FUEL CELLS ALEJANDRO AVENDAO

Transcription

1 FUEL CELLS ALEJANDRO AVENDAO 1

2 1) INTRODUCTION 3 2) BACKGROUND 3 Fuel Cell Basics 3 Fuel Cell types 4 A. Proton Exchange Membrane Fuel Cells (PEMFC) 4 B. Direct Methanol Fuel Cells (DMFC) 5 C. Phosphoric Acid Fuel Cells (PAFC) 6 D. Molten Carbonate Fuel Cells (MCFC) 6 E. Solid Oxide Fuel Cells (SOFC) 7 Fuel Cell System Components 8 A. System Configuration 8 B. Fuel Processor 8 C. Air Management 9 E. Thermal Management 9 F. Power Management 10 Fuel cell system characteristics 10 3) FUEL CELL SYSTEM APPLICATIONS IN DISTRIBUTED GENERATION 11 Stationary Power 11 PEMFC Internal Voltage and Voltage Drops 12 Double-Layer Charging Effect 12 Dynamic Model of PEMFC 13 DG Applications of the model 16 Hybrid FC systems 17 High-Temperature SOFC-Turbine Systems 18 PEM Fuel Cell-Wind Power Hybrid System 19 CONCLUSIONS 20 REFERENCES 21 2

3 1) Introduction Distributed generation (DG) is a small scale electric power source connected directly to the utility s distribution network or on the customer site of the meter, and it provides electric power at a site closer to customers than the central station generation. The capacity scale of DG ranges from several KW to 50 MW. DG provides a multitude of services to utilities and consumers, including standby generation, peak shaving capability, base-load generation, or cogeneration. For businesses, DG can reduce peak demand charges, reduce overall energy use, ensure greater power quality and reliability, and reduce emissions. For large utilities and power producers, DG can augment overall system reliability, avoid large investments in transmission system upgrades, reduce transmission losses, closely match capacity increases to demand growth, and open markets in remote or environmentally constrained areas. Various technologies are available for DG, including turbine generators, internal combustion engine/generators, micro-turbines, photovoltaic/solar panels, wind turbines, and fuel cells. The application of fuel cell technologies to advanced power generation systems portends the most significant advancement in energy efficiency, conservation, and environmental protection for the next decade. 2) Background Fuel Cell Basics Fuel Cells (FC) are widely recognized as one of the most promising technologies to meet the future power generation requirements. Since fuel cells directly convert fuel and an oxidant into electricity through an electrochemical process, they can achieve operating efficiencies approaching 60% nearly twice the efficiency of conventional internal combustion engines. Fuel cells produce very low levels of pollutant emissions (NO, SO, and CO). Fig.1 Fuel cell basics [1]. 3

4 Like a battery, a fuel cell consists of a pair of electrodes and an electrolyte. Unlike a battery, the species consumed during the electrochemical reactions are continuously replenished so that there is never a need to recharge the cell. The basic components of a fuel cell are illustrated in Fig.1. A fuel, usually hydrogen, is supplied to the fuel cell anode. At the anode, the fuel is oxidized, yielding electrons, which travel through the external circuit. At the cathode, the oxidant is reduced, consuming electrons from the external circuit. Ions travel through the electrolyte to balance the flow of electrons through the external circuit. The anode and cathode reactions and the composition and direction of flow of the mobile ion vary with the type of fuel cell. Fuel cells are generally characterized by the type of electrolyte. Presently, the most promising fuel cell types are: a) Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), which use a polymer membrane as the electrolyte b) Phosphoric acid fuel cells (PAFC), which use pure phosphoric acid as the electrolyte c) Molten carbonate fuel cells (MCFC), which use a molten mixture of lithium, sodium, and potassium carbonates as the electrolyte d) Solid oxide fuel cells (SOFC), which use a ceramic material as the electrolyte. Fig. 1 indicates the principle anode and cathode reactions and the mobile ion associated with the most common fuel cell types. For all cells except the DMFC, the net cell reaction is 1 + O2 H O (1) 2 H 2 2 Although these five major fuel cell types have similar structure and similar net chemical reactions, they are very different with respect to operating characteristics, materials of construction, and potential application. Fuel Cell types A. Proton Exchange Membrane Fuel Cells (PEMFC) A PEMFC has a solid polymer electrolyte made of a Teflon-like material, which is an excellent conductor of protons and an insulator of electrons. At the anode, with the help of a platinum catalyst, hydrogen atoms break into electrons and positive ions. The positive ions move through the membrane and are attracted to the cathode, while the electrons travel through the external load, producing a voltage across the load. Positive ions and electrons recombine at the cathode (again with the help of the catalyst). Steam is produced at both electrodes as a result of the chemical reactions. A schematic diagram of PEMFC, showing its overall chemical reaction, is given in Figure 2. Like other fuel cells, the PEMFC is very efficient. The efficiency for a PEMFC stack operating on hydrogen and pressurized air at typical operating current conditions is approximately 50%. The PEMFC also provides a very high power density. Automotive fuel cell systems based on PEMFC technology have demonstrated a power density as high as 1.35 kw/liter, which is comparable to that of the IC engine. 4

