Battery Hybrid

energies Article

System Design and Energy Management for a Fuel Cell/Battery Hybrid Forklift Zhiyu You 1 , Liwei Wang 1, *, Ying Han 2 1 2 3

*

and Firuz Zare 3

Key Laboratory of Electronic Information (Southwest Minzu University), State Ethnic Affairs Commission, Chengdu 610041, China; [email protected] School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, China; [email protected] Power and Energy Group, University of Queensland, Brisbane, QLD 4072, Australia; [email protected] Correspondence: [email protected]; Tel.: +86-28-8592-8384

Received: 12 November 2018; Accepted: 6 December 2018; Published: 8 December 2018







Abstract: Electric forklifts, dominantly powered by lead acid batteries, are widely used for material handling in factories, warehouses, and docks. The long charging time and short working time characteristics of the lead acid battery module results in the necessity of several battery modules to support one forklift. Compared with the cost and time consuming lead acid battery charging system, a fuel cell/battery hybrid power module could be more convenient for a forklift with fast hydrogen refueling and long working time. In this paper, based on the characteristics of a fuel cell and a battery, a prototype hybrid forklift with a fuel cell/battery hybrid power system is constructed, and its hardware and software are designed in detail. According to the power demand of driver cycles and the state of charge (SOC) of battery, an energy management strategy based on load current following for the hybrid forklift is proposed to improve system energy efficiency and dynamic response performance. The proposed energy management strategy will fulfill the power requirements under typical driving cycles, achieve reasonable power distribution between the fuel cell and battery and, thus, prolong its continuous working time. The proposed energy management strategy is implemented in the hybrid forklift prototype and its effectiveness is tested under different operating conditions. The results show that the forklift with the proposed hybrid powered strategy has good performance with different loads, both lifting and moving, in a smooth and steady way, and the output of the fuel cell meets the requirements of its output characteristics, its SOC of battery remaining at a reasonable level. Keywords: fuel cell; battery bank; hybrid forklift; buck-boost DC/DC converter; energy management strategy; state of charge

1. Introduction Forklifts are indispensable and widely used in factories, warehouses, docks, and distribution centers as goods handling vehicles. Electric forklifts, dominantly powered by lead acid batteries, have wide application in indoor operations and other conditions which have strict requirements, such as medicine and food industries. The weaknesses of lead acid battery modules are a long charging time and a short working time which can result in an increase of the user-cost under a multi-shift working condition [1,2]. Hydrogen/oxygen fuel cells, as a new energy device, have become the most promising choice for the energy of the vehicles due to its cleanliness, safety, sustainability, high efficiency, and high energy conversion efficiency [3–6]. The maximum theoretical efficiency of hydrogen/oxygen fuel cells is 83% [7], and the general efficiency is 40–60% [8]. The calculation of efficiency conversion and the polarization plot of typical losses for a hydrogen/oxygen fuel cell are Energies 2018, 11, 3440; doi:10.3390/en11123440

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shown in Equations (2.1)–(2.18) and Figure 2.1 in [7]. Forklift manufacturers, such as Crown, Plug manufacturers, such ashave Crown, Plug Power, have constructed pure fuel cell electric Power, and Raymond, constructed pureand fuelRaymond, cell electric forklifts, obtaining somewhere in the forklifts, obtaining somewhere in thefuel performance evaluation of hydrogen cell design forklifts, fuel performance evaluation of hydrogen cell forklifts, fuel reloading time, andfuel forklift [9–12]. reloading time, design [9–12]. However, such the pure fuelpower cell forklift has some However, the pureand fuel forklift cell forklift has some disadvantages, as a large configuration, high disadvantages, such as response, a large power high cost, and slow response, which cost, and slow dynamic whichconfiguration, cannot meet the requirements whendynamic the forklift is in operation cannot theconditions. requirements when the forklift is in operation in some special conditions. in somemeet special In paper, aa fuel fuel cell/battery cell/battery hybrid In this this paper, hybrid forklift forklift system system is is constructed constructed with with the the combination combination of of the strengths of the fuel cells and batteries, its hardware and software in each functional unit are the strengths of the fuel cells and batteries, its hardware and software in each functional unit are designed hybrid forklift forklift prototype prototype is is successfully successfully made. made. With designed in in detail detail and, and, finally, finally, aa hybrid With the the detailed detailed analysis ofoutput outputcharacteristics characteristics and energy thecell fuel cell and battery, a loadfollowing current analysis of and energy flowflow of theoffuel and battery, a load current following energy management is to proposed to realize the management and distribution the energythe management strategy isstrategy proposed realize the management and distribution of the fuel of the fuel cell/battery hybrid forklift so as to the improve the system energy efficiency, dynamic cell/battery hybrid forklift energy soenergy as to improve system energy efficiency, dynamic response response performance, and the The fuelexperimental cell. The experimental results that the performance, and extend theextend life ofthe thelife fuelofcell. results prove thatprove the proposed proposed practical andas effective, as itmeet can fully meet the power demand underconditions operating strategy isstrategy practicalisand effective, it can fully the power demand under operating conditions achieve stable output of the fuel cell.the However, the output power of the fuel does and achieveand stable output of the fuel cell. However, output power of the fuel cell does notcell change not change with respect to the load and the SOC of the battery is maintained at a reasonable level. with respect to the load and the SOC of the battery is maintained at a reasonable level. 2. Topology Topology of 2. of the the Fuel Fuel Cell/Battery Cell/Battery Hybrid Hybrid Power Power System System The power powersystem system a pure cell the hasdisadvantages the disadvantages of a large configuration, The of of a pure fuel fuel cell has of a large powerpower configuration, slow slow dynamic response, range of output voltage variation.InInorder ordertoto overcome overcome these these dynamic response, and and widewide range of output voltage variation. shortcomings, researchers researchers proposed proposed aa fuel fuel cell cell hybrid hybrid system system [13,14]. [13,14]. The The fuel fuel cell cell hybrid hybrid system system is is shortcomings, an electric-electric hybrid system. According to the different connection modes of the fuel cell and an electric-electric hybrid system. According to the different connection modes of the fuel cell and auxiliary power power battery battery systems, systems, the topology is auxiliary the topology is shown shown in in Figure Figure 1. 1. Fuel Cells System

Load

Fuel Cells System

Unidirectional DC/DC Converter

Lead-acid Battery

Lead-acid Battery

(a) Fuel Cells System

Load

(b) Load

Fuel Cells System

Unidirectional DC/DC Converter

Load

Bidirectional DC/DC Converter

Bidirectional DC/DC Converter

Lead-acid Battery

Lead-acid Battery

(c)

(d)

Figure 1. hybrid power power system. system. (a) (a) Simple Simple topology topology of of the the hybrid Figure 1. Topology Topology of of the the fuel fuel cell/battery cell/battery hybrid hybrid power system; system; (b) (b)topology topologyofofthe thehybrid hybridpower powersystem system with unidirectional DC/DC converter; power with anan unidirectional DC/DC converter; (c) (c) topology of hybrid power system with a bidirectional DC/DC converter; (d) topology the topology of topology of hybrid power system with a bidirectional DC/DC converter; andand (d) the of the the hybrid power system with unidirectional and bidirectional DC/DC converters. hybrid power system with unidirectional and bidirectional DC/DC converters.

