energy recovery and emission cutting in a mobile gantry … energy using... ·...

1077-2618/08/$25.00©2008 IEEE

Energy recovery and emission cutting in a mobile gantry crane

RUBBER-TIRED GANTRY (RTG) CRANES

are commonly used in shipping ports

around the world to move containers mass-

ing up to 40 metric

tons. These cranes are mobile and

derive their electrical power require-

ments for the hoist motor from a die-

sel engine and generator set rather

than from the utility system. Because these cranes are

independent of the utility system, energy regenerated via

the hoist motor as a container is lowered to the ground

and is typically wasted as heat in dissipator resistors.

In large coastal shipping ports, thousands of diesel

engines are operated within ships,

trains, trucks, and cranes. Although

these engines are the workhorse of

the industry, their exhaust is a

known pollutant that can cause can-

cer and other diseases. Around the vicinity of the ports of

Los Angeles and Long Beach, California, in particular, the

diesel exhaust is associated with 70% of pollution-related

health problems; moreover, diseases caused by the portDigital Object Identifier 10.1109/MIAS.2008.929351

BY MARK M. FLYNN,PATRICK MCMULLEN,

& OCTAVIO SOLIS

© ARTVILLE

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Authorized licensed use limited to: University of Texas at Austin. Downloaded on November 10, 2008 at 10:12 from IEEE Xplore. Restrictions apply.

pollution are responsible for hundredsof deaths annually in this area [1].Because of the problem, these twoports—the busiest in the UnitedStates—plan to reduce diesel emis-sions by 50% over the next five years.

Container cranes, such as the RTGcrane shown in Figure 1, are majorcontributors of port-based diesel emis-sions. These cranes employ conven-tional power trains consisting of adiesel engine coupled to an alternatorthat provides electrical power for a setof hoist, trolley, and gantry motors.The diesel engine prime mover allowsan RTG crane to be unencumbered by a utility mains con-nection as it moves in the shipyard.

When a shipping container is lifted by a conventionalRTG crane, the diesel engine provides the energydemanded by the hoist motor. When the container is low-ered, the container’s potential energy is converted by thehoist motor into electrical form, but the conventional

drive system has no means to store thisregenerated energy. Consequently, thisenergy is typically dissipated as heat inresistor banks, resulting in a reducedoverall system efficiency and increasedfuel consumption and emissions.

This article describes the results ofan experimental test during which theconventional power train of an RTGcrane was converted to a hybrid versionusing a pair of high-speed flywheels forenergy storage. High-speed flywheelsare ideal energy storage devices for usewith RTG cranes, as they are able toboth source and absorb large amounts

of power at the high cycle rates demanded by the hoistmotor. They have been used with much success in otherdemanding high-cycle environments when long life isrequired [2], [3]. This article will provide 1) an overview ofan RTG crane, 2) its electrical system performance whenlifting and lowering loads without flywheels, 3) an over-view of the flywheel and its associated motor drive, and4) the improvement in the operation of the crane whenflywheels are employed.

Conventional RTG Crane Overview

MechanicalRTG cranes such as the one shown in Figure 1 are knownas one-over-four RTG cranes because they are able to raiseone shipping container above a stack of four containershigh. The crane grabs hold of a container with a devicecalled a spreader, which has a mass of 10 t. The crane canlift a maximum shipping container mass of 40.6 t yieldinga total lift load of 50.6 t.

Using hoist, trolley, and gantry motors, RTG cranescan move containers in three degrees of freedom. The hoistmotor is used to raise and lower the container, trolleymotors move the container from one side of the crane tothe other, and gantry motors are used to reposition the

entire crane. This articlefocuses on the testing con-ducted with the hoist motoronly, because the trolley andgantry motors afford lesserregeneration opportunities.

ElectricalThe simplified power trainfor the crane tested is shownin Figure 2. The 455-kWrated diesel engine turns a500-kVA, 460-VLL,rms, three-phase alternator. The outputof the alternator is connectedto two independent hoistmotor drives, each of whichpowers an isolated three-phase winding in the hoistmotor. Each motor drive con-nects to the alternator via athree-phase, full-bridge, passive

2

AlternatorDieselEngine

InverterRectifier

Hoist Motor Drive 1

InverterRectifier

Hoist Motor Drive 2

dc Bus

dc Bus

ControlledResistor

HoistMotor

ControlledResistor

RTG crane hoist motor drive.

