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An energy-saving method to solve the mismatch between installedand demanded power in hydraulic pressLei Lia, Haihong Huanga,*, Zhifeng Liua, Xinyu Lia, Matthew J. Triebeb,c, Fu Zhaob,caSchool of Mechanical Engineering, Hefei University of Technology, Hefei, ChinabSchool of Mechanical Engineering, Purdue University, West Lafayette, IN 47907-2088, USAcDivision of Environmental and Ecological Engineering, Purdue University, West Lafayette, IN 47907-2088, USAa r t i c l ei n f oArticle history:Received 15 June 2015Received in revised form6 August 2016Accepted 14 August 2016Available online 17 August 2016Keywords:Energy-savingInstalled powerDemanded powerMatchHydraulic press groupa b s t r a c tImproving the energy efficiency of hydraulic presses has become an important field of research in low-carbon manufacturing systems. The mismatch between installed and demanded power is the main causeof low energy efficiency among hydraulic presses. This study presents an energy-saving method to solvethe problem, where a single drive system composed of several motor-pumps, is partitioned into severaldrive zones corresponding to load profiles. The system is used to supply power to several hydraulicpresses with approximately same installed power. Each drive zone is shared by grouped hydraulicpresses in the same operation. Furthermore, a method for scheduling drive zones is presented to sharedrive zone with no conflict and shorten their idle time. The composition of each drive zone is optimizedto 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 energyefficiency of a single hydraulic press in the group is increased by approximately 20% and the averageenergy consumption can be reduced by 43% compared with the traditional setup. 2016 Elsevier Ltd. All rights reserved.1. IntroductionHydraulic presses are widely used in the metal forming processbecause of their high power-to-mass ratio, high stiffness, and highload capability. Unfortunately, they are also known for their highenergy consumption and low energy efficiency. In 2013, the num-ber of metal forming presses was approximately two million inChina. 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 thevigorous promotion of low carbon and energy saving economies inmanufacturing 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 hydraulicpresses.The installed power of the drive system in hydraulic press isdesigned to meet the maximum power demand of pressing oper-ations. However, as the same drive also serves other operationswhich have lower power demands, mismatches between installedpower and demanded power occur. Valve-controlled hydraulicsystems have been widely applied in conventional hydraulic sys-tems to transform installed power into demanded power becauseof their low cost and simple structure. However, valve-controlledhydraulic systems have many drawbacks, such as considerableenergy and pressure loss (Grabbel and Ivantysynova, 2005).An energy-saving, pressure-compensated hydraulic systemwithan electrical approach was proposed to reduce the usage of con-trolling valves, while achieving pressure compensation functionand regeneration (Wang and Wang, 2014). A common approachused to circumvent mismatch is to control the flow based on loadsensing technique (Finzel et al., 2009). And a widely used system isthe volume control electrohydraulic system driven directly byvarious kinds of variable-speed motors, such as variable-frequencymotors (Camoirano and Dellepiane, 2005; Su et al., 2014) and servomotors (Zheng et al., 2009). The control of pressure, flow, and* Corresponding author. Hefei University of Technology, 193 Tunxi Road, HefeiCity, Anhui, China.E-mail address: huanghaihong (H. Huang).Contents lists available at ScienceDirectJournal of Cleaner Productionjournal homepage: /locate/jclepro/10.1016/j.jclepro.2016.08.0630959-6526/ 2016 Elsevier Ltd. All rights reserved.Journal of Cleaner Production 139 (2016) 636e645direction of working fluid achieved by controlling rotation speed(Zheng et al., 2010). However, conventional control approaches arebased on a linear model. This may not guarantee satisfactory con-trol performance for the servo motor direct drive volume controlsystem; therefore, considerable research has focused on adaptivecontrol approaches (Chen et al., 2008; Ferreira et al., 2006; Lin et al.,2013; Wang et al., 2012). However, these methods increase thecomplexity 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 achievedsignificant development recently (Locateli et al., 2014). Digitalpump concepts have been analyzed, in which individual cylindersin piston pumps can be switched