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Fundamentals of Biochemistry 4th Edition - Solutions, bio
Fabulous Creatures Mythical Monsters and Animal Power Symbols-A Handbook, Angielski
Frater Albertus - Alchemist's Handbook, Wisdom Ancient
Fenaroli's Handbook of Flavor Ingredients, INNE
Fuel injection ASY, Samochodowe, Zeszyty serwisowe fabia
Fenomen+Davida+Icke, HISTORIA MAJÓW
Fuel Cell Handbook (sixth edition, ogniwa paliwowe, Fuel.Cell.Handbook.6.edición.todoquimica.net
[ Pobierz całość w formacie PDF ]73.
Siemens Westinghouse Power Corporation, “A High Efficiency PSOFC/ATS-Gas Turbine
Power System,” Final Report for U.S. Department of Energy, February 2001
74.
“Switchmode: A design guide for switching power supply circuits & components,” Motorola
publications, Ref: SG79/D, REV5, 1993
75.
K. Rajashekara, “Propulsion system strategies for fuel cell vehicles,” Fuel cell power for
transportation 2000 conference, SAE 2000 World congress, March 2000, Ref: 2000-01-0369
76.
T. Matsumoto, et al, “Development of fuel cell hybrid vehicle,” Fuel cell power for
transportation 2002 conference, SAE 2002 World congress, March 2000, Ref: 2002-01-0096
8.4
System Optimization
The design and optimization of a fuel cell power system is very complex because of the number of
required systems, components, and functions. Many possible design options and trade-offs affect
unit capital cost, operating cost, efficiency, parasitic power consumption, complexity, reliability,
availability, fuel cell life, and operational flexibility. Although a detailed discussion of fuel cell
optimization and integration is not within the scope of this section, a few of the most common
system optimization areas are examined.
From Figure 8-53, it can be seen that the fuel cell itself has many trade-off options. A fundamental
trade-off is determining where along the current density voltage curve the cell should operate. As
the operating point moves up in voltage by moving (left) to a lower current density, the system
becomes more efficient but requires a greater fuel cell area to produce the same amount of power.
That is, by moving up the voltage current density line, the system will experience lower operating
costs at the expense of higher capital costs. Many other parameters can be varied simultaneously
to achieve the desired operating point. Some of the significant fuel cell parameters that can be
varied are pressure, temperature, fuel composition and utilization, and oxidant composition and
utilization. The system design team has a fair amount of freedom to manipulate design parameters
until the best combination of variables is found.
8.4.1
Pressure
Fuel cell pressurization is typical of many optimization issues, in that there are many interrelated
factors that can complicate the question of whether to pressurize the fuel cell. Pressurization
improves process performance at the cost of providing the pressurization. Fundamentally, the
question of pressurization is a trade-off between the improved performance (and/or reduced cell
area) and the reduced piping volume, insulation, and heat loss compared to the increased parasitic
load and capital cost of the compressor and pressure-rated equipment. However, other factors can
further complicate the issue. To address this issue in more detail, pressurization for an MCFC
system will be examined.
8-93
8-
Figure 8-53 Optimization Flexibility in a Fuel Cell Power System
In an MCFC power system, increased pressure can result in increased cathode corrosion. Cathode
corrosion is related to the acidity of the cell, which increases with the partial pressure of CO
2
, and
therefore with the cell pressure. Such corrosion is typified by cathode dissolution and nickel
precipitation, which can ultimately result in a shorted cell, causing cell failure (1). Thus, the
chosen pressure of the MCFC has a direct link to the cell life, economics, and commercial
viability.
Increasing the pressure in a MCFC system can also increase the likelihood of soot formation and
decrease the extent of methane reforming. Both are undesirable. Furthermore, the effect of
contaminants on the cell and their removal from a pressurized MCFC system have not been
quantified. The increased pressure also will challenge the fuel cell seals (1).
The selection of a specific fuel cell pressure will affect numerous design parameters and
considerations such as the current collector width, gas flow pattern, pressure vessel size, pipe and
insulation size, blower size and design, compressor auxiliary load, and the selection of a bottoming
cycle and its operating conditions.
These issues do not eliminate the possibility of a pressurized MCFC system, but they do favor the
selection of more moderate pressures. For external reforming systems sized near 1 MW, the
current practice is a pressurization of 3 atmospheres.
