Energy Efficiency Comparison of Hydraulic Accumulators And

energies

Article Energy Eﬃciency Comparison of Hydraulic Accumulators and Ultracapacitors

Jorge Leon-Quiroga 1, Brittany Newell 1, Mahesh Krishnamurthy 2, Andres Gonzalez-Mancera 3 and Jose Garcia-Bravo 1,*

1 Purdue Polytechnic School of Engineering Technology, Purdue University, West Lafayette, IN 47907, USA; [email protected] (J.L.-Q.); [email protected] (B.N.) 2 Department of Electrical and Computer Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA; [email protected] 3 Department of Mechanical Engineering, Universidad de los Andes, Bogota 111711, Colombia; [email protected] * Correspondence: [email protected]

Received: 21 February 2020; Accepted: 20 March 2020; Published: 2 April 2020

Abstract: Energy regeneration systems are a key factor for improving energy efficiency in electrohydraulic machinery. This paper is focused on the study of electric energy storage systems (EESS) and hydraulic energy storage systems (HESS) for energy regeneration applications. Two test benches were designed and implemented to compare the performance of the systems under similar operating conditions. The electrical system was configured with a set of ultracapacitors, and the hydraulic system used a hydraulic accumulator. Both systems were designed to have the same energy storage capacity. Charge and discharge cycle experiments were performed for the two systems in order to compare their power density, energy density, cost, and efficiency. According to the experimentally obtained results, the power density in the hydraulic accumulator was 21.7% higher when compared with the ultracapacitors. Moreover, the cost/power ($/Watt) ratio in the hydraulic accumulator was 2.9 times smaller than a set of ultracapacitors of the same energy storage capacity. On the other hand, the energy density in the set of ultracapacitors was 9.4 times higher, and the cost/energy ($/kWh) ratio was 2.9 times smaller when compared with the hydraulic accumulator. Under the tested conditions, the estimated overall energy efficiency for the hydraulic accumulator was 87.7%, and the overall energy efficiency for the ultracapacitor was 78.7%.

Keywords: eﬃciency; energy storage systems; electrical power systems; hydraulic power systems; hydraulic accumulator; ultracapacitor

1. Introduction Energy consumption in the transportation and the industrial sector represented 72% of the total energy consumption during 2018 in the United States. Most of this energy (around 87%) comes from petroleum-based sources and natural gas [1]. In the industrial sector, around 24% of the energy is spent by the agriculture, construction, and mining sectors. Many applications within these sectors use hydraulic equipment and hydraulic machinery, so improving the energy eﬃciency of the hydraulic systems that are already in use could have a large impact on the reduction of energy consumption and emissions. If the energy eﬃciency of the hydraulic machinery used in industrial applications is improved by 5%, based on the data provided by the U.S. Energy Department [1], it is possible to estimate an overall annual reduction of 1% in energy consumption in the US. Over the last 20 years, there has been interest in developing and improving systems for energy regeneration in hydraulic machinery [2–5]. Some of the options for energy storage in energy regeneration

Energies 2020, 13, 1632; doi:10.3390/en13071632 www.mdpi.com/journal/energies Energies 2020, 13, 1632 2 of 23 devices include flywheels, compressed air, electrical energy storage systems (EESS), and hydraulic energy storage systems (HESS). In the electrical energy regeneration system, an electric accumulator or capacitor is used to store energy [6]. This kind of system is used in electric hybrid machinery. The return line of a boom mechanism in an excavator is connected to a hydraulic motor that is used to move an electric generator that produces electrical energy that is stored in an electric accumulator (ultracapacitor or battery) [2,3,7]. The main benefit of this system is an improvement in energy efficiency, but the complexity that is added to the baseline hydraulic system is evident. Ultracapacitors are mostly used in electric applications that require a high power density such as wind turbines in remote areas [8], energy regeneration and engine size reduction in rubber wheeled gantry cranes [9], and even biomedical applications [10]. The hydraulic energy regeneration system is similar to its electric counterpart. Instead of having an electric accumulator (ultracapacitor or battery), the hydraulic energy regeneration system uses a hydraulic accumulator that works as the energy storage device [6]. One of the main benefits when using a hydraulic regenerative system is the relative ease of installation, since the baseline application already uses a hydraulic system. Moreover, no complex power controls are needed, which is a significant advantage. On the other hand, the energy storage density in hydraulic accumulators can be a drawback when compared to a traditional electric system. Hydraulic regenerative systems have been studied for applications in relief valves [11], where the flow in the return line of the valve is used to charge a hydraulic accumulator. Alternatives like digital hydraulics have been studied to improve energy efficiency in hydraulic systems [12,13]. In these studies, a network of valves is used to change the flow rate in the actuator and to reconfigure the system in order to use the flow from assistive loads to move actuators with resistive loads.

2. Relevance of This Work Most of the previous studies regarding EESS and HESS have focused on the characteristics of each energy storage system separately. Some studies have focused on the study of the performance of ultracapacitors [14–17] and others have focused on hydraulic accumulators. [18,19], but very little research has been done to compare the two storage systems (hydraulic accumulators and ultracapacitors) side by side. The main purpose of this study was to have a direct comparison of the performance characteristics of ultracapacitors and hydraulic accumulators when used as energy storage devices. Previous studies concerning EESS and HESS for hybrid applications have not compared the beneﬁts and drawbacks of both systems under similar operating conditions; these studies have been focused on each system individually. For this work, an experimental procedure was developed for measuring the charging and discharging cycles of a hydraulic accumulator. Likewise, a test bench using ultracapacitors of similar energy storage capabilities was tested. The estimated energy capacity of each system was modeled with the equations shown in the following section. The experimental procedure and experimental equipment are also described in that section. The results of this research can be used to stablish a control strategy to optimize systems that use hydraulic accumulators as energy storage systems or as a design strategy to select one over the other.

3. Test Bench Description The main purpose of the test benches developed in this study was to compare an electrical and a hydraulic energy storage system under similar operating conditions in order to determine the eﬃciency, power density, energy density, and cost by energy capacity. The main characteristics of the hydraulic accumulator and the ultracapacitor used are shown below in Tables1 and2. Energies 2020, 13, 1632 3 of 23

Table 1. Characteristics of the hydraulic accumulator used in this study.

Energies 2020, 13, x FOR PEER REVIEW Hydraulic Accumulator 3 of 23 Manufacturer Parker Hannifin Table 1. CharacteristicsReference of the hydraulic accumulator A2N0058D1K used in this study. Mass (kg) 4.53 Vol. CapacityHydraulic (cm3) Accumulator950 Max.Manufacturer Pressure (Bar) Parker 207 Hannifin Reference A2N0058D1K Table 2. CharacteristicsMass (kg) of the ultracapacitor4.53 used in this study. Vol. Capacity() 950 Ultracapacitor Max.Manufacturer Pressure (Bar) Maxwell207 Technologies Reference BCAP0050 P270 S01 Table 2. Characteristics of the ultracapacitor used in this study. Mass (g) 12.2 Energy Capacity (mWh)Ultracapacitor 50.6 Max. Voltage (V) 2.7 Manufacturer Maxwell Technologies Max. Current (A) 6.1 Reference BCAP0050 P270 S01 Mass (g) 12.2 To compare both devicesEnergy in Capacity a similar (mWh) way, two test benches50.6 were designed and built. Both test benches were designed to measureMax. Voltage charge and (V) discharge response2.7 of the systems, which in a hydraulic system are the pressure and flowMax. rate Current and in(A) the electric system6.1 are the voltage and current. 3.1. Hydraulic Test Bench 3.1. Hydraulic Test Bench The test bench for the hydraulic system was designed to measure the flow of energy through the The test bench for the hydraulic system was designed to measure the flow of energy through the accumulator while charging and discharging. The main components of the system were the battery, accumulator while charging and discharging. The main components of the system were the battery, the electric motor, the hydraulic pump, and the hydraulic accumulator. The battery was the main the electric motor, the hydraulic pump, and the hydraulic accumulator. The battery was the main source of power for the system, and it was connected to the electric motor with a DC/AC inverter. source of power for the system, and it was connected to the electric motor with a DC/AC inverter. The electric motor was used to drive the hydraulic pump, which moved the fluid from the reservoir to The electric motor was used to drive the hydraulic pump, which moved the fluid from the reservoir the hydraulic accumulator. The hydraulic testbench and the DC/AC inverter are shown in Figures1 to the hydraulic accumulator. The hydraulic testbench and the DC/AC inverter are shown in Figures and2, respectively. The schematic representation of the test bench that was designed and constructed 1 and 2, respectively. The schematic representation of the test bench that was designed and for the hydraulic accumulator is presented in Figure3. constructed for the hydraulic accumulator is presented in Figure 3.