on or off with valves, allowingflow distribution to occur in intervals among several outlets(Heitzig et al., 2012), thereby matching the demanded power of anoperation. Digital hydraulics have considerable advantages overanaloguetechnologywithregardtoefficiency,redundancy,robustness, and component standardization. Studies have shownthat digital hydraulics can significantly 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 moreactuators to reduce the amount of partial load, allowing a reductionof installed power (Heitzig et al., 2012). However, as the hydraulicpress itself is not a multi-actuator system, digital pump itself willnot improve energy efficiency significantly and is not commonlyused in hydraulic presses.In summary, although the hydraulic press is one of the mostcommonly used manufacturing systems, the work on mismatchingbetween 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 systemin grouped hydraulic presses. Matching between installed anddemanded power is achieved by coordinating operations andoptimizing the configuration of the drive systems. Meanwhile, thewaiting time of drive system is reduced by drive zone sharing,which significantly improves energy efficiency of the hydraulicsystem.2. Methods2.1. Energy characteristics of the hydraulic pressIn 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. Theenergy dissipation and load profiles of a simple drive system areshown 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), andoften part or all of these operations are included in a formingprocess. The function of each operation is shown in Table 1. Amongthem, FF, PF, FR, and SR are necessary operations, and the others areselected according to the requirements of the forming process. Theinstalled power of the drive of a hydraulic system is designed tomeet the maximum power requirement of PF, but the pressing timeis much shorter than that of the forming process and the demandedpower of other operations is much less than that of PF operation,usually leading to a mismatch between installed power anddemanded power, as shown in Fig. 2.Furthermore, between two successive forming processes, thereis a waiting (WT) time for loading and unloading the work piece,which is almost equal to the time of the forming process. When thedrive system is waiting, the demanded power is nonexistent and allpumps are in the unloaded state. As the motor-pumps cannotswitch on and off frequently, the total input energy is convertedinto heat and dissipated to the environment according to Fig. 1(a).The problems mentioned previously lead to low efficiency andhigh energy loss from the hydraulic press (Zhao et al., 2015). If theinstalled power of a hydraulic press could be changed duringdifferent operations to match the ideal installed power as in Fig. 2,then high efficiceny and low energy consumption would beachieved.Nomenclaturentotal number of the motor-pumpPioutput power of drive partP(t)demanded power of hydraulic pressPmioutput power of the motor-pump iPriunloading powerhbthe energy efficiency of operationbTbthe time length of operationbPDdemanded powerkbthe number of motor-pumps to supply energyhmithe efficiency of motor-pump iATopening area of the throttle valveCthe constant coefficientPtpower loss of the throttle valve.Pvpower loss of the valves.Pflinear loss and local loss.mindex of the throttle valveboperation of hydraulic pressahydraulic pressgnumber of hydraulic presses in a grouph(a,b)vertical displacement of operationbof hydraulic pressaA(a,b)corresponding slider area of operationbof hydraulicpressacbtime matching coefficient of operationbAT(a,b)area of the throttle valve of operationbof hydraulicpressaT(a, PF)pressing time of hydraulic pressa,Tworking period of the hydraulic groupT(a,b)time length of operationbof hydraulic pressaPC(t)active powerEinenergy consumption of hydraulic systemhconversion efficiency from the electric energy intoforming energyEbelectrical energy consumption of operationbafterusing this methodL. Li et al. / Journal of Cleaner Production 139 (2016) 636e6456372.2. Energy-saving method for hydraulic pressThe drive system is partitioned into several subsystems (eachsubsystem could contain one or more drives i.e. motor-pumps),each with different installed power to match the ideal installedpower. Thus the demanded power of each operation can be met.That is, different subsystems are selected to drive the hydraulicpress according to the needed power of each operation. However,only one subsystem is employed by the hydraulic press at any timewhile the other subsystems are in unloaded state, leading to moreenergy loss. The solution is to share the drive system by groupingseveralhydraulic pressestogether sothat these presses areworkingon different operations simultaneously. The energy supplied by allthe subsystems is equal to the energy demand of all hydraulicpresses in the group, as described in Equation (1):PPihitgZt0Ptdt(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, Piisthe input power of the drive i,hiis the energy efficiency of the drivei, P(t) is the demanded power in the forming process, andgis theFig. 