5 This power is produced while the cell operates at a relatively low temperature of 60 C 80 C (140 F 180 F). This low operating temperature permits the fuel cell to reach operating temperature quickly. The combination of high efficiency, high power density, and rapid start-up makes the PEMFC attractive as a replacement for conventional automobile engines. Fig. 2 A schematic diagram of PEMFC [2]. Unfortunately, the low operating temperature of the PEMFC leads to very slow chemical kinetics. Precious metal catalysts, typically platinum, must be used at the electrodes to facilitate the reactions. As recently as 10 years ago, the cost of the catalyst alone was as high as $184/kWe making the PEMFC too expensive for most applications. In recent years, dramatic advances in the design of the electrodes and the application of the catalyst have led to catalyst costs approaching the design goal of $3.50/kWe. However, further advances in technology and manufacturing are needed to reduce the cost of other cell components. B. Direct Methanol Fuel Cells (DMFC) Like the PEMFC, direct methanol fuel cells use a polymer membrane as the electrolyte. In the DMFC, however, the fuel is methanol, which is dissolved in liquid water and supplied to the anode. Since it is a liquid, methanol is easy to transport, and since the methanol is used directly in the stack, there is no need for a fuel processor. However, because the reaction rate for methanol on currently available catalysts is slow, DMFCs 5

6 have relatively low efficiencies and power densities. Furthermore, since methanol is soluble in the polymer membrane, it can cross over to the cathode where it reacts without producing electrical power, thus, further reducing efficiency. However, DMFCs can be competitive with batteries in terms of storage density. Presently, the most promising applications for DMFCs appear to be as replacements for batteries in small portable power applications where the simplicity of the DMFC system and the portability of the liquid methanol fuel outweigh the relatively low efficiency. C. Phosphoric Acid Fuel Cells (PAFC) Phosphoric acid fuel cells were the first fuel cells to be commercially available. The PAFC consists of porous carbon electrodes surrounding a porous matrix that retains the liquid phosphoric acid electrolyte. Except for the nature of the electrolyte, the PAFC structure resembles the PEMFC with porous carbon electrodes and carbon collector plates located on either side of the electrolyte-electrode assembly. A fluid such as air, water, or oil is circulated between the collector plates to cool the stack assembly. Phosphoric acid fuel cells operate with efficiencies that are comparable to PEMFCs but at power densities that are lower. The operating temperature of the PAFC is approximately 200 C (390 F). This temperature is hi gh enough to facilitate the recovery of heat produced within the stack for water and space heating in building applications. However, the operating temperature is not high enough to overcome the need for precious metal catalysts. D. Molten Carbonate Fuel Cells (MCFC) Molten carbonate fuel cells are typically designed for mid-size to large stationary (or shipboard) power applications. As illustrated in Fig. 3, the MCFC consists of nickel and nickel-oxide electrodes surrounding a porous substrate which retains the molten carbonate electrolyte. Fig. 3 MCFC [1]. 6