Figure 1. Hydraulic test bench.

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Figure 2. DC/AC inverter used in the hydraulic test bench. Figure 2. DC/AC inverter used in the hydraulic test bench. Figure 2. DC/AC inverter used in the hydraulic test bench.

. Figure 3. Schematic representation for the hydraulic accumulator test bench. Figure 3. Schematic representation for the hydraulic accumulator test. bench. A list of theFigure components 3. Schematic used representation in the test bench for the is hydraulic presented accumulator in Table3. test bench. A list of the components used in the test bench is presented in Table 3. A list of the componentsTable 3. usedList of in components the test bench used is in presented the hydraulic in Table test bench. 3. Table 3. List of components used in the hydraulic test bench. Index TableDevice 3. List of components used Reference in the hydraulic test bench. Characteristics Index Device Reference Characteristics 1 Electric motor Motenergy 0907 Max. Speed: 5000 rpm Index1 ElectricDevice motor MotenergyReference 0907 PeakMax. torque:Characteristics Speed: 38 5000 Nm rpm 1 2Electric Hydraulic motor pumpMotenergy GP-F20-12-P-A 0907 Disp:Max.Peak 12 Speed:torque: cm3/rev 5000 38 Nm rpm 2 Hydraulic pump GP‐F20‐12‐P‐A Max.PeakDisp: Flow: torque: 12 40 cm L 38/⁄min Nm 2 Hydraulic pump GP‐F20‐12‐P‐A Max.Disp:Max. Pressure: Flow:12 cm 40 252⁄ L/min Bar 3 Flow meter FlowTech FSC 375 Max. Pressure: 6 kpsi Max. Pressure: 252 Bar Max.Max. Flow: Flow: 26.45 40 L/min L/min 3 4 FlowV meter1 and V 2 FlowTechHydraforce FSC 12375 V DC NC Max.Max.Max. Flow: Pressure:Pressure: 56.7 L / 252min6 kpsi Bar 3 Flow meter FlowTechsolenoid FSC valve 375 Max.Max.Max. Pressure: Pressure:Flow: 26.45 3 kpsi 6 kpsi L/min 5 Pressure gage Wika A-10 Max. Pressure: 5 kpsi 4 V1 and V2 Hydraforce 12 V DC NC Max.Max. Flow:Flow: 26.4556.7 L/min L/min Signal output: 4 to 20 mA 4 V1 and V2 Hydraforcesolenoid valve 12 V DC NC Max.Max. Flow:Pressure: 56.7 3 L/min kpsi 6 Hydraulic accumulator Parker A2N0058D1K Capacity: 58 in3 5 Pressure gage solenoidWika A‐ 10valve Max.Max.Max. Pressure: Pressure:Pressure: 3 kpsi 35 kpsikpsi 5 Pressure gage Wika A‐10 PrechargeMax.Signal Pressure: output: pressure: 45 to 1kpsi kpsi 20 mA 6 Hydraulic Parker A2N0058D1K SignalCapacity: output: 58 in 4 to 20 mA The6 testHydraulicaccumulator bench could be operatedParker in either A2N0058D1K the charging or dischargingCapacity:Max. Pressure: modes. 58 in During3 kpsi charging Precharge pressure: 1 kpsi mode, the electricaccumulator AC motor was turned on and was used to move theMax. hydraulic Pressure: pump. 3 kpsi Valve V1 was Precharge pressure: 1 kpsi

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The test bench could be operated in either the charging or discharging modes. During charging mode, the electric AC motor was turned on and was used to move the hydraulic pump. Valve V1 was switched on, allowing flow to go through the accumulator while valve V2 was closed. The relief valve RV1 was closed unless a relief pressure of 3000 psi was reached. A schematic representation of the test bench during charging mode is presented in Figure 4. The red lines show the high‐pressure lines.

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Energies 2020, 13, 1632 5 of 23 The test bench could be operated in either the charging or discharging modes. During charging mode, the electric AC motor was turned on and was used to move the hydraulic pump. Valve V1 was

switchedswitched on, on, allowing allowing ﬂow flow to to go go through through the the accumulator accumulator while while valve valveV V2 2was was closed. closed. The The relief relief valve valve RV1RV1 was was closed closed unless unless a reliefa relief pressure pressure of 3000of 3000 psi psi was was reached. reached. A schematic A schematic representation representation of the of test the benchtest bench during during charging charging mode mode is presented is presented in Figure in Figure4. The 4. red The lines red lines show show the high-pressure the high‐pressure lines. lines.

Figure 4. Hydraulic test bench during charging mode.

During charging mode, the measured variables were the current. and voltage from the battery connected to the electric AC motorFigure powering 4. Hydraulic testthe bench pump during and charging the mode.pressure and flow rate going to the accumulator. The battery and theFigure AC 4. motor Hydraulic were test bench connected during charging through mode. a DC/AC inverter. With these During charging mode, the measured variables were the current and voltage from the battery variables, itconnected wasDuring possible to thecharging electric to mode,calculate AC motor the measured poweringthe power variables the pump input were and fromthe the current pressure the andbattery, and voltage ﬂow the rate from goingpower the battery to the going to the accumulator,accumulator.connected the consumption to Thethe electric battery of AC and electric motor the AC powering energy, motor were the and connectedpump the and energy throughthe pressure saved a DC and/ ACin flowthe inverter. accumulator.rate going With theseto the The speed of the electricvariables,accumulator. motor it was was The changed possible battery and to with calculate the ACa motor motor the power werecontroller, inputconnected from which through the battery, was a DC/AC connected the power inverter. going between With to these the the battery accumulator,variables, it thewas consumption possible to ofcalculate electric the energy, power and input the energy from savedthe battery, in the accumulator.the power going The speedto the and the electricofaccumulator, the motor. electric motor theThe consumption wasschematic changed of electric withrepresentation a motor energy, controller, and theof theenergy which electric saved was connected in system the accumulator. between used theto The power battery speed the pump is shown in andFigureof the the electric electric 5. The motor motor. technical was The changed schematic data with of representation athe motor battery, controller, of the the electric which inverter, was system connected and used the to between power electric the the pump motor battery is are shown in Table 4. shownand the in electric Figure5 motor.. The technical The schematic data of representation the battery, the of inverter, the electric and system the electric used motor to power are shown the pump in Tableis shown4. in Figure 5. The technical data of the battery, the inverter, and the electric motor are shown in Table 4.

Figure 5. Schematic representation of the electric system. . FigureTable 5. Schematic 4. Technical representation data of the of electric the electric system. system. Device Figure 5.Reference Schematic representation of theCharacteristics electric system.

Table 4. Technical data of the electric system.

Device Reference Characteristics

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Energies 2020, 13, x FOR PEER REVIEWTable 4. Technical data of the electric system. 6 of 23

Device Reference Characteristics Electric motor Motenergy 0907 Max. Speed: 5000 rpm Electric motor Motenergy 0907 Max. Speed:Peak 5000 torque: rpm 38 Nm Peak torque: 38 Nm ContinuousContinuous current of 80current Amps of AC 80 Amps AC InductanceInductance phase to phase: phase 0.1 to millihenry phase: 0.1 millihenry Inverter InverterKEB48600 KEB48600 Max. Power:Max. 6 kWPower: 6 kW Max. voltage:Max. 48 voltage: V 48 V Max. Current: 125 A Max. Current: 125 A Battery SUN-CYCLE Max. Voltage: 48 V Battery SUN‐CYCLELiFePO4 LiFePO4 48 V 48 V 24Max. Ah discharge Max. Voltage: current: 6048 A V 24 Ah Weight: 9.8Max. kg discharge current: 60 A Weight: 9.8 kg

To measure the efficiency eﬃciency of the hydraulic accumulator, a similar test for the discharge cycle was was developed. In In the the discharge discharge test, the output load was simulated with aa variablevariable oriﬁce,orifice, VV33. The load load applied to the hydraulic system increased when VV3 was progressively closed. A no no-load‐load condition was simulated when the valve was completely open. A A schematic schematic representation representation of the the system system during during discharge mode is presented in Figure6 6..

