A 2.5 ton forklift is employed in this study and it is driven by a 3-phase ... The technical specifications of the fuel cell/battery hybrid system can be summarized in ...
Fuel Cell and Battery Powered Forklifts Zhe Zhang1, Henrik H. Mortensen2, Jes Vestervang Jensen2, and Michael A. E. Andersen1 1: Dept. of Electrical Engineering, Technical University of Denmark, Kongens Lyngby, Denmark 2: H2 Logic A/S, DK-7400 Herning, Denmark
Abstract—A hydrogen-powered materials handling vehicle with a fuel cell combines the advantages of diesel/LPG and battery powered vehicles. Hydrogen provides the same consistent power and fast refuelling capability as diesel and LPG, whilst fuel cells provide energy efficient and zero emission electric propulsion similar to batteries. In this paper, the performance of a forklift powered by PEM fuel cells and lead acid batteries as an auxiliary energy source is introduced and investigated. In this electromechanical propulsion system with hybrid energy/power sources, fuel cells will deliver average power, whilst batteries will handle all the load dynamics, such as acceleration, lifting, climbing and so on. The electrical part of the whole propulsion system for forklift has been investigated in details. The energy management strategy is explained and verified through simulation. Finally, experimental results from a prototype are given, to present the validity of analysis and design. Keywords—Fuel cell, battery, forklift, converter, control
I.
INTRODUCTION
As fossil fuels such as diesel and liquefied petroleum gas (LPG) become more scarce and expensive, hydrogen can offer a sustainable supplement as it can be produced based on sustainable energy sources [1]-[3]. Proton exchange membrane (PEM) fuel cells are considered as a good candidate for automotive applications due to their zero emission, low operating temperature and high power density [4]-[5]. Fuel cells can be implemented in electrical vehicles either as a standalone system or in combination with other power sources such as a battery and/or a super-capacitor. Hybridization can distinctly improve the performance of a whole propulsion system on various aspects, such as decreasing fuel cell cost, isolating the fuel cell from load fluctuations and exploiting the regenerative power from traction motors. Hence, hybrid fuel cell power conversion systems are well suited for the applications where the average power demand is low whilst load dynamics is relatively high [6]-[8]. Forklift as a critical material handling vehicle has been used widely in various places, such as warehouses and distribution centres. In recent years, the forklift propulsion system is identified as a potential fuel cell application for the near-term markets. From low emission point of view, forklifts are used considerably more intensively than cars, so the environmental and CO2 reduction benefits are much greater compared to the number of vehicles. In the most intensively used forklifts, using hydrogen in one forklift can remove an amount of CO2 equivalent to the yearly emissions from approximately eight cars. Furthermore, forklifts are often used in large fleets in a localized area, where hydrogen refuelling stations and infrastructure can be concentrated. Apart from the environmental benefits, the improved degree of utilization This work is supported by European Demonstration of Hydrogen Powered Fuel Cell Forklifts, FCH JU Grant No. 256862.
compared to diesel and compressed natural gas can significantly reduce operating costs. On the other hand, compared to the battery-powered forklift trucks, the fuel cell powered ones can run up to three times longer than its battery driven counterpart, eliminating the lost productivity due to replace battery. The fuel cell produces constant voltage and maintains constant power output capacity throughout the shift. And by using flexible hydrogen dispensing, located conveniently throughout the warehouse, truck operators can refuel their lift trucks in 3 minutes or less. Through a literature review, it is found that only very few papers which have addressed the hybrid fuel cell/battery forklift systems. In [9], two triple-hybrid systems, including a fuel cell, battery and super-capacitor were investigated for a forklift system. The simulation results presented a significant reduction on the load variations of the fuel cell by using supercapacitors. But super-capacitors will increase the system cost and complexity of system control. In [10], by conducting a parametric study in simulation, various combinations of fuel cell size and battery capacity were employed to investigate their effect on hydrogen consumption and battery state-ofcharge (SOC). But, somehow, the measurement results are missing in that paper. Hence, a hybrid fuel cell/battery powered forklift propulsion system has been investigated and developed in this paper. The low temperature PEM fuel cells will deliver an average load power as well as charge the batteries when the SOC is low, whilst the batteries will handle all the load variations to increase the system dynamic response and at the same time protect the fuel cells from overvoltage and high di/dt. Following the introduction, the configuration of fuel cell and battery hybrid energy system is presented in Section II, where the characteristics of the main units, such as the fuel cell system, the battery bank and the DC-DC converters are analysed. The energy management strategy with battery voltage control is explained and then verified through simulation. Finally, experimental results from a prototype are given, to present the validity of analysis and design. II.
