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ADVANCED CONVERTERS FOR FUEL CELL POWER SYSTEMS

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ADVANCED CONVERTERS FOR FUEL CELL POWER SYSTEMS



Adriana FLORESCU, Dan Alexandru STOICHESCU, Alina OPREA

University POLITEHNICA of Bucharest

Faculty of Electronics, Telecommunications and Information Technology

Department of Applied Electronics and Information Engineering

Abstract - DC-DC converters for fuel cell power system are usualy bi-directional and have complicated circuits, needing many devices and switching configurations. This article presents the most advanced and new converter suitable for many applications (including FCHEVs applications), including both the DC/DC converter and DC/AC inverter. It proves that high efficiences, low costs and future developments in fuel cell power systems can and will be achieved with simple, ingenious, widespread converters.

Keywords - Fuel cell (FC), PWM inverter, boosted PWM inverter,  Z-source inverter, hibrid electrical vehicle (HEV).

1. INTRODUCTION

In clean electrical energy production from hydrogen or other fuels with high efficiency and low pollution emission, fuel cells currently represent the most advanced energy conversion system that attract the interest of more and more engineers, as shown by a growing number of firms investing in their research and development. A comparison of electrical efficiencies between fuel cell system and other conventional energy conversion systems [1] is given in Fig.1.

Fig. 1. A comparison of electrical system efficiencies between a fuel cell system and other

conventional energy conversion systems

Typical PEM fuel cell polarization curve

Fig.2. Overview of fuel cell types, parameters and possible 17217c224r application domains

Fuel cells are usually classified by the electrolyte employed in the cell. An exception to this classification is the DMFC (Direct Methanol Fuel Cell) that is a fuel cell in which methanol is directly fed to the anode. A second grouping is by their operating temperature: there are low temperature fuel cells - the Alkaline Fuel Cell (AFC), the Polymer Electrolyte Fuel Cell (PEMFC - that presents today the most interest of all types of fuell cells), the Direct Methanol Fuel Cell (DMFC), the Phosphoric Acid Fuel Cell (PAFC) - and high-temperature fuel cells that operates at 600-1000°C - the Molten Carbonate Fuel Cell (MCFC) and the Solid Oxide Fuel Cell (SOFC - also used in many applications).

An overview of fuel cell types, parameters and application domains [1] is given in fig.2. The main applications of fuel cells cover today stationary, mobile and portable fields.

2. BASIC FUEL CELL POWER SYSTEMS

Fig. 3 shows the basic fuel cell power systems for all the main applications that include the fuel cell - who needs to be controlled in flow rate, pressure, humidity, temperature etc -,                                                                                                                                                                                                                                                                                                                                                                               the power converters (DC-DC converters and/or DC-AC converters) that adapt the fuel cell to the load and the energy storage, such as a baterry or an supercapacitor, required by fuel cell's dynamic limitations.

Fig.3. Basic fuel cell power systems

Fig.4. Block diagram of fuel cell power system for stationary applications

Fig.5. Block diagram of fuel cell power system for mobile applications



Fig.6. Block diagram of fuel cell power system for portable applications

            Fig.4 represents the block diagram of a fuel cell power system for stationary applications, such as household applications. The main disadvantage of fuel cells is that they produce a low DC output

voltage with a wide range variation (fig.2) so an indirect DC/DC converter is connected in order to increase it. Multiple-stage power conversions including isolation are needed. A three phase inverter is placed at DC-DC converter's output in order to supply different types of loads. Fig. 5 represents the block diagram of a fuel cell power system for mobile applications such as hybrid vehicular applications. Besides the DC/DC converter and the traction inverter, the secondary battery guarantees the load leveling, assuring braking energy recovery and good performances in the transient operations. It also supplies with energy the air compressor, the hydrogen circulation pump and the cooling pump for inverter/motor etc. Fig.6 represents the block diagram of a fuel cell power system for portable applications such as laptops, cell phones or PDAs. It is similar to the block diagram in fig.4 excepting the inverter that is missing, taking account of the DC load. Isolation may be or may be not needed.

