The 13th Scandinavian Internatinal Cnference n Fluid Pwer, SICFP2013, June 35, 2013, Linköping, Sweden Universal Energy Strage and Recvery System A Nvel Apprach fr Hydraulic Hybrid M Erkkilä, F Bauer*, D Feld* Hydac Oy, Tampere, Finland *Hydac Internatinal GmbH, Sulzbach/Saar, Germany Email: mikk.erkkila@hydac.fi, frank.bauer@hydac.cm, daniel.feld@hydac.cm Abstract The paper presents the basic principles f hybrid systems. It gives a rugh apprximatin f factrs that can affect the efficiency f a hybrid system. A Shrt cmparisn between hydraulic, mechanic and electric energy string system is presented. In a mbile wrking machine, there are mainly three pssible surces fr energy strage r recvery which are: energy f the cmbustin engine, when it is wrking n part lad, braking energy f a vehicles kinetic energy and its hydraulic system. A nvel hydraulic system is intrduced that is capable t stre and recver energy frm all energy surces f mbile wrking machine. The pwer peaks during the wrking cycles f the machine can be cvered with energy stred in the hydraulic accumulatr and s the size f the cmbustin engine can be reduced and dimensined accrding t the average pwer. It als reduces the energy cnversin lsses. Simple and cst effective design makes it an attractive alternative fr many applicatins. Keywrds: Hydraulic hybrid 1 Intrductin In mbile wrking machines the lading f the cmbustin engine is ften nt cnstant during the wrking cycle. Maximum pwer is in many cases needed nly fr shrt perids. Nrmally the cmbustin engine must be dimensined accrding t the maximum peak pwer, which leads t versized engines cmpared t the average pwer during the wrking cycle and t lw efficiency, because the engine is running mst f the time n part lad. In the past it has nt been a prblem t increase the engine size, but the tightening f engine exhaust regulatins like Eur/TIER and increased fuel cst will frce the equipment manufacturer t search fr new slutins. During mvements kinetic energy is charged in the vehicle and it is nrmally extinguished and wasted in the brakes, causing wear and thermal lading. Mre sphisticated transmissins, like Cntinuusly Variable Transmissins (CVT) [1], hydrstatic, r electrical transmissins are able t transmit the braking energy back t the cmbustin engine. Unfrtunately the braking capacity f the new lw emissin diesel engines is limited. In sme applicatins, like in city busses r package delivery r garbage vehicles, in which the amunt f braking energy is sufficient; energy recuperatin systems have been tested and are in use. By lifting f a lad, ptential energy is accumulated in the wn mass f the lifting equipment. In a traditinal hydraulic system, when the equipment is lwered, this energy is lst in the return ntch f the cntrl valve. Balancing systems can be used t cmpensate the dead weight. There are als prpsals hw the returning energy culd be recvered. In mst f the studies nly ne f these three pssible energy string surces has been cnsidered and the energy recuperatin systems have been individually develped. 2 Hybrid systems In the past years a lt f effrt has been fcused t the develpment f hybrid drives. Car industry has already taken hybrid slutins in mass prductin, in rder t reduce fuel cnsumptin. In the branch f mbile wrking machines many studies and investigatins have been carried ut, regarding the pssibility t use hybrid technlgy. By side f energy saving ne interesting chance fr mbile wrking machines is the pssibility t reduce the engine size. Traditinally the engine has t be dimensined accrding t the peak pwer f the machine. In many mbile machines there is a big difference between required average and peak pwer. Figure 1 shw the s called Ywrking cycle fr a 45