7 Collector plates and cell separator plates are typically fabricated from stainless steel, which can be formed less expensively than the carbon plates in the PEMFC and PAFC cells. Thermal energy produced within the cell stack is transferred to the reactant and product gases and a separate cooling system is not usually required. The MCFC operates at a very high temperature of approximately 650 C (1200 F). At this temperature, precious metal catalysts are not required for the fuel cell reactions. In addition, the heat available from the stack can be used to produce steam and hot water in building cogeneration applications. Furthermore, at this temperature, fuel gases other than hydrogen can be used by reforming the fuel within the cell stack in a process called internal reforming. For example, with the proper catalysts, carbon monoxide introduced into the anode compartment of the fuel cell will react with the water produced by the fuel cell reaction (1) to produce hydrogen and carbon dioxide through the water gas shift reaction CO + H 2 O CO2 + H 2O (2) The resulting hydrogen gas can then fuel the cell reaction (1). Internal reforming can be accomplished with carbon monoxide and simple hydrocarbon fuels such as methane, which is the major component of natural gas. This greatly simplifies the balance of plant equipment required to operate the fuel cell. Heavier hydrocarbons may still require external fuel processing. E. Solid Oxide Fuel Cells (SOFC) Solid oxide fuel cells operate at the highest temperatures, 800 C 1000 C (1500 F 1800 F), of all fuel cell systems. These high tempe ratures simplify system configuration by permitting internal reforming and also facilitates the development of cogeneration systems as well as hybrid power systems that use fuel cells as topping cycles for gas turbines and/or steam cycles. Since the electrolyte is solid, the cell can be formed in a variety of configurations. One of the most common cell configurations has a tubular geometry as illustrated in Figure 4. Fig. 4 SOFC [1]. 7

8 In this configuration, the cathode is a hollow tube constructed from lanthanum manganite. The electrolyte, an yttria stabilized zirconia is supported by the cathode. The anode, a nickel cermet surrounds the electrolyte. Fuel gas enters the cell from the outer surface of the tube and air enters the cell from the inner surface. Development efforts for SOFCs are focused on reducing manufacturing cost, improving system integration, and lowering the operating temperature to the range of 550 C 750 C (1020 F 1380 F). The lower operating temperature would still provide the advantages of internal reforming while reducing the material problems associated with very high temperature operation. Fuel Cell System Components A. System Configuration The basic features of a fuel cell system are illustrated in Fig. 5. As this figure indicates, a fuel cell system is composed of six basic subsystems: the fuel cell stack discussed in the preceding section, the fuel processor, air management, water management, thermal management, and power conditioning subsystems. The design of each subsystem must be integrated with the characteristics of the fuel cell stack to provide a complete system. Optimal integration of these subsystems is key to the development of cost effective fuel cell systems. Fig. 5 Fuel cell system schematic [1]. B. Fuel Processor Since most fuel cells use hydrogen as a fuel and most primary energy sources are hydrocarbons, a fuel processor is required to convert the source fuel to a hydrogen rich fuel stream. The complexity of the fuel processor depends on the type of fuel cell system and the composition of the source fuel. For low temperature fuel cells such as PEMFCs and PAFCs, the fuel processor is relatively complex and usually includes a desulfurizer, a steam reformer or partial oxidation reactor, shift converters, and a gas clean-up system to remove carbon monoxide from the anode gas stream. 8