Figure 6. Hydraulic test bench during discharging mode. Figure 6. Hydraulic test bench during discharging mode. To discharge the accumulator, valves V1 and V2 were open, while the orifice was open just between 0% andTo 10%.discharge The measured the accumulator, variables valves during V the1 and discharge V2 were mode open, were while the the pressure orifice and was the open flow ratejust betweenjust before 0% the and orifice; 10%. The with measured these variables, variables it wasduring possible the discharge to calculate mode the were instantaneous the pressure hydraulic and the flowpower rate used just from before the the accumulator. orifice; with The these output variables, power could it was be possible calculated to bycalculate multiplying the instantaneous the flow rate hydraulicand the pressure, power used and the from effi theciency accumulator. of the hydraulic The output accumulator power could bebe derived.calculated Equations by multiplying (1) and the(2) wereflow usedrate and to calculate the pressure, the power and the input efficiency and power of the output hydraulic in the accumulator accumulator, could and Equationbe derived. (3) Equationswas used to (1) calculate and (2) were the e ffiusedciency, to calculate the nomenclature the power shown input and in Table power5 describes output in the the variables accumulator, used. andThe resultsEquation for (3) a single was used test areto calculate shown in the Figure efficiency,7 as a reference. the nomenclature shown in Table 5 describes the variables used. The results for a single test are shown in Figure 7 as a reference. PIn = QInpAcc,In (1) , (1) P = Q p (2) Out Out,Acc,Out (2) R QOutp,Acc,Outdt (3) ηAcc = R , (3) QInpAcc,Indt

Table 5. Variables used for determining the instantaneous efficiency of the accumulator.

Variable Description

(W) Power charging the accumulator (W) Power discharging the accumulator (gal/min) Volumetric flow charging the accumulator

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Table 5. Variables used for determining the instantaneous efficiency of the accumulator. Energies 2020, 13, x FOR PEER REVIEW 7 of 23 Variable Description Energies 2020, 13, x FOR PEER REVIEW 7 of 23 PIn (W) Power charging the accumulator (gal/min) Volumetric flow discharging the accumulator POut (W) Power discharging the accumulator , (psi) Pressure charging the accumulator Q In(gal/min)(gal/min) Volumetric Volumetric flowflow charging discharging the accumulator the accumulator (psi) Pressure discharging the accumulator ,Q,Out (gal (psi)/min) VolumetricPressure flow dischargingcharging the the accumulator accumulator p Efficiency of the accumulator ,Acc,In (psi)(psi) Pressure Pressure charging discharging the accumulator the accumulator pAcc,Out (psi) Pressure discharging the accumulator Efficiency of the accumulator ηAcc Efficiency of the accumulator

Figure 7. Pressure in the accumulator during the charging and discharging process. Figure 7. Pressure in the accumulator during the charging and discharging process. Figure 7. Pressure in the accumulator during the charging and discharging process. The results for pressure and flow rate in the accumulator during charging are shown in Figure The results for pressure and flow rate in the accumulator during charging are shown in Figure8. 8. This plot shows a sudden rise in pressure when V1 was opened and a steady increase in pressure This plotThe shows results a sudden for pressure rise in and pressure flow rate when inV thewas accumulator opened and during a steady charging increase are in shown pressure in onceFigure once the pre‐charge pressure was reached. It 1took approximately 17 seconds to fully charge the the8. pre-charge This plot shows pressure a sudden was reached. rise in It pressure took approximately when V1 was 17 opened seconds and to fully a steady charge increase the accumulator in pressure accumulator to its maximum pressure of 2800 psi. The flow rate plot showed a sudden increase in toonce its maximum the pre‐charge pressure pressure of 2800 psi.was Thereached. flow rateIt took plot approximately showed a sudden 17 increaseseconds into flowfully rate charge and athe flow rate and a decrease in flow rate once the pre‐charge pressure was reached. decreaseaccumulator in flow to rate its oncemaximum the pre-charge pressure pressureof 2800 psi. was The reached. flow rate plot showed a sudden increase in The results for pressure and flow rate during discharging are shown in Figure 9. The flowThe rate results and fora decrease pressure inp flowand flowrate rateonceQ theduring pre‐charge discharging pressure are shownwas reached. in Figure 9. The discharge discharge time for the accumulator2 was approximately2 3 seconds with the valve 25% open. time forThe the results accumulator for pressure was approximately and flow 3rate seconds during with the discharging valve 25% are open. shown in Figure 9. The discharge time for the accumulator was approximately 3 seconds with the valve 25% open.

FigureFigure 8. 8.Pressure Pressure and and ﬂow flow rate rate while while charging. charging. Figure 8. Pressure and flow rate while charging.

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FigureFigure 9. 9.Pressure Pressure and and ﬂow flow rate rate while while discharging. discharging.

SeveralSeveral experiments experiments like like the onethe describedone described in this in section this section were carried were out carried to calculate out to the calculate efficiency the andefficiency performance and performance of the hydraulic of the accumulator. hydraulic accumulator. Load conditions Load were conditions changed were in all changed the experiments in all the byexperiments changing the by orifice changing area. the A orifice list of thearea. experiments A list of the is experiments shown in Table is shown6. The in 100% Table value 6. The for the100% 2 orificevalue area for the was orifice 11.7 mm area. was 11.7 mm. Table 6. List of experiments performed in the hydraulic test bench. Table 6. List of experiments performed in the hydraulic test bench. Orifice Area While Orifice Area While Orifice Area While Orifice Area While Orifice ChargingArea While OrificeDischarging Area While OrificeCharging Area While OrificeDischarging Area While Charging Discharging3.1% Charging Discharging3.1% 3.1%6.2% 6.2% 3.1% 9.4% 9.4% 3.1% 12.5% 12.5%6.2% 12.5%6.2% 25.0%9.4% 25.0%9.4% 3.1% 100.0%12.5% 100.0% 12.5% 12.5% 3.1% 3.1% 25.0%6.2% 6.2%25.0% 9.4% 9.4% 6.2% 100.0% 25.0% 100.0% 12.5% 12.5% 25.0%3.1% 25.0%3.1% 100.0%6.2% 100.0%6.2% 9.4%3.1% 3.1% 9.4% 6.2% 6.2%25.0% 6.2% 12.5%9.4% 9.4%12.5% 9.4% 100.0% 25.0%12.5% 12.5%25.0% 25.0% 25.0% 100.0% 100.0%100.0% 3.1% 3.1% 3.2. Electric Test Bench 6.2% 6.2% To compare the hydraulic accumulator9.4% with the ultracapacitor set, an electric test9.4% bench was 9.4% 100.0% designed and constructed. A schematic12.5% representation of the test bench is presented in12.5% Figure 10. 25.0% 25.0% 100.0% 100.0%

3.2. Electric Test Bench To compare the hydraulic accumulator with the ultracapacitor set, an electric test bench was designed and constructed. A schematic representation of the test bench is presented in Figure 10.

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Figure 10. Schematic representation of the test bench used for ultracapacitors.

Figure 10. Schematic representation of the test bench used for ultracapacitors. The ultracapacitorsFigure Figure10. Schematic 10.and Schematic the rheostatsrepresentation representation are shown of thethe testin test Figures bench bench used 11 used forand ultracapacitors. for 12, ultracapacitors. respectively.

The ultracapacitorsThe ultracapacitors and and the the rheostats rheostats are are shown shown in Figures Figures 11 11 and and 12, 12 respectively., respectively. The ultracapacitors and the rheostats are shown in Figures 11 and 12, respectively.

Figure 11. Ultracapacitors used in the electric test bench. FigureFigure 11. 11.Ultracapacitors Ultracapacitors used in in the the electric electric test test bench. bench.

Figure 11. Ultracapacitors used in the electric test bench.

