Hybrid Fuel Cell Strategies for Clean Power Generation
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projectsofme
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09-10-2010, 01:02 PM



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Hybrid Fuel Cell Strategies for Clean Power
Generation




ABSTRACT:
A hybrid 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. Since fuel cells directly convert fuel and an oxidant into electricity through an electrochemical process, they produce very low emissions and have higher operating efficiencies. Hence combining fuel cells with other sources, the efficiency of the combined system can be further increased. In this paper, different types of fuel cell hybrid systems and their applications are presented. They are classified based on the need for improving the efficiency or extending the duration of the available power to the load as a back-up power.
INTRODUCTION
Fuel cells 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 (NOx, SOx, and CO2). There are several types of fuel cells, distinguished by the type of electrolyte material used. Among these, proton exchange membrane (PEM) fuel cell and the solid oxide fuel cell (SOFC) are being considered for automotive and on-site power generation applications. PEM fuel cells operate at low temperatures (less than 100 degrees Celsius), making them temperature-compatible with many of today's automotive systems and also allowing faster start-up. However, due to a relatively small temperature gradient to the ambient atmosphere the waste heat produced is low-grade and requires large heat exchangers. Solid oxide fuel cells operate at extremely high temperatures of the order of 700 to 1,000 degrees Celsius. As a result, they can tolerate relatively impure fuels, such as those obtained from the gasification of coal. Waste heat is high-grade, allowing for smaller heat exchangers and the possibility of co-generation to produce additional power. The power densities of both PEM and SOFC systems are of the order of about 500 mW/ cm2 under typical operating conditions. The peak power densities under idealized conditions have been reported to be greater than 1000 mW/cm2. The reformer system for SOFC is less complex than PEM reformers. This is because SOFC can use carbon monoxide along with hydrogen as fuel. In addition, SOFC demonstrates high tolerance to fuel impurities such as natural gas. Also, the operating temperature of the reformer and the stack are compatible in SOFC systems, whereas in PEM systems the stack operating temperature is about 80 – 100 C and that of the reformer is about 900 – 1000 C. The water management is not a concern in SOFC because the electrolyte is solid-state and does not require hydration. The by-product is steam rather than liquid water, which must be drained away in PEM systems. SOFC does not need precious metal catalysts. The relatively simple design (because of the solid electrolyte and fuel versatility), combined with the significant time required to reach operating temperature and to respond to changes in electricity demand, make SOFC suitable for large to very large stationary power applications. The start-up time for the SOFC is of the order of 20 to 30 minutes, where as the PEM system could be started in less than a minute. Hence the SOFC is not suitable for propulsion applications. However, as an Auxiliary Power Unit (APU), the starting time of SOFC is not a major issue. 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 efficiency across a broad power range for various power generation technologies are shown in Fig. 1. As can be seen, combining fuel cell with a gas turbine increases overall cycle efficiency while reducing per kilowatt emissions. In addition, the work is also going on to combine fuel cells with wind power and solar power generation for back–up power generation and energy storage. Getting higher efficiencies combined with low emissions, hybrid systems are likely to be the choice for next generation of advanced power generation systems. These systems, not only are used for stationary power generation, but also found application in Transportation systems.

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24-09-2012, 02:55 PM

Hybrid Fuel Cell Strategies for Clean Power Generation


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Abstract:

A hybrid 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. Since fuel cells directly convert fuel and an
oxidant into electricity through an electrochemical process,
they produce very low emissions and have higher
operating efficiencies. Hence combining fuel cells with
other sources, the efficiency of the combined system can be
further increased. In this paper, different types of fuel cell
hybrid systems and their applications are presented. They
are classified based on the need for improving the
efficiency or extending the duration of the available power
to the load as a back-up power.

INTRODUCTION

Fuel cells 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
(NOx, SOx, and CO2). There are several types of fuel cells,
distinguished by the type of electrolyte material used. Among
these, proton exchange membrane (PEM) fuel cell and the
solid oxide fuel cell (SOFC) are being considered for
automotive and on-site power generation applications.
PEM fuel cells operate at low temperatures (less than 100
degrees Celsius), making them temperature-compatible with
many of today's automotive systems and also allowing faster
start-up. However, due to a relatively small temperature
gradient to the ambient atmosphere the waste heat produced is
low-grade and requires large heat exchangers. Solid oxide
fuel cells operate at extremely high temperatures of the order
of 700 to 1,000 degrees Celsius. As a result, they can tolerate
relatively impure fuels, such as those obtained from the
gasification of coal. Waste heat is high-grade, allowing for
smaller heat exchangers and the possibility of co-generation to
produce additional power. The power densities of both PEM
and SOFC systems are of the order of about 500 mW/ cm2
under typical operating conditions.

IGH TEMPERATURE SOFC - TURBINE SYSTEM

The fuel cell- gas turbine hybrid systems are used in stationary
power generation applications [2]. They are also being
investigated for use as APU in commercial airplanes to provide
the power to all the electrical loads, and in railroad vehicles to
provide the power to all the accessory loads [3]. Since the
temperature of the exhaust air of SOFC is about 850C, the heat
and the pressure difference could be used to drive a downstream
turbine to generate more power without using additional heat.
Combination of a high temperature fuel cell with a
turbine/microturbine has several important ramifications to the
energy industry.

PEM FUEL CELL – WIND POWER HYBRID SYSTEM

Recently there is 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 stand alone power
generation system. Wind power present 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 the wind power. The wind power
is used to produce hydrogen by electrolysis of water. The
hydrogen produced is stored and can be as fuel for fuel cell
vehicles. It is possible to combine the wind power and fuel
cell as hybrid power system for power generation applications
(Type 2 hybrid system) as shown in Figure 5.

PEM FUEL CELL – SOLAR POWER HYBRID SYSTEM

Similar to the wind power, a solar- hydrogen hybrid system
produces hydrogen, stores it and then converts its energy to
electricity for further use as shown in figures 7 and 8. Fuel
cell - Solar energy hybrid system is particularly useful in
spaceship applications. During daytime, the photovoltaic cells
convert the solar energy to electricity that is directly used for
propelling the spaceship.

CONCLUSIONS

In this paper, the system architectures and the advantages of
some of the hybrid fuel cell systems are discussed. These
systems can be used for stationary power generation or for
transportation applications. For example, SOFC-gas turbine
hybrid systems could be used as APUs in airplanes or in trains
or ships. The key parameters to be optimized depend on the
applications. In addition to the systems described in this paper,
there are possibilities of combining fuel cell power with other
power sources, or two different types of fuel cells, to take
advantage of the best characteristics of the individual power
sources for a given application.
The major hurdle to the commercialization of the technology
is economics. The costs of the major components must be
significantly reduced. With the advancement in fuel cell,
microturbine, and Stirling engine technologies, there will be
increasing market potential for cogeneration and distribution
applications throughout the world
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