Energy Saving Estimation of Athens Trolleybuses Considering Regenerative Braking and Improved Control Scheme

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Article Energy Saving Estimation of Athens Trolleybuses Considering Regenerative Braking and Improved Control Scheme

Nena Apostolidou and Nick Papanikolaou * ID

Department of Electrical & Computer Engineering, Democritus University of Thrace, 67132 Xanthi, Greece; [email protected] * Correspondence: [email protected]; Tel.: +30-25410-79921

Received: 23 June 2018; Accepted: 19 July 2018; Published: 22 July 2018

Abstract: In this work, the electromechanical system of the 8000-series of Athens trolleybuses, based on data provided by OSY S.A., is analyzed. Those data were used to develop a valid model in order to estimate the total energy consumption of the vehicle under any possible operating conditions. In addition, an effort is made to estimate the energy saving potential if the wasted energy—in the form of heat—during braking or downhill courses is recovered (regenerative braking) and retroﬁtted during normal operation. This process requires the installation of appropriate electrical apparatus to recover and temporarily store this energy amount. Moreover, due to the fact that the main engine of the system is an asynchronous electric machine, its driving scheme is also of interest. This study assumes the current driving scheme, that is the direct vector control (DVC), and proposes an alternative control method, the direct torque control (DTC). Energy consumption/saving calculations highlight the effectiveness of incorporating regenerative braking infrastructure in trolleybuses transportation systems. Finally, a sustainable hybrid energy storage unit that supports regenerative braking is proposed.

Keywords: asynchronous machine; direct vector control; direct torque control; hybrid energy storage systems; regenerative braking; trolleybus energy consumption

1. Introduction Athens trolleybuses are a particularly widespread electric vehicle for mass transportation in Greece. OSY S.A. owns 91 single buses and 51 articulated buses, covering 25 route-lines in Athens center [1]. The main traction motor of the 8000-series single trolleybuses under study is an asynchronous machine of 210 kW rated power; from the perspective of limiting the trolleybus electric energy consumption, the focus is on investigating the regenerative energy saving potential and the efﬁciency of the machine driving scheme [1]. Although regenerative braking energy recovery has been widely used in hybrid electric vehicles [2–6], as well as in some public means of electrical transportation, such as metros and trains [7,8], it has not been studied and incorporated in the case of trolleybuses, in any east-European country and other countries world-wide. The reason for this is some technical restrictions in the electric system of those vehicles, which are being discussed in this study. Hence, under this frame, the main contribution of the current work is the comprehensive study of regenerative braking energy recovery application in trolleybuses. In addition, a hybrid energy storage system is proposed, in order to improve the energy efﬁciency of the regenerative braking scheme. Regarding the regenerative braking, the amount of energy that can be saved from the existing trolleybuses under real operating conditions (e.g., during speed reduction intervals or downhill

Resources 2018, 7, 43; doi:10.3390/resources7030043 www.mdpi.com/journal/resources Resources 2018, 7, 43 2 of 18

Resources 2018, 7, x FOR PEER REVIEW 2 of 19 traveling) is estimated [1]. However, this energy saving scheme cannot be directly applied to the existing powertraveling supply) is estimated system [1]. ofHowever, those th trolleybusesis energy saving because scheme therecannot is be notdirectly any applied energy to the storage unit available,existing as shown power in supplyFigure system1. However, of those modern trolleybuses power because electronic there is not technologies any energy storage of energy unit storage available, as shown in Figure 1. However, modern power electronic technologies of energy storage and retroﬁtand that retrofit have that been have proposed been proposed in recent in recent literature literature [9– 11[9–]11 can] can be be used used (under(under appropriate scaling) in order toscaling) implement in order regenerative to implement regenerative braking recovery, braking recovery as proposed, as proposed in Figure in Figure1. 1.

Overhead Line Electric Energy Current Electric Input Apparatus Inverter

Storage Device AC Current

Necessary Electric ~ Apparatus Electric Energy Motor (Asynchronous M Machine)

Kinetic Energy

Figure 1. Electric energy management in 8000-series of Athens trolleybuses. The energy flows strictly Figure 1. Electricunidirectional energyly from management the overhead in power 8000-series line to ofthe Athens trolleybus trolleybuses. electric apparatus The. energyDC—direct ﬂows strictly unidirectionallycurrent; fromAC—alternati the overheadng current. power line to the trolleybus electric apparatus. DC—direct current; AC—alternating current. In addition, besides implementing the existing direct vector control (DVC) scheme of the trolleybus asynchronous machine, the direct torque control (DTC), which is widely used for the In addition,dynamic besides control implementing of electric alternating the existing current ( directAC) m vectorachines control[12,13], is (DVC) also considered. scheme of Both the trolleybus asynchronousmethods machine, rely on the dynamic direct torquemodel of control the machine (DTC),, though which they is implement widely used different for control the dynamic logic control [14]. These driving schemes are applied to the electric trolleybuses of interest in order to highlight of electric alternating current (AC) machines [12,13], is also considered. Both methods rely on the the performance of each one across the total operating range for this particular application. dynamic modelIn the of the present machine, study, an though appropriate they behavioral implement model different of the trolleybus control logic electromechanical [14]. These driving schemes aresystem applied is developed, to the including electric trolleybusesthe machine driving of interest scheme. inExhaustive order to simulations highlight based the on performance this of each one acrossmodel a there performed total operating in Matlab/Simulink range for this environment, particular covering application. the total operating range of the trolleybus; the results lead to an analytical functional model of the energy consumption estimation, In the present study, an appropriate behavioral model of the trolleybus electromechanical system as well as of the regenerative energy recovery potential; energy consumption/saving calculations are is developed,based including on realistic the transportation machine driving conditions. scheme. Finally, Exhaustive sustainability simulations estimations regarding based on the this model are performednecessary in Matlab/Simulink energy recovery apparatus environment, are presented covering [15]. the total operating range of the trolleybus; the results leadIt is to noted an analyticalthat the current functional work focuses model on the of 8000 the-series energy of single consumption trolleybuses owned estimation, by OSY as well as S.A. Nevertheless, the proposed modelling and energy saving analysis is valid for any type of of the regenerative energy recovery potential; energy consumption/saving calculations are based on trolleybus. realistic transportation conditions. Finally, sustainability estimations regarding the necessary energy recovery apparatus2. Existing and are Proposed presented Driving [15]. Scheme It is noted that the current work focuses on the 8000-series of single trolleybuses owned by 2.1. Existing Driving Scheme—Direct Vector Control OSY S.A. Nevertheless, the proposed modelling and energy saving analysis is valid for any type The block diagram of the DVC that is currently used in the trolleybuses under study is depicted of trolleybus. in Figure 2 [1]. The dynamic equivalent model of the asynchronous machine applied to the stator reference frame is used for the calculation of the torque and flux producing currents. Specifically, two 2. Existingstator and phase Proposed currents Driving and rotor Scheme speed feedback serve as inputs to the rotor flux, electromagnetic torque, and rotor field angle estimator unit, based on the dynamic model of the machine. The 2.1. Existingestimated Driving values Scheme—Direct are compared to Vector the reference Control values to produce the PWM signals that drive the six-

The block diagram of the DVC that is currently used in the trolleybuses under study is depicted in Figure2[ 1]. The dynamic equivalent model of the asynchronous machine applied to the stator reference frame is used for the calculation of the torque and flux producing currents. Specifically, two stator phase currents and rotor speed feedback serve as inputs to the rotor flux, electromagnetic torque, and rotor field angle estimator unit, based on the dynamic model of the machine. The estimated values are compared to the reference values to produce the PWM signals that drive the six-pulse inverter. Park transformation is used to turn the time-variable (sinusoidal) quantities of the machine Resources 2018, 7, 43 3 of 18 Resources 2018, 7, x FOR PEER REVIEW 3 of 19

(current,pulse voltages, inverter. ﬂux, Park etc.)transformation into time-invariable is used to turn ones the time (direct-variabl currente (sinusoidal) (DC) quantities), quantities of tothe be used by errormachine controllers. (current, The voltages, implementation flux, etc.) into oftime closed-invariable loop ones control (direct (torque current and(DC) ﬂuxquantities), controller) to be of the used by error controllers. The implementation of closed loop control (torque and flux controller) of appropriate estimated DC quantities approximates the control of the separately excited DC machines; the appropriate estimated DC quantities approximates the control of the separately excited DC as a result,machines; fast dynamic as a result, response fast dynamic and response accuracy and of accuracy the control of the system control issystem achieved is achieved [16,17 [16].,17].