Figure 12. Resistor bank used in the electric test bench. Figure 12. Resistor bank used in the electric test bench. This test bench was designed to measure the flow of energy through the ultracapacitors during Thischarging test andbench discharging. was designed The resistance to measure or loadthe flow ﬂowwas madeof energy using through a bank of the resistors ultracapacitors connected during in charging and discharging. The resistance or load was made using a bank of resistors connected in chargingparallel, and whichdischarging. made theThe value resistance of the or resistance load was variable made usingand easy a bank to adjustof resistors manually. connected The in parallel,ultracapacitor whichwhich made made arrangement the the value value ofhad the sixof resistance cellsthe connectedresistance variable in variable and series easy in toanda balancing adjust easy manually. toboard adjust and The sixmanually. ultracapacitor of these The ultracapacitorarrangementboards connected had arrangement six cellsinFigure parallel, connected had 12. so six Resistor the cells in total series connected banknumber in used a balancing of inultracapacitorsin seriesthe electric board in a andbalancingtestused sixbench. was of 36. theseboard This boards numberand six connected wasof these boardsin parallel,selected connected so based the in totalon parallel, calculations number so the of for ultracapacitors total the numberenergy storage of used ultracapacitors capacity was 36. for This usedhydraulic number was accumulators36. was This selected number and based was ultracapacitors. The selection of 36 ultracapacitors made the energy storage capacity comparable selectedonThis calculations test based bench on for was calculations the designed energy for storage to measurethe energy capacity the storage flow for hydraulic of capacity energy accumulators forthrough hydraulic the and ultracapacitorsaccumulators ultracapacitors. and during between the two systems. Switches S1 to S6 were used to activate the boards connected in parallel. Theultracapacitors. selection of The 36 ultracapacitors selection of 36 made ultracapacitors the energy made storage the capacity energy comparablestorage capacity between comparable the two charging andAs discharging. mentioned previously, The resistance the number or of load ultracapacitors was made used using in the a test bank bench of was resistors based on connected the in systems. Switches S1 to S6 were used to activate the boards connected in parallel. parallel,between theoreticalwhich the twomade calculations systems. the Switchesvaluefor energy of S1 storagethe to S6resistance werecapacity used in variabletoboth activate systems. and the Energy boardseasy stored connectedto adjust in a hydraulic inmanually. parallel. The ultracapacitorAsaccumulator mentioned arrangement can previously, be calculated had thesix with numbercells the followingconnected of ultracapacitors equation: in series used in ina balancingthe test bench board was basedand six on ofthe these theoretical calculations for energy storage capacity in both systems. Energy stored in a hydraulic boards connected in parallel, so the total number of ultracapacitors used was 36. This number was accumulator can be calculated with the following equation: selected based on calculations for the energy storage capacity for hydraulic accumulators and ultracapacitors. The selection of 36 ultracapacitors made the energy storage capacity comparable between the two systems. Switches S1 to S6 were used to activate the boards connected in parallel. As mentioned previously, the number of ultracapacitors used in the test bench was based on the theoretical calculations for energy storage capacity in both systems. Energy stored in a hydraulic accumulator can be calculated with the following equation:

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As mentioned previously, the number of ultracapacitors used in the test bench was based on the theoretical calculations for energy storage capacity in both systems. Energy stored in a hydraulic accumulator can be calculated with the following equation:

Z v f Eacc = pdv (4) − vo

In Equation (4), Eacc is the total energy storage capacity of the hydraulic accumulator, p is the pressure, v0 is the initial volume, and v f is the ﬁnal volume. The charging and discharging process of the accumulator can be assumed as adiabatic, and the polytropic index of nitrogen can be assumed as 1.4, according to Rabie [20], so the relationship between pressure and the volume in the hydraulic accumulator can be expressed as follows:

n n pv = p0v0 (5)

Plugging Equation (5) into Equation (4) to obtain Equation (6):

Z V f n n Eacc = p0v0v− dv Vo

1 n v f n v − Eacc = p0v 0 1 n − v0 p vn 0 0 1 n 1 n Eacc = v − v − (6) 1 n f − 0 − The ﬁnal compressed volume can be expressed as a function of the maximum pressure in the accumulator. n = n pmaxv f p0v0

!1/n p0 v f = v0 (7) pmax The ﬁnal equation for energy in the accumulator can be obtained by plugging Equation (7) into Equation (6). Equation (8) is the expression for the energy in the hydraulic accumulator:

 ! 1 n  p v  p −n  0 0  0  Eacc =  1 (8) n 1 pmax −  −

In Equation (8), p0 is the precharge pressure in the hydraulic accumulator, v0 is the initial gas volume, pmax is the maximum pressure, and n is the ideal gas constant. The values used for the calculation of energy capacity in the hydraulic accumulator are shown in Table7.

Table 7. Estimated energy capacity of the hydraulic accumulator.

Variable Value

p0 (psi) 1000 pmax (psi) 3000 n 1.4 v0 (liter) 1 Eacc (Wh) 1.77 Energies 2020, 13, 1632 11 of 23

The energy storage capacity of the ultracapacitor arrangement needs to be approximately equal to the energy estimated from Equation (8) for the systems to be comparable. The energy in an ultracapacitor can be calculated with Equation (9):

1 E = CV 2 (9) ult 2 ut

In Equation (9), C is the capacitance and Vut is the voltage of the ultracapacitors. The capacitance and the voltage of the arrangement can be calculated as function of the number of cells in series (NC) and the number of boards in parallel (NB) with Equations (10) and (11):

NB C = Ccell (10) NC

Vut = NCVcell (11) The total energy in the ultracapacitor arrangement can be calculated with Equation (12):

1 E = C V 2(N N ) (12) ult 2 cell cell B C The energy capacity of the ultracapacitors is close to the energy capacity in an accumulator with six cells connected in series in a single board and six boards connected in parallel. The estimated energy storage capacity of the ultracapacitor arrangement is shown in Table8.

Table 8. Estimated energy capacity of the ultracapacitors.

Variable Value

Ccell (F) 50 Vcell (V) 2.7 NB 6 NC 6 Eult(Wh) 1.82

As mentioned previously, this test bench was designed to measure the flow of energy through the ultracapacitors while charging and discharging. During charging mode, switch SB was on (see Figure 10), while switch SR was off. The current in the circuit depended on the value of the resistance. During discharge mode, switch SB was turned off and switch SR was turned on, which allowed the current to flow from the ultracapacitors to the rheostats, where the energy stored was dissipated as heat. During the experiments, the measured variables included voltage across the ultracapacitors and the current flowing through them. With these variables, the instantaneous power could be calculated, and the energy stored in the ultracapacitors could be estimated. The results for a charge and discharge experiment are shown in Figure 13 to illustrate the output. Sixty experiments (30 for charge and 30 for discharge) like the one described in this section were made to calculate the efficiency and performance of the ultracapacitor arrangement. In the first round of experiments, just one board with six cells was connected, and five different values of resistance were tested. For the second round of experiments, two boards were connected and five different values of resistance were tested, this process was carried out until six boards were connected and tested with five different values of resistance. The same procedure was applied for discharge. The details for the tests performed in the electric testbench are summarized in Table9; the number of boards connected, the equivalent capacitance (C), and the equivalent resistance (R) are presented in the table. A detailed explanation of the results is included in the next sections. Energies 2020, 13, x FOR PEER REVIEW 11 of 23

(10)

(11) The total energy in the ultracapacitor arrangement can be calculated with Equation (12): 1 (12) 2 The energy capacity of the ultracapacitors is close to the energy capacity in an accumulator with six cells connected in series in a single board and six boards connected in parallel. The estimated energy storage capacity of the ultracapacitor arrangement is shown in Table 8.

Table 8. Estimated energy capacity of the ultracapacitors.

Variable Value

F 50

V 2.7

Wh 1.82 As mentioned previously, this test bench was designed to measure the flow of energy through the ultracapacitors while charging and discharging. During charging mode, switch SB was on (see Figure 10), while switch SR was off. The current in the circuit depended on the value of the resistance. During discharge mode, switch SB was turned off and switch SR was turned on, which allowed the current to flow from the ultracapacitors to the rheostats, where the energy stored was dissipated as heat. During the experiments, the measured variables included voltage across the ultracapacitors and the current flowing through them. With these variables, the instantaneous power could be calculated, and the energy stored in the ultracapacitors could be estimated. The results for a charge and discharge Energies 2020, 13, 1632 12 of 23 experiment are shown in Figure 13 to illustrate the output.