1RTG crane.

HIGH-SPEEDFLYWHEELS AREIDEAL ENERGY

STORAGEDEVICES FOR USE

WITH RTGCRANES.

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diode rectifier producing a nominal dc bus voltage of650 V. The maximum hoist motor power (measured at thedc bus) is limited by the motor drives to approximately300 kW (150 kW per motor drive).

During a regeneration event (i.e., when a container islowered), the voltage at each dc bus rises significantlycommensurate with the limited dc bus capacitance. Whenthe voltage on a particular bus equals 730 V, the respectivechopper-controlled resistor bank is activated to dissipatethe regenerated energy.

Crane Testing MethodDuring the operation at the port, crane operators attempt tomaintain a pace of moving one container per minute. A typi-cal operating scenario consists of the crane loading the con-tainers onto the awaiting trucks as shown in Figure 3. Insuch a case, the empty spreader is lowered to a stack of con-tainers that can be anywhere from one to four containershigh. The target container is acquired and lifted out of thestack. The crane trolleys the container to a position above thetruck and then lowers the container onto the truck. The cycleis completed when the empty spreader is raised away fromthe truck and is then trolleyed back to the container stack.

In the above scenario, the empty spreader is raised andlowered once; likewise, the combined spreader and con-tainer load is raised and lowered once. There are also twotrolley operations, one with an empty spreader and onewith a combined spreader and container load. From theseobservations, a testing method was devised that simulatesthe typical operating scenario and allows for the evaluationof the crane’s fuel consumption and emissions output bothwith and without the flywheel energy storage. The testmethod consists of raising and lowering the empty spreaderonce a minute for 30 min followed by raising and loweringa combined spreader and container load once a minute for30 min. Thus, at the end of a standard one-hour test, 60cycles are performed. All auxiliary loads such as the exteriorlighting and the operator car’s air-conditioning system werein use during the test. The trolley events were neglectedbecause the energy consumed and regenerated during thistime is small compared with that of the hoist motor. The testmethod is summarized in Table 1.

In practice, the target container for a given move can belocated at a variable height within the four-container-tallstack. The maximum fuel consumption and emissions occurwhen the target container is located on the ground levelbetween the full-height container stacks as shown in Fig-ure 3. These worst-case conditions were simulated duringtesting by raising and lowering the hoist to the height offour containers, which is approximately 10.6 m.

According to port authorities, the typical shippingcontainer has a mass of about 15 t. Thus, a container witha similar mass of 15.3 t was selected for use during testing.Throughout the testing, the fuel consumption and emis-sions were measured. In addition, the electrical systemperformance was recorded at the dc bus of each hoist motordrive; these locations correspond to the connection pointsfor the flywheel energy storage system described below.

Crane Operation Without Flywheels

Lifting EventDuring the first phase of testing, the baseline fuel con-sumption and emissions output from the diesel enginewere determined. The crane was operated according to theone-hour test described earlier without the flywheelenergy storage. Figures 4 and 5 show the dc bus powersupplied to each hoist motor drive during an emptyspreader lift and 15.3-t container lift, respectively. Whenthe hoist power is ramping toward its peak, the hoist isaccelerated; once the desired lifting speed has beenachieved, the hoist power demand is reduced and the loadis lifted at a constant speed with constant power.

3

T

Truck

SpreaderOperator Car

Hoist Motor

TargetContainer

T

T

(a) (b) (c)

Typical crane operating scenario: (a) Target container to be acquired with spreader. (b) Trolley container to awaitingtruck. (c) Lower container onto truck and release spreader.

TABLE 1. CRANE OPERATIONS PERFORMED DURINGSTANDARD ONE-HOUR FUEL CONSUMPTION ANDEMISSIONS TEST.