wheel lader, fr filling a lrry C frm a pile B. And figure 2 the crrespndent pwer demand frm the engine during this cycle. Figure 1: Y wrking cycle f a wheel lader Figure 5: 5: Serial hybrid system Discharge accumulatr Pwer In serial hybrid the engine is decupled fr frm m the pwer cnsumers like pwertrain pwertrain r wrking hydraulics, which utilizes the pssibility t ptimize the engine cntrl and efficiency nly accrding t ptimal energy prductin. Charge accumulatr The disadvantage f a serial hybrid is that the cmplete pwer in the main pwer train has t be transfrmed first t electrical r hydraulic pwer and then back t mecha mechannical pwer. The main pwer line pwer lss is then: Zeit time Recuperable energy 1 Figure 2: Pwer demand during the Y wrking cycle [2] Frm figure 2 we can see that the amunt f recverable energy is mderate. The main benefit fr hybrid system in this kind f machines is t dimensining the engine accrding t the average pwer and cvers the peak pwer demand with the stred energy. ɳ 1 ɳ (1),where: Pl is pwer lss Pe is actual engine pwer Hybrid drives can rughly be divided int int tw categries: parallel and serial hybrids ɳ,and efficiencies. 2.1 Serial hybrid 2.2 Parallel hybrid In the serial hybrid hybrid the ttal engine pwer is transfrmed t electrical r hydraulic pwer, frm which the energy i s stred and reused. Figure 5 shws a typical hydraulic serial hybrid design. In parallel hybrid system the energy string device is arranged in parallel t the main pwer line line,, and it has n influence t the basic functin f it. Figure 6 shws the typical hydraulic parallel hybrid design. 46 ɳ are the pwer transfrm
The energy string capacity f hydraulic accumulatrs is limited,, but they are able t restre and discharge high pwer. Figure 6: : Energy and pwer densities ies f energy strage device [3] Figure 4: : Parallel hybrid system In parallel hybrid system the speed f the engine has t be cntrlled due t the requirements f the main pwer line. The parallel hybrid can nly increase r decrease the engine trque. If the accumulatr is nt charged r discharged, the m main pwer line efficiency is nly affected by the idle pwer lss f the energy strage device. The main pwer line pwer lss is then, where P l is pwer lss. is pwer lss f the energy string device, which nrmally depends n the pressures s in high and lw pressure accumulatrs and running speed f the engine. When the accumulatr is charged and discharged,, the pwer lss can be defined as: where: Q is the flw that is charged r discharger frm r t the accumulatr, and p ha p la, 1ɳ, (3) is the pressure in high pressure accumulatr and is the pressure in lw pressure accumulatr. (2) 3 Hydraulic accumulatr as energy strage 3.1 Characteristics f a hydraulic accumulatr cmpared with ther energy strage devices Figure 3 shws the energy and pwer densities fr different kind f energy string devices. Table 1 shws a cmparisn f the characters f electrical, mechanical and electrical pwer string devices Technlgy Electrical Mechanical Hydraulic Energy strage device Duble layer capacitr Lithium in battery Flywheel Energy density Pwer density Ageing / Capacity lss Temperature sensitivity Self discharge Packaging Cst effectiveness Safety / Service Table 1: Cmparisn f energy string devices In the past the main fcus f hybrid drives has been cncentrated t the develpment f electric hybrids due t the better efficiency f the electric cmpnents. nts. A study made by Liebherr [4] ] shws that due t the high number f required energy transfrmatins the ttal efficiency f electric hybrid systems can drp n the level f the hydraulic systems. See figure 7 7. System/Generatr Energy transfer: Vltage cnversin Energy transfer: mechanical t n the level f electrical t chemical eletrical energy strin device 98 % 96 % Hydraulic accumulatr 94 % 88 % 90 % 92 % Energy transfer: Vltage Energy transfer: electrical t cnversin n the chemical t electrical mechanical system level Figure 7: : Pwer lsses ses in electrical energy strage [4]] Energy strage 47