1. (a) Energy dissipation of a simple drive system and (b) the corresponding load profiles.Table 1The function of each operation.OperationFunctionFFThe slider moves downward with a high speed to approach the workpiecePFForming workpiece with a lower speedPMKeep a high pressure in the cylinder for a whileUDReleasing the pressure in the cylinder before moving upwardFRThe slider moves upward with high speedSRThe slider moves upward with lower speed to the original positionFig. 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) 636e645638number of hydraulic presses.Now consider a drive system composes of several motor-pumps,which provides energy for all hydraulic presses in a group. Thedrive system is partitioned into several drive zones according totheload profiles of operations. Each drive zone is composed of severalidentical drive units designed to match the demanded power of thecorresponding operations, and each drive unit is composed ofseveral motor-pumps. According to the requirements of the form-ing process, an auxiliary zone maybe utilized toprovide the neededpressure and flow for the auxiliary operation. Here we take fourdrive zones (FF, PF, FR, and SR drive zone) as an example to describethe detailed composition of the drive system, as shown in Fig. 3.Among grouped hydraulic presses, operations with similardemanded power of different hydraulic presses are supported bythe same corresponding drive unit. Different hydraulic presses in agroup complete their forming process by sharing one drive system.The state of the drive zones and grouped hydraulic presses in aworking period is shown in Fig. 4.In Fig. 4, a series of colored rectangles in a line represents theforming process of a hydraulic press. Each colored rectangle in aline represents an operation of a hydraulic press which is supportedby the corresponding drive zone, as shown in the legend. The blankrectangle in the line indicates that the hydraulic press is waiting.When the hydraulic press groupstarts towork, the motor-pumps inthe drive system are activated one by one to reduce its effect on theelectric power system. After completing the FF operation supportedby the FF drive zone, hydraulic press I begins the PF operationsupported by PF drive zone. Meanwhile, hydraulic press II beginsthe FF operation, supported by the FF drive zone. All the hydraulicpresses complete their forming process one by one. When a hy-draulic press has completed an operation, but the next hydraulicpress is not ready for the same operation, the drive zone will be inidle state until the next hydraulic press is ready.2.2.1. Drive system modelingThe main components of the drive unit and actuator of a hy-draulic press are shown in Fig. 5. The number of motor-pumpsdepends on the maximum output power requirement in theforming process. Normally, not all motor-pumps supply energy foran operation, although all motor-pumps keep working in a formingprocess. n is the total number of motor-pumps, Pmiis the outputpower of motor-pumps i, Priis the power while in unloadedstate(i 1, 2, 3, n), ATis the opening area of the throttle valve, Auis the upper chamber area of the piston cylinder, Alis the lowerchamber area of the piston cylinder. p is the pressure in the upperchamber of the piston cylinder, ptis the pressure in the lowerchamber. v is the velocity of the piston and slider, and F is the forceacting on the slider, including gravity and forming force.Thus, the energy efficiency of operationb(hb) can be expressedas follows:hbZTb0PDdtZTb00Xkbi1PmihmiXnikb1Pri1Adt(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 tosupply energy, andhmiis the efficiency of motor-pump i.hbcan beimproved by reducing Pmiand Prias the demanded energy of eachoperation is certain.According to the model, the total output power of a hydraulicsystem is:Xkbi1Pmi Pt Pv Pf PDb FF;PF;PM;UD;FR;SR(3)where Ptis the power loss of the throttle valve, Pvis the power lossof the intermediate valves, and Pfis the friction loss and local loss ofpipelines. The power loss consists of three parts in the hydraulicsystem, which is categorized into two types: the unnecessary lossPvand Pf, which can be reduced or even eliminated with thedevelopment of technology, and