From all fig.4, 5 and 5 it results that all fuel cell systems typicaly include in their power electronics section a DC/DC converter for all applications and also a DC/AC inverter for stationary and mobile applications [2], [3], [4].

3. ADVANCED CONVERTERS FOR MOBILE APPLICATIONS

As it can be seen from the above analysis, the power inverter is the key component in the system to handle all power flow control. The inverter in FCHEVs has to output the requested power to the traction motor, capture excess power from the FC and absorb energy from regenerative braking.

There are typically two configurations available (fig.7 and fig.8) [5]. The FCHEV using the conventional PWM inverter (fig.7) must use a bi-directional DC/DC converter to control battery's state of charge because the modulation index is the inverter's only control freedom. Also, the conventional inverter is a buck (step-down) inverter, the output ac voltage being limited below the FC voltage. Because of the wide voltage range of the FC (fig.2), the conventional inverter imposes high stresses to the switching devices and motor and limits motor's constant power speed ratio.

Fig.7. Fuel cell power system with traditional PWM inverter supplied by a bidirectional DC/DC converter

The DC/DC boosted PWM inverter (fig.8) improve these stresses, at the price of higher cost and complexity. It is used to boost (step-up) the voltage from the FC, to a steady dc bus voltage and the inverter's output ac voltage is controlled by the modulation index. The system configuration using the DC/DC boosted inverter typically uses a bi-directional dc-dc converter to control battery's state of charge, too. Both configurations (fig.7 and fig.8) use an inverter bridge and at least one dc-dc converter, which increases the cost and system complexity and reduces the system reliability.

The recently presented Z-source inverter [6], [7] is suitable for many applications, including FCHEVs. The Z-source inverter is attractive for three main reasons. First, the traditional PWM inverter has only one control freedom, used to control the output ac voltage while the Z-source inverter has two independent control freedoms: shoot-through duty cycle and modulation index, providing the ability to produce any desired output ac voltage to the traction motor, regulate battery's state of charge and control FC output power (or voltage) simultaneously. Second, the Z-source inverter provides the same features of a dc-dc boosted inverter (i.e., buck/boost), yet its single stage is less complex and more cost effective. Third, the Z-source inverter has the benefit of enhanced reliability due to the fact that momentary shoot-through can no longer destroy the inverter (i.e., both devices of a phase leg can be on for a significant period of time).

a)

b)

Fig.8. Fuel cell power system with DC/DC boosted PWM inverter supplied by a bidirectional DC/DC converter: a) block diagram of DC/DC Boost converter; b) detail of DC/DC Boost converter

By replacing one of the capacitors in the Z-source network (LC) with a battery as shown in Fig. 9, the Z-source inverter can be used in FCHEVs. Traditional PWM inverter always requires an extra DC/DC converter to interface the battery in FCHEVs. The Z-source inverter eliminates the DC/DC converter and utilizes instead an exclusive Z-source (LC) network to link the main inverter circuit to the FC (or any dc power source). By substituting one of the capacitors in the Z-source with a battery and controlling the shoot through duty ratio and modulation index independently, one is able to control the FC power, output power, and battery's state of charge at the same time. These facts make the Z-source inverter highly desirable for use in FCHEVs, as the cost and complexity is greatly reduced when compared to traditional inverters.

Fig.9. Fuel cell power system with Z-source inverter supplied by a bidirectional DC/DC converter

The newly proposed Z-source inverter has the unique feature that it can boost the output voltage by introducing a shoot through operation mode, which is forbidden in traditional voltage source inverters. With this unique feature, the Z -source inverter provides a potential cheaper, simpler, single stage approach for fuel cell vehicles. Moreover, it highly enhances the reliability of the system because the inverter can handle momentary shoot through caused by electromagnetic interference (EMI) without interrupting the operation. It only slightly increase the passive component requirement.