Raise Lower

Empty spreader operations 30 30

Combined spreader andcontainer load operations

30 30

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At the beginning and end of thelifts, short-duration power fluctuationscan be observed. The fluctuations,although not desirable, are the signa-ture of the human operator at the con-trols of the crane and reflect one’spersonal style of manipulating the joy-stick controls. In addition, it is notedthat the two motor drives do not supplyexactly the same power. For instance,when the load is lifted under constantpower, Drive 1 supplies 59 and 131kW, respectively, and Drive 2 supplies54 and 119 kW, respectively (Fig-ures 4 and 5). The mismatch in eithercase is the same; specifically, Drive 1supplies 10% more power than Drive 2.The variance can be attributed tomotor drive controller settings and tolerances as well asslight mismatches between the two motor windings.However, neither of the above two anomalies is a causefor concern during the fuel consumption and emissions

test, because the hoist motor drivesperform similarly with or withoutthe flywheels and the same humanoperator is used for both portions ofthe test.

Lowering EventThe conventional RTG crane has noability to store the energy regeneratedwhen the hoist is lowered. This energyis thus dissipated as heat by the resis-tors shown in Figure 2. Figure 6 showsthe dc bus voltage of Drive 1 and thefiltered dc bus power regenerated fromboth motor drives during the loweringof the empty spreader. The waveformsare similar for the case when the con-tainer is lowered. During the constant

power portion of the waveforms, the hoist is lowered at aconstant speed. To stop the descent of the hoist, the powerregeneration from the motor drives is increased, therebyextracting the kinetic energy from the spreader and themechanical hoisting components as can be observed duringthe interval from 16 to 18.5 s. Note that while the hoist isbeing lowered and the resistor is dissipating power, thevoltage oscillates between 730 and 750 V as the chopperswitches the resistor into and out of the circuit. In addi-tion, the motor drive power mismatch during lowering isreversed when compared with the lifting case.

RTG Crane EfficiencyThe aggregate efficiency of the crane’s hoist system con-sisting of the mechanical hoisting components, the hoistmotor, and the hoist motor drives directly affects the por-tion of the load’s potential energy, which can be capturedby the flywheels and made available for reuse. It is thus amajor factor in determining the maximum possible fueland emissions savings. The hoist system’s efficiency ispresented below as calculated from the manufacturer’s dataand as measured during experimental testing.

1) Calculated efficiency: The efficiency of the hoist systemcan be computed using the hoisting speed providedby the manufacturer. For example, the manufacturer

5

0 5 10 15 20 250

50

100

150

200

250

300

Time (s)

Pow

er (

kW)

Drive 1

Drive 2

Total

DC bus hoist motor drive power during container lift.

6

0 5 10 15 20 250

25

50

75

100

125

150

175

200

Pow

er (

kW)

Time (s)

Power, Drive 1

Power, Drive 2

Power, Total

0

100

200

300

400

500

600

700

800

Vol

tage

(V

)

Voltage

Motor drive power and voltage during lowering of the spreader.

THECONVENTIONAL

RTG CRANECANNOT STORE

THE ENERGYREGENERATED

WHEN THE HOISTIS LOWERED.

4

0

50

100

150

200

Pow

er (

kW)

Drive 1

Drive 2

Total

0 5 10 15 20 25

Time (s)

DC bus hoist motor drive power during spreader lift.

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states that the RTG crane hoists a 40.6-t container(either up or down) at a rate of 0.383 m/s. If the con-tainer is being lifted, the power, PL, supplied to thecombined spreader and container load is

PL ¼ mgv

¼ 50, 600 kg 3 9:81 m=s2

3 0:383 m=s

¼ 190 kW, (1)

where m is the total load mass, g is the accelerationdue to gravity, and v is the hoisting speed.

After the acceleration phase is complete, loadsmassing more than 22 t are hoisted at a constantspeed dictated by the maximum continuous combineddc bus power (Pdc-max) of 250 kW. (The 250-kWcontinuous power limit can be observed in Figure 5for the case of a 25.3-t load lift.) Therefore, the one-way efficiency (g) of the hoist system when lifting a50.6-t load can be computed as

gone-way, lift ¼PL

Pdc-max

3 100%

¼ 190 kW

250 kW3 100%

¼ 76:0%: (2)

The manufacturer’s data indicate that the hoist-ing speed for a spreader-only load is 0.866 m/s.Using (1) with a 10-t spreader load, PL is foundto be 85.0 kW. The total dc bus power requiredduring the constant speed portion of the lift isfound from Figure 4 to be 113 kW. Thus, theone-way lifting efficiency is computed as

gone-way, lift ¼85:0 kW

113 kW3 100%

¼ 75:2%: (3)

It is possible to compute efficiency values forevery load mass in between the points given andalso for lowering events if desired. However, hereit is simply concluded that the hoist system effi-ciency is approximately constant over the loadrange. To confirm the above calculations, themeasured efficiency data are presented below.