On the ther hand electric energy transfer can be dangerus in mbile machines which are wrking in rugh envirnment. Fr example 60 kw means 600 V and 100 A r 100 V and 600 A. Cable breaks r damaged cable islatins can cause far bigger prblems than hydraulic hse breaks. Hydraulic accumulatrs have been used several decades in mbile wrking machines. Security standards have been develped n a high level and security devices are well knwn and available. In mbile wrking machines the required pwer is typically changing extremely during the wrking cycles and thse are typically shrt. Due t the gd pwer density hydraulic accumulatrs are suitable t cmpensate rapid pwer peaks. In practice the accumulatr has t perate n wide temperature range, which limits the theretical pwer charging capacity. Hydac s ASP Accumulatr Simulatin Prgram is an excellent tl fr dimensining accumulatrs fr hybrid slutins. It is based t real gas characteristics an therm dynamical prcesses during lading and unlading. In praxis a 50 L accumulatr with perating pressure range maximum 330 bar and minimum 100 bar is able t stre abut 360 kj energy. Figure 9 shws that with 360 kj energy can be laded, when a vehicle with weight f 10 tn is braked frm a velcity f 36 km/h t 19 km/h. With this stred energy it is pssible t lift a lad f 3,6 tn in a height f 10 m. 3.2 The energy string capacity f a hydraulic accumulatr The Energy capacity f a hydraulic accumulatr fr an ideal gas can be defined as [5]:! " # $" & ' % $ () &/' *1. ' 0 (4), Where; E is the energy capacity f the accumulatr, V 0 is the vlume f the accumulatr, p min is the minimum pressure p max is the maximum pressure p 0 is the pre charge pressure f the accumulatr Figure 9: Practical energy capacity f a 50L accumulatr 4 Cnventinal hydraulic energy string systems In the fllwing sme knwn hydraulic hybrid systems are shwn. 4.1 Adding an accumulatr t a hydraulic system The easiest pssibility t stre energy is t add an accumulatr t the hydraulic system as shwn in figure 10. K is the adiabatic cnstant fr nitrgen. The figure 8 shws the energy capacity fr a 10 Liter accumulatr, with a maximum pressure f 350 bar as a functin f P min, when the pre charge pressure is 0.9 * P min. Figure 8: Theretical energy string capacity f a 10 L accumulatr 48
Wrking hydraulics 2 1 Figure 10: Simple energy string system This kind f system is used in a frestry machine applicatin by the cmpany HSM. Where energy is stred in the accumulatr and used t accelerate the feeding rlls in a harvester head [6] see figure 11. Accrding t HSM it was pssible t reduce 20% fuel cnsumptin with this system. Figure 12: Usable energy capacity in a simple system, marked as blue 4.2 Using an extra pump Figure 13 shws a hydraulic system, where an extra pump is used t charge and discharge the accumulatr. The pump is mechanically driven frm the engine and the transmissin. It is then able t stre braking energy and als energy frm the engine, when the engine is nt fully laded. Figure 11: Serial hybrid system [6] The main disadvantage fr the general use f this kind f system is that the energy frm the accumulatr can nly be used, when the pressure in the accumulatr is higher than the lad pressure, see figure 12. The prelading and the minimum perating pressure f the accumulatr can f curse be chsen s that the accumulatr pressure is always higher than the maximum lad pressure, but als then the accumulatr energy capacity is limited. Als the pressure difference between accumulatr and system pressure is lst. Figure 13: Additinal pump/mtr unit with accumulatr as energy string unit This kind f energy string system has been prpsed by the Purdue university research team fr the MultiActuatr Hydraulic Hybrid Machine Systems [6] see figure 14. 49