Figure 1a 1a is is aa relatively The fuel fuel cell cell and and the the Figure relatively simple simple topology topology of of aa hybrid hybrid power power system system [15]. [15]. The battery systems systems are are directly directly connected connected to to the the load, load, and and the the bus bus voltage voltage will will be be determined by the the battery determined by battery terminal voltage. In this topology, the voltage level of the fuel cell and the battery is determined battery terminal voltage. In this topology, the voltage level of the fuel cell and the battery is by the ratedby voltage of the load.ofOnly whenOnly the voltage level of the DCof bus matched with the determined the rated voltage the load. when the voltage level theisDC bus is matched load the willload the system bestthe energy economic efficiency. In addition, according to the with will theobtain systemthe obtain best and energy and economic efficiency. In addition, according to the polarization curve of a fuel cell, the output voltage of the fuel cell changes with the output current, which is similar to the current source, with soft output characteristics and slow dynamic response. In most cases, the output voltage of a fuel cell is lower than the required voltage of the DC

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bus when the load changes, which makes thevoltage energyofmanagement of the hybrid polarization curve of a fuel cell, the output the fuel cell changes with thepower outputsystem current,very difficult and, hence,to reduces the efficiency and reliability of the system. which is similar the current source, with soft output characteristics and slow dynamic response. The fuel cellthe output connected incell parallel battery voltage via a unidirectional DC/DC In most cases, outputisvoltage of a fuel is lowerwith than the the required of the DC bus when the load which makes of the power system very difficult and, converter to changes, form a hybrid systemthe asenergy shownmanagement in Figure 1b. Thehybrid structure can control the output power hence, reduces the efficiency and reliability of the system. of the fuel cell indirectly by controlling the output current of the DC/DC converter, and solves the outputvoltage is connected parallel the battery viauncontrollable a unidirectional DC/DC converter problem The thatfuel thecell output and in power of with the fuel cell are in Figure 1a [16]. In to form a hybrid system as shown in Figure 1b. The structure can control the output power of the addition, the battery is directly connected in parallel with the DC bus in this topology, so its voltage fuel cell indirectly by controlling the output current of the DC/DC converter, and solves the problem level must be matched with the load voltage. that the output voltage and power of the fuel cell are uncontrollable in Figure 1a [16]. In addition, In Figure 1a,b, the battery is directly connected to the load, so the battery terminal voltage level the battery is directly connected in parallel with the DC bus in this topology, so its voltage level must is required to match with thevoltage. load. In order to solve the requirements of the battery voltage level, the be matched with the load battery can be connected to the bus via a bidirectional DC/DC converter. The voltage topology of the In Figure 1a,b, the battery DC is directly connected to the load, so the battery terminal level hybrid power system is with shown Figure 1c,d.toCompared with the topology in Figure 1a, in Figure is required to match the in load. In order solve the requirements of the battery voltage level, 1c, the battery toterminal voltage overcome and the converter. SOC of the battery can be the limitation battery canofbethe connected the DC bus via aisbidirectional DC/DC The topology of the[17,18]. hybrid power systempower is shown in Figure 1c,d. with the topology Figure 1a, the managed The output of the fuel cell canCompared be controlled indirectly byincontrolling in Figure 1c, the limitation of thevoltage battery of terminal voltageCompared is overcomewith and the the SOC of thein battery can1b, in discharge current and the output the battery. topology Figure be managed [17,18]. The output power of the fuel cell can be controlled indirectly by controlling the the Figure 1d, the voltage of the DC bus can be stabilized within the specified range of the load, and discharge current and the output voltage of the battery. Compared with the topology in Figure operation of charging and discharging, and the controlling of the output of the fuel cell, 1b, can be in Figure 1d, the voltage of the DC bus can be stabilized within the specified range of the load, and the realized. This topological structure can achieve fuel cell and battery energy control and management operation of charging and discharging, and the controlling of the output of the fuel cell, can be realized. well, but is relatively complicated. This topological structure can achieve fuel cell and battery energy control and management well, but is In this paper, the hybrid system topology shown in Figure 1b is selected as the system structure relatively complicated. of the fuel cell/battery forklift.topology It is easy for in this simple achieve only the In this paper, thehybrid hybrid system shown Figure 1b isstructure selected astothe system not structure hybrid energy management, butforklift. also the charging andsimple discharging oftothe battery. of the fuel cell/battery hybrid It is easy for this structure achieve not only the hybrid energy management, but also the charging and discharging of the battery.

3. Fuel Cell/Battery Hybrid Power System Design 3. Fuel Cell/Battery Hybrid Power System Design

3.1. Hybrid Power System Description

3.1. Hybrid Power System Description

According to to the showninin Figure 1b, paper this paper constructed the fuel cell/battery According thetopology topology shown Figure 1b, this constructed the fuel cell/battery hybrid hybrid power system, shown in Figure 2, by using hydrogen supply system, proton power system, shown in Figure 2, by using hydrogen supply system, proton exchange membraneexchange fuel membrane fuel stack cell and (PEMFC) stack and controller, DC/DC controller, energy cell (PEMFC) controller, unidirectional DC/DCunidirectional controller, energy management controller, management controller, and lead-acid battery. and lead-acid battery.

Energy Management Controller

PEMFC Conroller RS232

CAN Hydrogen Supply System

PEMFC Stack

Unidirectional Buck-Boost DC/DC Converter

Forklift Motor

Fuel Cell Power system Analog Signal

Controller Communication

Energy Flow

Lead-acid Battery

Figure 2. Fuel cell/battery hybrid power system. Figure 2. Fuel cell/battery hybrid power system.

The hydrogen supply system provides high-purity hydrogen fuel for the fuel cell, employing 30 MPa, 20 L high-pressure hydrogen storage tanks, equipped with high- and low-pressure regulating valves, pressure indicator, pressure detecting sensor, electromagnetic valve, and other auxiliary devices. The high-pressure hydrogen storage system is shown in Figure 3. Ballard’s 1020ACS series PEMFC stack (Ballard Power Systems Inc., Burnaby, BC, Canada) is adopted for the fuel cell as a

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The hydrogen supply system provides high-purity hydrogen fuel for the fuel cell, employing 30 MPa, 20 L high-pressure hydrogen storage tanks, equipped with high- and low-pressure regulating valves, pressure indicator, pressure detecting sensor, electromagnetic valve, and other auxiliary Energies 2018, 11, x FOR PEER REVIEW 4 of 24 devices. The high-pressure hydrogen storage system is shown in Figure 3. Ballard’s 1020ACS series PEMFC stack (Ballard Power Systems Inc., Burnaby, BC, Canada) is adopted for the fuel cell as a main source for the hybrid system. PEMFC receives issued point. Thepower real-time state parameters of the The PEMFC cancontroller be collected andinstructions uploaded to the by energy the energy management controller and controls the PEMFC stack to work at the optimal power management controller, and then are used as the operating parameters of the controller. The output point. The real-time stateconverter parameters of control the PEMFC be collected uploaded the of unidirectional buck-boost DC/DC can the can output voltage,and current, andtopower energy management controller, and then are used as the operating parameters of the controller. the PEMFC unit to improve the working performance of the PEMFC and prolong the life of the The unidirectional buck-boost DC/DC converter can control the output voltage, current, and power PEMFC. The lead-acid battery is the energy storage unit of the hybrid power system. When the of the PEMFC unit to improve the working performance of the PEMFC and prolong the life of the hybrid power system is started, it is used as the auxiliary power supply of the system. When the PEMFC. The lead-acid battery is the energy storage unit of the hybrid power system. When the hybrid PEMFC's output power is insufficient, it is used as the auxiliary power source to provide power power system is started, it is used as the auxiliary power supply of the system. When the PEMFC’s supplement for the forklift motor. Theasenergy management system controls the supplement PEMFC controller output power is insufficient, it is used the auxiliary power source to provide power for and the DC/DCmotor. converter in real-time according to the power of the forklift motor and the the forklift The energy management system controls thedemand PEMFC controller and the DC/DC SOCconverter state of the battery.according Thus, it controls the demand output power of themotor PEMFC andstate theofcharging in real-time to the power of the forklift andstack the SOC the and battery. discharging the battery to realize energy distribution and management of the hybrid Thus, of it controls the output powerthe of the PEMFC stack and the charging and discharging of the battery to realize the energy distribution and management of the hybrid system. system.

Figure3.3.Hydrogen Hydrogen supply Figure supplysystem. system.

3.2. Air-Cooled PEMFC System

3.2. Air-Cooled PEMFC System

The fuel cell system is composed of the PEMFC stack, cooling fan, inlet solenoid valve, exhaust The fuelvalve, cell system composed of the PEMFC cooling fan, inlet solenoid valve, exhaust solenoid and airis filter. The PEMFC stack adoptsstack, Ballard’s air-cooled self-humidifying PEMFC solenoid and air Thecells, PEMFC self-humidifying stack,valve, consisting of filter. 56 single withstack ratedadopts power Ballard’s at 2 kW, air-cooled output voltage range of 30–56PEMFC V, ◦ stack, consisting of 56 single cells, with rated power at 2 kW, output voltage range of 30 V–56 V, a a maximum output current of 75 A, and a maximum operating temperature of 75 C. The fuel for PEMFC hydrogen oxygen. Hydrogen the stackoffrom the anode of the maximum output current ofis75 A, and aand maximum operatingenters temperature 75 °C. PEMFC stack, which undergoes a chemical reaction under the action of the catalyst tothe generate , the The fuel for PEMFC is hydrogen and oxygen. Hydrogen enters the stack from anodee−of + − H , and release a large amountaofchemical heat. Thereaction e migrates to the viathe an catalyst external circuit and e−, PEMFC stack, which undergoes under the cathode action of to generate + migrates to the cathode via the proton exchange membrane, and combines generates a current. H H+, and release a large amount of heat. The e− migrates to the cathode via an external circuit and with O of the cathode and e− to form H2 O under the action of the catalyst. The remaining H2 of the generates a2 current. H+ migrates to the cathode via the proton exchange membrane, and combines reaction and the formed H2−O are discharged through the exhaust gas. The PEMFC system is shown in with O2 of the cathode and e to form H2O under the action of the catalyst. The remaining H2 of the Figure 4.

reaction and the formed H2O are discharged through the exhaust gas. The PEMFC system is shown in Figure 4.