2) Measured efficiency: By comparing Figures 4 and 6,an aggregate two-way efficiency estimate for lift-ing and lowering operations can be computed forthe hoist system. Specifically, from Figure 4, it wasobserved that an average total power of 113 kWwas required to lift the empty spreader at a con-stant speed of 0.866 m/s. The total power receivedduring regeneration when the spreader was loweredat the same speed was 61 kW. The efficiency isthus calculated as

gtwo-way ¼61 kW

113 kW3 100% ¼ 53:9%, (4)

gone-way ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigtwo-way

p ¼ 73:5%: (5)

The one-way efficiency obtained in (5) represents an aver-age value of the one-way lifting and lowering efficiencieswith a spreader load. It is lower than the value obtained in(3) because of the presence of the less efficient lowering eventin the average.

Figure 7 presents the measured one-way efficienciescomputed using the above method for a range of hoistloads. Each measurement point in the figure representsthe average of several measurements performed for therespective load. The measured efficiencies range from73.9–78.7%, with a mean value of 76%. While the linearfit suggests a modest efficiency improvement withincreasing load, the measured efficiencies for containermasses greater than 13 t lie in a narrow band that can beapproximated as constant. Of chief importance is thatbecause of the 76% mean hoist system efficiency, nearlyone-fourth of a given container’s potential energy cannotbe recovered as it is lowered. However, despite this disad-vantage, it is shown in the following sections that the fly-wheel storage system remains capable of greatly reducingthe fuel consumption and emissions output.

Overview of Flywheel and Motor DriveThe flywheel energy storage unit consists of a three-phasepermanent magnet synchronous motor-generator coupledto a steel flywheel on the same rotor. The rotor assembly islevitated on active magnetic bearings and spins in avacuum to minimize losses. A three-phase inverter func-tions as the flywheel’s motor drive; the drive and flywheelmotor are designed to transfer a maximum power of150 kW to and from the dc bus. Energy storage is 4.57 MJ(1.27 kWh) at a maximum speed of 36,000 rev/min.The mass of the unit is 320 kg yielding a specific energyof 14.2 kJ/kg. The flywheel unit is shown in Figure 8with a cutaway view of the rotor superimposed.

When flywheels are used with an RTG crane, two unitsare employed; a single unit provides isolated energy storageto an individual hoist motor drive as shown in Figure 9. Thetwo units are packaged together and installed underneath acrane support beam as shown in Figures 10 and 11. Totalenergy storage with two flywheels is 9.14 MJ, which is muchmore than the 5.26 MJ of potential energy released by a

7

0 5 10 15 20 2550

60

70

80

90

100

Container Mass Under Spreader (t)

Effi

cien

cy (

%)

MeasurementFit

One-way hoist system efficiency versus container mass.73

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maximum hoist load of 50.6 t as it traverses 10.6 m. When aminimum operating speed of 10,000 rev/min is taken intoaccount, the total usable energy storage is 8.43 MJ.

The excess energy storage was reduced by decreasingthe maximum operating speed to 20,000 rev/min yieldinga usable energy storage of 2.12 MJ (0.59 kWh) for the fly-wheel pair. This decrease is advantageous as it results indecreased losses and simplified motor drive and magneticbearing control. The final maximum operating speed wasselected by considering the typical container mass mostoften encountered in operation and the efficiencies of thesystems involved. For instance, typical containers mass15 t, and relatively few mass more than 20 t. Thus theability to capture the energy regenerated from lowering a20-t container is a desirable goal.

When lowered 10.6 m, a 20-t container (30 t includingspreader) releases 3.12 MJ, which is more than the capacityof the flywheel pair. However, when the efficiency of theenergy flow path from the container to the flywheel mass isincluded, the storage capacity is sufficient. For example, the

hoist system’s mean one-way efficiency was found to be76%. The average one-way efficiency of the flywheel motorand its drive is 89.5%. Using these two values, the amountof energy that can be captured as a combined 30-t spreaderand container load descends 10.6 m is

3:12 MJ 3 0:760 3 0:895 ¼ 2:12 MJ: (6)

Other forms of energy storage for RTG cranes such assupercapacitors, have been reported in the literature [4].However, compared with supercapacitors, high-speed fly-wheels can offer higher efficiency, higher cycle life withoutdegradation, reduced ambient temperature concerns, andincreased reliability owing to the absence of the hundredsof series connections in supercapacitor banks [5].