600 VDC ACCELERATION & BRAKING PEDAL Rbraking

REFERENCE T_REF TORQUE & PWM 6 PULSE IGBT VALUES FOR F_REF FLUX Asynchronous TORQUE & FLUX CONTROLLER INVERTER Machine

T F Ω SPEED ESTIMATOR MOTOR MODELING ia FLUX, TORQUE & FIELD ib ANGLE ESTIMATOR STATOR CURRENTS

Figure 2. Block-diagram of 8000-series Athens trolleybuses traction system [13]. VDC: DC voltage supply, Rbraking: Breaking Resistance. Figure 2. Block-diagram of 8000-series Athens trolleybuses traction system [13]. VDC: DC voltage supply, Rbraking: Breaking Resistance. 2.2. Proposed Control Scheme—Direct Torque Control 2.2. Proposed Control Scheme—Direct Torque Control The main difference of DTC with respect to DVC is that transformation from the static reference frame to theThe synchronously main difference rotatingof DTC with reference respect to frame DVC is is that not transformation necessary [ 14from]. Becausethe static refer of theence fact that frame to the synchronously rotating reference frame is not necessary [14]. Because of the fact that this this technique involves error limitation within a hysteresis zone, there is no need for handling DC technique involves error limitation within a hysteresis zone, there is no need for handling DC quantities,quantities which, which simpliﬁes simplifies the the control control scheme. scheme. InIn addition, there there is no is noneed need for a for speed a speed sensor. sensor. Figure3Fig depictsure 3 depicts the DTC the block DTC diagram, block diagram, where where electromagnetic electromagnetic ﬂux flux and torqueand torque values values estimated by the dynamicestimated by machine the dynamic model machine arecompared model are compared to their to reference their reference values. values The. The errors errors are are fedfed to two hysteresisto two controllers hysteresis andcontrollers the resulting and the resulting signals signals determine determine the inverterthe inverter pulsation, pulsation, withwith the the use use of an of an optimally operating state selection table. optimally operating state selection table. Resources 2018, 7, x FOR PEER REVIEW 4 of 19

DC Supply

Flux s dψs Phase A ψ s,ref Hysteresis Comparator Table of Optimum Inverter s Phase B ψ s Inverter Switching Torque dT e Phase C Te,ref Hysteresis States Comparator Te isα isb isc i Electromagnetic sa isb

Flux & Torque usa Estimator usb

3-Phase Asynchronous Machine

Figure 3. Block diagram of direct torque control (DTC) of an asynchronous machine.

3. Modeling of the Electromechanical System

3.1. Electric Supply Circuit Figure 4 illustrates the driving unit of the trolleybuses under study; it includes the vehicle power source (600 VDC), an LC filter, a braking resistance—combined with a unidirectional chopper— branch, the power inverter, and the asynchronous machine (M). During accelerations and constant speed vehicle courses, the energy flow is directed from the power supply to the machine, which operates as a motor. During deceleration/braking intervals and downhill traveling, the asynchronous machine operates as a generator, converting the kinetic energy of the trolleybus into electric energy, this time routed from the machine via the power inverter, to the inverter DC bus; this energy can, under appropriate conditions, be stored and retrofitted to the electric machine (when it requests energy supply). However, in the system under study, there is no storage unit available. Thus, the braking energy is consumed on the braking resistance, in the form of heat.

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3. Modeling of the Electromechanical System

3.1. Electric Supply Circuit Figure4 illustrates the driving unit of the trolleybuses under study; it includes the vehicle power source (600 VDC), an LC ﬁlter, a braking resistance—combined with a unidirectional chopper—branch, theResources power 2018 inverter,, 7, x FOR andPEER the REVIEW asynchronous machine (M). 5 of 19

B Supercapacitor A B Bidirectional Converter

Figure 4. Driving unit of Athens trolleybuses asynchronous machine (M)(M) [[1]1]..

3.2. Mechanical Modeling of the Trolleybus During accelerations and constant speed vehicle courses, the energy ﬂow is directed from the powerFigure supply 5 describes to the machine, the mechanical which operates part of asthe a trolleybus motor. [1,18]. The total rolling resistance of the vehicleDuring (ΣFtr) deceleration/brakingcomprises the rolling resistance intervals (F andr) and downhill the wind traveling, resistance the (Fw asynchronous); this force—tha machinet is, the operatesvehicle transport as a generator, load— convertingis calculated the andkinetic reduced energy to of athe torque trolleybus value into on electric the wheels energy, (Ttr this). Then, time routedaccording from to the trolley machinebus via drive the system power (transmission inverter, to the factor inverter ig), this DC torque bus; thisvalue energy is reduced can, underto the appropriateelectric machine conditions, shaft level be (mechanical stored and retroﬁttedload, Tm) [1 to9, the20]. electric machine (when it requests energy supply).The However,mathematical in the equations system of under the mechanical study, there model is no storagein Figure unit 5 are available. as follows Thus,: the braking energy is consumed on the braking resistance, in the form of heat. Fr =M v g  (f r  cosa+sina) (1)

3.2. Mechanical Modeling of the Trolleybus 2 Fw=A f k air V (2) Figure5 describes the mechanical part of the trolleybus [ 1,18]. The total rolling resistance of the m=evδM (3) vehicle (ΣFtr) comprises the rolling resistance (Fr) and the wind resistance (Fw); this force—that is, the vehicle transport load—is calculated andJ reduced i22 to a torque m value i on the wheels (Ttr). Then, JmWWm g m g according to the trolleybus driveδ=1+ system22 (transmission + =1+ factor + i ), this torque value is reduced to( the4) r M r M MMg WWVVVV electric machine shaft level (mechanical load, Tm)[19,20]. The mathematical equations of the mechanical model2 in Figure5 are as follows: Jew =m e r w (5)

Fr= Mv·g·(f r· cosJew a + sin a) (1) J=e,drive 2 (6) ig 2 Fw= Af·kair·V (2) where Mv is the passenger load; g is the acceleration of gravity; fr is the rolling resistance coefficient, me = δ·Mv (3) related to the type and shape of the vehicle; a is the road inclination angle; Af is the area of the vehicle face; kair is the aerodynamic coefficient; V is the linear velocity of the trolleybus; me is the equivalent vehicle mass; δ is the vehicle mass increase resulting from the individual moments of inertia of all its rotating masses (in this case, the rotating masses are considered to be the wheels and the electric motor); Jw is the inertia of the wheels; Jm is the motor inertia; mw is the mass of the wheels; mm is the motor mass; rw is the radius of the wheels; Jew is the equivalent vehicle moment of inertia; me is the equivalent total vehicle mass; and Je,drive is the total vehicle (equivalent) inertia reduced to the motor axis rotating speed.