SYSTEM CONFIGURATION
A. Forklift specifications A 2.5 ton forklift is employed in this study and it is driven by a 3-phase motor couple to the front wheels with a peak power rating of 35 kW. The forklift is powered by a fuel cell/battery hybrid system, which is illustrated in Fig. 1, where fuel cells are adopted as a primary energy source and batteries are employed as a power buffer unit in order to deliver the power during the fuel cell’s warm-up stage and all dynamic
of their output current is plotted in Fig. 2. Generally, as for the fuel cell itself, the lower output power is, the higher efficiency is. However, due to the balance-of-plant (BOP), it makes operation at low power too inefficient, and therefore the fuel cell system will be turned into standby state without any output power in order to keep relatively higher total system efficiency. Furthermore, in order to avoid damages from high cell voltage, the fuel cell can only operate from 20% of the total power to full power. Fig. 1: Block diagram of electrical system of the fuel cell and battery powered forklift.
C. Batteries: Batteries are used to support power load, during fuel cell start-up and also when there are fast current changing because of fast load variations of the forklift, for example, lifting, accelerating or regenerative braking. Seven 100 Ah lead acid batteries connected to the DC bus. In the simulation, the instantaneous SOC(t) of battery can be estimated by the wellknown estimation method, i.e. by integrating the battery current overtime, and then subtracting it from the initial battery SOC0, as ∙ (1) where battery efficiency ηbatt can be defined differently during charge and discharge, C represents the nominal battery capacity, and ibatt is the instantaneous battery current.
Fig. 2: Fuel cell terminal voltage as a function of its output current.
Fig. 3: One cycle of VDI 2198.
powers due to the load transients as well. The technical specifications of the fuel cell/battery hybrid system can be summarized in Table 1. TABLE I.
TECHNICAL SPECIFICATIONS
Average power output
~10 kW
Maximum power output
~35 kW for 15 sec.
System efficiency
>48% @10 kW
Hydrogen storage capacity
~1.5 kg @350 bar
Battery storage capacity
100 Ah
Run time on full tank
5-6 hours/ 1 working shift
Refuelling time
3-4 min.
D. DC-DC converter Due to the variable output voltage of the fuel cells as well as the variable dc-link voltage (battery voltage), a DC-DC converter, which can boost up or buck down its input voltage, is required to regulate and manage the output power of the fuel cell system. There are 10 flexible isolated EWiRaC dc converter units [11] connected in a special way in order to regulate the fuel cell output voltage to match the battery voltage. Here an average current control is adopted on the DCDC converters, so the fuel cell with the DC-DC converters can be treated as a controllable current source from the system control standpoint. E. VDI cycles VDI stands for ‘Verband Deutscher Ingenieure’, and it is an accepted standard for comparing the fuel consumption of the different forklift trucks. The VDI cycle is defined in terms of a physical track on which the forklift must perform a set of tasks within defined time constraints. However, in practice, how the truck is used, and, in particular, the way that the operator drives can, and usually does, make an even more significant difference. In this study the VDI cycles are used as the load variations in the simulation in order to compare different system control schemes. The parameter of one cycle of the VDI 2198 employed here is shown in Fig. 3 which presents a simplified measurement of the power draw during a VDI cycle. III.