9 In higher temperature fuel cells such as MCFCs and SOFCs, fuel processing for simple fuels such as methane may consist simply of desulfurizing and preheating the fuel stream before introducing it into the internally reforming anode compartment of the fuel cell stack. More complex fuels may require additional steps of clean-up and reforming before they can be used even by the high temperature cells. For all types of fuels, the higher operating temperatures associated with MCFC and SOFC systems provide better thermal integration of the fuel cell with the fuel processor. C. Air Management In addition to fuel, the fuel cell requires an oxidant, which is typically air. Air is provided to the fuel cell cathode at low pressure by a blower or at high pressure by an air compressor. The choice of whether to use low or high pressure air is a complicated one. Increasing the pressure of the air improves the kinetics of the electrochemical reactions and leads to higher power density and higher stack efficiency. Furthermore, in PEMFC stacks, increasing the air pressure reduces the capacity of the air for holding water and consequently reduces the humidification requirements. On the other hand, the power required to compress the air to a high pressure reduces the net available power from the fuel cell system. Some of this energy can be recovered by expanding the cathode exhaust through a turbine before exhausting it to the atmosphere. Nevertheless, the air compressor typically uses more power than any other auxiliary device in the system. Furthermore, while the fuel cell stack performance actually improves at low power, the performance of the air compressor is usually poor at very low loads. Currently, most fuel cell stack designs call for operating pressures in the range of 1 8 atm. To achieve high power densities and to improve water management, most automotive fuel cell systems based on PEMFC technology are operated at pressures of 2 3 atm. E. Thermal Management A fuel cell stack releases thermal energy at a rate that is roughly equivalent to the electrical power that it produces. This thermal energy can be used for a variety of purposes within the fuel cell system, transferred externally to meet the thermal needs of a particular application, or rejected to the surroundings. Low temperature fuel cell systems are cooled by either air or a circulating liquid. In some low temperature, low power 200 W systems, excess air flowing over the cathode is sufficient to transfer thermal energy from the cell. In larger low temperature systems, additional flow channels are provided within the cell stack and either air or a liquid coolant (typically deionized water) is circulated through the channels to remove thermal energy. If a liquid coolant is used, the stack will be more compact. Furthermore, with a liquid coolant, it is easier to transfer thermal energy for other purposes such as space heating or water heating in cogeneration applications. In high temperature systems such as the MCFC and SOFC, the fuel cell stack operates at such a high temperature that all of the thermal energy from the cell reaction can be transferred to the reactant gases without heating the exhaust beyond the operating temperature limit of the stack. With these systems, thermal energy is available at a high enough temperature to drive the reforming reaction either internal or external to the stack. Thermal energy from the stack exhaust can also be used to preheat the incoming air stream. Thermal energy that is not needed for reforming or air 9

10 preheating can be used to make steam or hot water for cogeneration in a heat recovery boiler. Proper integration of the fuel cell system is essential to insure that thermal energy available from the stack is used for the most appropriate application. F. Power Management The final component of the fuel cell system is the power management system. This system converts the electricity available from the fuel cell to a current and voltage that is suitable for a particular application and supplies power to the other auxiliary systems. Fuel cell stacks produce direct current at a voltage that varies with load. A switching power converter is used to match the voltage produced by the fuel cell to the needs of the application and to protect the fuel cell from overcurrent or undervoltage conditions. If the application requires alternating current, the electricity is processed through an inverter, which constructs single or three-phase waveforms as required by the application. If the application involves interconnection with the utility grid, then the power management system must also be able to synchronize the frequency of the fuel cell system power with the utility power and provide safety features to prevent the fuel cell system from feeding power back into the utility grid if the grid is off-line. Fuel cell system characteristics Fuel cell systems promise to provide a number of advantages when compared to conventional power systems. These advantages include modularity, high efficiency across a broad range of load conditions, and low environmental impact. These advantages coupled with projected cost reductions will make fuel cells attractive in a variety of applications. The major component of a fuel cell system, the fuel cell stack, is composed of individual fuel cells assembled in repetition. Thus, the fuel cell stack is modular and can be constructed in sizes ranging from a few watts to a megawatt or more. Other components of the fuel cell system, particularly the fuel processor, do not scale as well as the stack. However, even fuel cell systems incorporating fuel processors can be constructed to meet a variety of applications with power needs as small as 10 kw. Across the entire range of applicable sizes, fuel cell systems offer attractive electrical conversion efficiencies. Furthermore, the fuel cell system efficiency remains high even at off-design conditions. The efficiency for various fuel cell systems ranges from 40% 50% for simple systems in a broad range of sizes. Few small to medium-sized conventional systems can achieve efficiencies comparable to those provided by fuel cell systems at design conditions. Furthermore, no conventional system can maintain efficiencies comparable to fuel cell systems at part-load operation. More complex fuel cell systems can yield even higher efficiencies. For a combined system consisting of a pressurized SOFC with the exhaust gas driving a gas turbine, the overall electrical conversion efficiency can be as high as 60%. The combined gas turbine/steam cycle is the only conventional cycle that can approach, at least at design load, this level of efficiency. Since fuel cells can operate at high efficiency even in relatively small sizes, they are attractive in small-scale cogeneration applications such as buildings. By producing electricity and thermal energy for applications such as water heating or space heating, fuel cells can offer cogeneration efficiencies as high as 80% (from a first law of thermodynamics standpoint). 10

