power is nonexistent and all pumps are in the unloaded state. As the motor-pumps cannot switch on and off frequently, the total input energy is converted into heat and dissipated to the environment according to Fig. 1(a). The problems mentioned previously lead to low effi ciency and high energy loss from the hydraulic press (Zhao et al., 2015). If the installed power of a hydraulic press could be changed during different operations to match the ideal installed power as in Fig. 2, then high effi ciceny and low energy consumption would be achieved. Nomenclature ntotal number of the motor-pump Pioutput power of drive part P(t)demanded power of hydraulic press Pm i output power of the motor-pump i Pr i unloading power hb the energy effi ciency of operationb Tbthe time length of operationb PDdemanded power kbthe number of motor-pumps to supply energy hm i the effi ciency of motor-pump i ATopening area of the throttle valve C the constant coeffi cient Ptpower loss of the throttle valve. Pvpower loss of the valves. Pflinear loss and local loss. mindex of the throttle valve boperation of hydraulic press ahydraulic press gnumber of hydraulic presses in a group h(a,b)vertical displacement of operationbof hydraulic press a A(a,b)corresponding slider area of operationbof hydraulic pressa cb time matching coeffi cient of operationb AT(a,b)area of the throttle valve of operationbof hydraulic pressa T(a, PF)pressing time of hydraulic pressa, Tworking period of the hydraulic group T(a,b)time length of operationbof hydraulic pressa PC(t)active power Einenergy consumption of hydraulic system h conversion effi ciency from the electric energy into forming energy Ebelectrical energy consumption of operationbafter using this method L. Li et al. / Journal of Cleaner Production 139 (2016) 636e645637 2.2. Energy-saving method for hydraulic press The drive system is partitioned into several subsystems (each subsystem could contain one or more drives i.e. motor-pumps), each with different installed power to match the ideal installed power. Thus the demanded power of each operation can be met. That is, different subsystems are selected to drive the hydraulic press according to the needed power of each operation. However, only one subsystem is employed by the hydraulic press at any time while the other subsystems are in unloaded state, leading to more energy loss. The solution is to share the drive system by grouping severalhydraulic pressestogether sothat these presses areworking on different operations simultaneously. The energy supplied by all the subsystems is equal to the energy demand of all hydraulic presses in the group, as described in Equation (1): P Pihit g Zt 0 Ptdt(1) where t is the time of the forming process, i is the index of sub- systems corresponding to the operation of a hydraulic press, Piis the input power of the drive i,hi is the energy effi ciency of the drive i, P(t) is the demanded power in the forming process, andgis the Fig. 1. (a) Energy dissipation of a simple drive system and (b) the corresponding load profi les. Table 1 The function of each operation. OperationFunction FFThe slider moves downward with a high speed to approach the workpiece PFForming workpiece with a lower speed PMKeep a high pressure in the cylinder for a while UDReleasing the pressure in the cylinder before moving upward FRThe slider moves upward with high speed SRThe slider moves upward with lower speed to the original position Fig. 2. The comparison between the installed power and the demanded power of the hydraulic press in a forming process. L. Li et al. / Journal of Cleaner Production 139 (2016) 636e645638 number of hydraulic presses. Now consider a drive system composes of several motor-pumps, which provides energy for all hydraulic presses in a group. The drive system is partitioned into several drive zones according tothe load profi les of operations. Each drive zone is composed of several identical drive units designed to match the demanded power of the corresponding operations, and each drive unit is composed of several motor-pumps. According to the requirements of the form- ing process, an auxiliary zone maybe utilized toprovide the needed pressure and fl ow for the auxiliary operation. Here we take four drive zones (FF, PF, FR, and SR drive zone) as an example to describe the detailed composition of the drive system, as shown in Fig. 3. Among grouped hydraulic presses, operations with similar demanded power of different hydraulic presses are supported by the same corresponding drive unit. Different hydraulic presses in a group complete their forming process by sharing one drive system. The state of the drive zones and grouped hydraulic presses in a working period is shown in Fig. 4. In Fig. 4, a series of colored rectangles in a line represents