the essential loss Pt, which cannotbe eliminated if the drive method is not changed but can beminimized by optimizing design.The power loss of the throttle valve can be obtained as follows:Pt ptqt vAl1m?1CAT?m?1(4)where m is constant depending on the shape of the orifice(0.5 m 1), and C is a coefficient depending on the orifice and thefluid nature. The main factor affecting essential loss is the area ofthe throttle according to Equation (4), which is considerablygreaterthan the unnecessary loss. The opening area of throttle shall beoptimized according to the demanded power of the operation toreduce essential loss.Moreover, improving the efficiency of motor-pumphmicanalso improvehb(e.g. selecting appropriate motor-pumps thatworks with high efficiency). Based on the characteristics of thevariable pumps mentioned previously (Fig. 1(b), the relationshipbetween pressure and flow of the pump is expressed by theequation q f (p), and the relationship for pump i is expressed byequation q fi(p). The total output power of pumps in operationbis expressed as follows:Xkbi1PmiXkbi1pqiXkbi1pfip(5)The total output power of pumps in operationbis expressed in1pmuP- ro toM4me t sySev i rD12 Drive Zone 3 Drive Unit234FFRFFPSRFig. 3. Composition of the drive system.L. Li et al. / Journal of Cleaner Production 139 (2016) 636e645639Equation (5), based on which the motors and correspondinginstalled power of the drive system can be determined.Among the operations in a forming process, the energydemanded by PF is used to deform the workpiece, which is theuseful energy for a forming process. The energy efficiency of aforming process (h) can be calculated as follows:hZTPF0PDdtPb0BZTb0PDdt.hb1CA ZTWT0Xni1Pridt(6)where TPFis the time length of operation PF and TWTis the timelength of WT. The time length of WT is another factor affectingforming efficiency.In summary, each drive unit can be designed according to therequired power of hydraulic press operations based on Equations(2)e(5) to improve the degree of matching between the installedand demanded power. In addition, the waiting time can be short-ened to improve energy efficiency according to Equation (6).Scheduling drive zones and designing drive units will be discussedin the following subsections.2.2.2. Scheduling schemeThe time length of PF operation (which has maximum energyconsumption) is determined bythe requirements of forming, whichremains unchanged. The total PF operation time of all hydraulicpresses in a group is set as a working period time. To maximizeenergy efficiency, the PF drive zone supplies energy to groupedhydraulic presses successively without idling, which can be used toadjust time length of other operations. The working period time iscalculated as follows:Xga1Ta;PF T(7)where T (a, PF) is the PF operation time of hydraulic pressa,gis thenumber of hydraulic presses in a group, and T is the working periodof the hydraulic press group, during which each grouped hydraulicpress completes an entire forming process including WT.Determining time length of other operations on each hydraulicHydraulic Press IHydraulic Press IIHydraulic Press IIIHydraulic Press IVHydraulic Press VHydraulic Press VIFFPFSRWTTimeFRFig. 4. The state of hydraulic presses and drive zones in a working period.AuAlpqtATn.i1vptFConstant power variable displacement pump with the drive of AC asynchronous motorSliderq1qiqnFig. 5. Components of the drive model and acutator of the hydraulic press.FR ZonePF ZoneFF ZoneT ( , FR)T ( , PF)T ( +1, FF)T ( +1, FR)T ( +1, PF)Fig. 6. Time length constraints of FF and FR operations.L. Li et al. / Journal of Cleaner Production 139 (2016) 636e645640press is the next step. These time lengths can be categorized intotwo types: one type is process parameter as in the case of PM andUD, which have shorter time length but cannot be changed. Theothers are auxiliary operations, including FF, FR and SR. To guar-antee that no WT time exists between successive operations in aforming process and that grouped hydraulic presses can share drivezones without conflict, the time length of auxiliary operationsshould be adjusted, which is expressed as follows:?Ta;FR ? Ta 1;PFTa 1;FF ? Ta;PFa 1;2;g(8)whereT (a, FR) is the FRoperation time of hydraulic pressa, T (a, FF)is the FF operation time of hydraulic pressa. Hydraulic pressa 1implements the same operation following hydraulic pressa, andhydraulic pressg 1 is hydraulic press 1 in the second workingperiod which follows hydraulic pressg, as shown in Fig. 6.In the SR operation, the time length is short and the demandedpower is small. Thus there is no need for adjustment. After timelength is determined, the number of