Inlet Air Filter

PEMFC

Fan

H+, and release a large amount of heat. The e− migrates to the cathode via an external circuit and generates a current. H+ migrates to the cathode via the proton exchange membrane, and combines with O2 of the cathode and e− to form H2O under the action of the catalyst. The remaining H2 of the reaction and the formed H2O are discharged through the exhaust gas. The PEMFC system is shown Energies 2018, 11, 3440 5 of 24 in Figure 4.

Inlet Air Filter

H2 IN Supply Valve

PEMFC

Fan

H2 OUT Purge Valve

Figure 4. Diagram of the air-cooled PEMFC system.

Figure 4. Diagram of the air-cooled PEMFC system. 3.3. PEMFC Controller Design

3.3. PEMFC Controller Design The fuel cell power generation system is composed of a PEMFC stack and a PEMFC controller. The output performance of the stack is affected by some operating parameters, such as the operating temperature, air flow rate, and the exhaust gas interval. The output performance is inconsistent under different operating conditions. According to the theoretical modeling and analysis of the fuel cells in [19], the output voltage of the fuel cells can be approximately expressed as: Vcell = f ( TFC , IFC , PH2 , PO2 )

(1)

where TFC is the PEMFC stack temperature, IFC is the PEMFC stack output current, PH2 and PO2 are the hydrogen and oxygen partial pressures, respectively. Since the cathode of the air-cooled self-humidifying PEMFC adopts a through-open structure which is directly connected to the outside air, and the PO2 is close to a normal value. A precision proportional electrical regulating valve can be adopted at the front end of the anode inlet, which can automatically adjust the inlet H2 pressure and keep it at the set pressure point. Thus, the PH2 can be approximated as a constant value. The output voltage of the fuel cell can be approximately simplified as: Vcell = f ( TFC , IFC ) (2) In Equation (2), under certain conditions of output current IFC , the stack operating temperature TFC will affect the output voltage V cell . There is an optimal operating temperature TFC.opt , which enables the output voltage and output power of the stack to be the maximum, with optimal output performance [20–22]. In this paper, the air-cooled PEMFC power generation controller is designed to control the operating temperature of the PEMFC stack and enable it to work near the optimal operating temperature point, with the maximum output power. The functional block diagram is shown in Figure 5. The specific design process of the PEMFC controller is referred to in [23]. The detailed design and analysis of the realization circuit of each functional unit are given in [23]. In this paper, the PEMFC controller needs to accept the control commands of the energy management controller and controls the PEMFC stack operation according to the control instructions. The control flow chart is shown in Figure 5b [23]. Where V stack is the output voltage of the PEMFC. The optimal temperature control algorithm adopts segmented prediction negative feedback control (SPreNFC) [22].

The specific design process of the PEMFC controller is referred to in [23]. The detailed design and analysis of the realization circuit of each functional unit are given in [23]. In this paper, the PEMFC controller needs to accept the control commands of the energy management controller and controls the PEMFC stack operation according to the control instructions. The control flow chart is shown in Figure 5b [23]. Where Vstack is the output voltage of the PEMFC. The optimal temperature Energies 2018, 11, 3440 6 of 24 control algorithm adopts segmented prediction negative feedback control (SPreNFC) [22]. RS232 interface

RS232 Driver

Load control

Output Voltage Load Breaker Driver

Output Current PE MFC Stack

Stack tempreture H2 Pressure

Analog Signal Acquisition Circuit

DC48V

H2 control

MCU H2 Valve Driver STM32F10 3

PEMFC Purge Auxiliary control System

Purge Valve Driver

Fan control

Fan Driver

Power Supply

PEMFC Controller

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(a) Start System hardware initialization

Whether to trigger

AVailable control

N

command?

Y

Y Read control parameter

Y

Available parameters?

N

N

Whether to start

Stop PEMFC System

Y load breaker is closed？

UART/ADC/DAC initialization

N

Receive control command

Execute control command

Perform protection control

PEMFC?

Use default control parameters

Reset Valve and Load Breaker

N

protection

N Vstack>40V?

Y

Perform optimal temperature control

Execution status indication

Y load breaker closed

Upload parameters

(b) Figure 5. Functional diagramand andcontrol controlflowchart flowchart of (a)(a) Functional diagram of Figure 5. Functional diagram of the thePEMFC PEMFCcontroller. controller. Functional diagram thePEMFC PEMFC controller; controller; (b) the PEMFC controller. of the (b)Control Controlflowchart flowchartofof the PEMFC controller.

real-time parameters, suchasasthe theoutput outputvoltage, voltage, output output current, current, operating TheThe real-time parameters, such operatingtemperature, temperature, and hydrogen pressure of the stack are collected by the PEMFC controller. The optimal temperature and hydrogen pressure of the stack are collected by the PEMFC controller. The optimal temperature T , which needs to be controlled at present, is calculated by using the optimal temperature formula TFC.optFC.opt , which needs to be controlled at present, is calculated by using the optimal temperature in [22]. The SPreNFC temperature control algorithm is used to control the operation of the stack near formula in [22]. The SPreNFC temperature control algorithm is used to control the operation of the the optimum temperature point. At the same time, according to the datasheet of the PEMFC stack, stack near the optimum temperature point. At the same time, according to the datasheet of the the auxiliary system of the fuel cell is controlled to maximize net power output and optimal operation PEMFC stack, the auxiliary system of the fuel cell is controlled to maximize net power output and performance. The PEMFC controller has an RS232 communication interface which can accept the optimal operation performance. The PEMFC controller has an RS232 communication interface which control commands and control parameters, sent by the energy management controller, to realize the can accept the control commands and control parameters, sent by the energy management controller, to realize the operation control of the PEMFC and upload the PEMFC state parameters to the energy management controller. The designed PEMFC controller is shown in Figure 6. Figure 6a is the PEMFC controller prototype, Figure 6b is the control response curve of the PEMFC controller to control the PEMFC stack, TFC.opt is the optimal control temperature of the PEMFC stack, and TFC is

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operation control of the PEMFC and upload the PEMFC state parameters to the energy management controller. The designed PEMFC controller is shown in Figure 6. Figure 6a is the PEMFC controller prototype, Figure 6b is the control response curve of the PEMFC controller to control the PEMFC stack, TFC.opt is the optimal control temperature of the PEMFC stack, and TFC is the working temperature of PEMFC stack. It can be seen from the response curve that when the output current of PEMFC changes, the PEMFC controller immediately calculates a new TFC.opt , and then controls the cooling system of the stack to make the TFC gradually reach the optimal temperature point. The output voltage reaches its maximum value and the output power is optimal. When the TFC reaches the range that is near the optimal operating temperature point, the output voltage of the PEMFC reaches the maximum value and the output power becomes optimal. For the selection of the SPreNFC control parameters, please refer2018, to [20], optimal control parameters can be obtained through experimental testing. 7 of 24 Energies 11, xand FORthe PEER REVIEW

(a)

Voltage/V, Current/A, Temperature/。C

60

Output Voltage Output Current TFC.opt

50

TFC

40

30

20

10

0 0

1000

2000

3000

time/s

4000

5000

(b) Figure 6. PEMFC controller. (a) Prototype of the PEMFC controller; (b) Response curve of control.

Figure 6. PEMFC controller. (a) Prototype of the PEMFC controller; (b) Response curve of control.