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2 2 J Jm·ig m mm·ig δ = 1 + W + = 1 + W + (4) 2 · 2 · M M rW MV rW MV V V 2 Jew= me·rw (5) J = ew Je,drive 2 (6) ig where Mv is the passenger load; g is the acceleration of gravity; fr is the rolling resistance coefﬁcient, related to the type and shape of the vehicle; a is the road inclination angle; Af is the area of the vehicle face; kair is the aerodynamic coefﬁcient; V is the linear velocity of the trolleybus; me is the equivalent vehicle mass; δ is the vehicle mass increase resulting from the individual moments of inertia of all its rotating masses (in this case, the rotating masses are considered to be the wheels and the electric motor); Jw is the inertia of the wheels; Jm is the motor inertia; mw is the mass of the wheels; mm is the motor mass; rw is the radius of the wheels; Jew is the equivalent vehicle moment of inertia; me is the equivalent total vehicle mass; and Je,drive is the total vehicle (equivalent) inertia reduced to the motor axisResources rotating 2018, 7 speed., x FOR PEER REVIEW 6 of 19

Jeq, drive , ωm ,Tm Ttr=ΣFtr rw ΣFtr=Fr(a, Mv)+Fw(V) ωw Trolley Asynchronous Wheels Machine Tm=Ttr / ig rw Μv Jm , mm Jew , me

FigureFigure 5.5. ModelModel ofof thethe trolleybustrolleybus mechanicalmechanical part.part.

ItIt isis notednoted thatthat thethe aerodynamicaerodynamic coefficientcoefficient value,value, kkairair == 0.4,0.4, waswas calculatedcalculated accordingaccording toto datadata givengiven byby MANMAN TruckTruck && BusBus [[2121],], usingusing thethe curvecurve fittingfitting applicationapplication ofof MatlabMatlab software.software. Also,Also, forfor thethe calculationcalculation ofof thethe rollingrolling resistanceresistance coefficientcoefficient value,value, f rfr == 0.001,0.001, literatureliterature datadata werewere usedused [[2222].]. TableTable1 1summarizes summarizes the the main main technical technical data data of of the the trolleybuses trolleybuses under under study study [1 [1].].

Table 1.1. TechnicalTechnical datadata ofof 8000-series8000-series AthensAthens trolleybusestrolleybuses[ 1[1]]..

Technical Data of 8000-Series Athens Trolleybuses Technical Data of 8000-Series Athens Trolleybuses ig 11.02 ig 11.02 Mv 20,000 kg (max), 14,000 kg (min) Mv 20,000 kg (max), 14,000 kg (min) f 2 2 AA f 7.57.5 m m rwrw 0.540.54 cm cm 2 JJe,e,drivedrive 6060 kg kg·m·m2 MaximumMaximum linear linear velocity velocity 6565 km/h km/h Maximum acceleration 1.5 m/s2 2 MaximumMaximum acceleration deceleration 1.31.5 m/s m/s2 Maximum deceleration 1.3 m/s2

3.3. Electrical Modeling of the Trolleybus 3.3. Electrical Modeling of the Trolleybus With regard to the electrical losses of the power supply circuit and the machine itself [23], With regard to the electrical losses of the power supply circuit and the machine itself [23], all all technical and operational parameters of the trolleybus were taken into account [1]. In addition, technical and operational parameters of the trolleybus were taken into account [1]. In addition, the the switching losses of the device are estimated as a percentage of the machine rated output power and switching losses of the device are estimated as a percentage of the machine rated output power and modeled as a variable current source, depending on the system electromagnetic torque. The trolleybus modeled as a variable current source, depending on the system electromagnetic torque. The electrical losses model is shown in Figure6. trolleybus electrical losses model is shown in Figure 6.

Diode Rectifier / Chopper/Braking LC Filter Resistance Total Inverter Conduction Losses Conduction Losses Switching Conduction

R Losses L on Losses

VDF R

F, F, IGBT F, F, DIODE

isw F, DIODE V

600 V C F, IGBT

R br

RFE, ab Iron Core RFE, bc Losses RFE, ca

Machine Copper Losses

Figure 6. Trolleybus total electrical losses model.

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Jeq, drive , ωm ,Tm Ttr=ΣFtr rw ΣFtr=Fr(a, Mv)+Fw(V) ωw Trolley Asynchronous Wheels Machine Tm=Ttr / ig rw Μv Jm , mm Jew , me

Figure 5. Model of the trolleybus mechanical part.

It is noted that the aerodynamic coefficient value, kair = 0.4, was calculated according to data given by MAN Truck & Bus [21], using the curve fitting application of Matlab software. Also, for the calculation of the rolling resistance coefficient value, fr = 0.001, literature data were used [22]. Table 1 summarizes the main technical data of the trolleybuses under study [1].

Table 1. Technical data of 8000-series Athens trolleybuses [1].

Technical Data of 8000-Series Athens Trolleybuses ig 11.02 Mv 20,000 kg (max), 14,000 kg (min) Af 7.5 m2 rw 0.54 cm Je,drive 60 kg·m2 Maximum linear velocity 65 km/h Maximum acceleration 1.5 m/s2 Maximum deceleration 1.3 m/s2

3.3. Electrical Modeling of the Trolleybus With regard to the electrical losses of the power supply circuit and the machine itself [23], all technical and operational parameters of the trolleybus were taken into account [1]. In addition, the switching losses of the device are estimated as a percentage of the machine rated output power and modeled as a variable current source, depending on the system electromagnetic torque. The Resources 2018, 7, 43 6 of 18 trolleybus electrical losses model is shown in Figure 6.

Diode Rectifier / Chopper/Braking LC Filter Resistance Total Inverter Conduction Losses Conduction Losses Switching Conduction

R Losses L on Losses

VDF R

F, F, IGBT F, F, DIODE

isw F, DIODE V

600 V C F, IGBT

R br

RFE, ab Iron Core RFE, bc Losses RFE, ca

Machine Copper Losses

Figure 6. Trolleybus total electrical losses model. Figure 6. Trolleybus total electrical losses model. Resources 2018, 7, x FOR PEER REVIEW 7 of 19

In addition,addition, the total energy consumption is analyzed in the individual electrical losses of the line—includingline—including the the diode diode rectifier, rectifier, the the LC filter,LC filter and, theand braking the braking resistance resistance branch—the branch power—the inverter power losses,inverter the losses, asynchronous the asynchronous machine losses,machine the losses, load energy the load (that energy is, the (that transport is, the load, transport which load, includes which the rollingincludes resistance, the rolling the resistance, wind resistance, the wind and resistance the trolley, and passenger the trolley load), passenger and finally load) the, and kinetic finally energy the storedkinetic inenergy the rotating/moving stored in the rotating parts/moving of the vehicle parts (asof the shown vehicle in Figure (as shown7). in Figure 7).

ENERGY KINETIC 1/2JΩ2 (kWh) + ENERGY SOURCE ENERGY (kWh/km) LOAD (kWh/km) LINE INVERTER MOTOR LOSSES LOSSES LOSSES (kWh/km) (kWh/km) (kWh/km)

Figure 7. TrolleybusTrolleybus i individualndividual energy consumption components.components.

3.4.3.4. Matlab/ Matlab/SimulinkSimulink Behavioral Model The energy that is currently wasted in the above-mentionedabove-mentioned way can be estimated considering the vehicle vehicle total total energy energy consumption, consumption, that that is, the is, electricalthe electrical and mechanical and mechanical power power losses of losses the traction of the system,traction assystem, analyzed. as analyzed. To achieve To a achieve valid estimation, a valid estimation, a detailed a behavioral detailed behavioral Simulink model Simulink is developed, model is asdeveloped, shown in as Figure shown8. in Figure 8. Simulations have been performedperformed inin Matlab/SimulinkMatlab/Simulink environment, based based on the electromechanical model of Figure8 8,, withwith samplingsampling frequencyfrequency 200200 kHz;kHz; bothboth DVCDVC andand thethe proposedproposed DTC driving schemes are considered considered.. The switching frequency of the PWM unit used in the DVC is 2 kHz,kHz, coinciding with the one used in the trolleybusestrolleybuses underunder studystudy [[11].].

ACCELERATION & BRAKING PEDAL

DVC/DTC Overhead Line, 600 VDC Electric Inverter Equipment, Rbr Electric Energy Input V Jeq, drive , ωm ,Tm Trolley Μ v AM Jm , mm

Ttr=ΣFtr rw T =T / i r m tr g ω w w Jew , me ΣF =F (a, M )+F (V) tr r v w Figure 8. Athens trolleybuses electromechanical model. DVC—direct vector control, DTC—direct torque control.