11 3) Fuel Cell System Applications in Distributed Generation Stationary Power In many respects, stationary power applications are even more favorable for fuel cell systems than transportation applications. In stationary applications, most systems will operate continuously so the time to reach operating temperature from a cold start is not typically an important criterion. Thus, higher temperature systems including MCFC and SOFC systems can be considered in addition to PAFC and PEMFC systems. In addition, the first cost barrier is lower in stationary applications with fuel cell systems becoming attractive for some applications at costs as high as $1500/kWe. Finally, in stationary applications, the fuel source is likely to be natural gas. Natural gas is primarily methane which is a light hydrocarbon and relatively easy to reform. In addition, a distribution infrastructure for natural gas is already in place. Promising stationary applications include premium power systems; cogeneration systems for residences, commercial buildings, and industrial facilities; and distributed power generation for utilities. FCs are good energy sources to provide reliable power at steady state; however, due to their slow internal electrochemical and thermodynamic characteristics, they cannot respond to electrical load transients as quickly as desired. They are connected to the power grid through power electronic interfacing devices (dc/dc converters and inverters), and it is possible to control their performance by controlling the interfacing devices. Modeling of FCs can therefore be helpful in evaluating their performance and for designing controllers. Among the above types of FCs, PEMFC is the most widely used type that is already commercially available for portable, residential, and vehicular applications. This paper addresses the modeling and control of PEMFC power systems for DG applications. Figure 6 shows a block diagram of an FC power plant interfaced with the utility grid via boost dc/dc converters and a three-phase pulsewidth modulation (PWM) inverter. Fig. 6 A block diagram of a grid-connected FC power system [2]. 11

12 An energy storage device, e.g., a battery bank or super capacitor, is used to improve the performance of the FC power system under transient disturbances such as motor starting. An LC bandpass filter is used to eliminate (or at least reduce) undesired harmonics. A short transmission line is used to connect the PEMFC power system to the utility grid. The rest of this article discusses the modeling of PEMFC and control of real and reactive power flow in a grid-connected PEMFC power system. PEMFC Internal Voltage and Voltage Drops The internal voltage of an FC is a nonlinear function of the FC current, internal temperature, and pressure of oxygen and hydrogen gasses. Like in batteries, FC output voltage is the difference between its internal voltage and its internal voltage drops, namely the activation, ohmic, and concentration voltage drops. These voltage drops are nonlinear functions of FC current, temperature, and chemical reactions. The activation voltage drop is due to sluggish electrode kinetics, the ohmic voltage drop is due to the resistance to the flow of hydrogen electrons and positive ions, and the concentration voltage drop is due to the inability to move reactants and products fast enough through the electrolyte to and from the electrochemical reaction site. Activation voltage drop is dominant at low FC currents, concentration voltage drop is dominant at high FC currents, and the ohmic voltage drop (while occurring at all current levels) is more pronounced in the linear portion of the FC voltagecurrent (V-I) characteristic. Double-Layer Charging Effect The hydrogen positive ions and electrons traveling from the anode to the cathode (through the membrane and the load, respectively) are attracted to each other at the surface of the cathode. These two charged layers of opposite polarity are formed across the boundary between the porous cathode and the membrane. The layers, known as electrochemical doublelayer, can store electrical energy and behave like a supercapacitor. The equivalent electrical circuit of FC considering this effect is shown in Figure 7. Fig. 7 The equivalent electrical circuit of an FC, considering the double-layer charging effect inside the FC [2]. 12

13 In this circuit, E is the internally generated voltage of the FC, and C is the equivalent capacitor due to the double-layer charging effect. Since the electrodes of a PEMFC are porous, the capacitance C is very large for PEMFC and can be in the order of several Farads. The nonlinear resistances (Ract, Rconc and Rohmic ) represent the nonlinear voltage drops discussed previously. Due to the net heat generated by the chemical reaction inside FCs, their internal temperature may rise or fall. This temperature rise (or drop), which can be obtained as a function of time using the energy balance phenomena inside the FC, affects FCs output voltage and power delivering capability. Dynamic Model of PEMFC Figure 8 shows a block diagram for PEMFC, based on which a MATLAB/Simulink dynamic simulation model is developed. The FC output voltage is a function of the internal temperature of FC, its voltage drops, and the load current demand from the FC. The input quantities to the model are anode and cathode pressures, initial temperature of the FC, and room temperature. At any given load current and time, the internal temperature T is determined, and both the load current and temperature are fed back to different blocks (Figure 8) that take part in the calculation of the FC output voltage. Fig. 8 A diagram for building a dynamic model of PEMFC [2]. Steady-state and dynamic responses of the PEMFC model were obtained and validated in the laboratory for a 500-W Avista Labs SR-12 PEMFC stack. Figure 9 shows the experimental setup. The programmable electronic load was used as a current injection source to simulate different loads. The output voltage, current, and internal temperature of the FC are measured by voltage and current transducers and a thermocouple and transferred to a 12-b data acquisition card in a PC. Figures 10 and 11 show a comparison of the V-I and power-current (P-I) characteristics of the PEMFC stack, obtained both experimentally and from a dynamic model built in MATLAB/Simulink. These figures show very good agreement of the simulation results with the experimental data. The upper and lower ranges of the measured data give the upper and lower ranges of high-frequency ripples existing in the data. Figure 11 also shows that the FC 13