3.4. Unidirectional Buck-Boost DC/DC Converter Design In this study, the output voltage of the PEMFC varies between 30 and 56 V. The varying range of the output voltage is large, and when the load changes, the output voltage is unstable and cannot be directly connected to the load. In order to overcome the shortcomings of the PEMFC, the output voltage needs to be connected with a unidirectional buck-boost DC/DC converter to transform the output voltage of the PEMFC into the allowable range of the battery and forklift motor. Since the

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3.4. Unidirectional Buck-Boost DC/DC Converter Design In this study, the output voltage of the PEMFC varies between 30 and 56 V. The varying range of the output voltage is large, and when the load changes, the output voltage is unstable and cannot be directly connected to the load. In order to overcome the shortcomings of the PEMFC, the output voltage needs to be connected with a unidirectional buck-boost DC/DC converter to transform the output voltage of the PEMFC into the allowable range of the battery and forklift motor. Since the rated power of the PEMFC stack is 2 kW, a DC/DC converter that can meet the maximum power output of the fuel cell stack is required. In this paper, the DC/DC converter is designed based on the NiQor non-isolated buck-boost module (NiQor is the product model name), which is produced by SynQor, Inc., Boxborough, MA, USA. The input voltage range of the NiQor module is 9–60 V, the output voltage range is adjustable between 0 and 60 V, and the maximum input or output current is 40 A. According to the polarization curve in the PEMFC datasheet, when the PEMFC has the maximum power output, its output voltage is close to 35 V and the output current is close to 60 A. Therefore, two NiQor modules are connected in parallel, and the maximum output current can reach 80 A, which fully meets the output power requirements of the PEMFC stack. The NiQor module has 11 control ports, which are voltage setting V set , current setting Iset , enable control ON/OFF, current sharing control Ishare , synchronous control Syncin , voltage sensing reference positive V sense+ , voltage sensing reference negative V sense− , input V in+ and V in− , and output V out+ and V out− . V in− and V out− are internally connected together as the reference ground of the input and output. V sense+ and V sense− are used as the reference input for the internal output voltage sampling of the NiQor module, and are generally connected to the voltage output terminal V out+ and V out− . When it is necessary to synchronize with an external device, Syncin is used as the synchronous clock input port, otherwise it can be left floating. In this paper, it is not synchronized with the external device, so it is left floating. Ishare realizes the current sharing when multiple modules are used in parallel. In this paper, Ishare of the two modules are connected in parallel to realize the internal current sharing of the modules. ON/OFF enables and disables the NiQor module conversion, which is equivalent to a signal switch to control the module to start and stop. V set and Iset are used to set the expected output voltage value and output current limit of the module. It is controlled by the analog voltage. The corresponding relationship between control voltage value and output value is determined by Equation (3): VVset = 2.366 − 2.316 ×

Vset Iset VI = 0.0953 + 2.085 × Vmax set Imax

(3)

where V set is the expected output voltage value of DC/DC converter; V max is the maximum output voltage of DC/DC converter, and the default value is 60 V; Iset is the expected output current limit of the DC/DC converter; Imax is the maximum output current value of the DC/DC converter, and the default value is 40A; VVset is the control voltage corresponding to the expected output voltage; VIset is the control voltage corresponding to the expected output current limit. According to Equation (3), the control voltage range of VVset is 0.05–2.366 V, and the control voltage range of VIset is 0.0953–2.1803 V. In this paper, the parallel block diagram is shown in Figure 7a. Based on the NiQor module datasheet, the control unit of the DC/DC converter is designed by using a C8051F040 microcontroller (Silicon Laboratories, Inc., Austin, TX, USA), with the purpose of detecting the operating state parameters, such as DC/DC input and output, receiving the control commands issued by the energy management controller, and controlling the output of the NiQor module. The functional block diagram of the control unit is shown in Figure 7.

In this paper, the parallel block diagram is shown in Figure 7a. Based on the NiQor module datasheet, the control unit of the DC/DC converter is designed by using a C8051F040 microcontroller (Silicon Laboratories, Inc., Austin, TX, USA), with the purpose of detecting the operating state parameters, such as DC/DC input and output, receiving the control commands issued by the energy management controller, and controlling the output of the NiQor module. The functional block Energies 2018, 11, 3440 9 of 24 diagram of the control unit is shown in Figure 7.

Vin+

Vin+

Vin-

Vin-

NiQor （1）

ON/OFF Vset Iset

Vset Iset

Vin+ Vin-

NiQor （2）

Vout+ Vsense+ VoutIshare Vsense-

Vout+ Vout-

Vout+ Vsense+

VoutON/OFF Vset Iset Ishare Vsense-

ON/OFF

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(a)

Input Voltage Detection

CAN Driver

Output Voltage Detection

Power Supply

Input Current Detection

Main Controller

OutputT Current Detection

C8051F040

CAN Interface

Output Voltage SET

Output Current SET

Current Sharing Control

Relay Control

RUN ON/OFF Control

DC/DC Converter Control Board (b) Figure diagram of the DC/DC converter. (a) Parallel block diagram of the NiQor Figure7.7.Functional Functional diagram of the DC/DC converter. (a) Parallel block diagram of themodule. NiQor (b) Control functional diagram of the DC/DC module. (b)board Control board functional diagram of theconverter. DC/DC converter.

The convertercontrol controlboard boarddetects detectsthe theinput/output input/outputvoltage voltage value and current value The DC/DC DC/DC converter value and current value of of the DC/DC converter in real-time via the input voltage detection, output voltage detection, the DC/DC converter in real-time via the input voltage detection, output voltage detection, input input current current detection, detection, and and output output current currentdetection detectionunits. units. The The output output voltage voltage and andcurrent currentofofDC/DC DC/DC converter are set in real-time by the output voltage SET and output current SET units. The operation converter are set in real-time by the output voltage SET and output current SET units. The operation of DC/DCconverter converterisiscontrolled controlled current sharing control, RUN ON/OFF control, andrelay the of the the DC/DC byby current sharing control, RUN ON/OFF control, and the relay control unit. The real-time parameters collected by the control board are transmitted to the control unit. The real-time parameters collected by the control board are transmitted to the energy energy management controller through the controller area network (CAN) interface. The energy management controller through the controller area network (CAN) interface. The energy management management controller controller generates generates new new control control parameters parameters according according to to the the received received real-time real-time status status parameters, and sends control commands to the control board in real-time to control the DC/DC parameters, and sends control commands to the control board in real-time to control the DC/DC converter. converter.The Thedesigned designedDC/DC DC/DC converter converter is is shown shownin inFigure Figure8.8.The Thesimulation simulationcontroller controllerofofDC/DC DC/DC converter byby a USB to CAN converter (USBCAN) and computer, and the control converterisisconstructed constructed a USB to CAN converter (USBCAN) and computer, and theprogram control isprogram written according to the DC/DC converter CAN communication protocol to test the functions of the is written according to the DC/DC converter CAN communication protocol to test the DC/DC such as converter, start, stop, the of the output maximum functionsconverter, of the DC/DC suchsetting as start, stop, the voltage, setting and of the output current voltage,limit. and The test results are shown in Figure 8b. Theare efficiency DC/DC converter is tested by using a maximum current limit. The test results shown of in the Figure 8b. The efficiency of the DC/DC DC power supply and an electronic load. The test results are shown in Figure 8c. The efficiency of the converter is tested by using a DC power supply and an electronic load. The test results are shown in converter 94.3% when the output powerisis94.3% 2010.9when W. The includes DC/DC loss Figure 8c.isThe efficiency of the converter theefficiency output power is 2010.9 W. converter The efficiency and DC/DC converter cooling fan power. The cooling fan power is 15 W. If the cooling fan power is includes DC/DC converter loss and DC/DC converter cooling fan power. The cooling fan power is 15 not calculated, the efficiency of the DC/DC converter is 95.1%. W. If the cooling fan power is not calculated, the efficiency of the DC/DC converter is 95.1%.

functions of the DC/DC converter, such as start, stop, the setting of the output voltage, and maximum current limit. The test results are shown in Figure 8b. The efficiency of the DC/DC converter is tested by using a DC power supply and an electronic load. The test results are shown in Figure 8c. The efficiency of the converter is 94.3% when the output power is 2010.9 W. The efficiency includes DC/DC is 24 15 Energies 2018, 11, 3440 converter loss and DC/DC converter cooling fan power. The cooling fan power 10 of W. If the cooling fan power is not calculated, the efficiency of the DC/DC converter is 95.1%.