(a) (b)

FigureFigure 13.13. ResultsResults forfor chargingcharging ((aa)) andand dischargingdischarging ((bb).).

Sixty experiments (30 for charge and 30 for discharge) like the one described in this section were Table 9. Information of the tests performed in the ultracapacitor test bench. made to calculate the efficiency and performance of the ultracapacitor arrangement. In the first round ofNumber experiments, of Boards just Connected one board with C (F) six cells R (Ω was) connected,Number of and Boards five Connected different values C (F) of resistance R (Ω) were tested. For the second round of experiments,7.7 two boards were connected and five different7.7 6 6 values of resistance were tested, this process was carried out until six boards were connected and 1 8.33 4.3 4 33.32 4.3 tested with five different values of resistance.2.1 The same procedure was applied for discharge.2.1 The details for the tests performed in the electric1.6 testbench are summarized in Table 9; the number1.6 of boards connected, the equivalent capacitance7.7 (C), and the equivalent resistance (R) are presented7.7 in the table. A detailed explanation of the results6 is included in the next sections. 6 2 16.66 4.3 5 41.65 4.3 2.1 2.1 1.6 1.6 7.7 7.7 6 6 3 24.99 4.3 6 50 4.3 2.1 2.1 1.6 1.6

4. Results for the Hydraulic Accumulator Instantaneous power in the accumulator could be calculated as the product of pressure and flow rate. These variables were measured during charging and discharging. The results for energy calculation and energy efficiency for the hydraulic accumulator are shown in Table 10. The energy cycle efficiency is the ratio between the total energy released while discharging and the total energy stored while charging.

Table 10. Results for energy calculation in the hydraulic accumulator.

Variable Value Total Energy Stored While Charging 1.308 0.003 Wh ± Total energy released while discharging 1.147 0.005 Wh ± Energy cycle eﬃciency 87.7 0.6% ±

The efficiency of the hydraulic system in transferring power from the shaft of the hydraulic pump to the hydraulic accumulator is shown in Table 10. For vehicular application, the kinetic energy of the wheels would move the shaft of a hydraulic pump, which would move the fluid to the hydraulic accumulator in order to absorb the kinetic energy of the vehicle. In the test bench, the wheel was replaced by an electric system. In previous studies, it has been demonstrated that using hydraulic accumulators in vehicle drivetrains can have a positive impact in the efficiency of a vehicle. Wang et al. [19] demonstrated the advantages of a simulated drivetrain for a light passenger Energies 2020, 13, 1632 13 of 23 vehicle, where, although the energy used for the simulated drive cycle was better using the pure electric drivetrain, the acceleration performance was better for the hydraulic drivetrain thanks to its higher power density. Moreover, Hui et al. [18] studied the effect of using a hydraulic accumulator for extending the state of charge of a battery when hybridizing an electric drivetrain with a hydraulic regeneration with positive results due to the high efficiency of hydraulic accumulators. The power in theEnergies hydraulic 2020, 13 pump, x FOR shaftPEER REVIEW was calculated as the product of the shaft torque and the rotational speed.13 of 23 The torque was estimated based on the pressure at outlet of the pump. The pressure and the torque Energies 2020, 13, x FOR PEER REVIEW 13 of 23 were correlated according to the next expression. 퐷∆푝 푇 = (13) 휂 D퐷∆∆푚p푝 T푇== (13)(13) In Equation (13), 휂푚 is the mechanical efficiencyη휂m푚 of the pump. The efficiency is a function of the pressure and the flow rate in the system, so the efficiency changes throughout the experiment. The InIn Equation Equation (13),(13), η휂푚is is the the mechanical mechanical eefficiencyfficiency of the pump. pump. The The efficiency efficiency is isa function a function of ofthe pressure and the flow ratem were measured, and with the datasheets provided by the manufacturer of thepressure pressure and and the the flow flow rate rate in the in thesystem, system, so the so theefficiency efficiency changes changes throughout throughout the theexperiment. experiment. The the pump, it was possible to estimate the efficiency and the torque at any operating conditions. Thus, Thepressure pressure and and the the flow flow rate rate were were measured, measured, and and with with the the datasheets datasheets provided provided by by the the manufacturer manufacturer of at any time in the experiment, the input power in the hydraulic pump could be estimated, as could ofthe the pump, pump, it was it was possible possible to estimate to estimate the theefficiency efficiency and andthe torque the torque at any at operating any operating conditions. conditions. Thus, the power going to the accumulator. Then, the instantaneous efficiency of the system could be Thus,at any at time any timein the in experiment the experiment,, the input the input power power in the in hydraulic the hydraulic pump pump could could be estimated, be estimated, as could as obtained. At any instant of the experiment, the flow rate and the pressure could be measured. At the couldthe power the power going going to the to theaccumulator. accumulator. Then, Then, the the instantaneous instantaneous efficiency efficiency of ofthe the system system could could be same instant, the power input could be measured. With this information, it was possible to create an beobtained. obtained. At Atany any instant instant of the of theexperiment, experiment, the flow the flowrate and rate the and pressure the pressure could could be measured. be measured. At the efficiency map for the hydraulic accumulator, which is shown in Figure 14. The same map without Atsame the instant, same instant, the power thepower input could input be could measured. be measured. With this With information, this information, it was possible it was possibleto create toan the data points is shown in Figure 15. The map between datapoints was estimated with linear createefficiency an effi mapciency for mapthe hydraulic for the hydraulic accumulator, accumulator, which is which shown is in shown Figure in 1 Figure4. The 14same. The map same witho maput interpolation using the Matlab function griddata [21]. withoutthe data the points datapoints is shown is shown in Figure in Figure 15. 15 The. The map map between between datapoints datapoints was was estimated with with linear linear interpolationinterpolation using using the the Matlab Matlab function function griddata griddata [21 [21]]. .

Figure 14. Instantaneous efficiency of the hydraulic accumulator with data points. Figure 14. Instantaneous eﬃciency of the hydraulic accumulator with data points. Figure 14. Instantaneous efficiency of the hydraulic accumulator with data points.

Figure 15. Instantaneous eﬃciency of the hydraulic accumulator using a gear pump. Figure 15. Instantaneous efficiency of the hydraulic accumulator using a gear pump.

The mapFigure presented 15. Instantaneous in Figures efficiency 14 and of15 the is importanthydraulic accumulator in identifying using operating a gear pump. condition s that would be optimal for a system like the one proposed in this study. According to these plots, the The map presented in Figures 14 and 15 is important in identifying operating conditions that highest efficiency was around 80% and was obtained for flow rates of approximately 1 gpm and would be optimal for a system like the one proposed in this study. According to these plots, the pressures of approximately 1800 psi. highest efficiency was around 80% and was obtained for flow rates of approximately 1 gpm and The current and voltage of the electric system were also measured during the experiment. A pressures of approximately 1800 psi. similar map of efficiency could be made for the conversion of electric power to hydraulic power. The The current and voltage of the electric system were also measured during the experiment. A map is shown in Figure 16. similar map of efficiency could be made for the conversion of electric power to hydraulic power. The map is shown in Figure 16.