It is note worthy that in practical cases, for dc-dc boosted PWM inverter, the associated cost and volume increase of extra heat sinking effort and gate drive for an extra switch is also significant. In addition, great reliability enhancement of the Z-source inverter is a very important advantage.

The comparison effiency results in fig.10 [7] shows that the Z-source inverter has lower average switching device power in low boost ratio range (1-2) in which most fuel cells reside. In cases when a low voltage fuel cell is used and a boost ratio much higher than 2 is needed, the dc-dc boosted PWM inverter is the best configuration. Z-source inverter also provides higher efficiencies in most operation ranges. From the above comparison, the Z-source inverter provides the highest efficiency in most regions of the power range of the inverter itself.

Fig. 10. Calculated efficiency of inverters

Fig. 11. Inverter efficiency calculated using Mitsubishi average loss simulation software

To verify the efficiency calculation of the traditional inverters, Mitsubishi average loss simulation software was used in fig.11. The efficiencies of the conventional PWM inverter and dc-dc boosted PWM inverter calculated from the software under the same operation conditions are shown in Fig. 11. As can be seen from Figs. 10 and 11, the calculated efficiencies and simulated efficiencies are very close, fact that confirms the conclusions about the efficiences of the three types of analysed inverters.

4. CONCLUSIONS

This article presents the most advanced and new converter suitable for many applications, including FCHEVs applications. The Z-source inverter is very promising for use in FCHEVs because of the following unique features and advantages:

1) less complex, and more cost effective than a dc-dc boosted inverter, while providing the same function (i.e., buck boost);

2) greater reliability, because shoot-through can no longer destroy the inverter;

3) no need for any dc-dc converters to control the battery's state of charge, or boost the dc bus voltage, because the Z-source inverter has two independent control freedoms.

Through Z-source inverter, power from the FC, power to the motor and battery's state of charge are controlled. It provides high efficiency, low cost and future developments in fuel cell power systems.

REFERENCES

[1] V. C. Regep, E. Mamut, "Stand For The Experimental Study Of Pem Fuel Cells", Rom. Journ. Phys., Vol. 51, Nos. 1-2, P. 41-48, Bucharest, 2006

[2] C. Liu, A. Ridenour, J.S. Lai, "Modeling and Control of a Novel Six-Leg Three-Phase High-Power Converter for Low Voltage Fuel Cell Applications",  IEEE Transactions on Power Electronics, Vol. 21, No. 5, pp. 1292-1300,  Sept. 2006

[3] H.J. Chiu, and Li-Wei Lin, "A Bidirectional DC-DC Converter for Fuel Cell Electric Vehicle Driving System",  IEEE Transactions on Power Electronics, Vol. 21, No. 4, pp. 950-958,  July 2006

[4] M. Tekin, D. Hissel, M. C.Péra J. M. Kauffmann, , "Energy-Management Strategy for Embedded Fuel-Cell Systems Using Fuzzy Logic",  IEEE Transactions on Industrial Electronics, Vol. 54, No. 1, pp. 595-603, Febr. 2007

[5] C. Liu, and J.S. Lai, "Low Frequency Current Ripple Reduction Technique With Active Control in a Fuel Cell Power System With Inverter Load", IEEE Transactions on Power Electronics, vol. 22, no. 4, pp. 1429-1436, July 2007

[6]  F. Z.  Peng, M. Shen, K. Holland, "Application of Z-Source Inverter for Traction Drive of Fuel Cell-Battery Hybrid Electric Vehicles",  IEEE Transactions on Power Electronics, Vol. 22, No. 3, pp. 1054-1061,  May 2007.

[7] M. Shen, A. Joseph, J. Wang, F. Z. Peng, D. J. Adams,  "Comparison of Traditional Inverters and Z-Source Inverter for Fuel Cell Vehicles", IEEE Transactions on Power Electronics, vol. 22, no. 4, pp. 1453-1463, July 2007.













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