4. Energy Consumption Estimations Using DVC and DTC—Comparison of Results

4.1. Trolleybus Traveling Conditions under Study

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In addition, the total energy consumption is analyzed in the individual electrical losses of the line—including the diode rectifier, the LC filter, and the braking resistance branch—the power inverter losses, the asynchronous machine losses, the load energy (that is, the transport load, which includes the rolling resistance, the wind resistance, and the trolley passenger load), and finally the kinetic energy stored in the rotating/moving parts of the vehicle (as shown in Figure 7).

ENERGY KINETIC 1/2JΩ2 (kWh) + ENERGY SOURCE ENERGY (kWh/km) LOAD (kWh/km) LINE INVERTER MOTOR LOSSES LOSSES LOSSES (kWh/km) (kWh/km) (kWh/km)

Figure 7. Trolleybus individual energy consumption components.

3.4. Matlab/Simulink Behavioral Model The energy that is currently wasted in the above-mentioned way can be estimated considering the vehicle total energy consumption, that is, the electrical and mechanical power losses of the traction system, as analyzed. To achieve a valid estimation, a detailed behavioral Simulink model is developed, as shown in Figure 8. Simulations have been performed in Matlab/Simulink environment, based on the electromechanical model of Figure 8, with sampling frequency 200 kHz; both DVC and the proposed DTCResources driving2018, 7 schemes, 43 are considered. The switching frequency of the PWM unit used in the DVC7 of is 18 2 kHz, coinciding with the one used in the trolleybuses under study [1].

ACCELERATION & BRAKING PEDAL

DVC/DTC Overhead Line, 600 VDC Electric Inverter Equipment, Rbr Electric Energy Input V Jeq, drive , ωm ,Tm Trolley Μ v AM Jm , mm

Ttr=ΣFtr rw T =T / i r m tr g ω w w Jew , me ΣF =F (a, M )+F (V) tr r v w FigureFigure 8 8.. AthensAthens trolleybuses trolleybuses e electromechanicallectromechanical model. model. DVC DVC—direct—direct vector vector control control,, DTC DTC—direct—direct torquetorque control. control.

4.4. Energy Energy Consumption Consumption Estimation Estimationss Using Using DVC DVC and and DTC DTC—Comparison—Comparison of of Results Results

4.14.1.. Trolley Trolleybusbus Traveling Traveling Conditions under Study

The total energy consumption, as well as the amount of energy consumed on the dynamic braking resistance (which represents the percentage of the energy that can be recovered) are estimated under various operating conditions. These operating conditions are summarized below:

In all cases of acceleration, the maximum torque is constant and equal to 1708.1 Nm—achieved with a suitable torque limiter—resulting in the vehicle maximum acceleration limit (1.5 m/s2). Similarly, in all deceleration conditions, the maximum torque is constant at −1480.6 Nm, resulting in the trolley maximum deceleration limit (1.3 m/s2), according to data provided by OSY. S.A.

4.2. Energy Consumption Estimations—Constant Speed Course Figure9 depicts the trolleybus total energy consumption during constant-speed courses, as a function of vehicle speed, in case of maximum (FULL) and minimum (EMPTY) passenger load, assuming 0%, 1%, and 5% road inclination; both DVC and DTC schemes are considered. As derived from Figure9, in all examined cases, energy consumption is almost the same for DVC and DTC schemes (noted deviations are within the simulation accuracy limits). Resources 2018, 7, x FOR PEER REVIEW 8 of 19

• Minimum (Mv = 14,000 kg) and maximum (Mv = 20,000 kg) passenger load. • Accelerating from zero to different speeds—up to 65 km/h—assuming zero-inclined road. • Acceleration at intermediate speeds, assuming zero-inclined road. • Decelerating from maximum trolley speed (65 km/h) [1] to various speeds, assuming zero- inclined road. • Decelerations from intermediate speeds, assuming zero-inclined road. • Steady-speed courses (10, 20, 30, 40, and 50 km/h), assuming 0%, 1%, 5%, −1% and, −5% road inclination, respectively. • DVC and DTC driving schemes. • In DVC case, asynchronous machine slip is 1%. In all cases of acceleration, the maximum torque is constant and equal to 1708.1 Nm—achieved with a suitable torque limiter—resulting in the vehicle maximum acceleration limit (1.5 m/s2). Similarly, in all deceleration conditions, the maximum torque is constant at −1480.6 Nm, resulting in the trolley maximum deceleration limit (1.3 m/s2), according to data provided by OSY. S.A.

4.2. Energy Consumption Estimations—Constant Speed Course Figure 9 depicts the trolleybus total energy consumption during constant-speed courses, as a function of vehicle speed, in case of maximum (FULL) and minimum (EMPTY) passenger load, assuming 0%, 1%, and 5% road inclination; both DVC and DTC schemes are considered. As derived Resourcesfrom2018 Figure, 7, 43 9, in all examined cases, energy consumption is almost the same for DVC and DTC 8 of 18 schemes (noted deviations are within the simulation accuracy limits).

FigureFigure 9. Trolleybus 9. Trolleybus total total energy energy consumption consumption as as a a function function of of speed, speed, for forvarious various conditions conditions of of passengerpassenger load load (EMPTY, (EMPTY FULL),, FULL road), road inclination inclination (0%,(0%, 1%, 5%) 5%) and and driving driving scheme scheme (DV (DVC,C, DTC DTC).).

An interesting outcome is that, in all cases except the one of DVC on 5% road inclination with Anmaximum interesting passenger outcome load (FULL) is that,, higher in all casesenergy except consumption the one is observed of DVC onat low 5% speeds. road inclination This could with maximumbe explained passenger by the load fact (FULL), that at low higher speeds energy, the machine consumption operate iss with observed under at high low electromagnetic speeds. This could be explainedflux value bys, wh theereas fact over that atnominal low speeds, speed— thethat machine is, 28 km/h operates—the electromagnetic with under high flux electromagnetic reference value flux values,is reduced whereas inversely over nominal to speed speed—that(field weakening is,28 technique km/h—the). Also, electromagnetic at low speeds, the flux machine reference operat valuees is reducedwith inverselysmall amplitude to speed modulation (field weakening ratio compared technique). with that Also, at high at lowspeed speeds, operation the, which machine leads operates to withmachine small amplitude low power modulation factor and high ratio stator compared rms current with value thats.at high speed operation, which leads to machine lowFigure powers 10–12 factor depict and the hightrolley statorbus total rms energy current consumption values. allocation, according to Figure 7, in case of maximum (FULL) and minimum (EMPTY) passenger loads, 0%, 1%, and 5% road Figures 10–12 depict the trolleybus total energy consumption allocation, according to Figure7, inclination, with DVC and DTC implementation. in case of maximum (FULL) and minimum (EMPTY) passenger loads, 0%, 1%, and 5% road inclination, Resources 2018, 7, x FOR PEER REVIEW 9 of 19 with DVC and DTC implementation. InIn particular, particular, Figure 1010 depicts depicts the the motor motor energy energy consumption consumption as a function as a function of speed, of speed,under under variousvarious operating/control operating/control conditio conditions.ns.

Figure 10. Motor energy consumption as a function of speed, for various conditions of passenger load Figure 10. Motor energy consumption as a function of speed, for various conditions of passenger load (EMPTY, FULL) and road inclination (0%, 1%, 5%), with DVC and DTC implementation. (EMPTY, FULL) and road inclination (0%, 1%, 5%), with DVC and DTC implementation. From Figure 10, it appears that, in the case of 0% road inclination, motor energy consumption receivesFrom the Figure highest 10 portion, it appears of the that, total inenergy the caseconsumption of 0% road across inclination, the speed range. motor However, energy consumptionin the receivescase of the 1% roadhighest inclination portion, this of the portion total is energy significantly consumption decreased, across whereas the in speed case of range. 5% road However, ininclination the case of, it 1% is drastically road inclination, restricted this. portion is signiﬁcantly decreased, whereas in case of 5% road inclination,Figure it 11 is depicts drastically the mechanical restricted.–transport–load energy consumption as a function of speed for various operating/control conditions under study. Figure 11 depicts the mechanical–transport–load energy consumption as a function of speed for various operating/control conditions under study.

