Control optimization of an electric traction bus powered by PEMFC and PV I. Zamora1, J.I. San Martín2, F.J. Asensio1, J. García-Villalobos1, J.J. San Martín2, V. Aperribay2. Department of Electrical Engineering - University of the Basque Country (UPV/EHU) 1
Escuela Técnica Superior de Ingeniería de Bilbao Alda. Urquijo s/n, 48013 Bilbao (Spain) e_mail:
[email protected]
Abstract—This paper focuses on the design and optimization of the electrical system operation in an electric drive bus, powered by a PEM fuel cell and photovoltaic modules. Through the optimization developed, an improved performance and reduced operating costs for the system, in normal operation, is pursued. Aiming that objective, models of all components (fuel cell, solar modules, electric motors, batteries, controllers, etc.) involved in the system have been developed using the software MATLAB. Results obtained with these models have allowed the designing of an optimized control system. Index Terms—Clean Urban Transport, Fuel cells, Hydrogen, Modeling, Photovoltaic systems.
I.
INTRODUCTION
Currently, environmental legislation forces automobile manufacturers to replace those vehicles that produce large amounts of polluting emissions. Therefore, it is essential to develop new solutions to deal future challenges in the automotive field. In this sense, the technology of electric vehicles powered by hydrogen has ended the R&D phases performed by manufacturers and field testing has already begun to demonstrate the reliability and the dependability of the technology. The polymer electrolyte membrane (PEM) fuel cells are best suited to meet the requirements needed by this application. They present quiet operation, modularity and low maintenance.
2
Escuela de Ingeniería de Eibar Avda. Otaola, 29, 20600 Eibar (Spain) e_mail:
[email protected] acceleration levels and behaviour similar to a conventional diesel bus. This project has continued in HyFLEET: CUTE. ECTOS project‘s main objective was to feed three hydrogen fuel cell buses powered by a 250 kW PEM system from Ballard, in the regular public transport fleet of Reykjavik, for a test period of two years [4]. For that purpose, a hydrogen fuelling station completely integrated into an urban setting has been constructed. Finally, the purpose of the STEP project was to determine technical, environmental, economic and social factors that need consideration in the introduction of hydrogen fuel cell buses. In this project, three Mercedes Citaro fuel cell buses were used in Western Australia, for two years [5]. This paper presents the control system optimization of an electric drive bus powered with a fuel cell and photovoltaic modules, starting from the design presented in [6]. It also contains a description of the components used in modelling and results obtained in the simulations. II. STRUCTURE OF THE FUEL CELL BUS Figure 1 shows the block diagram of the H2 bus system, which gives an idea of how the electrical and electronic system of the electric traction bus is and works. Main power flows from the fuel cell, photovoltaic modules or batteries.
In recent years, several types of fuel cell based buses have been developed, using different propulsion structures and strategies for energy management [1], [2]. Thus, several projects have been carried out, among which the CUTE project (Clean Urban Transport for Europe), ECTOS (Ecological City Transport System), STEP (Sustainable Transport Energy Project) and HyFLEET: CUTE can be highlighted. The CUTE project was the first, which has simultaneously investigated hydrogen production, its supply in urban areas and the operation of fuel cell vehicles in commercial public transport networks. As part of the project, 27 buses powered by a 205 kW PEMFC had circulated in streets of nine European cities for two years (Amsterdam, Barcelona, Hamburg, London, Luxembourg, Madrid, Porto, Stockholm and Stuttgart) [3]. The fuel cell used has permitted
Figure 1. Electrical bus block diagram
Starting from the fuel cell, its output goes to a buck converter, because the output voltage of the fuel cell is not regulated and therefore must be stabilized. Most of the outgoing energy from the buck converter is used for feeding the electric motor while the rest can be used to charge the electric batteries through the charging regulator. The DC/AC inverter is responsible for adequately feed the electric drive
Considering the energy from the photovoltaic modules, a boost converter is required, due to the possible fluctuation of the unregulated voltage from the photovoltaic cells. Apart from this converter, a MPPT controller (Maximum Power Point Tracking) is needed to extract the maximum power of the photovoltaic modules. Once stabilized the voltage, energy is injected into the DC link. If the electric bus is running, this energy is used to supply auxiliary systems or to support the fuel cell. In case there is much solar radiation and the bus is idle, an electrolyser is used to generate hydrogen, which is injected again in the bus hydrogen tank. This electrolyser is fed continuously through a buck converter and auxiliary systems are fed from the DC link, with the corresponding prior conditioning. Regarding to the electric motor output, the power flow between the DC/AC inverter and this motor is bi-directional (Fig. 1). This way, it is possible to return energy to the DC link by regenerative braking. The energy from this process is used again for charging batteries or, if they are already charged, for on-board hydrogen generation. As can be seen, all this system requires a complex power management, which is key for optimizing the system itself and improving the overall efficiency. Hence, the importance of performing multiple analyses by simulation to quantify how the system behaves and, based on the obtained results, make improvements and optimization. III.