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Computer

Input Current

60

Voltage/V, Current/A

USB CAN

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(a)

Battery

Simulation Control Program

unidirectional DC/DC controller

50

Output Voltage

45 40

Input Voltage

Output Current

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Electronic Load

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(c) Figure 8. Unidirectional buck-boost DC/DC converter. (a) Prototype of the DC/DC converter. Figure 8. Unidirectional buck-boost DC/DC converter. (a) Prototype of the DC/DC converter. (b) (b) Program control and response test. (c) Efficiency test. Program control and response test. (c) Efficiency test.

3.5. Battery Energy Storage System 3.5. Battery Energy Storage System In this study, the Lead-acid battery is employed for the hybrid power system to be as the starting Inand thisauxiliary study, the Lead-acid is employed hybrid in power to be asbank the power power source.battery 24 lead-acid batteriesfor arethe connected seriessystem into a battery starting power and auxiliary power source. 24 lead-acid batteries are connected in series into with the rated voltage of 48 V and the rated capacity of 100 Ah. The SOC is defined as the ratio of thea battery bank withofthe voltage 48 V andofthe capacity of 100 Its Ah.value The range SOC isisdefined as residual capacity therated battery to theofcapacity therated fully-charged state. from 0 to the ratio of the residual capacity of the battery to the capacity of the fully-charged state. Its value 1. When SOC = 0, the battery is completely discharged. When SOC = 1, the battery is fully charged. range from 0the to SOC 1. When SOCas = 0, the battery isfor completely SOC 1, the battery In thisispaper, is used the condition selecting discharged. the chargingWhen current of=the battery to is fully charged. In this paper, the SOC is used as the condition for selecting the charging current of determine its charging current. The SOC estimation formula is given in [15], as shown in Equation (4): the battery to determine its charging current. The SOC estimation formula is given in [15], as shown in Equation (4):

SOCt  SOC0  1 / Qmax    i (t )dt 100%

(4)

where Qmax is the maximum capacity when the battery is fully charged (Ah), and i is the discharge current (A). SOCt is the current SOC, and SOC0 is the initial SOC. The energy management controller will collect the demand current of the forklift motor in real-time, combine the output current of the DC/DC converter to calculate the discharge current of the battery in real-time, and calculate the SOC of the battery bank in real-time according to Equation

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SOC t = SOC0 − (1/Qmax ) ·

Z

i (t)dt · 100%

(4)

where Qmax is the maximum capacity when the battery is fully charged (Ah), and i is the discharge current (A). SOCt is the current SOC, and SOC0 is the initial SOC. The energy management controller will collect the demand current of the forklift motor in real-time, combine the output current of the DC/DC converter to calculate the discharge current of the battery in real-time, and calculate the SOC of the battery bank in real-time according to Equation (4). The discharge rate of the battery is 1 C–3 C (100 A–300 A), and the maximum charging current of the battery is 0.3 C. In this paper, the energy management strategy will combine the SOC of the battery to select the charging current, and the specific parameters are shown in Table 1. Table 1. Relationship of the charging current and SOC. SOC Charging current

SOC < 60% 0.3 C (30 A)

60% < SOC < 80% 0.2 C (20 A)

80% < SOC < 90% 0.07 C (7 A)

SOC > 90% Floating charge

3.6. Energy Management Controller Design The energy management controller controls the output power of PEMFC on the premise of meeting the demand of load power, making the output current of PEMFC stable, preventing the output current and power mutation when the load mutation occurs, so as to improve the output performance of the PEMFC and prolong the life of the PEMFC. At the same time, the battery is effectively charged and discharged to prevent over-charge and over-discharge of the battery, ensuring stable and safe operation of power sources. In this study, the energy management controller communicates with the PEMFC controller through the RS232 communication interface to receive the state parameter uploaded by the PEMFC controller, and transmits the control command. It communicates with the DC/DC converter through the CAN communication interface to receive the output voltage, current and other state parameters of the DC/DC converter, and send the DC/DC converter control command. The energy management controller collects the request current of the forklift motor in real-time, estimates the battery SOC online, combines the operating parameters of the PEMFC and DC/DC converter, generates a new control parameter according to the hybrid energy management control strategy, and controls the operation of the PEMFC controller and DC/DC converter. The designed hybrid energy management controller is shown in Figure 9. 3.7. Hybrid Forklift System In this study, the fuel cell/battery hybrid forklift system structure consists of the designed PEMFC controller, DC/DC converter, and energy management controller which is shown in Figure 10. The parameters of the hybrid system are shown in Table 2. A fuel cell/battery hybrid forklift prototype is shown in Figure 11. The solenoid valve in the PEMFC stack is connected to the outside of the hybrid power system frame via the pipe to directly discharge the exhaust gas to the atmosphere. “Power Switch” is the power switch of the energy management controller. The “Start/Stop Button” is the start/stop switch for the hybrid power system. The energy management controller issues a control signal according to the state of the start/stop button, and control opened or closed of the relay to implement power supply control of the PEMFC controller.

of the DC/DC converter, and send the DC/DC converter control command. The energy management controller collects the request current of the forklift motor in real-time, estimates the battery SOC online, combines the operating parameters of the PEMFC and DC/DC converter, generates a new control parameter according to the hybrid energy management control strategy, and controls the operation the PEMFC controller and DC/DC converter. The designed hybrid energy management Energies 2018, 11,of3440 12 of 24 controller is shown in Figure 9.

LCD Display

RS232 Communication

PEMFC Controller

Power Supply

MCU CAN Communication

DC/DC Converter

Start/Stop Button

C8051F040 Load Current

48V

Load Current Detection

Fault indicator

Energy Management Controller (a)

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In this study, the fuel cell/battery hybrid forklift system structure consists of the designed PEMFC controller, DC/DC converter, and energy management controller which is shown in Figure 10. The parameters of the hybrid system are shown in Table 2. A fuel cell/battery hybrid forklift prototype is shown in Figure 11. The solenoid valve in the PEMFC stack is connected to the outside of the hybrid power system frame via the pipe to directly discharge the exhaust gas to the atmosphere. “Power Switch” is the power switch of the energy management controller. The “Start/Stop Button” is the start/stop switch for the (b) hybrid power system. The energy management controller issues a control signal according to the state of the start/stop button, and control opened Figure 9. Energy management controller. (a) Functional diagram of the management energy management Figure Energy controller. (a) Functional diagram ofPEMFC the energy controller; or closed of9.the relaymanagement to implement power supply control of the controller. Prototype the energy management (b) controller; Prototype (b) of the energyof management controller. controller.

3.7. Hybrid Forklift System

Energy Management Controller

Start/Stop Button

PEMFC Controller RS232

Relay

I Breaker

RS232

T V

CAN

I

Diode

Cool Fan

DC/DC Converter

P

H2 IN

H2 OUT Solenoid Valve

To Atmosphere

PEMFC Stack Lead-acid Battery Communication Signal Energy Flow Pressure Solenoid Proportional Regulating Valve Regulating Vale H2 valve

V Voltage Sensor I Current Sensor T Temperature Sensor

Hydrogen Supply System

P Pressure Sensor Forklift

Figure hybridforklift forkliftsystem system structure. structure. Figure 10. 10. Fuel Fuel cell/battery cell/battery hybrid

Table 2. Parameters of the hybrid system. Subsystems

Power Switch

Descriptions Rated power Cells

Values 2000 W 56 pieces

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Table 2. Parameters of the hybrid system. Subsystems

Descriptions

Values

Fuel Cell (PEMFC)

Rated power Cells Voltage range Maximum current Maximum temperature

2000 W 56 pieces 30–56 V 75 A 75 ◦ C

Lead-acid Battery

Rated voltage Rated capacity Set numbers

48 V 100 Ah 24 series

Moving motor Forklift Lifting motor Energies 2018, 11, x FOR PEER REVIEW

Rated voltage Rated power Rated voltage Rated power

48 V 6300 W 48 V 8600 W

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Figure 11. Prototype fuel cell/battery hybrid forklift. Figure 11. Prototype fuel cell/battery hybrid forklift.