Figure 11. Mechanical–transport–load energy consumption as a function of speed, for various conditions of passenger load (EMPTY, FULL) and road inclination (0%, 1%, 5%), with DVC and DTC implementation.

Figure 11 shows that in the case of 0% road inclination, the mechanical load consumption a receives limited portion of the total energy consumption at low speeds, which increases with speed. In the case of 1% road inclination, this portion increases significantly, whereas in the case of 5% road inclination, the increase becomes significant. Figure 12 depicts the inverter energy consumption as a function of speed, for various operating/control conditions under study.

Resources 2018, 7, x FOR PEER REVIEW 9 of 19

In particular, Figure 10 depicts the motor energy consumption as a function of speed, under various operating/control conditions.

Figure 10. Motor energy consumption as a function of speed, for various conditions of passenger load (EMPTY, FULL) and road inclination (0%, 1%, 5%), with DVC and DTC implementation.

From Figure 10, it appears that, in the case of 0% road inclination, motor energy consumption receives the highest portion of the total energy consumption across the speed range. However, in the case of 1% road inclination, this portion is significantly decreased, whereas in case of 5% road inclination, it is drastically restricted. Resources 2018Figure, 7, 43 11 depicts the mechanical–transport–load energy consumption as a function of speed for 9 of 18 various operating/control conditions under study.

Figure 11. Mechanical–transport–load energy consumption as a function of speed, for various Figurecondit 11. ionsMechanical–transport–load of passenger load (EMPTY, FULL energy) and road consumption inclination (0%,as 1%, a 5%), function with DVC of and speed, DTC for various conditionsimplementation. of passenger load (EMPTY, FULL) and road inclination (0%, 1%, 5%), with DVC and DTC implementation. Figure 11 shows that in the case of 0% road inclination, the mechanical load consumption a receives limited portion of the total energy consumption at low speeds, which increases with speed. FigureIn the case11 shows of 1% road that inclination in the, casethis portion of 0% increases road inclination, significantly, wh theereas mechanical in the case of load 5% road consumption a receivesinclination limited, portionthe increase of becomes the total significant energy. consumption at low speeds, which increases with speed. Figure 12 depicts the inverter energy consumption as a function of speed, for various In the case of 1% road inclination, this portion increases signiﬁcantly, whereas in the case of 5% road operating/control conditions under study. inclination, the increase becomes signiﬁcant. Figure 12 depicts the inverter energy consumption as a function of speed, for various operating/controlResources 2018, 7, x FOR conditions PEER REVIEW under study. 10 of 19

Figure 12. Inverter energy consumption as a function of speed, for different conditions of passenger Figure 12. Inverter energy consumption as a function of speed, for different conditions of passenger load (EMPTY, FULL) and road inclination (0%, 1%, 5%), with DVC and DTC implementation. load (EMPTY, FULL) and road inclination (0%, 1%, 5%), with DVC and DTC implementation. Figure 12 shows that the inverter energy consumption receives a limited portion of the total energyFigure consumption 12 shows across that the the inverterspeed range energy, which consumption is drastically decreased receives with a limitedroad inclination portion. In of the total addition, it decreases moderately with speed rise. energy consumption across the speed range, which is drastically decreased with road inclination. Figures 10–12 suggest that the allocation of individual system losses is mainly affected by road In addition,inclination it— decreaseswhich is moderatelythe main parameter with speed of total rise. transport load increase—while trolleybus passengerFigures 10load–12 has suggest limited thateffect thes on allocation it. Also, it is of noted individual that in all system cases, lossesline losses is mainly receive affecteda low, by road inclination—whichalmost constant (under is the 2%) main portion parameter of total energy of total consumption. transport load increase—while trolleybus passenger load has limited effects on it. Also, it is noted that in all cases, line losses receive a low, almost constant (under 2%) portion of total energy consumption.

4.3. Energy Consumption Estimations—Acceleration Figure 13 depicts the trolleybus total energy consumption during acceleration from zero to different speeds, as a function of vehicle speed, on 0% road inclination, in the case of maximum (FULL) and minimum (EMPTY) passenger load, with DVC and DTC implementation.

Resources 2018, 7, x FOR PEER REVIEW 11 of 19 Resources 2018, 7, x FOR PEER REVIEW 11 of 19

4.3. Energy4.3. Energy Consumption Consumption Estimations Estimations—Acceleration—Acceleration FigureFigure 13 depicts 13 depicts the thetrolley trolleybusbus total total energy energy consumption consumption during during acceleration acceleration from from zero zero to to Resourcesdiffere2018diffent speeds, 7re,nt 43 speeds, as a, as function a function of vehicleof vehicle speed, speed, on on 0% 0% road inclination, inclination, in inthe the case case of maximum of maximum 10 of 18 (FULL)(FULL) and minimum and minimum (EMPTY) (EMPTY) passenger passenger load, load, with with DVC DVC and DTCDTC implementation. implementation.

Figure 13. Trolleybus total energy consumption during acceleration from zero to a final speed, as a Figure 13.functionTrolleybus of the final total speed, energy for various consumption conditions of during passenger acceleration load (EMPTY, from FULL zero), with to D aVC final and speed, as a functionFigure D1 ofT3C. theTrolleybus implementation. final speed, total energy for various consumption conditions during of passengeracceleration load from (EMPTY, zero to a FULL),final speed, with as DVCa and DTCfunction implementation. of the final speed, for various conditions of passenger load (EMPTY, FULL), with DVC and Figure 13 suggests that during acceleration from zero to final speed, lower total consumption is DTC implementation. observed with DTC. Figure 13Figure suggests 14 depicts that the during trolleybus acceleration total energy fromconsumption zero to during final acceleration speed, lower at intermediate total consumption is Figure 13 suggests that during acceleration from zero to final speed, lower total consumption is observedspeeds, with on DTC. 0% road inclination, in case of maximum (FULL) and minimum (EMPTY) passenger load, observedwith with DVC DTC and .DTC implementation. FigureFigure 14 14 depicts depicts thethe trolleybustrolleybus total total energy energy consumption consumption during during acceleration acceleration at intermediate at intermediate speeds,speeds, on on 0% 0% road road inclination,inclination, in in case case of of maximum maximum (FULL) (FULL) and andminimum minimum (EMPTY) (EMPTY) passenger passenger load, load, withwith DVC DVC and and DTC DTC implementation.implementation.

Figure 14. Trolleybus total energy consumption during acceleration from an initial to a final speed, as a function of speed, for various conditions of passenger load (EMPTY, FULL), with DVC and DTC implementation.

From Figure 14, it seems that during acceleration intervals from an initial to a final speed, lower total consumption is observed with DTC, whereas the lower the machine slip in DVC, the lower the Figure 14. Trolleybus total energy consumption during acceleration from an initial to a final speed, Figuretotal 14. energyTrolleybus consumption total becomes energy consumption. during acceleration from an initial to a ﬁnal speed, as a function of speed, for various conditions of passenger load (EMPTY, FULL), with DVC and DTC as a function of speed, for various conditions of passenger load (EMPTY, FULL), with DVC and implementation. DTC implementation.

From Figure 14, it seems that during acceleration intervals from an initial to a final speed, lower totalFrom consumption Figure 14 is, itobserved seems thatwith during DTC, wh accelerationereas the lower intervals the machine from anslip initial in DVC, to athe final lower speed, the lower totaltotal consumption energy consumption is observed becomes with. DTC, whereas the lower the machine slip in DVC, the lower the total energy consumption becomes. All cases of acceleration under study suggest that, in the specific machine operation point (that is, 1708.1 Nm), DTC proves more effective, contrary to DVC. This is due to the fact that DTC is more efficient at high-torque transient operating conditions [14].