14 peak output power occurs near its rated output, beyond which the FC goes into the concentration zone. In this region, the FC output power will decrease with increasing load current due to a sharp decrease in the FC terminal voltage. The dynamic property of PEMFC depends mainly on the following three aspects: doublelayer charging effects, fuel and oxidant flow delays, and thermodynamic characteristics inside the FCs. Figure 12 shows a sample comparison of the experimental data and the transient response of the dynamic model. Simulation results agree well with the measured data. Fig. 9 Experimental setup for the SR-12 PEMFC stack [2]. Fig. 10 V-I characteristics of SR-12 and the model [2]. 14

15 Fig. 11 P-I characteristics of SR-12 and the model [2]. Fig. 12 The transient responses of SR-12 and the model [2]. 15

16 DG Applications of the model The validated 500-W Simulink model was used to build a 480-kW PEMFC power plant, shown in Figure 6. The power plant consists of 10 FC units connected in parallel, where each unit has 8 (series) 12 (parallel) 500-W PEMFC stacks. Boost converters adapt the output voltage of the FC units to the dc bus voltage. The dc bus voltage is determined mainly by the inverter output voltage and the voltage drop across the LC filter. The boost dc/dc converter adapts the FC output voltage to the dc bus voltage, and the voltage controller helps regulate the output voltage within a ±5% tolerance band under normal operation. Conventional PI controllers are used for the boost dc/dc converters to achieve the aforementioned goal. In practice, a load sharing controller can be applied on the converters, which connect the FC units to the dc bus, to achieve a uniform load distribution among them. Fig. 13 A block diagram of the control system for the inverter [2]. A two-loop (voltage, current) control scheme is used for the inverter (Figure 13) to control the real and reactive power delivered from the FC power system to the grid. The controllers make use of the transformation from stationary (abc) reference frame to synchronously rotating (dq) reference frame and vice versa. These transformations transform the sinusoidal three-phase voltage and current from a stationary (but timevarying) reference frame to a synchronously rotating (but constant) reference frame and vice versa. 16

17 Hybrid FC systems A hybrid power system consists of a combination of two or more power generation technologies to make best use of their operating characteristics and to obtain efficiencies higher than that could be obtained from a single power source. Hybrid fuel-cell systems are power generation systems in which a high-temperature fuel cell is combined with another power generation technology. The resulting system exhibits a synergism in which the combination has far greater efficiency than could be provided by either system operating alone. The efficiencies across a broad power range for various power generation technologies are shown in Fig. 14. As an example, combining SOFC or molten carbonate fuel cell with the gas turbine would increase the overall cycle efficiency while reducing per-kilowatt emissions. In some systems, combining fuel cells with wind or photovoltaic systems would extend the duration of the available power, which is of significance, rather than the overall efficiency. This type of system is used as a backup power or as an energy storage system. Getting higher efficiencies combined with low emissions, hybrid systems are likely to be the choice for the next generation of advanced power generation systems. These systems are not only used for stationary power generation, but also find application in transportation systems. Fig. 14 Efficiencies of various power generation technologies [3]. 17