B. Fuel Cell subsystem In this hybrid system, the fuel cells only delivery average power to the load, and also charges the battery bank when its SOC is low. The output voltage of the fuel cells as a function
HYBRID SYSTEM POWER MANAGEMENT AND CONTROL
Generally, a control system of the fuel cell powered forklift can be divided into three layers: 1) Component layer: the fuel cell BOP management, the DC-DC converter, regulation, the battery charge control, the
be protected from any noise and required load dynamics. The different powers are defined and denoted in this figure. Two different power control strategies have been investigated here as follows. A. SOC control scheme As the block diagram shown in Fig.5, the controller requests the battery to be charged with a certain power depending on the SOC and the rate of the SOC, whatever the load power is: If the SOC is lower than SOCref, the battery charging current reference is positive and the fuel cell current is necessary to charge the battery, i.e. the requested charge power is translated to a fuel cell power/current reference, and at low SOC higher power is requested and vice versa; if SOC is higher than SOCref, then correspondingly, the fuel cell current/power reference is reduced to zero. As a consequence, a transient in the load modifies the fuel cell power reference. While due to the BOP of the fuel cell, the fuel cell cannot response the transient conditions effectively, and thereby the battery supplies all load variations.
Fig. 4: Simplified block diagram of the hybrid power system.
Fig. 5: Block diagram of the battery SOC control scheme.
SOC
0.8 0.7 0.6
Power (pu.)
1.3 0.65 0
-0.65 -1.3 80
100
120
140
160
180
Time (s)
Fig. 6: Simulation results with SOC control scheme (SOCref=0.8).
Fig. 7: SOC regulation: SOCref step up from 0.5 to 0.8 (blue solid line).
traction and hydraulic pump motor control and the braking resistor control are included in this layer. 2) Power/energy management layer: in this layer, the control on SOC of the batteries, output power of the fuel cells, and power balancing between load and power sources will be implemented. 3) Supervisory layer: it consists of user interface, comunication and system monitor and protection. Regarding to the power regulation, a simplified block diagram of the hybrid fuel cell/battery powered forklift is shown in Fig.4. The fuel cell and battery are operated as an interactive power conditioning system so that the fuel cell can
The simulation results are given in Figs. 6 and 7. Specifically speaking, the SOCref (blue) and regulated SOC (red) are shown in the upper subplot; the load power Pload (blue), the battery power Pbat (red), the fuel cell power reference (black dashed line) and the fuel cell actual output power Pfc (black solid line) are illustrated in the lower subplot, respectively, in Fig. 6. As we can see, due to the maximum ramp rate of the fuel cell being limited, there is a distinct difference between the reference and real output of the fuel cell power, and the response dynamics of the fuel cell is consequently restricted which can effectively increase its lifetime. The battery bank as a power buffer unit balances the power difference between the required load power and the actual fuel cell power, so the hybrid system is regulated and the SOC is fully controlled. Moreover, the regulated system can also response to the changes from the reference signal, as the simulation results shown in Fig. 7, where the SOCref steps up from 0.5 to 0.8 and the battery SOC can follow it accordingly in the SOC control loop. While, the drawbacks of this SOC control strategy are: 1) the SOC is only one factor in defining the allowed charge power, where especially the temperature, stratification and sulfation can be very significant factors too; 2) the SOC estimates are not sufficiently reliable. Effectiveness of the control highly depends on the actual battery SOC estimation, but it is difficult to estimate the SOC precisely when batteries employed in the heavy-duty and intensively used vehicles, such as a forklift; 3) the battery voltage will vary up and down dramatically according to the load conditions, which is harmful to the batteries. The situation is even worse due to the fuel cell’s slow response, which can be seen in Fig. 6, where the batteries can see very high voltage caused by the regenerative energy coming from the load and plus the energy from the fuel cell since its current cannot ramp down fast enough. Therefore, in practice, the battery voltage control is needed. B. Voltage control scheme In this scheme, the voltage control loop is added, as shown in the Fig. 8, where GP_I, fc and Gv_P, bat represent the transfer
Power limit
Vbat,ref
PID Regulator
SOC Battery SOC Estimation
vbat
Pfc vfc
PBOP
Ramp rate limit
GP_I,fc
÷ SOC
Rbat
Pbat
Pload
ibat vdc
GV_P,bat
Power (pu.)