4.4. Energy Recovery Estimations—Deceleration Figure 15 depicts the energy recovery potential during deceleration from 65 km/h to a ﬁnal vehicle speed, as a function of vehicle speed, on 0% road inclination, in case of maximum (FULL) and minimum (EMPTY) passenger load, with DVC and DTC implementation. Resources 2018, 7, x FOR PEER REVIEW 12 of 19

All cases of acceleration under study suggest that, in the specific machine operation point (that Resources 2018, 7, x FOR PEER REVIEW 12 of 19 is, 1708.1 Nm), DTC proves more effective, contrary to DVC. This is due to the fact that DTC is more efficientAll cases at highof acceleration-torque transient under operatingstudy suggest conditions that, in [ 14the]. specific machine operation point (that is, 1708.1 Nm), DTC proves more effective, contrary to DVC. This is due to the fact that DTC is more 4.4. Energy Recovery Estimations—Deceleration efficient at high-torque transient operating conditions [14]. Figure 15 depicts the energy recovery potential during deceleration from 65 km/h to a final Resources4.4.vehicle Energy2018, 7speed, Recovery, 43 as Estimationsa function of— vehicleDeceleration speed, on 0% road inclination, in case of maximum (FULL) and11 of 18 minimumFigure 15 (EMPTY) depicts passenger the energy load, recovery with DVCpotential and duringDTC implementation. deceleration from 65 km/h to a final vehicle speed, as a function of vehicle speed, on 0% road inclination, in case of maximum (FULL) and minimum (EMPTY) passenger load, with DVC and DTC implementation.

Figure 15. Energy consumed on braking resistance (energy recovery potential) during deceleration Figure 15. Energy consumed on braking resistance (energy recovery potential) during deceleration from maximum speed (65 km/h) to different speeds, as a function of the final speed, for different fromconditions maximum of speed passenger (65 load km/h) (EMPTY, to different FULL), speeds,with DVC as and a function DTC implementation. of the final speed, for different Figure 15. Energy consumed on braking resistance (energy recovery potential) during deceleration conditions of passenger load (EMPTY, FULL), with DVC and DTC implementation. from maximum speed (65 km/h) to different speeds, as a function of the final speed, for different From Figure 15, it seems that during deceleration from 65 km/h to final speed, higher energy conditions of passenger load (EMPTY, FULL), with DVC and DTC implementation. Fromrecovery Figure is observed 15, it seemswith DTC. that This during result deceleration indicates that fromat the 65maximum km/h todeceleration final speed, working higher point energy (that is, −1480.6 Nm), DTC is proven to be more efficient than DVC. This is in agreement with the recoveryFrom is observed Figure 15, with it seems DTC. that This during result deceleration indicates that from at 65 the km/h maximum to final decelerationspeed, higher workingenergy point results in Section 4.3, where high-torque transient conditions are also present. Moreover, Figure 16 (thatrecovery is, −1480.6 is observed Nm), with DTC DTC. is provenThis result to indicates be more that efficient at the maximum than DVC. deceleration This is in working agreement point with the depicts the energy recovery potential during deceleration from an initial to a final speed, on 0% road results(that inis, Section−1480.6 Nm),4.3, whereDTC is high-torque proven to be transientmore efficient conditions than DVC. are This also is present. in agreement Moreover, with the Figure 16 resultsinclination, in Section in case4.3, ofwhere maximum high-torque (FULL) transient and minimum conditions (EMPTY) are also passenger present. load, Moreover, with DVC Figure and 16 DTC depicts the energy recovery potential during deceleration from an initial to a final speed, on 0% road depictsimplementation. the energy recovery potential during deceleration from an initial to a final speed, on 0% road inclination,inclination, in in casecase of of maximum maximum (FULL) (FULL) and andminimum minimum (EMPTY) (EMPTY) passenger passenger load, with load,DVC and with DTC DVC and DTCimplementation. implementation.

Figure 16. Energy consumed on braking resistance (energy recovery potential) during deceleration from an initial to a final speed, for various conditions of passenger load (EMPTY, FULL), with DVC and DTC implementation. Figure 16. Energy consumed on braking resistance (energy recovery potential) during deceleration Figure 16. Energy consumed on braking resistance (energy recovery potential) during deceleration from an initial to a final speed, for various conditions of passenger load (EMPTY, FULL), with DVC from an initial to a ﬁnal speed, for various conditions of passenger load (EMPTY, FULL), with DVC and DTC implementation. and DTC implementation.

Similar to Sections 4.3 and 4.4, higher energy recovery is observed with DTC during deceleration at intermediate speeds. Moreover, the higher energy recovery is observed in the region of high speeds, because of the higher kinetic energy of the system. According to the results in Figures 15 and 16 and the technical data in Table 1, the energy recovery efficiency in all cases of speed reduction ranges from roughly 66% to 73%. These results are in accordance with results found in literature regarding energy recovery efficiency of public means of electrical transportation (trains and metros) [24–26], which reinforces the reliability of the proposed modeling. Resources 2018, 7, 43 12 of 18 4.5. Energy Recovery Estimations—Downhill 4.5. EnergyFigure Recovery 17 depicts Estimations—Downhill the energy recovery potential as a function of vehicle speed, in the case of maximum (FULL) and minimum (EMPTY) passenger load, during constant speed course on road withFigure −1% 17 and depicts −5% road the inclination, energy recovery with DVC potential and DTC as implementation. a function of vehicle speed, in the case of maximumIt (FULL)is noted andthat minimumthe energy recovery (EMPTY) for passenger the empty load, trolley duringbus course constant on a speeddownhill course slope on of road−1% with −1%is and negligible,−5% road thus inclination,this case is not with mentioned DVC and in Figure DTC implementation. 17.

FigureFigure 17. Energy17. Energy consumed consumed on on braking braking resistanceresistance (energy (energy recovery recovery potential) potential) as a asfunction a function of speed, of speed, for various conditions of passenger load (EMPTY, FULL) and road inclination (−1%, −5%), with DVC for various conditions of passenger load (EMPTY, FULL) and road inclination (−1%, −5%), with DVC and DTC implementation. and DTC implementation. Figure 17 suggests that, during downhill courses, in the case of −1% road inclination, higher energyIt is noted recovery that theis observed energy recoverywith DVC. for Howe the emptyver, in trolleybusthe case of course−5% road on ainclination, downhill slopeover 30 of–35− 1% is negligible,km/h, thushigher this energy case is recovery not mentioned is observed in Figure with DTC.17. Those results are also justified by the correspondingFigure 17 suggests torque that,conditions. during Once downhill more, passen courses,ger inload the has case insignificant of −1% road effect inclination, on trolleybus higher energyenergy recovery behavior. is observed with DVC. However, in the case of −5% road inclination, over 30–35 km/h, higher energy recovery is observed with DTC. Those results are also justified by the corresponding 4.6. Energy Consumption/Recovery Overall Conclusions torque conditions. Once more, passenger load has insignificant effect on trolleybus energy behavior. In summary, the most important conclusions drawn from the energy consumption estimations 4.6. Energyare the Consumption/Recoveryfollowing: Overall Conclusions •In summary,In constant the-speed most course importants, energy conclusions consumption drawn is significantly from the energy increased consumption by increasing estimations road inclination, whereas the increase in the trolleybus passenger load, for the same road inclination, are the following: has limited effects. Also, the greater the downhill inclination, the greater the regenerated energy • In constant-speedamount. courses, energy consumption is significantly increased by increasing road •inclination, Correspondingly, whereas the the energy increase consumption in the trolleybus allocation passenger (see Figure load, 7) is significantly for the same influenced road inclination, by road inclination, whereas the trolleybus passenger load has limited effects on it. Also, it appears has limited effects. Also, the greater the downhill inclination, the greater the regenerated that as speed increases and load torque approaches the machine nominal value (1355 Nm), the energy amount. electrical losses of the machine become less relative to the mechanical ones. Therefore, the system • Correspondingly,performance is thehigher energy in this consumption operating range. allocation (see Figure7) is significantly influenced by road inclination, whereas the trolleybus passenger load has limited effects on it. Also, it appears that as speed increases and load torque approaches the machine nominal value (1355 Nm),