The performance of an internal reforming MCFC also would benefit from pressurization, but
unfortunately, the increase is accompanied by other problems. One problem that would need to be
overcome is the increased potential for poisoning the internal reforming catalyst resulting from the
8-94
increase in sulfur partial pressure. The current practice for internal reforming systems up to 3 MW
is atmospheric operation.
Pressurization of an SOFC yields a smaller gain in fuel cell performance than either the MCFC or
PAFC. For example, based on the pressure relationships presented earlier, changing the pressure
from one to ten atmospheres would change the cell voltage by ~150, ~80, and ~60 mV for the
PAFC, MCFC, and SOFC, respectively. In addition to the cell performance improvement,
pressurization of SOFC systems allows the thermal energy leaving the SOFC to be recovered in a
gas turbine, or gas turbine combined cycle, instead of just a steam bottoming cycle. Siemens
Westinghouse is investigating the possibilities associated with pressurizing the SOFC for cycles as
small as 1 to 5 MW.
Large plants benefit the most from pressurization, because of the economy of scale on equipment
such as compressors, turbines, and pressure vessels. Pressurizing small systems is not practical, as
the cost of the associated equipment outweighs the performance gains.
Pressurization in operating PAFC systems demonstrates the economy of scale at work. The
IFC 200 kWe and the Fuji Electric 500 kWe PAFC offerings have been designed for atmospheric
operation, while larger units operate at pressure. The 11 MWe plant at the Goi Thermal Power
Station operated at a pressure of 8.2 atmospheres (2), while a 5 MWe PAFC unit (NEDO /
PAFCTRA) operates at slightly less than 6 atmospheres (3). NEDO has three 1 MWe plants, two
of which are pressurized while one is atmospheric (3).
Although it is impossible to generalize at what size a plant would benefit by pressurization, when
plants increase in size to approximately 1 MW and larger, the question of pressurization should be
evaluated.
8.4.2
Temperature
Although the open circuit voltage decreases with increasing temperature, the performance at
operating current densities increases with increasing temperature due to reduced mass transfer
polarizations and ohmic losses. The increased temperature also yields higher quality rejected heat.
An additional benefit to an increased temperature in the PAFC is an increased tolerance to CO
levels, a catalyst poison. The temperatures at which the various fuel cells can operate are,
however, limited by material constraints. The PAFC and MCFC are both limited by life shortening
corrosion at higher temperatures. The SOFC has material property limitations. Again, the fuel cell
and system designers should evaluate what compromise will work best to meet their particular
requirements.
The PAFC is limited to temperatures in the neighborhood of 200ºC (390ºF) before corrosion and
lifetime loss become significant. The MCFC is limited to a cell average temperature of
approximately 650ºC (1200ºF) for similar reasons. Corrosion becomes significant in an MCFC
when local temperatures exceed 700ºC (1290ºF). With a cell temperature rise on the order of
100ºC (180ºF), an average MCFC temperature of 650ºC (1200ºF) will provide the longest life,
highest performance compromise. In fact, one reference (4) cites "the future target of the operating
temperature must be 650
°
C
+3
0
°
C (1290
°
F
+
55
°
F)."
8-95
The high operating temperature of the SOFC puts numerous requirements (phase and conductivity
stability, chemical compatibility, and thermal expansion) on material selection and
development (5). Many of these problems could be alleviated with lower operating temperatures.
However, a high temperature of approximately 1000
C (1830ºF), i.e., the present operating
temperature, is required in order to have sufficiently high ionic conductivities with the existing
materials and configurations (5).
°
8.4.3
Utilization
Both fuel and oxidant utilizations
51
involve trade-offs with respect to the optimum utilization for a
given system. High utilizations are considered to be desirable (particularly in smaller systems)
because they minimize the required fuel and oxidant flow, for a minimum fuel cost and
compressor/blower load and size. However, utilizations that are pushed too high result in
significant voltage drops. One study (6) cites that low utilizations can be advantageous in large
fuel cell power cycles with efficient bottoming cycles because the low utilization improves the
performance of the fuel cell and makes more heat available to the bottoming cycle. Like almost all
design parameters, the selection of optimum utilization requires an engineering trade-off that
considers the specifics of each case.
Fuel Utilization:
High fuel utilization is desirable in small power systems, because in such
systems the fuel cell is usually the sole power source. However, because the complete utilization
of the fuel is not practical, except for pure H
2
fuel, and other requirements for fuel exist, the
selection of utilization represents a balance between other fuel/heat requirements and the impact of
utilization on overall performance.