4. Energy Management Strategy Based on Load Current Following 4. Energy Management Strategy Based on Load Current Following 4.1. Analysis of Characteristics of Hybrid Energy 4.1. Analysis of Characteristics of Hybrid Energy In this paper, the fuel cell/battery hybrid forklift system includes two power sources: fuel cell In this The paper, cell/battery hybrid forklift two power sources: and battery. fuelthe cellfuel is the main power source andsystem batteryincludes is the auxiliary power source.fuel cell and From battery. The fuel cell is the main power source and battery is the auxiliary power source. the working principle of the fuel cell, the output characteristics of the fuel cell are relatively From working principle of the fuel cell, output characteristics of thethe fuel cell are soft and theythe cannot respond in a timely manner to thethe power request of the load when load power relatively soft and they cannot respond in a timely manner to the power request of the load when the suddenly increases. Thus, it is necessary to gradually increase the power output under the condition loadthe power suddenly increases. Thus, is to necessary gradually increase output under that fuel cells are fully activated, andittry keep thetooutput current of the the fuelpower cells stable during its the condition that the fuel cells of arethe fully and try toincreased keep the when outputthe current of the fuel cells operation. If the output current fuelactivated, cell is excessively load power suddenly stable during its operation. If the output current thethe fuel cellcell is with excessively increased when the increases, the fuel cell terminal voltage will drop,of and fuel too low a terminal voltage load power suddenly increases, the fuel cell terminal voltage will drop, and the fuel cell with too low a terminal voltage will enable the fuel cell to be reverse-polarized, causing irreparable damage. Therefore, when the load power suddenly increases, the fuel cell should not provide a large power output instantaneously. The fuel cell output current should be gradually increased according to the characteristics of the fuel cell, and the output power should be increased until the load power

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will enable the fuel cell to be reverse-polarized, causing irreparable damage. Therefore, when the load power suddenly increases, the fuel cell should not provide a large power output instantaneously. The fuel cell output current should be gradually increased according to the characteristics of the fuel cell, and the output power should be increased until the load power demand is met or the maximum output power is reached. In the system, the battery is connected in parallel with the DC/DC converter and the forklift motor. Without any independent charging and discharging control unit, the charging and discharging modes are determined by the load power. When the battery needs to access to the output power, it can be connected immediately. When the DC bus voltage is higher than the battery voltage, the battery enters the charging state at once. If the load power suddenly increases, the battery immediately supplements the demanded load power. After the fuel cell gradually increases the output power, the battery will gradually reduce the output power. When the output power of the fuel cell is gradually increased to more than the load power, the battery changes from the discharging state to the charging state, and gets into the charging or floating charging state according to the SOC state. According to the power of the forklift motor, the operation state of the hybrid power system can be divided into three cases without considering the loss: (1) Forklift motor is not running (forklift lifting and moving motor systems do not operate). In this mode, the total load of the hybrid power system is only the battery (the battery can be regarded as the load of the system), and the DC/DC converter can be controlled to charge the battery. At this time, the current and power balance of the hybrid power system are as follows:  ILoad = 0         IBattery < 0 IBattery = − IDC/DC.out    P   Load = 0    PFC = PBattery < PFC.max

(5)

where ILoad is the input current of the motor of forklift (A). IBattery is the battery discharge current of the battery (A), the discharge is the positive, the charge is the negative. IDC/DC.out is the DC/DC converter output current, which is consistent with the variation trend of the output current of the fuel cell (A); PLoad is the motor power of forklift (W); PFC is the output power of the fuel cell (W); PBattery is the discharge power of battery (W); and PFC.max is the maximum output power of the fuel cell. (2) The required power of forklift motor is less than the maximum output power of the fuel cell. In this mode, the hybrid power system load includes forklift motor and battery (at this point, the battery can be regarded as the load of the system). The output of the fuel cell provides power to forklift motor on the one hand, and on the other hand, it can be charged according to the SOC of battery. At this time, the current and power balance of the hybrid power system are as follows:  ILoad < IDC/DC.out      I Battery < 0   IBattery = ILoad − IDC/DC.out    PFC = PBattery + PLoad < PFC.max

(6)

(3) The required power of forklift motor is greater than the maximum output power of the fuel cell. In this mode, the hybrid system load is only the forklift motor. The fuel cell outputs at the maximum power, and at this point the battery is used as a power source to compensate for the insufficient power output of the fuel cell. At this time, the current and power balance of the hybrid power system are as follows:

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 ILoad > IDC/DC.out      I Battery > 0  IBattery = ILoad − IDC/DC.out     PLoad = PFC + PBattery

(7)

For these three cases, we can sum up that there are two cases: battery discharge (IBattery > 0) and charging (IBattery < 0). When the battery is in the discharge state, the energy management controller should gradually increase the output power according to the output characteristics of the fuel cell, reduce the discharge power of the battery, and put the battery in the shallow discharge state, to extend its life. When the battery is in the charge state, the energy management controller should estimate the charging current of the battery according to the SOC of the battery, enable the DC/DC converter to be at a constant current output, and charge the battery to prevent battery overcharging. At the same time, the output current of the fuel cell should be kept stable, so that the fuel cell can work as close to the optimal power point as possible. 4.2. Energy Management Strategy Based on Load Current Following For a hybrid power system, the energy management strategy will affect the energy utilization efficiency, dynamic response performance, fuel economy, and service life of the hybrid power system [2,13,14,17,24–28]. The main objectives of the energy management strategy for the proposed hybrid forklift is to effectively distribute the output power of each energy unit under the premise of meeting the power demand of the forklift, to reduce the hydrogen fuel consumption, to enhance the fuel utilization rate, to improve the performance of the power supply, to prolong the service life of the fuel cell and the battery, and to increase the cruising range of the forklift. The proposed energy management strategy is about to fulfil the following objectives: (1) (2) (3) (4)

To enable the DC Bus voltage to stay the allowable voltage range of the forklift motor. To prevent battery overcharge and over discharge, to keep the SOC of battery at a reasonable level, and to reserve enough energy for next start-up of the system. To keep the fuel cell working at the optimal working point in varying environments, the fuel cell output power optimal, and the fuel utilization efficiency the highest. To keep the output current and power of the fuel cell steady, prevent the output from changing with the load power, and to follow the change of load power gradually until the maximum power output is reached.

On the grounds of the characteristics of hybrid energy, this paper proposes a hybrid energy management control strategy based on load current following. The control strategy needs to continuously adjust the output current limit of DC/DC converter according to the request current of load, and gradually follow the request current of the load to meet the power demand of forklift. The control flow chart of energy management strategy based on load current following is shown in Figure 12. IDC/DC.set is the output current setting value of the DC/DC converter (A). IDC/DC.min is the minimum output current value of the DC/DC converter (A), which is converted from the minimum output current of the fuel cell (IFC.min ). IDC/DC.max is the maximum output current value of the DC/DC converter (A), which is converted from the maximum output power of the fuel cell (PFC.max ). ∆IFC.add is the adjustment threshold of the fuel cell when output current increasing, it is selected by PEMFC experimental test. When PEMFC operates stably, if the output current of the PEMFC suddenly increases ∆IFC.add , the voltage value of the single cell falls within the allowable range without excessive peak current, then this value can be used as the adjustment value of the current increase. According to the experiment, the value of ∆IFC.add is 3 A in this study.

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Get ILoad, IDC/DC.out values IBattery=ILoad-IDC/DC.out

N Y

IBattery>0

IDC/DC.set≤IDC/DC.min

IDC/DC.setIDC/DC.out+△IFC.dec

|TFC.err|IDC/DC.max

Y

Calculate IBattery Value

IDC/DC.set=IDC/DC.max

IDC/DC.set=ILoad+IBattery

IDC/DC.set>IDC/DC.max

Y

Y IDC/DC.set=IDC/DC.max

IDC/DC.set 0,a large this indicates that the ILoadwill is greater than the IDC/DC.out , and the battery in the lower than I0.5 V or peak current, which bring about abnormal performance of theisfuel cell discharged state. The energy management control strategy immediately determines whether controller, and then protection or shutdown accidents of the fuel cell power generation system. In this the of IDC/DC.max If converter IDC/DC.set is should equal to , keep the Ito DC/DC.set and the case,the theIDC/DC.set setting is value DC/DC beIDC/DC.max gradually reduced avoidunchanged, excessive increment output of of the unchanged. If IDC/DC.set is increasing less than Isuddenly DC/DC.set, the |TFC.err| of the outputpower current thefuel fuelcell cellremains when the request current of load According needs to be detected. If |T FC.err | is greater than ξ, it indicates that the fuel cell has not yet to the experiment, the value of ∆IFC.dec is 3 A in this study. reached optimal from the previous output state to the current output of state, |TFC.err |the is the erroroutput value of the optimal temperature and the stack temperature the and fuel the cell. stack activation is insufficient (increasing the output current when the stack activation ξ is the temperature error adjustment threshold, and this value is selected according to the optimal polarization is insufficient, may enable the voltage of the single cell to be reduced to less than 0.5 V, causing the stack to reverse-polarize and damage the stack), the output current of the