the electrical losses of the machine become less relative to the mechanical ones. Therefore, the system performance is higher in this operating range. • During constant speed courses, in the case of zero road inclination, the system mechanical load (for both empty and full trolleybus) ranges from about 10 Nm (0.66% of the nominal torque value) to 40 Nm (3% of the nominal load). These values are much lower than the machine nominal torque value (1355 Nm). In the case of 1% road inclination, the machine mechanical load ranges from about 130 Nm (9.6% of the nominal torque value) to 212 Nm (15.6% of the nominal torque value), whereas for 5% road inclination, the machine load ranges from about 619 Nm (45.62% of the nominal torque value) to 895 Nm (65.97% of the nominal torque value). This wide load torque deviation justiﬁes the fact that DVC and DTC are more effective under different operating conditions. Resources 2018, 7, 43 13 of 18

• During deceleration at intermediate speeds, a greater amount of energy recovery is achieved in the area of higher initial speeds (i.e., 40, 50, 60 km/h), as a result of the higher kinetic energy of the electromechanical system. • For constant-speed courses at low speeds (i.e., at 10 km/h), trolleybus consumption is greater than the energy consumption at higher speeds. This is due to the high electromagnetic flux and the small amplitude modulation ratio of the inverter. • During accelerations with the maximum permitted rate—it corresponds to a torque equal to 125% of the machine nominal torque value—DTC seems to outperform DVC in terms of energy consumption (as it has been already discussed). • Similarly, during decelerations with the maximum permitted rate—it corresponds to a braking torque equal to −109% of the machine nominal torque value—DTC seems to outperform DVC in terms of the brake energy recovery amount. • Finally, it appears that there is a possibility of recovering a significant amount of energy both during decelerations of the trolleybus, as well as during its downhill courses. In the latter case, the energy recovery is higher than in the first one. However, the first case is more frequent on actual trolleybus runs.

These results suggest that each driving method (DVC and DTC) is more efﬁcient—from an energy consumption point of view—at different machine operating points, for the given control parameters (DVC error controller gains, DTC hysteresis loop bands). Considering the energy consumption estimations, it seems that a diagram of machine working points (as a combination of machine speed and torque values) can be deﬁned, for the various trolleybus drive scenarios under study. In this context, Figure 18 depicts the machine operating range map, where the area of DTC outperformance (lower energy consumption) is marked in blue color, whereas the pink colored area stands for the DVC outperformance. In the hatched area, the performance of these two driving methods is almost Resourcesthe same.2018, 7, x FOR PEER REVIEW 15 of 19

Tm (Nm) Fluxnom Flux

DTC

64%

Energy Energy (kWh/km)

Consumption DVC

11% 69% 100% 138% 155% 172% 224% ω (rad/sec) -13% m

-44%

-59%

Energy Energy

Recovery Recovery (kWh/km)

-109%

FigureFigure 18. 18.OperatingOperating area area of of the the tro trolleybuslleybus asynchronous machine. machine. In In the the blue blue area, area DTC, DTC is more is more efficient,efﬁcient, whereas whereas in in the the pink pink area,area, DVC DVC is moreis more efﬁcient. efficient. In the In hatched the hatched area, the area, performance the performance of these of thesetwo two driving driving methods methods is almost is almost the same. the same.

Last, but not least, the optimum operating mode map in Figure 18 can be exploited by the owner company of the trolleybuses in a future upgrade of the driving scheme.

5. Energy Consumption/Saving Potential Estimations for a Typical Trolley Course Considering the developed energy consumption profile (based on exhaustive simulations) of the trolleybus, we may proceed with calculations regarding the total energy consumption of the vehicle and the corresponding energy recovery potential for a typical course scenario, including all the previously-mentioned road/travelling conditions. Figure 19 depicts the course under study.

Deceleration Acceleration (V V =30/20, 500m (V V =20/30, initial/ final (Speed, 20, initial/ final 40/30, 50/40 km/h) 30/40, 40/50 km/h) 30, 40, 50 0% km/h) Acceleration

(Vintial= 0, Vfinal= 20, 30, 40, 50 km/h)

Figure 19. Diagram of considered trolleybus course, including road/travelling conditions under study.

The travelling conditions of the course in Figure 19 are listed in Table 2. According to Figure 19 and Table 2, the total distance of the course is 14 km. In addition, Table 1 includes two additional scenarios of the route described in Figure 19. Table 3 summarizes the energy consumption results for those three alternative scenarios, as well as the energy recovery potential as a percentage of the consumed energy.

Resources 2018, 7, x FOR PEER REVIEW 15 of 19

Tm (Nm) Fluxnom Flux

DTC

64%

Energy Energy (kWh/km)

Consumption DVC

11% 69% 100% 138% 155% 172% 224% ω (rad/sec) -13% m

-44%

-59%

Energy Energy

Recovery Recovery (kWh/km)

-109%

Figure 18. Operating area of the trolleybus asynchronous machine. In the blue area, DTC is more efficient, whereas in the pink area, DVC is more efficient. In the hatched area, the performance of Resourcesthese2018 two, 7, driving 43 methods is almost the same. 14 of 18

Last, but not least, the optimum operating mode map in Figure 18 can be exploited by the owner Last, but not least, the optimum operating mode map in Figure 18 can be exploited by the owner company of the trolleybuses in a future upgrade of the driving scheme. company of the trolleybuses in a future upgrade of the driving scheme.

5. Energy Consumption Consumption/Saving/Saving Potential Estimations for a Typical TrolleyTrolley CourseCourse CConsideringonsidering the the developed developed energy energy consumption consumption profile proﬁle (based (based on onexhaustive exhaustive simulations) simulations) of the of trolleybusthe trolleybus,, we may we mayproceed proceed with calculation with calculationss regarding regarding the total the energy total energy consumption consumption of the vehicle of the vehicleand the and corresponding the corresponding energy energy recovery recovery potential potential for fora typical a typical course course scenario scenario,, including including all all the previously-mentionedpreviously-mentioned road/travelling conditions. conditions. Figure Figure 1199 depictsdepicts the the course course under under study study..

Deceleration Acceleration (V V =30/20, 500m (V V =20/30, initial/ final (Speed, 20, initial/ final 40/30, 50/40 km/h) 30/40, 40/50 km/h) 30, 40, 50 0% km/h) Acceleration

(Vintial= 0, Vfinal= 20, 30, 40, 50 km/h)

FigFigureure 19.19. DiagramDiagram of of considered considered trolleybus trolleybus course, course, including including road/travelling road/travelling conditions conditions under under study. study. The travelling conditions of the course in Figure 19 are listed in Table2. According to Figure 19 and TableThe travel2, theling total conditions distance of the course course in is Figure 14 km. 1 In9 are addition, listed in Table Table1 includes2. According two to additional Figure 19 andscenarios Table of 2,the the route total describeddistance of in the Figure course 19. is 14 km. In addition, Table 1 includes two additional scenarios of the route described in Figure 19.

Table 2.3 summarizesEnergy consumption the energy of the consumption trolleybus (considering results for maximum those three passenger alternative load, M scenarios,v = 20,000 kg)as well as thein energy assumed recovery routes, under potential low andas a heavy percentage trafﬁc congestion.of the consumed Scenarios energy. 1, 2, and 3.