Natural gas systems with endothermic steam reformers often make use of the residual fuel from the
anode in a reformer burner. Alternatively, the residual fuel could be combusted prior to a gas
expander to boost performance. In an MCFC system, the residual fuel often is combusted to
maximize the supply of CO
2
to the cathode while at the same time providing air preheating. In an
SOFC system, the residual fuel often is combusted to provide high-temperature air preheating.
The designer has the ability to increase the overall utilization of fuel (or the oxidant) by recycling a
portion of the spent stream back to the inlet. This increases the overall utilization while
maintaining a lower per pass utilization of reactants within the fuel cell to ensure good cell
performance. The disadvantage of recycling is the increased auxiliary power and capital cost of
the high temperature recycle fan or blower.
One study by Minkov, et al. (6) suggests that low fuel and oxidant utilizations yield the lowest
COE in large fuel cell power systems. By varying the fuel cell utilization, the electric power
generation split between the fuel cell, steam turbine, and gas turbine are changed. The low fuel
utilization decreases the percentage of power from the fuel cell while increasing the fuel cell
performance. The increased power output from the gas turbine and steam turbine also results in
their improved performance and economy of scale. The specific analysis results depend upon the
assumed stack costs. The optimal power production split between the fuel cell and the gas and
steam turbines is approximately 35%, 47%, and 17% for a 575 MW MCFC power plant. The
51
. Utilization - the amount of gases that are reacted within the fuel cell compared to that supplied.
8-96
associated fuel utilization is a relatively low 55%. It remains to be seen whether this trend will
continue to hold for the improved cells that have been developed since this 1988 report was issued.
Oxidant Utilization:
In addition to the obvious trade-off between cell performance and
compressor or blower auxiliary power, oxidant flow and utilization in the cell often are determined
by other design objectives. For example, in the MCFC and SOFC cells, the oxidant flow is
determined by the required cooling. This tends to yield oxidant utilizations that are fairly low
(~25%). In a water-cooled PAFC, the oxidant utilization based on cell performance and a
minimized auxiliary load and capital cost is in the range of 50 to 70%.
8.4.4
Heat Recovery
Although fuel cells are not heat engines, heat is still produced and must be removed. Depending
upon the size of the system, the temperature of the available heat, and the requirements of the
particular site, this thermal energy can be either rejected, used to produce steam or hot water, or
converted to electricity via a gas turbine or steam bottoming cycle or some combination thereof.
Cogeneration:
When small quantities of heat and/or low temperatures typify the waste heat, the
heat is either rejected or used to produce hot water or low-pressure steam. For example, in a PAFC
where the fuel cell operates at approximately 205
F), the highest pressure steam that could
be produced would be something less than 14 atmospheres (205 psia). This is obviously not
practical for a steam turbine bottoming cycle, regardless of the quantity of heat available. At the
other end of the spectrum is the TSOFC, which operates at ~1000
°
C (400
°
°
C (~1800
°
F) and often has a cell
exhaust temperature of approximately 815
F) after air preheating. Gas temperatures of
this level are capable of producing steam temperatures in excess of 540
°
C (1500
°
F), which makes
it more than suitable for a steam bottoming cycle. However, even in an SOFC power system, if the
quantity of waste heat is relatively small, the most that would be done with the heat would be to
make steam or hot water. In a study performed by Siemens Westinghouse of 50 to 2000 kW
TSOFC systems, the waste heat was simply used to generate 8 atmosphere (100 psig) steam (7).
°
C (1000
°
Bottoming Cycle Options:
Whenever significant quantities of high-temperature rejected heat are
available, a bottoming cycle can add significantly to the overall electric generation efficiency.
Should the heat be contained within a high-pressure gas stream, then a gas turbine potentially
followed by a heat recovery steam generator and steam turbine should be considered. If the hot gas
stream is at low pressure, then a steam bottoming cycle is logical.
If a steam bottoming cycle is appropriate, many design decisions need to be made, including the
selection of the turbine cycle (reheat or non-reheat) and the operating conditions. Usually, steam
turbines below 100 MW are non-reheat, while turbines above 150 MW are reheat turbines. This
generalization is subject to a few exceptions. In fact, a small (83 MW) modern reheat steam
turbine went into operation (June 1990) as a part of a gas turbine combined cycle repowering
project (8).
8-97
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