drive units in each drive zonecan be obtained as follows:cbXga1Ta;b,Xga1Ta;PF# 1 b FF;FR;SR(9)where cbis the time matching coefficient of operationband also isthe number of drive units in drive zoneb, and is an integer-valued function that indicates the nearest integer that is less thanthe variable.In order to reduce the number of drive zones, operations withapproximately the same demanded power can be put together andsupported byone drive zone. The time matching coefficient is then:cb1b2Xga1Ta;b1 Ta;b2,Xga1Ta;PF# 1 b1;b2 UD;FR;SR(10)Generally, each drive zone has only one drive unit and the valueof cbis 1.After scheduling, the sum time length of some identical opera-tions is shorter than the working period time, leading to drive zoneidle. During this idle time, all motor-pumps in the drive zone areunloaded to reduce energy loss from switching. The idle time iscalculated as follows:TWb T ?Xga1Ta;b b FF; PM; UD; SR(11)whereTW(b)is the idle time of drive zonebduring aworking period.WhenT (a 1, FF) T (a, PF) and T (a, FR) T (a 1, PF), TW(FF)andTW(FR)will be at the minimum. As far as Constraint (8) is satisfied,increasing T (a, FF) and T (a, FR) can improve the energy efficiencyaccording to Equation (6).2.2.3. Matching designAfter the time length of each operation is determined, drivezones can be designed to match demanded power. One drive zoneis set to follow the maximum power for UD, PM and SR operationbecause the demanded power of these operations is very small(approximately zero). If an operation has no power requirement,then no drive zone will be assigned to it, but the operation timeremains the same according to the requirement of each formingprocess. Thus, the matching design should focuse on the FR and FFdrive zone according to the scheduling scheme.In FR operation, the pumps play a major role in adjusting thevelocity of the slider. The pressure p(a, FR) generated by the gravityof the slider is sufficiently large to make the pumps work at highefficiency. According to the time length constraints of FR operationin Constraint (8), the demanded flow of each hydraulic press iscalculated as follows:Xkam1fFR;mpa;FR Aa;FRha;FR=Ta;FR a 1; 2g(12)where h(a, FR) is the vertical displacement of operation FR on hy-draulic pressa, A(a, FR) is the corresponding lower chamber area ofpistoncylinderin hydraulic pressa, p(a, FR) is the inlet pressureof acylinder for operation FR in hydraulic pressa, f(FR,m)(p(a, FR) is theflow of pumpmin drive zone FR with pressure p(a, FR), and kais therequired number of motor-pumps in hydraulic pressa.As the installed power of FR drive zone is designed to meet themaximum demanded power, the number of motor-pumps in FRdrive zone can be determined using the following equation:XkDFRm1Pm max8:Xkam1fFR;mpa;FR ? pa;FR9=;(13)where kDFRis the number of motor-pumps of FR drive zone, and P(m)is the output power of pumpm.In FF operation, the slider falls under its own weight and thevelocity is adjustedbychanging AT. Mostof the flowcomes fromtheupper oil tank through the prefill valves and the pumps only play asupporting role in supplying flow. Thus, the length of time isoptimized by changing ATto adjust the velocity of the slider:ha;FFAa;FFCATa;FFpm1a;FF Ta;FF(14)where h(a, FF) is the vertical displacement of operation FF in hy-draulic pressa, A(a, FF) is the corresponding area of piston cylinderin hydraulic pressa, AT(a, FF) is the area of the throttle valve, andp1(a, FF) is the outlet pressure of a cylinder, which is primarilyassociated with the weight of the slider. After AT(a, FF) and theprefill valve are designed, the composition of the FF drive zone iseasily determined. The pumps are selected to provide the flow thatthe prefill valve cannot reach.Based on the hydraulic system model presented in section 2.2.1,the matching design of each drive unit is completed according toEquations (13) and (14). The approach is to use a variabledisplacement pump or throttle valve to adjust velocity according toload profiles, followed by more accurate adjustments. The match-ing design of drive units also considers load profiles to make eachmotor-pump in the drive zone work at high efficiency.After each drive zone is designed,hbis improved because of thehigher matching degree. The WT time of the drive system iseliminated and the drive zones have less installed power, signifi-cantly reducing the energy consumption according to Equation (6).3. Experiments and resultsA group of five identical hydraulic presses, each with a nominalpressure of 20 MN and installed power of 510 kW, are selected asthe research object to validate the energy-saving method. The presshas typical mechanical structure, with