Initial/Final Road Inclination (%)/Scenario Distance Times of Traveled Distance Type of Course Speed (km/h) 1/2/3 (km) (Light/Heavy Trafﬁc)

0/20 20/60 0/30 40/120 0/40 20/60 Acceleration 0/50 0/0/0 ignored 20/60 20/30 20/60 30/40 40/120 40/50 20/60 30/20 20/60 Deceleration 40/30 0/0/0 ignored 40/120 50/40 20/60 20, 30, 40, 50 0/0/0 0.5 1 Constant Speed 20, 30, 40 1, 5, −1, −5/0, 1, −1, 0/0, 0, 0, 0 0.1 10

Table3 summarizes the energy consumption results for those three alternative scenarios, as well as the energy recovery potential as a percentage of the consumed energy. As shown in Table3, energy consumption with the implementation of DTC, in both trafﬁc cases, is slightly lower, whereas the energy recovery potential as a percentage of energy consumption is slightly higher, compared with DVC. However, the deviation of the results is small. In Scenario 2, where high road inclinations (5%, −5%) are omitted, energy consumption appears to be signiﬁcantly lower than in Scenario 1 and the percentage of the energy recovery potential is higher. In addition, it appears that energy consumption in both scenarios is not particularly affected by the driving method. Resources 2018, 7, 43 15 of 18

In Scenario 3, where road inclinations are completely omitted, it appears that in both trafﬁc conditions, with DTC implementation, energy consumption is slightly lower and the percentage of the energy recovery potential is higher. Compared with Scenarios 1 and 2, energy consumption in Scenario 3 is lower (as there are no road inclinations).

Table 3. Energy consumption/recovery potential of the trolleybus (considering maximum passenger load, Mv = 20,000 kg) in assumed routes, under low and heavy trafﬁc congestion. Scenarios 1, 2, and 3. DVC—direct vector control; DTC—direct torque control.

Energy Consumption (kWh/km)/Recovery Potential (%) Control Method/Traffic Case Scenario 1 Scenario 2 Scenario 3 DVC, Light Traffic 2.01/8.5 1.01/9.45 0.69/14.3 DVC, Heavy Traffic 2.74/13.46 1.8/15.55 1.42/20.9 DTC, Light Traffic 1.97/8.8 1.1/9.6 0.67/18.9 DTC, Heavy Traffic 2.69/14.2 1.82/17.22 1.39/24

Considering the results of energy consumption/saving potential calculations in Table2, it seems that a significant percentage of the braking energy can be recovered, that is, up to 24% of daily energy consumption (particularly under heavy traffic conditions, where the number of decelerations increases significantly). These outcomes are in line with results regarding regenerative energy savings found in literature [27–29].

6. Proposed Hybrid Energy Storage Apparatus In this paragraph, an initial estimation of the sustainability of a proposed hybrid energy storage apparatus, which is an optimal solution incorporated in many public means of electrical transportation worldwideResources 2018 [, 247, x,30 FOR], isPEER attempted; REVIEW this is illustrated in Figure 20. 17 of 19

Trolley dc Bus A

Bidirectional Bidirectional Converter 1 Converter 2

48 VDC

SUPERCAPACITORS BATTERIES

Figure 20. Proposed hybrid energy storage apparatus; pointspoints A, B refer toto FigureFigure4 4..

The components of the proposed apparatus of Figure 20 are the following: The components of the proposed apparatus of Figure 20 are the following: • A supercapacitor bank of 0.2 kWh maximum storage capability, in order to support the energy • Arecovery supercapacitor during deceleration bank of 0.2 intervals. kWh maximum This unit storage will also capability, support inacceleration order to support intervals, the in energy order recoveryto limit the during power deceleration losses of intervals. the overhead This unit supply will network. also support Its dimensioning acceleration intervals, is based in on order the toresults limit of the Figure powers losses15 and of 16. the overhead supply network. Its dimensioning is based on the results • ofA battery Figures bank15 and of 163.5. kWh maximum energy capability, in order to support the energy recovery • Aduring battery vehicle bank downhill of 3.5 kWh course maximums. Its dimensioning energy capability, is based in order on the to results support of theFigure energy 17. recovery • duringTwo bidirectional vehicle downhill DC/DC courses. converters, Its dimensioning one for each isenergy based stora on thege resultsbank. of Figure 17. As depicted in Figure 20, the assumed energy storage voltage level is 48 V and the selected battery technology is Li–ion, in order to have mature commercial solutions available [31]. According to these specifications and taking into account the current electric energy prices and the available commercial data, it turns out that the investment payoff is estimated at 30,000 km, which leads to a payback period of less than two years. This result highlights the sustainability potential of the proposed energy recovery scheme.

7. Conclusions In this article, the behavioral model of 8000-series of Athens trolleybuses is developed, with the use of data provided by OSY S.A., in order to achieve a reliable estimation of their energy consumption. At the same time, the amount of kinetic energy stored in the rotating/moving parts of the vehicle is estimated; the feasibility of saving this energy, by means of regenerative braking, is considered. Furthermore, the employed driving methods are the existing DVC and the DTC (as an interesting alternative). In this context, an energy consumption functional model of the trolleybus has been developed, based on exhaustive simulations in Matlab/Simulink environment under various travelling and transport loading conditions (using both DVC and DTC). Next, this energy consumption model is utilized for the energy saving potential estimation of these trolleybuses, under various realistic operating scenarios. The results of this study lead to the conclusion that there is a significant margin of energy saving of Athens trolleybuses; up to 24% of daily energy consumption. Furthermore, a sustainable hybrid energy storage apparatus is proposed and estimations regarding its payoff period suggest a less than two year payback time interval. Finally, the proposed energy consumption modelling and energy saving estimation procedure is applicable for any trolleybus system of public transportation.

Resources 2018, 7, 43 16 of 18

• Two bidirectional DC/DC converters, one for each energy storage bank.

As depicted in Figure 20, the assumed energy storage voltage level is 48 V and the selected battery technology is Li–ion, in order to have mature commercial solutions available [31]. According to these speciﬁcations and taking into account the current electric energy prices and the available commercial data, it turns out that the investment payoff is estimated at 30,000 km, which leads to a payback period of less than two years. This result highlights the sustainability potential of the proposed energy recovery scheme.

7. Conclusions In this article, the behavioral model of 8000-series of Athens trolleybuses is developed, with the use of data provided by OSY S.A., in order to achieve a reliable estimation of their energy consumption. At the same time, the amount of kinetic energy stored in the rotating/moving parts of the vehicle is estimated; the feasibility of saving this energy, by means of regenerative braking, is considered. Furthermore, the employed driving methods are the existing DVC and the DTC (as an interesting alternative). In this context, an energy consumption functional model of the trolleybus has been developed, based on exhaustive simulations in Matlab/Simulink environment under various travelling and transport loading conditions (using both DVC and DTC). Next, this energy consumption model is utilized for the energy saving potential estimation of these trolleybuses, under various realistic operating scenarios. The results of this study lead to the conclusion that there is a signiﬁcant margin of energy saving of Athens trolleybuses; up to 24% of daily energy consumption. Furthermore, a sustainable hybrid energy storage apparatus is proposed and estimations regarding its payoff period suggest a less than two year payback time interval. Finally, the proposed energy consumption modelling and energy saving estimation procedure is applicable for any trolleybus system of public transportation.

Author Contributions: Conceptualization, N.A. and N.P.; Methodology, N.P.; Software, N.A.; Validation, N.P. and N.A.; Formal Analysis, N.P.; Investigation, N.A.; Resources, N.A.; Data Curation, N.A.; Writing-Original Draft Preparation, N.A.; Writing-Review & Editing, N.P.; Visualization, N.A.; Supervision, N.P.; Project Administration, N.P.; Funding Acquisition, N.P. Funding: This research received no external funding. Acknowledgments: Authors would like to thank OSY S.A. and especially Ellias Kollias and Paraskevas Kampourakis for their support and for providing the necessary data regarding the 8000-series of Athens Trolleybuses. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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