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temperature control performance. According to the experiment, the value of |TFC.err | is 3 ◦ C in this study. After the starting the energy management strategy, the ILoad (request current of the forklift motor) is detected first, and the IBattery (discharge current of the battery) is calculated according to the IDC/DC.out (output current of DC/DC converter), entering the discharge or charge control flow according to the IBattery symbol: (1)

(2)

When IBattery > 0, this indicates that the ILoad is greater than the IDC/DC.out , and the battery is in the discharged state. The energy management control strategy immediately determines whether the IDC/DC.set is the IDC/DC.max If IDC/DC.set is equal to IDC/DC.max , keep the IDC/DC.set unchanged, and the output power of the fuel cell remains unchanged. If IDC/DC.set is less than IDC/DC.set , the |TFC.err | needs to be detected. If |TFC.err | is greater than ξ, it indicates that the fuel cell has not yet reached the optimal output from the previous output state to the current output state, and the stack activation is insufficient (increasing the output current when the stack activation polarization is insufficient, may enable the voltage of the single cell to be reduced to less than 0.5 V, causing the stack to reverse-polarize and damage the stack), the output current of the fuel cell is kept unchanged. If |TFC.err | is less than ξ, it indicates that the stack is fully activated, and the output current of the fuel cell can be increased by ∆IFC.add . Then the energy management control strategy modifies the IDC/DC.set to increase the fuel cell output current ∆IFC.add (when the adjusted value exceeds the maximum output value of the fuel cell, the IDC/DC.set is equal to the maximum output value of the fuel cell). This adjustment is repeated to enable the output current of the fuel cell to follow the request current of the forklift motor gradually, until the output current of the fuel cell reaches a maximum value. When IBattery < 0, this indicates that the ILoad is less than the IDC/DC.out , and the battery is in the charging state. The energy management control strategy immediately determines whether the IDC/DC.set is the IDC/DC.min . If IDC/DC.set is equal to IDC/DC.min , keep the IDC/DC.set unchanged, and the output power of the fuel cell remains unchanged. In order to prevent the sudden increase of ILoad causing the output current of the fuel cell to immediately increase to the IDC/DC.set , and the single cell voltage of the fuel cell is too low or a large peak current occurs affects the normal operation of the PEMFC controller, it is necessary to judge whether the difference between the IDC/DC.set and ILoad is greater than the reduction adjustment threshold ∆IFC.dec , if IDC/DC.set is greater than IDC/DC.min . If the difference is greater than the adjustment threshold ∆IFC.dec , the energy management controller recalculates the SOC of battery, and selects an appropriate charging current IBattery according to the current SOC of battery, and takes the sum of IBattery and ILoad as the new setting value of IDC/DC.set . With the increase of the SOC, the charging current of the battery will gradually decrease.

4.3. Test of Energy Management Strategy In this paper, the 2.6 kW electronic load is used instead of the forklift motor in Figure 10 to build a fuel cell/battery hybrid simulation test system, which tests the proposed hybrid energy management strategy to verify its effectiveness. During the test, the energy management controller obtains the output parameters of each energy source of the hybrid system through the CAN communication interface, and draws a response curve of current and power. (1) Response test when load power is slowly reduced. The electronic load is used to simulate the power request of load shown in Figure 13a, and the hybrid power system is controlled by the proposed load current following control strategy. The response curve is shown in Figure 13b.

communication interface, and draws a response curve of current and power. (1) Response test when load power is slowly reduced. The electronic load is used to simulate the power request of load shown in Figure 13a, and the hybrid power system is controlled by the proposed load current following control strategy. The Energies 2018, 11, 3440 18 of 24 response curve is shown in Figure 13b. 2600

Power Request Curve

2.5kW

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(b)

Figure 13. Response test when load power is slowly reduced. (a) Request curve of load power; Figure 13. Response test when load power is slowly reduced. (a) Request curve of load power; and and (b) the response curve of current and power. (b) the response curve of current and power. 1 According to the proposed energy management strategy, when the request power of load is 0

① According to the proposed energy management strategy, when the request power of load is kW, the IFC is equal to IFC.min , the output current of the fuel cell is stable, and the battery is charged. 0 kW, the IFC is equal to IFC.min, the output current of the fuel cell is stable, and the battery is charged. 2 At the moment when the load power suddenly increases to 2.5 kW, according to the current

② At the moment when the load power suddenly increases to 2.5 kW, according to the current activation situation of the stack, the proposed energy management control strategy gradually increases activation situation of the stack, the proposed energy management control strategy gradually the output current and power of the fuel cell, and maintains the output current and power output increases the output current and power of the fuel cell, and maintains the output current and power smoothly in each adjustment phase. During the adjustment process, the insufficiently part of the load output smoothly in each adjustment phase. During the adjustment process, the insufficiently part of request power is compensated by the battery. the load request power is compensated by the battery. ③ When the PFC reaches the maximum rated power, the proposed energy management control strategy controls the fuel cell output at the maximum rated power, and the insufficiently part of the load request power is compensated by the battery. ④ When the PLoad is suddenly reduced from 2.5 kW to 1 kW, since the PLoad is less than PFC, the battery is switched from the discharged state to the charged state. The energy management control strategy estimates the SOC value of battery and IBattery, and determines the output current limit of the DC/DC converter. Due to the discharge of the previous battery, the SOC is low, the estimated

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3 When the PFC reaches the maximum rated power, the proposed energy management control

strategy controls the fuel cell output at the maximum rated power, and the insufficiently part of the load request power is compensated by the battery. 4 When the PLoad is suddenly reduced from 2.5 kW to 1 kW, since the PLoad is less than PFC ,

the battery is switched from the discharged state to the charged state. The energy management control strategy estimates the SOC value of battery and IBattery , and determines the output current limit of the DC/DC converter. Due to the discharge of the previous battery, the SOC is low, the estimated charging current is large and the calculated setting current value of IDC/DC.set is greater than the maximum current limit value of the DC/DC converter, the setting value of IDC/DC.set is kept unchanged. 5 As IBattery decreases, when the difference between IDC/DC.set and IDC/DC.out reaches the

adjustment threshold ∆IFC.dec , the energy management control strategy re-estimates the output setting value and adjusts the output current limit value. When it satisfies the load power, at the same time, the battery can be continuously charged with a new charging current. 6 When the load continues to reduce power, the energy management control strategy immediately

estimates the SOC of battery, calculates a new output setting value of IDC/DC.set , and always keeps the battery in the constant current charging mode, keeping the output current and output power of the fuel cell stable while meeting the power demand of load. The current and power response curves shown in Figure 12 indicate that the proposed energy management control strategy has achieved the control objectives. During the entire operation, the output current of the fuel cell is stable, and the output power of the fuel cell gradually follows the load power request, preventing the impact of load abrupt changes on the fuel cell.

(2) Response test when the load power is frequently changing. The electronic load is used to simulate the power request of load shown in Figure 14a, and the hybrid power system is controlled by the proposed load current following control strategy. The response curve is shown in Figure 14b. 1 According to the proposed energy management strategy, when the request power of load is

0 kW, the IFC is equal to IFC.min , the output current of the fuel cell is stable, and the battery is charged. 2 At the moment when the load power suddenly increases, according to the activation situation

of the stack, the proposed energy management control strategy gradually increases the output of the fuel cell, and maintains the output current and power output smoothly in each adjustment phase. During the adjustment process, the insufficient part of the load request power is compensated by the battery. 3 When the PFC reaches the maximum rated power, the proposed energy management control

strategy controls the fuel cell output at the maximum rated power, and the insufficient part of the load request power is compensated by the battery. 4 When the load power drops suddenly, the energy management control strategy immediately

follows the load request current, adjusts the output setting value of the DC/DC converter, and makes the constant current output of the DC/DC converter ensure the output current of the fuel cell is stable. The current and power response curves shown in Figure 13 indicate that the proposed energy management control strategy has achieved the control objectives. During the entire operation, the output of the fuel cell is stable, meets the output characteristics of the fuel cell, and the control strategy is practical and effective.

load power request, preventing the impact of load abrupt changes on the fuel cell. (2) Response test when the load power is frequently changing. The electronic load is used to simulate the power request of load shown in Figure 14a, and the hybrid2018, power system is controlled by the proposed load current following control strategy. 20 The Energies 11, 3440 of 24 response curve is shown in Figure 14b. 2600 2400

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time/s (b) Figure curve of of load power; Figure 14. 14. Response Response test testwhen whenthe theload loadpower powerisisfrequent frequentchange. change.(a)(a)Request Request curve load power; and and (b) (b) the the response response curve curve of ofcurrent currentand andpower. power.