Crane Operation with Flywheels

InitializationWhen an RTG crane is turned on at the beginning of awork shift, the diesel engine is started and allowed towarm up. The alternator is then connected to the hoist

10Two flywheel units arranged in a single package.

11Flywheels installed on RTG crane.

9

Inverter

FlywheelFlywheel andMotor Drive

InverterRectifier

Hoist Motor Drive1 or 2

dcBus

ControlledResistor

Flywheel interface to an individual dc bus.

8Flywheel unit with rotor superimposed.

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motor drives and the dc buses are ener-gized. If the speed of an individual fly-wheel is below 10,000 rev/min, therespective unit is motored to thisminimum operating limit where itremains until the first lowering event.If the speed of a flywheel is otherwiseabove 10,000 rev/min, it is allowed tocoast at its present speed until the firstlowering event described below.

Lowering EventWhen not in service, the spreader isstowed at the maximum lift height.Thus, the first hoist operation per-formed by a crane operator is a lower-ing event of the empty spreader. Asthe hoist is lowered, the resultantvoltage rise at each dc bus is used by the respective fly-wheel controllers to compute an appropriate powercommand for the flywheel motor drives to capture theregenerated energy.

Figure 12 shows the total power regenerated by thehoist motor drives to the dc bus when the 15.3-t containeris lowered; the regenerated power during the lowering ofthe empty spreader was shown in Figure 6. The speed ofeach flywheel increases during this time, as the potentialand kinetic energy of the load are absorbed. The mismatchin flywheel speeds reflects the power mismatch of the hoistmotor drives and is of no concern during operation. If thecontainer is more massive than 20 t, the flywheels will befully charged to 20,000 rev/min before the hoist descent iscompleted. In this event, the excess energy will be dissi-pated by the chopper-controlled resistors.

Lifting EventEnergy stored in the flywheels during a lowering event isavailable for reuse during a subsequent lift. Figure 13shows the hoist power provided by the diesel engine andthe flywheel units during the lift of a 15.3-t container.The corresponding flywheel speeds are provided in Figure14. The waveforms for the spreader lift are similar.

During a lift, the flywheel control-ler measures the engine power at thedc bus and is programmed to limit theengine power to 50 kW by supplyingthe power difference required by thehoist while it is accelerating. In thismanner, the peak power demand onthe diesel engine is reduced, which inturn reduces the engine wear, fuel con-sumption, and emissions output. Afterthe power peak, the hoist is lifted at aconstant speed. The flywheel control-ler then gently reduces the maximumflywheel output power to 60 kW perflywheel unit (120 kW per flywheelpair), allowing the engine to ramp upin power gradually and thus avoidheavy particulate emissions.

As the flywheels approach a fully discharged state of10,000 rev/min, the remaining hoist power demand istransitioned to the engine gradually (Figure 13). Becauseof the power mismatch between the hoist motor drives,

14

0 2 4 6 8 10 12 14 168

10

12

14

16

18

20

Flywheel 1

Flywheel 2

Time (s)

Spe

ed (

Tho

usan

ds, r

ev/m

in)

Flywheel speeds during container lift.

13

0 2 4 6 8 10 12 14 160

50

100

150

200

250

300

Engine

Flywheels

Hoist

Time (s)

Pow

er (

kW)

DC bus powers during container lift with flywheels.

12

0 5 10 15 200

20

40

60

80

100

120

140

160

180

200

Pow

er (

kW)

Time (s)

Power

10

11

12

13

14

15

16

17

18

19

20

Spe

ed (

Tho

usan

ds, r

ev/m

in)

Speed,Flywheel 1

Speed,Flywheel 2

Regeneration during lowering of the container withflywheels.

THE ABILITY TOCAPTURE THE

ENERGYREGENERATED

FROM LOWERINGA 20-T

CONTAINER IS ADESIRABLE GOAL.