Valve blck p u LS A EL Engine Trans missin B p u Figure 15: : A new apprach fr hydraulic hybrid Figure 13: Additinal pump/mtr unit with accumulatr as energy string unit in MultiActuatr Hydraulic Hybrid Machine System [7] The main disadvantage f this system is that the energy frm accumulatr has t be first cnverted t mechanically energy, and then with the wrking hydraulic pump back t hydraulic energy. Due t the required energy cnversins the energy lsses are high and the verall efficiency is pr. 5 A Nvel Apprach fr Hydraulic Hybrid 5.1 System design In the new prpsed system (see figure 15) 1 a pump/mtr unit that can be pressurized n bth sides A and B is added t feed a hydraulic accumulatr frm the wrking hydraulic system. This kind f unit is nrmally used as pump in a clsed lp pwer transmissin system The pump/mtr unit can be driven as shwn n the picture, direct r via thrugh drive frm the main engine r frm gearbx that is cnnected t drive wheels. 5.2 Functin principle The hydraulic system can be designed as a nrmal Lad Sensing system, where nly a pump/mtr unit and an accumulatr are added. The functin f the pump/mtr unit is t cntrl the flw fr charging the accumulatr and cntrlling the flw frm the accumulatr t wrking hydraulics. It als wrks as pressure transfrmer between the accumulatr and system pressure. In the system there are als 2/2valves n the bth sides s f the pump/mtr r unit. With thse valves it is pssible discnnect the energy string system frm the wrking hydraulic system t avid pwer lses, when the energy string system is nt activated during the peratin f the machine. S the high pressure in accumulatr r in wrking hydraulics is nt increasing the hydrmechanical r vlumetric lses f the pump/mtr unit. The valves are wrking g als as safety devices, when the A side valve is pened nly accumulatr lading is pssible. When B side valve is pened the accumulatr can be discharged. 5.3 Energy string The pen lp Lad Sensing pump keeps the A side f the pump/mtr unit pressurized. When there is n flw cnsumptin fr the wrking hydraulics the Lad Sensing pump is n Stand By pressure, nrmally n the level f abut 20 bar. This is enugh t assure the needed suctin pressure and flw fr the pump/mtr unit. If there is a parallel need fr wrking hydraulics the Lad Sensing pump pressure is cntrlled accrding t the needed lad pressure. In this case the pump/mtr unit wrks as a pressure transfrmer between the Lad Sensing pump and accumulatr pressure. The pump/mtr unit can use the rest f the available flw capacity f the main pump t charge the accumulatr. 50
The amunt f the stred energy is cntrlled with the displacement cntrl f the pump/mtr unit. 5.3.1 Energy string frm the engine When during the wrking cycles f the machine, there are perids, when the engine pwer is nt fully used, the remaining energy can be stred t the accumulatr. Figure 16 shws the usability f the stred energy f a 50 liter accumulatr, with a ttal energy strage capacity f 350 kj. The amunt f stred pwer can be cntrlled with the displacement cntrl f the pump/mtr unit. 5.3.2 Braking energy Mdern driveline designs, like hydrstatic transmissins, r CVT are able t transmit the braking energy frm the drive wheels back t the diesel engine. The braking capacity f mdern diesel engines is limited, and the braking energy cannt be stred. With the prpsed system the braking energy frm the driveline can be stred t the accumulatr. The braking frce and pwer can be cntrlled by the displacement cntrl f the pump/mtr unit. One big advantage f the braking energy string system is that the wear and the heating f the vehicle brakes can be reduced. 5.3.3 Energy string frm the hydraulic system If the hydraulic system is capable t return pressurized flw frm the wrking hydraulic system, the returning il flw frm the main valve blck can be restred t the accumulatr. 