5. Test Result of Forklift ①and According to Hybrid the proposed energy management strategy, when the request power of load is 0 kW,Inthe I FC is equal to IFC.min, the output current of the fuel cell is stable, and the battery is charged. this paper, the energy management controller is realized by using the proposed energy management control strategy, and the fuel cell/battery hybrid forklift prototype is constructed by using the PEMFC controller, DC/DC converter, and energy management controller. In order to verify the effectiveness and control performance of the energy management strategy and fuel cell/battery hybrid forklift, an experimental test was carried out on the lifting and moving of the fuel cell/battery hybrid forklift prototype.

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(1) Lifting test of the hybrid forklift. When thex fuel current Energies 2018, 11, FOR cell/battery PEER REVIEWhybrid forklift lifts the cargo, the response curves of power and 21 of 24 of the fuel cell and battery are shown in Figure 15.

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time/s Figure 15. 15. Response Response curve curve of of the the hybrid hybrid forklift forklift while while lifting. lifting. Figure

Before the the lifting lifting motor motor is is operated, operated, the the fuel fuel cell cell charges charges the the battery battery and and its its output output power power is is less less Before than 2 kW. When the cargo is lifted, the power of load suddenly increases, and the fuel cell gradually than 2 kW. When the cargo is lifted, the power of load suddenly increases, and the fuel cell gradually increases the the output output power power under under the the control control of of the the energy energy management management controller controller until until the the rated rated increases power of 2 kW is reached, and the insufficient power of load is supplemented by the battery. In the power of 2 kW is reached, and the insufficient power of load is supplemented by the battery. In the course of lifting, the output current and power of the fuel cell are gradually increased, and the output course of lifting, the output current and power of the fuel cell are gradually increased, and the is generally stable, which to the output characteristics of the fuel cell. When lifting output is generally stable,conforms which conforms to the output characteristics of the fuel the cell.cargo When the is completed, the request power of load is instantaneously reduced to 0 kW. Under the control of the cargo lifting is completed, the request power of load is instantaneously reduced to 0 kW. Under the energy management controller, the output power of the fuel cell is gradually reduced, and the output control of the energy management controller, the output power of the fuel cell is gradually reduced, current the fuel cell isof gradually to the charging current of thecurrent battery.value During the and the of output current the fuel reduced cell is gradually reduced to thevalue charging of the whole operation process, the output current of the fuel cell is stable, and the output power changes battery. During the whole operation process, the output current of the fuel cell is stable, and the slowly,power which changes is consistent with the output characteristics the fuel cell. The battery charged and output slowly, which is consistent with theof output characteristics of theisfuel cell. The discharged according to the power of the load. The test results show that the fuel cell/battery hybrid battery is charged and discharged according to the power of the load. The test results show that the forklift prototypehybrid meets forklift the loadprototype conditionmeets demand the cargo. The energy distribution of fuel cell/battery thewhile load lifting condition demand while lifting the cargo. eachenergy powerdistribution unit is reasonable the control effect is good. The of eachand power unit is reasonable and the control effect is good. (2) Moving test of hybrid forklift. (2) Moving test of hybrid forklift. The fuel fuel cell/battery cell/batteryhybrid hybridforklift forkliftisis tested tested under under actual actual moving moving conditions. conditions. The The power power and and The current response response curves curves of of the the fuel fuel cell cell and and battery battery are are shown shown in in Figure current Figure 16. 16.

Power/W

Energies 2018, 11, 3440 Energies 2018, 11, x FOR PEER REVIEW

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In the the process distributes thethe output of In process of of moving, moving,the theenergy energymanagement managementcontroller controllerreasonably reasonably distributes output each power source. On the premise of ensuring the power demand of load, the output power of the of each power source. On the premise of ensuring the power demand of load, the output power of fuelfuel cell cell gradually increases or decreases withwith the the power of load, andand the the output is stable. When the the gradually increases or decreases power of load, output is stable. When power demand of load is lower than the output power power of the fuel cell,fuel the cell, battery theenters charging the power demand of load is lower than the output of the theenters battery the state, otherwise the battery enters the discharging state. The output current of the fuel cell gradually charging state, otherwise the battery enters the discharging state. The output current of the fuel cell increases or decreases the current of load, andofthe change is slow and stable, meets gradually increases orwith decreases with the current load, and process the change process is slowwhich and stable, the requirement the output characteristic the fuel cell. of The test results thatresults the performance which meets theof requirement of the outputofcharacteristic the fuel cell. show The test show that of the fuel cell/battery forklift is good, the forklift functional units of hybrid power the performance of thehybrid fuel cell/battery hybrid is good, thethe functional unitssystem of the operate hybrid normally, the energy is reasonable and the energy management strategy is effective. power system operatedistribution normally, the energy distribution is reasonable and the energy management strategy is effective. 6. Conclusions 6. Conclusions In this paper, a fuel cell/battery hybrid forklift is designed based on combined characteristics of lead-acid batteries fuel cells to reduce theforklift size and configuration power of the fuel cell, to improve In this paper, and a fuel cell/battery hybrid is designed based on combined characteristics of the dynamic response performance of the fuel cell, and to extend the cruising range the battery. lead-acid batteries and fuel cells to reduce the size and configuration power of theoffuel cell, to According the selected topology of the hybrid system, hardware units, range such as the improve thetodynamic response performance of the power fuel cell, and to the extend the cruising of the PEMFC According controller, unidirectional converter, and energy management controller, battery. to the selected buck-boost topology ofDC/DC the hybrid power system, the hardware units, such as are designed in detail, and a fuel cell/battery hybrid forklift prototype is built. An energy management the PEMFC controller, unidirectional buck-boost DC/DC converter, and energy management strategy based on load current following is proposed tohybrid manage the fuel cell/battery hybrid energy. controller, are designed in detail, and a fuel cell/battery forklift prototype is built. An energy Under the premise of meeting the power demand of the forklift, the output current and power of management strategy based on load current following is proposed to manage the fuel cell/battery the fuel cell are stable, the output performance is optimal, and the SOC of the battery is maintained hybrid energy. Under the premise of meeting the power demand of the forklift, the output current at a power reasonable Theare proposed energy management is tested bySOC using an electronic and of thelevel. fuel cell stable, the output performancestrategy is optimal, and the of the battery is load to simulate different power requests of the load. The test results show that the proposed energy maintained at a reasonable level. The proposed energy management strategy is tested by using an management strategy achieves the energy management control target, the output meets the output electronic load to simulate different power requests of the load. The test results show that the characteristics requirements of the fuel cell, the output is stable, and the request current of the can proposed energy management strategy achieves the energy management control target, theload output be followed during operation to gradually adjust the output current of the fuel cell. The hybrid forklift meets the output characteristics requirements of the fuel cell, the output is stable, and the request current of the load can be followed during operation to gradually adjust the output current of the

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prototype was tested for moving and lifting. The test results verify the proposed control strategy, the energy distribution, and the running performance of the system. Author Contributions: Conceptualization: Z.Y. and Y.H.; resources: Z.Y. and F.Z.; validation: Z.Y. and L.W.; writing—original draft: Z.Y., L.W., and Y.H.; writing—review and editing: Z.Y. and F.Z. Funding: This work was supported by the Fundamental Research Funds for the Central Universities (2016NZYQN12) and the open research subject of the Key Laboratory of Sichuan signal and information processing, Xihua University (SZJJ2016-094). Conflicts of Interest: The authors declare no conflict of interest. It should be noted that the whole work was accomplished by the authors collaboratively. All authors read and approved the final manuscript.

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