Energies 2020, 13, 1632 14 of 23

The map presented in Figures 14 and 15 is important in identifying operating conditions that would be optimal for a system like the one proposed in this study. According to these plots, the highest efficiency was around 80% and was obtained for flow rates of approximately 1 gpm and pressures of approximately 1800 psi. The current and voltage of the electric system were also measured during the experiment. A similar Energies 2020, 13, x FOR PEER REVIEW 14 of 23 map of efficiency could be made for the conversion of electric power to hydraulic power. The map is shownEnergies in2020 Figure, 13, x FOR16. PEER REVIEW 14 of 23

Figure 16. Instantaneous efficiency for conversion of electric power to hydraulic power. Figure 16. Instantaneous efficiency for conversion of electric power to hydraulic power. The maximumFigure 16. Instantaneous efficiency was efficiency around for 50%, conversion relatively of electric low becausepower to thishydraulic was thepower. efficiency of The maximum efficiency was around 50%, relatively low because this was the efficiency of converting the electric power taken from the battery to hydraulic power in the accumulator. The convertingThe maximum the electric efficiencypower taken was from around the battery 50%, to relatively hydraulic low power because in the this accumulator. was the efficiency The electric of electric power from the battery had to be converted into AC power in the inverter. After that, the AC powerconverting from thethe batteryelectric hadpower to be taken converted from the into battery AC power to hydraulic in the inverter. power After in the that, accumulator. the AC power The power was converted into mechanical power by the electric motor. This mechanical power was used waselectric converted power into from mechanical the battery power had to by be the converted electric motor. into AC This power mechanical in the inverter. power was After used that, to the move AC to move the shaft of the hydraulic pump, and the hydraulic pump moved the fluid from the reservoir thepower shaft was of theconverted hydraulic into pump, mechanical and the power hydraulic by the pump electric moved motor. the This fluid mechanical from the reservoirpower was to used the to the accumulator through the hydraulic system, which had some power losses due to friction. accumulatorto move the throughshaft of the the hydraulic hydraulic pump, system, and which the hydraulic had some pump power move lossesd duethe fluid to friction. from the reservoir Another important aspect to consider was that the current of the battery was very high during to theAnother accumulator important through aspect the to hydraulic consider was system, that thewhich current had ofsome the batterypower los wasses very due high to friction. during the the charging process, which was not the most efficient way to use the battery. The results for the chargingAnother process, important which wasaspect not to the consider most effi wascient that way the to current use the of battery. the battery The resultswas very for high the battery during battery for one of the charging experiments are shown in Figure 17. forthe one charging of the chargingprocess, experimentswhich was not are the shown most in efficient Figure 17 way. to use the battery. The results for the battery for one of the charging experiments are shown in Figure 17.

Figure 17. Experimental results for current, voltage, power, and load. Figure 17. Experimental results for current, voltage, power, and load. The maximum current during this experiment was 66 A, which was much higher than the continuousThe maximum current recommendedFigure current 17. Experimental during for this the e experimentresultsﬃcient for operation current, was voltage, 66 of thisA, which battery,power, was and according load.much higher to the technical than the datacontinuous presented current in Table recommended4. The e ﬃ ciencyfor the efficient results for operation a system of withthis battery a model, according of a piston to t pumphe technical were The maximum current during this experiment was 66 A, which was much higher than the estimated.data presented The modelin Table of the4. The piston efficiency pump results was made for a by system using commerciallywith a model availableof a piston datasheets pump were of continuous current recommended for the efficient operation of this battery, according to the technical estimated. The model of the piston pump was made by using commercially available datasheets of data presented in Table 4. The efficiency results for a system with a model of a piston pump were different piston pumps and then estimated with interpolation for a piston pump with a volumetric estimated. The model of the piston pump was made by using commercially available datasheets of displacement of 0.73 푖푛3⁄푟푒푣, which was 11.9 푐푐/푟푒푣. The results of the numerical estimation are different piston pumps and then estimated with interpolation for a piston pump with a volumetric shown in Figure 18. displacement of 0.73 푖푛3⁄푟푒푣, which was 11.9 푐푐/푟푒푣. The results of the numerical estimation are shown in Figure 18.

Energies 2020, 13, 1632 15 of 23 diﬀEnergieserent piston2020, 13, pumpsx FOR PEER and REVIEW then estimated with interpolation for a piston pump with a volumetric15 of 23 displacement of 0.73 in3/rev, which was 11.9 cc/rev. The results of the numerical estimation are shown Energies 2020, 13, x FOR PEER REVIEW 15 of 23 in FigureEnergies 18 2020. , 13, x FOR PEER REVIEW 15 of 23

Figure 18. Instantaneous efficiency of the hydraulic accumulator using a piston pump (estimated).

FigureFigure 18. 18.Instantaneous Instantaneous effi efficiencyciency of of the the hydraulic hydraulic accumulatoraccumulator using a a piston piston pump pump (estimated). (estimated). TheFigure estimated 18. Instantaneous results of efficiencyFigure 16 of show the hydraulic that the accumulator instantaneous using efficiency a piston pump of the (estimated). system using a modelThe estimated of a piston results pump of Figurecould be16 showimproved, that the mostly instantaneous because axial effi ciency piston of pumps the system had higher using a efficiencies.The estimated The results results for ofthe F efficiencyigure 16 show including that the the instantaneous electric motor efficiencyare shown of in the Figures system 19 usingand 20 a. model ofThe a piston estimated pump results could of be F improved,igure 16 show mostly that becausethe instantaneous axial piston efficiency pumps hadof the higher system effi usingciencies. a modelThese figures of a piston show the pump efficiency could of be conversion improved, of mostlyelectric becausepower to axial mechanical piston power pumps in had the ehigherlectric Themodel results of for a the piston efficiency pump including could be the improved, electric motor mostly are because shown in axial Figures piston 19 and pumps 20. Thesehad higher figures efficiencies.systemefficiencies. using TheThe a gear resultsresults pump forfor (experimental)thethe efficiencyefficiency includingincluding and a piston tthehe electric electricpump (estimated). motormotor areare shownshown inin FFiguresigures 1199 andand 2020.. show the efficiency of conversion of electric power to mechanical power in the electric system using a TheseThese figuresfigures showshow thethe efficiencyefficiency ofof conversionconversion ofof electricelectric powerpower toto mechanicalmechanical powerpower inin thethe eelectriclectric gear pump (experimental) and a piston pump (estimated). systemsystem usingusing aa geargear pumppump (experimental)(experimental) andand aa pistonpiston pumppump (estimated).(estimated).

Figure 19. Efficiency of the electric system in the charging process using a gear pump. Figure 19. Eﬃciency of the electric system in the charging process using a gear pump. Figure 19. Efficiency of the electric system in the charging process using a gear pump. Figure 19. Efficiency of the electric system in the charging process using a gear pump.

Figure 20. Eﬃciency of the electric system in the charging process using a piston pump (estimated). Figure 20. Efficiency of the electric system in the charging process using a piston pump (estimated).

Figure 20. Efficiency of the electric system in the charging process using a piston pump (estimated). Figure 20. Efficiency of the electric system in the charging process using a piston pump (estimated).

Energies 2020, 13, x FOR PEER REVIEW 16 of 23

EnergiesEnergiesFrom2020 20 F20, igures13, 1,3 1632, x FOR 19 andPEER 20 REVIEW, it can be observed that the electric system worked better when using1616 of ofa 23 23 gear pump. The difference in the results was due to the input torque needed to turn the shaft of the hydraulicFrom pump Figures, which 19 andin this 20 ,case it can is belower observed for the that gear the pump electric than system for the work pistoned better pump when used. using The a From Figures 19 and 20, it can be observed that the electric system worked better when using overallgear efficiencypump. The of difference the system in was the highlyresults dependentwas due to on the pump input efficiency. torque needed to turn the shaft of the ahydraulic gear pump. pump The, diwhichfference in this in thecase results is lower was for due the to gear the pump input torquethan for needed the piston to turn pump the used. shaft The of the hydraulic pump, which in this case is lower for the gear pump than for the piston pump used. 5. Resultsoverall efficiencyfor the U ltracaof thepacitors system was highly dependent on pump efficiency. The overall efficiency of the system was highly dependent on pump efficiency. 5. TheResults tests for in the the U ltraca ultracapacitorpacitors test bench were made by changing the number of boards connected5. Results and for the the resistance Ultracapacitors used in the circuit. For each of the six possible board configurations, five differentThe values tests of in resistance the ultracapacitor were used. test Starting bench with were one made board by of changingsix ultracapacitors, the number the ofcharge boards connectedThe tests and in the ultracapacitorresistance used test in the bench circuit. were For made each by of changing the six possible the number board of configurations, boards connected five andand discharge the resistance tests usedwere inperformed the circuit. five For times, each ofan thed each six possibletime, the board resistance configurations, selected was five different. different In differenttotal, thirty values experiments of resistance were wereconducted used. forStarting charging with and one thirty board experiments of six ultracapacitors, for discharging. the chargeThe valuesand discharge of resistance tests were were used. performed Starting five with times, one board and each of six time ultracapacitors,, the resistance the selected charge and was discharge different. resultstests were during performed charge and five discharge times, and cycles each are time, presented the resistance below. selected was different. In total, thirty In Fromtotal, Fthirtyigure experiments 21, it can be wereseen thatconducted the maximum for charging voltage and level thirty of experimentsthe system was for discharging.reached faster The experimentsresults during were charge conducted and discharge for charging cycles and are thirty presented experiments below. for discharging. The results during whencharge fewer and boards discharge were cycles used are—that presented is, when below. fewer ultracapacitors were energized. In Figures 21 and 22, eachFrom line represents Figure 21, oneit can experiment be seen that for the one maximum value of resistance. voltage level of the system was reached faster whenFrom fewer Figure boards 21, we it canre used be seen—that that is, the when maximum fewer ult voltageracapacitors level ofwe there energized. system was In reached Figures faster21 and when22, each fewer line boards represents were one used—that experiment is, whenfor one fewer value ultracapacitors of resistance. were energized. In Figures 21 and 22, each line represents one experiment for one value of resistance.