three beams and four pillars,as shown in Fig. 7(a). The slider is fixed on two pistons and aL. Li et al. / Journal of Cleaner Production 139 (2016) 636e645641plunger, which can move up and down together in their respectivecylinders, and the cylinders are fixed to the upper beam. The slidermoves down when the high-pressure liquid passes into the upperchamber of the cylinder, and moves up when the high pressureliquid passes intothe lowerchamber. The movementdirections andthe velocity of the slider are switched by changing the status of thesolenoid valves. This allows the FF, PF, PM, UD, FR, and SR opera-tions on the press.The drive system of the hydraulic press is composed of sevenconstant power variable displacement pumps driven by six motors,as shown in Fig. 7(b). Five of the motors have a rated power of75 kWand are connected toa pump with a maximum flowof 400 L/min. The sixth motor, with a rated power of 90 kW, is connected totwin pumps with a maximum flow of 402 L/min. Either of the twinpumps can provide flow for the hydraulic press individually.Whether pumps are available to access the hydraulic system or notdepends on the demanded power of different operations, and thepump that is redundant to an operation will be set to a state ofunloaded by switching the corresponding pressure relief valve.A deep drawing forming process takes 13 s to complete on thepress being studied. Only five operations are included in the pro-cess, FF (0se2s), PF (2se7s), UD (7se9s), FR (9se13s) and SR(13se14s) operation. The WT time (14se25s) is used to load andunload thework piece. The composition of the drive system and theusage of motor-pumps in different operations are shown in Table 2.In this table, “1” indicates, for the corresponding pump, flowandpressure for an operation is available to the hydraulic circuit,whereas “0” indicates that the pump is unloading.The active electrical energy comsumption of the motors ismeasured to scale hydraulic press energy consumption in theforming process. A power meter is selected to measure the powerby measuring voltage and current through the motors at any time.The real-time measured data is transmitted to a PC based on theMODBUS-RTU communication protocol. The schematic of the testsystem is shown in Fig. 8.3.1. Energy consumption of a single hydraulic pressThe active electrical energy consumption of each operation isobtained as follows:EbZtbTbtbPCtdt XtbTbttbPCtDt(15)where Ebis the electrical energy consumption of operationb, tbisthe start time of operationb,Dt is the sampling interval, and PC(t) isthe active power of each operation at the sampling point. Accordingto Equation (6), the energy conversion efficiency from the electricalenergy into forming energy is expressed as follows:hEPFPEb(16)The electrical energy consumption of motor-pumps is obtainedby the test according to Equation (15). The energy consumed by allmotors and the useful energy provided by available motors of eachoperation are obtained after several tests, as shown in Table 3.3.2. Energy consumption of the hydraulic groupThe time length of FF operation and FR operation is prolongedaccording to Constraint (8) and the corresponding drive zone ofeach operation is redesigned according to Equations (13) and (14)to minimize the installed power of a drive zone. The number ofmotor-pumps is reduced, and these drive zones have no idle time.As the demands for motor-pumps in the UD operation are thesame as that in the SR operation, these two drive zones are com-bined. The time matching coefficient is calculated asFig. 7. Hydraulic press and the drive system.Table 2Composition of the drive system and the usage of motor-pumps in differentoperations.Motor-pumpsState of different operationsNumberRated flowRated powerFFPFUDFRSRPump 1400 L/minP1 75 kW11010Pump 2400 L/minP2 75 kW11010Pump 3400 L/minP3 75 kW11010Pump 4400 L/minP4 75 kW11010Pump 5400 L/minP5 75 kW11010Pump 6402 L/minP6 90 kW01111Pump 701111L. Li et al. / Journal of Cleaner Production 139 (2016) 636e645642Fig. 8. Schematic of the test system.Table 3The time length and energy consumption of each operation.Operations of hydraulic press (b)FFPFUDFRSRWTTime (s)0 to 22 to 77 to 99 to 1313 to 1414 to 25Input Energy Eb(kJ)330.04358.45975.24409.54946.20206.671638.15Total Input Energy Ein(kJ)4864.29Useful energy (kJ)236.761333.6981.38946.2077.750hEPF/Ein? 100% 27.42%Einis the energy consumption of the hydraulic system in a forming process, and Ein SEb.Fig. 9. The working state of the drive zones and hydraulic presses in a working period. (Different rectangles of the same color represent different operations of a hydraulic press. Aseries of rectangles in a line indicates