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the flywheels are not likely to reach 10,000 rev/min at thesame instant. Therefore, the power transition can occur intwo steps as each flywheel is discharged.

Figure 15 illustrates the percentage of a container’spotential energy that can be captured by the flywheelsduring a lowering event and then returned to the con-tainer during a subsequent lifting event. As the figureshows, theoretically more than 46% of a container’s poten-tial energy can be reused; however, in practice, diesel sav-ings are somewhat lower because of additional systemlosses. For example, some activities, such as gantry andtrolley events and, most importantly, intervals when theengine is idling but the hoist is not in use, consume dieselfuel but do not provide significant opportunities forenergy regeneration. Power consumed by auxiliary systemssuch as air conditioning and lighting also increases fuelusage without the prospect of energy recovery.

Testing Results SummaryTable 2 summarizes the fuel and emissions results duringthe one-hour testing for the cases with and without theflywheel energy storage. Significant improvements weremade in all quantities measured. Although the nearly21% fuel consumption reduction is excellent, it is lowerthan the theoretical maximum as expected, which islargely because the engine was idling for 50% ofthe time during the one-hour test. Future enhancements

to the flywheel energystorage system are ex-pected to yield furtherimprovements.

ConclusionsThe flywheel system hasshown through experi-mental testing to greatlyreduce the fuel consump-tion and emissions output.In addition, the reducedpeak power demand fromthe diesel engine increasesthe engine life. Further-more, because of theenergy storage made avail-able by the flywheels, thediesel engine can be re-duced in size for greaterfuel and emissions savings.Initial field tests with areduced-size engine haveyielded fuel savings ofup to 35% and more. Asan added measure of reli-ability, if a single flywheelsystem were to becomeinoperable, the crane cancontinue operating withthe remaining flywheel. If

desired, this lone flywheel can be configured to perform atelevated power levels for a limited time until the servicecan be completed.

References[1] J. Wilson, ‘‘Long beach ports produce plan to reduce diesel emissions,’’

LA Times, B.1, Jun. 29, 2006.

[2] M. M. Flynn, J. J. Zierer, and R. C. Thompson, ‘‘Performance testing

of a vehicular flywheel energy system,’’ SAE 2005 Trans. J. PassengerCars—Mech. Syst., vol. 114, pp. 119–126, Feb. 2006.

[3] G. Reiner, ‘‘Current state in the application of magnetodynamic (fly-

wheel) storage systems in urban transport buses,’’ presented at the Fly-

wheel Storage Technology Workshop, Oak Ridge, TN, 1995.

[4] S. Kim and S. Sul, ‘‘Control of rubber tyred gantry crane with energy

storage based on supercapacitor bank,’’ IEEE Trans. Power Electron.,vol. 21, no. 5, pp. 1420–1427, Sept. 2006.

[5] S. R. Holm, H. Polinder, J. A. Ferreira, P. van Gelder, and R. Dill,

‘‘A comparison of energy storage technologies as energy buffer in

renewable energy sources with respect to power capability,’’ in Proc.IEEE Young Researchers Symp. Electrical Power Engineering, Leuven, Bel-

gium, 2002. [CD-ROM].

Mark M. Flynn (mm_ [email protected]) is with theCenter for Electromechanics, University of Texas, in Austin.Patrick McMullen and Octavio Solis are with VYCON, Inc.,Yorba Linda, California. This article first appeared as‘‘High-Speed Flywheel and Motor Drive Operation for EnergyRecovery in a Mobile Gantry Crane’’ at the 2007 IEEEApplied Power Electronics Conference.

TABLE 2. FUEL AND EMISSIONS RTG TEST RESULTSWITH AND WITHOUT FLYWHEELS.

NoFlywheels

WithFlywheels

%Reduction

Fuel consumption (L/h) 25.4 20.1 20.9

Nitrous oxides (kg/h) 0.386 0.286 25.9

Particulate emissions (kg/h) 0.0408 0.0136 66.7

15

68.0 %

76.0%

60.9% To Container

HoistMotor

Inverter

Flywheel and Motor Drive Hoist and Motor Drive

Inverter

dc Bus

Mechanics

From Container

46.3%

100%

Flywheel

Potential energy captured from and returned to the container.

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