COMPONENT DESCRIPTION AND MODELLING
In this section, a description of the system components is presented, providing the most relevant data which have been used to build simulation models in MATLAB. A. Fuel Cell An FCvelocity-HD6 fuel cell, from Ballard manufacturer, has been considered [7]. This fuel cell provides an ideal design for automotive applications and specifically for integration into buses. The power that can be supplied by the stack is 150 kW, with efficiency around 62-71%. The efficiency of the whole fuel cell system, taking into account the air compressor, cooling system and all control components, is 55%.
Where: R = 8.3145 J/(mol K); F = 96,485 A s/mol; z is the number of moving electrons; En is the Nernst voltage (V); Į is the charge transfer coefficient; PH2 is the partial pressure of hydrogen inside the stack (atm); PO2 is the partial pressure of oxygen inside the stack (atm); k is the Boltzmann's constant (J/K); h is the Planck's constant (J s); ǻG is the activation energy barrier (J); T is the temperature operation (K) and Kc is the voltage constant at nominal condition of operation. This fuel cell model has been implemented in Matlab/Simulink. To validate the correct operation, a sweep current to the fuel cell by varying the load resistance has been performed, obtaining the polarization curve of Figure 2. This polarization curve obtained in the simulation is similar to that provided by the manufacturer. 800
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and also manages the voltage/frequency parameters to maintain the torque constant at all times.
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Figure 2. I-V Polarization curve
B. Hydrogen storage Hydrogen storage is made by 10 ZM180 bottles of 178 litres each, which equals 4.31Kg of H2. So that, the total system capacity is 43.1 kg of H2. The pressure at which the hydrogen is storaged is 350 bars at 15°C [8]. To model the behaviour of the hydrogen bottles, the ideal gas equation and the universal gas constant is used. Tank pressure is a function of the consumption of hydrogen by the fuel cell and tank supply by the electrolyser. The tank model has been implemented in Simulink. Figure 3 shows the tank pressure variation depending on the net amount of hydrogen (slpm) that enters and leaves the tank, with an operating temperature of 25 °C. If only 0 to 100 s is considered, where the flow is only consumed by the fuel cell and is kept constant (1743.72 slpm), a pressure drop from 350 bar to 349.313 bar can be appreciated.
For the modelling, variations of pressure, temperature, composition and flow rates of air and fuel have been taken into account [7]. Variables that affect the open circuit voltage (EOC), the variation of current (i0) and the Tafel slope (A) are modified according to equations (1) to (3). E oc
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K c En § · zFk ¨ PH PO ¸ 'G 2 ¹ RT © 2 e Rh
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(1) (2) (3) Figure 3. Input and output hydrogen flow rate (top) and tank pressure (below)
To model photovoltaic modules, a current source dependent on solar irradiation, resistors, diodes and voltage sources have been used. Figure 4 shows the solar cell V-I polarization curve, for an irradiation of 1000 W/m2. It can be seen that the open circuit voltage (339 V), the short circuit current (10.6 A) and the voltage (273.75 V) and current (9.86 A) for maximum power match the data provided by the manufacturer.