Figure 21. Voltage while charging according to the number of boards connected. Figure 21. Voltage while charging according to the number of boards connected. Figure 21. Voltage while charging according to the number of boards connected.

Figure 22. Power while charging according to the number of boards connected. Figure 22. Power while charging according to the number of boards connected. Now the sameFigure results 22. Power are presented, while charging but theyaccording are presented to the number according of boards to connected. the number of the test (Figures 23 and 24). Test number 1 had the highest value of resistance, and test number 5 had the

Energies 2020, 13, x FOR PEER REVIEW 17 of 23

Now the same results are presented, but they are presented according to the number of the test Energies 2020, 13, x FOR PEER REVIEW 17 of 23 (Figures 23 and 24). Test number 1 had the highest value of resistance, and test number 5 had the lowest value of resistance. The individual lines represent the number of boards connected in the EnergiesNow2020, 13the, 1632 same results are presented, but they are presented according to the number of the17 of test 23 electric(Figures system. 23 and The 2 time4). Test to reachnumber maximum 1 had the voltage highest value value was of lower resistance, at a lower and resistance.test number The 5 valuehad the of lowestthe resistance value ofuse resistance.d in each test The is individual presented linesin Table represent 11. the number of boards connected in the lowestelectric value system. of resistance.The time to Thereach individual maximum lines voltage represent value was the lower number at a of lower boards resistance. connected The in value the Table 11. Resistance value for the tests. electricof the resistance system. The use timed in toeach reach test maximum is presented voltage in Table value 11 was. lower at a lower resistance. The value of the resistance used in each test is presented in Table 11. Test Number Resistance Value Test 1 Table 11. Resistance value for the 7tests..7 Ω Table 11. Resistance value for the tests. TestTest Number 2 Resistance6.0 Ω Value TestTest 3 1 Test Number Resistance Value4.37 Ω.7 Ω TestTest 4 2 Test 1 7.7 Ω 2.16 Ω.0 Ω TestTest 5 3 Test 2 6.0 Ω 1.64 Ω.3 Ω Test 3 4.3 Ω 2.1 Ω Test 4 Test 4 2.1 Ω Test 5 Test 5 1.6 Ω 1.6 Ω

Figure 23. Voltage while charging according to the value of resistance.

Figure 23. Voltage while charging according to the value of resistance. Figure 23. Voltage while charging according to the value of resistance.

FigureFigure 24. 24. PowerPower while while charging charging according according to to the the value value of of resistance. resistance.

A discharging experiment was conducted for each charging experiment. The results according to

the number of boardsFigure are presented 24. Power onwhile Figures charging 25 and according 26, and to thethe value results of accordingresistance. the number of the test are presented in Figures 27 and 28.

Energies 2020, 13, x FOR PEER REVIEW 18 of 23

A discharging experiment was conducted for each charging experiment. The results according Energies 2020, 13, x FOR PEER REVIEW 18 of 23 to the number of boards are presented on Figures 25 and 26, and the results according the number of the testA discharging are presented experiment in Figures was 27 andconducted 28. for each charging experiment. The results according to the number of boards are presented on Figures 25 and 26, and the results according the number of the test are presented in Figures 27 and 28. Energies 2020, 13, 1632 18 of 23

Figure 25. Voltage while discharging according to the number of boards connected.

Figure 25. Voltage while discharging according to the number of boards connected. Figure 25. Voltage while discharging according to the number of boards connected.

Energies 2020, 13, x FOR PEER REVIEW 19 of 23

Figure 26. Power while discharging according to the number of boards connected. Figure 26. Power while discharging according to the number of boards connected.

Figure 26. Power while discharging according to the number of boards connected.

Figure 27. Voltage while discharging according to the number of the test. Figure 27. Voltage while discharging according to the number of the test.

Figure 28. Power while discharging grouped according to the number of the test.

Using results presented in Figures 21–28, it was possible calculate the energy stored in the ultracapacitors and the efficiency. The results for energy calculations in the ultracapacitors are presented in Table 12.

Table 12. Results for energy calculation in the ultracapacitors.

Variable Number of Boards Connected in Parallel 1 2 3 Value Error Value Error Value Error Energy stored while 0.31 0.01 0.63 0.02 0.92 0.03 charging (Wh) Energy stored while 0.25 0.01 0.50 0.01 0.76 0.02 discharging (Wh) Energy efficiency 77.46 2.33 77.2 2.7 80.75 3.3

Energies 2020, 13, x FOR PEER REVIEW 19 of 23

Energies 2020, 13, 1632 Figure 27. Voltage while discharging according to the number of the test. 19 of 23

Figure 28. Power while discharging grouped according to the number of the test. Figure 28. Power while discharging grouped according to the number of the test. Using results presented in Figures 21–28, it was possible calculate the energy stored in the Using results presented in Figures 21–28, it was possible calculate the energy stored in the ultracapacitors and the eﬃciency. The results for energy calculations in the ultracapacitors are ultracapacitors and the efficiency. The results for energy calculations in the ultracapacitors are presented in Table 12. presented in Table 12. Table 12. Results for energy calculation in the ultracapacitors. Table 12. Results for energy calculation in the ultracapacitors. Variable Number of Boards Connected in Parallel Variable Number of Boards Connected in Parallel 1 12 2 33 Value ValueError ErrorValue Value Error Error Value ErrorError EnergyEnergy stored stored while while charging (Wh) 0.31 0.01 0.63 0.02 0.92 0.03 0.31 0.01 0.63 0.02 0.92 0.03 Energycharging stored while (Wh) discharging (Wh) 0.25 0.01 0.50 0.01 0.76 0.02 EnergyEnergy stored e ﬃwhileciency 77.46 2.33 77.2 2.7 80.75 3.3 0.25 0.01 0.50 0.01 0.76 0.02 dischargingVariable (Wh) Number of Boards Connected in Parallel Energy efficiency 77.46 2.334 77.2 52.7 80.75 6 3.3 Value Error Value Error Value Error

Energy saved while charging (Wh) 1.24 0.04 1.61 0.05 1.89 0.06 Energy stored while discharging (Wh) 0.97 0.03 1.29 0.04 1.47 0.05 Energy eﬃciency 78.99 3.91 80.4 4.73 77.74 5.42

6. Comparative Evaluation The results for energy density and power density are presented in Table 13. Three energy systems were compared: a battery, which is the principal source of energy, the ultracapacitor, and the hydraulic accumulator. Energies 2020, 13, 1632 20 of 23

Table 13. Energy density and power density.