18 High-Temperature SOFC-Turbine Systems Successful development and commercialization of fuel cell/turbine hybrid power generation will allow the following: Extremely high efficiency compared to other fossil fuel systems Ultra low emissions without additional cleanup Siting flexibility with environmentally friendly energy systems Fuel flexibility. Combination of a high-temperature fuel cell with a turbine/microturbine has several important ramifications to the energy and transportation industry. The SOFC systems are being developed in the range of 5 kw for automotive applications to several megawatts for power generation applications. The microturbines are being developed from 30 kw to 30 MW and the gas turbines power is in the range of MW. Fig. 15 SOFC-gas turbine hybrid system [3]. A fuel cell/gas turbine hybrid system of 500-kW power is depicted in Fig. 15. The fuel is first reformed to obtain the hydrogen rich reformate and fed to the anode of the SOFC. The ambient air is drawn using a compressor and pressurized to about kpa (3 4 atm). The compressed air is heated using the exhaust of the gas turbine with a heat exchanger and fed to the cathode. The cathode exhaust from the SOFC and the unused fuel from the anode are burned in a combustor to increase the temperature of the exhaust to about 1000 C to meet the requirements of the turbine. The heat and the pressure difference drives the downstream turbine to generate more power without using additional fuel. The turbine exhaust after heating the compressor exit air is also used for 18

19 heating the fuel that is going into the reformer. The turbine drives the generator and produces a three-phase ac output. This ac power is first converted to the dc power and then combined with the dc output from the fuel cell using the power conditioning system. This dc is converted to the ac output before feeding to the utility. For stationary power generation applications, generally, natural gas is used as the fuel. PEM Fuel Cell-Wind Power Hybrid System Recently, there has been a lot of emphasis on the electric power generation using wind energy. Wind turbines are being used not only for grid connection but also as standalone power generation systems. Wind power presents some challenges in producing continuous electric power. A significant problem is the intermittent nature of the wind, and the wind power generated depends on wind speed. Combining the wind power generation system with a fuel-cell system would solve some of the problems associated with wind power. The grid-connected wind hydrogen system provides off-peak hydrogen production and low-cost electricity. In Fig. 16 is shown a hybrid system based on wind power and PEM fuel cell. The wind power is used for generating hydrogen using the electrolysis of water and is stored in cylinders at a certain pressure. This hydrogen is used as the fuel to the fuel-cell stack. The stored hydrogen can also be used to fuel the fuel cell-vehicles. Fig 16. Wind fuel cell hybrid power system [3]. 19

20 The hybrid system could be configured in several ways: The wind power could be used to supply the power to the balance of the plant of the fuel-cell system, particularly during startup of the system, and the excess power could be used to supplement the power from the fuel cell. This is a fuel-cell dominant system and the wind generator supplements the fuel-cell power. Hydrogen is generated using electrolysis and stored during the peak power availability from the wind power generation system. The stored hydrogen is used for generating power using the fuel cell during the low output power operation of the wind unit. Electrolyzers can be used to reduce/eliminate surplus wind power generation. Fuel-cell power is generated only during daily peak load period to firm up the wind generation. This is a wind power dominant system and the fuel cell supplements the wind power. Conclusions Current worldwide electric power production is based on a centralized, grid-dependent network structure. This system has several disadvantages such as high emissions, transmission losses, long lead times for plant construction, and large and long term financing requirements. Distributed generation is an alternative that is gathering momentum, and modern technologies, such as fuel cells, are likely to play an increasing role in meeting ever-increasing power demands. Fuel cells have many advantages over conventional power generating equipment: High efficiency Low emissions Siting flexibility High reliability Low maintenance Modularity Multi-fuel capability. Because of their efficiency and environmental advantages, fuel cell technologies are viewed as an attractive 21st century solution to energy problems. Some negative aspects of fuel cell power include short operating life, high equipment costs, and lack of field experience. Despite this, fuel cells have a wide range of applications that can allow them to succeed in several markets. 20

21 References [1] Fuel Cell Systems: Efficient, Flexible Energy Conversion for the 21st Century Michael W. Ellis, Michael R. Von Spakovsky, And Douglas J. Nelson [2] Fuel Cells: Promising Devices for Distributed Generation M.Hashem Nehrir, Caisheng Wang and Steven R. Shaw IEEE Power and Energy Magazine [3] Hybrid Fuel-Cell Strategies for Clean Power Generation Kaushik Rajashekara IEEE Transactions On Industry Applications, Vol. 41, No. 3, May/June 2005 [4] Fuel Cells for Distributed Generation A Technology and Marketing Summary March