Figure 4. V-I polarization curve obtained with the model of a solar cell in Simulink
D. Buck converters Two of such converters have been used. One of them is located at the output of the fuel cell stack, which provides a voltage between 550 and 800 V, and a maximum current of 300A. This voltage is reduced and stabilized at 400V (DC link voltage) at the buck converter output. The other converter is between the DC link (400 V) and the electrolyser, fed to 50 V.
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C. Photovoltaic modules PV modules are made of monocrystalline silicon, the IS180 from Isofoton manufacturer [9]. Considering the useful area of the bus roof, 15 modules have been implemented, which provide a peak power of 2.7 kW. The voltage at the maximum power point is 273.75 V. This voltage value will be critical to the design of the power converter for MPPT and conditioning voltage for charging batteries.
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Figure 5. Input and output voltage of the Buck converter
E. Boost converter with MPPT The function of the boost converter is to increase and stabilize the unregulated voltage generated by the photovoltaic modules. This voltage is raised to 400 V to allow its injection into the DC link. The output power varies in relation to the solar radiation and temperature. The situation in the I-V curve can also be affected by the load connected to the photovoltaic modules. In order to extract maximum power, a MPPT control has been implemented in Simulink with current and voltage feedback. To validate this model, it is assumed that the boost output voltage is fixed, as the DC link voltage is controlled by the action of the buck converter of the fuel cell. Figure 6 shows the boost input voltage, as result of the control performed by the MPPT control. This voltage corresponds to the maximum output voltage (273.75 V) plus a small offset. Given that the irradiation used for the simulation has been 1,000 W/m2, properly operation of the control can be verified. As can be seen the controller acts quickly stabilizing the voltage practically to its final value within 5 ms.
To adjust the duty cycle depending on the dynamic characteristics of the system, a PWM control structure with a PID controller has been implemented in Simulink. A 100 kHz switching frequency is used. To verify the correct operation of the control algorithm implemented, three resistors connected in parallel at different instants of time, as a variable load, have been introduced. Each resistor has a value of 10 ȍ. Therefore, initially load seen by the converter is R1 (10 ȍ), then R1//R2 (5 ȍ) and finally R1//R2//R3 (3.33 ȍ). Figure 5 shows the input and output voltage for t1 and t2 activation instants in 0.02 s and 0.04 s, respectively. As can be seen, the buck output voltage remains constant to variations in the load and input voltage.
Figure 6. Transient volatge of the PV panel
F. Buck-Boost charge regulator Since the system has to work at the same time that the batteries work, but independently, a parallel configuration is required. Furthermore, the topology must be capable of transferring energy to and from the batteries to a maximum power, close to the maximum that can be supplied by the batteries (103.68 kW). Hence, a buck-boost configuration for interconnection between the batteries and the DC link is used. Figure 7 shows the battery pack charging voltage and current when the converter is operating in buck mode. It can
be seen that the controller acts quickly setting the voltage of the batteries to 394 V (voltage battery charge). Moreover, the current is set to a value of 140 A, reaching the value recommended by the manufacturer for charging batteries [10].
Figure 9 shows the behaviour of the lithium-ion battery in terms of voltage and current during battery charging. The voltage applied to the battery pack by the bidirectional converter is 394.2 V (27x14.6 V) and the charging current supplied 140 A (2x70 A). Both values correspond to the voltage and current recommended by the manufacturer for efficient battery charging. For the simulation, an initial state of charge (SOC) of 50% has been pre-set, to appreciate the evolution of this charge over time.