Energy/Vol Energy/Mass Cost/Energy Energy Storage System (Wh/m3) (Wh/kg) (US$/Wh) Battery 195144 115.2 0.45 Ultracapacitor 2539.7 2.72 138.67 Accumulator 1227 0.29 404.68 Power/Vol Power/Mass Cost/Power Energy Storage System (kW/m3) (kW/kg) (US$/kW) Battery 325.24 0.192 270.83 Ultracapacitor 2588 2.21 217 Accumulator 7548 2.69 75

The radar plot presented in Figure 29 better illustrates these results. The radar plot shows the results in a scale from 0 to 10 for the three energy storage systems used in this study. The “Score” value for each component of the radar plot was determined using this equation:

System Value Score = 10 (14) Best Value · In Equation (14), the “System Value” is the value of each system: battery, ultracapacitor, or accumulator, and “Best Value” is the best value among the three energy storage systems. This “Best Value” depends on the speciﬁc characteristic to be studied. For instance, in energy/volume, the “Best Value” would be the highest value among the three systems, but in cost/power, the “Best Value” will Energies 2020, 13, x FOR PEER REVIEW 21 of 23 be lowest.

Figure 29. Radar plot comparing three energy storage systems. Figure 29. Radar plot comparing three energy storage systems. From the radar plot, it is possible to see that the energy side of the plot is mostly covered by the From the radar plot, it is possible to see that the energy side of the plot is mostly covered by the battery. Neither of the other two systems could compete against the battery in terms of energy storage. battery. Neither of the other two systems could compete against the battery in terms of energy On the other hand, the hydraulic accumulator dominates the power part of the plot. This means that storage. On the other hand, the hydraulic accumulator dominates the power part of the plot. This the hydraulic accumulator is suited for high power applications. A comparison between the hydraulic means that the hydraulic accumulator is suited for high power applications. A comparison between accumulator and the ultracapacitor is shown in Figure 30. the hydraulic accumulator and the ultracapacitor is shown in Figure 30.

Figure 30. Radar plot comparing ultracapacitors and hydraulic accumulators.

From Figure 29, it is possible to see that the hydraulic accumulator is more suitable than the ultracapacitors in the power segment of the plot, while the ultracapacitor is better in the energy segment of the plot. However, the battery was better than these two systems for storing energy. It is important to note that the energy efficiency for the ultracapacitor was around 78.7%, and the energy efficiency of the hydraulic accumulator was 87.7%. This improvement in energy efficiency and the better power density compared with ultracapacitors could be a determining factor in choosing a hydraulic system over an electric system for a specific application when needing to rapidly charge or discharge energy storage devices, such as in the case for regenerative breaking. The net cost of each system is presented in Table 14, but the information of this table is not enough to make an effective cost analysis of the energy storage systems considered in this study. It is necessary to consider the results presented in Table 14 and in the radar plots of Figures 29 and 30.

Table 14. Cost of each system.

Energy Storage System Cost Battery 390 USD

Energies 2020, 13, x FOR PEER REVIEW 21 of 23

Figure 29. Radar plot comparing three energy storage systems.

From the radar plot, it is possible to see that the energy side of the plot is mostly covered by the battery. Neither of the other two systems could compete against the battery in terms of energy storage. On the other hand, the hydraulic accumulator dominates the power part of the plot. This meansEnergies 2020that, 13the, 1632 hydraulic accumulator is suited for high power applications. A comparison between21 of 23 the hydraulic accumulator and the ultracapacitor is shown in Figure 30.

Figure 30. Radar plot comparing ultracapacitors and hydraulic accumulators. Figure 30. Radar plot comparing ultracapacitors and hydraulic accumulators. From Figure 29, it is possible to see that the hydraulic accumulator is more suitable than the From Figure 29, it is possible to see that the hydraulic accumulator is more suitable than the ultracapacitors in the power segment of the plot, while the ultracapacitor is better in the energy segment ultracapacitors in the power segment of the plot, while the ultracapacitor is better in the energy of the plot. However, the battery was better than these two systems for storing energy. It is important segment of the plot. However, the battery was better than these two systems for storing energy. It is to note that the energy efficiency for the ultracapacitor was around 78.7%, and the energy efficiency of important to note that the energy efficiency for the ultracapacitor was around 78.7%, and the energy the hydraulic accumulator was 87.7%. This improvement in energy efficiency and the better power efficiency of the hydraulic accumulator was 87.7%. This improvement in energy efficiency and the density compared with ultracapacitors could be a determining factor in choosing a hydraulic system better power density compared with ultracapacitors could be a determining factor in choosing a over an electric system for a specific application when needing to rapidly charge or discharge energy hydraulic system over an electric system for a specific application when needing to rapidly charge or storage devices, such as in the case for regenerative breaking. discharge energy storage devices, such as in the case for regenerative breaking. The net cost of each system is presented in Table 14, but the information of this table is not enough The net cost of each system is presented in Table 14, but the information of this table is not to make an effective cost analysis of the energy storage systems considered in this study. It is necessary enough to make an effective cost analysis of the energy storage systems considered in this study. It to consider the results presented in Table 14 and in the radar plots of Figures 29 and 30. is necessary to consider the results presented in Table 14 and in the radar plots of Figures 29 and 30. Table 14. Cost of each system. Table 14. Cost of each system. Energy Storage System Cost Energy Storage System Cost BatteryBattery 390390 USDUSD Ultracapacitors 210 USD Hydraulic accumulator 391 USD

From the comparative results presented in the radar plots of Figures 29 and 30, it is possible to see that regarding cost/energy ratio, the battery was far better than ultracapacitors and hydraulic accumulators. The cost per unit of energy in the battery was 0.45 USD/Wh, the cost per unit of energy in the ultracapacitors was 138.7 USD/Wh, and the cost per unit of energy in the hydraulic accumulator was 404.7 USD/Wh. Regarding the cost per unit of power, the best system was the hydraulic accumulator with 75 USD/kW, the ultracapacitor had a cost/power ratio of 217 USD/kW, and the battery had a cost/power ratio of 270.8 USD. These results demonstrate the potential of hydraulic accumulators for applications that require a high power density. Nevertheless, it is necessary to consider how expensive an accumulator could be if it is required to store energy for a large period, because its cost per unit of energy is not the best.

7. Conclusions Two test benches were designed and built to test two different energy storage systems: a hydraulic accumulator and an ultracapacitor with identical energy capacities. The energy efficiency under the test conditions for the hydraulic accumulator was 87.7%, and the energy efficiency of the ultracapacitor was 78.7%. Energies 2020, 13, 1632 22 of 23

The efficiency map from this study can be used to determine a control strategy for a regenerative system with hydraulic accumulators. This efficiency map can be replicated for different hydraulic pumps by using numerical models for a pump. In addition, the analysis of the efficiency map for a piston pump shows that a hydraulic accumulator would be more efficient if a piston pump is used instead of a gear pump. This study also shows that energy segments of the radar plot were dominated by the ultracapacitor, while the power segments were dominated by the hydraulic accumulator. It is interesting to note that there were segments in the radar plot that were not covered by either of the three energy storage systems. In other words, none of the systems showed a good score in cost/power and energy/volume. This means that energy storage systems with high energy density can provide high power but at a high cost. Moreover, there was no system with a high score in power/volume and cost/energy, which means that the energy storage systems with good power density can be used to store energy but at a high cost. The higher energy efficiency in the hydraulic accumulator and the better power density compared with ultracapacitors could be determining factors in choosing a hydraulic system over an electric system for a specific application, where there is a need to rapidly charge or discharge energy storage devices, such as in the case of regenerative breaking.

Author Contributions: Conceptualization, J.L.-Q., J.G.-B. and M.K.; methodology, J.L.-Q., J.G.-B. and M.K.; software, J.L.-Q.; validation, J.L.-Q., B.N., M.K., A.G.-M., and J.G.-B.; formal analysis, J.L.-Q., and J.G.-B.; investigation, J.L.-Q., M.K., A.G.-M., and J.G.-B.; resources, B.N., and J.G.-B.; data curation, J.L.-Q.; writing—original draft preparation, J.L.-Q.; writing—review and editing, J.L.-Q., B.N., M.K., A.G.-M., and J.G.-B.; visualization, J.L.-Q., B.N., M.K., A.G.-M., and J.G.-B.; supervision, B.N., M.K., A.G.-M., and J.G.-B.; project administration, B.N., and J.G.-B.; funding acquisition, B.N., and J.G.-B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Purdue Polytechnic, office of the vice dean of research, and the school of Engineering Technology at Purdue University. Acknowledgments: Special thanks for Sun Hydraulics for providing testing components. Conflicts of Interest: The authors declare no conflict of interest.

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