the forming process of a hydraulic press. Each colored rectangle in a line represents an operation of a hydraulic press which is supported by the corresponding drive zone, as shown in the legend. The blank rectangle in the line indicates that the hydraulic press is waiting. When the hydraulic press groupstarts towork, the motor-pumps in the drive system are activated one by one to reduce its effect on the electric power system. After completing the FF operation supported by the FF drive zone, hydraulic press I begins the PF operation supported by PF drive zone. Meanwhile, hydraulic press II begins the FF operation, supported by the FF drive zone. All the hydraulic presses complete their forming process one by one. When a hy- draulic press has completed an operation, but the next hydraulic press is not ready for the same operation, the drive zone will be in idle state until the next hydraulic press is ready. 2.2.1. Drive system modeling The main components of the drive unit and actuator of a hy- draulic press are shown in Fig. 5. The number of motor-pumps depends on the maximum output power requirement in the forming process. Normally, not all motor-pumps supply energy for an operation, although all motor-pumps keep working in a forming process. n is the total number of motor-pumps, Pm i is the output power of motor-pumps i, Pr i is the power while in unloaded state(i 1, 2, 3, n), ATis the opening area of the throttle valve, Au is the upper chamber area of the piston cylinder, Alis the lower chamber area of the piston cylinder. p is the pressure in the upper chamber of the piston cylinder, ptis the pressure in the lower chamber. v is the velocity of the piston and slider, and F is the force acting on the slider, including gravity and forming force. Thus, the energy effi ciency of operationb(hb) can be expressed as follows: hb Z Tb 0 PDdt Z Tb 0 0 X kb i1 Pm i hm i X n ikb1 Pr i 1 Adt (2) where Tbis the time length of operationb(b FF, PF, PM,UD,FR,SR), PDis the demanded power, kbis the number of motor-pumps to supply energy, andhm i is the effi ciency of motor-pump i.hbcan be improved by reducing Pm i and Pr i as the demanded energy of each operation is certain. According to the model, the total output power of a hydraulic system is: X kb i1 Pm i Pt Pv Pf PDb FF;PF;PM;UD;FR;SR(3) where Ptis the power loss of the throttle valve, Pvis the power loss of the intermediate valves, and Pfis the friction loss and local loss of pipelines. The power loss consists of three parts in the hydraulic system, which is categorized into two types: the unnecessary loss Pvand Pf, which can be reduced or even eliminated with the development of technology, and the essential loss Pt, which cannot be eliminated if the drive method is not changed but can be minimized by optimizing design. The power loss of the throttle valve can be obtained as follows: Pt ptqt vAl1m ?1CA T?m ?1 (4) where m is constant depending on the shape of the orifi ce (0.5 m 1), and C is a coeffi cient depending on the orifi ce and the fl uid nature. The main factor affecting essential loss is the area of the throttle according to Equation (4), which is considerablygreater than the unnecessary loss. The opening area of throttle shall be optimized according to the demanded power of the operation to reduce essential loss. Moreover, improving the effi ciency of motor-pumphm i can also improvehb(e.g. selecting appropriate motor-pumps that works with high effi ciency). Based on the characteristics of the variable pumps mentioned previously (Fig. 1(b), the relationship between pressure and fl ow of the pump is expressed by the equation q f (p), and the relationship for pump i is expressed by equation q fi(p). The total output power of pumps in operation bis expressed as follows: X kb i1 Pm i X kb i1 pqi X kb i1 pfip(5) The total output power of pumps in operationbis expressed in 1 pmuP- ro toM4me t sySev i rD12 Drive Zone 3 Drive Unit 2 3 4 FF RFFP SR Fig. 3. Composition of the drive system. L. Li et al. / Journal of Cleaner Production 139 (2016) 636e645639 Equation (5), based on which the motors and corresponding installed power of the drive system can be determined. Among the operations in a forming process, the energy demanded by PF is used to deform the workpiece, which is the useful energy for a forming process. The energy effi ciency of a forming process (h) can be calculated as follows: h Z TPF 0 PDdt P b 0 B Z Tb 0 PDdt . hb 1 C A Z TWT 0 X n i1 Pr idt (6) where TPFis the time length of operation PF and TWTis the time length of WT. The time length of WT is another factor affecting forming effi ciency. In summary, each drive unit can be designed according to the required power of hydraulic press operations based on Equations (2)e(5) to improve the degree of matching betwe