Figure 7. Battery set voltage and current during buck operation
Figure 8 shows the DC link voltage and the battery pack voltage and current when the converter operates in boost mode. The battery voltage is maintained at the rated voltage (345.6 V) while the DC link voltage (converter output) reaches 400 V with a small ripple. The batteries provide an average current of 300 A (nominal discharge current) with 600 A current peaks, which is the acceptable maximum discharge current for the batteries, verifying that the design of the model meets the performance requirements. 400
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Figure 10 shows the behaviour of the lithium-ion battery in terms of voltage and current during battery discharge. The nominal voltage of the battery pack is 345.6 V (27x12.8 V) and the supplied average discharge current is 300 A (2x150 A). Both values correspond to the nominal voltage and current recommended by the manufacturer. For the simulation, the SOC of the battery has been pre-set at 100%, to appreciate the evolution of the discharge over time.
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Figure 8. DC link voltage and battery set voltage and current during boost operation
G. Batteries The modelled batteries are of Lithium Ion type. The U2712XP model from the Valence manufacturer has been used [10]. The energy to be supplied by this battery pack, to satisfy the needs required by the bus, is approximately 100 kWh. This way, two branches of 27 batteries each are placed to supply a total charge of 276 Ah and a total energy of 95.4 kWh. Depending on whether the battery current is negative or positive, the equations for the charge and discharge of the battery are defined. These equations have been implemented in Simulink.
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Figure 9. Voltage, current and SOC during battery charging
Figure 10. Voltage, current and SOC during battery discharging
H. Inverter This is a fully controlled three phase inverter, which allows bidirectional power flow. It is necessary to inject the regenerative braking energy from the drive, when working in generator mode (when the bus is braking). The control of this inverter has been performed using field-oriented control (FOC). This control has been implemented in Simulink.
Applying a second order low pass filter, a sinusoidal component of the line voltage at the inverter output is obtained. The filter cut-off frequency is 100 Hz, with a damping factor of 0.707. This allows passing the 50 Hz fundamental component.
This power is converted to hydrogen flow rate (slpm) generated by the electrolyser as shown in equation (5).
I.
In this model, the efficiency variation due to the pressure change of the hydrogen tank is not taken into account.
Electric motor The power required by the bus electric traction at peak load is 150 kW. However, to ensure that the motor is able to respond to any unexpected situation, such as a high slope, a permanent magnet motor with a mechanical power of 170 kW has been chosen. The model used for permanent magnet synchronous motor is included in the library SimPowerSystems/Simulink Machines 7.4. The electrical and mechanical parts of the machine are represented by a second order state-space model. Furthermore, the sinusoidal model assumes that the flux created by the permanent magnets in the stator is sinusoidal, implying that the electromotive forces are sinusoidal.
1Kg 1h 14,12 l h Poutput ( kW ) 33 kW 60min 1Kg
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Flow rate ( slpm )
(5)
SIMULATION RESULTS
Figure 12 shows the complete model of the bus system implemented in Matlab/Simulink. On-board power generation, distribution, regulation and power control systems have been considered. The purpose of the simulations developed is to understand the behaviour of each element and the overall system. From there, solutions to optimize both the design and control system can be identified.
To set the set-point speed, the turning circle of the bus tyre has been taken into account, so the change of angular velocity to linear velocity can be made. As reference, a bus wheel with a 22.5 inches rim diameter and 80 mm tire profile has been taken. Establishing a reference speed of 55 km/h, an angular velocity of 398.88 rpm is obtained. To perform this simulation an increasing torque over time has been applied (Figure 11). It is observed that the motor phase currents increase as the torque increases, keeping constant the speed set-point.
Figure 12. Electric bus model (Matlab/simulink implementation)
One of the problems detected is the large ripple voltage in the DC link, in certain operating regimes, which can cause premature aging of the devices (Fig. 13).
Figure 11. Motor torque applied and current of the phase A
J.
Electrolyzer To perform the necessary simulations, a 2.25 kW electrolyser is taken as reference, which has been used by the International Energy Agency in a hydrogen implementing agreement, in which integrated systems of hydrogen study cases are analysed [11]. The electrolyser produces hydrogen at 20 bar of pressure and has an efficiency of 45%. For the construction of the electrolyser model, a gain value is used with the value of the electrolyser-compressor assembly efficiency, so that for a given input power, it will have a useful power output, defined by (4). Poutput
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Figure 13. Fuel cell and DC link voltage
Moreover, Figure 14 shows how the fuel cell system is not able, by itself, to adequately supply the demand for some instantaneous power peaks, such as the motor starting. Another problem detected concerns to the hydrogen production, as current drifts are detected from the battery during the electrolysis process.