5.4 Reuse f stred energy Stred energy can be used fr the wrking hydraulics. The pressure level f the hydraulic system is cntrller with the Lad Sensing pressure. With the displacement cntrl f the pump/mtr unit the amunt f il that is taken frm the accumulatr r frm the LSpump can be cntrlled. The flw frm the accumulatr is used via the pump/mtr unit t the main hydraulic system. The mtr/pump unit transfrms the pressure between system and accumulatr pressure. When the pressure in the system is higher, than that in the accumulatr the clsed lp unit wrks as pump and takes the needed extra pwer frm the engine. When the system pressure is lwer, than the ne in the accumulatr the clsed lp unit wrks as mtr and releases energy t the engine, driveline r auxiliary devices. The whle accumulatr capacity can then be utilized. Figure 16: Energy usability in the prpsed hydraulic hybrid system If the LSpump is a unit that can wrk as a mtr (mring unit) the stred energy can be used fr bsting the drive line r fr auxiliary units. 5.5 Pwer lsses and efficiency The pwer lsses f the prpsed system can be defined as fllws:,, (5) where: P l is pwer lss P idle(p,n) is cnstant pwer lss, depending f the pressure and rtatin speed. Q is flw t (r frm) the accumulatr, p s is system pressure and p a is accumulatr pressure. The main pwer lss:, depends nly n the flw, which is stred r taken ut f the accumulatr and the pressure difference between accumulatr and system pressure. This ffers a remarkable advantage cmpared with traditinal serial r parallel hybrid systems. 5.6 Benefits The size f the engine can be reduced, because it nly needs t deliver the average pwer, nt the max peak pwer during the wrking cycle. 51
The hydraulic system can be designed as a nrmal Lad Sensing system, where the main pump pressure is cntrlled by the LS pressure. The flw frm the accumulatr is used via the pump/mtr unit t the hydraulic system. The LS pump size can then be chsen smaller r the wrking speed f the machine can be increased. The mtr/pump unit cntrls the flw rate that is stred r taken ut frm the accumulatr. The main advantages f the system are listed as fllws: Simple system design Universally usable fr restring energy N pressure lsses, as in a valve cntrlled system Min. energy transfrmatin lses Flw frm the accumulatr can be directly used fr the wrking hydraulics: Smaller main pump can be used Reduced tank flw: smaller tank can be used The accumulatr energy string capacity can be used n a wide pressure variatin range and the full energy strage capacity f the accumulatr can be used. mbilen Arbeitsmaschinen" 07./08. Juni 2011 in Friedrichshafen [4] Behm, D., Hlländer, C., Landman, T., Hybrid Antriebe bei Raubenbaggern Knzepte und Lösungen, Hybridantriebe für Arbeitsmaschinen, Karlsruher Schriftenreihe Fahrzeugsystemtechnik, 7:117124, 2011. [5] Sahlman, M., Valve cntrlled multichamber cylinder, Master s theses Tampere University f Technlgy,2012 [6] Prinz zu Hhenlhe, F., Phlegmatisierung als Tugend in der Mbilhydraulik Das Energiespeichersystem des Kranvllernters HSM 405H2, Hybridantriebe für mbile Arbeitsmaschinen, Karlsruher Schriftenreiche Fahrzeugsystemtechnik, 7:151163, 2011 [7] http://www.ccefp.rg/sites/all/files/dcuments/inf _1A.2_MultiActuatr_Hybrid.pdf Dwnladed 8.4.2013 6 Outlk and cnclusins Hydraulic hybrid is a gd alternative t imprve ttal efficiency and t reduce the size f the engine in a mbile wrking machine. In the design f hybrid systems, it is imprtant t reduce energy transfrming lsses t the minimum. The prpsed new hydraulic hybrid system can fulfill all requirements regarding energy string frm different restrable surces. It is als capable t use the full accumulatr capacity. And can minimize energy transfrmatin lses. The nvel system design has nly been shwn n a principle level. Fr real system sme kind f pwer management system is needed t cntrl the charging and the discharging functins. References [1] Erkkilä M, MdelBased Design f PwerSplit Drivelines, Dissertatin, September 2009 Tampere Finland [2] Bauer, F., Feld, D. Warum hydraulische Hybridsysteme? Het Hydrauliek Sympsium, Delft, 87104, 2010 [3] Thiebes, P., Geimer, M., Energy strage devices fr industrial vehicles with hybrid drive trainsquelle: 1. VDI Fachknferenz: "Getriebe in 52