Fig. 8: Block diagram of the battery voltage control scheme.
Fig. 9: Power balance: output power of hybrid system (solid line) and load power Pload (red dashed line).
Fig. 10: Comparison of the two control schemes in terms of the battery voltage: SOC control (blue dashed line), and voltage control (red solid line).
rate is also required. A set of boundaries for the stack power output will be re-calculated based on an off-line defined table of allowable current ramp rates with a certain current range, before each initialization of the controller. From the simulation results in Fig. 9, the balance between the load power and the output power of the hybrid system, which is one of the basic requirements, is satisfied, so that the effectiveness of the control scheme is verified. Comparing to the SOC control method, the voltage overshoot and undershoot across the batteries are restrained as the waveforms presented in Fig. 10. But the battery voltage has more severe variation than the battery SOC, and the only control freedom which can be applied to regulate the battery voltage is the fuel cell output power, which means the fuel cell must provide some dynamic response to support the system function; otherwise the power balance cannot be satisfied properly. For this reason, it is preferable to operate with a faster up/down current ramping of the fuel cell in the voltage control than that in SOC control. C. Protection In order to make sure the system can operate properly in any load conditions, different operating ranges with different system commands can be defined. An example, based on both SOC and battery voltage, is presented in Fig. 11. In the red area, the SOC is bigger than 1 and/or battery voltage is above Vmax, the system will completely shut down. When voltage is between Vbrake and Vmax, power resistors increase the pulse width gradually to protect the battery from overvoltage; Vdrive and Vstill are the battery voltage reference when the forklift is moving and standing still, respectively. On the other hand, when the SOC is too low, for instance, lower than 0.6, the fuel cell will start to charge the battery with its full power to avoid the battery’s operating with insufficient capacity. Simultaneously an over voltage protection circuit must monitor the stack cell potential which is a result of the output from the stacks current control loop. Whenever the fuel cell voltage exceeds 0.85 V/cell the system should be put in idle state and wait until a certain amount of energy has been drawn from the battery.
Fig. 11: Different operating ranges.
functions from current to power of the fuel cell and from power to voltage of the battery, respectively. The battery’s open circuit voltage and internal resistance Rbat, which are functions of the SOC, can be determined by the manufacturer-provided data. By voltage control, the feedback loop of the battery voltage will compensate for the voltage drop effectively. There will still be fed forward of the immediate power dissipation from the vehicle to ensure a quick calculated response to sudden power consumptions. The voltage reference Vref,bat can be made dynamically; for instance, a lower reference voltage allows more of the regenerative power to be sent to the batteries, and in contrast it can be higher when the vehicle is not moving, which means that the likelihood of regeneration is very small. Regarding the fuel cells, a function of limiting slew
IV.
EXPERIMENTAL RESULTS
With the voltage control method proposed above, a fuel cell and battery hybrid power conversion system for a 2.5 T forklift is manufactured and tested. The measured waveforms at a customer site are presented in Fig. 12, where all the values are normalized as ,
, ,
,
(2)
The battery voltage is effectively regulated by the voltage control scheme and thereby the voltage overshoots and undershoots are limited. It can be seen in Fig. 12 (e) that the braking resistor is switched ON and OFF in order to protect the battery from the overvoltage due to the high regenerative power. Also, the internal resistance of the battery is estimated at run-time based on the battery voltage response to the battery current and is plotted in Fig. 13. It can be seen that the value
0.8
(a)
Load Current (Normalized )
0.6 0.4 0.2 0
-0.2
(b) Battery Current (Normalized )
0.2 0 -0.2 -0.4
cases, the battery voltage control is more effective to attenuate the DC bus voltage overshoot in different operating scenarios and battery conditions such as sulfation, stratification and temperature etc., especially, when a large regenerative power is back from the load. While the major weakness of the SOC based control is that the SOC is the only factor in defining the allowed charge/discharge power, and unfortunately the actual SOC of the battery is difficult to estimate in heavy-duty vehicles, and furthermore other very significant factors of the battery such as temperature, stratification and state of health have not been taken into account in the SOC control scheme.