the working state of a drive zone as indicated by the legend. Every colored rectangle represents the time period when the drive zone isproviding energy to the hydraulic press of the same color. The blank rectangle in the working process represents the drive zone in the state of unloading. All same colored rectanglesrepresent a forming process in a hydraulic press.)Table 4Energy consumption change of each operation of a hydraulic press.Operations (b)FFPFUDFRSRWTEnergy ChangedDEb(kJ)?93.280?265.970?97.83?1638.15Input Energy Eb(kJ)236.761,333.69143.57946.20108.840Total Input Energy Ein(kJ)2769.06h0EPF/Ein? 100% 48.16%L. Li et al. / Journal of Cleaner Production 139 (2016) 636e645643cbXga1Ta;b=T# 1 ?TbTPF? 1b FF;UD;PM;FR;SR(17)And cFF cUDSR cFR 1, indicates that each drive zone hasonly one drive unit. The UD operation and SR operation share adrive zone.These two operationsdrive zones arecombined and thenumber of motor-pumps is unchanged because of the samedemanded power. The time length of the UD and SR operations,remains unchanged, leading to the minimal WT time for the drivezones. The working states of the drive zones and hydraulic pressesin a working period are shown in Fig. 9.Since all the hydraulic presses in this case study are identical,after measuring the power consumption of different operations(supported by different drive zones) on one single press it ispossible to predict the energy consumption of the group based onscheduling scheme without carrying out actual measurements.Assuming that useful energy supplied by available motor-pumpsremains unchanged, the composition of each drive zone is ob-tained from Table 2 and the energy consumption of hydraulicpresses is reduced by removing the motor-pumps in the unloadedstate. The energy consumption change in each hydraulic pressoperation is shown in Table 4.For forming process using the energy saving method, Ebis theelectrical energyconsumption of operationbin a hydraulic press,h0is the conversion efficiency of the electric energy into formingenergy, andDEbis the difference in electrical energy consumptionin operationb, which is calculated as follows:DEb Eb? Ebb FF;PF;UD;FR;SR(18)According to the data in Table 4, the average energy consump-tion of a hydraulic press is minimized in each operation, thus, theefficiency is increased. The details of energy reduction in eachoperation and the corresponding increase in forming efficiency in aforming process are shown in Fig. 10.Results from the case study suggest that the energy-savingmethod developed is quite promising. The energy saved reaches2095.23 kJ (from 4864.29 kJ to 2769.06 kJ) in a forming process andthe forming efficiency increased by 20.74% (from 27.42% to 48.16%),all of which are achieved by eliminating the WT time of the drivesystem and the redundant motor-pumps of each operation. Theenergy-saving effect is based on the hypothesis that the input en-ergy of each operation remains unchanged to match the drive zone,although in reality energy loss exists in each operation. The effi-ciency could be further improved if the conversion efficiency of themotor-pump is considered when designing each drive zone ac-cording to the load profiles of each operation.4. Summary and conclusionsIn the forming process using hydraulic press with conventionaldrive configuration, 40% of the input energy is consumed by idlemotors and only 27% is used for forming. The WT increases energyconsumption and the mismatch between installed and demandedpower leads to lower efficiency.Based on the analysis of operation load profiles, this paperproposed an energy-saving method for a group of hydraulicpresses. In this approach, the drive system is partitioned intoseveral drive zones, therebyeliminating the mismatch between theinstalled and demanded power of hydraulic presses. Drive zonesshared by grouped hydraulic presses are designed and a newschedule is developed to shorten the idle time of the drive systemwithout changing the forming parameters. The scheduling schememinimizes the idle time of the drive zones and the drive zones canhave less installed power. Total installed power was minimized tomore closely match the demanded power. The energyconsumptionof a single hydraulic press was measured and significant energy-saving effect was observed for a group of five presses.The energy-saving method was based on the load profiles ofgrouped hydraulic presses and worked well for multi-actuatorsystems with similar installed power. Considering the differenceof installed power for hydraulic presses in an automatic formingline, further studies should focus on the flexibility of the system.Other research directions include, osc