Figure 14. Fuel cell and DC link voltage under a peak demand
Figure 16. Control strategy implemented for the electrolysis process
ACKNOWLEDGMENT Considering the previous obtained results, one of the improvements made is the implementation of a set of ultracapacitors in the DC link. After placing four ultracapacitors in series, with a capacity of 63 F and a voltage of 125 V each, the bus has managed to maintain a stable DC link voltage against this type of disturbance (Figure 15). 800
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Figure 15. Fuel cell and DC link capacitor set voltage
Another aspect improved is the control of hydrogen production, as current drifts have been detected from the battery during the electrolysis process. To fix this, a change in the control strategy of the electrolyser buck has been made, introducing a current control loop (Figure 16). Thus, the hydrogen production system is able to work according to the irradiance at each instant. V.
CONCLUSIONS
This paper has presented the modelling of an electric bus based on fuel cell and PV modules. Also the control of the electrical system and production of hydrogen is presented. Through the simulations developed, undesirable behaviours in the electrical systems of the bus have been detected. Improvements to increase the electrical efficiency, to prevent possible degradation of power electronic devices and to increase the performance of the electrolyser and photovoltaic modules are proposed. These improvements include the installation of ultracapacitors in the DC link and a control strategy for a more efficient hydrogen production.
The work presented in this paper has been supported by the Basque Government (Ref. IT532-10) and by the University os the Basque Country UPV/EHU (UFI 11/28). REFERENCES [1] S.J. Andreasen,, L. Ashworth, I. Natanael, M. Remón, S.K Kær “Directly connected series coupled HTPEM fuel cell stacks to a Li-ion battery DC bus for a fuel cell electrical vehicle” International Journal of Hydrogen Energy, Vol.33, pp. 71377145, 2008. [2] L. Xiangjun, X. Liangfei, H. Jianfeng, L. Xinfan, L. Jianqiu, O. Minggao, “Power management strategy for vehicular-applied hybrid fuel cell/battery power system”, Journal of Power Sources, Vol.191, pp.542-549, 2009. [3] Clean Urban Transport for Europe. Project No.NNE5-200000113. Deliverable No.8. Final Report, 2006. [4] Maria H. Maack, Thomas H. Schucan, “Ecological City Transport System”. IEA Hydrogen Implmenting Agreement, 2006. [5] STEP (Sustainable Transport Energy Project) [Internet]: http://www.global-hydrogen-busplatform.com/About/History/STEP [6] I. Zamora, J.I. San Martín, J. García, F.J. Asensio, O. Oñederra, J.J. San Martín, V. Aperribay “PEM fuel cells in applications of urban public transport”. ICREPQ’11 – Las Palmas de Gran Canaria (Spain), 13-15 de April, 2011. [7] FCvelocity-HD6 Fuel cell data sheet [Internet]: http://www.ballard.com/files/PDF/Bus/HD6.pdf [8] ZM180 hydrogen storage bottle data sheet [Internet]: http://www.dynetek.com/pdf/350.pdf [9] IS-180 photovoltaic module data sheet [Internet]: http://www.solar-light.es/pdf/ISOFOTON.pdf [10] U27-12XP batteries data sheet [Internet]: http://www.valence.com/sites/default/files/u_charge_xp_modul e_datasheet_aug_12_2.pdf [11] Dutton, A. G.; Bleijs, J. A. M.; Dienhart, H.; Falchetta, M.; Hug, W.; Prischich, D.; Ruddell, A. J. Experience in the design, sizing, economics, and implementation of autonomous windpowered hydrogen production systems. Int. J. Hydrogen Energy 2000, 25, 705-722.