Braking Resistor Current (Normalized )
Output Current of DC converter (Normalized )
Battery Voltage (Normalized )
(c) 0.9
0.8
REFERENCES (d)
0.8 0.6 0.4 0.2
(e) 0.8 0.6 0.4 0.2 Time
Fig. 12: Measurement waveforms.
Fig. 13: Internal resistance of the battery.
changes with the different SOC, direction, temperature and so on.
V.
charging/discharging
CONCLUSIONS
A hybrid fuel cell/battery power propulsion system was investigated and applied for a forklift. Different system power management strategies were studied, i.e. the battery SOC control and the battery (DC bus) voltage control. In those two
A. Emadi, Y. Gao and M. Ehsan, “Modern electric, hybrid electric, and fuel cell vehicles: fundamentals, theort and design”, 2nd Edition, 2009, CRC Press. [2] Z. Zhang, P. Riccardo, M. A. E. Andersen and O. C. Thomsen, “A review and design of power electronics converters for fuel cell hybrid system applications” Energy Procedia, vol. 20, pp.301-310, 2012. [3] J. Hwang, J. Kuo, W. Wu and etc., “Lifecycle performance assessment of fuel cell/battery electric vehicles,” Int. Journal of Hydrogen Energy, vol.38, no8, pp.3433-3446, 2013. [4] G. Hoogers, “Fuel cell Technology-Handbook,” 2002, CRC Press. [5] J. Kim, J. Lee and B. H. Cho, “Equivalent circuit modeling of PEM fuel cell degradation combined with a LFRC,” IEEE Trans. Ind. Electron. vol.60, no.11, pp.5086-5094, Nov. 2013. [6] S. Yamaguchi, T. Yamazaki, Y. Namekawa and Y. Takahashi, “Hybrid energy control using DC-DC converter for single operator fuel cell vehicle,” in Proc. IEEE Vehicle Power and Propulsion Conference (VPPC), pp.1188-1193, 2012. [7] Z. Zhang, Z. Ouyang, O. C. Thomsen and M. A. E. Andersen, “Analysis and design of a bidirectional isolated DC-DC converter for fuel cell and super-capacitor hybrid system,” IEEE Trans. Power Electron. vol.27, no.2, pp.848-859, 2012. [8] Z. Zhang, O. C. Thomsen and M. A. E. Andersen, “Soft-switched dualinput dc-dc converter combining a boost-half-bridge and a voltage-fed full-bridge cell,” IEEE Trans. Power Electron. vol.28, no.11, 2013. [9] E. Hosseinzadeh, M. Rokni, S. G. Advani and A. K. Prasad, “Performance simulation and analysis of a fuel cell/battery hybrid forklift truck,” International Journal of Hydrogen Engergy, 38(2013), pp.4241-4249. [10] TM Keranen, H. Karimaki, J. Viitakangas, and etc. ”Development of integrated fuel cell hyrbid power source for electric forklift,” Journal of Power Sources, 196, pp.9058-9068, 2011. [11] H. Schneider, S. Bergendorff, L. Petersen and M. A. E. Andersen, “Isolated EWiRaC: a new low-stress single-stage isolated PFC coverter,” in Proc. 22nd Annual IEEE Applied Power Electronics Conference and Exposition (APEC), pp.159-164, 2007. [1]
